Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 142 - Part A IMPACT OF ZEOLITES AND OTHER POROUS MATERIALS ON THE NEW TECHNOLOGIES AT THE BEGINNING OF THE NEW MILLENNIUM

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S t u d i e s in S u r f a c e S c i e n c e a n d C a t a l y s i s Advisory Editors: B. Delmon and J.To Yates

Vol. 142

IMPACT OF ZEOLITES OUS MATERIALS ON THE NEW TECHNOLOGIES AT THE BEGINNING OF THE NEW MILLENNIUM PART A Proceedings of the 2 ~ International FEZA (Federation of the European Zeolite Associations) Conference Taormina, Italy, September 1-5, 2002

Organized by the ITALIAN ZEOLITE ASSOCIA TION under the auspices of the Federation of the European Zeolite Associations Edited by R. A i e l l o , G. G i o r d a n o a n d F. T e s t a

Dipartimento di Ingegnefia Chimica e dei Materiali, Universitd della Calabria Arcavacata di Rende, Italy

0

2002

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9 2002 Elsevier Science B.V, All rights reserved.

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PREFACE

It is a pleasure to present the Proceedings of the Second Conference of the Federation of the European Zeolite Associations where are collected the contributions of internationally renowned researchers in the field of the Science and Technology of micro and mesoporous materials. Aim of the Conference, organized by the Italian Zeolite Association, is to create an international forum where researchers from academia as well as from industry can bring and discuss ideas finalized to evaluate the impact of zeolites and other porous materials on the new technologies at the beginning of the new millennium. Among the others, in fact, the technologies for the production of chemicals, which will become always more important for maintaining our standard of life and our environment safe, will need substantial innovation and we hope that this book will be a source of new ideas for further fundamental and applied research work not only for the participants of the Conference but also for the whole scientific community. These proceedings report the oral and poster communications presented during the FEZA Conference, subdivided into 8 thematic sessions. The volume contains also the full text of the three plenary and two keynote lectures. The scientific contributions, coming from 35 countries both European and extra European, testify of the great vitality of the zeolite science in its various branches, from those always represented at the zeolite conferences (synthesis, catalysis, ion exchange and modification, natural zeolites... ) to the new emerging areas (mesoporous materials, environmental sciences, computational chemistry, advanced materials... ) and, at the same time, of the blend of multidisciplinary knowledge involved in this science in continuous evolution. The editors would like to acknowledge the dedication of the members of the Paper Selection Committee: A. Alberti, G. Centi, M. Derewinski, F. Fajula and J. B.Nagy, and express their gratitude to all the referees who contributed to the selection of the Conference papers. A special and grateful acknowledgment has to be addressed to Dr. A. Katovic (Treasurer) for her great involvement all along the Conference organization. Rosario Aiello Girolamo Giordano Flaviano Testa Editors

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vii The Federation of the European Zeolite Associations (FEZA) was constituted in Szombathely (Hungary) on 12 July 1995 by the representatives of the National Zeolite Associations from France, Germany, Hungary, Italy, the Netherlands and UK, plus Bulgaria and Spain, which were going at that time to constitute the respective national associations. The Constitution of the FEZA was approved on 25 January 1996. At the same date and in successive meetings of the FEZA Committee, other national associations were accepted, i.e., the Romanian Zeolite Association, the Georgian Association of Zeolites, the Polish Zeolite Association, the Czech Zeolite Group, and finally, in the course of last meeting in Montpellier, on 9 July 2001, the admission of Portugal and Slovakia was decided. Among the objects of the Federation, there is the task to arrange Specialist Workshops, Euroconferences or Meetings of an educational character. Accordingly, in the three-year period from 1996 to 1998, a series of six Euroworkshops on Zeolites have been organized, with the financial support of the European Union, on synthesis; ordered mesoporous materials; sorption, diffusion and separation; natural zeolites; application in catalysis, and modification and characterization. In the same frame, the FEZA originated the proposal of a cycle of Euresco Conferences on Zeolite Molecular Sieves. The first Euroconference of this cycle has been held in Obernai (France) during the last March on the "Isomorphous Substitution by Transition Metals". The proposal to organize an International Thematic Conference trader the auspices of the FEZA was made by the leading members of the Hungarian Zeolite Association during the FEZA Committee Meeting, held in Budapest in 1998. Although this type of Conference was not expressly considered in the FEZA Constitution, the proposal was accepted with enthusiasm by the members of the Committee. The 1st International FEZA Conference was therefore held in Eger (Hungary) on 1-4 September 1999, on the theme "Porous Materials in Environmentally Friendly Processes". The Eger Conference was a very successful Conference and this encouraged the FEZA Committee to continue on the same way. Now I have the particular pleasure and pride to present the volume constituting the Proceedings of the 2nd International FEZA Conference, which will be held in Taormina (Italy) on 1-5 September 2002 on the theme "Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium". The reading of the contents and the information directly gathered from the organizers makes me convinced that this will be a very successful Conference either for the richness of themes or for the quality of the contributions. In addition, I am sure that these Proceedings will be prepared by the Editors and printed by the Publisher with the usual care and attention to the printing quality. One last information for the reader. The next FEZA Conference, the 3rd of the series, will be held in Prague in August-September 2005, under the auspices of the Czech Zeolite Group on the theme of "Molecular Sieves from Basic Research to Industrial Applications". Carmine Colella Chairman of the FEZA Committee

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SUPPORT AND SPONSORING (as of May 30, 2002) The Organizing Committee wishes to thank various Institutions and Companies for their financial support to FEZA 2002.Their contributions allowed a reduced registration fee for students and a bursary program. INSTITUTIONS Universit~ della Calabria Dipartimento di Ingegneria Chimica e dei Materiali- Universith della Calabria Universit~ di Messina Universit~ di Catania Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM) COMPANIES EniTecnologie Sasol Italy Philips Netzsch Jeol Micromeritics UOP M.S. COECO Pirossigeno

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xi

ORGANIZING COMMITTEE

Chairman R. Aiello

University of Calabria, Italy

Vice-Chairman G. Giordano

University of Calabria, Italy

Secretary E Testa

University of Calabria, Italy

Treasurer A. Katovic

University of Calabria, Italy

Members E Crea S. Crisafulli S. Galvagno A. Parmaliana

University University University University

of of of of

Calabria, Italy Catania, Italy Messina, Italy Messina, Italy

INTERNATIONAL SCIENTIFIC ADVISORY BOARD (Council of FEZA)

C. Colella (President) H. Van Bekkum (Former President) E Di Renzo (Secretary) M.W. Anderson P. Behrens J. Ceika P. Ciambelli E Hudec I. Kiricsi D.C. Koningsberger S. Kowalak C. Minchev J. Perez-Pariente E Ramoa Ribeiro R. Russu G. Tsitsishvili

Italy The Netherlands France United Kingdom Germany Czech Republic Italy Slovakia Hungary The Netherlands Poland Bulgaria Spain Portugal Romania Georgia

xii PAPER SELECTION COMMITTEE R. Aiello A. Alberti G. Centi M. Derewinski E Fajula J.B. Nagy

University of Calabria, Italy University of Ferrara, Italy University of Messina, Italy Polish Academy of Science, Krakov, Poland CNRS-ENSCM, Montpellier, France University of Namur, Belgium

xiii

CONTENTS

ZEOLITE SYNTHESIS AND CHARACTERIZATION Zeolite characterization with spectroscopic methods A. Zecchina, G. Spoto, G. Ricchiardi, S. Bordiga, E Bonino, C. Prestipino and C. Lamberti (PLENARY LECTURE) Synthesis of alumino, boro, and gallosilicate zeolites by steam-assisted conversion method and their characterization R. Bandyopadhyay, Y. Kubota and Y. Sugi Aluminium distribution in MCM-22. The effect of framework aluminium content and synthesis procedure J. D~dedek, J. Cejka, M. Oberlinger and S. Ernst Grafting of aluminium on dealuminated H-BEA using alkoxides A. Omegna, M. Haouas, G. Pirngruber and R. Prins Influence of various synthesis parameters on the morphology and crystal size of zeolite Zn-MFI A. Katovic, G. Giordano and S. Kowalak In situ dynamic light scattering and synchrotron X-Ray powder diffraction study of the early stages of zeolite growth G. Artioli, R. Grizzetti, L. Carotenuto, C. Piccolo, C. Colella, B. Liguori, R. Aiello and P Frontera Synthesis of MCM-22 zeolite by the vapor-phase transport method S. Inagaki, M. Hoshino, E. Kikuchi and M. Matsukata Defect-flee MEL-type zeolites synthesized in the presence of an azoniaspiro-compound R. Millini, D. Berti, D. Ghisletti, W.O. Parker, Jr., L.C. Carluccio and G. Bellussi Chemical and structural aspects of the transformation of MCM-22 precursor into ITQ-2 R. Schenkel, J.-O. Barth, J. Kornatowski and J.A. Lercher Nanocrystalline ZSM-5: a highly active catalyst for polyolefin feedstock recycling D.P Serrano, J. Aguado, J.M. Escola and J.M. Rodriguez Modeling superoxide dismutase: immobilizing a Cu-Zn complex in porous matrices and activity testing in H202 decomposition K. Hernadi, D. M~hn, I. Labddi, I. Pdlink6, E. Sitkei and I. Kiricsi Crystal growth of zeolite Y studied by computer modelling and atomic force microscopy JR. Agger and M. W. Anderson Interaction of small molecules with transition metal ions in zeolites: the effect of the local environment P Nachtigall, M. Davidovd, M. Silhan and D. Nachtigallovd

3

15

23 31

39

45

53 61 69 77

85 93

101

xiv Preparation and characterization of mesoporous TS-1 catalyst K. Johannsen, A. Boisen, M. Brorson, 1. Schmidt and C.J.H. Jacobsen Observations of layer growth in synthetic zeolites by field emission scanning electron microscopy S. Bazzana, S. Dumrul, J. Warzywoda, L. Hsiao, L. Klass, M. Knapp, J.A. Rains, E.M. Stein, M.J Sullivan, C.M. West, J Y. Woo and A. Sacco, Jr. XANES and XPS studies of titanium aluminophosphate molecular sieves M.H. Zahedi-Niaki, E Beland, L. Bonneviot and S. Kaliaguine An investigation of the intermediate gel phases of A1PO4-11 synthesis by solid state NMR spectroscopy Y. Huang, R. Richer and C. Kirby The benzene molecule as a probe for steric hindrance at proton sites in zeolites: an IR study B. Onida, B. Bonelli, L. Borello, S. Fiorilli, E Geobaldo and E. Garrone Structural characterization of Co- and Si-substituted A1PO-34 synthesized in the presence of morpholine A. MartuccL A. AlbertL G. CrucianL A. Frache and L. Marchese Chemical linking of MFI-type colloidal zeolite crystals P. Agren, S. Thomson, Y. Ilhan, B. Zibrowius, W. Schmidt and E Schfith Synthesis and characterization of MCM-22 zeolites for the N20 oxidation of benzene to phenol D. MelonL R. MonacL E. RombL C. Guimon, H. Martinez, 1. Fechete and E. Dumitriu Novel solid strong base derived from zeolite supported CaO X. W. Han, G. Xie, Y. Chun, X.. W. Yan, Y. Wang, J Xue and JH. Zhu ZSM-5 spheres prepared by resin templating L. Tosheva and J Sterte Novel Nanocomposite Material A. Carati, C. Rizzo, L. Dalloro, B. Stocchi, R. Millini and C. Perego Vibrational and optical spectroscopic studies on copper-exchanged ferrierite G. Turnes Palomino, S. Bordiga, C. Lamberti, A. Zecchina and C. Otero Aredm Variable temperature FTIR spectroscopy of carbon monoxide adsorbed on protonic and rubidium-exchanged ZSM-5 zeolites C. Otero Are6n, M. Pe~arroya Mentruit, M. Rodriguez Delgado, G. Turnes Palomino, O. V. Manoilova, A.A. Tsyganenko and E. Garrone Preparation and characterization of Zn-MFI zeolites using short chain alkylamines as mineralizing agents S. Valange, B. Onida, E Geobaldo, E. Garrone and Z. Gabelica

109

117

125

135

143

151 159

167 175 183 191 199

207

215

Crystal growth of nanosized LTA zeolite from precursor colloids S. Mintova, B. Fieres and T. Bein

223

Synthesis of hybrid zeolite disc from layered silicate Y. Kiyozumi, M. Salou and E Mizukami

231

XV

Effect of alkali metal ions on synthesis of zeolites and layered compounds by solid-state transformation T. Nishide, H. Nakajima, Y. Kiyozumi and E Mizukami (A1)-ZSM-12: syhnthesis and modification of acid sites J. Cejka, G. Ko~ovd, N. Zilkov6 and I. Hrub6 Formation of new microporous silica phase in protonated kanemite-TMAOHwater system E KoolL Y. Kiyozumi, M. Salou and E Mizukami Raman spectroscopic studies of the templated synthesis of zeolites P.P.H.JM. Knops-Gerrits and M. Cuypers Preparation, characterization and catalytic activity of non-hydrothermally synthesized saponite-like materials R. Prihod'ko, M. Sychev, E.J.M. Hensen, J.A.R. van Veen and R.A. van Santen Self-bonded A1, B-ZSM-5 pellets C. Perri, P. De Luca, D. Vuono, M. Bruno, J. B.Nagy and A. Nastro Syntheses and characterization of A1, B-LEV type zeolite from systems containing methyl-quinuclidinium ions D. Violante, P. De Luca, C.V. Tuoto, L. Catanzaro, M. Bruno, J. B.Nagy and A. Nastro Synthesis and ion exchange properties of the ETS-4 and ETS-10 microporous crystalline titanosilicates C.C. Pavel, D. Vuono, A. Nastro, J. B.Nagy and N. Bilba Quasiisothermal degradation kinetics of tetrapropylammonium cations in silicalite-1 matrices O. Pachtova, M. Kodigik, B. Bernauer and E Bauer Cationic silver clusters in zeolite rho and sodalite J. Michalik, J. Sadlo, M. Danilczuk, J. Perlinska and H. Yamada The first example of a small-pore framework hafnium silicate Z. Lin and J. Rocha Synthesis, characterization and catalytic activity of vanadium-containing ETS-10 P. Brand, o, A.A. Valente, J. Rocha and M. W. Anderson Infrared evidence for the reversible protonation of acetonitrile at high temperature in mordenite J. Czyzniewska, S. Chenevarin and E Thibault-Starzyk Spectroscopic and catalytic studies on Cu-MCM-22: effect of copper loading A.J.S. Mascarenhas, H.O. Pastore, H.M.C. Andrade, A. Frache, M. Cadoni and L. Marchese Preparation and properties of MFI zincosilicate S. Kowalak, E. Szymkowiak, M. Gierczyfiska and G. Giordano Influence of Cs loading and carbonates on TPR profiles of PtCsBEA L. Stievano, C. Caldeira, M.E Ribeiro and P. Massiani New evidences for the fluoride contribution in synthesis of gallium phosphates V.1. Pdrvulescu, C.M. Visinescu, M.H. Zahedi-Niaki and S. Kaliaguine

239 247

255 263

271 279

287

295

303 311 319 327

335 343

351 359 367

xvi High-field ESR spectroscopy of Cu(I)-NO complexes in zeolite CuZSM-5 A. Pb'ppl and M. Hartmann

375

Characterization of acid sites in dehydrated H-Beta zeolite by solid state NMR E'. Montouillout, S. Aiello, E Fayon and C. Fernandez

383

Characterization and quantification of aluminum species in zeolites using high-resolution 27A1 solid state NMR A.A. Quoineaud, E Montouillout, S. Gautier, S. Lacombe and C. Fernandez Control of AFI type crystal synthesis with additional gel components J Kornatowski, G. Zadrozna and JA. Lercher

391 399

Synthesis and characterization of mordenite (MOR) zeolite derived from a layered silicate, Na-magadiite T. Selvam and W. Schwieger

407

Hydrothermal synthesis and characterization of new phosphate-based materials prepared in the presence of 1,4-dimethylpiperazine L. Josien, A. Simon-Masseron, S. Fleith, E Gramlich and J Patarin

415

Modeling of crystal growth at early stages of analcime synthesis from clear solutions B. Suboti~, R. Aiello, J. Bronik and E Testa

423

Synthesis of zincosilicate molecular sieve VPI-7 using vapor phase transport J Dong, C.E Xue and G. Liu

431

Combined IR and catalytic studies of the role of Lewis acid sites in creating acid sites of enhanced catalytic activity in steamed HZSM-5 J Datka, B. Gil, P. Baran and B. Staudte Heterogeneity of Cu + in CuZSM-5, TPD-IR studies of CO desorption J Datka and P. Kozyra

439 445

Speciation and structure of cobalt carbonyl and nitrosyl adducts in ZSM-5 zeolite investigated by EPR, IR and DFT techniques P. Pietrzyk, Z. Sojka, B. Gil, J Datka and E. Broctawik

453

Spectroscopic and catalytic behaviour of [015-CsHs)Rh(TI4-1,5-CsH12)] in Mt56Y and Hs6Y (M ' = Li, Na, K, Rb and Cs) E.C. de Oliveira, R.G. da Rosa, H. 0. Pastore

461

Improved synthesis procedure for Fe-BEA zeolite D. Aloi, E Testa, L. Pasqua, R. Aiello and J B.Nagy One-step benzene oxidation to phenol. Part I: preparation and characterization of Fe-(A1)MFI type catalysts G. Giordano, A. Katovic, S. Perathoner, E Pino, G. Centi, J B.Nagy, K. Lazar and P. Fejes

469

477

xvii CATALYSIS

From micro to mesoporous molecular sieves: adapting composition and structure for catalysis 487 A. Corma and M.T. Navarro (PLENARY LECTURE) One step benzene oxidation to phenol. Part II: catalytic behavior of Fe-(A1)MFI zeolites 503 S. Perathoner, F. Pino, G. CentL G. Giordano, A. Katovic, J. B.Nagy, K. Lazar and P. Fejes Synthesis, structure, and reactivity of iron-sulfur species in zeolite ZSM-5 511 R. W. Joyner, M. Stockenhuber and O.P. Tkachenko Characterization of FeMCM-41 and FeZSM-5 catalysts to styrene production 517 J.R.C. Bispo, A.C. Oliveira, M.L.S. Corr~a, J.L.G. Fierro, S.G. Marchetti and M. C. Rangel Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts 525 N.G. Gallegos, M.V. CagnolL J.E Bengoa, A.M. Aloarez, A.A. Yeramidm and S. G. Marchetti On the necessity of a basic revision of the redox properties of H-Zeolites 533 Z. Sobalik, P. Kubdmek, O. Bortnovsky, A. Vondrovdt, Z. Tva~Skovr, JE. Sponer and B. Wichterlovdt The role of zeotype catalyst support in the synthesis of carbon nanotubes by CCVD 541 K. Hernadi, Z. Krnya, A. Siska, J Kiss, A. Oszkr, J. B.Nagy and I. Kiricsi The influence of water on the activity of nitridated zeolites in base-catalyzed reactions 549 S. Ernst, M. Hartmann, T. Hecht, P. Cremades Ja~,n and S. Sauerbeck Selective catalytic reduction of N20 with light alkanes and N20 decomposition over Fe-BEA zeolite catalysts 557 T. Nobukawa, K. Kita, S. Tanaka, S. Ito, T. Miyadera, S. Kameoka, K. Tomishige and K. Kunimori Hydroxymethylation of 2-methoxyphenol catalyzed by H-mordenite: analysis of the reaction scheme 565 E Caoani, L. Dal Pozzo, L. Maselli and R. Mezzogori Unraveling the nature and location of the active sites for butene skeletal isomerization over aged H-Ferrierite 573 S. van Donk, E. Bus, A. Broersma, J.H. Bitter and K.P. de Jong Hydroconversion of aromatics over a Pt-Pd/USY catalyst 581 C. Petitto, G. Giordano, E Fajula and C. Moreau Hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of gas oils in one step on Pt,Pd/H-USY 587 Z Varga, J. Hancsrk, G. Tolvaj, W.I. Horvrth and D. Kall6 Reformate upgrading to produce enriched BTX using noble metal promoted zeolite catalyst 595 S.H. Oh, K.H. Seong, Y.S. Kim, S. ChoL B.S. Lim, J.H. Lee, J. Woltermann and Y.E Chu

xviii Dehydroisomerization of n-butane to isobutene over Pd/SAPO-11. The effect of Si content of SAPO-11, catalyst preparation and reaction condition Y. Wei, G. Wang, Z Liu, P Xie and L. Xu Vapor phase propylene epoxidation over Au/Ti-MCM-41 catalyst: influence of Ti grafting A.K. Sinha, T. Akita, S. Tsubota and M. Haruta Intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta U Wilkenh6ner, D.W. Gammon and E. van Steen The effect of zeolite pore size and channel dimensionality on the selective acylation of naphtalene with acetic anhydride J (~ejka, P Prokegovd, L. Ceroenfi and K. Mikulcovd Alkylation of phenol with methanol over zeolite H-MCM-22 for the formation of p-cresol G. Moon, K.P M6ller, W. B6hringer and C.T. O'Connor Relative stability of alkoxides and carbocations in zeolites. QM/MM embedding and QM calculations applying periodic boundary conditions L.A. Clark, M. Sierka and J. Sauer H-Beta zeolite for acylation processes: optimization of the catalyst properties and reaction conditions P Botella, A. Corma, E Rey and S. Valencia Aniline methylation on modified zeolites with acidic, basic and redox properties I.L Ioanova, O.A. Ponomoreva, E.B. Pomakhina, E.E. Knyazeva, V.V. Yuschenko, M. Hunger and J. Weitkamp Aldol condensation catalyzed by acidic zeolites T. Komatsu, M.Mitsuhashi and T. Yashima Role of intracrystalline tunnels of sepiolite for catalytic activity Y. Kitayama, K. Shimizu, T. Kodama, S. Murai, T. Mizusima, M. Hayakawa and M. Muraoka Catalytic wet oxidation of reactive dyes with H202 over mixed (A1-Cu) pillared clays S.-C. Kim, D.-S. Kim, G.-S. Lee, J.-K. Kang, D.-K. Lee and Y.K. Yang Application of zeolites as supports for catalysts of the ethylene and propylene polymerization I.N. Meshkova, T.A. Ladygina, T.M. Ushakova, N. Yu. Kovaleva and L.A. Novokshonova Catalytic properties of beta zeolite exchanged with Pd and Fe for toluene total oxidation J. Jacquemin, S. Siffert, J.-E Lamonier, E. Zhilinskaya, A.Aboukai's Hydroisomerization of n-Butane over Pd/HZSM-5 and Pd/Hmordenite with and without binder P Ca~izares, E Dorado, P Shnchez and R. Romero Butane isomerization on several H-zeolite catalysts S. De RossL G. Moretti, G. Ferraris and D. Gazzoli Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene JL. Valverde, A. de Lucas, P Sdnchez, E Dorado and A. Romero

603

611 619

627

635

643

651 659

667 675

683

691 699

707 715 723

xix Influence of cocations on the activity of Co-MOR for NO/N20 SCR by propene I. Asencio, E Dorado, JL. Valverde, A. De Lucas and P. Sdnchez Catalytic performance of mesoporous silica SBA-15-supported noble metals for thiopene hydrodesulfurization M. Sugioka, T. Aizawa, Y. Kanda, T. Kurosaka, Y. Uemichi and S. Namba

731

739

Skeletal isomerization of 1-hexene to isohexenes over zeolite catalysts Z. Wu, Q. Wang, L. Xu and S. Xie

747

Preparation and catalytic characterisation of Al-grafted MCM-48 materials M. Rozwadowsla', M. Lezanska, J Wloch, K. Erdmann and J Kornatowski

755

Photoreduction of incorporated molecules in zeolite X: methylviologen K.T. Ranjit and L. Kevan

763

Effective utilization of residual type feedstock to middle distillates by hydrocracking technology S.K. Saha, G.K. Biswas and D. Biswas

771

Direct analysis of deactivated catalysts in 1-pentene isomerization by high-resolution fast atom bombardment mass spectrometry J.M. Campelo, E Lafont and J.M. Marinas

781

Selection of an active zeolite catalyst and kinetics of vapor phase esterification of acetic acid with ethyl alcohol A.M. Aliyev, E.E. Sarijanoo, O. Tun 9 Sava~gi, R.Z. Mikailov, T.N. Shakhtakhtinsky, A. Sario~lan, P.E Poladly and A.R. Kuliyeo Hydrodesulfurization of dibenzothiophene over Mo-based catalysts supported by siliceous MCM-41 A. Wang, Y. Wang, Y. Chen, X. Li, P. Yao and T. Kabe Acylation of 2-methoxynaphtalene over ion-exchanged ~-Zeolite ]. C. Kantarh, L. Artok, H. Bulut, S. Ydmaz and S. Olkii Development of new ZSM-5 catalyst-additives in the fluid catalytic cracking process for the maximization of gaseous alkenes yield A.A. Lappas, C.S. Triantafillidis, Z.A. Tsagrasouli, V.A. Tsiatouras, I.A. Vasalos and N.P. Evmiridis Characterization of H and Cu mordenites with varying SIO2/A1203 ratios, by optical spectroscopy, MAS NMR of 29Si, 27A1 and 1H, temperature programmed desorption and catalytic activity for nitrogen oxide reduction V. Petranovskii, R.E Marzke, G. Diaz, A. Gomez, N. Bogdanchikova, S. Fuentes, N. Katada, A. Pestryakov and V. Gurin A comparison of SAPO, GaPSO, MgAPO and GaPO's as DeNOx catalysts V.I. Pdrvulescu, M. Alifanti, M.H. Zahedi-Niaki, P. Grange and S. Kaliaguine The influence of textural properties of MFI type catalysts on deactivation phenomena during oligomerization of butenes G. Giordano, E Cavani and E Trifir6

787

795 799

807

815

823

831

XX

Dehydrogenation of propane over various chromium-modified MFI-type zeolite catalysts V.A. Tsiatouras, T.K. Katranas, C.S. Triantafillidis, A.G. Vlessidis, E.G. Paulidou and N.P Evmiridis Effect of Pd addition on the catalytic performance of H-ZSM-5 zeolite in chlorinated VOCs combustion R. Lrpez-Fonseca, S. Cibridm, J.1. Guti~rrez-Ortiz and J.R. Gonzdtlez-Velasco Influence of the amount and the type of Zn species in ZSM-5 on the aromatisation of n-hexane A. Smie~kovLt, E. Rojasovr, P Hudec, L. Sabo and Z. Zidek Simultaneous desulfurization and isomerization of sulfur containing n-pentane fractions over Pt/H-mordenite catalyst J. Hancsrk, A. Holl6, I. Valkai, Gy. Szauer and D. Kall6 Propylene polymerization using various metal-containing MCM-41 as cocatalyst T. Miyazaki, Y. Oumi, T. Uozumi, H. Nakajima, S. Hosoda and T. Sano Oxidation of cyclohexene catalyzed by manganese(III) complexes encapsulated in two faujasites M. Silva, R. Ferreira, C. Freire, B. de Castro and JL. Figueiredo Heavy aromatics upgrading using noble metal promoted zeolite catalyst S.H. Oh, S.I. Lee, K.H. Seong, Y.S. Kim, JH. Lee, J Woltermann, WE. Cormier and Y.E Chu Preparation of iron-doped titania-pillared clays and their application to selective catalytic reduction of NO with ammonia D.-K. Lee, S.-C Kim, S.-J. Kim, J.-K. Kang, D.-S. Kim and S.-S. Oh Sulfated Zr-pillared saponite: preparation, properties and thermal stability L. Bergaoui, A. Ghorbel and J.-E Lambert Isomerization and hydrocracking of n-decane over Pt-Pd/A1MCM-41 catalysts S.P. Elangovan, C Bischof and M. Hartmann Influence of nickel metal distribution in Ni/Y-zeolite on the reactivity toward CO hydrogenation D.-S. Kim, S.-C. Kim, S.-J Kim and D.-K. Lee Hydrodechlorination of chlorinated compounds on different zeolites B. Imre, Z. Krnya, I. Hannus, J. Halrsz, J B.Nagy and I. Kiricsi Ammoxidation of ethylene into acetonitrile over Co-zeolites catalysts M. Mhamdi, S. Khaddar-Zine and A. Ghorbel Physicochemical characterization of vanadium-containing K10 epoxidation catalyst I. Khedher, A. Ghorbel and A. Tuel Conversion of aromatic hydrocarbons over MCM-22 and MCM-36 catalysts E. Dumitriu, I. Fechete, P. Caullet, H. Kessler, V. Hulea, C. Chelaru, T. Hulea and X. Bourdon HE-DE exchange and migration of Ga in H-ZSM5 and H-MOR zeolites M. Garcia-Sanchez, P. Magusin, E.JM. Hensen and R.A. van Santen

839

847

855

863 871

879 887

895 903 911

919 927 935 943 951

959

xxi Catalytic conversion of trichloroethylene over HY-zeolite E. Finocchio, C. Pistarino, P. Comite, E. Mazzei Justin, M. Baldi and G. Busca FT-IR studies of internal, external and extraframework sites of FER, MFI, BEA and MOR type protonic zeolite materials G. Busca, M. Beoilacqua, T. Armaroli and M. Trombetta NO reduction with isobutane on Fe/ZSM-5 catalysts prepared by different procedures M.S. Batista and E.A. Urquieta-Gonzdlez

967

975 983

Catalytic and infrared spectroscopic study of NO+CO reaction over iron-containing pillared montmorillonite 991 E L6nyi, J. Valyon and I. Kiricsi A study on alkylation of naphtalene with long chain olefins over zeolite catalyst 999 H. Guo, Y. Liang, W. Qiao, G. Wang and Z. Li Synthesis of anthraquinone from Phthalic Anhydride with Benzene over Zeolite Catalyst 1007 Y. Wang, W.-R. Miao, Q. Liu, L.-B. Cheng and G.-R. Wang Simultaneous hydrogenation and ring opening of aromatics for diesel upgrading on Pt/zeolite catalysts. The influence of zeolite pore topology and reactant on catalyst performance M.A. Arribas, A. Martinez and G. Sastre Catalytic combustion of chlorobenzene over Pt/zeolite catalysts S. Scird, S. Minicd, C. CrisafullL G. Burgio and V. Giuffrida Ag and Co exchanged ferrierite in lean NOx abatement with CH4 P. Ciambelli, D. Sannino, M.C. Gaudino and M. Flytzani-Stephanopoulos The effect of sulfate ion on the synthesis and stability of mesoporous materials M.L. Guzmdn-Castillo, H. Armend6riz-Herrera, A. Tob6n-Ceroantes, D.R. Acosta, P. Salas-Castillo, A. Montoya de la F. and A. Vfzquez-Rodriguez Catalytic behavior of Cd-clinoptilolite prepared by introduction of cadmium metal onto cationic sites G. Onyestydtk and D. Kall6

1015 1023 1031 1039

1047

MESOPOROUS M O L E C U L A R SIEVES

Confinement at nanometer scale: why and how? E Di Renzo, A. Galarneau, P. Trens, N. Tanchoux and E Fajula (PLENARY LECTURE)

1057

Anchorage of dye molecules and organic moieties to the inner surface of Si-MCM-41 Y. Rohlfing, D. W6hrle, J. Rathousk~, A. Zukal and M. Wark Mesocellular aluminosilicate foams (MSU-S/F) and large pore hexagonal mesostructures (MSU-S/H) assembled from zeolite seeds: hydrothermal stability and properties as cumene cracking catalysts Y. Liu and T.J Pinnaoaia

1067

1075

xxii Fabrication of large secondary mesopores in MCM-41 particles assisted by aminoacids and hydrophobic functional groups 1083 I. Diaz and J P~rez-Pariente

Hexagonal and cubic thermally stable mesoporous Tin(IV) phosphates with acidic, basic, and catalytic properties

1091

C. Serre, A. Auroux, A. Geruasini, M. Hervieu, G. Ferey

Characterization of [Cu]-MCM-41 by XPS and CO or NO adsorption heat measurements

1101

M. Broyer, J.P Bellat, O. Heintz, C. Paulin, S. Valange and Z Gabelica

Synthesis and characterisation of iron-containing SBA-15 mesoporous silica

1109

E Martinez, Y.-J Han, G. Stueky, JL. Sotelo, G. Ouejero and JA. Melero

Synthesis and characterization of mesoscopically ordered surfactant/co-surfactant templated metal oxides

1117

T. Czuryszla'ewicz, J. Rosenholm, E Kleitz and M. Linden

Preparation of novel organic-inorganic hybrid micelle templated silicas. Comparison of different routes for materials preparation

1125

D.J. Maequarrie, D.B. Jackson, B.L. King and A. Watson

Structure and catalytic performance of cobalt Fischer Tropsch catalysts supported by periodic mesoporous silicas

1133

A. E Khodakou, R. Bechara and A. Gribooal-Constant

Highly dispersed VOx species on mesoporous supports: promising catalysts for the oxidative dehydrogenation (ODH) of propane

1141

A. Briickner, P Rybarczyk, H. Kosslick, G.-U. Wolf and M. Baerns

Modelling mesoporous materials

1149

M. W. Anderson, C.C. Egger, G.JT. Tiddy and J.L. Casei

Acidity and thermal stability of mesoporous aluminosilicates synthesized by cationic surfactant route

1157

M. Derewinski, M. Machowska and P Sarv

Mesoporous silicate as matrix for drug delivery systems of non-steroidal antinflammatory drugs

1165

R. Aiello, G. Cauallaro, G. Giammona, L. Pasqua, P Pierro and E Testa

Aluminum incorporation and interracial structures in A1SBA-15 mesoporous solids: double resonance and optically pumped hyperpolarized 129XeNMR Studies

1173

E. Haddad, J.-B. d'Espinose, A. Nossov, E Guenneau and A. G~d~on

Tailoring the pore size of hexagonally ordered mesoporous materials containing acid sulfonic groups

1181

R. van Grieken, J.A. Melero and G. Morales

Novel vesicular mesoporous material templated by catanionic surfactant self-assembly 1189 X. W. Yan and J.H. Zhu

Preparation and characterization of Co-Fe-Cu mixed oxides via hydrotalcite-like precursors for toluene catalytic oxidation J. Carpentier, J.-E Lamonier, S. Siffert, H. Laversin, and A. Aboukai's

1197

xxiii Catalytic oxidation over transition metal doped MCM-48 molecular sieves C. WeL Q. Cai, X. Yang, 14(.Pang, Y. Bi and K. Zhen Highly selective oxidation of aromatic hydrocarbons (styrene, benzene and toluene) with H202 over Ni, Ni-Cr and Ni-Ru modified MCM-41 catalysts V..Parvulescu, C. Anastasescu, C. Constantin and B.L. Su Mesoporous materials as supports for heteropolyacid based catalysts M. Gulbihska, M. wrjtowski and M. Laniecki Synthesis and characterization of A1-MCM-48 type materials using coal fly ash P. Kumar, N.K. Mal, Y. OumL T Sano and K. Yamana Synthesis of well-aligned carbon nanotubes on MCM-41 W. Chen, A.M. Zhang, X. Yan and D. Han Synthesis and characterization of CuO and Fe203 nanoparticles within mesoporous MCM-41/-48 silica C. Minchev, R. Krhn, T. Tsoncheva, M. Dimitrov, 1. Mitov, D. Paneva, H. Huwe and M. Frrba Study of the porosity of montmorillonite pillared with aluminum/cerium M.J. Hernando, C. Blanco, C. Pesquera and E Gonzdlez X-ray absorption fine structure investigation of MCM-41 materials containing Pt and PtSn nanoparticles prepared via direct hydrothermal synthesis C. Pak, N. Yao and G.L. Hailer Ordered assembling of precursors of colloidal faujasite mediated by a cationic surfactant J Agfindez, 1. Diaz, C. Mdrquez-Alvarez, E. Sastre and J P~rez-Pariente Synthesis, characterisation and catalytic activity of SO3H-phenyl-MCM-41 materials F. Mohino, I. Diaz, J. POrez-Pariente and E. Sastre Synthesis of ordered mesoporous and microporous aluminas: strategies for tailoring texture and aluminum coordination V. Gonzdlez-Pe~a, C. Mdrquez-Alvarez, E. Sastre and J P~rez-Pariente Characterization of a heteropolyacid supported on mesoporous silica and its application in the aromatization of a-pinene H. Jaramillo, L.A. Palacio and L. Sierra Catalytic activity, deactivation and re-use of A1-MCM-41 for N-methylation of aniline J.M. Campelo, R.M. Leon, D. Luna, J.M. Marinas and A.A. Romero Restructured V-MCM-41 with non-leaching vanadium and improved hydrothermal stability prepared by secondary synthesis N.K. Mal, P. Kumar, M. Fujiwara and K. Kuraoka Comparative study of MCM-41 acidity by using the integrated molar extinction coefficients for infrared absorption bands of adsorbed ammonia A. Taouli and W. Reschetilowski Confinement of nematic liquid crystals in SBA mesoporous materials L. Frunza, S. Frunza, A. Schrnhals, U. Bentrup, R. Fricke, 1. Pitsch and H. Kosslick Synthesis and characterization of bimetallic Ga,A1-MCM-41 and Fe,A1-MCM-41 R. Bfrjega, C. Nenu, R. Ganea, Gr. Pop, S. "-erban and T. Blasco

1205

1213 1221 1229 1237

1245

1253

1261 1267 1275

1283

1291 1299

1307

1315 1323 1331

xxiv Fischer-Tropsch synthesis on iron catalysts supported on MCM-41 and MCM-41 modified with Cs A.M. Alvarez, J E Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramidn and S. G. Marchetti Coordination and oxidation states of iron incorporated into MCM-41 K. Ldzdr, G. Pdl-Borb~ly, A. Szegedi and H.K. Beyer Synthesis and characterization of In-MCM-41 mesoporous molecular sieves with different Si/In ratios W. Brhlmann, O. Klepel, D. Michel and H. Papp The effect of niobium source used in the synthesis on the properties of NbMCM-41 materials 1. Nowak Inclusion of europium(III) ~-diketonates in mesoporous MCM-41 silica A. Fernandes, J. Dexpert-Ghys, C. Brouca-Cabarrecq, E. Philippot, A. Gleizes, A. Galarneau and D. Brunel Synthesis, characterization and catalytic properties of mesoporous titanostanno silicate, Ti-Sn-MCM-41 N.K. Mal, P. Kumar, M. Fujiwara and K. Kuraoka Alternative synthetic routes for NiA1 layered double hydroxides with alkyl and alkylbenzene sulfonates R. Trujillano, M.J. Holgado and V. Rives Spectroscopic studies on aminopropyl-containing micelle templated silicas. Comparison of grafted and co-condensation routes D. Brunel, A.C. Blanc, E. Garrone, B. Onida, M. Rocchia, JB.Nagy and D. J Macquarrie Preparation, characterization, stability and catalytic reactivity of the 3d transition metals incorporated MCM-41 molecular sieves V. Pdrvulescu and B.L. Su Amine-functionalized SiMCM-41 as carrier for heteropolyacid structures L. Pizzio, P. Vdzquez, A.Kikot and E.Basaldella Acidity of mesoporous aluminophosphates and silicas MCM-41. A combined FTIR and UV-Vis-NIR study E. GianottL V. Dellarocca, E.C. Oliveira, S. Coluccia, H.O. Pastore and L. Marchese Modification of silica walls of mesoporous silicate and alumino-silicate by reaction with benzoyl chloride L. Pasqua, E Testa, R. Aiello, G. Madeo and J. B.Nagy

1339

1347

1355

1363 1371

1379

1387

1395

1403 1411

1419

1427

ADVANCED MATERIALS AND APPLICATIONS Options for the design of structured molecular sieve materials J. Sterte , J. Hedlund and L. Tosheoa (KEYNOTE)

1437

XXV

Chromium containing zeolite beta macrostructures V. Naydenov, L. Tosheva and J Sterte Semiconductor nanoparticles in the channels of mesoporous silica and titania thin films M. Wark, H. Wellmann, J Rathousk~ and A. Zukal Spin-coating induced self-assembly of pure silica and Fe-containing mesoporous films N. Petkov, S. Mintooa and T. Bein Guanidine catalysts supported on micelle templated silicas. New basic catalysts for organic chemistry D.J. Macquarrie, K.A. Utting, D. Brunel, G. Renard and A. Blanc Attempts on generating basic sites on mesoporous materials X.W. Yan, X. W. Han, W.Y. Huang, J.H. Zhu and K. Min Application of zeolite in the health science: novel additive for cigarette to remove N-nitrosamines in smoke Z Xu, Y Wang, JH. Zhu, L.L. Ma, L. Liu and J Xue Direct synthesis of ZSM-5 crystals on gold modified by zirconiumphosphonate multilayers S. Dumrul, J Warzywoda and A. Sacco, Jr. Square root relationship in growth kinetics of silicalite-1 membranes P. Nov6k, L. Brabec, O. Solcov6, O. Bortnovsky, A. Zik[mov6 and M. Ko6i~ik Transport characteristics of zeolite membrane from dynamic experiments A. Zikfnov6, B. Bernauer, V. Fila, P. Hrab6nek, J Hradil, V. Krystl and M. Ko6ifik Incorporation of zeolites in polyimide matrices P. Sysel, M. Fry6ov6, R. Hobzov6, V. Krystl, P. Hrab6nek, B. Bernauer, L. Brabec and M. Ko6i~ik The formation mechanism of ZSM-5 zeolite membranes Y LL J. ShL J Wang and D. Yan Mesoporous molecular sieves for albumin A.Y. Eltekov and N.A. Eltekova Characterizing the novel porous superbase K+/ZrO2 by probe adsorption: a Raman study W.Y. Huang, Y Wang, Q. Wang and Q. Yu The synthesis and characterization of zeolite ZSM-5 and ZSM-35 films by self-transformation of glass J. Dong, W. Fan, G. Liu and J. Li Preparation of mesoporous materials as a support for the immobilization of lipase A. Macario, V. Calabr~, S. Curcio, M. De Paola, G. Giordano, G. lorio and A. Katovic

1449 1457 1465

1473 1481

1489

1497 1505 1513 1521

1529 1537 1545

1553 1561

ADSORPTION, DIFFUSION, SEPARATION AND PERMEATION Adsorption and diffusion of linear and dibranched C6 paraffins in a ZSM-5 zeolite E. Lemaire, A. Decrette, JP. Bellat, JM. Simon, A. M~thioier and E. Jolimaftre

1571

xxvi Adsorption of indole and benzothiophene over zeolites with faujasite structure J.L. Sotelo, M.A. Uguina and V.1. Agueda

1579

Determination of microporous structure of zeolites by t-plot method - State-of-the-art 1587 P. Hudec, A. Smiegkovd, Z. Zidek, P. Schneider and O. Solcov6 Binary mixture adsorption of water and ethanol on silicalite Y. OumL A. Miyajima, J. Miyamoto and T. Sano

1595

Influence of water adsorption on zeolite Beta C. Flego, G. Pazzuconi and C. Perego

1603

Diffusion and adsorption of hydrocarbons from automotive engine exhaust in zeolitic adsorbents D. Caputo, M. Eik and C. Colella

1611

Kinetic processes during sorption and diffusion of aromatic molecules on medium pore zeolites studied by time resolved IR-spectroscopy H. Tanaka, S. Zheng, A. Jentys and J.A. Lercher

1619

Calorimetric study of C2H4 adsorption on synthetic zeolites with Na § and Ca 2§ cations 1627 1. V. Karetina, G.Ju. Zemljanova and S.S. Khvoshchev A1-MCM-48: synthesis and adsorption properties for water, benzene, and nitrogen M. Rozwadowski, M. Lezanska, R. Golembiewski, K. Erdmann and J. Kornatowsla"

1631

A frequency-response study of the kinetics of ammonia sorption in zeolite particles Gy. Onyestydk, J. Valyon and L. V.C. Rees

1639

An attempt to correlate the non-isothermal desorption behavior of heterocyclic compounds on a NaY zeolite B. Hunger, 1.A. Beta, C. Engler, E. Geidel, O. Klepel and H. B6hlig

1647

Determination of diffusion coefficient for Cu(II) retention on chemically activated clinoptilolite R. Pode, T. Todinca, A. lovL R. Radovet and G. Burticd

1655

Dynamics of sorption columns in dewatering of bioethanol using zeolites M. Boldi~, K. Melzoch, J. Pokorny and M. Ko~i~ik

1663

Adsorption properties of MCM-41 materials for the VOCs abatement G. Calleja, D.P. Serrano, J.A. Botas and EJ. Guti~rrez

1671

Adsorption of linear and branched paraffms in silicalite: thermodynamic and kinetic study I. Gener, J. Rigoreau, G. Joly, A. Renaud and S. Mignard

1679

Location and transport properties of ammonia molecules in a series of faujasite zeolite structures as studied by FT-IR and 2H-NMR spectroscopies 1687 E Gilles, J.-L. Blin, H. Toufar and B.L. Su Characterization of mesoporous solids: pore condensation and sorption hysteresis phenomena in mesoporous molecular sieves M. Thommes, R. K6hn and M. Fr6ba

1695

xxvii NATURAL ZEOLITES Ion exchange selectivity of phillipsite A.E GualtierL E. Passaglia and E. Galli Sorption of ammonia from gas streams on clinoptilolite impregnated with inorganic acids K. Ciahotn~, L. Melenov6, H. Jirglov6, M. Boldi~ and M. Ko6iHk Microtopographic features and dissolution behavior of natural zeolite surfaces studied by Atomic Force Microscopy (AFM) M. VoltolinL G. Artioli and M. Moret Occurrence and crystal structure of magnesian chabazite E. Passaglia and O. Ferro Treatment of urban dump leachates with natural zeolite packed bed column T. Rodriguez F., E. Acevedo del Monte, G. Mori and B. Rafuzzi Phosphorous removal from wastewater by bioaugmented activated sludge with different amounts of natural zeolite addition J. Hrenovic and D. Tibljas Zeolitized tufts as pedogenic substrate for soil re-building. Early evolution of zeolite/organic matter proto-horizons A. Buondonno, E. Coppola, M. BuccL G. Battaglia, A. Colella, A. Langella and C. Co#ella Neapolitan yellow tuff for the recovery of soils polluted by potential toxic elements in illegal dumps of Campania Region E. Coppola, G. Battaglia, M. BuccL D. Ceglie, A. Colella, A. Langella, A. Buondonno and C. Colella Application of Jordanian faujasite-phillipsite tuff in ammonium removal K.M. Ibrahim Evidence of the relationship occurring between zeolitization and lithification in the yellow facies of Campanian Ignimbrite (southern Italy) A. Langella, P. De Simone, D. Calcaterra, P. Cappelletti and M. de' Gennaro

1705

1713

1721 1729 1737

1743

1751

1759

1767

1775

ION E X C H A N G E AND MODIFICATION

Characterisation of iron containing molecular sieves - the effect of T-element on Fe species 1785 P. Decyk, M. Trejda, M. Ziolek and A. Lewandowska Thermal decomposition of sodium azide in various microporous materials 1793 Gy. Onyesty6k Modifying the acidic properties of PtZSM-5 and PtY zeolites by appropriately varying reduction methods 1801 A. Tam6sL K. Niesz, 1. P6link6, L. Guczi and 1. Kiricsi

xxviii Vibrational studies of iron phthalocyanines in zeolites P.P.H.JM. Knops-Gerrits, E Thibault-Starzyk and R. Parton

1809

The effect of dealumination on the A1 distribution in pentasil ring zeolites J Dgdedek, V. Grbov6 and B. Wichterlov6

1817

Pb(II) ion exchange on zeolite-supported magnetite. Characterization of process by effective diffusivity coefficient V. Pode, T. Todinca, R. Pode, V. Dalea and E. Popovici

1825

Galliation of beta zeolite by the pH control method Y. Oumi, S. KikuchL S. Nawata, T. Fukushima and T. Sano

1833

Ion exchange behaviour of two synthetic phillipsite-like phases C. Colella, B. de' Gennaro, B. Liguori and E. Torracca

1841

Competitive exchange of lead(II) and cadmium(II) from aqueous solution on clinoptilolite S. Berber-Mendoza, R. Leyva-Ramos, J. Mendoza-Barron and R.M. GuerreroCoronado Competitive ion exchange of transition metals in low silica zeolites C. Weidenthaler, Y Mao and W. Schmidt

1849

1857

STRUCTURE ANALYSIS AND MODELLING Computational methods for the design of zeolitic materials M. Elanany, K. Sasata, T. Yokosuka, S. Takami, M. Kubo and A. Miyamoto

1867

(KEYNOTE)

A theoretical investigation on pressure-induced changes in the vibrational spectrum of 1877 zeolite bikitaite E. Fois, A. Gamba, G. TabacchL O. Ferro, S. Quartieri and G. Vezzalini Flexible aluminium coordination of zeolites as function of temperature and water content, an in-situ method to determine aluminium coordinations J.A. van Bokhoven, A.M.J. van der Eerden and D.C. Koningsberger

1885

Structure analysis of boron-silicalite and of a "defect-free" MFI-silicalite by synchrotron radiation single crystal X-ray diffraction M. Milanesio, D. Viterbo, L. Palin, G.L. Marra, C. Lamberti, R. Aiello and E Testa

1891

Density functional theory modelling EPR spectra of Cu(II) in Y zeolite D. Berthomieu, J.M. Duc~r~ and A. Goursot

1899

Molecular modeling: a complement to experiment for designing porous materials used 1907 in separation technologies by adsorption S. Girard, C. Mellot-Draznieks, G. FOrey and P. Pullumbi NMR-crystallographic studies of aluminophosphate A1PO4-40 C.M. Morais, C. Fernandez, V. Montouillout, E Taulelle and J. Rocha

1915

xxix Structural characterization of borosilicates synthesized in the presence of ethylenediamine S. Zanardi, A. Alberti, R. Millini, G. Bellussi and G. Perego Molecular dynamics simulations of water confined in zeolites P. Demontis, G. Stara and G.B. Suffritti EXAFS and optical spectroscopy characterisation of silver within zeolite matrices S.G. Fiddy, N.E. Bogdanchikova, V.P. Petranooskii, J.S. Ogden and M.Avalos-Borja Molecular dynamics simulations of static and dynamic properties of water adsorbed in chabazite S. Jost, S. Fritzsche and R. Haberlandt Correlations in anisotropic diffusion of guest molecules in silicalite-1 S. Fritzsche and J. Kiirger A combined anomalous XRPD, EXAFS, IR, UV-Vis and photoluminescence study on isolated and clustered silver species in Y zeolite C. Prestipino, C. Lamberti, A. Zecchina, S. Cresi, S. Bordiga, L. Palin, A.N. Fitch, P. Perlo and G.L. Marra DFT and IR studies on copper sites in CuZSM-5: structure-redox conditionsdenox activity relationship E. Broctawik, J. Datka, B. Gil and P. Kozyra Diffusion of water in silicalite by molecular dynamics simulations: ab initio based interactions C. Bussai, S. Hannongbua, S.Fritzsche and R. Haberlandt Comparison of small size alumino- and borosilicates optimised by periodic Hartree-Fock A. V. Larin and D.P. Vercauteren Monte Carlo simulation of the temperature dependence of adsorption of nitrogen and oxygen by LiLSX zeolite S.R. Jale, D. Shen, M. Biilow and ER. Fitch Density functional theory calculations of Henry's constant for N2, 02 and Ar molecules in Ca-A and Ca-LSX zeolites G. De Luca, P. Pullumbi and N. Russo Author index Subject index

1923 1931 1939

1947 1955

1963

1971

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2011 2021

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Z E O L I T E SYNTHESIS AND C H A R A C T E R I Z A T I O N

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Studies in Surface Science and Catalysis 142 R. AieUo, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Zeolite characterization with spectroscopic m e t h o d s A. Zecchina 1'2, G. Spoto 1'2, G. Ricchiardi 1'2, S. Bordiga 1'2'3, F. Bonino 1, C. Prestipino 1'3 and C. Lamberti 1'2'3 1 Department of Inorganic, Physical and Material Chemistry, Via P. Giuria 7, 10125 Turin (I) 2 INSTM Unith di Torino 3 INFM Unit~ di Torino Universit~ (I)

Among the different spectroscopic techniques reported in the plenary lecture presented at the FEZA 2002 conference, in this work, we will focus on IR spectroscopy only, devoted to the characterization of the acid strength of the BrCnsted groups in zeolites In particular, in this brief review we will show how the systematic study of the spectroscopic manifestations observed by IR spectroscopy upon dosing to H-zeolites probe molecules with increasing proton affinity will allow to establish a spectroscopic acidity scale for the BrOnsted groups hosted in different zeolites. 1. INTRODUCTION It is universally recognized that the BrCnsted groups represent the most reactive species present in the channels and cavities and that most of the catalytic properties of the zeolites in the acidic form can be ascribed to these species [ 1-3]. The ability of the BrCnsted groups to interact with molecules entering the channels has been the subject of many investigations with physical techniques and, among them, vibrational spectroscopy has definitely played the major role in elucidating the interaction mechanisms and the structure of the formed species. In particular, one of the key questions associated with the activity of the BrCnsted groups present in the zeolite channels is related to the estimation of their acid strength and to the comparison with that of the most common mineral acids and superacids [4].To illustrate in a systematic way the results obtained by the application of vibrational spectroscopy, a useful approach is to describe the interaction of the BrCnsted site of zeolite 1~ (chosen as a prototype system) with molecules characterised by a proton affinity (PA) ranging in a very broad interval. To this end the following sequence of molecules (ordered following their PA) will be illustrated: N2 (PA = 118.2 kcal mol-1), CO (PA = 141.9 kcal mol-1), C2H4 (PA = 162.6 kcal mol-1), C3H6 (PA = 179.5 kcal mol-i), H20 (PA = 166.5 kcal mol-1), CH3CN (PA = 188.6 kcal mol-1), CH3OH (PA = 181.9 kcal mol-1), (CH3)20 (PA = 192.1 kcal mol-1), THF (PA = 196 kcal mol 1) and Py (PA = 204 kcal mo1-1) [5]. All these molecules have a basic character increasing along the sequence, and when appropriate temperature and pressure conditions are adopted, they can form hydrogen bonded adducts whit the BrCnsted groups, as shown in Scheme 1.

B!

i/i l

0,,.. /Ox, ..,,0

o':S

0 0

+B

0,,, /([Ix

o=S ,

OO

,,0 Scheme 1

The temperature conditions allowing the formation of the internal acid-base adducts change gradually in the above series from 77 K to 300 K. In fact the N2 and CO (which are very weak bases) adducts are stable only at very low T, while the adducts (or the salts) formed by interaction with the bases at the end of the series are stable at RT. The adducts formed by the acid-base reaction illustrated in scheme 1, have stretching and bending modes which differ from those of the original BrCnsted group because the hydrogen bonding perturbation is usually associated with profound modifications of the v(OH), 8(OH) and ),(OH) vibrational frequencies and minor changes of the internal modes of the bases B. In this brief review we want to show that the systematic study of these modifications form the basis of the so called spectroscopic method for the estimation of the acid strength of the BrCnsted groups in zeolites [6-15]. 2. D I S C U S S I O N

By starting from the molecules with lower PA, Figure la and lc report the modifications induced by the hydrogen bonding perturbation on the v(OH) stretching mode of the BrCnsted groups of 13 zeolite as function of the pressure of N2 and CO respectively. From the spectra it is clearly emerging that upon dosage of the base B the v(OH) mode of unperturbed groups (band at 3614 cm -1) is gradually consumed while that of the v(OH.-.B) vibration (shifted to lower frequency) simultaneously shows up. The clear isosbestic points observed in both spectra ensure that the 1:1 process illustrated in Scheme 1 is really occurring in a stoichiometric way. Other important observations are: (i) the negative shift A V(OH) increases on passing from N2 (AV = -126 c m -1) to C O (A~- = -319 cm-1), i.e. with the proton affinity of the base; (ii) the full width at half maximum (FWHM) of the v(OH) mode increases on passing from the unperturbed BrCnsted groups (FWHM -- 20 cm 1) to the N2 (FWHM -- 85 cm -1) and CO (FWHM -- 220 cm -1) adducts. It can be easily verified that the FWHM is roughly 90of the shift AV. The resuks illustrated in a) and b) are the typical ones expected for the presence of linear hydrogen bonds [16,17] and represent further and clear demonstration of the formation of 1:1 adducts. Following the immense literature on the IR spectroscopy of the hydrogen-bonded systems [ 16-18], the shift to lower frequency and the increase of the bandwidth are due to the decrease of the force constant induced by the polarisation of the O-H bond and by coupling of the v(OH) with the v(O...B) modes of the adducts, which consequently can be better expressed as v(OH.--B) + v (O.-.B). As briefly mentioned before, the formation of the hydrogen bonded adducts can be accompanied also by a perturbation of the internal modes of the base (in the present case the N-N or the C-O stretching modes).

(a)

(b) 0.02

!

3750 3 5 0 0 3250 W a v e n u m b e r (cm 4)

I

i Io. a.u. i 1

, (c)

2360

a.u.

23'40'23'20

Wavenumber (cm-1)

(d)

I'll

i

3750 3 5 0 0 3250 Wavenumber (crn4)

' 22'oo ' 2~'so ' 21'oo Wavenumber (crn4)

Fig.1. IR spectra of increasing equilibrium pressures of N2 and CO adsorbed at liquid nitrogen temperature on activated H-I3 zeolite, parts (a,b) and (c,d) respectively. (a), (c): O-H stretching region. (b): N-N stretching region. (d) C-O stretching region. In each part the dotted line spectrum is that recorded before gas dosage.

As far as carbon monoxide is concerned, the perturbation of the v(CO) mode upon formation of the OH...CO adduct is shown in Figure ld. It is clear that the stretching frequency undergoes a consistent blue shift (AV (CO) = +34 cm 1) with respect to the gas phase value. This is the result of the CO bond polarisation subsequent to the hydrogen bond formation. An analogous result has been obtained for the adducts of CO with Na +, K +, Cs +, Ag + cations in zeolites, a subject which will be extensively discussed in refs. [ 19-26]. At highest filling conditions, also the silanols located on the external surfaces of the microcrystals or at internal defects form hydrogen bonded adducts with CO. The shift induced on the v(OH) stretching frequency of the silanols is definitely smaller (AV = -90 cm -1) than that observed for the BrCnsted sites: this is simply the consequence of the fact the shift of the v(OH) mode of the acid centres caused by the interaction with a given base is related to the acid strength of the group itself. In other words this different response simply reflect the fact that the OH groups of the structural BrCnsted sites are much stronger acid than the OH groups of the silanols. We will demonstrate in the following that this observation can be supported by a large amount of experimental observations obtained with different bases and different zeolites, so proving its general validity. We anticipate that this general correlation, which is the extension to heterogeneous systems of the well known Bellamy-Hallam-Williams (BHW) relation extensively documented in homogeneous phase [27-28], will form the basis of the spectroscopic method for the acid strength evaluation of the BrCnsted groups of zeolitic systems. Finally, notice also the band at 2138 cm -1 in Figure ld, favored at the highest dosages, which is due to liquid-like CO physically adsorbed in the channels and only interacting with the hydroxyl free, homopolar part of the internal surface [19-21]. Although very weak, mention must be made also of the peaks at 2230 cm -1 of adsorbed CO (Figure ld), because it indicates that in the 13-zeolite treated under vacuum at 673 K a small fraction of sites with very large polarising character are present which can represent potential sites for acid catalysed reactions. These sites, whose concentration is strongly influenced by the thermal treatments and can vary from one sample to the other, are A13+ ions in trigonal coordination deriving from the thermally induced migration of framework Al atoms into partial framework position [19-21]. Coming back to adsorbed nitrogen (Figure lab), it is worth noticing that although the interaction of nitrogen with the structural BrCnsted groups is very weak, the induced polarisation of the N-N bond (only RAMAN active in the gas phase) is sufficient to make the v(NN) mode of the OH.-.NN adduct slightly IR active and hence to originate an appreciable absorption in the 2400-2300 cm -1 range. The v(NN) stretching frequencies of the adducts with structural BrCnsted groups (2330 cm 1) and with silanols (2325 cm -1) are upward shifted as expected [29]. On the basis of the literature concerning homogeneous systems the formation of hydrogen bonded adducts should be accompanied by an upward of the ~5 and ~, modes. Unfortunately, due the overshadowing effect of the skeletal vibrations, it was not possible to measure the effect of the formation of the hydrogen bonded species on the ~5and T modes. In this review article it not possible to continue in the same detailed way the description of the spectra obtained with molecules like ethene, propene, acethylene, etc. which come immediately after N2 and CO in the PA scale. We consequently move to acetonitrile (PA = 188.6 kcal mol-1). The reasons of this choice are twice: i) the acetonitrile probe (CH3CN and CD3CN) has been studied extensively over a great variety of zeolites [4,30-36]; ii) the acetonitrile-zeolite complex is characterised by a complex spectroscopy generated by Fermi resonance effects. As these effects are dominant in the spectra of the adducts of structural BrCnsted sites with bases of medium-strong PA, their detailed illustration for the acetonitrile

complexes can be useful for the comprehension of a great variety of experiments involving different and stronger bases. The spectra of increasing doses of deuterated acetonotrile adsorbed on [~-zeolite [36] are illustrated in Figure 2.

l'+

0.1 a.u.

II I 41

++

db +t, I, +.

41.

,l

9 .+

i

ii '%

.I ++41+

41111

i. 9 ,i+

,i,. ,i. l,

l,

,i.

+

::

3500

A ":. ..B " i

3000

2500

C

2000

1500

Wavenumber (crn Fig.2. IR spectra of increasing equilibrium pressures of CD3CN adsorbed on H-I~ zeolite. Solid line spectra 1-9 refer to CD3CN equilibrium pressures in the 0-10-1 Torr interval, while the dotted line one refers to a much higher pressure (30 Torr). Labels A, B and C denote the three components due to Fermi resonance effects (see text). As found before for N2 and CO we observe the progressive erosion of the structural BrCnsted groups because of the formation of hydrogen bonded adducts (full line spectra in Figure 2); at the highest filling conditions also the band due the silanol groups is eroded (dotted line spectrum). While upon interaction with the nitrile molecule the silanol band originates a broad peak shifted at lower frequency ( AV = -345 cm-1; FWHM = 260 cm-1), two absorptions with apparent maxima at 2856 and at 2452 cm -1 (hereafter named A and B respectively) originate from the structural BrCnsted peak (instead of the single one expected on the basis of the previous results). Other relevant features of the spectra illustrated in Figure 2 are: i) the v(CN) modes of the structural BrCnsted groups and of the weaker silanols are found at 2297 and 2275 cm 1 respectively (i.e. at frequencies higher than those of the free molecule); ii) a novel band at 1325 cm 1 shows up with coverage which can be ascribed to the 8 mode of the BrCnsted-acetonitrile group. The last result demonstrates that the interaction has become sufficiently strong to shift the 8 mode in a frequency range not dominated by the framework vibrations (a fact which makes it observable). The observation of the precise position of the 8 mode gives us the key for the explanation of the presence of A-B doublet. In fact as the minimum separating the A and B partners is observed at a frequency corresponding

to the twice of the 8 mode, it can be readily inferred that it corresponds to the Evans window generated by Fermi resonance effect between the v(OH...B) _+v(O-.-B) mode centred at 2680 cm "l (FWHM = 750 em "1) and the 28 overtone. This explanation finds justification in the abundant spectroscopic literature of hydrogen bonded systems [5,17,18,30] and on the observations concerning adduets with stronger bases (vide infra). In Figure 2 also a band at 1680 cm "1 (labeled with the symbol C) is clearly evident. A similar band is observed for aeetonitrile on H-ZSM-5 and H-MOR [30]. The assignment of this peak will be given in the following after a general introduction to the Fermi resonance effects in hydrogen bonded systems.

Fig.3. Qualitative representation of the IR spectra of weak (a-c), medium (d-f) and strong (g-h) A-H...B or A'---H-B§ H-bonded complexes. The grey areas correspond to regions obscured by the skeletal modes of the zeolite frameworks. For each spectrum the evolution of the proton potential as a function of the A-H distance is also schematically illustrated (fight).

To guide the understanding of the resuks obtained with other bases of larger proton affinity, we think that it is useful to represent schematically the dependence of the v, 8, )', 28, 2)' frequencies upon the O...B distance (Figure 3) used as measure of the strength of the acid base interaction. This dependence has been somewhat freely deduced from the literature data concerning homogeneous compounds and does not have fully quantitative meaning. Notwithstanding this fact, it can be successfully utilized to illustrate the IR spectroscopy of the hydrogen bonding interactions occurring in the zeolites. From this Figure 3 the following seven important points can be underlined: (i) The frequency of the v(OH.-.B) + v(O...B) mode decreases gradually following the well known curve established for homogenous compounds [15]; the shift AV is accompanied by a progressive broadening of the band (FWFM ___-90 AV); (ii) The frequencies of the ~5(OH-.B) and ),(OH...B) modes behaves in a opposite way and the same do the 28 and 2), overtones (the upward shifts are however definitely smaller); (iii) When the frequency of the 28 overtone falls within the stretching band a Fermi resonance occurs with formation of a Evans window and doubling of the peak (A and B bands) [37,38]; (iv) When both the frequencies of the 28 and 2), overtones fall within the v(OH...B) band, the broad v(OH-.-B) band is partitioned by the Fermi resonance into three peaks (named A, B and C) [38]. The relative intensity of the C band with respect to the A and B doublet increases as the strength of the hydrogen bond increases and becomes gradually dominant; (v) For the strongest hydrogen bonding interactions (i.e. for negative shifts of the stretching mode of the order of 2000 cm -1) the v, ~5 and y curves directly intersect. Under these circumstances, corresponding to a fiat potential well characterized by a single minimum [39], direct mixing is occurring and distinction between v, 8 and ), modes becomes impossible. Following the homogeneous literature [17,18] this condition corresponds to that of an hesitating proton; (vi) For strongest hydrogen bonds the downwards shifted v(OH) fall in the range typical of the internal modes of the base B: this fact can further complicate the assignment of the IR spectra; (vii) When the base B approaches proton affinity values near to 200 kcal mo1-1, proton tranfer occurs with formation of hydrogen bonded BH § This is for instance the case for NH3 [40] and Py [5,41] which lead to the formation of Z-.-.H-NH3 § and Z-...HPy+ adducts. As Z- is a weak base (the conjugated acid is strong) and NH4§ and PyH § are also weak acids, the acid-base hydrogen bonding interaction is weak and the shift A V of the v(NH) mode is consequently that typical of a weak interaction. As the PA of Z- is lower than that of NH3 and Py, it is quite conceivable that at the highest filling conditions also (H3N-H.-.NH3) § and (Py-H...Py) § dimers can be present in the zeolite channels. On the basis of these considerations we can now understand the essential features of the sequence of spectra reported in Figure 4, where the IR spectra of the interaction products of series of bases (ranging from N2 to Py) of increasing PA with zeolite H-13 are illustrated [5]. The gradual shift to lower frequency of the broad absorption associated with the perturbed OH group is well documented. The formation and the evolution of the A, B and C peaks and of their baricenter upon the change of the adsorbate and of the proton affinity is also clearly emerging. Notice how the greatest shift is occurring for THF (PA = 196 kcal mo1-1) and how the Py shifts is definitely smaller (because of protonation). Very similar resuks have been obtained on H-MOR, on HZSM-5 and on H-Y [30,41,42] so showing that the considerations illustrated before have general character. As for every adsorbate with proton affinity ranging from 118 to 196 kcal/mole we can determine the shift A V of the BrCnsted sites and of the silanols, we have the possibility to

10 plot them in a XY diagram (Figure 5) and to verify if the BHW relation, whose validity has been well established in solution [27,28], is also holding for the hydrogen bonding interactions occurring in the zeolite channels. 9

i

eta +

...

N2

CO H4

t J (D O t,-

[

CH,,CN _ /~

t~

L_

O r

OH

,<

HF Py

3500

3000

2500

2000

1500

Wavenumber (cm -1) Fig.4. Comparison of the background subtracted IR spectra of H-Beta/B adducts (B CO, C2H4, etc.). All the spectra were recorded at a H§ ratio equal to 1.

= N2,

11

15 16

2000 H-MOR H-ZSM-5 1600

H-Y 1200 10 1

17

800

H-F

89

J

400

SiOH

5 A

~ b . -tBBF" '

0

I

100

'

I

200

'

I

"

300

I

'

400

I

500

'

I

'

600

s (cm Fig.5. Plot of the shift (AV) of the v(OH)Br,~t~ frequencies in 1:10H...B complexes formed on H-Beta (v and v), H-ZSM-5 (O), H-Mor ( ) and H-Y (A) by interaction with different basis (B) v s the shift (AV) of SiOH groups in 1:1 complexes with the same basis. The data corresponding to FH.--B 1:1 adducts are also reported for comparison. Broken line correspond to the AVsioH v s AVsion plot. B is as follows: (1) 02; (2) N2; (3) N20; (4) CO2; (5) CO; (6) C4H4S, C2H2; (7) C2H4, C6H6, C4H6; (8) C4I-I40, C3H6; (9) HC2CH3; (10) H20; (11) CH3CN, CH3CO; (12) CH3OH; (13) CH3CH2OH; (14) (CH3)20; (16) THF; (17) NH3. The huge amount of data summarized in Figure 5 demonstrates that: i) the relation is linear for A V (BrCnsted sites) in the 0 - 1000 c m -1 interval; ii) the data obtained on H-13, H-ZSM-5, H-MOR are located on the same line, so indicating that the acid strength of the BrCnsted sites of these zeolites is identical or very similar; iii) the data obtained on H-Y are located on a line characterised by a smaller slope: this clearly shows that the acid strength of H-Y is smaller than that of the previous materials. It must be underlined that from the comparison of the slopes a quantification of the relative strength of the BrCnsted sites present in H-13, H-ZSM-5 and H-MOR on one side and H-Y on the other side can be estimated on the basis of the empirical relation first established by Paukshtis and Yurchenko [9] for the base CO. This relation is: PA (kJ mol 1) = 1390 - 442.5 log[ A V (OH)/A V (SiOH)]

(1)

12 where the ratio of the A V values is deduced from the slope of the straight lines of Figure 5 and is characteristic of each zeolite; iv) for shifts higher than 1000 cm 1 the data deviate from the straight line: this is not unexpected since the linear BHW plot is verified only for hydrogen bonds of small-medium strength. In turn deviation from the straight line can be considered as indication of presence of strong hydrogen bonds characterized by fiat potential walls where the proton is in so called "hesitating state". To qualitatively illustrate how the strength of the BrCnsted sites of zeolites are definitely higher than that of a common acid like HF, the HBW plot of the HF data obtained in Argon matrices [43-48] are also reported in Figure 5 A quantitative comparison of the slopes cannot be made in this case since the "solvents", i.e. the argon matrix on one side and the zeolitic framework on the other side, are too different. As a final comment of this brief review we shall dedicate a small space to the discussion of the IR spectroscopy of H20 adsorbed on BrCnsted sites and to the related question of whether and when proton transfer occurs. It is now well ascertained that the interaction of a Br0nsted site with a single molecule gives a hydrogen bonded species on H-ZSM-5, H-I3 and H-Mor, while the interaction with two or more molecules gives proton transfer with formation of solvated H30 § or H502§ [5,30]. This problem has been recalled because it shows clearly how co-operative effects between adsorbed molecules are able to promote reactions which are otherwise not possible with single molecules. This observation is related to the more general one concerning the cautions which must be always be considered when result obtained at low filling conditions are extrapolated to situations where the channels are filled with several species. 3. CONCLUSIONS In this brief review we have shown how, the acid strength of the strong Br0nsted sites of H-13,as probed by measuring the shift Av induced by the interaction with bases of proton affinity comprised in a wide interval, is found to be nearly identical to that of H-ZSM-5 and H-MORD, but higher than that of H-Y. Bases with PA < 200 kcal mo1-1 form hydrogenbonded 1:1 adducts, characterized by uncompleted proton transfer. Only for bases with PA > 200 kcal mo1-1 the true proton transfer is really observed with formation of ionic pairs. The basic IR spectroscopy of all of these complexes is discussed and compared with that of the analogous complexes in solution. The interaction of N2 and CO with the external OH of H20 adsorbed on strong Br0nsted sites indicates a substantial decrement of acid strength with respect to that of the original strong BrCnsted site of the zeolite. REFERENCES 1. W. H01derlich, M. Hesse and F. Niiumann, Angew. Chem., Int. Ed. Engl., 27 (1988) 226. 2. A. Corma, Chem. Rev., 95 (1995) 559. 3. A. Corma and A. Martinez, Adv. Mater., 7 (1995) 137. 4. R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto and A. Zecchina, J. Phys. Chem., 99 (1995) 11937. 5. C. Paz~, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina and G. Bellussi, J. Phys. Chem. B, 101 (1997)4740. 6. L. Kubelkovd, S. Beran and J. Lercher, Zeolites, 9 (1989) 539. 7. M.A. Makarova, A.F. Ojo, K. Karim, M. Hunger and J. Dwyer, J. Phys. Chem., 98 (1994) 3619.

13 8. M.A. Makarova, V.L. Zholobenko, K.M. A1-Ghefaill, N.E. Thompson, J. Dewing and J. Dwyer, J. Chem. Soc., Faraday Trans., 90 (1994) 1047. 9. E.A. Paukshits and E.N. Yurchenko, React. Kinet. Catal. Lett., 16 (1981) 131. 10. L.M. Kustov, V.B. Kazansky, S. Beran, L. Kubelkov~i and P. Jir6, J. Phys. Chem., 91 (1987) 5247. 11. E. Garrone, R. Chiappetta, G. Spoto, P. Ugliengo, A. Zecchina and F. Fajula, in 'Proceedings of the 9th International Zeolite Conference, Montreal 1992', (R. Von Ballmoos, J.B. Higgins and M.M.J. Tracy, Eds) Butterworth-Heinemann, London, (1993), p. 267. 12. A. Zecchina, C. Lamberti and S. Bordiga, Catal. Today, 41 (1998) 169. 13. H. Kn~zinger (1997) in 'Handbook of heterogeneous catalysis', (G. Ertl, H. Kn~3zinger and J. Weitkamp, Eds.), VCH, Weinheim (D), (1997), Vol. 2, p. 286. 14. H. Kn~3zinger and S. Huber, J. Chem. Soc. Faraday Trans., 94 (1988) 2047. 15. A. Zecchina, G. Spoto and S. Bordiga in "Handbook of vibrational spectroscopy" (J. M. Chalmers and P.R. Griffiths Eds.), John Wiley & Sons Ltd, Chichester, UK (2002). 16. G.C. Pimentel and A.L. McClellan 'The Hydrogen Bond', W.H. Freeman, San Francisco, (1960). 17. D. Hadzi and S. Bratos, in 'The Hydrogen Bond', (P. Shuster, G. Zundel and C. Sandorfy, Eds.) North Holland, Amsterdam, (1976) Vol. 2, p. 565. 18. 'The Hydrogen Bond', (P. Schuster, G. Zundel and C. Sandorfy, Eds.), North Holland, Amsterdam, (1976). 19. S. Bordiga, E. Escalona Platero, C. Otero Are~, C. Lamberti and A. Zecchina, J. Catal., 137 (1992) 179. 20. S. Bordiga, D. Scarano, G. Spoto, A. Zecchina, C. Lamberti and C. Otero Are~in, Vib. Spectrosc., 5 (1993) 69. 21. A. Zecchina, S. Bordiga, C. Lamberti, G. Spoto, L. Carnelli and C. Otero Are~in, J. Phys. Chem., 98 (1994) 9577. 22. S. Bordiga, E. Garrone, C. Lamberti, A. Zecchina, C. Otero Are~in, V.B. Kazansky and L.M. Kustov, J. Chem. Soc. Faraday Trans., 90 (1994) 3367. 23. S. Bordiga, C. Lamberti, F. Geobaldo, A. Zecchina, G. Turnes Palomino and C. Otero Are~in, Langmuir, 11 (1995) 527. 24. C. Lamberti, S. Bordiga, F. Geobaldo, A. Zecchina, and C. Otero Are~in, J. Chem. Phys., 103 (1995) 3185. 25. C. Otero AreLq, A. A. Tsyganenko, E. Escalona Platero, E. Garrone and A. Zecchaina, Angew. Chem. Int. Ed., 37 (1998) 3161. 26. S. Bordiga, G. Turnes Palomino, D. Arduino, C. Lamberti, A. Zecchina and C. Otero Are~in, J. Mol. Catal. A, 146 (1999) 97. 27. L.J. Bellamy, H.E. Hallam and R.L. Williams, Trans. Faraday Soc., 54 (1958) 1120. 28. L.J. Bellamy and R.J. Pace, Spectrochim. Acta, 25A (1969) 319. 29. F. Geobaldo, C. Lamberti, G. Ricchiardi, S. Bordiga, A. Zecchina, G. Tumes Palomino and C. Otero Are~n, J. Phys. Chem., 99 (1995) 11167. 30. A. Zecchina, F. Geobaldo, G. Spoto, S. Bordiga, G. Ricchiardi, R. Buzzoni and G. Petrini, J. Phys. Chem., 100 (1996) 16584. 31. A.G. Pelmenschikov, R.A. van Santen, J. J~inchen and E. Meijer, J. Phys. Chem., 97 (1993) 11071. 32. J.F. Haw, M.B. Hall, S.A.E. Alvarado, E.J. Munson, Z. Lin, L.W. Beck and T. Howard, J. Am. Chem. Soc., 116 (1994) 7308. 33. J. Flori~in and L. Kubelkov~i, J. Phys. Chem. 98 (1994) 8734.

14 34. A.G. Pelmenschikov, J.H.M.C. Wolput, J. Jiinchen and R.A. van Santen, J. Phys. Chem., 99 (1995) 3612. 35. L. Kubelkov~i, J. Koala and J. Flori~in, J. Phys. Chem., 99 (1995) 10285. 36. C. Paz~, A. Zecchina, S. Spera, A. Cosma, E. Merlo, G. Spanb and G. Girotti, Phys. Chem. Chem. Phys., 1 (1999) 2627. 37. G. Herzberg, in 'IR and Raman Spectra of Polyatomic Molecules', Van Nostrand, New York, (1945), p. 216. 38. S. E. Odinokov and A. V. Jogansen, Spectrochim. Acta, 28A (1972) 2343. 39. U. B/Shner and G. Zundel, J. Phys. Chem., 90 (1986) 964. 40. A. Zecchina, L. Marchese, S. Bordiga, C. Paz~ and E. Gianotti, J. Phys. Chem. B, 101 (1997) 10128. 41. R. Buzzoni, S. Bordiga, G. Ricchiardi, C. Lamberti, A. Zecchina and G. Bellussi, Langmuir, 12 (1996) 930. 42. A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Spanb and F. Geobaldo, J. Chem. Soc., Faraday Trans., 92 (1996) 4863. 43. G. L. Johnson and L. Andrews, J. Phys. Chem., 87 (1983) 1852. 44. L. Andrews and G.L. Johnson, J. Chem. Phys., 79 (1983) 3670. 45. L. Andrews and S. R. Davis, J. Chem. Phys., 83 (1985) 4983. 46. L. Andrews, J. Mol. Struct., 100 (1983) 281. 47. L. Andrews, J. Phys. Chem., 88 (1984) 2940. 48. L. Andrews, R.B. Bahn, R.T. Arlinhaus and R.D. Hunt, Chem. Phys. Lett., 158 (1989) 564.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 ElsevierScienceB.V. All rights reserved.

15

Synthesis of alumino, boro, and gallosilicate zeolites by steam-assisted conversion method and their characterization Rajib Bandyopadhyay a*, Yoshihiro Kubota b and Yoshihiro Sugi b aInstitut ftir Geologie, Mineralogie und Geophysik, Ruhr-Universitfit Bochum, D-44780 Bochum, Germany UDepartment of Chemistry, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan

Steam-assisted conversion method was utilized for the synthesis of different zeolites, namely alumino, boro, and gallosilicate zeolites. Synthesis behavior of the materials was studied by varying the gel composition and other parameters, and some of the characteristics were compared with the samples obtained by conventional hydrothermal method. Presence of small amount of external bulk water in the SAC method played significant role for the conversion of the dry gel. 1. INTRODUCTION Zeolites are synthesized by conventional hydrothermal synthesis method [1]. However, quest for new and convenient method of synthesis has always been a major interest of the scientific community. In recent years, a new type of synthetic route namely dry-gel conversion, and more precisely steam-assisted conversion (SAC) method has been utilized for the synthesis of zeolites [2-12]. In this method, the initial gel, which is prepared normally for zeolite synthesis, is dried and converted into crystalline products at desired temperature in presence of small amount of water as the source of steam. Although this method has been so far introduced for few specific zeolite structure types, this might lead to more general synthesis concept for porous structures. In addition, the solvent-free and environment friendly process in itself is of great interest. In our recent studies, we have utilized this method to synthesize isomorphously substituted zeolites with various structures such as MFI, BEA, MTW and SSZ-31 [7,8,10-12]. In the present study, we report an overview of synthesis of these structures by SAC method. Characterization of the samples made by SAC method, comparison with conventional hydrothermal method, and the investigation of advantages and limitations of this method are also part of interest in this study. 2. EXPERIMENTAL

2.1. Synthesis of materials by SAC method Synthesis of aluminosilicate zeolites In a typical synthetic procedure of [A1]-beta, appropriate amount tetraethylammonium hydroxide (TEAOH Aldrich, 35% solution in water) was mixed and stirred with colloidal

16 silica (Ludox AS 40, 40% solution in water) followed by the addition of NaOH (32% solution in water) with continuous stirring. A12(SO4)3 (Nacalai) was dissolved in de-ionized water and added to the above mixture. After stirring for 1 h, the gel was dried at 80 ~ over oil bath with continuous stirring, allowing evaporation of water. When the gel became thick and viscous, it was homogenized manually using a Teflon rod until it dried. The white powder was then poured in a small Teflon cup (20 x 20 mm I.D.) which was placed in a Teflon-lined autoclave (23 ml) with the support of a Teflon holder. At the bottom of the autoclave, small amount (ca. 0.2 g per 1 g of dry gel) of external bulk water was taken in such a manner that the external bulk water never came into the direct contact with the dried gel. Crystallization of the amorphous powder was carried out at 175 ~ and autogenous pressure for 3 days. Afterwards, the sample was removed from the autoclave, washed thoroughly with water and dried for further characterization. The synthesis conditions for the zeolites prepared by SAC are summarized in Table 1 and the schematic diagram of synthesis is depicted in Fig. 1. Table 1. Synthesis conditions of zeolites prepared by SAC method Sample

Molar gel composition Temperature SiO2 M203 a SDA b NaOH (~ [A1]-beta 1 0.01-0.033 0.37 0.056 175 [A1]-SSZ-31 1 0.0026-0.0054 0.2 0.05-0.12 175 [B]-beta 1 0.02-0.033 0.8-1.2 0.056-0.1 175 [B]-ZSM-5 1 0.005-0.01 0.36-1.0 0.056-0.1 175 [B]-ZSM-12 1 0.002-0.005 0.36 0.1 175 [Ga]-beta 1 0.0014-0.033 0.5 0.3 175 [Ga]-ZSM-5 1 0.005-0.01 0.2 0.1 175 [Ga]-ZSM-12 1 0.005-0.01 0.42 0.3 175 aM is A1, B or Ga for corresponding isomorphously substituted zeolite bSDA = Structure Directing Agent

Time (day) 3 2 3 3 3 3 4 3

For the synthesis of [A1]-SSZ-31 by SAC method, the structure directing agent (1,1,1,8,8,8-hexaethyl-l,8-diazoniaoctane dihydroxide) was first prepared by the procedure described earlier [10,11]. This was mixed with appropriate amount of NaOH, colloidal silica (Snowtex 40, Nissan Chemical Co.) and de-ionized water. Finally, A12(SO4)3 was dissolved in

Drygel(powder)

Tefloncup | (20X20mm) I Teflon holder

r. ~.

1 ii,~"

J Teflon-lined autoclave I...... iI ..~.... Water(23ml)

Figure 1. Schematic diagram of synthesis of zeolites by steam-assisted conversion method

17 water and mixed with the above mixture. The gel was stirred, dried in the same manner as described above, and crystallized at 175 ~ for 1 to 2 days.

Synthesis of borosilicate zeolites Borosilicate zeolites were synthesized by SAC method using similar technique as the aluminosilicate zeolites. Sodium tetraborate decahydrate was taken as the boron source and TEAOH as the SDA. Three structure types, namely BEA, MFI and MTW were synthesized by varying the initial gel composition.

Synthesis of gallosilicate zeolites Gallium-substituted zeolites with structure types BEA, MFI and MTW were synthesized by SAC method using tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), and methyltriethylammonium bromide (MTEABr), respectively as SDA.

2.2. Synthesis of materials by hydrothermal method For the comparison of synthesis behavior, most of the samples prepared by SAC method as described above were also synthesized by conventional hydrothermal method. [A1]-Beta and [B]-Beta were synthesized using TEAOH and DABCO, respectively as the SDA. [Ga]-Beta could not be synthesized by hydrothermal method following the identical composition and conditions as SAC method. Moreover, as [A1]-SSZ-31 was difficult to synthesize by hydrothermal method, [B]-SSZ-31 was prepared following our previous procedure [13]. [Ga]-ZSM-5 and [Ga]-ZSM-12 were also synthesized by hydrothermal method under similar conditions. 2.3. Characterization X-ray powder diffraction (XRD-6000 Shimadzu) was used to determine the phase purity and crystallinity of the samples. Inductively coupled plasma (JICP-PS-1000 UV, Leeman Labs Inc.) was performed for the elemental analyses. Crystal size and morphology of the samples were monitored by scanning electron microscopy (SEM) using a Philips XL30 microscope. Thermal analyses of the samples were carried out on a Shimadzu DTG-50 analyzer. 27A1, liB and 71Ga MAS NMR were performed on a Varian Inova 400 FT-NMR spectrometer. 3. RESULTS AND DISCUSSION

3.1. Synthesis and characterization Fig. 2 shows the XRD patterns of different as-synthesized beta samples prepared by SAC method. High intensity of the peaks and absence of any baseline drift indicated that the samples were highly crystalline, and comparable with the samples made by conventional hydrothermal method. [A1]-beta was obtained with SIO2/A1203 ratio 30-100, and [B]-beta was obtained with SIO2/B203 ratio 30-50. On the other hand, [Ga]-beta could be synthesized with SiO2/Ga203 = 30-700. 13C CP MAS NMR of all of the as-synthesized beta samples showed chemical shifts at 6-7 ppm and 52-53 ppm indicating the presence of tetraethylammonium ion and intactness of the SDA inside the zeolite pore. liB and 71Ga MAS NMR spectra of the [B l-beta and [Ga]-beta samples showed chemical shifts at -4.2 ppm and 156 ppm,

18

[Ga]-beta At~

.

J~l\^

SSZ-31(12h)~ BEA (6h)

2

7

12 17 22 27 32 37 42 47 52

2

7

2 0 ( ~)

2 0 ( ~)

Figure 2. XRD patterns of as-synthesized beta samples prepared by SAC method

..........

" ............................

12 17 22 27 32 37 42 47 52

' .........

Figure 3. XRD patterns showing formation of SSZ- 31 through phase change

l

~

r~

E

2

7

12 17 22 27 32 37 42 47 52

20(~

Figure 4. XRD patterns of as-synthesized ZSM-5 samples prepared by SAC method

2

7

12 17 22 27 32 37 42 47 52

2 0 ( ~)

Figure 5. XRD patterns of as-synthesized ZSM-12 samples prepared by SAC method

respectively, which revealed successful isomorphous substitution of A1 by B and Ga, and also the tetrahedral framework nature of the corresponding element [8,14]. In the synthesis of [A1]-SSZ-31 by SAC method, alkali concentration of the initial gel mainly governed the phase selection, and pure SSZ-31 phase was obtained with NaOH/SiO2 = 0.05-0.12 whereas MFI was mixed below or above this range. At an optimum ratio of NaOH/SiO2 = 0.084, SSZ-31 was formed through a phase change with time course. At shorter time BEA was formed and it was converted to SSZ-31 at later stage, and finally the phase changed to a mixture of MFI and SSZ-31 (Fig. 3). Rao et al. observed similar type of phase transformation of high-silica BEA to OU-1 [6]. 13C CP MAS NMR of as-synthesized sample mostly resembled that of the SDA supporting the presence of SDA inside the pore. CHN analysis of the as-synthesized sample also showed 1.5 N + or 0.75 SDA molecule per unit cell, which revealed close fitting of the SDA inside the pore. A single peak at 52.2 ppm in the 27A1 MAS NMR spectra showed the tetrahedral framework nature of A1 and absence of any octahedral species [ 11 ]. XRD patterns of boron and gallium-substituted ZSM-5 and ZSM-12 samples made by SAC method are shown in Fig. 4 and Fig. 5, respectively. Pure [B]-ZSM-5 and [B]-ZSM-12

19 were obtained during the synthesis of [B]-beta using TEAOH as the structure-directing agent. At lower TEAOH/SiO2 (0.36) and higher SIO2/B203 (100-200), pure MFI phase was synthesized. MFI was also obtained at higher TEAOH/SiO2 (1.0-1.2) when the NaOH/SiO2 ratio was increased from 0.056 to 0.1. On the other hand, at higher alkali concentration, pure MTW was obtained when 8iO2/B203 ratio was further increased to 200 and 500. Similar to beta samples, [B]-ZSM-5 and [B]-ZSM-12 also showed chemical shifts at -4.0 a n d - 3 . 8 ppm, respectively in the liB MAS NMR spectra, indicating the presence of tetrahedral B(OSi)4 entity in the framework. [Ga]-ZSM-5 and [Ga]-ZSM-12 were synthesized with SiO2/Ga203 ratio 100 and onwards in the initial gel. All-silica ZSM-5 and ZSM-12 (without any Ga) could also be synthesized by SAC method. Similar to the [Ga]-beta sample, [Ga]-ZSM-5 and [Ga]ZSM-12 also showed chemical shift at 150-155 ppm and absence of any peak at 0 ppm in the 27A1 MAS NMR, indicating presence of tetrahedral Ga in the framework and absence of any octahedral species.

3.2. Role of external bulk water in the synthesis by SAC method As described in the experimental section, conversion of the dried amorphous gel into crystalline zeolite material was achieved in presence of very small amount of external bulk water, which was taken at the bottom of the autoclave as the source of steam. In all cases, when the syntheses were carried out in absence of the external bulk water, crystallization of the dry gel failed and only amorphous phase was obtained (Fig. 6). Although the gel was dried as much as possible, presence of very minute amount of water in the dried powder could not be neglected. Thus, contribution from the apparently adhered water in the dry gel could not lead to successful crystallization. Keeping a fixed amount of dry gel (ca. 1 g), when the crystallization was studied in the presence of bulk water and by increasing the amount of this water, it was interestingly observed that a minimal amount of external bulk water as the source of steam was necessary for the successful crystallization. Similar observation during the synthesis of zeolites was also noticed previously by other researchers [3,4,9]. It was suggested that to initiate the crystallization, keeping certain saturated vapor pressure was not enough, and adsorption and condensation of water on or inside the dry gel was necessary for the crystallization [9]. Therefore, the profound effect of water vapor in SAC method could be visualized, although its exact mechanism in the nucleation and crystallization of the dry gel is yet to be fully understood.

[B]-beta

JAil-beta

~ t

{i

With H20

l

. . . . . . .

[Ga]-beta

1

With0u~H?~

]i

With H20

:

Without H20

2 7 12 17 22 27 32 37 42 47 52 2 7 12 17 22 27 32 37 42 47 52 2 7 12 17 22 27 32 37 42 47 2 0 ( ~) 2 0 ( ~) 2 0 ( ~)

Figure 6. XRD patterns of as-synthesized beta samples showing the role of external bulk water in SAC method

20

Figure 7. SEM images of samples prepared by hydrothermal and SAC methods

3.3. SAC versus hydrothermal method: comparison and advantages

During the synthesis of a series of zeolites with different structure types by SAC method, the synthesis behavior and the characteristics of the samples were compared with that of conventional hydrothermal method. Interestingly, a number of advantages of synthesis by SAC method could be noticed compared to hydrothermal method. First of all, it involved nearly complete conversion of gel to zeolite, and the yield of the as-synthesized samples made by SAC was usually more than that made by hydrothermal one. For example, the yield of the SSZ-31 samples made by SAC was higher (about 75 to 90%) than that of SSZ-31 synthesized by hydrothermal method (65%). The shorter time of crystallization was also a major advantage in SAC method. It took only 3 days to obtain fully crystalline beta (B, A1, and Gasubstituted) samples, whereas using hydrothermal method beta samples were obtained usually after one week. Similar crystallization period was also observed with [Ga]-ZSM-12 made by SAC and hydrothermal method. Higher temperature (ca. 175 ~ of crystallization could also be utilized for the synthesis by SAC, which was not favorable sometimes for hydrothermal method. The average size of the crystals obtained by SAC method was usually smaller than that obtained by hydrothermal method (Fig. 7). Apart from these advantages, synthesis by

21 steam-assisted conversion involves minimization of waste disposal, reduction in reactor volume, and could be used in the continuous production of zeolites.

3.4. Difficulties and limitations with synthesis by SAC method Although synthesis of zeolites by SAC method was convenient and it had some advantages over conventional hydrothermal method, it is worthwhile to briefly mention the difficulties experienced during the synthesis. Since the synthesis method involved drying of the gel over oil bath, at one stage the gel became thick and viscous and stirring by mechanical means was difficult. At this stage the gel had to be stirred manually using a Teflon rod. This process of drying was tedious and time taking. Depending on the organic template used, sometimes the gel became very sticky, and gave trouble in drying the gel and finally making the powder. One common problem that we often observed during the synthesis by SAC method was the problem of scaling up and homogeneity. The gel must be mixed homogeneously before drying to obtain fully crystalline material. From our experience we have seen that large-scale synthesis by SAC method often yielded partial amorphous or undesired products. Although the exact reason is not fully understood, it is believed that manual drying of large amount of viscous gel might give rise to partial non-homogeneity, and water vapor could not react deep inside the large amount of dry gel. Although interesting results and fully crystalline material was obtained during the large-scale synthesis of [A1]SSZ-31 [11], there are scopes for overcoming the technical difficulties and improvement in this synthesis method. 4. CONCLUSION Zeolites with various substitutions such as alumino, boro and gallosilicates, and of different structures such as BEA, MFI, MTW and SSZ-31 could be successfully synthesized by steam-assisted conversion (SAC) method. Very small amount of external bulk water showed immense effect on the crystallization of the dry gel. The samples obtained by SAC method showed similar characteristics to that prepared by conventional hydrothermal synthesis method. Synthesis of zeolites by SAC method showed advantages over hydrothermal methods in terms of conversion, yield, crystallization time, and so on. SAC method could be extended for the synthesis of other zeolites and molecular sieves also. ACKNOWLEDGEMENT The study was partly supported by grants from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and one of the authors (R.B.) thanks Japan Society for the Promotion of Science (JSPS) and Alexander von Humboldt Foundation, Germany, for research fellowships. REFERENCES 1. R. Szostak, Molecular Sieves: Principles of Synthesis and Identification, van Nostrand Reinhold, New York, 1988. 2. M.H. Kim, H.X. Li and M.E. Davis, Micropor. Mater., 1 (1993) 191. 3. P.R.H.P. Rao and M. Matsukata, Chem. Commun., (1996) 1441.

22 4. 5. 6. 7. 8.

T. Tatsumi, Q. Xia and N. Jappar, Chem. Lett., (1997) 677. T. Tatsumi and N. Jappar, J. Phys. Chem. B, 102 (1998) 7126. P . R . H . P . Rao, K. Ueyama, E. Kikuchi and M. Matsukata, Chem. Lett., (1998) 311. R. Bandyopadhyay, Y. Kubota and Y. Sugi, Chem. Lett., (1998) 813. R. Bandyopadhyay, Y. Kubota, N. Sugimoto, Y. Fukushima and Y. Sugi, Micropor. Mesopor. Mater., 32 (1999) 81. 9. M. Matsukata, M. Ogura, T. Osaki, P. R. H. P. Rao, M. Nomura and E. Kikuchi, Top. Catal., 9 (1999) 77. 10. R. Bandyopadhyay, Y. Kubota, M. Ogawa, N. Sugimoto, Y. Fukushima and Y. Sugi, Chem. Lett., (2000) 300. 11. R. Bandyopadhyay, R.K. Ahedi, Y. Kubota, M. Ogawa, Y. Goto, Y. Fukushima and Y. Sugi, J. Mater. Chem., 11 (2001) 1869. 12. R. Bandyopadhyay, Y. Kubota, S. Nakata and Y. Sugi, Stud. Surf. Sci. Catal., 135 (2001) 331. 13. R. Bandyopadhyay, Y. Kubota, S. Tawada, and Y. Sugi, Catal. Lett., 50 (1998) 153. 14. C.R. Bayense, A.P.M. Kentgens, J.W. de Haan, L.J.M. van de Ven and J.H.C. van Hoof, J. Phys. Chem., 96 (1992) 775.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 ElsevierScienceB.V. All rights reserved.

A l u m i n i u m Distribution in M C M - 2 2 . The A l u m i n i u m Content and Synthesis Procedure

23

Effect

of

Framework

Jifi D6de6ek a, Jifi 0ejka a, Matthias Oberlinger b and Stefan Ernst b a j. Heyrovsks~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic*

b Department of Chemistry, Chemical Technology, University of Kaiserslautern, Erwin Schr6dinger Strasse 54, D-67663 Kaiserslautern, Germany* The distribution of single aluminium atoms and aluminium pairs in the framework of zeolite MCM-22 was investigated using sodium and Co(n) ion-exchange and UV-Vis spectroscopy of Co(g) ions located in cationic positions at maximum Co loading in MCM-22 zeolite. Single (isolated) A1 atoms, unable to balance divalent cations, relatively closely located unpaired A1 atoms balancing Co(H) hexaaquo complexes and A1 pairs of A1-O-(Si-O)I,2-A1 types located in one ring and forming cationic sites for "bare" divalent cations are present in MCM-22. Aluminium distribution between individual A1 types in the MCM-22 structure depends on the framework aluminium content and conditions of zeolite synthesis. 1. INTRODUCTION Zeolites are at present widely used in the chemical industry as catalysts in refineries, petrochemical industry (particularly in transformations of aromatic hydrocarbons, methanol transformation and olefin isomerisation), for improvement in gasoline yield and/or production of cleaner fuels. Recently, their potential for application in "green chemistry" for environmental protection and fine chemical synthesis was investigated (1,2). High number of zeolite molecular sieves differing in the shape, size and dimensionality of their channel systems is promising for the further applications of zeolites as highly active and selective heterogeneous catalysts. To understand the role of these zeolite catalysts in various chemical transformations it is very important to be able to describe in detail their active sites. It is without any doubt that the properties of active sites " Financial support of the Grant Agency of the Academy of Sciences of the Czech Republic (A4040001) and Volkswagen-Stiftung (I/75 886) is highly appreciated. , S.E. and M.O. gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie and Max-BuchnerForschungsstiftung

24 in silicon rich zeolites (Si/A1 > 8) are closely connected to the distribution of aluminium atoms in their framework and their accessibility in the channel system (3). In general, aluminium can be distributed in the zeolite framework as single aluminium atoms, being relatively far from other aluminiums ((Si-O)n>3-Al-(Si-O)na3) and in the form of various "pairs" like A1-O-Si-O-A1 or A1-O-(Si-O)z-A1. The A1 pairs can be located in one and the same ring and only such A1 pairs create cationic sites for bare divalent cations in the dehydrated zeolite. Note that different cationic sites and, thus, different types of such A1 pairs can be usually found in zeolites. Also two single aluminium atoms ((Si-O)n>3-Al-(SiO)n~_3) can be close enough together to balance divalent Co(H) aquo complexes in as prepared zeolites (e.g. two aluminium atoms located on the opposite sides of the zeolite channel). Moreover, due to the zeolite channel structure and the low A1 content in silicon rich zeolites, also the spatial distribution of A1 atoms among the channel systems has to be taken in account. Recently, it was shown that the above described distribution of heteroatoms (A1, Fe, B) in the framework of ZSM-5 is not controlled by statistic rules and depends on the content of heteroatom in the framework, on the conditions of zeolite synthesis and on the nature of the heteroatom (4,5,6). MCM-22 (structural code MWW) represents a relatively new zeolite with interesting properties, which can be used e.g. for toluene alkylation with propylene to cymene or n-butene skeletal isomerization to isobutene (7,8). MCM-22 possesses a peculiar structure comprising two independent channel systems. The first pore system is defined by two-dimensional sinusoidal 10-membered ring channels while the second one consists of large supercages (free inner diameter of 0.71 nm, free height 1.82 nm) defined by 12-membered tings. These huge intracrystalline voids are accessible only through 10-membered tings apertures. A detailed characterization of Broensted and Lewis acid sites in this zeolite was presented recently (9). Preliminary results showed that single A1 atoms, A1-O-(Si-O)I,z-A1 pairs located in one ring and close unpaired A1 atoms are present in zeolite MCM-22 and single A1 atoms represent ca. 60 % of the framework A1 atoms. However, the number of individual types of A1 pairs/cationic sites, the distribution of A1 atoms between these sites and the effect of framework A1 content on this distribution is not known (9). In addition, there is a lack of information on the effect of synthesis procedure on the A1 distribution in MCM-22. The objective of this contribution is to derive information on the distribution of aluminium in the framework of MCM-22 and on the effect of A1 content and synthesis conditions on this distribution. For this purpose UV-Vis-NIR diffuse reflectance spectroscopy and ion exchange capacity of zeolite were investigated. The ion exchange capacity of the zeolites for Co(H) ions combined with Vis spectroscopy of as prepared Co-zeolites was employed to estimate the sum of A1-O-(Si-O)I,z-A1 pairs located in one ring and of close unpaired A1 atoms. UV-Vis-NIR spectroscopy of dehydrated Co-zeolite gives information on the number of A1 pairs and close non-paired A1 atoms and on the distribution of A1 in individual A1 pairs/cationic sites. Details of this method are described in Refs (4,5,6).

25 2. EXPERIMENTAL SECTION Samples of zeolite MCM-22 with Si/A1 ratios from 13.6 to 40 have been prepared using two different synthesis procedures. (A1)MCM-22/A samples were synthesized using aluminium nitrate and Cab-O-Sil M5 as silica source at 156 ~ for 10 days (9), while (A1)MCM-22/B from silica sol VP-AC 4039 (Bayer AG, 30 wt.-% SiO2 in water) and aluminium sulfate at 150 ~ for 4 - 9 days (longer time for lower Si/A1 ratios). In both types of synthesis, hexamethyleneimine was used as structure-directing agent. The crystallinity and phase purity of all synthesized zeolites were checked by X-ray powder diffraction (Siemens D5005 with Bragg-Brentano geometry using CuKa radiation) and scanning electron microscopy (Jeol). Sodium ion exchange capacity of zeolites was used as a measure of framework aluminium atoms. The ion exchange with Na or Co ions was carried out in the following way: Na-MCM-22 zeolites were prepared by three times repeated equilibration of calcined MCM-22 with a 1 M aqueous solution of sodium nitrate for 24 hours at ambient temperature, 100 ml of solution corresponded to 1 g of MCM-22. To obtain maximum loaded Co(II)-MCM-22, 1 g of MCM-22 was treated three times with 100 ml of a 0.05 M Co 2+ nitrate solution for 24 hours at ambient temperature to guarantee the exclusive presence of Co(II) hexaaquo complexes in ion exchanged zeolites. Samples were carefully washed with distilled water, dried at ambient temperature and ground. The chemical composition of calcined and Co exchanged MCM-22 samples was estimated after their dissolution by chelatometric titration (A1), gravimetry (Si), atomic absorption spectrometry (Co) and atomic emission spectrometry (Na). The chemical compositions of the maximum Co(H) exchanged zeolites are compiled in Table 1. The charge balance of these samples ((2Co+Na)/A1) is close to one (0.89 - 0.96). This evidences exclusive presence of divalent Co(H) hexaaquo complexes in maximum Co(R) loaded Co-MCM-22. Prior to the monitoring of the spectra of dehydrated Co(II)-MCM-22, the samples were calcined for 1 hour at 480 ~ under a flow of oxygen and then dehydrated for another 2 hours at the same temperature under vacuum at 7x10 -2 Pa. After dehydration, the samples were cooled down to ambient temperature, transferred under vacuum into the optical cell and sealed. Table 1 Chemical composition of ion exchanged Co(II)-MCM-22 samples. Zeolite Si/AI Co/AI Na/AI MCM-22/A 13.6 0.18 0.60 MCM-22/A 18.7 0.17 0.58 MCM-22/A 38 0.09 0.71 MCM-22/B 17 0.18 0.58 MCM-22/B 30 0.19 0.52 MCM-22/B 33 0.26 0.40 MCM-22/B 40 0.34 0.28 UV-Vis-NIR diffuse reflectance (DR) spectra of as-prepared and Co(lI)-exchanged MCM-22 were collected using a Perkin-Elmer UV-Vis-NIR spectrometer Lambda 19 equipped with a diffuse reflectance attachment with an integrating sphere coated by

26 BaSO4. Spectra were recorded against BaSO4 standard and in a differential mode with the parent zeolite treated at the same conditions as a reference. For details see ref. (10). The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(Roo) = (1-Roo)Z/2R~ , where ILo is the diffuse reflectance of a semi-infinite layer and F(tLo) is proportional to the absorption coefficient. 3. R E S U L T S AND DISCUSSION 3.1. Co(II) ion exchange capacity and Vis spectroscopy of as prepared Co-MCM-22 Only one absorption band with a maximum around 19 400 cm -1 possessing a shoulder at about 21 000 cm -1 is observed in the spectra of as prepared Co-MCM-22. These spectra were recently reported in ref. (5) and they are not shown in the Figures below. The exclusive presence of the 19400 cm -1 band indicates that only octahedrally coordinated Co(l]) ions, i.e. Co(II) hexaaquo complexes are present in as prepared Co-MCM-22 (11). Thus, a presence of oxidic Co species or monovalent complex ions of the [Co2+(HzO)sL]--type can be unambiguously ruled out. Note that the extinction coefficient for a Co(II) hexaaquo complex with octahedral symmetry (with symmetrically forbidden transitions) is significantly lower than the one for bipyramidal Co(li) complex. Therefore, the ion exchange capacity of MCM-22 for the Co 2+ ions reflects the number of A1 atoms arranged close enough to be balanced by divalent water complexes of metal cations, i.e. A1-O-Si-O-A1 and A1-O-(Si-O)z-A1 pairs located in one ring and unpaired A1 atoms, close enough together to balance large Co(lI) hexaaquo complex, for details see refs. (5,6). Thus, one Co(n) ion represents one aluminium pair or two single A1 atoms close enough to balance Co(H) hexaaquo complex. The difference between the total number of framework A1 atoms and A1 atoms balancing Co(H) ions corresponds to the number of single A1 atoms. 3.2. UV-Vis spectroscopy of bare Co 2+ ions The UV-Vis spectra of dehydrated Co-MCM-22 consist of a complex band in the range 14 000 - 25 000 cm -1 and a broad band centered around 30 000 cm -1 as it is shown in Fig. 1. The absorption at 30 000 cm 1 corresponds to the Co-O charge transfer (CT) band (of. ref. (11)) of some Co-O1,2-Co bridging species (4,6,11). These Co-O1,2-Co species are formed only in Co-MCM-22/A with low framework A1 content, which was synthesized using

a

./ ":./ ,......:...'. .....,...~.

i

N ~162

i

k"::

:

kl O

10000

20000

30000

wavenumber (cm -1)

40000

10000

20000

30000

wavenumber (cm -1)

40000

Fig. 1 Normalized UV-Vis spectrum of dehydrated Co-MCM-22. a) MCM-22/A: Si/A1 13.6( ); 17.8 ( - - ) and 38 ( . . . . . ); b) MCM-22/B, Si/A1 17 ( ); 30 ( - - ) and 40 ( .....

).

27 aluminium nitrate and Cab-O-Sil M5. On the other hand, only a negligible amount of these Co species is formed in MCM-22/B synthesised using aluminium sulfate and silica sol. Formation of these Co-O:,2-Co bridging species was not observed for MCM-22 zeolites with high concentration of aluminium. This is in agreement with the results on other Co-zeolites possessing significantly higher density of aluminium atoms and cationic sites for divalent cations as Co-mordenite and Co-ferrierite (Si/A1 ~ 8) (10,12). Thus, the Co-O1,2-Co bridging species are balanced by unpaired aluminium atoms. Recently, this type of close unpaired aluminium atoms and corresponding Co-O1,2-Co bridging species was reported also for zeolites ZSM-5 and Beta (4,13). However, it is necessary to point out that the attribution of the 30 000 cm-: absorption band to the Co-O:,2-Co bridging species is based only on the similarity to the CT absorption band and requires further confirmation. Moreover, the mechanism of the formation of this species during evacuation and the structure and coordination to the zeolite framework are not understood into much detail. The complex band in the range of 14 000 - 25 000 cm 1 is present in the spectra of all Co-MCM-22 samples and corresponds to the d-d transitions of the Co(H) ions in extraframework sites of silicon rich molecular sieves, cf. refs. (10,14,15,16). Because water molecules and Co-OH groups are not reflected in the NIR spectrum of dehydrated Co-MCM-22 samples (not shown in the Figures, for details see refs. (6,10)), these COOI) represent exclusively bare Co(H) ions located in cationic sites and balanced by two framework aluminium atoms, i.e. A1-O-Si-O-A1 and A1-O-(Si-O)2-A1 pairs. As follows from Fig. 2, where normalized Vis spectra of dehydrated Co-MCM-22/A and B zeolites with different framework aluminium content and maximum Co(H) loading are shown, the shape of Vis spectra of Co01) ions in MCM-22 depends both on the framework A1 content of the zeolite and on the synthesis procedure. The absorption band at 15 000 cm -1 is observed only with high Si/A1 for both synthesis procedures, while a complex band around 22 000 cm-: dominates at low Si/A1 ratios in MCM-22 synthesized using aluminium nitrate and Cab-O-Sil M5. A complex bands with a maximum around 17 000 cm -1 is present in the spectra of all Co-MCM-22. One can infer, therefore, that at least three types of bare Co(H) ions characterized by absorption around 15 000, 17 000 and 22 000 cm 1 are present in MCM-22. However, the estimation of the number of types of bare Co(if) ions, identification of the whole absorption spectra corresponding to these individual types of Co(H) ions and the description of the Co(H) coordination requires further studies, which are under progress. Thus, A1-O-Si-O-A1 and A1-O-(Si-O)2-A1 pairs are located in three or more different local arrangements in the MCM-22 framework. The differences of local arrangements of A1 pairs correspond to different geometry of tings accommodating A1 pairs and/or to different distribution of aluminum pair in one ring. The relative concentration of aluminium pairs possessing cationic sites for Co(1I) ions is reflected in the intensity of the absorption band at 15 000 cm-:, which decreases while relative concentrations of aluminium pairs characterized by Co01) ions exhibiting absorption bands around 18 000 and 22 000 cm-: increase with increasing framework aluminium content. In contrast to aluminium pairs characterized by Co(H) band at 15 000 cm-: the formation of which is independent on the synthesis conditions, the relative concentration of A1 pairs characterized by Co(N) ions exhibiting absorption bands around 18 000 and 22 000 cm 1 is significantly affected by the conditions of MCM-22 synthesis. The formation of A1 pairs characterized by Co0I) absorption around 22 000 c m -1 significantly increases in the case of synthesis using aluminium nitrate and Cab-O-Sil M5.

28 3.3. Aluminium distribution in MCM-22 Characteristic features of the formation of CoxOyspecies in zeolites, i.e. absorption in the Co(H) ions window around 12 000 cm -1 and steadily increasing absorption from NIR to UV region were not observed in the spectra of maximum Co(lI) loaded MCM-22/A and B. It indicates that significant formation of these CoxOy species in dehydrated maximum Co(H) loaded Co-MCM-22 can be excluded. Thus, only single and close unpaired aluminium atoms and their pairs are present in MCM-22. The extinction coefficient of the CT band at 30 000 cm -1 corresponding to the bridging Co-O-Co species was estimated for Co-beta zeolites. Its value is significantly higher (ca. 100 times) compared to the extinction coefficients corresponding to the d-d transitions of bare Co(H) ions (13). Thus, negligible minority of close unpaired A1 atoms (less 3% of total A1), characterised by CT band at 30 000 cm -1, corresponds to the maximum intensity of the Co(H) CT band in the spectrum of dehydrated Co-MCM-22/A (Si/A1 = 38, synthesis using aluminium nitrate and Cab-O-Sil M5). In this case, the ion exchange capacity of MCM-22 for Co(II) ions represent a measure of the concentration of A1-O-Si-O-A1 and A1-O-(Si-O)2-A1 pairs located in one ring in MCM-22 framework.

The effect of the framework aluminit~n content and the conditions of synthesis on the relative concentration of A1-O-Si-O-A1 and A1-O-(Si-O)z-A1 pairs and single isolated aluminium atoms in MCM-22 is depicted in Fig. 3. The aluminium distribution is dramatically affected both by the framework aluminium content and the conditions of zeolite synthesis. In the case of MCM-22/A, synthesized using aluminium nitrate and Cab-O-Sil, single aluminium atoms predominate in the whole aluminium concentration range ( 6 0 - 80 % of all aluminium atoms). The relative concentration of single aluminium atoms decreases with increasing framework aluminium content. On the other hand, the relative concentration of single aluminium atoms in MCM-22/B, synthesized using aluminium sulfate and silica sol, decreases with increasing framework aluminium content and A1 atoms in pairs represent the majority of A1 atoms in samples with low framework aluminium content in the zeolite (70 % of aluminium atoms for Si/A140).

a

.~

~J ~

'r N

10000

15l)00

20l)00

wavenumber (cm "z)

25l}00

10000

15600

20600

wavenumber (cm q)

25600

Fig. 2 Normalized Vis spectrum of dehydrated Co-MCM-22. a) MCM-22/A: Si/A1 13.6 ( ); 17.8 ( - - ) and38 (. . . . . ); b) MCM-22/B, Si/A117 ( ~ ) ; 30 ( - - ) and 40 (. . . . . ).

29 100 ...................

r~

E] 75 o

~

50

o ~

2-

o,

1-

O"'-. m

O

I-'l- -

25

.................. O

.

[]

...... [] ..... []

0

lo

2'0

3'0

4'0

Si/AI

Fig. 3 Relative concentrations of isolated single A1 atoms (e) and A1 in pairs (m) in MCM-22/A (empty), MCM-22/B (full), synthesised by different procedures.

0

lo

2'0

Si/AI

3'o

4'0

Fig. 4 Concentration of isolated single A1 atoms (e) and A1 in pairs ( I ) in MCM-22/A (empty), MCM-22/B (full) synthesised by different procedures.

The effect of the framework aluminium content and the conditions of synthesis on the concentration of aluminium pairs and single isolated aluminium atoms in MCM-22 is depicted in Fig. 4. The ratio between single aluminium atoms and A1 pairs increases with decreasing aluminium content in the framework. However, the decrease in the concentration of aluminium atoms in pairs in MCM-22/B, synthesized using aluminium sulfate and silica sol, is significantly lower as compared to those of MCM-22/A. Moreover, at low framework A1 content, there is a significantly higher concentration of A1 pairs in MCM/22B (3.5 times for Si/A1 ca. 40), synthesized using aluminium sulfate and silica sol than in MCM-22/A, synthesized using aluminium nitrate and Cab-O-Sil M5. The increase in the relative concentration of A1 atoms in pairs in MCM-22/B described above and the dramatic effect of synthesis conditions on aluminium distribution (20 and 60 % aluminium atoms in pairs in MCM-22/A and B, for Si/A1 = 40, respectively) clearly show that the aluminium distribution in MCM-22 is not controlled by statistic rules, but by the conditions of zeolite synthesis. This indicates that the distribution of aluminium in MCM-22 zeolites possessing very similar chemical composition but prepared by different synthesis procedures can be significantly different. We are still far from final conclusions concerning the key parameters of the synthesis procedure, which controls the aluminium distribution. However, it is possible at present to suggest that the final distribution of aluminium depends already on the early stages of the synthesis when the most simple structural blocks are being formed. On the other hand, it is evident that the differences in the aluminium distribution in zeolites will be reflected in the ion exchange capacity for polyvalent cations or cationic complexes, distances among acid centers and their positions in the zeolite channel system. Therefore, different catalytic behaviour of these zeolites could be expected. 4. CONCLUSIONS Isolated single aluminium atoms, single aluminium atoms close enough to balance Co(H) hexaaquo complexes but not bare Co(H) ions and various types of aluminium pairs

30 located in one ring and forming charge compensating sites for bare divalent cations are present in MCM-22. The aluminium distribution in MCM-22 is not controlled exclusively by statistic rules, rather it is dramatically affected by the content of aluminium in the framework and the conditions of zeolite synthesis. The aluminium distribution is most probably more affected by the synthesis conditions rather than by the zeolite composition. Single aluminium atoms, which are close enough together to balance Co(H) hexaaquo complexes are present only in MCM-22 with lower aluminium content and synthesized using aluminium sulfate as A1 source(MCM-22/B). Three (or more) types of tings containing aluminium pairs and forming charge compensating sites for bare divalent cations are formed in MCM-22. The relative concentration of aluminium pairs characterized by Co(H) absorption at 15000 cm -1 increases and the relative concentration of aluminium pairs characterized by Co(H) absorption in the region 20 000 -25 000 cm -1 decreases with decreasing framework aluminium content.

REFERENCES

10. 11. 12. 13. 14. 15. 16.

T.F. Degnan Jr., Topic. Catal. 13 (2000) 349. L. Cerven3?, K. Mikulcov~i and J. (~ejka, Appl. Catal. A, 223 (2002) 65. A. Corma, Chem. Rev. 95 (1995) 559. J. D~de~ek, D. Kauck2~ and B. Wichterlov~i, Chem. Comm., 11 (2001) 970. J. D~de~ek, M. Tudor and J. t~ejka, Zeolites and Mesoporous Materials at the Dawn of the 21 st Century, eds. A. Galarneau, F. Di Renzo, F. Fajula and J. Vrdrine, Stud. Surf. Sci. Catal. 135 (2001) 182. J. D~de~ek, D. Kauck3?, O. Gonsiorov~i and B. Wichterlov/l, Phys. Chem. Chem. Phys., submitted. J. (~ejka, A. Krej~i, N. Zilkov/l, J. Kotrla, S. Ernst, A. Weber, Micropor. Mesopor. Mater., in press. M.A. Asensi, A. Corma, A. Martinez, J. Catal. 158 (1996) 561. J. t~ejka, J. Drde~ek, M. Tudor, N. Zilkov/t, J. Kotrla, S. Ernst, Zeolites and Mesoporous Materials at the Dawn of the 21 st Century, eds. A. Galarneau, F. Di Renzo, F. Fajula and J. Vrdrine, Stud. Surf. Sci. Catal. 135 (2001) 352. J. D~de~ek, B. Wichterlov~i, J. Phys. Chem. B, 103 (1999) 1462. A.B.P Lever, "Inorganic Electronic Spectroscopy", Elsevier, (1984). D. Kauck2~, J. Drde~ek and B. Wichterlov/t, Micropor. Mesopor. Mater. 31 (1999) 75. Z. Sobalik, J. D~de~ek, D. Kauck2~ and B. Wichterlov~t, Micropor. Mesopor. Mater., submitted. J. D~de~ek, D. Kauck3? and B. Wichterlov~t, Micropor. Mesopor. Mater. 35-36 (2000) 483. K. Klier, Adv. Chem. Series, 101 (1971) 480 and references therein. A.A. Verbeckmoes, B.M. Weckhuysen, R.A. Schoonheydt, Micropor. Mesopor. Mater. 22 (1998) 165.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

31

Grafting of aluminium on dealuminated H-BEA using alkoxides Anna Omegna, Mohamed Haouas, Gerhard Pirngruber and Roel Prins Laboratory of Technical Chemistry, Swiss Federal Institute of Technology (ETH), CH-8093 Zurich, Switzerland The use of aluminium alkoxides for the incorporation/grafting of aluminium into the partially dealuminated lattice of zeolite-J3 was studied. By means of FTIR and 1H MAS NMR spectroscopy the formation of Bronsted acidity was observed after reaction and subsequent calcination. 27A1MQ MAS NMR was able to resolve aluminium sites with close isotropic chemical shifts, thus enabling to follow the process of aluminium insertion. In particular, it was shown that tetrahedral aluminium at framework position was formed during grafting. Highly distorted extraframework tetrahedral aluminium was formed as well, which belongs to an amorphous silica-alumina phase. 1. INTRODUCTION Dealumination is generally applied to increase the thermal stability of zeolites and to create mesopores, which enable shorter diffusion paths. However, since the activity in most of the zeolite-catalyzed reactions is primarily governed by the amount of aluminium in the framework, it would be desirable to re-insert aluminium into the partially dealuminated zeolites, thereby maximizing stability and activity. Several approaches have been undertaken, e.g. the reaction with aluminium halides [ 1-4]. But many questions are still open regarding the incorporation of aluminium into the zeolite lattice and the nature of the aluminium species formed during the reaction. Recently, the use of aluminium alkoxides for the grafting of aluminium into siliceous MCM-41 was reported [5]. In the present work the use of aluminium alkoxides for the post-synthetic incorporation of aluminium into the framework of dealuminated zeolite-[3 is described. The procedure using alkoxides is milder than the one using halides. The mineral acids formed in the grafting ofhalides can attack the zeolite lattice, but the alcohols formed in grafting of alkoxides cannot do this. A detailed characterization of the resulting materials is discussed. 2. EXPERIMENTAL

2.1 Materials Microcrystalline zeolite-[3 PB 13Na (CU Chemie Uetikon) was calcined in flowing air at 550~ for 8 h to remove the template. HI3 was then obtained by subsequent three-fold exchange with 1 M ammonium nitrate solution under reflux, followed by washing with deionized water and calcination at 550~ for 5 h. Dealumination of the HI3 was carried out at 600~ for 10 h in a muffle furnace (giving the sample H[3-T), followed by a two-fold washing with a 1 M hydrochloric acid solution at 25~ for 16 h, washing with deionized water and

32 drying at 120~ for 3 h (giving the sample H[3-TL). The dealuminated material H[~-TL was then dispersed in dry hexane and added to a solution of aluminium isopropoxide in dry hexane. The amount of AI(OPr)3 was adjusted in order to achieve a bulk Si/A1 ratio of 5 in the final material. The mixture was stirred for 24 h under nitrogen at room temperature. The resulting material was then washed with dry hexane and dried in air at 25~ (giving the sample H[3-TLA). The dried material H[3-TLA was calcined in air at 550~ for 4 h, to give the sample H[3-TLAT. 2.2 Characterization Elemental analysis was done by means of laser ablation coupled with inductivelycoupled plasma mass spectrometry (LA-ICP-MS) as described in ref. [6]. XRD powder diffraction patterns from 5 ~ to 60 ~ 20 were obtained on a Siemens D5000 diffractometer using CuKc~ radiation (L - 1.5406 A). N2 adsorption measurements were carried out a t - 1 9 6 ~ on a Micromeritics ASAP Tristar 3000 using the conventional volumetric technique. FTIR spectra were obtained at 4 cm -1 resolution with a Mattson Galaxy spectrometer equipped with an MCT detector. Prior to measurements, the samples were pressed into self-supporting wafers (10-15 mg/cm 2) and evacuated at 350~ for 6 h under a residual pressure of 10 -6 Pa. For quantitative comparison spectra were normalized using the integrated intensities of the Si---O vibration overtones. Solid state NMR spectra were recorded on a Bruker Avance AMX400 spectrometer operating at a static field of 9.4 T. 27A1 MAS NMR measurements were performed at a resonance frequency of 104.26 MHz. Spectra were recorded at a spinning rate of 10 kHz, a pulse length of 0.27 #s Qr/12) to insure quantitative measurements, and a delay time of 1 s. For quantitative evaluation, all samples were weighed, and the spectra were calibrated by measuring a known amount of (NH4)AI(SO4)2" 12H20 under identical conditions [7]. MQ MAS NMR experiments were recorded at a spinning rate of 15 kHz using a 4 mm probehead. The two-pulse z-filtered procedure was applied. The excitation pulse was n and the conversion pulse was n/3. In the tl dimension, 256 points were acquired with an increment of 33.6 ps.lH MAS NMR measurements were performed at a resonance frequency of 400.13 MHz. Spectra of H[3-TL loaded with deuterated pyridine as well as H[3-TLAT and Hi3-TLAT loaded with deuterated pyridine were recorded in a 4-mm rotor at a spinning rate of 10 kHz, pulse length of 2.7 #s (7r/4) and delay time of 10 s. The spectrum of H[3-TL was recorded in a 7-mm rotor at spinning rate of 7 kHz, pulse length of 3.5/~s (7r/4) and delay time of 10 s. All zeolites had been previously dehydrated under vacuum at 350~ for 6 h. Adsorption of deuterated pyridine was carried out on the dehydrated materials at room temperature. After 1 h contact, the samples were evacuated at 100~ to remove the physisorbed pyridine. For a quantitative comparison, all samples were weighed, and the spectra were calibrated by measuring a known amount of 1,1,1,3,3,3-hexafluoro-2-propanol under identical conditions [7]. 3. R E S U L T S AND D I S C U S S I O N

Results of the X-ray diffraction analysis (not shown) revealed that crystallinity was maintained after the subsequent treatments. Table 1 summarizes nomenclature and properties of the different beta samples. Elemental analysis of the sample H[3-T showed that the bulk composition did not change after thermal treatment compared to the parent H[~. The aluminium atoms that were extracted from the lattice remained in the zeolite in the form of

33 extraframework aluminium. The Si/A1 ratio increased appreciably after acid treatment, indicating that leaching of a considerable amount of aluminium took place. Nitrogen adsorption analysis revealed that the BET surface area as well as the micropore volume did not change appreciably upon dealumination. However, mesopore volume and external surface area increased, indicating that a secondary pore system developed during the treatments. Reaction of H[3-TL with aluminium isopropoxide led to a decrease of the Si/A1 ratio to 4.9, in agreement with the composition of the reaction mixture. This means that all the aluminium was retained in the final material. Reaction with AI(OPr)3 led to a decrease in BET surface area as well as micro- and mesopore volume. The decrease in mesopore volume was more pronounced than that in the micropore volume, suggesting that AI(OPr)3 went preferentially to the mesopores. The mesopore volume was partially restored after calcination. Table 1. Nomenclature and properties of the beta zeolites Sample

Subsequent treatments

HI3

Si/A1 (+0.5)

BET surface area (+5) (m 2g-I)

Micropore Volume (cm 3g-l) a

Mesopore Volume (cm 3g-l) a

External surface area (m 2g-1) a

12.1

585

0.137

0.168

165

HI3-T

Calcination, 600~

12.1

593

0.129

0.216

241

H[3-TL

(2x) 1M HC1, 25~

65.4

602

0.133

0.223

244

H~-TLA

AI(OPr)3, 25~

4.9

476

0.119

0.079

220

H[3-TLAT Calcination, 550~ 4.5 490 0.112 a Calculated with the t-plot method according to Lippens and De Boer.

0.141

189

FTIR spectra of the different samples are compared in Fig. 1. The IR spectrum of the parent zeolite HI3 shows four bands at 3780, 3743, 3665 and 3610 cm -1 (see Fig. 1a). The sharp peak centered at 3742 cm -1 is due to the OH vibration of free silanols SiOH present on the external and internal (low frequency tail) surfaces of the microcrystals. The band at 3610 cm ~ is attributed to strongly acidic bridged Si(OH)A1 hydroxyls [8-10]. The weak bands at 3665 and 3780 cm -~ are assigned to low acidity OH groups bonded to extralattice aluminium [11]. There is a large feature in the 3000-3500 cm -~ range which has been assigned either to SiOH groups in framework defect sites interacting through hydrogen bonds [12] and/or to Si(OH)A1 groups interacting with oxygen atoms of the framework [7]. After thermal treatment the intensity of the band at 3610 cm -~ decreased, indicating that some aluminium atoms were extracted from the framework (see Fig. l b). Acid treatment on H~ led to the spectrum of Fig. 1c. The signal due to silanols increased remarkably and shifted to lower frequency. The large feature centered at 3500 cm -1 became more intense. This suggested that a considerable amount of defect sites was created during the treatment of zeolite H[3-T with hydrochloric acid. After grafting with AI(OPr)3 the silanols band remained unaffected in position and intensity (see Fig. 1d). After calcination the band at 3610 cm -1 associated to Si(OH)A1 groups reappeared, indicating that aluminium was incorporated in the framework. At the same time the silanols band shifted back to the original value (3743 cm 1) and decreased in intensity (see

34 Fig. l e). This suggests that insertion of aluminium into the lattice proceeded through the reaction of aluminium with defect sites on the internal surface (nests).

3743 3730

~_J

i

(a)

36OO

cD

r~

(_b_) I

3800

'36'00'34'00' Wavenumbers (cml)

32'00

'

3000

Fig. 1. FTIR spectra in the VOH region of (a) HI3; (b) H[3-T; (c) HI3-TL; (d) H[3-TLA; (e) HI3-TLAT. In order to confirm the presence of Bronsted acidity in the final material, 1H MAS NMR spectra were recorded (see Fig. 2). In the spectrum of the dealuminated H[3-TL (spectrum 1, Fig. 2), only two peaks are visible, a narrow and intense signal at 1.8 ppm due to silanol groups [ 13] and a broad component between 3 and 7 ppm, which can be assigned to hydrogen bonded silanols [ 12] and/or to Bronsted groups interacting with the framework [7]. This broad component corresponds to the broad feature in the IR spectrum between 3000 and 3500 cm ~ [7]. After grafting of AI(OPr)3 and subsequent calcination, the ~H MAS NMR spectrum showed an additional signal at 4 ppm overlapping with the broad component at 3-7 ppm (spectrum 3, Fig. 2). At this frequency protons of Si(OH)A1 groups are expected to resonate [13]. This is in agreement with the IR results, which showed the formation of Bronsted acidity upon calcination of the aluminated zeolite-j3. In order to distinguish strong and weak acid sites and to clarify the assignment of the broad component at 3-7 ppm, adsorption of deuterated pyridine was carried out. Adsorption of deuterated pyridine on the dealuminated zeolite HI3-TL (line 2, Fig. 2) led to the appearance of a feature at about 10 ppm, due to silanols interacting via H-bond with the pyridine. At the same time, the band at 1.8 ppm decreased and the broad feature at 3-7 ppm disappeared. No peak of pyridinium ions adsorbed on Bronsted acid sites could be detected. When pyridine was adsorbed on the zeolite H[3-TLAT, however, the typical signal of pyridinium ions appeared a t - 1 5 ppm [13], in addition to the peak at 10 ppm. This unequivocally proves that Bronsted acid sites were formed upon reaction with AI(OPr)3 and subsequent calcination. Quantification of the 1H MAS NMR spectra revealed that about 360 pmol/g of Bronsted acid sites were created, corresponding to approximately

35 10% of the total aluminium content. This means that the remaining aluminium was present at extra-lattice positions, either as aluminium (hydr)oxide or as silica-alumina.

.

2\ ........................

|

18

I

10

9., ....

, .......... ,"

,..

I

2 8 (ppm)

1

i

-14

-22

Fig. 2. ZH MAS NMR spectra of interaction of zeolites [3 with deuterated py: Spectrum1" zeolite H[3-TL. Spectrum 2: H[3-TL loaded with deuterated py. Spectrum 3" zeolite HI3-TLAT. Spectrum 4: HI3-TLAT loaded with deuterated py. Asterisks denote spinning side bands.

27A1MAS NMR

spectra of the different zeolite samples are compared in Fig. 3. The spectrum of the parent zeolite H[~ (see Fig. 3a) is characterized by an intense signal at 50 ppm, due to aluminium in tetrahedral coordination. A broad weak feature is also present at ca. 0 ppm, assigned to octahedrally coordinated aluminium species. After thermal treatment followed by acid leaching, the signal at 50 ppm was strongly decreased and that at 0 ppm disappeared (see Fig. 3b), indicating that extraction of aluminium occurred. Treatment with AI(OPr)3 resulted in an increase of the overall A1 concentration, as can be seen in Fig. 3c. Tetrahedral as well as octahedral species were present in the sample. A third resonance was observed at ca. 30 ppm, which has been assigned to five-coordinated [ 14] as well as to highly distorted tetrahedral A1 species [15]. The resonance at 0 ppm was the most intense, indicating that the octahedral coordination was predominant. After calcination at 550~ the intensity ratio between 4- and 6-coordinated A1 species was reversed and the tetrahedral species became predominant (see Fig. 3d). An increase of the species responsible for the signal at 30 ppm was also observed. In the 27A1MAS NMR the lines are shifted and broadened due to quadrupolar effects. A deconvolution of the signals is impossible when they strongly overlap, as in the spectra of Fig. 3. Therefore, 27A1 MQ MAS NMR measurements were performed to separate the peaks of the different aluminium species and to allow the determination of their isotropic shifts and quadrupolar coupling constants. In the MQ MAS spectrum of the dealuminated H[3-TL two resonances were visible at 60.2 and 55.5 ppm, called AI(IV)a and AI(IV)b, respectively (see Table 2). They are narrow and experience a small quadrupolar interaction. This suggests that these signals are associated to tetrahedral framework aluminium species. Their intensity was very weak, which explains why the Bronsted acid sites corresponding to these framework aluminium atoms could not be detected by pyridine adsorption. In the MQMAS spectrum of H[~-TLA (Fig. 4a) four new resonances were detected. An additional 4-coordinated

36 aluminium site AI(W)c with a large quadrupolar broadening appeared at 69.5 ppm, which was assigned to extraframework aluminium EFA1 species.

(d) (c) (b) _

_

.

A

,

,

(a) 160

120

i0

40 6(ppm)

6

-,i0

-80

Fig. 3.27A1 MAS NMR spectra of: (a) HI3; (b) H[3-TL; (c) H[3-TLA; (d) HI3-TLAT. Moreover, in the region of 0-20 ppm two octahedrally coordinated sites, AI(VI)a and AI(VI)b, became visible. A sixth resonance was seen in the region of pentacoordinated aluminium, AI(V). Both sharp tetrahedral signals AI(IV)a and AI(IV)b increased upon grafting (see Table 2), suggesting that incorporation of aluminium into the framework took place already at room temperature. After calcination at 550~ (Fig. 4b) both resonances corresponding to octahedral aluminium decreased in intensity. Most of the intensity of octahedral aluminium signals was converted to highly distorted AI(IV)c and to pentacoordinated AI(V). The species AI(IV)b changed very little, whereas the tetrahedral species AI(IV)a almost disappeared. The fact that the total aluminium amount was the same in both spectra proves all aluminium was accounted for in the spectra. The increase of the framework species AI(IV)a and AI(IV)b shows that a partial incorporation of aluminium into the lattice already occurred during the grafting step. Since the material was not thermally treated, the bonds of the grafted aluminium to the framework were not fully condensed and probably still weak. This could explain why Bronsted acid sites corresponding to the created tetrahedral aluminium species were not well resolved in the IR spectrum of HI3-TLA (see Fig. ld). Calcination at 550~ led to a full incorporation of the aluminium into the lattice and, thus, also to the appearance of the IR band at 3610 cm -1. However, no additional framework aluminium was created during calcination. Extraframework AI(IV)c species as well as AI(V) were preferentially formed at the expense of octahedral aluminium. The species AI(IV)a was even partially extracted from the lattice, indicating that this aluminium species was weakly coordinated to the framework and not stable at high temperatures. Also van Bokhoven et al. [ 16] differentiated two types of T-sites in zeolite [3, one which easily dealuminates (with an isotropic shift of 60 ppm - AI(IV)a) and another one, which does not (with an isotropic shift of 55 ppm - AI(IV)b). This is in qualitative agreement with our results. The total framework aluminium content derived from 27A1 NMR is in good agreement with the 1H NMR data of Bronsted acidity for zeolite H[3-TLAT. Moreover, it can be noted that framework species only represent about 25% of the total amount of tetrahedral aluminium. The predominant tetrahedral species is the EFA1

37

AI(IV)c, which formed during grafting and increased by calcination. For reasons that will be explained in a separate publication, we believe that this species is associated with an amorphous phase of silica-alumina.

.....

F

Fl(ppm)

1

-,o o AI(VI)~. I ~ . A i ( V I ) 9

~9 i v ) ,

100

50

~"" 0

.. -50

AI(VI)a ~AI(VI)b

b

" AI(V) "" " Al(iV)aA ~ . a AI(IV)b ~

150

-40

40

Al(IV)~ ~ . ~ .Al(V). : Al(IV)b AI(IV)~

8o

120 -100

150 100

F2(ppm)

50

0

-50

120 -100

FZ(ppm)

Fig. 4. 27A1 MQ MAS NMR sheared spectra of (a) HI3-TLA and (b) H[3-TLAT. The F2 projection contains the quadrupolar lineshape, whereas the F1 projection shows the pure isotropic spectrum.

Table 2. Concentration and quadrupolar parameters of the different A1 species as derived from the 27A1 MQ MAS NMR. sample HI3-TL

HI3-TLA

H~-TLAT

8iso (ppm) Qcc(MHz) Conc. (gmol/g) 8iso (ppm) Qcc(MHz) Conc. (~tmol/g) 8iso (ppm) Qcc(MHz) Conc. ( tool/ )

AI(IV)c 69.5 5.3 480 65.8 5.6 770

AI(IV)a 60.2 2.7 10 60.2 2.7 130 60.2 2.7 40

AI(IV)b 55.5 2.2 70 55.5 2.2 200 55.5 2.3 200

AI(V). 42.4 5.2 250 38.2 5.2 540

AI(VI)a 7.5 5.5 820 9.2 6.9 660

AI(VI)b 12.4 3.8 940 11.6 4.5 480

38

4. CONCLUSIONS FTIR spectroscopy as well as 1H NMR spectroscopy showed that Bronsted acid sites were formed as a result of the reaction of dealuminated 13 with AI(OPr)3 followed by calcination. Evolution of the IR spectra showed that incorporation of aluminium into the lattice proceeded through the reaction of AI(OPr)3 with the silanol nests of the dealuminated zeolite-J3. By means of 27A1 MQ MAS NMR spectroscopy it was observed that aluminum atoms in framework position were formed already after grafting. The majority of the grafted aluminium was, however, not incorporated into the zeolite lattice, but formed an amorphous silicaalumina phase. ACKNOWLEDGMENTS

The authors thank Prof. D. Gtinther for carrying out the elemental analysis. REFERENCES

1. C.D. Chang, C.T.-W. Chu, J.N. Miale, R.F. Bridger and R.B. Calvert, J. Am. Chem. Soc., 106(1984) 8143. 2. R.M. Dessau and G.T. Kerr, Zeolites, 4 (1984) 315. 3. M.W. Anderson, J. Klinowski and L. Xinsheng, J. Chem. Soc., Chem. Comm., 1596 (1984). 4. K. Yamagishi, S. Namba and T. Yashima, J. Catal., 121 (1990) 47. 5. R. Mokaya and W. Jones, Phys. Chem. Chem. Phys., 1 (1999) 207. 6. A. Omegna, M. Haouas, A. Kogelbauer and R. Prins, Micropor. and Mesopor. Mater., 46 (2001) 177. 7. M. M~iller, G. Harvey and R. Prins, Micropor. and Mesopor. Mater., 34 (2000) 281. 8. G. Qin, L. Zheng, Y. Xie and C. Wu, J. Catal., 95 (1985) 609. 9. A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M.Padovan and C. Otero Are/m, J. Chem. Soc., Faraday Trans., 88 (1992) 2959. 10. H. Kn6zinger and S. Huber, J. Chem. Soc., Faraday Trans., 94 (1998) 2047. 11. L.M. Kustov, V.B. Kazansky, S. Beran, L. Kubelkov/l and P. Jim, J. Phys. Chem., 91 (1987) 5247. 12. V.L. Zholobenko, L.M. Kustov, V.Y. Borovkov and V.B. Kazansky, Zeolites, 8 (1988) 175. 13. M. Hunger, Catal. Rev.-Sci. Eng., 39 (1997) 345. 14. J. Sanz, V. Forn6s and A. Corma, J. Chem. Soc., Faraday Trans., 84 (1988) 3113. 15. A. Samoson, E. Lippmaa, G. Engelhardt, U. Lohse and H.G. Jerschkewitz, Chem. Phys. Lett., 134 (1987) 589. 16. J.A. van Bokhoven, D.C. Koningsberger, P. Kunkeler, H. van Bekkum and A.P.M. Kentgens, J. Am. Chem. Soc., 122 (2000) 18482.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

39

Influence of various synthesis parameters on the morphology and crystal size of zeolite Z n - M F I A. Katovic 1, G. Giordano 1 and S. Kowalak 2 1Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria via Bucci, 1-87030 Rende (CS), Italy e-mail: katovic @unical.it 2 Faculty of Chemistry, A. Mickiewicz University, 60-780 Poznan, Poland The use of the zinc-phosphate complex in the hydrothermal synthesis has been proven to be a good way for obtaining zinc-zeolite materials, especially in the case of the zeolite Zn-MFI that can be prepared in a large range of Si/Zn ratios [ 1, 2]. Moreover, the pH values of the starting hydrogel play an important role on the nature of the crystalline phases obtained. For that reason the first step in the synthesis optimization is the definition of the pH range that is specific for each zeolite type. The crystal size and morphology can be varied by altering the chemical synthesis parameters (zinc, organic and water content as the main ones) as well as the source of silica. Moreover, the isopropanol decomposition is carried out with the purpose to valuate the acidity of the prepared Zn-MFI samples.

1. INTRODUCTION The introduction of transition metals into the zeolites enlarges their application in catalysis [3, 4]. First of all, the isomorphic substitution of silicon and/or aluminium with other metals modulates the acidity of the zeolite and enables its use as a catalyst in acid catalysis. Secondly, the presence of transition metals in the zeolite structure opens the way to their application as redox and bifunctional catalysts. Zinc, as a component of catalysts used for hydrogenation-dehydrogentaion reaction, is introduced into the zeolite structure since it can be interesting in industrial processes like production of methanol [5, 6]. Another catalytic use of the zinc containing zeolites, especially zeolite MFI turned out to be very active, is in the aromatization of the light alkanes [7, 8]. Different ways are used for the introduction of metals into the zeolite framework: ion exchange, solid state reaction, chemical vapour deposition and direct synthesis. The preparation by direct hydrothemal synthesis utilizing zinc phosphate complexes of zinccontaining zeolites, such as MFI, MTW and TON, has already been presented and the catalytic activity in the cumene cracking has been verified [ 1, 2]. In this work the results obtained from a detailed study on the parameters affecting the synthesis of Zn-MFI zeolite-type are presented. A particular attention is made on the influence of the water and organic contents on the morphology and crystal size, as these are important in the catalysis application.

40 2. EXPERIMENTAL

2.1 Synthesis of Zn-MFI The molar composition of the starting hyrogels was the following: x N a 2 0 - y TPABr - q Zn(NO3)2/p

H3PO4 - SiO2 - w H20

where x = 0.08 - 0.5 (depends on the value of q); y = 0.02 - 0.20; w = 10 - 50 and q = 0 0.1 while the ratio p/q was always equal to 3. Three different silica sources were used: precipitated silica (A) - s.s.a. 180 m~/g (BDH), precipitated silica (B)- s.s.a. 550 mZ/g (BDH) and silica fumed (C) - s.s.a. 320 m2/g (Sigma-Aldrich). The synthesis procedure was the same as the previously published one [1]. The syntheses were carried out under autogenous pressure in static conditions at 170 ~ The crystallization time varied in accordance to the zinc, organic and water content. All the samples were firstly identified by XRD (Philips PW 1730/10 diffractometer, Cu K~I radiation) and the most interesting ones were further characterized by scanning electron microscopy (Stereoscan 360 Cambridge Instruments), atomic absorption spectrophotometry (GBC 932 AA) and thermal analysis TG-DTG-DTA (Netzsch STA 409).

2.2 Catalyst preparation and tests The zeolite samples subjected to the catalytic tests were prepared in four different modes: i) the as-synthetized Na-Zn-MFI form was calcinated at 550 ~ for 5 hours (T1); ii) the as-synthetized zeolite samples in Na-form were firstly treated with a 0.1 M NH4C1 solution and then calcinated as in i) (T2); iii) the samples were calcinated as in i) and then the procedure ii) was applied (T3); iv) the samples prepared as in ii) were additionally treated with a 0.1 NH4C1 solution (T4). The isopropanol decomposition catalytic tests were carried out on a pulse microreactor over 15 mg-catalyst samples that were activated at 400~ for half an hour in helium stream prior to the reaction. The test conditions were: substrate pulse lgl, gas flow 50 ml/min and the reaction temperature 230 ~ and 280 ~

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of the Zn-MFI The previously obtained results from the catalytic tests on the cumene cracking gave an input for the further optimisation of the direct hydrothermal synthesis of the Zn-MFI zeolite-type from the starting reaction mixtures containing zinc-phosphate complexes. This method was proven to be very successful in the case of iron incorporation into the various zeolite structure types. Its simple and economic use is justified on the basis of the chemical limits given by iron cation behaviour in the highly alkaline media. In the case of zinc cations, they can be introduce directly into the hydrogel by dissolving a suitable salt, but the restriction of the zeolite crystallization field is observed. The dense phases are favoured and the amounts of zinc that can be incorporated is limited. The advantages of utilizing zincphosphate complexes are the versatility of the preparation method that has an effect not only on the nature of the crystalline phases obtained but also on the tailoring of the morphological and dimensional properties of the same.

41 Table 1. Morphology and crystal size of some zeolite Zn-MFI samples. N~ x Org w Si/Zn morphology

cryst, size, gm

A1

0.08

0.08

10

100

brick-like

-- 8

B1

0.1

0.08

10

100

spheroid agg.

15 + 22

B2

0.1

0.08

20

100

spheroid agg.

18 + 24

B3

0.1

0.12

30

100

B4

0.1

0.08

30

100

C1

0.1

0.08

30

100

See Fig. 2.

C2

0.1

0.08

50

100

See Fig. 2.

C3

0.1

0.12

30

100

See Fig 1.

A2

0.3

0.08

10

33.3

spheres

8 + 14

B5

0.3

0.08

20

33.3

spheres

3+9

See Fig. 1. brick-like

28 + 40

The influence of the following chemical parameters on the synthesis of the zeolite ZnMFI was investigated: amount of zinc, nature of the silica source, organic compound and water content. The examined range of the Si/Zn ratios in the hydrogel is 100 - 10, the highest value of the zinc content was taken on the basis of the observed co-crystallisation with dense phases from the reaction mixtures prepared with a precipitated silica source having a s.s.a, of 180 m2/g. The pH value of the reaction mixture has to be in the range 9 - 12. Otherwise the crystallization process can not proceed and the obtained product remains completely amorphous or the crystallization is not complete. While in the case of high zinc contents in the hydrogel, the dense phases appear.

Figure 1: Scanning electron micro graphs of the Zn-MFI samples obtained from gels prepared with different silica sources: (a) precipitated silica: sample B3 and (b) silica fumed:sample C3 ( ~ = 20 gm).

42 Table 2. Bulk chemical analysis of the representative zeolite Zn-MFI samples. (hydrogel molar composition: x Na20 - 0.08 TPABr - q Zn(NO3)E/p HaPO4 - SiO2 - w H20) N~ x Na20 wH20 (Si/Zn)gel (Zn)uc (Si/Zn)zeolite B4

0.1

30

100

1.1

86

B5

0.3

20

33.3

2.4

39

C4

0.45

40

20

4

23

C5

0.7

40

10

5.8

16

As expected, neither of the previously mentioned chemical parameters influence the nature of the obtained zeolite type if the right pH value of the gel is chosen, only in the case of the silica source having the lowest s.s.a. (A) the co-crystallization with a dense phase is observed for zinc contents higher than 0.08 in the initial reaction mixture. The morphology and crystal sizes vary remarkably by changing one of the parameters (representative zeolite samples are presented in Table 1) and it can be useful in the preparation of the catalysts in the case of the diffusion controlled catalyzed reactions. On the other hand the zinc incorporation into the MFI structure depends only on its amount present in the starting hydrogel. In Table 2. the chemical analysis of the four samples chosen for the catalytic testing is shown. The cinetics of the crystallization process was not determined, so the influence of the organic cation content resulted negligible as the crystallization times for stopping the reaction, chosen only on the basis of the zinc content in the hydrogel, were always longer than actually needed. This was possible because the zeolite Zn-MFI exhibits a good thermodynamical stability; when the dense phase is present in the final product it is form simultaneously with the zeolite phase. On the other hand, the amount of organic cation found in the zeolite structure does not depend on its content in the initial reaction mixture and the found value is ca. 4 molecules per unit cell.

Figure 2: Scanning electron micrographs of the Zn-MFI samples obtained from gels prepared with different amounts of water: (a) sample CI: w= 30 and (b) sample C2: w= 50 ( ~ = 10 Bm).

43

3.2 Catalytic tests The isopropanol decomposition was chosen as a probe catalytic test in order to give an insight to the acid characteristics of the prepared zeolite Zn-MFI samples. Four samples containing different amounts of zinc (see Table 2.) were subjected to four different thermal and/or ion-exchange treatments. At this stage the behaviour of the Zn-MFI as a catalyst was taken into account only in quantitative terms. The isopropanol decomposition involves two pathways: dehydration to propene and dehydrogenation to acetone. The mechanisms that explain these reaction usually assume that the acidic centres (BrOnsted and Lewis sites) are involved in the formation of propene, while the redox sties (basic) are required for the production of acetone [9]. So the conversion of the alcohol into propene can be correlated to the acidity of the solid material studied. In Fig. 3. the catalytic activity of the chosen Zn-MFI samples is shown with respect to the zinc loading and post-synthesis treatments used. As well known, it is correlated, of course, to the reaction temperature applied for the test, so the production yield of propene and/or acetone is higher at 280 ~ It can be observed that the removal of the sodium cations is crucial for the Zn-MFI catalytic activity. For T1 the sodium cations remain in the zeolite structure, so the inactivity of the samples, with the exception of the sample with the lowest zinc content at 280 ~ suggest the interference of the inorganic cations with zinc sites. This is confirmed by the catalytic behaviour of the samples prepared by the procedure T3 where the samples are calcined before the ion-exchange. The activity of all studied samples are more or less insignificant (the results are not shown). One can suppose that during the first thermal treatment the sodium cations probably migrate into the framework position not accessible for the ion-exchange and/or they are bonded to the zinc species present in the zeolite structure in a way that make them completely inactive. 50 o= 40 "~ 30 = o 20 ~J 10 o

IT1 I

r'i acetone 230 ~ N propene 230 ~ II acetone 280 ~

_~ . 1

. 2

. Si/Zn

. 3

WIpropene 280 ~ 4

IT21

50 30 .~ 40

o 10 o

IT41

~ 50 ~ 40 30 .~

~ ~ 1

. _1~!. ,,,, . 2 3 Si/Zn

. 4

A

10 o

A

. 1

. 2

. 3

m

4

Si/Zn

Fig. 3. Histograms presenting the percentage conversion of isopropanol to acetone and/or propene over four different Zn-MFI samples (1= B4, 2= B5, 3= C4, 4= C5) prepared by three different thermal and/or ion-exchange procedures at 230 ~ and 280 ~ respectively.

44 In a way, it can be considered that the ion-exchange treatment performed prior to the removal of the organic cation from the zeolite structure is necessary for the configuration of the acid sites present in the Zn-MFI framework. The remarkable increase of the catalytic activity in the case of the T4 treatments procedure may be based on the re-introduction of Br0nsted active sites. So, the sample with the highest zinc content becomes catalytically active. For the later, one can suppose that the great part of the zinc species are not incorporated into the framework and that its "overloading" into the zeolite structure generates inactive sites from the catalysis point of view. The obtained results show that the activity of the zeolite Zn-MFI in the isopropanol decomposition depend on the post-synthesis thermal and ion-exchange treatments. Moreover, the higher amount of zinc found in the zeolite does not correspond to higher activity. As the amount of zinc was determined only as a bulk quantity, the "inactivity" of the sample with the highest zinc loading suggest that the exact kind and location of the zinc species are necessary in order to better understand its catalytic behaviour.

4. CONCLUSIONS From the presented results the influence of the chosen chemical parameters (zinc, organic compound, water and silica source) on the synthesis of the zeolite Zn-MFI can be seen as a tool for varying the morphology and crystal size of the crystals once the starting zinc content in the hydrogel is specified, i.e. none of the other chemical parameters under the same conditions is able to change the zinc incorporation into the zeolite structure. Although the nature and location of zinc species have not been studied in detail yet, this group of materials possess acid characteristics as seen from the catalytic tests on isopropanol decomposition. The obtained Zn-MFI materials result interesting and promising for catalytic applications. The detailed characterization of the zinc species incorporated into the zeolite framework as well as their localization and mobility require further study.

REFERENCES 1. A. Katovic, E. Szymkowiak, S. Kowalak, G. Giordano, A. Fonseca, J. B.Nagy, Stud. Surf. Sci. Catal., 135, 2001, (04-P-14) p. 337 2. A. Katovic, E. Szymkowiak, S. Kowalak and G. Giordano, Atti del Congresso AIZ-GIC 2000 (Ravello), pp. 53-56I.E. Maxwell and W.H.J. Stork, Stud. Surf. Sci. Catal., 58 (1991) 571 3. J.N. Armor, Microporous and Mesoporous Materials, 22 (1998) 451 4. W.M.W. Sachtler and Z.Zhang, Adv.Catal., 32 (1992) 129 5. T.E. Gier and G.D. Sucky, Nature 394 (1991) 508 6. I.E. Maxwell and W.H.J. Stork, Stud. Surf. Sci. Catal., 58 (1991) 571 7. J. Kanai and N. Kawata, J. Catal., 114 (198) 284 8. N. Wiswanadhan, A.R. Pradhan, N. Ray, S.C. Vishnoi, U. Shanker and T.S.R. Prasada Rao, App. Catal. A: General 137 (1996) 225 9. M.F. Gomez, L.A. Arrua and M.C. AbeUo, React.Kinet.Catal.Lett., 73 (2001) 143

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

45

In situ Dynamic Light Scattering and Synchrotron X - R a y P o w d e r Diffraction study of the early stages o f zeolite growth

G. Artiolia, 1L Grizzetti", L. Carotenuto b, C. Piccolob' c, C. Colellad, B. Liguorid, R. Aiello~ and P. Frontera~ a Dipartimento di Scienze della Terra, Universita Statale di Milano, Via S. Botticelli 23, 20133 Milano, Italy b MARS (Microgravity Advanced Research and Support) Center, Via E. Gianturco 31, 80146 Napoli, Italy c Dipartimento di Scienza e Ingegneria dello Spazio, Universith Federico II, Piazzale V. Tecchio 80, 80125, Napoti, Italy d Dipartimento di Ingegneria dei Materiali e della Produzione, Universit/t Federico II, Piazzale V. Tecchio 80, 80125, Napoli, Italy e Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria, Via P. Bucci, 87030, Rende (CS), Italy The synthesis of zeolite LTA from a clear solution of nominal composition 8.6Na200.18A1203-SIO2-1501-120 was investigated using a combination of techniques (Dynamic Light Scattering, DLS, and X-ray Powder Diffraction, XRPD, in situ experiments performed at the BM8 beam line of the ESRF facility in Grenoble, France) to obtain information on the early stages of the nucleation process. The experiments were performed at temperatures ranging between 60 and 100 ~ The DLS data clearly show at all temperatures the rapid development of an amorphous phase which consistently precedes the appearance of the LTA crystals. The observation of the amorphous precursor, the delayed formation of the crystalline zeolite, and the analysis of the kinetic parameters of the process indicate that the nucleation process of zeolite LTA from clear solution is heterogeneous and occurs at the interface between the solution and the amorphous precursor. 1. INTRODUCTION The nucleation and growth of zeolites and microporous materials are an area of active investigation. The proper interpretation of the early stage of aluminosilicate zeolite formation is of primary importance for the design, engineering and production of valuable industrial materials. As these processes are affected by a large number of physical and chemical

46 parameters, it is difficult to interpret univocally the basic mechanisms which might act differently in different systems. To contribute to the solution of this important problem, the investigation of the zeolite LTA synthesis from clear solutions [ 1] was selected. This system appears to be an appropriate example of zeolite synthesis occurring without the aggregation of pre-formed structural units. Actually, since the synthesis takes place at elevated pH, it is expected that condensation of the basic structural monomers and the formation of the complex secondary building units (SBU), that are generally detected in zeolite syntheses at lower pH, is prevented. Moreover, the use of a clear solution allows investigation with optical diagnostics. This paper reports the results of the in situ LTA synthesis experiments investigated by simultaneous DLS and synchrotron XRPD techniques at ESRF (Grenoble), using a microreactor cell and isothermal time-resolved modes.

2. EXPERIMENTAL 2.1. Materials Zeolite LTA is described by the ideal formula Na12[A112Si12048]-216H20 [2]. The structure is cubic with a = 24.6 A and space group symmetry Fm7c. The three-dimensional fourconnected framework is composed by tetrahedral SiO4 and AIO4 units in the 1:1 proportion, and it is generally described in terms of sodalite units (or ~-cages), and double 4-rings. The interconnection of these units produces a large cavity, 11.4 A in diameter (m-cage), and two intersecting channel systems. Electrical neutrality is achieved by the inclusion of Na § ions in the cages, besides a number of easily diffusing water molecules. From the thermodynamic point of view zeolite LTA is a metastable phase and tends to transform in hydroxylsodalite (SOD) with time, as evidenced by our long-term synthesis. The composition of the starting clear solution for zeolite LTA synthesis was 8.6Na200.18A1203- SIO2-150H20. Alumina solutions, prepared by dissolving solid NaAlO2 (Carlo Erba) in NaOH (Carlo Erba) solutions, and silica solutions (Aldrich sodium silicate solution plus NaOH solution) were prepared separately to obtain the solution components. They were cooled at room temperature and filtered through 0.5 ~tm PTFE filters before mixing the components to produce the final solution. Filtering was performed in order to prevent the presence of contaminant particles that, even at tow concentration, would affect both the nucleation process and the monitoring by light scattering. All the experiments reported were performed using fresh solutions inside quartz capillaries having a square section about 1 mm 2 in size, placed in ad hoc thermostatic sample holder. Isothermal experiments were performed in the temperature range 60-100~ 2.2. Diagnostics In situ time resolved XRPD-DLS experiments were performed at the ESRF (European Synchrotron Radiation Facility, Grenoble) BM8 GILDA beam line. The experimental set-up is shown in Figure 1. The capillary containing the starting solution was placed into a specifically designed thermostatic sample holder and it was carefully aligned simultaneously at the center of the X-ray beam and the laser beam of DLS apparatus. The hydrothermal treatment was carried out in the furnace shown in Figure 2.

47

Figure 2. Detail of the furnace employed for the synthesis experiments.

48 All XRD data were collected using a wavelength of 1.0401 A, calibrated against the lattice parameters of the NIST LaB6 standard (SRM 660; nominal a = 4.15695(6) A at RT). X,ray diffraction patterns were accumulated on a fiat Image Plate (IP) detector, using a Translating Image Plate System (TIPS) [3]. The IP was mounted on a computer controlled translating slide and moved behind a vertical steel slit with a horizontal opening of 3 mm. A continuos series of diffraction patterns were thus obtained during each experiment. The time-resolution for each run was defined by the translation speed of the tP detector combined with the width of vertical slit. The latent images collected on the IP were retrieved using the Fuji BAS2500 scanner. The isothermal XRPD patterns were analyzed by model independent single-peak profile fitting in order to integrate a sufficient number of Bragg peaks to follow the evolution of the crystalline zeolite in time. The General Structure Analysis System GSAS program was used [4]. The following peaks were integrated: (200), (220), (622), (642), (222), (820), (644), for zeolite LTA and (110), (211), (310) for zeolite SOD. The Dynamic Light Scattering apparatus, integrated on the beam line, was based on a laser source operating at 632 nm with a laser power of 22 mW. Intensity correlation function analysis was performed with a ALV digital correlator. The viscosity and the refractive index were measured for calculating the hydrodynamic particle size. Details of the DLS technique are reported in [5]. In order to evaluate the effects of the room temperature aging of the synthesis solutions before the hydrothermal treatment, the solutions were analyzed by the NMR technique. The instrument employed is a MSL Bruker 400.

3. RESULTS AND DISCUSSION

Typical 295i and 27A1spectra of the synthesis solutions before the hydrothermal treatment are reported in Figures 3 and 4. Figure 3 shows the NMR line of the monomeric silicate anion at 6 (ppm) = - 71.3 and the line of the dimeric-silicate anion at 5 (ppm) = - 79.8. The 27A1-NMR spectrum of the solution (Figure 4) exhibits, in addition to the AI(OH)4 line at about fi (ppm)= 77, two small lines which can be assigned to the aluminosilicate anions Al(OSi)m (OH). ( O ) L ( m + n ) .

8i(OHhO-

-O(OH)2 S i - O - Si(OH)20-

-20

-80 29

-I60

Figure 3. Si-NMR spectrum of the clear solution.

,~(osi)~(o~.(o)L(~.)

90

I0

Figure 4.27AI-NMR spectrum of the clear solution.

-90

ppm

49 The evolution of the "Si" and "AI" species during the aging at 25 ~ before the hydrothermal treatment is reported in Table t and shows that the line intensity does not substantially change with the aging time. Referring to the "Si" species, it can be noted:that, because of the high alkalinity of the solution, the monomer/dimer ratio is constantly high, and therefore the possible large SBU's cannot be detected. Table 1. NMR monitoring of the evolution of the 'Si' and 'AI' species during aging at 25 ~ Time of aging (hours)

Intensity species of Si (integrated line area)

Intensity species of AI (integrated line area)

Si(OH)30--O(OH)2Si-O-Si(OH)2085.09 14.91 85.83 14.17 85.27 t4.73 85.31 14.69 86.13 I3.87 85.45 14.65 85.68 14.32

8 16 24 48 72 96 120

~"

o

.~_ |

18 16

12 8 6

~ |~ .c_

4 2 0

Al(OSi)m(OH).(O)~_(m§~ 5.68 5.78 5.44 6.03 5.64 5.42 5.79

0

...............................................................................................

8

................................................... 0 0 ....eL.......................................................................................

6 E

O

10

.Ex ~= Q

AI(OH)~ 94.32 94.22 94.56 93.97 94.36 94.58 94.21

i-.

.

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)o,-rA(XRD>

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7 "~

s

4~Cl

S data } e D L............ 0

i

1

30

60

!

l

[

i

!

90

120

150

180

210

9

1 0

240

elapsed time (rain)

Figure 5 a. Time evolution of zeolite LTA crystallization in capillary at 70 ~ Figure 5 reports the time-evolution of zeolite LTA crystallization for three investigated synthesis temperatures: a) T = 70 ~ b) T = 90 ~ and c) T=100 ~ The decay time of the auto-correlation function measured by DLS is proportional to the average particle size, while the XRPD integrated intensity of the diffraction peaks, corrected for the incident beam decay, is proportional to the total amount of zeolite produced. At the highest temperature (100 ~

50 after 60 min (Figure 5c) the increasing amount of hydroxylsodalite clearly indicates 1the transformation of the system towards more stable phases.

~, < O

10

...................................

10

9 ..... e .....................................................................................

.@ =

8

-0

..... ~ ............................................................................ .......

7 ---= ............................................................. O ...................... @ 6

---.

5

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2

1

"~

i

'l I

0

,

.

30

60

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90

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.

=

5

.-..J

2

-o

4 "~-~

* DLS data

O

E

v

.

.-=~-4 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

9

1

i

.

0

120 150 180 210 240 270 300 330

elapsed time (rain)

14 o

=g

~ x

-~m

...........................

7

12

6

(xRo)' g

6 -}~-............................................................. [] S O D ( X R D ) i - ~ 3

o2]o. .................................... . . []. ......................... . 0

30

60

90

9DLS

120

o

data "!_0-~ 1 150

elapsed time (min)

Figure 5 b,c. Time evolution of zeolite LTA crystallization in capillary at 90 ~ (top)and 100 ~ (bottom). Comparison of the simultaneous XRPD and DLS data clearly indicates the presence of an amorphous phase formed during the early stages of nucleation (commonly referred to as the induction period). In fact, at the beginning, the DLS detected well visible signals, when no measurable diffraction signals were observed. The absence of detectable diffraction peaks indicates that the nature of the developing phase observed by DLS was not crystalline. The

51 substantial scattering contribution from the glass capillary and the solution prevents from the measurement of the diffuse scattering from the polymerizing phase. A full kinetic analysis of the isothermal XRPD data was performed and it is reported elsewhere [6]. The nucleation and growth process of zeolite LTA from clear solutions was followed by direct quantification of the crystalline product during isothermal in situ and ex situ experiments. The resulting kinetic model indicates that the nucleation mechanism is not controlled by diffusion in the liquid phase, but rather by surface processes on the gel precursor. The order of reaction is consistent with a mechanism involving heterogeneous nucleation. Moreover, the experimental values obtained for the apparent activation energy (Ea = 70.9 and 75.9 kJ/mol for the in situ and ex situ experiments, respectively) are in good agreement with the results of a number of recent investigations by dynamic light scattering [7], which are also consistent with the values expected for heterogeneous gel preparation [8]. Although some of the earlier studies interpreted the nucleation of zeolite LTA from clear solution as occurring homogeneously in the solution phase if the synthesis is purely inorganic (i.e., no organic structure directing agent, or SDA, is present) the ubiquitous presence of an amorphous precursor is observed and it is acknowledged that some rearrangement occur in the amorphous phase [7, 9]. The fundamental nucleation mechanism seems to be totally different if the zeolite forms in the presence of the organic SDA [ 10].

lO

A

w E

~6

iiiiiiiiiiiii

8 7

~

m

.E5

_'_ iiiiii .......................

............. f

4

,,, .....

~3

~

i

~

& T=80*C

......... 1

XT=90*C I T=100~

0

'~

o

i

~

f

i

5

10

15

20

elapsed t i m e

::

i

25

(rain)

Figure 6. Evolution of the DLS decay time at different temperatures. The DLS data shown in Figure 6 indicate for each isotherm at least two different regions of particle growth, separated by a small plateau. The two regions of particle growth (or coalescence) may be related to the growth of the amorphous and crystalline phases, respectively, since the second region of growth consistently coincides at all temperatures with the start of the detection of crystalline material by diffraction.

52 4. CONCLUSIONS The results obtained by truly simultaneous DLS and XRPD in situ experiments show the presence of an amorphous phase formed during the early stages of the hydrothermal synthesis of zeolite LTA from clear solution. The induction time decreases and the growth rate increases with temperature, as expected. The data are consistent with the results of the.full kinetic analysis performed on the same system [6], indicating a mechanism involving heterogeneous nucleation. 5. ACKNOWLEDGEMENTS This investigation was funded by ASI contracts I/RJ33/00 and I/R/118/01.

REFERENCES 1. R. Aiello, F. Testa, L. Maiorino and J.B. Nagy, Influence of the aging on the crystallization of zeolite A from clear solutions, submitted to Microgravity Quarterly. 2. V. Gramlich and W.M. Meier, Z. KristaUogr., 133 (1971)134. 3. C. Meneghini, G. Artioli, A. Baterna, A.F. Gualtieri, P. Norby and S. Mobilio, Joum. Synchrotron. Rad., 8 (2001) 1162. 4. A.C. Larson and R.B. Von Dreele, GSAS (General Structure Analysis System), Los Alamos National Laboratory, document LAUR 86-748 (1998). 5. R. Pecora (ed.), Dynamic Light Scattering, Plenum Press, New York, t985. 6. 1L Grizzetti and G. Artioli, Kinetics of nucleation and growth of zeolite LTA from clear solution by in situ and ex situ XRPD, Microporous Mesoporous Mater., accepted. 7. P.S. Singh, T.L. Dowling, J.N. Watson and J.W. White, Phys. Chem. Chem. Phys., 1 (1999) 4125. 8. A. Culfaz and L.B. Sand, Adv. Chem. Ser., 121 (1973) 140. 9. L. Gora, K. Streletzky, R.W. Thompson and G.D.J. Phillies, Zeolites, 18 (1997) 119. 10. M. Smaihi, S. Kallus, J.D.F. Ramsay, Stud. Surf. Sci. Catal., 135 (2001) 2.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

53

S y n t h e s i s o f M C M - 2 2 z e o l i t e b y the v a p o r - p h a s e t r a n s p o r t m e t h o d

S. Inagaki, M. Hoshino, E. Kikuchi and M. Matsukata*

Department of Applied Chemistry, Waseda University, 3-4-10kubo, Shinjuku, Tokyo 169-8555, Japan *[email protected] It is reported that the first synthesis of MCM-22 zeolite by the vapor-phase transport (VPT) method and the comparison in the morphologies of MCM-22 synthesized by the VPT and hydrothermal synthetic (HTS) methods. In the VPT method, pure MCM-22 zeolite was crystallized at 150~ for 5 days and at 175~ for 3 days. The morphology of MCM-22 zeolite crystallized by the VPT method was sphere which is an agglomerate of hexagonal thin plates having about 3-5 gm in width with less than 100 nm thick. Such morphology was similar to MCM-22 zeolite synthesized by the HTS method under static condition. The crystallization by the VPT method with the addition of HMI onto dry gel gave smaller MCM-22 crystals consisting of thin plates with 1-2 gm in width. 1. I N T R O D U C T I O N MCM-22 zeolite denoted as a MWW structure is synthesized hydrothermally from a silica-alumina mixture with various cyclic amines as structure-directing agents (SDAs) 1). This type of zeolite has the large 'pocket' (the inner diameter = 0.71 nm) on the external surface and two independent pore systems: one is defined by two-dimensional sinusoidal channel with 10-membered ring (10MR), and the other consists of large supercages delimited by 12MR. In addition to the characteristics of such a structure, increasing attention has been paid to MCM-22 from the view point of excellent acid catalysis. It has been reported that MCM-22 is active for toluene disproportionation 2) at lower temperature in comparison with ZSM-5 zeolite and has recently been commercialized as the catalyst for benzene alkylation by propylene 3). The vapor-phase transport (VPT) method, one of the dry gel conversion techniques for zeolite synthesis 4), has advantages over the hydrothermal synthetic (HTS) method as follows: the expansion of chemical compositions which can be crystallized 4), the reduction of the SDA concetration 4) and the improvement of catalytic activity and selectivity 5). In this study, we report the first synthesis of MCM-22 zeolite by the VPT method and the comparison of the morphologies of MCM-22 synthesized by the VPT and HTS methods. The crystallization by the VPT method with the addition of HMI onto dry gel at 175~ was also carried out in order to control the crystal size in the VPT method.

54

2. EXPERIMENTAL 2.1 Preparation of dry gel The hydrogel was prepared by mixing appropriate amounts of fumed silica (99.8 % purity, Aldrich), NaA102 (A1203:36.5 wt%, Na20:33 wt%, Kanto Chem.), NaOH (96.0 % purity, Kokusan Chem.) and distilled water. The mixture was composed of SiO2: Na20: A1203:H20 = 1.0: 0.05: 0.028: 44. The parent dry gel was obtained by drying the hydrogel on a hot stirrer at 80~ overnight after stirring the gel vigorously at room temperature for 30 min. 2.2 MCM-22 zeolite synthesis by the VPT method The dry gel weighing 0.5 g was crushed to powder and placed in a 57 ml autoclave. The autoclave was set up so as to separate the dry gel from the liquid mixture of hexamethylenimine (HMI; >97.0 % purity, Kanto Chem.) and distilled water, as shown in Figure la. The used amounts of HMI and distilled water were 0.5-1.5 g and 0.7-4.0 g, respectively. Upon heating, HMI and water were supplied to the dry gel from the vapor-phase. The dry gel was crystallized at 150~ or 175~ for 3-7 days under autogeneous pressure. After the crystallization, the products were filtered, washed and dried at 100~ overnight to obtain 'as-made' ones. 2.3 A VPT method with HMI addition onto dry gel In the usual VPT method, a structure-directing agent such as HMI was separated from the dry gel in an autoclave. In this study, HMI measuring 0.5 ml was added dropwise to the dry gel placed in an autoclave, after employing HMI (0.5 g) and water (2.0 g) in the bottom of the autoclave (Figure l b). The crystallization of 'as-made' product and its post-synthesis treatment were carried out the same procedures as those described above. 2.4 Hydrothermal synthesis of MCM-22 zeolite In this study, the conventional hydrothermal synthesis of MCM-22 zeolite was also carried out in order to compare the product with the one obtained by the VPT method. A mixture of fumed silica, NaA102, NaOH, HMI and distilled water was stirred vigorously at room temperature for 30 min. The composition of the mixture was SiO2: Na20: A1203: H20: HMI - 1.0: 0.05: 0.028: 44: 0.5. The mixture was introduced into a 23 ml autoclave and crystallized at 150~ or 175~ for 3-9 days under static and rotating (20 rpm) conditions. After the crystallization followed by the same procedure described above, 'as-made' product was obtained. 2.5 Characterization The structure of the 'as-made' products was characterized with X-Ray diffractometer (Rigaku, RINT-2000, 40 kV, 20 mA, CuI~). Those identified as the precursor of MCM-22 were calcined in flowing air at 580~ for 6 h (the ramping rate - I~ minl). The 'calcined' products were also characterized by XRD, SEM (Hitachi, S-2150, 15 kV), FE-SEM (Hitachi, S-4500S, 15 kV) and nitrogen adsorption (BELSORP 28SA, BEL Japan Inc.)

55

b)

Dry gel

Dry gel on which HMI was dropped

Teflon| holder

Teflon| holder

HMI, Water

HMI, Water

Teflon| steel autoclave

stainless

Teflon| steel autoclave

stainless

Figure 1 Setupof autoclaves for a) the vapor-phase transport (VPT) method, and b) the VPT method with hexamethylenimine (HMI) addition onto dry gel. 3. RESULTS AND DISCUSSION 3.1 The morphologies of products by the VPT and HTS methods Pure MWW was formed after 5 days of crystallization at 150~ with 2.0 g of H20 and 1.5 g of HMI (Figure la). The intensities of the XRD pattem for the calcined products by the VPT method was stronger than those by the HTS method under rotating and static conditions, as shown in Figures lb and 1c, respectively. Figure 2 shows the SEM images for the 'as-made' products synthesized at 150~ by the VPT method and the HTS methods under static and rotating conditions. The shape of the products obtained by the VPT method was sphere with a diameter of about 15 gm (Figure 2a). A higher magnification view shows that these spherical particles are actually an agglomerate of thin plates having about 2 gm in width with less than 100 nm thick (Figure 2b). On the other hand, the HTS method gave the products with a hexagonal shape under the rotating condition and they are composed of isolated thin plates with about 1 ~tm in width (Figures 2c and 2d). The morphology of MCM-22 obtained by the VPT method was similar to those by the HTS method under static condition, as shown in Figures 2e and 2f. Such morphology was also observed in the previous reports by Gfiray et al. 6) and Marques et al. 7). Since a little nucleation and slow crystal growth probably occurred due to a low concentration of HMI in the dry gel during the crystallization, the crystals formed by the VPT method could be larger than those by the HTS method. ,,

,,,,,

,,

r~

,j,,,

= =

5

10 15 20 25 30 35 40 20 (CuKt~) / degree Figure 1 XRD patterns for the products crystallized at 150~ followed by the calcination, a) After 5 days of crystallization by VPT method, b) after 5 days of by crystallization HTS method under rotating condition and c) after 9 days of crystallization by HTS method under static condition.

56

Figure 2 SEM images for the products crystallized at 150~ a, b) for 5 days by the VPT method, c, d) for 5 days by the HTS method under rotating condition and e, f) for 9 days by the HTS method under static condition.

57 Table 1 External surface area and micropore volume for calcined MCM-22 synthesized by the VPT method and the HTS method under rotating condition Micropore External Synthetic conditions volume surface area Temperature / ~ Method Period/days / mm3 g-1 / m2 g-1 VPT

175

7

154

HTS 175 7 99.2 Pore volume and external surface area were calculated by the t-plot method. (t -- 0.29)

57.2 306

Table 1 lists the micropore volumes and the external surface areas calculated from the nitrogen adsorption isotherms for calcined MCM-22 which was crystallized at 150~ for 7 days by the VPT method or the HTS method under the rotating condition. Compared with the product obtained by the HTS method, MCM-22 synthesized by the VPT method possessed a larger micropore volume and a smaller external surface area, which are 154 mm 3 g-1 and 57.2 m 2 g-l, respectively. These results from the nitrogen adsorption and the XRD indicate that the products by the VPT method have a higher crystallinity than the one by the HTS method. The smaller external surface area with the calcined MCM-22 by the VPT method was in agreement with that the product by the VPT method was composed of larger thin plates having a width of about 2 gm.

3.2 Crystallization of MCM-22 zeolite by the VPT method Product phase significantly depended on the amounts of H20 and HMI used in the case of the VPT method. Table 2 summarizes the crystallization results by the VPT method at 175~ The precursor of MWW structure was here denoted as MCM-22(P). We successfully obtained pure MCM-22(P) by the crystallization for 3 days with 2.0 g of H20 and 1.5 g of HMI (Run V-1 in Table 2). In other runs, ZSM-5 was formed concurrently with MCM-22(P), or the product was amorphous. The conditions for crystallizing pure MCM-22(P) were located in a narrow range. MWW structure was formed when not less than 2.0 g of H20 was used with 1.5 g of HMI. ZSM-5 was formed along with MCM-22(P) (V-4) with decreasing amount of H20, while a further decrease in the amount of H20, ZSM-5 was only obtained (V-3). On the other hand, as the amount of added HMI decreased, a mixture of ZSM-5 and MCM-22(P) phases was formed (V-7), the product synthesized with a less amount of HMI was amorphous (V-6). In the case of the crystallization with a less amount of HMI, we suppose that the amount of HMI supplied to the dry gel from the vapor-phase was insufficient to crystallize MWW structure. Table 2 shows also the synthetic conditions by the HTS method under rotating and static conditions, and the resultant products. In the hydrothermal synthesis at 175~ MWW structure was formed under rotating condition after 5 days of crystallization (H-l), while no crystal phase was indicated under static condition (H-2).

3.3 Synthesis of MCM-22 zeolite with small thin plates by the VPT method In the VPT method, it took 5 days to form MCM-22(P) in the crystallization temperature at 150~ as described above. The faster crystallization of the MCM-22(P) phase occurred at 175~ of crystallization. Pure MCM-22(P) was formed within 3 days (Run V-1 in Table 2). The SEM and FE-SEM images for MCM-22(P) crystallized at 175~ by the VPT method are shown in Figures 4a and 4b. The products crystallized at 175~ was spherical particles composed of thin plates having about 3-5 gm in width, and similar to those

58 crystallized at 150~ as shown in Figures 2a and 2b. On the othe hand, the product crystallized at 175~ by the HTS method was comprised of isolated hexagonal thin plates with a width of about 1-2 pm, as shown in Figure 5. In order to control the crystal size of MCM-22 zeolite by the VPT method, a dry gel onto which HMI was additionally dropped was crystallized at 175~ This method is expected to give smaller crystals of MCM-22 zeolite because much nucleation is possessed due to a large amount of HMI staying onto the dry gel during crystallization. Figure 6 illustrates the XRD patterns for the products synthesized by the VPT method with the addition of HMI to dry gel, comparing with the usual VPT method. When HMI was added to dry gel, though the intensities of XRD pattern for the products crystallized for 5 days were weak (Figure 6b), 7 days of crystallization led to the product showing the intensities of XRD pattern comparable to that crystallized for 5 days by the VPT method (Figure 6c). Figure 7 shows the SEM images for MCM-22(P) crystallized by VPT method with the addition of HMI to dry gel. The product was about 10-30 ~tm diameter spherical of thin plates, as shown in Figure 7a. Figure 7b shows that the thin plates was about 1-~2 lam in width with less than 100 nm thick and smaller than those by the usual VPT method, as expected.

Table 2 Run No.

Synthetic method

V- 1

VPT

Synthetic conditions and the resultant as-made products Amount added Crystallization in autoclave period Product H20 / g HMI / g / days 2.0

1.5

3

MCM-22(P)

V-2

VPT

2.0

1.5

5

MCM-22(P)

V-3

VPT

0.7

1.5

5

ZSM-5

V-4

VPT

1.0

1.5

5

MCM-22(P), ZSM-5

V-5

VPT

4.0

1.5

5

MCM-22(P)

V-6

VPT

2.0

0.5

5

Amorphous

V-7

VPT

2.0

1.0

5

MCM-22(P), ZSM-5

H-1

HTS (while rotation)

5

MCM-22(P)

H-2

HTS (under static condition)

9

Amorphous

*The amount of dry gel was 0.5 g. The crystallization was carried out at 175~

Figure 4 a, b) SEM images for the products crystallized at 175~ for 5 days by the VPT method.

59

Figure 7 a, b) SEM images for the products crystallized at 175~ for 7 days by the VPT method with the addition of HMI to dry gel.

4. CONCLUSIONS We successfully obtained pure MWW by the VPT at 175~ and the product was highly crystalline. MCM-22 zeolite synthesized by the VPT method was agglomerates of hexagonal thin plates having about 2 ~tm in width and less than 100 nm thick. On the other hand, the products by the HTS method under rotating condition was isolated hexagonal thin plates with a width of about 1 ~tm. The morphology of the products by the VPT method was similar to that synthesized by the HTS method under static condition. The crystallization by

60 the VPT method with the addition of HMI to dry gel was also carried out and MWW structure could be obtained. The size of thin plates observed in this product was smaller than the one by the usual VPT method. It is supposed that the formation of smaller thin plates occurred by the presence of a sufficient amount of HMI in the dry gel to nucleate a large number of the precursor of MWW structure.

REFERENCES

1. 2. 3. 4. 5. 6.

M. Rubin and R Chu, U.S. Pat. 4,954,325 (1990) E Chu, M.E. Landis and Q.N. Le, U.S. Pat. 5,334,795 (1994) P. Wu, T. Komatsu and T. Yashima, Micropor. Mesopor. Mater., 22 (1998) 343 X. Wu, J. Dong, J. Li and F. Wu, J. Chem. Soc., Chem. Commun., (1990) 755 M. Ogura, S. Nakata, E. Kikuchi and M. Matsukata, J. Catal., 199 (2001) 41 I. Gt~ray, J. Warzywoda, N. Ba9 and A. Scacco Jr., Micropor. Mesopor. Mater., 31 (1999) 241 7. A.L.S. Marques, J.L.F. Monterio and H.O. Pastore, Micropor. Mesopor. Mater., 32 (1999) 131

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Defect-free MEL-type azoniaspiro-compound

zeolites

61

synthesized

in

the

presence

of

an

Roberto Millini, Donatella Berti, Danila Ghisletti, Wallace O. Parker, Jr., Luciano C. Carluccio and Giuseppe Bellussi EniTecnologie S.p.A., Via F. Maritano 26, 1-20097 San Donato Milanese (MI - Italy); Pure, defect-free MEL type zeolites was synthesized by using 1-ethyl-6-azonia-spiro-[5,5]undecane (EASU) as a structure directing agent. The absence of intergrowth with MFI-type structure, which usually occurs when tetrabutylammonium (TBA) is used as a SDA, was confirmed by the observation of the weak and sharp (110) reflection in the XRD pattern of the calcined pure silica sample, as well as by the Rietveld refinement of the synchrotron powder diffraction pattern. Evidences derived from a molecular modeling study supported the high specificity of EASU towards the crystallization of defect-free MEL. 1. INTRODUCTION The first synthesis of ZSM-11 (IZA code: MEL) was described in 1973 using tetrabutylphosphonium or ammonium ions as structure directing agents (SDA's) [1]. Successively, several other synthesis procedures, based on the use of different SDA's, including C7 - C12 alkanediamines [2], octylamine [3], diquatemary [4] and alkyltrimethyl-ammonium compounds [5], were reported. ZSM-11 is closely related to ZSM-5 (MFI) being end-members of a family of zeolites with the same pentasil structure unit [6]. Both structures, in fact, consist of a stacking of pentasil layers related by inversion (i, MFI) or mirroring (t r, MEL). However, what was initially considered as pure MEL structure is, in reality, an intergrowth between the two end-members in which both i- and o--type of stacking co-exist [7]. In a detailed structural study performed on BOR-D, the borosilicate analog of ZSM-11, Perego et al. found that the probability of the ix-type of stacking is only 25% [7]. Successively, the same group demonstrated the possibility to control the degree of stacking by choosing the appropriate mixture of tetraalkyl-ammonium cations [8]. The first synthesis of defect-free MEL-type zeolite was reported by Fyfe et al., who used a mixture of benzyl-trimethyl-ammonium and tetrabutyl-ammonium chlorides as SDA's [9,10]. The samples were characterized by high resolution NMR and synchrotron X-ray powder diffraction (SXPD), confirming the absence of/-type of stacking [9-11 ]. A new and highly reproducible synthesis procedure based on the use of N,N-diethyl-3,5-dimethylpiperidinium as a SDA was proposed by Nakagawa [ 12] and the quality of the MEL samples was assessed by HREM observations and Rietveld refinement of the SXPD pattern [ 13] as well as by single crystal X-ray analysis [14]. More recently, Piccione and Davis reported the synthesis of defect-free MEL by using 2,2-diethyloxyethyltrimethylammonium (DEOTA) cation [ 15].

62 The use of a new SDA, 1-ethyl-6-azonia-spiro-[5,5]-undecane (EASU), for the synthesis of pure and defect-free MEL-type zeolites is here described. This SDA was identified during a systematic screening of the templating properties of azoniaspiro-compounds in zeolite synthesis. 2. EXPERIMENTAL

2.1. Synthesis To synthesize EASU, a solution of 1,5-dibromopentane (Fluka, 1.00 mol), 2ethylpiperidine (Fluka, 0.80 mol) and 400 g aqueous ammonia (30 wt%) was heated under reflux for six hours. The red-brownish solution obtained was roto-evaporated and the oily residue recrystallized from isopropanol to give the bromide salt in the form of a white solid. The yield of the reaction was 65% product with purity > 90% (from IH and 13C NMR). The bromide was then dissolved in deionized water, exchanged in O H form by electrodialysis, and roto-evaporated, after which the concentration was determined by acid titration. Zeolite syntheses were performed according to the following general recipe. The source of silica (Sylobloc 47) was added under vigorous stirring to an aqueous solution containing NaOH, EASU-OH, H2SO4 and, when required, A12(804)3" 16H20. The resulting homogeneous suspension was charged into a stainless steel oscillating autoclave and heated at 443 K for 7 14 days. The crystalline product was filtered, dried at 393 K and finally calcined at 823 K. 2.2. Characterization A1 content was determined by ICP-AES using a Thermo Jarrell Ash spectrometer; SiO2 was determined gravimetrically; quantitative C, H and N analyses were carded out with a Perkin Elmer 2400 analyzer. Sample morphology of the samples was examined by Scanning Electron Microscopy (SEM), using a Jeol LV5400 microscope operating at 25 keV accelerating voltage. The standard gold coating technique was used to avoid sample charging. Thermogravimetric (TG) analyses were carded out over the 293 - 1173 K interval with 10 K'min 1 heating rate and 300 ml-minl air flow using a Mettler TG50 thermobalance, controlled with a Mettler TC 11A microprocessor. Solid state NMR spectra (1H decoupled) were obtained with samples (200 mg) in 7 mm zirconia rotors, undergoing magic angle spinning (MAS) at 5 ld-tz on a Bruker ASX-300 spectrometer. 13C and 29Si shifts were referenced to adamantane (39.5, 29.5 ppm) and tetrakis(trimethylsilyl)silane (-9.8 and -135.2 ppm), respectively. X-Ray powder diffraction (XRD) data were recorded on a Philips X'PERT diffractometer over the 3 < 20 < 53 ~ angular region, with steps of 0.02 ~ 20 and 20 s/step accumulation time; the CuKo~ radiation (~, = 1.54178 A) was used. SXPD data were collected at room temperature on a calcined, pure-silica MEL sample loaded in a Lindemann capillary (0.3 mm i.d.), rotating at 1 Hz, at the GILDA beamline BM08 of the ESRF (Grenoble) during the experiment 08-02-174. Details about the setting of the beamline and of data collection and treatment are given elsewhere [ 16]. A wavelength of ~, = 0.82714(2) A was used and the data were collected on a Fuji Image Plate located 204.83 mm from the sample and perpendicular to the incident beam. The elaboration of the scanned digital image was carded out with the Fit2d software package [ 17] and the diffraction data finally converted to into a conventional 1/20 profile, covering the 3.5 < 20 < 55 ~ angular range

63 with fixed angular steps of 0.028452 ~ 20. Rietveld refinement of the SXPD pattern was carded out with the GSAS software package [18]. The structure model of the MEL framework was that reported in [ 13]. 2.3. C o m p u t a t i o n a l details

The search for the minimum energy location of the ASU and EASU cations within the MEL, MFI, MTW and MOR porous structures was performed with the Monte Carlo docking procedure proposed by Freeman et al. [19]. 100 SDA conformations were periodically extracted from a 0.2 ps long high temperature (1500 K) Molecular Dynamics (MD) trajectory of the isolated SDA molecule, and randomly docked within the zeolite model. Finally, energy minimization of the crudely docked structures gave a representation of the low energy sites and an estimation of the binding energy (B.E.). The search was successively refined using the recently developed Quench Dynamics protocol, which allows the complete inspection of the conformational space of the docked molecules to be performed [20]. For each Zeolite/SDA conformation, a 100 ps long MD simulation was run in the NVT ensemble at 3000 K; every 200 fs, the MD run was interrupted and the resulting conformation was minimized and stored in an archive file for successive elaboration. The simulation was run maintaining the purely siliceous MFI framework fixed. Monte Carlo docking calculations were performed with the MSI Catalysis 4.0.0 software package, employing the cflgl__czeo forcefield. Quench Dynamics calculations wee carded out with the MSI Cerius 2 program All the calculations were performed on a Silicon Graphics Octane workstation with the MSI Cerius 2 4.2 Materials Science software package, using the COMPASS forcefield [21 ].

Table 1 Description of the most representative syntheses. Sample 8iO2/A1203 Ri/SiO2 Na/SiO2 H20/SiO2 1 -0.2 0.1 45 2 -0.2 0.1 45 3 -0.2 0.1 45 4 100 0.2 0.1 45 5 100 0.2 0.1 45 6 100 0.2 0.1 45 7 50 0.2 0.1 45 8 25 0.2 0.1 45 9 ~ 0.2 0.1 20 10 ~ 0.2 0.1 30

pH 12.1 12.1 12.1 11.9 11.9 11.9 12.5 11.4 12.0 11.9

T (K) Time(d) Products 14 A M ii 428 443 7 AM + MEL 443 14 MEL 428 14 AM 443 7 AM + MEL 443 14 MEL 443 14 AM 443 14 AM 443 14 Quartz 443 14 Quartz 11 -0.2 0.1 60 11.7 443 14 AM + MEL 12 100 0.2 0.1 30 12.4 443 7 AM + MEL 13 100 0.2 0.1 30 12.4 443 14 MEL 14 ~ 0.2 -30 12.9 443 14 AM + MEL 15 -0.35 0.1 45 12.0 443 4 SSZ-31 + MEL 16 ~ 0.35 0.1 45 12.0 443 6 MEL + SSZ-31 17 ~ 0.35 0.1 45 12.0 443 7 MEL i 1-ethyl-6-azonia-spiro-[5,5]-undecane hydroxide; ii Amorphous

64

d

b

e~

C

k

H

g 3

i J

g

'

-'""- - i

.pp.,. e ' 0 " 6'o" " "4'0" " a ' 0 " " 6 Figure 1. ]3C CP-MAS NMR spectrmn of as-synthesized sample 3.

I

. . . .

I

. . . .

-105

I

. . . .

-110

I

"

-115

"'"

"

'/

"

-120

"""

"

I"

"

-125

" " ' ' "

i

zmm

Figure 2. 298i MAN NMR spectnnn of calcined sample 3.

3. RESULTS AND DISCUSSION 3.1. Synthesis Before describing the influence of the synthesis parameters, it must be pointed out that the crystallization of MEL is sensitive to the source of silica used, since it was preferentially obtained with Sylobloc 47 or TEOS. When Aerosil 200 or Ludox AS40 were used, amorphous and layered phases were mainly recovered, while with sodium silicate the crystallization of MOR was observed. All the representative syntheses described in Table 1 were performed using Sylobloc 47 as silica source. Pure MEL can be obtained both as purely siliceous and as aluminosilicate phase, provided that the molar SIO2/A1203ratio in the reaction mixture is kept _> 100 and the hydrothermal treatment is performed at 443 K for 14 days (Table 1). No crystalline phases were obtained from aluminum-reach reaction mixtures (runs 7 and 8) or by decreasing the temperature to 428 K (runs 1 and 4). Another critical parameter is the H20/SiO2 molar ratio, which should be close to 45. Quartz was in fact obtained from more concentrated aluminum-free reaction mixtures (runs 9 and 10) while partially crystallized MEL was recovered upon increasing the H20/SiO2 molar ratio to 60 (run 12). It is interesting to note that in the presence of aluminum the H20/SiO2 molar ratio can be decreased to 30 (run 13) without any influence, however, on the crystallization kinetics. Upon increasing the EASU/SiO2 molar ratio to 0.35 a different behavior was observed, in the sense that pure MEL is obtained only after 7 days of hydrothermal treatment and its formation occurs through the consumption of SSZ-31 which is formed first (runs 15-17). 3.2. Physico-chemical and structural analysis In the pure silica system, defect-free MEL crystallizes in the form of elongated tetragonal prisms terminating with tetragonal pyramids (Figure 1). When A1 is added to the synthesis gel, the shape of the crystals is maintained but the average dimensions decrease. TG analysis showed that -- 4 EASU molecules/unit cell are decomposed in an unique and sharp step in the range 6 0 0 - 800 K (peak temperature in the DTG pattern at 698 K) (Figure 2). The decomposition of the organic molecules occurs without any apparent loss of crystallinity.

65

100

i / ~

95

m

--0.002

,..1

g - -0.004

85 300

i 400

,

~'

~

500

600

700

"

Temp.

Figure. 3. SEM micrograph of pure silica MEL sample 3.

i

~

i

Ii

800

900

1000

1 00

-0.006

(K)

Figure 4. TG (--) and DTG ( ..... ) traces of pure silica MEL sample 3.

Solid state 13C NMR spectroscopy of the as-synthesized MEL catalyst confirmed that the SDA (EASU) remains intact within the pores of the crystalline product (Figure 3). The splitting of signal a in figure X is attributed to two different (chiral) orientations around this carbon. High-resolution 29Si MAS NMR studies of Fyfe and co-workers [9-11] revealed that between 283 and 333 K dealuminated ZSM-11 undergoes a displacive phase transition. This temperature dependence of the lattice structure gave six 29 Si peaks at 373 K and eight peaks at 263 K, implying a lowering of the unit cell symmetry with decreasing temperature from the I4m2 to the 1-4 space group. 29Si MAS NMR spectroscopy of the purely siliceous sample 3 revealed three composite peaks (centered at - 111.6, - 113.8 and - 116.3 ppm), broader than those reported previously [9-11 ]. The poor resolution of this spectrum (Figure 4), compared to Fyfe and co-workers, is presumably due to small deviations from perfect ordering not easily detected by XRD. Highly resolved spectra were obtained only aider steaming highly crystalline ZSM-11 at 1023 K [9-11]. According to them, at room temperature, the most suitable structural model would be the low-temperature one (space group 1-4). However, Terasaki et al. found that the SXPD pattern of a pure silica MEL sample can be nicely refined with both the I-4m2 and 1-4 models and that the refined structures display very similar

2-Theta

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Figure 5. (A) XRD pattem of calcined MEL sample 3; the inset shows the sharp (110) reflection. (B) Experimental (+), calculated (--) and difference (lower) SXPD profiles for calcined MEL sample 3. Vertical bars indicate the positions of Bragg reflections.

66 features [ 13]. van Koninsgveld et al. in the single crystal X-ray structure refinement of assynthesized MEL reached the same conclusion [ 14]. Figure 5A shows the typical XRD pattern of a calcined MEL sample. High crystallinity permitted the presence of the weak 110 reflections at 20 = 6.2 ~ to be monitored. Since the intensity of this reflection rapidly decreases with increasing amounts of MFI intergrowth, its observation indicates that the samples possess a defect-free MEL type structure [12]. This conclusion was confirmed by the Rietveld refinement of the SXPD data, carded out in the topological I-4m2 tetragonal space group, starting from the structural model reported by Terasaki et al. [13]. This model proved to be suitable for reproducing the experimental SXPD pattern, as demonstrated by the final Rietveld plot reported in Figure 5B. The main crystallographic and geometric data are summarized in Table 2. The fractional atomic coordinates (available upon request) are very close to those reported in ref. 13. It must be pointed out that the Si-O bond distances are spread over a narrow range (1.594- 1.602 A) as a consequence of the soft constraint imposed on these distances during the refinement ( d s i - o = 1.60 + 0.01). On the contrary, no constraints were imposed on the O-O distances and, in spite of that, the O-T-O angles are close to the value expected for the tetrahedral coordination (Table 2). Table 2 Main crystallographic Space Group a (A) c (,~) V (.~3) Rp Rwp R(F 2) Red. X2 No. Reflections

and geometric data. I-4m2 (#119) 20.0777(3) 13.4154(2) 5407.9(2) 0.0181 0.0276 0.0703 1.213 1156

T-O distance Mean: min / max: O-T-O angle Mean: min / max: T-O-T angle Mean: min / max:

1.599(2) 1.594 - 1.602,4, 109.5 ~ 105.0 - 111.6 ~ 152.6 ~ 138.0 - 173.7 ~

3.3. Modeling studies In a previous paper, we have shown that, depending from the SIO2/A1203 molar ratio in the reaction mixtm'e, 6-azonia-spiro-[5,5]-undecane (ASU), the unsubstituted parent SDA of EASU, favors the formation of MTW, MOR and of ERS-10, a zeolite with a still unknown framework structure [22]. Here we demonstrate how the simple substitution of a hydrogen atom by an ethyl group in {x-position produces completely different results, in the sense that, apart from the partial crystallization of SSZ-31, defect-free MEL is selectively favored by the new SDA. In order to understand the reasons for this selectivity, we have carded out a molecular modeling study to determine minimum energy location and energetic of EASU in the different microporous structures. The framework structures considered were MEL, MOR, MTW and MFI, the latter was included because of the structural analogy with MEL. The nonbonding (van der Waals) energies (Ewdw) for the EASU cation in the different zeolites are listed in Tab. 3. According to these data, the EASU cation strongly stabilizes the MEL

67

MEL

MH

Figure 6. Lowest energy conformations of EASU cation in MEL and MFI.

structure by 3 1 . 9 - 34.1 kJ-moll relative to MOR and MTW and 46.0 kJ.mol l relative to MFI. It is worth noting that, among the zeolite frameworks considered, MFI is predicted to be the least stabilized by EASU and this is in agreement with the lack of i-type of stacking in these crystals (see above). An inspection of the lowest energy conformations of EASU in MEL and MFI revealed that this cation nicely fits the intersections between the straight 10MR channels of MEL (Figure 6). The same is not true in the case of MFI, in which the cation lies again at the channel intersection but with the two rings developing along the linear 10MR channels (Figure 6).

Table 3 Non-bonding (van der Waals) energy for the different zeolite/EASU and ASU systems. Zeolites experimentally obtained with the two SDA's are marked in bold. ZEOLITE EASU ASU Evdw(kJ'mol"1) AEvdwi(kJ.mol"1) Evdw(kJ-mol"1) AEvdwi(kJ.mo1-1) MEL -132.2 0 -109.0 0 MOR - 100.3 +31.9 -100.3 +8.7 MTW -98.1 +34.1 -103.6 +5.4 MFI -86.2 +46.0 -87.0 +22.0 i Non-bonding energy relative to MEL/EASU and MEL/ASU systems

According to the data reported in Table 3, ASU seems to stabilize MEL better than the experimentally obtained MOR and MTW by 8.7 and 5.4 kJ.mol l, respectively. This contradiction can be explained by considering that the latter two zeolites are further stabilized by the effective packing of ASU molecules in the straight 12MR channels [23]. This stabilization is expected to be larger than that occurring in MEL, where the ASU cations are preferentially located at the channel intersections.

68 4. CONCLUSIONS A new SDA, 1-ethyl-6-azonia-spiro-[5,5]-undecane (EASU), was used for synthesizing pure defect-free MEL-type zeolites. The absence of intergrowth phenomena with MFI-type zeolite was confirmed by Rietveld refinement of the SXPD data and by the presence of the weak and sharp (110) reflection in the XRD pattern. The high specificity of EASU towards the crystallization of MEL was confi17ned by molecular modeling studies.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

P. Chu, U. S. Patent 3 709 979 (1973) assigned to Mobil. L.D. Rollmann and E. W. Valyoesik, U. S. Patent 4 108 881 (1978) assigned to Mobil. J.P. MeWilliams and M. K. Rubin, U. S. Patent 4 894 212 (1989) assigned to Mobil. E.W. Valyoesik, U. S. Patent 4 914 963 (1990) assigned to Mobil. J.S. Beck and J. D. Schlenker, U. S. Patent 5 213 786 (1993) assigned to Mobil. G.T. Kokotailo, P. Chu and S. L. Lawton, W. M. Meier, Nature 275 (1978) 199. G. Perego, M. Cesari and G. Allegra, J. Appl. Cryst. 17 (1984) 403. G. Perego, G. Bellussi, A. Carati, R. Millini and V. Fattore, in: Zeolite Synthesis. ACS Symposium Series no. 398 (M. L. Occelli and H. E. Robson Eds.), American Chemical Society, Washington DC, 1989, p. 360. 9. B.H. Toby, M. M. Eddy, C. A. Fyfe, G. T. Kokotailo, H. Strobl and D. E. Cox, J. Mater. Res. 3 (1988) 563. 10. C. A. Fyfe, H. Gies, G. T. Kokotailo, C. Pasztor, H. Strobl and D. E. Cox, J. Am. Chem. Soc. 111 (1989) 2470. 11. C. A. Fyfe, Y. Feng, H. Grondey, G. T. Kokotailo and A. Mar, J. Phys. Chem. 95 (1991) 3747. 12. Y. Nakagawa, WO Patent 95/09812 (1995) assigned to Chevron. 13. O. Terasaki, T. Ahsuna, H. Sakuma, D. Watanabe, Y. Nakagawa and R. C. Medrud, Chem. Mater. 8 (1996) 463. 14. H. van Koningsveld, M. J. Den Exter, J. H. Koegler, C. D. Laman, S. L. Njo and H. Graafsma, in Proc. 12th Intern. Zeolite Conf. (M. M. J. Treacy, B. K. Markus, M. E. Bisher and J. B. Higgins Eds.), MRS, Warrandale (PA), 1999, p. 2419. 15. P. M. Piccione and M. Davis, Microporous Mesoporous Mater. 49 (2001) 163. 16. R. Millini, G. Perego, D. Berti, W.O. Parker, Jr., A. Carati and G. Bellussi, Microporous Mesoporous Mater. 35-36 (2000) 387. 17. A. Larson and R. B. Von Dreele, GSAS Manual, Los Alamos Report No. LAUR-86-748, Los Alamos National Laboratory, USA, (1986) 18. A. P. Hammerslay, S. O. Svensson, M. Hanfland, A. N. Fitsch and D. H~iusermann, High Pressure Res. 14 (1996) 235. 19. C. M. Freeman, C. R. A. Catlow, J. M. Thomas and S. Brode, Chem. Phys. Lett. 186 (1994) 231. 20. R. Millini, Stud. Surf. Sci. Catal. 135 (2001) 264. 21. H. Sun, J. Phys. Chem., 102 (1998) 7338. 22. R. Millini, L. C. Carluccio, F. Frigerio, W. O. Parker, Jr., and G. Bellussi, Microporous Mesoporous Mater. 24 (1998) 199.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

69

Chemical and structural aspects of the transformation of the M C M - 2 2 precursor into ITQ-2 R. Schenkel, J.-O. Barth, J. Kornatowski, and J.A. Lercher Institut ~ r Technische Chemic, Technische Universitat M0nchen, 85747 Garching, Germany MCM-22 materials were prepared from precursors with varying Si/A1 ratios. During transformation of swollen MCM-22 into ITQ-2 strong desilication and amorphization were observed, which increased in intensity with decreasing aluminum concentration of the precursor. The ease of delamination increased in the same order. Exfoliation of the MCM-22 precursor to ITQ-2 was best performed at Si/A1 ratios above 20.

1. I N T R O D U C T I O N

Layered materials have a high potential as catalysts and supports, since two-dimensional slit-pores provide an easier access to organic molecules exceeding the typical size of zeolitic micropores. These crystalline layered structures can be intercalated [1,2], yielding materials which show advantages in comparison to amorphous oxides, i.e., (i) a high internal order and homogeneous distributions of the slit-pores due to the crystallinity of the layers, and (ii) well defined micro-/mesoporosity. MCM-36 [ 1] is a typical example of such a molecular sieve. Materials, similar to MCM-36, but with a completely disordered structure of layers, have been reported by Corma et al. [3,4], who observed that the zeolitic layers of the MCM-22 precursor can be fully separated by means of ultrasonic treatment. This procedure yields crystalline monolayers. The resulting material has been named ITQ-2 [3,4]. It consists of 2.5 nm thick sheets with a 10-membered ring channel system inside the sheets. Its specific surface area has been found [3,4] to be high (600-700 m2/g) and structurally well defined. However, this type of transformation depends subtly on the composition of the materials and the chemical methods employed. We report here on the influence of the synthesis routes, their modification, and the importance of chemical composition (A1 concentration) of the MCM-22 precursors on the formation and properties of lTQ-2. 2. EXPERIMENTAL MCM-22 precursors were synthesized using the procedure described by He et al.[5] (reaction gels with Si/A1 = 8, 12, and 50, under static conditions). One part of the precursors was filtered, washed with water, dried at room temperature in air and finally calcined at 823 K under flowing N2 with 8% 02 for 48 h to produce crystalline MCM-22. The other part of the non-dried and non-calcined precursors was swollen 5 with aqueous solutions of hexadecyltrimethylammonium chloride (CTMAC1, 25%) and tetrapropylammonium hydroxide (TPAOH, 40%) at relative weight ratios MCM-22 (P) / CTMAC1 / TPAOH equal

70 to 1:4:1.2, and pH adjusted to 13.5 with an NaOH solution. The swollen materials were filtered, washed, dried at room temperature and, in form of ca. 5 wt.% water slurries, treated in an ultrasonic bath [3,4] (120W, 35 kHz) at pH=12.5 [4] for 36 h. Then, several drops of concentrated HC1 were added to obtain pH below 2, which allowed an easier recovering of the resulting ITQ-2 solids by centrifugation. After drying at ambient conditions, the organic material was removed by calcination in a procedure analogous to that for calcining MCM-22. The overall Si/A1 ratio was determined from elemental analysis with atomic absorption spectroscopy (AAS, UNICAM 939). Powder XRD patterns were recorded with a Philips XPERT PRO diffractometer using CuI~ radiation. XRD simulations were performed using "Cerius 2" (Version 4.6, MSI). Nitrogen adsorption (PMI automated BET-sorptometer) was measured at 77.4 K for samples activated in situ at 673 K for 20 h. Scanning electron microscopy (SEM, JEOL 500) and transmission electron microscopy (TEM, JEOL 2010) were used to characterize crystal habitus and structural aspects. The adsorption of pyridine and 2,6-di-tert-butyl-pyridine (DTBPy) were followed by in situ IR spectroscopy (Bruker IFS-88, resolution 4 crn-1) at 373 K under partial pressures of 10-3 - 10-2 mbar. For this, the zeolite samples were pressed into self-supporting wafers (ca. 5 mg) and activated in vacuum (p<10 -6 mbar) with an increment of 10 K/min up to 723 K (holding for 30 min). After adsorption, the physisorbed bases were removed by evacuation (p < 10-6 mbar) for l h and desorbed by heating from 373 K to 723 K with an increment of 2 K/min. All spectra were baseline corrected between 3800 and 1100cm-1 and normalized to the integral peak area of the overtones of the framework vibrations in the range 2100 - 1735 crn1. 3. RESULTS AND DISCUSSION

The Si/A1 ratios of the calcined MCM-22 samples were 33.3, 11.4, and 7.6 for the samples A, B and C, respectively (see Table 1). After swelling, delamination, and calcination of the MCM-22 precursors, the Si/AI ratios of resulting ITQ-2 samples were 20.1, 9.4 and 7.3, respectively. As has been reported for MCM-36, the swelling and pillaring processes can result in a partial dealumination of the precursors [6]. A similar effect was expected for the transformation steps into ITQ-2. However, the opposite was observed, i.e., a significantly lower Si/AI ratio implying a strong desilication during the ultrasound treatment under basic conditions. This might result either from a partial destruction of the zeolite framework, or from a removal of various silica species present in the materials. The effect increased with decreasing A1 content of the materials. Samples A and B of MCM-22 had similar BET and micropore surface areas as well as Table 1. Elemental analysis of the studied materials. Sample MCM-22 A MCM-22 B MCM-22 C ITQ-2 A ITQ-2 B ITQ-2 C

MCM-22 and ITQ-2 samples in calcined form Si/A1 SiO2 [mol%] A1203[mol%] Na20 [mol%] 1.5 0.4 98 2 33.3 4.2 0.3 95.5 11.4 5.9 4.5 89.6 7.6 2.4 0 97.6 20.1 5.1 0 94.9 9.4 6.4 1.6 92.1 7.3

71 micropore volumes (Table 2). For sample C, the BET surface area and the micropore volume were significantly lower. This is attributed to the relatively low Si/AI ratio of this sample. In accordance with Mochida et al. [7] and GOray et al. [8], the synthesis of MCM-22 is best performed at Si/AI ratios of 12.5-15. The low Si/A1 ratio of sample C may, thus, have led to a partially collapsed structure. Transformation of MCM-22-A into ITQ-2-A led to an increase in all surface areas. This is attributed to the removal of extra framework species during the transformation (mainly increase of the micropore surface area) and the delamination of the layers (mainly increase of the meso-/macropore surface area). The effects of such transformations are in line with the constancy of the micropore volumes (see Table 2). This is likely the result of two opposite effects. A decrease in the micropore volume should be expected from the lower concentrations of supercages in the delaminated materials and the formation of 12-membered ring "cups" on the surface of the zeolite layers [4]. On the other hand, an increase in the micropore volume should be expected as the result of the removal of debris formed during the ultrasound treatment and/or of the extra framework species potentially present between the layers of the precursor materials. Note that the latter effect could also be a consequence of the desilication observed during the transformation from MCM-22 to ITQ-2. From the observations above, it can be concluded that the exfoliation depends critically on the aluminum concentration of the precursor. The transformation of swollen MCM-22 precursors into ITQ-2, performed for less than 36 h, is ineffective for Si/A1 ratios of 20. With decrease in the A1 concentration to the Si/A1 ratio of 50, the necessary time for ultrasound treatment becomes shorter [3,4]. The powder X-ray diffraction patterns of the three calcined MCM-22 precursor materials are in good agreement with literature data [9] and exhibit reflection patterns characteristic of the MWW topology. The highest crystallinity has been observed for sample B with Si/A1 = 11.4 (Fig. l a). The broad background observed for sample C indicates the presence of an amorphous phase. The XRD patterns of the ITQ-2 samples (Fig. l b) differ markedly from those of calcined MCM-22. ITQ-2-A hardly shows reflections characteristic of MCM-22 indicating its complete transformation [3-5]. The peaks are broader, which is typical for the reduction in particle size to the nm scale. That pattern also does not show the 001 and 002 reflections at 20 = 3.1 o and 6.5 ~ characteristic of the 2.5 nm periodicity of the MCM-22 precursor. In contrast Table 2. Structural properties of the studied materials derived form N2 sorption techniques. Sample

Si/A1 of calcined samples

BET surface area [m2/g]

Micropore surface area [m2/g]

MCM-22 A MCM-22 B MCM-22 C ITQ-2 A ITQ-2 B ITQ-2 C

33.3 11.4 7.6 20.1 9.4 7.3

360 386 157 523 432 354

191 175 89 284 188 173

Meso- and macropore surface area [m2/g] 169 211 68 239 244 181

Micropore volume [cmVg] 0.08 0.07 0.04 0.07 0.08 0.04

72 the XRD patterns of samples B and C display all, though broader and less intense, the characteristic peaks of MCM-22 in spite of the same length of the ultrasonic treatment (36 h in contrast to 1 h applied in refs.[3,4]). The XRD patterns of these samples resemble more those of MCM-56 type materials [10]. In contrast to ITQ-2 (sample A) with a completely disordered "house of cards" structure (accompanied by a significant amorphization), sample B and C present an arrangement of the MCM-22 zeolite layers, which is disordered along the crystallographic z-axis ("face to face"- MCM-56 vs. "edge to face"- ITQ-2). The lack of a relevant increase in the BET surface area for ITQ-2-B also suggests a re-aggregation of the layers. The present results, however, do not allow us to assign the additional reflections at 20 = 6.6 ~ 9.4 ~ and 13.6 ~ in the patterns of those two samples. Simulated X-ray di~action patterns of MCM-22 and the three MCM-22 samples correspond well to each other (Fig. 2). For ITQ-2, the simulated pattern exhibits sharp and well-resolved signals of the (hk0) planes due to the infinite x,y-dimensions used for the calculation (peak broadening factors in z-axis: 2.6 nm). This is not the case for the

a)

3.5 3 2.5

1.5

MCM-22 B

0.5 0 3

6

9

12

15

18

21

24

27

30

33

36

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36

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9

12

15

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30

2O Figure 1. Powder X-ray diffraction patterns of (a) MCM-22 (A: Si/AI = 33.3; B Si/AI= 11.4; C" Si/A1 = 7.6) and (b) ITQ-2 (A: Si/A1 = 20.1; B: Si/A1 = 9.4; C: Si/A1 = 7.3) samples.

73 synthesized sample; the lines 120 (100) characteristic of MCM-22 layers are hardly visible. 100 They have to be strongly 80 broadened, as a result of a (3to) reduction of the particle size .~. 60 it] (200) b) to nm scale and a very small ~ 40 o thickness of the layers. These observations indicate clearly that (i)exfoliafion by ultrasound treatment significantly reduces the long2 6 10 14 18 22 26 .30 34 38 range order in the ITQ-2 20 structure (ii) delamination of Figure 2. Simulated X-ray diffraction pattems of the precursors with higher A1 (a) MCM-22 (Si/A1 = 12) and (b) ITQ-2 (Si/A1 = 12). concentrations is more difficult, and (iii) a partial amorphization of the zeolite layers cannot also be ruled out. Scanning electron micrographs (not shown) prove that the MCM-22 samples A, B, and C synthesized under static conditions are mostly platelets with an approximate diameter of 0.5 1 pm. The platelets are agglomerated into disclike aggregates with an approximate diameter of 5 lam and height of 3 lam, while rotation of the autoclaves prevents the crystallites from aggregation [8]. SEM of ITQ-2-A exhibits particles of irregular shape and size, whereas ITQ-2-B and C are composed to a high extent of original aggregates destroyed only slightly by swelling and delamination. Transmission electron micrographs (Fig. 3) confirmed the well-resolved crystalline structure for all MCM-22 samples. The layer thickness can be estimated to be approximately 2.5 nm, which is consistent with the values reported by Roth et al. [1]. TEM of sample ITQ-2 A exhibits 2.5 nm thin single crystalline MCM-22 layers (Fig. 3a) and an amorphous phase. Figure 3b is the top view of a layer of this sample, which shows the projection of the array of 12-membered ring half cups of the MCM-22 structure with a unit cell length of 1.3 nm in the x,y-plane. Note that this micrographs demonstrate that the treatment Figure 3. TEM of ITQ-2 samples; a) and does not result in complete damage of the b) ITQ-2 A (Si/AI = 20.1), c) ITQ-2 C crystalline structure of the layers. The layer (Si/A1 = 9.4).

74 thickness in sample ITQ-2-B is about 2.5 nm, which is comparable to parent MCM22 (Fig. 3c). The presence of significant amounts of crystalline MCM-22 and possibly MCM-56 phases after ultrasonic treatment proves that the sample has not been completely delaminated. The comparison of all TEM-micrographs shows that the sample with the lowest aluminum concentration is the easiest one to exfoliate, while a lower aluminum content of the materials favors a higher amorphization. Infrared spectra of the investigated materials in the activated form (Fig. 4) show typical bands of OH-stretching vibrations of surface hydroxyl groups. Bands at 3743 and 3730 cm-1 are assigned to terminal and internal Si-OH groups, respectively, those at 3663cm -1 to OHgroups of extra framework aluminum species, while the bands at 3620 cm-1 are attributed to strong Bronsted acidic bridging hydroxyl groups. For MCM-22 B, the band at 3743 cm-1 is very weak, while

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3800 3700 3600 3500 3400 3300 3200 3100 3000

Wavenumber [cm-11 Figure 4. IR spectra of activated MCM-22 B and the ITQ-2 samples A, B and C; T = 308K, p < 10-6 mbar (activation at 723 K for 30 min).

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Wavenumber [ern-1]

Figure 5. Difference IR spectra of adsorbed (a) pyridine and (b) 2,6-DTBPy on ITQ-2 samples A, B and C after reaching adsorption/desorption equilibrium at 10-2 mbar partial pressure of the base and evacuation for 60 min.; T = 373 K, p < 10-6 mbar.

75 for ITQ-2-A it shows the highest intensity. During the transformation into ITQ-2, the concentration of terminal Si-OH groups increases significantly due to the strong increase of the external surface. The band corresponding to bridging hydroxyl groups at 3620 crn-1 appears to be weaker for ITQ--2 sample A than for MCM-22, due to the lower AI concentration of this sample. All spectra show the bands of structural overtone vibrations between 2100 and 1550 crn-1. IR spectra of sorbed pyridine and 2,6-di-tert-butylpyridine (2,6-DTBPy) have been used to probe the acidic properties and to distinguish between acid sites located in the zeolite pores and on the external surface/pore mouth of the layered materials (Fig. 5). During adsorption of pyridine, several bands appear in the regions of 3140-2970 cm-1 and 1700-1400 cm-1. The former group is assigned to C-H stretching vibrations of pyridine, while the bands at 1635, 1620, 1540 and 1450 cm-~ are attributed to ring vibrations of pyridinium ions on Bronsted acid sites and of pyridine coordinatively bound to Lewis acid sites [11,12]. In the spectrum of ITQ--2-C, sodium cations from the synthesis are concluded to cause the shoulder at 1442 cm-~ assigned to pyridine coordinated to Na § Adsorbed 2,6-DTBPy exhibits several bands in the regions of 3400-2800 crn-1 and 17101350 cm-1, assigned to stretching and bending vibrations, respectively, analogous to pyridine. The bands at 3365, 1616 and 1530 cm-1 are characteristic of the DTBPyH + ion. For the quantification of Bronsted acid sites on the external surface [13], the band at approximately 3365crn-1 assigned to the v(N-IT) stretching vibration of the DTBPyFF ion is used. Due to a larger kinetic diameter (Fig. 6), 2,6-DTBPy can adsorb only at the pore openings or at the external surface of the zeolitic material and cannot penetrate into the pore system of MCM-22. Pyridine, however, penetrates into the 10- and 12-membered ring channels and interacts with the acid sites in channels and cavities. The areas of the 1545 crn-1 band of pyridinium ions of the three ITQ-2 samples normalized to the molar concentrations of aluminum in the material (Fig. 6) show that the relative concentration of adsorbed pyridine is slightly lower for ITQ-2-A than the concentration adsorbed on ITQ-2-B and C. The largest amount of adsorbed 2,6-DTBPy observed for sample A indicates that a large number of Bronsted acid sites interacts with the bulky base due to the larger external surface area and the better accessibility to the sterically demanding probe molecules. The lower amount adsorbed on sample B and C fits well to lower surface areas obtained from BET measurements. In agreement with the other observations the varying accessibility of the acid sites on the external surface indicates a lower degree of exfoliation of the layered materials B and C.

Figure 6. Adsorption ofpyridine and 2,6--DTBPy on ITQ-2 samples A, B and C.

76 4. CONCL]JSIONS Delamination of swollen MCM-22 precursor to ITQ-2 is favored by decreasing aluminum concentration of the parent material. The observations strongly imply that a higher charge density at higher A1 contents of the framework results in stronger interactions between the zeolitic layers. This is speculated to cause a more difficult exfoliation. Different degrees of delamination with lower concentrations of aluminum in the materials are reflected in increasing surface areas and a better accessibility to acid sites located at the external surface of the ITQ-2 layers for 2,6-di-tert-butylpyridine. Delamination of the precursor materials is accompanied by significant amorphization and desilication. The extent of this amorphization increases with decreasing aluminum content of the zeolite precursor. For materials with Si/A1 ratio below 15, swelling and ultra sound treatment lead to molecular sieves with a structure similar to MCM-56. The transformation process of these materials does not yield a completely disordered layered structure ("edge to face"), but induces a disordering along the crystallographic z-axis ("face to face"). Exfoliation of MCM-22 precursor to ITQ-2 can be best performed with Si/A1 ratio higher than 20. ACKNOWLEDGEMENTS The work was partially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Sonderforschungsbereich 338. Thanks are due to Prof. S. Weinkauf and Dr. L. Simon for the help with TEM and SEM measurements. REFERENCES 1. W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, and S.B. McCullen, Stud. Surf. Sci. Catal., 94 (1995) 301. 2. J.-O. Barth, J. Kornatowski, and J.A. Lercher, J. Mater. Chem., 12 (2002), 369. 3. A. Corma, V. Fornes, S.B. Pergher, Th.L.M. Maesen, and J.G. Buglass, Nature, 396 (1998) 353. 4. A. Corma, V. Fornes, J.M. Guil, S.B. Pergher, Th.L.M. Maesen, and J.G. Buglass, Micropor. Mesopor. Mater., 38 (2000) 301. 5. Y.J. He, G.S. Nivarthy, F. Eder, K. Seshan, and J.A. Lercher, Micropor. Mesopor. Mater., 25 (1998) 207. 6. J.-O. Barth, R. Schenkel, J. Kornatowski, and J.A. Lercher, Stud. Surf. Sci. Catal., 135 (2001) 136. 7. I. Mochida, S. Eguchi, M. Hironaka, S. Nagao, K. Sakanishi, and D.D. Whitehurst, Zeolites, 18 (1997) 142. 8. I. Gfiray, J. Warzywoda, N. Bac, and A. Jr. Sacco, Micropor. Mesopor. Mater., 25 (1999) 207. 9. M.E. Leonowicz, J.A. Lawton, S.L. Lawton, and M.K. Rubin, Science, 264 (1994) 1910. 10. A.S. Fung, S.L. Lawton, and W.J. Roth, US Patent No. 5.362.697 (1994). 11. J.W. Ward, J. Catal., 9 (1967) 225. 12. J.A. Lercher, Ch. Gr~ndling, and G. Eder-Mirth, Catal. Today, 27 (1996) 353. 13. A. Corma, V. Forn6s, L. Forni, F. Marquez, J. Martinez-Triguero, and D. Moscotti, J. Catal., 179 (1998) 451.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

77

Nanocrystalline ZSM-5: a highly active catalyst for polyolefin feedstock recycling D. P. Serrano*, J. Aguado, J.M. Escola and J.M. Rodrfguez Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, c/Tulipfin s/n, 28933, M6stoles, Spain, e-mail: [email protected] Nanocrystalline HZSM-5 (crystal size around 25 - 60 nm) shows remarkable activity in the catalytic cracking of LDPE, HDPE and PP despite the low temperatures ( 3 4 0 - 380 ~ and catalyst loadings used (plastic / catalyst mass ratio - 100). Gaseous hydrocarbons are the main products with a maximum centered at C4, which are formed through a carbocationic end-chain cracking mechanism. The selectivity towards C3 - C5 hydrocarbons is enhanced on decreasing the cracking temperature, reaching a 95 - 100 % at 340 ~ PIONA analyses of the liquid hydrocarbons obtained at higher temperatures (380 ~ indicate that the highest amount of olefins (47 %) is obtained in the cracking of the linear HDPE. In contrast, the branched PP leads mainly towards aromatic compounds (38 %). 1. I N T R O D U C T I O N Conventional applications of zeolites take advantage of their shape-selectivity, which is determined by both the framework type and the pore size, whereas the presence of active sites on the extemal surface of the crystals has been usually considered as an undesirable property. Hence, zeolites are typically synthesized with a crystal size within the micrometer range, which leads to very low external surfaces (< 10 m 2 g-l). However, the synthesis of nanocrystalline zeolites, with crystal sizes below 100 nm, has arisen lately great interest [ 1], since new applications for these materials are envisaged, mainly in the catalytic conversion of bulky molecules [2]. Thermal and catalytic cracking of polyolefins into short-chain hydrocarbons has been widely investigated in recent years [3-7]. These polymers account for roughly 60 wt% of the total household plastic wastes. Thermal cracking of polyolefins proceeds with low yields towards the corresponding raw monomers [8]. Indeed, a wide product distribution with poor economical value is obtained, which should be further upgraded. An interesting alternative is based on the use of acid solid catalysts, mainly zeolites, to promote the polyolefin degradation, which yields hydrocarbon mixtures with higher commercial value. However, conventional zeolites lead to low activities due to their small pore size compared to the polymer molecules. Hence, high zeolite loadings and temperatures must be used to promote the polymer cracking. *

Corresponding author

This research has been funded by the regional government of Madrid through the Strategic Group Programme (Direcci6n General de Investigaci6n, Consejer{a de Educaci6n, Comunidad de Madrid)

78 A different approach is based on the use of nanocrystalline zeolites, having a high percentage of external acid sites fully accessible for the polymer macromolecules [2]. In this case, the cracking of the polymers initially would take place on the external acid sites, whereas subsequent secondary reactions may occur over both internal and external active sites. The present work reports the results obtained in the catalytic cracking of LDPE, HDPE and PP at different temperatures over nanocrystalline HZSM-5. 2. EXPERIMENTAL

2.1. Catalyst preparation Nanocrystalline ZSM-5 (n-HZSM-5) has been synthesized according to a procedure previously reported in literature [9], using tetrapropylammonium hydroxide (TPAOH) as structure directing agent, and tetraethylorthosilicate (TEOS) and aluminum isopropoxide (AIP) as silica and aluminum sources, respectively. After aging for 40 h, the crystallization took place under autogenous pressure at 170~ for 72 h. The zeolite was separated by centrifugation, washed with deionized water, dried at l l0~ for 12 h and finally calcined under static air for 5 h at 550~ A standard HZSM-5 (HZSM-5) was also synthesized for comparison following a procedure reported elsewhere [ 10].

2.2. Catalyst and polymer characterization XRD spectra of the zeolites were obtained with a Philips X'PERT MPD diffractometer using Cu K~ radiation. FTIR spectra were recorded using an ATI Mattson spectrometer. The sample was diluted in KBr (1 wt%) and the spectra were obtained with 64 scans and a resolution of 2 cm -1. Nitrogen adsorption-desorption isotherms at 77 K were performed in a Micromeritics TRYSTAR 3000 instrument. Previously, the samples were outgassed at 350 ~ C under vacuum. The overall surface area was calculated by applying the BET equation, whereas the external surface area was obtained by the t-plot method. Chemical compositions of the zeolite samples were determined by ICP-AES measurements with a VARIAN VISTA AX apparatus. The size and morphology of the crystals were determined by TEM using a JEOL 2000 electron microscope operating at 200 kV. The raw polyolefins used in this work were provided by REPSOL-YPF with the following properties: LDPE (Mw=416,000), HDPE (Mw=188,000) and PP (Mw=450,000, isotacticity index=93%).

2.3. Catalytic cracking experiments The polyolefin cracking experiments were carried out in a stainless steel batch reactor provided with a screw stirrer (Figure 1), operating at atmospheric pressure under a continuous nitrogen flow of 30 NmL min -1. In each experiment, 8 g of plastic and the suitable amount of catalyst (plastic / catalyst mass ratio = 100) were loaded into the reactor. The system was first heated up to the reaction temperature in 30 min, and thereafter the temperature is kept constant for the reaction time (3 - 6 h). Liquid and gaseous products leaving the reactor were separated in a condenser at 0 ~ C and collected

79

/

-

- \

'

/

/

~

N\\l

~

/

J

Figure 1. Scheme of the polyolefin cracking reaction system: (1) reactor, (2) condenser, (3) exit gas flowmeter, (4) gas sampling bag, (5) inlet gas flowmeter for GC analysis. PIONA analyses of the gasoline range fraction were performed in a Varian 3800 GC equipped with a Chrompack 100 m capillary column. 3. RESULTS AND DISCUSSION 3.1. P h y s i c o c h e m i c a l properties of the catalysts

XRD spectra of both nanocrystalline and micrometer HZSM-5 zeolites are shown in Figure 2. The positions of the peaks of n-HZSM-5 are coincident with those of the micrometer ZSM-5 zeolite (size around 1.5 ~m) and no broad bottom reflection is observed showing the absence of amorphous material and, therefore, the high crystallinity of the samples. However, the spectrum of the nanocrystalline zeolite presents less intense and broader reflections in regards to those of micrometer ZSM-5 zeolite. According to the Scherrer law, this fact confirms the nanocrystalline size of the sample (< 100 nm). Another indication of the crystallinity of the n-HZSM-5 zeolite proceeds from the FTIR spectrum (data not shown) wherein the presence of a band at 550 cm -1, typical of the atomic ordering of pentasil zeolites and absent in amorphous silica, can be clearly appreciated.

. t~

t--

"

= -

lo

'

20

3'0

'

4o

'

~0

2O

Figure 2. XRD spectra of both nanocrystalline and micrometer size HZSM-5 samples

80

Figure 3. N2 adsorption isotherms at 77 K

Figure 4. TEM micrograph of n-HZSM-5

N2 adsorption-desorption isotherms at 77 K of both zeolites are shown in Figure 3. The BET surface area of n-HZSM-5 zeolite is 438 mZ/g whereas its external surface area, calculated by the t-plot method, was 78 mZ/g, i.e. its external surface accounts for 18 % of the total BET surface area. In addition, the isotherm shows a steep jump at P/P0 - 0.95, describing a hysteresis cycle, which indicates the presence of interparticular mesoporosity. In contrast HZSM-5 zeolite sample shows the typical isotherm shape of microporous material without any jump at higher pressures and having a BET surface area of 362 m 2 g-~. Figure 4 illustrates a TEM micrograph of the n-HZSM-5 sample, which shows the presence of agglomerates made up of nanocrystallites with cubic-like morphology and a size within the 25 - 60 nm range. Electron diffraction, carried out during the TEM measurements, confirmed the high crystallinity and stability of this sample. 3.2. Catalytic cracking of the polyolefins The cracking of pure HDPE, LDPE and PP over the n-HZSM-5 zeolite, with a Si/A1 ratio of 30.1, has been studied at temperatures within the range 340 - 380 ~ C for different reaction times. In all cases, a plastic/catalyst mass ratio (P/C) of 100 was used. For both polyethylenes, these temperatures are clearly below the threshold temperatures corresponding to their thermal cracking (400 - 420~ Thus, the conversions obtained in their respective blank reactions were negligible. In contrast, the PP thermal cracking reactions showed that at 380 ~ C this polymer is degraded with a meaningful conversion of 23 % after a 3 h reaction. The results obtained in the catalytic cracking experiments are summarized in Tables 1, 2 and 3 for the three polyolefins investigated, respectively. Almost complete conversions were attained with both HDPE and LDPE at 360 and 380~ A high conversion (-- 90 %) was also obtained in the PP cracking at 380~ Interestingly, significant conversions were obtained with the three polymers at a temperature as low as 340 ~ C. In order to compare the performance of the nanocrystalline and micrometer HZSM-5 samples, the latter was also used as a catalyst for the HDPE cracking at 360~ for 3 h. The micrometer HZSM-5 presents a similar aluminum content (Si/A1 = 31) to that of n-HZSM-5. The conversion obtained over the standard HZSM-5 sample was 43.8%, clearly lower than the value corresponding to nHZSM-5 (92.8 %). For the three polymers, the high catalytic activity exhibited by the n-

81 HZSM-5 material is remarkable taking into account both the low amount of catalyst used and the reduced temperature of reaction. In addition, these results point out that the n-HZSM-5 zeolite catalyzes directly the cracking of the polymer molecules, without the need for previous thermal cracking steps. An important characteristic of a catalyst regarding its possible commercial application is its resistance to deactivation. In the particular case of cracking reactions over zeolites, deactivation is caused mainly by coke deposition. From the results shown on Tables 1-3, it can be appreciated that on increasing the reaction time, the conversion is raised in the same extent for the cracking of the three polyolefins. This behaviour suggests that deactivation does not take place significantly at least during the 6 first hours of reaction and indicates the high stability of n-HZSM-5 against coking. Table 1 Catalytic cracking of HDPE over n-HZSM-5 zeolite Temperature (~ 380 360 340 340

time (min) 180 180 180 360

Conversion (wt%) 98.6 92.8 18.8 45.3

Product selectivity (wt %) C1-C5 C6-C12 C13-C40 63.3 72.3 100.0 100.0

35.9 27.5 0.0 0.0

0.8 0.2 0.0 0.0

Table 2 Catalytic cracking of LDPE over n-HZSM-5 zeolite T emperature (~ 380 360 340 340

time (min) 180 180 180 360

Con vers ion (wt %) 98.6 99.4 31.6 62.4

Product selectivity (wt %) C 1-C5 C6-C 12 C 13-C40 63.6 72.3 100.0 100.0

31.4 25.4 0.0 0.0

5.0 2.3 0.0 0.0

Table 3 Catalytic cracking of PP over n-HZSM-5 zeolite Temperature (~ 380 360 340 340

time (min) 180 180 180 360

Conversion (wt %) 89.5 35.8 7.1 15.9

Product selectivity (wt %) C1-C5 C6-C12 C13-C40 50.2 66.1 100.0 100.0

23.3 16.5 0.0 0.0

26.5 17.4 0.0 0.0

82 The product distributions obtained in the cracking experiments are also shown in Tables 1-3. In all cases, the main products are light hydrocarbons (C~ - C5 fractions), which are formed by catalytic end-chain scission reactions. In addition, the selectivity towards the C~ - C 5 fraction increases as the temperature is reduced, reaching values close to 100% at 340 ~ C. In the case of PP cracking at 360 and 380~ a significant amount of heavier products (C13 - C40) is also observed, in contrast with the results obtained with both LDPE and HDPE, wherein this fraction is almost negligible. This is likely due to the significant extent of thermal degradation reactions with this polymer, that occur simultaneously with the catalytic cracking leading to a broad product distribution. Figure 5 compares the product distributions per carbon atom number obtained in the catalytic cracking of HDPE at T = 340 and 380 ~ C, respectively. The maximum of the product distribution per atom carbon number is placed at C4 compounds. A second maximum is observed centered at C8 at 380 ~ C, presumably formed as a result of secondary oligomerization reactions of the C4 hydrocarbons. These secondary reactions seem to be inhibited by decreasing the reaction temperature. Hence, heavier hydrocarbons than the C5 fraction are not obtained at the lowest temperature (340~ whereas the selectivity towards C4 hydrocarbons is over 50 %. These results are extremely interesting since they show the possibility of addressing the zeolite selectivity towards a reduced group of gaseous hydrocarbons.

60 Ill

50

--o~T=

380 ~

- - A , - - T = 340 -0 C

o•40 4d

>,

30

4--. om ~9

20

~t r

10

i

2

I

4

I

6

I

8

i

10

i

12

i

14

16

Carbon atom n u m b e r Figure 5. Selectivity by carbon atom number obtained in the catalytic cracking of HDPE over n-HZSM-5

83 Table 4. Composition of the C1-C5fraction obtained in the catalytic cracking of HDPE, LDPE and PP over n-HZSM-5 zeolite at 340~ Product selectivity (wt %) Carbon atom HDPE LDPE PP number olefins paraffins olefins paraffins olefins paraffins 2 1.4 0.0 2.3 0.0 1.8 0.0 3 14.4 3.4 4.0 2.4 4.2 12.1 4 40.4 14.0 33.4 23.1 12.9 44.1 5 26.3 0.1 23.2 11.6 4.2 20.7 Table 4 shows the content of olefins and paraffins present in the products corresponding to the cracking reactions carried out at 340~ with 100% selectivity towards the light fraction C1-C5. It is noteworthy the high amount of olefins obtained with HDPE (more than 80 %) whereas for LDPE they account for 63% of the products. In contrast, paraffins are the main components of the gaseous fraction obtained in PP cracking reactions (close to 80 %). Consequently, a less olefinic product is obtained with increasing the degree of branching of the raw polymer. PIONA analyses of the liquid fraction (C6 - Cl2) are shown in Figure 6. Considerable differences can be observed among the liquid products obtained from the three plastics. Thus, PP cracking leads to a very high proportion of aromatic hydrocarbons in the liquid fraction (almost 40 %). Therefore, the above commented paraffinic character of the gases formed from PP cracking actually stems from cyclization, aromatization and hydrogen transfer reactions occurring with this polymer. In contrast, the catalytic cracking of the most linear polymer (HDPE) yields the highest amount of olefins in the liquids, which is in line with the previous observations for the gaseous fraction. The presence of significant amounts of isoparaffins and naphtenes in the gasoline range hydrocarbons obtained from the three polyolefins is also remarkable, as these compounds are known to improve the octane number of this fraction. 4. CONCLUSIONS The results reported and discussed in this work indicate that the n-HZSM-5 zeolite, with nanometer crystal size, presents remarkable properties for the catalytic cracking of polyolefins: HDPE, LDPE and PP are converted with high conversions at low temperature (340-380~ to yield mainly light hydrocarbons (C3- C5). The catalytic activity is noteworthy despite the low temperatures and catalyst loadings used. In addition, the activity of the n-HZSM-5 sample remains constant along the reaction times investigated, suggesting that its deactivation under those conditions is negligible.

84

,-,

c

o ,_

o u

4O

30

20

9 0

10

0

@ Figure 6. PIONA analyses of the C6-C12fractions obtained at 380~ This catalyst allows the polyolefin cracking reactions to be directed towards the selective formation of gaseous hydrocarbons by end-chain scission reactions. In the case of both HDPE and LDPE, gaseous olefins are obtained with high selectivity, which is an interesting achievement in regards to promote the development of new alternatives for the feedstock recycling of plastic wastes. 5. REFERENCES

1. B.J. Schoeman, Microporous Mater., 9 (1997) 267. 2. D.P. Serrano, J. Aguado and J. M. Escola, Ind. Eng. Chem. Res., 39 (2000) 1177. 3. J. Aguado, J. L. Sotelo, D. P. Serrano, J. A. Calles, J. M. Escola, Energy Fuels, 11 (1997) 1225. 4. Y. Uemichi, J. Nakamura, T. Itoh, M. Sugioka, A. A. Garforth and J. Dwyer, Ind. Eng. Chem. Res., 38 (1999) 385. 5. W. Kaminsky and J. S. Kim, J. Anal. Appl. Pyrolysis, 51 (1999) 127 6. J. M. Arandes, I. Abajo, D. L6pez-Valerio, I. Fernfindez, M. J. Azkoiti, M. Olazar and J. Bilbao, Ind. Eng. Chem. Res., 36 (1997) 4523. 7. S.C. Cardona and A. Corma, Appl. Catal. B, 25 (2000) 151. 8. T.P. Wampler, J. Anal. Appl. Pyrolysis, 15 (1989) 187. 9. R. Van Grieken, J. L. Sotelo, J. M. Men6ndez and J. A. Melero, Microporous Mesoporous Mater., 39 (2000) 135. 10. E. Costa, M. A. Uguina, A. De Lucas and J. Blanes, J. Catal., 107 (1987) 317.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

85

Modeling superoxide dismutase: Immobilizing a Cu-Zn complex in porous matrices and activity testing in H202 decomposition K. Hernadi a'*, D. M6hn a, I. Lab~idib, I. P~ilink6c, E. Sitkeib and I. Kiricsi a aDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich B. t6r 1, Szeged, H-6720 Hungary bDepartment of Inorganic and Analytical Chemistry, University of Szeged, D6m t6r 6, Szeged, H-6720 Hungary CDepartment of Organic Chemistry, University of Szeged, D6m t6r 8, Szeged, H-6720 Hungary A bimetallic bridged complex, the

Cu(II)-diethylene-triamino-i~-imidazolato-Zn(II)-tris-

aminoethylamine triperchlorato complex, was prepared and immobilized in A1MCM-41 and montmorillonite. The host-guest materials were intended to be superoxide dismutase mimics, however, only the complex intercalated in montmorillonite proved to be active in oxygen transfer reaction. The inability of the complex immobilized in A1MCM-41 to decompose an aqueous solution of H202 might be due to diffusional constraints exerted on the reactants and/or the products by the relatively more regulated environment of molecular sieve than that of the layered clay. 1. INTRODUCTION Homogeneous catalysts are generally more selective and occasionally more active than their heterogeneous counterparts. Nevertheless, they also have several disadvantages. Just two of them are their tedious and often unsuccessful separation from the reaction mixture and their shorter lifetime. Both of these unfavorable features may be eliminated if the homogeneous complexes are heterogenized. Complexes immobilized in solid or semisolid matrices may mimic enzymes [1, 2], which may mean high activity at low temperature and Research leading to this contribution was financed through grants from the National Science Foundation of Hungary (OTKA T034793). The support is gratefully acknowledged.

86 very high selectivity: typical features of environmentally friendly procedures. When complexes are built within zeolite cavities catalysts may be produced, which have the same or occasionally superior activities and selectivities as homogeneous catalysts have, with distinct advantages, such easy separation and recycling. The complexes often resemble the prosthetic groups, the zeolite hosts have similarity to apoenzymes, thus, these materials are often termed zeozymes [3]. The methods of introducing complexes to zeolites have been reviewed [4]. On the basis of the review and original contributions four procedures may be distinguished, (i) preparation of the complex in the intrazeolite space, (ii) introducing the preformed complex into the voids of the zeolite (called as the ex situ method later on) (iii) introducing the ligands into the zeolite and letting the complex form with the ion-exchanged transition metal ion (denoted as in situ method) (iv) using the preformed complex as template for zeolite synthesis. The first three methods require intrazeolite space large enough to accommodate the complexes and/or windows large enough to allow ligand precursors or ligands or the preformed complexes to enter into channels or cavities of the molecular sieve. The fourth procedure may be the most flexible by excluding or minimizing space constraints. In this work we got around size constraints by using montmorillonite, a swellable layered silicate, and A1MCM-41, a mesoporous zeotype as hosts. In trying to model the superoxide dismutase enzyme (in its active center Cu(II) and Zn(II) ions are coordinated by histidine ligands), we describe the synthesis of a Cu-Zn complex immobilized in montmorillonite or A1MCM-41.

The

preformed

complex

was

introduced

in-between

the

layers

(montmorillonite) or into the channels (MCM-41) of the hosts and the catalysts were tested in H202 decomposition. 2. EXPERIMENTAL For the ex situ preparation of the immobilized materials the guest molecule, the Cu(II)diethylene-triamino-#-imidazolato-Zn(II)-tris-aminoethylamine triperchlorato complex (Cu-

Zn complex, for short) was prepared following the general recipe published in ref. [5]. In this procedure the zinc or the copper perchlorate was dissolved in water, then, the ligands (imidazole and diethylene triamine for the copper salt solution and tris(2-aminoethyl)amine for the zinc salt solution) were added under continuous stirring. For preparing the bridged binuclear complex, the two complex solutions were added in 1" 1 molar ratio to a quarter of that amount of free Zn(II) in ethanol under vigorous stirring. Stirring was maintained for 24 hours and then, the bridged binuclear complex was allowed to crystallize helped by cooling

87 and evaporating the excess solvent. The hosts were either montmorillonite (Bentolit-H, Laporte) or A1MCM-41 (Si/A1 = 42). The latter was synthesized in our laboratory following recipes published in the literature [69]. The immobilized complex catalysts were prepared by using isopropanol. Calculated amount of the preformed complex was dissolved in 50 ml of isopropanol and from this solution it was introduced into MCM-41 or montmorillonite (0.5-0.5 g) suspended in isopropanol. This mixture was stirred for 24 hours at room temperature, then filtered and dried. The host-free complexes were examined by derivatography and IR spectroscopy. The host materials and the immobilized materials were characterized by X-ray diffraction and IR spectroscopy, thermal analysis and BET measurements. The thermal behavior of the substances was investigated by a Derivatograph Q instrument. The powdered samples were placed on a platinum sample holder and studied under the following conditions: mass sample 100 mg, heating rate 10 degree/min, temperature range 300 to 1000 K in air. X-ray diffractograms were registered on wellpowdered samples with a DRON 3 diffractometer. BET measurements were performed in a conventional volumetric adsorption apparatus cooled to the temperature of liquid nitrogen (77.4 K). Prior to measurements the host samples were pretreated in vacuum at 573 K for 1 h. Neither BET surface areas nor basal distances were significantly altered compared to those of the host materials (montmorillonite: 90 m2/g, 1.46 nm [as received], A1MCM-41: 932 m2/g, 4.059 nm [calcined at 773 K for 10 hi). For IR spectroscopy the KBr technique was applied (2 mg sample in 200 mg KBr). Spectra were taken in the mid infrared region (Matson Genesis or BIO-RAD FTS-65A/896 spectrometers, 32 scans). The catalyst samples as well as the effects of host-free complexes were tested in the decomposition of hydrogen peroxide. The reaction was performed in the liquid phase at 333 K. Actual concentration of hydrogen peroxide was determined by the formation of its titanil sulfate complex, followed by UV-VIS spectroscopy (PERKIN ELMER). The well-stirred reaction mixture contained 100 mg of the catalysts and 20 cm 3 of 1.5 % hydrogen peroxide aqueous solution. UV-VIS spectroscopy was also used for studying the structure of the host-free as well as the host-guest complexes.

88 3. RESULTS

3.1. Host-free complex UV-VIS spectroscopy results revealed that the transition metal ions could be introduced successfully in their individual mononuclear complexes (absorptions relevant to the free ions disappeared form the spectra of these complexes). The structure of the binuclear complex is the following [5]:

q

.

_

[0104-]3

The thermal behavior of the complex was relatively simple (Figure la). Up to 473 K water was removed in an endothermic process (probably it is the physisorbed and the crystal water). At 523 K the complex decomposed vigorously. Above 573 K the residual material underwent gradual and continuous weight loss. The FT-IR spectrum of the complex was recorded and as it was expected it contained many absorption bands. Their assignation was not done, the spectrum was only used to compare with those of the intercalated structures.

3.2. Host materials A great deal of information is available concerning the structural characteristics of these materials [10, 11]. Let us summarize in a couple sentences the main features relevant to this work. Montmorillonite is a layered material of cation exchange capability. It is capable of swelling in a large variety of solvents, such as water, various alcohols, etc., therefore, it can accommodate the complex in various positions" either in a way when the Cu-imidazole-Zn distance is parallel with the clay sheet or when it is perpendicular (or stands in an angle) to it. If intercalation is too dense free traffic of the reacting species may be hindered, thus, optimization of pillaring is of importance. MCM-41 is an ultra large pore molecular sieve, capable of accommodating the complex within its structure. Again, depending on the position and the density of the intercalated complex, plugging the channels may or may not occur.

89 3.3. Intercalated substances When the complex investigated were incorporated into any of the porous matrices studied they became more stabile, they lost all ligands at higher temperature (around 1100 K) than the complexes alone (around 900 K). Between 373 K and 473 K dehydration could be observed for both host materials. From around 573 K organic ligands were removed from the A1MCM-41 intercalated sample. From montmorillonite host, ligands were removed in steps (they were combusted since measurements were performed in air). For an illustrative example, see Figure lb. 10

20

10

10

0

5

~10 -10 -10

-20

-20

-30

-30 1000

-40

-20

-30

i

0

w

1

i

200

i

i

i

i

400

~

I

!

,

600

i

i

'

~'"~

800

i

,"

T/oC

DTA

-10 -15

0

.................... 200 400 600

800

20 1000

T/oc

Fig. 1. TG (thermogravimetric) and DTA (differential thermal analytic) curves of (a) the bare Cu(II)-diethylene-triamino-#-imidazolato-Zn(II)-tris-aminoethylamine triperchlorato complex and (b) the Cu-Zn complex immobilized in montmorillonite TG curve reveals that in the first step tris-aminoethylamine ligand bonded to Zn ion was removed (530 K) followed by the decomposition of diethylene-triamino ligand at 723 K. Decomposition was over by 923 K. The final step was the removal of the imidazolato bridge. This sequence is in accordance with the general observation that the stability of Cu complexes exceeds that of Zn complexes [12] on one hand, and the measurable weight losses on the other. Comparing the thermogravimetric curveS, it can be concluded that both host materials stabilizes the Cu-Zn complex. Instead of explosion, total degradation of the complex ends up at 673 K in A1MCM-41 and at 923 K in montmorillonite. It may be presumed that the Cu-Zn complex is immobilized mainly with ion-exchange. Further stabilization is achieved via interaction of the nitrogens in the ligands with silanol groups of

90 the host. The FT-IR spectra of the intercalated materials displayed many of the features found in the host-free complex, indicating that the complex was in the structure of the host materials, indeed.

3.4. Decomposition of hydrogen peroxide First, blank reactions were made using both the host-free complexes and the complex-free hosts. Then, the two intercalated materials were investigated in the decomposition of H2OE solution. While both montmorillonite and the molecular sieve were inactive in the decomposition reaction of hydrogen peroxide (H202 = H20 + 1/2 O2) at the reaction temperature, the bare Cu-Zn complex was found to be active, although only after 30 min induction period. By the end of the reaction (gas evolution ceased), the complex decomposed, its blue color turned green. The complex intercalated in montmorillonite was also found to be active in the decomposition reaction, the induction period, however, was 70 min. Nevertheless, the turnover rate for the immobilized material was higher (97.6 h 1) than that of the host-free complex (71.6 hl). Moreover, the immobilized substance could be reused at least three times without appreciable loss of activity, while the bare complex decomposed and lost its activity by the end of the first run. To much of our surprise the complex immobilized in MCM-41 turned out to be completely inactive. 4. DISCUSSION Experimental results revealed that seamless introduction of the Cu(II)-diethylene-

triamino-#-imidazolato-Zn(II)-tris-aminoethylamine triperchlorato complex was feasible into either the mesoporous molecular sieve or the layered silicate. There were no size constraints and considerable channel diameter enhancement or increase in basal spacings could not be measured. This is a strong indication that in the intercalated material the Cu-Zn axis is parallel with the clay sheets or the channels of MCM-41. Immobilization increased the stability of the complex considerably. Decomposition took place at significantly higher temperatures than in the case of the host-free complex. In the intercalated material the original structure of the complex was largely preserved, since infrared bands of the bare complex could be recognized in those of host-guest substances. The preformed complex is kept in-between the layer of the clay or in the tubular structure of the molecular sieve mainly via electrostatic forces (due to ion exchange). Additionally,

91 hydrogen bonds between the structural OH groups of the hosts and the nitrogen of the various ligands may be expected. Even though these bonds are not weak, some mobility for the guest complex, especially for the ligands, is still probably allowed. This flexibility may make possible for the central ion(s) to change the coordination arrangement and the ligands to assume the conformation most suitable for the reaction to be catalyzed. As far as the guest materials are concerned, montmorillonite, due to its quasi twodimensional structure provides ample space for the unhindered diffusion of the reactant molecule, therefore the immobilized complex is catalytically active. In the tubular (more closed) structure of A1MCM-41, overwhelming proportion of the Cu-Zn complex is presumably not accessible as catalytically active site, the immobilized complex was found to be inactive. When reaction takes place the fourfold coordination changes and the hydrogen peroxide coordinates to the transition metal ion. Computations with similar model system [13] revealed that this way somewhat distorted square pyramidal configuration might be formed. The actual reaction takes place at the Cu(II) center, however, Zn(II) also must have a role. The complex of this study is taken as a model of the active center of bovine erythrocyte superoxide dismutase, which has been shown to consist of a Cu(II) and Zn(II) ion bridged by histidyl imidazolate residue by X-ray crystallographic studies [ 14]. The Cu(II) was shown to play a direct role in the catalytic electron transfer mechanism [15], however, the role of Zn(II) is not so clear. Quite possibly it organizes the peptide chain in the real enzyme [ 16] and it influences the electronic properties of Cu(II) through the imidazolate bridge. In our immobilized system, the latter property may only prevail. It is certain that the Zn(II) does not take part directly in H202 decomposition. 5. CONCLUSIONS The superoxide dismutase enzyme mimic complex could be immobilized in both A1MCM41 and montmorillonite, however, only the montmorillonite intercalated material displayed activity in H202 decomposition. In turn this activity is superior to the host-free Cu-Zn complex. The AIMCM-41 intercalated material was less stabile and it was not active at all. The latter fact is probably due to the restricted diffusion of the reactant molecules to the active sites and the products from the active site.

92 REFERENCES

1. A.J. Kirby, Angew. Chem. Int. Ed. Engl., 35 (1996) 707. 2. H. Gr6ger, J. Wilken, Angew. Chem., Intl. Ed. Eng. 40 (2001) 529. 3. R. Parton, D.E. de Vos and P.A. Jacobs, in Proceeding of the NATO Advanced Institute on Zeolite Microporous Solids" Synthesis, Structure and Reactivity, (E.G. Derouane, F. Lemos, C. Naccache and F.R. Riberio. eds.), Kluwer, Dordrecht, p. 555 (1992). 4. K.J. Balk-us, Jr and A.G. Gabrielov, J. Inclusion Phenom. Mol. Recognit. Chem., 21 (1995) 159. 5. M. Sato, S. Nagae, M. Uehara and J. Nakaya, J. Chem. Soc., Chem. Commun. (1984) 1661. 6. J.M. Kim, J.H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 7. J.S. Beck, J.C. Vartuli, G.J. Kennedy, C.T. Kresge, W.J. Roth and S.E. Schramm, Chem. Mater., 6 (1994) 1816. 8. R. Schmidt, D. Akporiaye, M. St6cker and O.H. Ellestad, J. Chem. Soc., Chem. Commun., (1994) 1493. 9. K.M. Reddy and C. Song, Catal. Lett., 36 (1984) 103. 10. E. Nemecz in Clay Minerals, Akad6miai Kiad6, Budapest, Chapter I (1981). 11. J.C. Jansen, M. St6cker, H.G. Karge and J. Weitkamp (eds.), Advanced Zeolite Science and Applications, Elsevier, p.338 (1994). 12. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th edition, John Wiley & Sons, New York, 1988, p. 596 and p.755. 13. K. Hernadi, I. P~ilink6, E. B6ngyik and I. Kiricsi, Stud. Surf. Sci. Catal. 135 (2001) 366, CD-ROM edition 27P10. 14. J.S. Richardson, K.A. Thomas, B.H. Rubin and D.C. Richardson, Proc. Natl. Acad. Sci. USA, 72 (1975) 1349; J.A. Tainer, E.D. Getzoff, J.S. Richardson and D.C. Richardson, Nature, 306 (1983) 284. 15. E.M. Fielden, P.B. Roberts, R.C. Bray, D.J. Lowe, G.N. Mautner, G. Rotilio and L. Calabrese, Biochem. J. 139 (1974) 49. 16. S.J. Lippard, A.R. Burger, K. Ugurbil, M.W. Pantoliano and J.S. Valentine, Biochemistry, 16 (1977) 1136.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

93

Crystal Growth of Zeolite Y Studied by Computer Modelling and Atomic Force Micoscopy J.R. Agger and M.W. Anderson* UMIST Centre for Microporous Materials, PO Box 88, Manchester, M60 1QD, UK.

1. INTRODUCTION The advent of atomic force microscopy I has revolutionised our ability to image zeolite crystals. 2 Careful interpretation of such images provides a wealth of clues about zeolite crystal growth and has led to the proposition of a layer upon layer growth mechanism. 36 The clues take many forms: generally uniform layer heights; well defined layer topographies; mathematically defined individual terrace front separations and both similarities and differences between the layers observed on different crystal facets. Globally these factors have enabled the proposal of initial growth mechanisms in a number of important zeolites. 37 Simulation of such growth by computer has allowed access to probabilities of attachment that are inextricably linked to energies of attachment for various sites on the surface of zeolite A. s Zeolite A was chosen for its cubic symmetry, that facilitates creation of the model, its industrial importance and the availability of high quality AFM images to validate the simulation produced. In this paper we present a computer simulation of the growth of a second industrially important zeolite with cubic symmetry. However, the trigonal nature of the actual growth layers of this zeolite necessitates a new set of site definitions.

~

M4

Figure 1. Schematic representation of the growth of zeolite Y. Nucleation occurs on each of the eight { 111 } faces. Triangular growth terraces merge at the edges of the crystal eventually forming the characteristic octahedral morphology. The orientation of growth terraces is rotated by 60 ~ with respect to the triangular facets expressed in the final crystal morphology. *The authors thank the EPSRC for the award of an advancedfellowshipto JRA, and John Thorntonfor taking the AFM image.

94

Figure 2. AFM image of a (111) face of a l l.tm zeolite Y crystal. Crystal orientation is shown i n s e t the shaded face is imaged.

Figure 3. a) FAU layer viewed down [111] showing pores and beta cage orientations within the layer and b) The same layer viewed side on.

The zeolite Y structure comprises a face-centred-cubic array of beta cages tetrahedrally connected by D6Rs. The pore system comprises a three dimensional array of 13A diameter supercages tetrahedrally linked through 7.4A diameter, 12-ring windows. The unit cell possesses Fd3m symmetry and has the following parameters: a = b = c = 24.74 ~ ot = 13= 7 = 90~ Crystal growth of zeolite Y, shown schematically in figure 1, 3 o c c u r s via nucleation on each of the eight (111) faces. The precise nature of any given surface nucleation remains unknown but growth occurs perpendicular to the surface, along <111 >, until a minimum surface energy is reached, creating a layer. Layers advance parallel to the surface in all directions governed by a simple set of attachment energies for various sites. The terraces have uniform height of 1.4 nm and this correlates well with the thickness of a faujasite sheet. 9 A faujasite sheet, as shown in figure 2, comprises beta cages tetrahedrally co-ordinated via D6Rs. Each beta cage is co-ordinated to three other cages within the layer and to a fourth cage situated either in the layer above or the layer below in alternating fashion. 2. EXPERIMENTAL Zeolite Y was synthesised with the following gel composition: 10SiO2:1.0A1203:2.4Na20:1401-120:1.0 crown ether

95 The material sources were 40wt% colloidal silica (HS-40 Ludox); 40wt% sodium aluminate solution; 15-crown-5 and 18-crown-6 (Aldrich). The gel was aged for two days at room temperature followed by crystallisation in Teflon bottles for 8 days at 95~ The AFM image was recorded on a Nanoscope III Multimode Microscope from Digital Instruments operating in TappingMode TM. The sample was secured in a thermo plastic resin in order to prevent lateral movement. A first order planefit was conducted on the image in the x and y directions to level the crystal terraces and simulated illumination is used to emphasise the crystal steps. 3. COMPUTATION The difficulty in modelling zeolite Y arises from the trigonal symmetry of the faujasite layers. The following discussion pertains to figure 4 (for simplicity the figure concerns only a 5 x 5 growth array, however the logical steps detailed are applicable to much larger arrays). Each layer may be broken down into three basic elements: beta cages that connect to the layer above, labelled u; beta cages that connect to the layer below, labelled d; and super-cage pores, labelled p. The growing surface is modelled as a (3n+2) x (3n+2) array, where n is any integer. This allows the correct connectivity throughout the entire layer. Simple algorithms are used to determine both geometry and spatial position within the trigonal symmetry of any given site. Prior to nucleation, the entire array is filled with zeroes, however, the nine positions that connect to the layer below, ie. 01, 04, 07, 10, 13, 16, 19, 22 and 25 are tagged as sites for possibile nucleation. At this point a discussion of the possible site geometries is necessary. Modelling the growth of a single faujasite layer requires the definition of seven different site geometries, as shown in figure 5. Sites oriented downwards within their layer will connect to the layer below and up to three upwards-oriented beta cages within their layer. Such geometries are arbitrarily named Dd, Ddu, Dduuand Dd~uu. Sites oriented upwards within their layer will connect to up to three downwards-oriented beta cages within their layer. Such geometries are arbitrarily named Ud, Udd, and Uddd. The nine aforementioned sites that connect to the layer below are thus Dd sites.

Figure 4. Modelling a faujasite layer as an array: a) layer annotated with cage orientations and pores, b) layer annotated with array positions, c) sheafing the layer results in a square array that may be used in the actual program.

96

Dd

Udd

Figure 5. The seven different site geometries: The capitalised first letter of each code denotes the orientation of the site under consideration within its own FAU layer. The secondary letters denote existing connections within the same layer and the layer below. The program works by allocating a growth probability to each different kind of site geometry, in a similar fashion to allocation of probabilities for SURFACE, EDGE, KINK and U site geometries in zeolite A. s Each iteration of the program represents one time unit. From the different site geometries present on the surface at any given time, the program selects one at random, taking into account the relative probability for each geometry. Once a geometry has been selected, a randomly chosen site with that geometry is grown at. The program then adjusts vicinal site geometries based on the orientation of the site. For downwards-oriented sites it adjusts site geometries to the left and both up and down to the fight. For upwards-oriented sites it adjusts site geometries to the fight and both up and down to the left. Inspection of figure 4a) should help to clarify this. Having adjusted all vicinal site geometries the program augments the time counter and the growth process is repeated. The program, written in Fortran 77 (MIPSpro), is compiled and run on a Silicon Graphics 02 workstation. The program outputs (x,y) co-ordinates for successive growth sites that are then read into a small routine written in Mathematica 4.0 (Wolfram Research Inc.) that plots them. Calculations have been performed both on 101 x 101 grids and 1001 x 1001 grids. In the former, both the trigonal layer symmetry and the 12 ring super cage access windows are evident. The latter gives a more accurate picture of overall terrace morphology and regularity. 4. RESULTS AND DISCUSSION

The AFM image shown in figure 3 serves as a measure of the accuracy of any simulation generated by the program. The growth layer must be slightly irregular whilst maintaining a strong triangular habit and must grow from one or possibly a small number of nucleation points. The form of growth depends strongly on the inter-relation of the chosen site type probabilities. It should be noted that the probabilities discussed in subsequent sections are not normalised to one.

97

a)

.."

9

9

,.

~

9

9

p.

~'."

9

b)

9

9

.

~jo

9 oo

I

c)

9

,

ad)

P

~P

,d~"e

"r

4

b."

,,

Figure 6. Site probabilities that generate a) multiply-nucleated triangular terraces, b) mono-nucleated triangular terraces, c) multiply nucleated isotropic terraces and d) mono-nucleated isotropic terraces. Array size: 101 x 101. The rate of surface nucleation is principally governed by the ratio of the Ud site probability to the Dd site probability. Both sites involve connection to only one other beta cage and are hence least favourable. The smaller the ratio, the more likely surface nucleation becomes. This can clearly be seen in Figure 6. Figure 6a) shows a simulation generated with equal probability for Ud and Dd sites. Figure 6b) shows the simulation after scaling all probabilities by a factor of 1000 relative to the Dd site probability. The effect is the total inhibition of multiple nucleation. The phenomenon is also observed in the comparison of figures 6c) and 6d). The specific probabilities used are given in table 1. In both figures 6a) and 6b) one of the triangular terraces shows a kink in the edge and this is in accordance with a previously proposed growth mechanism. 3

98 Overall terrace shape is governed by the ratio of the Dau site probability to the Ua site probability. Small ratios cause the layer to grow isotropically with no preferred direction. Large ratios strongly impose three-fold symmetry causing triangular habit. This may be understood intuitively as follows. First, consider nuclation at a Da site. The three possible connections to this surface nucleation are all Ua sites. Growth at one of the three will generate two Da~ sites. Dau sites connect to two beta cages and are hence more favourable connections. Thus as soon as growth occurs at one of the three Ua sites it will swit'dy be followed by growth at the two subsequent Da~ sites and triangular habit is generated, as shown in figure 7.

11r Figure 7. Formation of triangular habit. Figure 8 shows two simulations calculated on a 1001 x 1001 array. These show that the probability for growth at Uaa sites serves to control layer regularity. As this probability approaches the probabilities for Da~u and Uaaa,the layer becomes increasingly irregular.

a)

_

a)

! I

Figure 8. Site probabilities that generate triangular terraces of varying regularity. Array size: 1001 x 1001.

99 Table 1 Relative site probabilities used to simulate growth patterns observed in figures 7 and 8. Site type

Fig. 6a)

Fig. 6b)

Fig. 6c)

Fig. 6d)

Fig.8a)

Fig. 8b)

Dd

1

1

1

1

1

1

D d~

1 000

1 000 000

7

1 000

1 000 000

1 000 000

D duu

2 000

2 000 000

7

1 000

1 000 000

1 000 000

D duuu

10 000

000 000

7

1 000

9

000 000

9 000 000

10

Ud

1

1 000

7

1 000

1 000 000

1 000 000

Udd

1 000

1 000 000

7

1 000

200 000

400 000

U ddd

2 000

2 000 000

7

1 000

1 000 000

1 000 000

5. CONCLUSIONS A program has been written that models the growth of a faujasite layer on the (111) surface of a cubic zeolite Y crystal. Seven distinct surface sites have been defined, though more will be needed to create a multi-layer model. The sites have been arbitrarily named based on a system that takes account of their own orientation within the layer and their possible connectivities. Definition of probabilities for growth at these distinct sites controls the topography of the layer produced. The importance of various probability ratios and their specific topographical effects have been highlighted. Careful selection of the probabilities enables very accurate simulation of a typical growth layer, in terms of both shape and regularity. A modified version of this program will be written to generate a multi-layer simulation similar to that produced for zeolite A. It will however, be considerably more complex owing to the increased number of site definitions, the trigonal layer symmetry and the lateral shi~ing of successive layers to create an ABCABC stacking sequence. Such programs generate data on fundamental zeolite growth processes and are invaluable tools to further our limited understanding of the complex phenomenon that is zeolite growth.

REFERENCES

1. 2.

3. 4.

Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. (1986), 56, 930-933. Weisenhorn, A. L., MacDougall, J. E.; Gould, S. A. C.; Cox, S. D., Wise, W. S., Massie, J.; Maivald, P.; Elings, V. B.; Stucky, G. D.; Hansma, P. K. Science (1990), 247, 13301333. Anderson, M. W.; Agger, J. R.; Thornton, J. T.; F orsyth, N. Angew. Chem. Int. Ed. Engl., (1996), 35, 1211-1213. Agger, J. R.; Pervaiz, N.; Cheetham, A. K.; Anderson, M. W. d. Am. Chem. Soc. (1998), 120, 10754-10759.

100 5. 6. 7. 8. 9.

Anderson, M. W.; Agger, J. R., Pervaiz, N.; Weigel, S. J.; Cheetham, A. K. in Proc. 12th Int. Zeol. Confi, Baltimore, Treacy, M. M. J.; Marcus, B. K.; Bisher, M. E.; Higgins, J. B., Eds.; MRS, (1998), pp 1487-1494. Sugiyama, S.; Yamamoto, S.; Matsuoka, O.; Nozoye, H.; Yu, J.; Zhu, G.; Qiu, S., Terasaki, O. Microporous and Mesoporous Materials (1999), 28, 1-7. Agger, J. R.; Hanif, N.; Cundy, C. S.; Wade, A.; Anderson, M. W. inprep. Agger, J. R.; Hanif, N., Anderson, M. W. Angew. Chem. Int. Ed. Engl., (2001), 40, 40654067. Oshuna, T.; Terasaki, O.; Alfredsson V.; Bovin J.-O.; Watanabe D.; Carr, S. W.; Anderson, M. W. Proc. R. Soc. Lond. A (1996), 452, 715-740.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

101

Interaction of Small Molecules with Transition Metal Ions in Zeolites: The Effect of the Local Environment P. Nachtigall, M. Davidovfi, M. Silhan and D. Nachtigallovfi J. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic and Center for Complex Molecular Systems and Biomolecules, Dolej~kova 3, 182 23 Prague 8, Czech Republic The interaction of small molecules (CO, NO, NO2, N2, H20) with transition metal ions inside the zeolite matrix was studied with the combined quantum mechanics / interatomic potential function (QM-Pot) technique. Various sites of the transition metal ions located on the channel wall or on the channel intersection were investigated for ZSM-5 and FER matrices. The interaction of small molecules with transition metal ions leads to changes in structure and coordination of active sites. The extent of these changes depends on the character of the transition metal ion site. The structure, coordination, vibrational analysis, and photoluminescence spectra were investigated and compared with available experimental data (EXAFS, IR, UV-VIS, and EPR). The explanation of differences in catalytic activities of Cu/ZSM-5 and Cu~ER systems is also suggested based on the computational results.

1. INTRODUCTION Knowledge about the structure and siting of transition metal ions (TMI) in zeolites is essential in order to explain unusual catalytic activity and selectivity of TMI/zeolite systems. For direct decomposition of NO to molecular nitrogen and oxygen the Cu/ZSM-5 system has been shown to have the highest catalytic activity.[1] Since this discovery research has been focused on the characterization of the active sites and on the mechanism of the NO conversion. The catalytic activity of copper ions differs for various zeolite structures, Si/A1 ratios, and Cu loadings.[2,3] Based on numerous experimental studies (employing EXAFS, EPR, IR, and UV-VIS techniques) it was shown that two dominant types of copper ion sites occurs (see, e.g., Refs [4] and [5]). However, the detail information about the structure and coordination of Cu + ions in ZSM-5 matrix are not experimentally accessible. Recently we studied the structure and coordination of Cu- and Ag-species in high-silica zeolite matrices using a combined quantum mechanics/interatomic potential function (QM-Pot) method.[6-8] In agreement with experimental results two dominant types of monovalent metal sites were found: In type I

102 sites, the Cu + ions are coordinated to 3 or 4 oxygen atoms of the six (or five) membered aluminosilicate ring on the wall of one of the channels. In type II sites the Cu + ions are coordinated to two oxygen atoms of single A104 tetrahedron located at the intersection of two channels. In this site the Cu + ions occupy the open space on the channel intersection. The same site types were found for both ZSM-5 and FER frameworks,[7] however, the relative stabilities of individual sites differ in these two zeolites. The aim of the present contribution is to investigate the properties of individual TMI site types in high-silica zeolite frameworks when interacting with small molecules (CO, NO, NO2, N2, H20). The implications for catalytical activities of these TMI/zeolite systems are also discussed.

2. M E T H O D

The combined quantum mechanics/interatomic potential function technique[9] was used in the study of small molecules interacting with Cu and Ag ion sites in ZSM-5 and FER matrices. This methods treats the transition metal ion and its surroundings (tens of zeolite framework atoms) at the ab initio level and it treats the interactions with more distant atoms at the interatomic potential function level. The QM-Pot method is therefore suitable for investigation of subtle differences among various zeolite frameworks. Periodic unite cells containing 96 and 72 T-atoms were adopted for ZSM-5 and FER, respectively. QM part of the model was represented by clusters of sizes ranging from 3-T to 10-T including transition metal and adsorbed small molecules. B3LYP functional was used for description of electronic structure of the inner part of the model and core-shell model potential was used for description of the interactions outside the inner part. This model respects the details of individual zeolite framework matrices. The detail description of QM-Pot method can be found in Ref. [ 10] and the detail description of the model can be found in Ref. [6] The calculations were carried out with the QMPOT program, which makes use of the TURBODFT[ 11 ] and GULP[ 12] programs for DFT and core-shell model potential calculations, respectively. The interaction of small molecules with the Cu + and Ag + ions was investigated for type I and type II sites of copper in both ZSM-5 and FER matrices. For the interaction with copper at type I site in ZSM-5 the six-membered ring on the wall of zig-zag channel ("Z6" according to the notation introduced in Ref. [6]) was used with A1 atom at T4 position. The interaction of small molecules with the Cu + in the vicinity of two A1 atoms was studied on the same ring with A1 atoms placed in T4 and T10 positions and the proton located on the T10 tetrahedron. (For T-sites we use the numbering scheme introduced for orthorhombic symmetry by Koningsveld.[13]) Type I site in FER was represented by a six-membered ring on the wall of perpendicular channel ("P6" according to the notation introduced in Ref. [7]) with A1 atom in T1 site. (T-sites numbering of Ref. [14] was adopted.) For type II sites (on the intersection of two channels) aluminum atom was placed either to T12 or to T6 position in ZSM-5 and to T2 or T4 positions in FER. The Cu + sites considered in this study are depicted at Figure 1.

103

Figure 1. Definition of type I and type II sites in ZSM-5 and FER matrices. Transition metal ion is depicted as a ball. View along the direction of the main channel.

3. RESULTS AND DISCUSSION The interaction of copper and silver ions with ZSM-5 and FER frameworks were studied previously in our laboratory by means of combined QM-pot technique.[6-8,15] Two dominant types of the Cu § ion sites were identified in both zeolite frameworks: sites where the Cu § ions are coordinated to 3 or 4 oxygen atoms of six-membered ring on the wall of one of the channels were denoted "type I" sites, and, sites where the Cu § ions are coordinated to only two oxygen atoms of the A104 tetrahedron located on the channel intersection were denoted "type II" sites (Figure 1). On the contrary, a strong preference for type II sites was found for larger Ag § ions in ZSM-5 framework. The interaction energies of small molecules with Cu § and Ag § ion sites in ZSM-5 and FER depend on the site type and zeolite framework. However, coordination of monovalent transition metal ion (TMI) to the zeolite framework is very similar for both sites and both zeolites upon the interaction with single small molecule as CO, NO, N2, and NO2. Somewhat different results were obtained for interaction of TMI/zeolite with water molecule.

3.1. Interaction of Cu+/zeolite with CO, NO, N2, and NO2 molecules The coordination environment for type I and type II sites is rather different in highsilica zeolites. However, upon the interaction with any of the studied molecules the coordination environment of TMI becomes rather similar for both site types: TMI is coordinated to two oxygen atom of framework A104 tetrahedron and to one atom of adsorbed molecule (two in case of NO2). As a result, the vibrational frequencies of adsorbed molecules were found very similar for both TMI site types.

104

Figure 2. Coordintaion of the Cu + ions inside the ZSM-5 and FER framework before (top part) and after (bottom part) adsorption of NO molecule. The Cu + ion and atoms of NO molecules are depicted as balls. View along the direction of zig-zag and perpendicular channels of ZSM-5 and FER, respectively.

The coordination of Cu § ions with adsorbed NO molecule are depicted in Figure 2 for several sites under investigation. Upon the interaction with NO radical the Cu § ions keeps coordination to only two oxygen atoms of A104 tetrahedron, with r(Cu-O) bond lengths in the range of 2.02-2.08 A. Also the r(Cu-N) bond lengths are very similar for all Cu § sites, about 0.1 A shorter than for the gas phase Cu+(NO) ion. The N-O bond is almost identical for all sites considered (1.16 A). This is about 0.025 A longer than in the gas phase Cu+(NO) species, indicating the smaller charge transfer from NO to Cu in zeolite than in the gas phase species. The coordination of two oxygen and one nitrogen atom ligating the Cu + ion is planar for type II sites and almost planar (up to 20 ~ deviation from planarity) for type I sites. This is due to the fact that type II sites located inside the cavity at the channel intersection allow for planar coordination environment on the Cu + ion, while for the type I sites the repulsive interaction between the channel walls and ligand molecule forces the nonplanar coordination environment on the Cu + ion. In addition, the NO molecule is always bonded to the Cu + ion in a direction where it can reach the open space at the channel intersection. Such coordination can be easily reached for the Cu + ions located on the intersection of two channels (type II) without significant changes of (Cu+... zeolite) coordination. On the contrary, the NO molecule cannot bind to Cu + ions on the channel wall (type I) without major changes in (Cu + ... zeolite) coordination (Figure 2).

105 For type I sites, where the interaction with NO leads to partial loss of Cu + coordination with the zeolite framework the interaction energies of NO and Cu+/zeolite are smaller than for type II sites where the Cu+... framework coordination is not changed upon the NO adsorption (Table 1). Very similar results were obtained also for CO, N2, and NO2 molecules. The results are summarized in Table 1. It should be pointed out that when ZPE and temperature is taken into account the interaction energies are significantly lowered, e.g., AG~ kcal/mol for CO interaction with type II site of Cu+/ZSM-5.

Table 1 Interaction energies of CO, NO, N2, and NO2 molecules with Cu+/zeolite (in kcal/mol) a. Type I Type II ZSM-5 FER ZSM-5 FER CO 36 28 42 40 N2 20 13 26 24 NO 28 20 35 33 NO2 35 26 43 38 a At 0 K temperature, ZPE not inclueded.

The interaction with type II sites (sites on the channel intersection) is stronger by 6-8 and 11-13 kcal/mol in ZSM-5 and FER, respectively, than interaction with type I sites (sites on the channel wall). The interacction energies are rather similar for type II sites in ZSM-5 and FER, while the interaction with type I sites is stronger in ZSM-5 than in FER. Upon the interaction with one molecule the coordination of TMI to zeolite framework is unchanged for type II sites while it is lowered significantly for type I sites. For type I site the interaction Of TMI with any of the studied molecule leads to the loss of the coordination of TMI to non-A104 tetrahedron framework oxygen atoms and TMI is moved farther form the channel wall. The significant differences between type I and type II sites were also found for interaction with two or three molecules. For example, the Cu + ion in type II site can bind two or even three CO molecules (in agreement with experimental observation) while the Cu + ion in type I site can bind two CO molecules at most. The results obtained in this study are in reasonable agreement with available experimental data. Due to the space limitation we only briefly compare with some of them obtained for CO interaction with Cu+/zeolites. Zecchina et al. found that all Cu + sites in ZSM-5 are accessible for CO, contrary to other zeolite matrices.[ 16] Sarkany observed the frequency shift of zeolite asymmetric stretching vibration due to the interaction with Cu+(CO)n species.[ 17] Using XAFS technique Kumashiro et al. also found changes in Cu + coordination environment upon adsorption of CO.[ 18] The average Cu-C distance 1.89 A was found and average distance between Cu + and framework oxygen atoms increases from 1.98 to 2.05 A when CO is adsorbed. Using the combination of XAFS, IR, and adsorption calorimetry techniques Kumashiro et al. concluded that two types of Cu + sites differing in CO adsorption energy can be distinguished in ZSM-5: (i) strongly interacting Cu + site

106 where the Cu t ion is coordinated to two framework oxygen atoms and (ii) Cu t site where the Cu t ion is coordinated to three framework oxygen atoms.

Figure 3. Structure and coordination of Cu + ion in ZSM-5 upon the interaction with water molecule. Resulting structures as obtained with large cluster model embedded in full periodic environment (part a) and with small 1-T and 5-T cluster models (parts b and c, respectively).

3.2. Interaction of water with Cut/ZSM-5 The structure and coordination of the Cut/ZSM-5 with water molecules are influenced by the strong tendency of water molecules to create a hydrogen bond with the zeolite framework oxygen atoms in addition to the interaction with transition metal ion. A typical

107 structure obtained with combined QM-pot technique (including large cluster treated at ab initio level) is depicted in Figure 3a. As a result of this strong hydrogen bonding the Cu + ion stays strongly coordinated to only one oxygen atom of A104 tetrahedraon (rcu_o=l.94 /~) while the interaction of Cu + with the second oxygen atom of A104 tetrahedron is significantly weaker (rcu_o=2.47 A). The tendency of adsorbed water molecule to create a hydrogen bond with framework oxygen atom requires the use of sufficiently large models for description of water interaction with TMI/zeolite systems. This is documented in Figure 3b and 3c where the results of cluster model calculations employing the small clusters are shown. Small cluster models lead to structures in which the hydrogen bond closes the six-atom ring, while larger cluster model leads to different structures. In fact, for larger Ag + ions we have obtained the structures were adsorbed water molecules creates a hydrogen bond with the framework oxygen atoms from the oposite site of the channel.

4. CONCLUSIONS Combined quantum mechanics/interatomic potential function study of the interaction of Cu + and Ag § ions with MFI shows that both transition metal ions tends to occupy the open space at the channel intersection when the location of framework A1 atom allows. Due to the larger size of Ag + ion the sites at the channel intersection (type I sites) are always preferred, regardless of the location of framework A104 tetrahedron. The Cu + ions preferably occupy the positions at the channel intersection only if the A104 tetrahedron is located at the channel intersection. Rather different results were found for Cu+/FER system, in which the sites on the wall of one of the channel (type II sites) are always preferred. The interaction of small molecules with transition metal ions leads to changes in structure and coordination of active sites. The extent of these changes depends on the character of the transition metal ion site. Upon the interaction with any of the studied molecules the coordination environment of TMI becomes rather similar for both site types and both ZSM-5 and FER frameworks: TMI is coordinated to two oxygen atom of framework A104 tetrahedron and to one atom of adsorbed molecule (two in case of NO2).

Acknowledgement: P.N. acknowledges support from the Granting Agency of the Czech Republic, Grant No. 203/00/0637. M.D., M.S., and D.N. wish to thank to the Czech Ministry of Education for support to the Center for Complex Molecular Systems and Biomolecules (Grant No. LN00A032). We thank to J. Sauer and M. Sierka for QM-Pot computer code any numerous helpful discussion. We would also like to thank J. Gale for providing GULP code.

108 REFERENCES

1. M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya, and S. Kagawa, J. Chem. Sot., Chem. Commun. (1986) 1272. 2. B. Wichterlova, J. Dedecek, Z. Sobalik, A. Vondrova, and K. Klier, J. Catal. 169 (1997) 194. 3. G. Moretti, Catal. Lett. 23 (1994) 135. 4. J. Dedecek, Z. Sobalik, Z. Tvaruzkova, D. Kaucky, and B. Wichterlova, J. Phys. Chem. 99 (1995) 16327. 5. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo, G. Vlaic, and M. Bellatreccia, J. Phys. Chem. B 1 O1 (1997) 344. 6. D. Nachtigallova, P. Nachtigall, M. Sierka, and J. Sauer, Phys. Chem. Chem. Phys. 1 (1999) 2019. 7. P. Nachtigall, M. Davidova, and D. Nachtigallova, J. Phys. Chem. B 105 (2001) 3510. 8. M. Silhan, D. Nachtigallova, and P. NachtigaU, Phys. Chem. Chem. Phys. 3 (2001) 4791. 9. M. Sierka and J. Sauer, Faraday Discuss. (1997) 41. 10. M. Sierka and J. Sauer, J. Chem. Phys. 112 (2000) 6983. 11. O. Treutler and R. Ahlrichs, J. Chem. Phys. 102 (1995) 346. 12. J.D. Gale, J. Chem. Soc.-Faraday Trans. 93 (1997) 629. 13. H. v. Koningsveld, J. C. Jansen, and H. v. Bekkum, Acta Crystalographyca 7 (1987) 564. 14. P. A. Vaughan, Acta Crystalographyca 21 (1966) 983. 15. D. Nachtigallova, P. Nachtigall, and J. Sauer, Phys. Chem. Chem. Phys. 3 (2001) 1552. 16. A. Zecchina, S. Bordiga, G. T. Palomino, D. Scarano, C. Lamberti, and M. Salvalaggio, J. Phys. Chem. B 103 (1999) 3833. 17. J. Sarkany, J. Mol. Struct. 410 (1997) 95. 18. P. Kumashiro, Y. Kuroda, and M. Nagao, J. Phys. Chem. B 103 (1999) 89.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

109

Preparation and characterization o f m e s o p o r o u s TS-1 catalyst Kim Johannsen a, Astrid Boisen a, Michael Brorson a, Iver Schmidt a and Claus J.H. Jacobsen a aHaldor Topsoe A/S, Nymollevej 55, DK-2800 Lyngby, Denmark

Highly crystalline titanium-containing zeolite TS-1 with an intracrystalline mesopore system has been prepared and characterized. The TS-1 zeolite is crystallized in a carbon nanoparticle matrix (18 nm particles), which is subsequently removed by heating, leaving a mesopore system with the same diameter as the carbon nanoparticles. The resulting mesoporous TS-1 zeolite is characterized using X-Ray Diffraction, Inductively Coupled Plasma emission spectroscopy, UV/vis-spectroscopy, Raman spectroscopy, Hg-porosimetry and Scanning Electron Microscopy. The results show, that the TS-1 is highly crystalline, anatase-free and highly mesoporous with an average crystal size o f - 5 pm. The mesoporous TS-1 is shown to exhibit a smaller degree of diffusion limitations in the epoxidation of cyclohexene with aqueous hydrogen peroxide than conventional TS-1.

1. INTRODUCTION 1.1 The titanium silicalite TS-1

In 1983 Taramasso et al. [1] were the first to report fabrication and use of the titanium containing silicalite zeolite now known as TS-1 by incorporation of titanium ions into silicalite-1. TS-1 shows a great potential in catalysis, since it was soon learned, that it can be used as a catalyst for selective oxidation and epoxidation of a number of organic compounds with aqueous hydrogen peroxide as oxidant, examples being hydroxylation of phenol to dihydroxybenzene [21, epoxidation of propene and other alkenes 131, ammoxidation reactions [41 and the oxidation of organic compounds with dioxygen in biomimetic systems [51. TS-1 is one of the only known catalysts facilitating the use of the inexpensive hydrogen peroxide instead of more costly organic peroxides, which normally are required in selective oxidations. 1.2 Diffusion limitations

One of the main drawbacks of using TS-1 for catalysis is the small size of the micropores, giving rise to intracrystalline diffusion limitations for bulky substrates, reducing the yield and conversion in some catalyzed reactions. Mainly two different approaches have been made to alleviate this problem: 1) Increase in pore size. Titanium containing zeolites with larger pores (BEA and UTD-1) have been prepared and tested for selective oxidation with hydrogen peroxide. However, the catalytic properties of these zeolites are substantially different from those of TS-1 [6]. 2) Reduction in crystal size. Instead of increasing the pore size attempts have been made to prepare nanosized crystals of TS-1 with improved diffusional properties. An increase in surface area/volume ratio would be achieved, and a larger percentage of the zeolite volume would thus be available to catalysis (since the diffusion length is unaltered). This was

110 shown to be possible, eg. using Confined Space Synthesis [71, with which highly crystalline TS-1 crystals can be prepared with crystal sizes down to -20 nm. Unfortunately there are difficulties involved in separating the nanosized crystals from the product mixture in the preparation and use of nanosized zeolite. 1.3 Anatase When preparing TS-1 catalyst it is imperative to avoid formation of TiO2 (anatase), since anatase catalyzes the thermal decomposition of H202 to H20 and 02 and thus lowers the selectivity and yield of the oxidation process. Conventional TS-1 can now easily8 be prepared without the presence of anatase by avoiding alkali ions in the synthesis tl and using appropriate titanium and silicon alkoxides t91. 1.4 Aim of this work In light of the above-mentioned factors the preparation, characterization and use of an anatase-free micronsized TS-1 with an intracrystalline mesoporosity, giving a larger surface area/volume ratio, seems desirable. This was the aim of the work presented here.

2. EXPERIMENTAL Chemicals used are all reagent grade if possible. The following chemicals were used for preparation of mesoporous TS- 1: 9 Carbon Black Pearls BP2000| (Cabot Corp.- average particle diameter 18 nm) 9 Tetrapropylammonium hydroxide (CH3CH2CH2)4NOH(TPAOH), 40% solution in water 9 Ethanol CH3CH20H(EtOH), 99% 9 Tetrabutyl orthotitanate Ti(OBu)4(TBOT), 97% 9 Tetraethyl orthosilicate Si(OCH2CH3)4(TEOS), 99% 2.1 Preparation of mesoporous TS-1 Carbon Black Pearls (having a pore volume o f - 3 . 5 ml/g) are impregnated to incipient wetness with a solution of TPAOH, EtOH and water. The ethanol is then evaporated at room temperature, leaving free pore volume. Then the carbon black pearls are re-impregnated with a mixture of TBOT and TEOS to give the correct synthesis gel composition within the carbon pores. The carbon is left for 3 hours at room temperature in order to hydrolyze the alkoxides and is subsequently placed in a stainless steel autoclave. It is heated at 170~ for 72 hours to crystallize the zeolite around the carbon nanoparticles, creating micronsized zeolite crystals with carbon nanoparticles included into the crystal. The black carbon/zeolite mixture is then removed from the autoclave, washed 3 times with water and once with EtOH and dried at 110~ to remove excess of water and EtOH. It is placed in an oven and slowly heated to 550~ the temperature is maintained for 8 hours, burning off the carbon as CO2 and leaving the mesoporous TS-1 (see figure 1). 2.2 Characterization of the product The resulting white powdery product is examined using X-Ray Diffraction (XRD) to determine the crystallinity and to establish the structure of the zeolite, Inductively Coupled Plasma emission spectroscopy (ICP) to establish Si/Ti-ratio, UV/vis and Raman spectroscopy to determine the presence or absence of extraframework titanium in the form of TiO2

111 (anatase), Scanning Electron Microscopy (SEM) to determine morphology, Hg-porosimetry to measure pore volume and pore radius, and N2-adsorption measurements to determine surface area.

Figure 1: Principle of carbon nanoparticle removal by burn-off, leaving a mesoporous zeolite crystal; the dark grey dots symbolizing carbon nanoparticles. 2.3 Epoxidation of 1-octene and cyclohexene TS-1 zeolite as mentioned can be used as catalyst in selective partial oxidations with aqueous hydrogen peroxide. One of the well-known examples is in the epoxidation of alkenes TM. It was chosen to test the catalytic properties of the mesoporous TS-1 as compared to conventional TS-1 (prepared in the lab by a similar procedure as described above for mesoporous TS-1) in the epoxidation of 1-octene and cyclohexene. It is expected that diffusion limitations occur for the epoxidation of the bulky cyclohexene, thus the use of mesoporous TS-1 is expected to increase the yield for this reaction. The same is not expected in the epoxidation of the linear 1-octene, instead similar catalytic properties of the conventional and mesoporous TS-1 is expected. In these experiments the alkene was dissolved in methanol and placed in a roundbottomed flask with a reflux condenser. TS-1 (mesoporous or conventional) was suspended in the solution by stirring, and the set-up was heated to 40~ (the reaction temperature). At 40~ H202 was inserted and the experiment started. Samples were taken at regular intervals and analyzed by gas chromatography for the alkene and the epoxide, as well as the three possible ring-opening side-products: the alkanediol and the two alkoxyethers. Reagent grade chemicals were used where possible.

3. R E S U L T S 3.1 Characterization of mesoporous TS-1 The Carbon Black Pearls was impregnated and treated as described in section 2.1. After carbon burn-off a white powdery product was recovered. It was examined by XRD, confirming that it was highly crystalline TS-1 zeolite. ICP results showed that the compound had the expected Si/Ti-ratio of 50, corresponding to the molar ratio of the reactants. In an effort to determine the presence or absence of anatase, UV/vis and Raman spectroscopy was employed (an example of UV/vis comparing an anatase-free and anatase-containing mesoporous TS-1 can be seen in figure 2). From figure 2 it can be seen, that the mesoporous TS-1 exhibits two peaks, one at 220 nm and one at 270 rim. The peak at 220 nm is normally attributed to framework-titanium species in TS-1 and the latter to hydrated frameworktitanium t1~ If anatase is present, a third peak arises at 330 nm, which can be attributed to the anatase ~lz~. Thus UV/vis spectroscopy can in some cases determine if anatase is present.

112 However, as it can be seen there is a significant overlap between the peaks, and another technique may be necessary to confirm the findings from UV/vis. In Raman, several peaks will be present from silicalite-1 (i.e. the silicon species in the framework), TS-1 (titanium species in the framework) and anatase (if present), some of them coinciding. 1,000

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Figure 2: UV/vis spectre of anatase-containing and anatase-free sample of mesoporous TS-1. Several different peak patterns have been presented in the literature, not all of them agree, eg. Gao et al. [~21 and Can Li et a/. [13] present slightly different peak patterns for TS-1 zeolite. Gao et al. [121 gives the following peaks" Silicalite-l" 295 cm "l, 377 cm -1, 435 cm 1, 469 cm -1, 800829 cm -l TS-I' the above and two new at 960 c m "1 and 1128 cm -1. Anatase: 145 cm l 397 cm1,515 cm -1, 638 cm -1. Can Li e t a / . [13] gives the following peaks: Silicalite-l" 290 c m -1, 380 cm -1 960 cm -1 1170 cm 1 TS-I" the above and also 490 cm -1, 530 c m l 1125 cm l Anatase: 144 cm -1, 3 9 0 cm -1, 516 cm -1, 637 c m -1. Agreement can be seen for the peaks of anatase. However, Can Li finds the peak at 960 c m 1 for both TS-1 and silicalite-1, and there are some disagreements on some other peaks in TS-1 and silicalite-1. But it can be seen, that the peak at 637 cm 1 from anatase is distinct and highly separated from any TS-1 peaks, and the presence of it is a positive indication for anatase. The mesoporous TS-1 prepared was examined with both UV/vis and Raman spectroscopy. The results show, that the samples prepared according to the procedure described in section 2.1 were anatase-free, no peak was seen at 637 cm -1. The results furthermore agree with the report by Gao et al. No peaks were found at 490 cm ~ or 530 cm ~. Hg-porosimetry showed the existence of a mesopore system in the TS-1 crystals. The mesopore volume was - 450 ml/kg and the average mesopore diameter was 220 ,X,. Scanning Electron Microscopy (SEM) of the two samples can be seen in figure 3. The SEM micrographs show, that both TS-1 samples consist of well-shaped rectangular crystals with an average crystal size of a few ~tm. Furthermore the SEM micrograph of the mesoporous TS-1 prepared clearly shows the mesopores created according to figure 1. ,

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Figure 3: SEM micrographs of the mesoporous TS-1 (left) and conventional TS-1 (right).

3.2 Epoxidation experiments The catalytic properties of the mesoporous TS-1 was examined by performing a selective epoxidation of 1-octene and cyclohexene with aqueous hydrogen peroxide in methanol using conventional and mesoporous TS-1 as catalyst (figure 4) H202

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Figure 4: Epoxidation reactions of 1-octene (top) and cyclohexene (bottom) In both reactions a selectivity towards the epoxides of >90% was achieved at the reaction temperature of 40~ The main byproducts were the addition products of the epoxides, i.e. 1,2-octanediol and 1,2-cyclohexanediol. To evaluate the catalytic properties of the mesoporous TS-1 as compared to conventional TS-1 in the experiments, the ratio of the epoxide concentration with mesoporous and conventional TS-1 (Cmesoporous/Cconventional) was plotted against time. This can be seen in figure 5 (taken from ref. 14 with permission). Figure 5 shows that the mesoporous TS-1 has similar catalytic properties for the epoxidation of the linear 1-octene as conventional TS-1, thus the introduction of a mesopore system does not decrease the catalytic ability of the TS-1. For epoxidation of the more bulky cyclohexene the mesoporous TS-1 shows a significant increase, a tenfold higher epoxide concentration is seen without any change in product distribution. And since the results with 1-octene showed, that the two samples have similar intrinsic catalytic activity (the samples had equal Si/Ti-ratios), the increase for cyclohexene epoxidation must be due to a better accessibility of the active sites, i.e. improved diffusional properties of the mesoporous TS-1 as compared to conventional TS- 1.

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Figure 5: The relative amount of epoxide formed using mesoporous TS-1 compared to conventional TS-1 as a function of time for 1-octene (rT) and cyclohexene (o). 4. CONCLUSIONS Highly crystalline mesoporous TS-1 zeolite was prepared by impregnating a carbon nanoparticle matrix with the TS-1 synthesis gel in two steps. The TS-1 crystallized around the carbon nanoparticles, which were subsequently removed by heating, leaving a mesopore system. The mesoporous TS-1 thus prepared was examined by X-ray diffraction, UV/vis and Raman spectroscopy, Hg-porosimetry and scanning electron microscopy. The mesoporous TS-1 was highly crystalline, anatase-free rectangular crystals with an average size of a few gm. The crystals had a mesopore system with an average mesopore size of 220 A and a mesopore volume of 450 ml/kg not including the micropores. Epoxidation experiments with TS-1 as catalyst showed, that the mesoporous TS-1 exhibited significantly lower diffusion limitations. The improved diffusion properties gave a tenfold increase in epoxide yield for cyclohexene as compared to conventional TS-1 with the same Si/Ti-ratio without any change in product distribution. This is a significant advantage of using mesoporous TS-1 instead of larger-pore titanium zeolites, where a change in product distribution is normally seen. This carbon nanoparticle procedure for producing mesoporous zeolites can easily be adapted to other zeolite types, as long as the synthesis gel can be impregnated in one or more steps into the carbon matrix. The technique thus has potential use in a large number of applications, where zeolites are used as catalysts and diffusion limitations are seen. Great potential can also be seen in the field of bifunctional catalysts, where it appears attractive to disperse another catalytic functionality within the individual zeolite crystals.

REFERENCES 1. M. Taramasso, G. Perego and B. Notari, US Pat. 4 410 501 (1983) 2. B. Kraushaar-Czarnetzki and J.H.C. Van Hooff, Catal. Lett. 2 (1989) p. 43 3. M.G. Clerici, G. Bellussi and U. Romano, J. Catal. 129 (1991) p. 159

115 4. P. Roffia, M. Padovan, E. Moretti and G. de Alberti, Eur. Pat. 208 311 (1987) 5. G. Lu, H. Gao, J. Suo and S. Li, J. Chem. Soc., Chem. Commun. (1994) p. 2423 6. A. Carati, C. Flego, E. Previde Massara, R. Millini, L. Carluccio, W.O. Parker Jr. and G. Bellussi, Microporous Mesoporous Mater. 30 (1999) p. 137 7. C. Madsen and C.H. Jacobsen, Chem. Commun. (1999) p. 673 8. H. van Bekkum, E.M. Flanigen, P.A. Jacobs and J.C. Jansen (editors), "Introduction to Zeolite Science and Practice", 2nd edition, Stud. Surf. Sci. Catal. 137, page 916-917, Elsevier Science (2001) 9. A. Thangaraj and S. Sivasanker, J. Chem. Soc. Chem. Commun. (1992) p.123 10. A. Thangaraj, R. Kumar, S.P. Mirajkar and P. Ratsanamy, J. Catal. 130 (1991) p. 1 11. Xiang-Sheng Wang and Xin-Wen Guo, Catal. Today 51 (1999) p. 177 12. Huanxin Gao, Wenkui Lu and Qingling Chen, Micropor. Mesopor. Mat. 34 (2000) p.307 13. Can Li, Guang Xiong, Jianke Liu, Pinliang Ying, Qin Xin and Zhaochi Feng, J. Phys. Chem. B 105 (2001) p.2993 14. I. Schmidt, A. Krogh, K. Wienberg, A. Carlsson, M. Brorson and C.J.H. Jacobsen, Chem. Commun. (2000) p.2157

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

117

Observations o f layer growth in synthetic zeolites by field emission scanning electron microscopy S. Bazzana, S. Dumrul, J. Warzywoda, L. Hsiao, L. Klass, M. Knapp, J.A. Rains, E.M. Stein, M.J. Sullivan, C.M. West, J.Y. Woo and A. Sacco, Jr. Center for Advanced Microgravity Materials Processing, Chemical Engineering Department, 147 Snell Engineering Center, Northeastern University, Boston, MA 02115, USA Field emission scanning electron microscopy (FE-SEM) has been used to image the crystal growth-induced surface features of synthetic zeolites. The capability of FE-SEM to image layers of nanometer scale thickness in zeolites A, X, and Silicalite has been demonstrated by comparing atomic force microscope (AFM) and FE-SEM images of their surface topography. Layers on the {100} faces of zeolite A, {111} faces of zeolite X, and {100} and {010} faces of Silicalite with AFM measured thickness of~l.2, ~1.5, ~1-2, and ~1-2 nm, respectively were successfully imaged by FE-SEM using crystals coated with thin Cr films. Also, FE-SEM could discern layers on the uncoated crystals but the resulting images were characterized by much poorer topographic contrast. In addition, layers on the {110} zeolite A surfaces hitherto not reported have been imaged by FE-SEM. The presence of growth terraces on the {110} faces has been related to the high ratio of the rate of surface nucleation to the rate of lateral propagation of layers occurring on these surfaces near the end of crystallization. The FE-SEM images of layers on the basal planes of cylindrical zeolite L crystals have been also acquired and also discussed in context with the previously hypothesized spiral growth mechanism on these surfaces. 1. INTRODUCTION Recent atomic force microscope (AFM) observations of the fine surface features of synthetic zeolites A, Y, hexagonal faujasite (EMT), and Silicalite showed the evidence of layer-by-layer growth [1-5]. Individual layers had thickness that depended on the zeolite type, but in all cases studied did not exceed a few nanometers (~1.2-2 nm). Breck [6] first reported layer growth, showing square terraces on a (100) face of a 1 Bm zeolite A crystal. AFM is an ideal tool for imaging the nanometer scale vertical topographic features in zeolites due to its atomic vertical resolution. However, AFM observations of crystals that are very small and/or inclined to the horizontal are difficult owing to the problems with controlling their orientation and peculiarities of image analysis [2]. Although its vertical resolution does not match that of AFM, scanning electron microscopy (SEM) offers much greater imaging flexibility and can be used to image randomly oriented, small, and/or intricately shaped crystals (surfaces). Modem field emission (FE) SEMs, permitting routine operation at low beam energies (<5 keV), make it possible to successfully image poorly conductive samples,

118 such as zeolites, without the need for the traditional, relatively thick Au-Pd coatings. Operations at low beam energies improve the signal-to-noise ratio and are desirable for high resolution SEM imaging [7]. Application of very thin (1-2 nm) Cr coatings to enhance the secondary electron yield (i.e., improve resolution) without altering a sample's surface structure has been shown to be effective [7]. In this study low voltage FE-SEM was applied to observe fine surface structures of both the tmcoated and Cr-coated zeolite crystals. The capability of FE-SEM to image nanometer thick layers in zeolites A, X, and Silicalite is demonstrated by comparing the AFM and FE-SEM images of their surface topography. Application of FE-SEM to better understand zeolite crystal growth mechanisms is illustrated by presenting and discussing growth terraces imaged for the first time on the {110} zeolite A surfaces and by showing the images of layers on the basal planes of cylindrical zeolite L and discussing them in the context of the previously suggested spiral growth on these surfaces. 2. EXPERIMENTAL Zeolite crystals were grown statically in hydrothermal syntheses, or were purchased commercial products (AD40NX hydrated zeolite Na-X powder from The PQ Corp.). Gel compositions, temperatures, and times used in these syntheses are given in Table 1. Crystals were examined in the as-synthesized, or as-received form. As-synthesized crystals were obtained by separating crystallization products from their mother liquors by vacuum filtration followed by washing with deionized water and air-drying at 80 ~ Crystals used in all FESEM and AFM analyses were sprinkled onto double-sided carbon tape mounted on SEM specimen stubs. When required, crystals to be imaged by FE-SEM were coated with a thin Cr film using a Denton Vacuum Hi-Res 100 Chromium Sputtering System (sputtering rate 0.1 .Ads at 0.66 Pa of ultra high purity Ar (99.999%, Matheson)). FE-SEM images were acquired on a Hitachi S-4700 FE-SEM in the secondary electron imaging mode using the upper detector, 3 mm working distance, 1-2 kV accelerating voltage, and 10-15 BA emission current. The lateral resolution under these conditions is ~2.5 nm [8]. Cr-coated crystals were imaged using the conventional slow speed scanning. The images of the uncoated crystals could not be obtained using this method due to charging phenomena occurring even at this low accelerating voltage. Thus, images were acquired by averaging 128 frames obtained by fast scanning (TV rates). AFM images were recorded in ambient air in TappingMode on a Digital Instruments Dimension 3000 scanning probe microscope (SPM) equipped with a NanoScope IIIa SPM controller. Some of the images, which were collected on inclined surfaces, were "leveled" by applying a first-order planefit correction in the x and y direction. 3. RESULTS AND DISCUSSION Figure la illustrates typical growth terraces imaged by the AFM on the {100} faces of large ~15-30 ~tm zeolite A crystals. Such topography is due to layer growth following the terrace-ledge-kink (TLK) mechanism [2]. The terraces shown in Figure la generate four vicinal faces on the {100} zeolite A surfaces. Vicinal faces are defined as "plane-like" surfaces which appear to be crystallographic planes being symmetrical in accordance with the symmetry of the crystal, but which cannot be described by simple indices [9]. Evidence

119 of the pyramidal structure of the { 100} surfaces of zeolite A observed by AFM (Figure la) also could be seen on the FE-SEM images. As illustrated in Figure l b, the more pronounced terraces on the { 100} surfaces could be imaged on the Cr-coated crystals by FE-SEM using magnifications as low as 3 500-5 000X. Higher resolution (magnifications 80 000-130 000X) FE-SEM images of the { 100} surfaces (Figure 2a) could discern individual terraces. AFM analysis of square terraces on a (100) surface, illustrated in Figures 2b and 2c, clearly showed terrace height-1.2 nm. Comparison of the FE-SEM and AFM images shown in Figures 2a-c illustrates that FE-SEM can image the nanometer-scale vertical details on zeolite surfaces coated with a thin Cr film. Terraces could be also observed on the uncoated crystals at intermediate magnifications (up to 35 000X), however, the FE-SEM images of such crystals were generally characterized by poor topographic contrast. Without Cr coatings only crystal surfaces highly inclined to the incident beam at magnifications up to 100 000X showed good topographic contrast.

Table 1 Crystallization conditions used to grow crystals analyzed (gel compositions: m Na20: n K 2 0 : k (NH4)20: x A1203: y SiO2: w Triethanolamine: q Tetrapropylammonium Bromide: z H20) Zeolite Crystal m n k x y w q z Temp. Time Type Size (~tm) (~ (days) A -2-3 1.94 0 0 1 0.84 0 0 194 95 2 A -15-30 1.94 0 0 1 0.84 5.5 0 194 95 12 X -100 4.76 0 0 1 3.5 6 0 454 100 12 Silicalite -300x50x50 0 0 60 0 90 0 8 750 180 16 L -1-9 0 2.35 0 1 10 0 0 160 175 5

Figure 1. (a) AFM, (b) FE-SEM image of growth terraces on the { 100} faces of-20-25 gm zeolite A crystals. Crystal shown in (b) was coated with a thin Cr film.

120

Figure 2. (a) FE-SEM, (b) AFM image of a series of vertices of square shaped terraces on the { 100} zeolite A faces with (c) showing height of individual terraces measured along the bold line in (b) by AFM. Crystal shown in (a) was coated with a thin Cr film. Figure 3 illustrates the FE-SEM images of small-1-3 gm pyramidal structures frequently observed on the vicinal faces of large -15-30 gm zeolite A crystals. Such pyramids were shown before by AFM to be formed by layers with thickness -0.5, -0.7, and-1.2 nm [4, 5]. As shown in Figure 3, FE-SEM could image the terraces generating the small pyramids on both the uncoated and Cr-coated surfaces without difficulty. Each imaged small pyramid had a growth protrusion or crystallographic fault at its apex. The "growth protrusions" and the larger bow of parabolic cross-sectional profiles of small pyramids compared to these of the vicinal faces [5] indicate a different growth mechanism of small pyramidal structures compared to vicinal faces. It is hypothesized that near the end of crystallization crystal growth can proceed by a mechanism where locally ordered aluminosilicate species act as faulted two-dimensional embryos becoming in time critical twin nuclei. Such nuclei would generate layers by a twin-plane reentrant corner mechanism [10], which lowers the energy of formation of surface nuclei, thus resulting in the accelerated growth normal to the { 100} planes. The relative ease of imaging the terraces on the uncoated small pyramids is likely due to the relatively high bow of their surfaces because the higher inclination of the local surface to the incident beam results in higher topographic contrast [7]. Figure 4a shows the triangular habit of terraces imaged by FE-SEM on the { 111 } surfaces of small -1-2 gm commercial zeolite X crystals. These triangles appear to be rotated by 60 ~ from the crystal edges. This is in agreement with the previous AFM observations of the { 111 } surfaces of zeolite Y crystals [1] possessing the same cubic faujasite (FAU) structure. The mechanism of terrace growth in FAU crystals, which explains the rotated character of the triangular terraces, has been postulated before [1]. Figures 4b-d show a

121 comparison of the FE-SEM and AFM images of a series of terrace edges on the { 111 } surfaces of large -100 gm zeolite X crystals. The individual layers on zeolite X with thickness-1.5 nm measured by AFM could be imaged by FE-SEM using Cr-coated crystals.

Figure 4. (a) FE-SEM image of a (111) face o f - 1 - 2 gm commercial zeolite X crystal, (b) FE-SEM, (c) AFM image of a series of terrace edges on the { 111 } faces of-100 gm zeolite X crystals, with (d) showing height of individual terraces measured along the bold line in (c) by AFM. Crystals shown in (a) and (b) were coated with a thin Cr film.

122 Figure 5a shows the FE-SEM image recorded on the (010) surface of a large Silicalite crystal. The image reveals a series of larger terraces composed of sub-terraces. Comparable AFM images of a (010) Silicalite surface showed that sub-terraces have height -1-2 nm (Figures 5c and 5d), what is consistent with the findings of previous AFM study [3]. Larger terraces could be imaged by FE-SEM without coating the surface with Cr film, but individual sub-terraces were better imaged using Cr-coated crystals. It is hypothesized that Cr coating improved contrast by enhancing the secondary electron yield [7]. Terraces consisting of subterraces could be also imaged by FE-SEM on the { 100} Silicalite surfaces (Figure 5b), however, these surfaces were considerably smoother than the {010} surfaces. Identical results have been observed by Anderson et al. [3], who explained the presence of larger terraces by growth defects interrupting layer growth of Silicalite and piling up many individual layers with height equivalent to the breadth of one double 5-ring MFI chain.

Figure 5. FE-SEM images of a series of larger terraces consisting of sub-terraces on (a) {010}, (b) { 100} Silicalite faces coated with a thin Cr film. Comparable AFM image of terraces with sub-terraces on a (010) Silicalite face is shown in (c), with (d) illustrating height of terraces and sub-terraces measured along the bold line in (c) by AFM.

123 Figure 6 shows typical growth terraces on the { 100} and { 110} faces of small -2-3 gm zeolite A crystals imaged by FE-SEM (note the similarity of the appearance of the { 100} faces in Figure 6a to the image of a (100) face shown by Breck [6]). Terraces on the { 110} faces result in rectangular pyramidal structure of these surfaces. The rectangular shape of terraces on the { 110} faces is consistent with the two-fold symmetry of these faces. No surface topography has been observed by AFM on the { 110} faces of larger-10 ~tm zeolite A crystals [2]. FE-SEM analysis of the poorly developed { 110} faces of large -15-30 gm crystals investigated in the present study did not reveal any topography either. The topography of a surface is determined by the relative rates of the two processes: the supply of layers and the motion of layers [10]. The higher the surface nucleation rate relative to the surface lateral spreading rate, the more bowed the appearance of the crystal faces should be [2], i.e., the more growth terraces should be seen on the crystal face. This suggests (assuming that growth mechanism of the { 110} and { 100} faces is similar) that the growth habit seen on the { 110 } surfaces of small zeolite A crystals imaged in the present study is caused by a high ratio of the rate of surface nucleation to the rate of lateral propagation of layers occurring on these surfaces near the end of crystallization. It is worth noting that crystals shown in Figure 6 have well developed { 111 } faces. As illustrated in Figure 6b, no terrace topography could be discerned on these surfaces.

Figure 7. FE-SEM images of the basal surfaces of cylindrical zeolite L crystals coated with a thin Cr film.

124 Images shown in Figure 7 illustrate the appearance of circular shaped layers on the basal surfaces of cylindrical zeolite L crystals. Although the FE-SEM image in Figure 7a could suggest the spiral step growth on these surfaces as implied by some authors [ 11], the image in Figure 7b clearly indicates the absence of growth spirals on the basal surfaces of crystals studied here. Instead, layers forming straight steps are clearly seen in Figure 7b. This is consistent with the results of HREM study of the surface structure of zeolite L, which revealed surface steps with minimum height of 0.75 nm on the basal planes [ 12]. 4. CONCLUSIONS Layers on the {100} faces of zeolite A, {111} faces of zeolite X, and {100} and {010} faces of Silicalite with the AFM measured thickness of ~1.2, ~1.5, ~1-2, and ~1-2 nm, respectively were successfully imaged by FE-SEM using crystals coated with thin Cr films. FE-SEM could also distinguish layers on the uncoated crystals, but the images exhibited poor topographic contrast. Terraces on the {110} zeolite A surfaces have been imaged for the first time using FE-SEM. Their presence has been explained by the high ratio of the rate of surface nucleation to the rate of lateral propagation of layers occurring on these surfaces near the end of crystallization. The FE-SEM images of layers on the basal planes of cylindrical zeolite L showed no evidence of the previously suggested spiral growth on these surfaces. These results demonstrate the potential of FE-SEM used in concert with AFM to study zeolite crystallization.

REFERENCES

1. M.W. Anderson, J.R. Agger, J.T. Thornton and N. Forsyth, Angew. Chem. Int. Ed. Engl., 35 (1996) 1210. 2. J.R. Agger, N. Pervaiz, A.K. Cheetham and M.W. Anderson, J. Am. Chem. Soc., 120 (1998) 10754. 3. M.W. Anderson, J.R. Agger, N. Pervaiz, S.J. Weigel, A.K. Cheetham, Proc. 12th IZC, Materials Research Society (1999), p. 1487. 4. S. Sugiyama, S. Yamamoto, O. Matsuoka, H. Nozoye, J.Yu, G. Zhu, S. Qiu, O. Terasaki, Micropor. Mesopor. Mater., 28 (1999) 1. 5. S. Dumrul, S. Bazzana, J. Warzywoda, R.R. Biederman, A. Sacco, Jr., submitted to Microporous Mesoporous Mater. 6. D.W. Breck, Journal of Chemical Education, 41 (1964) 678. 7. J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, A.D. Romig, Jr., C.E. Lyman, C. Fiori and E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, 2 nd ed., Plenum Press, New York, 1992. 8. Customer test report, Scanning electron microscope S-4700-I, Hitachi, Ltd. 9. J.W. Mullin, Crystallization, 3rd ed., Butterworth-Heineman, Oxford, 1993. 10. W.A. Tiller, The Science of Crystallization: Microscopic Interfacial Phenomena, Cambridge University Press, Cambridge, 1991. 11. J.P. Verduijn, U.S. Patent No. 5 486 498 (1996). 12. T. Ohsuna, Y. Horikawa, K. Hiraga and O. Terasaki, Chem. Mater., 10 (1998) 688.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

125

XANES and XPS studies of titanium aluminophosphate molecular sieves M. Hassan Zahedi-Niaki ~, Francois Beland b'~, Laurent Bonneviot b'~ and Serge Kaliaguine a *

a Department of Chemical Engineering, Laval University, Quebec, PQ, Canada * b Department of Chemistry, Laval University, Quebec, PQ, Canada, G1K 7P4 The environment of titanium in titanium aluminophosphate molecular sieves, TAPO-5, -11, -31 and 36 was studied using X-ray absorption (XANES) and X-ray photoelectron spectroscopy. The XPS results revealed that three different titanium species with Ti in isolated tetrahedral, isolated pentahedral and polymeric TiO~ in octahedral environments existed on the surface of all TAPO samples. The XANES results showed that the titanium species in TAPO36 and TAPO-5 possess different environments compared to that in TAPO-11 and TAPO-31. INTRODUCTION TAPO molecular sieves were reported [1] to be active in liquid-phase oxidation and epoxidation of different substrates using dilute hydrogen peroxide as the oxidant. In titanium silicalites (TS-1 and TS-2) by using various characterization techniques, it was found that the Ti-species active in liquid-phase oxidation were the isolated framework tetrahedral titanium whereas extra-framework Ti did not contribute to the catalytic properties of these materials [2]. On the other hand the isomorphous substitution of titanium in several aluminophosphate lattices was recently evidenced using characterization techniques such as ESR and ESEM [3 ], FTIR and proton NMR spectroscopies [4]. Moreover similar observations have also been reported by others for TAPO-5 a n d - 1 1 [5,6], TAPSO-37 [7]. Despite the wide panel of techniques applied up to now, there is not yet an overall picture of the Ti distribution within the framework of these systems. X-ray absorption spectroscopy is a technique which proved to be very useful in the study of the environment of Ti ions in titanium silicalites [8-11 ]. It seems that very little work was performed [6] in trying to apply this spectroscopy to TAPOs. This paper is the continuation of our contribution to the investigation of TAPO materials, which presents X-ray absorption and X-ray photoelectron investigations of TAPO-5, -11, -31 and -36. 1- EXPERIMENTAL The details of the hydrothermal synthesis were reported elsewhere [12,13]. Table 1 shows the composition of all TAPO samples. The surface chemical properties of TAPO particles were studied using a V.G. Scientific Escalab Mark II system with a hemispherical analyzer operated in the constant pass energy mode (20 eV). An Mg I ~ X-ray source (hv =1253.6 eV) was operated at 20 mA and 15 kV. The PIXAS program [14] that was developed in the surface science laboratory of Laval University was used for the Ti2p peak treatment. Prior to each spectrum simulation, the background and oxygen satellite peaks were subtracted from the experimental spectra. The * Correspondingauthor,[email protected],[email protected] wCurrentaddress:ImtitutdeRecherchesur la Catalyst,2 Av.AlbertEinstein69626Villcurbannr Cedex,France s Currentaddress:SiliCyclr 1200,St-Jean-Ba~str Suite114,Quebec(Queboc),G2E 5E8,Canada

126

Table 1 The chemical composition and micropore volume of TAPO samples. Sample Chemical Composition a B.E.T Surface Micropore bulk surface Area Volume b Ti A1 P Ti2p Al2p P2p (m2 ~-1) . . . . (ml g-l),,, TAPO-5 0 . 0 2 0.49 0 . 4 8 0.06 0.54 0.40 305 0.111 TAPO-11[A] 0 . 0 1 0.46 0.53 0.01 0.52 0.47 166 0.022 TAPO-11[B] 0.02 0.46 0.52 0.03 0.57 0.40 350 0.074 TAPO-11[S] 0 . 0 1 0.50 0.49 0.003 0.49 0.50 225 0.08 TAPO-31 0.02 0.50 0 . 4 8 0.02 0.52 0.46 225 0.102 TAPO-36 0.04 0 . 4 8 0.48 0.04 0.58 0.38 373 0.147 a Fractionof T atoms, b Fromn-butane adsorption binding energy was corrected by referring to the Cls = 284.6 eV. The spectra were then deconvoluted using mixed Gauss-Lorentz functions and the least square minimization method. The X-ray absorption spectra were recorded in transmission mode at station IV at LURE (Orsay-France). The detail of XANES experiment was reported elsewhere [15] 2- RESULTS A complete characterization of TAPO samples was given in references [3,4,12,13]. These characterization results are summarized in the following paragraph. The XRD spectroscopy showed that each structure-type was well crystallized and the spectra were in accord with their corresponding patterns reported in the literature [12,13]. The UV-Visible spectra of all TAPOs showed a single peak with maximum at 230 nm assigned to tetrahedral Ti and a small shoulder at 280 nm due to some highly dispersed extraframework species [1 ]. The FTIR spectra ofpyridine adsorbed TAPO-5 and TAPO-36 show Bronsted acid sites not present in the corresponding AIPOs [4]. The presence of acidic sites in these TAPOs was also confirmed by proton MAS NMR spectroscopy. These acidic sites are associated with isomorphously substituted Ti 4+ in framework at the position of pS+ ions. Evidence for titanium substitution for phosphorous was obtained by using ESR and ESEM spectroscopies [3]. The ESR spectra of Ti 3§ in all ~t - irradiated TAPO samples except TAPO-11 [13] show a very well resolved ESR spectrum. By contrast the TAPO-11[B] ESR spectrum showed a broad background probably due to extra-framework Ti species. Figure 1 shows the Ti2p XPS spectra of the TAPO-5. Three different species with low (Ti2p3/2= 457.8-458.0 eV), medium (458.8 - 459.0 eV) and high (460.0-460.2 eV) binding energies were detected on the surface of all TAPO samples. The peak positions and their relative intensities are summarized in Table 2. Table 2 The details ofXPS deconvoluted spectra of TAPOs and reference materials. Sample Ti2p3a Binding Energya(eV) Fractional Area (%) Peak1 P e a k 2 Peak3 PeakI P e a k 2 Peak3 TAPO-5 457.8 458.8 460.2 23.2 63.2 13.6 TAPO- 11 [A] 457.8 458.8 460.1 32.2 61.3 6.5 TAPO-11[B] 457.8 458.9 460.2 33.5 63.4 3.1 TAPO- 11 [S] 457.9 458.8 460.2 28.1 51.1 20.8 TAPO-31 457.9 458.9 460.1 20.5 74.6 4.9 TAPO-36 457.6 458.6 460.0 37.0 43.5 19.5 TS-1 . . . . . . 460.0 . . . . . . 100 TiO2(anatase) 457.8 . . . . . . 100 . . . . . . a BE referencedto Cls =284.6 eV; FWHM=I.8 _+0.2 eV, Ti2p5/2/Ti2p3/2= 0.475+ 0.025,Ti2ps/2-Ti2p3/2=5.50+ 0.15. ,

ii

,,,

i,

127

The XANES spectrum of dehydrated TAPO-36 samples together with spectra of TS-1 and anatase samples are presented in Figure 2. The pre-edge spectra of TAPOs are intermediate between those of TS-1 and anatase. TAPO-11 [B] and TAPO-31 spectra are those which exhibit the most similarities with anatase(Figure 3a). The same trend is observed at the top of the edges (Figure. 3b). The most water sensitive TAPOs are TAPO-36 and TAPO-5 according to the pre-edge region evolution upon hydration dehydration treatments (Figure 4).

O 0

< U

.N

i 0

4960 Binding Energy(eV)

5000 Energy (eV)

5040

Figure 2 -Ti K-edge XANES profiles of dehydrated 3% TS-I (top); dehydrated TAPO-36 (middle) and anatase (bottom)

Figure l-The XPS spectra of T APO-5

O .,=

C3. t= o

< N N ..9 r

i

E t,.s O

i Z

Z ,.,,I,,..l,,,l|t_

4960

4970 Energy (eV)

4980

4980 4990 5000 5010

Energy (eV)

Figure 3 - a) XANES prepeaks and b) top of the edge XANES profiles of dehydrated (from top to bottom) TAPO-36, TAPO-5, TAPO-11[S], TAPO-11[B], TAPO-31 and anatase.

4960

4965

4970

4975

Energy (eV)

Figure 4 - Ti K-edge XANES profiles of dehydrated (dotted line) and hydrated (full line) of TAPO-36 (top) and TAPO-5 (bottom)

128 2- DISCUSION 2-1 DRS-UV-Visible

The extraframework titanium oxides such as anatase or ruffle which are frequently formed during the synthesis of titanium-containing molecular sieves can easily be distinguished using the DRS-UV-Visible spectra as a finger print. For example fully developed anatase phase normally appears as a peak at 330 nm in the DRS-UV-Visible spectra. The smaller clusters of TiO2 appearing as dispersed nano-phases are associated with absorption between 280 nm to 330 nm. The isolated Ti4+ cation in tetrahedral environment of siliceous materials possesses a characteristic peak between 200-220 nm. Upon coordination with other ligands such as H20 and NH3 the framework Ti 4+ cation increases its coordination to 5- or 6fold and then shows absorption features between 220-280 nm. The cluster size and the dispersion of the extraframework titanium oxides materials depend on the hydrolysis rate of the titanium-source (normally a titanium alkoxide). A rapid hydrolysis of Ti-alkoxide would cause an increase of TiO2 polymerization and give larger oxide clusters, whereas a very slow and controlled hydrolysis of Ti-alkoxide yields isolated TiO~ species [16,17]. On the other hand the rate of Ti-alkoxide hydrolysis can be controlled by the pH and temperature of the gel. In hydrothermal syntheses of Ti-containing silicalites generally the high pH causes the formation of polymeric TiO2 clusters which at high titanium loading appear as TiO2 crystallites of anatase or rutile. In the synthesis of Ti-containing A1POs, the gel contains a strong acid (H3PO4) and the initial gel pH is around 1-2. At this pH titanium oxide is dissolved in the gel solution. This is specially significant because at high Ti loading (over 2.0 wt %) still no anatase-like TiO2 species can be detected [12,26] whereas in similar Ti loading of the Ti-silicalites synthesis such as TS-1 the formation ofanatase is very probable. The synthesis ofTi-containing mesoporous molecular sieves (MCM-41) can be carried out both in acidic and alkaline media. Recently the DRS l.N-Visible spectra of the Ti-MCM-41 prepared by the acidic route was reported by Zhang et al [18] and Corma et al [27] and indeed resembles that of the TAPOs [1,14] and TAPSO-37 [7] with an absorption band peak at 220 nm and shoulder at 260 nm. Using other characterization results, they have suggested that titanium more likely occupied framework site. The DRS-UV-Visible spectra of TAPO-5, -11, 31, 36 [1,12] show similar absorption band but with a maximum at 230 nm and shoulder at about 270-280 nm, the shoulder is more pronounced for TAPO-31. The presence of framework Ti in TAPO-5, 11, 31, and TAPO-36 as mentioned earlier was very well established by combined ESR and ESEM techniques [3] as well as more recently by FTIR-pyridine adsorption and 1H MAS NMR characterization techniques [4]. The small shift of about 10 nm of the major peak and shoulder in TAPOs compared to that observed in DRS UV-Visible spectra of Ti-MCM-41 will be discussed below. Gao et al [19], based on their DRS-UV-NIR results of some Ti-containing reference compounds with known Ti-coordination, have shown that the ligands can play a major role in determining the band maximum of ligand-metal charge transfer (LMCT) transitions of Ti cations. Ba2TiO4 possesses isolated TiO4 tetrahedra linked by Ba atoms [22]. JDF-L1 (Na4Ti2 Si 022 41-I20) and Ti -umbite (K2Ti 8i309 n20) contain isolated TiO5 square pyramids and isolated TiO6 octahedra with Ti-O-Si linkages, respectively [23,24]. According to DRS-UVNIR results of Gao et al [19], the LMCT transitions of Ti 4+ (energy) decrease in the order of JDF-1 (with penta-coordinated Ti4+) > Ti-umbite > Ba2TiO4. This is in contrast to the expectation that Ba2 TiO4 with 4-fold coordination would display the highest LMCT transitions (or absorption at lower wavelength). They suggested that the actual lower LMCT transitions of the Ti atoms in Ba2TiO4 can be accounted for by Ba atoms as the second coordination sphere

129 that are much less electronegative than Si atoms as the second coordination sphere in JDF-L1 and Ti -umbite. In titanium aluminophosphate molecular sieves (TAPOs) the presence of A1 in the second coordination sphere (O-A1 ligand), can explain the shift of the DRS-UV-Visible band maximum from 220 nm in Ti-MCM-41 to 230 nm in TAPOs, since the AI atom is less electronegative than Si. Obviously from the above discussion it appears that the DRS results alone are insufficient to determine the coordination of Ti atoms in TAPOs because of the mixed effects of both the coordination and the ligands on to the LMCT transition. The combined XPS and XANES results provide more structural information about the environment of Ti in TAPO materials.

2-2 X-ray photoelectron spectroscopy The deconvoluted XPS spectra of all TAPOs show the presence of three different Ti species with their Ti2p3/2binding energies separated by about 1 eV (Table 2). The first species with the lowest binding energy arising at 457.8 _+ 1 eV is characteristic of octahedral titanium species [20]. TAPO-36 sample with 4 atomic % bulk titanium possesses the higher surface concentration of octahedral species (peak 1). This may be explained by a higher Ti content and a higher hydrophilic character (more surface hydroxyl) of the TAPO-36 compared to the other TAPOs. Among the three TAPO-11 samples, the TAPO-11[S] with the lowest Ti content shows the lowest surface concentration of octahedral species. TAPO-5 and TAPO-31 samples with the same bulk Ti content (2 atomic %) contain almost similar amounts of surface octahedral species. It may be significant to note that the order found here for the relative intensity of peak 1, namely TAPO-31 ~ TAPO-5 < TAPO-11 [B] < TAPO-36 is essentially opposite to the order of catalytic activity in five different oxidation and epoxidation reactions [1, 26]; TAPO-31 ~ TAPO-5 > TAPO-11 [B] > TAPO-36. The above orders suggest that the other two species besides the one associated to peak 1 (extraframework octahedral Ti) may both be active in liquid phase catalytic oxidation of cyclohexane, 2-hexanol and epoxidation of 1-octene [1, 26]. In agreement with previous studies, the third peak at 460.1 +_ 1 eV belongs to isolated Ti4§ in tetrahedral position [20,28]. It is tempting in that context to assign the second Ti2p peak to Ti species in some intermediate coordination environment. Peaks similar to peak 2 and 3 were also reported for TS-1 [20a] and TiOx thin films deposited on SiO2 glasses [25] respectively. Trong On et al [20a] reported that the Ti2p3/2XPS spectrum ofa TS-1 sample with the small amount of extraframework TiO2 species (Ti in octahedral coordination) showed a small shoulder at about 457.0 eV. Upon ball-milling the cluster size of this species decreased and its Tizp XPS peak position shifted to higher binding energies with an increase in peak intensity. They argued that these changes can be explained by the appearance of lower coordinated Ti species on the surface of milled TS-1 now in the form of amorphous TiO2-SiO2 mixed phases. In TAPOs synthesis (as earlier discussed) due to low initial pH of the gels, the extraframework TiOx species can be formed in smaller clusters on TAPOs surface. The lowest binding energy doublet in the Ti2pXPS spectra of TAPO samples (see Figure 1 and Table 2) is likely associated with this octahedral TiOx species. Gao et al [19] by using DRS-UV-NIR, Raman, XPS and XANES spectroscopies also suggested that two dimensional polymerized TiO5 units, possessing at least one Ti-O-Si bond and three or more Ti-O-Ti bonds per Ti atom, exist on the surface of TiO2/SiO2 supported oxide samples with more than 5 wt % Ti. Their results include a Ti2p3;2 XPS spectrum showing a maximum at about 459.0 eV and the DRS-UV-NIR spectrum with a shoulder at 260 nm. Similar species with Ti-O-AI bonds may

130 also be formed on TAPOs surfaces giving rise to the intermediate binding energy doublet in Ti2p XPS spectra.

2-3 X-ray Absorption spectroscopy To further document the coordination state of titanium in TAPOs the same series of samples was investigated using X-ray absorption techniques at the Ti K edge. The XANES profiles of the hydrated or dehydrated TAPOs were found intermediate between that of anatase taken as a reference of bulky aggregated TiO6 ootahedra, and TS-1 containing 1.5 % Ti taken as a reference for isolated tetrahedral species [Figure 2]. Such intermediate profiles were also observed for various titanium silicates (TS-1 [28], TS-2 [11] and Ti-Silicalite-48 [29]) or for titanium boralite [15]. In the later case, it was possible to simulate the entire edge profiles using a linear combination of the profiles of the two above mentioned species [15]. Such a calculation provides very poor fitting of the TAPOs profiles particularly in the pre-edge region (Figures 3a). There are at least two possible reasons that may explain this difficulty. First there may be more than two species in TAPOs or assuming a mixture of only two Ti environments, there may be no available references to rely on for these two species. For instance, it is impossible to reproduce the strong pre-edge peak at 3.5 eV found for TAPO-36 (Table 3). In fact such a pre-edge position was observed to arise from penta coordinated Ti in phtalocyanine [31] or from distorted octahedral Ti in the silicate kaersutite [32]. Sometimes more than one pre-edge peak is observed as for TAPO-31 and TAPO-11[B]. For TAPO-31 three of these peaksare in almost perfect match with those of anatase at 2.3, 5.0 and 7.8 eV. However for the TAPO-11 the satellite peaks at 2.1 and 7.2 eV do not match that of anatase nor that ofrutile. Theoretical calculations demonstrated that the position of these peaks indeed depends on the size of the oxide aggregates [32,33]. Accordingly the shit~ of the satellite peaks observed in TAPO-11[B] would be consistent with aggregates of oxide smaller than twice the Ti-Ti distance namely smaller than -~ 7 A, that would fit inside the cavity of the structure. By contrast in TAPO-31, the peak positions suggest the formation of a bulky anatase phase most likely expelled from the lattice. For the present TAPOs series, the decrease in intensity of the preedge peaks and the change in the satellite peak position (Figure 3a and Table 3) is consistent with a lower concentration of bulk anatase and a larger amount of small TiO6 clusters. The top of the edges (Figure 3b) with smoother features along the same series of TAPOs also ordered as TAPO-31>TAPO-11[B]>TAPO-11[S]>TAPO-5_=_TAPO-36 is consistent with a high concentration of small clusters and eventually isolated octahedral species for the TAPO-36. The rational for the weak sensitivity to dehydration for the TAPO-31 and TAPO-11 is the large concentration of bulk anatase or large oxide aggregate for these TAPOs. Therefore it is not surprising to find at the other end ofthe series the most water sensitive TAPOs (TAPO-5 and -36) (Figure 4). TAPO-5 exhibits a larger pre-edge evolution upon dehydration: a shift toward lower energy, from 4.8 to 3.8 eV and the appearance of a shoulder at about 3.0 eV. The appearance of this shoulder is consistent with the formation of tetrahedral sites equivalent to those of TS-1 (Table 3). In comparison TAPO-36 exhibits a smaller shift of ca. 0.2 eV and increase in intensity of about 20%. The peak at 3.8 eV or 3.5 eV in both dehydrated TAPO-5 and -36 is likely related to an intermediate state of coordination of Ti which is created by the removal of water molecules. This could account either for highly distorted octahedra generated by a structure rearrangement of the cluster or for a decrease in coordination number from 6 down to 5. Whatever the exact coordination of Ti, in this intermediate state, the difference between TAPO-5 and TAPO-36 is attributed to a deeper effect of the dehydration treatment on the former which was indeed found less hydrophilic than the latter.

131

Table 3 The pre-edge characteristic of the dehydrated TAPOs. Pre-edge positiona (eV) TAPO-5 TAPO-11[A] TAPO-11[B] TAPO-11[S] TAPO-31 TAPO-36 TS-1 Anatase Ruffle Ba2TiOb a The zero energyis taken at 4964.2 eV., ,

3.8 / 3.0 3.8 2.1/4.2/7.2 2.1/4.1 2.3/4.1/5.0/7.8 3.5 3.0 2.3/4.7/7.8 2.1/4.8/7.8 2.5 b Fromreference[31]

i|l

Intensity 0.18 0.16 0.08/0.17/0.10 0.08/0.17 0.10/0.15/0.15/0.14 0.24 0.83 0.13/0.22/0.21 0.05/0.20/0.18 0.84 ii

ll|

At this stage of the discussion, it is important m relate the information provided by both XANES and XPS techniques being aware that XANES is a bulk technique while the depth analysis of XPS is 10-20 A. According to the interpretation of the XPS spectra made above, TAPO-31 and TAPO-36 possess respectively the least (20.5%) and the highest (37%) surface concentration of anatase-like species compared to the other TAPO samples. This is contrary m the observation made from the Ti K-edges. If the first interpretation is correct such discrepancy should be attributed to a very different distribution of Ti sites in the bulk and at the surface of the TAPOs crystallites. Table 1 gives both the bulk composition obtained by atomic absorption and the surface composition determined by XPS. The comparison between these two sets of data allows some discussion of the spatial distribution of Ti in all samples. Samples TAPO-5 and TAPO-11 [B] show a significant surface segregation of Ti, whereas samples TAPO-11 [A], TAPO-31 and TAPO-36 have surface Ti contents apparently similar m the bulk ones and TAPO-11 IS] seems to be surface depleted. It is also notic~ that samples TAPO-36 and TAPO-11[B] are significantly surface enriched with At. interestingly these two samples are also the least active in catalytic liquid phase oxidation of cyclohexane and 1-octene [1,26]. This observation suggests that the lower activity could be linked m the partial pore mouth plugging by an extraframework A1 rich phase. The highest surface area fraction of peak 1 would then be associated with a higher content of octahedml Ti co-germinated with this phase. Obviously however this Ti phase would also be surface segregated since both XANES and DRS-UVVisible indicate that the bulk content ofoctahedral Ti is low specially in TAPO-36. TAPO-5 and TAPO-31 have the highest two catalytic activities and they also show very different spatial distributions of Ti. In TAPO-5, the surface / bulk Ti atomic ratio is 3 whereas it is equal to 1 in TAPO-31. Thus the distribution ofsurface species giving rise to XPS peaks 1, 2 and 3 is likely also different from the bulk. The same is likely to be true in TAPO-31 (in spite of a surface / bulk ratio of 1) because the high catalytic activity of TAPO-31 suggests a bulk content of tetra-coordinated species (which at the surface yield peak 3) at least equal m the one of TAPO-5. The high surface concentration observed systematically for the intermediate coordination Ti species which gives rise to XPS peak 2 indicates that this Ti phase is surface segregated in all samples. It is dominating in the bulk of TAPO-5 but decreases progressively until essentially absent in the bulk of TAPO-31 as shown by the XANES results in Figure 3a. The tetrahedml framework Ti highlighted by the previous ESR and ESEM study [3] is likely to be in rather low concentration in the bulk of all these samples. It is however not clear

132 if this species alone yields active catalytic oxidation centers or if the intermediate coordination species also contributes to this activity. 3- C O N C L U S I O N In the present study, titanium aluminophosphate molecular sieves TAPO-5, -11, 31 and 36 have been characterized using X-ray photoelectron and X-ray absorption spectroscopies. Both techniques revealed that more than two titanium species are present in these materials. Isolated tetracoordinated and bulky extraframework titanium were found by XPS and XANES in hydrated TAPOs. Another species with intermediate coordination seems also to exist in TAPOs. This intermediate Ti species seems to be in higher content in TAPO-5 and TAPO-36 compared to the other TAPO samples (TAPO-11 and-31). XANES also showed that dehydration has some effect on TAPO-5 and TAPO-36, whereas almost no evident change was observed in TAPO-11 and TAPO-31 samples, which contain more hexacoordinated Ti. The relation between the reactivity of different TAPOs and the relative concentration of each Ti species is not straightforward and seems to be affected by their inhomogeneous surface/bulk distribution of Ti and AI atoms and likely associated to a pore plugging effect for the low reactive TAPO-11 and TAPO-36 samples. ACKNOWLEDGMENT

This work was financially supported by the National Science and Engineering Research Council of Canada. MHZN thanks the Ministry of Culture and Higher Education oflRAN for a scholarship. Dr. ~ Adnot (Laval University) is gratefully acknowledged for the XPS measurements. REFRENCES 1 Zahedi-Niaki, M. H.; Kapoor, M. P.; Kaliaguine S. Jr. Catal. 1998, 177, 231. 2 B. Notari, Adv. Catal. 1996, 41,253. 3 Prakash, A. M.; Kevan, L." Zahedi-Niaki, M. H.; Kaliaguine, S. Jr. Phys. Chem. B, 1998, 105, 831. 4 Zahedi-Niaki, M. H.;Zaidi, S. M. J.; Kaliaguine, S. Micropor. Mesopor. Mater. 1999, 32, 251. 5 Ulagappan N.; Krishnasamy, V. Jr. Chem. Soc., Chem. Commun., 1995, 3 73. 6 Akolekar, D. B.; Ryoo 1L Jr. Chem. Soc., Faraday Tran., 1996, 92, 4617. 7 Jappar, N.; Tanaka, Y.; Nakata, S.; Tatsumi, T. Micro. Meso. Mater., 1998, 23, 169. 8 Thomas, J.M., and Sankar, G.,Acc. Chem. Res., 2001, 34, 571. 9 Mountjoy, G., Pickup, D.M., Wallidge, G.W., Cole, J.M., Newport, 1LJ., and Smith, M.E., Chem. Phys. Lett., 1999, 304, 150. 10 Ricchiardi, G.; Damin,/k;Bordiga, S.; Lamberti, C.; Spano, G.; Rivetti, F; Zecchina, A., d. Am. Chem. Soc. 2001, 123, 11409. 11 Trong On, D.; Bonneviot, L.; Bittar,/k; Sayari, A.; Kaliaguine, S. Jr. Mol. Catal. 1992, 74, 233. 12 Zahedi-Niaki, M. H.; Joshi, P. N.; Kaliaguine, S. Proc. on the 11 th Inter Zeolites Conf., (Chon, H., Ihm, S.-K., Uh, Y. S., Eds.) Stud. Surf. Sci. Catal.,Vol 101, p1013, 1996 Elsevier, Amsterdam. 13 Zahedi-Niaki, M. H.; Joshi P. N.; Kaliaguine, S. Jr. Chem. Soc., Chem. Commun. 1996, 47. 14 Parry, L.; Adnot, A. PIXAS Data Processing Software for XPS-AUGER-SIMS, Surface Analysis Laboratory, Dept. of Chem. Eng., Laval University, 1994, Quebec, PQ, Canada.

133 15 16 17 18

Trong On, D.; Kaliaguine, S.; Bonneviot, L. J. Catal. 1995,157, 235. Srinivasan, S.; Datye, A. K.; Smith, M. H.; Peden, C. H. F. J. CataL, 1994, 145, 565. Castillo, R.; Koch, B.; Ruiz, P.; Delmon, B. ,I. Catal. 1996, 161, 524. Zhang, W.; Fr6ba, M.; Wang, J.; Tanev, P.; Wong, J.; Pinnavaia, T. J. Jr. Am. Chem. Soc., 1996, 9164. 19 Gao, X." Bare, S. R.; Fierro, J. L. G." Banares, M. A.; Wachs, I. E. Jr. Phys. Chem. B. 1998, 102, 5653. 20 a) Trong On, D.; Kapoor, M. P.; Thibault, E.; Gallot, J. E.; Lemay, G.; Kaliaguine, S. Micropor. Mesopor. Mater 1998, 20, 107. b) Kaliaguine, S. Recent Advances and New Horizons in Zeolite Sciences and Technology, (Chon, H., Ihm, S.-K., Uh, Y. S., Eds.) Stud. Surf. Sci. Catal., Elsevier, Amsterdam; 1996, Vol. 102 p. 221 and references therein. 21 A. Yu. Stakheev, E. S. Shpiro, and J. Apijok, J. phys. Chem., 1993, 97, 5668. 22 Wu, K. K.; Brown, I. D. Acta Crystallogr. Sect. B. 1973, 29, 2009. 23 Roberts, M. A.; Sankar, G.; Thomas, J. M.; Jones, R. H.; Du, H.; Chela, J.; Pang, W.; Xu, R. Nature, 1996, 381, 401. 24 Lin, Z.; Rocha, J.; Brandao, P.; Ferreira, A.; Esculcas, A. P.; Pedrosa de Jesus, J. D. jr. Phys. Chem. B., 1997, 101, 7120. 25 Xu, W.-X.; Zhu, S.; Fu, X.-C. Appl. Surf. ScL, 1998, 136, 194. 26 Zahedi-Niaki, M. H.; Kapoor, M. P.; Kaliaguine, S. Proc. of 12 th Inter. Zeolite Conf., Treaty, M. M. J.; Marcus, B. K.; Bisher, M. E.; Higgins J. B. (editors), Vol. II, p. 817-824, Materials Research Society (MRS), Warrendale, USA, 1999. pp. 1221-1226. 27 Corma, A.; Jorda, J. L.; Navarro, M. T.; P6rez-Pariente, J.; Rey, F.; Proc. of 12th Inter. Zeolite Conf., Treaty, M. M. J.; Marcus, B. K.; Bisher, M. E.; Higgins J. B. (editors), Materials Research Society (MRS), Warrendale, USA, 1999, Vol. I~ pp. 817-824. 28 Le Noc, L.; Trong On, D.; Solomykina, S.; Echchahed, B.; B61and, F.; Cartier dit Moulin, C.; Bormeviot, L. Stud. Sur)f. Sci Catal., Elsevie, Amsterdam, 1996,101, 611. 29 Reddy, K. M. Kaliaguine, S.; Sayari, A.; Ramaswamy, A. V.; Reddy V. S.; Bonneviot, L. Catal. Lett. 1994, 23, 175. 30 Yarker, C. b_; Johnson, P. A. V.; Wright, A. C.; Wong, J.; Ge~gor, 1L B.; Lyric, F. W.; Sindair, R. N. J. Non-Cryst. Solids1996, 79, 117. 31 Behrens, P.; Felshe, J.; Vetter, F.; Schultz-Elkoff, G.; Jaeger, N. I.; Niemann, W. J. Chem. Soc., Chem. Commun. 1991,678. 32 Aifa, Y.; Poumella, B.; Jeanne-Rox, V.; Cortes, 1L; Veduinski, K V.; Kraizman, V. L. jr. Phys., 1997, IV, 7, C2-217. 33 Wu, Z. Y.; Ouvard G.; Natoli, C. K. J. Phys. 1997, 7, C2-199.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) . 9 2002 Elsevier Science B.V. All rights reserved.

135

An investigation of the intermediate gel phases of AIPO4-11 synthesis by solid-state NMR spectroscopy Yining Huang,* Roger Richer and Chris Kirby University of Western Ontario, Department of Chemistry, London, ON, Canada N6A 5B7 In this work, we report our recent solid-state NMR results on the investigations of the intermediate gel phases formed during the formation of crystalline A1PO4-11 molecular sieves. In additional to 31p and 27AI magic angle spinning (MAS) methods, we have used the 31p/27A1 double-resonance experiments such as cross polarization (CP) to obtain valuable information on the A1-O-P connectivity in the gel phases. 1. INTRODUCTION Aluminophosphate (A1PO) based molecular sieves are normally synthesized under hydrothermal conditions. Although much of the data on synthesis conditions have been accumulated in literature [1], little is known about the mechanisms of the crystallization. The crystallization processes usually involve the formation of the intermediate gel phases at the early stages of the reaction. The gel phases eventually transform into crystalline molecular sieves. However, the structure of these intermediate gels is usually poorly characterized due to their amorphous nature. 31p and 27AI MAS experiments have been employed to study the gel structure. Although these methods do yield valuable information on the local environments of P and AI atoms, simple MAS techniques do not provide AI-O-P connectivity information. Recently, we have demonstrated that the heteronuclear dipolar coupling based techniques such as one- and two-dimensional A1/P CP can be used to map out the AI-O-P connectivity in the gel phases of AIPO-based molecular sieve synthesis [2]. In the present work, we have characterized the intermediate gel phases formed during the synthesis of AIPO4-11 by combination of MAS and A1/P CP techniques. A1PO4-11 was first synthesized by Union Carbide [3]. It has AEL structure containing unidimensional channel systems with noncircular 10-ring pores [4]. 2. EXPERIMENTAL SECTION The aluminum and phosphorus sources were AI(OH)3 and 85% H3PO4, respectively. Din-propylamine was used as structural directing agent. The crystalline A1PO4-11 and corresponding gel phases were prepared with gel composition: 1.0A1203:1.0P203:1.0Pr2NH:40H20 according to Tapp et al. [5]. The crystallization was carried out at 200 ~ after initial gel was aged at 90 ~ for 24 hrs. The intermediate gel samples were obtained by quenching the reaction at various stages of the crystallization.

136 All the NMR experiments were performed on a Varian/Chemagnetics Infinity-Plus 400 MHz WB spectrometer equipped with three rf channels. A Varain/Chemagnetics 7.5-mm triple tuned T3 MAS probe was used to acquire the spectra. The spinning rates were between 5 and 7 kHz. The 90 ~ pulse lengths for Ill, 31p, and27A1 (central transition only) were 6, 12 and 17 ~ , respectively. The one- and two-dimensional A1/P CP experiments performed in the present work were described in Ref. [6]. The 27A1 --9 31p CP experimental optimization was carried out using crystalline AIPO4-11. The optimal contact time for 27A1 --9 31p CP was 1 ms and pulse delay was 200 ms. For 27A1 --->31p 2D CP, 6000 scans were acquired for each of the 64 tl slices. The 31p and 27A1 shifts were referenced to 85% H3PO4 and 1M AI(NO3)3 aqueous solution, respectively. For ~H --->31p CP experiments, the Hartman-Hahn condition was determined using (NI-h)H2PO4. The repetition time of 5 s was used and 160 scans were acquired for each spectrum. The powder XRD patterns of the gels were recorded on a Rigaku diffractometer using Co Ka radiation (a wavelength of 1.7902/~). 3. RESULTS and DISCUSSION

The powder XRD patterns of the intermediate gel phases are shown in Figure 1. The initial gel (without heating), the gel aged at 90 ~ for 24 hrs and the solid material produced by heating the aged gel at 200 ~ for 50 min. are all amorphous. The first evidence of A1PO4-11 crystals in the diffraction patterns was observed after heating the aged reaction mixture at 200 ~ for 60 min. Heating the aged gel at 200 ~ for more than 80 min. yields the pure crystalline AIPO4-11. We did not observe any metavariscite and variscite phases in the early stage of the reaction as reported by Tapp and co-workers [5]. The 31p and 27A1 MAS spectra (Figure 2) were obtained to probe the local chemical environments of P and AI atoms. The spectra of the gel heated at 200 *C for more than 80 minutes are identical to those of pure crystalline A1PO4-11, which is consistent with the powder XRD patterns. For the initial gel, the 27A1 MAS spectrum contains a very weak peak at around 45 ppm and a strong peak a t - 7 ppm with a prominent shoulder on the low-field side at about 4 ppm. Based on the shift value, the 45 ppm peak may be assigned to tetrahedral A1 sites in the aluminophosphate gel. As shown in Figure 2, the intensity of this peak increases with increasing the heating time. The position of this tetrahedral peak gradually shifted towards high-field side with increasing heating time, indicating that there is a slight change in the chemical environment. The assignments for the main peak at -7 ppm and its low-field shoulder at 4 ppm are ambiguous. The -7 ppm peak may be assigned to octahedral AI sites in the aluminophosphate gel. The shoulder at 4 ppm could be due to either the octahedral A1 in unreacted alumina or five-coordinated AI in an aluminophosphate gel. It is unclear if these two maximums at 4 and -7 ppm are due to two separate resonance signals or just one A1 site with asymmetric lineshape resulting from a large quadrupolar coupling constant. The 31p spectrum of the initial gel has a sharp peak at 3 ppm and a broad peak a t - 1 1 ppm. After aged at 90 ~ for 24 hrs, the sharp peak at 3 ppm disappeared and the spectrum only has a very broad resonance centered a t - 1 4 ppm. The assignment of the broad peak at around-14 ppm is also worth mentioning. In previous studies of A1PO synthesis, a broad peak in the range between-10 a n d - 2 0 ppm has also been observed in the gel samples

137

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-40

-60

139 obtained in the early stages of the crystallization [7]. This peak has been assumed to be the amorphous aluminophosphate species. However, there is no direct proof that this P peak is actually connected to the AI sites. Several other P species such as mono-, di- and polyphosphates, hydrogen monophosphates and dihydrogen phosphates can also appear in this region [8]. Heating the gel at 200 ~ for 60 min. resulted in the appearance of a very weak shoulder at about -26 ppm on the high-field side of t h e - 1 4 ppm main peak. This shoulder is more prominent in the spectrum of the sample heated at 200 ~ for 80 min. Since the position of the weak shoulder is in the region where two P peaks of pure A1PO4-11 (-27 and-31 ppm) also appear, we may assume that this broad shoulder is due to a small amount of A1PO4-11 crystallites, which is also seen in powder XRD pattern. As discussed above, the 31p and 27A1 MAS spectra only provide limited structural information and ambiguities do exist in spectral assignments. To further characterize the structure of the intermediate gel phases, we have carried out 27A1 ---->31p cross polarization experiments. The cross polarization is mediated by heteronuclear dipolar interactions [9]. Thus, this technique provides connectivity information through the distance dependence of the CP process. Previously, Fyfe and co-workers have shown that 27A1/31P CP can be used to characterize the crystal structure of AlPO-based molecular sieves [6]. This method has also been employed to study aluminophosphate glasses [10]. Very recently, we have shown, for the first time, that 27A1 ---->31p CP can also be used to extract the AI-O-P connectivity in the gel phases of AIPOs and SAPOs synthesis [2]. We first examined the initial gel without heating. A previous study suggested that mixing the aluminum oxide with phosphoric acid and organic amine at room temperature results in precipitation of aluminum oxide hydrate in a more reactive colloidal form that serves as a precursor for further reaction and that the subsequent pretreatment of the gel processor at about 90 ~ converts aluminum oxide hydrate to aluminophosphate complexes which eventually lead to microporous AIPO4-11 [11]. If this was the case, there should be no aluminophsphate species existing in the initial gel. Our 31p MAS spectrum shows two peaks at 3 and -11 ppm, respectively. Tapp et al. reported that amine phosphate might form in the initial gel [5]. Since some unprotonated monophosphates do have chemical shifts near 0 ppm [8], the sharp peak at 3 ppm may be assigned to amine phosphate. However, the chemical environment o f - 1 1 ppm peak is not clear at all. Figure 3 shows the twodimensional 27A1 ~ 31p CP spectrum of initial gel without heating, from which several interesting features are immediately apparent. The P projection only contains a broad CP 3~ signal at-14 ppm. The sharp resonance at 3 ppm seen in P MAS spectrum does not appear in the projection indicating that this P species is not linked to any of the AI sites observed in 27A1 MAS spectrum. This result confirms that the 3 ppm peak is likely due to amine phosphate which is not connected to any AI species. In AI projection, two peaks are observed at 41 and -7 ppm. The shoulder at 4 ppm appearing in the corresponding A1 MAS spectrum does not show up in the projection, suggesting that this resonance is due to the unreacted aluminum oxide and not five-coordinated AI in the aluminophsphate gel. The broad peak a t - 1 4 ppm in P projection is correlated to both 41 and -7 ppm peaks in A1 projection. Our results clearly indicate that mixing AI and P sources together with the structure directing agent at room temperature yield a solid mixture containing amine phosphate, unreacted aluminum oxide and amorphous aluminophosphate species. Both tetrahedral and octahedral A1 sites are present in the aluminophosphate gel, and both are

140 -11

' PMAs-JI L - - 31p project'

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~:

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f r o m 85% H3PO 4

Fig. 3 2D 27A1 to 31p CP spectrum of initial gel without heating

long contacttime

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from 85% H3PO 4

Fig. 4 2D 17AIto 3tp CP spectrum of the gel heated at 200~ min.

Fig. 5 ~H to 31p CP spectrum of the gel heated at 200~ min.

141 connected to the P. Careful inspection of 2D spectrum reveals that the broad peak a t - 1 4 ppm in P projection actually encompasses two components. The first component is a sharp P peak centered a t - 1 8 ppm, which is only connected to AI peak at 41 ppm. It represents a small amount of P atoms with only tetrahedral AI as the nearest neighbors. The second component is a broad peak a t - 1 4 ppm connected to both tetrahedral and octahedral A1 sites. These connectvities can be seen more clearly in the P slices taken through the tetrahedral and octahedral A1 sites (Figure 3). These two components are apparently due to the P sites with different degrees of condensation. We have further characterized the gel heated at 200 ~ for 60 minutes by twodimensional 27A1 --~ 31p CP (Figure 4). Two peaks were observed in the A1 projection corresponding to the tetrahedral (39 ppm) and octahedral (-7 ppm) A1 sites, both of which are connected to P sites. The 4 ppm peak seen in the AI MAS spectrum did not appear, implying that this A1 site is not part of the aluminophosphate gel and probably due to the unreacted A1 source. A broad P peak positioned a t - 1 6 ppm is connected to both tetrahedral and octahedral AI sites, indicating that this broad resonance represents P atoms, which are not fully condensed. The P projection also contains a well defined broad shoulder at about-27 ppm and this shoulder is connected to tetrahedral AI site only. The chemical shift of this shoulder is very close to the frequencies of the peaks in crystalline A1PO4-11. These results suggest that the chemical environment of this P site is P(-OA1)4. Additional evidence for this assignment is that compared to the same shoulder in the P MAS spectrum the intensity of the shoulder a t - 2 7 ppm was significantly enhanced relative to t h e - 1 6 ppm main peak. Fyfe and co-workers have carried out the 27A1 ~ 29Si CP on various zeolites and found the found that the relative enhancement of Si signals could be related to the number of AI atoms in neighboring T sites [ 12]. In our case, the enhancement of 31p at -27 ppm is consistent with our argument that -27 ppm P peak is due to fully condensed P(-OAI)4 environment, whereas -16 ppm is due to partially condensed P sites (where the number of AI atoms in the first coordination sphere is less than 4). However, care must be taken when interpreting CP intensity since cross polarization from a quadrupolar nucleus such as 27A1 (1=5/2) is usually very inefficient due to the difficulty in spin locking [13]. For this reason, we also conducted the IH ~ 31p CP on the same gel sample. Figure 5 shows that the short contact time (0.1 ms) favors only-16 ppm main peak, suggesting strong dipolar interactions between proton and P a t - 1 6 ppm. This result implies that the broad-16 ppm peak indeed represents P sites which are not fully polymerized and of mixed coordination, P(OH)x(OAI)n.x (where x is 1-3). The CP spectrum obtained with long contact time of 10 ms contains mainly a weak peak a t - 2 7 ppm. The fact that the very long contact time is needed to cross polarize the -27 ppm peak indicates indirectly that the this peak is due to fully condensed P site with P(-OA1)4 environment. In summary, we have examined the evolution of the gel phases as a function of crystallization time by solid-state NMR. We have also carefully characterized several gel phases obtained at several different stages of the reaction. In contrast to previous reports, we found that aluminophsphate species exists in the gel phases formed at room temperature. The different P and AI sites were identified and their connectivity mapped out unambiguously by A1/P CP experiments. The solid sample corresponding to the beginning of the nucleation (200 ~ min.) was also characterized. Valuable structural information

142 regarding the gel structure was obtained by CP method and this information is not readily available from simple AI and P MAS experiments.

Acknowledgements Y.H acknowledges the financial support from Natural Science and Engineering Research Council of Canada for a research grant and the Canada Foundation for Innovation for the award of a solid-state NMR spectrometer. R.R. thanks OGSST for a scholarship. REFERENCES 1. For reviewers see a) H. Gies, B. Marler, U. Werthmann, in Molecular Sieves: Science and Technology, Vol. 1, (Eds: H.G. Karge and J. Weitkamp), pp35-64, Springer, Berlin, 1998; b) M.E. Davis, R.F. Lobo, Chem. Mater., 4 (1992) 756; c) R. J. Francis and D. O'Hare, J. Chem. Soc. Dalton Trans., (1998) 3133; d) S. Oliver, A. Kuperman, G.A. Ozin, Angew. Chem. Int. Ed., 37 (1998) 46. 2. Y. Huang, D. Machado, Micropor. Mesopor. Mater., 47 (2001) 195. 3. S.T. Wilson, B.M. Lock, E.M. Flanigen, US Patent No. 4 310 440 (1982). 4. J.M. Bennett, J.W. Richardson Jr., J.J. Pluth, J.V. Smith, Zeolites, 7 (1987) 160. 5. N.J. Tapp, N.B. Milestone, D.M. Bibby, Zeolites, 8 (1988) 183. 6. C.A. Fyfe, K.T. Mueller, H. Grondey, K.C. Wong-Moon, J. Phys. Chem., 97 (1993) 13484. 7. a) M.E. Davis, C. Monte, P.E. Hathaway, J.M. Grace, in Zeolites: Facts, Figures, Future (Eds: P.A. Jacobs and R.A. van Santen), Elsevier, Amsterdam, 1989, pp 199-215. b) H. He, J. Klinowski, J. Phys. Chem., 98 (1994) 1192; c) E. Jahn, D. Mueller, J. Richter-Mendau in Synthesis of Microporous Materials, Vol. I (Eds: M.L. Occelli, H.E. Robson),Van Nostrand Reinhold, New York, 1992, pp249-265; d).S. Prasad, S.B. Liu, Chem. Mater., 6 (1994) 633. 8. a) I.L. Mudrakovskii, V.P. Shmachkova, N.S. Kotsarenko, V.M. Mastikhin, J. Phys. Chem. Solids, 47 (1986) 335; b) P. Hartmann, J. Vogel, B. Schnabel, J. Magn. Reson., 111 (1994) 110. 9. A. Pines, M.G. Gibby, J.S. Waugh, J. Chem. Phys., 59 (1973) 569. 10. R.M. Wenslow, K. Fiske, K.T. Mueller, in Sold-State NMR Spectroscopy of Inorganic Materials (Ed: J.J. Fitzgerald), ACS Symposium Series 717, 1999. 11. X. Ren, S. Komameni, D.M. Roy, Zeolites, 11 (1991) 142. 12. C.A. Fyfe, K.C. Woog-Moon, Y. Huang, H. Grondey, K.T. Mueller, J. Phys. Chem., 99 (1995) 8707. 13. a) A. J. Vega, Solid State NMR, 1 (1992) 17; b) A. J. Vega, J. Magn, Reson., 96 (1992) 50.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 9 2002 Elsevier Science B.V. All rights reserved.

143

The benzene molecule as a probe for steric hindrance at proton sites in zeolites: an IR study. B. Onida, B. Bonelli, L. Borello, S. FioriUi, F. Geobaldo and E. Garrone* Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24 - 10129 Torino, Italy. When a molecule, like benzene, is engaged in H-bonding with an acidic proton in zeolites, secondary interactions with the surrounding walls may occur, orienting the molecule and effecting the spectroscopic measure of the acidity. Information is gained on the geometry of the site from an otherwise disturbing phenomenon. 1. INTRODUCTION Probe molecules are extensively used to study H-bond formation with acidic hydroxyls in solids, the bathochromic shift of the stretching frequency (Avoi0 being assumed as measure of the acidic strength of the OH species [1]. In recent years Bellarny-Hallam-William (BHW) plots [2, 3], where shifts in frequency caused by H-bonding of two acidic groups to a set of mildly basic molecules are plotted one against the other, have been used: a straight line is observed, the slope of which is the measure sought. With microporous solids, such as zeolites, the probe molecule must h e small, so to be able to diffuse inside the pore system and reach the sites. A widely used candidate is CO [1], which, however, must be adsorbed at low temperature, because of the weakness of its interaction. Benzene may be used instead: adsorption occurs at room temperature and similar AVOH are observed [ 1, 4]. Though bulkier than CO, benzene can~still diffuse inside most of zeolitic channels, even if sometimes diffusion may be slow. When bulky molecules are used~ the possibility arises that steric hindrance be exerted by the surroundings of the acidic hydroxyl species, so hampering the interaction, and affecting the measure of acidity. This aspect is dealt with in the present paper by scrutinizing a set of data concerning different molecules and solids. Data reported concern the interaction of the isolated SiOH species in severely dehydrated Aerosil, and the Bronsted site Si(OH)A1 in a few zeolitic systems: ZSM-5, MCM-22, theta, SAPO-40. Data for silica and ZSM-5 are from literature [1, 5]. Also considered is the mesoporous silica MCM-41, exhibiting the same isolated hydroxyl species as Aerosil, in order to check whether any steric hindrance is introduced by mesoporosity. The set of weakly basic molecules employed are: N2, CO, ethylene, benzene, propene, toluene, 1,3,5- trimethylbenzene (TMB), the strength of which as bases is in the order listed

[5].

Ethene and propene can both engage in H-bonding and act as proton acceptors. At RT proton transfer to ethene is slow with all zeolites considered, so that the H-bonded ~*Corresponding author, E-mail: [email protected], FAX: +39-011-5644699

144 complex can be readily observed. Proton transfer to propene is faster, but only with ZSM-5 the process requires fast time-resolved experiments [6]. 2. EXPERIMENTAL

SiO2 and ZSM-5 (Si/A1 = 25) were from Degussa and Zeolist, respectively. SAPO-40 (Si/AI = 0.12) and MCM-22 (Si/AI = 14) samples were prepared according to the literature [7, 8]. MCM-41 was prepared according to [_9], treated in flowing air up to 823 K and maintained at the same temperature for six hours, in order to remove the template. Theta (Si/A1 = 20) was prepared at the University of Calabria [10]~ For FT-IR measurements, the powders were pressed into thin, self-supporting wafers; spectra were collected, at a resolution of 2 cm 1, in the 4000-500 cm "1 range, on a Bruker FTIR Equinox 55 spectrometer, equipped with a MCT cryodetector (128 scan). Pre-treatments were carried out using a standard vacuum frame, in a IR cell equipped with KBr windows. Wafers were outgassed at 773 K. Adsorption of CO and N2 wascarried outat the nominal temperature of 77 K. The sollware 'Moldraw' [ 11 ] has been used to define the bulkiness of the probes. 3. RESULTS AND DISCUSSION

Figure 1 reports some experiments for the zeolite MCM,22_ Curve a is the spectrum, in the OH stretching region, of the bare sample, after outgassing in vacuo at 773 K. The other curves refer to the contact of some of the probe molecules employed. H-bonding shills the O-H vibration to frequencies the lower, the more basic is the probe, i. e. the order of basicity is N2 < CO < C2H4 0.2 a.u.

----________&

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30b0

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2800

Figure 1 Curve a: MCM-22 outgassed at 773 K. Curve b: after adsorption of N2; curve c: after adsorption of CO; curve d after adsorption of C2H4.

145 Table 1- AVoHvalues observed with N2, CO, ethen_e, propene, benzene, toluene and TMB on different systems.

C2H4-

40 40 115 125" 120 120

CO 90 90 290 330 a 320 316

0.148

0.154

0.364

N2

system AEROSIL ~ MCM-41 SAPO-40 ZSM5 MCM-22 theta CROSSSECTION (am) a Reference [ 1]. bReference [6].

,i

A (oa)

104 / 360 390 b 390 320

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C7Hs 147

500 450

120 120 330 360 a 310 285

0.482

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,

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,

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152 152 503 54o ~

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400 360 0.645

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/ 0.674

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oo

37'00 36'00 35b0 3~00 wavenumber

33~0 3~0

31'00 3(~0

c m -~

Figure 2. MCM-22 and MCM-41 under CO at a nominal temperature of 77 K (curve 1 and 3, respectively) and under benzene at room temperature (curve 2 and 4, respectively). Data for the other systems are only reportedas numerical Av values in Table 1, together with those related to the other probes. Inspection of the Table reveals some irregularities, concerning e.g. the values for benzene. With silica, ho~ Aerosil and MCM-41, the benzene shift (120 cml ) is larger than that of ethene (104 cm'l). With zeolites the ethene shifts is some 20-60 crn1 larger. With MCM-22 and theta, the benzene shift is not only lower than the ethene shift, but even lower than that observed with

146 CO: with most zeolites (Y, ZSM-5, mordenite, beta), as well as Aerosil and MCM-41, the opposite takes place. Figure 2 illustrates this fact by comparing the CO and benzene shiits with MCM-41 and MCM-22. We ascribe these irregularities to the presence of zeolitic walls surrounding the acidic site, with which relatively big molecules like benzene (at variance with ethene and CO) may interact, not allowing the H-bonded adduct to assume the optimal conformation. H-bonding is strongly sensitive to the geometry and influenced_by even small perturbations of the B---H distance or the O-H-..B angle [2]. This explanation is in line with the work of Su and Barthomeuf, reporting the interaction of C-H groups of benzene with basic oxygen atoms of the framework in faujasites [4], and with the very recent computational work by Kenmer et al. [12], showing that van der Waals interactions with walls orient ferrocene molecules in the cavities of NaY. The secondary interactions with the surrounding walls, preventing the optimal conformation of the H-bonded adduct, may be regarded to as a steric hindrance at the proton site, the extent of which may be studied by means of BHW plo~. Because of the non-porous nature of the solid, the isolated SiOH species in Aerosil may be assumed to be non-hindered: for this reason we have adopted in all. BHW plots SiOH values as independent variables. With MCM-41, a straight line passing through the origin with unit slope is observed (figure not reported), proving that the silanol species is the same in the two samples, and that mesoporosity does not cause any hindrance to the acidic centre. Figure 3 shows the BHW plot for zeolitic systems:_ the broken straight lines have been drawn discarding the points deviating from linearity, and applying the least square method. From the slopes, the known scale of acidity is obtained- SiO2(Aerosil) - SiO2 (MCM-41) < SAPO-40 < H-zeolites (theta, MCM-22, ZSM-5). The acidity of the H-zeolites show indeed marginal differences, not comm__e_ntedhere. With SAPO-40 (Figure 3a) and ZSM-5 (Figure 3b) hindrance at the Bronsted sites is observed for aromatic tings. With MCM-22 (Figure 3c) and theta (Figure 3d) hindra_n_ee is observed for smaller molecules~ i.e olefins. With MCM-22, propene does not lie on the straight line, whereas ethene does. With theta also ethene deviates from linearity: indeed, the ethene value is close to that of CO, whereas it is definitely higher in all other cases. To define a measure of hindrance at the Bronsted site,_we have considered the percentage difference between the value expected in its absence, as inferred by the straight lines, and that actually observed. To measure the bulkiness of the probe_ molecules, wehave chosen the largest molecular diameter in a direction orthogonal to the probable O-H axis in the adduct formed, e.g. the cross-section of CO and N~ perpendicular to the molecular axis. Corresponding values are reported in Table 1. Figure 4 reports plots of such deviations from linearity as a function of molecular diameter for the four zeolitic systems. Relatively small molecules do not cause deviation, whereas the larger ones do, and_ a monotonic increase in deviation is seen with increasing cross section. This is strong evidence that secondary interactions is the cause of deviations in BHW plots. As a guide to the eye, broken straight lines have been drawn through the points pertaining to the same zeolite. Hindrance is seen first with benzene in SAPO-40 (curve a) and ZSM-5 (curve b), with propene in MCM-22 (curve c) and with ethene in theta (curve d). As the size of the smallest molecule showing deviation from the linearity in the BHW plot decreases, hindrance at the site seems to be in the order SAPO-40 ~ ZSM-5 < MCM-22 < theta. Support to this comes from the observation that, for the same probe, showing hindrance in all cases (benzene), the percentage of deviation increases in the same order, i.e. interaction with the walls are larger in the case of theta than with ZSM-5.

147 600

600

C3H6

C3H 6 500

500-

I 0

~ ~

400 1 -|

C 2 H

4

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40O

O

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SlOHAerosil_(cm

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o

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20 40 60 80 100 120 140 160 180 AV

SiOHAerosU (cm-~)

0

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20 40

'610'

I

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80 1 0 0 1 2 0 1 4 0 1 6 0 1

Av SiOHA~, (cm 1)

Figure 3. Bellamy-Hallam-William plot for the various zeolite systems taking as reference the Av values for the isolated silanol of Aerosil. Section a: SAPO-40, section b: ZSM-5, section c: MCM-22, section c: theta.

148 35

35

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TMB ' e a

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 molecular hindrance (nm)

i

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 mnler.ulAr hindrance. (nm~ 35

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0.0-0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 molecular hindrance (nm)

Figure 4. Correlation between extent of hindrance and largest transverse molecular diameter o f probe molecule for SAPO-40 (a), ZSM-5 (b), MCM-22 (c) and theta (d).

149 4. CONCLUSIONS When secondary interactions with the walls take place, the strength of H-bonding is under-evaluated. On the other hand, the study of H-bonding with relatively bulky molecules, like benzene, may give a measure of hindrance at the site, and information can be drawn from an otherwise disturbing phenomenon. Surroundings may have an orientating effect of molecules at the proton site [12], and influence the reactivity of the molecule at the active site. Indeed, with zeolites where significant hindrance at the Bronsted site is observed, like MCM-22 [ 13] and theta [ 14], proton transfer to light olefins at room temperature is slower than that observed for zeolites which show smaller hindrance at the proton site (ZSM-5, mordenite) [6, 15] in spite of the fact that the spectroscopically measured acidity is the same. ACKNOWLEDGMENT We thank Drr Girolamo Giordano for theta samples, Dr. Flaviano Testa and Prof Rosario Aiello for MCM-22 samples, Prof. Piero Ugliengo for Moldraw software and for fruitful discussions. REFERENCES

1. E. A. Paukshtis and E. NrYurchenko, React~ Kinek Catal. Lett., 16 (1981) 131_ ; A. Zecchina and C. Otero Aregn, Chem. Soc. Rev., (1996) 287 and references therein; J. A. Lercher, C. Gr0ndling and G_ Eder-Mirth, Cat_zL Today, 27 (1996) 353 and references therein; L. M. Kustov, V. B. Kasansky, S. Beran, L. Kubelkovh, and P. Jim, J. Phys. Chem., 91(20) (1987) 5247;_tL Knt~uzinger aM_S. Huher, 1. Chem. Sot., Faraday Trans, 94(15) (1998) 2047 and references therein; E. Garrone, A. Barbaglia, B. Onida, B. Civalleri and P. Ugliengo, Phys. Chem. Chem_ Phys., 1 (1999) 4649 and references therein. 2. G. C. Pimentel and A. L. McClellan, in The Hydrogen Bond, W. H. Freeman and Co., San Francisco, 1960. 3. H. Kn6zinger, in The H-bond: recent advances in theory and experiment, P~ Schuster, G. Zundel and C. Sandorfy (eds), North Holland, Amsterdam, 1976, p. 1269. 4. D. Barthomeuf, Zeolite, 1993, 13, 626; B. L. Su and D. Barthomeuf, J. Catal., 139 (1993) 81. 5. C. Paz~, S. Bordiga, C. Lamberti, M. Salvataggio andA. Zecchina, J. Phys. Chem. B, 101 (1997) 4740 6. G. Spoto, S. Bordiga, G. Ricchiardi, D. Scarano, A. Zecchina and E. Borello, J. Chem. Soc. Faraday Trans., 90 (1994) 2827. 7. N. Dumont, Z. Gabelica, E. (3. Derouane andL. B. McCusker, Microporous Materials, 1 (1993) 149. 8. Corma A., C. Corell and J. P&ez-Pariente, Zeolites 15 (1995) 2. 9. Q. Cai, W. Y. Lin, F. S. Xiao, W. Q. Pang, x. H. Chen and B. S. Zou, Microp. Mesop. Materials, 32 (1999) 1. 10. G. Giordano, A. Katovic, A. Fonseca and L B_ Nagy, Stud. Surf. Sci. Catal. 135 (2001) 175. 11. P. Ugliengo, D. Viterbo and G. Chiari, Z. Kristallogr. 9 (1993) 207. 12. E. Kemner, I. M. de Schepper and G. J.Kearley, Chem. Commun., (2001) 2466.

150 13. B. Onida, F. Geobaldo, F. Testa, R. Aiello and E. Garrone, J. Phys. Chem., 106 (2002) 16840 14. F. Geobaldo, S. Fiorilli, B. Onida, A. Katovic, G. Giordano and E. Garrone, in preparation. 15. Geobaldo, F., Spoto, G., Bordiga, S., Lamberti, C. and Zecchina, A., J. Chem. Soc. Faraday Trans., 93(6) (1997) 1243.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Structural characterization of Co- and synthesized in the presence of morpholine.

151

Si-substituted

AIPO-34

A. Martucci l*, A. Alberti l, G. Cruciani 1, A. Frache 2 and L. Marchese 2. 1Dipartimento di Scienze della Terra, Sezione di Mineralogia, Petrografia e Geofisica, Corso Ercole I d'Este, 32, I- 44100 Ferrara, Italy 2Dipartimento di Scienze e Tecnologie Avanzate, Universit/t del Piemonte Orientale "A. Avogadro", C.so Borsalino 54, I-15100 Alessandria, Italy.

SAPO-34, CoAPO-34 and CoAPSO-34 materials, synthesized using morpholine as structure directing agent, were structurally characterized by single-crystal and X-ray powder diffraction (XRD) data. HF was also used for the crystallization of CoAPO-34. The incorporation of Co and/or Si atoms in the framework sites was determined by different routes in the structure refinement. In CoAPO-34, at room temperature one of the A1 framework sites was sixfold-coordinated with four O and two F atoms. Fluorine bridged two A1 atoms. The loss of fluorine occurred at around 450~ and restored the tetrahedral coordination of the 6-coordinated A1 atom. In agreement with TGA analysis, the presence of two morpholine molecules in each cage of all these chabazite-type materials was assessed by structural studies. -NH2 vibrations in the IR spectra of assynthesized samples indicated that some morpholine molecules are protonated and act as counterbalancing charges of the negative framework. Structural modifications under heating, followed by in-situ high temperature XRD, showed that CoAPO-34 materials start to lose crystallinity at around 730~ Bridging hydroxyls [Co2+-O(H)-P] were found by FTIR, and the presence of tetrahedral Co 2+ ions was inferred by UV-Vis-NIR spectroscopy. 1 INTRODUCTION When cobalt ions are incorporated into the tetrahedral framework sites of microporous aluminophosphates (A1POs) or silicoaluminophosphates (SAPOs), selective heterogeneous catalysts are produced [1]. Co-substituted alumino- or silicoaluminophosphates are materials which may combine both redox and acid properties. SAPO-34, for instance, a material with chabazite-type framework, is a particularly good shape-selective catalyst for methanol to light-olefin conversion [2]. A1PO-34 type structures have recently been synthesized in the presence of morpholine as structure-directing agent; HF was also used for crystallization [3]. The aim of this work is to determine the structures of Co-substituted A1PO-34 and SAPO-34 in the *to whom correspondence should be addressed: [email protected]; [email protected]

152 presence of morpholine in order to have direct evidence of the metal substitution in the A1 framework sites, and to localize the organic species in the chabazite cages. CoAPO34, including F, was also studied via X-ray structure determination in order to understand its ability to coordinate A13+ [4], and to follow the structure modifications of F-CoAPO-34 under heating. FTIR and DR UV-Vis spectroscopies were also used to monitor the presence of cobalt ions in framework positions. 2. E X P E R I M E N T A L

SAPO-34 and CoAPSO-34 were synthesized by mixing the appropriate amounts of AI(OH)3, H3PO4 and distilled water. The mixture was stirred until a uniform gel was obtained. SiO2 and/or Co(CH3COO)2 were then added, followed by morpholine. The resulting gels (0.08Co:0.92Al:0.25Si:0.90P:50HzO:l.25Morph.) were crystallized in a Teflon-lined autoclave under autogenous pressure at 195~ for 10 days. CoAPO-34 was synthesized in a similar way, but without SiO2 and with the addition of HF (F-CoAPO34; 0.08Co:0.92Al:lP:0.35F:50H20 :l.25Morph.). TGA analysis was carried out on a TA instrument (SDT2960 mod). The weight loss percentages from 200 to 700~ of all as-synthesised materials was in the range 18-20%, this means that two molecules of morpholine could be present within the chabasite cages. Single crystal X-ray diffraction data were collected at room temperature on three crystals of SAPO-34 and two of CoAPSO-34 (with different Co/A1 ratios), using a Nonius KappaCCD diffractometer equipped with a CCD detector. The chemical compositions of one SAPO-34, and two CoAPSO-34 crystals were determined by an ARL-SEMQ electron microprobe in the wavelength dispersive mode, at 15 K V , with 20nA sample current and defocused beam (20~tm), on the same crystals used for the Xray data collection. Powder diffraction data of F-CoAPO-34 were collected at high resolution at the Swiss-Norwegian beamline (SNBL) at the X-ray synchrotron source of ESRF (Grenoble). A sample of F-CoAPO-34 was also packed in an open capillary and heated "in situ" using a hot air stream, at the GILDA beamline at ESRF. During the heating process powder diffraction patterns were collected on a translating image plate in the temperature range 50-850~ FTIR spectra were recorded on a Bruker IFS 88 spectrometer, and diffuse reflectance (DR) UV-Vis-Nir spectra on a Perking Elmer Lambda 19 equipped with a reflectance sphere attachment.

3. RESULTS AND DISCUSSION

Single-crystal structure refinements of SAPO-34 and CoAPSO-34 were carried out, starting from the crystallographic data reported for the CoAPSO-34 structure [5] in space group R-3. The position of the morpholine molecules was determined by Fourier and difference Fourier maps. Incorporation of Si and Co in tetrahedral sites was assessed by: a) analysis of the T-O bond distances. For Si-O we assumed the value of 1.603A found in the purely siliceous chabazite [6], for A1-O the value of 1.725 A found in the structurally-related SAPO-47 structure [7], and for Co-O and P-O the values 1.93 and 1.52 A respectively [8].

153 b) the Co and A1 occupancies in sites T1 and T2 obtained by scattering curve refinements. The refinements of SAPO-34 crystals showed slight differences in the S i - P substitutions and, in accordance with criterion a), the T2 site was occupied by about 78% P and 22% Si, in excellent agreement with the results of Pluth and Smith [7], whereas chemical analyses gave about 15% Si. The two crystals of CoAPSO-34 differed mainly in their Co and Si contents in the tetrahedral sites. One of these, in accordance with criteria a) and b), showed about 15% of Co in T1, but no Si in T2, whereas the other crystal was characterized by about 10% Co in T1 and 5% Si in T2. According to the chemical analyses, the A1 tetrahedron was occupied by 19% Co and the P tetrahedron by 5% Si in the first crystal, whereas in the second one the two tetrahedral were occupied by 15% Co and 9% Si, respectively. Structure refinements showed the presence of two morpholine molecules related by the inversion centre. Each of these occupied three different positions with 1/3 of occupancy rotated around the threefold axis and the oxygen atom placed, as a pivot, on the triad (Figure 1).

Figure 1. Projection along [111] of the chabazite cage, showing the morpholine molecules in SAPO-34 and CoAPSO-34.

154 The distances from the framework oxygens of all atoms of the organic molecule were always greater than 3.3A, thus indicating that morpholine was only weakly bonded, if at all, to the framework. This behaviour is quite unusual: however, the presence of water molecules near the center of the 8-rings suggested that different morpholine molecules could be connected by means of hydrogen bonds through water molecules to form morpholine-H20 chains (Figure 2). The structure refinement of F-CoAPO-34, obtained by high resolution X-ray diffraction data at the SNBL beamline of ESRF, strongly resembles that of A1PO-34 performed by Harding and Kariuki [9]; the symmetry is triclinic, with space group P-1. The framework in both cases in the asymmetric unit is composed of three tetrahedral P atoms, two tetrahedrally coordinated A1 atoms and one octahedrally coordinated A1 atom. This framework site is coordinated to four O and two F atoms. Therefore, the sixfold coordination of an A1 atom found in the F-A1PO-34 structure is maintained also when Co atoms are located in the framework (see spectroscopic results). We observed that fluorine bridged two A1 atoms, which were found closer together than other framework atoms. The presence of bridging fluorine led to a distortion of the trigonal symmetry of the CHA framework.

Figure 2. Projection along [100] of the chabazite cage, showing the chains of morpholine and water molecules in SAPO-34 and CoAPSO-34.

155 Two fully occupied sites of morpholine were found at the intersection of the six- and eight-rings (Figure 3).

Figure 3. Projection along [010] of the chabazite cage of CoAPO-34 containing two morpholine molecules. The N atom of the morpholine molecules is suitably located for hydrogen bonding (2.63A) with one framework oxygen. The "in situ" analysis of the powder diffraction data collected at the GILDA beamline at the X-ray synchrotron source of ESRF showed that, when CoAPO-34 was heated to around 450~ it lost both fluorine and morpholine molecules. The unit cell parameters became strongly rhombohedral with a ~ b ~ c 9.33A and ct ~ [~ ~ 7 ~ 94.2~ but the symmetry remained triclinic P-1. The original sixcoordinated A1 atoms became four-coordinated after removing the fluorine atoms, and this restored the three-dimensional topology of the chabazite-type structures. A memory of the coordination and distortion was nevertheless maintained after template and fluorine removal, and the threefold axis of the rhombohedral symmetry was not restored. The results of the X-ray structure refinements for Si- and Co- containing materials, as well as for A1PO-34 [9], indicated clearly that two morpholine molecules are always present in the chabazite cage of A1PO-34-type structures. This figure perfectly fits the TGA data reported in the experimental section. As a result, morpholine can be either protonated or unprotonated in these structures. IR and Raman measurements, however,

156 showed that protonated morpholine molecules are indeed present in both SAPO-34 and A1PO-34 [3]. More recent spectroscopic studies (see below) suggest that, also in Cocontaining materials, protonate morpholine molecules are present. 3.1 Spectroscopic results Fig. 4 shows the IR spectra (1800-1500 cm "l) of CoAPO-34 taken under vacuum at temperatures ranging from 200 to 550~ Absorption at 1604 cm 1, which progressively disappeared as the temperature increased, is assigned to a bending mode of +NH2 groups in protonated morpholine. FTIR studies (figure not reported for the sake of brevity) revealed that, like to structurally related CoAPO-18 materials, acid and redox centres are present in CoAPO-34 materials, confirming that Cobalt ions isomorphously substituted some A13+ ions in the A1PO-34 framework [10,11]. The acid sites were related to Bronsted hydroxyl groups bridged between Co 2+ and p5+ and associated with the presence of Co(II)/Co(III) redox couples. In fact, two vibrational absorption at ca. 3575 and 905 cm -1, assigned respectively to the stretching and bending modes of bridged OH groups, were present in the FTIR spectra. These hydroxyl groups disappear after calcinations in 02, and were restored by reducing in H2 at 400~ Finally, DR UVVis-NIR spectra (figure not reported) showed the presence of bands in the visible (20.000-15.000 cm -1, triplet) and in the near infrared (10000-4000 cml), attributable to 4Tl(P) --~ 4A2(F) and 4Tl(F) --+ 4A2(F) ligand field transition of tetrahedrally coordinated Co 2§ centres which should be located at framework sites.

30

0 t""

20

E r

2/

L,.

I-- 10

0

L

1800

,

I

1700

,

I

1600

1500

Wavenumbers [cm1]

Figure 4." FTIR spectra of CoAPO-34 recorded after evacuating the samples at increasing temperatures" a) 200~ b) 400~ c) 550~

157 REFERENCES

1. J.M. Thomas, Angew. Chem. Int. Ed. Engl. 33 (1994) 913. 2. Y. Xu, C.P. Grey, J.M. Thomas, A.K. Cheetham, Catal. Lett., 4 (1990) 251. 3. L. Marchese, A. Frache, E. Gianotti, G. Martra, M. Caus/t, S. Coluccia Micro. Meso. Mat. 30 (1999) 145. 4. P.F. Feng, X. Bu, T.E. Gier, G.D. Stucky, Micro. Meso. Mat. 23 (1998) 221. 5. G. Nardin, L. Randaccio, V. Kaucic, N. Raijic, Zeolites 11 (1991) 192. 6. M.J. Diaz_Cabanas, P.A. Barrett, M. A. Camblor, Chem. Commun. (1998) 1881. 7. J.J. Pluth, J.V. Smith, J. Phys. Chem. 93 (1989) 6516. 8. R.D. Shannon, Acta Crystallogr. A32 (1976) 751. 9. M.M. Harding, B.M. Kariuki, Acta Crystallogr. C50 (1994) 852. 10. L. Marchese, G. Martra, N. Damilano, S. Coluccia, J.M. Thomas, Stud. Surf. Sci. Catal., 101 (1996) 861 11. E. Gianotti, L. Marchese, G. Martra, S. Coluccia, Cat. Today, 54 (1999) 547

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

159

Chemical linking of MFl-type colloidal zeolite c r y s t a l s P. Agren*, S. Thomson t, Y. Ilhan, B. Zibrowius, W. Schmidt, F. Schiith MPI ftir Kohlenforschung, 45470 MOlheim, Germany Nanoparticles were synthesized using solutions of hydrolyzed TEOS and aluminate TPAOH at room temperature. These nanoparticles were crosslinked by reaction with 1,7dichlorooctamethyltetrasiloxane. The resulting solid after calcinations is X-ray amorphous, but they show NMR and IR signatures akin to MFI type zeolites. They have mieroporous properties like a zeolitic structure, but in addition provide a mesopore system resulting from the crosslinking. The acid site strength of aluminosilicate materials is lower than that of HZSM-5, as shown by pyridine desorption at different temperatures followed by IRspectroscopy. 1. INTRODUCTION Colloidal zeolite crystals have been intensively investigated over the last years (for instance [1,2]). However, the colloidal dimension is rather wide, ranging from the nanometer to the thousand nanometer scale. Conventionally, zeolite crystals are termed "colloidal", if they have sizes of several ten nanometers. It has also been known for quite some time, that even smaller entities of only several nanometers in size exist in zeolite synthesis solutions under specific conditions which are assumed to be the precursors of MFI type zeolites [3,4]. These entities which were primarily detected by small angle scattering, typically had sizes of 3-5 nm. At their first detection, their internal structure remained fairly unclear, although it was thought that they contain some structural elements of the zeolites which eventually are formed from them. More far reaching conclusions, based on NMR spectroscopy, IRspectroscopy, small angle X-ray scattering, electron microscopy, gel permeation chromatography and other techniques, with respect of the structures of such entities in clear solution systems were drawn by Martens and coworkers. The authors presented evidence for the presence of MFI "nano slab s" at intermediate stages of a clear solution synthesis [5] which upon heating assemble into macroscopic zeolite crystals [6]. For some reactions it has been proposed that they occur advantageously on the external crystallite surface of zeolites. This includes the Beckmann rearrangement of cyclohexanone oxime to e-caproclactame [7] in the gas phase, although this has recently been disputed for the liquid phase reaction [8] as well as for the gas phase reaction, where it was shown that both educt and product have access to the pore system of the MFI structure [9]. The higher activity of smaller crystallite size material was explained by an increased effectiveness factor of such small particle catalysts. In other systems, however, the reacting molecules are clearly too bulky to enter the pore system, and their conversion over zeolitic catalysts could be improved, if particles with a high fraction of external surface area were used [10]. It seems thus interesting to produce zeolitic materials with a maximum external surface area and investigate their properties in such reactions. Present address: Vaisala Oyj, P.O. Box 26, FIN-00421 Helsinki, Finland t Present address: Materials Division, Building 3, Australian Nuclear Science and TechnologyOrganisation (ANSTO), PMB 1, Menai, 2234, New South Wales, Australia

160 If the zeolite nanoslabs proposed by Martens and coworkers could be used in the form in which they are assumed to be present in clear solution, a maximum of external surface area could be achieved, since in such materials essentially the whole surface area would be external or could at least be accessible to reactant molecules. However, if these very small entities existing in pre-crystallizing solutions are removed by extraction or by solvent evaporation, or if these solutions are heated, agglomeration of the primary units takes place with a concomitant loss of external surface area. To maintain a high external surface area, we have attempted to capture the nanosized particles in a matrix which provides access to their external surface. For this purpose we have used silicones with reactive groups on both ends or aluminum Keggin ions. The following discussion will focus on samples which where crosslinked with the silicone 1,7-dichloro-octamethyltetrasiloxane. 2. EXPERIMENTAL 2.1. Materials

For the preparation of the nanosized zeolites clear solution syntheses were carried out according to published procedures, but without the aging step to keep the particles as small as possible. The molar composition of the solution was 12 TPAOH : 25 SiO2 : 1.67 A1203 : 480 1-120 : 100 EtOH. The alumosilicate solution was prepared by combining to precursor solutions. For solution 1, TEOS was hydrolyzed by stirring with two thirds of the TPAOH solution (40wt.%) for 24 h at room temperature. Solution 2 was prepared by dissolving freshly precipitated AI(OH)3 gel (prepared according to Schoeman et al. [ 11]) in one third of the TPAOH solution and the rest of the water. After combining the two solutions they were transferred to sealed polypropylene bottles and aged for different times at room temperature. After the desired aging time, 4 ml were removed and put in a polyprolylene beaker with a stirring bar. 1 ml of 1,7-dichlorooctamethyltetrasiloxane was added under vigorous stirring. A gel was formed after approximately 5 h. The gel was aged for another 24 h at room temperature, then dried at 363 K for 24 h. Calcination was carried out by heating to 823 K with a rate of 1 K/min and kept at 823 K for 5 h. A reference sample of unlinked nanoparticles was prepared by adding THF to the solution after the appropriate aging time, particles were salted out into the organic phase by NaCI addition, and the product was obtained by evaporation of the THF, according to the procedure described in [12]. In the following the discussion will focus on three sets of samples, non-crosslinked material SO (uncalcined) and S0C (calcined), crosslinked after 3.5 h of aging $3.5 and $3.5C and crosslinked after 24 h of aging $24 and $24 C. 2.2. Characterization

Samples were characterized by nitrogen adsorption with a Micromeritics ASAP 2010 at liquid nitrogen temperature. Prior to the measurements, samples were pretreated overnight under vacuum at 473 K. 29Si and 27A1 MAS NMR measurements were carried out using a Bruker Avance 500 spectrometer at a spinning rate of 15 kHz. XRD measurements were performed on a STOE STADI P transmission diffractometer with a position sensitive detector. Selected samples were analyzed by transmission electron microscopy on a Hitachi HF 2000 microscope equipped with a cold field emitter gun. IR spectra of adsorbed pyridine were recorded using a Nicolet spectrometer and a home built in situ cell. Prior to pyridine admission, samples were outgassed in vacuum at 673 K for two hours. Then samples were exposed to 4 mbar of pyridine at 413 K and evacuated, before spectra were taken. To assess acid site strength, samples were heated to temperatures of 473 K, 523 K, 598 K and 673 K and after each step cooled to 413 K to record the spectra.

161 3. RESULTS AND DISCUSSION 3.1. Coordination of silicon and aluminum centers MAS NMR spectroscopy provides information on the coordination state and the connectivities of silicon and aluminum in the samples. In Fig. 1, the 29SiNMR spectra of the samples SO and $24 are shown. The spectrum of SO shows three distinct signals at 91.5, 101.3 and 109.6 ppm, which can be assigned to Q2, Q3 and Q4 silicon atoms. The spectra are quite similar to those reported earlier for isolated "nanoslabs" by Ravishankar et al. [5]. In the crosslinked sample $24 the signal corresponding to Q2 silicon has vanished and the relative intensity of the Q3 signal is strongly decreased. Two new signals are visible at 18.9 and 22.4 ppm which are caused by silicon connected to the methyl groups in the silicone-linker. The reduction of the Q2 and Q3 signals in the crosslinked material can be attributed to the reaction of the Si-C1 group of the silicone with surface hydroxyl groups of the nanoparticles. The strong reduction of the NMR signals corresponding to the not fully connected silicon atoms shows, that the reaction with the silicone is quite efficient. After calcinations the signals at 18.9 and 22.4 ppm have disappeared, due to the removal of the Si-20 -40 .4]0 -80 -100 -120 -140 CH3 groups. The relative intensity of the Q4 signal has still increased slightly. This means that also the backbone of the silicone consists of almost fully Figure 1" 29SiMAS NMR spectra of connected silicon atoms, resulting from a ZSM-5 nanoparticles, SO, (top trace) condensation process during calcinations. and of crosslinked nanoparticles aged 27A1 MAS NMR spectra of all samples studied for 24 tl, $24, (bottom trace). typically showed two signals at 52 ppm (tetrahedral aluminum) and 1 ppm (octahedral aluminum) corresponding to about equal amounts of octahedral and tetrahedral aluminum. For low aluminum concentrations (Si/A1 = 50 and smaller), complete incorporation even in nanosized crystals has been proven [11]. However, the amount of aluminum offered in our syntheses was rather high (Si/A1 - 7.5) to create a high concentration of acid sites. It could therefore be expected that some of the aluminum was not incorporated in the zeolite framework but is present in the sample as extraframework alumina. The NMR data show that the reaction of the silicone with the surface silanols proceeds smoothly and goes to completion. The amount of silicone added (2.85 mmol) is approximately sufficient to react with all free silanol groups (7.3 mmol silicon in the aliquot taken). Since the medium in which the reaction is carried out, is aqueous, some hydrolysis and condensation of the silicone with other silicone molecules can occur as well. This would be detected in the NMR spectra as a high fraction of Q3 species left. However, since the hydrolysis and condensation are acid catalyzed and the surface silanols are moderately acidic, preferential reaction with these is expected and also detected by the reduction in Q3 intensity. ................................................................................

i .........

,..i,,.,,,.t,.,I.,,..,,.,,,,,,,,.,,.,...,,,,,,,

3.2. Textural properties Nitrogen physisorption isotherms of the three samples SOC, $3.5C and $24C are shown in Fig. 2. Strong differences can be detected between the three samples. SOC has a micorpore volume of 0.19 cm3/g in good agreement with data on nanocrystalline ZSM-5 and also bulk ZSM-5 [13]. However, different to bulk materials with MFI structure, there is still a slight increase of the volume adsorbed up to about p/p0 = 0.4. This is attributed to supermicropores between the individual agglomerated nanocrystals and to adsorption on the appreciable

162

450

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400

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,._, 350 r

SO $3.5C $24C

300-

co 2 5 0 O3

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Figure 2: Nitrogen adsorption isotherms of the samples SOC, $3.5C and $24C. external surface area of the materials. In addition, the isotherm is not fully reversible at low pressures. In order to rule out the possibility of artifacts, we used samples from another batch, different equilibration times, another adsorptive (Ar) and another instrument. However, in all cases the non-reversibility was observed. The isotherm for $3.5C has a totally different character. There is no indication of microporosity, the isotherm increases smoothly, as for the adsorption on nonporous samples and then increases steeply beyond p/p0 = 0.8. This is indicative of a sample with porosity in the meso- to macropore range. As for SOC, non-reversibility of the isotherm was observed. The isotherm of $24C shows the presence of a microporous material with an additional mesopore system, as targeted for in the synthesis. The steep in crease at low relative pressures indicates microporosity, while the increase at higher relative pressures and the high pressure hysteresis proves the presence of mesopores with a rather wide size distribution. The BET equivalent surface area corresponds to 600 m2/g, although this value has to be interpreted with caution, since the BET algorithm can not directly be applied to samples with microporosity. Nevertheless, the appreciable slope in the range of relative pressures between 0.1 and 0.4 is indicative of a rather high surface area in mesopores and on the external surface. Other than for S0C and $3.5C, the isotherm of $24C was fully reversible. The differences in the isotherms of the samples $3.5C and $24C can be explained by the different degree of crystalline organization of the nanoparticles. Full organization of the silicate framework to a crystalline material is expected to take some time, as has been reported for the crystallization of colloidal zeolite A [2]. The nanoparticles which were formed in that study were not immediately crystalline, but started to crystallize from the middle towards the shell. A similar process is most likely occurring in the ZSM-5 system studied here as well. After aging at 3.5 h, the colloidal particles are unporous gel particles, which are linked with the silicone, creating the meso/macropore-system after calcinations. The sample aged for 24 h, in contrast, has been convened to the MFI structure, which creates the zeolite-like microporosity in addition to the meso/macropore system due to crosslinking. Further increases in the aging time from 4 to 14 days did not lead to further increase of the micropore system nor did it have an observable effect on the secondary pore structure, nor did aging at 368 K for 3 days before crosslinking. Similar observations were reported by Persson

163 et al. [ 11 ], who reported that the size of the colloidal zeolite crystal reached a maximum after 48 h. The lack of reversibility of the nitrogen sorption isotherms for samples S0C and $3.5C is not fully clear, yet. A tentative explanation takes into account the fact that in these samples relatively loose packed or not well organized particles are present, which might swell during nitrogen adsorption and thus lead to a non-closure of the isotherm. In contrast, $24C consists of strongly linked, well organized particles, for which no textural changes during nitrogen adsorption are expected to occur. 3.3. Structural Properties

All samples were completely amorphous according to X R analysis. Only the broad feature around 20-25 ~ (2| could be observed, which is characteristic for amorphous silica. TEM analysis of dry sols, however, revealed lattice fringes in some small particles of a few nanometers in size, indicative of crystalline material. This, however, is no contradiction to the XRD data, since very small crystalline domains will lead to so strongly broadened reflections, that they can not be distinguished from the background. Such lack of Bragg reflections has been observed before for similar colloidal zeolite crystals consisting of very small crystalline domains [ 12]. For MFI type materials with a lack of long range periodicity, the IR band around 550 to 580 cm4 assigned to a double five-ring vibration has been interpreted as the most conclusive evidence for the presence of MFI structural elements at least on a local scale [ 14]. This band is also observed for the crosslinked sample $24 (Fig. 3). The sharp bands at 806, 1263 and 2962 cm4 are assigned to vibrations of the silicone linkers. T h ~ were only observed in crosslinked, uncalcined samples, corroborating the results of the Si NMR analysis. Thus, although no Bragg reflections were detected for the samples, TEM and IR data reveal, that the materials consist of small ZSM-5 crystallites, which are linked by silicone moieties.

50-

u 9 40--

f

E ~ 30c

f

f

f

I--

N 2o10-

bO 0

35'oo

30'00

25'oo

20'00

Wavenumbers (cm-l)

15'oo

o'oo

Figure 3 FTIR spectrum of sample $24 in KBr. The band at 563 cm" is indicative for ZSM5 in the crosslinked material. The sharp bands at 806, 1263 and 2962 cm-1 are assigned to vibrations of the silicone linker

164 3.4. Acidity

The acidity of sample $24C was considerably different compared to a commercial HZSM5 sample (SM-27, calcined to give the H-form, Alsipenta). The spectra of the OH-region of $24C after water desorption at 673 K have their prominent features in the range above 3700 cm 4 (Fig. 4). No band at 3610 cm4 which is characteristic for Si-OH-AI bridges is observed, in contrast to normal HZSM-5 which could indicate that aluminum was not incorporated in the crystals in spite of the detection of tetrahedraUy coordinated aluminum in the w 29Si NMR spectra. The spectrum of ~ (a) { 4M' ,v{ I~ _~., I sample $24C shows three bands in the silanol region, an intense one at 3743 cm1' and two shoulders at 3733 and 3724 cmwhich become visible only after dehydration. A band at about 3680 cm-1 which vanishes upon heating, is attributed to vicinal silanols, which are easily dehydrated [15] The 3743 cm 1 band is (c) .i attributed to silanol groups on the external ai00 a~0 a~0 a~0 ~ surface of the particles, which explains the 39o0 ~0 Wavenumber(cm-1) high intensity of this band, since the colloidal particles have a high external Figure 4: IR spectra of sample $24C in the surface area. The other bands at lower OH-stretching region. (a) at 413 K (b) after wavenumbers are attributed to internal evacuation at 573 K (c) after evacuation at silanol groups [16] or tentatively to 673 K. Spectra are offset for clarity. asymmetric hydrogen bonded silanol on the external surface [ 15]. Upon heating to 673 K there is a substantial reduction in the intensity of the silanol bands, which indicates substantial dehydroxylation. Using adsorbed pyridine as probe molecule, Broensted as well as Lewis acidity could be detected in the samples (Fig. 5). The bands correspond to those typically observed for such types of acid sites, i.e. 1454 cm-1 (Lewis) 1491 cm"1 (Lewis and Broensted), 1543 cm 1 (Broensted), 1620 cm"1 (Lewis) and 1635 cm-1 03roensted). The intensity of the 1453 crn-1 .,... .4-

8

3-

.~ .2-

.1-

O1700

18'5o

16'oo

15'5o

I~'oo

~&0

14'00

Wavenumbers (cm- 1)

Figure 5: Spectra of pyridine adsorbed on $24C after different desorption temperature. From top: 413K, 473 K, 523 K, 593 K, 673 K. Spectra are offset for clarity.

~350

165 band is substantial and comparable to that observed for the commercial HZSM-5 reference sample, indicating that the external silanols have Broensted acidic character. Upon heating, most of the Broensted adsorbed pyridine is removed upon heating at 593 K, but a substantial reduction is observed already at appreciably lower temperature. In comparison, the commercial reference sample still had substantial amounts of pyridine adsorbed to Broensted acid sites after heating at 673 K and only at 723 K all pyridine was completely desorbed from the Broensted acid sites. Corresponding to the absence of an OH-vibration around 3610 crn1 the acid site strength detected by thermal desorption of adsorbed pyridine is thus substantially lower than for HZSM-5. The crosslinked colloidal MFI type material has thus a quite different acidity compared to HZSM-5 which should affect the catalytic properties. The altered acidity can be attributed to several factors. (i) the high external surface area and the correspondingly high concentration of terminal silanol groups will provide a high number of moderately acidic sites (ii) the apparent lack of aluminum incorporation to give rise to strong bridging hydroxyl groups and (iii) possibly Si-OH groups resulting from the silicone crosslinker which also might have differing acidity compared to regular silanol groups in zeolites or amorphous silica. 4. CONCLUSIONS Crosslinking of colloidal zeolite crystals with a silicone spacer provides an interesting possibility to create an additional pore system in such materials. With properly aged precursor solutions, in which colloidal particles with the MFI framework topology are present, a solid having both micro- and meso/macroporosity could be produced with the micropore volume corresponding to that expected for an MFI type zeolite. Such materials might be interesting for reactions, in which a high external surface area of zeolitic materials in combination with an additional pore system minimizing diffusion resistances is desired. However, several factors still need to be fine-tuned and better understood. (i) the question why no strong acid sites could be produced in these materials has to be addressed; (ii) the fraction of surface silanols which are used for crosslinking and those which are used for the catalytic reaction need to be optimized; (iii) the possibilities for tailoring the meso/macropore system by using silicone linkers with differently long polysiloxane backbone need to be explored; (iv) the reason for the non-complete reversibility of the sorption isotherm of samples aged for shorter times before crosslinking should be better understood. If these and related questions can be solved, the route could give access for materials with interesting properties. One could, for instance, use them even in the uncalcined form, if applications at low temperatures are envisaged. For a reaction to proceed on the external surface, template molecules in the pore system are not critical and need not be removed. In addition, the silicone molecules could provide a special environment, the properties of which could be tuned by different substituents on the silicon atoms. Catalytic tests of such and related materials are in progress, and results will be available at the time of the conference. ACKNOWLEDGEMENTS We would like to thank B. Spliethoff for recording the TEM. This work was partially supported by the EU (HPRN-CT- 1999-00025) which is gratefully acknowledged REFERENCES 1. B.J. Schoeman, J. Sterte, J.E. Otterstedt, J.Chem.Soc.Chem.Commun., (1993) 994. 2. S. Mintova, N.H. Olson, V. Valtchev, T. Bein, Science, 283 (1999) 958.

166 3. W.H. Dokter, H.F. van Garderen, T.P.M. Beelen, R.A. van Santen and W. Bras, Angew.Chem.Int.Ed.Engl.. 34 (1995) 73. 4. P.P.E.A. de Moor, T.P.M. Beelen, B.U. Komanschek, O. Diat and R.A. van Santen, J.Phys. Chem.B, 101 (1997) 11077. 5. R. Ravishankar, C.E.A. Kirschhock, P.P Knops-Gerrits, E.J.P. Feijen, P.J. Grobet, P. Vanoppen, F.C. De Schryver, G. Miehe, H. Fuess, B.J. Schoeman, P.A. Jacobs and J.A. Martens, J.Phys.Chem.B, 103 (1999) 4960. 6. C.E.A. Kirschhock, V. Buschmann, S. Kremer, R. Ravishankar, C.J.Y. Houssin, B.L. Mojet, R.A. van Santen, P.J. Grobet, P.A. Jacobs and J.A. Martens, Angew.Chem.Int.Ed.Engl., 40 (2001) 2637. 7. H. Ichihashi and H. Sato, Appl.Catal. A - General, 221 (2001) 359 with further references. 8. M.A. Camblor, A. Corma, H. Garcia, V. Semmer-Herledan and S. Valencia, J.Catal. 177 (1998) 267. 9. H. Kath, R. Gl~iser and J. Weitkamp, Chem.Eng.Technol., 24 (2001) 150. 10. For instance: A. Chica and A. Corma, J.Catal., 187 (1999) 167. 11. A.E. Persson, B.J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites, 15 (1995) 611. 12. R. Ravishankar, C. Kirschhock, B.J. Schoeman, D. De Vos, P.J. Grobet, P.A. Jacobs and J.A. Martens, in: M.M.J. Treaty, B.K. Marcus, M.E. Bisher, J.B. Higgins (eds.), Proceedings of the 12th International Zeolite Conference, MRS, Pennsylvania 1999, p. 1825. 13. R. Van Grieken, J.L. Sotelo, J.M. Menendez, and J.A. Melero, Micropor.Mesopor.Mater., 39 (2000) 135. 14. R.F. Howe, Stud.Surf.Sci.Catal., 102 (1996) 97. 15. G.P. Heitmann, G. Dahlhoff, and W.F. HOlderich, J.Catal. 186 (1999) 12. 16. M. Trombetta and G. Busca, J.Catal. 187 (1999) 521.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

167

Synthesis and characterization of MCM-22 zeolites for the N20 oxidation of benzene to phenol D. Meloni a, R. Monaci a, E. Rombi a, C. Guimon b , H. Martinez b, I. Fechete ~ and E. Dumitriu ~ a Dipartimento di Scienze Chimiche, Universit~ di Cagliari, Complesso Universitario di Monserrato, S.S. 554 Bivio per Sestu, 09042 Cagliari, Italy Tel.: +39 070 6754422; Fax: +39 070 6754388 b Laboratoire de Physico-Chimie Moltculaire, UMR (CNRS) 5624, Htlioparc Pau-Pyrtntes, 2 Av. P.Angot, 64000 Pau, France Laboratory of Catalysis, Technical University oflasi, 71 D. Mangeron Ave., 6600-Iasi, Romania MCM-22 zeolites with various Si/A1 and/or Si/Fe ratios were synthesized and characterized by XRD, microcalorimetry and other techniques. The catalytic activity and selectivity of MCM22 zeolites were investigated in the gas-phase oxidation of benzene with N20 at 673 K. The oxidation of benzene produces phenol as main product. The influence of the nature of catalysts (i.e., the content of iron, the nature of active sites) on catalytic activity of MCM-22 was investigated. In addition, some results about the nature of coke, which causes the Fe-MCM-22 deactivation, are exposed.

1. INTRODUCT!QN The one step oxidation of benzene to phenol with N20 seems to be an interesting altemative to the current commercial processes for the synthesis of phenol. Various materials have been tested as catalysts (mixed metal oxides, zeolites), but the most promising results were obtained when zeolites of MFI structure, like Fe-silicalite or H-ZSM-5 [1,2], have been used (selectivities greater than 90%, almost 100%). Until now the mechanism of benzene hydroxylation has not been elucidated. Panov et al. [3] proposed that a particular type of extra-framework iron is responsible for the hydroxylation reaction, whereas others proposed that Brtnsted acid sites [4] or the extra-framework aluminum playing the role of Lewis acid sites [5] are involved in this reaction. Recently, Juttu et al. [6] reported the catalytic activity of MCM-zeolites for the gas phase hydroxylation of benzene by dinitrogen monoxide (15% benzene conversion at 603 K and nearly 100% selectivity towards phenol), and they found that the presence of Br6nsted acidity is critical for this reaction, but there is no correlation of the activity with the iron content The last assertion agrees with [4], but it is contradictory to the results reported previously by others authors. Therefore, the aim of this work is to study the oxidation of benzene to phenol with N20 at 673 K and atmospheric pressure, over different samples of Fe-MCM-22 zeolite and to investigate the influence of iron content on their catalytic activity, as well as the influence of the acidity of samples. In addition, some results about the nature of coke, which causes the Fe-MCM22 deactivation, are exposed.

168 2. EXPERIMENTAL

2.1. Catalyst preparation MCM-22 zeolites were hydrothermally synthesized according to a slightly modified procedure given by Corma et al. [7]. Aerosil 200 (Degussa) and sodium aluminate (56 % A1203, 37,5% Na20, Carlo Erba) were used as Si and A1 sources, respectively, and hexamethyleneimine (99% HMI, Aldrich) as a template, while sodium hydroxide was used for the adjustment of pH. A typical gel composition, expressed as molar ratio of the oxides, was 2.7Na20-1.0A1203-30SiO215HMI-1340H20. After aged with stirring for 60 min at room temperature the resulting gel was transferred into Teflon-lined autoclave. The gel was crystallized at 423 K for 7 days under stirring (60 rpm). The crystalline product was washed thoroughly with deionized water until pH<9, and dried at 403 K overnight. Then, the solid was calcined in air at 823 K for 10 h with a temperature ramping rate of 2 K min ~, to remove the template and to perfect the formation of 3D structure. By varying the Al content in the gel, MCM-22 samples with different S i/Al atomic ratios were obtained (Table 1). Iron-containing MCM-22 zeolites were hydrothermally synthesized according to a slightly modified procedure given by Yashima et al. [8]. Na-form zeolites thus obtained were transformed into H-form by an ion exchange with 2 M NHaNO3 followed by calcination in air at 773 K for 3 h. Since MCM-22 is relatively difficult to be dealuminated [9], three procedures: acid reflux, steaming, and steaming followed by acid reflux, were adopted for the H-form MCM-22 to obtain dealuminated and/or deferrated MCM-22. The acid reflux was carried out using 6 M HNO3 at 358 K for 10 h, while the steaming were performed at 600~ for 8 to 16 h as described in Table 1. It could be noted that the samples treated only by steaming were subsequently washed with dilute acid solution (0.01 N HC1, 5 h) in order to remove the extra-framework aluminum and/or iron atoms.

2.2. Characterization The chemical composition was determined by elemental analysis (LIBERTY 200 ICP,

VA~AN).

X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1800 diffractometer using nickel-filtered CuK.~(1= 1.5406 A) radiation. The BET surface areas and pore size distributions of the prepared materials were determined from N2 adsorption isotherms at 77 K (Micromeritics ASAP 2100, sample activation at 523K for 18h). The main physicochemical properties of the catalysts are collected in Table 1. The acidity of samples was determined by microcalorimetry using NH3 as probe molecule. A Tian-Calvet heat flow calorimeter (Setaram) equipped with a volumetric vacuum line was used for microcalorimetric measurements [10]. Each sample (100 mg) was pretreated overnight at 673 K under vacuum (10 -3 Pa). Adsorption was carried out at 353 K by admitting successive doses of probe molecule and recording the thermal effect. The run was stopped at a final pressure of 133.3 Pa. After outgassing (10 -3 Pa) for 1 hour at 353 K, a second adsorption was carried out. From the difference between the two parallel adsorption isotherms, the concentration of irreversibly chemisorbed species was evaluated.

169 Table 1. Elemental analysis and textural properties of the MCM-22 catalysts. SA~ Vtot.b Vmicroc Sample A1 Fe Si/A1 Si/Fe Dealuminationprocedure (m2/g) (cm3/g) (cm3/g) % wt % wt steaming acid 559 0.43 0.20 FN1300 2.10 0.81 18.6 99.3 FN1301 1.49 0.69 26.9 459 0.34 0.16 119.1 358 0.37 0.13 FN1303 1.46 0.43 26.7 186.5 FN1304 1.40 0.35 28.5 235.2 321 0.24 0.11 546 0.30 0.19 FN1250dl 2.80 13.9 873 K, 8 h 0.43 0.18 FN1250d3 2.62 14.9 873K, 8h 358 I<2,10h 497 488 0.37 0.17 FNI250d4 2.32 18.2 358 K, 10 h 487 0.41 0.17 10.3 FN1254d5 7.35 873 K, 8 h 432 0.56 0.15 27.7 FN1254d6 2.72 358K, 10h 463 0.51 0.16 19.8 FN1254d7 3.81 873K, 8h 358K, 10h "BET technique; %alues measured at P/Po=0.998; cvalues obtained by Horvath-Kawazoeequation. 2.3. Catalyst testing The catalytic runs were performed at atmospheric pressure in a continuous fixed-bed quartzglass microreactor at 673 K. Prior to reaction, the catalyst was activated under air flow at 773 K for 5h, followed by cooling to the reaction temperature under helium flow. A pre-heated feed of helium containing the reagents at benzene/N20 = 1/4 molar feeding ratio was continuously passed through the catalyst, contact time x = 0.5 s. Obviously the partial oxidation of benzene by N20 is carried out at contact time ranging from 2 to 4 s [2,3], but it was chosen this small contact time in order to assure the differential running of the reactor (small conversions, below 5%). Samples of the effluent were collected in Pyrex glass trap and condensed at 233 K through a cryostat and liquid products were analyzed by a FID HP-6890 gas-chromatograph, equipped with a fused-silica capillary column SUPELCO Petrocol DH 0.25 mm O.D. and 50 m long. GC-MS identification of the various products was performed on a HP-5989A apparatus. The chemical composition of coke was determined by dissolution at room temperature of the coked zeolite samples with a solution of HF (40%), recovery of the soluble part of the carbonaceous compounds in methylene chloride, and analysis through GC-MS. The procedure is reported in detail elsewhere [11 ]. 3. R E S U L T S AND D I S C U S S I O N 3.1. Characterization of catalysts The structure of MCM-22 has been shown to consist of layers linked together along the c-axis by oxygen bridges and contains two independent pore systems [12]. Within the layers are twodimensional sinusoidal 10-M ring channels, and between two adjacent layers are 12-M ring supercages (-~ 0.71 x 0.71 x 1.82 nm) communicating with each other through 10-M ring apertures. The unusual structure of zeolite MCM-22 is formed from a layered precursor designated as an MCM-22(P) [13], which is able to condensate the silanol groups present on the layer surfaces by calcination, and leading to the formation of the 3D structure. Several MCM-22(P) samples were synthesized with various SIO2/A1203 and/or Si/Fe ratios (Table 1). The diffraction patterns of these materials agree well with those previously reported [13] (Figures 1 A a n d B).

170

t...i... i ,! l

0.0

10.0

20.0 30.0 2 theta

40.0

50.0

0.0

1A

I 10.0

, tfl.i!,

I 20.0

I 30.0 2theta

I 40.0

50.0

1B

Figure 1. Powder XRD patterns of the (a) as-synthesized sample and (b) the calcined sample (FNI 250, Si/AI = 15.0*) (A) and (FN1254, Si/Fe = 15*) (B). (*gel mixture) Results of the three procedures of dealumination over two different samples of H(A1,Fe)MCM-22 are also reported in Table 1. As it could be seen the reflux treatment on MCM22 with 6 M HNO3 at 363 K for 8 h caused only slight dealumination, and these results are in agreement with those previously reported [9]. However, a combination of steaming at 873 K with the subsequent acid reflux increased the Si/A1 and Si/Fe ratios [14]. Surface area and micropore volume for all the dealuminated MCM-22 samples did not change significantly from those of the parents. Therefore, it could be concluded that the microporous structure of MCM-22 was retained fully after the dealumination/deferration treatments mentioned above. In order to obtain information concerning acidity before/after the dealumination, the total concentration and the acid strength distribution of active sites were determined by microcalorimetry using NH3 as probe molecule (Table 2). The acid strength was considered as follows: strong acid sites (ns) correspond to Qd~r. > 150 kJ/mol, medium (nm) to 150 < Qd~r. < 120 kJ/mol), and weak acid sites (nw) correspond to 120 < Qd~r. < 70 kJ/mol, respectively. The total concentration of acid sites is designed as (nT). Table 2. Acidity of the MCM-22 samples at 3 53 K. 'Sample r~ (lamol/g) FN1300 10 FN1303 31 FN1304 25 FN1250 dl 68 FN1250 d4 55 FN1254 d6 11 FN1254 d7 15

nm (lamol/g) 112 64 68 96 252 21 35

nw (lamol/g) 1178 316 452 872 1127 338 252

nT (~mol/g) 1300 411 545 1036 1434 370 302

171 It could be observed a significant decrease of the acidity from FNI 300 sample to FNI 303304 samples, in agreement with the increase in the Si/A1 ratio. An increase in the acid strength and the total acidity can be observed when the samples were dealuminated. Calorimetric measurements carried out on the FNI d6 and FNI d7 samples, show that their acidity is almost the same, indicating that MCM-22 is practically not influenced by treatments to obtain deferrated form.

3.2. Catalytic activity Activity comparison runs were performed on fresh catalyst samples at the previously mentioned reaction conditions. It was chosen the differential mode of operation for the catalytic reactor from the following reasons: (i) this mode of operation allows a better comparison of the catalyst samples, and (ii) the deactivation rate of catalysts by coke formation is slower. Indeed, under the differential operating conditions of the reactor, the catalytic results show that the conversion of benzene is very low for all the samples (< 5%). The conversion decreases with time-on-stream (t.o.s.), whereas the selectivity to phenol increases until 95%. The results are not shown here. The catalytic activity of MCM-22 catalysts for the benzene hydroxylation by N20 depends on the iron content 0~igure 2). These results are in agreement with the previous findings on (A1,Fe)ZSM-5 [2,3]. Moreover, it could be observed from the same figure that the catalytic activity is shifted toward small iron content, which is also in agreement with the previous reports [2,3]. Higher Fe contents mainly favour the complete oxidation at the expense of the phenol formation. Some authors [2,15], assert that the selectivity towards phenol could to be correlated with extraframork ferrous sites, which would be able to provide nucleophilic oxygen species responsible for the partial oxidation. The experiments carried out on such treated samples proved that the FN1250 dl, d3 and d4 zeolites, without iron, are inactive. The presence of the iron is necessary to have catalytic activity, as it was demonstrated by sample FNI 254 d7. Moreover, calorimetric measurements show that the acidity of the samples seemed not to influence the catalytic activity. 0,5 t~ 0,4 "5 0,3 t,-

"13. 0,2 0

E 0,1 E 0

0,35

0,43

0,69

0,81

%Fe

Figure 2. Amount of phenol formed vs. the iron content.

172

3.3. Catalyst deactivation Coke formation was investigated during benzene oxidation at 673 K over (A1,Fe)MCM-22 samples (FN1300-304). The amount of coke deposited over the zeolite FN1304 (Si/AI=30) was determined for different time on-stream values and for x equal to 0.5 s (Table 3), through dissolution of the zeolite in hydrofluoric acid and recovering of coke deposit in two parts: one soluble and the other insoluble in methylene chloride (CH2C12). Table 3. Coke composition of the FN1304 sample at different times on-stream.

.

t.o.s (min)

15

ct~. (wt.%)

4.9

5.2

C~d.(wt.%) C~ot (wt.%)

3.3 1.6

0.5 4.7

.

.

.

30

.

240 5.3 0.3 5.0

The solubility of coke in methylene chroride (C,o0 decreases with increasing the time onstream (hence with the increase in coke content). Initially, all the coke is soluble in CH2C12, whereas after 30 minutes on-stream, the main part of coke is insoluble (Ci~ol) in the CH2C12, hence constitued of very polyaromatic compounds (Figure 3). The maximum observed in the percentage of soluble coke and the secondary formation of insoluble coke indicate that insoluble coke results, as it is demonstrated with many zeolite catalysts [16] from transformation of soluble coke. Probably, the formation of insoluble coke results from the growth of coke molecules located in cages near the outer surface of the cristallites or from a dehydrogenative coupling of coke molecules located in the supercages [ 16]. This second mode could be predominant because of the proximity (3.1/~) of the supercages along the oblique channels. The composition of soluble coke was established from GC and GC-MS analysis. The coke components can be classified into six (A-F, Table 4) families with CnH2n-zand C~-I2r~O= as the general formula. Main constituents of soluble coke belong to the CNH2,-12(B) family, C,H2,-160 (D) and Cnn2n-180 (E) families. 25

0 Soluble

20

O

coke

="Insoluble coke

10

o

5

0

1

2

3

4

5

6

7

8

Total coke (wt.%)

Figure 3. Change in the amount of soluble and insoluble coke vs. the total coke content.

173 Table 4. Main components of the soluble coke for FN1304. Famiry General Coke molecules Developed Formulae Formulae C~2.-60

Phenol

CnH2n-12

methylNaphthalene x = 1-2

CnH2n-14

C.I-I2..160 C.H2.-180

CnH2n-1802

OH

6

~

'H3)x

(CH3)

Biphenyl x =0-1 Dibenzofuran O

9H-Fluoren-9-one

O

Xanthone

The nature of the coke shows that both benzene and phenol can undergo carbonaceous compounds (coke). According to Gate et al. [17] dehydrogenative coupling would occur on acid catalysts, as MCM-22 zeolite, through a mechanism with benzene as the reactant, scheme I. D and E compounds, formed directly through the N20 oxidation of benzene to phenol, are transformed into compounds of family F, scheme IL which is transformed into insoluble coke.

C

9

"~

Insoluble coke

OH

Scheme I

0

~

\o/ Insoluble coke

F

Scheme II

174 4. CONCLUSIONS The best results were obtained with the Fe-containing MCM-22 zeolites. Therefore, the catalytic activity for benzene hydroxylation could be correlated with ferrous sites. The experiments carried out on the dealuminated/deferrated samples shown that the acidity is not responsible for activity, but they are sometimes involved in the coke formation. The coke is constituted by polyaromatics hydrocarbons and/or polycondensated oxygenated compounds. ACKNOWLEDGEMENTS

The financial support ofMURST (PRIN 2000) is gratefully acknowledged. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

E. Suzuki, K. Nakashiro and Y. Ono, Chem. Lett., 6 (1988) 953. G.I. Panov, V.I. Sobolev and A.S. Kharitonov, J. Mol. Catal., 61 (1990) 85. V.I. Sobolev, A.S. Kharitonov, Y.A. Paukshitsand G.I. Panov, J. Mol. Catal., 84 (1993) 117. R. Burch and C, Howitt, Appl. Catal. A: Gen., 103 (1993) 135. J.L. Motz, H. Heinichen and W.F. H61derich, J. Appl. Catal. A: Cchem., 136 (1998) 175. G. Juttu and R. Lobo, 13th International Zeolite Conference, session 27 (27-R-02), Montpellier, July 8-13 (2001). A. Corma, C. Corell, M. Camblor, J. P6rez-Pariente, J.M. Guil, R. Guil-L6pez, S. Nicolopoulos, J. Gonz~lez Calvet and M. Vallet-Regi, Zeolites 16 (1996) 8. P. Wu, H. Lin, T. Komatsu and T. Yashima, Chem. Commun., (1997) 663. S. Unverricht, M. Hunger, S. Ernst, H.G. Karger and J. Weitkamp, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 84 (1994) 3 7; P. Wu, T. Komatsu and T. Yashima, Micropor. Mesopor. Mater., 22 (1998) 343. V. Solinas and I. Ferino, Catal. Today, 41 (1998) 179. P. Magnoux, P. Roger, C. Canaff, V. Fouch6, N.S. Gnep, M. Guisnet, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 34 (1987) 317. M.E. Leonowicz, J.A. Lawton, S.L. Lawton and M.K. Rubin, Science, 264 (1994) 1910; S.L. Lawton, M.E. Leonowicz, R.D. Partidge, P. Chu, M.K. Rubin, Microp. Mesopor. Mater. 23 (1998) 109. M.K. Rubin and P. Chu, US Patent No. 4 954 325 (1990). P.Wu, T. Komatsu and T. Yashima, Micropor. Mesopor. Mater. 22 (1998) 343. R. Monaci, E. Rombi, M.G. Cutrufello, V. Solinas, G. Bedier and G. Spoto, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 130 (2000) 1679. M. Guisnet, P. Magnoux and D. Martin, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 111 (1997) 1. B.C. Gates, J.R. Katzer and G.C.A. Schuit, in "Chemistry of Catalytic Processes", Chemical Engineering Series, McGraw-Hill, New York, 1979.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

175

Novel solid strong base derived from zeolite supported CaO* X.W. Han, G. Xie, Y. Chun, X.W. Yan, Y. Wang, J. Xue and J.H. Zhu** Department of Chemistry, Nanjing University, Nanjing 210093, China

Calcium oxide could be dispersed on zeolite NaY or NaZSM-5 to form strong basic sites on the host by the use of microwave radiation. After activation at 873 K, some CaO/NaY and CaO/NaZSM-5 composites possess an extraordinary high basic strength with H_ of 27. Moreover, these strong basic materials had a high water-stability.

1. I N T R O D U C T I O N Strong basic zeolites are extremely desirable for developing shape-selective basic catalysis in mild conditions. However, the strong basic sites, especial those with a basic strength (/-/_) of 26.5, are difficult to create on zeolites. It seems impossible to generate these sites through adjusting the chemical composition of zeolite framework, because of the lack of strong basicity on the zeolite X even when it was exchanged with Cs ion [ 1]. Therefore introducing a basic guest into zeolite host became the practical way. Unfortunately, the strong basicity generated on zeolite by deposition of alkali metals vapors or loading alkali metal compounds such as K(NH3) 2 or KNO 3 [2-3] would lose soon in the catalytic processes involving water in reactants or products. To avoid this disadvantage, MgO was thus dispersed on zeolite by microwave radiation in order to prepare strong basic zeolites that are stable in water [4], due to its insolubility in water. Although microwave radiation has been used to disperse metal oxides such as zirconia on zeolite to form functional catalyst [5], the resulting basic strength (/-/_) of MgO/NaY or MgO/KL was about 22 instead of 26.5, regardless of how the amount of basic guest has changed from 5% to 20%. Suspicion thus exists if the super basic materials can be prepared through the use of microwave radiation, which prompts us to pursue this research further. Consult the report that calcium oxide (CaO) could possess an unusually strong basicity after it is evacuated at a high temperature [6], we try to load CaO onto zeolite by the use of *National Natural Science Foundation of China under Grant 29773020, Shanghai petrochemical company and Analysis Center of Nanjing University financially support this subject. ** Corresponding author, E-mail: [email protected], FAX: 0086-25-3317761.

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Time of microwave radiation (min)

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2'0 Amount of CaO loaded (wt.-%)

Figure 1. The influence of (A) time of microwave radiation and (B) amount of CaO loaded on the dispersion of CaO onto zeolite NaY and NaZSM-5.

microwave radiation, in order to obtain the new zeolite-like material with a high basic strength along with water-resistance. 2. E X P E R I M E N T A L Zeolite NaY (Si/A1 = 2.86) with surface area of 766mR/g and NaZSM-5 (Si/A1 = 23) with surface area of 354 m2/g were used for the host material. CaO was purchased from Shanghai Chemical Agent Company and calcined at 773 K before use. To disperse CaO on zeolite, the CaO was first ground along with the support at a given weight ratio for about 20 minutes, then radiated in a non-modified domestic microwave oven with a frequency of 2.45 GHz and output power of 0.8 kW for 5-20 minutes. For comparison the other samples were impregnated with K2CO3 or KOH solution [ 1]. X-ray diffraction was used to examine the dispersion of CaO and the X-ray tube was operated at 40 kV and 100 mA as described previously [3]. The XRD peak intensity ratio of CaO (d = 2.62) to the strongest peak in the XRD pattern of the parent zeolite, e.g., the NaY ( l l l ) ( d = 14.4), was denoted as factor R and used to characterize the proportion of the residual bulk CaO in the sample. Adsorption isotherms of n-hexane on CaO/NaY or CaO/NaZSM-5 were performed at 298 K after the adsorbent was activated at 873 K. To evaluate the basicity o f samples, titration combined CO2-TPD and decomposition of 2propanol was employed [7]. The temperature for activation of sample in CO2-TPD and the probe reaction was 873K, and blank TPD was carried out from room temperature to 873K confirming there was no any desorption occurred prior to the adsorption of CO2. Catalytic decomposition of 2-propanol was carried out using a conventional flow reactor at atmospheric pressure with a WHSV of 2.40 h -1 [3]. Basic strength of these samples was measured by use of a Hammett indicator [8]; samples were first pretreated at 873 K in a N2 (99.99%) flow, then transferred in cyclohexane under a dry N2 atmosphere and tested in the process as that reported by Kikuchi and Yoneda [8]. For assessing the water stability of solid base, sample was washed

177

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20 Figure 2. The XRD patterns of (a) NaY, (b) 20%K2CO3/NaY, (c) sample B calcined at 873 K, (d) 20%CaO/NaY, (e) sample D calcined at 873 K.

by distilled water and filtered until the filter liquor was proven to be neutral in titration [4], then the residual basicity of sample was titrated with a standard HC1 solution [3]. 3. RESULTS AND DISCUSSION Figure 1 shows the dispersion of CaO on the zeolite host under microwave radiation. As the radiation time increased from 0 to 20 min, the XRD peak intensity ratio of CaO to zeolite NaY decreased from 0.32 to 0.23 on the spectra of 20%CaO/NaY. This means the dispersion of CaO on zeolite because in the microwave irradiation where the electric field applies a force on charged particles as a result of which the charged particles start to migrate or rotate [9]. Prolong the radiation time to 40 min only decreases the ratio to 0.22 (Fig. 1A), which means that dispersion of CaO, no more than 20 wt.-% on NaY could come to a balance nearly by irradiation of microwave within 20 min. One might argue that the formation of CaNaY through cation exchange could not be excluded in these experiments, but this formation, if existed, seemed to have a negligible influence on the XRD peak strength of CaO, because the XRD peak intensity ratio of CaO to zeolite only changed from 0.23 to 0.22 in the case of prolonging the radiation time from 20 to 40 minutes. Otherwise, the ratio should be significantly lowered since the crystallinity of NaY zeolite kept unchanged in such radiation conditions [10]. Both 5%CaO/NaY and 10%CaO/NaY samples had identical XRD patterns as the parent zeolite after an irradiation of 20 minutes, indicating that the loading of CaO up to 10 wt.-% did not caus e detectable damage in the structure of zeolite and all the guests were dispersed. As the amount of CaO exceeds 10 wt.-% however, the characteristic XRD peak of CaO began to appear, and its intensity increased with the increasing amount of CaO loaded (Fig. 1B). Based on these results, it is very likely that CaO can be dispersed on zeolite NaY by microwave radiation, and the dispersion threshold is around 10 wt.-%. Similar results were

178 also observed on CaO/NaZSM-5 and the dispersion threshold was found to be about 7 wt.-% (Fig. 1B). Figure 2 reveals the effect of calcination on the XRD patterns of zeolite NaY loaded CaO or K2CO3. No considerable distortion was found in the bulk structure of 20%CaO/NaY, even when the sample was calcined at 873 K for 2h. In contrary, the sample of 20%K2COJNaY became amorphous after calcination at 873 K, probably resulting from the interaction between alkali ions and the framework of zeolite at high temperature. In the adsorption of n-hexane at 298 K, the amount of adsorbate on CaO/NaY sample decreased as the loading amount of CaO increased, as shown in Figure 3. This fact indicates that CaO dispersed on the zeolite and decreased the pore volume. However, the adsorption isotherm on CaO/NaY sample still kept its original Langmuir type as that on zeolite NaY, demonstrating the existence of the uniform channels in the composite that is very important for selective catalysis or adsorption. Similar results were obtained on CaO/NaZSM-5 sample on

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Table 1. Basicity and basic strength of CaO/NaY samples

....

~

,

Sample

,,

Amount of CaO Basicity (meq/g) Basic strength (/-/) (mmol/g) A 1) B 2) C 3) B/A B/C 5%CaO/NaY 0.89 1.79 1.63 1.50 0.92 1.08 18.4 10%CaO/NaY 1.78 3.57 2.74 1.26 0.77 2.17 27.0 20%CaO/NaY 3.56 7.14 5.85 1.32 0.82 4.43 27.0 5%CaO/NaZSM-5 0.89 1.79 1.51 1.39 0.84 1.09 22.5 10%CaO/NaZSM-5 1.78 3.56 2.78 1.26 0.78 2.21 27.0 1) This was calculated value based on one CaO molecule reacted with two HC1 molecules. 2) This was tested by titration method and the sample was activated at 873 K. 3) This was measured by use of CO2-TPD method.

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Figure 4. The remained basicity of modified zeolite v s . the amount of water passed. (a) 20%CaO/NaY, (b) 20%KOH/NaY, (c) 20%KRCO3/NaY, (d) 5%CaO/NaZSM-5, (e) 10%Na2CO3/NaZSM-5 and (f) 10%KOH/NaZSM-5.

which the adsorption isotherm of n-hexane also kept the Langmuir type even though the adsorption amount was decreased, in comparison with that of the host (Fig. 3). Table 1 lists the basicity of samples evaluated by titration. Loading CaO of 5 wt.-% on zeolite NaY increased the basicity from 0.14 meq/g to 1.63 meq/g, close to the calculated value of 1.79 meq/g and proven the efficiency of generating basicity by dispersion of CaO. As expected, the sample loaded with CaO of 10 wt.-% or 20 wt.-% exhibited a larger basicity, and the ratio of evaluated basicity to the calculated value was around 0.8, obviously larger than the ratio observed on MgO/NaY (lower than 0.7) [ 10]. In the case of loading same amount of guest material (10 wt.-%), CaO could generate a basicity on the host (2.74 mmol/g), slightly larger than that of K2CO3 (2.36 mmol/g) but close to that of KOH (2.89 mmol/g). Moreover, about 71% of the basicity remained on the sample of 10%CaO/NaY after calcination at 873 K, mirroring the XRD results demonstrated in Figure 2. In contrary, only 11% of the original basicity remained on 10%K2CO3/NaY and 46% on 10%KOH/NaY samples under the same conditions, probably due to the evaporation or sublimation of alkali metal oxide species at high temperature. In addition, 20%CaO/NaY had a surface area of 491 m2/g, noticeably larger than that of NaY supported CsAc (460 mZ/g)[11]. Figure 4 demonstrates the water-resistance of the basic sample derived from zeolite NaY. Dispersing 20 wt.-% CaO on the zeolite increased the basicity from 0.14 meq/g to 5.85 meq/g, close to that of 20%KOH/NaY (5.43 meq/g) or 20%K2CO3/NaY (3.40 meq/g). However, when the water of 600 ml/g passed, 20%CaO/NaY sample kept 82% of the basicity but about 52% or 42% remained on 20%KOH/NaY or 20%K2CO3/NaY. Clearly CaO/NaY composite has a higher water-stability than the other two samples, resulting from the extremely low solubility of CaO in water. As a result, CaO/NaY can be used in the catalytic processes involving water in reactants or products. Figure 4 also shows the water-stability of CaO modified zeolite NaZSM-5. About 92% of the basicity could be remained on 5%CaO/NaZSM-5 after the water

180

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Figure 5. CO2-TPD profiles of (a) 20%CaO/NaY; (b) 10%CaO/NaY; (c) 5%CaO/NaY; (d) mixture of CaO to NaY (1:4 w/w), (e) CaO and (f) NaY zeolite.

of 700 ml/g passed, meanwhile 65% survived on 10%Na2CO3/NaZSM-5 and 56% on 10%KOH/NaZSM-5 sample (see the curve d-f in Fig.4). The water resistance of solid bases derived from NaZSM-5 seems higher than that of those from NaY. One of the reasons might be the difference in pore structure including the pore size and zigzag of the channel, and the hydrophobic surface of ZSM-5 zeolite, which hinders the water from accessing the basic site. Figure 5 shows the CO2-TPD spectra of zeolite NaY and the basic derivatives. No CO2 was desorbed from NaY above 373 K during the process, but on the mixture sample of NaY and CaO, CO2 desorption was observed at 654 K and 822 K, because CO2 adsorb on CaO and desorbed at 654 K, 755 K and 822K. Desorption of CO2 from CaO (0.54 meq/g) and the mixture (0.48 meq/g) was similar though the amount of CaO in the two samples was different. CaO might disperse on the support when the mixture sample is heated at 873 K so that the surface area of CaO is extended and more CO2 thus adsorbed. CO2 desorbed from 5%CaO/NaY sample at 347 K, 508 K and 799 K with the amount of 1.50 meq/g, more than that from either CaO (0.54 meq/g) or the mixture sample (0.48 meq/g). No doubt the porous structure and large surface area of zeolite are beneficial for dispersion of CaO, therefore more basic sites are generated on the composite. Desorption of CO2 on 10%CaO/NaY occurred at 355 K, 508 K and 848 K, but the amount was slightly decreased (1.26 meq/g). There were three peaks on the CO2-TPD spectrum, the third emerged at around 848 K instead of 799 K and its area was clearly increased. Loading more CaO on zeolite did not cause significant change on the CO2-TPD spectrum of 20%CaO/NaY. The amount of CO2 desorbed was slightly increased tol.32 meq/g, but the third peak of CO2-TPD was shifted around 790 K with a shoulder at 745 K, close to that on CaO sample, coinciding with the XRD results that revealed existence of residual phase of CaO on the 20%CaO/NaY sample. As evident in Table 1, the basicity measured by titration is not the same as that measured by

181

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Reactiontemperature(K)

Figure 6. Yield of acetone formed on (a) NaY, (b) 5%CaO/NaY, (c) 20%CaO/NaY and (d) 10%CaO/NaY, (e) CaO, (f) NaZSM-5, (g) 5%CaO/NaZSM-5 and (h) 10%CaO/NaZSM-5 samples in the catalytic decomposition of 2-propanol via reaction temperature.

CO2-TPD, and the more CaO are dispersed on zeolite NaY, the more different it will be. Such difference was found on KNO3/A1203 samples [12], and attributed to the formation of overlapped structure of basic species. In titration, the surface basic species could be taken away by the acid solution; therefore the inside was exposed to react with acid. During the CO2TPDprocess, however, CO2 did not reach inside basic species and only could be caught by superficial ones, so that the basicity measured will be different even on the same sample. The basicity of 5%CaO/NaY measured by titration (1.63 meq/g) was close to that tested by CO2TPD (1.50 meq/g), meaning the well dispersion of CaO in the sample after microwave radiation and the activation at 873 K and coinciding with the XRD result. For the sample of 10%CaO/NaY and 20%CaO/NaY, the basicity measured by titration and CO2-TPD methods was rather different, indicating the formation of overlapped structure of basic species on the two samples. However, 5%CaO/NaY sample possessed a strength (H_) of 18.4 (Table 1), but 10%CaO/NaY and 20%CaO/NaY exhibited the basic strength (H) of 27.0 that is characteristic of super basic material [ 13]. Existence of super basic sites seems to relate with the formation of overlapped structure of basic species, because only those samples, loaded guest material with the amount near or slight over the dispersion threshold, exhibited the unusual high basic strength [1,3,7, 12]. For instance only the 10%CaO/NaZSM-5 possessed the basic strength (/-/_) of 27 (Table 1) meanwhile the sample of 5%CaO/NaZSM-5 or 7%CaO/NaZSM-5 showed a basic strength (/-/_) of 22.5, though the dispersion threshold of CaO was about 7 wt.-%. Figure 6 shows the yield of acetone formed on zeolite NaY, NaZSM-5 and their basic derivatives in decomposition of 2-propanol. Since basic site was involved for catalysis of 2propanol dehydrogenation [14-15], acetone yield in the reaction products could be used to characterize the basic catalytic ability of the sample [1,3]. No acetone was detected in the

182 products on zeolite NaY due to lack of basic catalytic center. Loading CaO of 5 wt.-% on NaY provoked the appearance of acetone as shown in Fig. 6 and the yield was enlarged as the loading amount rose from 5% to 10 wt.-%. Further addition of CaO on NaY slightly increased acetone yields below 600 K, but decreased the yield above 623 K. For the basic zeolite NaZSM-5, acetone formed in product and more than that on CaO. This is not unusual because weak and moderate basic sites are active for the dehydrogenation of 2-propanol [14-16]. Loading CaO dramatically enhanced acetone yield on zeolite NaZSM-5 above 643 K, but CaO/NaZSM-5 also showed a considerable activity for dehydration of 2-propanol, for instance the water yield on 10%CaO/NaZSM-5 was 32.4% at 673 K though the sample possessed the high basic strength (H) of 27. Different from the increase of acidity upon exchange of Na + by Ca R+,which is a well-known phenomenon in zeolite by simple cation exchange in solution and attributed to hydrolysis of one of the bonds with reformation of H § the observed acidity of CaO/NaZSM-5 was decreased implying no new acid sites formed but only part of the original ones remained. Existence of both acid and basic sites on the zeolite supported CaO might result from the especial preparation process using microwave radiation. Unlike impregnation in which aqueous basic solution exteriorly immersed the host and suppressed the acid sites on zeolite, in the preparation of microwave radiation the basic guest was mixed with the host in the style of solid phase, which is not dispersed as uniformly as that of the basic solution. Consequently, these particles of guest were dispersed or migrated in a limited area instead of the total surface during the radiation of microwave. As a result, basic sites exist in some area of the host meanwhile acid sites remained in another area, forming the functional materials useful for the reactions requiring cooperation of acid and basic active centers. REFERENCES 1. J.H. Zhu, Y. Chun, Y. Wang and Q.H. Xu, Chin. Sci. Bull., 44(1999) 1926. 2. T. Baba, H. Handa and Y. Ono, J.C.S. Faraday Trans., 90(1994), 187. 3. J. H. Zhu, Y. Chun, Y. Wang and Q.H. Xu, Catal. Today, 51 (1999) 103. 4. J.H. Zhu, Y. Wang, Y. Chun, Z. Xing and Q.H. Xu, Mater. Lett., 35(1998) 177. 5. J.H. Zhu, J.R. Xia, Y. Wang, G. Xie, J. Xue and Y. Chun, Stud. Surf. Sci. Catal., 135(2001) 320. 6. T. Iizuka, Y. Endo, H. Hattori and K. Tanabe, Chem. Lett., (1976) 803. 7. Y. Wang, W.Y. Huang, Y. Chun, J.R. Xia and J.H. Zhu, Chem. Mater., 13(2001) 670. 8. N. Kikuchi and Y. Yoneda, J. Catal., 21 (1971) 164. 9. S.A. Galema, Chem. Soc. Rev., 26(1997) 233. 10. Y. Wang, J.H. Zhu, Y. Chun and Q.H. Xu, Micropr. Mesopr. Mater., 26(1998) 175. 11. F. Yagi F, H. Kanuka, H. Tsuji and H. Hottori, Stud. Surf. Sci. Catal., 90(1994) 349. 12. J.H. Zhu, Y. Wang, Y. Chun and X.S. Wang, J.C.S. Faraday Trans., 94(1998) 1163. 13. K. Tanabe and R. Noyori, Chokyo-san, Chokyo-enki, Kodansha, Tokyo, 1980, 114. 14. H. Pines and W.O. Haag, J. Am. Chem. Soc., 82(1960) 2471. 15. H. Noller and G. Ritter, J.C.S. Faraday Trans., 80(1984) 275.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

183

Z S M - 5 spheres prepared by resin templating Lubomira Tosheva and Johan Sterte

Division of Chemical Technology, Lule& University of Technology, S-971 87 Lulegt, Sweden, luto @km. luth.se ZSM-5 spherical macrostructures were prepared using macroporous anion exchange resin beads as shape-directing templates. Resin particles and ZSM-5 clear synthesis solutions were mixed in autoclaves and treated at 170~ for different times. After the synthesis the resinzeolite composite beads obtained were separated from the zeolite crystallized in the bulk solution, dried and calcined. The resin as well as the structure-directing agent used for the zeolite synthesis were removed during the calcination leaving highly porous ZSM-5 spherical particles. The influence of different parameters such as treatment time, aging, aluminum content and synthesis solution to resin weight ratio on the properties of the product particles was investigated by X-ray diffraction, scanning electron microscopy and nitrogen adsorption measurements.

1. INTRODUCTION Porous materials are widely used in areas such as catalysis, separation and purification, sorption and chromatography. A common approach to prepare porous materials is to use templates, which, upon removal, determine the pore structure of the products. At present, novel techniques are being developed to synthesize materials of controlled porosity and, preferably, of controlled macro shape. Macroporous monoliths of silicalite-1 have been synthesized by the layer-by-layer method by organizing silicalite-polymer multilayers around latex beads followed by an assembly of the particles into macroscopic structures and removal of the organic components [1]. Hollow zeolite spheres have also been fabricated by this method [2]. Large monolithic zeolite foams have been synthesized using polyurethane foams as a template [3]. Preformed zeolite nanocrystals have been employed to form a variety of ordered porous structures, e.g., zeolite fibers using bacterial threads or a glass capillary as templates [4,5]. Mesoporous single crystals of ZSM-5 and TS-1 have been obtained by growing the crystals around carbon black template particles [6,7]. Recently, a novel approach for the preparation of spherical macrostructures employing anion exchange resin beads as templates was reported. Amorphous silica as well as silicalite-1 and zeolite beta macrostructures have been prepared by the method [8-10]. Generally, the method consists of two steps: (i) synthesis of silica/zeolite within the resin beads followed by (ii) removal of the resin and the zeolite structure-directing agent by calcination. The first step is facilitated by the ion exchange capacity of the resin and the presence of negatively charged

184

anionic species in the synthesis solutions employed. The zeolite crystallization within macroporous resins is a complex process that differs from the zeolite bulk crystallization [ 10]. Despite the general similarities of regarding the influence of various synthesis parameters on the properties of different zeolite spheres, separate experiments are needed for each zeolite system in order to determine the property limits of the particular zeolite spheres. The present work describes the preparation and properties of ZSM-5 spheres.

2. EXPERIMENTAL SECTION Clear ZSM-5 synthesis solutions with the molar composition 0.31 Na20 : 9TPAOH : xA1203 (x=0, 0.25 or 0.5) : 25 SiO2 : 405H20 were prepared from silica sol (Bindzil 30/220, freeze dried prior to its use, 96.1 wt.% SIO2, 1.23 wt.% Na20, Eka Chemicals AB), aluminum isopropylate (Sigma) and tetrapropylammonium hydroxide (TPAOH) (20 wt.% aqueous solution, Merck). The synthesis solutions were prepared by mixing the silica and alumina solutions obtained after dissolving the silica sol and the aluminum isopropylate in portions of the TPAOH solution. The synthesis solutions were added to resin beads (Dowex MSA-1, Sigma) in PTFE-lined stainless steel autoclaves at solution to resin weight ratios between 1 and 10 and the mixtures were treated at 170~ for different times. In a series of experiments, the mixtures were aged in the autoclaves for 24 h at room temperature before the hydrothermal treatment. After the synthesis, the resin-zeolite composite was separated from the zeolite crystallized in the bulk by decanting, treated in a 0.1 M ammonia solution in an ultrasonic bath for 5 min, rinsed repeatedly with distilled water and dried at room temperature. Finally, the resin and the TPA structure-directing agent were removed by calcination at 600~ for 5 h after reaching this temperature at a rate of 1~ min -1. The calcined samples were characterized by X-ray diffraction (XRD, Siemens D5000 diffractometer, Cu Ka radiation), scanning electron microscopy (SEM, Philips XL 30) and nitrogen adsorption measurements (Micromeritics ASAP 2010). Before the XRD analysis the samples were ground into a powder. The degree of crystallinity (%) was calculated from the area of the peaks in the range 22-25 ~ 20 using the most crystalline sample as a reference. The samples were slowly scanned over the area selected using identical conditions. Samples were outgassed at 300~ overnight prior to the nitrogen adsorption measurements. Specific surface areas were determined by the BET equation and pore size distributions were calculated by the BJH method using the desorption branch of the isotherms.

3. RESULTS AND DISCUSSION Fig. 1a shows XRD patterns of calcined samples prepared for treatment times between 6 and 24 h using a synthesis solution to resin ratio of 10 and a SIO2/A1203 ratio of 100. The crystallinity of the samples increased with an increase in the time of treatment and the appearance of the spherical particles changed. The spheres containing a certain amount of amorphous material prepared for treatment times of up to 10 h were solid and hard. Those spheres were identical in shape and size to the original resin beads (Fig. l b) or with similar size but differing appearance (Fig. lc). The highly crystalline samples prepared for treatment

185

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d

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20

30 20/degrees

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Figure 1. XRD patterns of calcined spheres prepared for different times of treatment (a) and the corresponding SEM images of the macro particles (b-d). times longer than 10 h were of inferior stability, fragile spheres resulted after 15 h of treatment and the particles obtained after 24 h of treatment were hollow (Fig. l d). Similar trends regarding the dependence of crystallinity and mechanical strength of the spheres on the time of treatment were observed for samples prepared at decreased synthesis solution to resin weight ratios, namely 5, 2 and 1 (SIO2/A1203 ratio of 100). However, the particles prepared with ratios equal to 2 and 1 shrunk upon calcination. This shrinkage is probably related to the fact that the pore volume provided by the macroporous resin beads for zeolite growth is larger than the amount of zeolite that can actually be crystallized from the synthesis solution. The luck of bulk crystallization for both systems as well as the higher degree of shrinkage for the particles prepared using a ratio of 1 are consistent with this conclusion. The fully crystalline sample prepared after 24 h of treatment using a ratio equal to 1 consisted of agglomerates of ZSM-5 crystals, most probably resulted from the disintegration of the spheres. A comparison with the hollow and fragile but still spherical particles obtained after 24 h of treatment with a ratio of 10 (using the same SIO2/A1203 ratio, namely 100)

186

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40

50

Figure 2. Two pairs of XRD patterns of samples prepared after 8 and 9 h of treatment without and with (*) aging. indicates that the stability of the final product beads depends not only on the treatment time but also on the synthesis solution to resin weight ratio used. Further, the effect of aging of the synthesis solution - resin mixture was studied by comparing samples prepared without aging and after 24 h of aging at room temperature using a synthesis solution to resin weight ratio of 10 and a SIO2/A1203 ratio of 100. Two pairs of XRD patterns of calcined ZSM-5 spheres prepared after 8 and 9 h of treatment with and without aging are shown in Fig. 2. The samples prepared with aging were of inferior crystallinity compared to the non-aged ones. The effect of aging on the properties of molecular sieve films has been studied by Yan et al. [11]. They found that by aging the synthesis gel at 50~ the concentration of soluble silicate species is increased and a smaller and more homogeneous crystal layer on the support is obtained. A similar effect was observed in the present study: the sphere surfaces of the samples prepared with aging were more homogeneous according to the SEM study (not shown). However, the ion exchange resin used as a support in this work is more complex compared to the inert supports usually used for the preparation of zeolite films and other factors should also be taken into account. Thus, the nutrients for the zeolite synthesis are introduced into the resin by ion exchange and a difference might exist when the ion exchange is performed at room temperature (during aging) and at 170~ (direct crystallization). The chemistry of the resin itself may also have an influence. The effect of the aluminum content on the ZSM-5 crystallization within the resin was investigated by preparing samples of varying SIO2/A1203 ratios. Fig. 3 shows XRD patterns of ZSM-5 spheres prepared with SIO2/A1203 ratios equal to 50 (i), 100 (ii) and in the absence of aluminum (iii) for treatment times of 8 h (a) and 24 h (b). The crystallinity of the samples prepared for 8 h of treatment increased with an increase of the SIO2/A1203 ratio (Fig. 3a). After 24 h of treatment the samples prepared with a ratio of 100 and in the absence of

187 (a)

(b)

~1 1t

10

20

30 20/degrees

40

50

10

11

20

30 2e/degrees

(iii) ]

40

50

Figure 3. XRD patterns of calcined ZSM-5 spheres prepared with a SIO2/A1203 ratio equal to 50 (i), 100 (ii) and in the absence of aluminum for treatment times of 8 h (a) and 24 h (b). aluminum were highly crystalline and some amorphous material was still present in the sample prepared with a ratio of 50 (Fig. 3b). SEM was used to study the morphology of the sphere surfaces of the samples prepared with varying SIO2/A1203 ratios. SEM images of the sphere surfaces of samples prepared for 24 h of treatment in the absence of aluminum and using a SIO2/A1203 ratio of 50 are shown in Fig. 4. The size of the surface MFI crystals increased with increasing aluminum content from the sample prepared in the absence of aluminum (Fig. 4a) to the sample prepared with a SIO2/A1203 ratio of 50 (Fig. 4b). These results are in agreement with the results reported by Persson et al. for the effect of alumina on the ZSM-5 crystallization from clear solutions [12]. They found that by increasing

Figure 4. SEM images of the surface particles of calcined ZSM-5 spheres prepared for 24 h of treatment in the absence of aluminum (a) and with a SIO2/A1203 ratio of 50 (b).

188

0.8

350

:: :::: :::::::: .. .. .. .. .. .. ..

300 ~o m~250

i i i fill

,-

0.6i

.I

i i

i i liii i l i i l l

i i i iili . . . . . . .

,x~ ~200 9 ~150

O_(3F.(3F.O--O--O--O--O--O--O'OO"

F

0.4i

~100

0.2

9 ~> 50 O.0

0.2 6 ' 0' 8 ' ' 0'4'0' Relative p~essure'(p/po )" '

1.0

0.0 10

i

I

i

i

i i i i

1 O0

. . . . . . . | i i | i i i

Pore d i a m e t e r (D)/A

1000

Figure 5. A typical nitrogen adsorption/desorption isotherm for the ZSM-5 spheres prepared (a) and the corresponding BJH pore size distribution (b). Open symbols, adsorption; solid symbols, desorption. the SIO2/A1203 ratio of the synthesis solution, the crystal growth rate, the particle concentration and the conversion of metal oxides to MFI are increased whereas the average particle size is decreased. The pore structure of the ZSM-5 spheres prepared comprised of the micropores of the ZSM-5 and mesopores emanating from the removal of the ion exchanger. A typical nitrogen adsorption/desorption isotherm (exemplified with the one for ZSM-5 spheres prepared with a SIO2/A1203 ratio of 100 for 9 h of treatment without aging at a synthesis solution to resin weight ratio of 10) is shown in Fig. 5a. The isotherm is of type IV, characteristic of mesoporous materials but with a relatively steep increase at low relative pressures indicating a substantial microporosity. The dual pore size system is evident from the BJH pore size distribution shown in Fig. 5b. Table 1. Influence of various parameters on the crystallinity (cryst, %) and the BET surface area (SA, m2g-1) of the ZSM-5 spheres prepared. Parameter Treatment time cryst/S A Aging cryst/S A SIO2/A1203 ratio cryst/SA SIO2/A1203 ratio cryst/SA Synthesis solution to resin weight ratio cryst/S A

Sample 7h 10/847

50, 8h 5/740 50, 24h 39/421 1:1, 12h 13/933

8h 24/642 8h 18/660 100, 8h 24/642 100, 24h 76/302 1:2, 12h 18/775

24h 9h 40/641 76/302 9h 22/693 noA1, 8h 66/351/ noA1, 24h 100/324 1:5, 12h 52/629

189 Data on the degree of crystallinity and the BET surface area of ZSM-5 samples is given in Table 1. Generally, the BET surface areas decreased with an increase in the sample crystallinity. From this table the influence of various synthesis parameters on the crystallinity and the BET surface areas can be seen. Thus, samples prepared with aging have lower crystallinity and higher surface areas compared to non-aged ones. Decreasing the SIO2/A1203 ratio and the synthesis solution to resin weight ratio also leads to a decrease of crystallinity and an increase of the BET surface area values.

4. CONCLUSIONS ZSM-5 spheres were prepared employing macroporous strongly basic resin beads as shapedirecting templates. Resin beads were mixed with ZSM-5 clear synthesis solutions and after a hydrothermal treatment at 170~ ZSM-5 was crystallized within the pore system of the ion exchanger. The resin was removed thereafter by calcination leaving behind ZSM-5 spherical particles. Crystallinity of the ZSM-5 spheres was dependent on the treatment time, aging of the synthesis solution, the SIO2/A1203 ratio of the synthesis solution and the synthesis solution to resin weight ratio used. The crystallinity increased with an increase in the time of treatment, the SIO2/A1203 ratio of the synthesis solution and the synthesis solution to resin weight ratio. Also, samples of inferior crystallinity were obtained after aging the synthesis solution - resin mixture for 24 h at room temperature prior to synthesis. The BET surface areas of the ZSM-5 spheres were decreasing with an increase in the sample crystallinity in the range 850 - 300 m 2 g-J. The pore structure of the samples prepared consisted of both microporous (ZSM-5) and mesopores (from the removal of the ion exchanger).

ACKNOWLEDGEMENTS The partial financial support from the Swedish Research Council for Engineering Sciences (VR) is gratefully acknowledged.

REFERENCES 1. K . H . Rhodes, S. A. Davis, F. Caruso, B. Zhang and S. Mann, Chem. Mater. 12 (2000) 2832. 2. X . D . Wang, W. L. Yang, Y. Tang, Y. J. Wang, S. K. Fu and Z. Gao, Chem. Commun. (2000) 2161. 3. Y.-J. Lee, S. Lee, Y. S. Park, K. B. Yoon, Adv. Mater. 13 (2001) 1259 4. B. Zhang, S. A. Davis, N. H. Mendelson and S. Mann, Chem. Commun. (2000) 781. 5. L. Huang, Z. Wang, J. Sun, L. Miao, Q. Li, Y. Yah and D. Zhao, J. Am. Chem. Soc. 122 (2000) 3530. 6. C . J . H . Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt and A. Carlsson, J. Am. Chem. Soc. 122 (2000) 7116. 7. I. Schmidt, A. Krogh, K. Wienberg, A. Carlsson, M. Brorson and C. J. H. Jacobsen, Chem. Commun. (2000) 2157. 8. L. Tosheva, V. Valtchev and J. Sterte, J. Mater. Chem. 10 (2000) 2330.

190 9.

L. Tosheva, B. Mihailova, V. Valtchev and J. Sterte, Microporous Mesoporous Mater. 39 (2000) 91. 10. L. Tosheva, B. Mihailova, V. Valtchev and J. Sterte, Microporous Mesoporous Mater., 48 (2001) 31. 11. Y. Yah, S. R. Chaudhuri and A. Sarkar, Chem. Mater. 8 (1996) 473. 12. A. E. Persson, B. J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites 15 (1995) 611.

Studies in Surface Science and Catalysis 142 R. Aielio, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

191

N o v e l nanocomposite material A. Caratia, C. RIZZO a, L. Dalloro b, B. Stocchia, R. Millinia, C. Perego a a EniTecnologie, Via Maritano 26, 20097 San Donato Mil. (MI), Italy

b Polimeri Europa, Istituto Guido Donegani, Via Fauser 4, 28100 Novara, Italy A spherical nanocomposite S-1/SiO2 has been prepared, starting from a milky slurry of silicalite-1 (S-I) crystallites and a fluid siliceous gel, precursor of an amorphous silica, with a narrow pore size distribution in the mesopore region (MSA). The spherical particles show a core of nanocrystalline S-1 and an outer shell of nanoporous MSA, so the novel composite material is formed by two different nanostructured units, each preserving their individuality. The performances of nanocomposite material in the Beckmann rearrangement of cyclohexanone oxime are comparable with those of S-1, while the MSA binder behaves as an inert. 1.

INTRODUCTION

Nanomaterials, characterized by at least one dimension (i.e. length, width, thickness, pore diameter) in the nanometer size range, usually defined as 1 to 100 ran, are of high interest in catalysis. Indeed their very high surface-to-volume ratio plays a key role in catalysis, being related to accessibility and availability of active sites. The ability to tailor the materials at the nanostructure level opens new opportunities in the optimization of catalysts. For example, the performances of zeolite catalysts are heavily affected by their crystal size. Generally small crystals reduce diflhsion limitation, so increasing catalytic activity and stability. Several examples are reported in literature. For hydrocarbon transformations, the stability of ZSM-5 increases for crystal size of 5-100 nm [1]. For phenol hydroxylation, the catalytic activity of TS-1 decreases as crystal dimensions increase [2]. For Beckmann rearrangement, zeolitic metal-silicates having an external surface area > 5 m2/g and crystallite dimensions < 500 nm give high yields to ~caprolactam [3 ]. Therefore nanocrystalline zeolites (i.e. crystal size < 100 nm) are preferred for catalyst formulation, but tmfortunately, as crystal size decreases, separation of zeolite from its mother liquor becomes more difficult [4]. Another important role in industrial catalysts is played by the binder, indeed zeolites can not be used as made in catalytic reactor, but they have to be mixed with an inorganic binder in order to increase their particle dimension and to permit their use in industrial plant. Binder can modify the performance of the final catalyst, due to its own activity and/or causing variation in diffusion, for example due to pore blockage.

192 In this work the preparation of a novel nanocomposite material based on a zeolite (S-l) and a nanoporous binder (MSA) is described [5]. MSA is mainly mesoporous with a lower contribution of micropores, it shows a narrow pore size distribution that can be modulated in mesoporous region [6]. The catalytic activity of the nanocomposite material is compared with that of the individual components in the Beckmann rearrangement of cyclohexanone oxime.

2. EXPERIMENTAL

2.1. Synthesis

Synthesis ofS-1. S-1 was prepared by adding the solution of 417 g of Si(OC2H5)4 to 102 g of tetrapropyl ammonium hydroxide (TPAOH) and 152 g of water. The so obtained sol had the following molar composition: TPAOH/SiO2 = 0.25, H20/SiO2- 4. It was transferred to 1 liter stainless-steel autoclave and heated at 190 ~ under autogenous pressure and stirred for 2 h. After hydrothermal treatment a milky slurry was obtained. A known amount of slurry was centrifuged to give a white solid, which was dried overnight at 120 ~ and calcined at 550 ~ for 5 h. Synthesis of mesoporous siliceous MSA. 485 g of deionized water were charged in 1 liter stainless-steel autoclave. 92 g TPA-OH (40 wt % in aqueous solution free from alkali cations) and 417 g of Si(OC2H5)4 were added in sequence. The sol was heated at 60~ for lh, obtaining a homogeneous fluid gel with the following molar composition: TPAOH/SiO2 = 0.09, H20/SiO2 = 15. A small part of the gel was aged 15 h at room temperature, dried at 100 ~ and calcined 8 h in air at 550 ~ Synthesis of nanocomposite S-1/MSA. A mixture 30:70 wt/wt (calculated on SiO2) of siliceous MSA gel and milky S-1 slurry was prepared. 4 wt % of polyvinylalcohol (PVA) was added. The mixture was heated at 70~ for 3h and aged for 15 h at room temperature. Then it was pumped to a pressure nozzle of a spray dryer Niro-Mobile HI-TEC, at a rate of 5 1/min. The outlet temperature was about 100 ~ The sample is reported as "Spray A". For comparison a milky slurry of S-l, added with 4 wt % PVA, was sprayed in the same operative conditions. The sample is further reported as "Spray B". 2.2. Characterization X-ray powder diffraction (XRD) analyses were performed with a Philips X'PERT vertical diffractometer equipped with a proportional detector and a secondary monochromator. Data were collected stepwise in the 3 < 20 < 53 ~ angular region with 0.02 ~ 20 step and 20 s/step accumulation time; the CuKa (~ = 1.54178 A) radiation was used. Morphology of the samples was investigated with a Jeol JSM-5400LV Scanning Electron Microscope (SEM), using the Back Scattered Electrons (BSE) technique, and with a Philips EM420 Transmission Electron Microscope (TEM) operating at 120 kV accelerating voltage. For SEM-BSE analysis, a small amount of sample was embedded in epoxy resin and the surface ground with sandpaper until internal sections of the particles were obtained. The surface was finally polished with diamond past in water. To avoid sample charging, a thin layer of carbon was deposited by sputtering on the surface of the sample. For TEM observations, a small amount of powdered sample was embedded in epoxy resin and ultrathin sections ( 5 0 0 - 1000 A thick) were successively cut with a Reichert-Jung ultramicrotome equipped with a diamond knife.

193 Textural properties of all samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples (-~ 0.2 g) were outgassed for 16 h at 350 ~ under vacuum. The specific surface area was evaluated by 2parameters linear BET plot in the range p/pO 0.01-0.2. The total pore volume was evaluated by Gurvitsch rule.

2.3. Catalytic activity The catalysts were tested in a tubular fixed-bed glass microreactor (length: 200 mm, internal diameter: 11.5 mm, axial sheath for thermocouple with external diameter of 4 mm). The catalyst (size: 42+80 mesh, charge: 0.5 g) was diluted with granular quartz up to a volume of 2 cm 3 and positioned in the reactor between two layers of quartz. The cyclohexanone oxime was dissolved in methanol and toluene, preheated and introduced into the reactor together with a nitrogen stream. The reactant mixture was vaporized before coming into contact with the catalyst. Before the test the catalyst was dried in a nitrogen stream for 1 h at a temperature higher than 300 ~ and accustomed to the reaction by feeding only the solvents mixture for at least 30 minutes. Operating condition: Temperature = 350 ~ WHSV (referred to the cyclohexanone oxime feeding and to the catalyst active phase) = 4.5 h -1, methanol/toluene/N2/cyclohexanone oxime molar ratio = 10:l 0:8:1. The mixture of effluent vapors from the reactor was condensed and samples were collected for evaluation of catalytic performances by GC analysis.

3.

RESULTS

S-1 is obtained in quantitative yield with respect to SiO2 in form of a white powder. XRD analysis indicates the presence of a pure and well-crystallized MFI-type structure with the expected monoclinic symmetry. SEM analysis shows the typical morphology of the S-1 crystals, i.e. well formed elongated hexagonal prisms, with dimensions in the range (400-700) x (100-300) x (40-70) nm. Since one dimension of the crystals is lower than 100 nm, this S-1 sample can be classified as nanomaterial. As reported in the experimental section, the milky slurry obtained from the S-1 synthesis was mixed with a siliceous MSA gel and PVA to give the spray-dried sample Spray A. A second sample, Spray B, was also prepared without the addition of the MSA gel to the mixture. Representative SEM-BSE micrographs of the two samples are shown in Figures 1 and 2, respectively. Both samples are constituted by spherical particles with diameter in the range 50 - 120 ~tm, but with different textural properties. In Spray A the particles are constituted by a compact outer shell, 3 - 6 ~tm thick, which includes an inner low density phase (Figure 1). On the contrary, only this low density phase was observed in Spray B, which was obtained without the addition of the binder (Figure 2). A more detailed insight in the particle morphology was obtained by TEM analysis. Figure 3a shows a representative micrograph of sample Spray A, corresponding to the boundary zone between the outer shell and the low density inner phase. It is evident that the outer shell is constituted by the amorphous silica phase (MSA) while the S-1 crystals are mainly present in the core of the particles.

194 The presence of S-1 crystals is unambiguously confirmed both for their typical morphology (see inset in Figure 3a) and for the observation of the lattice fringes at highest magnifications (Figure 3b).

Figure 3 a, b. TEM micrographs of sample Spray A

195

250 - o - s-i -"

..o- MSA

200

Spray A

(b)

Spray B "~ 150 0 r~

100 E

50

0

~

1.E-06

1 .E-05

1 .E-04

1 .E-03

l .E-02

1 .E-01

1 .E+00

Relative Pressure (p/pO) Figure 4 a, b. N2 adsorption/desorption isotherms for the samples under investigation. The relative pressure is reported both in linear (a) and logaritmic (b) scale.

196 In Table 1 the textural properties of Spray A and B are summarized and compared with those of S-1 and siliceous MSA. The isotherms are reported in Figure 4 a. Table 1 Textural properties of S-l, MSA, Spray A, Spray B

Sample

Specific surface area

Total pore volume

(m2/g)

(mUg)

I

390

0.25

MSA

IV + (I)

460

0.34

Spray A

I +(IV)

540

0.37

Spray B

I + (IV)

390

0.31

S-1

Isotherm type

S-1 is characterized by a reversible Type Ia isotherm distinctive of primary micropore filling. Furthermore, the horizontal plateau over a wide range of high p/pO is indicative of a relatively small amount of multilayer adsorption on the open surface. The Spray B isotherm is very similar to S-1 as far as p/pO _ 0.85, indeed they are characterized by the same specific surface area, but Spray B shows a total pore volume higher than S-1. That is justified by the presence of the porosity arising from interparticle voids evidenced by the hysteresis loop at high relative pressure (p/pO > 0.9) and the asymptotic approach to the saturation vapor pressure. MSA shows a Type IV + (I) with a H2 hysteresis loop. So the material is mainly mesoporous with a low contribution of micropores responsible of the adsorption observed at very low relative pressure, p/pO < 0.1. The H2 hysteresis type is usually attributed to different size of pore mouth and pore body (this is the case of ink-bottle shaped pores) or to a different behavior in adsorption and desorption in near cylindrical through pores. Spray A isotherm is a combination of the isotherms of the other three samples. An irreversible Type I + (IV) isotherm with two hysteresis loop is observed for Spray A: the first one shows a lower area with respect to MSA sample, due to the low content of binder in the sample, the second one is quite similar of that observed for Spray B and it is related to the presence of the porosity arising from interparticle voids. In order to better compare the micropore region, the isotherms are also displayed as logarithmic plots (Figure 4 b). In agreement with their uniform micropore size both S-1 and Spray B show a clear inflection point at very low p/p o (10"6 - 10".5,). The inflection point at very low p/pO is not detectable in the MSA, due to the small contribution of micropores. The Spray A shows all the pore size contributions. In Table 2 the catalytic behaviors of S-l, MSA and Spray A are compared after 20 h of time on stream. Spray A and S-1 show the same catalytic performances: a very high conversion of cyclohexanone oxime (Cox) with a high selectivity to e-caprolactam (ScPL). By contrast, MSA shows a very low catalytic activity.

197 Table 2 Catalytic perfong,antes in Beckmann rearrangement. % Cox

o~ SCPL

(after 20h)

(after 20h)

S-1

99

96

MSA

5

70

Spray A

98

95

Sample

See experimental

4. DISCUSSION The proposed formulation process can be performed avoiding the separation of solid phase from mother liquor, that constitutes a problem for nanocrystalline zeolitic materials. Spherical particles of Spray A are formed by a low density core containing the S-1 crystals, while MSA binder forms the outer shell. According to this Spray B, obtained by spray drying the S-1 sltm~ without binder, shows only the low density phase. Nanocomposite Spray A sample exhibits a high surface-to-volume ratio, as described for nanostructured solids, related to both the tridimensional organization of internal surfaces (micro/mesoporosity of S-1 and siliceous MSA, respectively) and to the external exposed faces (nanometric S-1 particles). In the Beckmann rearrangement of cyclohexanone oxime, the siliceous MSA shows the required performances of a good binder: very low catalytic activity and quite good selectivity. In the nanocomposite material the performances of S-1 are not reduced by the presence of the binder. This behavior can be related to the peculiar morphology obtained that plays down the interactions between crystalline and amorphous phases. Amorphous silica can play its role of binder, without modifications of the catalytic behavior of zeolite. Thank to its controlled pore distribution in mesopore region no diffusion problems are evidenced. The described nanocomposite preparation is suitable also for other zeolite structures. These nanocomposites can be used as active phase for preparation of extrudate catalysts or as such as catalyst for fluid or transported bed reactor [6]. 5.

CONCLUSION

A spherical nanocomposite S-l/siliceous MSA can be prepared, starting from the S-1 mother liquor, avoiding the liquid-solid separation. The nanocomposite shows a very peculiar morphology, with a low density core of S-1 and an outer shell of nanoporous amorphous silica, so the interactions between zeolite and binder are minimized. The peculiarity of this nanocomposite is that both the components are nanomaterials characterized by a well defined morphology: S-1 particles exhibit one dimension lower than 100 nm and nanoporous silica has a narrow pore size distribution in the mesopore region. The nanocomposite described has been tested in Beckmann rearrangement, showing the same activity of S-l, no effects of binder is observed.

198

Acknowledgements The authors would like to acknowledge Carlo Barabino and Giuseppe Botti for their precious technical support. REFERENCES 1. E.J. Rosinski, A.B. Schwartz, C.J. Plank, US 3,926,782 (1975). 2. A.J.P.H. van der Pol, J.H.C. van Hooff, Appl. Catal. A, 92 (1992) 113. 3. H. Sato, K. Hirosa, M. Kitamura, Y. Umada, N. Ishii, H. Tojima, EP 242,960 (1987). 4. I. Schmidt, A. Krogh, K. Weinberg, A. Carlsson, M. Brorson, C.J.H. Jacobsen, Chem. Commun., (2000) 2157. 5. A. Carati, G. Botti, L. Dalloro, EP 1,106,576 (2001). 6. A. Carati, C. Rizzo, M. Tagliabue, C. Perego, Stud. Surf. Sci. Catal., 130B (2000) 1085.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

199

V i b r a t i o n a l and optical spectroscopic studies on c o p p e r - e x c h a n g e d ferrierite G. Turnes Palomino, a S. Bordiga, b C. Lamberti, b A. Zecchina b and C. Otero Arefina Departamento de Qufmica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain Dipartimento di Chimica IFM, Universit~t di Torino, 10125 Torino, Italy CuLferrierite was prepared by reaction between the protonic form of the zeolite and CuC1 vapour. UV-Vis spectroscopy showed that copper was present in the zeolite mainly in the form of Cu ~ ions; no evidence was found for the presence of Cu I~ ions or copper species in microaggregates. FTIR spectroscopy showed that the zeolite adsorbs CO at liquid nitrogen temperature with formation of CuX(CO)n adducts, mainly mono- and dicarbonyls. After dosing with dioxygen at room temperature CuLferrierite was partially oxidized. This partially oxidized sample was found to be capable of oxidizing carbon monoxide. 1. INTRODUCTION Cu-exchanged ZSM-5 zeolites were found to display high catalytic activity towards the decomposition of nitrogen oxides, including direct decomposition of NO to N2 and O2 and the selective catalytic reduction of NO by hydrocarbons in the presence of excess oxygen [1-9]. Both of these processes are of considerable practical interest, since nitrogen oxides are regarded as being major air pollutants. Because of its potential use in the solution of air pollution problems, Cu-ZSM-5 and other copper-exchanged zeolites are currently under active investigation. The reason why these materials are active for the decomposition of nitrogen oxides is still a matter of discussion and active research, and characterization of the active copper species is a prime requirement for understanding their catalytic activity. In this sense, numerous studies have been carried out in order to elucidate the nature of copper sites in zeolites and to correlate structural data with redox chemistry [10-16] and catalytic activity [3,4,7,11,17,18]. Although most research has been focussed on Cu-ZSM-5, an MFI type zeolite, Cu-exchanged zeolites belonging to other structural types have also been investigated [4,8,16,18,19-25]. These studies have shown that the catalytic activity for the decomposition of nitrogen oxides differs depending on the zeolite structure type [16,25]. This different catalytic behaviour could probably be correlated with the different coordination environment and accessibility of the sites occupied by extraframework copper cations in the different zeolites, but more research is desirable in order to enlarge the available set of experimental results, which would hopefully help in understanding catalytic activity. As a contribution to this field, and following previous studies on Cu-ZSM-5 [17,26], Cu-MOR [27] and Cu-Y [28] zeolites, we report here on spectroscopic characterization of Cuferrierite (Cu-FER) obtained by reacting H-FER with gas phase CuC1. We have used IR and UV-Vis spectroscopies to obtain information on the oxidation state of copper ions, their coordination environment and their redox behaviour.

200 2. E X P E R I M E N T A L

The starting ferrierite zeolite used in this study was a commercial alkaline sample (Na,KFER) supplied by Engelhard Corporation (Iselin, NJ, USA); it had a Si/A1 ratio of 8. From this sample, the corresponding ammonium form was obtained by ion exchange with an ammonium nitrate solution. The NH4-FER sample thus obtained was thermolysed and outgassed at 473 K, to yield H-FER; this protonic form was then reacted with CuC1 vapour at 573 K and finally outgassed at 823 K to remove excess CuC1. More details on the experimental set-up and procedures used were reported (for CuI-ZSM-5) elsewhere [17]. To check ion exchange, Fourier transform IR spectroscopy was used. Nearly complete exchange was obtained, as shown in Figure 1, were IR spectra in the O-H stretching region (before and after ion exchange) are reported. After ion exchange, the band at 3601 cm l , which corresponds to bridged Si(OH)A1 hydroxyls, disappears, thus proving that all acidic protons were replaced by copper ions. Further IR spectroscopic characterization of Cu-FER 0.4was carried out by using adsorbed CO as a probe molecule. For these infrared measurements, a thin selfsupported wafer was prepared and introduced in an IR cell which allowed in situ thermal treatments, gas dosage and v low-temperature measurements o 0.2 (,to be made. The spectra were .,Q collected with a resolution of 2 0 cm ~ on a Bruker FTIR IFS66 ..Q ,< spectrometer equipped with an MCT cryodetector. Although for low temperature measurements the IR cell was permanently cooled with liquid 2 nitrogen, the actual sample temperature (under the IR 0.0 beam) was likely to be ca. IO03800 36;0 34;0 ll0K. Waven um be r/cm -1 UV-Vis diffuse reflectance spectra were obtained at room temperature on a Varian Cary5 spectrometer using a quartz Fig. 1. IR spectra in the O-H stretching region cell designed to allow in situ of ferrierite before (spectrum 1) and after high temperature treatments (spectrum 2) the exchange procedure. and gas dosage.

201 3.

RESULTS AND DISCUSSION

3.1. Outline of the ferrierite structure Ferrierite, structure type FER in the IUPAC nomenclature [29], is a zeolite with orthorombic structure. In the framework, five-membered ring building units are connected to form 10-membered ring channels running along the [001] direction (main channels). These channels have an elliptical cross section, 4.2x5.4 ,~ in diameter, and are intersected at right angles by 8-membered ring channels (3.5x4.8 A) running parallel to the [010] direction. Cation location in Na,Mg-FER has been studied crystallographically by Vaughan [30] and by Barrer and Marshall [31], among others, who found two occupied cation sites. Of these, only one, located in the main channel, is accessible to adsorbed molecules. Recent synchrotron X-ray diffraction [32], luminescence [33] and computational [34] studies on copper-exchanged ferrierite are substantially in agreement with the previous crystallographic (X-ray diffraction) work. According to the synchrotron radiation studies of Attfield et al. [32], the accessible copper site is located at the intersection of 10-membered ring and 8-membered ring channels, and the copper ion is coordinated to only two oxygen atoms of the zeolite framework. This low coordination could presumably lead to a high catalytic activity. Computational studies [34], however, seem to suggest that, besides the site at the channel intersection, Cu I ions (in CuLFER) could also be located on the wall of the main channel, where they would have a higher coordination number: 3 to 4. Although details are not yet completely settled, the situation does not seem to be much different from that found with Cu IZSM-5 for which both EXAFS measurements [26] and theoretical studies [35,36] have shown that there are two different (accessible) sites for extraframework Cu I ions; which can be coordinated to either two or three to four oxygen atoms of the zeolite framework. This structural analogy between CuLFER and CuLZSM-5 suggests that CuLFER can be active in the catalytic decomposition of nitrogen oxides. 3.2. UV-Vis spectroscopy Figure 2 shows the UV-Vis diffuse reflectance spectrum of CuI-FER outgassed at 823 K.

2.5

2.0

--~ -1

1.5

v .~

1.0

v

0.5

0.0 50000

'

i 40000

,

i 30000

'

Wavenumber/cm

I 20000

,

i 10000

-1

Fig. 2. Diffuse reflectance UV-Vis spectrum of Cu-FER outgassed at 823 K.

202 The intense bands observed in the 50000-30000 cm -1 region can be assigned, on the basis of the known spectra of copper complexes [37-39], to metal-to-ligand charge transfer transitions. The absence of any absorption in the 20000-5000 cm -~ range, where d--->d transitions of CUIIwould appear [37-39], proves that copper ions are present as Cu I. Possible formation of reduced species, as copper metal clusters, which would give rise an absorption edge at c a . 17000 cm -~ [40,41], can also be excluded. Therefore, these results from UV-Vis spectroscopy confirm that copper in the Cu-FER sample here described is present, at least mainly, in the monovalent oxidation state; no divalent Cu H species or zerovalent copper aggregates have been detected. 3.3. IR spectroscopy of adsorbed CO Figure 3 shows the IR spectra, in the C - O stretching region, of carbon monoxide adsorbed, at liquid nitrogen temperature and increasing dosage, on the CuI-FER sample previously outgassed at 823 K. At the lowest CO equilibrium pressure the spectrum shows a single infrared absorption band at 2157 cm -1. According to data reported in the literature for other CuLcontaining zeolites [17,19,21,26-28,42,43], the 2157-cm -1 band is assigned to monocarbonyl CuI...CO species. Bands at nearly the same frequency were reported for CO adsorbed on CuI-ZSM-5 (2157 cm -1) [17,26], CuI-MOR (2159 cm -1) [27], CuLY (2159 cm -1) [28] and CuI-~ (2157 cm -l) [44]; they were invariably assigned to monocarbonyl Cu[...CO adducts.

0.8 .

D

//11

0.15.

4

///,.~~p

////

0.6

(1) t-" t~

/

0.4

2170 cm.1 j 2178 cm 1

O

r/

\\\\

o.o5 ________________~______________~/ __,__

0

/

2175

< 0.2

0.0' 2200

' Wavenumber/cm

2100

'

2000

1

Fig. 3. FTIR spectra, at 77 K, of CO adsorbed on Cu-FER. Increasing equilibrium pressure from c a . 0.1 to 20 Torr (spectra 1 to 4). Inset shows an expanded view of the 2225-2150 cm -l region.

203 Upon increasing the CO equilibrium pressure the band at 2157 cm -I is gradually eroded and simultaneously two new bands develop at 2178 and at 2149 cm -~. These facts strongly suggest that copper monocarbonyl species add a second CO ligand to yield CuI(CO)2 dicarbonyl adduct. In agreement with previous reports [17,26-28], the bands at 2178 and 2149 cm -~ are assigned, respectively, to the symmetric and the asymmetric C - O stretching modes of the CuI(CO)2 adducts. At the highest CO equilibrium pressure two new, and weak, IR absorption bands appear at 2191 and 2170 cm -j. These weak bands should be assigned [17,26-28] to tricarbonyl CuI(CO)3 species. This result suggests that a small portion of copper (I) ions can accept a third CO ligand, although most of them remain, even at the highest CO equilibrium pressure used, as CuI(CO)2 adducts. Qualitatively, these results are similar to those found for the CO/CuI-ZSM-5 system [ 17,26], where successive formation of Cu I mono-, di- and tricarbonyl species was also observed upon increasing the CO equilibrium pressure. However, the proportion of tricarbonyl species (as compared to that of dicarbonyls) seems to be smaller in the case of CO adsorbed on CuI-FER, thus suggesting that ferrierite contains a smaller proportion of Cu I ion in a low coordination state. Note that Cu ~ ions which have only two nearby framework oxygens would be more likely to from tricarbonyls than those having three or four oxygen neighbours.

3.4. Characterization and properties of the oxidized copper ferrierite In order to investigate the redox behaviour of Cu I ions present in copper-exchanged ferrierite, a sample previously outgassed at 823 K was put in contact with 20 Torr of oxygen at room temperature. Immediately after dosing with oxygen the sample colour was seen to change from white to grey. This change is reflected in the corresponding UV-Vis spectrum, depicted in Figure 4, which shows the development of a new (and broad) absorption band at about 24000 cm -z. According to published data on copper oxides, this new absorption band can be assigned to optical (d-d) and charge transfer transitions involving Cu n ions [37-39,45]. These results prove that contact with oxygen at room temperature causes oxidation of part of the Cu ~ ions. Most of them, however, should remain in the monovalent state, since the UVVis spectrum (Figure 4) still displays strong features of the non oxidized material (bands in the 50000 to 30000 cm -1 range). Following contact with oxygen, CO was dosed on the sample (at room temperature) and an FTIR spectrum was run; this spectrum is shown in Figure 5. It displays the IR absorption bands corresponding to the CuI(CO)2 species (2149 and 2178 cm-l), and also a complex band which has a sharp peak at 2349 cm -l. The spectrum of adsorbed CO (Figure 5) gives further evidence that most copper ions remain in the monovalent oxidation state even after contact with oxygen. Note, however, that Cu I tricarbonyls are not formed at room temperature. A relevant feature of the full line spectrum in Figure 5 is the presence of a distinctive band which peaks at 2349 cm -~ and has a shoulder at about 2355 cm -~. The sharp peak at 2349 cm -~ corresponds to gas phase carbon dioxide (asymmetric stretching vibration mode ~)3), while the shoulder at about 2355 cm -I should be assigned [46] to the same ~)3 mode of CO2 slightly perturbed by extraframework cations in the zeolite sample. The point to remark is that the above IR measurements prove that the oxidized copper ferrierite sample is capable of oxidizing CO to CO2. This is in consonance with the known ability of Cu H oxides to act as oxidation catalysts [45-50], but it should be noted that CO2 was not produced after CO adsorption on a CuI-ZSM-5 sample previously dosed with oxygen [ 15]. This different behaviour of Cu~-ZSM-5 and CuLFER is worth of further investigation, since it could have relevant consequences in the potential use of these materials as catalysts for air

204 2.5

0.8

2.0 0.6 5

'~1.5

o9

"~ 1.0

0.4

o

0.2

0.5

. .................................................. i 0.0 50000

i

i

,

i

,

t

40000 30000 20000 10000 Wavenumber/cm 1

Fig. 4. Diffuse reflectance UV-Vis spectra of CuI-FER before (dotted line) and after (solid line) contact with oxygen at room temperature.

3.0 2400

23~00'

22~00'

i 21;0

Wavenumber/cm

' 2000

-1

Fig. 5. FTIR spectra of CO (ca. 50 Torr) adsorbed on Cu-FER before (dotted line) and after (solid line) contact with oxygen at room temperature.

pollution control (e.g. in automobile exhaust catalytic converters). Further experimental research is in progress in order to address this interesting point, as well as in an attempt to characterize the oxidized copper species; to this end, EPR spectroscopy is now being applied.

Acknowledgments This contribution has been supported by the Italian MURST: COFIN Area 03 bando 2000, and by the Spanish DGESIC" Project No. PB97-0147.

REFERENCES 1 2 3 4 5 6 7

M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya and S. Kagawa, J. Chem. Soc., Chem. Commun., (1986) 1272. Y. Li and W. K. Hall, J. Phys. Chem., 94 (1990) 6145. M. Iwamoto and H. Hamada, Catal. Today, 10 (1991) 57. J. Valyon and W. K. Hall, J. Phys. Chem., 97 (1993) 1204. B. Wichterlovfi, Z. SobalN and M. Skokenek, Appl. Catal. A., 103 (1993) 269. M. Iwamoto and H. Yahiro, Catal. Today, 22 (1994) 5. M. Shelef, Chem. Rev., 95 (1995) 209.

205 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

34 35 36 37 38

A. Corma, V. Fornds and E. Palomares, Appl. Catal. B, 11 (1997) 233. A. Fritz and V. Pitchon, Appl. Catal. B, 13 (1997) 1. J. Sfirkfiny, J. L. D'Itri and W. M. H. Sachtler, Catal. Lett., 16 (1992) 241. D.J. Liu and H. Robota, Appl. Catal. B, 4 (1994) 155. B. Wichterlovfi and J. Dedecek, J. Phys. Chem., 99 (1995) 16327. T. Beutel, J. S~irkfiny, G. D. Lei, J. Y. Yan and W. M. H. Sachtler, J. Phys. Chem., 100 (1996) 845. B. Wichterlov~i, J. Dedecek, Z. Sobalfk, A. Vondrova and K. Klier, J. Catal., 169 (1997) 194. G. Turnes Palomino, P. Fisicaro, E. Giamello, S. Bordiga, C. Lamberti and A. Zecchina, J. Phys. Chem. B, 104 (2000) 4064. R. Bul~nek, B. Wichterlovfi, Z. Sobal~ and J. Tichs), Appl. Catal. B, 31 (2001) 13. G. Spoto, A. Zecchina, S. Bordiga, G. Ricchiardi, G. Martra, G. Leofanti and G. Petrini, Appl. Catal. B, 3 (1994) 151. A. Corma, A. Palomares and F. Mfirquez, J. Catal., 170 (1997) 132. Y. Huang, J. Am. Chem. Soc., 95 (1973) 6636. J. Dedecek, Z. Sobal~, Z. Tvaruzkova, D. Kaucky and B. Wichterlovfi, J. Phys. Chem., 99 (1995) 16327. Y. Kuroda, H. Maeda, Y. Yoshikawa, R. Kumashiro and M. Nagao, J. Phys. Chem. B, 101 (1997) 1312. H. Miessner, H. Landmesser and K. Richter, J. Chem. Soc., Faraday Trans., 93 (1997) 3417. Z. Sobah'k, J. Dedecek, I. Ikonnikov and B. Wichterlovfi, Microporous Mesoporous Mater., 21 (1998) 525. P.J. Carl and S. C. Larsen, J. Phys. Chem. B., 104 (2000) 6568. A.E. Palomares, F. Mfirquez, S. Valencia and A. Corma, J. Mol. Catal. A, 162 (2000) 175. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo, G. Vlaic and M. Bellatreccia, J. Phys. Chem. B, 101 (1997) 344. C. Lamberti, S. Bordiga, A. Zecchina, M. Salvalaggio, F. Geobaldo and C. Otero Arefin, J. Chem. Soc., Faraday Trans., 94 (1998) 1519. G. Turnes Palomino, S. Bordiga, A. Zecchina, G. L. Marra and C. Lamberti, J. Phys. Chem. B, 104 (2000) 8641. Ch. Baerlocher, W. M. Meier and D. H. Olson, Atlas of Zeolite Framework Types, 5th Ed., Elsevier, Amsterdam (2001). P.A. Vaughan, Acta Cryst., 21 (1966) 983. R.M. Barrer and D. J. Marshall, J. Chem. Soc., 2296 (1964) 1964. M.P. Attfield, S. J. Weigel and A. K. Cheetham, J. Catal., 172 (1997) 274. B. Wichterlovfi, J. Dedecek and Z. Soball'k, "Proceedings 12th International Zeolite Conference", M. M. J. Treacy, B. K. Marcus, M. E. Bisher, E. Higgins, eds; Materials Research Society: Warrendale, PA, 1999, p. 941. P. Nachtigall, M. Davidovfi and D. Nachtigallovfi, J. Phys. Chem. B, 105 (2001) 3510. D. Nachtigallovfi, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 1 (1999) 2019. P. Nachtigall, D. Nachtigallovfi and J. Sauer, J. Phys. Chem. B, 104 (2000) 1738. B.N. Figgis, Introduction to Ligand Fields, Interscience, London, 1966. G. Ferrandi and S. Murlidharan, Coord. Chem. Rev., 36 (1981) 45.

206 39 B. J. Hathaway, in Comprehensive Coordination Chemistry (G. F. Wilkinson, R. D. Gillard and J. A. McCleverty, Eds.), Pergamon Press, Elmsford, N.Y., 1987, Vol. 5, p. 533. 40 J. Texter, D. H. Strome, R. G. Herman and K. Klier, J. Phys. Chem., 81 (1977) 333. 41 C. Lamberti, G. Spoto, D. Scarano, C. Paz6, M. Salvalaggio, S. Bordiga, A. Zecchina, G. Turnes Palomino and F. D'Acapito, Chem. Phys. Lett., 269 (1997) 500. 42 M. Iwamoto, H. Yahiro, K. Tanda, N. Mizumo, Y. Mine and S. Kagawa, J. Phys. Chem., 95 ( 1991) 3727. 43 J. Sfirkfiny, J. L. D'Itri and W. M. H. Sachtler, Catal. Lett., 16 (1992) 241. 44 G. Turnes Palomino, A. Zecchina, E. Giamello, P. Fisicaro, G. Berlier, C. Lamberti and S. Bordiga, Stud. Surf. Sci. Catal., 130 (2000) 2915. 45 L. Gang, J. Van Grondelle, B. G. Anderson and R. A. van Santen, J. Catal., 186 (1999) 100. 46 B. Bonelli, B. Civalleri, B. Fubini, P. Ugliengo, C. Otero Arefin and E. Garrone, J. Phys. Chem. B, 104 (2000) 10978. 47 N.N. Sazonova, A. V. Simakov and H. Veringa, React. Kinet. Catal. Lett., 57 (1996) 71. 48 F. Dannevang, US patent 5,587,134 (1996). 49 A. Wollner and F. Lange, Appl. Catal. A, 94 (1993) 181. 50 T. Curtin, F. O'Regan, C. Deconinck, N. Kntittle and B. K. Hochneff, Catal. Today, 55 (2000) 189.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

207

Variable temperature FTIR spectroscopy of carbon monoxide adsorbed on protonic and rubidium-exchanged ZSM-5 zeolites C. Otero Arena, a M. Pefiarroya Mentruit, a M. Rodriguez Delgado, a G. Turnes Palomino, a O. V. Manoilova, b A. A. Tsyganenkob and E. Garrone c a Departamento de Quimica, Universidad de las Islas Baleares, 07071 Palma de MaUorca, Spain b Institute of Physics, St. Petersburg University, 198904 St. Petersburg, Russia c Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, 10126 Torino, Italy Infrared spectroscopic studies have shown that adsorbed carbon monoxide interacts with Bronsted acid Si(OH)A1 groups of the zeolite H-ZSM-5 forming hydrogen bonded H...CO and H...OC species, which were characterized by C-O stretching IR absorption bands at 2174 and 2120 cm1, respectively. In the rubidium-exchanged zeolite, corresponding Rb+--.CO and Rb§ adducts gave characteristic bands at 2161 and 2119 cm~, respectively. Variabletemperature FTIR spectroscopy has shown that, in both cases, C- and O-bonded species are interrelated through a temperature dependent equilibrium in which the proportion of Obonded adducts increases with increasing temperature. By means of van't Hoff plots, the corresponding enthalpy change was found to be AH~ 4.6 kJ mol"l for the CO/H-ZSM-5 system and AH~ 1.8 kJ mol"~ for CO/Rb-ZSM-5. 1. INTRODUCTION Interaction of carbon monoxide with zeolites in their protonic (acid) form is well known [1-3] to give rise to hydrogen bonded OH.-.CO adducts which involve the bridging Si(OH)A1 hydroxy groups of the zeolite (Bronsted acid sites). Similarly, when CO is adsorbed on zeolites exchanged with alkali metal cations, formation of M+.'-CO (M = Li, Na, K, Rb, Cs) adducts occurs. Low-temperature IR spectra of adsorbed CO invariably show a main C--O stretching band which is upward shifted with respect to that of free CO (2143 cm~), and which corresponds to the above adducts [2,3]. For the series of alkali-metal exchanged M +ZSM-5 zeolites, this cation-specific high frequency (HF) band was observed at wave numbers which gradually decrease from 2195 cm"1 for Li§ [4] down to 2157 cm"~ for Cs+ [5]; while for CO adsorbed on H-ZSM-5 it was observed at about 2173 cm"~ [2,6]. Besides the HF band, a minor low-frequency (LF) band was also noted in many infrared spectra of CO adsorbed on cation exchanged zeolites [7-9]. This LF band appears below 2143 crn"1 and is also cation-specific. For the above series of M+-ZSM-5 zeolites it was observed at wave numbers gradually increasing from 2100 crn! for Li+ [4] up to 2122 cm! for Cs+ [5]. Quantum chemical calculations have shown [10-12] that this LF band corresponds to the fundamental C-O stretching mode of M+.--OC adducts, where the CO molecule interacts with the cation through the oxygen atom. It was also recently documented [13] that CO adsorbed on the protonic faujasite-type zeolite H-Y forms both OH...CO and OH...OC hydrogenbonded species upon interaction with the zeolite Bronsted acid sites; which is in consonance

208 with the known ability of CO to form hydrogen bonded OC...HF and CO---HF adducts with hydrogen fluoride [14]. Previous variable-temperature infrared spectroscopic studies [15-17] have shown that Cand O-bonded species are in a temperature-dependent equilibrium which can be described by equation (1), where Z stands for the zeolite framework and M + is an alkali-metal cation or a proton: ZM+...CO = ZM+...OC

(1)

The enthalpy change, AH ~ involved in the above linkage isomerization equilibrium was determined for a few systems. In kJ mol -~, it amounts to 3.8 for CO adsorbed on Na-ZSM-5 [15], 2.4 forCO on Na-Y [16], 3.2 forCO on K-ZSM-5 [18] and 4.3 forCO on H-Y [13]. It is clear that the value of AH ~ depends on the specific cation considered, on the zeolite structure type and (most likely) on the nature of the interaction between CO and the adsorbing centre: hydrogen-bonded or purely electrostatic. However, it seems clear that more experimental measurements are needed before general trends can be established. The aim of this paper is to analyse temperature-dependent infrared spectra of CO adsorbed on H-ZSM-5 and Rb-ZSM-5, so as to extend previous work and to advance towards a more detailed understanding of the factors involved in the interaction between carbon monoxide and zeolites. Some preliminary results on H-ZSM-5 were reported elsewhere [19]; the CO/Rb-ZSM-5 system is documented here for the first time. 2. EXPERIMENTAL The zeolite H-ZSM-5, structure type MFI [20], was synthesized following standard procedures [21]. It had a nominal Si/A1 ratio of 25. Powder X-ray diffraction showed good crystallinity, and absence of any diffraction lines not assignable to the corresponding structure type [22]. From H-ZSM-5, the rubidium-exchanged sample was obtained by ion exchange with an aqueous solution of rubidium nitrate. The process was monitored by disappearance of the original IR absorption band corresponding to bridged Si(OH)A1 hydroxy groups. After ion exchange, the Rb-ZSM-5 sample was also checked by powder X-ray diffraction. For infrared studies, a thin self-supported wafer of each sample was prepared and outgassed (activated) in a dynamic vacuum (residual pressure < 10 -4 Torr) for 2 h at 700 K inside an IR cell which allowed in situ high-temperature activation, gas dosage, and variabletemperature measurements to be carried out. Details on the design and performance of this home-made infrared cell were given elsewhere [23,24]. Liquid nitrogen was used for refrigeration and temperature was measured by means of a platinum resistance thermometer inserted close to the sample wafer. For better thermal contact between the zeolite wafer and the refrigerated cell body, about 0.3 Torr of helium was admitted into the sample compartment before running the background spectrum at liquid nitrogen temperature. Carbon monoxide was then dosed to an equilibrium pressure of about 1.2 Torr, the cell was closed and infrared spectra were recorded at 77 K and on gradual warming up of the infrared cell following removal of liquid nitrogen. A series of spectra was thus taken (for each sample) at about 10 K intervals in the range going from 77 K to room temperature. These transmission FTIR spectra were recorded at 3 cm -I resolution by means of a Bruker IFS66 FTIR instrument. The zeolite blank spectrum taken at 77 K before CO admission, was used as a background; all the spectra shown in this work are background subtracted.

209 3. RESULTS AND DISCUSSION In the O - H stretching region, the background infrared spectrum of the H-ZSM-5 sample showed the characteristic absorption bands at 3748 cm -t (silanols) and at 3615 cm 1 (bridging Si(OH)A1 hydroxy groups). The silanol band was not affected by adsorbed CO (1.2 Torr), while that corresponding to the BrOnsted acid OH group was shifted to 3300 cm l (and enlarged) as a consequence of hydrogen bonding [2,25]. Fig. 1 shows some selected variabletemperature IR spectra in the C - O stretching region. Interaction of CO with the zeolite BrOnsted acid groups gives rise to a major IR absorption band at 2174 cm -~, and also to a minor (and complex) band in the 2115-2130 cm l region.

1.0-

1

J

2

0.9 0.8 0.7 0.6 0 c.Q 0 o9 ,.Q

<

3 0.5 0.4

4

0.3

2

xlO

0.2 0.1 0.0 2200

'

21'80

'

2160

2140

2120

Wavenumber (cm")

Fig. 1. Selected FTIR spectra of CO (ca. 1.2 Torr) adsorbed on H-ZSM-5 at variable temperatures: 1,157" 2, 169; 3, 190; 4, 205; 5,218 K.

The band at 2174 cm -~ (HF band) corresponds to the C - O stretching mode of CO interacting, trough the carbon atom, with the zeolite Br0nsted acid groups: OH...CO adducts. The weak and complex IR absorption band in the low frequency region can be resolved into two components, which peak at about 2126 and 2120 cm -~. The component at 2126 cm -~ is the 13CO counterpart of the HF band, while the remaining weak band at 2120 cm -1, hereafter termed LF band, is assigned to the C - O stretching mode of OH...OC adducts where the CO molecule is hydrogen bonded to the zeolite Br0nsted acid sites through the oxygen atom. At

210 77 K the intensity of the LF band was extremely low. However, on raising the temperature, when the HF band starts to decrease the LF bands becomes comparatively more intense. Note, by contrast, that the band at 2126 cm -~ rapidly decreases.

0.91

0.8 0.7 0.6 0.5

..Q ~ 0.4 .Q < 0.3

x2 J!

0.2

4

0.1

0.0

2200

2180

2160

2140

2120

Wavenumber (cm 1)

Fig. 2. Selected FTIR spectra of CO (ca. 1.2 Torr) adsorbed on Rb-ZSM-5 at variable temperatures: 1, 119" 2, 149; 3, 170; 4, 178 K.

Selected variable-temperature spectra of CO adsorbed on Rb-ZSM-5 are shown in Fig. 2. The HF band, which corresponds to the C - O stretching of Rb+...CO adducts, is observed at 2161 cm -l. The LF band, assigned to Rb+...OC species, appears at 2119 cm -1. Besides these two bands, another one (shoulder) is also seen at about 2148 cm -~. The nature of this latter band is not completely clear; it cannot be due to interaction of CO with silanols, since at the small CO dose used for IR measurements (about 1.2 Torr) no effect was observed on the silanol band (at 3748 cm-l). We tentatively assign the band at 2148 cm -~ to the C - O stretching of Rb+...CO adducts formed by CO adsorbed on Rb + ions which are more shielded by zeolite framework oxygens than those giving rise to the band at 2161 cm -1. This assignment finds support in the fact that both EXAFS measurements [26] and theoretical studies [27,28] have shown that in Cu+-ZSM-5 there are two different sites for extra-framework Cu + ions, which can be coordinated to either two or three to four oxygen atoms of the zeolite framework; the Rb + ion could well be in a similar situation. Other tentative explanations have been proposed for the band at 2148 cm ~ and for a similar one (at 2150 cm -l) observed for CO adsorbed on K-ZSM-5 [ 18,29]. They include the simultaneous adsorption of two CO molecules on a single

211 cation [29] and the C - O stretching of carbon monoxide C-bonded to the alkali-metal cation at an angle with the corresponding axial electric field [30]. While the precise nature of the 2148 cm -~ band is still open to debate, we note that this does not affect the main purpose of our work, which concerns correlation between HF and LF bands. Fig. 2 shows that when the temperature is increased from 119 to 149 K the intensity of the HF band decreases, whereas that of the LF band increases. At higher temperatures both bands decrease in intensity, because the net amount of adsorbed CO decreases, but the ratio of integrated intensities, ALF/AHF, was found to increase over the whole temperature range. Note that the integrated intensity of the HF band, AHF, was measured after deconvolution and subtraction of the shoulder at 2148 cm -~. Regarding the LF band, it should be noted that the 13CO counterpart of the HF band overlaps the same frequency region; for this reason, integrated ALF values were corrected by subtracting 1% of the corresponding AHF values (1% is approximately the natural abundance of the 13CO isotope). For the CO/H-ZSM-5 system, integrated intensities of the HF and LF bands were also determined, and the ratio ALF/AHF was found to increase over the whole temperature range; in a similar way as that found for the CO/Rb-ZSM-5 system. Qualitatively, the same behaviour of the corresponding HF and LF bands was also reported for CO adsorbed on Na-ZSM-5, KZSM-5 and H-Y [13,15,18]. This behaviour can be explained as follows. The equilibrium constant, K, of Eq. (1) in the Introduction section should be equal to the ratio 0oc/0co, where 0oc and 0co are the fractional coverages of O- and C-bonded species, respectively. Hence, K=

(ALF/AHF)(EHF/ELF)

(2)

Where EHF and eLF are the molar absorption coefficients of the corresponding IR absorption bands. In the isomerization equilibrium described by Eq. (1), the temperature dependence of K should be given by the van't Hoff equation, In K= (-AH~

(3)

+ (AS~

Combination of Eqs. (2) and (3) yields, In (ALF/AHF)= (-AH~

+ (AS~

+ In (eLF/eHF)

(4)

Fig. 3 shows that Eq. (4) is obeyed for both systems over the whole temperature range studied (from about 100 K to room temperature). It is therefore concluded that C- and Obonded species are in a temperature-dependent equilibrium. Corresponding values of AH ~ the enthalpy change in the isomerization equilibrium, can be directly deduced from the linear plots in Fig. 3. The values found were AH~ 4.6 kJ tool -1 for the CO/H-ZSM-5 system and AH~ 1.8 kJ mol 1 for CO adsorbed on Rb-ZSM-5. Note that these enthalpy values do not depend on molar absorption coefficients of HF and LF bands, which would only affect the vertical intercepts of the van't Hoff plots. For the case of the CO/Rb-ZSM-5 system, the enthalpy difference, at 77 K, between Cand O-bonded species was also determined by means of quantum chemical calculations on the interaction between CO and the cluster model [HAI(OH)3]-Rb +, which was used to simulate cation sites in zeolites [ 11 ]. The reported result is 2.5 kJ mol -~, which is not too different from

212 the present value of 1.8 kJ mol -~. In further agreement with the present experimental results, quantum chemical calculations have also proved [11,12] that C-bonded adducts show a higher interaction energy than O-bonded species. SD =0.014

-1.0 -

R 2 = 0.997

-1.5

-2.0 -2.5 u_ "i"

-3

<:

_=

-3.0

9 Rb-ZSM-5 9 H-ZSM-5

-3.5 SD =0.034

-4.0

R 2 = 0.998

-4.5 -5.0 ,

4

!

,

6

( l / T ) 10 3 / K "1

Fig. 3. The van't Hoff plots of the natural logarithm of the intensity ratios of the LF and HF bands, for CO/H-ZSM-5 and CO/Rb-ZSM-5, versus reciprocal temperature. R: linear regression coefficient ; SD: standard deviation.

As already quoted in the Introduction, previous experimental measurements on the CO/Na-ZSM-5 and CO/K-ZSM-5 systems gave corresponding AH ~ values (in kJ mol -~) of 3.8 and 3.2, respectively. When taken together with the present value of 1.8 for CO/Rb-ZSM-5, it becomes clear that the enthalpy involved in the isomerization equilibrium between M+...CO and M+...OC species decreases as the size of the cation increases; a fact which is also in agreement with quantum chemical calculations on model systems [10-12]. However, for CO adsorbed on the faujasite-type zeolite Na-Y the corresponding AH ~ value was found to be 2.4 kJ mol -~ (as compared to 3.8 in CO/Na-ZSM-5); this clearly shows that AH ~ depends not only on the cation considered but also on the zeolite structure type. In Na-Y, the Na § ion is more shielded by framework oxygen atoms than in Na-ZSM-5 [3]; hence, the net electric field is smaller. Although further confirmation is desirable, the magnitude of the electric field created by the extra-framework cation and nearby framework oxygens appears to be a major factor determining the energy involved in reversing the orientation of the dipolar CO molecule. For CO adsorbed on protonic zeolites, the present value of AH~ 4.6 kJ mol ~ for the enthalpy involved in the isomerization equilibrium between hydrogen-bonded OH...CO and OH...OC species is to be compared with that of AH~ 4.3 kJ mol -R reported for the CO/HY system. Both values are nearly coincident within experimental error, which is estimated to be about +0.5 kJ mol -~. However, it should be noted that H-ZSM-5 is known to have a stronger BrOnsted acidity than H-Y [21,31 ]; and this factor could, in principle, affect the magnitude of AH ~ The whole subject, which is of considerable interest for both theoretical aspects of

213 hydrogen bonding [32-34] and technological applications of zeolites and related materials [35-38], can hopefully be further developed by systematic experimental studies on other zeolites and related periodic porous solids, combined with appropriate quantum chemical calculations.

REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

23 24

J.C. Lavalley, Catal. Today, 27 (1996) 377. A. Zecchina and C. Otero Arefin, Chem. Soc. Rev., 25 (1996) 377. H. Kn6zinger and S. Huber, J. Chem. Soc., Faraday Trans., 94 (1998) 2047. C. Otero Are~in, O. V. Manoilova, M. Rodrfguez Delgado, A. A. Tsyganenko and E. Garrone, Phys. Chem. Chem. Phys., 3 (2001) 4187. A. Zecchina, S. Bordiga, C. Lamberti, G. Spoto, L. Carnelli and C. Otero Arefin, J. Phys. Chem., 98 (1994) 9577. C. Otero Are~in, G. Turnes Palomino, E. Escalona Platero and M. Pefiarroya Mentruit, J. Chem. Soc., Dalton Trans., (1997) 873. H. B6se and H. F6rster, J. Mol. Stmct., 218 (1990) 393. M. Katoh, T. Yamazaki and S. Ozawa, Bull. Chem. Soc. Jpn., 67 (1994) 1246. G. Turnes Palomino, C. Otero Are~in, F. Geobaldo, G. Ricchiardi, S. Bordiga and A. Zecchina, J. Chem. Soc., Faraday Trans., 93 (1997) 189. A.M. Ferrari, P. Ugliengo and E. Garrone, J. Chem. Phys., 105 (1996) 4129. A.M. Ferrari, K. M. Neyman and N. R6sch, J. Phys. Chem., 101 (1997) 9292. P. Ugliengo, E. Garrone, A. M. Ferrari, A. Zecchina and C. Otero Arefin, J. Phys. Chem. B, 103 (1999) 4839. C. Otero Arefin, A. A. Tsyganenko, O. V. Manoilova, G. Turnes Palomino, M. Pefiarroya Mentruit and E. Garrone, Chem. Commun., (2001) 455. G. Schatte, H. Willner, D. Hoge, E. Kn0zinger and O. Schrems, J. Phys. Chem., 93 (1989) 6025. C. Otero Arefin, A. A. Tsyganenko, E. Escalona Platero, E. Garrone and A. Zecchina, Angew. Chem. Int. Ed., 37 (1998) 3161. A. A. Tsyganenko, E. Escalona Platero, C. Otero Are~in, E. Garrone and A. Zecchina, Catal. Lett., 61 (1999) 187. A.A. Tsyganenko, C. Otero Arefin and E. Escalona Platero, Stud. Surf. Sci. Catal., 130 (2000) 3143. O.V. Manoilova, M. Pefiarroya Mentruit, G. Turnes Palomino, A. A. Tsyganenko and C. Otero Are~in, Vib. Spectrosc., 26 (2001) 107. G. Turnes Palomino, M. Pefiarroya Mentruit, A. A. Tsyganenko, E. Escalona Platero and C. Otero Are~in, Stud. Surf. Sci. Catal., 135 (2001) 219. W . M . Meier and D. H. Olson, Atlas of Zeolite Structure Types, Butterworth, London, 1992. R. Szostak, Molecular Sieves, Van Nostrand Reinhold, New York, 1989. M . M . J . Treacy and J. B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Elsevier, Amsterdam, 2001. M.A. Babaeva, D. S. Bystrov, A. Yu. Kovalin and A. A. Tsyganenko, J. Catal., 123 (1990) 396. C. Otero Are~in, O. V. Manoilova, A. A. Tsyganenko, G. Turnes Palomino, M. Pefiarroya Mentruit, F. Geobaldo and E. Garrone, Eur. J. Inorg. Chem., (2001) 1739.

214 25 26 27 28 29 30 31 32 33 34 35 36 37 38

G.C. Pimentel and A. McClellan, The Hydrogen Bond, W. H. Freeman, San Francisco, 1960. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo, G. Vlaic and M. Bellatreccia, J. Phys. Chem. B, 101 (1997) 344. D. Nachtigallov~, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 1 (1999) 2019. P. Nachtigall, D. Nachtigallov~i and J. Sauer, J. Phys. Chem. B, 104 (2000) 1738. K. Hadjiivanov, P. Massiani and H. Kn6zinger, Phys. Chem. Chem. Phys., 1 (1999) 3831. M. Katoh, T. Yamazaki and S. Ozawa, J. Colloid Interface Sci., 203 (1998) 447. A.G. Pelmenshchikov, E. A. Paukshtis, V. G. Stepanov, V. I. Pavlov, E. N. Yurchenko, K. G. Ione, G. M. Zhidomirov and S. Beran, J. Phys. Chem., 93 (1989) 6725. M.J. Calhorda, Chem. Commun., (2000) 801. D. Hadzi, Theoretical Treatments of Hydrogen Bonding, Wiley, Chichester, 1997. D. A. Smith, Modeling the Hydrogen Bond, ACS Syrup. Series No. 569, American Chemical Society, Washington D.C., 1994. C. Otero Are~in, Comments Inorg. Chem., 22 (2000) 241. G.A. Ozin, A. Kuperman and A. Stein, Angew. Chem. Int. Ed., 28 (1989) 359. G.D. Stucky, Progr. Inorg. Chem., 40 (1992) 99. K. Moller and T. Bein, Chem. Mater., 10 (1998) 2950.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

P r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of Z n - M F I chain a l k y l a m i n e s as mineralizing agents

215

zeolites

using

short

S. Valange a'b*, B. Onida c, F. Geobaldo c, E. Garrone c and Z. Gabelica ~ a ENSCMu, Groupe S6curit6 et Ecologie Chimiques, 3, Rue A. Werner, F-68093, Mulhouse Cedex, France b Present address: LACCO, UMR CNRS 6503, ESIP, 40, Av. du Recteur Pineau, F-86022 Poitiers, France c Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy We report in detail the direct synthesis and the full characterization (XRD, SEM, microprobe analysis) of a series of MFI zincosilicates prepared through the shortchain alkylamine route. Special attention was devoted to the description of the local environment of the Zn species in the MFI lattice after exposing the as synthesized zeolite to various thermal treatments, by means of FTIR. A model depicting the structural state of the various Zn species in our Zn-MFI is presented and discussed. 1. I N T R O D U C T I O N Recently, Zn-containing zeolites, and especially ZnHZSM-5 (or HZn-MFI), attracted much attention because of their remarkable catalytic activity in dehydrogenation and aromatization reactions [1]. Such catalysts are currently prepared by indirect methods, essentially by modifying H-MFI through various post-syntheis treatments such as ionic exchange [2], incipient wetness impregnation [3], Zn or ZnC12 sublimation [4] or solid state reaction with ZnO [5] or metallic Zn [6]. Since the first patent recipe describing the direct crystallization of Zn-MFI in NaOH medium [7], very few studies were conducted in that direction probably because of the inevitable formation of Zn oxides or hydroxides that readily co-precipitate in mild alkaline media. Such a drawback was recently attempted to overcome with variable success, by using various complexing agents such as fluoride [8] or phosphate [9] ions, or citric acid (in the case of Zn-MOR) [10]. In a preliminary attempt [11], we have tried to s o l u b i ~ e Zn 2+ and Cu 2+ ions within the gel precursor through complexation with the mineralizing agent, prior to the hydrothermal crystallization of the zeolite. Short chain alkylamines, knowing to efficiently form soluble and relatively stable complexes with a large variety of metallic ions [12], proved very efficient to mobilize both ions. Here we report in detail the synthesis and the full characterization by XRD, microprobe analysis, SEM and spectroscopic methods of a series of MFI zincosilicates prepared through the amine route. FTIR was

216 specifically used to describe the behavior of Zn in the MFI lattice and to monitor its structural rearrangements after exposing the as-synthesized Zn-MFI to various thermal treatments. 2. E X P E R I M E N T A L

Zinc-containing MFI zeolites were synthesized by direct crystallization of hydrogels involving zinc nitrate, TPABr and Aerosil-200 as silica source, in the presence of various alkylamines, namely methylamine (MA), ethylamine (EA), diethylamine (DEA) or ammonia. Heating was achieved at 185~ in Teflon autoclaves and in static conditions for periods ranging from 1 to 13 days. The nature, purity and crystallinity of the phases were determined by XRD (Philips PW 1800 diffractometer, Cu-I~). Crystal size and morphology analyses by SEM were carried out using a Philips XL30 microscope coupled to an electronic microprobe for quantitative analyses. The Zn content was determined using bulk chemical analysis. Two particular samples, prepared in the presence of methylamine, were fully characterized by FTIR. These are an as-synthesized, 100% crystalline Zn-containing Al-free zeolite (Zn-MFI-a) and the corresponding calcined (air, 600~ sample (Zn-MFI-c). FTIR spectra were obtained on a Bruker IFS 55 Equinox spectrometer equipped with a MCT cryodetector, at a resolution of 2 cm -1. Thin self-supporting wafers of the zeolite were prepared and placed in a cell allowing thermal treatments either in vacuum or in controlled atmosphere. For CO adsorption experiments, the temperature of the cell was decreased to 196~ by cooling the sample with liquid nitrogen. Zn-MFI-a was first directly calcined under vacuum inside the IR cell before re-activation. Both the in situ calcined Zn-MFI-a and Zn-MFI-c were then re-activated in vacuum inside the IR cell at 500~ prior to the IR measurements. 3. R E S U L T S AND D I S C U S S I O N 3.1. S y n t h e s i s o f Zn-MFI w i t h v a r i o u s a l k y l a m i n e s or a m m o n i a . As part of a research aiming at preparing Cu-MFI and mixed (Cu,Zn)bearing MFI zeolites used as efficient catalysts for various oxidation reactions [13], we decided to prepare pure Zn-MFI as reference, by direct synthesis following the same conditions as for the analogous Cu-MFI [14], namely by using the same alkylamine route. We have already partially mentioned the potential successful preparation of MFI zincosilicates from alkylamine-based alkali-free media [15]. By forming medium strong soluble complexes with alkylamines, we have shown that zinc ions can be readily mobilized, so that their incorporation in the MFI lattice is a p r i o r i easy. In the present approach, besides the detailed synthesis description, special attention is devoted to the study of the behavior of zinc in the MFI lattice and its eventual changes and structural rearrangements upon calcination. Table 1 shows the XRD and SEM characteristics of Zn-MFI intermediate and final phases prepared in methylamine medium, for variable heating times. It appears that the crystallization rate of Zn-MFI is quite low if compared to that of

217 other heteroelement-bearing MFI prepared through the alkylamine route [12]. Indeed, a heating time longer t h a n 6 days is required to obtain a purely crystalline Zn-MFI without traces of amorphous phase. In contrast to the syntheses of silicalite, A1-MFI, B-MFI or even Ga-MFI in the presence of methylamine, the crystallization of Zn-MFI is far longer, as it was also observed for Ti-MFI [16] and seems to depend on the intrinsic properties of the cations (e.g. ionic radius) but also on the ability of methylamine to bring the metallic ion into the Si-bearing hydrogel and form various metallosilicate oligomers (induction period) t h a t would further start to crystallize [17-18]. Table 1 Characterization by XRD and SEM of various Zn-MFI precursor and final phases crystallized hydrothermally after variable heating times.

Heating time (days)

Nature of the intermediate phases (XRD)

Description of the SEM micrographs

1

Amorphous + ~ MFI

Amorphous phase + ~ prismatic crystals (< 20 ~m) Amorphous phase + ~ prismatic crystals + 8 crystal aggregates (30-40 ~m) Not investigated Zn-MFI (see below) + ~ amorphous phase Large and thick Zn-MFI crystals, sometimes twinned into bow-tie shaped single crystals (50-60 ~tm)

Amorphous + ~ MFI 3 6

MFI + amorphous phase MFI + ~ amorphous phase MFI

13

MFI zincosilicates with no more than about two Zn per MFI unit cell could be obtained from the short-chain alkylamine route. The nature of the base and its complexing power were among the critical variables that could be controlled to monitor an optimized incorporation of Zn into the zeolite. Methylamine, ethylamine and ammonia proved to be the most efficient mobilizing-complexing agents, while diethylamine appeared to be far less suitable in incorporating zinc, as ascertained by chemical analysis (Table 2) and also as suggested by the relatively elongated prismatic morphology of the crystals along the c axis, analogous to that of silicalite (SEM not shown). Table 2 Bulk chemical analysis and Si/Zn mol ratios of Zn-MFI zeolites prepared with various alkylamines and ammonia

Nature of the amine

Si]Zn mol. ratio(gel)

Si (wt. %)

Zn (wt. %)

Si/Zn mol. ratio (crystal)

MA NH3 EA DEA

47 47 47 47

39.6 39.8 39.9 39.4

1.87 1.74 1.85 1.58

47.7 51.5 48.6 56.2

218 In this particular case, the zinc incorporation in the MFI network is somewhat hindered possibly because of the rather strong Zn amino-complexes formed in the gel precursor. The relatively strong Zn-diethylamine associations probably do not release easily the appropriate Zn species for a possible condensation with silica so as to form a Zn bearing hydrogel and/or MFI framework. Conversely, the use of ammonia and the other alkylamines resulted in the formation of large and thick Zn-MFI crystals, sometimes twinned into bow-tie single crystals or polycrystalline lamellar aggregates (Figs 1-3).

Fig.2. Scanning electron micrograph of the Zn-MFI crystals prepared in the presence of ammonia ( = 100, 20 and 50 ~m respectively). Microprobe mapping of the phase prepared in methylamine medium, ZnMFI(MA), indicates that the Zn 2§ ions were not homogeneously partitioned throughout the 60-65 ~m sized crystallites. A Zn depleted core and an Zn enriched outer rim are observed (Fig 4a), suggesting that Zn integrated the zeolite at the end of the crystallization process. This result can not be explained by only considering the relatively high ionic radius of Zn 2§ ions, as, in some cases, very large ions such as Ge 4§ are rapidly incorporated [19]. As sometimes observed in hydrothermal synthesis in the presence of metallic ions complexed by amines, some of such mineralizing agents can form very strong metal complexes, and therefore prevent their stepwise dissociation with the release of the metallic ionic species before their incorporation in the growing zeolite crystallite [15]. In the

219 present case, the metal complex will decompose and release the metallic species more readily at the end of the crystallization, which will result in a Zn-enriched outer rim of the crystals. Other attempts to prepare MFI zincosilicates with larger Zn contents proved inefficient probably partially due to the inappropriate size (ionic radius) of the zinc ions. Indeed, zinc as well as some other bulky elements like Cd or Ti, are less readily accommodated in tetrahedral zeolitic sites because of the Pauling's rule [12] and thus are not good candidates for an extended incorporation, in zeolitic lattices.

Fig.4. X-ray emission mapping of Zn (left) and Si (right) of a Zn-MFI crystallite prepared in methylamine medium.

3.3. Investigation of the local e n v i r o n m e n t variously heat treated Zn-MFI(MA).

of the zinc species

in

The detailed nature and localization of the Zn species in Zn-MFI-a and ZnMFI-c were investigated by FTIR. In a first step, the zeolite acidity was evaluated by probing the OH stretching region and the CO adsorption region. The FTIR spectrum in the OH stretching region (Fig 5) of Zn-MFI-a, of silicalite and Zn-MFI-c shows two bands at about 3730 cm 1 and 2690 cm -1, as well as a broad absorption in the range 3500-3200 cm -~. The band at 3730 cm -~ is due to terminal SiOH groups corresponding to lattice defects, while the poorly resolved broad band occurring in the range 3500-3200 cm -~ is due to H-bonded

220 silanols present in lattice nanodefects due to T missing sites (hydroxyl nests) [20]. The assignment of the 3690 cm -1 band is less straightforward, as this vibration was never observed in silicalite samples so far. This band, systematically observed in various MFI metallosilicate samples prepared via the methylamine route, was tentatively ascribed to isolated internal silanols characterized in particular geometric environments, as discussed earlier [21]. The presence of Zn in Zn-MFI-a gives a new medium-strong band at 3640 cm -1 (arrow in the figure), which we tentatively ascribe to isolated ZnOH species partly anchored to the zeolite framework. Indeed, although such a band is very difficult to observe in variously prepared Zn-MFI materials [22], Scarano et al. [23] have reported t h a t IR bands corresponding to the stretching mode of hydroxyl groups linked to isolated Zn 2§ ions in various Zn hydroxy-oxidic type compounds should appear in the 3620-3674 cm -1 spectral region. However, the absence of this band in the OH vibrational range of Zn-MFI-c, indicates t h a t calcination in air at 600~ mainly destroys these new Zn-OH species.

1.0

o t-

0 gg

0.5-

0.0

38()0

'

3 oo

'

3,;oo

'

wavenumbers (cm~)

32'oo

'

3ooo

Fig.5. FTIR spectra of various zeolite samples prepared via the methylamine route and outgassed at 500~ curve a: Zn-MFI-a as-made, then activated in vacuum in the IR cell; curve b: calcined silicalite and curve c: calcined Zn-MFI-c On the other hand, in Zn-MFI-a, these species readily interact with CO through H-bonding, indicating some acidic character (OH band at 3450 cm -1, spectrum not shown), although they do not behave as true BrSnsted acid centers (absence of any interaction with ammonia [21]). We have indeed already shown t h a t the shift observed is of about 190 cm -1 and is far larger t h a n the shift observed (45 cm 1) for the most acidic ZnOH species present in ZnO and even larger t h a n the shift observed (about 100 cm -1) for the acidic silanols in silicalite

221 [21]. This therefore means t h a t these ZnOH species in Zn-MFI-a, which are more acidic t h a n silanols groups in silicalite, represent a new species t h a t should not be related to the presence of any ZnO type phase but r a t h e r belong to a species truly anchored onto the zeolite lattice. Moreover, the acidity of these ZnOH species is comparable to t h a t of A1OH species found in some dealuminated zeolites and still partly anchored to the zeolite framework [24]. Assuming t h a t ZnOH species are similarly anchored to the zeolite surface and hence not truly positioned on the t e t r a h e d r a l framework sites, we tentatively propose the following structural state for such species (scheme a):

\ /

Si

/_

Sit

~o/

~

N

\ /

Si

/_

~o/

Si

\

OH

I

/ O

Zn,.

......

/

.....O"

I

I

Si

Si

/I \ /I \ Scheme a

H

0

J

Zn

I

0

I

Si

Si

/l\Jl

\

Scheme b

Considering the CO stretching region of the same Zn-MFI-a sample, it comes out t h a t the adsorption of CO leads to the appearance of a new band. Indeed, besides the presence of the intense CO stretching b a n d at 2165 cm -1 corresponding to the OH stretching frequency at 3450 cm -1 and ascribed to CO interacting with ZnOH species t h r o u g h H-bonding, a new wide band around 2200 cm -1 was observed. It is assigned to exposed zinc (isolated Zn linked to two Si-O- groups thus behaving as Lewis type acid sites) t h a t can interact with CO (scheme b). At the extreme limit, both species can also be considered as partly belonging to a very defected MFI framework, as already observed in the case of Ge-MFI [19]. While the absorption at 2165 cm 1 in Zn-MFI-c, due to CO interacting with acidic ZnOH, has almost disappeared, two other distinct bands above 2200 cm 1 (2211 and 2204 cm -1 at high CO coverage) are seen. They are logically assigned to dicarbonylic Zn species generated t h r o u g h extensive carbonylation of the dehydrated species in scheme (b). Surprisingly, the spectra obtained u n d e r

222 different CO coverages (not shown here) do not suggest the steady sintering of such "ZnO" species expected to occur at high temperatures. This unexpected behavior is currently more in depth investigated elsewhere [25]. REFERENCES 1. M.V. Frash, R.A. van Santen, Phys. Chem. Chem. Phys., 2 (2000) 1085 and references cited therein. 2. P. Laidlaw, D. Berthell, S.M. Brown, G. Watson; D.J. Willock, G.J. Hutchings, J. Mol. Catal. A, 178 (2002) 205 and references cited therein. 3. V.R. Choudhary, S.K. Jana, Appl. Catal. A, 224 (2002) 51 and references cited therein. 4. A. Seidel, F. Rittner, B. Boddenberg, J. Phys. Chem. B, 102 (1998) 7176. 5. K. Osako, K. Nakashiro, Y. Ono, Bull. Chem.Soc. Jpn., 66 (1993) 755. 6. J. Heemsoth, E. Tegeler, F. Roessner, A. Hagen, Microporous Mesoporous Mater., 46 (2001) 185 and references cited therein. 7. S.A.B. Barri, R. Tahir, Chemical Process and Catalyst, Eur. Patent Appl. N ~ 0 351 067 (1989) ; ibid, US Patent N ~ 5 208 201 (1993). 8. S. Kowalak, E. Szymkowiak, I. Lehman, G. Giordano, Stud. Surf. Sci. Catal., 135 (2001) 311. 9. A. Katovic, E. Szymkowiak, G. Giordano, S. Kowalak, A. Fonseca, J. B.Nagy, Stud. Surf. Sci. Catal. 135 (2001), 337. 10. M. Dong, J. Wang, Y. Sun, Microporous Mesoporous Mater., 43 (2001) 237. 11. S. Valange, Z. Gabelica, M. Lopez Granados, S. Rojas, J.L.G. Fierro, in Book of Abstracts, 11th Intern. Zeolite Conf., Seoul, Korea, RP 39 (1996). 12. Z. Gabelica, S. Valange, Res. Chem. Intermed., 24 (1998) 227. 13. S. Valange, Z. Gabelica, M. Abdellaoui, J.M. Clacens, J. Barrault, Microporous Mesoporous Mater., 30 (1999) 177. 14. S. Valange, F. Di Renzo, E. Garrone, F. Geobaldo, B. Onida, Z. Gabelica Stud. Surf. Sci. Calal., 135 (2001) 175. 15. Z. Gabelica, S. Valange, Microporous Mesoporous Mater., 30 (1999) 57. 16. M. Shibata, Z. Gabelica, Zeolites, 19 (1997) 246. 17. M. Shibata, Z. Gabelica, Microporous Mater., 12 (1997) 141. 18. M. Shibata, Z. Gabelica, Microporous Mater., 11 (1997) 237. 19. Z. Gabelica, J.L Guth, Angew. Chem. Int. Ed., 28 (1989) 81. 20. A. Zecchina, S. Bordiga, G. Spoto, L. Marchese, G. Petrini, G. Leofanti and M. Padovan, J. Phys. Chem., 96 (1992) 4991. 21. S. Valange, Z. Gabelica, B. Onida, E. Garrone, in Proc. 12th Intern. Zeolite Conference, Baltimore, 1998, (Eds.) M.M.J. Treacy et al., MRS, Warrendale, Pennsylvania, USA, Vol. IV (1999) 2711. 22. V.B. Kazansky, V.Yu. Borovkov, A.I. Serykh, R.A. van Santen, B. Anderson Catal. Lett., 66 (2000) 39. 23. D. Scarano, G. Spoto, S. Bordiga, A. Zecchina, C. Lamberti, Surface Science, 276 (1992) 281. 24. B. Onida, F. Geoblado, F. Testa, F. Crea, E. Garrone, Microporous Mesoporous Mater., 30 (1999) 119. 25. B. Onida, F. Geobaldo, S. Valange, Z. Gabelica, E. Garrone, in preparation.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

223

Crystal growth o f nanosized L T A zeolite from precursor colloids S. Mintova*, B. Fieres and T. Bein Department of Chemistry, University of Munich, Butenandtstr. 5-13 (E), 81377 Munich, Germany The crystal growth of nanosized LTA type zeolite from a colloidal precursor solution at different temperatures was investigated. Dynamic light scattering (DLS) in backscattering mode monitors the evolution of particle size in synthesis solutions during aging and subsequent heating at 50~ 70~ and 90~ Complete transformation from amorphous aluminosilicate colloidal particles into crystalline LTA type zeolite is observed atler extended hydrothermal treatment of the aged precursor solution. The resulted LTA crystals have mean hydrodynamic radius of about 50 nm, which corresponds to the size of the amorphous entities formed in the precursor solutions spontaneously a~er mixing of the initial substances. The size of the amorphous particles formed in the precursor mixture affect the ultimate size of the LTA nanocrystals, while the synthesis temperature influence on the rate of zeolite crystallization. On heating of these suspensions for longer time, the Ostwald ripening resulted in additional substantial crystal growth and formation of LTA crystals with bigger particle size (-~230 nm). 1. INTRODUCTION The chemistry of zeolite formation depends on many factors including the type of the initial precursor solutions, solubility of various species, aging time, crystallization temperature, time of heating, etc. [1]. Usually studies of the zeolite nucleation and crystallization mechanism are based on investigations of the amorphous precursor suspensions and crystalline particles by ex-situ methods such as X-ray diffraction, SEM, IR spectroscopy, and etc. Since the events in the early stages are of critical importance in determining the course of the subsequent crystallization, it is clear that a detailed understanding of these phenomena must be obtained if the goal of rational control of the synthesis of these materials could be achieved. The synthesis of microporous nanocrystals in the form of stable colloidal suspensions was one of the important events in the zeolite science during the past decade. Several microporous materials, amongst others MFI-, LTA-, FAU-, BEA-type materials, have been prepared in the form of colloidal suspensions with narrow particle size distribution [2-6]. The synthesis of zeolite crystals with equal particle size requires homogeneous distribution of the viable nuclei in the system. Therefore, the homogeneity of the starting system and simultaneity of the Corresponding author. E-mail: [email protected]

224 events leading to formation of precursor gel particles and their transformation into crystalline zeolitic material is of primary importance. In order to obtain such homogeneous starting systems abundant amounts of tetraalkylammonium hydroxides and water were employed [710]. On the other hand the content of alkaline cations is very limited. All these factors together with the careful choice of the reactants allow the stabilization of "clear" starting mixtures where only discrete gel particles are present. The absence of microscopic gel phase in the precursor mixture makes it possible to conduct in-situ measurements during the crystal growth of molecular sieves from clear precursor solutions [11-13]. In-situ measurements have provided information about the evolution of the crystalline zeolite phase, average particle size, particle size distribution, and the ratio between amorphous and crystalline species. In the present paper, we report on in situ DLS investigation of the crystal growth of LTA type zeolite from colloidal precursor solutions at three different temperatures. The evolution of particle size is monitored by the backscattering of light in the synthesis solution during hydrothermal treatment. 2. EXPERIMENTAL Nanosized LTA crystals were synthesized from colloidal precursor solution with the following molar composition: 0.4Na20: 11.25SIO2: 1.8Al203: 14.0(TMA)20: 2001-120, by mixing colloidal silica sol (30 wt. %), tetramethylammonium hydroxide pentahydrate (TMAOH x 51-/20), sodium hydroxide, aluminium isopropoxide, and doubly distilled water. The crystallization of LTA zeolite was performed at 50~ 70~ and 90~ in a quartz cuvette mounted in a heating sub-unit in a DLS instrument (ALV-NIBS/HPPS) and in a forced air oven. The resulting nanosized particles were separated from the mother liquid by two-step centrifugation at 20,000 rpm for 2 h. Dynamic light scattering was used (ALV-NIBS/HPPS) to investigate the precursor solutions at room temperature (RT), and upon hydrothermal (HT) treatment. The back scattering geometry (scattering angle 173 ~ HeNe laser with 3 mW output power at 632.8 nm wavelength) allows measurements at high sample concentration, since a complete penetration of the incident light through the sample was not required. All samples were characterized with both the cumulant and distribution function analyses (DFA). By applying the DFA the particles were classified in logarithmic radius classes and for each of them the relative number or mass content was determined. The Rayleigh-Debye model [ 13] was used for the DFA. The results from DFA were displayed as an unweighted particle size distribution, which shows the scattered intensity per particle size classes. X-ray diffraction (XRD) powder data of the samples were collected on a Scintag XDS 2000. FT-IR spectra of samples in KBr pallets were obtained on a Bruker Equinox spectrometer. Images of the LTA zeolite particles were taken on a Philips XL 40 scanning electron microscope (SEM).

225 3. RESULTS AND DISCUSSION A clear precursor solution, prepared as described above, was aged at RT on an orbital shaker and after 24 h was transferred into a quartz cuvette and subjected to DLS measurements. The back-scattering of light produced from these X-ray amorphous samples was strong enough to be monitored as a function of the particle numbers and particle mass. The DFA results from initial mixture subjected to hydrothermal (HT) treatment at 50 ~ are depicted in Fig. 1. The presence of only one class colloidal particles with mean radius of about 10 nm was found in the solution after 24 h aging at RT. No changes were observed in the mean radius and particle size distribution in this sample alter 3 h HT treatment (see Fig. 1). The solid extracted from this sample was amorphous according to XRD analyses. Further increase of the HT treatment to 19 h leads to the formation of a second generation of particles with a mean radius of about 50 nm (Fig. 1). Most of the small fraction of particles (~-10 nm) is consumed and transformed into 50 nm particles under prolonged heating up to 24 h (Fig. 1).

1

~

....

3

2

Fig. 1. DLS data of precursor mixture subjected to

~. ,9~~i!~~ l~ 1

HT treatment at 50 ~ for 3 h (1), and 19 h (2) and

'~ .~

ii, ii!!

24 h (3). The DFA is displayed as scattering

~

iii. ~i!.

' '~ ~.iL ' ~!!' ~ ~ii,

~ir,

9

.!i'

:'

t

I

1

10

!i~,

,

L

. !~. .

9 s

1

intensity per unweighted particle size classes.

#,

'ii.

t

ji

~

I ' I'

.

.

.

.

.

...... i- ~

1 O0

!

1000

Particle size, nm The corresponding particle mass distribution and particle number distribution functions show that during the first 5 h HT treatment, the content of the 10 nm colloidal particles is much higher than that of the 50 nm colloidal particles. A milky suspension is obtained alter HT treatment of the solution for 24 h at 50 ~ and sedimentation of the crystalline product is visible alter 40 h. The average hydrodynamic radius of the amorphous and crystalline particles based on the cumulant analysis does not change for the sample heated up to 24 h, and particles with mean radius about 50 nm has been observed in the X-ray amorphous (up to 10 h HT) and crystalline (alter 19 h HT) samples. Increasing the temperature for HT treatment of the initial precursor solution from 50 ~ to 70 ~ leads to a considerable decrease of the crystallization time necessary for complete transformation from amorphous into crystalline LTA nanoparticles, i.e., from 24 h to 10 h, respectively (Fig. 2). Monomodal particle size distribution has been measured in the freshly prepared solution and samples heated up to 2 h (the mean radius of colloids is about 10 nm).

226 1

iiiii~"! t

2

, ;,/ "~ e~

iii

'

!i!i I

,

Fig. 2. DLS data of precursor mixture !

'

10

100

subjected to HT treatment at 70 ~ for

I

'

I

2h(1),7h(2),9h(3)and24h(4). The DFA is displayed as scattering

|

|

intensity per unweighted particle size

|

1

classes.

9

', ~

i

1

4

,

'

9

3

1000

Particle radius, nm

Two populations of particles are recorded in the sample subjected to 7 h heating, the first with a size around 10 nm and the second with a size about 50 nm (Fig. 2). The amount of the first generation of particles (- 10 nm) is decreased with increasing crystallization time resulting in fully crystalline LTA sample alter 10 h heating. The particle size distribution in this sample is similar to the one observed under heating at 50 ~ except that the transformation process from the first particle generation (-10 nm) into second one (--50 nm) occurs faster at higher temperature (within 4 h). Fig. 2 clearly demonstrates that the content of the colloidal particles (-~10 nm) sharply decreases alter 9 h HT treatment. At the same time, an increase of the crystallinity of the LTA-type material was observed. Alter 10 h HT treatment, single crystals without any aggregates were obtained from this mixture (70 ~ while increase in the particle size was observed alter prolonged heating, which results in the new population of crystals with mean radius of about 230 nm (Fig. 2). The additional increase of the particle size is probably due to the Ostwald ripening, and the solution mass transfer is the dominant mechanism for the substantial crystal growth of LTA particles in the same solution but for longer heating time. This observation has been made based on m situ TEM investigation of the crystal growth of LTA zeolite from a precursor solution synthesized at RT and additional HT treatment [6]. The particle size of the LTA samples synthesized at different temperatures determined from the SEM is in a good agreement with the DLS data and the Ostwald ripening is demonstrated in Fig. 3. The purified crystalline sample obtained alter 10 h HT treatment of the solution reveals the presence of almost spheroidal particles with monomodal distribution (--50 nm), while formation of larger particles with size about 230 nm is visible alter longer heating (24 h). The particles exhibit crystalline morphology and no amorphous material is detected atter 10 h HT, although the typical cubic shape for LTA zeolite is not observed in this sample. All particles collected from solutions heated for 7 h, 9 h and 24 h show X-ray scattering suggesting the onset of crystalline order of the LTA structure (Fig. 4). Further supporting evidence related to the structure of the nanosized particles in the precursor solutions is obtained from IR spectroscopy. Two structural vibrational bands in the region of

227 400 - 700 c m "1 characterize the IR spectra of amorphous and crystalline silicalite samples, respectively 9 Distinct absorption appears at 450 crn1 and at 560 cm-1 (indicative of the double-four and six-rings existing in the LTA zeolite structure) in the spectra of the amorphous and crystalline zeolite samples. These data confirm that already at a very early stage of the synthesis (3 h), the topology of LTA is possibly formed but the sample exhibit Xray amorphous pattern.

Fig. 3. SEM images of LTA crystals synthesiszed at 70 ~ for (a) 10 h and (b) 24 h.

The initial mixture subjected to heating at 90 ~ for an hour shows only monodisperse particles with mean radius of about 10 nm, while a multimodal particle size distribution is observed in the sample after 3 h heating (Fig. 5). Complete transformation from amorphous to LTA crystalline colloidal particles is observed after hydrothermal treatment of the precursor solution for 6 h.

(c) o ,,,,4

(b)

(a) r

T

1

r

5

15

25

35

T ~

45

2 Theta, degrees Fig. 4. XRD patterns ofLTA nanocrystals obtained under HT treatment at 70 ~ for (a) 7 h, (b) 9 h and (c) 24 h.

228 The average hydrodynamic radius of the crystalline particles is about 50 nm and corresponds to the second generation of particles formed in all precursor mixtures prior to the appearance of the crystalline LTA type phase. The particle radii measured with the DLS technique is consistent with the size of the nanocrystals determined from the SEM images. The corresponding evolution of the light scattering results suggests that the rate of zeolite crystallization depends on the synthesis temperature, and the size of the crystalline particles is comparable to the size of the amorphous units formed immediately after mixing of the initial compounds.

r~ %lt~ i i I r~ # ii. = ?"~i , i~~i~. . . . . . ,. .", 9-= ili , i~ ji, ' :!~

9~

~ij~

~

i!i'

1

~

~ii,

10

/e~3

Fig. 5. DLS data of precursor mixture subjected to HT treatment at 90 oC for 1 h (1), 3 h (2),

/I

and 6 h (3). The DFA is displayed as scattering

ti'~~l

intensity per unweighted particle size classes.

100

1000

Particle size, nm

4. CONCLUSION The nanoscopic entities provided by mixing of the initial substances for the formation of clear precursor solutions and the particle size distribution during the transformation from amorphous into crystalline LTA crystals were studied by in situ DLS at different temperatures. The particle size distribution data provide evidence about the formation and consumption of species in the synthesis mixtures during the course of aging and heating processes. The small particles about 10 nm are consumed during the aging process and transformed into crystalline LTA particles with mean radius about 50 nm during hydrothermal treatment. The crystallization rate is increased with increasing the synthesis temperature but the ultimate crystalline particle size was constant. On heating of the suspension for longer time, solution mediated transport resulted in additional substantial crystal growth. Acknowledgement: Support for this work was provided by the BFHZ.

REFERENCES 1. R. Szostak, Molecular Sieves. Principles of synthesis and identification, Blakie Academic & Professional, London-Weinheim-New York-Tokyo-Melbourne-Madras, 1998. 2. A.E. Persson, B. J. Schoeman, J. Sterte, J.-E. Otterstedt, Zeolites 14 (1994) 557.

229 3. M. Tsapatsis, M. Lovallo, T. Okubo, M. E. Davis, Mater. Res. Soc. Symp. Proc. 371 (1995)21. 4. C. E. A. Kirschhock, R. Ravishankar, L. van Looveren, P. A. Jacobs, J. A. Martens, J. Phys. Chem. B 103 (1999) 4972. 5. Q. Li, D. Creaser, J. Sterte, Micropor. Mesopor. Mater. 31 (1999) 141. 6. S. Mintova, N. Olson, V. Valtchev, T. Bein, Science 283 (1999) 958. 7. S. Mintova, N. Olson, T. Bein, Angew. Chem. 38 (1999) 3201. 8. B.J. Schoeman, Micropor. Mesopor. Mater. 22 (1998) 9. 9. Q. Li, B. Mihailova, D. Creaser, J. Sterte, Microporous Mesoporous Mater. 43 (2001) 51. 10. B. J. Schoeman, E. Babouchkina, S. Mintova, V. Valtchev, J. Sterte, J. Porous Mater. 8 (2001) 13. 11. S. Mintova, N. Petkov, K. Karaghiosoff, T. Bein, Micropor. Mesopor. Mater. 50 (2001) 121. 12. B. J. Schoeman, Zeolites, 18 (1997) 97. 13. P. C. Hiemanz, R. Rajagopalan, Principles of colloid and surface chemistry, Marcel Dekker Inc, New York-Basel-Hong Kong, 1997.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

231

Synthesis o f hybrid zeolite disc from layered silicate Y. Kiyozumia, M. Salou b and F. Mizukami ~ a National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1, Higashi, Ibaraki 305-8565, Japan b Department of Chemistry, Toyohashi University of Technology Tempaku-Cho, Toyahashi 441-8580, Japan Silicalite / mordenite hybrid disc-shaped self-supporting membrane was synthesized from layered silicate such as kanemite by using solid state transformation. Mechanical strength (compression strength) of the disc showed over 10kg/cm:. Large differences in morphology and SiO2/AlzO3 ratio between both sides of the membrane were observed. The upper side (silicalite-side) of the membrane was made up of intergrowths of prism-like crystal morphology (ca. 1-21xm), the down side (mordenite-side) was composed of scale-like crystals (ca. >1 pan). The SiOz/A1203 ratios for upper and down sides of the disc were <600 and 15, respectively. The XRD patterns were comparable to silicalite and mordenite structure. I. INTRODUCTION Inorganic materials are generally much superior in thermal, mechanical and structural stability, and in chemical resistance compared with organic materials. In recent years, much attention has been paid to the preparation of zeolitic membranes because of their technological importance in catalysis, separation, sensing, optics and so on [1]. Zeolitic membranes have been prepared in various manners, in-situ hydrothermal synthesis or dry-gel conversion, with or without seed, their combinations or modifications, etc. Most of these zeolitic membranes have been used for separation of water-organic, organic-organic and gasorganic mixtures. Zeolites show different atYmities to hydrocarbon molecules depending on the composition of elements constituting the frameworks. If the hybrid zeolite membrane can be synthesized, the application to the highly selective separation membrane can be expected. Santamaria [2] et. al. reported that a-A1203 supported MOR/MFI/CHA hydrophilic tubular membrane

is

much

effective

for

the

separation

of

a

water-propanol

232 mixture.

They synthesized such membrane by in-situ hydrothermal method.

We have

reported that zeolites can be synthesized from a layered silicate such as kanemite by solid-state transformations [3,4]. High p-xylene selectivity was observed for the disc-shaped ZSM-5 catalyst in the alkylation of toluene with methanol [5]. It is necessary to establish various synthesis methods to fabricate zeolite molding depending on the uses, for example, separation membranes and membrane reactors. This work deals with the synthesis of binder-free MFI/MOR hybrid zeolite discs from layered silicates by solid-state transformation and their properties.

2. EXPERIMENTAL 2.1

Synthesis The starting material, kanemite, was prepared according to the procedure previously

described [6]. Silicalite precursor was prepared by dispersing lg of kanemite in an aqueous solution of tetrapropylammonium hydroxide [TPAOH] (20 wt%). The pH was adjusted to 11 with 6 mol dm 3 HC1 and the mixture was heated at 70 ~ for 3h with stirring. After cooling to room temperature, the pH of the suspension was adjusted to 8.5 (a gel was obtained) in order to exchange the remaining Na cations with protones and form Si-OH groups. The resulting product was filtered and washed with 3001111 of deionized water and dried at 50 ~ for 20 h to yield kanemite-TPAOH powder. The powder was shaped into a disc using an uniaxial compression molder at a pressure of 443 MPa. The disc was 20 n'll'n in diameter and ca. lmm thick. Crystallization was carried out in a Teflon-linning autoclave (Parr co. No.4749, 23 ml in volume) at 130 ~ for 0-46 h under dry condition. In order to remove TPA, the silicalite disc was calcined at 550 ~ for 20 h in air. Mordenite (MOR) precursor gel was prepared from kanemite and NaA102 (20 wt%).

(SiO2/A1203 ratio = 14) in aqueous solution of piperidine [Pip]

The pH was adjusted to similar silicalite precusor.

MOR precursor gel was

coated on the silicalite disc by dip coat method or using a spin coating apparatus (Yamato, SC-18). The thichless of coating layer was adjusted by changing the amount of gel and the rotational frequency. After the disc was dried at 50 ~ for 20 h, it was put into the autoclave. Then, the autoclave was heated at 150 ~ for 0 - 5 days under dry conditions.

233

Kanemite + NaAIO2 Piperidine aq.

Gelation

,

2>

pH=12, 70 ~ for 3h

I st x- -e....I

Silicalite disc

pH=8.5, cooling

Gel coating

Heating in an autoclave

Figure 1. Synthesis method of hybrid zeolite disc from kanemite

2.2

Analysis

The products were identified by X-ray diffraction (XRD) with monochromatic Cu Kct radiation (Mac Sci. MXP-18). TG-DTA measurements were can'ied out in a dry air flow (100ml min -1 ) at a heating rate of 10 ~ rain 1 ( Mac Sci. TG-DTA 2000).

29Si and 13C MAS

NMR spectroscopic characteristics of the samples were measured using a Brucker AMX-500 spectrometer operating at 99.36 MHz, fitting the samples in a 4 mm ZrO2 rotor, spinning at 4 kHz. The morphology of the samples was investigated by scalming electron microscopy (SEM) using a Hitachi S-800 operated at 15kV.

3. RESULTS AND DISCUSSION

XRD patterns of kanemite and TPA-silicalite precursors with increasing reaction time m'e represented in Figure 2. The patterns of intercalated kanemite and silicalite precursor after heating at 130 ~ for 5 min. are quasi-amorphous and a broad peak starts to appear between 20 = 15 - 30 ~ however, all the peaks of kanemite are still present, indicating that the crystallinity decreases and loss of layered structure proceeds [7]. After l h 50 min. of heating, crystallization starts and the line width of the peaks becomes narrow with increasing reaction

234 time, indicating that the regularity of the framework increases. The crystallinity increases drastically between lh 50 min. and 4h of heating.

No noticeable increase is observed

between 4 and 36 h.

,I

16

m 14 U3

c

12

Q

c~ 10

CO

oT "

8

r

>" c

6 4

.c_

2

10

15

20

25

30

35

40

45

50

55

60

2e/degrees Figure 2. X-ray diffraction patterns of kanemite (1), intercalated kanemite at 3h after the cation exchange (2), silicalite precursors heated at 130 ~ for 5 min. (3), 20 min. (4), 1 h 50 rain. (5), 2 h (6), 4 h (7), 16 h (8) and 46 h (9). To measure the amount of TPA + in the silicalite disc, TG-DTA measurement was done. The weight loss calculated from the exothermic peak due to the combustion decomposition of TPA + are summarized in Table 1 together with product. when the silicalite disc was calcined at 0 to 400~

The hybrid disc was not obtained

because TPA + still remained in the

silicalite discs (Table 1, Runs 1-3 : 4.1 - 9.8 wt %), and TPA + may penetrate from the silicalite side to MOR precursor side during the MOR synthesis process.

Therefore, the

calcination process of the silicalite disc is required to synthesize the hybrid disc.

In the case

of after the TPA + is almost perfectly removed from the silicalite disc (Table 1, Run 4 : 0.3 wt%), MOR was formed on the silicalite disc.

235 Table 1. Weight loss of silicalite disc and product Run No.

Calcination

Weight loss

Product

(~ )

( wt %)

1

no

9.8

( upper / down) MFI / MFI

2

300

8.3

MFI / MFI

3

400

4.1

MFI / MFI

4

550

0.3

MFI / MOR

Figure 3. Hybrid zeolite disc from kanemite (shaped disc with 20 mm in diameter) Hybrid zeolite disc from kanemite are shown in Figure 3. The shaped disc with 20 mm in diameter and observed

ca.

1 mm in thick was obtained.

during the transformation.

The reduction of the volume was not

In kanemite, water molecules are present at the

external surface, between the layers around the Na cations and within the hexagonal rings constituting the layers [7]. d-spacings were

ca.

From XRD data, kanemite and TPA intercalated kanemite

10.2 and 12.7A, respectively. This result suggests that the silicate sheets

of kanemite are intercalated by monolayer of TPA +. intercalated kanemite gave 1.1 of

29Si-MAS-NMR spectra of TPA

Q4/Q3ratio, then the intensity of Q4 peak is higher than that

of original kanemite and increases during the solid state transformation. These results show that silicate layers of kanemite start condensing during the intercalation of TPA + and the phenomenon continues during the solid state transformation, leading to the formation of a three-dimensional network.

236 Moreover, the disc showed good mechanical strength (compression strength) over 10 kg/cm 2. Figure 4 shows compression strengths [8] of hybrid disc and FCC catalyst ( Nikki. Co.:CK-300, including the binder ). In comparison with the commercial product, there is no inferiority on the mechanical strength. Physical properties of the hybrid zeolite disc are summarized in Table 2.

Large

differences in morphology and SiO2 / A1203 ratio between two sides of the disc obtained finally were observed.

The silicalite-side of the disc took an intergrowth structure of

prism-like crystals (ca. 1-2~tm), the other side (mordenite-side) was composed of scale-like crystals (ca. > 1lam). The SiO2 / A1203 ratios for the silicalite- and mordenite-sides of the disc were <600 and 15, respectively.

In the MOR-side, the aluminum stocked as a starting

material can be almost inserted in the product, because of the SiO2/A1203 ratio of MOR precursor was 14.0. The original orientation from the layer structure of kanemite reflected the shape of the zeolites obtained by the transformation.

The zeolite crystals are more

uniform and smaller in size than those obtained by the hydrothermal synthesis, due to the crystallization in a restricted enviromnent (Figure 5 SEM).

= 2 P / rc D L

20

cy 9Compression strength ( kg / cm 2) P 9Max. weight load (kg)

i

15

D 9Diameter of test sample (cm)

n

L 9Length of test sample (cm)

~0 ID

-D~

o

10

-

-

El"

y-----t-

p

(D

:"

i

L

o

c,.) -

I

I

I

10

20

30

I

40

I

- r'l--

FCC catalyst as a reference (Nikki Co., CK-300)

50

Crystallization time (h) Figure 4. Compression strength of hybrid zeolite disc and FCC catalyst

237 Table 2. Physical properties of hybrid zeolite disc Distance from disc surface Phase 0 - 200 ~.tm

SiO2/A1203 ratio

Morphology

< 600

MFI prism-like crystals (ca. 1-2gm) MFI prism-like crystals (ca. 1-2gm) MOR scale-like crystals (ca. > 1Jam)

2 0 0 - 800 gm 800 - 1000 jam

300 - 600 15

The XRD patterns of both the sides were characteristics of silicalite and mordenite, respectively.

Silicalite is most siliceous zeolite with 10 membered-rings (straight chamaels :

5.3 x 5.6~, sinusoidal channels : 5.1 x 5.5 ~), which constitutes the upper side of the disc. On the other hands, down side of the disc is formed by MOR (12 membered-rings with 7.0 x 6.5 k).

8000

1.51 O*

7000 6000

#

1 10"

5000

.,.,,

4000 3000

2000 lOOO o

I0

20

30

2 Theata(deg)

40

I0

30

20

2 Theata (deg)

Figure 5. SEM images and XRD profiles of hybrid zeolite disc ( left" silicalite-side, right 9mordenite-side)

40

50

238 Figure 6 shows 27A1-MAS-NMR spectra of MOR detached from down side of hybrid disc. The MOR was heated at 550~ to remove the piperidine molecules from pores of MOR. It gave only the signal at 54.4 ppm indicative of tetrahedrally coordinated framework A1. I

I

I

100

50

0

-50

Chemical shift (ppm)

Figure 6. 27A1-MAS-NMR spectra of MOR 4. CONCLUSIONS A disc shaped from kanemite-TPA and kanemite-Pip powder was transformed to a binder-free hybrid zeolite disc (silicalite / MOR) in a dry system. The SiOz/A1203 ratios for upper and down sides of the disc were <600 and 15 respectively. In comparison with the commercial product, there is no inferiority on the mechanical strength. References

[ 1] F. Mizukami, Stud. Sulf Sci. Catal., Vo1.125, 1 (1999). [2] M. Salomon, J. Coronas, M. Menendez, J. Santamaria, Chem. Commun., 125 (1998). [3] S. Shimizu, Y. Kiyozumi, K. Maeda, F. Mizukami, G. Pal-Borbely, R. M. Mihalyi, H.K. Beyer, Adv. Mater. 8, 759 (1996). [4] S. Shimizu, Y. Kiyozumi, F. Mizukami, Chem.Lett. 403 (1996). [5] I. Kiricsi, S. Shimizu, Y. Kiyozumi, M. Toba, S. Niwa, F. Mizukami, Micropolvus mesoporous Mater., 21,453 (1998). [6] S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc., Chem. Commun., 680 (1993)., T. Yanagisawa, T. Shimizu, K. Kuroda, and C. Kato, Bull. Chem. Soc. Jpn., 63, 988 (1990). [7] Z. Johan, G. F. Maglione, Bull.Soc.Mineral.Cristallogr., 95, 371 (1972). [8] Japanese Industrial Standards, Q-4653.

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordano and F. Testa(Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

E f f e c t of a l k a l i m e t a l i o n s o n s y n t h e s i s compounds by solid-state transformation

239

of z e o l i t e s

and

layered

T. Nishide a, H. Nakajima a, Y. Kiyozumi b, and F. Mizukami b a Nihon University, College of Engineering, Department of Materials Chemistry and Engineering, Tamura, Koriyama 963-8642, J a p a n b National Institute of Advanced Industrial Science and Technology, Institute for Materials and Chemical Process, Tsukuba Central 5, 1"1"1, Higashi Tsukuba, Ibaraki 305-8565, J a p a n The effect of alkali metal ions on the crystal structure of zeolites and layered silicates synthesized by a solid-state transformation was investigated. The compounds were prepared using a silica sol, t e t r a b u t y l a m m o n i u m hydroxide (TBAOH) and alkali metal ions. Addition of a small amount of alkali ions to a silica gel containing TBAOH resulted in silicalite'l, silicalite-1/silicalite-2 intergrowth or an unknown phase. However, addition of a large amount of K, Rb or Cs ions led to the formation of fibrous layered silicates (CsLSN, RbLSN and KLSN), which were composed of the alkali metal ions and novel linear Si-O-Si-O chains, characterized by their 29Si MAS NMR spectra and XRD patterns. 1. INTRODUCTION Zeolites and layered silicate compounds containing mesoporous molecular sieves are of much interest, not only because of their high performances with respect to ion exchange, solid acidities and transport of gases owing to their similar-size micro and meso pores, but also on account of their superior thermal and mechanical properties and structural stabilities as inorganic materials [1-13]. There are m a n y reports in the literature concerning these compounds, which have been mostly prepared by hydrothermal processes [1"5,10-13]. Recently, they have been synthesized and processed into disks and films in a solid state, that is, by a solid-state transformation, or by a vapor-phase transport method [5-9]. In these synthesis processes, alkali metal ions such as Na and K ions can function as mineralizing agents [14-16], and there are several reports about the effect of alkali metal ions such as Na, K or Cs ions on the crystal phases of zeolites obtained in hydrothermal syntheses [16-20]. Barre and Mainwarim [20] reported t h a t the addition of Cs ions leads to the formation of zeolite and non-zeolite compounds when metakaolinite is treated with silica and CsOH in the hydrothermal synthesis. Gabelica et al. [16] reported t h a t Cs ions act as certain structure-breaking cations in hydrothermal MFI syntheses. However, there are no reports of studies t h a t systematically examined the effect of the amount and

240 kinds of alkali metal ions added on the crystal structures of the resulting zeolite and layered compounds. Two of the authors have reported that a novel helical layered silicate (HLS) is synthesized from a reaction mixture of 1,4-dioxane-NaOH-amorphous silica-TMAOH-H20 under a hydrothermal condition [21], where TMAOH denotes t e t r a m e t h y l a m m o n i u m hydroxide. They also indicated t h a t a new layered silicate can be prepared by a hydrothermal conversion of a Na-magaditeTMAOH-1,4"dioxane-H20 mixture [22]. This paper explains the effect of alkali metal (Na, K, Rb and Cs) ions on the crystal structures of zeolites and layered silicates formed by a solid-state transformation, and describes the novel silicates whose 29Si MAS NMR spectra show Q2 signals. 2. EXPERIMENTAL 2.1. Synthesis of zeolites and layered silicates containing alkali metal ions A mixture of H20 (0.26 tool) and 60% HNOa (0.034 tool) was added to a solution of tetraethoxysilane (0.124 tool) in 99.5% ethanol (220 ml) to obtain a silica sol. To the diluted silica sol (Si: 1.8• 10 .2 tool), t e t r a b u t y l a m m o n i u m hydroxide (TBAOH) (1.40 • 10 .2 mole) and a solution of an alkali metal ion (Na +, K +, Rb + or Cs +) (1.80• 10 .3 ~ 1 . 8 0 • 10 .2 mole) were added. The alkali metal compounds used were NaOC2Hs, KOC(CHa)a, Rb2COa and Cs2COa. The mixed solution was dried to a gel with a rotary evaporator. The gel was heated in an autoclave at 170~ for 28 hr in the absence and presence of water (0.1 or 0.2 ml), which was introduced separately. Products obtained were filtered, washed with deionized water and ethanol, and dried in air. 2.2. Measurements The crystal phases of the products were determined with a Rigaku RAD-2 X-ray diffractometer using graphite-filtered CuK a radiation. 29Si MAS NMR spectra of the products were measured with a Bruker AMX 500 spectrometer. The Fourier transform infrared (F~IR) spectra of the products were recorded on Perkin-Elmer PARAGON 1000PC and 1600 spectrometers. Elemental analysis of the products was carried out with a Hitachi atomic absorbance spectrometer Z'8100 and gravimetric analysis. Scanning electron microscope (SEM) images of the products were measured with a J E O L 6700F spectrometer. Solid basicity of the products was measured by the titration method using 4-phenylazo-l-naphtylamine (pKa: 4.0), neutral red (pKa: 6.8), bromothymol blue (pKa: 7.2), phenolphthalein (pKa: 9.3) and 2, 4- dinitroaniline (pKa: 15.0) as indictors.

3. RESULTS AND DISCUSSION 3.1. Structure of the products Several white solid products were obtained under these experimental conditions. The structures were evaluated based on the XRD patterns of the products obtained in the presence of H20 (0.2 ml) because zeolites and layered silicates

241 were only produced under t h a t condition. The structures are listed in Table 1. Silicalite-2 was synthesized even when the silica gel did not contain any alkali metal ions. TBAOH leads to the preferential formation of silicalite-2 in the Table 1

Compounds synthesized by solid-state transformation (H20:0.2 ml) with the addition of alkali metal ions

M+/Si molar ratio

Na §

K§

Rb §

Cs §

0

Silicalite-2

Silicalite-2

Silicalite-2

Silicalite-2

0.10

Silicalite-1/ Silicalite-2 Cristobalite

Phase A

Silicalite- 1

0.25

RbLSN

Amorphous

0.50

Silicalite-1/ Silicalite-2 Amorphous

RbLSN

CsLSN

0.75

Amorphous

RbLSN

CsLSN

1.00

Amorphous

Silicalite-1/ Silicalite'2 KLSN Phase B KLSN Phase B KLSN Phase B KLSN Phase B KLSN

RbLSN

CsLSN

KLSN: a layered silicate containing K ions, RbLSN: a layered silicate containing Rb ions, CsLSN: a layered silicate containing Cs ions solid-state transformation as well as in the hydrothermal synthesis [23]. When a small amount of alkali metal ions was added, zeolites were predominantly produced. The XRD pattern of the b product obtained with a 0.1 molar ratio of Na to Si (Fig. la) is similar to t h a t of the MFI/MEL intergrowth [18] "~ a l!l in the region measured, whereas cristobalite (Q) is r j J! iit included in the product. This result indicates t h a t the addition of Na ions 5 10 20 30 40 50 leads to the silicalite-1 20 /deg. (Si-MFI)/silicalite-2 Fig.1 XRD patterns of the products synthesized with (Si-MEL) intergrowth. (a) a 0.10 and (b) a 1.00 molar ratio of Na ions The XRD pattern of the to Si. product obtained with a ~

.r -,,,~,,,,.,-.,,~,,,...~-,~ ~ ' ' ~ ' " +

~"""~"'~""~*"~'"~',"~--,u.,~.~

:ii

~ . , , ,

....

242 0.10 molar ratio of K to Si is r a t h e r complicated (Fig. 2a). The reflections marked with O a r e similar to those of the silicalite- 1/ a , ti i O 1 silicalite-2 intergrowth. .". o o t o ' I,~, t o ! However, the reflections ., !i ,,;:/": == y=: ~ n . i marked with /~ are not 50 10 20 30 40 ascribed to any zeolites 20 / deg. and silicates. These reflections are Fig. 2 XRD patterns of the products synthesized attributed to a novel with (a) a 0.10 and (b) a 1.00 molar ratio of K potassium layered ions to Si. silicate (KLSN) that will be discussed later. The intergrowthes obtained with Na or K ions are similar to those seen in hydrothermal syntheses using TPAOH d or TBAOH together with K or Na ions [18,24], where TPAOH denotes tetrapropylammonium hydroxide. The product obtained with a 0.10 10 20 30 40 50 molar ratio of Rb ions to 20 / deg. Si (Fig. 3a) is an Fig. 3 XRD patterns of the products synthesized unknown phase (phase with (a) a 0.10 and (b) a 1.00 molar ratio of A) since the XRD pattern Rb ions to Si. is not attributed to any zeolite, silicate, or layered silicate. On the other hand, the XRD pattern of the product synthesized with a 0.10 molar ratio of Cs ions to Si (Fig. 4) is ascribed to silicalite-1 [25], indicating that silicalite-1 tends to be formed in the solid-state transformation when Cs ions are added to the silica gel. A template such as TPAOH or TBAOH interacts with a silica gel surface, leading to the formation of a specific zeolite structure. Koegler et al. [25] estimated the configuration of TPA ions as a template on the silica gel surface in the hydrothermal synthesis. The TPA molecules lie in a zigzag p a t t e r n on the surface, resulting in the formation of silicalite-1. Gabelica et al. [16] noted t h a t zeolite frameworks depend on the nature of the alkali metal ions and t h a t the larger alkali ions such as Cs ions show a stronger structure-breaking effect t h a n the small ones such as Na ions. Thus, without any alkali ions in the silica gel, ,x

A

..

lh

243 TBA ions are presumed to be arranged in a straight pattern, leading to silicalite-2 in the solid'state transformation. However, with the addition of alkali metal ions to the gel, it is presumed t h a t both the TBA and alkali ions interact with the silica 5 10 20 30 40 50 gel and t h a t the TBA 20 / deg. molecules also lie in a Fig. 4 XRD pattern of the product synthesized with zigzag pattern as well a 0.10 molar ratio of Cs ions to Si as in the straight one. In the case of Na and K ions, the TBA ions lie in both the straight and zigzag patterns, resulting in the silicalite- 1/silicalite- 2 intergrowth. On the other hand, Cs ions .= seem to lead to only the zigzag p a t t e r n because of their structure-bre aking 5 10 20 30 40 50 effect, resulting in the 20 / deg. formation of only silicalite-1, whereas Rb ions are presumed to Fig. 5 XRD pattern of the product synthesized with lead to a different a 1.00 molar ratio of Cs ions to Si. pattern, resulting in the unknown phase A. On the other hand, different products were obtained by adding a large amount of alkali metal ions to the silica gel. The XRD pattern of the compound produced with a 1.00 molar ratio of Na to Si shows no reflections in the region measured (Fig. lb), indicating t h a t the product is amorphous. However, the addition of K, Rb, or Cs ions (a 0.25 - 1.00 molar ratio of the ions to Si) led to products that showed many reflections in their XRD patterns. The XRD p a t t e r n of the product obtained with Cs ions (a 1.00 molar ratio of Cs to Si) is shown in Fig. 5. This pattern is not ascribed to any zeolites, layered silicates, cesium silicates or silicates t h a t have been reported so far. An elemental analysis of this compound revealed t h a t it contained no organic |

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,

|

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,

244

compounds but only Cs, Si and O, leading to the formula of SiCso.46502.23"0.6H20. The FTIR spectrum showed no absorbance ascribed to organic compounds. This fact supports the result of the elemental analysis. To clarify the structure of the product, 29Si MAS NMR spectra, SEM images and high-resolution synchrotron powder diffraction patterns were measured. The 29Si MAS NMR spectrum of the product (Fig. 6) shows only one Q2 peak at "95.4 ppm. This peak is ascribed to Q2 of the Si-O unit :::i [26]. This result indicates that a silicon ion in the product bonds two -O-Si- groups and two -O-or "OH groups, leading to the formation of novel linear Si'O'Si-O chains. The SEM -70 -co -90 -loo -1!o -12o images of the product (Fig. 7) r si / ppm show log-like crystals composed Fig. 6 298i MAS NMR spectrum of of many fibers (a) and layered the product prepared with a 1.00 crystals consisting of many molar ratio of Cs ions to Si. fibrous components (b). These images suggest that the fibrous crystals consist of linear chains corresponding to the Q2 peak in the 298i MAS NMR spectrum. A high-resolution synchrotron powder diffraction analysis revealed that the fibrous layered silicate was composed of novel linear Si-O-Si-O chains and Cs ions held in the chains [27]. Thus, a new cesium layered silicate (CsLSN) could be synthesized under the experimental conditions mentioned above, that is, in the solidtransformation. The formation process is presumed to be as follows. As a result of adding a large amount of Cs ions, the silica gel surfaces are covered with a layer of Cs ions. The SiO4 moieties separated from the other gel part interact with the Cs ions and are rearranged into linear Si-O-Si-O chains without any bond with the other part. The XRD pattern of the product Fig. 7 SEM images of the product obtained with a 1.00 molar ratio of Rb to synthesized with a 1.00 Si is shown in Fig. 3b. This pattern is molar ratio of Cs ions to Si. ....

.

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245 also ascribed to no zeolites or layered silicates. The 29Si MAS NMR spectrum of the product showed one Q2 signal ('95.0 ppm) and one Q0 signal (-62.1 ppm). These results indicate t h a t the Si ions in the product form linear Si-O-Si-O chains as well as CsLSN and are also present as SiO4 moieties, which are generated in the preparation process. An elemental analysis of this compound revealed t h a t it contained only Rb, Si and O and no organic compounds, leading to the formula of SiRb0.~lsO2.2~9"0.7H20. The FTIR spectrum showed no absorbance ascribed to organic compounds. These results indicate t h a t a new rubidium linear silicate (RbLSN) can be synthesized as well as CsLSN by this solid'transformation process. The XRD p a t t e r n of the product produced with a 1.00 molar ratio of K to Si is shown in Fig. 2b. M a n y reflections are observed in the region measured. All reflections, m a r k e d with A, are not ascribed to any zeolites or silicates. The 29Si MAS NMR spectrum showed a peak at "93.2 ppm ascribed to a Q2 signal and two peaks at "100.6 ppm and -63.2 ppm ascribed to Q3 and Q0 signals, respectively [26]. The Q2 and Q0 signals indicate t h a t the product contained linear Si-O-Si-O chains and SiO4 moieties, as well as the case of Rb salt. The Q3 signal is attributed to the bonding between two linear Si'O-Si'O chains. The Si-O moieties in the linear chain were able to bond with those in neighboring chains because the ionic radii of K ions are smaller t h a n Rb or Cs ions. These results indicate t h a t the product includes a linear silicate (KLSN) consisting of Si'O-Si'O chains and their partial cross'linking, with K ions present between the chains. The product synthesized by adding a 0.25 - 0.75 molar ratio of K ions to Si showed a new reflection at 2 0 of 23.8 ~ , which is attributed to an unknown phase B. 3.2. Solid basicity of CsLSN and RbLSN The solid basicity of CsLSN and RbLSN was determined by the titration method using several indicators, to be H0,max of 12.5 and 12.0, respectively. Both CsLSN and RbLSN showed similar basicity and were found to be solid bases. After CsLSN was kept in a CO2 atmosphere for one day at ambient temperature, its FTIR spectrum (Fig. 8) shows bands at 1640, 1340 and 1050 i' cm 1 (arrows), which are ascribed to <~ adsorbed CO2. This result reveals that CsLSN adsorbed CO2. The FTIR .~ ~. spectra of RbLSN after being kept in a C02 atmosphere also showed bands at 1640, 1350 and 1040 cm "1, indicating that RbLSN adsorbed CO2 as well. ,,

4. CONCLUSION It is concluded that, depending on the kinds and a m o u n t of alkali metal ions added, the structure of the

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FTIR spectrum of CsLSN.

246 p r o d u c t s f o r m e d in a solid-state t r a n s f o r m a t i o n changes. As a result, the a d d i t i o n of a large a m o u n t of K, Rb or Cs ions to a silica gel leads to novel l a y e r e d silicates composed of l i n e a r S i ' O ' S i - O chains a n d s h o w i n g Q2 s i g n a l s in t h e i r 29Si MAS N M R spectra.

ACKNOWLEDGEMENTS This w o r k w a s s u p p o r t e d by a g r a n t from the P r o m o t i o n s a n d M u t u a l Aid C o o p e r a t i o n for P r i v a t e Schools of J a p a n . REFERENCES 1. F. Mizukami, Studies in Surface Science and Catalysis, 125, (1999) 1. 2. G.A. Ozin, A. Kuperman and A. Stein, Angew. Chem. Int. Ed. Engl., 28 (1989) 359. 3. M.E. Davis, Chem. Eur. J., 3 (1997) 1745. 4. I . W . C . E . Arends, R. A. Sheldon, M. Wallau and U. Schuchardt, Angew. Chem. Int. Ed. Engl., 36 (1997) 1144. 5. R.W. Thompson, "Recent advances in the understanding of zeolite synthesis, (Molecular sieves. Vol. 1) H. G. Karge and J. Weitkamp (Editors), (Springer-Verlag, Berlin) 1998, pp.l'33. 6. S. Shimizu, Y. Kiyozumi, K. Maeda, F. Mizukami, G. P.'Borb61y, R. M. Mih~lyi and H. K. Beyer, Adv. Mater., 8 (1996) 759. 7. M. Salou, Y. Kiyozumi, F. Mizukami, P. Nair and K. Maeda, J. Mater. Chem., 8 (1998) 2125. 8. W. Xu, J. Dong, J. Li, J. Li and F. Wu, J. Chem. Soc. Chem. Commum., 1990, 755. 9. M. Matsukata, N. Nishiyama and K. Ueyama, Microporous Mater., 7 (1996) 109. 10. T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 11. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc. Chem. Commum., 680 (1993). 12. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. -W. Chu. D. H. Olsen, E. W. Sheppard, S. B. McCullen, J. B. Higgins, and J. L. Schlenker, J. Chem. Soc., 114 (1992) 10834. 13. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 14. J. H. Koegler, H. van Bekkum and J. C. Jansen, Zeolites, 19 (1997) 262. 15. D. M. Bibby, N. B. Milestone and L. P. Aldridge, Nature, 280 (1979) 664. 16. Z. Gabelica, N. Blom and E. G. Derouane, Appl. Catal., 5 (1983) 227. 17. J. B. Nagy, Z. Gabelica and E. G. Derouane, Zeolites, 3 (1983) 43. 18. G. A. Jablonski, L. B. Sand and J. A. Gard, Zeolites, 6 (1986) 396. 19. R. M. Barrer and N. McCallum, J. Chem. Soc., 4029 (1953). 20. R. M. Barrer and D. E. Mainwaring, J. Chem. Soc., 2534 (1975). 21. Y. Akiyama, F. Mizukami, Y. Kiyozumi, K. Maeda, H. Izutsu and K. Sakaguchi, Angew. Chem. Int. Eds. Engl., 38 (1999) 1420. 22. F. Kooli, F. Mizukami, Y. Kiyozumi and Y. Akiyama, J. Mater. Chem., 11 (2001) 1946. 23. D. M. Bibby, N. B. Milestone and L. P. Aldridge, Nature, 280 (1979) 664. 24. E. Moretti, S. Contessa and M. Padovan, Chemica e Industria, 67 (1985) 21. 25. M. M. J. Treacy and J. B. Higgins, "Collection of Simulated XRD Powder Patterns for Zeolites", The Structure Commission of the International Zeolite Association, 4th ed., (Elsevier, Amsterdam) 2001, p. 236. 26. G. Engelhardt and D. Michel, "High resolution solid-state NMR of silicates and zeolites", (John Wiley & Sons, NY) 1987, pp. 106"330. 27. T. Ikeda, T. Nishide, H. Nakajima and K. Futazawa, Proceedings of the Conference of Japan Association of Zeolite, C26 (2001) p. 101.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

247

(AI)-ZSM-12: Synthesis and modification of acid sites v

.

1"

Jif-i Cejka , Gabriela Kogov~ 1, N a d ~ d a Zilkovfi I and Irena Hrubfi2 1j. Heyrovsk?7 Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, CZ-182 23 Prague 8, Czech Republic 2 Department of Inorganic Technology, Institute of Chemical Technology, Technickfi 5, CZ-166 28 Prague 6, Czech Republic

Zeolite (A1)-ZSM-12 was synthesised in a broad range of Si/A1 ratio using methyltriethylammonium bromide as structure-directing agent with the aim to characterize the type and concentration of acid sites in dependence on Si/A1 ratio and calcinations procedure. It was shown that the minimum Si/A1 ratio, which can be achieved for ZSM-12, is around 37-40. FTIR spectroscopy used to investigate the adsorption of d3-acetonitrile on Broensted and Lewis acid sites revealed that both sites are present in a significant concentration after all calcination procedures. However, the amount of Lewis acid sites can be decreased using the proper way of calcination. The highest concentration of Broensted sites particularly at lower Si/A1 ratios was achieved via calcinations of (A1)-ZSM-12 in a stream of ammonia followed by a repeated sodium ion exchange and further calcinations in a stream of air.

1. INTRODUCTION Zeolites are industrially used in a high number of acid catalyzed reactions due to their strong acidity and shape-selective properties. The number of large-scale processes employing various zeolite based catalysts is steadily increasing (1). It is evident that the acid strength and concentration of active sites play a very important role in the catalytic performance of zeolites and, thus, the possibility to prepare zeolites with a given type and number of acid sites is highly required. In the case of zeolites, the concentration of Lewis sites usually increases with calcination or activation temperature and number of examples can be found in the literature describing the negative role of these sites in deactivation of zeolites, e.g. (2). It is well known that the concentration and acid strength distribution can be varied with the aluminium content during the synthesis or even further changed by various dealumination procedures. For the characterization of the amount and different types of acid sites temperature-programmed desorption is usually used, however, this technique does not allow us to distinguish between Broensted and Lewis sites. For this reason infrared spectroscopy is advantageously used

This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (A4040001, $4040017) and Volkswagen-Stiftung 0/75886).

248 in combination with the adsorption of proper probe molecules like d3-acetonitrile or pyridine (3). Zeolite ZSM-12 belongs to high silica zeolites with one-dimensional pore structure consisting of linear, non-penetrating, channels formed by 12-membered rings with a diameter of about 0.57 x 0.61 nm. ZSM-12 can be synthesized in the range of Si/A1 ratio from about 25 up to at least 100 and it seems that the optimum template for its synthesis is methyltriethylammonium bromide (4). The catalytic properties of this zeolite were investigated e.g. in cumene synthesis via benzene alkylation with propylene or isopropyl alcohol (5,6). The objective of this contribution is to describe the optimum synthesis procedure for aluminium containing ZSM-12 with a different range of Si/A1 ratio and to characterize in detail the concentration of the individual types of acid sites. As (A1)-ZSM-12 possesses usually a significant concentration of Lewis we aimed also to use different calcination procedures to investigate the possibility of decreasing the concentration of these sites in calcined zeolite.

2. E X P E R I M E N T A L SECTION (A1)ZSM-12 zeolite with different Si/A1 ratios was synthesized using sodium silicate, aluminium nitrate, sulphuric acid and methyltriethylammonium bromide (MTEABr) as structure-directing agent. In a typical preparation 1.33 g of Al(NO3)3.9H20 (p.a., Fluka) in 37.6 g distilled H20 with 2.8 g HzSO4 was added. Then 31.6 g of sodium silicate (Riedel de Hahn) dissolved in 37.6 g of distilled H20 were added under vigorous stirring. The mixture was stirred for 60 min. Later on 11.52 g of organic template MTEABr (99%, Fluka) dissolved in 37.6 g distilled H20 was added to the resultant gel, which was finally stirred for another 60 min. The synthesis was carried out under static conditions in a 30 ml Teflon-lined stainless steel autoclaves at 140 ~ for 12 to 300 hours under autogeneous pressure. After crystallization, the product was filtered, washed with distilled water and dried at 100 ~ for 5 h. Template removal from one batch of as-synthesized zeolite was performed under four different calcination procedures as described below: 1. zeolites (A1)-ZSM-12/1 were calcined at 550 ~ for 8 hours in a stream of air (temperature ramp used = 1~ 2. zeolites (A1)-ZSM-12/2 were calcined at 500 ~ for 16-32 hours in a stream of air (temperature ramp used = 1~ 3. zeolites (A1)-ZSM-12/3 were calcined at 350 ~ for 2 hours in a stream of air, then the samples was cooled down to the ambient temperature followed by two ionexchanges with 0.5 M NaNO3 for 8 hours and one with 1 M NaNO3 for 8 hours using 100 ml of solution for 1 g of zeolite. After that zeolites were calcined at 550 ~ for 8 hours (temperature ramp u s e d - 1~ 4. zeolites (A1)-ZSM-12/4 were calcined at 460 ~ for 4 hours in a stream of ammonia, then the samples were ion exchanged twice with 0.5 M NaNO3 for 8 hours once with 1 M NaNO3 for 8 hours using 100 ml of solution for 1 g of zeolite at room temperature and calcined again at 550 ~ for 8 hours in a stream of air (temperature ramp used - 1~

249 Ammonium forms of all ZSM-12 zeolites were prepared via four times repeated ion exchange with 0.5 M ammonium nitrate at ambient temperature (100 ml of solution for 1 g of zeolite, 4 hours). After the ion exchange the zeolites were washed thoroughly by distilled water and dried at 50 ~ for 5 hours. All as-synthesised, calcined and ion-exchanged zeolites were checked for their crystallinity and phase purity by X-ray powder diffraction on a Siemens D5005 diffractometer equipped with graphite monochromator and scintillation counter and using CuKa radiation in a Bragg-Brentano geometry. The shape and size of the crystals was determined using scanning electron microscopy (Jeol). Atomic absorption spectroscopy was used to determine the chemical composition of synthesized (A1)-ZSM-12. The concentrations of Broensted and Lewis sites in zeolite (A1)-ZSM-12 were determined after the adsorption of d3-acetonitrile followed by FTIR spectroscopy using a Nicolet FTIR Prot6g6 460 spectrometer. For this purpose, the zeolite powders were pressed into self-supporting wafers with a density of 4.0-12.0 mg/cm 2. d3-Acetonitrile was degassed by repeated freezing and thawing cycles before its use. Zeolites were activated "in-situ" by overnight evacuation at 400 ~ All measured spectra were normalized to a wafer thickness of 10 mg/cm 2. For a quantitative characterization of the Broensted acid sites (B), the C N-B vibration at 2296 cm 1 was used with an extinction coefficient of ~(B) = 2.05 + 0.1 cm.~tmo1-1. For a quantitative evaluation of Lewis acid sites (L), the C N-L vibration at 2323 cm -1 was used with an extinction coefficient of ~(L) = 3.6 + 0.1 cm.gmol 1 (3) (see Table 1).

3. R E S U L T S A N D D I S C U S S I O N

Aluminium containing zeolite ZSM-12 was readily synthesized with structure directing agent methyltriethylammonium bromide. The composition of the reaction mixture was changed in the following r a n g e - Na20 : MTEABr : A1203 : SiO2 : H20 = 22-66 : 33-99 : 1 : 80-240 : 3 500-10 300. Optimizing the synthesis time (between 48 and 300 hours) at 140 ~ it was possible to synthesise reproducibly well-crystalline zeolites (A1)-ZSM-12 with Si/A1 ratio from 35 till about 110 without the presence of any further amorphous or crystalline phase. The effect of crystallisation time on the crystallinity of (A1)-ZSM-12 with initial Si/A1 molar ratio = 40 is depicted in Fig. 1. During this synthesis, only X-ray amorphous materials was obtained up to about 80 hours and the first diffraction lines, which belong to the ZSM-12, were observed after 96 hours. Well-crystalline zeolite material was formed after at least 144 hours and from this picture it is evident that pure crystalline materials was synthesized. Ernst et al. obtained ZSM-12 contaminated by small amounts of crystobalite with further prolongation of the synthesis time and particularly at higher synthesis temperatures (characteristic diffraction line is around 20 = 22.0 o) (4). At synthesis temperature of 140 ~ we have observed the formation of crystobalite after more than 300 hours.

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2Theta (*) Fig. 1 Powder XRD patterns of (A1)-ZSM-12, Si/A1 (in the reaction mixture) = 40. Crystallization time 9A - 72 h, B - 96 h, C - 120 h, D - 144 h, E - 168 h. Fig. 2 confirms that in the case of the synthesis of (A1)-ZSM-12 with different Si/A1 ratios carried out under the same synthesis conditions, the higher the Si/A1 ratio the shorter time needed for the complete synthesis of this zeolite. The relative intensity of the diffraction line at 20 = 20.99 o was used for comparison of the crystallinity of different zeolites in this study. While for (A1)-ZSM-12 with Si/A1 ratio = 90 after 72 hours the intensity of this diffraction line achieved already at least 80 % of the highest intensity among all samples synthesized, the intensity of (A1)-ZSM-12 with Si/A1 = 40 was only around 35 % (Fig. 2). This indicates that with increasing concentration of aluminium in the reaction mixture the time required for complete crystallization is longer, which is in a good agreement with data obtained for other high-silica zeolites. Table 1 Synthesis of (A1)-ZSM-12 with different Si/A1 ratio after the typical calcinations procedure Zeolite Si/A1 Si/A1(can) Si/A1 (IR) Broensted sites Lewis sites (rm) (mmol/g) (mmol/g) (A1)ZSM-12/1 40 35 45 0.18 0.07 (A1)ZSM-12/1 60 46 52 0.20 0.06 (A1)ZSM-12/1 90 71 82 0.13 0.03 (A1)ZSM-12/2 40 35 43 0.22 0.08 (A1)ZSM- 12/2 60 46 53 0.20 0.06 (A1)ZSM-12/2 90 71 77 0.15 0.03 (A1)ZSM-12/3 40 35 42 0.24 0.07 (A1)ZSM-12/3 60 46 50 0.19 0.05 (A1)ZSM-12/3 90 71 86 0.12 0.04 (A1)ZSM- 12/4 40 35 37 0.35 0.06 (A1)ZSM-12/4 60 46 51 0.22 0.04 (A1)ZSM-12/4 90 71 74 0.14 0.02 rm - reaction mixture, can - chemical analysis, IR - adsorption of d3-acetonitrile

251

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Time (h) Fig. 2 Influence of the Si/A1 ratio in the reaction mixture on the crystallization time of(A1)-ZSM-12 at 140~ (Si/A1 = 40 (O), 60 (m) and 90 (A)).

Fig. 3 Scanning electron micrographs of zeolites (A1)-ZSM-12 synthesized at 140 ~ with different Si/A1 in the reaction mixture (A = 40, B = 60, C = 90, D = 120).

252 Scanning electron microscopy revealed the typical rice-shape of ZSM-12 crystals, which is, for (A1)-ZSM-12 zeolites with different Si/A1 ratios, depicted in Fig. 3. Under the same synthesis conditions the size of the crystals increases with the decreasing concentration of aluminium in the zeolite as was already described by Ernst et al. (4). While the crystals of (A1)-ZSM-12 synthesised from the reaction gel with Si/A1 - 40 are formed from regular particles of diameter about 0.5-0.6 ~tm and length around 2-3 ~tm, the both parameters are increasing in the sequence of Si/A1- 40 < 60 < 90 < 120. In the case of (A1)-ZSM-12 with Si/A1 = 120 size of the zeolite crystals reaches about 1.0-1.5 pm in a diameter and around 6-8 ~tm in length. Comparing the Si/A1 ratio from reaction mixture with Si/A1 ratio based on chemical analysis of individual (A1)-ZSM-12 zeolites, it is evident that only in the case of Si/A1 = 40 these two values correspond, however, at higher Si/A1 ratios the value obtained from the chemical analysis is always of about 10-15 % lower compared to the Si/A1 in the reaction mixture. Only traces of aluminium were analysed after filtration of solid phase after the synthesis, which probably means that some portion of silicium species remained in the liquid phase after the synthesis.

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Fig. 4 IR spectra of hydroxyl vibration region of(A1)ZSM-12 with Si/A1 = 40 (I) and 60 (II) before and after d3-acetonitrile adsorption (A) and spectra of acetonitrile region after its adsorption (B). Infrared spectrum of (A1)-ZSM-12 in the region of OH vibrations consists of 3 typical bands, one of them is assigned to the vibrations of terminal silanol groups (3 735 cm -1) and the others to two types of bridging Si-OH-A1 groups at 3 612 and 3 580 cm 1 (Fig. 4). These two signals were tentatively attributed to the vibrations of bridging OH groups located in the main channel and in the 6-membered rings, respectively (7). The adsorption of d3-acetonitrile followed by FTIR spectroscopy evidenced that all aluminium incorporated into the (A1)-ZSM-12 is accessible for this probe molecule. In

253 addition, the quantitative data of the concentration of Broensted and Lewis sites were in a good agreement with the chemical analysis. The concentration of Broensted and Lewis sites strongly depends on the Si/A1 ratio in the synthesized zeolite (A1)-ZSM-12 (see Table 1). With increasing amount of aluminium in the framework the concentration of Lewis acid sites increases. E.g. the concentration of Lewis sites was about 30 % for Si/A1 = 90 and it increased up to more than 40 % for Si/A1 ratio of 40. Significant concentration of Lewis sites in (A1)-ZSM-12 was described already by Chiche et al. (7) via pyridine adsorption followed by IR spectroscopy. Therefore, it is very surprising that in the experiments performed by Zhang et al. (8) the authors did not find almost any Lewis sites not only for ZSM-12 but also for zeolite Beta, for which the high concentration of Lewis sites is typical. As the presence of Lewis sites in the zeolites can significantly increase the rate of deactivation due to the formation of various hydrocarbon deposits, one of the aims of this contribution was to investigate the ways of preparation of (A1)-ZSM-12 zeolites with different concentration of Broensted and Lewis acid sites. To suppress the formation of significant amount of Lewis sites several calcinations procedures were tested (for details see Experimental Section). Two important trends are evident from Fig. 5. The typical calcination procedure (in a stream of air at 550 ~ for 8 h) and the increasing overall concentration of acid sites (with decreasing Si/A1 ratio) led to the increase in the concentration of Lewis sites, which is in agreement with the behaviour of other zeolites like ferrierite (3). The calcination of (A1)-ZSM-12 performed at 500 ~ for 16-32 h resulted in a similar concentration of Lewis sites like in the previous procedure. It means that at temperatures higher than 500 ~ it is possible to remove completely methyltriethylammonium bromide from the channel system of zeolite (A1)ZSM-12, however, relatively long time is required, which supports the formation of a significant amount of Lewis sites. In a similar way, the high concentration of Lewis acid sites was achieved after calcinations performed only at 350 ~ followed by a repeated ion-exchange with sodium nitrate. Further calcinations at 550 ~ led again to the comparable concentration of Lewis acid sites (cf. Table 1).

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ratio Si/AI

Fig. 5 Dependence of the concentration of Broensted (A) and Lewis (B) sites on the ratio of Si/A1 for different calcinations procedures (1 - O, 2 - A, 3 - n, 4 - o).

254 The only calcination procedure, which led to a decrease in the concentration of Lewis sites, was that one described by van Bekkum et al. for zeolite Beta (9) using a stream of ammonia instead of stream of air (oxygen) during the first stages of the calcination procedure. In this case, after calcination in a stream of ammonia at 460 ~ some part of the template was already removed and after repeated ion-exchange with sodium nitrate a partly sodium form of (A1)-ZSM-12 was obtained. Therefore, further calcinations at 550 ~ in a stream of air did not result in a partial dehydroxylation leading to the formation of a high concentration of Lewis sites. Thus, using this combined procedure, we were able to prepare (A1)-ZSM-12 with Si/A1 ratio around 40 possessing about 20 % of Lewis sites.

4. CONCLUSIONS Zeolite (A1)-ZSM-12 was synthesized with the Si/A1 ranging from 40 to 120 at 140 ~ under static conditions. In spite of the Si/A1 ratio in the synthesized zeolite, all samples exhibited very high crystallinity and the absence of other amorphous and crystalline phases. The rate of the crystallisation of (A1)-ZSM-12 strongly depended on the Si/A1 ratio in the reaction mixture. With increasing Si/A1 ratio in the reaction mixture the crystallisation rate increased as well. The adsorption of d3-acetonitrile over the ammonium forms of (A1)-ZSM-12 followed by IR spectroscopy revealed that (A1)-ZSM-12 possesses a high concentration of Lewis sites (35-45 %) after typical calcination in a stream of air at 550 ~ When the assynthesised zeolite was calcined in a stream of ammonia followed by sodium ionexchange and further calcination in air, the concentration of Lewis sites can be diminished to about 20 % for (A1)-ZSM-12 with Si/A1 ratio equaled to 40.

REFERENCES

K. Tanabe and W.F. Hrlderich, Appl. Catal. A 181 (1999) 399. B. Wichterlovfi, N. 2;ilkovfi, E. Uvarova, J. Cejka, P. Sarv, C. Paganini and J.A. Lercher, Appl. Catal. A 182 (1999) 297. B. Wichterlov~, Z. Tvarfi~kovg, Z. Sobalik, P. Sarv, Micropor. Mesopor. Mater. 24 (1998) 223. S. Ernst, P.A. Jacobs, J.A. Martens and J. Weitkamp, Zeolites 7 (1987) 458. C. Perego, S. Amarilli, R. Millini, G. Bellussi, G. Girotti and G. Terzoni, Micropor. Mater. 6 (1996) 395. B. Wichterlov~, J. 0ejka and N. Zilkov~, Micropor. Mater. 6 (1996) 405 B.H. Chiche, R. Dutartre, F. di Renzo, F. Fajula, A. Katovic, A. Regina and G. Giordano, Catal. Lett. 31 (1995) 359. W. Zhang, P.G. Smirniotis, M. Gangoda and R.N. Bose, J. Phys. Chem. B 104 (2000) 4122. P.J. Kunkeler, B.J. Zuurgeed, J.C. van de Waal, J.A. van Bokhoven, D.C. Koningsberger, H. van Bekkum, J. Catal. 180 (1998) 234.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

255

F o r m a t i o n o f n e w m i c r o p o r o u s silica p h a s e in p r o t o n a t e d k a n e m i t e - T M A O H water system F. Kooli, a Y. Kiyozumi ,'~ M. Salou u and F. Mizukami" "~National hastitute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan bDepartment of Materials Science, Toyohashi University of Technology Tempaku-Cho, Toyohashi 441-8580, Japan

By hydrothennal transfornaation of protonated kanemite in teralnethylamnaoniun hydroxide (TMAOH) and water at temperatures above 130 ~ for 5 days, a new phase was formed and called FLS. Upon calcination up to 1000 ~ this phase was transfonned to a crystalline microporous silica material with high thermal stability. The calcined phase has a large surface area 335 na2 g-I and micropore volume of 0.100 mL glat 500 ~ The solid state transfonaaation of the as synthesized FLS to TMA-containing sodalite structure in sodium aluminate and TMAOH.5H20 mixture at 150 ~ for l h is also reported. 1. INTRODUCTION Kanemite (NaHSi2Os.3H20) can be easily synthesized in the laboratory [1], the sheets consist of single layers of SiO4 tetrahedra. The flexibility of the kalaemite layers enables the preparation of mesoporous silica [2]. It was assumed that the flexible silicate layers wind around the aggregated surfactants, and the condensation of the silanol groups occurred during calcination [3]. This material was called a folded sheets mesoporous material, FSM. ha the other hand, the use of short-chained alkylammonium cations such as tetrapropylammonium (TPA) and tetrabuthylammonium (TBA) cations leads to the transformation of kanemite to a silicalite-1 and -2 via solid state transformation, respectively [4,5]. Recently, tetralnethylalnlnonium (TMA) cations are used to prepare a new silicate layered structure and crystalline microporous silica material from sodium and protonated magadiites, respectively [6,7]. To the best of our knowledge, no studies concerning the use of protonated kanemite (Hkanemite) were reported, as host for pillared materials or precursors for micro- or mesoporous laaatelials. In certain cases, the presence of significant cross-linking of kanemite layers inhibited the synthesis of mesoporous materials [8]. We describe here, the preparation and characterization of a new phase ffona H-kanemite and TMAOH. The textural properties in term of external surface area, micropore volume and pore size distribution of calcined FLS are also reported. The solid state transformation of the as-synthesized FLS to TMA-containing sodalite (SOD) is described. F. K. gratefully thanks the NEDO organization (Japan) for a research fellow.

256 2. EXPERIMENTAL SECTION 2.1. Synthesis Na-kanemite was prepared from a &Na2Si205 (SKS6) supplied by Clariant (Japan). In order to produce H-kanemite, 3 g of Na-kanemite was treated with 300 ml of 1N hydrochloric acid (HC1) at room temperature for 2h. The solid was filtrated and washed with deionized water then dried at 40 ~ ovenaight. To obtain FLS material, 1g of H-kanemite was added to a mixture of TMAOH solution (15%, 2.3 g) and water (0.5 g), then hydrothennally treated in stainless steel Teflon lined autoclave at 150 ~ over 5 days. The product was collected by filtration, washed with acetone and dried in air overnight at 40 ~ The effect of different reaction variables such as H-kanemite, water and TMAOH contents was also investigated at 150 ~ for a reaction time of 5 days, each parameter was varied individually. The calcination of the obtained products was performed in air at different temperatures from 300 to 1000 ~ for 1Oh with using heating rate of 3 ~ rain -1. The solid state transfornaation of FLS phase to SOD structure was achieved by grinding a mixture of FLS, sodium aluminate (NaA102) and TMAOH.5H20 (1:0.5:0.5 in weight) in agate mortar. The sample was ground for 10 rain. Then the powder was charged in a Teflonlined autoclave and heated at 150 ~ for different reaction times. The product was washed with 400 cm 3 of deionized water and air-dried overnight at 40 ~ 2.2. Characterization Powder X-ray diffraction (XRD) was performed between 2.5-50 ~ (20) on a MXP 18 difffactometer (Mac Science Co., Ltd) using Cu-K~x radiation (~= 0.15418 nm). Themaogravinaetry (TG) and differential thermal analysis (DTA) were measured with a Macscience TG-DTA 2000 analyzer. The samples were heated to 1000 ~ in air at heating rate of 5 ~ rain q. C, H, N analyses were cm-ried out on CE instruments model EA 1110 CHN analyzer. Fourier transform infrared (FT-IR) spectra were recorded on a BIORAD FTS-45RD spectrometer, using the K_Br pellet technique. The solid state 29Si magic-angle spimaing (MAS) nuclear magmetic resonance (NMR) spectra of the samples were measured on a Varian, VNMR400P spectrometer with interpulse delay of 800 seconds. Typically, 100 scans were accumulated. Nitrogen adsorption isotherms were measured at liquid nitrogen temperature on a Belsorp 28 SP instrument. All samples were outgassed at 200 ~ for 5 h prior to the adsorption measurements. 3. RESULTS AND DISCUSSION 3.1. Synthesis The starting H-kanemite showed a similar XRD pattern (Fig. 1) to crystalline silicic acid reported by Johan and Maglione [9]. The reaction of H-kanemite with TMAOH and water at 150 ~ over 5 days resulted in the formation of a white crystalline product, assigned as FLS. The powder XRD pattern of this material (Fig. 1) was significantly different to that of Hkanemite, but bore some similarity to that prepared from H-magadiite under identical reaction conditions [7]. H-kanemite was also converted into FLS over longer reaction times of up to 30 days, suggesting that the FLS is a stable phase. We noticed that the same phase was formed even for reaction times of 2 days. However, TMA cations were intercalated between the silicate layers in addition to silica phase were detected for shorter period of 1 day (Fig. 1).

257

j

9"; --

""silica phase

I

1

~

3d

~

/1.o2

/1.o2

Id=_

,-

120

too

o fo 2'0 3'o 4:o (20) 0 I'0 2'0 3'0 40 (20) Fig. 2. Powder XRD patterns of HFig. I. Powder X R D patterns of Hkanemite reacted with TMAOH and water at kanemite reacted with TMAOH and water at 150 ~ for different reaction periods, different temperatures (~ over 5 days When H-kanemite was treated with TMAOH and water at different temperatures over 5 days, it was transformed completely into FLS at temperatures between 130 and 190 ~ (Fig.2). At lower temperatures of 100 ~ however, a new phase with basal spacing of 1.60 nm was obtained, and related to the intercalation of TMA cations between silicate layers [ 10]. We noted that traces of FLS phase appeared at reaction temperature of 120 ~ Effect of TMAOH concentration. For TMAOH amounts between 2.3 to 5g, the conversion of H-kanemite to FLS was achieved with high crystallinity of obtained phase at TMAOH content of 2.3 g. When TMAOH content was lower than 2.3 g, H-kanenaite was not transforlned to FLS phase, and amorphous silica phase was obtained. On the other hand, Further increase of TMAOH content up to 5 g in the mixture indicates that H-kanemite was converted to another new phase different than FLS one. Effect of water. The FLS phase was obtained when H-kanemite was reacted only with TMAOH solution, and increasing the water amount up to 1g el-d~anced the formation of FLS phase. However the crystallinity of FLS phase was deteriorated by further increase of water in addition an amorphous silica phase was also fonned. Effect of silica content. Different amounts H-kanemite were added to the mixture of TMAOH and water. The powder XRD data indicated that FLS phase was formed when 1 to 1.5 g of H-kanemite were used. For amounts of H-kanemite lower than l g and higher than 1.5 g, silica phases were fol-med. 3.2. Characterization From the C, H, N analyses, the empirical molar ratio of C:H:N corresponds to 4.1:15:1, and indicated that TMA cations were incorporated intact in the as synthesized FLS phases.

258 The thermogravimetric (TG) and differential thermal analysis (DTA) measurements showed a weight loss of 11.7% at temperatures above 290 ~ with an exothennic effect at 400 ~ associated to the combustion of the organic species. Two other weight losses (of 12.2 %) below 200 ~ are also observed and due to removal of physiadsorbed and occhtded water molecules, respectively. The weight loss of FLS phase above 290 ~ is close to the total amount of carbon and nitrogen (11.8 % in wt.) detennined by elemental analysis, and it is reasonable to associate with TMA cations incorporated into the FLS material. The FT-IR spectrum of as-synthesized FLS confirmed the presence of TMA cations with absorption bands at about 1489, 1008 and 946 cm l [11] (Fig.3). The framework of FLS phase was different than the H-kanemite precursor, and new bands were detected, empirically assigned to asymmetrical Si-O "'external" stretching and internal vibrations, in the region of 9 5 0 - 1250 cm 1 [12]. The band at 1235 cm -1 could be assigned to asymmetric stretching vibrations of Si-O-Si (v,~(Si-O-Si)), characteristic of five-membered rings in zeolites [13]. The band at 806 cm ~ is attributed to Vsym(Si-O-Si) while the band at 602 cm 1 was ascribed to the "'crystallinity peak" in zeolite synthesis, resulting from the formation of silica five ring during hydrothermal treatment [14]. We tentatively assigned the bands in the range below 500 cm z to the bending vibrations [12]. For H-kanemite itself, 29Si MAS NMR spectrum (Fig. 4) showed an intense signal a t 99.8 ppm corresponding to Q3 silicon enviromnents [Si(OSi)3(OH)] in addition to signals a t 92.1 and-112.1 ppm, assigned to Q2 [Si(OSi)2(OH)2] and Q4 [Si(OSi)4] silicon types, respectively. These Q2 and Q3 silicon types arise frona the interlayer condensation upon protonation [ 15]. oo o

I

I

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9oo

-114.8

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-99.8

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~ ,

I

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,

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1500 1200 900 600 wavenumber / cm~ Fig. 3. FT-IR spectra of FLS material calcined at different temperatures (~ 1800

l L

]._,/V ~^. H-ka..~neln it e,. A

-,.,-

1 - 0

I

2.1

i

-120

i

i

-160

Chemicalshift/ppITl

Fig. 4. 29Si NMR spectra of H-kanemite and FLS material calcined at different temperatures (~

259 Differences in the fundamental structures of H-kanemite and FLS phase became readily apparent. The 29Si MAS NMR of FLS (Fig. 4) phase exhibited an intense signal at-113.6 ppm, related to Q4 silicon type [Si(OSi)4]. Small resomaances a t - 1 0 1 . 7 and -82.0 ppm conesponding to Q3 and Q2 enviromnents were also observed [ 16]. The important decrease of Q3 Si signal indicated that FLS is rich in Q4 Si species (with a Q4/Q3ratio of 2.7) compared to the starting H-kanemite. The peal( of Q3 sigmal in FLS material implies the presence of =-SiOH groups. 3.3. Thermal properties The as synthesized FLS was heated at 200 ~ for 10 hours, after the removal of different types of water, the corresponding powder XRD patterns remained practically unchanged. At higher temperatures above 300 ~ the FLS was transformed to another new phase (Fig. 5). The calcined FLS phase is found to be stable up to 1000 ~ with an amorphous phase being formed at this temperature (Fig. 5). The crystallinity was increased as the heating temperature increased above 500 ~ During the calcination, the basal spacing of FLS decreased from 1.02 mn to 0.90 nm. This contraction is most probably due to the elimination of TMA cations and to the loss of silanol density via condensation of Si-OH groups. After calcination of FLS material at 500 ~ the 29Si MAS NMR spectruna showed mainly a sigmal a t - 1 1 4 . 1 ppm . . . . 1000 assigned to Q4Si species (Fig. 4). No further signal of Q3 Si type was obselwed, .900 indicating that a complete condensation of Q3 Si species occuned during heating, and the silicate layers of FLS phase were cross?oo linked to each other [7]. Similar behavior 0.90 was reported for the transformation of twodimensional layered aluminosilicate 5oo (assigned as PREFER) to tl~'eedimensional fi'amework of FER-type material [17]. The detection of sigmal a t 4oo 90.2 ppm was still obseawed even after heating at 700 ~ (Fig. 4). This sigalal was ~._ 300 not detected frequently seen in zeolites, and could be related to the starting host 200 materials. 1.02 The FT-IR indicated that calcined FLS did not contain TMA cations, and exhibited a strong absorbance at 1088 cm -1 with a shoulder at 1227 cm-~; weaker bands at 814 and 609 cm ~ and two moderate I'0 2'0 3'0 4'0 (20) absorbances near 465 - 4 4 0 cnal (Fig. 3). Fig. 5. Powder XRD patterns of FLS The shift of the broad at 1072 to high value phase calcined at different temperatures at 1088 cm -~ was consequence of further (~ development of O4 Si species [18]. .

.

.

.

.

.

.

.

,,,,-4 rae~ r

. ,.,.~

.

.

.

.

.

.

.

.

.

.

260 3.4. Surface area and microporosity

The shape of isotherm for the as s3mthesized FLS material corresponds to a non porous material. For the calcined FLS material, however, the shape of the isotherm in the low relative pressure range corresponds to type I, thus indicating a microporous character [ 19]. The shape of these isothenns is similar to that reported for zeolite materials [20]. Specific surface areas of the calcined materials, estimated on the basis of BET equation, are collected in table 1. The FLS material has a value of 70 m2g 1, and indicated that the adsorption occurred mainly at the external surface. After calcination at temperatures above 400 ~ there is an increase in the measured BET surface area up to 335 na2g1. The large surface area was due to the formation of micropores during calcination and which were blocked by TMA cations before calcination. Further increase of calcination temperature to 1000 ~ leads to the collapse of FLS structure accompanied with a decrease of surface area at 47 m2g-~. The CBET constant derived from the BET equation for calcined FLS materials is negative (Table 1), and confirms a lnicroporous character of these materials [20], which not the case for nonporous as synthesized FLS phase with a positive CBET. According to the BET theory, CBET is related exponentially to the enthalpy (heat) of absorption on the first adsorbed layer. A negative value for CBET has no physical meaning, indicating that the BET method is not suitable for the evaluation of specific area of these materials with primary micropore filling. We have used another method, therefore, to calculate the lnicropore volume and external surface area of calcined FLS materials, which has recently applied to lnicroporous materials [21, 22]. Table 1 reports the external surface areas and micropore volumes detennined by this method. The micropore volume increased as the calcination temperature did, then sharply decreasing. The increase in micropore volume was related to the elimination of TMA cations, lnaking accessible the nitrogen molecules to the pores; the decrease of lnicropore volume was related to the collapse of the pores and the formation of amorphous phase. The value of the external surface areas is mainly constant mad close to 90 m2g-1, and it was reduced to 47 m2g-1 after the calcination at 1000 ~ due to the destruction of the FLS structure. A further characterization of the microporous character of calcined FLS salnples was realized by studying their lnicropore size distribution. All materials calcined at temperatures higher than 400 ~ presented essentially a micropore size with a 0.88 mn in diameter, with a second pore type population of 1.56 mn in diameter. Table l. Characteristic N2 adsoprtion/desorption data for calcined FL samples Total pore Samples SBET* CBET External Micropore Surface* Volume + Volume + FLS (25) 70 108 70 .... 0.124 FLS (300)" 147 217 75 0.031 0.144 FLS (400) 316 -474 94 0.090 0.212 FLS (500) 335 -432 90 0.110 0.243 FLS (700) 307 -436 92 0.093 0.242 FLS (900) 241 -503 88 0.072 0.195 FLS (1000) 47 98 47 .... 0.110 %alue between parenthesis indicate the calcination temperature (~ * m2g l , + mLg -1.

261 For FLS calcined at 900 ~ the distribution of pore size was shifted to relatively high values. The dimensions of TMA was estimated about 0.62 to 0.78 mn [23,24], the average pore size of 0.88 nm indicated that the TMA cations were accommodated easily in the material and TMA played a pore-filling role in the as-synthesized FLS. 3.5. Reaction sequences for the formation of FLS A plausible reaction consequence for the fonnation of the FLS phase thus is proposed: Hkanemite was intercalated by TMA cations, and during the exchange of protons by TMA, a part of silicate layers were dissolved and transfornaed to crystalline silica phases. The TMA intercalated kanemite was then transfonned to FLS phase and silica phase was completely dissolved after longer reaction periods. Indeed, the product yields were in the range of 70 to 85 %. The solution-rnediated transfon-nation of TMA intercalated kanemite to FLS during hydrothemaal treatment is not yet clear. However, in the unlmown structure of FLS material, the presence of five membered rings chains is possible, since the absorption at 1242 cm z was detected in the FT-IR spectrum and it was not detected in the starting H-kanelnite. The presence of five-membered rings chains could result from a dissolution of TMA intercalated kanemite and reorganization of silica species around the TMA cations. 3.6. Transformation of as-synthesized FLS to TMA containing Sodalite

. .,i-.,

r~ .,,.~

,_;o

1.02

to

20

30

40 (20)

Fig. 6. Powder XRD patterns of (a) FLS reacted with NaA102 and solid TMAOH.5H20 at (b) 25 ~ and 150 ~ for (c) lh and (b) 24 h, respectively.

The solid state transformation of intercalated silicates was firstly proposed as an efficient and alternative route for the preparation of silicalite type zeolites at relatively low temperatures. Recently the synthesis of TMA-containing sodalite (SOD) from a mixture of layered silicate (HLS) and sodium aluminate was reported using solid state transfonnation method [25]. ha our preliminary tests, we have succeeded to transform the FLS material to a SOD phase in the mixture of almninate and solid TMAOH.5H20. The SOD phase was obtained after 1 h of reaction at 150 ~ and its crystallinity was improved for a reaction time of 24 h (Fig. 6). However, the SOD phase was not obtained when the FLS material was reacted with sodium aluminate. At room temperature, the basal spacing of FLS phase was increased 1.02 to 1.66 mn prior the solid state transformation (Fig. 6). This expansion could confil'm indirectly the layered character of FLS phase.

262 4. CONCLUSIONS H-kanemite was converted at temperatures above 130 ~ to a new layered silicate containing five membered ring polyhedrons and TMA cations. Upon calcination, a threedimensional framework was formed resulting from the condensation of silicate layers, and mainly Q4silicon species was detected. The calcined FLS phase was stable at temperatures above 900 ~ with microporous character and surface area of 335 m 2 g-l. REFERENCES

~

4.

~

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

K. Beneke and G. Lagaly, Am. Miner. 62 (1977) 763. S. haagaki, A. Koiwai, N. Suzuki, Y. Fukushima and K. Kuroda, Bull. Chem. Soc. Jpn., 69 (1996) 1449. K. Kuroda, J. Porous. Mater., 3 (1996) 107. S. Shimizu, Y. Kiyozumi, K. Maeda, F. Mizukami, G. Pfil-Borb61y, R. M. Mihalyi and H. Beyer, Adv. Mater., 8-9 (1996) 759. M. Salou, Y. Kiyozumi, R Nair, F. Mizukami, K. Maeda an d S. Niwa, J. Mater. Chem., 8(1998) 2125. F. Kooli, F. Mizukami, Y. Kiyozmni and Y. Akiyama, J. Mater. Chem., 11 (2001) 1946. F. Kooli, Y. Kiyozuani and F. Mizukami, ChemPhysChem., 7-8 (2001) 549. C.-Y. Cheng, S.-Q. Xian and M. E. Davis, Microporous Mater., 4 (1995) 1 Z. Johan and G. F. Magoline, Bull. Soc. Fr. Mineral. CristallogT., 95 (1972) 372. J. J. Stevens and S. J. Anderson, Clays Clay Miner., 44 (1996) 142. X. Yang, J.-L. Paillaud, H. F. W. J. van Breukelen, H. Kessler and E. Duprey, Microporous Mesoporous Mater., 46 (2001) 1. E. M. Flanigen, J. Khatami and M. Szymanski, Adv. Chem. Ser., 101 (1971) 201. J. C. Jansen, F. J. van der Gaag and H. van Beld~um, Zeolites, 4 (1984) 369. A. Zecchina, S. Bordiga, G. Spoto, L. Marchese, G. Petrinin, G. Loefanti and M. Padovan, J. Phys. Chem., 96 (1992) 4985. D. C. Apperley, M. J. Hudson, M. T. J. Keene, and J. A. Knowles, J. Mater. Chem., 5 (1995) 577. E. Lipmaaa, M. Magi, A. Somoson, G. Engelhardt, and A.-R. Grimmer, J. Am. Chem. Soc., 102 (1980) 4489. L. Schreyeck, P. Caullet, J. C. Mougenel, J. L. Guth and B. Marler, Microporous Mater., 6 (1996) 259. Q. Kan, V. Fornes, F. Rey, and A. Corma, J. Mater. Chem., 10 (2000) 993. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, T. Rouquerol and T. Siemienewska, Pure Appl. Chem., 57 (1985) 603. A. Gervasini, Appl. Catal., A: General, 180 (1999) 71. M. J. Remy, A. C. Vieira Coelho and G. Poncelet, Micropouos Mater., 7 (1996) 287. E Kooli, T. Sasaki, M. Watanabe, C. Martin and V. Rives, Langmuir, 15 (1999) 1090. V. R Valtchev, J. Chem. Soc., Chem. Commun., (1994) 261. S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen. J. Am. Chem. Soc., 104 (1982) 1146. Y. Kiyozumi, Y. Akiyama, F. Mizukami, and T. Ikeda. T. Stud. Surf. Sci. Catal. 135 (2001) 02P25.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Raman

Spectroscopic Studies

263

o f the Templated Synthesis o f Zeolites

P.P.H.J.M. Knops-Gerrits*, M.Cuypers D6partement de Chimie, Universit6 Catholique de Louvain (UCL), Batiment LAVOISIER, Place L. Pasteur n~ B-1348 Louvain-la-Neuve, Belgium T61:(32)10 -47 29 39 Fax: (32)10- 47 23 30 E-mail : [email protected] * To whom correspondence should be addressed. The application of confocal Raman and FT-Raman in the study of zeolite synthesis and structural characterisation of zeolite framework vibrations, zeolite synthesis with structure directing agents and adsorption on zeolites is illustrated by a number of examples, such as in ZSM-5 synthesis, the tetrapropylammonium ion (TPA+), in EMT synthesis the Na-18-Crown-6 that can direct hypo- (and hyper-)cage formation, Can Li and co-workers performed zeolite X synthesis studies and FAU (X,Y), MFI, MOR and BETA characterization with UV-Raman. The FT-Raman properties of zeolite-entrapped [Ru(bpy)3] 2+ and [Mn(bpy)2]2+-NaY catalysts are also reviewed. 1. INTRODUCTION In comparison with this vast amount of infrared spectroscopy work, the field of zeolite Raman spectroscopy seems rather evolving [1-2]. The examination of highly characterised surfaces such as single crystals to which are sorbed sub-monolayers of reactive gases or vapours must obviously be far more likely to yield results of significance on catalytic processes [3-4]. If we wish to carry out a simple transflectance measurement the sample thickness would be ~2 angstroms i.e. -10 .5 of the conventional infrared pathlength. If one planned to try Raman the sample size is absurdly small. So the outlook looks bleak before one starts studying microporous oxide materials but in the presence of zeolites the good sorption capacities allow to study adsorbed or ship-in-the-bottle complexes quite well. Many catalysts are highly porous. Compounds such as silica aluminas or zeolites have huge areas to which molecules can sorb. In microporous materials the holes may be of molecular size and limit access to sites nearer the surface, but the surface area of these materials can be huge indeed - values of 100 to 1000m2g"1 are not unusual. Sorbing a monolayer to these materials does provide a decem amount of material and so vibrational spectroscopy started with this type of catalyst system. Pioneers such as Norman Sheppard adsorbed methane to silicas and using infrared were able to show how the molecules interacted [3-4]. In the development of zeolite science, IR has been one of the major tools for structure and reactivity characterization. The reason for this choice is that there is considerable difficulty in obtaining Raman spectra with acceptable signal-to-noise ratios from highly dispersed materials such as zeolites [5-6]. The Raman effect is intrinsically a weak phenomenon. In order to pass about the low sensitivity an increase of the signal can be obtained ( v 4 law) by an increase of the excitation frequency [7-9]. Raman spectra of zeolites are often obscured by a broad fluorescence. Several causes for fluorescence have been identified. (1) small amounts of aromatic, strongly luminescent molecules might be present in the samples. (2) the presence of (reduced) transition metal ions or Fe impurities in the lattice is known to cause luminescence. The latter problem can be overcome via high-purity synthesis, starting e.g. from metallic AI. (3) proton super-polarizability and basic surface OH groups are two other reasons.

264 Dehydroxylation can only be used if the sample resists such a treatment. The advent of Fourier transform Raman 07T-Raman) spectroscopy with excitation in the near infrared (NIR) domain offers new perspectives in reducing background fluorescence. 2. EXPERIMENTAL

2.1.Materials Commercial samples of ZSM-5 (PQ Company Si/A1 - 81, ALSI-PENTA Si/A1- 11), MOR (TOSOH with Si/A1 - 10), FER (PQ Company Si/A1-~ 10), NaY (Zeocat; Si/A1 ratio of 2.47) were obtained. Intergrowths of FAU/EMT were synthesised using crown-ethers as templating agents. The quantification of the FAU and EMT phases is done by independent calibration with pure 1,4,7,10,13,16-hexaoxacyclooctadecane 18-Crown-6 containing EMT and 1,4,7,10,13pentaoxacyclopentadecane 15-Crown-5 containing FAU. In EMT only Na-18-Crown-6 can direct hypo- (and hyper-)cage formation, whereas in a FAU phase both Na complexes of 18Crown-6 or 15-Crown-5 can direct supercage formation. [26-27]. 2.2.Spectroscopy Raman spectra were recorded on a Renishaw Raman Microscope. The zeolite samples, in powder form, were placed on a glass microscope slide. The power on the sample was about 2mW/mm2. The collection time varied from sample to sample and was between 15 and 20 minutes. The spectra were background corrected and a Fourier deconvolution procedure, described elsewhere3, was applied to resolve the overlapping bands in the OH stretching region. FT-Raman spectra were recorded on a Bruker IFS 100. The zeolite samples, in powder form, were pressed in metal sample-holders. FT-Raman spectroscopy has been used to characterise FAU/EMT intergrowths. The vibrational properties of the crown-ethers are discussed in the context of their symmetry and complexation. In the CH stretching region planar or puckered coordination around the Na + cations can occur. The conformation of the cation 15-Crown-5 and 18-Crown-6 complexes in the different faujasite structures is discussed. FT-IR spectra were recorded on a Nicolet F-730 spectrometer. 3. RESULTS AND DISCUSSION

3.1. Raman studies of zeolite synthesis. Just like IR spectroscopy, [28] Raman can detect small, X-ray amorphous zeolite particles. Therefore Raman has been used to examine both the liquid and the solid phase of zeolite synthesis mixtures. Ex situ methods (with separation of solid and liquid) and/n situ methods have been applied. In studying the liquid phase, one should remember that (i) minimum concentrations for detection of spontaneous Raman from liquids are typically 0.05 - 0.1 M, [ 17, 29] (t0 that the cross-section of the AI(OH)4 species is much stronger than e.g. for silicate or aluminosilicate anions [30]. Thus species which are present in low concentration or with variable structures may easily be overlooked in Raman spectra of the synthesis liquors. When zeolites are synthesized starting from a silica sol, the Raman spectrum of the initially formed solid phase resembles that of vitreous silica, with a broad band around 450-460 cm1 [29, 31,32]. For zeolites A, X and Y, aluminate is available in solution, as shown by a Raman band at 620 cm1 [30-34]. AI can be rapidly incorporated into the solid phase. In particular for zeolites X and Y, this process is accompanied by the appearance in solution of monomeric silicate species, such as SiO2(OH)22- (780 cm1) or dimeric silicates [30,32,34]. There is no Raman evidence for the presence of aluminosilicate ions in solution.

265

6,t8 a m ~

2911, 3~ilD and S'I4 ~ m "~ Bm

e,~aklP

Zeoletm X

Figure 1. Scheme of the formation mechanism of zeolite X (after Ref. 6 ). Analysis of the 400-550 c m "1 spectrum during heating or eventual aging of the synthesis gel provides information on the building blocks present in the still relatively disordered solid phase. Again this analysis is based on the correlation between ring-size and vs(Tow)frequency. Thus for zeolites A and X, the band close to 500 c m "1 indicates the presence of 4MR at the beginning of the gel heating [31]. In zeolite Y synthesis, the gel aging causes a shift towards lower frequencies (440, 361 cm-1), showing the formation of 6MR. During the subsequent heating, sodalite units with 4MR are formed and the 500 cm-1 band gains intensity again [32]. Syntheses from Si sources other than colloidal silica have also been studied [34]. UV Raman spectra of the liquid phase of the synthesis of the framework of zeolite X indicate that AI(OH)4 species are incorporated into silicate species, and the polymeric silicate species are depolymerized into monomeric silicate species during the early stage of zeolite formation. An intermediate species possessing Raman bands at 307, 503, 858 and 1020 cm 1 is detected during the crystallization in the solid phase transformation. The intermediate species is attributed to the ffcage, the secondary building unit of zeolite X. A model for the formation of zeolite X is proposed, which involves four-membered tings connecting to each other via six-membered ring to form ~cages, then the ,fl cages interconnect via double six-membered tings to form the framework of zeolite X[6,8]. Mordenite and ZSM-5 are synthesized at lower pH values, and it is not surprizing that in these conditions silicate species are not usually observed in solution [27,29]. In both cases, the solid initially resembles vitreous silica. In the case of ZSM-5, new vibrations in the solid phase are only observed when ZSM-5 crystals appear [27]. More information concerning the intermediate synthesis steps can be retrieved from mordenite synthesis spectra [29]. In mordenite synthesis, a 495 crn"1 band is observed at an early stage in the spectrum of the solid fraction of the gel. In analogy to what is observed for A and X zeolites, this band is ascribed to 4MR aluminosilicate units. Only at a later stage, broader bands appear at 402 and 465 cm-1, indicative of the formation of still rather disordered 5MR mordenite-like units. These bands eventually sharpen into the characteristic framework vibrations of mordenite. 3.2. Template observation via Raman.

Observation of the organic template against the weak zeolite background is highly facilitated in Raman in comparison with IR. In some particular cases, the incorporation and conformation of the organic template can be followed. In ZSM-5 synthesis, the tetrapropylammonium ion (TPA § is initially incorporated into the solid amorphous aluminosilicate in the all trans configuration, which it also possesses in solution. The eventual ZSM-5 crystals however contain a high-energy TPA § conformer, in which one of the N-C bonds is rotated [27,3 5].

266 Table. 1. Characteristic differences in the FT-Raman spectra of the FAU/EMT containing the crown-ethers 15-Crown-5 and 18-Crown-6. FAU EMT MIX (15-Crown-5) (18-Crown-6) 348 347 vfNa-O)lattice 354 355 v(Na-O)lattice 503 503 503 v(Si-O-Si)sym lattice I 860 860sh 15-Crown-5 v(C-O-C)sym 869 869 18-Crown-6 v(C-O-C)sym II 1001 1002 15-Crown-5 CH2 rocking complex III 1040 1040 15-Crown-5 v(C-O-C)asym 1082 1092 18-Crown-6 v(C-O-C)asym IV 1137 1137 18-Crown-6 CH2 wagging complex 1152 1152 15-Crown-5 CH2 wagging complex V 1264 1270sh 15-Crown-5 CH2 twisting complex 1294sh 1296 1294 18-C-6/15-C-5 CH2 twisting complex VI 2859 18-Crown-6 CH stretching complex The strong non-bonded interactions between-CH2- groups of different propyl groups are evidenced by the changed rocking and wagging modes. This conformation has also been proved in XRD studies. In syntheses of LTA zeolites, a tetramethylammonium ion (TMA +) can be incorporated in the sodalite cages. The tightness of this fit induces a frequency increase in the symmetric C-N stretching vibration of the template [33-36]. Raman studies indicate that at a fixed pH, organic cations (e.g. TMA +) and alkali ions stabilize different silicate species [37]. This clarifies the role ofTMA + in synthesizing LTA zeolites with high Si/A1 ratio, e.g., ZK-4. Crown-ether templates are used in the synthesis of cubic and hexagonal faujasites. The Na + forms of the cyclic ethers 18-crown-6 (18C6) and 15-crown-5 (15C5) can be used to direct the synthesis of faujasites towards the cubic FAU or the hexagonal EMT topology [26-27]. FTRaman spectra of as-synthesized FAU, EMT and a structural intergrowth of both topologies (MIX) are shown in Table 1. the intergrowth was synthesized with with a 3 : 1 18C6 : 15C5 mixture. The FT-Raman technique yields superior spectra compared to visible excitation, based on other Raman reports on this system [39-40]. In the spectra, features of framework and crown ether template are superimposed. The band at 503 cm"1 is the framework symmetric bending vibration; most other bands are crown-ether vibrations. There are a few differences between the spectra of Na+-15C5 in FAU and Na+-18C6 in EMT. Both ethers have a characteristic intense band between 800 and 900 cm1, which has C-O stretching and CH2 rocking character. A sharp band is observed at 1001 cm-1 in FAU and to a lesser extent in the intergrowth. This band is lacking in as-synthesized pure EMT, for 18-crown-6 and its complexes, the relation between Raman spectra and the symmetry of a compound (Dsd, Ci or C1) has been studied in depth. K § 18C6 always assumes D3d symmetry, and in crystalline (Na§ 8C6)(SCN) it has an envelope-like C1 structure. For dissolved Na§ 8C6, the geometry may fluctuate between different conformers (Dsd or C1) depending on the solvent. While Na§ 8C6 undergoes some distortion in the EMT hypocage, the EMT hypercage provides sufficient space for a more relaxed crown ether conformation, similar to solution structures [39-40].

267 3.3. Extra-framework metal complexes via Raman. In UV-Vis the bis-bipyridyl complexes are generally observed to vary significantly in their properties from the tris coordinated complexes. For the Ru tris and bis bipy complexes shown in Figure 3. The 480 nm MLCT band is generally split into two bands. For divalent transition metal bis complexes the second rt-rt* band of bipyridyl is split. The rt-rt* bands occur at 235nm, 280300nm and 290-310nm. This splitting is not always observed in zeolite occluded samples. For Mn bis-bipy the LMCT occurs at 358 and 415nm the MLCT bands at 440 and 530 nm. In FT-Raman a superposition based on the interaction between the vibrations of the lattice superstructure and the transition metal complex is seen. The Raman scattering cross-section of zeolites, mainly due the structure sensitive symmetrical SiOSi and SiOA1 bending modes (486521 cm1 for frameworks with a combination of 4,6,8,10 and 12 membered rings) and the Si-O and A1-O stretching modes (asym stretch at 1250-950 cm1, sym stretch at 950-700 cm1) coupled in the ring modes is very low. This is however not the case for the FT-IR spectra since here the structural vibrations (asym stretch at 1250-950 cm-1, sym stretch at 820-650 cm-1, double ring vibrations at 650-500 cm-1, T-O-T bending at 500-420 cm"1) that be divided into structure sensitive and insensitive modes in the spectra of the NaY zeolite limit the characterization of complexes with FT-IR and make any serious efforts to collect all normal modes impossible. Bipyridine complexes in zeolites. The properties of zeolite-entrapped [Ru(bpy)3] 2+ and related complexes have been studied by resonance Raman spectroscopy [45-52]. For similar compounds the Fourier transform technique allows to obtain spectra with excellent signal to noise ratios, even if the signals are not enhanced by a resonance effect. Table 2 shows the FT-Raman data with 1064 nm excitation for [Mn(bpy)2]2+-NaY and for the related crystalline compound [Mn(bpy)2(NO3)2]. [Mn(bpy)2]2+-NaY was recently discovered to display an unprecedented activity in epoxidation catalysis [53-55]. Differences between the spectra of zeolite-entrapped and crystalline [Mn(bpy)2] 2+ are due to the presence of different charge-compensating anions. In the zeolite spectrum, the band at 503 cm-1 is characteristic for the 4MR in the faujasite structure. The NO3- anion in the crystalline complex shows a strong band of the symmetric stretching vibration at 1041-1033 cml[38]. The Raman spectra of these bpy complexes are usually interpreted based on a local symmetry, i.e. a single bipyridine ligand is considered instead of the whole complex. While such an approach does not allow for geometry effects in the overall complex (e.g., cis vs. trans conformation in a bis bpy complex see Figure 2.), it facilitates the analysis of the normal modes [56]. Crystalline bpy has a C2h symmetry, with the nitrogen atoms in trans with respect to the interring C2-C2' bond. For bpy adsorbed on NaY [57], for [Mn(bpy)2]2+-NaY and for [Mn(bpy)2(NO3)2], the bpy molecule is in the cis form, with C2v symmetry. As a result, highly similar spectra are obtained for the latter three materials. There is however a clear difference between the spectra of bpy containing NaY and the [Mn(bpy)2] 2+NaY zeolite. For bpy adsorbed on NaY, the intense symmetric ring breathing mode Vl occurs at 1001 cm~[57]. In the [Mn(bpy)2]2+-NaY zeolite, this band shifts to 1015 cm~, due to the interaction of the lone pairs in the bpy ligand with the metal ion. This type of binding is obviously similar to the complexation of pyridine on exchanged cations. For [Mn(bpy)2]2§ the 1001 cm1 band is only a weak shoulder on the main 1015 cm1 band. This implies that the large majority of the bpy molecules in the[Mn(bpy)2]2+-NaY catalyst is interacting with Mn 2+.

Figure 2. Possible arrangements of bidentate ligands (bpy, phen, dmp,...) around a central metal ion : cis or trans bis- and tris-ligation.

268 Table 2. Comparative FT-Raman and FT-IR study of [Mn(bpy)2(NO3)2] {MnB2N2} and cis[Mn(bp),)2] 2+ -Y (MnB2Y) FTFTFT-IR FT-IR Modes Raman Raman MnB2N2 MnB2Y MnB2N2 MnB2Y A1 157 153 v(Mn-N), 13(C3-C2-C2'),~(C-N-C), ot(N-C2-C3) A1 241 243 8(Mn-N-C), v(Mn-N), 13(N-C2-C2') A1 352 354 v(C2-C2'), or(C-C-C), v(C-N) 462 ring breathing, Si-O-Si and Si-O-AI 503 504 TOT bending, Si-O-Si and Si-O-A1 B2 552 554 13(C3-C2-C2'), ]3 (N-C2-C2'), 8(Mn-N-C) v(C2-C3) 587 ring breathing, Si-O-Si and Si-O-A1 B2 627 628 623 634 ct(C-C-C), ot(N-C2-C3) B2 650 667 668 ~(c-c-c), v(Mn-~, ~(C-C-H) A1 735 737 ~(C-C-C), 8(C-C-H), v(C2-C3), oop 757 757 8(C-C-H), oop#1 768 A1 764 765 8(C-C-H), oop 772 772 8(C-C-H), oop#2 817 818 v(Mn-N) N of NO3" B2 1007 1015 1010 v(C2-C3), v(C-N), v(C3-C4), v(C4-C5) 1033 1030 v(Mn-N) N of NO3" A1 1041 1040 v(C4-C5), v(C2-C3), v(C3-C4), 5(C-C-H) A1 1060 1057 1058 5(C-C-H), v(C2-C3), c~(C-C-C), v(C3-C4) B2 1102 1097 8(C-C-H), v(C4-C5), v(C2-C3), v(C-N) B2 1160 1154 1152 or(C-C-C), 8(C-C-H), o~(C-N-C), v(C2-C3) A1 1178 1176 1172 s(c-c-I-i) A1 1253 1244 1258 8(C-C-H), v(C-N), 8(C2-C3-H) B2 1264 1265 1265 1266 v(C-N), 8(C2-C3-H), 5(C-C-H) A1 1285 1281 1287 v(C2-C3), v(C-N), v(C3-C4), v(C4-C5) B2 1310 1311 1305 1317 v(C2-C3), 8(C-C-H), v(C3-C4) v(C-N), v(C4-C5) B2 1436 1430 1435 8(C-C-H), v(C2-C3), 8(C2-C3-H) A1 1440 1442 8(C-C-H), v(C3-C4), v(C-N) B2 1478 1475 8(C2-C3-H), 8(C-C-H),v(C4-C5), v(C-N) A1 1491 1488 1490 1493 8(C-C-H), v(C2-C3), v(C2-C2'), v(C-N), 8(C2-C3-H) B2 1565 1568 1568 v(C-N), v(C3-C4), v(C4-C5), 8(C-C-H) A1 1573 1574 1577 v(C4-C5), v(C-N), 8(C-C-I-I) B2 1594 1591 1594 1597 v(C2-C3), v(C2-N), A1 1607 1606 v(C2-C3), v(C2-C2'), v(C-N), v(C3-C4) ~: ring bending vibration, 13 : ring-ring bending vibration, y : CH wagging vibration, 8 : CCH and MnNC out-of-plane (oop) vibrations, v: MnN, CH, CC and CN stretching vibrations. The 20 totally symmetrical normal modes comprise four in plane C-H deformations which can hardly be observed, four in plane ring angle deformations and 12 stretching modes: one interring C-C stretch, one Me-N stretch, four C-H, four C- C and 2 C-N stretching modes. In FT-

269

Figure 3. Structure of Ru(bpy)2 2+ -NaY and Ru(bpy)3 2+ -NaY Raman the Me-N bands occur in the 180-290 c m "1 region for metals with partially filled eg orbitals such as Mn(II) [59]. For [Mn(bpy)212+-Y this band which is a combination mode of the Mn-N-C deformation and the Mn-N stretch mode, is observed at 243 crn-1. Due to the steric constraint in [Mn(phenh]2+-Y the Mn-N stretch frequency is shifted to 251 cm1. The four ringangle deformations ct(CCC) below 1000 cm1 are weaker than these above 1000 cm1. The stretching modes are highly mixed as the C-H stretching modes contribute to the modes below 1200 cm1. Modes above 1300 cm1 are therefore more intense. The lattice affects the contributions of the out-of-plane 5(CCH) bending and v(C2C3) stretching modes. Its strain causes the shortening of these bonds resulting in shifts in the Raman frequency : 5(CCH) occurs at e.g. 1565 cm-1 in salts, and at 1573 crn-1 in the zeolite. The shortening of the v(C2C3) stretching results after occlusion in shifts of the bands at 1160, 1285 and 1594 cm-~ to 1154, 1281 and 1591 cm "1, respectively. ACKNOWLEDGEMENTS PPKG thanks the UCL and ESA PRODEX for a research grant. REFERENCES 1. I.R. Lewis, H.G.M. Edwards, Handbook of Raman Spectroscopy, 2001, Marcel Dekker. 2. N.J.Ortinis, T.A. Kruger & P.J.Dutta, Anal. Applies of Raman Spect. Ed., M.J. Pelletier Blackwell Science Oxford (1999) and refs contained therein. 3. R. Ferwerda, J.H. van der Maas & P.J. Hendra, J. Phys. Chem, 97 (1993) 7331. 4. P.J. Hendra. Intemet J. Vib. Spec.[www.ijvs.com] 5, 2 (2001) 4. 5. W. Pilz, Z. Phys. Chem. (Leipzig), 271 (1990) 219. 6. G. Xiong, Yi Yu, Z-C. Feng, Q. Xin, F-S. Xiao, C. Li Micropor. Mesopor. Mater, 42 (2001) 317. 7. P.-P. Knops-Gerrits, D.E. De Vos, E.J.P.Feijen, P.A.Jacobs. Micropor. Mater. 8 (1997) 3. 8. C. Li and P.C. Stair. Catal. Today 33 (1997) 353. 9. Yi Yu, G. Xiong, C. Li, Feng-S. Xiao, Micropor. Mesopor. Mater., 46 (2001) 23. 10. P.K. Dutta and B. Del Barco, J. Phys. Chem., 92 (1988) 354. 11. P.K. Dutta and J. Twu, J. Phys. Chem., 95 (1991) 2498. 12. C. Br6mard and M. Le Maire, J. Phys. Chem., 97 (1993) 9695.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation, characterization and s y n t h e s i z e d saponite-like materials

271

catalytic

activity

of

non-hydrothermally

R. Prihod'ko a, M. Sychev a, E.J.M. Hensen b, J.A.R. van Veen c and R.A. van Santen b aFaculty of Chemical Technology, National Technical University of Ukraine, 03056, Kiev, pr. Peremogy 37, Ukraine bSchuit Institute of Catalysis, Eindhoven University of Technology, Den Dolech 2, PO Box 513 5600 MB Eindhoven The Netherlands CShell International Chemicals B.V., Badhuisweg 3, 1031 CM Amsterdam, The Netherlands Saponite-like materials were non-hydrothermally synthesized from a Si/A1 gel and a solution containing the nitrate of a divalent octahedral cation (Zn, Mg, Ni and Co) and urea. The products were characterized by a variety of methods. The composition of the octahedral layer influences the structure and textural properties of these materials. Saponites having A13+predominantly in tetrahedral coordination can be obtained for Si/A1 _> 12. The acidic form of these saponites prepared via acid treatment with diluted HC1 show high activity in alcohol decomposition, cumene dealkylation and n-heptane isomerisation reactions. The Ni and Co-containing solids possess both acid and redox active sites. 1. INTRODUCTION Regardless of the fact that the use of natural clays in acid catalysis has been known for a long time, their large scale application is still hampered. Relatively low thermal stability and heterogeneity in chemical composition and textural properties of these solids are the main problems. With the use of synthetic clays it is possible to avoid indicated problems. However, the preparation of these materials, usually demanding hydrothermal treatment, has severe limitations for industrial-scale manufacturing and utilization. Recently, a novel nonhydrothermal synthesis of the saponite, a tetrahedrally charged trioctahedral smectite, was proposed [1-4]. Key points of this approach are the fourfold coordination of A13§ in the starting gel and controlled pH trajectory achieved via urea hydrolysis. The resulting materials possess high thermal stability, homogeneously distributed active sites, and exhibit promising catalytic properties in a wide range of reactions [ 1-4]. Incorporation of transition-metal ions, among them Ni and Co, into the framework of molecular sieves provides these materials with versatile catalytic properties [5]. By analogy, the incorporation of those metal ions into the lattice of claylike materials offers potentially a promising route for the preparation of new heterogeneous catalysts. Continuing previous studies [2-4], here we describe detailed preparation features, characterization and catalytic properties of the non-hydrothermally synthesized saponites differing in chemical composition.

272

2. E X P E R I M E N T A L

Saponite-like materials with Si/A1 ratio between 8.0 and 26 were synthesized on the basis of the theoretical composition of the saponite [Nax+(M2+6){Si8_xAlx}O20(OH)4]. The preparation of the sample with the Si/A1 ratio of 12 is given here as an example. An AI(OH)4- sol obtained by dissolving 4.22 g (11.25 mmol) AI(NO3) 3 ~ in a 28 cm 3 2.0 M NaOH was gradually added under vigorous stirring (1600 rpm) within approximately 1 min to the Si-containing solution prepared by dilution 30 g (0.135 mol) of a Na2SiO3~ (27 wt% SiO2) in 75 cm 3 of deionized water. The resulting gel was kept at 296 K without stirring for 1 h, and then suspended under stirring in 600 cm 3 H20 and heated to 363 K. Next, 500 crn 3 of solution containing 0.109 mol of the nitrate of octahedral cation (M 2+= Zn, Mg, Ni, Co) and 0.15 - 0.80 mol of CO(NH2)2 was slowly added to the stirred Si/A1 gel. This was taken as the starting point for synthesis. Typical synthesis duration was 12 to 72 hours, after which the resulting materials were separated by filtration, washed with deionized water 5 times, and dried overnight at 383 K. Once prepared, all materials were exchanged with NH4 § (1 M NH4C1) in order to remove interlayer cations. The saponites with Zn 2+, Mg 2+, Ni 2+ and Co 2+ in octahedral positions were denoted as ZnSP, MgSP, NiSP and CoSP, respectively. Their H-forms were prepared via decomposition of the NH4§ form at 673 K (as described in Ref. [6]) or through the treatment of saponites with 0.05 M HC1 at room temperature followed by washing and drying at 383 K. Equipment used and conditions of measurements. XRD: DRON-3 diffractometer, CuKoc-, CoKc~-radiation. XRF: high-vacuum device, Si(Li) detector (180 eV at 5.9 keV), sample weight = 200 mg. 27A1 and 298i MAS NMR: Bruker AMX 300 WB spectrometer, references [Al(H20)6] 3+ and TMS, respectively. UV-visible diffuse reflectance spectroscopy (DRS): Shimadzu UV 2401, reference BaSO4. FTIR: Perkin-Elmer 2000, KBr technique. N2 adsorption: ASAP 2010 (Micromeritics), degassing at 403 K, 10 -4 mbar, 5h, pore volume calculated by the C~s-method. Temperature-programmed reduction (TPR): 66% H2 in Ar, flowrate 20 ml/min, temperature range 298-1073 K, heating rate of 5 K/min, thermal conductivity detector. Catalytic tests" the decomposition of 2-methyl-3-butyn-2-ol (MBOH), 2-propanol, cumene dealkylation were performed at respectively 423,473 and 623 K as reported elsewhere [2,7,8]. Detailed operatory conditions of n-heptane isomerisation were described in ref. [8]. Prior to the testing, catalysts were preheated at 673 K in an Ar flow for 2 h. 3. RESULTS AND DISCUSSION 3.1. Characterization The XRD patterns of the samples obtained after 24 h synthesis time and an urea concentration of 0.15 mol/L resemble those previously reported [1,2] and evidence the formation of trioctahedral smectites. An important indication is the (060/-332) reflection at 1.54 A. The intensity of the reflections differ between the various samples due to variations in crystallinity, particle size and/or stacking. Important factors are the Si/A1 ratio with more crystalline materials resulting from ratios exceeding 12.0 and the use of Zn as octahedral cation. All samples show the typical smectite ability to swell in ethylene glycol. FTIR spectra of all samples contain lattice vibration bands at about 1050-, 986-, 820-, 690-, 457-440-cm -1 respectively assigned to Si(A1)-O

273 stretching, Si-O-A1, A1-OH-M 2+, Si-O-A1 and Si-O-M 2+ bending characteristic of the saponite [9,10]. Except for ZnSP, the samples do not suffer from amorphisation upon heat treatment at 773-873 K (IR, XRD). Diffuse reflectance spectra of NiSP display bands at around 379, 670 and 735 nm which originate from octahedrally coordinated Ni 2+ cations due to the transitions from 3A2g(F) to 3Tlg(P) 3T]g (F) and 3T2g(F) [11]. The DR spectra of CoSP show a band at 524 nm and shoulders at 500, 646 nm assigned to the transition v3 (4A2g--)4Tlg) and v2 (4Tlg(P)--)4Tlg), respectively, typical for Co 2+ in octahedral coordination [12]. Since Ni 2+ and Co 2+ admittedly accommodated in the non-framework positions were removed by the ion-exchange with NH4 § the observed DR bands can be attributed to the cations located in the octahedral sheets of the saponite structure. The discussed experimental results point to a saponite-type structure for the prepared materials. NiSP and CoSP have Ni 2+ and Co 2+ situated in the octahedral sheets. To obtain a more detailed picture on the nature of these materials we additionally performed 27A1 and 298i MAS NMR (including of some of the starting gels), N2 adsorption and TPR. The starting gels of both ZnSP and MgSP only contain the four-fold coordinated A13+, Al(tet), as evidenced by a band at 54 ppm in 27A1 MAS NMR spectra (Fig. 1A for ZnSP). After 12 h of synthesis for ZnSP, new resonances at around 64 and 9 ppm have appeared. These are assigned to tetrahedral and octahedral A13+ species in the saponite structure [10]. While here the band at 54 ppm is present as a shoulder, it has completely disappeared after 24 h pointing to complete conversion. The formation rate for MgSP is slower and the typical resonance at 64 ppm only appears after 30 h of synthesis. '~

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~5 [ppm] Figure 1. :TA1 (A,B) and :9Si (C) MAS NMR spectra as a function of synthesis duration. (A) ZnSP with (1) starting Si/A1 gel; (2) after 12 h; (3) after 24 h. (B) MgSP with (1) after 30 h; (2) after 36 h; (3) after 36 h with 0.40 mol/L urea. (C) (1) ZnSP after 24 h; (2) MgSP after 30 h; (3) MgSP after 36 h with 0.40 mol/L urea.

274 The Si/A1 ratio strongly governs the A13+distribution in the saponite structure. A13+is primary placed in tetrahedral positions for Si/A1 >__12. The CO(NH2)2 concentration does not affect the chemical shift (6) and the width of the Al(tet)band in ZnSP. However, it clearly is important for MgSP (Fig. 1B): the narrowest Al(tet) signal is detected for the solid produced with an increased urea concentration of 0.40 mol/L. This also resulted in increased crystallinity for NiSP and CoSP. Therefore, the materials prepared in the presence of 0.40 mol/L CO(NH2)2 were subjected for the further investigations. 298i MAS NMR spectra of ZnSP and MgSP with Si/A1 = 12 (Fig. 1C) exhibit three resonances of decreasing intensities at about -96, -92 and -87 ppm (Q3 Si(0A1), Q3 Si(1A1) and Q3 Si(2A1), respectively) originating from Si coordinated with A13+ in the tetrahedral sheets [ 1]. The lack of the resonance at -102 ppm evidences the absence of amorphous silica. However, the band at -87 ppm cannot be assigned unambiguously, since Q2 Si(0A1) also shows a resonance at comparable value. The synthetic saponites, especially MgSP, are composed of small platelets [ 1] with consequently a high amount of Si4+ situated at their edges. These atoms might produce a signal in the region of Q2 Si. This surmise is strengthened by the observation that ZnSP composed of a larger particles [ 1] indeed exhibits the band at -86.7 ppm with a lower intensity. Acidic smectites are frequently used as a catalysts. Consequently, we paid attention to proper preparation procedures to convert the materials to their acidic form. Figure 2 shows the 27A1 MAS NMR spectra of H-form of MgSP obtained via acid treatment and calcination of its NH4+-form. After treatment of MgSP with 0.05 M HC1, a new resonance in 27A1 NMR appeared at 55-56 ppm (Fig. 2) due to the non-framework tetrahedral A1 caused by the leaching. The band intensity increased with treatment time. Simultaneously, the relative content of the Al(oct) remains practically invariable (Fig. 2), Hence, some structural 55.5 66.1: distortion occurred upon the acid treatment. Calcination of (NH4)MgSP at 673 K drastically ~" modified its 27A1 NMR spectrum, i.e., the band at 55 ;~ i 8.3 ,,, ppm becomes dominant, and an "-i \ :',.,/;, 3 additional band of the Al(oct) at ~= :: ",.,. ,,/5.1 "-,., around-5.2 ppm appears (Fig. 2), indicating complete structural collapse. Similar results were obtained for ZnSP. i The N2 adsorption isotherm of ZnSP after 24 h of (Fig. 3) is 100 50 0 -50 almost of Type II (typical of the mesoporous solids) [13], with ~5 [ppm] H2 hysteresis, being insignificantly altered with the synthesis time. Figure 2:27A1 MAS NMR spectra of MgSP treated with 0.05 M HC1 during (1) 3 h; (2) 18 h; (3) (NH4)MgSP calcined at 673 K.

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Figure 3. N2 adsorption-desorption isotherms of saponites after different synthesis times: (1") 12 h, (1) 24 h and (2) 36 h. (3) Mesoporous silica included as a reference. For the other saponites, the isotherm shape transforms from ahnost Type IIb to Type Ib for the solids obtained for 36 h. This points to the development of micropores. The hysteresis loop is also changed from Type H1 to almost Type H4, characteristic of the slit-like pores [13]. Table 1 Chemical composition and surface area (SBET), total pore volume ( V t o t a l ) , m i c r o p o r e mes0pore ( V m e s o ) volumes of samples after synthesis for 36 h and 0.40 mol/L of urea Sample Si A1 M 2+ aBET Vtotal Vmicro (m2/g) (cc/g) (cc/g) ZnSP 7.38 0.62 7.62 218 0.183 0.004 MgSP 7.43 0.57 5.24 592 0.323 0.303 NiSP 7.39 0.61 6.40 540 0.340 0.250 CoSP 7.39 0.6i 5.12 405 0.270 0.239

(Vmicro)

Vmeso

(cc/g) 0.179 0.019 0.090 0.031

and LL

276 The comparison of these isotherms with that of mesoporous silica (Type II + H1 hysteresis) (Fig. 3, sample 3), suggests that Mg-, Ni, and Co-saponites prepared during 24 hours contain some amorphous admixture. The Si/A1 ratios and the synthesis times (up to 72 h) do not notably affect the saponite textural properties. However, the saponite textural properties strongly depend on the nature of octahedral cations (Table 1). The chemical composition of saponite-like materials (XRF, in molar ratios) reflects the similarity of the Si/A1 ratios in starting gels and in the resulting materials. The amount of the octahedral cations slightly deviates from the ideal saponite composition (Table 1). Temperature programmed reduction of NiSP obtained after 24 h proceeds in two steps with maxima at 570 and 778 K (Fig. 4). The first small TPR peak agrees with the reduction of Ni(OH)2 impurity, while the second one might be attributed to the reduction of the Ni 2+ located in the lattice, since interlayer Ni 2+ cations were reduced at a lower temperature (728 K) as determined for Ni-exchanged MgSP (Fig. 4). The synthesis duration of 36 h lead to the first peak disappearance, and the TPR pattern contains a single reduction step centred at 800 K (Fig. 4) indicating complete Ni 2+ incorporation. The sharper reduction peak relates to increased crystallinity obtained by prolonged synthesis. While in the case of CoSP virtually no Co(OH)2 was detected, the reduction takes place at considerably higher temperatures. The sharper main reduction step comparing with the NiSP (Fig. 4) shows that Co 2+ cations are better distributed in the saponite lattice. All data collected clearly indicate that the nature of octahedral cation has a strong effect on the formation rate of non-hydrothermally synthesized saponite-like materials. Based on the crystallization model proposed by Kloprogge [9], one may assume that the saponite crystallization initiates from separated sheets. Since the main fraction of tetrahedral layers was arranged in the Si/A1 gel prior to the synthesis starting point, the formation of octahedral brucitetype sheets becomes the rate-determining factor of the structure formation. In this case, the ability of octahedral cation to form such sheets at appointed pH plays a decisive role. One such example, here, is the remarkable difference in crystallization rate for the Zn 2+ cation compared to Mg 2+, Ni 2+ and Co 2+. Additional study is necessary to fully understand the underlying mechanism.

f

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400

600

800

1000

400

996 600

800

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Temperature [K] Figure 4. TPR patterns of NiSP and CoSP after different synthesis duration (1) 24 h, (2) 36 h; (3), (4) Ni- and Co- exchanged MgSP, respectively.

277 3.2. Catalysis The catalyst acid-base properties can be characterized by different methods and here we applied 2-methyl-3-butyne-2-ol (MBOH) and 2-propanol (iPrOH) decomposition [14]. The MBOH transformation over NiSP and CoSP containing H + and Na + charge-compensating cations undergoes the acid and redox catalyzed cleavage reaction giving 3-methyl-3-buten-l-in (Mbyne), acetylene and acetone. The same is also observed for iPrOH decomposition. These findings indicate that Ni 2+ and Co 2+ located in the octahedral positions of the saponite lattice, mainly exposed at the crystal can generate redox active sites. The alcohol decomposition over H-forms of ZnSP and MgSP proceeds only through the acid catalyzed pathway. The activity of these catalysts in the used reactions depends on the preparation procedure (Fig. 5), as could be expected on the basis of the NMR data. The acid-treated MgSP and ZnSP exhibit higher activity than that of the H-form prepared therlnal decomposition of the NH4§ form. This can be explained by the structural collapse of this material. The occurrence of an optimum in the catalyst activity with the acid treatment time may be attributed to the optimal ratio between the formed Bronsted and Lewis (non-framework A13+ cations) acid sites. Formation of the non-framework A1, which yields the Lewis sites, was clearly evidenced by 27A1 MAS NMR in the acid activated saponite. Since cumene dealkylation and n-heptane isomerisation require participation of the stronger acid sites compared with the alcohol decomposition, they show the presence of those sites in the H-form of Zn- and Mgcontaining saponites. The Si/A1 ratio influences the catalyst activity in following sequence: 12 > 26 > 8, and by analogy to zeolites, is most likely related to the Next Nearest Neighbour effect.

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Figure 5. Quasi steady-state activity (2 h run-time) of MgSP treated with 0.05 M HC1 during (1) 3 h, (2) 6 h, (3) 18 h, respectively, and (4) (NH4)MgSP calcined at 673 K in reaction under study. Left the activity in MBOH and iPrOH decomposition and right the activity in n-heptane isomerisation and cumene dealkylation.

278 4. C O N C L U S I O N S In the present study, we have shown that a wide variety of parameters influence the properties of Zn-, Mg-, Ni- and Co- containing saponite-like materials prepared under non-hydrothennal conditions. Most importantly, the nature of the divalent octahedral cation has a profound effect on the saponite crystallization. Especially, the use of Zn 2+ results in the efficient formation of crystallized materials within 24 h as opposed for Mg 2+, Ni 2+ and Co 2+ for which synthesis times of at least 36 h were required. The octahedral layer composition also leads to variations in the textural properties. Whereas MgSP is mainly microporous, NiSP and CoSP contain a small fraction of mesopores and ZnSP is mainly mesoporous. The materials with predominantly tetrahedrally coordinated A13+ can be obtained at Si/A1 >_ 12. The H-form of Zn and Mgsaponites can be prepared via their acid treatment with diluted HC1. The Ni- and Co-containing solids possess both acid and redox active sites. Furthermore, saponites with Zn and Mg in the octahedral sheets being ion-exchanged with protons display a high catalytic activity in alcohol decomposition, cumene cracking and n-heptane isomerisation. 5. A C K N O W L E D G E M E N T S The authors thank Ms. M.C. Mittelmeijer-Hazeleger (University of Amsterdam, The Netherlands) for the adsorption measurements and Dr. K. Erdmann (Nicholas Copernicus University, Torun, Poland) for some NMR experiments. These investigations were supported in part by Ukrainian Ministry of Education and Science and by a Spinoza grant (to R.A.v.S.) from the Dutch Science Foundation. REFERENCES

1. 2. 3 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

R..J.M.J. Vogels, M.J.H.V. Kerkhoffs, J.W. Geus, Stud. Surf. Sci. Catal. 91 (1995) p. 1153. M. Sychev, R. Prihod'ko, Stud. Surf. Sci. Catal.118 (1998) p. 967. M. Sychev, R. Prihod'ko, I. Astrelin, P.J. Stobbelaar, R.A. van Santen, Book Abstract, EUROCLAY'99 Conf., Krakow, Poland (1999) 135.. R. Prihod'ko, M. Sychev, I. Astrelin, K. Erdmann, E.J.M Hensen, R.A. van Santen, Rus. J. Appl. Chem. (2001), accepted for publication R.A. Sheldon and R.A. van Santen (eds.), Catalytic Oxidation, World Sci. Publ., Singapore, 1995. R.G. Leliveld, W.C.A. Huyben, A.J. van Dillen, J.W. Geus, D.C. Koningsberger, Stud. Surf. Sci. Catal.106 (1997) p. 137. D. Bassett, H.W. Habgood, J. Phys. Chem. 64 (1960) 769. E. Booij, J.T. Kloprogge, J.A.R. van Veen, Clays and Clay Minerals 44 (1996) 774. J.T. Kloprogge, Thesis, University of Utrecht, Utrecht, The Netherlands,1992. L.Li, X Liu, Y.Ge, R.Xu, J. Rocha, J. Klinowski, J. Phys. Chem. 97 (1993) 10389. A.P. Hagan, M.G. Lofthouse, F.S. Stone, M.A. Trevethan, Stud. Surf. Sci. Catal.3 (1979) p.417. J. Dedecek, B, Wichterlova, J.Phys. Chem. B. 103 (1999) 1462. F. Rouquerol, J. Rouquerol, K.Sing, Adsorption by Powder and Porous Solids. Principles, Methodology and Applications, Acad. Press, San Diego, 1999, pp.439-441. C. Lahousse, J. Bachelier, J.-C. Lavalley, H. Lauron-Pernot, A.-M. Le Govic, J. Mol. Catal. 87 (1994) 329.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

279

Self-bonded AI, B-ZSM-5 pellets. C. Perri a, P. De Luca a, D. Vuono a, M. Bruno a, J. B.Nagyb and A. Nastroa aDipartimento di Pianificazione Territoriale, Universit/t della Calabria, Arcavacata di Rende, 87030 Rende (Cs) Italy. bLaboratoire de R.M.N, Facult6s Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium. Synthesis of self-bonded ZSM-5 pellets is carried out using Li salts containing gels of composition: xLi20-(y+z)Na20-yAl203-zB203-30TPABr- 150 SIO2-480 H20 where z= 18,36; 5
2. EXPERIMENTAL The following materials were used to prepare the initial gels: solution of LiOH (1.5 molal, 2.0molal, 3.0 molal Baker), sodium aluminate (Carlo Erba), sodium tetraborate (Na2B407 Carlo Erba), tetrapropylammonium bromide (TPABr, Fluka), precipitated silica (BDH) and distilled water. The pellets of B-ZSM-5 studied were obtained from the systems: xLi20-(y+z)Na20-yA1203-zB203-30TPABr-150 SIO2-480 H20 where z = 18,36; 5
280 The synthesis gels were obtained by mixing an aqueous solution of LiOH, NaA102, tetrapropylammonium bromide (TPABr) and Na2B407. The mixture obtained was homogenized in the mortar. Precipitated silica was then added and the mixture was homogenized to obtain a uniform gel. The amorphous gels were then prepared in form of pellets using pressures of 0.1-10 kgcm 2. Pellets were put into special autoclaves where the water phase was separated from the pellets[3]. Synthesis runs were carried out 170+2~ under static condition and autogeneous pressure. The syntheses were interrupted after various times, and the corresponding so-formed intermediate phases removed, filtered, washed with cold water and dried overnight at 100~ The pure crystalline phases were obtained by ultrasound treatment from the final samples, separating them from the remaining amorphous phase. The products were identified and their crystallinity was determined using powder X-ray diffraction patterns, recorded on a Philips PW 1830 diffractometer using CuKot radiation and 0.5 ~ 20 min 1. The crystalline samples obtained as self-bonded pellets were characterised by thermal analysis TG, DTG, DSC. The combined thermal analyses were carried out at a heating rate of 10~ 1 under an air flow of 15 mlmin 1, using a Netzsch STA 429. The crystal sizes and morphologies were investigated by optical microscopy and scanning electron microscopy (SEM- Stereoscan- Cambridge). The NMR spectra were recorded on either a Bruker MSL 400 or a CXP 200 spectrometer. For 29Si (39.7 MHz) a 5.0~s (0=7t/2) pulse was used with a repetition time of 4.0 s. For ~3C (50.3 MHz), cross polarisation was used with 5.0 ~ts (0=n/2) pulses and a single contact sequence (5.0 ms contact time and a recycle time of 4.0s). For 27A1 (104.3 MHz) a 1.0 ~ts (0=n/12) was used with a repetition time of 0.2s. For the NMR measurements the pelleted samples were simply crushed manually in an agath mortar until a fine powder was obtained.

3. RESULTS AND DISCUSSION 3.1. Crystallisation fields and crystallisation curves. It is very important to determine the crystallisation fields in order to obtain pure zeolitic material. This is even more important for the preparation of self-bonded pellets, because the region where they can have enough rigidity is even more restricted[3]. The crystallisation fields determined by XRD data after three days of synthesis are illustrated for systems containing different amounts of boron in Fig. 1. B-ZSM-5 pellets are obtained in all systems at high alkalinities and SiO2/AlzO3 ratio greater than 10. A greater amount of silicon is probably necessary to form an inorganic binder. Increasing amount of boron in the initial gel is accompanied by a decrease of the analcime formed as cocrystallized phase. Crystallisation curves of B-ZSM-5 pellets obtained from systems containing different amounts of boron and having different values of SIO2/A1203 ratio have been determined (Fig.2). It can be seen from Table 1 that the rate of crystallisation is higher with 18 moles of B203.The induction period is not influenced in a first approximation by an increase of the BzO3 content. The induction period was measured at the appearance of ca 4% crystallinity.

281

B-ZSIVl-5 Pellets

~

B-Z,gM-5

B-Z~M-5

~ E~ZSM-5+Ana lO.

(~%~)

o

,

,

\

B

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o

,

0,0

i 4,5

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,

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Fig. 1 Crystallisation fields obtained after three days of synthesis at 170~ with different amount of boron from gels:xLi20-(y+l 8) Na20- y A1203-zB203-30TPABr-150 SIO2480H20 where 5 <__y_<15; 2.25<x<13.5 and Z=18; 36, respectively, in figure A and B. 100

100 -

80

80-

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100

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282 Table 1 Induction time (h) and crystallisation rates (%h) for various B-ZSM-5 pellets synthesized from gels: 13.5Li20-(y+z)Na20- y A1203-zB203-30TPABr-150 SIO2480H20 where 5
8 7

1.4 1.3

3.2 Chemical analysis by Atomic Absorption The analysis also shows that TPA/u.c. is systematically lower than A1/u.c. (Table2), hence part of alkali cations also neutralizes the (A1OSi)-and (BOSi)species. It is interesting to note that the sum of negative charges ((A1OSi)- and (BOSi)) and the sum of positive cations (Li +, Na + and TPA +) are almost equal suggesting that these cations are responsible for neutralization of framework negative charges. Table 2 Chemical composition of B-ZSM-5 self bonded pellets obtained from initial composition: 13.5Li20-(y+z) Na20- y A1203-zB203-30TPABr-150 SiO2- 480H20; where 5
1.77 2.14 1.43 2.12

6.04 5.02 8.13 6.64

1.04 0.96 2.94 2.82

charges

7.06 5.75 11.02 9.14

charges

7.08 5.98 11.07 9.46

3.3. Thermal characterization The thermal decomposition of TPA + ions occluded in B-ZSM-5 pellets was studied by TG, DTG and DSC analyses on samples obtained at different times. The DTG curves, reported in Figure 3, suggest that TPA + ions are essentially decomposed at around 420-480~ in crystalline compounds obtained at a reaction time of 2-3 days and hence it can be supposed that these ions neutralize the (A1OSi) species[8]. The low temperature peak at ca 250~ is present only for products obtained at a shorter reaction time (24 hours) and so with a lower crystallinity. DSC peaks are at higher temperatures for B-ZSM-5 pellets obtained with 18 moles of B203 (around 476-478~ than for B-ZSM-5 pellets obtained with 36 moles peaks DSC (around 465-469 ~ This could stem from different interactions between TPA + and boron or aluminium. Further experiments are needed in order to find a clear answer at this question.

283

20

220

420

20

62(

220

420

620

T [*C]

T FC]

72h

20

220

420

620

T [*Cl

Fig.3. DTG curves of B-ZSM-5 pellets obtained at different reaction times from systems: (A) 13.5Li20-(15+ 18) Na20- 15 A1203-18B203-30TPABr- 150 SiO2- 480H20. (B) 13.5Li20-(5+36) Na20- 5 A1203-36B203-30TPABr-150 SiO2- 480H20 (C) I3.5Li20-(7.5+18) Na20- 7.5 A1203-36B203-30TPABr-150 SiO2- 480H20

3.4. Hardness of pellets In order to better understand the origin of the difference in mechanical properties, and the modification of the hardness of the pellets, samples of B-ZSM-5 pellets were studied with a hardness tester, VANDER KAMP VK 200. Table 3 shows the hardness data expressed as Kp (kilo ponds) of B-ZSM-5 pellets obtained from systems with different amounts of boron and different values of SIO2/A1203 ratio. The reported data show that increasing amount of boron in the initial gel results in an increase in the hardness of the pellets. No clear out variation can be found as a function of SIO2/A1203 ratio.

284 Table 3 Hardness data expresses like Kp (Kilo ponds) of B-ZSM-5 pellets obtained from systems: 13.5Li20-(y+z) Na20y A1203-zB203-30TPABr-150 SiO2-4801-/20; where 5
1.8 1.7 2

3.5. N M R characterization The 13C-NMR spectra show clearly that the TPA + cations are incorporated intact in the MFI channels. Boron is incorporated in tetrahedral framework position (6=-3.6ppm), but a rather high amount remains in extraframework trigonal form (maximum at ca 0.0ppm). The relative amount of boron in tetrahedral position is higher for samples containing less boron in the initial gels ( B 2 0 3 = 18 moles), i.e. ca 55-60%, while this number decreases to ca 35-45% for the samples synthesized with higher initial boron content (]3203=36 moles). Aluminium is in both tetrahedral framework position (6=54 ppm) and extraframework octahedral position (6=5.5 ppm). It seems, that their is a competition between aluminium and boron for tetrahedral framework position. Indeed, when the relative framework boron is high, the relative amount of framework aluminium is lower (30-45%), while it is higher for lower framework boron content (ca 60%). The 29 Si-NMR spectra of all samples are characteristic of the ZSM-5 structure, with a maximum a t - 1 1 2 ppm with a shoulder at c a - 1 0 5 p p m stemming from Si(OA1) configuration and SiO- defect groups. In addition, all spectra contain an NMR line at -90 ppm due to Si(OH)2 defect groups [9].

3.6. SEM characteristics of the samples The SEM photographs were carried out on B-ZSM-5 pellets obtained at different reaction times. Figures 4 shows clearly that the growth of the crystals increases with increasing reaction time and the bigger crystallites form better pellets. Indeed the total compactness depends on the adhesion of the crystallites to the amorphous binder.

285

Fig. 4. SEM photographs of B-ZSM-5 self-bonded pellets obtained from system: SIO2/A1203=20 ;B203 = 18; Li20=13.5 (a) 12 h, (b)48h, (c)48h, inside pellets, (d) 120 h (e) 120 h inside pellets.

286 4. CONCLUSION Aluminium and boron containing ZSM-5 self-bonded pellets can be easily prepared from Li-containing gels. The hardness is very good for all samples. Both boron and aluminium atoms are partly incorporated in tetrahedral framework positions. ACKNOWLEDGEMENTS This work was partly supported by the Belgian Programme on Inter University Poles of Attraction initiated by the Belgian State, Prime Minister's Office for Scientific Technical and Cultural Affairs (OSTC-PAI IUAP n ~ 4/10 on Reduced Dimensionality Systems). REFERENCES 1.J.P. Gilson, US Pat. 4 537 899 (1985) 2.R.Aiello, J.B. Nagy, F. Crea and A.Nastro, Brevetto Ital.47891A89 (1989) 3. F. Crea, R. Aiello, A. Nastro and J. B.Nagy, Zeolites, 11 (1991) 521. 4. R. Aiello, A. Nastro, F. Crea, C. Colella, Zeolites 2(1982) 290. 5. R. Aiello, C. Colella, A. Nastro, R. Sersale, in D. Olson, A. Bisio (Eds.), Proc. Int. Zeolite Conf., Reno, 1983, Butherworths, Guildford, UK, 1984, p. 957. 6. R. Aiello, C. Colella, Brevetto Ital. 47735A84, 1984. 7. D.W.Breck, Zeolite Molecular Sieve, structure, Chemistry and use, Wiley, New York, 1974, p. 742. 8. J. E1-Hage A1-Asswed, N. Dewaele, J. B.Nagy, R.A.Hubert, Z. Gabelica, E.G. Derouane, F. Crea, R. Aiello, A. Nastro, Zeolites 8 (1988) 221. 9. R. Mostowicz, F. Testa, F. Crea, R. Aiello, A. Fonseca, J.B.Nagy, Zeolites 18 (1997) 308.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

287

Syntheses and characterization of AI, B-LEV type zeolite from systems containing methyt-quinuclidinium ions D. Violante a, P. De Luea a , C.V.Tuoto a, L. Catanzaro a, M. Bruno ~, J. B.Nagyb and A. Nastro a ~Dipartimento di Piam"fieazione Territoriale, Universith della Calabria, Arcavacata di Reade, 87030 Reade (Cs). bLaboratoire de RMN. Faeult~s Universitaires N.D. de la Paix, Rue de Bruxetles 61, B5000 Namur, Belgium. The aim of this research is to study systematically the synthesis of levyne zeolites from gels containing boron, aluminium and methyl-quinuclidinium as organic templating cation. In particular, the study is devoted to define the area of existence and the kinetics of crystaUisation of B-LZ and A1,B-LZ zeolites. 1. INTRODUCTION The levynite was one of the first zeolitic structures discovered and synthesizeA using organic compounds[ 1]. Its crystal structure can be described by a sequence of single six- membered rings and the diminsions of the pores are 4.8x3.6 ~ . The unit cell has a trigonal symmetry and contains 54 tetrahedral sites[2]. The chemical composition of the levyne is Ca3(AllsSi36Oltm)50H20. Recently, the isomorphous substitution of A1 by B, Ga and Fe has been reported. The syntheses were carried out using methyl-quinuelidinium (MeQI) as organic templating cation. In 1983, two patents reported the synthesis of LZ132 [3] and LZ133 [4], both obtained from the same batch composition but at different temperatures. In this study we report the syntheses of both LZ zeolites, discussing the crystallisation fields devoted to define the area of existence of each zeolite, and the crystallisation kinetic as a function of the composition of the initial batches and temperature. 2. EXPERIMENTAL PART The composition of the initial gels was: 4.5Na20-6MeQI-xB203-30SiO2-yA1203-500H20 where 0<x
288 The nature of the solid and the degree of crystallinity were determined by X-ray powder diffraction using a Philips PW 1730/10 diffractometer. The reference samples for quantitative measurement were obtained from the final crystalline products treated with ultrasounds in order to seperate the well crystallized material from the remaining amorphous gel. Thermal analyses (DSC, TG and DTG ) of dried gels and of the crystalline products were made on a Netzsch STA 409 thermal analyzer scanning from 20~ to 800~ at a rate of 10~ under a 15ml/min nitrogen flow. The size and shape of the crystalites were measured by SEM on a Stereo SCAN 360. The NMR spectra were recorded on either a Bruker MSL 400 or a Bruker CXP 200 Spectrometers. For 29Si (39.7MHz) a 6 las (0 =rt/6) pulse was used with a repetition time of 6.0 s. For 13C (50.3MHz), a 5.0 las (0=rt/2) pulse, a single contact time and recycle time of 4.0s were used. For 27A1 ( 104.3 MHz) , a 1.0 gs (0=rt/12) pulse was used with a repetition time of 0.2 s. For 11B ( 128.3 MHz), 1.0gs (0=rt/12) pulse was used with a repetition time of 0.2 s. 3. RESULTS AND DISCUSSION

Table 1 shows the crystalline phases obtained varying the amounts of B203 and A1203, the reaction temperature and the source of boron. Utilising Na2B407 as source of boron and A1203 as source of aluminium, B,A1-LZ132 is obtained without co-crystallising phases when the amount of A1203 is higher than 0.6. Table 1 Phases obtained, varying the amounts of B203 and A1203 in the initial gels, reaction temperature, reaction time and the source of boron. Gel compositions are: 4.5 Na206MeQI-xB203-30SiO2-yA1203-500H20; x y Source of Reaction Reaction Phase obtained Moles of Moles of Boron time (days) temperature B203 A1203 0.0<x
0.0 0.0 0.0

H3BO3 H3BO3 H3BO3

8 8 8

150~ 170~ 190~

Unreacted B-LZ132 B-LZ132

10 10 10

0.0 0.0 0.0

H3BO3 H3BO3 H3BO3

8 8 8

150~ 170~ 190~

Unreacted B-LZ132 B-LZ132

15 15 15

0.0 0.0 0.0

H3BO3 H3BO3 H3BO3

8 8 8

150~ 170~ 190~

Unreacted B-LZ132+B-LZ133 B-LZ132+B-LZ133

289 When the amounts of A1203 is higher than 0.6, the amounts of B203 have no influence on the crystallisation of A1,B-LZ132. From systems containing 0.2< B203_<0.6 and 0.3 _< A1203<0.7 the co-crystallisation of both phases A1,B-LZ132, A1,B-LZ133 and quartz is favoured. B-LZ132 is obtained, without co-crystallising phases when the amount of 7.5
290 lOO 8O .m

~ 9 60 U~

~" 40 o-e, 20 o

~ . . . ~

I

o

" 90 - ' "

1.0AI203-O.OB203

~

1.0AI203-0.6B203

....

0

5

1. . . . . . . . . .

10

15

Time/days

Fig. 1 Kinetics curves determined at 170 ~ of AI-LZ132 obtained from system: 4.5Na20-6MeQI--30SiO2-1.0A1203-500H20 and A1,B-LZ132 obtained from system: 4.5Na20-6MeQI-0.gB203-30SiO2-1.0A1203-500H20 100

100

~8o

Z"

~6o

,m t-

c-

==60

Z',AO L) 0"9"2O

I+, 1

O

--I--. 9 1 ~

0 0

80

5

10

20

+7

0 15

lirrWdays

A

==

0

...., . . . . . . . . . . . . 1

5

5EL;O3

i

1

10

15

~r~d~s B

Fig. 2 Kinetics curves of B, LZ132 obtained from sytems 4.5Na20-6MeQI-x B20330SIO2-500H20 where x= 7.5 10 at 170~ (A) and 190~ (B).

5. THERMAL CHARACTERIZATION Thermal analyses (TG, DTG and DSC) were performed on the basis of 20mg samples. Before thermal analysis the crystals were purified by ultrasounds. It is interesting to note that the thermal decomposition of MeQ + ions shows a low temperature peak at about 480-500~ not always well defined, and a high temperature peak at about 580-625~ 3). It can be interpreted, as was shown previously for TPA + in ZSM-5 [5] or MeQ + in levyne of high Si/A1 ratio [6] that the peak at 480~ is due to the decomposition of MeQ + neutralizing SiO defect groups, while the 580~ peak is due to the decomposition of MeQ + neutralizing (SiOA1) or (SiOB) negative charges and to the composition of aromatic products formed during the first decomposition step.

291 Table 3 Thermal analysis data of LZ132 obtained from systems: 4.5Na20-6MeQI-xB20330SiO2-yA1203-500H20 Phase x y Reaction % Total % H20 % MeQ + Significant obtained moles moles temperature weight loss peaks of of (~ (0-700~ (0-300~ (300~ DSC/~ A1203 B203 700~ A1-LZ132 0.8 0.0 170 19.85 1.96 17.43 486.2-585 A1,B-LZ 132 A1,B-LZ132 A1,B-LZ132 A1,B-LZ132 A1,B-LZ132 A1,B-LZ132

0.6 0.6 0.8 0.8 0.8 0.8

0.4 0.8 0.2 0.4 0.6 0.8

170 170 170 170 170 170

19.35 17.80 20.53 20.49 18.90 18.63

0.87 0.71 1.44 0.97 0.87 0.75

18.48 17.09 19.09 18.99 17.93 17.88

583 590 502.6-589.1 500-582.0 570 592.2

B-LZ132 B-LZ132 B-LZ132 B-LZ132

0.0 0.0 0.0 0.0

7.5 10 7.5 10

170 170 190 190

10.24 8.95 10.57 9.52

1.5 0.4 0.4 0.1

8.74 8.55 10.17 9.42

478.6-584.7 584 503.4-625.8 501.7-623.0

19,5 ~8,5 b

~

.~ 1,5~

0

9

0

or

9

'r

7,5

1 0

0

00

~Cl5

16,5

. . . . . . .

0

[ ........

1

!........

i

2

3

AI203/B2C)3

A

-

i

4

. . . .

i

5

0

1

"l . . . . . . . . .

i '

2

3

'i

. . . . . . . . . .

4

B

Figure 3 Variation of% MeQ + loss and % H20 loss of A1,B-LZ132 as a function of A1203/B203 ratio in the initial gel corresponding, respectively to figure 4A and 4B. The amounts of water and MeQ + ions incorporated in the framework of B,AL-LZ132 increase with increasing of ratio A1203/B203 present in the initial gel (Fig.3 A and 3B).

5

292 6. NMR CHARACTERIZATION

The 13C-NMRresults show clearly that methyl-quinuclidinium ions are incorporated intact in the levynite channels [7]. The l lB-NMR data are very revealing. In presence of aluminium, the total amount of boron is incorporated as framework tetrahedral atom, as it is shown by the NMR line at-3.3ppm[8]. However, when only boron source is present in the initial gels, only ca 50% of boron can be found at tetrahedral framework positions, while the other 50% are in the form of extraframework trigonal boron ( maximum at c a 1.6ppm). Aluminium is also incorporated essentially in tetrahedral framework position (8=55ppm) and only a small amount (ca 15%) remains in octahedral extraframework position. The amount of aluminium ( 2.2 A1/u.c.) does not depend on the amount of boron in the sample. The 29Si-NMR spectra is typical of levyne structure. Two main lines are at -115 and 107 ppm in both aluminium and boron containing samples. In presence of aluminium two additional lines are present at-103 and-98 ppm, while in presence of boron alone, only the-98 ppm line is present. 7. SEM CHARACTERIZATION

The size and shape of the A1-LZ132, A1,B-LZ132 and B-LZ132 crystallites of as-made samples are illustrated, respectively in Table 4 and in Figure 4. Table 4 Size of crystals obtained from different systems: 4.5Na20-6MeQI-xB203-30SiO2yA12Oa-500H20 Phase obtained Amounts Amounts Reaction Size/~tm of B203 of A1203 temperature/~ A1-LZ132 0.0 0.8 170 1.9 A1,B-LZ132 A1,B-LZ132

0.4 0.6

0.8 0.8

170 170

2.75 3.25

B-LZ132 B-LZ132 B-LZ132 B-LZ 132

7.5 7.5 10 10

0.0 0.0 0.0 0.0

170 190 170 190

28.6 31 28.8 35

It can be seen that the size of the crystal increases with increasing boron content in the initial gel. As the nucleation and the crystallization rates are decreasing with increasing boron concentration, less nuclei are formed in the beginning of the reaction with high boron contents, leading to the formation of larger crystals.

293

Figure 4 SEM micrographs, obtained at same enlargement, of levyne crystals synthesized with various amount of B203 and A1203 in the initial gels and at different reaction temperatures.

294 8. CONCLUSION Levyne crystals of various boron contents were successfully synthesized from gels containing MeQI. The low temperature decomposition DSC peak was attribuited to MeQ + ions neutralizing the SiO defect groups, while the high temperature decomposition peak was attribuited to MeQ + ions neutralizing the framework (SiOA1) or (SiOB) negative charges and to the aromatic products formed during the first decomposition. Boron is incorporated essentially in tetrahedral framework position in presence of aluminium, and only partially when only boron is present in the initial gels. REFERENCES

1. G. T. Kerr, US Patent 3 459 676 (1969) 2. W. M. Meier, D. H. Olson, Atlas of Zeolites Structure Types, Butterworth-Heinemann, 1992, p 114. 3. T.R.Cannan, M. T. Brent, E.M. Flanigen, E.P.0091048A1, 1983 4. M.T. Brent, M.T. Lok, T.R.Cannan, E.M. Flanigen, E.P. 0091049A1, 1983 5. J. E1 Hage-A1 Asswad, N.Dewaele, J.B.Nagy, R.A. Hubert, Z. Gabelica, E.G. Derouane, F. Crea, R.Aiello, A. Nastro, Zeolites 8 (1988)221. 6. Y.L. Casci, T.V. Whittam, Stud. Surf. Sci. Catal. 24 (1985)39. 7. C.V. Tuoto, J.B.Nagy, A. Nastro, Stud. Surf. Sci. Catal 105A (1996) 213. 8. F. Testa, R. Chiappetta, F. Crea, R. Aiello, A. Fonseca, J. B.Nagy, Stud. Surf Sci. Catal. 94 (1995) 349 and Colloids Surf. A. 115 (1996) 223.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

295

Synthesis and ion exchange properties of the ETS-4 and ETS-10 microporous crystalline titanosilicates C.C. Pavel a, D. Vuonob, A. Nastrob, J.B. Nagy c and N. Bilba a Department of Chemical Technology and Materials Chemistry, Faculty of Chemistry, "A1. I. Cuza" University of Iasi, Bd. D. Mangeron 71 A, 6600-Iasi, Romania a

bDepartment ofPianificazione Territoriale, University of Calabria, Arcavacata di Rende, 87030 Rende (CS), Italy r Laboratoire de R.M.N., Facult6s Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium This paper deals with the ETS-4 and ETS-10 titanosilicates synthesised hydrothermally using titanium (IV) chloride as titanium source, without any organic additives and no seeding. The starting materials were gels of molar composition: 1.49SiO2:xNa20:yTiO2:0.6KF: 1.28.xHCl:39.5H20, where: 0.5_<x_<2.5 and 0.1_y_<0.4. By varying the amount of Na20 and TiO2 respectively, the crystallization domains of the titanosilicates mentioned above were found. The optimal gel composition for crystallization was found kinetically. A high purity and crystallinity ETS-10 was obtained for 1.0 mole Na20 and 0.2 mole TiO2. The ETS-4 and ETS-10 phase mixture is due to a co-crystallization process. The physico-chemical characterisation of these synthesised materials was carried out by XRD, SEM, TG/TDG/DSC, EDS and sorption of 137Cs and 6~ radionuclides. 1. INTRODUCTION The incorporation of tetrahedrally coordinated titanium in the framework of micro and mesoporous materials of ZSM-5, ZSM-11, Beta, ZSM-48, NU-1, MCM-41 and HMS type gave TS-i and TS-2 [1-3], Ti-Beta [4], TS-48 [5], Ti-NU-1 [6], Ti-MCM-41 [7] and Ti-HMS [8] structures, respectively, with notable catalytic properties, especially in the selective oxidation of some organic compounds with aqueous H202 solution. Microporous crystalline titanosilicates with comer-sharing [TiO6]-2 octahedra and [SiO4] tetrahedra as building units constitute a novel zeotype family. Some of these structures are analogous to natural minerals (ETS-4,--~zorite [9], AM-3 and ETS-14~-~penkvilksite [10,11], AM-2, STS and UND-l*-~umbite [10,12,13] and unnamed materials, pharmacosiderite [14] and nenadkevichite [15]), others are new structures (ETS-10 [16], AM-4 [10])or unnamed structures [17,18]. Among crystallized microporous titanosilicates having Ti octahedral and Si tetrahedral, ETS-4 and ETS-10 are the most interested structures discovered by Kuznicki in 1989 [16,19]. Due to their chemical composition and particular framework structure, these materials show a remarkable catalytic activity and selectivity and properties of molecular and/or ionic sieves that make them very useful as catalysts, adsorbents or ion exchangers.

296 ETS-4 structure is similar to that of zorite [9] and that of ETS-10 was established by Anderson [20,21]. While ETS-4 has a disordered structure with intergrowths in two dimensions and pores of 3.7 A (small-pored materials), ETS-10 possesses a network of [SiO4] tetrahedra linked with [YiO6]-2 octahedra forming a three-dimensional wide-pore channel system whose minimum diameter defined by the distorted 12 members ring is 7.6x4.9 A (large-pored materials) [20,22]. During the last decade, numerous studies on the synthesis of titanosilicates were carried out. According to Kttmicki's patent [16] for the synthesis of ETS-4 and ETS-10, TIC13 is used as titanium source. Under different working conditions, using new titanium sources, for example, YiCh [23], TIC13 and YiO2 [24], Yi(SO4)2 [25], Ti2(SO4)3 [26], YiF4 and YiO2 [27], some organic additives and seeding the synthesis gel, high purity and crystallinity ETS materials were obtained. Since the crystallization domain of ETS-10 is very narrow further studies are necessary for the reaction parameters optimization in order to obtain pure phases. Due to octahedral titanium [TiO6]2- and relatively high amounts of titanium in their structure (TiO2/SiO2=0.5 in ETS-4 and -0.2 in ETS-10) these materials have high ion exchange capacities: -6.3 meq.g l and-4.5 meq.g -1 for ETS-4 and ETS-10, respectively, when dried. Their ion exchange, chemical stability, framework resistance to radiation, the framework type and the pore size (3 -7.5 A, i.e. the same as metal hydrated ions ones) these materials are suitable for the retention of toxic and radioactive metal ions from the waste waters. This paper deals with our studies on the ETS-4 and ETS-10 hydrothermal synthesis without seeds and organic additives and characterisation of the obtained products by powder XRD, SEM, TG/DTG/DSC and by sorption of 137Cs and 6~ from aqueous solutions.

2. EXPERIMENTAL SECTION 2.1. Experimental procedure The hydrothermal synthesis of ETS-4 and ETS-10 titanosilicates was carried out according to P. De Luca's procedure [28], starting from gels with following molar composition: 1.49SiO2:xNa20:yTiO2:0.6KF:l.28-xHCl:39.5H20, where: 0.5<x<2.5 and 0.1
Ka(mL/g)=[(A~-At)/Ad'V/M

(1)

297 where: Ai and At are the initial and final radioactive solution activities, respectively, V is the volume (mL) of the solution and M is the initial dry weight of the sorbent material. 2.2. Characterization The X-ray diffraction (XRD) patterns were recorded on a PHILIPS PW 1830 X-ray powder diffractometer using CuI~ radiation. The scan rate was 0.02 ~ in a 20 range of 5-45 ~ The morphology and crystal dimensions were evaluated by scanning electron microscopy (SEM) on a MICROSPEC WDX-2A microscope at 25 kV. The TG/DTG/DSC analysis was performed on a NETZSCH STA 429 instrument in order to determine the thermal stability, endo/exothermic effects and the weight loss of products within the temperature range 2 0 - 700 ~ The heating rate was 10 ~ under static air stream. The elemental chemical analysis, for the determination of Si/Ti and M+/Ti ratios was performed using a SHIMADZU AA-660 and a EDS ZAF -4/FLS instrument, respectively. The radioactivity measurements were carried out using a G e i g e r - Muller detector connected to a VALUTRONIK SCALLER 16M particle counter.

3. RESULTS AND DISCUSSION 3.1. Influence of sodium ions concentration on ETS-4 and ETS-10 phases crystallization The crystallization fields of phases obtained by variation the TiO2 and Na20 amounts in the initial gel, 1.49SiO2:xNa20:yTiO2:0.6KF:l.28"xHCl:39.5H20, where 0.5<x<2.5 and 0.1
13

0,4

12,8 80 J

0,3

/

//

12,6

-

12,4 12,2

ETS4

12 11,8 El'S-10 4

20 u

0,1 0,5

1,5

Na20, moles

2,0

2,5

Figure 1. Crystallization fields of the phases for the 1.49SiO2:xNazO:yTiO2: 0.6KF: 1.28"xHCl:39.5H20 system; 0.5<x<2.5; 0.1
- Q*

- ~-. El-S-4 pHi

0,25

0,75

1,25 1,75 NazO, moles

2,25

11,6 11,4 11,2 11 2,75

Figure 2. Crystallization degree of phases vs. Na20 content and initial pHi of the 1.49SIO2 :xNa20: 0.2TIO2: 0.6KF: 1.28.xHCl: 39.5H20 system; 0.5<x<2.5.

The data in Figure 1 show that the crystallization domain of ETS-10 phase is much more limited comparatively to that of ETS-4 and the ETS-10 crystallization is drastically influenced by the sodium and titanium amounts in the initial gel. The optimal titanium and sodium amounts needed to obtain ETS-10 with high purity and crystallinity are 0.2 mole TiO2 and 1.0 mole Na20, respectively. Variation of Na20 amount

298 determines a modification of gel pH. This is a controlling factor for the crystallization of the two titanosilicates, especially for ETS-10. The crystallinity degree (%) of obtained phases vs. Na20 content (mole) corresponding to the 1.49SiO2:xNa20:0.2TiO2:0.6KF:l.28.xHCl:39.5H20 systems, where 0.5<x<2.5 and the initial gel pHi are presented in Figure 2. The best ETS-10 sample was obtained at pHi=l 1.7. For values of pHi smaller than 11.7, respectively xl.5, ETS-4 is the main product. The Xray diffractograms of the crystalline products obtained for a constant initial titanium amount (0.2 mole TiO2) and different sodium amounts (0.75 - 1.5 mole Na20) are presented in Figure 3. The DSC curves for these samples are shown in Figure 4. 610"C

A e= ::)

x=1.5 ,~~,~~~.2sETS-IO + ETS-4 ~ \

W r O r. .m,

~

~27~

~-

2

2

"

2;0

a;0

4;0

1

x=1.25 ~

x=1.0 , ~,

ETS-IO + o~-quart2 121"C

5

;0

1% 2'0

25

3'o

2 theta

3'5 4'0

45

Figure 3. XRD diffractograms of products obtained from 1.49SiO2:xNa20:0.2TiO2: 0.6KF: 1.28.xHCl:39.5H20 system; x=0.75, x=l.0, x=1.25 and x = 1.5. Asterisks (*) indicate the second phase ETS-4, respectively Q*.

0

~;0

653"C

5;0

Ternperature, *C

6;0

z;o

Figure 4. DSC curves of products obtained from 1.49SiO2:xNa20:0.2TiO2:0.6KF: 1.28.xHCl:39.5H20 system; x = 0.75, x=l.0, x = 1.25 and x = 1.5.

The pure ETS-10 (1.0 mole Na20, 0.2 mole TiO2) shows one single endothermic peak at 121 ~ associated to the loss of physically sorbed water (13.18 % H20). ETS-4 (1.5 mole Na20, 0.2 mole TiO2) shows two endothermic peaks: the former at 165 ~ due to the loss of zeolitic water (8.3 % H20) and the latter at 233 ~ due to the loss of structural water (6.06 % H20), which is accompanied by a decrease in crystallinity with about 70 % [29]. ETS-10 titanosilicate is much more stable and its crystallinity begins to decrease above 480 ~ reaching the complete vitrification at 650 ~ The morphology of phases obtained for different initial Na20 amount (x=0.75, 1.0, 1.25 and 1.5) is shown in Figure 5. The Q* phase shows a spherulitic agglomerates morphology (Figure 5a) and ETS-10 mixed crystals are quasi-cubic in an elongated form (Figure 5b). When ETS-10 is mixed with ETS-4, the crystals of ETS-10 are bigger and have a cubic form, while the ETS-4 crystals have an elliptic form (Figure 5c). The same morphology is found also for 100 % ETS-4 (Figure 5d). From the kinetic study on crystallization of the phases for different sodium content in the initial gel, the crystallization parameters were estimated, namely the induction time corresponding to 4 % crystallinity (ti, hours) and crystallization rate (R, %/la). For pure ETS-10, the crystallization parameters are ti=12.5 h and R=9.68 %/h.

299

Figure 5. SEM image of the products obtained in the reacting system 1.49SiO2:xNa20: 0.2TiO2:0.6KF: 1.28.xHCl:39.5H20, where x = 0.75 (a), 1.0 (b), 1.25 (c) and 1.5 (d). An interesting aspect is the appearance of ETS-4 phase in the mixtures resulted during crystallization of ETS-10 (Figure 6) and an increase of pH liquid phase. Working at a pHi =12.15 to crystallise the ETS- 10 for about 30 h, the final pHf is higher than 13 and the cocrystallization of ETS-4 begins as shown in Figure 7.

Figure 6. Crystallization curves ofETS-10 and ETS-4 in the 1.49SIO2:1.25NazO:0.2TiO2: 0.6KF: 1.6HCl:39.5H20 system.

Figure 7. SEM image of ETS-4 cocristallized on ETS-10.

300 3.2. The influence of sodium and titanium ion concentration on the ETS-4 crystallization

In order to determine the crystallization parameters corresponding to ETS-4, two systems were studied, namely: 1.49SiO2:xNazO:0.3TiOz:0.6KF: 1.28.xHCl:39.5H20, x-1.25, 1.5, 2.0, 2.5 and 1.49SiO2:2.0NazO:yTiOz:0.6KF:2.56HCl:39.5H20, y-0.1, 0.2, 0.3, 0.4. The kinetic data and the crystal dimensions for ETS-4 are presented in Table 1 in comparison to those of ETS- 10. Table 1. Kinetics parameters of synthesis, crystallite sizes and Si/Ti ratio of samples crystallized in the 1.49SiO2:xNazO:yTiOz:0.6KF:l.28.xHCl:39.5H20 system. Kinetic parameters System Crystals Crystalline Si/Ti phases Nucleation Crystallization dimension, (molar ratio) (DRX) time, ti (h) rate, R (%/h) (gm) xNa20:0.3TiO2 1.25 2.2 ETS-4 9.12 3.78 16.50x6.65 1.50 3.8 5.74 14.37x6.32 2.0 ETS-4 2.00 2.47 61.82 12.76x9.84 2.0 ETS-4 2.50 4.1 38.15 21.67x18.85 1.6 ETS-4 2.0Na20:yTiO2 0.1 4.0 2.89 5.00x3.46 2.1 ETS-4 0.2 3.7 22.44 13.94x8.54 2.1 ETS-4 0.3 2.47 61.82 12.76x9.84 2.0 ETS-4 0.4 2.15 20.26 12.58x12.35 1.9 ETS-4 1.0NazO:0.2TiO2 12.5 9.68 4.34x3.17 4.9 ETS- 10 The experimental crystallization curves are shown in Figure 8. 100 t

~r

"-

801 i~jr-z---= ]

II

.~.0l I I / O / Ht/'

2Ol I/J' 0

10

/

/

~

v

-

~

/

~

"

20

30

40 50 Time, h

100]

.

80

1.25Na20 9 1.5Na20

60

Z0Na~O 70

a

80

"-.

.~40 O / TI

20-1 I I 0

9A

/ 10

9

/

20

=~

"

30

40 50 Time, h

60

9 0.1TiO2 0"2TIO2

0.3T!O~ 70

b

80

Figure 8. Crystallization curves of the phases obtained in the 1.49SiO;:xNa20:yTiOz:0.6KF: 1.28.xHCl:39.5H20 system, where (a) x=1.25, 1.5, 2.0 and 2.5 Na20; (b) y=0.1, 0.2, 0.3 and 0.4 TiO2. The nucleation times, ti and the crystal dimensions decrease with increasing sodium content till 2.0 mole Na20, while the crystallization rate, R increases. The ETS-4 phase, obtained from an initial mixture containing 2.5 mole Na20, shows an opposite behaviour, but its final crystallinity is higher (Figure 8a). The Na20 excess seems to have an inhibitor effect on the crystallization rate.

301 The increase in titanium concentration leads to decrease of the nucleation time and to increase of the crystallization rate (Figure 8b). The crystal dimensions and morphology are less affected by changing the TiO2 amount than Na20 one. As in the case of Na20 excess, there is also a "saturation" amount of TiO2 (0.4 mole) above which the crystallization rate decreases. In conclusion, the optimal amounts of TiO2 and Na20 in the starting gel for a short induction time and a high crystallization rate of ETS-4 are 2.0 mole Na20 and 0.3 mole TiO2. 3.3. Retention of radioactive =37Csand 6~ ions on ETS-4 and ETS-10 The radioactive 137Cs+ and 6~ ions sorption was performed at room temperature. The 137Cs+ concentration in the initial solution was 685 mg/L with a batch factor (V:M) of 2000 mL/g and the 6~ concentration was 300 mg/L with a batch factor of 1000 mL/g. The variation of Kd (mL/g) vs. contact time between the radioactive solutions and titanosilicates is presented in Figures 9 and 10. 1800 1600

900

ETS-10

ii

f

9

800,

f

70o'

1400

600 '

ETS4

o~ 1200

ETS-10

i

E 500

E 1000

/

~= 400

~'~ 800

300

6OO

ETS4

200

4OO

100

2OO

~'o

2'o

3'0

4'o

Time, h

5'o

6'o

io

Figure 9. Variation of Ko for ETS-4 and ETS-10 v s . contact time, when [Cs+]=685 mg/L and V:M=2000 mL/g.

0

o

1'o

2'0

3'o

4'0

Time, h

5'0

6'0

7'0

Figure 10. Variation of Kd for ETS-4 and ETS- 10 v s . contact time, when [Co2+]-300 mg/L and V:M=1000 mL/g.

The ETS-4 and ETS-10 samples sorb maximum 36.83 % 137Cs (3.6 m e q 137Cs+/g) and 18.86 % + (1.8 meq 6~ and 45.03 % 137Cs (4.5 meq 137Cs/g) and 42.97 o~ 60 Co (4.2 meq respectively, after 72 h. Although ETS-4 has a higher theoretical exchange capacity (-6.3 meq/g) than ETS-10 (-4.5 meq/g), the latter shows a higher sorption capacity for both radioactive cations. This behaviour can be attributed to the differences between their pore size (3.7 and 8A, respectively) or to stronger electrostatic field of ETS-4 that has a high electric charge density (Ti/Si=0.5) in comparison with ETS-10 (Ti/Si=0.2). 6~ 6~

4. CONCLUSION The above results on the titanosilicates have shown that the domain of ETS-10 formation is much narrow in comparison with ETS-4. The crystallization conditions for ETS-10 are much more restrictive. The pHi of the gel is an important factor in crystallization of both phases and it is mainly due to the amount of Na + ion existent in the gel. The optimal pHi value for the crystallization of ETS-10 pure phase is 11.7. At higher value of pHi, ETS-4 is the main phase obtained. Due to the increase of the pH during the crystallization process, ETS-4 crystallizes from ETS-10 yielding a mixture of phases. From the kinetic study on crystallization, the

302 optimal parameter values have been obtained: ti=2.47 h and R=61.82 %/h for ETS-4 and ti=12.5 h and R=9.68 %/h for ETS-10. In spite of the smaller ion exchange capacity of ETS-10 than of ETS-4, ETS-10 shows a higher retention capacity for both 137Csand 6~ radioactive ions. This difference in sorption capacities is attributed to differences existing between the structures of these two materials.

Acknowledgements. This research was supported by the Dipartimento di Pianificazione Territoriale, Trasporti ed Ambiente, University of Calabria, Rende, Italy.

REFERENCES 1. M. Taramasso, G. Perego and B. Notari, US Patent 4 410 501 (1983). 2. G. Perego, G. Bellusi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, Stud. Surf. Sci. Catal. 28 (1986) 129. 3. J.S. Reddy, R. Kumar and T. Ratnasamy, Appl. Catal. 58 (1990) L 1. 4. M.A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. Soc. Chem. Commun. (1992) 589. 5. K.M. Reddy, S. Kaliaguine, A. Sayari, A.V. Ramaswamy, J.S. Reddy and L. Bonneviot, Catal. Lett. 23 (1994) 175. 6. R.K. Ahedi, S.S. Shevade and A.N. Kotasthane, Zeolites 18 (1997) 361. 7. A. Corma, M.T. Navarro and J. Perez-Pariente, J. Chem. Soc. Chem. Commun. (1994) 147. 8. P.T. Tanev, M.Chibwe and T.J. Pinnavaia, Nature 368 (1994) 321. 9. A. Phillipou and M. Anderson, Zeolites 16 (1996) 9. 10. Z. Lin, J. Rocha, P. Brand~.o, A. Ferreira, A.P. Esculcas, J.D. Pedrosa de Jesus, A. Phillipou and M.W. Anderson, J. Phys. Chem. B 101 (1997) 7114. 11. S.M. Kuznicki, J.S. Curran and X.Yang, PCT W098/32695 (1998). 12. B. Mihailova, V. Valtchev, S. Mintova and L. Konstantinov, J. Mater. Sci. Lett. 16 (1997) 1303. 13. X. Liu, M. Shang and J.K. Thomas, Microp. Mater. 10 (1997) 273. 14. W.T.A. Harrison, T.E. Gier and G.D. Stucky, Zeolites 15 (1995) 408. 15. J. Rocha, P. Brand~.o, Z. Lin, A. Kharlamov and M.W. Anderson, Chem. Commun. (1996) 669. 16. S.M. Kuznicki, US Patent 4 853 202 (1989). 17. D.M. Poogary, R.A. Cahill and A. Clearfield, Chem. Mater. 6 (1994) 2364. 18. H.J. Hess, M.L. Balmer, B.C. Buker and S.D. Conradson, J. Solid State Chem. 129 (1997) 206. 19. S.M. Kuznicki and A.K. Thrush, Euro. Pat. 0405978A1, 1990. 20. M.W. Anderson, O. Terasaki, T. Obsuna, A. Phillipou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Nature 367 (1994) 347. 21. M.W. Anderson, O. Terasaki, T. Ohsuna, P.J.O. Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Philos. Mag. B 71 (1995) 401. 22. G. Cruciani, P. De Luca, A. Nastro and P. Pattison, Micropor. Mesopor. Mater. 21 (1998) 143. 23. T.K. Das, A.J. Chandwadkar and S.Sivasanker, Chem. Commun. (1996) 1105. 24. J. Rocha, A. Ferreira, Z. Lin and M.W. Anderson, Micropor. Mesopor. Mater. 23 (1998) 253. 25. W.J. Kim, M.C. Lee, J.C. Yoo and D.T. Hayhurst, Micropor. Mesopor. Mater. 41 (2000) 79. 26. N. Bilba, C.C. Pavel and A. Vasile, Anal. Stiin. Univ. "Al.I.Cuza" Iasi, Chimie VIII-2 (2000) 345. 27. X. Yang, J.L. Paillaud, H.F.W.J. van Breukelen, H. Kessler and E. Duprey, Micropor. Mesopor. Mater. 46 (2001) 1. 28. P. De Luca, S. Kuznicki and A. Nastro, Stud. Surf. Sci. Catal. 97 (1995) 443. 29. M. Naderi and M.W. Anderson, Zeolites 17 (1996) 437.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

303

Quasiisothermal degradation kinetics of tetrapropylammonium cations in silicalite-1 matrices 9

"

a*

Olga Pachtova a, Milan Kocmk , Bohumil Bernauerb, Frank Bauer r a j. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, 182 23 Prague 8, Czech Republic, e-mail: [email protected], cz

b Institute of Chemical Technology, Technick/t 5, 166 28 Praha 6, Czech Republic c Institut for Oberflachenmodifizierung e.V., Permoserstr. 15, Leipzig, Germany The present work deals with the kinetics of template removal investigated via thermogravimetric measurements both in the flow of non-oxidative and oxidative agent. Apart from linear temperature programs, quasiisothermal conditions were used which consisted in keeping the sample at the temperature of measurement Tm~x. A simple kinetic model was formulated which involves the effect of mobilisation reaction, rate of mass transfer from crystals to intercrystalline space, intercrystalline di~sion and the rate of mass transfer from the layer of crystals into the main stream of template removing agent. The model qualitatively appreciates all the principal features of template removal process including the effect of the percolation threshold. 1

INTRODUCTION

Zeolitic membranes are designed to separate chemical mixtures with targets of performance that are usually guided by economic considerations. Permeability and selectivity, which are as a rule inversely related, determine the performance of the membranes. These properties are determined by the continuity of the zeolitic phase, the thickness of the membrane, obstructions and/or leakage paths caused by grain boundaries in the zeolite layer, the orientation of the pores and the success in the calcination step. The calcination step is important for obtaining membrane free of cracks and organic residues. There is a series of studies about mechanism of tetrapropylammonium removal from small beds of MFI zeolite crystals. TG, DTG or DTA experimental arrangements are usually used [1,2]. The mechanismus of template degradation in pyrolytic atmosphere was suggested by Soulard et al [3,4]. With membranes the situation would be completely changed: (i) prevailing hydrodynamic regime would be a flow of gaseous agents in parallel to a composite membrane containing both zeolite crystals and continuous zeolite layer (ii) dimension of industrially applicable tubular membranes is supposed to be even of 2-3 meter in length. This is a contrast to tubular laboratory membranes which are usually by order of magnitude shorter [5,6]. The homogeneity of template removal in the membrane both in axial and the radial direction is another important aspect that has been considered not yet. Thus, the understanding the role of hydrodynamics in the template removal process is of interest. A simulation of template

304 removal process may give an insight into the template removal procedure. The aim of the present study has been the investigation of degradation kinetics of tetrapropylammonium cations in silicalite-1. The template degradation was investigated by thermogravimetry (TG) using two commercially TG-apparatuses. For an analysis of quasiisothermal degradation kinetics of tetrapropylammonium cations a mathematical model will be presented which is based on the assumption of the existence of percolation threshold for mobility of template degradation products. This is because the zeolitic ~amework of MFI type zeolites contains a system of intersecting 3D- channels and nitrogen atoms of template are situated in channel intersections. The tetrapropylammonium cations and tripropylamine molecules are reported to be in the zeolitic famework completely immobile [ 1,8,9]. 2

EXPERIMENTAL

2.1

Material

r .

.

.

Career gas

.

i

tting

All experiments were performed with large silicatite-90 ~ intergrowths (Si/A1 ~ 350) synthesized in our laboratory using the protocol by Kornatowski [7]. Crystals were prepared in the presence of tetrapropylammonium bromide and all experiments were done on crystals stemming from one synthesis batch with crystal size ~ 30 lam x 30 pm x 120 lam.

2;2 Experimental arrangement Two apparatuses for TG experiments with optimized flow hydrodynamics in the neighborhood of the "CA)FItID[~ sample were used: Figure 1 shows details of the Vr arrangement used by a Stanton-Redcrofi TG~750 instrument. The mass of silicalite-1 sample was about 15 mg and the linear flow velocity of air/nitrogen was i qr i 0.4 cm/s. cgf is the inlet concentration of the component j, cj,. is the concentration of the component j in the reactor volume Vr as well as at the reactor 9d = 1 8 outlet, d is the diameter of the reactor tube. The other TG-instrument was coupled with FTIR spectrometer which made possible a qualitative Figure 1. Reactor space in Stanton- analysis of released gaseous products of template degradation. The TG-FTIR equipment consists of a Redcrot~ TG-750 instrument Netzsch TG 209 thermobalance and a FTIR spectrometer Vector 22 (Bruker), connected with each other by a heated transfer line. The effluent from the thermobalance was passed through the gas cell of the spectrometer, kept at 200~ with neither splitting nor dilution. The IR spectra were recorded with a resolution of 4 crn1 in the range 4000-400 cmq. Quasiisothermal kinetics in TG arrangement was realized by heating the sample 50 ~ up to temperature Tin,= and then keeping it at this temperature. Duration of all experiments was between 6-7 hours. Table 1 summarizes conditions of the kinetic experiments. The template removal degree c~ was defined as: II

9

|

o~ = [re(O) - re(t)1~ re(O)

(1)

305 where re(O), and re(t) is the mass of the zeolite sample at time t = 0 and t = t, respectively. Table 1 C0nd!ti0ns of template decomposition Experimental Cartier gas arrangement

Linear velocity of Target temperature carrier gas [~

[cm/s]

a)

N2 0.4 cm/s 330 N: 0.4cm/s 370 N2 0.4 cm/s 380 N2 0.4 cm/s 390 N2 0.4 cm/s 430 Air 0.4 cm/s 330 Air 0.4 cm/s 370 Air 0.4 cm/s 380 Air 0.4 crn/s 390 Air 0A cm/s 430 Air 0.4 cm/s 480 Air 0.4 cm/s 530 b) N2 0.2 cm/s 330 Synthetic air 0.2 crrds 370 N2 0.7 cm/s 530 a)-stant0n-Rederoft TG-750 instrument, b) Netzsch TG 209 thermobalance 2.3. Kinetic model

For a semiquantitative description of the process the section of the tube containing a pan with the sample (of. the volume V~ in Figure 1) will be considered to be a continuous stirred tank reactor (CSTR). Reactor balanced space Vr was divided into 4 subspaees: (i) the reactor void space of the volume V~, (it does not include the pan with the zeolite layer), (ii) intererystaline space of the zeolite layer of the volume V~, (iii) space of the immobile phase (tetrapropylammonium hydroxide and/or tripropylamine) and (iv) space of the mobile phase (mobile pyrolysis products) in the zeolite crystals. The immobile phase is characterised by the crystal volume Vz and an actual concentration of the irranobite molecules aim. The mobile phase is characterised by the crystal volume Vz and the actual concentration of the mobile molecules bj in crystals (this is the concentration averaged over the volume of crystals in CSTR), of. Figure 2. At the beginning of the heating process the concentration of immobile molecules aim = am~x and its value is equal to the number of channel intersections [mol] per crystal volume unit. The mass balance of the component j in the volume Vr can be expressed by the following equation: d Cjr o Vr - - ~ : u S r c j r - u S r C j r +Ssjj~ (2) cjr is the molar concentration of the component j in the free volume of the reactor Vr, u is the linear velocity of the cartier gas, Sr is the effective cross-section area of the reactor, cjr~ and cjr

is the concentration of component j at reactor inlet and in the reactor free space (and at the same time at the reactor outlet), respectively. S, denotes the effective cross-section area of

306

v,

the intercrystalline space of the crystal layer andjj, is the flow density of the component j from the intercrystalline space into the reactor free vohmae. It is assumed that the contnl~ution of & is, to the total volumetric t_______~ Stream of carrier gas flow rate in the reactor would be negligible [t0]. / ~ the concentrations q 0 = 0 except [~ for those of oxygen and nitrogen. The mass balance of mobile component j in the intercrystalline volume V, can be C expressed by equation (3):

i 2

.../

,.---"

m m a

%%

,

'---._

1

dc.

v~ at =&Jjz-&Jj~

where cs, is the average concentration of the component j in the intercrystalline space of crystal layer, Sz is a total crystal surface area and Jjz is the flow density of the component j from crystal volume V~ into the intercrystalline space V,. The mass balance of the component j in the immobile phase of the crystals can be expressed by equation (4): dam - r m dt

Figure 2. Schematic representation of balanced spaces in the reactor and an example of the situation close the percolation threshold, 1-immobile molecules, 2-mobile molecules, 3vacancy, C-crystal layer

(3)

(4),

where rm is a mobilisation rate of immobile molecules in the crystal volume unit. The mass balance of the component j in the mobile phase in the crystals is described by equation (5):

db;

Z~ -a-7 = v, ~.,- s. jj~

(5)

The flowjj, can be expressed using the model of a linear driving force in the form: jj~=hjdcje- cj~)

(6),

where hi, is a mass transfer coefficient of component j from intercrystalline space into reactor volume /7, and its value depends mainly on diffusion coefficient of the component j in the intercrystalline space. The flow j~z represents the flow of the component j through the interface at crystal surface and it points into intercrystalline space~ It can be expressed using the linear driving force model of. equation (7):

jj~ = hj/bj - bj0

(7),

where bj, is a concentration of the component j on the crystal surface and hjz is a mass transfer coefficient of eomponentj from crystal volume Vzinto intercrystalline space.

307 Desorption of the degradation products starts at a certain concentration ap of mobilised atoms in the crystal. The corresponding fraction ap/am~ is called a percolation threshold [ 11]. For ap < aim equation (4) alone controls the kinetics of template degradation. During the time necessary to reach the concentration level ap there is no release of organic species from the crystals except for some template molecules located close to the crystal surface. For as synthesised silicalite-1 the tetrapropylamrnonium or tripropylamine block the channel intersections and therefore the mobilisation can be described by a site-percolation model [ 12]. As the first approximation we assumed that mobilised reactions are irreversible of the first order and mobilisation rate rm is thus independ of by_In the most simple ease: (8),

rm : kmaim

where km is the rate constant of the mobilisation reaetiorL After substitution from equation (8) in equation (4) and integration equation (4) with initial condition am(O) = am~ one obtains for concentration aim of the immobile molecules= (9)

aim : amaxeXp(-kmO

Using equation (7) for the flow jjz through the interface at crystal surface one obtains after substitution of equations (8), (9) and (7) in equation (5) the following expression for mass balance of the componentj in the mobile phase:

v. -dr: v. , j m OXp(-kmt)-s.

(10)

)

The most simple way to consider the effect of percolation threshold on the mobilization of template is to introduce into equation (7) a stepwise change for hjz in the form: h~ = 0 hjz = hjz*

for amax >--aim -~ ap for 0 _
(11)

The instant t* at which ap is reached can be expressed from equation (10) as: hv_

t

= (I/km). ln(am~,/ap)

(12)

Thus, the time development of the coeficient hjz can be obtained by the modification of relations (11):

le

hjz = O hjz = hjz* I

-

for O _ t "

(13)

aim

0 ap an~x Figure 3 Dependence of hjz on aim with consideration of percolation threshold ap.

Using relations (13) one can take into account the percolation threshold and

308 express the kinetic equation (I 1) as:

dbj =a~k, exp(-k,,t) for 0_
for t > t *

(14)

Assuming that there is an adsorption equilibrium between surface concentration bj, and concentration c/, of component j in the intercrystalline space one can express the surface concentration bj, in the mobile phase as: (15)

bj, = (amax- aim)Kjb cjJ(1 + Kjb cj,) 3.

RESULTS

AND

DISCUSSION

Figures 4 and 5 show quasiisothermal TG-curves measured in the experimental arrangement a) with as-synthesised silicalite-1 in non-oxidative and oxidative atmosphere It can be seen that only small mass changes occurred during 1.0the fast heating rate 50 K/min

(X

s~~uss~s~~s~

0.8. m~

0.6.

a~

0.4-

0.2.

~

~

up and tOinTm,=--330 nitrogen.~ The both curves in air

Tm~ have nearly linear shape. The _ [] m[] - o - 330~ form of the curves. In the = [] [] = = [] = ---n-- 370~ temperature region 370 ~ < = mmm [] = [] [] 9 ~176 .~176162

+ ~ 3~~176176plateauT"~x < at390~

etAstartsSaturati~be

|174 ,,, ~~" ....... ~ 4300C perceptible at the beginning of . * ~ 1 7 6 1- 7=*6....**-*~*-* * ~ * * * * template decomposition. At '~7"*-* 0-*-=-== 0.0 ~ * ,-,-,-,-,-*-,-,-**-,-*-*-**-*-*-*-* longer times the saturation plateau is followed by an .....b ' 1 6 0 ' 2 ~ ' 3 ~ ' 41~ ' , ~ ' increase of ct. The curves exhibit an inflex and approach Time lmin] the plateau at r = 1. When Fig.6 a vs time plots for template removal in N2, heating nitrogen was the carrier gas the 50 K/min up to Tm~xin the experimental arrangement (a) inflex was less perceptible. With increasing temperature the inflex was more pronounced and it was displaced to lower times. Starting from the temperature about 430 ~ the sigmoid shape of curves vanished and the dependencies 1- a vs t approached the exponential function 1- a - exp(-kmO. The development of TG-curves with increasing Tm~ can be qualitatively discussed using the above kinetic model. The limiting cases for high and low temperature region are as follows: (i)High temperature region: as temperature increases the kinetics constant k,, increases and the time t* decreases. During this period the mobile molecules can only be released from a subsurface crystal layer. With increasing temperature an equilibrium constant ~ b decreases according to: K yb = K~ expl-Atl//RTI (16) U

309 where (- AH) > 0 is the enthalpy change of pyrolysis product j associated with its adsorption. The diminution of Kj-b with increasing temperature results in+ K jb Cj, << 1. Thus, equation (15) reduces to: bjs = (am,= - a ,m) K /, cjs (17) In the next discussion we assume that sorption equilibrium is established according to equation (17). A further assumption is that one can neglect accumulation terms in equations (2) and (3). It makes possible to eliminate surface concentration bj+ from differential equation (14) for t > t*: db + SEhj~u S, t* V~ - - ~ + S:hj:" S:hj:* (a.,~ - a,n )Kjb ..(uS; +SEhj+ .)+ S g h j , . S r bj = V: am.~k,,, exp(- k,nt ) t I>

(18) Based on this model the concentration cj, of component j in the intercrystalline volume can be determined:

S=hj," (uSr + S,h.i, ) cj, = S, hj ,am= (1- exp<-k=t))K,+ (uS, + S+hj+) b/

(19)

When air is used as carrier gas it makes possible to estimate whether the concentration of the componentj exceeds the lower and higher flammable limit. (ii)

Low temperature region At low temperature expression ~b cj~ much higher then 1 and adsorption isotherm (15) becomes rectangular. At these conditions desorption may start to be the rate determining step of decomposition process. It means that the transport of component j through the intercrystalline space V. as well as ~

0.8-

~.

0.2

0.0

+

_ -A :&--A+

+m-

A

e

im ,+ Im

+; e

BI I~

.o

m

Q

9

e

Q,

.. El-

+..--='
6 .......... 16o

26o

.. a +'u.+

.

.

~

.

m + 430oc

.

4-480~

o -- 0 +

~000000000000000000 ii"

throughreactoris themuch freemoreSpace fasterVrthen~

T the desorption from the crystal. += =-=-=-'_~_ 330oc Concentration cj~ in the e .m. m 9, m +e+..~o__~_370oc intercrystalfine space would be m+ .+ . "++ _ | 380oc nearly zero and jTz would be [!t e m . *" ~-m.... 390oc d e s c r i b e d by equation (20)

e

.i

--

~ ~ .~_~-8-~-m-~-~-~ m-+-~.....

mm

0.6-

0.4-

:~

0000000000

~00

~

s30~

, bo

F i g . 7 a vs time plots for template removal in air, heating 50 K/rain uo to T,,~ in the e x l ~ r i m e n t a l arrangement (a)

jjz = kid bj

(20)

where kid is the rate constant of desorption of the corresponding template decomposition product. In this ease one can write similar equation to equation (10) using equation (20) forjj.

db/ (21) - - + kjab j = a ~ k m exp(-k,, t) for t* _< t dt It follows from equation (20) that the kinetics of template removal in the. low temperature

310 may depend only on kinetic constant k,, and rate constant of desorption kid. The constant km decides about the time of reaching t*. When some degradaton product has a rectangular shape of adsorption isotherm then even avery small amount of this species in the gas phase would slow down its desorption. The presence of oxygen in the cartier gas during template degradation has no significant effect on the position of the inflex of TG curves. This is consistent with our assumption that reaching of percolation threshold is determined only by pyrolytic reactions. 4. CONCLUSIONS A simple kinetic model was formulated to discuss the quasiisothermal template removal kinetics and the corresponding rate-determining step in the low and high temperature region. The principal feature of the model is the existence of the percolation threshold and the effect of temperature and gas composition on time t* necessary to reach the percolation threshold. With increasing temperature the mobilisation kinetic constant km increases and the time t* decreases. During the period t* the stress of zeolitic framework can be considerable due to the existence of a sharp boundary between a subsurface_ layer flee of the template and the bulk of the crystal. ACKNOWLEDGEMENT This research was supported by the Grant Agency of the Czech Republic as Grants No. 203/99/0522 and 104/01/0945 and by the Grant Agency of the Academy of Sciences of the Czech Republic as Grant No A 1040101. REFERENCES

1. E.R. Geus, H. van Bekkum, Zeolites 15, 1995, 333, 341. 2. K.R. Franklin and B.M. Lowe, Thermochimica Acta, 127, 1988, 319-327. 3. M. Soulard, S. Bilger, H. Kessler and J.L. Guth, Themaoehimica Acta, 204, 1992, 167178. 4. M. Soulard, S. Bilger, H. Kessler and J i , Guth~ Zeolites, 7,1987,463-469. 5. C. Bai et al., Journal of Membrane Science 105, 1995, 79-87 6. S. Morooka, T. Kuroda and K. Kusakabe, in: Proc. 5th Int. Conf. Inorg. Membr., Nagoya, 1988, p. 136 7. M. Soulard, S. Bilger, H. Kessler and J.L. Guth, kZeolites, 11, 1991, 784-791. 8. L.M. Parker, D.M. Bibby, J.E. Patterson, Zeolites 4, 1984,168-174. 9. J. Komatowski, Zeolites 8, 77, 1988. 10. O. Pachtovh, The development of channel system accessibility in sorption and membrane materials of siliealite-1 type prepared from precursors containing tetrapropylammonium hydroxide, Ph.D. Thesis, Institute of Chemical Technology, Prague, Czech republic, 2000 (in czech). 11. D. Stauffer and A. Aharony, Introduction to Percolation Theory, Taylor & Francis, London, 1992, p. 65. 12. G. Grimmet, Percolation, Springer, Berlin, 1999, p.24.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

311

Cationic silver clusters in zeolite rho and sodalite J. Michalik a, J. Sadlo a, M. Danilczuk a, J. Perlinska a and H. Yamada b alnstitute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland bNational Institute for Materials Science, Namiki I-1 Tsukuba, Ibaraki 305-0044, Japan

The silver agglomeration process in y-irradiated AgNaCs-rho zeolites and AgNa-sodalites has been investigated by electron paramagnetic resonance spectroscopy. In dehydrated rho zeolites the subsequent formation of Ag ~ Ag2+, Ag32+ and Ag43+ was observed, mg43+ cluster, stable at room temperature, after NH3 adsorption is coordinated by six ammonia ligands, two in a first coordination sphere and four in a second one forming the multicore complex Agn3+(NH3')2(NH3")4. In dehydrated sodalite the Ag65+ cluster is produced in the course of low temperature radiolysis as the result of electron trapping by the assemble of six Ag + cations located in the same sodalite cage. 1. INTRODUCTION Metal clusters exhibit novel electronic, optical and chemical properties owing to quantum size effect which results from dramatic reduction of the number of free electrons [ 1]. Small cationic silver clusters have been efficiently stabilized in zeolites. In y-irradiated AgNa-A zeolite depending on silver loadings and dehydration conditions the formation of Ag2+, Ag32+ and Ag6n+ has been proved by electron spin resonance (ESR) spectroscopy [2-6]. It was speculated that the clusters are trapped inside sodalite cages which is capable to accommodate up to six silver atoms. However, in fully exchanged Ag-ZK-4 zeolite, isostructural with zeolite A but with Si/A1 ratio higher than 1, silver hexamers are not stabilized at all [7]. It was concluded therefore that silver agglomeration processs in zeolites depends not only on lattice structure but also on total cation capacity which is controlled by Si/A1 ratio in the framework. Silver tetramers were efficiently produced by y-irradiation of hydrated Ag-ZK-4 zeolites [7] and by hydrogen reduction of dehydrated AgNaCs-rho zeolites [8]. It was postulated that they are also formed in silver sodalites irradiated with UV laser beam. The sodalite system containing oxalate anions as the internal reducing agents has shown that it could be reversibly marked with a laser beam for many cycles. Thus, potentially it could be used as a novel material for reversible storage of optical data. It was proposed, basing on diffuse reflectance results, that reversible changes in this material involve electron transfer between two silver tetramers trapped in adjacent sodalite cages [9]. In this paper we compare the radiation-induced silver agglomeration process in two different types of aluminosilica lattices - in zeolite rho and in sodalite. In zeolite rho the cages

312 are easily accessible through octagonal prisms, whereas in sodalites with cages interconnected by narrow hexagonal windows the transfer of cations or molecules is much more difficult.

2. E X P E R I M E N T A L

Sodium-cesium form of zeolite rho, NaCs-rho was synthesized by the modified method described by Robson [10]. Sodium sodalites with hydroxyl guest anions were synthesized under mild hydrothermal conditions at 150~ and autogenous pressure for 7 days. The concentration of NaOH in gel solution was 0.1 M to synthesize class B sodalites in which only some cages are filled with OH- anions. Silver cations were loaded into the framework cages by room temperature cation exchange with AgNO3 solution kept in dark. The silver loading was controlled by Ag + concentration in solution. After exchange was finished the samples were repeatedly washed with distilled water and dried in air. The AgNaCs-rho samples were dehydrated gradually under vacuum by increasing the temperature till 200~ and next oxidized with static oxygen at 300~ for 3h. Thereafter, 02 has been pumped out at the same temperature for 30 min. Ammonia was adsorbed under the pressure of 300 Torr for lh. The AgNa-sodalites were degassed at room temperature or dehydrated at 250~ in v a c u o . The stoichiometry compositions were calculated based on the results of inductively coupled plasma (ICP) analysis. All samples were y-irradiated in a 6~ at liquid nitrogen temperature (77 K) with a dose of 4 kGy. In addition AgNaCs-rho zeolite with high silver loading was also irradiated using excimer XeC1 laser ()~=312 nm, pulse duration-25ns) at 77 K. The ESR spectra were recorded with an X-band Bruker ESP-300E spectrometer equipped with variable temperature unit operating in the temperature range 100-350 K. 3. RESULTS AND DISCUSSION 3.1 Structure of zeolite frameworks

The rho structure consists of a body centered cubic arrangements of m-cages joined together through double eight-member ring. The m-cages which are composed of eight, six and four rings are the same 26-hedra as in zeolite A. However, in zeolite rho each m-cage is connected to six other m-cages by octahedral prisms, whereas in zeolite A the m-cages are joined by co-sharing single eight ring. The arrangement of cages in zeolite rho leads to an identical but interwined system of channels and cages accessed via eight tings of free diameter 0.39 x 0.51 rim. This makes rho catalytically attractive because all cation sites are equally accessible for adsorbate molecules. Materials with sodalite structure, while not generally consider as belonging to the family of zeolites, are nonetheless very closely related. The building block of the sodalite structure is cubooctahedron of corner-connected tetrahedra or sodalite cage, which is also a component of zeolite A, X and Y. The unit cell of sodalite consists of two cages with single cage stoichiometry Na4X(A1SiO4)3"2H20, where X represents a negative ion (e.g. hydroxyl or halogen) and has tetrahedral coordination with four sodium cations as the nearest neighbours. The sodalite structure is chemically highly adaptable in terms of substitution in both the

313

Figure 1. The framework structure of zeolite rho (a) and sodalite (b). framework and cubooctahedral interstices (ionic sites). The center of sodalite cages can be occupied by halides, chalcogenides and oxyanions which can be surrounded by alkali metal cations but also Ag § Ca 2§ Cd 2+ and other cations. Both frameworks are presented in Figure 1. 3.2. Silver agglomaration in zeolite rho The agglomeration of silver in zeolite rho has been studied for two silver loadings differing very much - Ag6Cs-rho and Ag0.3Cs-rho. Figure 2 shows the ESR spectra of both samples recorded at different temperatures after 7-irradiation at 77 K. In Ag0.3NaCs-rho with very low silver loading two types of ESR signals associated with silver species are recorded at 110 K: anisotropic doublet denoted B with A• mT, gl1=2.327 and g• representing Ag 2§ cation and two isotropic doublets due to silver atoms: l~176 : Aiso=61.5 mT, giso=2.0015 and l~176 Aiso=53.6 mT, giso=2.0015. Both silver isotopes l~ and l~ abundant in nature at comparable percentage have nuclear spins 1/2 and slightly different nuclear magnetic moments. Strong line in g = 2 region derives from radiation-induced lattice paramagnetic centers (Fig.2a). The silver signals stay unchanged till 270 K. At 290 K a weak triplet denoted D: Also-30.0 mT, giso=l.980 starts to appear. High field line of triplet shows the additional splitting due to the presence of silver isotopomers: l~ l~176 + and 1~ with slightly different magnetic properties (Fig.2b). If sample is heated at 350 K triplet D is replaced by quartet T: Also=20 mT and giso=l.980, representing silver trimer Ag32+ (Fig.2c). In zeolite Ag6NaCs-rho with silver loading 20-fold larger the changes in the ESR spectrum are similar but they occur at lower temperatures. Triplet of Ag2+ is already recorded at 110 K and quartet T of Ag32+ appears at 270 K (Fig.2d). However, at 350 K completely different s i g n a l - isotropic quintet Q 9Aiso=13.9 roT, g~so=l.973 of tetrameric cluster is observed. There are two possible paramagnetic silver tetramers - Ag4+ and Ag43+. Spin Hamiltonian parameters of both clusters were calculated by Arratia-Perez and Malli using the Dirac scattered-wave method [11]. Whereas monovalent cluster exhibits strong anisotropy

314

109A

a

0

107A

Ag 1~176

}

0,,

-/-kg// 109 112K

fEXCIMERLASER

!

275

I

350 K I

c

'

300

'

325

'

I

350

,

3;5 [roT]

I

275

'

I

300

'

I

325

~ { ~'--x3

1011

'

I

350

'

I

375 [mT]

Figure 2. EPR spectra of AgNaCs-rho zeolite irradiated at 77 K by y-rays (a - e) and UV laser beam (f). I - Ago 3NaCs-rho; EPR at 110 K (a), 290 K (b), 350 K (c), II - AgoNaCs-rho; EPR at 270 K (d), 350 K(e), 110 K (f) (A• mT, A,=I 0.86 mT) the trivalent one has almost isotropic character (A• 6.77 mT, A,=16.73 mT). Based on those calculations we assigned pentet Q to Ag43+ cluster. It shows remarkable stability and in deaerated sample was still recorded alter 1 year storage at room temperature. It seems that the lattice of zeolite rho provides very efficient structural trapping sites for silver tetramers. It was postulated earlier that Ag43+ clusters are located in octagonal prisms and bulky Cs + cations play a crucial role in cluster stabilization by blocking prism openings [8]. Silver clusters in zeolites were mainly produced by hydrogen or radiolytic reduction. Recently we proved that they can be also generated photochemically. After half an hour irradiation at 77 K of Ag6NaCs-rho zeolite with XeC1 excimer laser beam (9~=312 nm) followed by the annealing at 110 K we observed weak signal of Ag43+ cluster showing the same EPR parameters as tetrameric silver produced radiolytically. Although the ESR intensity of Ag43+ generated by laser beam is low because of shallow penetration depth potentially this method can be attractive for further applications in optics and electronics. The ESR results univocally indicate that in zeolite rho silver agglomeration process initiated by gamma reduction of Ag + cations requires the mobility of silver species - atoms at low temperatures and cations above 270 K. In zeolites with high silver loading the dimers are formed at 110 K because both Ag o atom and Ag + cation are located in the same octagonal cage and Ag o migration distance is very short. In contrast, when Ag + content is low Ag o atoms have to travel a long distance visiting several cages before they meet an Ag + cation. Long distance migration requires higher activation energy and therefore, the formation of clusters

315

a

Q

Q

Q Q

Q'

Q,

.,-A;~' "--

exl:

iii~I

o^i

.~

3

~,,~".-"" sire I

I

300

I

I

325

I

I

350

I

I

375

mT

Figure 3. The effect of NH3 adsorption on the EPR spectra 0 fAg4 3+cluster in Ag6NaCs-rho zeolite. with nuclearity higher than two, occurs only within the room temperature range when Ag + cations are mobile. 3.3. Zeolite rho with ammonia

When Ag6NaCs-rho zeolite containing radiolytically produced Ag43+ clusters (Fig.3a) is exposed to N H 3 at 220 K one can observe significant spectral changes on thermal annealing in, the temperature range 2 2 0 - 270 K. Pentet Q is gradually replaced by a new signal denoted Q which shows also five isotropic lines but with distinctly different hyperfine splitting Aiso-10.7 mT (Fig3.b). The substantial decrease of Ag hyperfine splitting can be only interpreted as a shift of spin density from silver tetramer to NH3 ligands indicating the formation of a multicore complex Ag43+(NH3)n 9The ligands are chemically bonded to silver tetramer because the ESR spectrum does not change by pumping ammonia out of the sample on a vacuum line at 60~

316

Figure 4. The structure and location of Agn3+(NH3)'2(NH0"4 complex stabilized in AgNaCs-rho zeolite. Signal Q' is also recorded when dehydrated rho zeolite is first exposed to ammonia and subsequently 3,-irradiated (Fig.3c). The outer lines of pentet Q' show then a well-resolved additional structure due to superhyperfine interaction of Ag43+ with ammonia molecules. The absorption of 15NH3 proves univocally that superhyperfine structure results from the interaction with nitrogen nuclei. Two inserts above and below the ESR spectrum in Fig.3c show the patterns on high field line of the ESR pentet recorded in the presence of 14NH3and 15NH3. The number of superhyperfine lines is clearly different for both ammonias reflecting the fact that both nitrogen isotopes have different nuclear spins - 1 and 1/2 for 14N and ~SN, respectively. The superhyperfine patterns were simulated and the best fits to experimental spectra were obtained with the following parameters, for 14NH3: Aiso'(2N) = 0.76 mT, Aiso"(4N) = 0.36 mT and for 15NH3: Aiso'(2N) =1.16 mT, Aiso"(4N) = 0.56 mT indicating that tetrameric silver interacts with two close and four more distant ammonia molecules. Based on simulation results we propose the complex structure as Ag43+(NH3')2(NH3")4 with tetramer located in octagonal prism whereas close ammonia ligands are situated in octagonal windows of two neighboring a-cages. Four more distant ammonia molecules are located in four other a-cages connected with the prism by square windows. The proposed model is shown schematically in Fig.4.

317 3.4. Silver agglomeration in sodalite. In contrast to zeolite rho paramagnetic silver clusters in sodalites are formed at much lower temperature. Fig.5a shows the ESR spectrum of dehydrated Ag3Nas-OH-sodalite T-irradiated at 77 K and annealed to 110 K. It consists of isotropic septet: Aiso = 8.0 mT, g = 1.997 of silver hexamer and low intensity doublet: Aiso=54.mT and giso=2.002 of Ag o atoms. Because silver atoms and cations are not mobile in liquid nitrogen temperature range the agglomeration mechanism in sodalites must be different than in zeolite rho. We found that in hydrated form of Ag3Nas-OH-sodalite hexameric silver is not formed. At 110 K the EPR spectrum shows mainly the anisotropic doublet: A_L=3.0 mT, g_L= 2,048, AI1=4.5 mT, gl 1=2,244 of divalent silver Ag 2+ and weak doublet with hyperfine splitting larger than 50 mT of Ag o atoms (Fig.5b).Dehydrated but unirradiated sodalite samples are ESR silent. This prompts us to postulate that during dehydration Ag + cations migrate and agglomerate in such a way that some cages are loaded with 6 Ag + leaving the others empty or containing only one Ag + cation. Such arrangements of close cations constitute the perfect trapping sites for electrons generated during low temperature radiolysis. The g value of isotropic septet (Fig.5a) supports our interpretation because g values of electron traps in various media are always lower than 2. The electron trapped by 6 Ag + cations can be also envisaged as Ag65+ cluster. The similar experiments carried out with hydroxyl sodalites containing different amount of Ag + cations did not prove the formation of silver tetramers which stabilization was postulated earlier in Ag-oxalate-sodalites irradiated by UV laser beam [9]. It might indicate that

AgNa-OH-SOD T = 110 K Ag o

dehydrated /~ /

I

I

I

I

Ag2+

_1

_i Ag~'+

}~

b

hydrated

o

I

I

300

I

I

320

I

I

340

'

I

360

I

I

380 [mT]

Figure 5. EPR spectra of Ag3Nas-OH-sodalite T-irradiated at 77 K and annealed at 110 K; sample dehydrated at 250 K (a), hydrated sample (b).

318 agglomeration processes induced photochemically in Ag-oxalate-sodalites and initiated radiolytically in Ag-OH- sodalites proceed by different reaction paths.

4. CONCLUSIONS Two different mechanisms of silver agglomeration were found in dehydrated and T-irradiated molecular sieves. For zeolites rho the EPR results unequivocally document that the agglomeration process is initiated by radiolytic formation of Ag o atoms and involves the reactions between silver atoms and Ag* cations followed by the reactions between paramagnetic silver clusters and Ag + cations. The cluster formation depends on the mobility of Ag atoms and cations and thus is controlled by temperature. In contrast the agglomeration of silver in sodalites proceeds in the course of dehydration filling some sodalite cages with six Ag § cations. In this case T-irradiation of zeolite does not play an active role but only enables the attachment of paramagnetic label - trapped electron, to the preexisting assembles of six Ag + cations making them active in ESR. Tetrameric silver clusters in rho zeolites can be attractive for catalytic processes. In spite of their remarkable stability they are accessible to molecular adsorbates as was proved by the formation of Agn3+(NH3')2(NH3")4 multicore complex in the zeolite exposed to NH3. Such complexes with different molecular ligands can be considered as the reactive complex in which electron transfer reaction from the core to the ligand initiates a catalytic process.

REFERENCES

1. 2. 3. 4. 5. 6.

W.P. Halperin, Rev. Mod. Phys., 58 (1986), 533. D. Hermerschmidt and R. Haul, Ber. Bunsen Ges Phys. Chem., 84 (1980) 902. J. Michalik and L. Kevan J., Am. Chem. Soc. 108 (1986) 4247. J.R. Morton, K.F. Preston, A. Saryari and J.S. Tse, J. Phys. Chem., 91 (1987) 2117. R.A. Schoonheydt and H. Leeman, J. Phys. Chem., 93 (1989) 2048. A.van der Pol, E.J.Reijerse, E. de Boer, T. Wasowicz, and J. Michalik, Mol. Phys., 75, (1992) 37. 7. J. Michalik, J-S. Yu and J. Sadlo, B Pol Acad Sci-Chem., 48 (2000) 293. 8. B. Xu and L. Kevan, J. Phys. Chem, 95 (1991) 1147. 9. A. Stein, G.A. Ozin, and G.D. Stucky, J. Soc. Photogr. Sci. Technol. Japan, 53 (1990) 322. 10. H.E. Robson, US Patent No. 3 904 738 (1975). 11. R. Arratia-Perez and G.Malli, J. Magn. Reson., 73, (1987) 134.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

319

The First Example of a Small-pore F r a m e w o r k Hafnium Silicate Zhi Lin and Jo~o Rocha* Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal

The first example of a mieroporous framework potassium hafnium silicate (AV-12, Aveiro microporous solid no.12) with a known structure is reported. AV-12 and the rare potassium zirconium silicate mineral umbite possess similar structures, composed of comer sharing MO6 octahedra and SiO4 tetrahedra forming a three-dimensional framework with eightmembered ring channels. The material has been characterized by powder X-ray diffraction, scanning electron microscopy, 29Si magic-angle spinning NMR spectroscopy, FT Raman and infrared spectroscopies and thermogravimetry.

1. I N T R O D U C T I O N In recent years, there has been an upsurge of interest in the synthesis and characterization of novel titanosilicates and their zirconium and tin analogues. TM These materials possess novel structures and display important physical and chemical properties. The isomorphous substitution of various metal ions into crystalline inorganic frameworks has been a very active area of research in the last two decades, allowing the fine tuning of properties of a given material. Although hafnium and zirconium ions have similar radii, it is of interest to prepare microporous hafnium silicates because the pore sizes and ion-exchange and adsorption properties (among others) of the materials may be slightly different. However, no research is presently available on the synthesis of such materials. In the course of a systematic study, we have obtained (to the best of our knowledge) the first example of a microporous hafnium silicate. AV-12 possesses the structure of the rare zirconium silicate mineral umbite. Umbite occurs in the Khibiny alkaline massif (Russia) and has ideal formula K2ZrSi309.H20. 5 Titanium and tin analogues of umbite have been prepared in the laboratory. 3'6'7 Here, we wish to report preliminary results of the synthesis and characterization of AV-12 by powder XRD, SEM, 29Si MAS NMR, TGA, FTIR and Raman. The natural occurrence of a hafnium silicate analogue of umbite is unknown. * Corresponding author. E-mail: [email protected]; Fax: 351 34 370084

320 2. E X P E R I M E N T A L

2.1. Synthesis Typical A V-12 synthesis. An alkaline solution was made by dissolving 1.50 g of precipitated silica (93 m/m%, Riedel-deHa6n) and 5.71 g KOH (85 m/m%, Aldrich) into 16.04 g n20. 2.55 g nf~14 (98 m/m%, Aldrich) was added to the alkaline solution while stirring thoroughly. This gel, with a molar composition 5.5 K20 : 3.0 SiO2 : 1.0 Hff)2 : 120 1-120, was transferred to a Teflon-lined autoclave and treated at 230 ~ for 1-4 days under autogenous pressure without agitation. The sample synthesized at 230~ for 1 day contains significant amount of amorphous materials. The crystalline product was filtered off, washed at room temperature with distilled water, and dried at 70 ~ overnight, the final product being an offwhite microcrystaUine powder.

2.2. Techniques Powder X-ray diffraction (XRD) was performed between 5-50~ in 20 on a Philips X'pert MPD diffractometer using CuKt~ radiation. The unit cell parameters were refined with programs PowderX s and Powder Cell. 9 Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis were carried out on a Hitachi S-4100 microscope. Thermogravimetry curves (TGA) were measured with a TG-50 Shimadzu analyser. The samples were heated in air with a rate of 5 ~ min1. 29Si MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer. Samples were spun at the magic-angle in double-bearing zirconia rotors. Spectra were recorded at 79.49 MHz using 40 ~ radiofrequency pulses, interpulse delays of 60 s and spinning rates of 5 kHz. Before measurement, the samples were fully hydrated. Chemical shifts are quoted in ppm from tetramethylsilane (TMS). Fourier transformed infrared (FTIR) transmission spectra were measured on a Mattson 7000 FTIR spectrometer in the range of 400-4000 cm1 using a KBr wafer, resolution 1 cm1 and 32 scans. Fourier transform Raman spectra were measured on a Bruker RFS 100/S spectrometer in the range of 50-3600 cm"1 using a Nd : YAG laser (1064 nm), resolution 4 cm 1 and 200 scans.

3. R E S U L T S A N D D I S C U S S I O N The SEM micrograph in Figure 1 shows that AV-12 consists of plate crystals ca. 5x5xl.5 l.tm. The powder XRD patterns of this material (Figure 2) and the previously reported potassium zirconium silicate umbite are very similar.2 Within experimental error, chemical analysis by EDS indicates a K/Hf ratio of 2. Because of Hf and Si peak overlap in

321

Figure 1. SEM image of AV-12.

EDS spectra, the Si/Hf ratio is difficult to estimate. On the other hand, the yield calculated according to the formula K2HfSi309 is nearly 100% based on the Hf source. The following AV-12 unit cell parameters have been calculated using the first 20 wellresolved powder X R reflections and program PowderX: a = 13.2665, b = 10.2333, c = 7.1726 A, with reasonable figures of merit (M20 = 14, F20 = 22 and M30 = 10, F30 = 19). Using these unit cell parameters, the atomic coordinates and space group (P212121) of mineral umbite, 5 the powder XRD pattern of AV-12 material has been simulated with program Powder

Experimental

.

I

10

20

I

20/~

I

I

I

I

30

Figure 2. Experimental and simulated powder XRD patterns of AV-12.

40

322 Cell. The axes a and b were exchanged in agreement with the unit cell ofumbite. The unit cell parameters have been refined between 10 and 50~ 20: a = 10.2768, b = 13.3084, c = 7.1999 A, similar to the cell parameters of mineral umbite 5 (a = 10.207, b = 13.241, c = 7.174 A). The experimental and simulated powder XRD patterns of AV-12, shown in Figure 2, are in good accord. Figure 3 shows polyhedral representations of the umbite 5 and, hence, AV-12 structure viewed along [001] and [100]. In this structure the metal M octahedra, MO6, and the T tetrahedra, SiO4, form a three-dimensional MT-condensed framework. The M oetahedron is coordinated by six T. In addition to the M-O-T bonds, these tetrahedra form also T-O-T links with each other. The resulting T radical has an identity period of three T tetrahedra and forms infinite chains, along the c axis, which are connected by M06 octahedra. Three infinite chains connect one, two and three Si04 tetrahedra, respectively, to a M06 octahedron. Channels form along the c axis with eight-membered tings comaining -O-Si-O-M-O- linkages. Potassium ions and water molecules (omitted in Figure 3 for clarity) reside in these channels.

Figure 3. Polyhedral representations of the umbite and AV-12 structure viewed along [001] and [ 100]. For clarity, the water molecules have been omitted.

323

'~imulated .

.

.

.

.

.

.

.

-82

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

-84 ~i (ppm)

.

.

.

.

.

.

.

.

.

.

.

.

-86

Figure 4. Experimental and simulated 29Si MAS NMR spectra of AV-12.

The 29Si MAS NMR spectrum of AV-12 material, shown in Figure 4, contains two overlapping peaks, which can be deconvoluted in two resonances at ca. 8 -83.8 and -84.4 in a 2 : 1 intensity ratio. Now, the crystal structure of umbite calls for the presence of three unique Si sites with on a 1 : 1 : 1 intensity ratio and, therefore, two peaks probably overlap in the resonance at ca. ~ -83.8. Ti-, Zr- and Sn-umbite materials give 29Si MAS NMR resonances between-84.6 and -87.3. 2-3 Peak overlapping was also observed for Zr and Ti-umbite: the former gives only one broad peak while the latter exhibits two resolved peaks. 2 The FTIR spectra of zirconium umbite and AV-12, shown in Figure 5, are very similar. Only small shitts and band intensity changes are observed. Above 900 cm 1, the spectrum of zirconium umbite contains bands at 1097, 1035, 964, 948, 938 and 905 cm 1. These bands are most likely attributed to the asymmetric and symmetric vibrations of the SiO4 framework polyhedm, v The specmun of AV-12 displays all these bands but they are slightly (ca. 5 cm1) shifted to high frequency. Bands under 900 cm1 are essentially unchanged. FTIR and Raman (not shown) spectra exlfibit similar band shiits. The total AV-12 mass loss, ascertained by TGA analysis, between 30 and 700 ~ is ca. 4.9 wt%. The TGA curves in Figure 6 and powder XRD patterns of AV-12 rehydrated for 24 hours after dehydration at 500 ~ show an almost reversible water loss. The small differences observed in the TGA curves may result from slow rehydmtion or silanol group on defect sites.

324

AV-12

I

I

I

| I

I

I

I

1150

i

I I

i

i

i

,

I I

i

i

900 650 Wavenumber (cm "1)

I

I

I

400

Figure 5. FTIR spectra of zirconium umbite and AV-12.

,.-. -1

o'1

O -3 t~

r -5 25

175

325

475

625

775

Temperature (~ Figure 6. TGA of (a) as-synthesized AV-12; (b) rehydrated sample after dehydration at 500 ~ for 2 hours.

325 Dehydration begins immediately on heating and is completed by c a . 500 ~ between 250 and 400 ~ as in synthetic zirconium umbite.

Most water is lost

4. C O N C L U S I O N S

The synthesis and characterization of the first example of a microporous potassium hafnium silicate, possessing the structure of the rare zirconium silicate mineral umbite, has been reported. The synthesis conditions are very similar to those reported for zirconium and titanium silicate umbite. 2"6 This suggests the possibility of preparing solid solutions among Zr, Ti and Hf, perhaps allowing the fine tuning of ion-exchange and adsorption properties. This work and the Rietveld refinement of the AM-12 structure are in progress in our laboratory.

5. A C K N O W L E D G M E N T S This work was supported by FCT, PRAXIS XXI, SAPIENS and FEDER.

6. REFERENCES 1 J. Rocha and M. W. Anderson, Eur. J. Inorg. Chem., (2000) 801. 2 Z. Lin, J. Rocha, P. Ferreira, A. Thursfield, J. R. Agger and M. W. Anderson, J. Phys. Chem., B 103 (1999) 957. 3 Z. Lin, J. Rocha and A. Valente, Chem. Commun., (1999) 2489. 4 Z. Lin, J. Rocha, Jfilio D. Pedrosa de Jesus and A. Ferreira, J. Mater. Chem., 10 (2000) 1353. 5 G. D. Ilyushin, Inorg. Mater., 29 (1993) 853. 6 Z. Lin, J. Rocha, P. Brand,o, A. Ferreira, Ana. P. Esculcas, Jfilio D. Pedrosa de Jesus, A. Philippou, M. W. Anderson, J. Phys. Chem. B 101 (1997) 7114. 7 A. I. Bortun, L. N. Bortun, D. M. Poojary, O. Xiang and A. Clearfield, Chem. Mater., 12 (2000) 294. 8 C. Dong, J. Appl. Cryst., 32 (1999) 838. 9 W. Kraus and G. Nolze, Federal Institute for Materials Research and Testing, Rudower Chaussee 5, 12489, Berlin, Germany. 10 S. R. Jale, A. Ojo and F. R. Fitch, Chem. Commun., (1999) 411.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

327

Synthesis, characterization and catalytic activity of v a n a d i u m - c o n t a i n i n g E T S - 1 0 Paula Brand~o a, Anabela A. Valente a, Jo~o Rocha a* and Michael W. Anderson b aDepartment of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal; *[email protected] bDepartment of Chemistry, UMIST, PO Box 88, Manchester M60 1QD, UK A series of microporous vanadium-containing ETS-10 (ETVS-10) with V/Ti molar ratios of 0.32, 0.87 and 0.95, have been prepared by direct hydrothermal method. Powder XRD patterns of the as-synthesised materials are almost identical. DR UV-vis data have shown that these materials contain essentially hexacoordinated V(IV). All resonances in the 29Si MAS NMR spectra of ETVS-10 samples are considerably broader than those of ETS-10, which may result from isomorphous substitution of V for Ti in the ETS-10 framework. ETVS-10 is active for phenol oxidation with H202 yielding hydroquinone, catechol, para-benzoquinone and tars. Phenol conversion increases with solvent polarity. Acetonitrile is the most effective solvent for the production of dihydroxybenzenes (ca. 22% yield) and catalytic results were successfully reproduced on repeated use of the catalyst. Leaching tests and ICP-AES analyses suggest that the reaction is catalysed by small amounts of soluble vanadium leached from the surface. 1. INTRODUCTION There has been an increasing interest in isomorphous substitution of transition metals ions, particularly titanium and vanadium, in molecular sieves lattices. These materials are catalytically active in several shape selective oxidation reactions useful in the synthesis of fine chemicals [ 1]. Because of this a large number of contributions have already been reported on the synthesis of vanadium-containing microporous-mesoporous materials [2,3]. ETS-10 (Engelhard titanosilicate structure 10) is one of a small group of zeotype materials, which contain a two-dimensional 12-ring pore system of octahedral titanium (IV) and tetrahedral silicon. In an attempt to enhance the catalytic performance of ETS-10, silicon has been isomorphously substituted by A1, Ga or B, and titanium by Nb [4]. A structural analogue of ETS-10, called AM-6, where titanium was fully replaced by vanadium, has also been reported [5]. Recently, it has been reported that the incorporation of chromium or iron in ETS-10 confers it redox properties useful for liquid-phase oxidation using H202 that ETS-10 alone does not possess [6, 7]. A major drawback of these materials is metal leaching during oxidative transformations. In our line of research for efficient ETS-10 related catalysts we have prepared vanadium-containing ETS-10 (ETVS-10) and investigated its catalytic performance in liquid phase oxidation of phenol by dilute hydrogen peroxide.

328

2. EXPERIMENTAL 2.1. Syntheses of ETVS-10 materials Three ETV(x)S-10 samples were prepared with x = V/Ti molar ratios of 0.32; 0.87 and 0.95, as ascertained by Energy Dispersive Absorption of X-rays (EDAX). Typically, two solutions, A and B, were prepared according to Table 1, then mixed and seeded with 0.10 g of ETS-10, and stirred thoroughly to give a gel (composition shown in Table 1), which was heated in an autoclave under autogeneous pressure for 1 day at 230 ~ The crystalline product was cooled down to room temperature, filtered, washed with distilled water and dried at 120 ~ An impregnated sample with V/Ti-0.32 was prepared by adding 1.0 g ETS-10 to an aqueous solution of VOSO4.5H20 (15.0 g H20 plus 0.15 g VSO4-5H20) and adjusting the pH to 10.0 with NaOH. This solution was stirred for 1 day at room temperature, followed by water evaporation in an oven at 120 ~ The ETS-10 sample was prepared according to the method described in ref. [8]. All vanadium-containing samples showed green colour. 2.2. Characterisation 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded at 79.49 MHz (9.4T) on Bruker Avance 400 spectrometer with 40 ~ pulses, spinning rates of 5.0 kHz and 35 s recycle delays. Powder X-ray diffraction (XRD) data were collected on a Rigaku diffractometer using CuK~ radiation filtered by Ni. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4100 microscope. Diffuse- reflectance UVvisible (DR-UV) spectra were recorded on Shimadzu 3101PC spectrometer using BaSO4 as Table 1 Composition of the solutions used in the syntheses of ETV(x)S-10 samples. x = V/Ti (mol)

Solution A (g): Sodium silicate solution a

H20

NaOH b NaC1 c KC1 d TIC13 e Solution B (g): VOSO4.5H20 f

H20

0.32

0.87

0.95

9.52 7.52 1.14 1.65 1.00 4.12

9.52 7.61 1.10 1.65 0.98 4.12

9.50 7.80 1.16 1.68 1.01 4.12

0.55 7.93

0.82 7.53

1.11 7.52

Gel composition 7.1" 1.7: 10.8" 1.0: 7.0: 1.7: 10.8" 1.0: Na20: K20: SiO2: TiO2: 0.3" 214.6 0.4:210.3 V205:H20 V/Ti in gel (molar ratio) 0.56 0.80 a Na20 8% m/m, SiO2 27% m/m, Merck; b Merck; c Aldrich; d Panreac; TIC13 in 10% m/m HC1, Merck; f Merck.

7.9: 1.7: 10.8: 1.0: 0.6:212.8

e

1.20 15% rn/m solution of

329 the reference material. Raman spectra were measured with a Renishaw imaging microscope 2000, using a 25 mW Spectra Physics 127 HeNe excitation laser (632.8 nm), a resolution of 1 cm-1, and an air cooled detector. Nitrogen adsorption at 77 K was measured using a gravimetric adsorption apparatus, equipped with a CI electronic MK2-M5 microbalance and an Edwards Barocel pressure sensor.

2.3. Catalysis The liquid phase oxidation of phenol was carried out in the presence of ETVS-10 at atmospheric pressure, using H202 (30 % in water) as oxidant. In a typical experiment 0.1 g of catalyst, 10 ml of solvent and a molar ratio phenol/H202 of 0.5 were used. The solvent, reactants and catalyst were added to a 25 ml round bottom flask with a magnetic stirrer, connected to a condenser and immersed into a thermostatic oil bath. Finally, the reaction mixture was cooled and the catalyst was separated. The reaction samples were analysed by high performance gas chromatography (Gilson HPLC) using a BarSpec Chrom-A-Scope detector, operating at 280 nm, and Spherisorb HPLC column (S 10 ODS2). The products were quantified using calibration curves determined with standard samples.

3. RESULTS AND DISCUSSION

3.1. Powder XRD Powder XRD patterns of ETS-10 and ETVS-10 are almost identical, the only difference being a slight shift of the ETS-10 reflection at 20 24.63 ~ to lower d-values with increasing vanadium content (20 24.70 ~ for V/Ti = 0.87), Fig. 1. All ETVS-10 samples contain a single phase and no other phases particularly silicon or vanadium oxides were detected.

3.2. SEM and EDAX Typical micrographs of ETVS-10 and (for comparison) ETS-10 are shown in Fig. 2. SEM images show that crystal morphology and size do not change upon vanadium substitution. EDAX analyses of different crystals in each ETVS-10 sample reveal similar composition.

3.3. Nitrogen adsorption Nitrogen adsorption at 77 K on ETVS-10 samples gives type I isotherms, characteristic of microporous materials. The values of specific surface area (ca. 400 m2/g) and total pore volume (ca. 0.14 cm3/g) of ETVS-10 materials are roughly the same, independently of vanadium loading, suggesting that vanadium is mainly in the framework and not in the pores (Table 2).

,,%.

ETVS-10

~

ETS-10 i

3

8

13

18

23

28

33

38

20/~ Figure 1. Powder XRD patterns of ETS-10 and ETV(0.87)S- 10.

330

Figure 2. SEM images of (a) ETS-10 and (b) ETV(0.87)S-10 crystals.

3.4 DRUV-vis spectroscopy Table 2 DR UV-vis spectroscopy Textural properties of ETS- 10 and ETVS- 10. has been used extensively to gain information on the Sample Langmuir surface Total Pore Volume (m2/g) (cm3/g) different vanadium species present in molecular sieves ETV(0.32)S- 10 413 0.15 ETV(0.87)S-10 430 0.15 [9-16]. AM-6, a homologue ETV(0.95)S- 10 390 0.14 of ETS-10, where all ETS-10 411 0.15 titanium is substituted by vanadium, contains hexacoordinated V(IV) and hence is a suitable DR UV-vis reference compound. The DR UV-vis spectrum of AM-6 shows broad bands centered at c a . 600, 420 - 430 and 270 nm (Fig. 3). The absorption bands at 600 and 420 - 430 nm are assigned to the d-d transitions of V(IV) responsible for the green colour. The charge transfer associated with O to V(IV) electron transfer occurs at c a . AM-6 300 nm. AM-6 and ETVS-10 show similar DR UV-vis absorption bands. These results suggest that ETVS-10 materials contain V(IV). o~,,~ r~

3.5. Raman spectroscopy The Raman spectrum of ETS-10 contains a strong and sharp band at c a . 730cm -1 associated with the TiO6 octahedra (Fig. 4). ETVS-10 samples display similar spectra, but the line at 730cm 1 broadens and shifts to higher frequencies as the vanadium content increases. For the impregnated ETS-10 the Raman spectrum shows no broadening.

I

240

I

340

m

I

440

m

I

540

i

I

640

m

I

I

740

Wavelength (nm) Figure 3. DR UV-vis spectra of as-synthesised ETVS- 10 and AM-6.

331

3.6. 29Si MAS NMR ETS-10 possesses two types of silicon chemical environments, Si(3Si, 1Ti) and Si(4Si, 0Ti), which give rise to the two groups of resonances at-94 to -97 and c a . - 103 ppm, respectively (Fig. 5). As the amount of vanadium in ETVS-10 increases, all 29Si MAS NMR resonances broaden considerably. Since SEM images and powder X-ray patterns did not reveal the presence of amorphous materials or crystalline impurity phases in the samples, the broadening of resonances may relate to isomorphous substitution of V for Ti in the ETS-10 framework. Furthermore, the impregnated ETS-10 samples with V(IV) in octahedral coordination do not show any broadening of 29Si MAS NMR peaks. These spectroscopic results suggest that vanadium is in the framework of the as-prepared ETVS-10 materials.

r/l

N==I

~ . ETS-IO I

I

200

a

400

I

600

i

I

800

i

I

'

1000

Raman (cm-1) Figure 4. Raman spectra of ETS-10, assynthesised ETVS- 10 and vanadium-impregnated ETS-10.

(a)

V/Ti=O.95

I

(b)

/ as-

V/Ti=O87

ETS-~O __~_.j

i

.... -80 ' -i35 :90 -~ir, ' -ldd iris -l'id i iS ....

(ppm)

impregnatedETS-IO i

. . . .

,

. . . . . . . . .

-80

-85

,

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

-90

-95 -100-105 -110-115

(ppm)

Figure 5.29Si MAS NMR spectra of (a) ETS-10 and as-synthesised ETVS-10 materials, and (b) as-synthesised ETV(0.32)S- 10 and impregnated ETS- 10 (V/Ti=0.32).

332

3.7. Catalysis The oxidation of phenol using H 2 0 2 (30 wt% aqueous) at 80 ~ in the presence of ETVS10 yields a mixture of dihydroxybenzenes (hydroquinone and catechol), para-benzoquinone (BQ) and unidentified brown tar compounds (Eq. 1); the parent material ETS-10 is inactive. Using acetonitrile as solvent, 35% phenol conversion is achieved after 6 h and selectivity towards catechol (CAT) and hydroquinone (HQ) is 35% and 29%, respectively (Table 3). Phenol conversion drops to less than 3% at 45 ~ No reaction occurred when using molecular oxygen or more bulky tert-butyl hydroperoxide (70 wt% aqueous) as oxygen donor.

OH

OH

Q)

OH

O

H

ETVS- 10 / H202

+

+ H20 + Tars

solvent / reflux

phenol

OH hydroquinone

catechol

(1)

O benzoquinone

Solvent effects are investigated by employing water, ethanol or dichloroethane instead of acetonitrile. The reactivity trend of ETVS-10 increases in the following order (TOF, mmol.hX.gcat-1), Table 3: dichloroethane (0.6) < ethanol (0.9) < acetonitrile (5.4) < water (6.7). The same trend is verified for the dielectric constant of the solvents (e, at 298 K): dichloroethane (10.42) < ethanol (25.30) < acetonitrile (36.64) < water (80.10). Hence, phenol conversion increases with solvent polarity, which is surprising since ETS-10 type materials can be assimilated to a (hydrophilic) solid solvent where polarity effects govern selective adsorption of more polar compounds, in which case the adsorption of water (as solvent) would be an inhibiting process. However, a similar solvent effect has been reported for vanadium silicalite VS-2, used as catalyst for phenol oxidation with H202, where maximum conversion was attained with water as solvent and a radical mechanism was proposed [ 17]. Acetonitrile, a weakly basic solvent, is found to be the most effective for the production of dihydroxybenzenes (ca. 64% selectivity at 35% conversion). It was also the best solvent for a V-Zr-O catalyst (active sites are vanadium species) used under similar reaction conditions [18]. With water as solvent oxidations are "deeper", leading to maximum tar formation. It is known that solvents can influence catalytic performance through the ability to: dissolve tarry deposits (which are negligibly soluble in water) on the catalyst [19]; change of redox potentials of vanadium active species [20]; change stability of intermediates formed in the reaction sequence [17]; interact with H202 to generate reactive species, orientating the reaction mechanism in different directions [21]. Further studies on solvent effect are required to clarify this complex phenomenon. Catalyst stability was checked by using the solid in a second reaction cycle after thorough washing with acetone and acetonitrile and heating to 300~ for 24 hours under air. Phenol

333 Table 3 Liquid phase oxidation of phenol in presence of ETV(0.87)S- 10. Product selectivity c Conversion a TOF b (mol %) (mol %) (mmol.h-l.gcat 1) CAT HQ BQ Acetonitrile 35 5.4 35 29 Acetonitrile (2 nd run) d 31 4.8 34 25 Ethanol 6 0.9 1 1 1 Dichloroethane 4 0.6 9 6 11 Water 43 6.7 3 6 3 a moles of phenol converted/moles of phenol in feed x 100; b estimated for 6 hours; c molar ratio of formed product to converted phenol; d activated catalyst used in a second cycle. Solvent

conversion and selectivity towards hydroquinone and catechol remain roughly the same (Table 3). The XRD patterns of the fresh and used ETVS-10 samples are identical, indicating that the catalysts retain their structure, but crystallinity decreases 23% (considering 100% crystallinity for the fresh sample). Vanadium-substituted molecular sieves have been shown as unstable towards leaching by H202 [22]. Metal leaching from ETVS-10 was checked through quantification of vanadium in the sample before and after the reaction. The solid was separated from the reaction mixture by filtration at reaction temperature and analysed by ICP-AES. After 1 h of reaction the vanadium present in the catalyst is ca. 11% of the original value. The filtrate was allowed to react for another 5 hours. The conversion of phenol thus obtained is similar to that obtained in the presence of the catalyst suggesting that the reaction is, to a certain extent, homogeneously catalysed. ICP-AES analysis also confirmed the presence of vanadium in the filtrate. The reaction may take place through heterolytic and/or homolytic mechanisms since vanadium has strong Lewis character and is a relatively strong "one-electron" oxidant [22]. Under typical reaction conditions (acetonitrile, 80~ the catalytic performance of ETVS10 is superior to that of chromium-containing ETS-10 in that yields towards the desirable dihydroxybenzenes are higher (22% and 1%, with ETVS-10 and ETCrS-10, respectively) and vanadium leaching at 80~ is 5 times less than that observed for chromium at 40~ [7].

4. CONCLUSIONS Vanadium-containing ETS-10 (ETVS-10) with different V/Ti molar ratios have been prepared and characterised. DR UV-vis data reveal that as-prepared ETVS-10 materials contain, essentially, V(IV) in octahedral coordination, as is the case of titanium in the ETS-10 framework. Both the Raman peak at 730 cm -1 and all the 29Si MAS NMR resonances of ETVS-10 materials are broadened upon vanadium substitution, an effect that was not observed for vanadium-impregnated ETS-10. Textural parameters are independent of the vanadium loading. Taken together, these data indicate the ETS-10 framework-substitution of vanadium for titanium. ETVS-10 oxidises phenol by H202 to hydroquinone, catechol, para-benzoquinone and tars. Activity increases with solvent polarity. Maximum yield of dihydroxybenzenes (ca. 22%) is

334 attained using acetonitrile as solvent while water leads to significant tar formation. The activity and selectivity remain roughly the same upon repeated use of the catalyst. Leaching tests and ICP-AES analyses suggest that the reaction is homogeneously catalysed.

REFERENCES

1. ,

.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

J . I . W . C . E . Arends, R. A. Sheldon, M. W. Wallau and V. Schuchardt, Angew. Chem. Int. Ed. Engl., 36 (1997) 1144. T. Sen, V. Ramaswamy, S. Ganapathy, P. R. Rajamohanan and S. Sivasanker, J. Phys. Chem., 100 (1996) 3809 and references therein. Z. Luan, J. Y. Bae and L. Kevan, Chem. Mater., 12 (2000) 3202 and references therein. J. Rocha and M. W. Anderson, Eur. J. Inorg. Chem., (2000) 801. J. Rocha, P.Brand~o, Z. Lin, M. W. Anderson, V. Alfredsson and O. Terasaki, Angew. Chem. Int. Ed. Engl., 36 (1997) 100. A. Valente, P. Brand,o, Z. Lin, F. Gon~alves, I. Portugal, M. Anderson and J. Rocha, Stud. Surf. Sci. Catal., 135 (2001) 27-P-15. A. Valente, P. Brand,o, I. Portugal, M. Anderson and J. Rocha, 4 th World Congress on Oxidation Catalysis, H. Geiling (ed.), DECHEMA e. V., Frankfurt am Main, 2001, 1-325. M . W . Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S. P. Mackay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347. D . C . M . Dutoit, M. Schneider, P. Fabrizioli and A. Baiker, Chem. Mater., 8 (1996) 734. Z. Luan, J. Xu, H. He, J. Klinowski and L. Kevan, J. Phys. Chem., 100 (1996) 19595. Z. Luan, D. Zhao and L. Kevan, Microp. Mesop. Mater., 20 (1998) 93. G. Catana, R. R. Rao, B. M. Weckhuysen, P. Van Der Voort, E. Vansant and R. A. Schoonheydt, J. Phys. Chem. B, 102 (1998) 8005. D. Wei, H. Wang, X. Feng, W. Chuch, P. Ravikovitch, M. Lyubovsky, C. Li, T. Takeguchi and G. L. Haller, J. Phys. Chem. B, 103 (1999) 2113. S. Dzwigaj, P. Massiani, A. Davidson and M. Che, J. Molec. Catal. A: Chemical, 155 (2000) 169. M. H. Zahedi-Niaki, S. M. Zaidi and S. Kaliaguine, Appl. Catal. A: General, 196 (2000) 9. L. X. Dai, K. Tabata, E. Suzuki and T. Tatsumi, Chem. Mater., 13 (2001) 208. A. V. Ramaswamy and S. Sivasanker, Catal. Lett., 22 (1993) 239. R. Yu, F.-S. Xiao, D. Wang, J. Sun, Y. Liu, G. Pang, S. Feng, S. Qiu, R. Xu and C. Fang, Catal. Today, 51 (1999) 39. A. Tuel and Y. B. Taarit, Appl. Catal. A, 102 (1993) 69. A. V. Ramaswamy, S. Sivasanker and P. Ratnasamy, Microp. Mater., 2 (1994) 451. J. O. Edwards and R. Curci, in: Catalytic Oxidations with Hydrogen Peroxide as Oxidant, G. Strukul (ed.), Kluwer Academic Press, Dordrecht, 1992. R. A. Sheldon, I. W. C. E. Arends and A. Dijksman, Catal. Today, 57 (2000) 157.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

335

I n f r a r e d e v i d e n c e f o r t h e r e v e r s i b l e p r o t o n a t i o n o f a c e t o n i t r i l e at h i g h t e m p e r a t u r e in m o r d e n i t e J. Czyzniewska ~'2, S. Chenevarin ~ and F. Thibault-Starzyk ~ Laboratoire Catalyse et Spectrochimie, UMR 6506 - ISMRA - CNRS, 6 Bd du Mardchal Juin, 14050 Caen Cedex, France e-mail: fts @ismra.fr 2 Central Laboratory of the Batteries and Cells, ul. Forteczna 12, 61-362 Poznan, Poland

The influence of temperature on the infrared spectrum of activated mordenites was studied, together with the influence on the state of adsorbed acetonitrile. The relative intensities of v(OH) vibration bands on the pure zeolite was shown to depend on the temperature. The reversible protonation of monomeric acetonitrile in mordenite at high temperature was evidenced, leading to a band at approximately the same vibrational frequency (2325 c m -1) as in the case of acetonitrile on Lewis sites (at room temperature).

1. I N T R O D U C T I O N Infrared spectroscopy is a major tool for in situ studies of zeolites, and for the characterisation of their surface properties. Zeolites acidity is characterised using probe molecules, among which acetonitrile is gaining increasing importance. Its basicity is between that of carbon monoxide and pyridine, which leads to a specific use: the probe not being protonated at room temperature, it allows a thorough study of the H-bond between the probe and the acid sites [ 1]. Acidity scales measured at room or low temperature can usually not be linked with catalytic activity on zeolites with different structures. We had however shown some time ago that acetonitrile could be used to build an acidity scale that could be correlated with catalytic activity in strongly demanding reaction such as saturated hydrocarbon conversion, with such different zeolites as MOR, FAU and MFI [2]. The idea was to use acetonitrile protonation as a test reaction, and to follow it by infrared spectroscopy in the micropores of the zeolite. The knowledge of the adsorption state of acetonitrile on the surface is therefore a crucial point, and we thought that protonation of acetonitrile on zeolites could only happen at high temperature. The protonation of acetonitrile at room temperature on mordenites was later proposed [3], but it was further shown that what was observed was a particularly strong Hbond taking place inside the side pockets of mordenite structure [4]. The initial study was done in diffuse reflection, which is not a good technique if the temperature is not kept

336 constant. We reinvestigate here the protonation of acetonitrile on zeolites at high temperature in transmission infrared, which is a more delicate but efficient technique, much more reliable for quantitative data at changing temperature. We show that the temperature has already a striking effect on the OH groups of mordenite itself, without any adsorbed probe molecule. With adsorbed acetonitrile, we find several phenomena taking place depending on the concentration of acetonitrile on the surface and on the temperature. The protonation of acetonitrile in mordenite upon heating was confirmed. 2. EXPERIMENTAL PART CD3CN was 99% pure from Aldrich. The H-MOR samples were prepared by ion exchange of Na-MOR (Si/A1 = 10.7, provided by IFP, Rueil-Malmaison, France) with 0.1 M NH4C1 solution. Two samples were obtained: containing 99% and 61% of protons denoted in the text as HMOR and NaHMOR, respectively. The sample HMOR contains protons in the main and in the side channels, whereas the sample NaHMOR contains protons only in the main channels, as shown previously [4]. The high temperature cell was used which allowed to register the spectra at the real reaction temperature. Infrared spectra were recorded with a Bruker IFS 66/S Fourier transform spectrometer, at a resolution of 4 cm -1. Self-supporting wafers of the solid (2 cm 2, 10-15 mg) were activated by heating in vacuum up to 110 C (with a heating rate of 0.5~ keeping at 110 C for 2 hours and heating again under vacuum up to 450 C (with a heating rate of 1~ and keeping for 3 hours at this temperature. Afterwards the sample was cooled to room temperature and the reference spectrum was registered. The results of pyridine adsorption showed that there are no Lewis acid sites on so activated samples. For studies of the v(OH) region after activation and cooling of the sample to room temperature the temperature (under vacuum) was increased to 400-450 C, and cooled again to room temperature using 25 C steps, at which spectra were recorded. For acetonitrile studies 1.4 Torr of d3-acetonitrile at equilibrium was introduced into the infrared cell at room temperature, pumped under vacuum, and the sample was heated up to 250-300 C (25 C steps, IR recording at each step), with or without evacuation. 3. RESULTS 3.1. Influence of the temperature on the spectrum of the pure zeolite After activation at 450 C and cooling the sample of HMOR to room temperature, the spectrum shows the v(OH) vibration band at 3607 cm -1 (HF-OH) with a shoulder at -3590 cm -1 (LF-OH), assigned to the v(OH) vibrations in the main channels and side pockets, respectively (Fig. 1). Increasing the temperature up to 450 C (25 C steps) the bands due to hydroxyl groups shift to lower wavenumbers, reaching at 450C the values 3588 and 3570 c m -1 for HF-OH and LF-OH, resp. The bands are broadened, and the maximum intensity decreases considerably. The HF-OH/LF-OH intensity ratio is also affected by temperature, changing from --2:1 at room temperature to 1.2:1 at 450 C. The total integrated absorption however remains nearly

337 constant. This effect is reversible when the temperature is decreased from 450 C to room temperature. Similar changes are observed over NaHMOR sample, only containing hydroxyl groups in the main channels, and presenting only the HF-OH band. The HF-OH band is shifted to lower wavenumbers and broadened at increasing temperature. This band stays symmetrical up to 400 C, and no LF-OH band can be detected at high temperature.

3607

25~

I 0"2ua

588

E~

0.2ua

~

3609

ff///t//~

EI

HMOR

3700

3dso

3600

3~;0

Wavenumber,

35'oo 3&o

3700

36'50 3600

c m -~

35;0

35o0

Wavenumber,

34;c

c m -~

Figure 1. FTIR spectra of HMOR and NaHMOR samples after activation, and cooling to room temperature, collected at increasing temperatures from 25 to 450 C (over HMOR sample) or to 400 C (over NaHMOR sample) (each spectrum recorded at 25 C step). 3.2. CD3CN

adsorption

0.2ua

0.5 u a

2650

2323 2, t14. 2286

1387

0.5 u a

J

o

i

I

75oc /

,,

RT,~@~L_--~.... "'i

i

i

|

|

3700 3650 3600 3550 3500 W a v e n u m b e r , cm -1

i

2700

2600

2500

2400

2300

2200

W a v e n u m b e r , cm -~

2100

1

1400

i

i

i'

1360

W a v e n u m b e r , cm -1

Figure 2. Adsorption of CD3CN, followed by evacuation at room temperature, and heating to 240 C over HMOR sample. The dotted line shows the spectrum after activation.

338 Introduction of 1.4 Torr CD3CN at equilibrium into the infrared cell leads to the consumption of all the BrCnsted acid sites and the appearance of the ABC bands system typical for strong H-bonds. The intensity of the ABC bands is lower over NaHMOR sample than over HMOR sample. After CD3CN adsorption over HMOR sample (Fig. 2) the v(CN) region shows a band at 2314 cm -1 (strongly H-bonded acetonitrile in the side pockets [4]), and two overlapping bands at 2295 and 2285 cm -~ (CD3CN on BrCnsted acid sites and on SiOH groups [5]). For NaHMOR the band at 2312 cm -~ is of very low intensity; interaction with HF-OH leads to a band at 2295 cm -1, and a band is observed at 2280 cm -1, assigned to interaction of CD3CN with Na + ions in the side pockets [4]. Over both catalysts the v(CD) vibration bands are observed at 2125 and 2113 cm -~. The band at 1387 cm -~ is due to 8(OH) vibrations of the perturbed OH groups H-bonded to acetonitrile in the side pockets [4].

3.2.1. Heating Heating of the samples with adsorbed acetonitrile leads to some modifications in the infrared spectra. The band at 2314 cm -1 diminishes in intensity and shifts to lower wavenumbers. At 200 C a new weak band appears at 2323 cm -1, and its intensity increases with temperature (Figs.2 and 3). The intensity of this band also increases upon increasing acetonitrile concentration (Fig. 4). During heating of the sample the band at 2286 cm -1 also diminishes in intensity. At the same time the band due to 8(OH) vibrations diminishes and at 225 C finally disappears. In the v(OH) vibrational region hydroxyl groups are recovered (positive band, increasing intensity at elevated temperatures). All these transformations are accompanied by an H/D exchange between the zeolite and CD3CN, as the bands in the v(CD) region start to disappear and a v(OD) band 0.2 u a

2320

2650

~

~~.~__~

] 0.1ua

_ _ _ _ - -

2320

2280

2240

Wavenumber, cm -1

Figure 3. Heating of adsorbed CD3CN from 200 to 240 C over HMOR sample. at 2650 cm -1 appears (Fig. 2).

2283

0.2 u a

~

+3.2Torr 300~

240~ 3h 240~ C 2700

' 26;0

' 2620

Wavenumber,

'

c m -1

2;40

"

2300

"

2260

Wavenumber,

'

2220

'

c m -1

Figure 4. FTIR spectra of CD3CN heated at different temperatures, different time, and after introduction of increasing doses of CD3CN over HMOR sample.

339

3.2.2. Reversibility To check if the changes are reversible we heated the sample with non deuterated acetonitrile (to avoid deuteration of the zeolite) to 250 C, and cooled it back to room temperature (Fig. 5). Although the assignment of the bands is much more complicated than in the case of deuterated acetonitrile, the perfect superposition of spectra recorded upon heating and cooling shows that the change upon heating is perfectly reversible.

0.2ua

2340

2320

2300

2280

2260

2240

W a v e n u r n b e r , c m -1

Figure 5. FTIR spectra of adsorbed CH3CN in the temperature range 75-250 C (each spectrum recorded at 25 C step) over HMOR sample.

3.2.3. Stability To check the stability of the corresponding species we heated the sample with adsorbed acetonitrile under vacuum (Fig. 6).

0.1 u a

I

I

I

I

I

2340

2320

2300

2280

2260

W a v e n u m b e r cm -1

Figure 6. Adsorption of CD3CN, and evacuation in the temperature range 50-175 C (each spectrum recorded at 25 C step) over HMOR sample. Increasing the evacuation temperature the 2325 cm -1 band is getting more clear. The band due to adsorption of acetonitrile on BrCnsted acid sites disappears much faster than the band at c a .

340 2325 c m -l. At 175 C almost all molecules of acetonitrile adsorbed on BrCnsted acid sites desorbed. The band at 2325 c m -1 is stable up to 250 C under vacuum over HMOR sample and disappears at 225 C over NaHMOR sample (spectra not shown). 4. D I S C U S S I O N The H forms of zeolites are very important catalysts for acid-catalysed reactions requiring high Br0nsted acidity [6]. Mordenites are among the most studied zeolites because they present interesting activities for many processes in petroleum refining. In a test n-hexane cracking reaction at 400 C on H-MOR, H-MFI and H-Y with similar Si/A1 ratios, it was shown that mordenite had the strongest initial activity [7]. There are two typles of BrCnsted acid sites in H-mordenite, located in 12-ring channels (IR band at 3612 cm- ) and in 8-ring channels (IR band at 3 5 8 5 c m -1) [8]. These two sites are supposed to have a significant difference in their catalytic properties. For example, the acid sites in the small channels displayed much higher (-- 10 times) protolytic cracking activity than the acid sites in the large channels [9]. Many basic molecules, e.g. pyridine, ammonia, carbon monoxide, acetonitrile are employed to characterise acidity of the catalysts [10,11,12],. Recently, d3-acetonitrile has been widely employed due to its medium proton affinity (187.4 kcal/mol [12]), its relatively small size and the particular sensitivity of the v(CN) stretching mode to the interaction with acid sites of different strength. CD3CN is preferred over CH3CN for infrared studies, because in the case of CH3CN the v(CN) spectral region is strongly complicated by Fermi resonance between the v(CN) and the combination iSs(CH3) + v(CC) frequencies [13]. The v(CN) stretching mode of CD3CN enables a distinction between molecules adsorbed onto strong and weak Lewis alumina sites (bands at 2332 and 2323 cm -1, respectively), bridging OH groups (band at 2300 cm-1), and terminal SiOH groups (band at 2280 cm -~) [5]. Acetonitrile interacts with the hydroxyl groups of the catalyst by H-bond formation [11,12]. Protonation of acetonitrile was also observed on some catalysts (e.g. at room temperature on the H-Nation membrane at the highest dosages [14,15]), and it was also proposed in the adsorption of increasing doses of CD3CN at 100 C on an acidic mordenite (giving a v(CN) band at 2314 cm-1). However, supplementary studies performed over H-mordenite showed that there is no proton transfer from the zeolite to the nitrile at room temperature and the band at 2314 cm -1 can be assigned to strongly H-bonded acetonitrile in the side pockets. We had used protonation of acetonitrile at high temperature as an acidity measurement for the prediction of catalytic activity in a series of zeolites with different structures, but this was done using DRIFTS [2]. We want here, using transmission spectroscopy as a more efficient technique, to reinvestigate the influence of temperature on adsorbed acetonitrile on mordenite.

4.1. OH groups The infrared spectra of our samples at room temperature after activation (Fig. 1) are well known and typical for H-mordenite samples [8]. However when recording the spectra at increasing temperature (up to 450 C) we observe important changes. The general decrease of the maximum intensity, broadening and shift towards lower wavenumbers are all often encountered on similar materials (e.g. HY zeolites [16,17]). This is a complex phenomenon, most of it is usually assigned to the anharmonicity of the zeolite as a vibrator: all vibrations being coupled, the deformation of the framework upon vibration at high temperature is responsible for shifting and deformation of structure vibrations, and thus of OH vibrations (by

341 coupling). Some authors even proposed that these changes are due to proton delocalization [ 17] (1H MAS NMR studies of activated zeolites carried out at elevated temperatures indicate the delocalization of protons [18,19]). Fripiat et al. [20] showed that the decrease in intensity observed on heating of boehmite is accounted for by a tunnel effect and by an increase in the populations of the higher vibrational energy levels. The most striking change in our spectra is however the change of relative maximum intensities for the two v(OH) vibration bands, HF-OH and LF-OH. In the case of NaH-MOR, containing no OH groups in the side pockets, and Na + in the main channels, no mobility of the Na + ions or OH groups exist even at high temperature (it would lead to OH groups in the side pockets, and to a v(OH) band at low frequency). It should be noticed that this was in the absence of any water (the cell is at 10.7 Torr). We therefore propose that the change of relative intensity between both bands is due to a difference in the effect of temperature on the extinction coefficient of the infrared band, probably because of a great difference in the local environment. This will be checked in further work by computer simulation of the vibration of the zeolite at high temperature. 4.2.

CD3CN

The presence of ABC bands in the infrared spectra after adsorption of CD3CN at room temperature shows that acetonitrile is hydrogen-bonded to hydroxyl groups as was observed by many authors [5,11]. However after heating at elevated temperatures the new infrared bands indicate that other types of interactions exist. The bands at 1387 cm -1 (perturbed OH groups H-bonded with acetonitrile) and the band at 2314 cm -I (strongly H-bonded acetonitrile in the side pockets) diminish, and completely disappear at 225 C, showing that there are less H-bonded species. Also less acetonitrile molecules are adsorbed on BrCnsted acid sites (lowering of the 2286 cm -l band intensity). At the same time the band at ca. 2325 cm -I becomes more intense (Fig. 2). This band already appears at lower temperature, but is more easily detected under vacuum, when the amount of other (more weakly) adsorbed species is lower. In the experiments performed without vacuum the band at c a 2325 cm -1 can be hidden by the band at 2314 cm -1. Bands in the 2320-2330 cm -1 region were observed previously on various zeolites and were assigned to the adsorption of acetonitrile on Lewis alumina sites [5,12]. We checked the absence of Lewis sites on our samples by pyridine adsorption experiments, and we therefore have to propose another assignment. Paz6 et al. studied the interaction of CD3CN with dehydrated H3PW~2040 in solid form at room temperature and observed a band at 2330 cm -1 [21], which they assigned to the formation of C D 3 C N H + N C C D 3 dimers. Under our conditions this explanation has also to be rejected because the band is stable under vacuum, with a very low amount of adsorbed molecules (to low for dimmer formation), whereas Zecchina et al. [15] showed that on their H-Nation membrane, the formation of the protonated dimmer occurs after the formation of the hydrogen-bonded species is completed, and when some more acetonitrile is sent on the surface. In our case, the 2325 cm -~ band appears in very small amounts (strongly adsorbed species) at room temperature. Other types of adsorbed acetonitrile are formed on the surface when the surface concentration is increased [4]. When the temperature increases, all adsorbed species are desorbed, and the correspond infrared bands diminish in intensity, except the species corresponding to the 2325 cm -l band (2320 cm -I at 300 C). This is the most strongly adsorbed species, its concentration increases with increasing temperature. A possible explanation for this is the formation of protonated acetonitrile (as an activated stated), which,

342 being ionic, is very strongly adsorbed on the surface (up to 450 C). This protonation is reversible: upon decreasing the temperature, the same spectra are recorded than during the heating phase. 5. CONCLUSION The influence of temperature on the infrared spectrum of activated mordenites was studied, together with the influence on the state of adsorbed acetonitrile. The relative intensities of v(OH) vibration bands on the pure zeolite was shown to depend on the temperature, which is not fully understood yet. The reversible protonation of acetonitrile in mordenite at high temperature was also evidenced, leading to a band at approximately the same vibrational frequency as in the case of acetonitrile on Lewis sites (at room temperature). Acknowledgements S. Chenevarin grateful acknowledges Bruker for financial support. REFERENCES 1. W. Daniell, N.-Y. TopsCe and H. Kn6zinger, Langmuir, 17 (2001) 6233. 2. F. Thibault-Starzyk, A. Travert, J. Saussey and J.C. Lavalley, Top. Catal., 6 (1998) 111. 3. R. Anquetil, J. Saussey and J.C. Lavalley, Phys. Chem. Chem. Phys., 1 (1999) 555. 4. O. Marie, F. Thibault-Starzyk and J.C. Lavalley, Phys. Chem. Chem. Phys., 2 (2000) 5341. 5. A.G. Pelmenschikov, R.A. van Santen and J. J~inchen, E. Meijer, J. Phys. Chem., 97 (1993) 11071. 6. A. Corma, Chem. Rev., 95 (1995) 559. 7. S. Jolly, J. Saussey, M.-M. Bettahar, J.-C. Lavalley and E. Benazzi, Apl. Catal. A, 156 (1997) 71. 8. P.A. Jacobs and W.J. Mortier, Zeolites, 2 (1982) 226. 9. D.B. Lukyanov, T. Vazhnova, J.L. Casci and J.J. Birtill, Europacat V, European Federation of Catalysis Societies Irish Catalysis Society, 2nd-7th September 2001, 6-P82. 10. E. Brunner, Catal. Today, 38 (1997) 361. 11. L. Kubelkovfi, J. Kotrla and J. Flori~in, J. Phys. Chem., 99 (1995) 10285. 12. C. Paz6, A. Zecchina, S. Spera, G. Spano and F. Rivetti, Phys. Chem. Chem. Phys., 2 (2000) 5756. 13. C.L. Angel and M.V. Howell, J. Phys. Chem., 73 (1969) 2551. 14. R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto and A. Zecchina, J. Phys. Chem., 99 (1995) 11937. 15. A. Zecchina, F. Geobaldo, G. Spoto, S. Bordiga, G. Ricchiardi, R. Buzzoni and G. Petrini, J. Phys. Chem., 100 (1996) 16584. 16. N.W. Cant and W.K. Hall, Trans. Faraday Soc., 64 (1968) 1093. 17. J.W. Ward, J. Catal., 9 (1967) 396. 18. T. Baba, N. Komatsu, Y. Ono and H. Sugisawa, J. Phys. Chem. B, 102 (1998) 804. 19. P. Sarv, T. Tuherm, E. Lippmaa, K. Keskinen and A. Root, J. Phys. Chem., 99 (1995) 13763. 20. J.J. Fripiat, H. Bosmans and P.G. Rouxhet, J. Phys. Chem, 71 (1967) 1097. 21. C. Paz6, S. Bordiga, B. Civalleri and A. Zecchina, Phys. Chem. Chem. Phys., 3 (2001) 1345.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

343

Spectroscopic and Catalytic Studies on Cu-MCM-22: Effect of Copper Loading. A. J. S. Mascarenhas a, H. O. Pastore a~*, H. M. C. Andrade b, A. Frache c, M. Cadoni c and L. Marchese c,* Grupo de Peneiras Moleculares Micro- e Mesoporosas, Instituto de Quimica, Depto. Quimica Inorg~nica, Universidade Estadual de Campinas, CP 6154, CEP 13083-970, Campinas- SP, Brazil. a

b Instituto de Quimica, Depto. Quimica Geral e Inorg~aica, Universidade Federal da Bahia, Campus UniversitY.rio de Ondina, s/n, CEP 40170-290, Salvador, BA, Brazil. c Dipart. di Scienze e Tecnologie Avanzate, Universith del Piemonte Orientale "Amedeo Avogadro", C.so Borsalino, 54, 15100, Alessandria, Italy, [email protected]. Copper exchanged MCM-22 zeolites with varying copper contents were prepared, characterized and tested in the nitrous oxide decomposition. Over-exchanged samples showed high activity, comparable to very active Cu-ZSM-5 catalysts. The nature of the copper species was investigated by T P R - H2, F T I R - CO and FTIR - NO, evidencing at least three different species: exchanged Cu 2+ ions, cationic oligomeric species and oxidic phases. Upon NO adsorption, mononitrosilic complexes Cu I - (NO) were formed, and completely oxidized to Cu u - (NO) with increasing NO pressure. Nitrate complexes were extensively formed by reaction with the oxidic phases in the sample with 183% cation exchange level. 1. INTRODUCTION Over-exchanged Cu-ZSM-5 is a very active catalyst in the decomposition of NO and N20 and also in the S C R - Selective Catalytic Reduction with hydrocarbons, however it presents serious limitations for practical applications [ 1], one of the most important being its dealumination under hydrothermal conditions. MCM-22 is a promising alternative zeolite, since it presents high hydrothermal stability and low level of dealumination. Its structure is composed of two independent channel systems, both accessed by 10 membered rings windows [2]: a 10-MR sinusoidal bidimensional system very much like the ZSM-5, and a 12-MR system formed by interconnected supercavities. Copper exchanged-MCM-22 have already been proposed as an active catalyst in the SCR with hydrocarbons [3] and in the decomposition of nitrous oxide [4], but in spite of these studies, certain details of siting and reactivity of the copper ions in Cu-MCM-22 catalysts are still not clear. The first attempt in this direction was the investigation of a Cu-MCM-22 with 194% of cation exchange level by means of infrared spectroscopy of adsorbed CO and NO [5], showing that there are certain characteristics of "Corresponding author: [email protected]

344 copper species and their surface reactivity that are typical of Cu-MCM-22 when compared to Cu-ZSM-5 and to other copper exchanged-zeolites. However, it was not clear if the observed Cu-MCM-22 behavior would be the same for lower cation exchange levels. This work aims at studying siting, coordination, oxidation and aggregation states of copper ions in Cu-MCM-22 as a function of copper content, and their influence in the nitrous oxide decomposition will be enlightened. 2. EXPERIMENTAL SECTION Zeolite MCM-22 (Si/A1 = 15) was prepared as described elsewhere [4]. The calcined sample was exchanged for 24 h at room temperature with solutions of Cu(NO3)2.3H20 in view of obtaining samples with cation exchange levels varying from 50 to 200%. The final pH was adjusted to 7.5 with NHaOH. The materials were washed, dried and calcined at 773 K under oxygen. Comparatively, a sample of ZSM-5 (Si/A1 = 15) was prepared by a method described in the literature [6], calcined at 773 K and copper exchanged (200%) as described above. Samples were characterized by XRD (Shimadzu XRD6000), ICP-AES (Perkin-Elmer 300-DV Spectrometer), BET (Micromeritics Flowsorb 2300), TPR (in a homebuilt device) and tested in the nitrous oxide decomposition (0.5 wt. % of N20) in a continuous flow microreactor, operating at a temperature range of 573 - 823 K, and the effluents were analyzed by gas-chromatography on a CG-3537, operating with TCD and an arrangement of 5A Molecular Sieve and Porapak Q columns. FTIR experiments on pelletized samples were performed with an ATI Mattson Research Series FTIR spectrometer at resolution of 4 cm "1 and by means of specially designed cells which were permanently connected to a vacuum line (ultimate pressure < 10-5 Torr) to make adsorption-desorption experiments. The samples were pretreated heating at 823 K (2 K.min 1) under vacuum for 4 h. CO and NO adsorptions were performed at 300 K. 3. RESULTS AND DISCUSSION

>., tO

t I

10

20

|

30

20 / degrees

i

40

,

50

Figure 1. X-Ray diffractograms of MCM-22 catalysts: A. as-synthesized MCM-22; B. calcined H,Na-MCM-22; C. Cu(50)-MCM-22; D. Cu(100)-MCM-22; E. Cu(150)-MCM-22 and F. Cu(200)-MCM-22.

345 Table 1 Elemental analyses and surface areas of the Cu-MCM-22 catalysts.

Sample

SiOrJAlzO3

%C.E. a

SnET

Unit Cell b

(m2.g -1)

(experimental)

H-MCM-22

30

~

471

H3,29Nal,17[A14,46Si67,540144]

Cu(50)-MCM-22

34

68

472

Nal,27Cu1,40[A14,07Si67,930144]

Cu(100)-MCM-22

33

127

436

Cu2,04[A14,08Si67,920144].0,2 75 CuO

Cu(150)-MCM-22

30

141

421

Cu2,27[A14,54Si67,460144].0,47CuO

Cu(200)-MCM-22

30

183

408

Cu2,23[Ala,a6Si67,540144].0,92CuO

a Cation exchange level calculated considering Cu/A1 = 0.5 = 100%. b Unit cell composition calculated on dry basis: Mx/n[AlxSi72-xO144], where M n§ are chargebalancing cations. The as-synthesized sample (Fig. 1.A) presented the x-ray pattem of the MCM-22 layered precursor and the calcined (Fig. 1.B) and exchanged samples (Fig. 1 . C - F) presented more defined peaks, characteristic of the tri-dimensional zeolite structure [7]. The elemental analyses and BET results are shown in Table 1. The experimental cation exchange levels, 68, 127, 141 and 183 %, are approximately those desired. The surface area of the samples decreased when the copper content increased, suggesting that MCM-22 porous systems might be partially occupied by copper species. In the sample with higher Cu loading oxidic-like species are in fact present as determined by TPR and FTIR measurements (vide infra). However, no peaks of CuO or Cu20 have been observed in the XRD pattems, indicating that if they were formed, should be of very small dimensions and well dispersed in the MCM-22 channels and/or cavities. The nature of these copper oxide species, that are probably formed in the porous system, were characterized by TPR and the results are shown in Figure 2. When compared to the results previously reported on the literature for Cu-ZSM-5 [4], three types of copper species were found in the Cu-MCM-22 catalysts: i) isolated Cu(II) ions, reduced at ca. 653 K; ii) [ C u - O - Cu] 2+ oligomeric species reduced in the 523 - 593 K range; iii) oxide-like species, occluded within the channels and/or cavities of MWW framework, reduced at 508 K. The cationic oligomeric species are the most important species in the over-exchanged samples, but are present even in the 68% cation exchanged. This is an effect of the pH adjust to 7.5 with NH4OH, that favors the formation of Cu(OH) +, Cuz(OH) 3+, Cu2(OH)22+, Cu3(OH)24+, etc. in aqueous solution [8]. Another interesting fact is that all species observed by TPR in Cu-MCM-22 samples are reduced at lower temperatures than in the Cu-ZSM-5 system (Fig. 2E) and this might be relevant from a catalytic point of view, since the proposed mechanism for the nitrous oxide decomposition involves redox reactions [9].

346

E

E=

D C B

o i

400

i

i

600

800

910'00

Temperature / K Figure 2. TPR profiles of Cu-MCM-22 catalysts: A. 68%; B. 127%; C. 141% e D. 183%, compared to E. Cu-ZSM-5, 200%.

CO adsorption at 300 K

Infrared monitoring of probe-molecules adsorption revealed intimate details of the copper species formed in these systems. CO adsorption (50 Torr) at 300K led to the formation of dicarbonylic complexes of copper(I), CuL(CO)2, whose symmetric and asymmetric absorptions appears at 2178 and 2151 cm 1, respectively, as shown in Figure 3.A. for the Cu(200)-MCM-22 sample. Under evacuation, these bands disappeared and a new band, due to the monocarbonylic Cu I - (CO) complexes emerged at 2159 cm -1 [10], that is stable under vacuum conditions. This band is highly asymmetric in the low wavenumber region, 21502125 cm ~, suggesting that additional species contribute to that absorption.

2151

ai

-0.25

(~

2159 I

0.2

c d

(B)

O

ID O t-

.Q L_

O

O

.Q

<

< ,

22'50

,

i

,

i

22'00 2 50 2 00 -1 Wavenumber / cm

2050

2250

2 2 ' 0 0 21'50 21'00 -1 Wavenumber / cm

2050

Figure 3. FTIR spectra of adsorbed CO: (A) on Cu(200)-MCM-22, from 50 Torr (curve 1) to vacuum condition (curve 16, residual pressure = 1,0 x 104 Torr); (B) after CO adsorption and evacuation for Cu-MCM-22 samples: a. 68%; b. 127%; c. 141% and d. 183%.

347 In order to explain the origin of the asymmetry of the band at 2159 cm -1 in Cu-MCM-22, it should be reminded that besides a main band at 2159 cm l , a shoulder at 2135 cm -1 was found on Cu-ZSM-5 as well [11 ], and this was assigned to CO adsorbed on Cu(I) ions in different coordination to the framework [12]. Moreover, in the case of Cu-Y it was shown that CO adsorption on different sites results in distinct bands at ca. 2135, 2145, 2160 cm -1 [11, 13, 14]. Although at least four different sites were proposed for the Cu-MCM-22 zeolite [15], in the experiments performed here it was not possible to observe distinct bands, but only the above mentioned asymmetry on the low wavenumber side of the main band at 2159 cm -~, which might be due to the result of the absorption of different sites. In highloading samples (compare spectra a and b with c and d) the asymmetry is more pronounced and the tail arises probably from the presence of oxide-like copper species. On the lower loading samples, however, the asymmetry arises from the absorption of isolated Cu sites [ 16]. This is in favour of the presence of copper ions in small oxidic aggregates, either cationic oligomeric or oxide-like copper species in higher loading samples, which was confirmed by TPR and by NO adsorption at 300K (vide infra). Samples with lower exchange level however, show a contribution at around 2150 cm -1 which may account for the presence of isolated copper ions at different cation sites. This effect has been studied in detail by separating the different contributions of the 2159 cm -~ band with a fitting and simulating procedure and the results will be presented in a separate contribution [16].

NO adsorption NO adsorption at 300 K was performed over the catalysts Cu-MCM-2 with 183% cation exchange level (Figure 4), from low (0,05 Torr) to high NO pressures (30 Torr). At low NO doses (0.05 Torr) two main bands were observed at 1813 and 1899 cm -1, this last one with a shoulder at 1908 cm l (curve 1). The band at 1813 cm 1 is commonly assigned to the stretching of mononitrosyl complexes on copper(I) and the band at higher frequency (1908-1899 cm -1) is in the region where the mononitrosylic complexes on copper(II) absorb [ 11, 17, 18].

1899 I

5

16121 ..-..j1612

0 t-

1813

..Q t__

0

i -Z-....t.

?

< 22'00

' 20'00

' ldoo

'

6'00

Wavenumber / cm

'

-1

4'00

'

Figure 4. FTIR spectra of NO adsorbed at 300 K on Cu(200)-MCM-22 (183%) from 0.05 Torr (curve 1) to 30 Torr (curve 17), and after 45 minutes of contact (curve 18).

348 Upon increasing NO pressure the band at 1813 cm l of the Cu I - (NO) complexes decreased, and simultaneously the band at higher wavenumbers related to the Cu n - (NO) species increased, showing that an oxidation process is taking place and is complete after the 45 min period of time. There is a clear NO pressure dependence on the Cu(II) species that originates the band at 1899 cm -~ and those that absorb at 1908 cm -1 ones; at high NO pressures the band at 1899 cm -1 become predominant (curve 18). This is an indication that there are at least two distinct Cu(II) sites present in Cu-MCM-22 which may differ by their coordination to the framework oxygens [ 16]. Another important family of bands in the region between 1650 and 1300 cm 1 was formed and increased under high pressure conditions. These bands are related to different vibration modes of nitrate complexes on CuII ions: the band at 1621 cm 1 and the shoulder at 1612 cm l are attributed to bridging nitrate species and the bands at 1571 and 1517 cm -1 due to chelating nitrate species [19]. The band at 1291 cm -~ is attributed to nitro or bidentate nitrito species. There are also bands related to NO2 species in the 2150-2050 cm -1 region of [20]. The behavior of the samples with different copper content is very similar to that showed for Cu-MCM-22 with 183%. However, the formation of nitrates seems to be a function of the copper exchange level, as denoted by the picture in Figure 5, and is in favor of the existence of cationic oligomeric and/or oxide like copper species in this zeolite, as shown by the TPR [4]. For the samples with cation exchange levels varying from 68 to 141%, the formation of nitrates is not so extensive as observed for the sample with 183%, where the family of bands in the region of 1650 - 1250 cm -1 is very intense. This suggests that oxidelike copper species, more abundant on the Cu(200)-MCM-22 sample, are responsible for the formation of the nitro, nitrate and nitrito species.

o= o a

22'00

' 20'00

' 18'00

Wavenumbers

' 16'00 / cm

' 14'00 -1

'

Figure 5. FTIR spectra of 10 Torr of NO adsorbed at 300 K on Cu-MCM-22 catalysts" a. 68%; b. 127%; c. 141% e d. 183% (Spectra recorded after 45 min of contact with NO).

349

Nitrous Oxide Decomposition

120 -

--1

o-.9, 1 0 0 ,-o

"~

]

80-

60-

=>

40-

8

20-

D

0r 573

623

673

723

773

823

Temperature / K

Figure 6. Conversion-temperature plot for the decomposition of 0.5% N20 in helium (GHSV = 45000 hq): , , Cu(50)-MCM-22; 0 , Cu(100)-MCM-22; A, Cu(150)-MCM-22; 0 , Cu(200)MCM-22 and O, Cu(200)-ZSM-5. Figure 6 shows the catalytic performances in the nitrous oxide decomposition as a function of the temperature for all Cu-MCM-22 samples discussed in this work. The activity increased as a function of the copper content until an exchange level of 141%, that presented exactly the same catalytic behavior of Cu(200)-ZSM-5, reaching 100% of conversion (5,0 x 10 -7 mo1N20.gcat-l.s"l) at 773 K. Cu(200)-MCM-22 (cation exchange level of 183%) is slightly less active than Cu(150)-MCM-22 (141%) and reached the maximum activity only at 823 K. This difference can be accounted for by the presence of a larger amount of copperbased oxidic phases in the first one, as shown by TPR, F T I R - CO and F T I R - NO, and confirms that oligomeric species are the most active for the decomposition of nitrous oxide. Isolated copper species do not seem to be as active as the oligomeric cationic species, since the under-exchanged sample, Cu(50)-MCM-22, is nearly inactive in the range of investigated temperatures, reaching only 7% of conversion at 550~ It is known that the adsorption of NO on Cu-ZSM-5 originates bands related to monoand dinitrosilic complexes of Cu(I) that are partially oxidized to mononitrosilic complexes of Cu(II), while for Cu-MCM-22 the experiments show that the oxidation process is fast and complete. There is also formation of nitro, nitrito and nitrate complexes in Cu-ZSM-5, but in lower amounts than that observed for Cu(200)-MCM-22. These results show that, in spite of the different copper species formed in the MWW and MFI structures, it is possible to attain high activities in MCM-22 zeolite as well.

4. CONCLUSIONS Over-exchanged Cu-MCM-22 is a catalyst as active as Cu-ZSM-5 in the decomposition of nitrous oxide (N20). Its catalytic performances can be interpreted on the basis of the different copper species formed in the MWW structure. TPR experiments showed

350 the existence of isolated Cu2+ ions in ion-exchange sites, cationic oligomeric species, such as [ C u - O - Cu] z+, and copper oxide species deposited in the cavities and/or channels of the MCM-22. FTIR of adsorbed CO confirmed the existence of well dispersed oxidic phases. Upon NO adsorption it was observed the formation of mononitrosylic complexes of copper(I), that were oxidized to mononitrosylic copper(II) complexes. This process was complete and NO pressure dependent for the over-exchanged sample with 183% of cation exchange, where an extensive formation of bands in the region of nitro, nitrito and nitrate complexes, was also observed, and this attributed to the reactivity of copper oxidic phases towards NO. ACKNOWLEDGEMENTS

This work was supported by the Brasilian "Fundaq~o de Amparo ~t Pesquisa do Estado de Sao Paulo" (FAPESP, grant number 99/10391-8) and by the Italian "Ministero della Ricerca Scientifica e Tecnologica" (MURST, "Progetti di Rilevante Interesse Nazionale", Cofinanziamento 2000). M.C. thanks "Associazione per lo Sviluppo Scientifico e Tecnologico del Piemonte" for a grant-in-aid. REFERENCES

1. J. N. Armor, CataL Today 26 (1995) 99. 2. M. E. Leonowicz, J. A. Lawton, S. L. Lawton, M. K. Rubin, Science 266 (1994) 1910. 3. A. Corma, A. E. O. Palomares, V. Forn6s, Res. Chem. Intermed. 24 (1998) 613. 4. A. J. S. Mascarenhas, H. M. C. Andrade, H. O. Pastore, Stud. Surf Sci. Catal. 135 (2001) 322. 5. A. Fraehe, M. Cadoni, C. Bisio, L. Marchese, A. J. S. Mascarenhas, H. O. Pastore, J. Phys. Chem. Submitted. 6. H. Robson, Micropor. Mesopor. Mater. 22 (1998) 551. 7. A. L. S. Marques, J. L. F. Monteiro, H. O. Pastore, Micropor. Mesopor. Mater. 32 (1999) 131. 8. M. Iwamoto, H. Yahiro, Y. Torikai, T. Yoshoka, N. Mizuno. Chem. Lett. (1990) 1967. 9. F. Kapteijn; G. Marbhn; J. Rodriguez-Mirasol; A. J. Moulijn. J. Catal. 167 (1997) 271. 10. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo,M. Vlaic, M. Belatreccia, J. Phys. Chem. B: 101 (1997) 344. 11. C. M~quez-Alvarez, G. S. McDougall, A. Guerrero-Ruiz, I. Rodriguez-Ramos, Appl. Surf Sci. 78 (1994) 477. 12. B. Wichterlovfi, J. Dedecek, Z. Sobalik, A. Vondrov~i, K. Klier, J. Catal. 169 (1997) 194. 13. G. D. Borgard, S. Molvik, P. Balaraman, T. W. Root, J. A. Dumesie, Langmuir 11 (1995) 2065. 14. V. M. Rakic, R. V. Hercigonja, V. T. Dondur, Micropor. Mesopor. Mater. 27 (1999) 27. 15. T. Wasowicz, A. M. Prakash, L. Kevan, Microporous Mater. 12 (1997)107. 16. A. Frache, A. Mascarenhas, M. Cadoni, H.O. Pastore, L. Marchese, in preparation. 17. H. J. Jang, W. K. Hall, J. L. d'Itri, J. Phys Chem. 100 (1996) 9416. 18. G. Spoto, S. Bordiga, D. Scarano, A. Zeeehina, Catal. Lett. 13 (1992) 39. 19. G. T. Palomino, S. Bordiga, A. Zecchina, G. L. Marra, C. Lamberti, J. Phys. Chem. B: 104 (2000) 8641. 20. M. V. Konduru, S. S. C. Chuang, J. Catal. 196 (2000) 271.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

351

Preparation and properties of M F I zincosilicate Stanistaw Kowalak, Ewa Szymkowiak, Monika Gierczyflska, Girolamo Giordano a Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznafi, Poland, e-mail: skowalak@ main.amu.edu.pl a Department of Chemical Engineering and Materials, University of Calabria, via P. Bucci, 1-87030 Rende (CS), Italy The following study demonstrates a possibility to synthesize zincosilicate MFI by hydrothermal crystallization using sodium zincate as zinc source. It reminds the conventional method of hydrothermal zeolite synthesis. The earlier syntheses involved mostly complexed Zn cations. This way relatively high Zn loading (Si/Zn = 33) can be attained. The properties of the resulting samples depend on the origin of sodium zincate and on zinc content. The resulting products (particularly after ion-exchange modification) show remarkable catalytic activity for cumene cracking, 2-propanol decomposition and cyclohexane transformation towards benzene. 1. INTRODUCTION The syntheses of the crystalline metallosilicate molecular sieves often require protection of the metal cations to be introduced against the precipitation in highly alkaline initial mixture [1]. Usually anions such as oxalates, phosphates or amines [2] were admitted into the initial mixtures in order to coordinate the metal cations. We always [3-5] admitted a substantial amount of phosphoric acid to the initial gels containing Zn(NO3)2, silicon sources (such as Ludox, BDH silica gel, silicic and fluorosilicic acid, sodium silicate), sodium hydroxide and tetrapropylammonium bromide (TPAB). We have noticed a considerable influence of silicon source on properties of resulting materials. The samples obtained with contribution of BDH silica gel were active for cumene cracking [5], whereas the samples of similar Zn content prepared with silicic acid indicated only the presence of weak acid sites and hence enabled to catalyze 2-propanol decomposition [3]. The zinc containing ZSM-5 zeolites were reported as active catalysts for aromatization of light paraffins (Cyclar process) [7], regardless the localization of Zn atoms in the zeolite phase. The localization of zinc atoms in the MFI zincosilicates is still not quite clear [3-5]. Although the MFI zincosilicates indicate ionexchange properties, there is no linear correlation between ion-exchange capacity and zinc loading. The NMR data suggest a migration of Zn atoms from the framework position upon thermal treatment [5]. The following study comprises syntheses of series of MFI zincosilicates of various Zn content. The principal difference in preparation procedure compared to earlier [2-5] works consists in employing sodium zincate as a zinc source. The procedure [6] is analogous to the conventional synthesis of aluminosilicate zeolites that always commences with forming a gel by merging respective sodium silicate and aluminate.

352 2. EXPERIMENTAL

2.1 Synthesis The principal materials used for syntheses were as follows: water glass (SIO2=27.25%, Na20=13.64%, 1-120=59.11%) as silicon source, NaOH, TPABr (Aldrich) as template, zinc was added in form of sodium zincate. The latter was prepared either from Zn(NO3)26 1-120 (Aldrich) or ZnO (POCh, Poland) by dissolving in concentrated NaOH. In the case of Zn(NO3)2 6H20 two different procedures were applied. One comprised the alkalization of zinc nitrate solution with NHaOH and forming of Zn(OH)2, which was then carefully washed in order to remove the nitrates. Then the resulting Zn(OH)2 was dissolved with adequate amount of NaOH solution. In other case the zinc nitrates were treated with stoichiometric amount of NaOH without any nitrate removal. The Zn loading in prepared samples varied in range of Si/Zn 1000 - 33. The initial gel was formed by mixing of water glass and TPABr solutions and then adding the zinc and sodium sources. Some amount of phosphoric acid was used for adjusting pH of the gel to the value of 11. Other acids (e.g. HF, H2SO4) were applied in some experiments and they also resulted in forming of MFI structure. The mixtures were magnetically stirred for an hour at room temperature then placed in the Teflon lined autoclave and heated at 170~ The crystallization period was 23 hours for the samples of lowest Zn content and 46 hours for the others. The crystallization products were washed with distilled water and dried. Then they were calcined in air at 450~ for 16 h. The composition of the mixtures and synthesis conditions are listed in Table 1. Some of the resulting samples were modified by means of ion-exchange with various cations (NH4 +, Na § Ca2+ Cu2+, Zn2+, Ce3§ Al3+). The aqueous solutions (0.1M) of respective salts were applied for modification conducted at room temperature. The ammonium modifications were calcined at 350~ in air in order to obtain the hydrogen forms. 2.2. Sample characterization The resulting samples were characterized by means of XRD (TUR M-62), SEM (Philips SEM 515), FTIR (Bruker Vector 22), DTA, DTG were conducted in air atmosphere with the temperature increase of 5~ ( SETARAM SETSYS 12 ). Adsorption measurements involved nitrogen adsorption isotherm, BET surface area measurement (Quantachrome AUTOSORB-1). ICP-AES (ARL 3410) was used for elemental analysis, TPR with hydrogen (Asap ChemSorb 2705). The catalytic tests for cumene cracking and for 2-propanol decomposition were conducted in a pulse micro-reactor attached to the gas-chromatograph. The catalyst samples (powder 0.015g) were activated prior to reaction at 400~ for 30 min in helium stream. The cumene reaction was carried out at 350~ and 2-propanol decomposition at 230~ respectively. The pulses of 1ttl were injected. The reaction of cyclohexene was carried out in a flow system [8]. The catalyst was pretreated in nitrogen at 500~ and the reaction was conducted at the same temperature. The results of catalytic tests were compared with activity of aluminosilicate zeolites ZSM-5 (Si/Al=27, A1SiPenta) modified with H and Zn cations. 3. RESULTS AND DISCUSSION As shown in Table 1 the crystallization time of 46 hours was sufficient for attaining high crystallinity. The samples of lowest Zn loading attained high crystallinity already after

353 23 hours of heating. The products were easy to be recovered by filtration. The crystallization period longer than 46 hours usually resulted in very hard monolith, which was very difficult to be removed from the autoclave vessel. The XRD data indicate poor crystallinity of the samples of highest Zn loading. The influence of applied zinc source on structure of products is conspicuous. The samples prepared with Zn(NO3)2 treated with N a O H without nitrate anions removal showed lower crystallinity then those involving sodium zincates obtained from zinc oxide or pure hydroxide. Table 1 Gel composition and crystallization condition Gel molar ratio Si 9 Zn 9 T 9 Na 9 P 9 H20

Zn source

Crystallization time (h)

1 1 1 1

: : : :

0,001 0,01 0,02 0,03

: : : :

0,08 0,08 0,08 0,08

: : : :

0,12 0,24 0,24 0,48

: : : :

0,32: 0,32: 0,32: 0,32:

40 40 40 40

Zn(OH)2 Zn(OH)2 Zn(OH)2 Zn(OH)2

23 46 46 46

1 1 1 1

: : : :

0,001 0,01 0,02 0,03

: : : :

0,08 0,08 0,08 0,08

: : : :

0,12 0,24 0,24 0,24

: : : :

0,32: 0,32: 0,32: 0,32:

40 40 40 40

ZnO ZnO ZnO ZnO

23 46 46 46

1 1 1 1

: : : :

0,001 0,01 0,02 0,03

: : : :

0,08 0,08 0,08 0,08

: : : :

0,12 0,24 0,24 0,48

: : : :

0,32: 0,32: 0,32: 0,32:

40 40 40 40

Zn(NO3)2 Zn(NO3)2 Zn(NO3)2 Zn(NO3)2

23 46 46 46

T-template, P- phosphoric acid Typical X R

data are given in Fig. 1.

Source of zinc- ZnO Si/Zn

I....... ]

]1

Source of zinc- Zn(N03)2 ~

Si/Zn :~

33i 4 Fig. 1. X R

142 Th et 24

34

4

14 2 Th et 24

patterns for the as synthesized samples of different zinc origin.

34

354 The calcination at 450~ does not bring any noticeable changes in crystallinity of the samples. The influence of zinc source on the morphology of the resulting samples is even more noticeable in the electron microscopy pictures. The presented SEM photographs (Fig. 2) show that crystallites obtained from Zn(OH)2 are very fine (about l l.tm) and very uniform. The samples prepared in the presence of nitrates are also fine but not as uniform as in the earlier series. In the case of ZnO used for preparing sodium zincate the resulting samples form circular agglomerates of about 5 lam in diameter. The loading of zinc also affects the morphology of the products. The contribution of amorphous phase is seen for the samples of highest Zn content (Fig.3).

Fig.3. SEM photographs of series prepared with ZnO of indicated Zn loading. Thermal analyses (TG and DTA) conducted in air indicate some influence of zinc loading on the course of the curves. The weight loss in the temperature range up to 400~ is almost insignificant for the sample of low Zn content. It becomes more substantial for the samples of higher Zn loading. The weight loss in this range results from desorption of water, but it is not reflected in distinctive DTA peaks. The second step of weight loss starts at about 400~ and is attributed to thermal decomposition of template. The contribution of this step in total weight loss decreases with the Zn content. The decomposition of organic template comprises oxidation which is reflected in very distinctive exothermic effect at about 400~ The temperature of this peak is 400~ for the samples of lowest Zn content and it attains the temperature 414~ for the other samples. The increase in decomposition temperature might result from contribution of the ZnO42- tetrahedra in the framework, that provides stronger bonding of alkylammonium cations. The exothermic effect is proceeded with the endothermic

355 effect at -440~ (only for the sample of lowest Zn content the peak is at 420~ - ~ ~ The effect can be attributed to desorption of template oxidation products. The Ill DTA endothermic effect is overlapped by a J]440oc ~~,~.,,,,,-~ subsequent small exothermic peak at ~-- . . . . ~ . . . . . . 450~ which can reflect a further oxidation. There is no considerable 42~~ TG difference in profiles of DTA curves for the MFI zineosilieate samples of different zinc origin. Thermoprogrammed Si/Zn=100 reduction (TPR) in hydrogen stream (Fig. 5) indicates significant influence of zinc source on reduction ability of the resulting samples. The zinc atoms are the most susceptible to be reduced in hydrogen among all the atoms in sodium forms of the investigated samples. The sample obtained from ZnO does not show any distinctive sharp peak on the TPR curve. Only a very broad and insignificant signal reflecting a hydrogen consumption is noticeable in the range 500 - 850~ The sample prepared with Zn(OH)2 indicates two distinct peaks at 620 and 700~ In the case of the sample prepared with Zn(NO3)2 the profile of the TPR is even more complex. Besides the mentioned peaks at 620 and 700~ there are two additional signals at 650 and 730~ It is 445"C not easy to interpret the results of TPR in details, but it is unambiguous that 414 ~C Si/Zn=33 different behavior of the studied samples results from different zinc surrounding in each case. The nitrogen adsorption measurements DTA (Fig. 6) indicate that specific surface area decreases with increasing zinc content. The samples of lowest Zn loading show the surface area about 300m2/g (BET). , 438oC 465~ TG The decrease in surface area is most drastic for the samples prepared in the 0 200 400 600 800 1000 presence of nitrates. 4000c

Si/Zn=1000

i

Temperature

i

[~

Fig.4. DTA and TG curves (in air, temperature rise 5~ for indicated samples prepared with Zn(OH)2.

356 Si/Zn=lO0

zinc source: ~ 003)2

-

50

r

!

i

i

250

450

650

850

Temperature

[~

Fig. 5. TPR profiles for the sodium forms of indicated samples. The temperature rise was 10~ gas (Ar 90%, H2 10%) flow 100ml/min, sample weight 0.03g.

"

4OO

-.- z,,(om.2

t -i- ZnO

~. 3oo

. . . . .

i

0 1000

100

50

33

Si/Zn

Fig. 6. Correlation between zinc content and surface area (BET).

The catalytic tests for 2-propanol decomposition (Fig.7) indicate much lower activity of the samples (of Si/Zn = 100) obtained in the presence of nitrates. The activity depends very much on cation introduced by means of ion exchange. The samples containing sodium or calcium are practically inactive. The modification with A1 cations always results in highest activity of all series. It is possible that some of introduced A1 ions replace zinc from the framework and contribute in generation of relatively strong acid sites. The hydrogen and cerium modifications of the samples of different origination showed also high activity. It is peculiar

Fig. 7. Activity (expressed as conversion) of indicated samples for 2-propanol decomposition.

357 that modification with Zn cations of the sample prepared with Zn(OH)2 results in very low activity whereas the other two samples modified with Zn are among the most active ones. The decomposition of 2-propanol results mostly in generation of propene. Only in the case of modification with Cu and Ce cations the contribution of acetone in the products is noticeable. The correlation between the Zn loading and the catalytic activity is not very clear. The zincfree silicalite-1 does not show any activity for the reactions applied for the catalytic tests. The introduction of some zinc into the structure (Si/Zn = 1000) is reflected in remarkable activity. Further increase in Zn content (Si/Zn = 100) brings about enhanced activity, but then the activity declines with growing Zn loading (Si/Zn = 50). It is even more dramatic for the Zn richest samples (Si/Zn = 33). In the case of the latter it is likely that low activity can result from lower crystallinity and some structure defects. The obtained samples show some activity for cumene cracking (Fig. 8). The activity of the samples obtained from Zn(OH)2 is distinctively prevailing for the samples of low Zn content and it decreases for the sample of Si/Zn = 50. The activity of series prepared with ZnO increases with Zn loading up to Si/Zn = 50. The last sample is even more active than the analogous sample prepared with Zn(OH)2. All the samples of Si/Zn = 33 are inactive for this reaction. Similarly to the 2-propanol reaction the samples modified with Na and Ca are inactive. 10-

..............................................................................S~i~Zii=i~............. 9

,_8-

10

~'6

%'~ "~6-

~4-

"~4-

r~2-

~2

0

H

Zn

Cu

AI

Ce

Ca

Na form

0

J~................................................................................. AI form Zn(OrI~ ZnO

lOOO

,

lOO

~

50

,

,

33 Si/Zn

Fig. 8. Activity of the indicated samples for cumene cracking. The preliminary test reactions of cyclohexane transformation [8] were conducted over investigated zincosoilicate series modified either with Zn cations or with protons. The same cation modifications of zeolites ZSM-5 were used for comparison. The reaction led to benzene as a result of dehydrogenation as well as to a variety of hydrocarbons as a result of cracking and subsequent oligomerization of cracking products. The conversion of cyclohexene over the samples under study was usually much lower than that obtained over zeolite ZSM-5. The selectivity of zeolites towards benzene was-~7% for the H form and -74% for the Zn form. Similar selectivity to that of Zn-ZSM-5 was noticed for H-form of zincosilicates. The respective Zn-modifications exhibited higher selectivity towards benzene. The zincosilicate MFI samples of intermediate zinc loading (i.e. Si/Zn 100 and 50) showed the highest activity and highest selectivity. The series prepared with Zn(2qO3)2 indicated again much lower activity than the other ones. The results confirm the earlier observation [7] that the origin and localization of zinc inside the MFI structure is not decisive for the catalytic activity for dehydrogenation and Cyclar process.

358 4. CONCLUSIONS The presented series of samples indicate that zincosilicate MFI can be obtained not only with Zn cations introduced into the initial mixture in form of simple salt solution where the ZnZ+ions are complexed by phosphate anions [3-5] or amines [2]. We have demonstrated that sodium zincate can be used as effective source of zinc for the MFI zincosilicate synthesis and the resulting samples can attain a substantial content of Zn. Although some amount of n3PO4 was used for adjusting pH of the gel in the studied series, the high crystallinity could be also attained with other acids such as HF or H2SO4. It is very conspicuous that the origin of applied zincate affects significantly the results of syntheses. The samples that involved zincates obtained by dissolving of ZnO or freshly precipitated pure Zn(OH)2 in NaOH show always better crystallinity and much higher catalytic activity than the series prepared from Zn(NO3)2 treated with NaOH and containing residual nitrates. The presence of nitrates in the crystallization mixtures affects very much the properties of the resulting products. The attempts to introduce the amounts of zinc higher than Si/Zn = 33 usually led to amorphous products. The correlation between zinc loading and catalytic activity is not very clear. The samples of the highest loading are always least active. It is likely that the structure of these samples are partially defected what is reflected in XRD data and in low surface areas. The modification of zincosilicate MFI with various cation influences very much their catalytic properties. It is interesting that aluminum cations introduced by means of ion exchange result in relatively high activity for the reactions involving acid sites. It is likely that some Al ions can replace the framework zinc. REFERENCES:

1. R. Szostak, Molecular Sieves, Principles of Synthesis and Identification, Van Nostrand Reinhold Catalysis Series, 1988. 2. S. Valange, Z. Gabelica, B. Onida, E. Garrone, Proc. 12th Int. Zeolite Conf. Baltimore 1998 (Eds. M.M.J. Treaty et al.), MRS, Warrendale, Penn., IV, (1999) 2711. 3. S. Kowalak, E. Szymkowiak, A. Jankowska, G. Giordano, Proc. 9th Int. Symp. Heterogeneous Catal., Varna, Inst. of Catal., Bulg. Acad. of Sci., Sofia 2000 (p. 229). 4. S. Kowalak, E. Szymkowiak, I. Lehmann, G. Giordano, Stud. Surf. Sci. Catal., 135 (2001) 311. 5. A. Katovic, E. Szyrnkowiak, G. Giordano, S. Kowalak, A. Fonseca and J. B.Nagy, Stud. Surf. Sci. Catal., 135 (2001) 337. 6. S. Kowalak, E. Szymkowiak, M. Gierczyfiska, Pol. Pat. pending, WP 362 (2001). 7. I.E. Maxwell and WH.J. Stork, Stud. Surf. Sci. Catal., 85 (1991) 571. 8. F. Roessner, E. Szymkowiak, to be published.

Studies in SurfaceScienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa(Editors) 9 2002 ElsevierScienceB.V.All rightsreserved.

359

I n f l u e n c e of Cs l o a d i n g a n d c a r b o n a t e s on T P R profiles of P t C s B E A * L. Stievano a, C. Caldeira b, M. F. Ribeiro b and P. Massiani ~, aLaboratoire de R6activit6 de Surface (CNRS-UMR 7609), Universit6 Pierre et Marie Curie, 4 Place Jussieu, casier 178, 75252 Paris CEDEX 05, France, e-mail: [email protected] bDepartemento de Engenharia Quimica, Instituto Superior T6cnico, Av. Rovisco Pals, 1049-001 Lisboa, Portugal The study by Temperature Programmed Reduction (TPR) of calcined PtCsBEA containing increasing Cs loadings, and therefore possessing increasing basicities, shows that the amount of Cs strongly influences the reducibility. The simultaneous mass spectrometry analysis suggests that a special care should be taken in the interpretation of the TPR profiles, especially for basic supports on which carbonates may form upon simple exposure of the samples to the CO2 of the air. Their reduction during TPR produces additional peaks, which alter the patterns and the quantitative evaluation of the consumption of hydrogen. 1. I N T R O D U C T I O N Pt supported on zeolites is involved in numerous catalytic processes. Even though the zeolite support is mostly used in its acidic form [1], the use of basic zeolites is being developed with the aim of controlling the electronic and hence the catalytic properties of the Pt metal particles [2]. A well-known example is that of basic KL that led in the 80's to the development of the industrial PtKL catalyst for dehydrocyclisation of n - p a r ~ s [3]. Three main interpretations were proposed to explain the specific activity of Pt on KL, which were recently reviewed [2,4-5]. They are (i) a molecular die effect, that involves the geometric constraints imposed by the rigid network of L (linear zeolitic channel), (ii) an electron transfer from the negatively charged oxygen atoms of the basic zeolitic framework to the metal particles (metal-support interaction) and/or (iii) the presence of very small Pt particles which might as well influence the electronic structure of the unsaturated Pt atoms at the surface. The high selectivity to aromatisation for Pt supported on other basic oxides with varying pore systems, * Financial support from CNRS, ICCTI CERC3 and the Embassy of France in Portugal is gratefully acknowledged.

360 as in particular Cs-exchanged BEA (three-dimensional large-pore zeolitic framework) [6] and Mg(AI)O [7] suggests that the die effect is, in fact, not determining. On the contrary, the role played by the Pt nanoparticles is supported by the observation on basic PtBEA, that the higher the basicity of the zeolite, the higher the dispersion of Pt and the selectivity towards aromatics in the conversion of n-hexane [6]. The reason for the appearance of small Pt particles in the presence of a strongly basic support, however, is still uncertain. The high dispersion, in fact, depends upon the nature and the location of the Pt species formed along the different steps of the preparation. In the case of PdNaX, it was shown that the acido-base properties of the support influence the stabilisation of the metal cations during the calcination step prior to reduction [8]. This generates specific microenvironments at the internal surface of the zeolite that may affect the final size of the noble metal particles. Temperature Programmed Reduction (TPR) is one of the most commonly used techniques to i d e n t ~ the chemical nature and the location of the calcined Pt species in the porosity; it is often presented as a routine one. However, a survey of literature shows many discrepancies and/or uncertainties about peak assignments. For instance, peaks at 200-250~ in the TPR patterns of calcined PtKL were attributed to the reduction of either Pt09 [9] or Pt ~-+ions [10], and the attributions varied also in the case of PtBEA [11-12]. Moreover, phenomena such as He spillover were taken as responsible for additional H2 consumptions [13]. In the present paper, TPR is employed to study the nature of the Pt species supported on CsBEAs having increasing Cs loadings, and therefore increasing basicities. In view of the controversial literature data, and in order to improve the comprehension of the TPR profiles of Pt supported on basic zeolites [6], the reduction of the catalysts is followed simultaneously by TPR and mass spectrometry (MS). 2. EXPERIMENTAL

2.1. Catalyst preparation Two parent acidic HBEAs, with same Si/A1 ratios (13 • 1), were provided after template removal at 550~ in air (Enitecnologie and RIPP); they are henceforth abbreviated as H~EM and H~RwP. They were ion-exchanged in a 0.5 N aqueous solution of CsC1 (50~ under stirring for 2 hours), the procedure being repeated three times with the aim to fully exchange the protons by Cs cations (nominal Cs/A1 molar ratio of 1.0). The so-obtained CsBEAs (samples Cs-exch.~ENi and Cs-exch.~RiPP) were subsequently exchanged with appropriate amounts of a 2.5.10 .2 M [Pt(NH3)4]C12 aqueous solution (weight of Pt per weight of dry zeolite in the suspension of 1%). The basicity of the Pt-containing CsBEAs was further increased by wetness impregnation with appropriate solutions of CsOH giving samples with nominal Cs/A1 molar ratios of 1.2, 1.4 and 1.6. The samples were finally calcined at 300~ for 2 hours using a high air flow (800 ml/min) and a low heating rate (50~ in order to prevent the

361 autoreduction of the Pt precursor by the NH3 ligands, which would lead to the direct formation of large particles of metallic Pt with no interest un catalysis. They were subsequently stored in room atmosphere before characterisations. All calcined Pt-containing samples will be henceforth referred to as PtCSx~ENI and PtCsx~Ripp, where x is the Cs/A1 molar ratio determined by chemical analysis.

2.2. T e c h n i q u e s of c h a r a c t e r i s a t i o n The contents of Cs, A1, Si, and Pt were measured by atomic absorption (Central Analysis Service of the CNRS, France). The porosity was studied by N2 physisorption on a Micromeritics ASAP 2010 instrument after evacuation in vacuo at 300~ The micropore volumes were determined using the t-plot method. The quantitative TPR profiles were obtained on about 80 mg of sample by using a gas mixture of 5% Hz in At, a gas flow rate of 20 ml/min and a heating rate of 7.5~ Before TPR, the samples were dehydrated in situ in flowing Ar at 200~ for i h to remove most of the adsorbed water. The He consumption was measured by a differential thermal conductivity detector (TCD). In order to analyse the nature of the gases evolved during reduction, a mass spectrometer (Hiden Analytical) was coupled to the TPR apparatus. The mass spectrometer was connected at the exit of the reduction reactor in order to allow the analysis of all gases evolved (mass range 3-50). A desiccator was placed before the TCD in order to adsorb water and/or ammonia that would influence the TPR profiles. 3. R E S U L T S AND D I S C U S S I O N

3.1. I n f l u e n c e o f the s u p p o r t on Cs l o a d i n g For all Pt-containing samples, the Pt loading is between 1.0 to 1.1 wt %, showing that all the Pt(NH3)42+ ions present in solution were exchanged in the 2.0-

-

,i

"/

~= 1.5

.f-

i

i

O

1.0-

C

o

e 0.5-

0.0 0.0

0.4

0.8

1~2

1.6

2.0

Nominal Cs/AI molar ratio

Fig. 1. Experimental Cs loading as a function of the nominal one

362

E'

I-03

g

~)0

'

'

_ ,.~

4oo-

' ........

'

'

!

'

'

'

I

'

'

' 1

'

'

'

HPEN I

~" 300-

0

I

I

I

0.0

0.2

0.4

"

I

I

0.6

0.8

Relative Pressure [P/P~

I

1.0

Fig. 2. N~. physisorption isotherms of the two parent HBEAs zeolite, whatever the parent BEA. Moreover, the close Si/A1 ratios measured for all materials (from 12 to 14) suggest that none of the BEA supports was dealuminated during the exchange treatments. On the contrary, the parent BEA strongly influenced the Cs loadings, as shown by Fig. 1, on which the experimental Cs/A1 molar ratios are reported as a function of the nominal ones. For the PtCsx~RiPP series, the two values are close, indicating that the protons were fully exchanged by Cs § cations during exchange in solution, and confirming the presence of over-exchanged Cs after the subsequent impregnation treatments. At variance, only about haft of the starting protons were substituted by Cs § cations in the case of H~ENI, although the exchange procedure was the same as above. As a result, some Bronsted acidity is left in Cs-exchanged PtCso.4~ENI (nominal Cs/AI=I.0), in agreement with preliminary FT-IR results of the adsorption of NH3 (G. Martra et al., to be published.) The residual acidity is progressively removed by the increasing addition of Cs by impregnation, the experimental Cs/A1 molar ratio reaching a maximum value close to 1 (sample PtCs0.93~ENI, nominal Cs/A1 = 1.6). 3.2. I n f l u e n c e o f t h e p a r e n t B E A o n p o r o s i t y

The lower Cs-exchange capacity of H~ENIis most probably related to a lower crystallinity of this sample, as reflected by broader and less intense X-ray diffraction peaks (not shown) and illustrated by the different shapes of the N2 physisorption isotherms for the two parent HBEAs (Fig. 2). Indeed, although both zeolites have rather similar micropore volumes (0.20 and 0.24 cm3-g"1 for H~ENI and H~RIPP, respectively), the steeper slope of the isotherm for H~ENI reveals additional physisorption of nitrogen by meso- and macroporous surfaces, which is in line with the larger external surface area expected for the much smaller crystallites of H~ENIthan those of H~RIPP(sizes below 80 nm and between 200 to 500 nm, respectively).

363

{

0.3

,

.

.

.

,

.

.

.

,

.

.

.

,

O.2-

0.~

-

PtCsxp~

I

0

o.o

I

0.0

'

,

'

!

,

'

0.4 Experimental

,

I

,

0.8 Cs/AI

molar

,

,

I

t .2 ratio

Fig. 3. Micropore volumes for the two BEA series as a function of the Cs loading.

For the two BEA supports, the addition of Cs has the effect of decreasing progressively the micropore volume, which becomes nearly half of the original one for the highest Cs loadings (Fig. 3). The micropore volume thus reaches a value as low as 0.10 ml/g in the case of PtCs0.93~ENI (nominal Cs/AI molar ratio of 1.6) which puts a limit to a further addition of Cs since the active internal surface of the zeolite would become to small for catalytic applications. 3.3. I n f l u e n c e o f t h e Cs l o a d i n g on r e d u c i b i l i t y The TPR profiles of the two series of samples are shown in Fig. 4 and Fig. 5, together with the MS signals of masses 15 and 18 that are representative of the mare MS phenomena occurring during reduction. Masses 16 and 17 (not shown) were also observed, with profiles almost identical to those of masses 15 and 18, respectively. In addition, very weak peaks were detected in the 28-30 mass range, but they are not discussed due to their very low signal to noise ratio. In the case of the Cs-exch.~ENi support, the consumption of I-I2 is barely identified, as expected from the absence of Pt in this sample (Fig. 5). However, a broad TPR signal with low intensity is detected for Pt-free Cs-exch.BRmP, revealing a non-negligible H2 consumption in the temperature range 250-600~ (Fig. 4). Interestingly, this signal is still identified when Pt is added (PtCso.85~RmP), and an additional broad TPR peak appears at low temperature (100-250~ related to a reduction of the platinum species. The Pt-reduction peak is slightly shifted to higher temperatures for Cs-richer PtCs126~RIPP and PtCsl.42~RIPP (maximum at about 250~ the main effect of the higher Cs loading being a dramatic increase of the intensity. For the Cs over-exchanged samples, a H2 consumption still takes place at high temperatures (400-600~ but its contribution is secondary compared to the total reduction process. The influence of the Cs loading on the shape of the TPR patterns is also well shown by the behaviour of the PtCSx~ENI series (Fig. 5). PtCs0.42~ENI, which has the lowest Cs loading, exhibits a single broad peak centred at 380~ Its shape

364

and position are rather different from those observed for the previous samples, but they are similar to those reported by Lee et al. [14] for Pt supported on HBEA, in line with the residual Bronsted acidity left in PtCso.42~ENI. These authors attributed this peak to a reduction of Pt ~+ ions. However, the high H9 consumption in our case (molar ratio H/Pt = 4.1) would rather suggest the reduction of Pt 4§ On increasing the Cs loading (samples PtCs0.63~ENI to PtCs0.93~ENI), the peak at 400~ strongly decreases in intensity whereas a peak similar to that observed for PtCso.85~RiPP with comparable Cs loading (temperature range 100-250~ becomes dominant. Simultaneously, the He consumption first decreases and then increases again (total molar H/Pt = 1.9, 2.4 and 4.3, on going from PtCs0.63~ENI to PtCs0.93~ENI). Such total H/Pt values, which could agree with the reduction of Pt ~+ and/or Pt 4§ species, are significantly lower than those measured for Cs over-exchanged PtCsl.26~RIPP and PtCs1.42~RIPP (total molar H/Pt = 6.6 and 7.1, respectively). 3.4. N a t u r e o f the r e d u c i b l e s p e c i e s The abnormally high H/Pt molar ratios (higher than 4) for the Cs overexchanged samples cannot be explained by assuming the simple reduction of Pt 2§ or Pt 4§ Therefore, it must involve the contemporaneous reduction of non platinic species, as was proposed in a previous paper [6]. For the two samples, the deconvolution of the TPR profiles shows, on the one hand, that the dominant H2 consumption (intense peak centred at 200~ actually corresponds to a H/Pt ratio slightly larger than 4, in line with the observation of PtO2 by EXAFS (P. Massiani et al., to be published). On the other hand, the residual consumption

~mass

15

........... mass 18

20 A PtCSl.42~'pJpp

"~15 I o E

PtCsl.~l~R~op CO C

C 0

~0

r =

5

-r"

0

m

09 PtCs o ~13.~p 9 0

200

400

Temperature[~

600

800

I [-

0

l

200

l

400

l

600

. , 800

T e m p e r a t u r e [~

Fig. 4. TPR (left) and MS profiles (right) for the samples prepared with H~RIPP

365

occurring above 250~ (molar H/Pt close to 2) is analogous to that measured for Pt-free Cs-exch.~RiPP, suggesting reduction processes in which Pt is not involved. The hypothesis that species other than Pt are reduced during TPR is supported by the MS data. Indeed, an evolution of mass 15 (same for mass 16) is observed for all samples between 250 ~ and 600~ and both the intensity and the shape of the mass 15's signals parallel those of the TPR peaks in this temperature range, as especially well illustrated for C s - e x c h . ~ R i p p (broad signal between 250 ~ and 600~ Fig. 4) and PtCs0.42~ENI (strong TPR and MS signals centred at 400~ Fig. 5). On the contrary, the mass 15's patterns significantly differ from those of mass 18 and must correspond to different phenomena. The parallel observation of masses 15 and 16 could be due to the desorption of ammonia. However, the fact that they are not associated to a parallel evolution of mass 17 (same as mass 18) indicates that the formers must rather be assigned to an evolution of methane, which most probably results from the reductive decomposition of carbonates, possibly formed on the supports by exposition to air. This conclusion is stromgly supported by the He consumption found on Pt-free Cs-exch.~RiPP, Oil which ammonia cannot be present (initial calcination at 550~ no exchange by Pt(NH3)4~+). In this hypothesis, however, the good correlation between the TPR and mass 15 profiles as well as the high intensity of the peaks for partially acidic PtCs0.42~ENI is rather unexpected, since carbonates should better form on the most basic supports. However, other effects such as steric hindrance and size of the crystallites may also play a role in the formation of these compounds. Work is currently being performed to confirm our hypothesis. The association of masses 18 and 17 is typical of the evolution of water, resulting either from reduction phenomena or from the direct desorption of water. For the two Cs-exchanged supports, two peaks of mass 18 are observed. The first one, located below 200~ cannot correspond to a reduction, since no corresponding TPR signal is observed. Most probably, it reveals the presence of

20 el)

PtCso.~l~

0")

~15 -'r" ,..,, O

5

PtCso.721~E.t

E

,~ ~0

PtCso 631~

.m

O

03

09

PtCso.421~F_~

ffJ e'O

mass 15 Cs-exch. PEN~ .......... mass 18

fro

' ' 6 o " 80o Temperature [~

o""'2oo""4oo

6oo"" '" "8oo

....

Temperature ['C]

Fig. 5 TPR (left) and MS profiles (right) for the samples prepared with H~ENI

366 residual water in the Ar used for the pre-treatment, which could have been reabsorbed onto the support during the cooling prior to reduction. Such a reabsorption cannot be excluded for the other samples, for which a similar peak is always observed. The attribution of the second peak of mass 18 (250-600~ is more complex, since it can reveal reduction processes, but could also be partly related to the dehydroxilation of the zeolitic support. The latter effect is expected to occur especially at temperatures higher t h a n 400~ at which a weak but regular production of water is observed for all samples.

4. C O N C L U S I O N S The TPR profiles of calcined PtCsBEAs strongly depend on the Cs loading, which is itself influenced by the zeolite morphology. Moreover, these profiles are affected by the possible reductive decomposition of carbonate species, as suggested by the MS observation of mass 15, attributable to the evolution of CH4 during the reduction of both Pt-free and Pt-containing samples. As a consequence, the TPR data should not be quantitatively interpreted on the basis of the reduction of solely platinum species, but additional consumptions of H2 occurring in a large temperature range have to be taken into account.

REFERENCES

1. P.A. Jacobs, Stud. Surf. Sci. Catal., 12 (1982) 71. 2. D. Barthomeuf, Catal. Rev. Sci. Eng., 38 (1996) 521. 3. J . R . Bernard, in "Proc. 5th Int. Zeol. Conf., Naples" (L.V.C. Rees, Ed.), Heyden, London (1980) 686. 4. R.J. Davis, Heterog. Chem. Rev., 1 (1994) 41. 5. P. Meriaudeau, C. Naccache, Catal. Rev. Sci. Eng., 39 (1997) 5. 6. T. Becue, F. J. Maldonado, A. P. Antunes, J. M. Silva, M. F. Ribeiro, P. Massiani, M. Kermarec, J. Catal. 181 (1999) 244. R. J. Davis, and E. G. Derouane, J. Catal., 132 (1991) 269. 8. A. Sauvage, M. Oberson de Souza, M. J. Peltre, P. Massiani, D. Barthomeuf, , J. Chem. Soc., Chem. Commun. (1996) 1325. D. J. Ostgard, L. Kustov, K. R. Poeppelmeier, W. M. H. Sachtler, J. Catal. 133 (1992) 342. 10. J. Zheng, J. Dong, Q. Xu, C. Hu, Catal. Lett. 37 (1996) 25. 11. L. W. Ho, C. P. Hwang, J. F. Lee, I. Wang, C. T. Yeh, J. Mol. Catal. A: Chemical, 136 (1998) 293. 12. A. B. da Silva, E. Jordao, M. J. Mendes, P. Fouilloux, Appl. Catal. A: General, 148 (1997) 253. 13. G. Martra, S. Coluccia, P. Davit, E. Gianotti, L. Marchese, H. Tsuji, H. Hattori, Res. Chem. Intermed., 25 (1999) 77. 14. J. K. Lee, H. K., Rhee, Catal. Today, 19 (1997) 235. .

.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

367

N e w evidences for the fluoride contribution in synthesis of gallium phosphates V. I. P~rvulescu a, C. M. Visinescu a, M. H. Zahedi-Niaki b and S. Kaliaguine b aUniversity of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: [email protected] bDepartment of Chemical Engineering Laval University, Quebec (Quebec), Canada, G1K 7P4 Gallium phosphates were prepared by hydrothermal synthesis under autogeneous pressure. Gel compositions with formulas Ga203. x P2Os.y HF.z Py(Pr3N).70 1-120 with x-- 0.5 till 1; y= 1 till 5, z-- 1.7 till 3.4 for Py and 1.5 till 3 for Pr3N were obtained under vigorous stirring by successive dissolution in a phosphoric acid/water solution of Ga203, hydrofluoric acid and template. These materials were characterized by XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTA, F T I ~ NH3-DRWT, XPS, SEM, MAS-NMR. The increase of the F/Ga gel composition ratios was found to enhance the crystallinity of the samples and, in certain limits, to improve the thermal stability. 1. I N T R O D U C T I O N Open-framework materials such as aluminum- and aluminum/gallium phosphates raise an increasing interest because of their potential applications as catalysts and of their specific structural architectures. Since the discovery of microporous aluminophosphates synthesized by using organic amines or quaternary ammonium cations as templates [1] and of microporous gallophosphates [2-4], numerous phosphates-based crystalline materials have been reported [5]. The phosphates have provided new microporous frameworks, since the constituent metal polyhedra are not limited to tetrahedra like the aluminosilicate zeolites [6]. Guth et al. [7] were the first who developed the fluoride synthesis method. The use of this method in combination with the ability of gallium to adopt various coordinations, namely, 4, 5 or 6, allowed the synthesis of new structures like cloverite, ULM, ZON, M ~ , MU etc. [8-12]. The addition of fluoride ions in the synthesis medium was reported to favor mineralization, allowing rapid assimilation of other reagents into a soluble, highly reactive form [13]. Fluorine remains incorporated in the framework. The diversity of these new materials also results from the nature of the organic molecules used as ligands (1,3-diaminopropane, 1,4-diaminobutane, methylamine [13 ], 1-methylimidazole [14], cyclohexylamine [ 15], 1,6-diaminohexane [16], pyridine [ 17], N, N, N', N'-tetramethylethylenediamine [18] etc.). From the studies published to date about the synthesis of these gallium phosphates the complexity of these syntheses emerges. Several factors including temperature, reaction time, reagents and solvents percentage of vessel filling could exert a crucial effect on the product characteristics. Gallium phosphates exhibit however only a poor thermal stability. In spite of the

368 numerous examples of synthesized structures, very scarce information was reported about the effect of the gel fluorine content on the thermal stability of the products. The aim of this contribution is to provide additional information on the role of fluorine in the synthesis of gallium phosphates. For such a purpose, gallium phosphates were prepared using various gel compositions, and Pr3N and Py as templates. These were characterized by XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTA, FTIR, NH3-DRIFT, XPS, MAS-NMR. 2. EXPERIMENTAL Gallium phosphates were prepared by hydrothermal synthesis under autogeneous pressure. Ga203 (Alfa AESAR, 99%+), phosphoric acid (85% H3PO4, BDH), hydrofluoric acid (40%, BDH), pyridine (Py) and tripropyl amine (Pr3N) both from Aldrich were used as reactants. Gel compositions with formulas Ga203. x P2Os.y HF.z Py(Pr3N).70 H20 with x-- 0.5 till 1; y= 1 till 5, z = 1.7 till 3.4 for Py and 1.5 till 3 for Pr3N were obtained under vigorous stirring by successive dissolution in a phosphoric acid/water solution of Ga203, hydrofluoric acid and template. Each dissolving step was controlled by measuring the pH value. After 24h stirring the gel was placed without stirring in a teflon lined autoclave, heated at 445 K for 24 h, and then cooled instantaneously to room temperature. After several washings, the material was dried and analyzed using several techniques: XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTADSC, FTIR, NH3-DRIFT, XPS, MAS NMR. The same analyses were also made for catalysts calcined at 623 K. Crystallinity of the prepared samples was evidenced by X-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer with CuKa radiation source and scintillation counter. N2 adsorption-desorption isotherms were recorded at 77 K, in the relative pressure range 104 - 0.99, with a Micromeritics ASAP 2000 equipment, after the samples were degassed at 323 K for 12 h, at 10 -4 Pa. TG-DTA-DSC curves were collected using a SETARAM 92 16.18 equipment. In situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were collected using a Bruker IFS88 infrared spectrometer with KBr optics and a DTGS detector. Pure samples were placed inside a commercial environmental chamber (Spectra-Tech 0030-103) attached to a diffuse reflectance accessory (Spectra-Tech collector). To investigate the stability of the adsorbed ammonia species during temperature elevation, the spectra were recorded under helium (30ml/min) at room temperature, 373,523 and 573 K, after exposure to flowing ammonia for 30 min at room temperature. Infrared Fourier Transform spectra were recorded with the same apparatus. Spectra in the lattice vibrations range were recorded for wafers of sample mixed with KBr in a Pyrex IR cell, with KBr windows. The XPS spectra were obtained using a SSI X probe FISONS spectrometer (SSX -100/ 206) with monochromated Al-Kot radiation. The spectrometer energy scale was calibrated using the Au 4f7/2 peak (binding energy 84.0 eV). For the calculation ofthe binding energies, the Cls peak of the C-(C, H) component at 284.8 eV was used as an internal standard. The composite peaks were decomposed by a fitting routine included in the ESCA 8,3 D software. The superficial composition of the investigated samples was determined using the same software. The Ga, P, Fls and NI~ peaks were investigated.

369 Table 1. Textural characteristics of the investigated samples Sample Gel composition

VP14 VP16 VP17 VP 18 VP 19 VP20 VP 15

Surface area, m 2 g-1 as-synthesized, calcined degassed at 323 K 9 4 12 6 14 7 14 9 14 11 12 9 8 3

Ga203. P205.1.5 HF.1.7 Py.70 H20 Ga203. P205.2 HF.1.7 Py.70 H20 Ga203. P205.3 HF. 1.7 Py.70 H20 Ga203. P205.4 HF. 1.7 Py.70 1-t20 Ga203. P205.5 HF. 1.7 Py.70 H20 Ga203. P205.5 HF.3.4 Py.70 H20 Ga203. P205.1.5 HF. 1.5 Pr3N.70 H20

The 3Zp and ~3C NMR analyses were conducted at room temperature using a Bruker ASX-300 spectrometer. The chemical shifts were referenced to ml(NO3)3 solution in water and adamantane for 31p and 13C, respectively. Spinning rate was 10 KHz and 31p MAS NMR spectra were recorded with both 1H decoupling and CP MAS method. A 90 degree pulse with 4.8 ms duration was used for both 3Zp and ~3C NMR experiments. 3. RESULTS 3. 1. Textural characteristics

Table 1 compiles the surface areas of as-synthesized gallophosphates and calcined samples. All the as-synthesized samples after degassing at 323 K exhibit a very low surface area. The calcination at 623 K led to an important decrease of the surface area, which except for the samples prepared with a Ga203/HF ratio of 1:5, was about 50%. For the samples VP18-VP20, the loss in the surface area was less than 30%.

VP 16

sample

"'|"

15

20

25

211~a

3O

35

4O

45

5O

692

|

J

|

|

w

690

i

w

w

|

w

688

|

|

w

!

w

686

|

|

w

!

~

684

w

|

i

!

w

682

Binding energy, eV

Fig. 1. XRD pattems of as-synthesized samples Fig.2. XPS spectra in the region of Fls energies

w

|

|

680

370 3.2. XRD Fig. 1 shows the X R pattems of two as-synthesized gallophosphates VP-16 and VP-18 samples. The same peaks were detected atter calcining at 625K, but under these conditions their intensity was decreased. The increase of the F/Ga gel composition ratios was found to enhance the crystallinity of these samples and, in certain limits, to improve the thermal stability as was appreciated from the modification in the intensity of the main peaks. A similar XRD pattern was previously reported by Schott-Darie et al. [14] as a novel structure without giving any assignment. These authors also used Py as template agent. After calcining at 773 K, the peaks in the range 2theta 5-10 disappeared, the resulted material being a mixture of amorphous and non-porous crystallized phases. 3.3. XPS XPS analysis (Fig. 2) suggested a different superficial packing of the materials as a function of the gel composition and the thermal treatment (Table 1). This become very evident for the calcined samples where the chemical composition may suggest a surface enrichment in F, while the XPS spectra recorded in the region of Fls binding energies clearly show the disappearance of the species binded to the N containing organic molecule. The two bands in the region Of Fls binding energies may suggest two types of fluorine as Loiseau and Ferey [16] already proposed: one corresponding to the CJa framework, which is strongly bound, and another corresponding to fluorine bound to N species. Fig. 2 show that the species corresponding to the framework F remained unchanged. XPS analysis suggest a different location of fluorine comparing to data reported in the literature [16]. Higher C_ra/F and Ga/N ratios than chemical ones indicate that these elements are not surface species. On the contrary, for P, the chemical and XPS ratios are in a very good concordance. Analysis of the Ga2p3 species gave additional information about the transformations occurring during calcination. In non-calcined microporous phosphates gallium, as Ga2p3, was found to be located at 1116.5 eV. After calcining, a second species was appeared at around 1118.7 eV. The relative ratios of these bands depended on the calcination temperature, but also on the fluorine content in the gel composition. Table 2. Comparative chemical and XPS atomic ratios Sample Chemical analysis of fresh samples C_ra/P Cra/F C_rafN VP14 1.0 1.33 118 VP16 10 1.00 118 VP16 ~ 10 1.00 118 VP17 ~ 10 0.66 118 VP18 ~ 10 0.50 118 VP19 1.0 0.40 118 VP20 1.0 0.40 0.59 VP15 1.0 1.33 1.33 c: calcined

Ga/P 0.96 1.11 1.14 1.03 0.97 0.92 0.87 1.30

XPS analysis Gaff 4.26 3.95 15.68 21.51 35.46 3.95 3.84 13.08

Cra/N 0.57 0.43 0.43 0.41 0.43 0.51 0.56 0.52

371

3.4. TG-DTA-DSC analysis The thermograms exhibited several sequences: two exothermic below 553 K, attributed to water desorption and template degradation, and other three after 553 K up to 993 K due to oxidation and burning of the template and to structure rearrangements caused by the template elimination. 3.5. MAS NMR

31p MAS NMR spectra of both as-synthesized form of gallium phosphate VP16 and VP18 samples are presented in Fig. 3. Two separate peaks resulted at-10.7 and -4.5 ppm These two resonance peaks that are assigned to tetrahedral phosphorous in different crystallografic positions were observed in other gallium phosphates [15]. The calcined form of both these two samples has provided essentially the same 31p NMR spectra with a single broad peak at-10.7 ppm This suggests that organic template molecules are interacting in different manner with lattice phosphorous atoms compared to water molecules. The ~3C MAS NMR spectra of both assynthesized gallium phosphates samples are identical (Fig. 4). These show two resonance peaks at 145.7 and 127.9 ppm, which correspond to Py. The 13C MAS NMR results show that Py molecules are more likely protonated and immobile in GAPO's structures. These two peaks were completely vanished after removing of template by calcination. 3.6. NH3-DRIFT Fig 5 shows typical NH3-DRIFT spectra for gallium phosphates. DRIFT lattice vibration spectra of these samples provided evidence for the structure of the framework of these materials. The main peaks in the range 500-1300 cm -1 correspond to the vibrations ofthe gallium phosphate framework, to the bending of the TO 4 tetrahedra, and to the symmetric and asymmetric stretching vibrations of the T-O-T linkages, respectively. The bands in the range 1400-1600 cm 1 are assigned to various N species. These may correspond to the different bonds in which is involved pyridinic N A large band due to OH and NH species is also present in the range 29003600 cm -"

I

,
100

.

50

J,,

ppm

.

0

.

-50

-1 O0

Fig. 3.31p CP MAS NMR spectra ofVP 16 andVP 18

200

,

.

.

180

160

. ,

.......

9

140PP#lr1120

.

.,,

100

Fig. 4.13C CP MAS NMR spectra of VP 16 and VP 18

372

i" .....

.I

i

. .................. 9 . .....

t ........

;J

.---------

.... ....

373 K, NH3/."t !L :.;..,~-,,,",;-"",;.~,~'~ ".........................

" .....

.', t i

,,"

' ",.

..2'.""

273 K, NH3 ..."~..........1",. ,,""" .ix,,."!

--J-........ / ....... "...... 273 K, He

x

,~,,""I,,'

.,,,.'%c..:.,

.........,.................

3500

2500

',,--"

i

1500 500 Wavenumbers, cm l

Fig. 5. NH3-DRWT spectra for VP16

................. i

373 K, NH3 /"~ ..../:-.. ;'~ "/..-._.,"-"~ .....

........................

"

,

/'"

..../ ...................... 273 K, NH3 ' ........ /, ....... .................. 273 K He

......

/

3500

..............

--~?-

2500

.........

. . . . . . . . ""

/

" /.,~,~,

/'

"

1500 500 Wavenumbers, cm 1

Fig. 6. NH3-DRIFT spectra for calcined VP16

The introduction of ammonia after degassing the samples at 423 K does not modify these spectra (Fig. 5). No bands due to either Bronsted or Lewis acid sites were detected, irrespective of the fluorine content. This is a clear indication that these materials exhibit no acidity. The calcination at 623 K of these catalysts changes the shape of the spectra in the region of lattice vibration, as a direct consequence of the changes occurred in the structure of the gallium phosphates. But even under these conditions no acidity has been evidenced (Fig. 6). 4. DISCUSSIONS The preparation of gallium phosphates using Py or Pr3N as templates following the fluorine route led to the synthesis of materials with similar XRD patterns (see Fig. 1). The surface area of these materials is very small suggesting that these are not tridimensional structures, but more probably bidimensional open structures. The stability of these phosphates is low. Calcining at 623 K generated an important loss in surface area, even though the original values were already very small. This process also occurrs with an important loss in the crystallinity. However, at'ter calcination at 773 K, the microporous structure evidenced by the patterns given in Fig. 1 was completely removed, these materials crystallizing in non-porous materials, with a surface area corresponding to the geometrical one. During this process part of material passed in an amorphous phase. These transformations were confirmed from TG-DTA analysis. The change of the fluorine to gallium ratio in the gel composition, indicated that the role of fluorine is more complex than to provide a simple advanced dissolution ofthe reactants. More fluorine in the composition gel was consistent with an increase of the crystallinity, as it was characterized from the relative intensity of the peaks located at 2 theta smaller than 10. Previous work of Loiseau and Ferey [16] proposed a model in which part of fluorine is bound directly to gallium and another to template. The XPS analysis we made on these samples confirmed such an interaction. Fluorine bound on gallium is stable, and remains in this position after calcination too. On the contrary, the same analysis evidenced that the fluorine bound on the template is eliminated during the calcination and its replacement parallels the drop in the crystallinity. Such a behavior comes to suggest that the mechanism involved in the construction of these microstructures is not a templating mechanism but an scaffolding one. The partial removal of Py or Pr3N was consistent with a loss of crystalinity, and complete removal with a total damage of the microporous texture.

373 However, the process occurring during the thermal treatment is more complex. XPS analysis evidenced that at high temperature some changes also occurred in the gallium structure. The calcination caused the formation of a new species, which was evidenced in the XPS spectra by a new band located at cca. 1118.7 eV. The difference of about 2 eV between the two gallium species is rather high. It is difficult to assign this new species to a different coordination, but also it is very difficult to assign it to a more oxidized state. The effect of a high fluorine/gallium ratio to the crystallinity and stability of these materials is also complicate to be explained. It is only possible to speculate that the interaction of fluorine with the template generated fluorinated species, which are more difficult to be decomposed than the pure templates. At the same time, to explain the improved crystallinity we may speculate that the polarity of the fluorinated templates may better order the gallium phosphate structures. 5.CONCLUSIONS

In conclusion all these data suggest a scaffolding mechanism more than a template one. The removal of the organic molecule causes a collapse, which leads to a decrease of the crystallinity and also of the surface area. Part of fluorine leaves the material during this process. Another part of fluorine is inserted in the framework, and the increase of the F/Ga ratio till around 4 is consistent with an increase in the stability ofthese materials. This seems to be caused by an increase of the amount of fluorine that is incorporated in the material framework. The use of pyridine or tripropyl amine did dot influenced the thermal stability, but only the structure of the resulted gallium phosphates.

REFERENCES

1. S. T. Wilson, B. M. Lock, C. A. Messina, T. R. Cannan and E. M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. 2. J.B. Parise, J. Chem. Soc., Chem. Commun., (1985) 606. 3. S.T. Wilson, N. A. Woodward, E. M. Flanigen and H. G. Eggert, Eur. Patent Appl. 0226219, 1987. 4. G. Yang, S. Feng and R. Xu, J. Chem. Soc., Chem. Commun., (1987) 1254. 5. A.K. Cheetham, G. Ferey and T. Loiseau, Angew. Chem. Int. Ed. 38 (1999) 3268. 6. M.T. Weller and S. E. Dann, Curr. Opin. Solid. State Mater. Sci. 3 (1998) 137. 7. J.L. Guth, H. Kessler and R. Wey, Stud. Surf. Sci. Catal. 28 (1986) 121. 8. M. Esterman, L. B. McCusker, Ch. Baelocher, A. Merrouche and H. Kessler, Nature, 352 (1991) 320, G. Ferey, J. Fluorine Chem., 72 (1995) 187. 9. F. Serpaggi, T. Loiseau, F. Taulelle and G. Ferey, Microp. Mesop. Mater. 20 (1998) 197. 10. S. J. Weigel, R. E. Morris, G. D. Stucky and A. K. Chetham, J. Mater. Chem. 8 (1998) 1607. 11. T. Wessels, L. B. McCusker, Ch. Baerlocher, P. Reinert and J. Patarin, Microp. Mesop. Mater. 23 (1998) 67. 12. P. Reinert, J. Patarin and B. Marler, Eur. J. Solid State Inorg. Chem. 35 (1998) 389. 13. R. I. Walton, T. Loiseau, D. O'Hare and G. Ferey, Chem. Mater. 11 (1999) 3201. 14. C. Schott-Darie, H. Kessler, M. Soulard, V. Gramlich and E. Benazzi, Stud. Surf. Sci. Catal. 84 (1994) 101. 15. T. Loiseau, D. Riou, F. Taulelle and G. Ferey, Stud. Surf. Sci. Catal. 84 (1994) 3 95.

374 16. T. Loiseau and G. Ferey, J. Mater. Chem. 6 (1996) 1073. 17. D. S. Wragg and R. E. Morris, J. Am. Chem. Soc. 122 (2000) 11246. 18. A. M. Chippindale, R. I. Walton and C. Turner, J. Chem. Soc. Chem Commun. (1995) 1261

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

375

High-field E S R spectroscopy o f Cu(I)-NO complexes in zeolite C u Z S M - 5 Andreas P6ppl a) and Martin Hartmannb)* a) Fakult~it f'tir Physik und Geowissenschaften, Universit~it Leipzig, D-04157 Leipzig, Germany b) Department of Chemistry, Chemical Technology, University of Kaiserslautern, P. O. Box 3049, D-67653 Kaiserslautern, Germany ESR spectroscopy using nitric monoxide as probe molecule is very suitable for characterizing Cu(I) cations supported on zeolites. These studies are especially useful if different ESR resonance frequencies (X-band, Q-band, W-band) are applied. In particular, it was possible to perform W-band ESR measurements of NO interacting with Cu(I) centers supported on zeolites for the first time, which require a special sample preparation under high vacuum conditions. In the W-band ESR spectra complete separation of the Cu(I)-NO signal from the signal of the residual Cu(II) cations was achieved allowing unambiguous assignment of the signals and determination of the spin hamiltonian parameters. 1. INTRODUCTION Despite its thermodynamic instability, nitric oxide is kinetically inert with respect to its decomposition into N2 and O2, which requires a catalyst. Copper ions dispersed at the surface of metal oxides or within the cavities of zeolites have shown a surprising activity for the direct decomposition of NO. It has been shown that CuZSM-5 catalyzes the thermal composition of NO [ 1-4], but the individual steps and the exact nature of the species involved are still a matter of debate. However there is some agreement on Cu(I) as the catalytically active center [2,3,5]. The high catalytic activity of copper-containing ZSM-5 zeolites in the NO decomposition and the desire to understand the mechanism of this catalytic transformation triggered several spectroscopic studies aiming at the structure of the Cu(I)-NO complexes. Several monomer (Cu+-NO, Cu+-NO+) and dimer (Cu+-(NO) +, Cu+(NO)2, Cu+(NO)z+) Cu(I)-NO complexes have been identified by IR spectroscopy [5-8] and quantumchemical calculations [9-12]. However, the detailed structure of these complexes is largely unknown. ESR spectroscopy would allow direct access to the symmetry and structure of these complexes. The formation of a monomer Cu(I)-NO complex in zeolites ZSM-5 [8,13,14] and Y [15,16] is evident from their respective cw-ESR spectra at X-band (9.4 GHz). However, spectral resolution does not allow the extraction of the tensor components with sufficient accuracy to obtain structural information. Analysis of the X-band spectra of the Cu(I)-NO complexes is furthermore hampered by the superimposition with the signals of residual Cu2+. In the present study, Cu(I)-NO complexes have also been studied at Q-band (33.2 GHz) and W-band (94 GHz). The increased spectral resolution and the separation of the Cu(I)-NO signals from the Cu2+ signal will allow a detailed structure determination of the formed complexes.

376 2. E X P E R I M E N T A L S E C T I O N

Zeolite ZSM-5 was synthesized with a nsi/nAl ratio of 17 as described previously [17], calcined and ion-exchanged with C u ( C H 3 C O O ) 2 . Prior to the adsorption of NO, the samples were activated at 400 ~ under vacuum (p < 3.10 .7 Pa) in a 100 % steel vacuum line. This prevents NO from reacting with small amounts of residual oxygen, which was, from our point of view, the most critical part in sample preparation. Subsequently, the samples were cooled to 25 ~ and exposed to a defined quantity of NO (defined gas volume of ca. 4 ml and pressures of PNO = 0.5 mbar for W-, Q- and X-band samples, respectively). Thereafter, the samples were cooled to liquid nitrogen temperature and sealed off. It is worth mentioning that the Cu(I)-NO adduct is unstable at room temperature and fades away with time. The ESR spectra were recorded at X-band (Vmv = 9.366 GHz), Q-band (Vmw = 33.0315 GHz) and Wband (Vmw = 93.9 GHz) at temperatures of 77 K (X-band) and 10 K (Q-band, W-band), respectively. The X- and Q-band measurements were carried out on the Bruker ESP 380 and EMX EPR spectrometers, respectively. The W-band experiments were performed using the Bruker E680 system. The magnetic flux density B0 of the magnet coils used for the X- and Qband ESR was calibrated by 1H NMR measurements. A LiF standard with gLiF = 2.002288(4) was used for the B0 calibration of the cryomagnet (W-band). All ESR measurements were made with the same B0 modulation amplitude of Bmoa ~ 0.4 mT and a modulation frequency of Vmoa = 100 kHz. The microwave power for the X- band was Pmw "~ 1.0 mW, for Q- band frequency ~ 0.1 mW and about 5 910-4 mW at 94 GHz (W-band). The microwave power is low enough to prevent saturation of the spin systems.

3. R E S U L T S AND D I S C U S S I O N

Figure l a shows the X-band ESR spectrum of CuZSM-5 after activation at 400 ~ in vacuum and adsorption of 0.5 mbar of NO at 25 ~ Although autoreduction of Cu(II) to Cu(I) takes place to a large extent, the signal of Cu(II) is still clearly visible. This signal is superimposed by the spectra of the Cu(I)-NO complexes and, therefore, an exact determination of the spin hamiltonian parameters of these complex is hardly possible. In the Q-band spectrum (Figures 1b and 2), the Cu(II) signal is almost separated from the signals of two different Cu(I)-NO complexes. Finally, at W-band frequency, complete separation between the Cu(II) signal and the signals of the Cu(I)-NO complexes is achieved (Figure 1c). In the W-band spectrum, the 63Cu (I = 3/2) hyperfine splitting in four lines is clearly visible in the gzz as well as in the gxx/yyarea. This confirms the formation of the Cu(I)-NO complexes. The spectral parameters of the dominant species A have been obtained by simulation of the spectra and are summarized in Table 1. Comparable g-tensor parameters (gxx = 1.999, gyy = 2.003, gzz = 1.889) have been reported by Sojka et al. [13] as well as by Park et al. [14] (gll = 1.92, gj_ = 2.012) from their experiments at X-band frequency. The Euler angle 13, which is the angle between the z-axes of the Cu hyperfine tensor and the g-tensor, was estimated to 35 ~ It was not possible to calculate the Euler angles o~ and Y from the powder ESR spectra. Surprisingly, the 14N (I = 1) hyperfine splitting was not observed for a loading of 1 NO molecule per 40 unit cells. A decrease of the NO loading to 1 molecule per 400 unit cells revealed a nitrogen hyperfine splitting of ANyy = 29 910-4 cm -1 in the respective Q-band spectrum (Figure 2). This confirms clearly that the unpaired electron is localized in the roy* NO molecular orbital with a probability of 50 %. Furthermore, the Q-band

377

c I

,

...... " ~i ~. .~i ! ~

~'-,""

..g k: ~./: :=.

...

Ill . l l l l

]

--

~

~

--

_ _ l

l

I

Z

--

L-

~. S

b

aJ ~

,

-100

0

I

,

100

I

200

,

I

,

I

300

B-Bo (mT) -) and calculated ( ...... ) ESR spectra of CuZSM-5 (1 NO per 40 Fig. 1" Experimental ( unit cells) at a) X-band, b) Q-band and c) W-band frequency.

378

AN AN

rhrh

x5

I I

I

I

I

I

cu

I

IACUz

gB

zz

I A~y~

g xx/yy I

1150

,

I

1200

,

I

1250

,

I

1300

,,

,,,

,,

I

1350

g (mm) Fig. 2: Q-band ESR spectrum of CuZSM-5 (1 NO molecule per 400 unit cells) at 10 K. spectrum shows the existence of a second Cu(I)-NO complex (species B) with slightly different g-tensor parameters (Table 1). Two different Cu(I)-NO complexes are also suggested from recent quantum-chemical calculations by Nachtigall et al. [18,19]: In type I sites the Cu(I) ions are coordinated to two oxygen atoms of a single A104 tetrahedron located at the interaction of two channels. In these sites, the Cu(I) ions reside in the open space at the channel intersection. In the type II sites, the Cu(I) ions are coordinated to three or four oxygen atoms of the six (or five) membered aluminosilicate rings forming the channel walls [19]. From the combined simulation of the X-, Q- and W-band spectra, an axially symmetric gtensor is derived for the Cu(I)-NO complex (species A), which in first approximation would suggest a linear Cu(I)-NO complex. However, from a number of reasons a linear complex is not a valid approximation: 1) The tensors Acu and g are not coaxial, which implies a bend complex. 2) Admixtures of the Cu 4s orbitals to the molecular orbital of the unpaired electron with substantial roy* contributions are for symmetry reasons not possible for a linear complex.

379 Table 1 ESR parameters for Cu(I)-NO species supported on ZSM-5 in comparison with Na(I)-NO species supported on zeolites A and ZSM-5 [20]. Species

Cu(I)-NO

Cu(I)-NO Na(I)-NO

Na(I)-NO

A

B

Support

ZSM-5

ZSM-5

ZSM-5

NaA

gxx

2.005

2.005

1.994

1.999

gyy

2.005

2.005

1.991

1.994

gzz

1.891

1.917

1.846

1.884

ACUxx / (10 -4 cm -1)

158

158

-

-

ACUyy/ ( 10 -4 cm 1)

130

130

-

-

ACUzz/ ( 10 .4 cm -1)

216

242

-

-

ANyy / (10 -4 cm -1)

29

29

32

35

6gzz

4.3" 10 -3

4.3-10 -3

28.8" 10 -3

5.0" 10 -3

50 9N a ( I ) - N O / Z S M - 5

40 E

4

[] N a ( I ) - N O / A

o

A Cu(I)-NO)/ZSM-5

30

tb

~

rn

20 4

10

4

. . E " """

9

0

9

I

20

"

...................... ~

I

40

"

I

60

v (GHz)

........ ~ . . . . . . . .

"

I

80

"

I

100

Fig. 3" Comparison of the frequency dependence of the linewidth ABzz of supported Cu(I)-NO and Na(I)-NO complexes

380

x2_y 2

Hy* UX*

xzyz Z2

03 xy ~

07

Z

05

Fig. 4" Molecular orbital (MO) scheme for a Cu(I)-NO complex. 3) For NO coordinated to Cu(I) (3d l~ configuration) we expect a removal of the degeneracy of the nx* and ny* NO molecular orbitals due to the Jan-Teller effect, which can only be realized by assuming a bend complex. Quantum-chemical calculation are currently underway with the aim of understanding our experimental results. on the resonance frequency Vres is found for all three systems A linear dependence of •t~ Na(I)-NO/A, Na(I)-NO/ZSM-5 and Cu(I)-NO/ZSM-5. This is shown for three ESR

381 frequencies in Figure 3. The linear frequency dependency of ABzz indicates that the main line broadening contributions arises from a distribution 5gzz in the g tensor principal values. The distributions 6gzz in the field dependent linewidth distribution have tumed out to be substantially larger for Na(I)-NO/ZSM-5 than for Na(I)-NO/A and Cu(I)-NO/ZSM-5. One possible explanation of these results is that in the case of Na(I)-NO adducts, the linewidth is mainly dominated by the electric field of the zeolite matrix. Presumably the inhomogeneous distribution of the aluminum atoms in the ZSM-5 framework result in a strong variance in the local electric fields at the adsorption sites in NaZSM-5 (nsi/nA1 = 17) and, hence, in their local chemical properties [20]. In zeolite A (nsi/nA1 = 1) this variance is expected to be significantly lower. In the Cu(I)-NO complex the Cu-N bond is shielded and the influence of the zeolite matrix is significantly lower, which renders the supported Cu(I)-NO complex resemble closely an isolated complex. Comparison of the g-tensor parameters of Cu(I)-NO complex with our data for Na+-NO complexes in ZSM-5 and zeolite A [20] (Table 1) shows that g• < ge in these systems, while gzz < ge,gxx,gyy is found in the present study for Cu(I)-NO. To interpret these ESR results, the molecular orbital scheme of the Cu(I)-NO complex was developed (Figure 4) [after ref. 13]. Our results indicate that for the Na+-NO complex only the LUMO ~1 mixes into the SOMO ~0 with the unpaired electron by spin orbital coupling. Otherwise, a substantial contribution of lower lying molecular orbitals such as dxz-y2 to the SOMO ~0 due to spin-orbital coupling must be assumed for Cu(I)-NO to explain g• > ge. This indicates that the SOMO-dxz.y2 energy splitting is comparable with the SOMO-LUMO splitting. Note that a small SOMO-dx2_y2 splitting may result in a destabilization of the N-O bond [ 13], which might support the catalytic activity of the Cu(I) ions.

4. CONCLUSIONS The formation and structure of monomer Cu(I)-NO complexes in CuZSM-5 have been investigated with X-, Q- and W-band ESR spectroscopy. The combination of these experiments allows an unambiguous assignment of the signals and a determination of the spin hamiltonian parameters due to complete separation of the gzz area from the gxx / gyy spectral area at W-band frequency.

ACKNOWLEDGEMENTS Financial support by Deutsche Forschungsgemeinschaft (DFG) in the flame of the priority program 1051 is gratefully acknowledged.

REFERENCES 1. M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya, S. Kagawa, J. Chem. Soc., Chem. Commun. (1986) 1272. 2. M. Iwamoto, H. Hamada, Catal. Today 10 (1991) 57. 3. M. Shelef, Chem. Rev. 95 (1995) 209.

382 4. G. Centi, C. Nigro, S. Perathoner, G. Stella, in "Environmental Catalysis", J. N. Armor (ed.); ACS Symposium Series 552, p. 7, American Chemical Society: Washington DC (1994). 5. J .Valyon, W.K. Hall, J. Catal. 143 (1993) 520. 6. T. Beutel, J. Sarkany, G.-D. Lei, J.Y. Yan, W.H.M. Sachtler, J. Phys. Chem. 100 (1996) 845. 7. V.I. Parvulescu, P. Grange, B. Delmon, J. Phys. Chem. 101 (1997) 6933. 8. E. Giamello, D. Murphy, G. Magnacca, C. Morterra, Y. Shioya, T. Nomura, M. Anpo, J. Catal. 136 (1992) 510. 9. Y. Yokomichi, T. Yamabe, H. Ohtsuka, T. Kakumoto, J. Phys. Chem. 100 (1996) 14424. 10. B. L. Trout, A.K. Chakraborty, A.T. Bell, J. Phys. Chem. 100 (1996) 17582. 11. R. Ramprasad, K.C. Hass, W.F. Schneider, J.B. Adams, J. Phys. Chem. 101 (1997) 6903. 12. H.V. Brand, A. Redondo, P.J. Hay, J. Phys. Chem. 101 (1997) 7691. 13. Z. Sojka, M. Che, E. Giamello, J. Phys. Chem. 101 (1997) 4831. 14. S.-K. Park, V. Kurshev, Z. Luan, C.W. Lee, L. Kevan, Microporous Mesoporous Mater. 38 (2000) 255. 15. C.C. Chao, J.H. Lunsford, J. Phys. Chem. 76 (1972) 1546. 16. C. Naccache, M. Che, Y. Ben Taarit, Chem. Phys. Lett. 13 (1972) 2. 17. S. Wellach, M. Hartmann, S. Ernst and J. Weitkamp, in "Proceedings of the 12th International Zeolite Conference", M.M.J. Treacy, B.K. Marcus, M.E. Bisher and J.B. Higgins (eds.), Materials Research Society, Warrandale, PA, pp 795-800 (1999). 18. P. Nachtigall, M. Davidova, M. Silhan, D. Nachtigallova, in: "Zeolites and Mesoporous Materials at the Dawn of the 21 st Century", A. Galarneau, F. Di Renzo, F. Fajula and J. V6drine (Eds.), Studies in Surface Science and Catalysis, Vol. 135, Elsevier, Amsterdam (2001), p. 14-O-03. 19. D. Nachtigallova, P. Nachtigall, J. Sauer, Phys. Chem. Chem. Phys. 3 (2001) 1552. 20. T. Rudolf, A. P6ppl, W. Hofbauer, D. Michel, Phys. Chem. Chem. Phys. 3 (2001) 2167.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

383

Characterization o f acid sites in dehydrated H-Beta zeolite by solid state N M R V. Montouillout 1, S. AieUo1, F. Fayon2 and C. Fernandez 1 1Laboratoire de Catalyse et Spectrochimie, CNRS-UMR6506 ISMRA / Universit6 de Caen Basse-Normandie, 6 bd du Mar6chal Juin 14050 Caen (France) 2Centre de Recherches sur les Mat6riaux/t Haute Temp6rature, CNRS-UPR4212 1D av. de la Recherche Scientifique 45071 Od6ans cedex 2 (France) A large panel of solid-state NMR techniques is used to characterize acid sites in steamed H-Beta. The structural environment of aluminum in hydrated H-Beta zeolite can be classically probed by 27A1 MAS NMR. In the case of dehydrated samples, structural information extracted from 1H MAS and double resonance NMR experiments is used to propose a deconvolution of the 27A1Hahn-Echo and QPASS spectra. This provides an estimation of the number and the geometry of aluminum sites. 1. INTRODUCTION As the catalytic active sites in zeolites are generally connected to framework or extraframework aluminum, the characterization of both aluminum and proton environment would provide important information to understand the solid acidity of these materials. Many spectroscopic studies OR and NMR) [1,2] have shown that the aluminum state in the framework of Beta zeolite is very complex and strongly depends on the nature and severity of the activation treatment. Thanks to a large number of high-resolution solid-state 27A1NMR studies [2-5], the nature of aluminum species in hydrated H-Beta zeolite is now quite well characterized. On the contrary, it remains difficult to probe the chemical and structural environment of aluminum in dehydrated zeolites because, in this case, the 27A1 quadrupole coupling constant (CQ) is very large. Indeed, quadrupolar nuclei like 27A1(spin 1=5/2), possess an electric quadrupolar moment that interacts with the electric field gradient (EFG) at the nucleus. Depending on the strength of the EFG, the quadrupolar interaction may be very large, and sometimes the NMR signal can disappear beyond detection, leading to the so-called "invisible" aluminum especially in dehydrated zeolite samples [6]. The electric field gradients strongly depends on the geometry and electric charge distribution around aluminum atoms, so that the knowledge of the quadrupolar coupling constant can give very interesting information about the zeolite structure and acidity. Even if it is now possible to detect this type of wide 27A1 NMR signal by different techniques such as Hahn-echo sequence [7], or more recently QPASS (Quadrupolar Phase Adjusted Spinning Sidebands) experiments [8], their interpretation remains hazardous without any a priori knowledge of the kind of expected sites. Interestingly, 1H MAS spectral resolution benefits from the dehydration process and allows distinguishing between different hydroxyl groups. The indirect observation of aluminum signal via 1H spectrum emerged then as an alternative method to overcome the problem of low resolution. It is possible to

384 discriminate the protons adjacent to aluminum from other OH groups using double resonance experiments based on the analysis of the effect of dipolar 1H/27Al interaction on MAS proton spectra. Moreover, Grey and Vega recently proposed the indirect estimation of large quadrupole coupling constant of aluminum with 1H-{27Al} TRAPDOR (TRAnsfer of Populations in DOuble Resonance) [9]. This method extends the capabilities of NMR to the characterization of catalytically interesting aluminum atoms. Nevertheless, 27A1 chemical shift, which remains one of the most important parameter for discriminating the coordination number and the nature of aluminum atoms can not be obtained from 1H-{27Al} TRAPDOR experiment. The analysis of direct 27A1 signal is thus mandatory. In this work, we describe one of the two complementary NMR approaches that allow the characterization of the active sites of dehydrated steamed H-Beta zeolite. Indeed, in a work that we are reporting elsewhere [10], we give a complete assignment of 1H NMR spectra and we propose an indirect analysis of 27Al NMR signal by using 1H-{27Al} TRAPDOR experiment. These information, as well as those ~ven by the characterization of hydrated sample are used in the present paper to analyze the 2 A1 Hahn-echo and QPASS spectra. 2. EXPERIMENTAL SECTION 2.l.Materials The parent sample is a commercial Beta zeolite (PQ, Si/Al~obal= 12.5), calcined under flowing air to final temperature of 500~ following the industrial regeneration processing. Hydrated H-Beta zeolite was a sample whose hydration level was equilibrated in air. Dehydrated samples were prepared by calcination under vacuum for 6h at 250~ (heating rate 2~ They were transferred in ZrO2 rotors directly in the vacuum line and the rotors were closed with tightly fitting Kel-F caps. No signal attributed to residual water has been observed on 1H MAS NMR spectra, indicating that the zeolite were fully dehydrated. Moreover, the 1H MAS M R spectra remained unchanged over several months, showing that the vacuum transfer system used in this work insures a perfect tightness of the rotors. 2.2. NMR Spectroscopy All solid state NMR experiments were performed on a Bruker Avance 400 spectrometer (vlH=400.33MHz, v27Al = 104.23MHz. 27A1 chemical shifts were reported relative to aqueous AI(NO3)3 1N solution.27Al MAS NMR spectra of the hydrated samples were recorded using a 30 kHz spinning speed and a single short-pulse excitation (typically 0.51as at --7r/18) which ensured quantitative excitation [11]. In the case of the dehydrated samples, the use of 4mm-ZrO2 NMR rotor limited the spinning rate at 15 kHz. 27A1 central transition have then been acquired using whole echo experiments (static or rotor synchronized) and selective conditions of irradiation. QPASS experiment was carried out with acquisition and processing protocols described by Massiot et al. [8]. The sequence was composed of an initial 90 ~ excitation pulse followed by nine 7t-pulses. Two rotor periods were added to the time spent between the last two rr-pulses in order to shift the signal and acquire a full echo. The data were acquired under the following conditions: 10 kHz spinning rate, 1.2~ts 7t pulse, 16 pitch slices. Successive Fourier transforms along both axes give the 2D QPASS spectrum in which each spinning sideband appears in a different slice, according to its order. The 1D QPASS infinite spinning rate or sideband free spectrum, is obtained by sheafing the spectra along the horizontal direction accordingly to the spinning speed and summing of all the slices. It is worthwhile noting that the non-uniform excitation and the cumulative pulse imperfections

385 may greatly affect the actual observed shape. Hence, the quantitative analysis of such QPASS spectra needs to take into account these effects. 2.3. Simulations

All the 27Al NMR spectra have been fitted to retrieve isotropic chemical shifts and mean quadrupole coupling constant using our homemade MASAI software [12, 13]. This software is based on the PULSAR kernel which predicts the NMR response resulting from any pulse sequence applied on quadrupolar and spin 89nuclei in powder samples using the Average Hamiltonian Theory (AHT). It must be noted that this program does not take into account possible relaxation phenomena due to molecular motions or dipolar flip-flop terms. When the samples are not perfectly crystallized, the NMR spectrum of a given crystallographic site is not represented by a unique set of isotropic chemical shift and quadrupolar parameters {5cs, C Q, rl}, but conversely by a distribution I-I(5cs, C Q, T1) of these parameters. The distribution, from one site to another, yields a total lineshape G1D(8) of NMR spectrum corresponding to the sum of an infinite number of individual lineshape glD(5 .. tics, C Q, rl)for which the set of parameters slightly differs: G 1D(/5)= III glD (/5"" 5 es , C Q, ~)l-I(8 es , C Q, rl)" d6 cs .dCQ .d~l

(1)

Consequently, the envelope of the NMR spectra is smoothed and the singularities, which are so characteristic of quadrupolar nuclei spectra, are lost. It is well known that there is a linear correlation between the structural parameter given by the mean T-O-T angles and the isotropic chemical shifts in aluminosilicates [14]. For a given aluminum nucleus, we will then simply assume that the bond angles are randomly distributed around an average value, so that it can be described by a gaussian distribution. Therefore, this implies that the distribution lies of the isotropic chemical shift can be characterized by a gaussian function. Hcs (Scs ) = ~ e 1x p cs ~r

[-

12 (Scs - < 8cs >)2 ] 2acs

(2)

where C~csis the variance of the chemical shift distribution around its average value <scs>. The problem of the distribution of quadrupolar parameters liQ (CQ, 11) has been already addressed by Czjzek et al. [15] in the case of solids isotropic on the average. With these assumptions, Czjzek et al. have shown that the corresponding distribution may be calculated by the following equation:

rlQ(CQ,n)-~-CQ-n-(1--6-)-exp-:-T-Q.(I+ C~Q

2C~Q

)

(3)

In this model, the function liQ (CQ,rl) only depends on one parameter ~Q, which represents the variance of each components of the EFG tensor. The total distribution function that appears in eq. 1 is finally the product of the two chemical shift and quadrupolar coupling distributions.

386

Al, v

I

I

I

I

I

120 lOG 80

!

I

I

60

I

40

I

I

20

I

I

0

I

I

-20

I

I

-40

I

I

-60

I

I

I

--80 (ppm)

Figure 1: Experimental and simulated 27A1MAS spectra (rot = 30 kHz) of hydrated H-Beta zeolite

3. RESULTS AND DISCUSSION

3.1.27Ai MAS NMR experiments on hydrated H-Beta zeolite The 27A1 MAS spectrum of the hydrated H-Beta sample is shown in figure 1. It exhibits at least four types of aluminum sites: two narrow and well-resolved signals at 58 and 1 ppm both flanked by a broader shoulder on the fight side. The two groups of peaks can be directly attributed to tetrahedral and octahedral aluminum, respectively. The narrow peaks correspond to aluminum with relatively symmetric environment (referred to as Alw-1 and Alvi-1), and the broad shoulders are characteristic of more distorted and environment distributed sites (Alw-2 and Al~-2). The NMR parameters obtained from the simulation of these spectra, taking into account distribution of quadnJpolar coupling constant, are given in table 1. The narrow signal AIIv-I is unambiguously assigned to aluminum at tetrahedral sites in the regular zeolite framework. The second narrow signal Al,a-1 have been attributed to an octahedrally coordinated framework aluminum, consisting of an aluminum atom connected to four lattice oxygen atoms, the oxygen of a water molecule and the oxygen of a hydronium ion [2-5, 16, 17]. This species is induced by the presence of water and only observable in the protonated zeolite. It reverses back to tetrahedral aluminum after an ammonia treatment. The presence of the more distorted tetrahedraJ and octahedraJ coordination should be related to the strong hydrolysis treatment experienced by the zeolite. Indeed, the steaming at 773 K causes complete hydrolysis of a part of the tetrahedral framework aluminum, which is then extracted from the framework.

387 Table 1 NMR parameters determined from 27A1 MAS spectra: " 6cs in ppm, b CO in MHz, c relative intensity ,,

tetrahedral aluminum

sample

,,

,

,,,,

,

,,

octahedral aluminum

58 a (2.7) b

~ 58 (5.9)

2 (1.9)

~-3 (6.6)

[38 %1c

[33 %]

[7 %1

[2 3%]

dehydrated

42 (8.4)

59 (15.3)

53 (2.1)

H-Beta

[52 %]

[45 %]

[3 %]

H-Beta

,,,,,,

The aluminum species corresponding to AlIv-2 and Alvi-2 are then assigned to extraframework tetrahedral and octahedral aluminum. These distorted sites represent about 50% of the total intensity in the sample. From the present study of aluminum in the hydrated zeolite, and from literature results, it must be concluded that the calcination of the Beta zeolite leads to the formation of both tetrahedral and octahedral extra-framework species, corresponding to 50% of the observable aluminum atoms. Nevertheless, both octahedral aluminum species are also expected to transform back to a tetrahedral coordination upon dehydration that should have similar effect than ion-exchange [5]. 3.2. 27A! MAS NMR experiments on dehydrated H-Beta zeolite Dehydration causes dramatic distortion of aluminum coordination geometry.

(b)

'

'

I

(ppm) 400

'

'

'

I

200

'

'

'

I

0

'

'

'

'1

-200

'

'

'

I

-400

'

Figure 2:27A1NMR experiments in fully dehydrated H-Beta zeolite, (a) and (b) static and rotor-synchronized (rot: 10 kHz) whole-echo spectra (c) infinite rate spectrum obtained from the QPASS experiment (rot: 10 kHz).

388 The aluminum atoms are then subject to very large quadrupole interactions which broaden their signal so that many authors consider them as "NMR invisible" [6]. Figure 2(a-c) shows 27A1 whole-echo spectra of the fully dehydrated H-Beta recorded in static conditions and with a 10 kHz spinning speed as well as the infinite rate spectrum obtained from the QPASS experiment. As expected, the static spectrum is broad, -100 kHz, and poorly resolved. We do not distinguish any quadrupolar lineshape, indicating strongly distributed environments and no-averaged residual dipolar interactions or chemical shift anisotropy. With magic angle spinning, the signal splits in at least 7 spinning sidebands, without any characteristic lineshape. The sideband free QPASS spectrum is more resolved and consists in, at least, two components. The main site (maximum around -0 ppm) exhibits an asymmetric lineshape with a steep low-field edge and a trailing high-field edge, characteristic of a distribution of Electric Field Gradient (EFG). All the spectra were fitted to retrieve isotropic chemical shifts and population with MASAI. The spectra were considered simultaneously in order to get the best agreement. The finite spinning rates and pulses train efficiency were taken into account by the sottware. The proposed deconvolution is based on some hypotheses. According to TRAPDOR experiments reported elsewhere [ 10], we postulated the presence of two different aluminum sites with quadrupole coupling constant in the range of 10 and 16 MHz. As the lines are characteristic of NMR parameters distribution, a Czjzek model was used for the distribution of EFG [15]. Finally, even if no aluminum with lower CQ constant could be detected in TRAPDOR experiment, the 10 kHz echo spectrum clearly shows the presence of a third weaker component. This signal could be attributed to aluminum not connected to proton. Simulated and experimental spectra are shown in figure 2, and corresponding NMR parameters are given in table 1. According to this simulation, aluminum atoms are found at least in three different environments in the dehydrated H-Beta zeolite. All of them are tetracoordinated to oxygen atoms, which agrees with the hypothesis concerning the transformation of octahedral to tetrahedral aluminum upon dehydration. Each of the two main components represents approximately 50% of the total intensity.

J

:

I

9

j t'~

./

.....

//-<-.

,~,,...:,I

I

400

.

. \~:,.,,

" I

j ~ I

200

I

',~,.~., k "~-~._ -~

...... I

0

I

-......._.~~. ~ I

-200

I

(pp rn)

Figure 3: Experimental and simulated 27A1 Hahn-echo MAS spectra (rot = 10 kHz) of dehydrated H-Beta zeolite

389 They both exhibit a large quadrupole coupling constant with an important distribution indicating a strong deviation from ideal tetrahedral symmetry as suggested by Hunger in the case of Br6nsted sites [13]. According to IH-{27Al} TRAPDOR experiments, the signal at 42 ppm can be attributed to Bri3nsted sites. In the same way, it seems that the 27A1 signal at 59 ppm corresponds to aluminum connected to proton attributed to extra-framework hydroxyl groups [10]. This signal can then be attributed to a tetracoordinated extra-framework aluminum species. Its large quadrupole coupling constant indicates a very distorted site. Eventually, the third signal at 53 ppm corresponds to a tetra-coordinated aluminum, likely not connected to proton which exhibits a quite symmetric environment. 4. CONCLUSION The decomposition of the 27A1MAS NMR spectra of hydrated and dehydrated steamed H-Beta zeolites leads to the identification of different types of aluminum sites. The aluminum sites in the dehydrated form of the H-Beta are all found in tetrahedral environment, which confirms the tetrahedral-octahedral reversible transformation also observed in previous ionexchange experiments. The average quadrupolar coupling constant found for the aluminum species in the dehydrated sample, are in the same order than those found in 1H-{27AI} TRAPDOR experiment and a relationship between the aluminum and the proton (Br6nsted) sites can thus be established in this basis. We show that the use of the adequate NMR techniques allows the detection of even very large NMR signal that can not be directly observed using conventional single pulse experiment. It is also possible to give a quantitative analysis of such signal using simulation sof~are including distribution model and taken into account irradiation and spinning effects. We thus anticipate that the use of QPASS/Hahn-echo experiments will be very interesting tools for the future studies of aluminum in strongly dehydrated zeolite or in working zeolite catalyst, in conjunction with TRAPDOR type proton experiments.

REFERENCES

1 2

A Vimont, F. Thibault-Starzyk and J.C. LavaUey, J. Phys. Chem. B, 104, 286-291 (2000). E. Bourgeat-Lami, P. Massiani, F. Di Renzo, P. Espiau and F. Fajula, Applied Catalysis, 72, 139-152 (1991). 3 L. Beck, and J. F. Haw, J. Phys. Chem. ,99, 1076 (1995) 4 L.C. de Menorval, W. Buckermann, F. Figueras and F. Fajula, J. Phys. Chem., 100, 465 (1996) 5 J.A. van Bokhoven, D.C. Koningsberger, P. Kunkeler, H. van Bekkum and A. P. M. Kentgens, J. Am. Chem. Soc., 122, 12842-12847 (2000) 6 D. Freude, H. Ernst and I. Wolf, Solid State Nucl. Magn. Reson., 3 (1993) 271 7 H. Ernst, D. Freud, I. Wolf, Chem. Phys. Lett., 2112, 518 (1993) 8 D. Massiot, V. Montouillout, F. Fayon, P. Florian, C. Bessada, Chem. Phys. Lett., 272, 295-300 (1997) 9 C. Grey and A. Vega, J. Am. Chem. Soc., 117, 8232-8242 (1995) 10 V. Montouillout, A. Vimont, S. Aiello, F. Thibault-Starzyck, C. Fernandez and F. Fayon, in preparation 11 A. Samoson, E. Lippmaa, Phys. Rev. B: Condens. Matter., 28 6567-70 (1983)

390 12 A.A. Quoineaud, V. Montouillout, C. Fernandez, S. Gautier, S. Lacombe, in preparationl3 C. Fernandez, A.A. Quoineaud, V. Montouillout, S. Gautier, S. Lacombe, Proceedings of the 13th International Zeolite Conference, MontpeUier, July 2001, eds. A. Galarneau et al. ELSEVIER Publ 14 E. Lippmaa, A. Samoson and M. Magi, J. Am. Chem. Soc., 108, 1730 (1986) 15 G. Czjzek, J. Fink, F. Gotz and H. Schmidt, J.M.D.Coey, J.P. Rebouillat, A. Lienard, Phys. Rev. B. 23, 2513 (1981) 16 C. Jia, P. Massiani, D. Barthomeuf, J. Chem. Soc. Faraday Trans., 89, 3659 (1993) 17 B.H. Wouters, T.-H. Chen, P. Grobet, J. Am. Chem. Soc., 120, 11419 (1998)

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

391

Characterization and Quantification o f Aluminum Species in Zeolites Using High-Resolution 27A1 Solid State N M R A.A. Quoineaud a, V. Montouillout a, S. Gautierb, S. Lacombeb and C. Fernandez a aLaboratoire de Catalyse et Spectrochimie CNRS-UMR 6506-ISMRA-Universit6 Caen BasseNormandie 6 Boulevard Mar6chal Juin 14052 Caen(France) bInstitut Frangais du P6trole, 1-4 Avenue de Bois-Pr6au 92852 Rueil-Malmaison (France)

27A1 MAS and MQMAS NMR have been used to identify and quantify framework and extra-framework (EFAL) aluminum in Y zeolites stabilized by steaming and EFAL extraction using various leaching agents. It is useful to characterize contributions of different A1 sites in USY. The decomposition of MAS and MQMAS was carried out with the use of a homemade software called MASAI. It takes benefits from the simultaneously analysis of a high resolved 2D-MQMAS and a quantitative MAS spectra. It allows a quantitative assignment of the framework and extra-framework aluminum species. Global and framework Si/A1 ratios have been calculated by 27A1 NMR and compared with Si/A1 ratio found by X-Ray diffraction and X fluorescence.

I. INTRODUCTION Zeolite Y is the active phase of most widely used commercial catalysts in catalytic cracking and hydrocracking processes. Its dealuminated ultrastable form (USY) is useful for its porous system and its activity which can be adapted to its catalytic applications. The activity and thus the acidity of the Y zeolite is closely related to the presence of aluminum atoms. A good way to control acidity and to increase the stability of zeolite is to modify the framework Si/A1 ratio. Hydrothermal steaming is generally used. The nature of extraframework aluminum (EFAL) created during steaming has been the subject of several research projects. Different forms for EFAL species have been proposed [1] which can be present as cations (A13+, AI(OH)2+...) or as neutral or polymerized (amorphous) species (A10(OH), AI(OH)3, A1203). It is shown that a presence of large amounts of EFAL has a negative effect on catalytic activity. Therefore, EFAL are eventually removed by chemical attack. Mineral acid is usually used to extract the EFAL but other agents have been proposed such as ethylenediaminetetraacetic acid (EDTA) [2] and ammonium hexafluorosilicate (HFS) [3]. A. Gola et al. [4] have suggested that the nature of EFAL removed (ionic or amorphous) is dependant of the nature of the agent used. It is then possible to extract progressively and selectively the ionic or amorphous EFAL. The stability of zeolite can be enhanced by the incorporation of multivalent cations, like rare-earth dements. The cations have a long-range effect on the structure of zeolite and a

392 distortion of bond angles is observed. Van Bokhoven et al. [5] is suggested that ionic EFAL species interact with framework aluminum as charge-compensating cations. The 27A1 MAS NMR technique has been widely used to probe the local environment of aluminum, providing useful information about the aluminum coordination as a function of the applied treatment. Tetra-, penta- and hexa-coordinated aluminum are created during steaming treatment. EFAL have been located in cationic sites by X-ray diffraction (XRD) structure refinement and by neutron diffraction [6, 7]. However, a drawback of 27A1MAS NMR comes from the second-order quadrupolar interactions that strongly broaden the spectra and may prevent the correct quantification of NMR spectra. It is often claimed that as much as 30% of the aluminum atoms, the so-called "invisible aluminum", cannot be observed in conventional 27A1 MAS NMR spectra [8, 9]. Selective conditions could prevent this effect [ 10].The validity of this assumption is verified using a reference samples. A major improvement in solid-state NMR of quadrupolar nuclei has been obtained by the recent introduction of the Multiple Quantum MAS (MQMAS) technique by Frydman et al. [11, 12]. This two-dimensional method produces highly resolved 27A1NMR spectra [13]. It allows for an unambiguous assignment of the aluminum coordination and gives access to the determination of characteristic site parameters such as quadrupolar coupling constant (QCC), chemical shift (Scs) [14, 15]. These parameters and their distributions [ 16] can be linked up to structural data by using the relationships between the 27A1 isotropic chemical shift and the T-O-T angle [17, 18] (with T=AI,Si) and between the quadrupolar coupling constant and the O-T-O angles [ 19, 20- 21 ]. The aim of this work is to characterize and quantify framework and extra-framework aluminum species in zeolites. 27A1MAS and MQMAS NMR spectra have been carried out on native and modified zeolites to identify as clearly as possible the aluminum environments. A homemade simulation software called MASAI has been used to decompose NMR spectra [22, 23]. It allows us to obtain main site parameters (QCC, 6cs) so as to correct the intensity of the MAS spectra to its quadrupolar dependence yielding an accurate quantification.

2. EXPERIMENTAL 2.1. Material

NaY zeolite (Si/AI=2.6), a commercial sample from Tosoh Corporation, was submitted to several ionic exchanges. Ammonia form of Y zeolite was obtained by multiple ion exchanges in ammonium nitrate solution. Each exchange was performed in a 5mL/g of I0N NHaNO3 solution and heated with reflux to 373K for 4 hours. The suspension was filtered, washed with distillated water and dried at 393K for 12 hours. After four exchanges, the residual amount of Na was determined by atomic adsorption to be bellow 500 ppm. Steaming was performed under 100% water vapour at 923K as described elsewhere [4]. Acid attack was carried out by heating a suspension of the steamed zeolite in a 0.1N nitric acid solution (5mL/g) with reflux for 2h. The suspension was filtered, washed with distillated water and dried at 393K for 12h. EDTA treatment was carried out by heating a suspension of the steamed zeolite in a 0.3N EDTA solution (0.05mL/g) with reflux, filtering and washing with distillated water. In order to eliminate the sodium introduced by EDTA ,the zeolite was then subjected to two ion exchanges in ammonium nitrate solution.

393 For the HFS treatment, steamed zeolite was mixed with an acetate sodium solution (0.1g/mL) and heated at 353K. A 0.4N HFS solution was then added at a rate of 0.25mL/min per gram of zeolite and stirred for 2h. The suspension was filtered, washed with distillated water and dried at 3 93K for 12h. 2.2 NMR experiments 27A1 MAS NMR and 27A1 MQMAS NMR experiments were performed on a Bruker Avance DSX-400 (B0=9.4T) spectrometer at a spinning speed of 14kHz. For MAS spectra, the applied RF field strength was about 30kHz. In order to obtain quantitative data, selective excitation pulses of 1 Its were used, corresponding to a x/16 flip angle [ 10]. Recycling delays of 0.5s were long enough to allow quantitative M R analysis. Two hundred scans were recorded for the 27AlMAS experiments Triple Quantum 27Al MAS M R (27Al MQMAS) were carried out using a specially made probehead in order to be able to apply high RF field (about 250-300kHz) with a rotor spinning speed of about 15kHz. 27A13QMAS were performed by means of two high power pulses (2.7 and 0.7 ~ts) and an additional soft and selective z-filter x/2 pulse [24]. A recycling delay of 0.5s was used. Chemical-shifts, 6, were reported relative to aqueous Ai(NO3)3 IN solution. To quantify aluminum species in zeolites, sample weights were balanced for water content. The method used to decompose MAS and MQMAS has been previously described [22, 23]. It is based on the simultaneous decomposition of 1D (MAS) and 2D (MQMAS) spectra. Observing that position and form of lineshape in MAS and MQMAS can be directly linked up to the quadrupolar and chemical shift parameters, it is possible to determine the quadrupolar coupling constant (QCC) and the isotropic chemical shift (6cs). This method allows us to calculate the correct intensities of MAS spectra and therefore to obtain a quantitative data.

3. RESULTS AND DISCUSSION 3.1. Chemical Characterization Table 1 summarizes the global and framework Si/A1 ratios determined by X-ray diffraction and X fluorescence. The number of EFAL per unit cell was determined as the difference between the global number of aluminum atoms in the sample obtained by X-ray fluorescence and the framework number of aluminum atoms obtained by XRD. Aluminum amount per unit cell was calculated from the Si/AI ratios on the basis of 192 T atoms (Si or Al) per unit cell. Calculated values of EFAL per unit cell are reported in table 1. As expected, the framework Si/A1 ratio indicates no dealumination occurs during the leaching treatment and the amount of EFAL decrease.

394 Table 1 Aluminum composition of steamed Y sample and treated with nitric acid, EDTA or HFS Global S i / A I Framework Si/AI EFAL sample (mol/mol) (mol/mol) per unit cell USY 2.6 11.6 35.3 USY-HNO3 7.7 13.8 9.1 USY-EDTA 4.8 11.6 17.9 USY-HFS 6.1 12.1 12.4

3.2. 27A1 MAS and MQMAS NMR Figure l a represents 27A1MAS of steamed and leached Y sample and the evolution of aluminum species with treatments. Three types of aluminum can be observed : tetra-, pentaand hexa-coordinated aluminum atoms. The steamed sample, leached by nitric acid, presents only two types of aluminum : tetra- and hexa-coordinated AI. The decomposition of MAS and MQMAS spectra of USY-HNO3 sample using the MASAI program is represented in figure 2. It shows four aluminum sites at 61.4, 63.4, 3.9, 0.9 ppm with the average quadrupolar coupling constant 2.6, 5.9, 1.8, 5.5 MHz corresponding respectively to the framework AI(IV), distorted Al(IV) and two hexa-coordinated EFAL

AL(VI).

Table 2 summarizes the main site parameters for all aluminum species (framework and extra-framework) present in USY-HNO3 zeolite. Using quadrupolar and chemical shift corrections, it is possible to calculate the amount of aluminum in each type of site. Global and framework Si/AI ratios can be determined and compared to global XRF Si/AI and framework XRD Si/A1 (reported Table 3). A rather good agreement is observed between the different method. (a)

b) p p m 0 2C +

4C

__.d i

i

150

t

'1

100

o

t

50

i

i

C

1

i

-50

i

i

-100

i

i

-150(ppm)

~80

40

I

0

'

I

'

1

-40 ppm

Figure 1" (a) 27A1MAS spectra of steamed Y zeolite (USY) and treated by nitric acid, EDTA and I-IFS respectively.- (b) 27Al MQMAS spectrum of steamed and leached by HNO3 sample

395 ppm

(a) 20 40

60 "

I

I

!

120 80

40

'

80

i

I

O

.40

"

'1

..........

................i " '

120

ppm

80

l 40

~" 0

ppm ,'.,~

i" ppm

a, _-'2N,

|| .|

(b)

I ' .40

20-

|||i. .II |'1|

,, ,~

":t r If

'1 I

it

! !\

l l, tt uqq

Ill.

40I

L

I

60-

~.7.,~",

801 120 80

40

0

-40

ppm

120

~......

80

I

,~,

40

6

I

-40

I

ppm

Figure 2" 27A1MAS and MQMAS experimental (a) and simulated (b) spectra of steamed and leached by HNO3 Y zeolite

Table 2 Characteristics of aluminum sites in the steamed and leached by HN03 Y zeolite determined by 27A1MAS and MQMAS NMR spectra decomposition Site 1 Site 2 Site 3 Site 4 Framework Distorted AI(IV) EFAL(VI) EFAL(VI) AI(IV) ,, 8cs (ppm) 61.4 63.4 3.9 0.9 Ocs (ppm) 2.1 5.0 0.5 5.0 QCC (M~) {~Q( M H z ) m~a (10 2~g)

2.6

5.9

1.8

5.5

1.2

3.1

1.4

2.6

1.6

1.4

0.9

1

396 Table 3 Comparison of global and framework Si/AI ratios determined by 27A1 MAS and MQMAS NMR, XRD and XRF method Framework Si/A1 Global Si/AI 27A1RMN 14.0 8.1 XRD 7.7 XRF 13.8

4. CONCLUSION Dealumination of steamed Y has occurred in different ways for acid leaching and EDTA and HFS treatments. 27A1 MAS NMR and 27A1 MQMAS N~IR can be used to give a quantitative assignment of framework and extra-framework aluminum species in USY. The identification of aluminum sites is improved by the simultaneous decomposition of a 27A1 MAS NMR acquired under selective conditions and a high resolved bidimensional 27A1 MQMAS NMR spectra. The determination of main site parameters (chemical shitt, quadrupolar coupling constant and their distribution) allows us to calculate the amount of tetra-, penta- and hexa-coordinated aluminum atoms.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

J. Scherzer, in : T.E. White, R.A. Della Betta, E.G. Derouane, R.T.K. Baker (Eds), Catalytic Materials " Relationship Between Structure and Activity, A.C.S. Symp. Ser. 248, American Chemical Society, Washington D.C. (1984) p. 157 N.P. Rhodes, R. Rudham, J. Chem. Soc. Faraday Trans. 89 (1993) 2551 A. Corma, V. Formes, F. Rey, Appl. Catal. 59 (1990) 267 A. Gola, B. Rebours, E. Milazzo, J. Lynch, E. Benazzi, S. Lacombe, L. Delevoye, C. Fernandez, Micr. Mes. Mat. 40 (2000) 73 J.A. Van Bokhoven, A.L. Roost, D.C. Koningsberger, J.T. Miller, G.H. Nachtegaal, A.P.M. Kentgens, J. Phys. Chem. B 104 (2000) 6743 P.K. Maher, F.D. Hunter, J. Scherze, in R.F. Gould (ED.), Molecular Sieve Zeolites-I, Advances in Chemistry, Vol. 101, American Chemical Society, Washington D.C., 1971, p 266 J.B. Parise, D.R. Corbin, L. Abrams, D.E. Cox, Acta Cryst. C40 (1984) 1493 D. Freude, H. Ernst and I. Wolf, Solid State Nucl. Magn. Reson., 3 (1993) 271 C.A. Fyfe, J. L. Bretherton, L. Y. Lam, Chem. Comm. (2000) 1575 E. Lippmaa, A. Samoson, M. Magi, J. Am. Chem. Soc. 108 (1986) 1730 L. Frydman, J.S. Harwood, J.Am. Chem. Soc. 117 (1995) 5367 A. Medek, J.S. Harwood, L. Frydman, J.Am. Chem. Soc. 117 (1995) 12779 C. Fernandez, J.P. Amoureux, Chem. Phys. Letters 242 (1995) 511 J.P. Amoureux and C. Fernandez, Solid State Nucl. Magn. Reson., 10 (1998) 211 G. Engelhart, A.P.M. Kentgens, H. Koller, A. Samoson, Solid State Nucl. Magn. Reson., 15 (1999) 171

397 16. F. Angeli, T. Charpentier, P. Faucon and J.-C. Petit. J. Phys. Chem. B. 103 (1999) 10356 17. E. Lippmaa, A. Samoson and M. M~igi. J. Am. Chem. Soc. 108 (1986) 1730 18. F. Angeli, J. -M. Delaye, T. Charpentier, J. -C. Petit, D. Ghaleb and P. Faucon, Chem. Phys. Letters, 320 (2000) 681 19. S. Ghose, T. Tsang, Am. Min. 58 (1973) 748 20. H. Hunger, T. Horvath, J. Am. Chem. Soc. 118 (1996) 12302 21. M.T. Weller, M.E. Brenchley, D.C. Appedey, N.A. Davies, Solid State Nuclear Magn. Reson., 3 (1994) 103 22. C. Fernandez, A.A. Quoineaud, V. Montouillout, S. Gautier, S. Lacombe, Proceedings of the 13th International Zeolite Conference, Montpellier, July 2001, eds. A. Galarneau et al. ELSEVIER Publ. 23. A.A. Quoineaud, C. Fernandez, V. Montouillout, S. Gautier, S. Lacombe, in preparation 24. J.P. Amoureux, C. Fernandez, S. Steuernagel, J. Magn. Reson. A 123 (1996) 116

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

399

Control o f A F I type crystal synthesis with additional gel c o m p o n e n t s J. Komatowski, G. Zadrozna and J.A. Lercher Technische Universit/it Mttnchen, Lehrstuhl II Fttr Technische Chemie, D-85747 Garching, Germany Morphology of the AFI type crystals and substitution of transition metals such as Cr can be controlled by additional components used in the synthesis gel. Although these components are not necessary for formation of the AFI type structure, they play an active role in both the synthesis reaction and the crystal growth. This is likely effect of formation of intermediate compounds able to modify the reaction course and influence the crystal morphology. Due to the active role of the additives, the summarizing term co-template is suggested.

1. INTRODUCTION

Applications of the molecular sieves in physics 1 have led to highly demanding specifications with respect to composition, morphology and structural perfection of the crystals. Thus, micro-, meso- and macroscopic parameters such as substitution of transition metals, crystal dimensions and morphology, including the form/shape of the external surface have to be precisely controlled. These requirements caused substantial changes in the experimental realization of the syntheses. The oldest trend in this direction was the development of methods for synthesis of large crystals2. Then, the growth of perfect single crystals, needed for, e.g., crystallographic measurements 3, diffusion studies ~, or sorption investigations 5 followed. The utilization of zeolites for optics or as laser materials 6 required additionally the control of the crystal morphology 7, which can be realized by the control of the growth rates into particular crystallographic directions, and is a lately evolved trend. On the basis of careful analysis of the synthesis routes for various aluminophosphate molecular sieves substituted with metals 7'8, such as, e.g., MeAPO-118, we noticed that the substituting metals and, in a broader sense, the synthesis conditions influence not only the chemical properties but also the crystallographic parameters of the products. Therefore, we initiated broader investigations searching for synthesis conditions that allow the control of the morphology and dimensions of AFI type crystals (AIPO4-5 and derivatives) 9q2. The characteristic common morphology of the AFI type crystals is a pencil-like hexagonal rod. The crystals can be grown to the length (c-axis) of up to over 1 mm with the width limited to about 50 txm13. In contrast to that, the new synthesis method developed in our group 9"12, yields flat hexagonal platelets with a short c-axis and a larger width. The method is based on use of additional components in the reaction gel, which results in the products of a desired morphology and a higher perfection of the structure via control of the elementary

400 chemical reactions and the crystal growth. As outlined above, large crystals or crystals of special features, such as materials having a particular morphology, aspect ratios, perfectly smooth walls or arranged assembles (oriented crystals) require the control over a large variety of parameters. In the present contribution, we carry out a comprehensive discussion dealing with the use of additional components in the reaction gels (co-templates) and with their active role in the syntheses of Cr- or Me-A1PO4-5. Considering this activity and influence of the additives on the crystallization process, their role is summarized for the synthesis of CrAPO-5 in comparison with those of A1PO4-5 and several other MeAPO-5 and it is suggested to term the chemicals as co-templates.

2. EXPERIMENTAL

The modified synthesis procedure has been developed from our general method for growing large crystals of the aluminophosphate molecular sieves 13. The reaction gels had the following molar ratios of components given as oxides: a A1203 : 1.00 P205 : b [Cr203 or Fe203 or 2MeO] : 1.55 TEA : 270 1-120 : 0.1-1.0 X, where: a + b = 1.00 and b = 0 or 0.05-0.2 for A1PO4-5 or [Cr,Me]APO-5, respectively, TEA (triethylamine) was the main template and X represented additional component(s) 1~ An A1 compound, co-template(s), and a Cr or Me salt were dissolved/dispensed in water under vigorous stirring (initial mixture A) 11,13. Diluted H3PO4 was reacted with TEA under stirring and cooling (mixture B). Then, (B) was admixed to (A) and homogenized by stirring at room temperature. The gels were placed in PTFE-lined autoclaves and treated thermally at 463 K for 16 to 72 h. The resulting crystals were decanted, filtered, dried, and finally calcined at 773 K in air for at least 48 h. The structure of the materials was investigated by powder X-ray diffractometry. The chemical composition was determined with AAS. The morphology and dimensions of the crystals were studied by SEM. The sorption properties and pore structure were characterized by measurements of sorption isotherms for water and benzene (a vacuum device equipped with quartz spring McBain balance and MKS Baratron gauge, 298 K, equilibrium state at each measurement point) and for nitrogen (a Micromeritics ASAP 2010 automated volumetric analyser, 77 K).

3. RESULTS AND DISCUSSION 3.1. Experimental observations on MeAPO-5 materials As co-templates in the reaction gels, a large group of organic and inorganic compounds has been tested, including alcohols, organic and inorganic acids, salts of alkaline metals and ammonia, alkylamines and quaternary ammonium salts, buffers, and media increasing the viscosity or density of the reaction gels (glycol, glycerol, starch, etc.). The effect of these compounds on the syntheses varied from the complete collapse (e.g., higher amounts of inorganic acids or salts), via no or only a slight effect (e.g., some additional amines and viscous media) to significant and positive changes in the crystalline phases obtained. Organic acids showed the strongest influence on the morphology of the crystalline phases, especially on shortening the crystal length. Alcohols had generally a favorable, although much more subtle effect. Interestingly, the influence of the organic acids was usually increased by addition of small amounts of inorganic acids 11'12, which may be an effect of buffeting or of a higher ionic strength of the reaction mixture.

401

Fig. 1. Two examples of the CrAPO-5 crystals synthesized without co-templates. The influence of metals used in the reaction gels for isomorphous substitution for the original A1 atoms is often quite subtle. Without a thorough study and exactly comparable synthesis conditions, the effect can easily be missed (e.g., influence of the Co 2+ and Mg 2+ ions). However, the metal ions introduced into the framework positions are able to cause remarkable changes in the dimensions and morphology of the AFI type crystals. The general tendency is that the metals present in the framework reduce the crystal length along the c-axis, i.e., they lower the aspect ratio l:d (length:width). The strongest effects on the morphology have been achieved by using the combination of an organic acid (as co-template) and a metal able of isomorphous substitution for A1. Among the metals tested (Cr, Fe, Co, Cu, Mn, Mg), Cr 3+ showed the strongest influence. However, one has to stress here the synergetic effect of both factors, i.e., influence of co-template and the metal. Without a co-template, the Cr 3+ ions can be incorporated into the framework positions only in very small amounts and remain unstable. In these materials, the effect of Cr 3+ on the morphology is none or negligible as such CrAPO-5 crystals contain extraframework Cr and are typical elongated hexagonal prisms. The content of Cr incorporated into the framework positions can modestly be increased by controlling some other parameters than the addition of the co-templates, e.g., a thorough choice of A1 and Cr compounds. Under these conditions, the crystals exhibit an aspect ratio of ca. 1:1 (Fig. 1). The use of cotemplates and its synergetic effect with the metal lead to flat hexagonal platelets with aspect ratios (l:d) being sometimes lower than 0.1 (Fig. 2). The micrographs of Fig. 2 illustrate the differences between various co-templates. A detailed study on influence of different organic acids on morphology of the CrAPO-5 crystals has been reported elsewhere 1~ In the group of favorably acting organic acids, the differences are clear, though not very large. Thus, the CrAPO-5 crystals can be grown up to 120 lam wide and below 10 ~m long. These crystals have mostly smooth, flat walls (also the hexagonal a-b surfaces), i.e., an almost perfect crystal habitus. All these products reveal very high sorption capacities for water, benzene and nitrogen, suggesting, thus, a perfect intemal order 9. Note that the use of organic acids as the co-templates appears to be the necessary condition for stable incorporation of Cr 3+ in higher amounts 11 (up to 0.5 Cr atoms per u.c.). Non-substituted A1PO4-5 has appeared to be less susceptible to the morphology control. The use of acids results in the aspect ratios (l:d) of ca. 1:1 and maximum dimensions of ca.

402 50x50 jam, i.e., not wider than the common hexagonal prisms (Fig. 3). Flatter crystals occur only rarely and not in high amounts. Another difference with CrAPO-5 is that the hexagonal walls are always slightly convex, suggesting an apparent aggregation (Fig. 3). Nevertheless, it has to be underlined that such short crystals exhibit distinctly higher sorption capacities than the long ones, synthesized without acids. This indicates clearly that the structure of nonsubstituted A1PO4-5 also reacts positively on the presence of suitable co-templates in the reaction gel and confirms the existence of the synergetic effect between the co-templates and the metal. It should be emphasized that the morphology control and structure improvement are not possible without the acids for the both CrAPO-5 and A1PO4-5 materials. The effects described above have been confirmed by the experiments with other metals. Although the presented syntheses have not been optimized, these preliminary results show an astonishing accordance with the observations for CrAPO-5 and A1PO4-5. The crystals synthesized with Fe, Cu, Mn (Fig. 4), and Co (not shown) exhibit tendencies similar to Cr. Syntheses without the co-templates yield crystals with the aspect ratios within the range of 3:1 to 1:1 (not shown). The introduction of organic acids into the reaction gel causes a significant shortening of the length in the direction of the c-axis (Fig. 4). Especially the FeAPO-5 crystals behave very similarly to CrAPO-5 and can be grown as flat hexagonal platelets. The

Fig. 2. SEM micrographs of CrAPO-5 synthesized with the following acids: acetic (top left), propionic (top fight), methacrylic (bottom left), succinic (bottom fight).

403

Fig. 3. SEM micrographs of A1PO4-5 synthesized from pseudo-boehmite with following acids: acetic (top left), acetic + ethanol (top fight), propionic (middle left), acrylic (middle fight), acetic [from an amorphous AI(OH)3 gel] (bottom left), and propiolic (bottom fight).

404 present observations do not allow us to determine exact reasons for this similarity. On the one hand, it can result from the same 3+ charge of the metal ions and lack of the framework charge in the crystals. However, it is not certain whether or not Fe is present in the 3+ oxidation state in the reaction mixture and, even if so, does it keep this state, entering the framework positions. On the other hand, the similar behavior can also be the effect of the ionic radius or of a specific disturbance in the crystal framework. All the divalent cations gave smaller effects (Fig. 4) on the morphology, which appears to support these considerations.

Fig. 4. SEM micrographs of FeAPO-5 (top two), CuAPO-5 (bottom left), and MnAPO-5 (bottom fight). All samples synthesized with the acetic acid.

3.2. Role of the co-templates The described observations of the syntheses of A1PO4-5 and its derivatives substituted with various metals show clearly that the additional components given into the reaction gels play an active role in both the synthesis reaction and the crystal growth. We speculate that this activity results from formation of unstable complex species which change reactivity and/or equilibrium between the main components of the reaction mixture. These effects can be equal or similar, on the one hand, to an appropriate choice of the reactant materials (e.g., A1 or Me compounds) or, on the other hand, to the control of the reaction rate between particular

405 reactants. The additional components determine also the overall ionic strength of the mixture as well as exert a non negligible influence on pH and its changes during the reaction. All these parameters/values can significantly control the course of the synthesis reaction and, therefore, also the crystal growth. The latter can be influenced in two ways: (i) by shifting the equilibrium between the chemical reaction rate and the crystal growth rate and (ii) by entering the molecules of the additional components together with the main template into the pore system of the growing crystals. Especially the last effect is important for the morphology of the crystals. This can be convincingly concluded from comparison between the effects observed for non substituted A1PO4-5 and the MeAPO-5 materials. No differences should occur in the resulting crystals, if the additional molecules would not enter the pore system. The observed synergetic effect between the organic acids and the metals as well as the clear positive influence of small amounts of inorganic acids added together with the organic ones give a strong support for the above argumentation.

3.3. Summarizing the additional components under the term co-template Precisely, the term "template", refers to a substance that allows a particular structure to be formed around it during synthesis. For many substances, it appears clear that, while being incorporated into the pores of the structure, the structure induced by this template may not be the exact negative of the templating molecule. Thus, these molecules act as structure directing agents rather than templates in the strict sense. This becomes obvious considering that (i) syntheses of zeolitic materials are possible without organic templates (ii) numerous different structures can be synthesized with use of the same template and (iii) the same structure can be synthesized using templates differing strongly in physical shape. If we decide, however, to stick to the simpler term template, it should include substances that influence structural, compositional, and morphological aspects of the crystallization process. Thus, all substances, influencing the process of MeAPO-5 crystallization in the sense of a desired controlling the product properties, are summarized under the term co-template in order to emphasize their assisting, though indispensable (for the crystal properties) role in the synthesis. This has been successfully shown for organic acids and alcohols.

4. CONCLUSIONS The morphology, dimensions, aspect ratio, and structure of the AFI type crystals are controlled by the use of additional co-templates in the reaction gel. These components are dispensable for the formation of the AFI type structure, but play an active role in the reaction, which is reflected in their significant influence on the resulting products. The additional components enable to tailor the crystal growth in particular crystallographic directions by influencing the particular growth rates. Using organic acids together with Cr 3+ in the reaction gel, perfect AFI type crystals were synthesized in the form of flat hexagonal platelets instead of elongated hexagonal rods. However, the co-templates effect not only the changes in the morphology of the crystals, but their presence causes also both a proper coordination of Cr 3+ to these molecules and the incorporation of the transitional metal cations into the molecular sieve framework. The principle of combining a suitable metal cation with an appropriately coordinating organic molecule seems to be a promising tool to achieve new goals in synthesis, especially in isomorphous substitution of further metals and in preparation of cleaner crystals of desired morphology. Formation of new structures seems to be also possible.

406 ACKNOWLEDGEMENTS

The work was partially supported by the Deutsche Forschungsgemeinschaft (DFG) within the program "Nanostructured Host-Guest Systems".

REFERENCES

1. a) G.A. Ozin, S. 0zkar, Adv. Mater., 4 (1992) 11; b) J. Caro, G. Finger, J. Kornatowski, J. Richter-Mendau, L. Werner, B. Zibrowius, ibid, 4 (1992) 273; c) C. Pereira, G.T. Kokotailo, R.J. Gorte, J. Phys. Chem., 95 (1991) 705; d) M. Ehrl, F.W. Deeg, C. Brauchle, O. Franke, A. Sobbi, G. Schulz-Ekloff, D. W6hrle, J. Phys. Chem., 98 (1994) 47. 2. a) R. Mostowicz, L.B. Sand, Zeolites, 3 (1983) 57; b) J. Kornatowski, ibid, 8 (1988) 77; c) T. Kodaira, K. Miyazawa, T. Ikeda, Y. Kiyozumi, Microp. Mesopor. Mater., 29 (1999) 329. 3. a) S. Radaev, W. Joswig., W.H. Baur, J. Mater. Chem., 6 (1996) 1413; b) W.H. Baur, W. Joswig, D. Kassner, J. Kornatowski, G. Finger, Acta Cryst., B50 (1994) 290. 4. a) S. Feng, T. Bein, Science, 265 (1994) 1839; b) M. Noack, P. K61sch, D. Venzke, P. Toussaint, J. Caro, Microp. Mater., 3 (1994) 201; c) V. Kukla, J. Kornatowski, D. Demuth, I. Girnus, H. Pfeifer, L.V.C. Rees, S. Schunk, K.K. Unger, J. Karger, Science, 272 (1996) 702. 5. a) J. J~inchen, M.J. Haanepen, M.P.J. Peeters, J.H.M.C. van Wolput, J.P. Wolthuizen, J.H.C. van Hoof, Stud. Surf. Sci. Catal., 84 (1994) 373; b) G. MUller, E. Bodis, J. Kornatowski, J.A. Lercher, PCCP, 1 (1999) 571. 6. a) S.D. Cox, T.E. Gier, G.D. Stucky, Chem. Mater., 2 (1990) 609; b) F. Marlow, J. Caro, L. Werner, J. Kornatowski, S. Dahne, J. Phys. Chem., 97 (1993) 11286;. 7. a) S. Ahn, H. Chon, Micropor. Mater. 8 (1997) 113; b) J. Caro, F. Marlow, K. Hoffmann, C. Striebel, J. Kornatowski, I. Girnus, M. Noack, P. K61sch, Stud. Surf. Sci. Catal., 105C (1997) 2171. 8. a) Y. Xu, P.J. Maddox, J.M. Thomas, Polyhedron, 8 (1989) 819; b) J. Kornatowski, G. Finger, K. Jancke, J. Richter-Mendau, D. Schultze, W. Joswig, W.H. Baur, J. Chem. Soc., Faraday Trans., 90 (1994) 2141. 9. J. Kornatowski, G. Zadrozna, J. Wloch, M. Rozwadowski, Langmuir, 15 (1999) 5863. 10. a) J. Kornatowski, G. Zadrozna, in Proc. of the 12th Intern. Zeolite Conf., Treacy M.M.J., Marcus B.K., Bisher M.E., Higgins J.B., Eds., Materials Research Soc., Warrendale, 1999, vol. III, p. 1577; b) J. Kornatowski, G. Zadrozna, J.A. Lercher, M. Rozwadowski, Stud. Surf. Sci. Catal., 135 (2001)04-P-12. 11. J. Komatowski, G. Zadrozna, M. Rozwadowski, B. Zibrowius, F. Marlow, J.A. Lercher, Chem. Mater., 13 (2001) 4447. 12. J. Kornatowski, G. Zadrozna, J.A. Lercher, to be published. 13. a) D. Demuth, K.K. Unger, F. Schtith, G.D. Stucky, V.I. Sradnov, Adv. Mater., 6 (1994) 931; b) G. Finger, J. Komatowski, Zeolites, 10 (1990) 615; Bull. Soc. Chim. Belg., 99 (1990) 857.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

407

Synthesis and c h a r a c t e r i z a t i o n o f m o r d e n i t e ( M O R ) zeolite derived from a layered silicate, N a - m a g a d i i t e T. Selvam and W. Schwieger* Institute of Technical Chemistry I, University of Erlangen-Nuremberg, EgerlandstraJ3e 3, D-91058 Erlangen, Germany Mordenite type zeolite has been synthesized by hydrothermal transformation of Na-magadiite using tetraethylammonium hydroxide (TEAOH) as an intercalating agent and/or structure directing agent. A mixed cation system (TEA +, Na + and K +) was applied to investigate this transformation at 175 ~ Mordenite samples with different SIO2/A1203 ratios were synthesized and characterized by means of XRD, TG-DTA, SEM, TEM, MAS NMR and N2 adsorption measurements. TEM indicates that the mordenite crystallites obtained by hydrothermal transformation of Na-magadiite are more uniform and in the nano size (80-120 nm) range, much smaller than the mordenite crystallites (5-6 ~tm) synthesized by conventional hydrothermal crystallization with and without the aid of organic structure directing agent. I. I N T R O D U C T I O N There has been considerable interest in layered silicates because of their ability for intercalation and swelling [ 1-4]. Several papers have been reported on the synthesis of porous materials from layered silicates, such as kanemite [5, 6], ilerite [7] and magadiite [8], by intercalation of surfactants because of the presence of interlamellar silanol groups. We earlier reported the preparation of porous materials with a basal spacing of 36 A from Na-kanemite (10 ~), by ion exchange of the interlayer sodium ions with long-chain surfactants [9]. This route is particularly useful for the development of cross-linked layered materials with a twodimensional pore system and high surface areas of about 300 to 400 m2g-1. In addition, there has been considerable work on the hydrothermal transformation and/or recrystallization of layered silicates into known zeolites. For example, aluminium-containing magadiite was found to be a precursor for the synthesis of MFI, MEL and FER type zeolites in the presence of an organic structure-directing agent by thermal treatment in dry state and in aqueous suspension as well [10-12]. Indeed, such a transformation of magadiite into commercially important zeolites is very attractive, both practically and economically, because of the minimum amounts of template used in the reaction mixture. The manganese substituted silicalite-1 (MFI) from Mn 2+.ion-exchanged magadiite has also been reported in the literature [13]. This approach has proven efficient for framework substitution of Mn into the MFI structure. We have recently reported the hydrothermal transformation of a layered silicate, Na-magadiite, into mordenite (MOR) zeolite, using TEAOH as an intercalating agent [ 14]. In the present study, the effect of various cations (TEAOH, potassium and sodium) in the hydrothermal transformation of Na-magadiite into mordenite is studied. Four mordenite samples with different 8iO2/A1203 ratios were synthesized from Na-magadiite and their characterization were performed by XRD, TG/DTA, SEM, TEM, A127- and 298i-MAS-NMR

408 and N2 adsorption measurements. We have found that the mordenite crystallites obtained by this new route are more uniform and in the nano size (80-120 nm) range. 2. EXPERIMENTAL 2.1 Hydrothermal transformation of Na-magadiite into mordenite

Na-magadiite was synthesized hydrothermally according to the published procedure [15]. From the chemical and thermogravimetric (TG) analysis, it was found that the chemical composition of the Na-magadiite used in this study was 13.8 SiO2 : Na20 : 9.8 H20. Hydrothermal transformation of Na-magadiite into mordenite was performed with the following molar composition: 0.33 Na20 : 0.08 K20 : x A1203 : SiO2 : 0.35 TEAOH : 35 H20 where x = 0.025, 0.016, 0.012 and 0.008. The influence of TEAOH, K20 and Na20 mole fractions in the hydrothermal transformation of Na-magadiite into mordenite was also studied by keeping the SIO2/A1203and H20/SiO2 ratios at a constant value of 40 and 35, respectively. The synthesis mixture was prepared by mixing the reagents in the following order: deionized water, sodium hydroxide (99%, Fluka), potassium hydroxide (85%, Fluka), tetraethylammonium hydroxide (TEAOH, 30% aqueous), Na-magadiite and stirred for 45 min. Then, required amount of sodium aluminate (19.93% A1203 and 19.11% Na20, Tricat Zeolite GmbH) was added dropwise and stirring continued for another 1 h. The resultant synthesis mixture was loaded into a Teflon-lined autoclave (50 ml) and heated at 175 ~ under autogenous pressure and static conditions for 24 h to complete the transformation of Na-magadiite into mordenite. The resultant product was filtered, washed thoroughly with deionized water, dried at room temperature and then calcined at 550 ~ for 12 h. In order to get the protonic form of mordenite (H-mordenite), the calcined samples (Na-mordenite) were treated with 1M NH4NO3 solution at 60 ~ for 12 h and then recalcined at 550 ~ for 12 h. 2.2 Characterization

Unless otherwise stated physico-chemical characterization were performed on protonic form of mordenite samples. X-ray diffraction patterns were recorded on a Philips X-ray diffractometer using Cu-Ka radiation. Scanning Electron Microscopy (SEM) images were obtained using a JEOL JSM 6400 Scanning Electron Microscope operated at 20 kV. Thermogravimetric-differential thermal analyses (TG-DTA) were performed using a TGDTA instrument (TA Instruments, SDT 2960) at a heating rate of 10 ~ min 1 in N2 atmosphere. Measurements of N2 adsorption at 77K were carried out using a Micromeritics ASAP 2010 apparatus. Chemical analyses of the samples were determined by Inductively Coupled Plasma Emission analysis (Perkin-Elmer 400). Selected samples were investigated by Transmission Electron Microscopy (Philips CM30T) and 27A1-and 29Si-MAS NMR spectroscopy (MSL 400, Bruker). 3. RESULTS AND DISCUSSION The XRD pattems of both the Na-magadiite and mordenite zeolite obtained from the hydrothermal transformation of Na-magadiite are shown in Figure 1. The recorded XRD pattem of Na-magadiite is typical of magadiite varieties and indicates that the product is free from impurities. The basal spacing obtained was 15.41 A, very close to the reported data [ 16]. The XRD pattem of the mordenite sample obtained by hydrothermal transformation of Na-magadiite is in good agreement with the reported profiles for the MOR structure [17], indicating the formation of a pure and highly crystalline mordenite sample.

409 The course of hydrothermal transformation of Na-magadiite into mordenite was followed by XRD. The XRD crystallinities of as-synthesized mordenite and the layered silicate, Na-magadiite, are plotted against time (Figure 2). The relative crystallinity

A

e" e-

20

2O

Figure 1.

X-ray powder diffraction pattems of: (a) Na-magadiite and (b) as-synthesized mordenite sample obtained by hydrothermal transformation of Na-magadiite at 175 ~ for 24 h [0.33 Na20 : 0.08 K20 : 0.025 A1203 : SiO2 : 0.35 TEAOH : 35 H20].

of the sample was determined from the sum of the areas of the peaks between 20 = 4 ~ and 6~ (for Na-magadiite) and 20 = 9 ~ and 10~ (for mordenite), respectively. It is known that the conventional hydrothermal crystallization kinetics of zeolites show typical sigrnoid curve [18]. However, our hydrothermal transformation kinetics of Na-magadiite into mordenite shows a different type of curve. The process of transformation of Na-magadiite into mordenite takes place as follows: At the beginning of the process, entire amount of TEAOH was present in the liquid phase of the reaction mixture. Heating of the reaction mixture causes dissolution of Na-magadiite in alkaline solution containing TEA + ions and aluminate anions, and thus makes the conditions for nucleation and crystal growth of mordenite via solution-mediated process. Presence of TEA + ions in the liquid phase makes possible a relatively fast starting crystallization of mordenite (0-6 h). On the other hand, during the same time interval, Na + ions from Na-magadiite were exchanged by TEA + ions from solution (intercalation), and thus decrease in the crystallinity of magadiite. Since the intercalation of Na- magadiite is faster than the crystallization of mordenite, this process causes a decrease of the concentration of TEA + ions in the liquid phase, and thus slowing down the crystallization of mordenite (4-12 h). It may be assumed that at t - 12 h, the most of TEA + ions are contained in TEA-intercalated magadiite, so that the amount of TEA + ions in the liquid phase is insufficient for further growth of mordenite crystals. However, slow dissolution of TEAintercalated magadiite at t-~ 12 h increases concentration of both silicon and TEA + ions in the liquid phase which makes conditions for further growth of already formed mordenite crystals. Further course of the crystallization is typical for crystallization of zeolites from amorphous aluminosilicate precursors (here TEA-intercalated magadiite). Continuing the hydrothermal treatment for a total of 48 h yielded no evidence of any other crystalline phase. However, prolonged hydrothermal treatment (48 h) resulted in a 6% decrease in the crystallinity of mordenite, caused by the slow dissolution of mordenite in alkaline solution

410 and precipitation of a layer of amorphous SiO2 by reaction of silicate anions from the liquid phase on the surfaces of mordenite crystals as previously observed during the dissolution of ZSM-5 crystals in alkaline solutions [ 19].

60

g

o

9

20 0 0

10

20

30

40

50

Time, hours

Figure 2.

The course of hydrothermal transformation of Na-magadiite into mordenite (as-synthesized) from a molar composition: 0.33 Na20 : 0.08 K20 : 0.016 A1203 :SiO2:0.35 TEAOH : 35 H20

In order to examine the influence of different cations (TEA +, Na + and K +) in the hydrothermal transformation of Na-magadiite into mordenite, additional series of experiments

0,8

1,00 0,00

0,25

~

~

0,50

0,75

,

0,0

1,00

TEAOH

Figure 3. The influence of TEAOH, K20 and Na20 mole fractions in the hydrothermal transformation of Na-magadiite into mordenite at 175 ~ for 24 h: [SIO2/A1203 = 40 and H20/SiO2 = 35]. were carried out by keeping the SIO2/A1203 and H20/SiO2 ratios at a constant value of 40 and 35, respectively. All the compositions and the results are summarized in the ternary diagram (Figure 3). Control experiment (in the absence of TEAOH) showed that the transformation Na-magadiite into mordenite occurred together with c~-quartz and gismondine (GIS). However, the impurities, such as c~-quartz and GIS are suppressed by the addition of TEAOH in the synthesis mixture. Highly crystalline and impurity-flee mordenite is obtained at intermediate TEAOH content in the synthesis mixture. Thus, the intercalating agent and/or structure-directing agent TEAOH is essential for the hydrothermal transformation of

411 Na-magadiite into mordenite. Further increase of TEAOH in the synthesis mixture leads to the formation offretite (OFF) type zeolite along with traces of mordenite, while from TEAOH rich synthesis mixture (sodium and potassium sources were not added), intercalated TEA § magadiite is obtained as the main product. The influence of K20 and Na20 mole fractions (Figure 3; from bottom to the top) on the hydrothermal transformation of Na-magadiite into mordenite is also studied. When K20 is not used in the synthesis mixture, mordenite is obtained with traces of analcime (ANA). A high K20 mole fraction leads to the formation of mordenite with (x-quartz and GIS. When no extra Na20 is used in the synthesis mixture, GIS is obtained with some unidentified phases. These experiments suggest that all three cations (TEA § Na § and K +) are necessary for the hydrothermal transformation of Na-magadiite into phase-pure mordenite. Physico-chemical properties of the mordenite samples with different SIO2/A1203 ratios are given in Table 1. Phase-pure mordenite samples with different SIO2/A1203 ratios were prepared by varying the SIO2/A1203 ratios in the gel by keeping the OH-/SiO2 ratios between 1.19-1.21 by the addition of appropriate amount of sodium hydroxide. It was found from the chemical analysis of the products formed that the SIO2/A1203 ratios are 13.4, 16.2, 17.0 and 19.4 for SIO2/A1203 input ratios of 40, 60, 80 and 120, respectively. This indicates that highly crystalline mordenite can be obtained with a narrow range of SIO2/A1203 ratios (13.419.4) without impurity phases. The BET surface areas (325-369 m2g-a) and micropore volumes (0.12-0.14 cm3g-1) are indicative of the purity of the mordenite samples obtained by hydrothermal transformation of Na-magadiite. This compares closely to similar values reported for mordenite synthesized by conventional crystallization method [20, 21]. Further evidence for the transformation of Na-magadiite into mordenite under hydrothermal treatment is provided by the TG-DTA analysis. It can be seen from the Table 1, that there is a significant amount of TEAOH incorporated in the as-synthesized mordenite zeolite independent from the starting SIO2/A1203 ratios (40-120) in the gel composition. This result suggests that the intercalating agent (TEAOH) and/or structure directing agent, which is intercalated in between the layers of Na-magadiite, is indeed necessary for the hydrothermal transformation of Na-magadiite into pure mordenite type zeolite. Table 1. Physico-chemical properties of mordenite samples derived from Na-magadiite a, b. Sample SiO2/A1203 Phase no. Gel c Product ~ 1 2 3 4

40 60 80 120

13.4 16.2 17.0 19.4

MOR MOR MOR MOR

Crystallinity (%)e 100 97 96 94

TEA+/ BET surface u.cf area (m 2g-l) 3.6 3.8 3.9 3.9

340 366 369 325

Micropore volume (cm 3g-1) 0.13 0.14 0.14 0.12

aChemical composition of the Na-magadiite was 13.8 SiO2 " N a 2 0 " 9.8 H20, bHydrothermal transformation time, 24 h; CGel composition: 0.33-0.36 Na20 90.08 K20 90.025-0.008 A1203 " SiO2 90.35 TEAOH 935 H20. aCalcined, exchanged and recalcined (H-mordenite). eThe relative crystallinity of the sample was determined from the sum of the areas of the peaks between 20 = 25 ~ and 28~ fNumber of template (TEA +) molecules per unit cell was calculated from the thermogravimetric analysis of the as-synthesized samples.

412 SEM micrographs of the mordenite samples prepared from the hydrothermal transformation of Na-magadiite, with and without the addition of the intercalating agent, TEAOH, are shown in Figure 4. In the presence of TEAOH (Figure 4a), very-small crystallites with a size of approximately 0.1 ~tm are observed. No amorphous material was present in the sample. The crystals are considerably smaller than the mordenite crystals (5-6 ~tm) synthesized by conventional hydrothermal crystallization with and without the aid of organic structure directing agent [22]. One of the main reasons for the formation of very small crystals is mainly due to large number of mordenite nuclei formed at the very start of the transformation process. Hence, the quick transformation of the TEA+-intercalated magadiite into mordenite, is a consequence of the growth of the large number of mordenite nuclei formed in the system. The morphology and size of the crystallites are quite similar for the mordenite samples (figure not shown) with different SIO2/A1203 ratios (13.4-19.4). On the other hand, in the absence of TEAOH, Figure 4b reveals the formation of rectangle shaped large crystallites (5 ~tm) with an appreciable amount of rice shaped agglomerates or impurity phases ((z-quartz and GIS). The morphology of this mordenite sample is quite different from the hydrothermally synthesized mordenite crystallites, which are very often hexagonal in nature [22]. This result clearly indicates that the presence of the intercalating agent is indispensable for the hydrothermal transformation of Na-magadiite into a pure mordenite. Transmission electron microscopy (TEM) investigations were also carried out to verify the small crystal size of the mordenite sample obtained from the hydrothermal transformation of Na-magadiite using TEAOH as the intercalating and/or structure-directing agent. The TEM image of the mordenite (sample 1 in Table 1) sample is shown in Figure 5. It can be observed from the micrograph that the majority of mordenite crystallites are cubic and spherical and are in the size range of 80 to 120 nm. Moreover, as is shown in Figure 5, a considerable distribution of crystallite sizes can also be observed. The 27A1 MAS NMR spectrum (Figure 6a) of a mordenite (sample 1 in Table 1) sample shows a single line at 51.6 ppm, as already observed for mordenite zeolites and assigned to A13+ in tetrahedral coordination [23]. No signal is observed between 50 and 0 ppm. This confirms that there is no extra framework A1 species (penta- or hexa- coordinated) in the mordenite sample. The 29Si-MAS NMR spectrum of this mordenite sample is given in Figure 6b. The 29Si spectrum exhibits strong lines a t - 1 0 5 a n d - 1 1 3 ppm, which are ascribed to Si(0A1) and (SilA1), respectively. A smaller and broader signal can also be seen at

Figure 4.

SEM micrographs of mordenite obtained from the hydrothermal transformation of Na-magadiite: (a) in the presence of TEAOH (sample 1 in Table 1) and (b) in the absence of TEAOH (control experiment in Figure 3).

413

Figure 5.

TEM micrograph of mordenite (sample 1 in Table 1) sample obtained from the hydrothermal transformation of Na-magadiite.

t . . . 1-*-* . : i r,-r-v-,-r,-,-;',

. .!. . .

-r-,-~-r~-r

-"',"~"-'-

-,-,-"-

""

......

'-I ~

, ~ ' I "'

' ~ i ....

r ....

1

-75 -80 -85 .-90 -.95 -100 -105 -110 -115 -120 -125 -130 -I35 -140 ~lppm

8/ppm

(a)

(b)

Figure 6. MAS NMR spectra of the as-synthesized mordenite (sample 1 in Table 1) sample: (a) 27A1 and (b) 29Si -99 ppm, which is attributed to the presence of (Si2A1) [23]. We are currently investigating in more detail the effect of SIO2/A1203 ratios and also the thermal treatment on the transformation of small- to large-port mordenite. 4. CONCLUSIONS Highly crystalline mordenite zeolite can be obtained at 175 ~ by hydrothermal transformation of Na-magadiite using TEAOH as the intercalating and/or structure-directing agent. XRD reveals that the mordenite samples obtained by this new route are pure and highly crystalline. Interestingly, it is found that this route requires all three cations (TEA +, Na + and

414 K +) for the hydrothermal transformation of Na-magadiite into phase-pure mordenite. TGDTA indicates that there is a significant amount of TEA + incorporated in the as-synthesized mordenite samples. This suggests that the intercalating and/or structure-directing agent (TEAOH), which is intercalated in between the layers of Na-magadiite, is indeed necessary for the hydrothermal transformation of Na-magadiite into mordenite. TEM confirms that mordenite crystallites are in the nano size (80-120 nm) range. We believe that improved catalytic properties can be expected from the smaller crystallites of these mordenite samples. Work is in progress to investigate this supposition. ACKNOWLEDGEMENTS We thank the Deutsche Forschungsgemeinschaft (Schw 478) and the Fonds der Chemischen Industrie (FCI) for generous financial assistance. REFERENCES ~

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

G. Lagaly, K. Beneke and A. Weiss, Am. Mineral. 60 (1975) 642. M. Ogawa, M. Miyoshi and K. Kuroda, Chem. Mater. 10 (1998) 3787. M. Ogawa and Y. Takizawa, J. Phys. Chem. B. 103 (1999) 5005. U. Brenn, W. Schwieger and K. Wuttig, Colloid Polym. Sci. 277 (1999) 394. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc. Chem. Commun. (1993) 680. K. Kuroda, J. Porous Mater. 3 (1996) 107. K. Kosuge and A. Tsunashima, J. Chem. Soc. Chem. Commun. (1995) 2427. J. S. Daily and T. J. Pinnavaia, Chem. Mater. 4 (1992) 855. U. Brenn, W. Schwieger and H. G. Karge, Proceedings of 12th International Zeolite Conference (M. M. J. Treacy, B. K. Marcus, M. E. Bisher and J. B. Higgins, eds.), Baltimore, USA, (1999) 847. S. Shimizu, Y. Kiyozumi, K. Maeda, F. Mizukami, G. Pal-Borbely, R. M. Mihalyi and H. K. Beyer, Adv. Mater. 8 (1996) 759. G. Pal-Borbely, H. K. Beyer, Y. Kiyozumi and F. Mizukami, Microporous Mesoporous Mater. 11 (1997) 45. G. Pal-Borbely, H. K. Beyer, Y. Kiyozumi and F. Mizukami, Microporous Mesoporous Mater. 22 (1998) 57. Y. Ko, S. J. Kim, M. H. Kim, J-H. Park, J. B. Parise and Y. S. Uh, Microporous Mesoporous Mater. 30 (1999) 213. T. Selvam and W. Schwieger, Stud. Surf. Sci. Catal. 135 (2001) 411. W. Schwieger, W. Heyer and K-H. Bergk, Z. Anorg. Allg. Chem. 559 (1988) 191. J. S. Daily and T. J. Pinnavaia, J. Inclu. Pheno. Mol. Recog. Chem. 13 (1992) 47. W. M. Meier, Z. Kristallogr. 115 (1961) 439. R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. A. Cizmek, L. Komunjer, B. Subotic, R. Aiello, F. Crea and A. Nastro, Zeolites, 14 (1994) 182. V. R. Chumbhale, A. J. Chandwadkar and B. S. Rao, Zeolites, 12 (1992) 63. A. A. Shaikh, P. N. Joshi, H. S. Soni and V. P. Shiralkar, Adsorpt. Sci. Technol. 16 (1998), 529. G. J. Kim and W. S. Ahn, Zeolites, 11 (1991) 745 T. Sano, S. Wakabayashi, Y. Oumi and T. Uozumi, Microporous and Mesoporous Mater. 46 (2001) 67.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

415

H y d r o t h e r m a l synthesis and characterization o f n e w p h o s p h a t e - b a s e d materials p r e p a r e d in the p r e s e n c e o f 1,4-dimethylpiperazine L. Josien a, A. Simon-Masseron a, S. Fleith a, V. Gramlich b and J. Patarin a aLaboratoire de Mat6riaux Min6raux, ENSCMu, Universit6 de Haute-Alsace, CNRS UMR 7016, 3 rue Alfred Wemer, 68093 Mulhouse Cedex, France bLaboratorium fiir Kristallographie, ETH-Zentrum, 8092 Ztirich, Switzerland Two new phosphate-based materials, Mu-23 and Mu-25, are prepared with 1,4dimethylpiperazine as structure directing agent. Mu-23 is the first two-dimensional fluorinated gallophosphate with a Ga/P molar ratio of 5/4. This new phosphate-based material, with the chemical formula [(C6H15N2)(C6HI6N2)GasF6(H20)2(PO4)a].4H20, crystallizes in the triclinic space group P-1 with the following unit cell parameters : a = 8.735(11) A, b = 8.864(5) A, c = 12.636(10) A, ot = 98.36(5) ~ 13= 100.18(8) ~ ~/= 115.84(7) ~ The synunetry of the three-dimensional zincophosphate Mu-25, [(C6HI6N2)Znz(HPO4)3],is monoclinic (space group P 21/n) with a = 8.641(5) A, b - 14.364(9) A, c = 12.581(11) A, [3 = 96.39(6) ~ The layers of the gallophosphate consist of GaOzF3(H20), GaO4F2 octahedra and GaO4, PO4 tetrahedra which share their oxygen and some of their fluorine atoms. The connectivity scheme of these different polyhedra leads to the formation of 8-membered tings. The framework of Mu-25 results from tetrahedrally coordinated zinc and phosphorus atoms linked through oxygen vertices. The interrupted framework (presence of P-OH and P=O bondings) displays channels with 4-, 8- and 12-membered ring openings. Structures were determined by single crystal X-ray diffraction and characterized by Solid State NMR spectroscopy, thermogravimetric (TGA) and differential thermal (DTA) analyses. 1. INTRODUCTION Beside the well known zeolites (i.e., microporous aluminosilicates), open-framework metal phosphates have attracted research activities due to their potential applications [1]. 3-D frameworks with large pore openings have been investigated such as the aluminophosphate JDF-20 with 20 membered-ring openings (20 MR) [2], the gallophosphates cloverite (20 MR) [3] and NTHU-1 (24 MR) [4] or the zincophosphate ND-1 (24 MR) [5]. The organic template introduced in the starting mixture plays a crucial role in the assembly of building units. On the other hand, an amine such as ethylenediamine is able to lead to more than 15 different phosphate-based materials. In most of gallophosphates and whatever the dimensionality of the inorganic network, the Ga/P molar ratio is below or equal to 1. Only five 3-D structures display a Ga/P molar ratio higher than 1 (see Table 1). To our knowledge, no layered or chain-like structure with Ga/P molar ratio higher than 1 has been reported in the literature. Here we report the synthesis and characterization of the first layered fluorinated gallophosphate with a Ga/P higher than 1 (Mu-23). It was prepared in the presence of

416 1,4-dimethylpiperazine (DMPIP) as structure-directing agent. Using the same organic template the 3-D zincophosphate (Mu-25) was obtained and its structure is also reported. Table 1 Summary of the gallophosphates with a Name Structure directing species n.s. [6] sodium n.s. [7] 4,4'-bipyridine Mu-5 [8] cyclam ULM-2 [9] DABCO DIPYR-GaPO [10] 4,4'-bipyridine Mu-23 [this work] DMPIP

Ga/P ratio higher than 1 Chemical formula [Na3Gas(PO4)402(OH)2] .2H20 [Gas(OH)2(C204)(POn)4(C10H9N2)].2HzO

[Ga16PI6064Fs(OH)4(GaC10H24N4)4] [GasP6024F4(OH)4(C6N2H14)]e4H20

[GavP6028F3(C10N2HI6)] [(C6H15Nz)(C6H16Nz)GasF6(HzO)2(PO4)4] 9 4H20

Ga/P ratio 5/4 5/4 5 /4 4/3 7/6 5 /4

n.s. 9Not specified Cyclam = 1,4,8,11-tetraazacyclotetradecane; DABCO = diazabicyclo[2,2,2]octane; DMPIP = 1,4-dimethylpiperazine

2. EXPERIMENTAL SECTION The synthesis was performed at 130~ and 170~ for the gallophosphate Mu-23 and the zincophosphate Mu-25 respectively (see below). Single crystal X-Ray diffraction was carried out with MoKc~ radiation in omega scan mode on a SYNTEX P21 4-circle diffractometer equipped with a graphite monochromator. Single crystals with dimensions of 0.4 x 0.4 x 0.2 mm and 0.1 x 0.1 x 0.1 mm were selected for Mu-23 and Mu-25 respectively. The structures were solved by direct methods with SHELXS86 [11] and refined with SHELXS-93 [12]. For both materials the hydrogen atoms were placed with geometrical constraints. The experimental and crystallographic data relative-to these two phosphate-based materials are reported in Table 2. The morphology and size of the crystals were determined by scanning electron microscopy using a Philips XL30 microscope. The powder XRD patterns were obtained with CuK~I radiation on a STOE STADI-P diffractometer equipped with a curved germanium (111) primary monochromator and a linear position sensitive detector. Thermogravimetric (TGA) and differential thermal analyses (DTA) were performed under air on a Setaram Labsys thermoanalyser with a heating rate of 5~ min 1 until 850~ In some cases, a calcination at 1000~ was necessary to obtain the total amount of the organic template. The 31p MAS NMR spectra of Mu-23 and [Mu-25] were recorded on a Bruker DSX 400 spectrometer with a pulse width of 4.5 and [1.5] ~ts for a n/2 and [n/6] flip angle, with a recycle time of 500 and [90] s and a spinning rate of 8 and [10] kHz. Mu-23 was also characterized by t9F MAS NMR. The spectrum was recorded with a pulse width, a flip angle, a recycle time and a spinning rate of 8.2 ~s, rt/2, 20 s and 14 kHz respectively.

417 Table 2 Summary of the experimental and crystallographic data of the fluorogallophosphate Mu-23 [(C6H15N2)(C6H16N2)GasF6(H20)2(PO4)4]'4H20 and of the zincophosphate Mu-25 [(C6H16N2)Zn2(HPO4)3] Mu-23 Mu-25 Mu-23 Mu-25 4.280 3.469 Fw (g mo1-1) 1181.97 534.92 Ix (mm q) -10 < h < 10 -10_
8.735(11)

8.641 (5)

collection degrees Data collection 293(2) temperature (K)

293(2)

b (A)

8.864(5)

14.364(9)

Observed

2432

1897

2975/18/247

2745/0/247

reflections lI>2o(!)l

c (.~)

12.636(10)

12.581(11)

Data/restraints/ parameters

ot 13 y V (A3)

98.36(5) 100.18(8) 115.84(7) 838.9(13)

90 96.39(6) 90 1552(2)

Z

1

4 2.285

R1, wR2 [1>2o(1)] 0.0255, 0.0641 (all data) 0.0304, 0.0652 Largest diff. peak 0.659 and --0.905 Refinement Full matrix leastmethod squares on F 2 F(000) 588

D calc (~ cm "3) 2.340

0.0307, 0.0720 0.0440, 0.0732 0.906 and -0.736 Full matrix leastsquares on F 2 1076

3. R E S U L T S AND D I S C U S S I O N

3.1. Synthesis The gallophosphate Mu-23 crystallizes in a narrow range of synthesis conditions. It is obtained from a mixture carried out under static conditions at 130~ for 6 days with the following molar composition : 1 Ga203 : 4 P205 : 8 HF : 640 H20 : 8 DMPIP. The Ga/P ratio in the crystallized material is quite different from the one introduced in the starting mixture (SM) i.e., (Ga/P)sM = 88 The amounts of gallium and amine are critical to obtain a pure phase. Attempts to increase the (Ga/P)sM ratio induce the co-crystallization of a non-identified phase (INC phase), which is obtained as pure phase when (Ga/P)sM = 1/1. Experiments with large amounts of DMPIP (Ga203 / DMPIP = 1/16) lead to amorphous material which shows that DMPIP looses its templating effect when it is introduced in high concentrations. The zincophosphate Mu-25 is prepared in aqueous medium without fluoride ion. A typical molar composition for the starting mixture is : 1 ZnO : 2 P205 : 50 H20 : 2 DMPIP. The latter is introduced in a Teflon| lined stainless steel autoclave and heated at 170~ under autogenous pressure for 13 days.

3.2. Crystal morphology The gallophosphate sample consist of large spheres of aggregated crystals (sphere up to 2 mm) characterized by a rhomboidal prism morphology with size ranges from 75 x 50 x30 pm to 700 x 500 x 200 pm (Figure 1a). For Mu-25, the crystals have also a rhomboidal prism morphology with size of ca. 4 x 2 x 1 mm (Figure lb).

418

Figure 1. Scanning electron micrographs of the fluorogallophosphate Mu-23 a) and the zincophosphate Mu-25 b). 3.3. Structure determination

The powder XRD patterns of Mu-23, the INC phase and Mu-25 are shown in Figures 2a, 2b and 2c respectively. According to the elemental and microprobe analyses, the as-synthesized Mu-23 and Mu-25 samples have the following composition (wt %): Ga 29.2, P 10.5, F 10.0, N 4.6, C 12.2 and Zn 28.1 , P 17.6, N 5.1, C 13.5 respectively. The results of these analyses are in good agreement with the unit cell formula obtained by structure determination i.e., [(C6H15N2) (C6HI6Nz)GasF6(H20)2(POn)4].4H20(Mu-23) and [(C6HI6N2)Zn2(HPO4)3](Mu-25). The thermal behavior of these materials was investigated by high-temperature XRD analysis and TG/DTA thermal analyses. For Mu-23 the total weight loss occurs in three steps. The first one before 200~ (10 wt%) corresponds to the removal of water. It is confirmed by the presence of endothermic peaks on the DTA curve in the same range of temperature. The second step, between 200 and 850~ (25 wt%) is associated to the decomposition of the organic template and removal of hydrofluoric acid. The removal of the amine results irt a collapse of the structure, and an amorphization is observed at 250~ from high-temperature XRD analysis. It should be noticed that after calcination at 850~ traces of organic compounds are still present. They are removed at ca. 1000~ (3 wt%).

a'A

. . . . . . . . . . . . . .

b__L_)

5

10

15

20

25

30

35

40

45

Figure 1. Experimental XRD patterns of a) Mu-23, b) INC phase and c) Mu-25.

50 o/20

419 For Mu-25, the first weight loss is observed at a higher temperature. Removal of water resulting from condensation of hydroxyl groups is observed between 180 and 300~ (2.3 wt%) and between 375 and 395~ with the beginning of the removal of the organic species (12.2 wt%). The latter is completely removed after heating the sample to 1000~ (13.4 wt%). The high temperature XRD analysis indicates that the structure collapses at c a . 260~

3.4. Structure description For both materials, direct methods revealed the position of Ga, P and Zn atoms. All the remaining atoms except the hydrogen ones were located from successive difference Fourier maps. The resulting atomic coordinates are reported in Table 3. The non-hydrogen atoms were refined with anisotropic displacement parameters. Mu-23 consists of amine and water molecules intercalated between inorganic sheets. Except for Ga(1), which is located on the inversion center, all the remaining atoms are in general position. The inorganic sheet consists of 4-membered ring ladder chains running along [ 100] direction (bold lines, Figure 2a) and 8-membered rings. This type of zigzag chain has previously been described as [Z(3,3)]oo chain by Ozin et al.[13] The originality of Mu-23 is the connectivity scheme of the [Z(3,3)]oo chains. The gallium atoms Ga(1) bonded through fluorine to two Ga(3) gallium atoms ensure the connection of the chains and lead to the formation of the layers. This connection unit is built of Ga2PO2F groups and forms 3membered rings (hatched part, Figure 2).

Figure 2. a) Projection of the structure of Mu-23 in the plane (a, b) showing the 8-membered tings and the 4-membered ring ladder chains (thick lines), b) View of the structure of Mu-25 showing the 8- and 12-membered ring openings. For clarity, the amine, water molecule, the framework oxygen and fluorine atoms are omitted. Three terminal X groups are bonded to the gallium Ga(3). Taking into account the bond lengths, and a bond valence calculation [14], these X groups were assigned to two F and one water molecule. Such an assignment (2F and H20) seems to be confirmed by the scheme of the hydrogen bondings (Figure 3). Moreover, a F/H20 molar ratio equal to two on these terminal positions is expected from the chemical and microprobe analyses (total of six fluorine atoms per unit cell). However, a mixture of fluorine and water molecule on these three terminal positions cannot be excluded. Therefore, layers display GaO2F3(H20) and

420 GaO4F2 octahedra, GaO4 and two crystallographically different PO4 tetrahedra. GaO4F2, GaO4 and PO4 polyhedra are quite regular. Their average bond lengths are d ~ 1.94 A, d ~ 1.82/~, d ~ 1.53/~ and d = 1.53 A. The GaO2F3(H20) octahedron is more distorted due to the presence of terminal groups (d (X = F or H20) ranges from 1.85 to 1.92 A). Table 3 Atomic coordinates (• and equivalent isotropic displacement parameters (A 2 x 103) for Mu-23. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor Mu-25

Mu-23 ,w,

Atom

x

y

Ga(1)

0 2745.6(5) 7384.4(5) 6056.3(11) 934.3(11) 718(3)

,

,

y

z

U(eq)

U(eq)

Atom

X

5000 0 308.0(4) 64.2(3) 3777.4(5) -2768.1(3) 2235.1(11)-788.6(7) 2459.9(10) 1089.2(7) 3 2 1 6 ( 3 ) 113(2)

8.93(12) 97.1(10) 10.65(11) 9.7(2) 9.3(2) 12.1(5)

Zn(1) Zn(2) P(3) P(1) P(2) 0(9)

1402(1) 6199(1) 3745(1) 6061(1) 8857(1) 4629(4)

-936(3) 1911(3)

1177(3) 3793(3)

1159(2) 2189(2)

12.7(5) 13.6(5)

1917(3) 3882(3) 4394(3) 0(6) 5859(3) 0(7) o(w) b 6693(3) 2324(3) 0(9) O(lW) c 6952(4) O(2W) c 8770(4) 9331(2) F(1) 9032(3) F(2) 5600(3) F(3) -1090(4) N(1)

1410(3) -501(3) 1950(3) 2800(3) 1434(3) 6451(3) 253(4) 2280(4) 4031(2) 4362(3) 3730(3)

939(2) 988(2) -410(2) -1855(2) -3506(2) -122(2) 4511(2) 3290(3) -1601(2) -3618(2) -3836(2)

15.5(5) 15.1(5) 14.8(5) 14.2(5) 17.7(6) 12.6(5) 31.1(7) 39.9(8) 13.6(4) 18.4(5) 18.7(4)

1610(2) -1448(3) 22(1) -503(2) 4240(3 23(1) -142(2) 2334(3) 27(1) 0(5) 0(8) 10241(4) -838(2) 3554(3) 22(1) O(11')" 2987(12) -481(6) 3082(9) 29(2) O(11)" 2357(8) -209(5) 2488(6) 32(2) O(1) 6924(4) 841(2) 318(3) 20(1) O(10) 2926(5) 1199(2) 3504(3) 41(1) N(1) 1189(5) 1824(3) 1437(4) 29(1) C(8) -297(6) 2276(4) 1639(5) 33(1) C(3) 623(6) 1486(4) -487(4) 28(1) C(7) 1461(7) 2531(4)-1834(4) 32(1) C(6) 2416(6) 2474(4) 1170(4) 33(1)

7169(4)

3029(3)

23.5(7)

N(4)

1832(4) 2149(3) -739(3)

-4156(4) -2517(5)

4343(4) 5414(5)

3329(3) 4258(3)

20.3(7) 23.4(9)

C(5) 0(7)

2031(7) 2888(4) 84(4) 28(1) 9489(4) 832(2) 3764(3) 24(1) 9 7 8 ( 7 ) 1082(4) 616(5) 37(2)

Ga(2) Ga(3) P(1) P(2)

O(1) 0(2) o(3) 0(4)

o(5)

N(4) C(S) C(2)

z

-

0(4) 0(6)

1270(1) 4545(1) 16(1) 100(1) 1432(1) 15(1) 389(1) 3064(1) 19(1) 1508(1 ) -468(1) 14(1) -125(1) 3466(1) 14(1) 684(2) 2163(3) 30(1)

7002(4)

7776(4) 9092(4)

-2720(5)

6120(5)

2109(3)

26.1(9)

C(2)

C(3)

-3707(5)

4324(5)

2243(3)

25.7(9)

O(12) a 4635(7) -317(5) 3830(5)

c(6) C(7) c(8)

-1551(5) -5160(6) -124(6)

7212(5) 2550(5) 8945(6)

4114(3) 3455(4) 2886(5) ,,

17(1)

34(2) 24.9(9) O(12') a 5146(11) 481(7) 4072(7) 34(2) 32.6(10) 0(2) 5945(4) 2478(2) -16(3) 23(1) 41.7(12) O(3) 4439(4) 1127(2) -847(3) 21(1) ,

a Occupancy factor. 0.5 b related to water molecule bonded to Ga(3) c related to water molecules located into the interlayered space

421 Water molecules and the mono- and diprotonated amines located in the interlayered space are in strong interaction with the inorganic layer by hydrogen bonding. A scheme of these hydrogen interactions is given in Figure 3.

Figure 3. Structure of Mu-23 : view towards (a,b) plane showing the hydrogen bonds (dashed lines). The structure of Mu-25 displays three types of HPO4 tetrahedra: one PO2OOH and two PO3OH. Two ZnO4 tetrahedra are also present sharing their oxygen atoms with the POzOOH and PO3OH tetrahedra. The Zn-O and P-O bond lengths range form 1.914 to 1.965 A, and 1.412 to 1.658 A with an average bond length of 1.937 and 1.523 A respectively. The structure results of the stacking of [4,8] layers connected by PO2OOH units (Figure 2b). The diprotonated amine is located in the pore system delimited by an interconnected threedimensional channel system with 8-, 10- and 12-membered ring openings. Such a structure was previously obtained by Ahmadi et al [ 15] but in that case dabconium cations were used as organic templates. 3.5. Solid State NMR Spectroscopy The 31p MAS NMR spectrtma of Mu-23 displays two signals at -7.2 and -9.2 ppm. The intensity ratio between the 2 peaks is close to 1 : 1, revealing in agreement with the structure determination, the existence of two distinct crystallographic phosphorus sites with the same multiplicity. Surprisingly, the 31p MAS NMR spectrum of Mu-25 reveals only one peak centered at 2.9 ppm. It is quite different from the one observed for the zincophosphate synthesized by Ahmadi et al for which the 3 distinct crystallographic sites have clearly been observed at 7.4, 3.3 and-1.0 ppm [19]. In the 19F MAS NMR spectrum of Mu-23, the main signal is observed at -130.7 ppm and a shoulder is also clearly evidenced at about-139 ppm. Such chemical shift values correspond to those usually observed for bridging and terminal fluorine atoms. However from this spectrum, it is difficult to distinguish the different types of fluorine present in the material since the relative intensity ratio between the two components i.e., 5 : 1 does not fit with the one expected from the structure analysis i.e., 2 : 1.

422 REFERENCES 1. S.J. Miller (Chevron Research Co.), Eur. Patent No. 209997 (1987) [Chem. Abstr., 106 (1987) 105235r]. 2. Q. Huo, R. Xu, S. Li, Z. Ma, J.M. Thomas, R.H. Jones and A.M. Chippindale, J. Chem. Soc. Chem. Commun. (1992) 875. 3. A. Merrouche, J. Patarin, H. Kessler, M. Soulard, L. Delmotte, J.L. Guth and J.F. Joly, Zeolites, 12 (1992) 226. 4. C.H. Lin, S.L. Wang and K.H. Lii, J. Am. Chem. Soc., 123 (2001) 4649. 5. G.Y. Yang and S.C. Sevov, J. Am. Chem. Soc., 121 (1999) 8389. 6. M.P. Attfield, R.E. Morris, E. Gutierrez-Puebla, A. Monge-Bravo and A.K. Cheetham, J. Chem. Soc. Chem. Commun., (1995) 843. 7. C.Y. Chen, P. Chu and K.H. Lii, Chem. Commun., (1999) 1473. 8. T. Wessels, L.B. MeCusker, C. Baerloeher, P. Reinert and J. Patarin, Mieroporous Mesoporous Mater., 23 (1998) 67. 9. T. Loiseau and G. F&ey, Eur. J. Solid State Inorg. Chem., 30 (1993) 369. 10. S.J. Weigel, R.E. Morris, G.D. Stucky and A.K. Cheetham, J. Mater. Chem., 8 (1998) 1607. 11. G.M. Sheldrick, SHELXS-86, Program for the Solution of Crystal Structures, University of G6ttingen, Germany, (1986). 12. G.M. Sheldrick, SHELXL-93, Program for Crystal Structure Determination, University of G6ttingen, Germany, (1993). 13. S. Oliver, A. Kuperman and G.A. Ozin, Angew. Chem., 110 (1998) 48; Angew. Chem. Int. Ed., 37 (1998) 47. 14. I.D. Brown, in Structure and Bonding in Crystals, M. O'Keeffe and A. Navrotsky (Eds.), Academic Press, New York, Vol. 2, Chap. 14, 1981. 15. K. Ahmadi, A. Hardy, J. Patarin and L. Huve, Eur. J. Solid State Inorg. Chem., 23 (1995) 209.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science.B.V. All rights reserved.

423

Modeling of crystal growth at early stages of analcime synthesis from clear solutions B. Suboti6a, R. Aiello b, J. Broni6a and F. Testab " Laboratory for the Synthesis of New Materials, Ruder Bo~kovi6 Institute, Bijenicka 54, 10000 Zagreb, Croatia. E-mail: bronic/[email protected] Dipartimento di Ingegneria Chimica e dei Materiali, Universifft della Calabria, 87030 Rende (CS) Italy. E-mail: r.aieUo/[email protected]

b

A population balance model of crystallization of zeolites from clear aluminosilicate solutions is developed. The influence of the warming up the crystallizing system, from the ambient temperature to the crystallization one, on the kinetics of the crystal growth and overall crystallization, is included in the model. The validity of the developed model is evaluated by correlation of simulated (calculated) kinetics of crystal growth and crystallization curves with the measured ones, during crystallization of analcime from clear solutions of constant composition, but prepared with different silica sources. 1. INTRODUCTION Based on the assumption that: (i) the most of the clear solution synthesis produce a single burst of nuclei in a short time, so the crystal populations from such systems are monodisperse [1], (ii) surface integration kinetics, rather than diffusion limitations, govern the rate of zeolite crystal growth [2,3] and (iii) earlier derived kinetics of crystal growth of zeolites [3], a population balance model of crystallization of zeolites from clear (alumino)silicate solutions is developed. The influence of the warming up the crystallizing system, from the ambient temperature to the crystallization one [4] on the kinetics of the crystal growth and overall crystallization, is included in the model. The validity of the developed model is evaluated by testing of the early stage of crystallization of analcime from the clear solutions, as model system.

2. MODEL SYSTEM Analcime was crystallized from an aluminosilicate solution having the batch molar composition: 87 Na20 A1203 84 SiO2 2560 1-120 [1]. The solution is prepared at 25~ using Cab-O-Sil, (system 1), puratronic silica (system 2), sodium silicate nonahydrate (system 3) and sodium silicate pentahydrate (system 4) as the silica sources, and sodium aluminate (prepared by dissolution of aluminum wire in NaOH solution) as aluminum source, in all syntheses [1]. All solutions were heated at 160~ to crystallize analcime [1].

424 3.POPULATION BALANCE OF THE MODEL SYSTEM

Based on the general principles of the population balance [5], of zeolite crystallization under isothermal, constant volume conditions [4,6-9], the changes of the particulate properties during zeolite crystallization from a clear (alumino)silicate solutions (see equations (1) - (11)), may be defined by a set of ordinary differential equations: dmoldt c = dNIdt c = B

(1)

d m l l d t c = Q mo

(2)

dm21dt c = 2 O m 1

(3)

dm3/dt c =3 Q m 2

(4)

where mi = ~Li(dN/dL)dL is i-th (i = 0, 1, 2, and 3) moment of the particle size distribution of zeolite crystals at crystallization time tc, N is the number of crystals with size L at the crystallization time tc, dN/dt c = B is the rate of nucleation, and Q = dL/dt c is the rate of crystal growth. Formation of monodisperse crystal population in the crystalline end products obtained by crystallization of zeolites from clear aluminosilicate solutions [1,2,10-13] indicates that all zeolite nuclei are formed in a short time [1] at the very start of the crystallization process. Assuming that the time interval Atn under which nuclei are formed, is much shorter than the entire time of crystallization tc (i.e., Atn << tc), and there is not formation of new nuclei during crystallization, it may be postulated that: N t = N(t c > Atn) = N O= constant, and hence, dNIdt c = B -- 0

(5)

where, N t = N O is number of nuclei formed at tc > Atn -- 0 as well as the number of zeolite crystals present in the system at any crystallization time tc < Atn, and B = dN/dt c is the rate of nucleation. Recent studies of the kinetics of the growth of zeolite microcrystals have shown that the growth rate is size-independent and governed by the reaction of monomeric and/or lowmolecular aluminates, silicate and aluminosilicate anions from the liquid phase on the surfaces of growing zeolite crystals [2,3,14]. The crystal growth takes place in accordance with the Davies and Jones model of growth and dissolution [15,16]. Taking into consideration particularities of zeolite crystallizing systems, the kinetics of crystal growth of zeolites may be defined as [3,14]: dL/dt c = kg (CA1- CAI )(Csi- C~i )r = kg f(C)

(6)

where L is the dimension of growing zeolite crystals at the crystallization time tc, dL/dt c is the growth rate of zeolite crystals, kg is the constant of the linear rate of crystal growth, CA1 and Csi are the concentrations of aluminum and silicon in the liquid phase during the crystallization process, CA1 and C~i are the concentrations of aluminum and silicon in the liquid phase corresponding to the solubility of the crystallized zeolite under given

425 crystallization conditions, r is the molar ratio Si/A1 of the crystallized zeolite and f(C) = (CA1 * )(Csi- Csi * ) r is the concentration factor. On the other hand, absence of the solid - CA1 precursor in the clear solutions stipulates that the changes in the concentrations CA1 and Csi of aluminum and silicon in the liquid phase are proportional to the amount of crystallized zeolite, i.e.,

(7a) (7b)

dCAl/dt c = - bl dmz/dt c dCsildt c = - bzdmzldt c

where, mz is the mass of zeolite crystallized from a given amount mcs of clear (alumino)silicate solution up to the time tc, b 1 = constant is the amount of aluminum contained in a unit amount of the crystallized zeolite and b2 = constant is the amount of silicon contained in a unit amount of the crystallized zeolite. Hence, o

CA1 = CA1 - b 1 mz

(8a)

o

CSi = Csi - b2 mz o

(8b)

o

where CAI and Csi are total amounts of aluminum and silicon in the amount mcs of the clear (alumino)silicate solution. Combination of eqs. (6), (8a) and (8b) gives: dL/dt c = Q = kg (C~

- CA1- b 1 mz)(C~i- Csi- b2 mz)r

(9)

It is well known that the growth rate of zeolite microcrystals is strongly influenced by the crystallization temperature, and that the growth rate constant kg changes with the crystallization temperature Tc (expressed in centigrade) in accordance with Arrhenius relation [4,17], i.e., kg= A exp{- Eal[R(273 + To)l} = A e x p ( B l T A)

(10)

were, E a is the apparent activation energy of the crystal growth, R = 8.34314 J K -1 mo1-1 is the gas constant, TA = 273 + Tc is the absolute temperature of the crystallization process, B -- Ea(K)IR and A is the slope and the intersection with the Y axis of the In kg vs. 1/T. The temperature T may be calculated by the solution of the empirical differential equation [4]: R h = d T i d t c = R ~ { 1- exp[Kh(T- TR)I}

(11)

where, R~ is the initial rate of heating-up of the reaction mixture, TR is the (maximum) reaction temperature, and Kh is a factor which determines the deviation of the T vs. tc function from linearity. Behaviour of systems 1-4 during crystallization of analcime [ 1] was simulated by simultaneous solution of differential equations (2)-(5), (7a), (7b), (9), and (11) by a fourthorder Runge-Kutta method using the corresponding numerical values of constants [1], i.e.,

426 G = rd6 (spheres), p = 2.25 g/cm 3,

CA1 = 0.1132 g A1203/(kg system), C~i= 0.1343 g

SiO2/(kg system), b 1 = 0.338, b 2 = 0.398, r = 1 (Si/A1 = 1 in analcime), A = 10500 cm/h, B=Ea/R = 9020.56 K, R~ = 137 - 10000 ~

K h = 2 and TR = 160 ~ C. Initial values mi(t c =

O) = N(t c = 0)[L(t c = 0)] i, where mo(tc = 0) = N(t c = 0) are listed in Table 1.

Table 1 Initial values (solution of differential equations (2)-(5) in t c = 0) of the moments m o, ml, m 2 , and m 3 of the crystal size distribution which correspond to the Systems 1-4. Intitial values

System- 1

System-2

System-3

System-4

too(0) (kg 1)

5.19x105

3.75x104

5.01x105

1.04x106

MI(0) (cm kg -1)

5.19x10 -4

3.75x10 -5

5.01x10 -4

1.04x10 -3

M2(0) (cm 2 kg -1)

5.19x10 -13

3.75x10 -14

5.01x10-13

1.04•

M3(0) (cm 3 kg -1)

5.19x10 -22

3.75x10-23

5.01x10-22

1.04x10-21

The initial values: CAI(0) = C AI o = 1.801 g A1203/(kg system) [eq. (7a)], Csi(0 ) = CS~ = 89.133 g SiO2/(kg system) [eq. (7b)], L(0) = 10 -9 cm [eq. (9)], and T(0) = 25 ~ C [eq. (11)], are the same for all four systems (1-4). 3.RESULTS AND DISCUSSION

Taking into consideration influences of kg and f(C) on the rate of crystal growth (see eq. (9)) and on the rate of crystallization (see eq. (4)), as well as the influence of crystallization temperature Tc on the values of kg and f(C) (see eq. (10)), it is evident that a considerable influence of the heating rate dTc/dtc on both the rate of crystal growth and the rate of crystallization may be expected [4,14]. The results of simulations presented in Fig. 1 confirm that expectation; namely, both the mass mAN of analcime crystallized (Fig. 1A) and the size L m of the largest analcime crystals (Fig. 1B) increases with increasing rate of heating, for a given crystallization time t c. Here, it is interesting that the slope Kg = kg f(C) (see eq. (6)) of the "linear" part of the L m vs. tc functions (dashed straight lines in Fig. 2B) does not depend on the heating rateR~ (Kg = 13.95 ~trn/h for R~ = 137, 275, and 10000 ~

but the

intersection ly of the straight line with the Y axis decreases with increasing heating rate, i.e., ly = - 11.77 ~tm for R~ = 137 ~ min), Iy = 0 forR~ = 10000 ~

(th --- 60 min), Iy = - 0.5.76 ~tm for R~ = 275 ~

(th --- 30

(th --- 0: th is time of heating of the reaction mixture from the

ambient Ta = 25~ to the reaction temperature Tmax = 160~

427

f

/ 7;/;"

E

/L

1.5

;';

LOf A Z F Jf o.o~____..,,,~ ""

% O

I.I. 0

2

4

6

tr (h)

I , I 8

I

, I

10

Fig. 1. Simulation of the influence of heating rate R~ on changes of: (A) amount mAN of analcime crystallized and (B) size L m of the largest analcime crystals, during the crystallization of analcime from systems 1 (O), 2 (A), 3 (V), and 4 (El). tc is time of crystallization. The solid curves (from left to right) correspond to the mAN vs. tc (Fig. A) and L m vs. tc functions (Fig. B) calculated for R~ = 10000, 1650, 800, 550, 410, 330, 275, 184, and 137 ~

The dashed curves represent the "linear" part of the L m vs. tc functions

calculated forR~ = 10000 ~

(left), R~ = 275 ~

(middle), and R~ = 137 ~

(fight).

The comparison of the measured values of L m (symbols in Fig. 1B) and the values of L m calculated for R~ - 10000 ~

(left-side curve in Fig. 2A) to R~ = 137 ~

(fight-side

curve in Fig. 2A) shows that the measured values lie in the region limited by the L m vs. t c curves calculated for R~ = 10000 ~ estimated rate of heating, R~ = 548~

O

(th --- 0) and R h = 275 ~

(th - 30 min). So, an

is needed to increase temperature of the solutions from

the ambient Ta = 25~ to the reaction temperature Tmax = 160~ in 15 min (= th). Thus, the values R~ = 548 ~ C (th = 15 min), and K h = 2 were used for simulation of temperature change (see eq. (11)) during crystallization of analcime in systems 1-4. Figure 2 shows that the values of L m calculated (simulated) on the basis of the proposed model (curves) are in excellent agreement with the values of L m measured (symbols) during the crystallization of analcime from the clear aluminosilicate solutions, prepared by silicon used from different sources [ 1]. This indicates that (i) the crystallization of analcime in system takes place in accordance with the proposed model, mathematically described by eqs. (1)-(11) and (ii) that the time th needed for heating of the systems from the ambient temperature, Ta = 25 ~ C to the reaction temperature, Tc = 160 ~ C is about 15 min (R~ = 548~ concluded by an analysis of the data presented in Fig. lB.

as it was

428 140

//

120 ~.

100

/ ~176176176

~ ~ ~ o'r ~ o ~ 1 7 6 1 7 6 1 7 6

8O 6O 4O 20

11

, 1 , 1 , 1 , 1 ,

2

I,

4

I,

6

I,

I,

8

I , I

10

t c (min) Fig. 2. Change of the size Lm of the largest analcime crystals during its crystallization from the systems 1 (O), 2 (A), 3 (V), and 4 (El). tc is time of crystallization. The curves were simulated by the proposed model using appropriate constants (see Text) and initial conditions (Table 1), and symbols represent the measured values of Lm. All simulations were performed for R~ = 548~

Symbols O, A, V, and El in Fig. 4B correspond to the symbols El, A, O, and

+ in Fig. 3 of ref. 1. The differences in the values of Lm for tc > 5 h arise from different number of nuclei formed in systems prepared by different silica sources (see values of mo(0) = N(0) in Table 1 and Table 3 in ref. 1). Figure 3A shows a comparison of simulated (curves) and measured (symbols) amounts mAN of analcime crystallized in systems 1 (O, solid curve), 2 (A, dashed curve), 3 (V, dash-dotted curve), and 4 (El, dotted curve). As already mentioned, the differences in the kinetics of crystallization (Fig. 3A) and crystal growth (Fig. 3B), are caused by differences in the number of nuclei present in the solutions formed by different silicon sources (see Table 3 in Ref. 1). In contrast to excellent agreement between the calculated and measured values of Lm (see Figs. 2 and 3B) and very well agreement between the calculated and measured values of mAN (see Fig. 3A) for tc < 6 h, the agreement between the calculated (curves) and measured (symbols) values of mAN for tc > 20 are not in satisfactory agreement (see Fig. 3A). Fig. 3A (see also Fig. 4 in ref. 1) shows that the measured values of mAN increased rather rapidly at early times and slowed down after about 20 h. The final yield was close to 5 g of analcime per kg of synthesis solution (mAN(tOt) = 5 g per kg of system in all of the experiments, although some results are scattered around that value [ 1]. The possible reason is that between 5 and 100 h of synthesis, the first generation of analcime crystals, which had grown to approximately 50 ~t in dimension, settled to the bottom of the autoclave, and began to grow as aggregates, losing their individual crystal identity, while another generation of nuclei was formed in the liquid phase above this settled population [15]. Hence it is reasonable to expect that the real growth rate of the settled analcime crystals (at tc > 5 h) is lower than the growth rate calculated on the basis of the growth of the individual unsettled

429 crystals (curves in Fig. 3B). Consequently, the real rate of analcime crystallization (symbols in Fig. 3A) is lower than the rate of crystallization calculated on the basis of the proposed model (curves in Fig. 3A).

5.0 '7',

0//Yk

4.0

r

3.0 2.0 1.0

/

/

!

!

A

A

#

O.O 400~

i oof

"

/

ii1

t

[

--" " " - "

...-

13 /

~ 20~ t l o ~ I ~,., , , ,_L_;. ., , .._,.7(,._, ._, L_2. , , :, ~,........ , _.....:,:2.~ , / 0

50

100

t c (min)

Fig. 3. Changes in (A) amount mAN of analcime crystallized in 1 kg of system of clear solution and (B) size Lm of largest crystals, during crystallization of analcime at 160 ~ C in systems 1 (O, solid curves), 2 (A, dashed curves), 3 (V, dash-dotted curves), and 4 (l-1, dotted curves), tc is the time of crystallization. Symbols represent the measured data, and curves represent were made by simulation using initial conditions from Table 2. All simulations were performed for th = 15 min (a = 550). Symbols O, A, V, and v1 in Fig. 6A correspond to the symbols rq, A, O, and + in Fig. 4 of ref. 1. 4.CONCLUSION A model of crystallization of zeolites from the clear solutions is designed. According to this model, crystallization is governed by the crystal growth of a constant number of nuclei (crystals) formed at the beginning of the crystallization process. All relevant critical processes, which occur at molecular level during the crystallization, including the warming up from the ambient to the crystallization temperature, are described by appropriate population balance equations. The validity of the developed model is evaluated by simulation of the crystallization of analcime from clear solutions (model system), and by correlation of the simulated (calculated) kinetics of crystal growth and crystallization with the measured ones. Changes in the characteristic parameters (mass of crystallized zeolite, size of the largest crystals, concentration factor, growth rate constant) during crystallization of analcime from clear aluminosilicate solution of constant composition (87 Na20 Al203 84 SiO2 2560 H20 ), but prepared using different silica sources (Cab-O-Sil, puratronic silica, sodium silicate nonahydrate, and sodium silicate pentahydrate), were simulated by simultaneous solutions of the model equations. Using numerical values of the corresponding constants and initial

430 conditions, equations were solved numerically by a fourth order Runge-Kutta method. Very well or even excellent agreement between the measured and calculated (simulated) kinetics of crystal growth and crystallization, at early stage of the crystallization of analcime show that crystallization of this zeolite from clear solutions takes place in accordance with the proposed model.

REFERENCES

1. 2. 3. 4.

G. S. Wiersema and R.W. Thompson, J. Mater Che~ 6 (1996) 1693. BJ Schoeman, J Sterte and J-E Otterstedt, Zeolites 14 (1994) 568. S. Bosnar and B. Suboti6, Microporous Mesoporous Mater. 28 (1999) 483. B. Suboti6, T Antoni6 and J. Broni6 in: Proc. 13th Int. Conf. Zeolites, A. Galarnueau, F. di Renzo, F. Fajula and J. Vedrine (eds.), Montpellier, 2001, poster 02-P-24. 5. A.D. Randolph and M.A. Larsen, Theory of Particulate Processes, Academic Press, New York and London, 1971. 6. R.W. Thompson and A. Dyer, Zeolites 5 (1985) 292.7. 7. C.L. Huang, W.C. Yu and T.Y. Lee, Chem. Eng. Sci. 41 (1986) 625. 8. A.Y. Skeikh, A.G. Jones and P. Graham, Zeolites 16 (1996) 164. 9. C. Falamaki, M. Edrissi and M. Sohrabi, Zeolites 19 (1997) 2. 10. L. Gora, K. Streletzky, R.W. Thompson and G.J.D. Phillies, Zeolites 18 (1997) 119. 11. C.S. Cundy, B. Lowe and D. Sinclair, J. Cryst. Growth 100 (1990) 189. 12. B.J. Schoeman, J. Sterte and J.-E. Otterstedt, J. Colloid Interface Sci. 170 (1995) 449. 13. T.A.M. Twomey, M. Mackay, H.P.C.E. Kuipers and R.W. Thompson, Zeolites 14 (1994) 162. 14. T. Antoni6 and B. Suboti6 in: Proc. 12th Int. Conf. Zeolites, M.J.M. Treacy, B.K. Marcus, M.E. Bisher and J.B. Higgins (eds.), Material Research Society, Warrendale, PA, 1999, p. 2049. 15. C.W. Davies and A.L. Jones, Trans. Faraday Soc. 51 (1955) 812. 16. A.L. Jones and H.G. Linge, Z. Phys. Chem. N.F. 95 (1975) 293. 17. S. Bosnar, J. Broni6 and B. Suboti6, Stud. Surf. Sci.Catal., 125 (1999) 69.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

431

S y n t h e s i s o f Z i n c o s i l i c a t e M o l e c u l a r Sieve V P I - 7 U s i n g Vapor P h a s e T r a n s p o r t Jinxiang Dong, Chun Feng Xue, Guanghuan Liu Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, P.R.China. Tel: +86-351-6010550-8, E-mail: jxdong@ public.ty.sx.cn

The synthesis and crystallization kinetics of zincosilicate molecular sieve VPI-7 were studied using a vapor phase transport (VPT) method. The influences of aging, components of raw materials and reaction temperature on the synthesis of VPI-7 were investigated. The characterization of crystallization kinetics of VPI-7 synthesized using VPT method and hydrothermal method respectively was compared. The samples were characterized by XRD, SEM, FT-IR and chemical analysis. The results showed that VPI-7 could be synthesized using VPT method from the reaction gel of the ratio of the raw materials of (0.35-0.85) Na20: (0.2-0.5)ZnO: SiO2: 6.9H20. The aging had greatly picked up the crystallization reaction speed of the two synthesis methods. 1. I N T R O D U C T I O N Traditional zeolites are kind of microporous materials with variant framework composed of silica and alumina tetrahedron. As all to known, the more large pore volume zeolite frameworks are benefit to expose catalysis active center and adsorb more molecular to participate in the reactions and alleviate the jug of the pore opening so that prolong the life-span of catalyst. Brunner and Meier have shown that there is close correlation between the minimum ring size in the framework structure and the framework density [1 ]. It may be essential for synthesis of large pore volume zeolite to stabilizing three-ring built units in the framework structure. Thus so far, frameworks of molecular sieves containing the divalent elements are rare. Although the beryllosilicate zeolite hydrothermally synthesized from the reaction system of Na20-BeO-SiO2-HaO-NaC1, its framework structure doesn't contain 3-rings just as that of analcime. Among zeolite minerals the beryllosilicate lovdarite was of particular interest because its structure contains the spiro-5 unit built from 3-rings, which occur only very rare in the tetrahedral framework [2]. However, the high toxicity of beryllium compounds limited their application. Fortunately, the beryllium may be replaced by zinc because of the similarity of berryllosilicates and zincosilicates [3,4]. The zincosilicate molecular sieves VPI-7 [5], VPI-9 [6,7] and RUB-17 [8] containing 3-ring units were hydrothermally synthesized from the sol containing NaOH, RbOH or RbOH and KOH, NaOH and KOH, respectively. They all have a relatively high framework density (16.7

432 T-atoms/1000A 3) and relatively small pore openings, and have 4.82 layers with the same orientation of tetrahedral and unit cells with one very long axis. Although the zinc atom in all three structures are associated with 3-rings, their position in the framework and the ratio of Si : Zn are different. For example, in VPI-7 and RUB-17 the Zn atoms are located in the 4.82 layers and framework structures contain sprio-5 traits with a Si atom in the central position and the Zn in each of the two 3-rings, whereas those in VPI-9 does not. In the addition, the Si : Zn ratio in both VPI-7 and RUB-17 is 3.5:1, while that for VPI-9 is 4:1 [9]. Their thermal stability are also different from each other since the network of alkali metal cations and water molecular plays a decisive role in the stabilization of RUB-17 and VPI-7, their thermal stability is poorer than that of VPI-9. Although a promising structural model of VPI-10 contains 3-rings and a relatively high framework density had been proposed, the topology of it has not been confirmed. The VPT method was first reported in 1990 as a new synthetic technology as a means to reduce the consumption of organic materials and the environment pollution. The zeolite ZSM-5 [ 10] had been crystallized from an amorphous aluminosilicate gel under the vapors of ethylenediamine, triethylamine and distilled water at the bottom of autoclave. The VPT method was investigated in detail by Kim and Nishiyama [11,12,13] to synthesis different kinds of zeolites and confirmed the important potential of this synthesis technique. The VPT also has been considered as a very useful method to synthesis zeolitic membrane on the supports having complicated structure like honeycombs. Dong [14] and N. Nishiyama [12] have applied the VPT method to prepare the membranes on a porous alumina support. In 1991, a hydrogen-storage composite material composed of alloy based on TiFe and mordenite (NaM) m TiFeCr/NaM was successfully synthesized using the VPT method [ 15]. In the paper, the effects of aging, reaction time and the ratio of raw materials on the synthesis of zincosilicate molecular sieves VPI-7 using vapor phase transport method were studied, and the crystallization kinetics curve and that of hydrothermal method were compared. The samples were characterized by the XRD, SEM, FT-IR and chemical analysis.

2. EXPERIMENTAL SECTION

A. Synthesis In our experiments, the reagents used were the following: zinc nitrate hexahydrate, silica fume, distilled water and sodium hydroxide. In a typical run, firstly, a starting mixture was prepared by combining zinc nitrate hexahydrate and silica fume with distilled water and drying at 423K for 24 hours until all the water was removed. Secondly, the reaction gel was obtained by combining the sodium hydroxide solution with the starting mixture. The final molar constitute of the gel was as followings: 0.65Na20: 0.2ZnO: SiO2: 6.9H20. And 15 mL distilled water was added in the autoclave, whose volume is 600mL. The autoclave was sealed and heated in the water vapor coming from the liquid phase at the bottom of the autoclave for certain time at 493K. To compare the crystallization kinetics, VPI-7 was also

433 hydrothermal synthesized from a reaction sol with molar composition of raw materials: 0.65Na20: 0.2ZnO: SiO2:44H20 under the condition of 493K and the same reaction time. The product was collected by centrifugation, washed with distilled water, and dried in air at room temperature. The aged gel or sol obtained from static keeping the fresh one at room temperature for 24 h.

B. Analysis X-ray powder diffraction patterns were obtained on a Rigaku D/Max 2500 powder diffractometer fitted with a fine-focus copper X-ray tube and the operation conditions were the speed of scanning 20 = 1~ per minute and the voltage and electric current equal to 100V, 40mA, respectively. The relative crystallization intensity of was calculated as following: the characterized diffraction peaks of the best sample (see the typical run) at 20 = 8.94, 13.98, 16.56, 17.78, 26.72, 26.78, 27.54, 28.04, 28.12, 28.34 and 31.68 were selected and the summation of their intensity considered as a reference, the ratio of the intensity of other sample to the summation was the relative intensity. The SEM photograph were acquired on a JEOL, JSM-35C scanning electron microscope under the operation voltage of 25 KV. The element composition of the product was also obtained by chemical analyses. The content of silica was analyzed by the gravitational method and that of zinc was measured by volume. 3. RESULTS

A. Synthesis In the reaction system of the same ratio of ZnO/SiO2=0.2 and heating at 493K for 120 h. VPI-7 could be synthesized from the composition with the ratio of Na20/SiO2 within range from 0.35 to 0.85. The highest relative crystallization intensity could be achieved at the ratio of Na20/SiO2=0.65 (Table 1). In the reaction system of the same ratio of Na20/SiO2=0.65 and heating at 493K for 120 h. VPI-7 molecular sieves could be synthesized from the composition with the ratio of ZnO/SiO2 within range from 0.1 to 0.4. The highest relative crystallization intensity could be achieved at the ratio of ZnO/SiO2=0.2 (Table 2). In the reaction system of the same molar ratio of raw materials: 0.65Na20: 0.2ZnO: SiO2:6.9H20 and heating at different temperature for 120 h (Table 3). The higher crystallization intensity of VPI-7 could be obtained at higher temperature. Although the different crystallization VPI-7 could also been acquired at other lower temperature, the crystallization temperature should not be lower than 423K. This fact is consistent with the principle of zeolite high temperature hydrothermal synthesis and the property of microporous of VPI-7. Table 1. The influences of Na20/SiO2 on the products synthesized using VPT Na20/SiO2 0.3 0.35 0.55 0.65 0.75 20.6% 64.8% 100% 56.6% VPI-7 VPI-7 VPI-7 Crystallinity Amorphous VPI-7

0.85 48% VPI-7

434

Table 2. The influences of ZnO/SiO2 on the synthesized product using VPT ZnO/SiO2(mol) 0.05 0.1 0.2 0.3 0.4 Phase Amorphous VPI-7 VPI-7 VPI-7 VPI-7 Relative crystallinity

74.16%

100%

69.2%

0.5 Unknown phase

33.7%

Table 3. The influences of temperature on the synthesized product using VPT Temperature (K) 493K 473K 453K Phase VPI-7 VPI-7 VPI-7 Relative crystallinty 100 % 86% 76.8 %

i

t

,,

5

|

I

10

t5

:

i

t

1

20

25

30

_ ..,l_ __

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I

:_

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40

423K Amorphous

........ I

45

50

(20)

(a) 1800

1800

1600

1600

1400

1400

1200

1200

1000

1000

800

800

600

600 400

4O0

200

200

0

0 fi

10

15

20

25

30

35

40

45

50

5

10

(20) (b) Fig. 1. The XRD Patterns of VPI-7 samples :(a). Literature method (c). Hydrothermal synthesis

15

20

25

30

35

40

45

(20)

(c) (b).Vapor phase transport

50

435

Fig. 2. The SEM of VPI-7 samples (a) Vapor phase transport method (b) Hydrothermal synthesis B. Characterization The typical XRD patterns of the product were showed in the Fig.1 (b) and Fig.l.(c). The diffraction peak positions and relative intensity of the XRD patterns of the sample were corresponded to the results of literature [5] Fig.1 (a). We could concluded that the product was the VPI-7 molecular sieve. The SEM photographs showed that the samples synthesized were irregular plate-like crystals. The samples synthesized using vapor phase transport are resemble to that of samples obtained by hydrothermal synthesis (Fig. 2 (a) and Fig.2 (b)). It is an effective approach to investigate the framework vibrate of zeolite by the FT-IR technique. We classified the FT-IR spectrum of VPI-7 molecular sieve (Fig. 3.) as follows: the peak at the 439 cm 1 is the twine vibration of the T-O bonds and the peaks at 510 cm 1 and 645 c m 1 a r e the vibration of double rings, and those at 725 c m -1 and 793 c m 1 a r e the symmetry flex vibration of external tetrahedron, and those at 1005 cm -1 and 1110 cm -1 are the dissymmetry flex vibration of the internal tetrahedron. Furthermore, the other peaks appear at the 926 cm -1, 1685 cm -1 and 3557 cm -1 need further determined. The results of chemical analysis showed that the element contents of the product have little convertibility (Table 4). The results of chemical analysis showed the Si:Zn ratio of framework have no distinguish changes with the change of Si:Zn in the gel or sol. At the same time, the results were approximately consistent with the analysis results in the literature [16]. Within the zincosilicate framework, the mode of Na cations connecting to two oxygen atoms of the zinc [ZnO4] unit is different from that of zeolite. And an infinite network formed the Na ions and water molecular plays an important role in the stabilization of VPI-7 structure. So the zinc atoms are difficult to be replaced by the Si atom. Unlike in the aluminosilicate zeolite, the A1 atom could be easily isomorphous substitution replaced by the Si atoms. The Si:A1 ratio of the product in zeolite framework are close correlation to the constitute of the initial

gel.

436 Table 4. The chemical analysis result Sample SiO2/ZnO(mol) ratio in gel 1 5 2 6.67 3 10

of the product synthesized using VPT Content of the product (wt%) S i O2/ZnO(mo 1)ratio SiO2 ZnO in the product 49.28 19.37 3.44 50.00 18.27 3.65 49.84 18.22 3.69

r u

4000

Fig.3. 4.

350;I

3000

2500

:2300

~ a v e n u ~ b ~r era-1

1500

I000

50:0

FT-IR spectrum of sample

DISCUSSIONS

Under the gel with the composition of 0.65Na20: 0.2ZnO: SiO2:6.9H20 and heating for certain time at 493 K, the influences of aging and reaction time on the products were investigated. The experiments showed that molecular sieve VPI-7 could be synthesized from fresh gel heated 120 h or the aged one heated for 24 h using vapor phase transport method. On the other hand, VPI-7 could also be hydrothermally synthesized from the fresh sol with composition of 0.65Na20: 0.2ZnO: SiO2:44H20 heated for 96 h or the aged one heated for 48 h at 493K (Fig. 4). The results showed that the crystallization rate of hydrothermal method was faster than that of the vapor phase transport method. After the gel or sol aging for 24 h, the crystallization rate of the latter exceeded that of the former. It showed that the aging had greater influences on the latter than the former. The fact could be ascribed to the difference between the concentration of alkali of the gel and that of the sol. In addition, the framework of the product had partially

437

100 80 oq

60 40 (D

20 ....

0

24

48

72

I

96

. . . . .

J

i

_

120

144

i

I

. . . . . .

168

192

I

I

216

240

Reation time (hour) ;

Aged gel, vapor phase transport

+

Fresh gel, vapor phase transport

Aged sol, hydrothermal synthesis

+

Fresh sol, hydrothermal synthesis

Fig. 4. The crystallization kinetics curves of the samples obtained under different conditions collapsed with the extending of reaction time using vapor phase transport method. The poor thermal stability of the products in the water vapor might be caused by the high concentration of sodium cations of the gel. 5. CONCLUSIONS VPI-7 molecular sieve was crystallized by transporting water in the vapor phase to a sodium zincosilicate gels using the VPT method. The suitable constitutes of the raw materials for synthesizing VPI-7 using VPT method were: (0.35-0.85)Na20: (0.2-0.5)ZnO: SiOa: 6.9H20. The aging of gel or sol could enhanced the crystallization rate for two methods. ACKNOWLEDGEMENT The study was financially supported by the Foundation of Youth Header Cultivating Project of Shanxi Province.

REFERENCES 1. G.O. Brunner and W.M. Meier, Nature, 337 (1989) 146. 2. S. Merlino, Eur. J. Mineral, 2 (1990) 809. 3. M.J.Annen, M.E.Davis, J.B.Higgins. J.L.Schlenker, Mat.Res.Soc.Symp.Proc. 223 (1991) 245.

438 4. M.J.Annen, M.E.Davis, Preprints - Division of Petroleum Chemistry, American Chemical Society, Publ by ACS, Vol.36 No.2 (1991) 385. 5. M.J.Annen, M.E.Davis, J.B.Higgins. J.L.Schlenker. J. Chem.Soc.Chem.Commun., (1991) 1175. 6. M.J.Annen and M.E.Davis, Microporous Material, 1 (1993) 57. 7. M.A.Camblor and M.E.Davis, J. Phys. Chem. , 98 (1994) 13151. 8. C. R0hrig and H. Gies, Angew. Chem., Int Ed.Engl., 34 (1995) 63. 9. M.A. McCusker, R.W. Grosse-Kunstleve, C. Baerlocher, Microporous Material, 6 (1996) 295. 10. W.Y. Xu, J.X. Dong, J.P. Li, J.Q. Li and F. Wu, J. Chem.Soc. Chem.Commun., (1990) 755. 11. M.H.Kim, H.X. Li and M.E.Davis, Microporous Material, 1 (1993)191. 12. N. Nishiyama,K. Ueyama and M. Matsukata, Microporous Material, 7 (1996) 299. 13. S.G. Thoma and T.M. Nenoff, Microporous and Mesoporous Material, 41 (2000)295. 14. J.X. Dong, T. Dou, X.G. Zhao, L.H. Gao, J. Chem.Soc.Chem.Commun., (1992) 1056. 15. W.Y. Xu, J.R Li, J.X. Dong, Adv. Mater., Vol.3 No.9 (1991) 442. 16. C.R6hrig, H.Gies and B.Marler, Zeolites, 14 (1994) 498.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

439

C o m b i n e d I R and catalytic studies of the role o f Lewis acid sites in creating acid sites o f enhanced catalytic activity in steamed H Z S M - 5 J. Datka a, B. Gil a, P. Baran a and B. Staudte b aFaculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krak6w, Poland bDept, of Physics & Earth Sciences, Leipzig University, 04103 Leipzig, Linnestr. 5, Germany The properties of Lewis acid sites in mildly and severely steamed HZSM-5 were followed by low temperature IR studies of CO adsorption. We performed also combined IR and catalytic studies in which butene-1 isomerisation was followed in steamed HZSM-5 in which Lewis sites were blocked by CO (CO adsorption was controlled by IR). The information on relative acid strength of Lewis acid sites was obtained in IR experiments of ammonia desorption followed by CO adsorption. All the results suggest that the less electroacceptor Lewis sites, characterized by IR band of adsorbed CO 2190 cm~, are responsible for the formation of strongly acidic and highly active Bronsted sites. Their amount increases upon mild steaming and decreases upon severe steaming parallel to the catalytic activity of zeolite. Our IR results agree with 27Al MAS NMR results and with EPR data of NO adsorption. 1. INTRODUCTION Among the variety of reactions catalysed by zeolites, those the most important for industry are catalysed by zeo|itic acid sites. It is well known, that mild steaming increases the catalytic activity of HZSM-5, whereas more severe steaming results in lowering the activity [ 1-6]. It was suggested that the reason for the enhanced activity was the presence of small amount of Bronsted sites of very high acidity. Those bridging hydroxyls interact with electroacceptor sites being A1 species partially extracted from the framework by steaming at mild conditions. This results in the delocalisation of negative charge upon the dissociation of proton from hydroxyl group. Our previous IR studies revealed the presence of such strongly acidic hydroxyls characterized by IR band at 3590 cm 1 in HZSM-5 steamed in mild conditions [7]. Recently Shigeishi et al. [8] who studied steamed H-mazzite reported the presence of strongly acidic 3590 and 3567 cml hydroxyls being OH groups interacting with strongly acidic Lewis acid sites characterized by the bands of adsorbed CO at 2229 and 2188 cm1. Up to now, the electroacceptor A1 species (Lewis type) which could be responsible for high acidity were not detected in steamed HZSM-5. The goal of the present study was to find such sites. We performed IR studies of CO adsorption in non steamed, mildly and severely steamed HZSM-5 as well as in partially dehydroxylated one. The information on relative acid strength of Lewis acid sites was obtained in IR experiments of ammonia desorption followed by CO adsorption We also carried out combined IR and catalytic study in which we followed the catalytic activity of zeolites with preadsorbed CO blocking selectively Lewis sites. CO adsorption and desorption were controlled by IR. The reaction studied was butene-1 isomerisation which occurred at temperature as low as 213 K, low enough to maintain CO bonded to Lewis sites.

440 2. E X P E R I M E N T A L HZSM-5 of Si/AI=15 (without template, Chemie AG Bitterfeld/Wolfen). The hydrothermal treatment was performed in an apparatus consisting of a horizontal quartz tube surrounded by a furnace. About 1.5 g of zeolite was placed in the tube in bed depth of 1 mm and heated up to 813 K (heating rate 10 K/min) under a pressure of 10 Pa. Steaming was performed by the water vapour with a pressure of 20 kPa or 85 kPa in nitrogen stream (flow rate 40 l/h) for 2.5 h. Such treated zeolites will be called in this paper "mildly steamed" and severely steamed" respectively. The partial dehydroxylation was realized by the calcination of mildly steamed zeolite at vacuum at 1100 K for 1 h. Ammonia (Linde 99.9%), CO (Linde 99.9%) and butene-1 (Fluka 99.5%) were used. Ammonia was adsorbed at 370 K, CO at 170 K and butene-1 at 213 K. The isomerisation of butene-1 adsorbed in zeolites was studied at 213 K by following the decrease of C=C - band at 1630 cm~ as a function of time. The isomerisation was found to be first order reaction, and rate constants were calculated. The spectra were recorded with a BRUKER 48IFS spectrometer equipped with an MCT detector. 3. RESULTS AND DISCUSSION The results of catalytic test isomerisation of butene-1 at 213 K are presented in the Table. Similarly to other authors observations [1-6] mild steaming results in a distinct increase of catalytic activity of HZSM-5 whereas severe steaming in a decrease of activity (the reaction was too slow to be observed at 213 K). Table The rate constants ofbutene-1 isomerisation at 213 K in non steamed, mildly and severely steamed HZSM-5 zeolite, as well as of mildly steamed with preadsorbed CO. HZSM-5 sample non steamed steamed in mild conditions steamed in severe conditions steamed in mild conditions with preadsorbed CO

Rate constant / s"~ 3.6 - 1 0 -4 2.8 - 1 0 -3 too low to be measured 1.4 91 0 -4

As mentioned in the Introduction, we undertook 1R studies of CO adsorption on Lewis sites in HZSM-5 at 170 K. CO is a very good probe molecule for Lewis sites in zeolites because of three reasons: (i) the bands of C-O bonded to Lewis sites are narrow and well resolved in the spectrum, (ii) the wavenumber of CO bands increases with the electroacceptor properties of sites and (iii) molecules interact much stronger with Lewis sites than with Bronsted sites (so CO interact with Bronsted sites only upon saturation of all the Lewis sites). Similarly, desorption at low temperature removes CO from Bronsted sites in the first order. This is demonstrated in Fig. 1 in which the spectra recorded upon the adsorption of small doses of CO at 170 K (Fig. 1 A) and upon the desorption of CO at 1 8 0 - 210 K (Fig. 1 B) are shown. 2190 - 2130 cm"~ bands of CO bonded to Lewis acid sites appear first at CO adsorption and are stable at the desorption. The fact that CO reacts preferentially with Lewis

441 sites makes possible the studies of the properties of small amounts of Lewis sites which are accompanied by a large amount of Bronsted sites.

0.03

8r..13 L_ 0 ..Q

<

olbO

A

0.02

0.01 0.00

,

,

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0.03

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O4

(D

O

c- 0.02 ..13 !,,-o

<

o 0

0.01

"

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04~

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0.00 22'40

'

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v [cm -1] Figure 1. A - the spectra of CO bonded to Lewis sites and Bronsted sites in mildly steamed HZSM-5 at 170 K. Spectra a, b, c were recorded upon the adsorption of increasing amounts of CO. B - the spectra recorded upon the adsorption of CO and subsequent desorption at 180 (a), 200 (b) and 210 K (c). The spectrum of CO adsorbed on Lewis sites in non steamed, mildly and severely steamed, as well as in partially dehydroxylated zeolite are presented in Fig. 2. The bands at: 2190, 2197, 2222 and 2230 cm "1 are present. Similar bands were observed by Kustov et al. [8]. The most electroacceptor sites represented by the bands at 2222 and 2230 cm 1 were formed by dehydroxylation (spectrum d ) - their amount increases distinctly upon calcination at 1100 K. On the other hand, the bands at 2190 and 2197 cm I represent less acidic electroacceptor sites.

442

0"06t 0

e'-

!,.,,. 0

0044

o

1

~

O4

~

/

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~

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,.',

I~

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,--

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I

~

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< 0.00 2240

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2160

v [cm "1] Figure 2. The spectra recorded upon the adsorption of CO and desorption at 210 K in ZSM-5 non steamed (a), mildly steamed (b), severely steamed (c), and partially dehydroxylated (d). Further information on the relative acid strength of Lewis acid sites was obtained in ammonia desorption experiments followed by CO adsorption. The excess of ammonia (sufficient to neutralize all the acid sites) was adsorbed at 370 K in non steamed HZSM-5. Ammonia was subsequently desorbed by evacuation at: 550, 563, 580 and 630 K and CO was adsorbed at 170 K upon each desorption step. The spectra of CO adsorbed upon ammonia desorption as well as adsorbed on zeolite without ammonia are presented in Fig. 3.

o.o64 (1.) or

I

o

~

~

o

r

u3

0.04

l,,.,.

o

.13

<

0.02 0.00

.

2240

,

2200

.

2160

v [cm 1] Figure 3. The spectra of CO adsorbed at 170 K on the non steamed HZSM-5 (a) and in the same zeolite in which ammonia was adsorbed and subsequently desorbed at 550 (b), 565 (c), 580 (d) and 630 K (e). At relatively low temperature (550 K) ammonia desorbs from OH groups and from weakly acidic Lewis sites: the band of CO at 2175 cm"1 (OH) and at 2190 cm "1 (Lewis sites) reappear. At higher temperatures (565 - 630 K) ammonia desorbs from weakly acidic Lewis sites characterized by CO bands at 2190 cmI, and also more acidic ones (2197 cml). The most acidic Lewis sites characterized by CO bands at 2122 and 2130 cm ] still keep adsorbed ammonia and these bands are absent upon the desorption at 630 K. All these results evidence, that the Lewis sites characterized by the IR bands of adsorbed CO at 2190 cm "] are the less

443 acidic, those characterized by CO band at 2197 c m "l a r e more acidic and those with CO bands at 2222 and 2230 cm l (formed by dehydroxylation) are the most acidic. This agrees with the conclusion based the comparison of the frequency of adsorbed CO" the higher is C-O frequency, the higher is the acid strength of Lewis sites. The role of Lewis acid sites in the formation of strongly acidic, catalytically active Bronsted sites was studied by performing combined IR and catalytic experiments. For thata purpose butene-1 isomerisation was followed in mildly steamed HZSM-5 in which Lewis acid sites were blocked by CO. CO doses were adsorbed to cover all the Lewis and part of Bronsted sites, CO was then desorbed at 200 K until CO was removed from all the Bronsted sites (2175 cm -1 band disappeared) and all Lewis sites were still keeping CO (Fig. 1 B, spectrum b). Butene-1 isomerisation was followed at 213 K in such pretreated zeolite, and the rate constant (the Table) was found to be distinctly lower than for the same zeolite without CO. This result is an evidence that Lewis acid sites are indispensable in the creation strongly acidic Bronsted sites of the enhanced activity. However, basing on the catalytic data it cannot be decided which of four kinds of Lewis sites is responsible for creation of strongly acidic Bronsted sites. Such an information can be obtained by comparing the spectra of CO bonded to Lewis sites in HZSM-5 non steamed, mildly and severely steamed presented in Fig. 2. Three of four IR bands of CO bonded to Lewis sites (2197, 2222 and 2230 cmq) decrease upon mild steaming and continue to decrease upon severe steaming. The only band which increased upon mild steaming and decreased upon severe steaming, i.e. behave parallel to the catalytic activity of zeolite is the band at 2190 cm 1 characterizing the less acidic Lewis sites. It is therefore possible, that the Lewis sites characterized by this band are responsible for the formation of strongly acidic and very active Bronsted sites. Such strongly acidic Bronsted sites could not be detected in our IR studies of CO adsorption because of their very low concentration Our IR data concerning the properties of Lewis acid sites in steamed HZSM-5 will be now compared with 27A1MASNMR [10] and EPR data of NO adsorption [11]. 27A1 MAS NMR experiments [ 10] have shown, that steaming of HZSM-5 (the same as used in this study) gave raise of 30 ppm signal not observed in non steamed zeolites. This was assigned to both extraframework AI species which were soluble in HCOOH (with formation of octahedrally coordinated species) as well as to framework A1 species, acid insoluble (which could not be transformed into octahedral species). In mildly steamed HZSM-5, the same as used by us, the contribution of framework A1 species to 30 ppm signal was much higher than of acid soluble extraframework species. It was suggested [ 10] that such framework AI species contributing to 30 ppm signal were partially hydrolysed A1 still strongly interacting with the framework. It is possible that weakly acidic Lewis sites, observed in our study in steamed HZSM-5 (CO band at 2190 cm~), may be such partially hydrolysed A1 framework species contributing to 30 ppm 27A1 NMR signal. It should be noted that, according to NMR results, [10] the concentration of partially hydrolysed AI species increases upon mild steaming and decreases upon severe steaming i.e. parallel to as the concentration of weakly electroacceptor Lewis sites (2190 cm -1 CO band) and parallel to as the catalytic activity. Our results agree also with the EPR results of NO adsorption [ 11 ] on the same samples as used in our present study. NO adsorbed in non steamed HZSM-5 activated at 770 K showed a signal at g = 1.98 of NO bonded to strong Lewis sites. Such strong Lewis sites were also detected in our study (CO bands at 2222 and 2230 cm"1 in Fig. 2). Steamed HZSM-5 showed much lower concentration of these strong Lewis sites (Fig. 2) and NO adsorption did not give raise of signal at g = 1.98 [ 11 ]. Weak Lewis sites characterised by CO band at 2190 c m "1 w e r e probably too weak to produce NO species responsible for g = 1.98. Strong Lewis

444

acid sites giving NO signal of g = 1.98 were formed only at calcination of steamed HZSM-5 to 1073 K i.e. upon some dehydroxylation- they are also seen in our IR spectra- Fig. 2. 4. CONCLUSIONS Summarising, it seems, that very weakly acidic Lewis sites, characterised by CO band at 2190 c m "1 in steamed HZSM-5 are partially hydrolysed A1 species non soluble in HCOOH giving contribution to 27A1MAS NMR signal at 30 ppm [ 10], and adsorbing NO without EPR signal at g = 1.98 [11]. It is possible, that these sites generate strong Bronsted acidity in steamed HZSM and very high catalytic activity. The amount of weak Lewis sites increases upon mild steaming and decreased upon severe steaming i.e. parallel to with the catalytic activity. 5. ACKNOWLEDGEMENT This research was supported by a grant of State Committee for Scientific Researches (grant no 3 T09A 010 17) REFERENCES

1. R.M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt, G.T. Kerr, in: Y. Murakami, A. Iijama, J.W. Ward (Eds.), New Developments in Zeolite Science and Technology, Studies in Surface Science and Catalysis, vol. 28, Elsevier, Amsterdam, 1986, p. 677 2. H.G. Karge, in: G. Oehlmann, H. Pfeifer, R. Fricke (Eds.), Catalysis and Adsorption by Zeolites, Studies in Surface Science and Catalysis, vol. 65, Elsevier, Amsterdam, 1991, p. 133 3. W. O. Haag, in : J. Weitkamp, H.G. Karge, H. Pfeifer, W. Hoelderich (Eds.), Zeolite and Related Materials: State of the Art 1994, Studies in Surface Science and Catalysis, vol. 84 Part C, Elsevier, Amsterdam, 1994, p. 1375 4. E. Brunner, K. Beck, M. Koch, H. Pfeifer, B. Staudte and D. Zscherpel, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. Hoelderich (Eds.), Zeolite and Related Materials: State of the Art 1994, Studies in Surface Science and Catalysis, vol. 84 Part A, Elsevier, Amsterdam, 1994, p. 357 5. E. Brunner, Proc. of the DGMK Conference "Catalysis on Solid Acids and Bases", Berlin, 1996, p. 106 6. J. Datka, S. Marschmeyer, T. Neubauer, J. Meusinger, H. Papp, F.-W. Schuetze and I. Szpyt, J. Phys. Chem. 100 (1996) 14451 7. J. Datka, B. Gil, P. Baran, B. Staudte, submitted 8. R. A. Shieishi, B. H. Chiche, and F. Fajula, Microporous and Mesoporous Materials, 43 (2001) 211 9. L.M. Kustov, V.B. Kazansky, S. Beran, L. Kubelkova and P. Jiru, J. Phys. Chem. 91 (1987) 5247 10. E.. Brunner, Proc. of the DGMK Conference "Catalysis on Solid Acids and Bases", Berlin, 1996 p. 106 11. B. Staudte, A. Gutsze, W. B0hlman, H.Pfeifer and B. Pietrewicz, Microporous and Mesoporous Meterials, 49 (2000) 1

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

445

H e t e r o g e n e i t y o f Cu + in C u Z S M - 5 , T P D - I R studies o f C O desorption J. Datka and P. Kozyra Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krak6w, Poland TPD-IR experiments of CO desorption were performed. The process of CO adsorption was followed by IR spectroscopy and the process of desorption was studied both by TPD and by IR. TPD-IR studies evidenced that three kinds of Cu + sites of increasing energy of CO adsorption were present in CuZSM-5. They are characterised by IR bands of adsorbed CO at 2165, 2160 and 2155 cm 1 respectively. The same three kinds of Cu + sites were found also in NaCuZSM-5 zeolites of lower Cu contents (NaJCu exchange degrees 20 and 40%). If small amounts of CO is adsorbed at room temperature, CO adsorbs on all three kinds of Cu + species without selecting the most favourable ones. At higher temperatures (340 - 460 K) CO desorbs from less favourable sites of higher CO stretching frequencies (2165 and 2160 cm~) and readsorbs on most favourable ones (CO band 2155 cm'~). The sites of the lowest C-O stretching frequency (2155 cm"~) show the strongest electrodonor properties and the strongest effect of 7t-backdonation of d-electrons of copper to 7: antibonding molecular orbital of CO. This effect weakens the C-O bonding. Cu + sites of the lowest C-O frequency are probably the most effective in activation of NO molecule and therefore the most active in "denox" process.

1. INTRODUCTION Since CuZSM-5 zeolites were found to be active catalysts in "denox" reaction [1-3], they attract a great deal of attention [4-11 ]. Besides of"denox" activity, Cu cations in ZSM-5 framework show some interesting properties such as bonding of nitrogen at room temperature, the possibility of variation the oxidation state and ability of bonding two NO or three CO molecules. Our laboratory has studied the properties of Cu + and Cu > ions in CuZSM-5 by IR spectroscopy using CO, NO and N2 as probe molecules [12-18]. We performed also quantumchemical DFT calculations concerning the properties of cations and their interaction with CO, NO and N2. The frequencies of C-O, N-O and N-N vibrations interacting with Cu + and Cu 2+ in CuZSM-5 calculated by DFT were practically the same as those obtained from IR experiments [15]. DFT calculations have explained also high efficiency of Cu + in CuZSM-5 in the activation of NO by the n-backdonation of d-electrons of Cu + to antibonding n" orbital of NO molecule [17,18]. The location of Cu + in ZSM-5 framework increases the energy of HOMO orbital of Cu + what facilitates the electron transfer to NO molecule. Moreover, the calculation has evidenced the migration of Cu cations upon the reduction and also upon NO adsorption [ 14]. The present study concerned the problem of heterogeneity of Cu + cations in CuZSM-5. The first information on this subject was reported by D6de6ek and Wichterlov/t [11] who revealed three bands in the luminescence spectrum of CuZSM-5 (Cu + species, characterised

446 by luminescence at 540 nm, were claimed to be the most active in "denox"). We studied the problem of heterogeneity of Cu § in TPD-IR experiments of CO desorption. In this study the status of zeolite and adsorbed molecules at the start of TPD procedure was controlled by IR and the process of thermoprogrammed desorption was followed both by IR and mass spectrometry. We studied NaCuZSM-5 zeolites of various Na/Cu exchange degrees i.e. of various Cu contents. 2. EXPERIMENTAL NaHZSM-5 zeolite, the composition of which can be represented by the formula: Na2.0H0.5(SiO2)93.5(A102)2.5], was a parent material. NaCuZSM-5 of various exchange degrees were obtained by a classical ion exchange in Cu(CH3COO)2 solution at 80~ The samples of exchange degrees of 20, 40 and 106% were obtained using C u ( C H 3 C O O ) 2 solutions of various concentrations. The Na/Cu exchange degrees were determined using AAS spectroscopy. CO (99.99% Linde) was used in adsorption experiments. For TPD-IR studies the zeolites were pressed into this wafers (5-8 mg/cm) and activated in situ in an IR cell at vacuum at 770 K for 1 h. The doses of CO were adsorbed at room temperature and IR spectra were recorded upon each adsorption. The desorption was realised at vacuum 10-5 mbar with the temperature increase 5 K/min. The detection of desorbing CO molecules was done by using mass spectrometer. The shape of TPD diagram obtained with the 12CO signal of m/e = 28 was the same as with much weaker signal 13CO of m/e = 29. IR spectra were recorded each 10 K during the desorption. IR spectra were recorded with a BRUKER 48IFS spectrometer equipped with an MCT detector. MS spectra were recorded with a PFEIFFER PRISMA QMS 200 spectrometer. 3. RESULTS AND DISCUSSION 3.1 Dependence of C-O stretching vibration on temperature In our TPD-IR experiments we recorded IR spectra at various temperatures. As the C-O stretching frequency may depend on temperature, we followed this dependence by performing the experiment in which relatively small amount of CO (15% of covering of all Cu +) was" adsorbed at room temperature on CuZSM-5 (exchange degree 106%), then the cell was heated to 570 K and subsequently cooled down to room temperature. The results are presented in Fig. 1 A. The position of C-O band at 2157 cm1 was practically independent of the temperature: the band did not shift in the temperature region: 3 0 0 - 490 K and shifted only by 0.5 cm t between 490 and 570 K. This result indicates that there is no coupling between C-O vibration the frequency of which is practically temperature independent dependent (because of relatively large distance between vibrational levels) and other vibration the frequency of which could be temperature. It indicates also that the shift of C-O band observed at the temperature increase (vide infra) were not due to thermal effect and could be attributed to the rearrangement of adsorbed molecules. 3.2. TPD-IR studies of CO desorption from CuZSM-5 The results obtained with CuZSM-5 of exchange degree 106% will be now presented. The adsorption of CO on CuZSM-5 results in the appearance of 2157 cm~ band of Cu+-CO monocarbonyls and also bands at 2134 and 2143 c m "1 [12], which were attributed to CO bonded to oxygen containing Cu species [19]. At higher CO loadings Cu+(CO)2 dicarbonyls are formed (IR bands at 2150 and 2180 cmt). Tricarbonyls were observed only at low temperatures [ 12, 20].

447

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<

0.0

0.0,

21'80 " 21'60 ' 21'40 ' 2120

21'80 ' 21'60 ' 21'40 ' 2120

v [ c m "1] v [ c m "1] Figure 1. A - IR spectra of CO adsorbed (15% of covering) in CuZSM-5 zeolite of exchange degree 106%. Spectra were recorded at (from bottom to top) 570, 520, 470, 420, 370 and 300 K during the cooling of zeolite with adsorbed CO. CO was adsorbed at room temperature, than it was heated to 460 K and subsequently cooled down to room temperature. B - IR spectra of CO adsorbed in CuZSM-5 a - amount of CO adsorbed larger than the amount of Cu § ions b - upon the evacuation at room temperature (decomposition of dicarbonyls) c - amount of CO adsorbed 10% of the amount of Cu § ions

In our TPD-IR experiments, the goal of which was to provide information on heterogeneity of Cu § ions in CuZSM-5, it was important to start TPD procedure at the moment in which each of Cu § ions bonds one CO molecule while dicarbonyls are absent. In order to reach such a state, the doses of CO were adsorbed at room temperature until all Cu § ions formed monocarbonyls and small amount of dicarbonyls appeared. (Fig. 1 B, spectrum a). The cell was next evacuated at room temperature until the bands of dicarbonyls disappeared and monocarbonyl band increased a little (spectrum b). This was a starting point for temperature programmed desorption. In another series of experiments much smaller amount of CO was adsorbed and only 10% of Cu § ions formed monocarbonyls (spectrum c). The TPD-IR results obtained with CuZSM-5 in which all the Cu § formed monocarbonyls are presented on Fig. 2. The TPD diagram is shown in Fig. 2 A, IR spectra in Fig. 2B and difference spectra in Fig. 2 C. IR spectra were recorded each 10 K, but for the clarity of figures only some of them are presented. The TPD diagram (Fig. 2 A) depicts two maxima at 320 K and 590 K. Maximum at 320 K in TPD diagram corresponds to disappearing of IR bands at 2134 and 2143 cm 1, what is the best seen in difference spectra (Fig. 2 C, top spectrum). As mentioned above, these maxima may correspond to CO bonded to oxygen-containing Cu § species. The main maximum at 590 K is broad and asymmetrical, what suggests heterogeneity of Cu + species. More information on heterogeneity of Cu § sites in CuZSM-5 was obtained from IR results. According to the data presented in Fig. 2 C the band of Cu+-CO decomposing at CO desorption (seen as minimum) shiits from 2160 to 2155 cm 1. The resorption above 500 K does not shift CO band. This result suggests that several kinds of Cu § sites bonding CO with different energies and also with different C-O stretching frequencies are present. CO molecules which are bonded more weakly show higher stretching frequency (2160 cm 1) than molecules bonded more strongly and desorbing at higher temperatures (above 500 K) which show lower stretching frequency (2155 cml).

448

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2120

Fig. 2. TPD-IR results of CO desorption from CuZSM-5 in which all the Cu § ions were covered by CO. A - TPD diagram B - IR spectra recorded during the heating at 300, 400, 450, 500 and 550 K C - difference spectra: 400-300 K, 450-400 K, 500-450 K and 550-500 K

449

8.0x10 "1~ 7.0x104~ 6.0x104~ 5.0x104~ 4.0x104~ 3.0x104~ 2.0x104~ 1.0xlO 4~ '1[)0'2[)0'300'4(]0

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Fig. 3. TPD-IR results of CO desorption from CuZSM-5 in which 10% of Cu + ions were covered by CO. A - TPD diagram B - IR spectra recorded during the heating at 300, 340, 380, 420 and 460 K C - difference spectra: 340-300 K, 380-340 K, 420-380 K, 460-420 K and 500-460 K (spectrum at 500 K not shown in Fig. A for the reason of clarity).

450 Interesting results were obtained in TPD-IR experiments in which only 10% of Cu § ions were covered by CO (Fig. 3). TPD diagram shows two peaks: at 320 K (the same as observed for 100% coverage- Fig. 2 A and attributed to CO bonded to oxygen-containing Cu species) and a peak at 590 K. The latter is, however, more narrow than in the case of CuZSM-5 with 100% loading. Contrary to the situation observed for 100% loading, in the case of 10% loading there is no desorption below 460 K. This can be seen both in TPD diagram (Fig. 3 A) and in IR spectra: these "recorded directly" (Fig. 3B) and difference spectra (Fig. 3 C). Below 460 K the band of Cu+-CO does not diminish (Fig. 3 B), but shifts to lower frequency. The frequency shift is the best seen in the difference spectra (Fig. 3 C). The analysis of difference spectra show that some rearrangement of CO molecules among Cu § sites takes place. At 340 K (Fig. 3 C - top spectrum) the decrease of 2165 cm 1 band is accompanied by the increase of 2155 cm1 band. At higher temperature (380 K) not only 2165 cm -1 but also 2160 cm1 band decreases, what is also accompanied by the increase of 2155 cm 1 band. At 420 - 460 K, only 2160 cm] band decreases (the band 2155 cm1 does still increase). Above 460 K only the desorption of CO from Cu § ions takes place, what is seen both in TPD diagram and in IR spectra (as the decrease of 2155 cm 1 band). All the results presented in Fig. 3 C indicate that three kinds of Cu § sites exist in CuZSM-5. They are characterised by the bands of adsorbed CO at 2155, 2160 and 2165 cm "1. If small amount of CO is adsorbed at room temperature the molecules react with all three kinds of Cu § sites without selection of the most favourable energetically. At higher temperatures, CO desorbs from weakly bonding sites (bands at 2165 and 2160 cm 1 decrease) and adsorbs on more strongly bonding ones (band at 2155 cm"] increase) but without leaving zeolitic channels. 0.14 0.12 0.10

b

0.08 0.06 0.04 0.02

0.00

21'80 " 21'60 ' 21~40 ' 21 20

v [cm 1] Figure 4. IR spectra of CO adsorbed at room temperature in CuZSM-5 (a) and upon heating to 460 K and subsequent cooling to room temperature (b) normalised to the same band area. The redistribution of CO among Cu § sites of various energies is also seen if comparing the spectra of CO (normalised to the same band area) adsorbed in CuZSM-5 (10% of covering all Cu § ions) at room temperature (Fig. 4 spectrum a) with the spectrum recorded upon heating to 460 K and subsequent cooling to room temperature (spectrum b). Heating resulted

451 in a small shift of Cu-CO band to lower frequency and in its narrowing of lower C-O stretching frequency. Narrowing is the effect of CO redistribution and occupation of only the sites most strongly bonding. It is worth mentioning that both high frequency submaxima 2155 and2160 cm"l and low frequency ones around 2 1 3 0 - 2140 cm~ (of CO adsorbed on oxygencontaining Cu species) disappeared. These pieces of observation agree with the data presented in Fig. 3 evidencing the desorption of CO from the less favourable sites and its readsorption on the most favourable (2155 cm"l band). If 100% of Cu + sites are occupied by CO, CO desorbs from sites of lower bonding energy but cannot readsorb, because more favourable sites are also occupied. Such molecules leave zeolitic channels, what is seen both in TPD diagram (Fig. 2 A) and in the IR spectra as the decrease of the CO band (Figs. 2 B, C). As mentioned, the presented above results were obtained with CuZSM-5 of exchange degree 106%. Our experiments (the same as presented above) evidenced that generally the same three kinds of Cu + sites (represented by the bands of adsorbed CO 2155, 2160 and 2165 cm~) are present in NaCuZSM-5 zeolites of lower Cu content i.e. of exchange degrees 20 and 40%. This result indicates that all three kinds of Cu + sites will be occupied independently of Cu § content (Cu exchange degree) It is high likely that our three kinds of Cu § sites characterised by C-O bands 2165, 2160 and 2155 cm "~ are ~, 13 and ~, sites detected by Drderek and Wichterlov~, [11] and attributed to Cu § ions in various locations in MFI framework. Generally, two main factors influence stretching frequency of CO interacting with adsorption sites" o-donation and ~t-backdonation. The o-donation is the interaction of o molecular orbital of CO which has a little antibonding character with electroacceptor adsorption site, it strengthens C-O bond. The 7t-backdonation is the donation of d-electrons of Cu § cation to n* antibonding orbital of CO molecule, it weakens C-O bond. The Cu + ions which adsorb CO with the lowest stretching frequency (2155 cm "~) show the strongest electrodonor properties and therefore the strongest effect of 7t-back donation. The effect of 7t-back donation of electrons from Cu § ions to NO molecule has a great importance in the activation of NO molecule. This is because of lower energy of LUMO (SOMO) orbital of NO comparing to CO molecule. NO is therefore more prone to be electron acceptor and to be activated. It seems probable that the sites of the lowest CO stretching frequency (2155 cm~) are therefore the most effective in NO activation and also the most active in "denox" process. 4. CONCLUSIONS 1. TPD-IR studies of CO desorption from CuZSM-5 evidenced, that three kinds of Cu + sites of increasing energy of CO adsorption are present in CuZSM-5. They are characterized by IR bands of adsorbed CO at 2165, 2160 and 2155 cm~ respectively. 2. If small amounts of CO were adsorbed at room temperature, CO adsorbed on all three kinds of Cu + species without selecting the most favourable ones. At higher temperatures (340 - 460 K) CO desorbs from the less favourable sites of higher CO stretching frequencies (2165 and 2160 cm1) and readsorbs on the most favourable ones (CO band 2155 cm~). 3. Three kinds of Cu + sites (CO frequencies at 2160 and 2165 cm ~) are present in NaCuZSM-5 of various exchange degrees: 20, 40 and 106%. 4. Cu + sites of the lowest C-O vibration 2155 cm"~ show the strongest effect of 7t-backdonation from d-orbitals of cation to 7t* antibonding molecular orbital of CO. These sites can be the most effective in NO activation (in "denox process"), so more, NO molecules

452 has lower energy of LUMO (SOMO) orbital than CO and therefore is more electroacceptor than CO. 5. ACKNOWLEDGEMENT

This research was supported by a grant of State Committee for Scientific Researches (grant no 3 T09A 010 17). REFERENCES

1. M. Iwamoto, H. Yahiro, Y. Mine and S. Kagawa, Chem. Lett., (1989) 213 2. M. Iwamoto, H. Yahiro, T. Kustuno, S. Bunyu and S. Kagawa, Bull. Chem. Soc. Jpn., 62 (1989) 583 3. M. Iwamoto and H. Hamada, Catal. Today, 10 (1991) 57 4. Y. Li and W.K. Hall, J. Phys. Chem., 94 (1990) 6145 5. J. D6de6ek and B. Wichterlovd, J. Phys. Chem., 98 (1994) 5721 6. M. Anpo, M. Matsuoka, Y. Shioya, H. Yamashita, E. Giamello, C. Morterra, M. Che, H. H. Patterson, S. Webber, S. Ouellette and M. A. Fox, J. Phys. Chem. 98 (1994) 5744 7. W. Grunert, N. W. Hayes, R. W. Joyner, E. S. Shpiro, M. R. H. Siddiqui and G.N. Baeva, J. Phys. Chem., 98 (1994) 10832 8. S.C. Larsen, A. Aylor, A. T. Bell and J. A. Reimer, J. Phys. Chem., 98 (1994) 11533 9. M. SheleI~ Chem. Rev., 95 (1995) 209 10. Y. Kuroda, R. Kumashiro, T. Yoshimoto and M. Nagao, Phys. Chem. Chem. Phys., 1 (1999) 649 11. J. 1)6de6ek and B. Wichterlov~, Phys. Chem. Chem. Phys., 1 (1999) 629 12. E. Broetawik, J. Datka and B. Gil, Studies on Surface Science and Catalysis, I. Kiricsi, G. Pal-Borbely, J. B. Nagy, H. G. Karge, Eds., 125 (1999) 409 13. E. Broctawik, J. Datka, B. Gil and P. Kozyra, Phys. Chem. Chem. Phys. 2 (2000) 401 14. B. Gil, J. Datkeg S. Witkowski, Z. Sojka and E. Brociawik, Studies on Surface and Catalysis. A. Corma, F. V. Melo, S. Mendioroz, J. L. G. Fierro, Eds., 130 (2000) 3249 15. E. Broctawik, J. Datka, B. Gil, W. Piskorz and P. Kozyra, Topics in Catalysis, 11/12 (2000) 335 16. E. Broetawik, J. Datka, B. Gil and P. Kozyra, Studies in Surface Science and Catalysis, A. Galaeneau, F. DiRenzo, F. Fajula, J. Vedrine, Eds., 134 (2001) 15 P 13 17. E. Broctawik, J. Datka, B. Gil and P. Kozyra, Catalysis Today, submitted 18. E. Broclawik, J. Datka, B. Gil and P. Kozyra, J. Phys. Chem., submitted 19. F. Geobaldo, B. Onida, M. Rocehia, S. Valange, Z. Gabelica and E. Garrone, Proceedings of the 3rd European Workshop on Environmental Catalysis, Maiori (2001) 261 20. A. Zecchina, S. Bordiga, M. Salvalaggio, G. Spoto, D. Searano and C. Lamberti, J. Catal. 173 (1998) 540

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

453

Speciation and structure of cobalt carbonyl and nitrosyl adducts in Z S M - 5 zeolite investigated by EPR, IR and D F T techniques P. Pietrzyka, Z. Sojkaa, B. Gil a, J. Datka a, E. Broctawikb, "Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krak6w, Poland blnstitute of Catalysis, PAN, ul. Niezapominajek, 30-239 Krak6w, Poland Adsorption of CO and NO on CoZSM-5 leads to the formation of various carbonyl and nitrosyl adducts. Their speciation and structure was studied by joint use of EPR and IR spectroscopies corroborated by DFT calculation. Depending on the temperature and pressure the following species were observed {COCO}7, {Co(CO)3}7 carbonyls and {CoNO}8 {Co(NO)2}9, {CoNO}7 nitrosyls. The bounding of CO and NO leads to nucleophilic activation of the diatomic ligands with distinctly different electron density redistribution within the Co-CO and Co-NO moieties. 1. INTRODUCTION The CoZSM-5 zeolites have recently received a great deal of attention as catalysts for many important reactions like selective catalytic reduction (SCR) of NOx [1,2,3], ammoxidation of alkenes [4] or Fischer-Tropsch process [5]. They also exhibit pronounced activity in catalytic oxidation of benzyl alcohol [6] and in spectacular regioselective oxidation of linear hydrocarbons [7]. Elucidation of the intimate mechanisms of those reactions implicates detailed knowledge of the structure of cobalt active sites and the elementary processes occurring within the Co coordination sphere upon contact with the reactants [8]. Due to their partly filled d orbitals and coordinative versatility the intrazeolite open-shell Co 2+ ions play an important role in mediating the activation of 02, NOx or CO• co-reactants. For a long time the complexation of such molecules has attracted considerable interest and is well documented [9], especially when the oxygen cartier properties were involved [ 10,11 ]. The framework of ZSM-5 zeolite provides three kinds of exchangeable cationic sites denoted as a, f3 and 7 [12,13], amongst them the two first are more abundant and preferentially occupied by Co 2+ ions. This gives rise to a distinct speciation of the intrazeolite cobalt reflected in its adsorption properties and reactivity. The specific local structure of Co species, primarily related with the reduced coordination along with the supramolecular effects due to space confinement, results in particular interfacial coordination chemistry not encountered in homogeneous Co complexes. Another characteristic feature of divalent cobalt traced to its d 7 electronic configuration is the presence of two spin states, a doublet 2Co (S = 1/2) and a quartet 4Co (S = 3/2) one, that can b e assumed depending on the nature of surrounding ligands. A change of the spin state that may be caused by the coordination or detachment of the reactants is a considerable factor affecting both the thermodynamics and kinetics of the catalytic reactions which involve Co 2+ ions. Although there is a growing understanding of the chemical transformations the reactant molecules undergo while they are attached to transition metal ions constituting the active

454 sites, effects related to spin state change upon coordination and their impact on the reactivity are not so widely appreciated [ 14]. This study concerns the interaction of Co 2§ in ZSM-5 zeolite with CO and NO molecules. For our purpose they can act also as probe molecules. The focus has been devoted to elucidation of speciation and molecular structure of the intrazeolite adducts formed upon adsorption and the spin processes accompanying the coordination. These processes have been investigated by combined use ofEPR, IR and DFT methods. 2. EXPERIMENTAL Parent NaHZSM-5 zeolite (of Si/A1 = 37) was treated by cobalt sulfate solution at room temperature. The procedure was repeated three times until finally the exchange degree of 40% was obtained. The CoZSM-5 samples were activated in vacuum at 770 K for 2 h. We studied also partially dehydroxylated HZSM-5 zeolite (activated at 970 K). The IR and EPR spectra were recorded with a BRUKER 48 IFS and ELEXYS-500 X-band spectrometers, respectively at 295, 170 or 77 K. In our experiments NO (Linde 99%) and CO (Linde 99.9%) gases were adsorbed in progressively increasing amounts. The geometry optimization with constraints imposed on the positions of the terminating hydrogen atoms was performed at the DFT level with the DMol program implemented in the InsightlI package developed by MSI [15]. A local potential in Vosko, Wilk and Nusair parameterization was used together with DNP basis set. The hyperfine coupling tensors were calculated with the Gaussian 98 program with the same cluster geometry using the B3LYP functional and LANL2DZ basis set. 3. RESULTS AND DISCUSSION 3.1. Intrazeolite cobalt centers

The EPR spectrum of dehydrated CoZSM-5 showed a broad signal at g = 5.4 and g = 2.0 attributed to a high spin state (S = 3/2) of Co 2+. Because of the large linewidth of this signal, possible speciation of cobalt into o~ and [3 sites could not be revealed. Therefore more insight into the structure of both Co centers was obtained from parallel DFT calculations performed on two different cluster models Co[SisA12Os(OH)12] and Co[Si4A1206(OH)12], which adequately mimic the seven-membered ring (M7) structure of the c~ sites and sixmembered ring structure (M6) of the 13sites, respectively [ 16]. According to the results of our calculations the structure of pentacoordinated cobalt in the ~ sites is essentially determined by four basal and one apical oxygen ligands, giving rise to a distorted rectangular pyramid. It exhibits three shorter (2.01-2.07/~) and two longer (2.13-2.16 A~) bonds with the framework oxygens and the average Co-Oz distance = 2.08 A~. This structure changes towards angularly distorted square planar pyramid in the low spin state of Co 2§ The distances to basal oxygen ligands are now shorter and more uniform (1.97-2.02/~), while the apical oxygen is elongated to 2.25 A.. As it can be expected the mean bond length decreases to = 1.99A~ on passing from high to low spin state. In the case of 13 sites cobalt ions are tetracoordinated forming a nearly planar rectangular structure. For the high spin cobalt the Co-Oz bond lengths are in the range of 1.94-2.21 ( = 2.06 .~), decreasing to 1.93-2.06 A_( = 1.99 A~) for doublet cobalt. The Mulliken partial charge on cobalt, equal to qco = 0.8 for 4Co/M7and 4Co/M6, decreases to 0.68 and 0.7 for 2Co/M7 and 2Co/M6 species, respectively. Akin to the average bond length , it is primarily determined by the spin state of Co ions rather then by their coordination.

455 3.2. Adsorption of C O As mentioned in the Introduction the experiments with CO and NO adsorption were performed, La., to provide information about speciation of cobalt in CoZSM-5 zeolite. In the case of IR studies, adsorption of both molecules was carried out at low pressure p < 0.1 Torr in order to avoid any intrusion from the gas phase spectra while EPR experiments were carried out at higher pressures of CO (p -~ 40- 60 Torr) and NO (p -~ 1 - 2 Torr). The IR spectrum of carbon monoxide adsorbed at 170 K on CoZSM-5 is shown in I Fig. 1. Several distinct CO bands at 2178, 2194, ~ ~ A m o~ ~ ~ ,, 2204, 2209, 2220 and 2230 cm "l are present, making their assignment to be more involved. To 0.04 distinguish the bands of CO adsorbed on Co 2§ from those due to the adsorption on Lewis and [ ......... , , Bronsted acid sites and Na § centers that were also present in the CoZSM-5 samples, we have 0.00 adsorbed CO additionally on NaZSM-5 and on 2: ~50 2210 2170 partially dehydroxylated HZSM-5 zeolites 0.04 (activated at 970 K). The corresponding spectra -B are presented in Fig 1A (b, c). The bands at 2194, 2220 and 2230 cm l have been assigned to CO o~ tN bound to Lewis acid sites, because they were o~ 0.02 present in partially dehydroxylated HZSM-5 (spectrum b). The band at 2178 cm l , which position is intermediate between those due to CO interacting with Si-OH-A1 groups (v = 2175 cm -1, 0.00 spectrum b) and with Na § cations ( v - 2180 cm l, 2250 22'10 2170 spectrum c), is just their superposition. The maxima at 2204 and 2209 cm l, absent 0.03 C " in the case of CO adsorbed on HZSM-5 and NaZSM-5 were assigned to CO interacting with 0.02 Co 2§ ions. The fact that two distinct Co2+-CO bands were present can be associated simply with two kinds of cobalt centers corresponding to ot 0.01 and 13 sites. The IR results indicated that these Co 2§ ions differ in electron donor-acceptor 21 properties. Roughly, the more electrodonor is the 0.00 a adsorption site the weaker is the O C - , Co 2250 ' 22'1o ' 2;7o -1 ~-donation and the stronger is the C o - , C O v/cm rt-backdonation. Thus we may reasonably assume Figure 1. A - IR spectra of CO adsorbed that the cobalt centers that give rise to the CO at 170K on CoZSM-5 activated at 770K 2204 cm l band are apparently more electrodonor (a), HZSM-5 activated at 970K (b), that those characterized by the band at 2209 cm l. NaZSM-5 (c) B - IR spectra of increasing amounts of The fact that both frequencies are greater than for CO adsorbed at 170 K on CoZSM-5 gaseous CO (v = 2143 cm l ) indicate that, as it can be expected for cationic centers, the observed activated at 770 K. can be classified as C - (a-f) differences between two {COCO}7 adducts nonclassical metal carbonyls [ 17]. consecutive adsorption steps

~176 I

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|

J\

456 As mentioned above, the maximum at 2194 cm 1 was present in the spectrum of CO adsorbed on CoZSM-5, besides of these at 2204 and 2209 cm 1 associated with Co sites, Fig. 1A (spectrum a). The 2194 cm "1 band is the same as for partially dehydroxylated HZSM-5 (spectrum b) and representing CO bonded to Lewis sites. However recently Geobaldo et al. [18] reported the presence of such a band (2196 cm "1) not only for Co-AI-BEA but also for Co-B-BEA (not containing the A1-Lewis acid sites) and had claimed that it is also due to Co 2+ species. We cannot exclude that the maximum at 2194 cm 1 is the superposition of CO adduct on cobalt and Lewis type sites. In order to get some information which of Co sites react preferentially with carbon monoxide, small doses of CO were successively introduced into the sample at 170 K. The corresponding IR spectra recorded aRer each adsorption step are shown in Fig. 1B, while the consecutive difference spectra (of the two adjacent runs) are shown in Fig. 1C. From inspection of the latter figure it is clear that the band at 2204 cm "1 appeared immediately after the first dose of CO has been adsorbed, whereas the 2194 and 2209 cm 1 band could be observed only after few doses (spectra c and d). These observations were complemented by the DFT calculations. Coordination of carbon monoxide to cobalt is exoenergetic by about 40 kcal/mol. It leads to the formation of a slightly bent 1"1~ adducts (the Co-C-O angle /9 = 172-173 ~ with the Co-C distance that is shorter for 13 sites (dco-c = 1.73 ,~) than for ot sites (dco-c = 1.76 A). In all the cases the C-O bond length was equal to 1.15 A and is slightly larger than that calculated for the free carbon monoxide (1.14 A). The structures of the ct and 13 monocarbonyl adducts are shown in Fig. 2 a, b. Although the geometry of the Co-C-O moiety is very similar for both adducts, they exhibit distinctly different electron density redistribution within the CO ligand. The results of the Mulliken population analysis revealed that upon the coordination partial charge on the cobalt center qco is decreased to 0.53, while on carbon atom it increased to qc = 0.24 and 0.28 for the at and 13 forms, respectively. This indicates that the OC ~ Co o-donation, besides the electrostatic interactions, dominates the bonding in both cases. However, in the case of the ot adducts the ligand to metal o-donation is distinctly more compensated by the n-backdonation than for the 13 monocarbonyls. As a result, the former should exhibit lower CO stretching than the latter one, providing a hint for the tentative assignment of the bands at 2204 cm -1 and 2209 cm 1 to ct and 13 carbonyls, respectively. Such attribution needs to be confirmed more adequately by quantum chemical calculation of the vco frequencies for both

Figure 2. DFT-optimized structures of cobalt monocarbonyl adducts (a) in ct sites, (b) in 13 sites and (c) mononitrosyl adduct in 13 site.

457 species, which is now in progress. Accumulation of the positive charge on carbon leads to a , !i , "-'i, [i Ay' g~ .. nucleophilic activation of CO upon the coordination. Adsorption of CO at higher pressure (pco > -~40 Torr) and low temperature (77 K) led to reversible formation of a new low spin {2Co(CO), }7 adduct, as it can readily be deduced from the EPR , ~ , i: spectrum (Fig. 3) with clear b: hyperfine structure due to the I I I i l i , ,I Alz....j coupling of the unpaired electron BImT with the 59Co nucleus (I = 7/2, 100%). Enhancement of the 280 300 320 340 360 380 Figure 3. X-band EPR spectrum of {Co(CO)3}7 resolution achieved by calculating adducts registered at 77 K, solid l i n e - first the 3rd derivative (dotted line) derivative, dotted line- third derivative. allowed to distinguish three different signals with g• = 2.222, gyl = 2.184, gz 1 = 2.011, [A• = 3.8, [hyll-- 3.2, lAz~l = 7.9 mT, gx2 = 2.234, gy2 = 2.172, gz2 = 2.012, lax21 = 3.8, ~Ay2l= 3.2, [Az21= 7.5 mT and gx3 = 2.234, gy3 = 2.173, gz3 = 2.018, ~lx3l = 3.8, ~ly3[ = 3.2, lAz31 = 7.1 mT, contributing to this EPR spectrum. Evidently, this reflects speciation of the cobalt high pressure adducts {2Co(CO),}7 into three closely related species. The observed magnetic parameters are consistent with an effective C2v symmetry and a Iz2,2A~> ground state of Co in all three complexes, but being primarily associated with Co they are less informative concerning the number of the coordinated CO molecules. However, from the comparison of the experimental anisotropic part of the cobalt hyperfine tensor of the most abundant species (Txx = - 4 . 1 , Tyy = - 3 . 5 , Tzz = 7.6 mT) with that calculated for di- (T,= = -11.3, Tyy = 2.6, Tzz = 8.7 mT ) and tricarbonyl (Txx= -4.5, Tyy = -3.7, Tzz = 8.2 mT) in ~ sites it can be deduced that the observed adducts most probably correspond to low spin cobalt {2Co(CO)3} species. Analogous adducts with the high spin cobalt as well as the adducts with n > 3 regardless of the spin state of Co, were found to be energetically unstable. These results indicate that the formation of intrazeolite tricarbonyl adducts involves an internal spin conversion of cobalt from quartet to doublet state, induced by the coordination of carbon monoxide 4Co + 3CO ~ {2C0(CO)3} 7. Indeed, because the framework oxygens are rather weak ligands, splitting of cobalt d- levels is not so large in the parent cage complex and remains in delicate balance with the spin pairing energy. Though initially the cobalt ions exhibit high spin state, the difference between the quartet and doublet energy is small. Therefore a strong rt acceptor and G-donating CO ligands that create a sizable energy gap between the cobalt d~ and d~ levels may readily ensure the low spin configuration of Co upon the coordination. The structure of Co-C-O moieties in the tricarbonyl complexes differs significantly from those of the monocarbonyl ones. For instance in ot sites both lateral CO ligands are highly bent (01,2 -- 135 ~ and at shorter distance to cobalt (dco-c = 1.95-1.96 A) than the central CO (dco-c = 2.53 A), which in turn is more straight (03 = 170~ In contrast to the

gx

.

.

.

.

i

.

.

.

t

i'

!

!

1

I

!

i

,

!

A2z,

i

! ........

458 monocarbonyl adducts, for tricarbonyls the coordination leads to significant polarization of all three bound CO molecules, as it can be deduced from the corresponding Mulliken partial charge distribution (qc 1 = 0.23, qo I = -0.19, qc 2 = 0.29, qo 2 = -0.24, qc 3 = 0.23, qo 3 = -0.20). This is accompanied by an increase of the C-O bond length to dc-o = 1.19 and 1.21 A for the lateral and central ligands, respectively. The results of EPR studies, which revealed the formation of three kinds of Co-tricarbonyls agree with the IR experiments, which also revealed three kinds of Co 2+ ions forming monocarbonyls (with the bands at 2204, 2209 and most probably also at 2194 cm l ) at low pressures.

3.3. Adsorption of NO IR spectra recorded upon the adsorption of NO at room temperature are presented in Fig.4. Two distinct bands at 1892 and 1941 cm 1 were present in the spectrum recorded at low NO loading (spectrum a). At higher coverage (spectrum b), the bands at 1896 and 1812 cm "1 appeared. Adsorption of NO in excess amounts at room temperature followed by evacuation resulted in disappearance of the 1941 cm "1 band, and the only bands left where those at 1812 and 1896 cm 1 (spectrum c) The second derivatives of the spectra a and b recorded at low coverage (lines d and e) show that the 1941 cm 1 maximum exhibited a shoulder at 1954 cm ~, and that an additional band at 1918 cm 1 was present, even though it is hardly seen in original spectra. At higher NO coverage dinitrosyls are favored, and accordingly the mononitrosyl bands were replaced by the bands at 1812 and 1896 cm "~ assigned to the cobalt dinitrosyl adducts. All these results revealed, that four kinds of Co mononitrosyls were present in the CoZSM-5 zeolite. They are characterized by N-O stretching bands at 1892, 1918, 1941 and 1954 cm 1. The position of the 1892 cm ~ mononitrosyl maximum is practically the same as that at 1896 cm ~ due to the dinitrosyl, but at low coverage the 1892 cm "1 band is present without 1812 cm ~ component. The positions of the four mononitrosyl bands observed in this study were practically the same as those reported earlier by Geobaldo et al. [18] for cobaltcontaining beta zeolites (1954, 1939, 1915 and 1895 cm~). These authors assigned them to NO bonded to Co 3+ (1954 cml), Co 2+ in counterionic positions (1939 cm "1) and Co 2 + grained to surface in defect positions (1915 and 1895 cml).

1

I

0.0 tO

2000

~

~

O')

1900

vlcm

T--

-1

1800

Figure 4. IR spectra of NO adsorbed at RT on CoZSM-5. a, b - adsorption of small portions of NO, c- adsorption of excess of NO followed by the evacuation at RT.

459 Assuming (aRer Geobaldo et al. [18]), that the band at 1954 cm 1 is due to NO bonded to Co 3+ ions, the bands at 1892, 1915 and 1941 cm1 can be related to Co 2+ ions. It seems, that contrary to Co-A1-BEA [18], in our CoZSM-5 samples of the exchange degree 40% practically all Co 2+ ions are in exchangeable positions. Three different IR bands of NO adsorbed (Fig. 4) associated with three different Co 2+ species correspond well to the three kind of Co 2+ tricarbonyls seen in EPR (Fig. 3) and three IR band of the Co 2+ monocarbonyls observed in IR spectrum (Fig. 1). According to the IR results presented in Figs. 4 and 1, the difference between the highest and lowest N-O stretching frequency (49 cm l ) is distinctly higher than in the case of CO bands (15 cm~). It indicates, that NO is more sensitive to the variation of the electron donor-acceptor properties of adsorption site than CO. It may be due to lower energy of the SOMO orbital of NO, which makes it more prone to rt back donation resulting in weakening of the N-O bond. From the DFT calculations it was found that the formation of both mono- and dinitrosyl species is energetically favorable. For instance, in the case of 13 sites ATE({CoNO }8) = -61 kcal/mol and AfE({Co(NO)2} 9) = - 9 9 kcal/mol, indicating that the complexation of nitric oxide by CoZSM-5 is considerably more exoenergetic than that of carbon monoxide. This is accordingly reflected in a quite different structure of both CO and NO adducts (Fig. 2b,c). The [3-mononitrosyl exhibits a bent 1"!1 coordination mode with the Co-N-O angle 0 = 141 ~ and the Co-N distance of 1.66 A. In contrast to the pentacoordinated cobalt monocarbonyl, ligation of NO leads to a tetracoordinated species of distorted trigonal pyramid structure. The partial charge on nitrogen increases from the initial value of-0.0082 in the free molecule to qN = 0.13 in the bound state, at the same time on cobalt center it is reduced to qco = 0.58, indicating a smaller net electron density transfer from the NO ligand to cobalt, as compared to CO. Contrary to the EPR silent non-Kramers {CoNO}8 species, dinitrosyl {Co(NO)2}9 complex gave rise to an EPR spectrum with the clear hyperfine structure due to 59Co (Fig. 5). Unfortunately, because of rather poor resolution of this spectrum any possible speciation of the dinitrosyl adduct could not be revealed. Notably, the speciation of Co 2+ dinitrosyls could not be seen in IR spectra (Fig. 4, c) as well. The parameters of the EPR spectrum, gl = 2.120, g2 = 2.105, g3 = 2.095, [ h i [ = 8.3, [A2I = 16.7, ~ 3 [ -- 6.1 mT, were determined by the computer simulation and are shown in the associated stick diagram. They are indicative for S = 89 species, yet are quite distinct _ _ g/ from those observed for the AI {2C0(CO)3} 7 adduct. This can be rationalized in terms of a A3 spin pairing mechanism of NO coordination. The two 21-I1/2radical ligands attaching to the quartet cobalt center 4C0 + 2NO {2Co(NO) 2}7 lead to reduction of its formal oxidation BImT state to Co(0). Such. assignment of the cobalt dinitrosyl species 250 290 330 370 410 was proposed earlier by Kevan Figure 5. X-band EPR spectrum of {Co(NO)2}9 et al. [19]. adduct registered at 77 K. ,

i

...............

i

i

,

,

,

i

,

,

i

,

i

i

l

460 4. CONCLUSIONS Adsorption of CO and NO on CoZSM-5 leads to the formation of several kinds of cobalt carbonyl and nitrosyl species characterized by the different electron density redistribution within the Co-CO and Co-NO moieties, respectively. Three types of {COCO}7 and {C0(CO)3 }7 carbonyl species, three kinds of nitrosyl {CoNO }8 and one kind of {CoNO }7 were observed. However, in the case of dinitrosyl {Co(NO)2}9 no speciation was detected. The formation of tricarbonyls involves CO spin crossing, while formation of dinitrosyls involves cobalt-nitric oxide spin paring. The presence of a positive charge on the coordinated molecules indicates that they are prone to a nucleophilic attack at the carbon and nitrogen centers. ACKNOWLEDGMENTS This study was supported by the grant of KBN (3 T09A 010 17). Z.S. thanks the Pruszyflski Foundation for the A. Krzyzanowski stipend. REFERENCES

1. K. Klier, R.G. Herman and S. Hou, Stud. Surf. Sci. Catal., 84 (1994) 1507. 2. J.N. Armor, Catal. Today, 26 (1995) 147. 3. T. Sun, M.D. Fokema and J.Y. Ying, Catal. Today 33, (1997) 251. 4. Y. Li and J.N. Armor, J. Catal., 176 (1998) 495. 5. S. Bessel, Appl. Catal. A, 126 (1995) 235. 6. S. Tsuruya, H. Miamoto, T. Sakae and M. Masai, J. Catal., 64 (1980) 260. 7. J.M. Thomas, Angew. Chem. Int. Ed., 38 (1999) 3588. 8. Z. Sojka and M. Che, Colloids Surf. A, 158 (1999) 165. 9. K. Klier, Langmuir, 4 (1988) 13. 10. R.F. Howe and J.H. Lunsford, J. Am. Chem. Soc., 97 (1975) 5156. 11. E. Giamello, Z. Sojka, M. Che and A. Zecchina, J. Phys. Chem., 90 (1986) 6084. 12. B. Wichterlova, J. D6d6cek, Z. Sobalik, Proc. 12 th Int. Zeolite Conference in Baltimore, M.M.J. Treacy, B.K. Marcus, M.E. Bisher and J.B. Higgins, Editors, MRS, 1999, p. 941. 13. J. D6d6cek, D. Kaucky, B. Wichterlova, Micropor. Mesopor. Mater., 35-36 (2000) 483. 14. R. Poli, Chem. Rev., 96 (1996) 2135. 15. DeMol, InsightlI release 95.0, Biosym/MSI, San Diego, 1995. 16. E. Broctawik, J. Datka, B. Gil, W. Piskorz, P. Kozyra, Topics in Catal., 11 (2000) 335. 17. A. J. Lupinetti, S. Fau, G. Frenking, S. H. Strauss, J. Phys. Chem. A, 101 (1997) 9551. 18. F. Geobaldo, B. Onida, P. Rivolo, F. Di Renzo, F. Fajula and E. Garrone, Catal. Today, 70 (2001) 107. 19. S. K. Park, V. Krushev, C. W. Lee and L. Kevan, Appl. Mag. Res., 19 (2000) 21.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

461

Spectroscopic and catalytic behaviour ofl(r I S-CsHs)Rh(rl 4-1,5-C8H12)] in M's6Y and H~Y (M'= Li, Na, K, Rb and Cs) E. C. de Oliveiraa, R. G. da Rosa b and H. O. Pastore"* aGrupo de Peneiras Moleculares Micro- e Mesoporosas, Instituto de Quimica, Universidade Estadual de Campinas, CP 6154, CEP 13083-970, Campinas, SP, Brasil, bInstituto de Quimica, Univ. Federal do Rio Grande do Sul, Porto Alegre, RS - Brasil.

Samples of {[(rlS-CsHs)Rh(r14-1,5-CsH12)]}-M'56Y (M '= Li, Na, K, Rb, Cs) and {[(rlS-CsHs)Rh(r14-1,5-CsH12)]}-H56Y were prepared from sublimation of the organometallic, [(rls-C5Hs)Rh(rl4-1,5-CsH12)], in alkaline and acid forms of zeolite Y, M'56Y, and H56Y respectively. The evolution of the organometallic species under thermal treatments in vacuum has been followed by infrared spectroscopy. The reactivity of the product was examined by the C-H activation reaction on cyclohexene. 1. INTRODUCTION Zeolite structures are being used, with a lot of interest, in the development of heterogeneous catalytic systems due to their defined crystalline structure, with channels and cages, their high thermal stability, large internal area and molecular sieving effects[ 1]. Zeolite Y has been specially focused. The systems formed from anchoring special chemicals at the inner surface of zeolites aim at joining the advantages of homogeneous and heterogeneous catalysts, and minimizing their disadvantages. They might be used for catalysis with shape selectivity, separation and purification of gases, artificial photosynthesis, photo and eletrocatalysis, to name but a few processes. Rhodium-based catalysts encapsulated in the zeolite Y have been frequently developped for conversion[2] and hydrogenolysis[3] of hydrocarbons, in the hydrogenation of carbon monoxide[4, 5], and in propylene hydroformylation[6- 8]. Their catalytic activities are not only due to the small dimensions of reactors that control the access of molecules to be transformed but also to the electronic features of the zeolite structure that affect the electronic properties of the catalyst lodged in the cages and channels[9]. The ion exchange in zeolite Na56Y is used to prepare other alkaline forms of this zeolite, Li56Y, K56Y, Rb56Y and Cs56Y, and its ammonium form (NH4)56Y that generates the acidic one, Hs6Y. In zeolites Li56Y, Rb56Y and Cs56Y, the ion exchange is not complete in the hexagonal prisms and B-cages, because these alkaline cations are very large when hydrated. In *This work was supportedby Funda~Aode Amparo ~ Pesquisa no Estado de S~o Paulo, grant numbers 98/10980-0 and 99/10391-8. Correspondingauthor e-mail: [email protected]

462 the exchange process of these large hydrated alkali cations, sodium ions are removed from the small cages and the charge compensation for their sites is made from the a-cage, generating another cation site in these cages, the site III. The existence of these sites characterizes zeolites Rb56Y and Cs56Y as class B zeolites, while the others are class A where sites III are empty. The number and localization of these sites are important because the organometallics anchoring in the a-cage occurs in these extraframework cations. 2. EXPERIMENTAL SECTION The organometallic, [(rls-CsHs)Rh(r14-1,5-CsH12)] or [(Cp)Rh(COD)], was prepared by the method described by Kang[10]. Zeolites M'56Y and (NH4)56Y were prepared by ion exchange of commercial Na56Y (Aldrich). The solids were calcined and were characterized by powder X-rays diffraction (Shimadzu, XRD 6000, CuK~, 30 kV, 40 mA, 2 ~ 20 minl), 29SiMAS NMR and 27A1-MAS NMR were obtained at 59,6 and 78,2 MHz, respectively (Bruker AC 300/P, MAS speed of 4,5 kHz, contact time of 50 ms, and TMS as a reference for 29SiMAS NMR and acid aqueous solution of AI(NO3), 1 mol.L"1 for 27A1-MAS NMR), and Si, Al, Na, K, Rb, Cs and Li elemental analysis by atomic absorption. The organometallic compound was encapsulated by sublimation and annealing onto a previously dehydrated self-supporting wafer of the above described zeolites. The systems so prepared were heated under vacuum (10-4-10-5 Torr) to provoke the loss of the COD ligand and to force the organometallic species to anchor onto the inner walls of the zeolitic cages. All the reactions were monitored by infrared spectra collected with a Nicolet 520 SX spectrometer, with 16 scans at a resolution of 2 cm1. The maximum anchoring capacity of the zeolite was determined by chemical analysis (ICP-AES Perkin-Elmer 300-DV, 233,477 for rhodium) of repeatedly loaded pellets. Catalytic tests on {(Cp)Rb(OZ)2}-Rb56Y and -Na56Y systems were made on a catalytic line coupled to a Hewlett Packard 5890 series II Gas chromatograph with a HP-1 column (50 meters and 0.20 mm of internal diameter). Cyclohexene was brought into contact with the catalyst carried by helium (10 mL.min1 unless otherwise stated).

3. RESULTS AND DISCUSSION

The materials prepared by ion exchange presented X-rays diffractograms characteristic of a faujasite structure. The process of ion exchange does not affect the crystalline structure of the Y zeolite. The elemental analyses of the alkaline forms of zeolite Y indicates that the ion exchange provided a degree of 90% exchange for (NH4)56Y and K56Y, 45% for Li56Y, e approximately 50% for zeolites Rb56Y and Cs56Y. Taking into account the occupancy of cation sites, one observes that even for the less exchanged samples, the a-cage cationic sites are occupied by each of the metal ions. Comparison of Far-IR spectra for dehydrated Na56Y e Cs56Y zeolites, Figure 1A e Bcurves 00, shows that not all the Na + cationic sites were substituted by Cs+in Cs56Y during ion exchange. Cs56Y spectrum seems to be a mixture of Cs+ in sites II e III[ 11] and Na + in sites I and II[ 11].

463 Thermal treatment of {(Cp)Rh(COD)}-Na56Y and {(Cp)Rh(COD)}-Cs56Y monitored by Far-lR, Figure 1, shows that after organometallic sublimation in Na56Y zeolite, Figure 1Acurves 01 e 02, the Na+iiband at 191 cm-l[11] is displaced by 5 cm1 to higher wavenumbers. Na+ni band is also displaced. However, after annealing and thermal/vacuum treatments the initial Nas6Y spectrum is restored, Figure 1-curve 05. These data show that anchoring after annealing occurs in the intrazeolite or-cage cations but thermal/vacuum treatments releases these cations as Far-IR indicates that they are no more disturbed by the organometallic which probably anchors to the oxygen ions on the walls. On Cs56Y zeolite the organometallic sublimation causes intensity changes in the band corresponding to Gt-cage Cs+H cations and displacement to higher wavenumbers of the one of Gt-cage Cs+m cations, Figure 1B-curve 01. After annealing and thermal/vacuum treatments only a partial restoration of spectra takes place, different of what was observed for Na56Y, probably because of the smaller space in the cages of Cs56Y zeolite.

Na+.~ Na*I

~00

9

2;0

2;0

i

= o~'~

Na+ltj

~;0

Wavenumber/cm"~

....

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\

50

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cr

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~

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,~

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Wavenumber/em4

A B Figure. 1 Far-IR spectra os (A) {(Cp)Rh(COD)}-Na56Y (Hg lamp): 00- dehydrated Na56Y zeolite; 02- after organometallic sublimation; 05- after 17 h annealing followed by thermal treatment at 250~ under vacuum for 3 h. 01- dehydrated Na56Y zeolite (globar lamp); (B) {(Cp)Rh(COD)}-Cs56Y: 00- dehydrated Cs56Y zeolite; 01- after organometallic sublimation; 03- after 5 h annealing followed by thermal treatment at 250~ under vacuum for another 5 h period. 27A1-MAS M R was used to check for dealumination upon calcination and indicated that no extraframework aluminum phase was formed in none of the M'56Y zeolites. 29Si-MAS NMR confirmed the Si/A1 molar ratio determined by elemental analysis. Figure 2 shows the spectra of {(Cp)Rh(COD)}-Na56Y and Li56Y after sublimation and annealing. The band at approximately 3100 cm1 was assigned to the organometallic anchoring from the Cp ring to the intrazeolitic cation, site II[ 12]. For {(Cp)Rh(COD)}-K56Y, Rb56Y and Cs56Y, a shoulder at 3072 cm1 also appears. The thermal decomposition after annealing causes the intensity decrease of the bands in the region of 2800-2900 cm-1 due to the release of the COD ligand. Simultaneously, the shoulder at 3072 cm4 disappears probably because the organometaUic fragment has more space to anchor in the cages.

464 Even after 6 h thermal treatment at 250~ under vacuum, weak bands in the region of 2800-2900 cml, due to the C-H bonds of Cp ligand[ 12] are still seen, characterizing a strong intrazeolitic anchoring. Higher temperatures bring about the total disappearance of the CH bands and darkening of the pellet. Probably coke is formed. Thus the duration of the thermal treatment was fixed at 250 ~ until complete removal of the COD ligand. The anchoring reaction and the proposed structure for the anchored organometallic are displayed in Figure 3.

//\~:\

09

J

~

/

~ ,,-'~

07

r,

/

v

~'~

~'----'---------

9 Wavenumber

/ c m "~

] Wavenumber

/ c m "~

Figure. 2. Thermal treatment of samples: (A) {(Cp)Rh(COD)}-NaseY: 01- after sublimation; 03- 8h annealing; 05- 12h annealing; 07-09- decomposition 2h, 250~ vaccum; l 1decomposition, lh, 300~ vacuum. (B) {(Cp)Rh(COD)}-Rb56Y: 01- after sublimation; 03l lh annealing; 05- decomposition, 4h, 250~ vacuum; 07- decomposition, 21% 250~ vacuum; 09-decomposition, lh, 300~ vacuum.

250 ~ C

I I

---M --. I !

v

{[(Cp)Rh(COD)I}-M'~Y

- COD M

!

~-

~

{[(Cp)RhI}-M'56Y

Figure. 3. Thermal decomposition reaction of {(Cp)Rh(COD)}-M'56Y (M' = Li, Na, K, Rb, ou Cs), and the proposed structure for the product.

465 Zeolite (NH4)56Y was thermally transformed in situ into H56Y zeolite where the o~- and B-cage protons are easily identified by infrared spectroscopy[13, 14].[(Cp)Rh(COD)] was sublimed on to a pellet of the acidic zeolite, thermally annealed for 12h and decomposed for another 12h period at 250~ and higher temperatures. The results are displayed in Figure 4. The organometallic sublimation, Figure 4, curve 01, causes a decrease in intensity in the band corresponding to protons from the m-cage, while B-cage protons are barely affected except for a slight displacement (3 cm -1) to the lower wavenumber side. These results indicate that the organometallic interacts directly with m-cage protons, anchoring to them much the same way as it anchors to alkali cations in the ~-cage[15, 16], Figure 3. The anchoring process somehow alters the interaction of 13-cage protons with structure oxygen atoms. No Hbonding from (~-cage acidic protons with the organometallic is evident[15] indicating that a proton transfer from zeolite to [(Cp)Rh(COD)] is possibly occurring.

5 (D (J csf.o (I) .o

,,"/",,'.i!":

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,, ,, ....

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35'00

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Wavenumber / cm

-1

34'00

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'3300

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o2

0

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/ cm ~

Figure. 4. Thermal treatmem of {(CpPJI(COD)}-H56Y: 00- a~er dehydration and in situ calcination; 01- after organometallic sublimation; 02- 10h annealing; 03- decomposition, 2h, 250~ vacuum; 04- decomposition, 2h, 250~ vacuum ; 05- decomposition, 21% 280~ vacuum; 06- decomposition, 13h, 280~ vacuum; 07- decomposition, 2h, 300~ vacuum.

466 As for the alkaline forms of zeolite Y, thermal decomposition leads to the total loss of COD ligand, indicated by the decrease in the intensity of the bands at around 2900-2800 cm1, Figure 4, 02 to 07, bands remaining after thermal treatment at 300~ C are due to the Cp ligand. Again, in the same way as for the alkaline forms of Y zeolite, COD release allows the organometallic fragment to bind directly to the oxygen atoms in the walls of the cages, releasing the a-cage protons and restoring the OH band at 3643 cm1 as already observed in the literature[ 15]. However this effect was not observed in this work, indicating again that a irreversible H-transfer from the OH acidic group in the cavity to the organometallic. Total consuption of acidic a-cage protons by the organometallic through repeated sublimation did not provoke the appearance of a hydrogen bonding band either, only a weak band at 3609 cm1, that disappears after thermal treatment, and a slight widening of the 13-cage acidic protons band, Figure 5.

12 11 10 09

O8

3800

3700

3600

3500

3400

3300

Wavenumber / cm 1

Figure. 5. Thermal treatment and organometallic saturation of {(Cp)Rh(COD)}-H56Y: 08aider another sublimation; 09- 4h annealing; 10- 14h annealing; 11- 3h, at 280 ~ C, vacuum; 12- another sublimation; 13- 18h annealing; 14- 3h, at 280 ~ C, vacuum. Spectra in Figures 4 and 5 show that although the anchoring in {(Cp)Rh(COD)}- H56Y is similar to the other alkaline forms of Y zeolite, the elimination of COD do not release the proton as it did for alkaline cations because this would bring the restoration of OH bonds and hence the infrared band corresponding to them. This might indicate that the acidic proton reacted with the organometallic turning it into a cationic species that becomes the charge compensating entity. To ascertain the intrazeolite localization of the organometallic, {[(Cp)Rh(COD)]}Na56Y was heated at 250 ~ C and then the wall-anchored organometallic fragment, {[(Cp)Rh]}- Na56Y reacted with trimethyl- (PMe3) or tricyclohexylphosphine (PCy3). The presence of PMe3 was indicated by the bands at 2986, 2973 e 2918 cm1 and of PCy3 at 2927 e 2957 cm1. The comparison of these spectra with the ones of pure phosphines anchored in Na56Y shows that PMe3 has reacted with organometallic and sodium cations while PCy3 reacted only with sodium cations. After annealing and reaction, dynamic vacuum eliminates PCy3 immediately while PMe3 is only eliminated ager exhaustive thermal treatment/vacuum cycles.

467 These results along with the Far-IR, the sublimation of the organometallic in H56Y zeolite, all indicate that the anchoring occurs on the intrazeolite space and not on the external surface of the crystals. Continuous catalytic tests on {[(Cp)Rh]}-Na56Y and {[(Cp)Rh]}-Rb56Y show that these systems are active in converting cyclohexene into benzene and cyclohexane. Table 1 shows the obtained results. {[(Cp)Rh]}-Na56Y seems to be a little more active than {[(Cp)Rh]}-Rb56Y since it presents benzene and cyclohexane formation at 100~ C, while the rubidium system begins to show a comparable activity only at 150~ C. Pure Na56Y and Rb56Y are not active in the reaction (Table 1) indicating that the active phase is formed by the {[(Cp)Rh]}-Na56Y and {[(Cp)Rh]}-Rb56Y due to the wall-anchored organometallic fragments. Table 1 Catalytic performance for {[(Cp)Rh]}-Na56Y and {[(Cp)Rh]}-Rb56Y a . Catalyst % de benzene % de cyclohexane 100~ 150~ 250~ 100~ 150~ 250~ Na56Y Rb56Y

a

{[CpRh]}-Na56Y

23

50

ne

33

45

ne

{[CpRh]}-Rb56Y

-~ 0

45

75

- 0

48

20

Helium flow: 10 mL.min"1, ne= not examined

Reactions were also run at lower contact time, at a helium flow of 20 mL.mim-~ which caused the formation of benzene and cyclohexane to drop to their half. Both catalysts were also tested for the cyclohexene hydrogenation under a continuos flow of hydrogen. Results indicate 95% formation of cyclohexane at 100~ C for {[(Cp)Rh]}Rb56Y, and 98% at 30 ~ C for {[(Cp)Rh]}-Na56Y. The results obtained here indicate that {[(Cp)Rh]}-Rb56Y and {[(Cp)Rh]}-Na56Y prepared in this work are capable to activate C-H bond in cyclohexene leading to the formation of benzene, however they also presented even better performances in cyclohexene hydrogenation at low temperatures. Therefore, the mechanism for this reaction must pass by the formation of a metal hydride capable of hydrogenating both the substrate, cyclohexene, as well as the intermediates in the formation of benzene, as shown in the tentative mechanism displayed in Figure 6. 8(ZO2)Rh(Cp) + 2C6H10 ~

8(ZO2)Rh(Cp)(H)+ 2C6H 6

2(ZO2)Rh(Cp)0--I) + C6H10

~

2(ZO2)Rh(Cp) + C6H12

6(ZO2)Rh(Cp)(H) + C6H6

~

6(ZO2)Rh(Cp) + C6H12

Figure. 6: Proposed mechanism for the C-H bond activation on cyclohexene with {(Cp)Rh}Rb56Y and {(Cp)Rh}-Na56Y.

468 CONCLUSIONS This work shows that the organometallic [(Cp)Rh(COD)] was anchored in the cages of zeolites M'56Y (M ' = Li, Na, K, Rb and Cs) and H56Y. Reaction with trialkylphophines, the consuption of acidic internal protons by the organometallic and spectroscopic Far-IR show that the anchoring occurs in the internal surface of the zeolite. The thermal treatment for removal of COD ligand leads to the anchoring of fragment organometallic, [(Cp)Rh], in the oxygen atoms of the M'56Y framework , while that in zeolite H56Y there seems to be a chemical reaction between the organometallic and the protons from the c~-cage generating a new species still not identified. The catalysts {[(Cp)Rh]}-Rb56Y and {[(Cp)Rh]}-Na56Y prepared in this work are able to activate the C-H bond of the cyclohexene at 150 and 100~ C, respectively, leading to the formation of benzene and cyclohexane. They are also active in the hydrogenation of the cyclohexene in temperatures as low as 30~ for {[(Cp)Rh]}-Na56Y. REFERENCES

1. D. W. Breck, Zeolite Molecular Sieves, Wiley, New York (1974). 2. T. J. McCarthy, G. D. Lei, W. M. H. Sachtler, J. Catal. 159 (1996) 90. 3. T. T. T. Wong, W. M. H. Sachtler, J. Catal. 141 (1993) 407. 4. T. J. Lee, B. C. Gates, J. Mol. Catal. 71 (1992)335. 5. W. M. H. Sachtler, Y.-Y. Huang, Appl. Catal. A 191 (2000) 35. 6. E. J. Rodes, M. E. Davis, B. C. Hanson, J. Catal. 96 (1985) 563 and 574. 7. I. Burkhardt, D. Gutschick, U. Lohse, H. Miessner, J. Chem. Soc., Chem. Commun. (1987) 291. 8. H. Miessner, I. Burkhardt, D. Gutschick, A. Zecchina, C. Monterra, G. Spoto, J. Chem. Soc., Faraday Trans. 1 85 (1989) 2113. 9. M. Boudart, A. W. Aldag, L. D. Ptak, J. E. Banson, J. Catal. 11 (1968) 35. 10. J. W. Kang, K. Moseley, P. M. Maitlis, J. Am. Chem. Soc. 91(1969) 5970. 11. S. Ozkar, G. A. Ozin, K. Moiler, T. Bein, J. Am. Chem. Soc. 112 (1990) 9575. 12. G. A. Ozin, M. M. Haddleton, C. Gil, J. Phys. Chem. 93 (1989) 6710. 13. J.W. Ward, Adv. Chem. Ser. 101 (1971) 380. 14. J. W. Ward, Zeolite Chemistry and Catalysis, Ed., American Chemical Society, New York, ACS Monogr., 171 (1976) 124. 15. S. Abdo, R. F. Howe, J. Phys. Chem. 87 (1983) 1713. 16. K. J. Balkus, Jr., K. Nowinska, Microporous. Mater. 3 (1995) 665.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

469

I m p r o v e d synthesis procedure for F e - B E A zeolite D.Aloia, F. Testa a, L. Pasqua b, R. Aiello a and J. B.Nagyr aDipartimento di Ingegneria Chimica e dei Materiali, Universit~ degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy bDipartimento di Ingegneria dei Materiali e della Produzione, Universifft Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy CLaboratoire de R. M. N., Facult6s Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium An improved and reproducible synthesis of BEA zeolite containing iron is reported. Starting from methods of preparation of Fe-BEA previously reported in the literature, compositions of gel systems, time of ageing and crystallization temperature were varied. The Si/Fe of the products obtained is lower than those prepared with other methods. The Fe-BEA samples have been characterized showing that essentially all iron is incorporated in the framework in tetrahedral position. 1. INTRODUCTION The isomorphous substitution of silicon for iron is considered as a very important step in the formation of phenol from benzene using Fe-ZSM-5 zeolite [1]. In addition, the ethylbenzene is also easily dehydrogenated into styrene on the same zeolite [2]. Fe-ZSM-5 zeolite was also proposed for the removal of NOx from exhaust automotive gases [3, 4]. The Fe-BEA zeolite has a high activity and selectivity in the m-xylene isomerization [5] as well as in the alkylation of benzene with propene [6]. A quite low SYFe ratio [6, 7] was claimed by La Pierre and Partridge [8]. The Fe-BEA is generally synthesized from tetraethyl orthosilicate (TEOS) as silicon source, tetraethylammonium hydroxide (TEAOH) as template molecules, Fe(NO3)3.9H20 as iron source and NaOH aqueous solution [5]. The time of ageing is generally 24 hours and the crystallization time 12 days at 120~ More recently, a very high iron-content Fe-Bea was synthesized (Si/Fe=7.5) using 25% methanolic TEAOH solution and a shorter ageing time (ca. 16 hours) at 120~ [9]. However the latter sample revealed to be not well crystallized. The present paper reports on an improved and reproducible synthesis of Fe-BEA, although the iron content (Si/Fe=25) is lower than in the above reported examples. 2. EXPERIMENTAL

2.1 Synthesis

The following reagents were used: tetraethylammonium hydroxide (TEAOH) 20% in H20 (Aldrich), TEAOH 25% in methanol (Aldrich), tetraethyl orthosilicate (TEOS, Aldrich) as

470 silicon source, Fe(NO3)3.9H20 (Merck), AI(OH)3 (Pfakz & Bauer), NaOH (Carlo Erba) and ultradistilled water. The following general composition were used for Fe-BEA: 40S iO2-xFe(NO3)3.9H20-yTEAOH-4NaOH-676H20 with x=0.38, 0.40, 0.44, 0.45, 0.49, 0.51, 0.60, 0.76, 1.02 y=10.88, 13.6, 16.3 and 19.04. The experimental procedure was carried out as follows: solution A is prepared by adding to aqueous TEAOH 20% solution under magnetic stirring during one hour TEOS. Solution B is prepared by adding NaOH to aqueous TEAOH 20% solution under magnetic stirring. Solution C is obtained by introducing Fe(NO3)3.9H20 into ultra distilled water under magnetic stirring during ten minutes. Solution A is added drop by drop to solution C. It is important to dissolve carefully the total amount of solution A and C. The so obtained solution is solution D. Finally, solution D is added slowly to solution B under magnetic stirring leading to a light yellow gel. The system is let in air under stirring during 24 hours. This allowed the ethanol formed during the hydrolysis of TEOS to evaporate. The so-obtained gels were put in PTFE-lined 25 cm 3 stainless steel autoclaves. The samples were obtained by hydrothermal synthesis at 120~ or 150~ for pref'Lxed times in static conditions. After quenching of the autoclaves the products were recovered, filtered, washed with distilled water and finally dried at 80~ for 24 hours. The syntheses of A1-BEA and Fe,A1-BEA were carded out in the conditions optimised for FeBEA. The gel compositions were the following: 40S iO2-xFe(NO3)3.9H20-yAI(OH)3-16.3TEAOH-4NaOH-676H20 with x=0, 0.06, 0.18 and 0.30 and y=0.30, 0.42, 0.54, and 0.60; note that x+y=0.60 in all the syntheses. The synthesis procedure was identical to that described above, except that solution C contained either both iron and aluminium sources or aluminium source alone. The reaction temperature was 150~ 2.2 Characterization The powder X-Ray diffraction patterns were collected using CuK~ radiation (Philips PW 1730/10 generator equipped with a PW1050/70 vertical goniometer). The amounts of iron, aluminium and sodium in the crystals were determined by atomic absorption spectrophotometry (GBC 932 AA). The amount of TEA occluded in the crystals was measured by TG analysis. The DSC curves allowed one to determine the behaviour of TEA + ions in the channels. The measurements were carried out with a Netzsch STA 409 instrument between 20 and 850~ at a ramp of 10~ in air with a flow rate of 5ml/min. The scanning electron microscope (SEM) micrographs were collected on a JEOL JSTM 330A. Surface area measurements were carried out on Micromeritics ASAP 2010 system in liquid nitrogen temperature. The calcined samples were degassed at 330~ and 10-5 Torr for 10 hours. 3. RESULTS AND DISCUSSION

Using the reagents shown in the experimental part the synthesis of Fe-BEA proposed in ref. 5 could not be reproduced. Indeed, starting from a gel composition of 40SIO2-1.02

471 Fe(NO3)3.9H20-19.04TEAOH-4NaOH-676H20 using a 24 hours ageing time only amorphous phase was obtained at 120~ after 12 days. Similar unsuccessful tentative was made if the ageing was done at 273~ for 3 hours. The reaction temperature of 150~ did not lead to any Fe-BEA zeolite. Finally, even if TEAOH 25% in methanol solution was used no Fe-BEA zeolite could be obtained following the method proposed in ref. 9. In order to find a well reproducible method using our reagents, we have systematically varied the amount of iron source, the amount of TEAOH, the ageing time of the gel, the reaction time and the temperature of the reaction. The general composition of the gel was the following: 40SiO2-xFe(NO3)3.9HzO-yTEAOH4NaOH-676H20 with x-0.38, 0.49, 0.51, y=19.04, ageing time=2, 18, or 24 hours, reaction time=7, 9, 10, 20, 21, 22, 26, 28, or 30 days. Only one synthesis led to Fe-Beta, with x=0.49, y=19.04, 24 hours ageing and 20 days of synthesis time at 120~ Even this synthesis was not reproducible. Note that using fumed silica instead of TEOS as silica source, some unidentified layered compounds were obtained. For x=0.45, 0.49 and 0.60 and y=10.88, 13.6 and 16.3, ageing time =2 or 24 hours, reaction time=16, 18, 20, 21, 22, 25, 29 or 40 days, T=120~ Fe-BEA co-cristallyze in most of the cases with an unknown phase having a diffraction peak at 5.6 20. Note that a similar peak was Table 1 Synthesis conditions for the Fe-BEA obtained from gels of composition 40SiO2xFe(NO3)3.9HzO-yTEAOH-4NaOH-676H20 at 150 ~ Reaction time (days) Product Sample Y Fe-BEA + U b 10, 15 19.04 1 0.51 Fe-BEA + U b 19.04 13 0.49 Fe-BEA + U b 19.04 11 0.44 Fe-BEA + Amorphous 16.3 0.76 16.3 Fe-BEA + U + Amorphous 0.76 16.3 Fe-BEA + U 0.76 7 0.60 16.3 4, 5, 6, 7, 8 Fe-BEA 8 0.45 16.3 8 Fe-BEA 9 0.45 16.3 25 Fe-BEA + U 10 0.45 13.6 8 Fe-BEA 11 0.45 13.6 15 Fe-BEA + U 0.49 10.88 12 Fe-BEA + U aAgeing time: 24 hours; bUnidentified phase Table 2 Synthesis conditions for (Fe,A1)-BEA and A1-BEA obtained from gels of compostion 40SiO2xFe(NO3)3.9H20-zAl(OH)3-16.3TEAOH-4NaOH-676H20 at 150 ~ Reaction time (days) Run Product (Fe,AI)-BEA 13 0.30 0.30 14 0.18 0.42 4, 5, 6 (Fe,A1)-BEA 0.06 0.54 3, 4, 5 (Fe,A1)-BEA 15 0 0.60 4, 5, 6 A1-BEA 16 aAgeing time: 24 hours

472 0.8

9 ooo o /"

0.7 0.6 0.5 0.4-

AMORPHOUS 9

0.3"

lO

9

'

1'2

'

9

114

'

1'6

i

'

1'8

'

20

TEAOH(n~les) Figure 1 Crystallization fields of Fe-BEA zeolite from gels of composition 40SiO2-xFe(NO3)3.9H20yTEAOH-4NaOH-676H20 at 150 ~ obtained during the synthesis of a low A1 content Beta zeolite [10]. This peak disappears during calcinations at 450~ As the reaction temperature of 120~ was not adequate to obtain pure Fe-BEA in a reproducible way, the reaction temperature was raised to 150~ At this temperature the amounts of Fe(NOa)a.9H20 and TEAOH are also varied in order to optimize the synthesis conditions. The data are reported in Table 1. It is clearly seen from Table 1 that in particular conditions, pure Fe-BEA can be obtained in a reproducible manner at 150~ The conditions are 0.45 or 0.60 Fe(NO3)3.9H20, 13.6 or 16.3 TEAOH, 24 hours ageing time and reaction time 4-8 days. The narrow crystallization fields are reported in Figure 1, where the crystallization field of Fe-BEA is surrounded at higher Fe content by a phase where Fe-BEA coexists with an unknown phase U. Note that at low ageing time of the gels (2 hours), only an amorphous phase was obtained in all cases. The white colour of all the final crystalline Fe-BEA zeolite samples shows unambiguously that Fe(III) occupies framework tetrahedral sites in the structure. In the zone of crystallization of Fe-BEA-i.e. 0.60 Fe(NOa)3.9H20 and 16.3 TEAOH-(Fe, A1)-BEA and Al-BEA were also synthesized maintaining the same moles of iron and aluminium source equal to 6. The data are reported in Table 2. It is seen that only the crystalline phases (Fe,A1)-BEA or A1-BEA were obtained in all synthetic runs already at 3 or 4 days crystallization time. These results reinforce the existence of the zone where pure FeBEA zeolite samples could be obtained. The d values of the XRD patterns of Fe-, (Fe,A1)- and A1-BEA samples are reported in Table 3. The d values are slightly higher in the Fe-BEA sample showing the expansion of the zeolitic structure due to the larger ionic radius of Fe (III) (0.063 nm) with respect to that of A1 (III) (0.053 nm). Note that the crystallographic data of (Fe,A1)-BEA sample are very close to those of pure A1-BEA sample suggesting that the Al-content of the (Fe,A1)-BEA sample is substantially higher than the Fe-content.

473

M-BETA

(Fe,A1)-BETA

Fe-BETA

10

20

'

3'0

'

4~0

2o Figure 2. X-ray diffractogram of A1-BEA (n ~ 16), (Fe,A1)-BEA (n ~ 13) and Fe-BEA (n ~ 7) Table 3 XRD data of A1,- (Fe,A1)-BEA and Fe-BEA zeolites (Fe,A1)-BEA M-BEA d (nm) I/Io (%) 20 I/lo(%) d (nm) 20 27.5 7.83 1.1320 24.2 7.82 1.130 21.64 0.410 18.7 21.66 0.410 23.3 22.66 0.392 100.0 22.68 0.392 100.0 25.52 0.349 10.0 25.54 0.348 13.9 26.98 0.330 17.0 27.02 0.330 18.1 29.70 0.301 15.6 29.72 0.300 18.1 30.74 0.291 6.8 30.70 0.291 10.6 33.62 0.266 8.1 33.62 0.266 9.0 43.96 0.206 7.4 43.92 0.206 9.9

20 7.80 21.48 22.50 25.34 26.84 29.54 30.52 33.48 43.72

Fe-BEA d (nm) 1.132 0.413 0.395 0.351 0.332 0.302 0.293 0.267 0.207

I/Io (%) 28.8 23.8 100.0 15.6 20.6 21.4 14.2 11.9 12.8

474

385.4~

100 -

95-

i

331.2~ ~

85-

80-

lli '

20o

400 ' 6o0 ~PERAa~R~ oc

'

80o

Figure 3 TG curve of the Fe-BEA sample obtained from the synthesis n ~ 8 having molar composition 40SiO2-0.6Fe(NO3)3.9H2016.3TEAOH-4NaOH-676H20 at 150 ~

2()0

4(1o 60o TEmERA~RE~

8oo

Figure 4 DSC curve of the Fe-BEA sample obtained from the synthesis n ~ 8 having molar composition 40SiO2-0.6Fe(NO3)3.9H2016.3TEAOH-4NaOH-676H20 at 150 ~

The TG and the DSC data of three characteristic Fe-, (Fe,A1)- and A1-BEA samples are reported in Fig. 3 and Fig. 4. The elimination of the organics is made in an air flow. The first region is characteristics of the water loss (from 20 to 180~ The second weight loss characterizes the elimination of TEA + ions countercations to SiO- defect groups [5, 9, 10, 11] or TEAOH species (from 180 to ca 400~ and the third weight loss is attributed to TEA + ions neutralizing the framework negative charges linked to the presence of iron and/or aluminium (from 400 to 850~ The weight loss due to water is the highest for the (Fe,A1)-BEA sample, it decreases for the FeBEA sample and it is the lowest for the A1-BEA sample. While the relative amounts of TEA § (Si-O)- and TEA+-(Si-O-A1Fe) - species decrease and increase, respectively in the series (Fe,A1)-BEA, Fe-BEA and A1-BEA the total amount of TEA § ions remains costant as it was shown previously [10, 11]. The DSC curves show three peaks at ca 330~ 385~ and 489~ in the Fe-BEA sample (Fig. 4a). The first two peaks correspond to the oxidation of the TEA § ions counterions to SiOdefects groups and the 489~ peak corresponds to the oxidation of the TEA § ions neutralizing the (SiOFe) negative charges. The DSC curves of the (Fe,A1)- and A1-BEA samples are more similar. It can be seen, however, that the maxima of the DSC peaks are lower for the (Fe,AI)BEA zeolite than for the A1-BEA zeolite, suggesting that the TEA + ions interact more strongly with the A1 sites than with the Fe sites. The nitrogen adsorption isotherms of the Fe-, (Fe,A1)- and A1-BEA zeolite samples all show characteristics Type I isotherms which reveal an essentially microporous nature of these materials. The nitrogen BET surface is higher for the Fe-BEA zeolite (618 m2/g) than for the A1-BEA sample (578 m2/g). The (Fe,A1)-BEA sample has an intermediate surface equal to 572 m2/g. The presence of a hysteresis loop between the adsorption and desorption branches of (Fe,A1)-BEA isotherms indicates the presence of impurities which give rise to desorption in the mesopore region (15-100/~ diameter).

475

Figure5a Scanning electron micrograph of the products obtained from the synthesis having molar composition 40SiO2-0.45Fe(NO3)3.9H2016.3TEAOH-4NaOH-676H20 at 150 ~

l~igure5b Scanning electron micrograph of the products obtained from the synthesis having molar composition 40SiO2-0.45Fe(NOs)3.9HEO16.3TEAOH-4NaOH-676H20 at 150 ~ calcined at 440 ~ for 8 hours

The SEM micrographs show that the morphology of all the samples is spheroidal suggesting the presence of mierocrystals in all particles. The average size of the particles is ca. 0.7 ~tm for the Fe-BEA, ca. 1 pm for the (Fe,A1)-BEA, ca. 0.4 pan for the A1-Beta samples. Fig. 5a shows, in addition, the SEM mierographs of the mixture of Fe-BEA with the unidentified crystalline phase (run n ~ 9 after 25 hours). The needle-like crystals can be easily identified. These crystals disappear under calcination at 440~ for 8 hours (Fig. 5b). At the same time the characteristic X-Ray peak at 5.6 20 also disappears. The 13C-NMR spectra show clearly that TEA+ cations are incorporated intact in the BEA zeolite channels: 8=53.3 ppm for -CHz- groups and 84.4 ppm for CH3-groups in Fe-BEA. The 27Al_NMR spectra show that aluminium is incorporated mostly in framework tetrahedral sites (~i=52.7 ppm), ca 60-70 % and probably as framework octahedral species (8=13.0 ppm), ca 30-40% in Fe,AI-BEA samples. In the pure AI-BEA the tetrahedral and octahedral species are 82% and 12%, respectively, the chemical shifts being equal. The 29Si-NMR spectra are most revealing. In all Fe-, Fe,AI- and AI-BEA samples three main NMR lines characterize the Beta zeolite structure. T h e - 1 0 2 ppm line stems from both Si(1AI) configurations and some possible SiOM (M=H, Na or TEA) defect groups [12]. The other two NMR lines at -109 ppm and -111 ppm (shoulder) are due to Si(OA1) configurations of two crystallographically different sites [11]. It seems that the coerystallizing phase with FeBEA when the 5.6 20 is present could be an MCM-22 containing phase [13]. 4. CONCLUSIONS A synthetic procedure previously proposed [5, 9] had to be modified in order to synthesize Fe-BEA zeolite in a reproducible manner using our reagents. Gels of the following compositions were prepared: 40SiO2-xFe(NO3)3.9H20-yTEAOH-4NaOH-676H20 with 0.3.~_x<1.02 and 10.88
476 narrow. The optimal conditions are the following: Fe(NOa)3.9H20 is comprised between 0.45 and 0.60 moles and the TEAOH 20 wt % aqueous solution has to have between 13.6 and 16.3 moles. At a temperature of 150~ the Fe-BEA zeolite can be obtained in 4 days instead of 12 days at 120~ The optimal ageing time is 24 hours. The optimal conditions were also used to prepare (Fe,Al)- and A1-BEA zeolites. All the Fe-containing Beta zeolites are white showing that Fe(III) occupies essentially framework tetrahedral sites. 5. ACKNOWLEDGMENTS

The present work is a part of a project coordinated by A. Zecchina and cof'manced by the Italian MURST (Cof'm 98, Area 03). REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

A. S. Kharatinov, G.A. Sheveleva, G.A. Panov, V.I. Sobolev, P.A. Ye, V.N. Romannikov, Appl. Catal. A 98 (1993) 33. C. Zhang, Z. Wu, Q. Kan, Cuihua Xuebao 17 (1996) 34. S. Iwamoto, S. Shimizu, T. Inui, Stud. Surf. Sci. Catal. 125 (1994) 1523. G. Fierro, G. Ferraris, M. Inversi, M. Lo Jacono, G. Moretti, Stud. Surf. Sci. Catal. 135 (2001) 30-P- 14. R. Kumar, A. Thangaraj, R.N. Bhat, P. Ratnasamy, Zeolites 10 (1990) 85. A.V. Smirnov, F. Di Renzo, E. Lebedeva, D. Brunel, B. Chiche, A. Tavolaro, B.V. Romanovsky, G. Giordano, F. Fajula, I. Ivanova, Stud. Surf. Sci. Catal. 105 (1997) 1325. G. Zi, T. Dake, Z. Ruiming, Zeolites 8 (1988) 453. R.B. La Pierre, R.D. Partridge, Eur. Pat. Appl. 94827 (1983). R.B. Borade, A. Clearfield, Microporous Mater. 2 (1994) 167. P.R. Hari Prasad Rao, C.A. Leony Leon, K. Ueyama, M. Matsukata, Mic. Mes. Mat. 21 (1998) 305. Z. Gabelica, N. Dewaele, L. Maistrian, J. B.Nagy, E.G. Derouane, in ACS Symposium Series 398, Zeolite Synthesis, M.L. Occelli and H.E. Robson, eds., American Chemical Society, Washington, DC, 1989, p. 518. R. Mostowicz, Zeolites 18 (1997) 308. R. Aiello, F. Crea, F. Testa, G. Demortier, P. Lentz, M. Wiame, J. B.Nagy, Microporus Mesoporous Mater. 35-36 (2000) 585.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

477

One-step benzene oxidation to phenol. Part I: Preparation and characterization of Fe-(A1)MFI type catalysts. G. Giordano 1, A. Katovic 1, S. Perathoner 2, F. Pino 2, G. Centi 2, J. B.Nagy 3, K. Lazar 4 and P. Fejes 5

1 Dipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, 87030 Rende (CS), Italy 2 Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italy 3 Laboratoire de RMN, FUNDP, 61 rue de Bruxelles, 5000 Namur, Belgium 4 Institute of Isotope and Surface Chemistry, 1525 Budapest, P.O. Box 77, Hungary 5 Applied Chemistry Department, University of Szeged, Rerrich B~la t~r 1, 6720 Szeged, Hungary The characteristics of iron-containing MFI type zeolites synthesized by different methods (direct synthesis, chemical vapor deposition, solid state reaction and ion exchange), in relation to their application for the one-step synthesis of benzene from phenol, were investigated by MSssbauer and NMR techniques. In the direct synthesis, the amount of iron incorporated in the zeolite framework depends on both its amount in the initial hydrogel and the TPABr template concentration. Part of these framework iron ions migrate to extra-framework position during catalytic tests and/or catalyst hydrothermal pretreatment forming active ( F e , A l ) f r a. . . . . k-O-(Fe,A1)extra-fr . . . . . . k pair sites in which iron is in a distorted octahedral coordination. D u r i n g long-term catalytic tests in benzene hydroxylation the iron ions migrate to more stable positions in the zeolite which is one of the cause of catalyst deactivation together with the formation of carbonaceous species and strongly chemisorbed phenol as detected by 13C-NMR. Introduction of iron by post-synthesis methods leads to a lower dispersion of iron and less stable species during the catalytic reaction. 1. I N T R O D U C T I O N Metal-substituted or metal-containing zeolites are used in many industrial processes, such as the production of caprolactam and direct oxidation of cyclohexanone, the isomerization of linear alkanes, the isomerization of linear C4 and C5 olefins into iso-olefines (Shell processes) and the liquid phase alkylation of benzene (Eni and Exxon-Mobil processes) [1-4]. Recently, the possibility of their

478 use as s u l p h u r r e s i s t a n t catalysts for the conversion of polycyclic aromatic hydrocarbons have been also shown [5-8]. A new field of interesting application is also related to the possibility of introducing well (atomically) dispersed transition metals having redox properties. The unique coordination given to the metal ions by coordination to the zeolite framework acting as coordinating ligand allows to obtain a peculiar reactivity [9]. Most of the studies have been focused on the use of these materials in liquid phase oxidation or NOx reduction in the gas phase, but recently the interest also focused on the selective oxidation of benzene to phenol in gas phase using N20 as the oxidant [9-12] and (Fe,A1)-containing MFI zeolite. In fact, N20 decomposed on the iron sites forming specific oxygen species (called a-oxygen) which can directly hydroxylate benzene to phenol. No other type of iron-containing catalyst has been found to m a t c h such a peculiar behaviour and n o t w i t h s t a n d i n g the intense research activity on these catalysts, the exact nature of the sites responsible for the activity is a matter of question. Different procedures are reported for the partial or total substitution of A1 in the zeolite or for the introduction of other metals. Ionic exchange is one of the most used methods [13], but also solid state reaction and t h e r m a l vapor deposition are becoming widely used preparation methods [14-17]. In alternative to these post synthesis methods leading essentially to extra-framework species (if part of the framework A1 ions are not removed before the addition of the metal), the introduction by direct synthesis [18-21] leads to framework substitution. However, by careful thermal or chemical p r e t r e a t m e n t it is possible to have a controlled partial migration of the T-metal such as Fe from framework to extraframework positions. Limited attention has been generally given in literature to analyze how the m e t h o d of introducing the metal affects the n a t u r e and distribution of the transition metal ion species. In this contribution the characteristics of Fe-(A1)-MFI type zeolites prepared by direct synthesis starting from a hydrogel containing iron complexes (oxalate or phosphate) and tetrapropylammonium bromide (TPABr) are compared with those of samples in which iron has been introduced by post synthesis methods: ion exchange, chemical vapor deposition and solid state reaction. The characteristics of the samples and their change during the catalytic reaction are investigated in relation to the use of these catalysts in the direct oxidation of benzene to phenol (see part II in this Volume) [22]. The location of iron in the zeolite have been studied by NMR and MSssbauer techniques. MSssbauer spectroscopy is an appropriate tool for identifying the oxidation and coordination states of iron. For instance, presence and inter-conversion of various ferrous and ferric states were demonstrated in Fe-MFI during reduction and oxidation treatments [23]. 2. E X P E R I M E N T A L

The zeolite samples of Fe-MFI and Fe,A1-MFI were p r e p a r e d in static conditions and under autogeneous pressure at 170 ~ The molar composition of the starting hydrogel was: x Na20 - y T P A B r - z A1203 - SiO2 - q Fe2OJp H A - 20 H20 where x = 0.1-0.32; y = 0.02 - 0.08; z = 0.0 - 0.05; q = 0.0-0.025; the ratio p/q = 3 and HA stays for HzC204 or H3PO4.

479 The synthesis procedure was the following: sodium aluminate (Carlo Erba) is added to a sodium-hydroxide (Carlo Erba) solution and after the homogenization the organic compound (TPABr from Fluka) and the silica source (silica-gel BDH) are added. In a n o t h e r b a k e r a solution of iron complex w i t h oxalic or ortophosphoric acid, starting from iron nitrate (Carlo Erba) and the acid (Carlo Erba) is prepared. This solution was slowly added to the hydrogel and after 1 hour of homogenization was transferred into the autoclaves. For the synthesis of the silica form, the procedure is identical but without the introduction of the aluminum source. Post-synthesis introduction of iron was made with the following methods: i) a direct ionic exchange of NH4+-A1-MFI form with a 0.1 M iron ammonium sulphate solution (stirred for 2 hrs at 60~ and then calcination at 550~ ii) chemical vapour deposition was carried out at 300 ~ by mixing the A1MFI samples with an anhydrous FeC13 in controlled atmosphere (twice for 2 hours). iii) a solid state exchange reaction was performed by mixing FeC13 (5% wt.) with H-MFI and treating the mechanical mixture for 8 hr at 600 ~ The solid products were recovered in a usual m a n n e r and checked by powder X-ray diffraction (XRD). The zeolite samples were f u r t h e r characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), thermal analysis, chemical analysis by atomic absorption, NMR and MSssbauer spectroscopies. MSssbauer spectra were obtained on samples in as received state at 77 K. Spectra are decomposed to Lorentzian lines. Isomer shift data are related to the center of metallic a-iron. The accuracy of positional parameters is ca.+_ 0.03 mm/s. Other experimental details are reported in [23]. 3. R E S U L T S AND D I S C U S S I O N 3.1 T h e r o l e o f t h e p r e p a r a t i o n

The main p a r a m e t e r s t h a t affect the preparation of Fe-MFI type zeolite are summarized in Table 1. Pure MFI type zeolite may be obtained even for a pH close to 11 and large ranges of Si/Fe and Si/A1 ratios. The crystallization time increases (from 1 day to 12 days) with the a m o u n t of iron and a l u m i n i u m present in the initial reaction mixture. The presence of A1 in the s t a r t i n g hydrogel containing Fe leads to a shorter crystallization time (see sample 2 and 10, Table1). Apparently the amount of TPABr in the starting hydrogel does not affect the reaction products. In fact, pure MFI type zeolite was obtained also in absence of organic molecules (see Table 1 samples 1-8, 9 and 10). The samples obtained in the presence of 0.08 moles of TPABr show a very good t h e r m a l stability. As a m a t t e r of fact, no transformation of MFI phase is observed after longer reaction times and even after a thermal t r e a t m e n t at 850 ~ The chemical analyses of as-made MFI samples and those after t r e a t m e n t are reported in Table 2. It may be noted that (i) in TPABr-rich hydrogel the amount of iron incorporated in the zeolitic framework is related to its content in the initial reaction mixture, and (ii) the iron is incorporated preferentially with respect to the A1 (see samples 3, 7 and 8, Table 2).

480 Table 1. Influence of the m a i n p a r a m e t e r s on the products of the system: x N a 2 0 - y T P A B r - z A1203 - SiO2 - q Fe2Ogp H A - 20 H20 Sample Si/A1 gel Si/Fe gel TPABr pH Time (d) Product 1 oo 100 0.08 11.5 1 MFI 2 ~ 10 0.08 11.2 12 MFI 3 100 100 0.08 11.5 1 MFI 4 100 50 0.08 11.5 2 MFI 5 50 oo 0.08 11.6 1 MFI 6 50 100 0.08 11.4 2 MFI 7 50 50 0.08 11.5 2 MFI 8 25 25 0.08 11.3 4 MFI 9 10 20 0.02 11.2 7 MFI 10 10 10 0.0 10.8 9 MFI Samples 4, 5, 6 and 7 corresponding, respectively to the samples Fe2.3MFIht-A90, Feo.sMFIssr-A62, Fel.lMFIht-A55, Fe2.2MFIht-A54 of Table 1 p a r t II of this work [22]. Instead, the a l u m i n u m a t o m s are p r e f e r e n t i a l l y i n c o r p o r a t e d into the MFI s t r u c t u r e w h e n the TPABr content decreases or in its absence (sample 9 and 10, Table 2). Consequently, a small a m o u n t of iron is detected in the final products. Table 2. Chemical analyses of MFI samples as-made and after t r e a t m e n t . Sample Si/A1 gel Si/Fe gel TPABr Si/A1 zeol

Si/Fe zeol

1 oo 100 0.08 106.5 2 oo 10 0.08 13.8 3 100 100 0.08 93.0 73.8 4 100 50 0.08 91.5 35.9 4 calc. 100 50 0.08 90.1 39.1 5 50 oo 0.08 57.2 5 ssr 50 oo 0.08 62.1 121.7 6 50 100 0.08 56.8 80.3 6 calc. 50 100 0.08 55.1 81.7 7 50 50 0.08 51.6 37.3 7 calc. 50 50 0.08 54.1 40.4 8 25 25 0.08 42.3 29.6 9 10 20 0.02 25.0 307 10 10 10 0.0 18.1 269 ll*ie 13 23.8 12*cvd 13 222 calc.= calcination at 600 ~ ssr= solid state reaction; ie= ion exchange; cvd= chemical v a p o u r deposition. * commercial s a m p l e s from Alsi P e n t a (SN27). Sample 11 e 12 corresponding respectively to Fe3.6MFIet-A13 and Feo.4MFIcvd-A13 of Table 1 p a r t II of this work [22].

481 An explanation for this behaviour can be attributed to the different preference showed by the solution cations for the iron- and aluminium- containing anions. Different authors have demonstrated [24] that Na § interacts better with [Si-OA1], whereas TPA § prefers [Si-O-Fe] groups. The calcination process does not affect the amount of iron incorporated in the zeolite structure. In fact, a small amount of iron is lost in the samples after the calcination (see samples 4, 4 calc., 6, 6 calc., 7 and 7 calc., Table 2). The incorporation of iron by post-synthesis methods results in large introductions of iron only with the ion exchange method, while solid state reaction and chemical vapor deposition exhibit a small iron incorporation (see sample 5ssr, l l i e and 12cvd, Table2). As reported in the part II of this work [22] the method of iron incorporation strongly affects the catalytic behaviour of the samples, especially their stability. 3.2 T h e n a t u r e of t h e i r o n s p e c i e s

The MSssbauer spectra at 77 K of selected samples and related parameters after deconvolution are reported in Figure 1 and Table 3, respectively. The spectrum of the as-synthesized sample exhibits a broad Fe 3§ singlet, with a comparatively low isomer shift (IS) value. The singlet (lack of quadrupole splitting, QS) indicates a symmetric environment, whereas the low isomer shift (IS77 K < 0.4 ram/s) is indicative of tetrahedral coordination. Thus, this spectrum is a typical one characteristic for the isomorphously substituted ferric ions in the as synthesized sample, prior to the removal of template molecules used in the synthesis (see e.g. in [25]). The activation of the sample results in a partial removal of iron from the framework. Combined (Fe, A])framework-O-(Fe, A1)extra_framework pairs may be formed as reflected in the appearance of quadrupole splitting [23]. The symmetry is extended to a distorted octahedral one, as shown by the increase of the IS and (QS values as well).

Table 3. MSssbauer parameters extracted from 77 K spectra QS Sample Component IS 7 as-made 7 after 3 h of reaction 7 after 3 cycles of reaction/regeneration IS= isomer shift, related FWHM= full line width contribution, %

Fe3+~tr Fea§ Fe 2+ Fe3+oct

0.33 0.45 1.10 0.42

1.08 3.04 0.92

FWHM

RI

2.10 0.68 0.34 0.87

100 96 4 100

to (z-iron, mnds; QS= quadrupole splitting, mm/s; at half maximum, mm/s; RI= relative spectral

482 During long term catalytic tests in benzene hydroxylation and r e p e a t e d activation-reaction cycles, a change in the iron species is noted which can be i n t e r p r e t e d as a migration of iron from the initial positions in the zeolite framework deriving from framework to extra-framework migration to more stable locations. This change is indicated by a decrease of line width (FWHM). Presence of Fe 2§ component detected in a minor amount (RI - 4 %) in the sample after exposure to the reaction mixture attests that a reversible Fe2*+~Fe 3+ cycle is involved in the reaction. It may be noted, that the samples were exposed to air after being removed from the catalytic reactor, and therefore a p a r t i a l reoxidation of Fe 2+ probably occurred. More complete information is expected from in situ MSssbauer measurements, but the detection of Fe 2* suggests t h a t iron ions are reduced during the catalytic reaction.

9

9

--

n

I

-6

'

I

-4

'

,,ipl , u m

[]

9

I

-2

I

'

I

0

'

I

2

'

I

4

'

I

6

Figure 1. 77 K MSssbauer spectra of as-synthesized sample (bottom); after 3 cycles of activation/reaction (middle); and exposed to 3 h of reaction (top).

483 A further aspect worth to be noted is the absence of magnetically split components in the spectra. This indicates that there is no superparamagnetic relaxation at 77 K, i.e. neither presence of extended oxidic (antiferromagnetic) Fe-O-Fe clusters, nor carbidic (ferromagnetic) Fe-C-Fe chains are detected. It may be noted that the threshold size of particles sufficient to exhibit magnetic component is a few nanometers. Thus, high dispersion of iron ions, most probably close to ionic one, is additionally proven. The 29Si-NMR spectra of the as made MFI samples and of the spent Fe-MFI catalysts show interesting behaviours. The as-synthesized MFI samples are white showing clearly that there are no extra-framework ions species in the samples. In these cases, 1H-29Si cross polarized spectra could be taken, where the intensities of the -103 ppm and-106 ppm lines are smaller in the cross polarized spectra. Oppositely, in the colored samples, where extra-framework ion species are present no cross polarized spectra could be measured. It was also the case of the spent samples. In the spent catalysts, the intensity of the -100 ppm line is quite small showing that the defect groups due to the silanol groups have been eliminated during the reaction. The spent catalysts certainly contain extra-framework ion species. After three hours of reaction, the catalyst becomes black and shows a 13CNMR spectrum centered at ca 140 ppm which is quite broad (A -- 4000 Hz) and could be due to carbonaceous species or coke. The 13C-NMR spectrum of the catalyst after three cycles of reaction-regeneration shows a broad line (A = 1000 Hz) at ca 126 ppm which can be attributed to strongly adsorbed phenol. 4. CONCLUSIONS The preparation procedure utilizing iron complexes is a good method for introducing iron into MFI type zeolite in a large crystallization field. The amount of the iron incorporated into the zeolitic framework depends on its content in the initial hydrogel for the TPABr-rich systems. In absence or in poor TPABr systems A1 is preferentially incorporated and only a small amount of Fe is detected in the crystals. This suggests that with a control of the amount of TPABr in the hydrogel, or with the addition of other organic molecules [24], it is possible to modulate the incorporation of iron into the MFI framework. The NMR and MSssbauer data indicate that from direct synthesis the iron is incorporated in tetrahedral position into the zeolitic framework. After the catalytic reaction extra-framework iron is detected. The deactivation of catalysts is due to the formation of carbonaceous species or coke. The addition of iron by post-synthesis methods is also possible, but the catalytic behaviour is strongly affected by the procedure of iron incorporation. REFERENCES

1 2 3 4

H. Ichihashi, M. Kitamura, Catalysis Today 1-6 (2002) 2617. H. Sato, K. Irose, N. Ishii, Y. Umada, USP 4,709,024 (1986). H. Sato, N. Ishii, K. Irose, S. Nakamura, Stud. Surf. Sci. Catal. 28 (1986) 775. P. Roffia, M. Padovan, E. Moretti, G. De Alberti, EP 208,311 (1986).

484

5

T. Fujikawa, K. Idei, T. Ebihara, H. Mizuguchi, K. Isui, Appl. Catal. A 192 (2000) 253. 6 R.M. Navarro, B. Pawelec, J.M. Trejo, R. Mariscal, J.L.G. Fierro, J. Catal. 189 (2000) 184. 7 M.A. Arribas, A. Martinez, Stud. Surf. Sci. Catal. 130 (2000) 2585. 8 C. Petitto, G. Giordano, F. Fajula, C. Moreau, Catal. Comm. 3 (2002)15. 9 A. Bell, G. Centi, B. Wichterlowa (Eds.), Catalysis by Unique Ion Structures in Solid Matrices. NATO Science Series II: Mathematics, Physics and Chemistry, Vol. 13, Kluwer/Academic Press Pub.: New York (2001). 10 G. Centi, G. Grasso, F. Vazzana, F. Arena, Stud. Surf. Sci. Catal. 130 (2000) 635. 11 L.V. Pirutko, A.K. Uriate, V.S. Chernyavky, A.S. Kharitonov, G.I. Panov, Micr. Mesop. Materials 48 (2001) 345. 12 L.V. Pirutko, V.S. Chernyavky, A.K. Uriate, G.I. Panov, Appl. Catal. A, 227 (2002) 143. 13 R.P. Townsend, in Introduction to zeolite science and practice, H. van Bekkum, E.M. Flanigen, J.C. Jansen (eds), Elsevier, Amsterdam, 1991, p 359~ 14 H.G. Karge and H.K. Beyer, Stud. Surf. Sci. Catal. 69 (1991) 43. 15 J. Varga, A. Fudala, J. Halasz, Gy. Schobel and I. Kiricsi, Stud. Surf. Sci. Catal. 94 (1995) 665. 16 F. Vazzana and G. Centi, Catal. Today 53 (1999) 683. 17 F. Cosentino, A. Katovic, G. Giordano, P. Lentz, J. B.Nagy, Stud. Surf. Sci. Catal. 125 (1999) 109. 18 P. Ratnasamy and R. Kumar, Catal. Today, 9 (1991), 329. 19 R. Kumar, A. Raj, S. Baran Kumar and P. Ratnasamy, Stud. Surf. Sci. Catal. 84 (1994) 109. 20 G. Giordano, A. Katovic, A. Fonseca, J. B.Nagy, Stud. Surf. Sci. Catal. 135 (2001) 175. 21 G. Giordano, A. Katovic, D. Caputo, Stud. Surf. Sci. Catal. 140 (2001) 307. 22 S. Perathoner, F. Pino, G. Centi, G. Giordano, A. Katovic, J. B.Nagy, K. L~z~r, P. Fejes, Stud. Surf. Sci. Catal. part II, this issue. 23 P. Fejes, J. B. Nagy, K. L~z~r, J. Hal~sz, Appl. Catal., A:General 190 (2000) 117. 24 G. Giordano, A. Katovic, Stud. Surf. Sci. Catal. 140 (2001) 297 and references therin. 25 K. L~z~r, G. Borb~ly, H. Beyer, Zeolites 11 (1991) 214.

CATALYSIS

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

487

From Micro to Mesoporous Molecular Sieves: Adapting Composition and Structure for Catalysis A. Corma and M.T. Navarro Instituto de Tecnologia Quimica, UPV-CSIC, Universidad Polit6cnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain. Different possibilities for application of molecular sieves in catalytic processes involving large size molecules are described. Preparation and reactivity of zeolites with mesopores, nanocrystalline zeolites, delaminated zeolites and amorphous and partially ordered mesoporous molecular sieves are presented.

1. M E S O P O R O S I T Y AND Z E O L I T E NANOCRYSTALS Microporous molecular sieves have shown widespread use in industry as heterogeneous catalysts, in oil refining and petrochemistry as solid acid catalysts, as base and redox catalysts for carrying out organic reactions. However, their applications are limited because of the sizes of their channels and cavities. Attempts to overcome this difficulty have been the topic of many studies. One possibility consists in increasing the pore size of the zeolite using new synthesis routes. In a general way, the different strategies employed towards the synthesis of ultralarge pore zeolites have been: The use of specific spacing units to build the inorganic framework (ALPO4-5) (1), the use of different oxide systems (VPI-5) (2), and specially designed templates (Cloverite) (3). Unfortunately, up to now, most of the extralarge pore molecular sieves cannot be used in catalytic process because they are thermally unstable. There is an indirect way to increase the accessibility of larger molecules to the zeolitic active sites that involve post-synthesis treatments in order to generate mesopores in the crystallites of the microporous zeolites. It is known that during the dealumination of the zeolite Y by steam, mesopores of sizes between 10-20 nm are formed (4,5). If a large number of defects are produced in a small area it could lead to coalescence of mesopores, with the formation of channels and cracks in the crystallites of the zeolite (Figure 1), that will increase the accessibility of large molecules to the external opening of the micropores. It was observed that when the zeolite Y was dealuminated by SiCI4 it generated little mesoporosity and preserved most of the microporosity of the zeolite. However, zeolite Y dealuminated by steam treatment presented more mesoporous and increased the catalytic activity towards large reactant molecules (6).

488

~ii

9

~

% <3

-

20

40

60

Pore B , ~

Figure 1. Schematic representation of mesopores formed in steamed zeolites Mesopores have also been created in MFI zeolite by alkali treatment. With this method Si atoms were removed from the framework, and mesopores with uniform size were created (7). In a simplified way, one may consider that the objective for the generation of mesopores is to increase the ratio between the external and internal surface of the zeolite. If this is so the same effect can be obtained by decreasing the crystal size of the zeolite. Thus, there is also a considerable interest in the field of synthetic zeolites, for decreasing the particle size from the micrometer to the nanometer scale and, following this, several zeolites have been synthesized with crystallites in the range of 10-200 nm (8-20). The synthesis of nanometer-sized zeolites has received much attention recently owing to their utility in fundamental studies of zeolite crystal growth, in the preparation of ultra-thin zeolite film and nano-composites and for catalytic and photochemical uses (21-28). Moreover, a lot of reports have been published about catalytic activity using zeolite catalysts with a small crystal size for different reactions (29). In the last years, the preparation of very small zeolite and zeotypes crystals with a controlled size distribution (called Confined Space Synthesis) has been described (26,30,31). This method involves the crystallization of the zeolite inside the pore system of an inert carbon matrix, and the removal of the carbon matrix by controlled combustion led to the isolation of very small crystals (20 nm) with high crystallinity. In Figure 2 the synthesis scheme is shown. Carbon particles

ca. 12.nrn 9,

ca. "lprn

. ,,, ,

crystal grown fn pore system of carbon

Pores created by

combustion of __ f_5___._v_ /P.~rhnn ~.articles n

~,.Mesoporous zeolite single crystal

Figure 2. Growth of zeolite crystals around carbon particles. The zeolite is nucleated between the carbon particles; the pores are sufficiently large, and the gel is sufficiently concentrated to allow growth to continue within the pore system.

489 2. M E S O P O R O U S M O L E C U L A R SIEVES

In 1992 researchers from Mobil (today, ExxonMobil) carried out the synthesis of materials named generically M41S, and show that they were mesoporous molecular sieves, with pore diameters in the 1.5 to 10 nm range (32-34). These materials can be synthesized by using surfactants as structure directing agents (35), and since then more than 22 new structures of mesoporous molecular sieve materials have been synthesized and among them we can highlight : MCM-50 (36), HMS (37,38), SBA-3 (39,40), SBA-2 (40), MSU-1 (41), MSU-2 (41), MSU-3 (41), SBA-12 (42), SBA-15 (42) and SBA-16 (42). Independently of the synthesis procedure and the reactants used, the surfactant remains occluded within the pores in the synthesized materials. In order to leave the pores free of organic, the materials can be submitted to calcination or extraction procedures (43-46). It was clear since the first moment that even though these materials were amorphous at a short range distance, i.e., the walls were amorphous, it should be possible to introduce catalytic active sites within the walls, and in this way to prepare molecular sieve catalysts in where the active sites were accessible to large reactant molecules. For instance, when the walls were formed by aluminosilicate, Br6nsted and Lewis acid sites were generated in MCM-41, in where the Br6nsted acid sites associated to tetrahedrally coordinated aluminium presented lower stability and milder acidity than those present in zeolites (34,36,44-54). Later, A1TM has also been incorporated in other structures such as MCM-48, SBA-15, A1-MSU-1 and A1-SBA-1 (55-61). These materials have shown interesting results as FCC catalysts, but their hydrothermal stability was too low (62-69). Much effort has been carried out to improve the hydrothermal stability by means of improved synthesis procedures (67-69). Recently, the synthesis of A1-MCM-41 stable under treatments at 800~ in the presence of steam has been described (70), but the resultant samples present Si/A1 ratios too high for many catalytic purposes. Better results have been achieved by Zhao et al. (71), who have prepared a mesoporous aluminosilicate similar to MCM41, using a polimer of aluminum and silicon as precursor and a cationic surfactant. For other reactions such as hydrocracking, HDS and HDN that require less hydrothermally stable catalysts than FCC, Mo-Ni/A1-MCM-41 has given good results owing to the high surface area and regular pore dimensions (72-77). Other processes in the refining industry in where mesoporous materials can be of interest are: n-paraffin isomerization, olefin oligomerization, olefin disproportionation, cracking of polyethylene and polypropylene (78-83). In the field of fine chemicals production, Pt supported on MCM-41 has been used for enantioselective hydrogenations (84). The acid forms of these materials can allow the diffusion of many of the large reactants and products in fine chemicals. In this sense, Al-mesoporous materials have shown good catalytic activities for the Friedel-Crafts alkylation of 2,4ditertbutylfenol with cinnamic alcohol (85-86), acylation of 2-methoxynaphthalene (87), alkylation of naphthalene (88), acylation of benzene (89), alkylation of sugars with alcohols for the production of alkylglucosides (90-93), and condensation of aldehydes .(94-95). When Lewis acids such as Ti, Sn, Cr, Mn, Fe and others have been introduced instead of A1, the mesoporous molecular sieves can be used as oxidation catalysts. In this sense, Ti was the first metal incorporated in the structure (96). This catalyst was active and selective for the

490 epoxidation of olefins with H202 (96), but the intrinsic activity is lower than for TS-1 and TiBeta when reacting small molecules (96,97). However, when larger molecules such as norbornene (97), limonene (98), or cholesterol (99) are oxidized with tertbutyl hydroperoxide, TS-1 is not active, and the activity of Ti-Beta is lower than that of Ti-MCM-41. The incorporation of Ti can be achieved by direct synthesis (96) or by a post-synthesis procedure (100), and the isomorphic substitution of Ti by Si has been proved by UV-Visible, EXAFS and photoluminescence techniques (101-103). Other reactions for which Ti-MCM-41 materials have shown activity are oxidation of arylamines (104), mercaptanes to sulfoxides and sulfones (105), hydroxylation of aromatics and other organic reaction of interest (106-112). Besides MCM-41, Ti has also been incorporated in other mesoporous structures such as for instance, HMS (113-116), SBA- 15 (117-119), MCM-48 (118-122) and MSU-X (123,124). Other metals such as V, Zr, Cr, Mn, Fe, Sn have also shown activity as oxidation catalysts when incorporated in the walls of mesoporous materials (125-149). However, most of those metals may leach when the catalysts are used in liquid phase reactions (111), and only Zr and Sn show a better stability (150,151). We want to point out here a recent application of Sn-MCM-41 for the Baeyer-Villiger oxidation of cyclic ketones to lactones using H202 as oxidant (137). Post-synthesis treatments can be complementary with direct synthesis methods to obtain strong acid, base, and redox catalysts. In the case of acid catalysts the synthesis of mesoporous materials with inorganic groups can generate strong sulfonic acid centers upon oxidation of the thiols (152-155). This type of material as direct or even better with a more hydrophobic surface are active catalysts of esterification of fatty acids with glycerin, showing good selectivity to the formation of monoesters. Heteropolyacids can be supported on mesoporous materials with the result of strong acid catalysts (156-159). However, leaching of the heteropolyacid can occur, specially if reactions are performed in liquid phase. Basic sites can be generated on the surface of mesoporous materials either by exchange with alkali cations, and formation of alkaline oxide clusters (160-162), or by anchoring amines (163-168), alkylammonium hydroxides (169, 170), and proton sponges (171). Schiff' s bases with Cr or Mn (172), flalocianines of Zn, Cu or Co (173, 174) or Fe porfirines (175) anchored on MCM-41 are active in redox process. From the point of view of redox catalysts, there is a post-synthesis treatment of Ti-MCM41 that has an important impact on the final activity. This has been the modification of the surface by silyl-organic reactants that have rendered the surface extremely hydrophobic properties, having this a big impact on the epoxidation activity of Ti-MCM-41 (176-180). Even if Ti-MCM-41 samples containing surface methyl groups have been produced by direct synthesis with the co-polymerization of methyltriethoxysylane and silicon tetramethoxyde (181), better results are obtained with the post-treatment procedure. The sylilated materials present also a better mechanical and hydrothermal stability (182-184). The incorporation of functional groups is an active field of research that may have an important impact in fields as significant as enantioselective catalysis, chromatography and separation (185-188), metal trapping (189-193), and for the preparation of nanofilament,; of graphite, polianiline and nanocables of Pt (194-198).

491 3. INCREASING ACIDITY AND STABILITY OF MESOPOROUS STRUCTURES We have said before that acidity and stability, specially hydrothermal stability, can be important issues with mesoporous materials, at least for some applications (199). One way to improve both the stability and acidity of these materials could be if zeolitelike order was introduced into the pore walls. This idea could be possible through the assembly of nanoclustered precursors that normally nucleate the crystallization of microporous zeolites. These precursors also known as zeolite seeds are presumed to promote zeolite nucleation by adopting A104 and SiO4 tetrahedral connectivities that resemble the secondary structural elements in a crystalline zeolite (200). Kloetstra et al. (201) transformed the preassembled walls of A1-MCM-41 and A1-HMS into zeolitic structures by post-assembly treatment with a microporous zeolite structure director, such as tetrapropylammonium cations. The composites formed, denoted as porous nanocrystalline aluminosilicate (PNAs) were more active for the cracking of cumene than the parent materials. The improved activity was assigned to enhanced Br6nsted acidity. Also, the PNA samples showed no significant deactivation during 3 h on stream in the cumene reaction and presented similar activity after regeneration at high temperature indicating that the framework does not collapse and the acid sites are still accessible. Moreover, it was observed that the PNA samples retained a well-ordered hexagonal structure with high BET area (796 m2/g) and pore size (23 A), measured by N2 adsorption. 27A1 MAS NMR measurements of PNA materials showed that the framework A1 environments are more symmetric. Recently, it was described the synthesis and properties of a hexagonal aluminosilicate mesostrucmre (called 10% A1-MSU-S) (202); Si/AI: 9:1) obtained from seeds that nucleate the crystallization of faujasitic zeolite type (200,203) with cetyltrimethylammonium bromide as surfactant. It was observed that the 10%A1-MSU-S sample retained a well-ordered hexagonal structured upon steaming at 800~ A1-MSU-S(A) sample is more stable than the mesostructures prepared from conventional silicate and aluminate precursors (B) or by ultrastable grafting reaction (C) upon steaming at 800~ The authors claim that the unique hydrothermal stability of A1-MSU-S is owing in part to the retention of a zeolite-like connectivity of A104 and SiO4 tetrahedra upon assembling the zeolite seeds into a mesostructure. However, they suggest that occluded carbon, formed through the cracking of surfactant during calcination, contributes to the structural stability. It is possible to prepare A1-MSU-S materials containing 0.01-38% A1 by using zeolite Y seeds as precursors. Other zeolitic units have been incorporated in the framework walls, and MSU-S mesostructures from zeolite MFI and Beta seeds (called MSU-S(MFI) and MSU-S(BEA) (204) with Si/A1 ratios in the range ~ 20:1 -300:1 have been prepared. These materials are intrinsically stable and do not require the presence of occluded carbon for steam stability. Also, these materials are more active acid catalysts than the equivalent A1-MCM-41 prepared from conventional aluminosilicate precursors (Table 1) (204).

492 Table 1 Textural properties and cumene conversions for mesoporous aluminosilicate sieves. Sample Unit cell Surface area Pore volume Pore Cumene Dimension (ma.g1) (cm3.g1) diameter conversion (a~

ao (A.)

(A0

(%)

1.06

36.8

32.3

1192 849

0.93 0.44

34.7 24.3

47.3

1124

1.06

39.1

Steamed 6000C, 5h Steamed 8000C, 5h

46.7 37.0

1065 885

0.94 0.46

38.0 26.4

1.5%A1-MCM-41 (b~ calcined

46.4

1013

1.08

38.7

1.5% A1-MSUS~F0 calcined

45.3

1231

Steamed 6000C, 5h Steamed 800~ 5h

44.5 36.6

1.5%A1-MSUS(BEA)calcined

31.5

11.7

Steamed 600~ 5h 35.2 639 0.39 20.1 Steamed 800~ 5h 55 (a) Reaction conditions: 6 mm i.d. fixed bed quartz reactor: 200 mg catalyst; cumene flow rate, 4.1 ~tmol minl: N2 carrier gas, 20 cm3min-1; conversions reported after 60 min on stream at 300~ (b) 1.5%A1-MCM-41 was prepared by the direct assembly of conventional alminosilicate anions formed from sodium aluminate, fumed SiO2, and TMAOH.

4. D E L A M I N A T E D ZEOLITES AS M I C R O - M E S O P O R O U S M A T E R I A L S

Another approximation to achieve accessibility of large size reactant molecules to active sites of zeolitic nature has been achieved by delaminating the laminar precursors of some zeolites. By doing this, one should be able to control the number of layers of the final structure and in this way to achieve crystals formed by only 4, 3, 2 or even single layers. This leads to materials formed by a well structural layer or layers of zeolitic nature that are arranged in a house of cart type structure, leaving surface areas in the order of 700 m2.g-1. These are stable materials that can have acidity close to that of the zeolites. They can contain framework Lewis acid sites such as Ti, Sn, Fe, etc. in a similar way as zeolites do, and can also act as supports for metals, oxides, enzymes or sensors (205-218). These materials can be prepared in the following way: the zeolite precursor is firstly expanded by refluxing in an aqueous solution of hexadecyltrimethylammonium or tetrabutylammonium at a pH = 9.0. The alkyl ammonium cations are introduced into the interlaminar space and the expansion of the layers can be followed by X-ray diffraction. After this, the layers are forced apart by subjecting

493 the slurry to an ultrasound bath or to a fast and energetic stirring. The resultant material can be formed mainly by single layers or mainly by two or three layers, depending on the delamination conditions. In Figure 3, we present the structure of the delaminated materials (ITQ-2 and ITQ-6) formed by starting from laminar precursors of MWW and a Ferrierite, respectively.

(')11 ()It

O11 ()11

OI1 ()H

{)11 ()11

()II {)11

()11 O1-t

()tt

(')11 OH

OH O I l

OH t ) l t

Hexaeonal Pr[.&m

()H

l I} M R

SwolleI~ER n

~ iTQ-6~

Figure 3. Structure of the delaminated materials (ITQ-2 and ITQ-6) formed by starting from laminar precursors of MWW and a Ferrierite. The resultant samples are active and selective catalytic materials for oil refining, petrochemistry and fine chemicals production. For instance, they show excellent activities for cracking and hydrocracking large molecules, aromatic alkylations with olefins, condensation and acetalization reactions, and selective epoxidation of olefins and oxidation of alcohols (205-219). These materials are stable under calcinations at 7000C and hydrothermally stable at 750~ in presence of steam when formed with 2 or more layers. Even if the number of layered zeolites is still small today, we have seen that new layered zeolite precursors can be synthesized if one looks specifically for this type of material (220). This certainly will enlarge the number and possibility of these materials.

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503

One step benzene oxidation to phenol. Part II: Catalytic behavior of Fe(AI)MFI zeolites S. Perathoner 1, F. Pin01, G. Centi l, G. Giordano 2, A. Katovic 2, J. B.Nagy3, K. Lazar 4, P. Fejes5 1 Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Salita Sperone 31, 98166 Messina, Italy 2 Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria, 87030 Rende (CS), Italy 3 Laboratoire de RMN, FUNDP, 61 rue de Bruxelles, 5000 Namur, Belgium 4 Institute of Isotope and Surface Chemistry, 1525 Budapest, P.O. Box 77, Hungary 5 Applied Chemistry Department, University of Szeged, Rerrich B61a t6r 1, 6720 Szeged, Hungary

The catalytic behavior in gas phase hydroxylation of benzene to phenol with nitrous oxide on Fe-(A1)MFI zeolites prepared by different methods is reported. The amount of iron, thermal treatment, presence of aluminium ions and method of iron addition to the zeolite are all important parameters which determine the catalyst life time as well as the selectivity and productivity to phenol. Control of these parameters make it possible to improve the catalyst life-time and productivity to phenol. 1. INTRODUCTION Phenol is a large volume chemical (current worldwide production is about 7 million tons) with an expected market growth of about 6% per year, presently being used for various applications such as production of (i) phenolic resins (used in various areas expected to grow in the future such as the plywood, construction, automotive, and appliance industries) and (ii) of caprolactam and bisphenol A (intermediates in the manufacture of nylon and epoxy resins, respectively). The projected expansion of the market, however, is limited by the coproduction of acetone in the current commercial process via cumene as the intermediate, because the projected market for acetone is stationary or even decreasing. Therefore there is industrial interest in developing new processes of direct synthesis of phenol from benzene. There are two possible benzene hydroxylation processes, one in the liquid phase using H202 and the other one in the gas phase using N20 (the productivity to phenol using 02 as the oxidant is very low). A new recent suggestion is the one-step catalytic oxidation of benzene to phenol in the presence of a H2-O2 feed over palladium membrane catalysts [1 ]. H202 is generated over the Pd catalyst in the presence of a the H2-O2 mixture. The gas phase process using N20 has the advantage over the liquid phase process of using a reactant (N20) which can be recovered from chemical processes (adipic acid

504 production, in particular) or produced from waste by-products (NH4NO3). N20 is a powerful greenhouse gas [2] and its emission will soon be restricted. Therefore, its use as a reactant in phenol synthesis provides the multiple environmental benefit of (i) improved ecocompatibility of phenol production (better atom economy, and reduction of process complexity, waste and risks), (ii) reduction of greenhouse gas emissions and (iii) the reuse of waste. The AlphOx Process from Solutia [3,4] uses N20 produced from adipic acid to hydroxylate benzene to phenol which is then hydrogenated to cyclohexanone and reintroduced into the adipic acid synthesis cycle. The process thus offers the double benefit of avoiding N20 emissions and increasing the productivity to adipic acid. Fe-MFI catalysts are used in the reaction of benzene hydroxylation with N20, but a main limitation is the relatively fast deactivation of the catalysts. The development of the process is still in the pilot plant phase. No other classes of catalysts have been reported in literature having comparable performances to Fe-MFI catalysts. The main problem in developing Fe-MFI type catalysts for benzene hydroxylation with N20 is understanding the factors controlling the catalyst deactivation in order to improve catalyst lifetime. Even so, most part of studies reported in the literature on this reaction and class of catalysts, have been focused on the mechanistic investigation of the nature of the active sites responsible for the reaction of benzene hydroxylation with N20. Since deactivation is quite fast (of the order of hours) and reasonably different in terms of rate from catalyst to catalyst, comparison of the catalytic behavior without considering this factor is not reasonable. In addition, the fact that iron can be present as an impurity in the MFI catalysts has not been always taken into consideration. As a consequence, different and contrasting hypotheses have been reported on the nature of the sites responsible for the reaction of benzene hydroxylation with N20. Suzuki et al. [5], Burch et al., [6] and Gubelmann et al. [7] indicated that Bronsted acid sites are necessary for this reaction to proceed. Sobolev et al. [8] concluded,on the contrary, that Bronsted acidity is not required for this reaction, and attributed the catalyst activity only to iron sites in the extra-framework positions (indicated as or-sites). Panov et al. [9] showed that the number of or-sites and the catalyst reactivity in the benzene to phenol reaction are directly related to the amount of iron present in the catalyst, whereas Zhobolobenko [10] proposed that structural defects in the MFI zeolite framework (defects generated during the heat pretreatment of the catalyst) are the active centres which generate the active u-oxygen species upon interaction with N20. They also pointed out that extra-framework aluminium species form during the calcination of MFI catalysts. Motz et al. [11 ], studying MFI catalysts with a higher content of extra-framework aluminium species created by a steaming treatment, attributed the catalyst activity to the presence of Lewis acidity associated with this extraframework aluminium. Recently Pirutko et al. [12], studying the catalytic activity of catalysts prepared by introducing iron in pentasil-type zeolites with different compositions (B-MFI, A1-MFI, Ga-MFI, Ti-MFI), concluded that A1-MFI and Ga-MFI exhibit high activity even in the presence of 0.01-0.03% by wt. Fe, while B-MFI and Ti-MFI zeolites need 10 to 100 times more Fe. In order to clarify this question of the nature of the active sites as well as to obtain a better understanding of the factors controlling the rate of deactivation, the present study was focused on understanding the relationship between catalyst composition and rate of deactivation, but also taking into account the role of the reaction conditions which may also play a fundamental part in determining the deactivation behavior. Comparison of the data

505 obtained in this study with that on the nature of the iron species in the same catalysts (see part 1 [ 13]) should provide some indications on the role of iron species in the catalytic reaction. 2. EXPERIMENTAL 2.1 Catalyst Syntheses

Details on the method of preparation of the Fe-(A1)MFI samples have been previously reported [14] or described in part I of this work [13]. Fe was introduced both during hydrothermal synthesis of the zeolite, or by different post-synthesis methods. The characteristics of the samples and the code used for indicating them through out the text are summarized in Table 1. In the code, the subscript after the Fe symbol indicates the % w/w iron content of the zeolite and the subscript after the formula indicates the method used to introduce the iron into the zeolite structure: i.e. stands for "ion exchange", CVD for "Chemical Vapor Deposition" (often indicated as sublimation of FeC13), and ssr for "solid state reaction" while h.t. indicates that iron was directly introduced during hydrothermal synthesis. Post-synthesis introduction of iron was made using, as the parent zeolite, a commercial NH4+-MFI zeolite (from Alsi-Penta) with a SiO2/A1203 ratio of 25. H-ZSM5 was obtained from the same parent zeolite by calcination. The iron content in these samples is around 250 ppm. Usual practice was to calcine the dried catalysts at 550~ in air. Table 1. Characteristics of the Fe-(A1)MFI catab rsts used for the tests of benzene hydroxylation. Sample Code ~

Method o f addition o f Fe

Si/AI

H-ZSM5-C

-

13

H-MFI-S

-

> 1000

Fel.IMFIht-S

hydrothermal synthesis

> 1000

-

Fel.IMFIht-A55

hydrothermal synthesis

55

Fe2.2MFIht-A54

hydrothermal synthesis

54

Fe2.3MFIht-A90

hydrothermal synthesis

Fel.sMFIie-A13

ion exchange

Fe3.6MFIie-A13

Fe0.4MFIcvD-A13

AI/Fe

%Fe

(wt.)

Note

< 0.05

1.1

Commercial samples fi"omAlsi Penta (SN27) Pure Fe-fi,ee Hsilicalite-1 Pure Al-free Fesilicalite-1

1.48

1.1

(Fe-A1)ZSM5 sample

0.75

2.2

(Fe-A1)ZSM5 sample

90

0.91

2.2

(Fe-A1)ZSM5 sample

13

3.71

1.8

ion exchange

13

1.85

3.6

CVD*

13

16.7

0.4

200

....

Parent zeolite is commercial Alsi Penta (SN27)

FeC13 is mixed with the zeolite and heated in air * 9FeC13 sublimation. # C indicates commercial sample, S silicalite and A indicates the Si/A1 ratio.

Fe0.sMFIss~-A62

solid state reaction

62

1.97

0.8

2.2 Catalytic Tests

Before the catalytic tests the Fe-(A1)MFI samples were activated in-situ at a temperature ranging from 600 to 700~ in the presence or absence of steam. The catalytic tests were made in a fixed-bed reactor at a temperature typically of 400~ feeding a mixture containing (i) 3% benzene and 6% N 2 0 in h e l i u m o r (ii) 2 0 % benzene and 3% N 2 0 in helium.

506 Sometimes steam (up to about 3%) was also added to the feed. The total flow rate was 3 L/h and the amount of catalyst 0.Sg (contact time of 0.6 s g/ml). The feed was prepared using an already calibrated mixture of N20 in helium and adding benzene using an infusion pump and a vaporizer chamber. The feed could be sent either to the reactor or to a by-pass for its analysis. The feed coming out of the reactor or from the bypass could be sent to vent or to one of two parallel absorbers containing pure toluene as the solvent (plus calibrated amounts of tetrahydrofuran as the internal standard) cooled at about 15~ in order to condense all organic products. The line to the absorbers was heated at about 200~ in order to prevent condensation of the products. The vent, after condensation of the organic products, was sent to a sampling valve for analysis of the residual gas composition. The reactor outlet stream was sent alternatively to the two parallel absorbers for a given time (typically 3 or 5 rain), in order to monitor the change in the catalytic activity averaged over this time. N20, O2, N2 and total oxidation products (CO and CO2) were analyzed using TCD-Gas chromatography and a 60/80 Carboxen-1000 column, whereas benzene and phenol (as well as other minor aromatic by-products) were determined by FID-Gas chromatography using a ECONO-CAP SE-30 "wide bore" column or a Mass-GC equipped with a capillary Chrompack CP-Sil 5CB-MS Fused Silica column.

3. RESULTS AND DISCUSSION

3.1 Role of catalyst composition Reported in Figure 1 is the catalytic behavior (productivity and selectivity to phenol in benzene hydroxylation with N20) as a function of the time-on-stream of a series of Fe(Al)MFI type catalysts prepared by hydrothermal synthesis. The characteristics and composition of these samples are reported in Table 1. The samples were selected in order to summarize the influence of the composition of the catalyst on the activity and rate of deactivation. A commercial H-ZSM5 sample (H-ZSMS-C) showed effectively good activity, in agreement with literature data. However, it deactivated very quickly and in about 1 h the productivity decreased by a factor of about 50. The selectivity to phenol progressively decreases from an initial value of around 65-70% to less than 10%. It should be noted that similarly to most of the commercial samples, H-ZSMS-C contains traces of iron (see Table 1) due to contamination during the industrial preparation. When a pure Fe-free Silicalite-1 catalyst (H-MFI-S) was tested, no formation of phenol at all could be observed (Figure 1). Silicalite-1 has the same MFI structure as ZSM-5, but is A1flee. The synthesis of Silicalite leads to the presence of structural defects (hydroxyl nests), as confirmed by the presence of silanol groups evidenced by FTIR spectroscopy. Pretreatment of the sample at 700~ in helium before the catalytic tests causes dehydroxylation of these hydroxyl nests forming oxygen vacancies which may activate N20, because they behave as F-centres. In agreement, the catalyst shows an initial activity in N20 decomposition (around 22% N20 conversion aiter 5-10 min of time-on-stream). This suggests that defects in the zeolite can activate N20, but are unable to either form active oxygen species in benzene hydroxylation and/or to activate the organic substrate (benzene) for this reaction.

507 1,6 ,-'r "7 ..c -~

1,4

+ --O-~ --v--

1,2

F: E 1,0 ...... or-. 9 0,8 o

+ Fel.IMFIht-S - - 0 - - Fe 2 aMFI~A90

0,6

._> "o

H-ZSM5-C Fez2MFI~-A54 H-MFI-S Fel.lMFI~-A55

0,4

~o

13. 0,2 0,0 0,0

0,5

1,0

1,5

2,0

2,5

Time, h

I

100

9

I

I

I

I

80 -.

~

0

60

o eeQ.

~

40

2o 0

0,o

.= ~v,

,

h

i 0,5

,

L~,

,

i

1,0

L

L

,

L

J

1,5

L

,

,

,

i

2,0

,

,

,

,v

i

J_

2,5

Time, h

Figure 1. Behavior of the catalysts as a function of the time-on-stream in the productivity and selectivity to phenol by benzene hydroxylation with N20. Catalysts pretreated at 700~ in helium for two hours. Reaction conditions: T=400~ 3% benzene, 6% N20, balance He. When an Al-free Fe-Silicalite (Fel.IMFIht-S) is used the productivity to phenol is low, but quite stable. No change in the productivity was observed up to about 20 hours of time-onstream. The selectivity to phenol is initially low and progressively increases up to final constant values of about 50%. The conversion of N20 is low (about 5%) and nearly constant with increasing time-on-stream. Therefore, in the absence of A1 the activity of the catalyst towards phenol synthesis is low, but remarkably stable, suggesting that A1 sites may play a role in the synthesis of phenol, but also in the side reactions leading to catalyst deactivation. When both Fe and A1 are present in the zeolite (Fel.lMFIht-A55 and Fe2.2MFIht-A54) the productivity to phenol increases by a factor of about 10 or 20, respectively and the selectivity

508 to phenol also markedly increases reaching values higher than 90% for Fel.lMFIht-A55. The productivity to phenol is approximately proportional to the amount of iron (compare phenol productivities for Fe~.tMFIht-A55 and Fe2.EMFIht-A54; the latter has twice the iron content and double the productivity of the former, while the Si/A1 content is the same). The selectivity to phenol, on the contrary, is higher in the case of the sample having the lower iron content. The trend with time-on-stream is quite similar in the two catalysts. The productivity to phenol decreases, although much less dramatically than in the case of HZSMS-C, while selectivity to phenol increases. The conversion of N20 is initially around 40% and then decreases to about 10-15% for Fe1.lMFIht-A55. When this result is compared with that of the sample having a comparable iron content, but no aluminium ions (Fel.lM~Iht-S), it is evident that the presence of A1 ions (or probably better sites related to A1, such as AI-OH Bronsted acid) together with iron ions is a condition necessary for efficient activation of N20. However, sites for the decomposition of N20 are also present which progressively become inactivated by the carbonaceous-type species formed on the catalyst, causing its deactivation. In fact, the rate of N20 conversion in Fel.lMFIht-A55 decreased by a factor of about 6-8 during the 2.5 hours of the experiments (Figure 1), while the productivity to phenol decreased by a factor of about 2. The side decomposition of N20 produces 02 (which is also detected in the reactor outlet) and probably 02 determines the total combustion of benzene, phenol or reaction intermediates and thus the formation of CO2 (CO is usually observed only in traces). This explains why in parallel to the decrease in N20 conversion, an increase in the selectivity to phenol is observed. With increasing iron content (Fe2.2MFIht-A54) the initial conversion of N20 is about 65% and then decreases to about 20-25% after 2.5h of time-on-stream. This indicates that the increase in iron determines an increase in the number of active sites for phenol synthesis (the productivity is about twice), but probably also increases the formation of a higher amount of a second type of iron species. Reasonably these species are small iron-oxide particles within the zeolite cavities which form in the process of partial migration of iron from framework to non-framework positions during the initial catalyst pretreatment. The increased amount of iron favours the process of aggregation of these iron species and thus a higher amount of aggregated iron oxide which is responsible for the decomposition of N20 to N2 + 02 instead of N2 + c~-O. As a consequence, the productivity of phenol in Fe2.2MFIht-A54 is higher than in Fel.lMFIht-A55, but the selectivity to phenol lower. Decreasing the amount of A1, while maintaining constant the amount of iron (compare Fe~.3MFIht-A90 with Fe2.2MFIht-A54) leads to an increase in both the productivity and selectivity, although the general trend remains again similar. This indicates that probably an A1/Fe ratio close to 1 is the optimal compromise to maximize both productivity and selectivity to phenol, suggesting that the active sites for phenol synthesis comprise both A1 and Fe. 3.2 Role of methodology in iron introduction in Fe-MFI catalysts In order to analyze in a concise way the effect of different methods of addition of iron in Fe-(A1)MFI samples on the catalytic performances (activity and deactivation), catalytic data have been summarized in Table 2. The following parameters are reported: (i) the maximum observed phenol productivity, which is generally obtained at the beginning of reaction or aiter about 20 minutes of time-on-stream (see Figure 1), (ii) the phenol productivity alter 2 hours, which gives an indication of the catalyst stability, and (iii) the selectivity in correspondence with the maximum phenol productivity and after 2 hours of time on stream.

509 Table 2 compares the behavior of some selected samples in which Fe was introduced postsynthesis by (i) an ion exchange method (Fel.sMFIie-A13 and Fe3.6MFIie-A13) using an aqueous solution of iron-ammonium-sulphate, (ii) CVD (contacting the anhydrated zeolite at 300~ with a flow of FeC13 in N2) and (iii) solid state reaction (FeC13 and the zeolite are mixed homogeneously and heated to 400~ For better comparison, data for two samples prepared by the hydrothermal method and containing both Fe and A1 (Fe1.1MFIht-A55 and Fe2.2MFIht-A54) as well as data for the parent zeolite used for ion-exchange and CVD (HZSM5-C) are also reported. Table 2. Phenol productivity and selectivity for Fe-(A1)MFI samples in which iron was introduced using different methods. Before benzene oxidation the catalysts were pretreated at 700~ in helium for two hours. Reaction conditions as in Figure 1. Selectivity3 Phenol productivity Sample Max1 2 h2 Max3 2h3 Fel.1MFIht-A54

0.38

0.18

81

92

Fe2.2MFIht-A55

0.81

0.62

56

69

H-ZSM5-C

0.44

0.02

61

11

Fel.sMFIie-A13

0.21

0.01

28

12

Fe3.6MFIie-A13

1.03

0#

78

17#

Fe0.aMFIcvo-A13

0.29

0.135

38

12

0.04 0# 7 1 Fe0.sMFIssr-A62 1 Maximumphenol productivity (mmolh"1g-l) at 400~ 2 Phenol productivity(mmol h~ g-l) at 400~ after 2 h of reaction 3 Phenol selectivity (%) at 400~ expressed on the total product basis in corrispondence to the maximum productivity and alter 2h of reaction. # The catalyst is completelydeactivatedjust after 1 h of reaction. The selectivitycorresponds therefore to a reaction time of 50 minutes $ The productivityto phenol does not further decrease in tests up to 20h of time-on-stream. The post-synthesis introduction of iron in the Fe-(A1)MFI catalysts generally leads to lower selectivities and productivities to phenol, although in some cases (e.g. Fe3.6MFIie-A13) good initial behavior is obtained. With respect to the behavior of the parent zeolite (HZSM5-C), the productivity to phenol increases more than twice and an increase in the selectivity to phenol could also be noted. However, similarly to H-ZSM5-C, after 1 h the productivity to phenol becomes negligible. When the iron content is lower, much poorer performances were observed, being higher the rate of side decomposition of N20. Therefore, the addition of iron by ion exchange could lead to catalysts having an initial activity better than the parent zeolite, but the effect of fast deactivation discussed before did not change. The addition of iron by CVD leads to not very selective catalysts due to a high rate of side N20 decomposition, but stable activity was noted after an initial decrease in phenol productivity. According to previous characterization, the CVD (sublimation) method leads to the formation of isolated or binuclear iron species [15], probably having characteristics similar to those of the active species responsible for the generation of the selective ~-O species during the interaction with N20. However, especially when the feed or the zeolite is

510 not fully anhydrated, nanoparticles of iron oxides also form [16], responsible for the decomposition of N20 and lowering of the selectivity. In agreement, the sample prepared by solid state reaction and in which the formation of the latter species is enhanced leads to very few selective and active catalysts in phenol formation. 4. CONCLUSIONS The Si/A1 and Si/Fe ratio in Fe-MFI, as well as also the method of addition of iron to the catalyst, have a marked influence on the catalyst activity and rate of deactivation during onestep oxidation of benzene to phenol. If deactivation is not taken into consideration, the comparison of different catalysts and the analysis of the structure-activity relationship may not be correct. Other factors not reported here due to space limitations determine the catalytic activity and stability, namely the temperature and atmosphere of pre-activation of the catalyst, the zeolite structure and the feed composition (N20/benzene ratio and their concentrations, and presence of H20 in the feed). These factors will be discussed in a subsequent manuscript. 5. REFERENCES

1. S. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Shoji, T. Namba, F. Mizukami, SCIENCE, 295 (2002): 105. 2. G. Centi, S. Perathoner, F. Vazzana, CHEMTECH, 29(12) (1999) 48. 3. A.S. Kharitonov, G.I. Panov, K.G. Ione, V.N. Romannikov, G.A. Sheveleva, L.A. Vostrikova, V.I. Sobolev, US Patent 5 110995 (1992). 4. P.P. Nott6, Topics in Catal., 13 (2000) 387. 5. E.Suzuki, K. Nakashiro, Y. Ono, Chem. Lett. (1988) 953. 6. R. Burch, C. Howitt, Appl. Catal. 86 (1992) 135. 7. M. Gubelmann, P.-J. Tirel, US Patent 5 001 280 and E.P. 0 341 113, both assigned to Rhone-Poulenc. (1988) 8. V.I. Sobolev, K.A. Dubkov, E.A. Paukshits, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov, G.I. Panov, Appl. Catal. A 141 (1996) 185. 9. G.I. Panov, V.I. Sobolev, A.S. Kharitonov, J. Mol. Catal. 61 (1990) 85. 10. Zhobolobenko, Mendeleev Commun., 28 (1993). 11. J.L. Motz, H. Heiniche, W. HSlderich, Studies in Surf. Sci. Catal., 105 (1997) 1053. 12. L.V. Pirutko, V.S. Chemyavsky, A.K. Uriate, G.I. Panov, Appl. Catal. A, 227 (2002) 143 13. G. Giordano, A. Katovic, S. Perathoner, F. Pino, G. Centi, J. B.Nagy, K. Lazar, P. Fejes, Stud. Surf. Sci. Catal., part I, this issue. 14. G. Centi, S. Perathoner, G. Romeo, Stud. Surf Sci. Catal., 135 (2001) 181. 15. B. Wichterlova, J. Dedecek, Z. Sobalik, in Catalysis by Unique Metal Ion Structures in Solid Matrices, G. Centi, B. Wichterlova, A. Bell Eds., NATO Science Series II Vol. 13, Kluwer Acad. Pub.: Dordrecht (The Netherlands), Ch. 3, p. 31. 16. G. Centi, F. Vazzana, Catal. Today, 53 (1999) 683.

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier ScienceB.V. All rightsreserved.

511

S y n t h e s i s , s t r u c t u r e , a n d r e a c t i v i t y of i r o n - s u l f u r s p e c i e s in z e o l i t e ZSM-5 Richard W. Joyner a, Michael Stockenhuber" and Olga P. Tkachenko b aThe Catalysis Research Laboratory, Department of Chemistry and Physics, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK. bPermanent address: Zelinsky Institute of Organic Chemistry, 47 Leninskii Prospect, Moscow, Russia. Iron-oxo species involving isolated iron atoms and nanoclusters can be formed within the pore structure of the zeolites ZSM-5 by a range of procedures, including aqueous exchange and chemical vapour deposition. We report that these can be sulfided by exposure to hydrogen sulfide at 623 K, forming analogous iron- sulfur entities. Characterisation by XPS and in particular EXAFS spectroscopy shows that there is almost complete replacement of oxygen by sulfur in the immediate iron coordination sphere, with an Fe - S distance of 2.26 A. The iron-sulfur materials are weakly active for the hydration of acrylonitrile to acrylamide, with the best materials achieving about 0.4 turnovers per iron species present. The catalyst appears to be subject to extensive hydrolysis during reaction. 1. INTRODUCTION A variety of oxo-species are produced when iron is introduced by different methods into the channels of microporous and mesoporous materials. The formation of oxo-bridged iron dimers is claimed to occur in ZSM-5 when a chemical vapour deposition route is used [1], while monomeric species predominate in MCM-41 [2]. When aqueous exchange is the method of introduction, we have reported extensive evidence that unusual oxo-nanoclusters are formed in ZSM-5 [3] and there is also some clustering when iron is introduced into zeolite beta by this method [4]. In ZSM-5 the average cluster contains four iron atoms and a similar number of oxygen atoms. The clusters are characterised by Fe - O distances in the range 1.93 - 2.10 A and also by unusually short iron iron distances of ca 2.55 .~. Because of these short iron - iron distances, we have drawn analogies with the structures of small sulfur-containing clusters that are know to occur in nature, and which can also be synthesised in the laboratory. Specifically we have drawn analogies with high-potential iron proteins (HIPIP) [5] and ferredoxins [6]. It is thus of interest to study whether any of'the iron-exchanged materials can be sulfided, and to examine what the structure and properties of the resultant materials might be. In particular we would like to mimic the enzyme

512

pseudomonas chlororaphis. This enzyme is thought to contain an iron-sulfur entity

as its active site, and it is able to catalyse hydration reactions, including the industrially significant hydration of acrylamide [7]. Our structural and reactivity results are the subject of this paper. 2. EXPERIMENTAL

The preparation and characterisation of the iron-oxo nanoclusters by aqueous exchange with H-ZSM-5 has been fully described [3]. Samples were also prepared by chemical vapour deposition, using the method first used by Chen and Sachtler [8]. About 500 mg of each sample was heated to 373 K in helium overnight and then exposed to hydrogen sulfide (Air Products, > 99.9%, 5% in helium) at 20 ml min -1 for 2 hours. The samples, which were originally yellowbrown in colour, were found to be black after sulfiding. The sulfided materials were characterised by X-ray photoelectron spectroscopy (XPS) using a VG ESCA-3 spectrometer, and by in situ iron K edge X-ray absorption spectroscopy (EXAFS) with fluorescence detection, at the Daresbury synchrotron radiation source. Standard experimental and data analysis methods were employed for both of these techniques, which have been fully described previously [3]. 3. RESULTS A N D DISCUSSION 3.1 Characterisation of sulfided materials

The sulfided materials were principally characterised by XPS and EXAFS, and some XPS results are listed in Table 1. Table 1: XPS results Catalyst 2.45% Fe-ZSM-51 iron (II) sulfide 2

Sulfur 2p binding energy/eV 163.6 and 170.6 162.5

Iron 2p3/2 binding energy/eV

Fe/S Ratio

710.8

1.2

710.0

1.2

1 Prepared by chemical vapour deposition followed by sulfiding. 2 BDH Technical Grade.

Figure 1 shows a typical iron K-edge EXAFS spectrum for the CVD sample after sulfidation, together with a spectrum calculated using the parameters listed in Table 2. The spectrum is dominated by an iron - sulfur interatomic distance of 2.26 + 0.02 A with a coordination number of 2.4 + 0.5. Before sulfiding, the main feature was an iron- oxygen distance with a mean interatomic value of ca 1.95 .~ and a co-ordination number of ca 4. There are, however three other shells of

513

i

i

|

i

A 1

i 4,

6

8

10

K:.Angstrom I J

9

.

.

,

lo

B

t

1="-4

It

o 1

3

5

7

p

Distance/A Figure 1. Experimental (solid line) and calculated iron K edge EXAFS for a catalyst prepared by chemical vapour deposi(ion and subsequently sulfide& A) Reciprocal space: B) Fourier transform. For details see the text, and for the parameters used for the calculated spectrum see Table 2.

514 neighbours that contribute to the calculated EXAFS in a statistically significant way. There is a residual oxygen shell at 1.93 A with a coordination number of about 0.4. The significance of the Fe - A1 and Fe - Fe shells of neighbours will be discussed elsewhere. Table 2 Parameters used to model the experimental EXAFS spectrum shown in Figure 1. Neighbour Oxygen Sulfur Iron Aluminium

Interatomic Distance/.A 1.93 2.26 2.74 3.20

Coordination Number 0.4 2.4 0.5 1.0

Debye-Waller Factor / A 2 0.008 0.015 0.020 0.014

The colour change on sulfidation and the XPS results show that sulfur has been introduced into these materials. The EXAFS results go further, in that they demonstrate almost complete displacement of oxygen by sulfur in the first iron coordination shell. This is indicated by the reported distance, 2.26 A, which is typical of the iron-sulfur interatomic separation in sulfides. The thermodynamic driving force for sulfidation is probably the formation of water, through the reaction: Fe- O + HzS .... > Fe- S + H20 For the bulk compounds, this reaction is thermodynamically favourable for iron (II) oxide (AHo29s = - 91 kJ tool-l), but not for Fe203. The sulfur 2p binding energy of ca 163 eV is consistent with the presence of sulfide. The higher binding energy observed is believed to be the result of atmospheric oxidation to sulfate during transfer to the spectrometer, indicating the lability of the sulfur species present. Similar behaviour has previously been observed in studies of sulfided chromia catalysts [9].

3.2 Reactivity The catalytic hydration of acrylonitrile to yield acrylamide is a reaction of industrial importance, which is usually catalysed by Raney copper. Since this Cu/A1 alloy catalyst undergoes slow decay and is subject to fouling by thermally polymerised product [10], there is interest in developing alternative catalysts. Several enzymes are used industrially in Japan [7], including the iron containing pseudomonas chlororaphis. We have therefore tested sulfided Fe-ZSM-5 materials for catalytic activity in the hydration of acrylonitrile. Reaction has been carried out in aqueous solution (1 ml acrylonitrile in 12.5 ml water) at temperatures in the range 338- 363 K, with product analysis by off-line gas chromatography. The results of a typical experiment with sulfided Fe-ZSM-5 prepared initially by the CVD route are shown in Figure 2. Some acrylamide is produced, but the activity is not high and we calculate that the maximum yield observed represents

515 only about 0.4 turnovers per iron atom. No other products were detected, so selectivity to acrylamide is 100%. We are therefore still some way away from mimicking the activity of the enzyme P. chlororaphis, which achieves its saturation yield of 45% acrylamide after a few hours at 20~ [7]. However other iron containing materials that we tested showed even lower activity than sulfided FeZSM-5. With the unsulfided iron containing zeolite the maximum yield found was < 0.2%, and no activity at all was detected with powdered iron sulfide. Our material gradually loses sulfur during reaction, as evidenced by the loss of its black colour. We believe that prolonged hydrolysis reverses the sulfidation reaction, since the black colour is also lost over several hours in boiling water. By contrast, the material is not reoxidised in air after many hours at > 100~ It is therefore possible that the formation of acrylamide is indeed catalytic (i.e. producing more than one turnover per active site), but that the catalyst is destroyed by a parallel hydrolysis reaction. We are currently investigating this.

0.7

0.6

0.5

e- 0.4 o (n L_

r 0 0

0.3

T increased to 95 C

0.2 T increased to 75~ 0.1

0

10

20

30

40

50

60

70

80

90

100

Time/h

Figure 2. Results of the hydration of acrylonitrile using sulfided Fe-ZSM-5 and the reaction conditions indicated in the text. The reaction was started at 65~ and the temperature was increased at the points indicated by the arrows. REFERENCES

1. 2.

P. Martura, L. Drozdova, A. Kogelbauer and R. Prins, J. Catal., 192 (2000) 236. M. Stockenhuber, M.J. Hudson and R.W. Joyner, J. Phys. Chem. B., 104 (2000) 3370.

516 3. R.W. Joyner and M. Stockenhuber, J. Phys. Chem. B., 103 (1999) 5963. 4. M. Stockenhuber, R.W. Joyner, G.S. Paine, unpublished results. 5. See e.g.S.P. Cramer in X-ray absorption, Principles, applications and techniques of EXAFS, Ed. D.C. Koningsberger and R. Prins, J. Wiley New York, 1988 and references therein. 6. C.R. Kissinger, E.T. Adman, L.C. Sieker and L.H. Jensen, J. Amer. Chem. Soc., 110 (1988) 8721. 7. See the review by H. Yamada and M. Kobayashi, Biosci. Biotech. Biochem., 60 (1996) 1391. 8. H.-Y. Chen and W.M.H. Sachtler, Catal. Today, 42 (1998) 73. 9. B.W.L. Southward, G.J. Hutchings, R.W. Joyner and R.A. Stewart, Catal. Lett., 68 (2000) 75. 10. M.S. Wainwright, Preparation and utilisation of Raney copper catalysts, in Catalysis of Organic Reactions, Ed. R.E. Malz Jr., pub Marcel Dekker, New York 1996, pp. 213.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Characterization of FeMCM-41 and FeZSM-5 Catalysts Production

517

to Styrene

J. R. C. Bispo a, A. C. Oliveiraa, M. L. S. Corr~a a , J. L. G. Fierro b, S. G. Marchetti ~ and M. C. Rangel a aInstituto de Quimica, Universidade Federal da Bahia. Campus Universithrio de Ondina, Federag~o. 40 170-280, Salvador, Bahia, Brazil, e-mail: [email protected] bInstituto de Catalisis y Petroleoquimica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain ~ Facultad de Ciencias Exactas, Universidad National de La Plata, 1900, 47 y 115, La Plata, Argentina FeMCM-41 and FeZSM-5 catalysts have been prepared and tested in the dehydrogenation of ethylbenzene to produce styrene. These new catalytic systems have high specific areas and can stabilize the trivalent state of iron. It was found that the FeZSM-5 catalysts are more active and selective than the FeMCM-41 ones and this behavior is explained in terms of the higher amount of the active oxidation state (Fe 3+) on the catalyst surface. These catalysts also result more active and selective than t~Fe203 (hematite) and are less toxic than the chromium-containing commercial catalysts used for this purpose.

1. INTRODUCTION Styrene monomer is one of the most important high value chemical used extensively for the manufacture of plastics, including crystalline polystyrene and styrene-butadiene rubber (SBR) [1]. Direct catalytic dehydrogenation of ethylbenzene has been the dominant technology for styrene production since its first commercial application [2-4]. In this process ethylbenzene is dehydrogenated to styrene and hydrogen over a catalyst in the presence of steam; toluene and benzene are formed as by-products. The overall reaction is highly endothermic and thermodynamically limited and thus the conversion is increased when increasing the temperature. The feed is much diluted with steam for limiting the coking rate of the catalyst, decreasing the temperature fall due to the reaction and improving the conversion at the equilibrium [5]. The most widely used industrial catalysts comprise iron oxide and promoters such as K2CO3, Cr203, CeO2, MOO3, V205 and so on [2]. Other promoters like aluminum, cadmium, magnesium, manganese, nickel and uranium oxides as well as rare-earths

518 have been used [6]. However, potassium-promoted iron oxide is better than any other catalyst known for ethylbenzene dehydrogenation in the presence of steam [2,3]. It is generally believed that potassium acts as a chemical promoter in the catalyst, whereas chromium oxide is a textural promoter stabilizing the high surface area of the active phase [2, 3, 7]. Despite its high activity and selectivity, the commercial catalyst still has some disadvantages which need to be improved in order to minimize the manufacturing costs of styrene. On the one hand, the active oxidation state is unstable; hematite (~Fe203) is preferred for styrene production, but it tends to go into oxides with lower oxidation states and even to elemental iron, and they catalyze carbon formation and dealkylation [3]. On the other hand, the iron-based catalysts have low specific areas and deactivate with reaction time being susceptible to poisoning by halides and residual organic chlorine impurities [2]. The most serious deactivation is caused by the loss of potassium promoter, which migrates in two directions as the catalyst ages. Potassium chloride is found downstream in the water layer of the condensed product as well as in the center of the catalyst pellets [2,3]. Besides, the large amounts of steam used in commercial operations increase the operational costs. The catalyst has the additional disadvantage of being toxic causing damage to the humans and to the environment. Therefore, the investigation for new systems which have high specific area, can stabilize the trivalent state of iron and are potassium and chromium free is much needed. With this goal in mind, this work deals with the evaluation of FeMCM-41 and FeZSM-5 as catalysts to the styrene production. 2. EXPERIMENTAL The FeZSM-5 sample was prepared by mixing an aqueous solution of ferric sulfate (0.09 mol.L -1) with an aqueous solution of sodium metasilicate (2.5 mol.L 1) and a tetrapropylammonium bromide (template) solution (0.2 mol.L 1) under stirring. The resulting solution was kept in an autoclave at 170 ~ for 72 h. Then, the sample was rinsed with water, centrifuged and dried at 120 ~ for 2h. After this, the solid was calcined at 500 ~ under nitrogen flow (3 h) and under air flow (5 h). The FeMCM-41 sample was prepared from a gel with sodium metasilicate (2.5 mol.Ll), tetramethylcethylammonium bromide (0.02 mol.Ll), ferric sulfate (0.1 mol.L1) and tetramethylammonium hydroxide, which was aged for 4 h under stirring and then kept under hydrothermal treatment in autoclave. The pH was adjusted to 12 with ammonium hydroxide, aged for 4 h under stirring and kept in an autoclave under hydrostatic pressure in an oven at 140 ~ for 16 h. The sample was rinsed with water, centrifuged and dried at 90 ~ for 12 h. The material was calcined at 500 ~ under nitrogen flow (lh) and under air flow (6 h). An iron oxide sample (hematite) was also prepared to be used as a reference catalyst. This solid was prepared by adding, under stirring, an aqueous solution of iron nitrate (1.0 mol.L 1) and a concentrated (25% w/w) solution of ammonium hydroxide to a beaker with water. The sol produced was centrifuged, rinsed with water, dried in an oven at 120 ~ and calcined at 500 ~ under nitrogen flow (2 h).

519 The iron contents were determined by inductively coupled plasma atomic emission spectroscopy (ICP/AES) by using an Arl 3410 model equipment. The absence of the templates in the catalysts was confirmed by Fourier transform infrared spectroscopy in the range of 4000-400 cm-1 using a model Valor II Jasco spectrometer and KBr discs. The structure of the FeZSM-5 and of the FeMCM-41 was confirmed by X-ray diffractometry experiments performed at room temperature with a Shimadzu model XD3A instrument using CuKa radiation generated at 30 kV and 20 mA. The specific area was measured (BET method) in a Micromeritics model ASAP 2000C equipment on samples previously heated under nitrogen (150 ~ 2 h). The temperature programmed reduction (TPR) was performed in a Micromeritics model TPD/TPO 2900 equipment, using a 5% H2/N2 mixture. The M6ssbauer spectra were obtained in transmission geometry, with a 512- channel constant acceleration spectrometer at 25 ~ A source of 57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performed against a 121am thick ct-Fe foil. All isomer shifts mentioned in this paper are referred to this standard at 25~ The spectra were evaluated by using a least-squares nonlinear computer fitting program with constraints. Lorentzian lines were considered with equals widths for each spectrum component. The spectra were folded to minimize the geometric effects. X ray photoelectron spectra were obtained with a VG ESCALAB 200R spectrometer equipped with a MgKet X-ray radiation source (hv = 1253.6 eV) and a hemispherical electron analyzer. The powder samples were pressed into small stainless steel cylinders and mounted onto a manipulator which allowed the transfer from the preparation chamber into the spectrometer. Before the analysis, they were outgassed (10 .9 mbar) or reduced in hydrogen at 500 ~ (1 h). The Si2p peak was chosen as an internal reference. This reference was in all cases in good agreement with the BE of the C ls peak, arising from contamination, at 284.9 eV. This reference gave an accuracy of + 0.1 eV. The catalyst performance was evaluated using 0.2 g of powder within 50 and 325 mesh size, and a fixed-bed microreactor, providing there is no diffusion effect. The experiments were carried out under isothermal condition (530 ~ and at atmospheric pressure, employing a steam to ethylbenzene molar ratio of 10. The reactor, containing the catalyst, was heated under nitrogen flow (60 ml.s 1) up to the reaction temperature. Then the feed was interrupted and the reaction mixture was introduced. The reaction mixture was obtained by passing a nitrogen stream through a saturator with ethylbenzene and then through a chamber where it was mixed with steam. The gaseous effluent was collected in a condenser and the organic phase was analyzed by gas chromatography, using a CG-35 instrument. In order to save the energy related to the steam consumption, the catalysts were also evaluated in the absence of steam.

3. RESULTS AND DISCUSSION Table 1 compiles the amount of iron in the catalysts, their specific area and their rates of ethylbenzene conversion and selectivity towards styrene in the steady state. It can be seen that the FeZSM-5 sample is active only in the presence of steam. This can

520 Tablel. Amount of iron (%Fe) and specific area of the catalysts (Sg) and their activity (a), activity per weight of iron (a/g) and selectivity (S) to styrene of the catalysts in the dehydrogenation of ethylbenzene Sample % Fe Sg a.lO 3 (mol.hl.g q) a/g (mol.hl.g Fe q) s (%) (m2.g1) With Without With Without steam steam steam steam FeZSM-5 1.32 425 3.5 0.0 4.7 100 0 FeMCM-41 1.04 1112 1.8 0.9 3.1 52 33 c~-Fe203 71.32 17 2.4 0.0 4.4. 10 -3 92 0

be related to the role of steam in reacting with the carbonaceous deposits according to the Boudart reaction. On the other hand, the FeMCM-41 sample was active both in the presence and in the absence of steam, although the activity and selectivity strongly decreased without it As FeMCM-41 has larger pores than FeZSM-5, one can suppose that it can afford a large amount of coke and then it can work even without steam. The other role of steam is to keep the trivalent state of iron highly selective to styrene [2,3]. As MCM-41 and ZSM-5 did not show any activity towards the reaction, the performance of the catalysts may be directly related to iron oxides. The FeZSM-5 catalyst was more active than the hematite-based sample. As hematite has larger amount of iron, and then larger number of active sites than the zeolite, one can conclude that iron is much more active in the zeolite structure, as confirmed by the values of activity per gram of iron. The FeZSM-5 catalyst was the most selective whereas the FeMCM-41 sample was the least one. Figure 1 shows the activity and the selectivity to styrene as a function of time. In all eases, the activity varied in the first hours of reaction and reached stables values after 4h. Concerning the selectivity, FeZSM-5 and hematite show stable values since the beginning of the reaction, while FeMCM-41 showed stable values only after 6h. ~,,

8

120

~.

6

,-~ 100 ~ 80

"~

4

",=, 60

2

40 20

9

0 ................. 0 2 4 Time (h)

(a)

6

8

0,

0

~ 1

~ 2

, , 3 4 5 Time (h)

6

,

7

(b)

Figure 1. (a) Activity and (b) selectivity to styrene of the catalysts in the dehydrogentaion ofethylbenzene. -4k- e~-Fe203; -" F e M C M - 4 1 ; - - I FeZSM-5

521 The TPR profile of pure hematite showed two peaks around 400 ~ and 740 ~ ascribed to the reduction of Fe 3+ and Fe 2+, respectively [8]. The TPR curve of FeMCM41 showed only a peak centered at 500 ~ whereas the curve of FeZSM-5 displayed a large peak beginning at the same temperature. As stated early [9] this peak is due to Fe 3+ reduction in zeolite structure. Therefore, it can conclude that the iron reduction is more difficult on both MCM-41 and ZSM-5 structures as compared to hematite. The M6ssbauer spectra of FeMCM-41 samples were poorly-defined with a low signal-to-noise ratio probably due to the low amount of iron. In the fresh sample a signal related to Fe 3+ species exchanged and/or superparamagnetic Fe203 was found. After the reaction, carried out with or without steam, a fraction of iron went into 7Fe203 and the sample was attracted by a magnet. This can be explained by considering that during the reaction very small crystallites of magnetite were produced which went into maghemite under the oxidizing conditions of the Mossbauer experiments. Table 2 shows the M6ssbauer parameters of the FeMCM-41 catalysts. Table 2. M6ssbauer hyperfine parameters catalyst tested with steam and WS Species Parameters 7-Fe203 H(T) 8(mm/s) 2e(mm/s) Fe 3+ exchanged 5(mm/s) and/or Fe203 sp A(mm/s)

at 25 ~ of FeMCM-41 catalysts. S represents the the catalyst tested without steam FeMCM-41 FeMCM-41 (S) FeMCM-4 I(WS) 48 + 1 47.5 + 0.4 0.4 + 0.1 0.41 + 0.06 0.03 (*) 0.1 + 0.1 0.32+0.03 0.33 (*) 0.33 + 0.02 0.78i0.04 0.9 + O.1 0.94 + 0.03

*parameter held fixed while fitting; sp: superparamagnetic; H: hyperfine field; 8: isomer shift; 2e: quadrupole shift; A: quadrupole splitting. The flesh FeZSM-5 catalyst shows the M6ssbauer parameters which fitted well to hematite.Using the magnetic excitation model [10], the average diameter of the crystallites can be estimated as 17.2 nm, which means that they are outside the channels of the zeolite, in accordance with previous work [ 11]. After the reaction, performed with steam, these crystallites grew up to 20.1 nm and no other phase, besides hematite, was noted. On the other hand, the reaction carried out without steam caused a phase change, producing maghemite detected by the strong reduction of the hyperfine filed (H) and the zero value of the quadrupole shift (2e). Again, this phase is related to the small crystallites of magnetite that were produced under the reaction conditions. In all spectra, there is a very weak central signal which may correspond to Fe 3§ species exchanged with the zeolite and/or superparamagnetic hematite related to a fraction of very small crystallites which could be located inside the channels. Table 3 shows the M6ssbauer parameters. These results show that magnetite is produced more easily in the FeMCM-41 structure than in the FeZSM-5 one, in the presence of steam. Therefore, MCM-41 is not able to avoid magnetite formation even in the presence of steam. On the

522 Table 3. M6ssbauer hyperfine parameters at 25 ~ of FeZSM-5 catalysts. S represents the catalyst tested with steam and WS the catalyst tested without steam. Species Parameters FeZSM-5 FeZSM-5(S) FeZSM-5(WS) H(T) 50.9 + 0.10 51.2 + 0.10 ~-Fe203 8(mm/s) 0.37 + 0.02 0.37 + 0.01 2e(mm/s) -0.19 + 0.03 -0.20 _+0.01 H(T) 49.5 + 0.10 y-Fe203 5(ram/s) 0.32 + 0.01 - 0.06 + 0.03 2e(mm/s) Fe 3+ exchanged 0.37(*) 8(mm/s) 0.37(*) 0.37(*) and/or Fe203 sp *Parameter held fixed while fitting; sp: superparamagnetic; H: hyperfine field; ~: isomer shift; 2e: quadrupole shift; A: quadrupole splitting. other hand, the FeZSM-5 catalysts showed only hematite after the reaction (with steam), showing that the zeolite structure is able to stabilize hematite. Without steam, hematite changed to magnetite which went into maghemite during the M6ssbauer experiments. Table 4 shows the binding energies (BE) of some characteristic core levels of Fe, O and Si in the FeMCM-41 samples as well as the surface amount of Fe 3+ species. The fresh catalyst showed a binding energy of 710.5 eV which is typical ofFer+species in hematite [ 11]. After the catalytic test, carried out with steam, the catalyst surface still showed only Fe 3+ species. When the reaction was performed without steam, however, a fraction of Fe 3+ was reduced to Fe 2+ (45%) as inferred by the BE of 709.7 eV [11]. After the reduction, carried out in the XPS equipment, some Fe 3+ is reduced to Fe 2+ in all samples, showing that the Fe 3§ state was stabilized during the reaction. The surface atomic ratio Fe/Si of the catalyst is also shown in Table 4. It can be seen that the reaction causes an enrichment of iron on the surface. The reduction performed in the XPS equipment increased the amount of iron on the catalyst surface even more. The binding energies (BE) of some characteristic core-levels of Fe, O, A1 and Si in the FeZSM-5 samples as well as the surface amount of Fe 3§ species are shown in Table 5. The flesh catalyst showed binding energies of 710.0 and 711.7 eV which are typical of Fe 2+ and Fe 3+ species respectively [11]. The presence of the reduced species can be explained by an effect of vacuum during the treatment of the samples, in the XPS equipment. The Fe 3§ reduction, produced by the outgassing, could be due to the dehydroxilation of the zeolite. Similar results have been found by other authors in Fe-zeolite L [12] and Fe-zeolite X [13] systems. After the catalytic tests, the amount of Fe 3§ species as well as the total amount of iron on the catalyst surface increased. After the reduction, carried out in the XPS equipment, a part of Fe 3+ is reduced to Fe 2+ in all samples and the amount of iron on the catalyst surface increased even more. By comparing the Fe/Si ratio of the catalysts, we see that the FeZSM-5 samples have much more iron on the surface, and then more Fe 3+ species, than the FeMCM-41 catalysts.

523 Table 4. Binding energies (eV), surface atomic ratios and surface amount of Fe 3§ of fresh and spent FeMCM-41 catalysts taken on samples previously treated under vacuum or under hydrogen at 500 ~ S represents the catalyst tested with steam and WS the catalyst tested without steam. Sample Cls Si2p Ols Fe2p3/2 Fe 3+ Fe/Si (%) (atom) FeMCM-41 (vac) 284.6 103.4 532.9 710.5 100 0.0054 (H2, 500~ 284.6 103.4 532.0 709.6 0.0069 711.5 40 FeMCM-41 (S) (vac) 284.6 103.4 532.9 709.7 0.0075 711.5 55 (H2, 500~ 284.6 103.3 532.0 709.5 0.0078 711.6 35 FeMCM-41 (WS)(vac) 284.6 103.4 532.9 711.3 100 0.0068 (H2, 500~ 284.6 103.4 532.9 709.7 0.0079 711.7 56 Table 5. Binding energies (eV), surface atomic ratios and surface amount of Fe 3+ of fresh and spent FeZSM-5 catalysts taken on samples previously treated under vacuum or under hydrogen at 500 ~ S represents the catalyst tested with steam and WS the catalyst tested without steam Sample Cls Si2p A12p Fe2p3/2 Fe 3+ Fe/Si (atom) FeZSM-5 (vac) 284.6 103.5 74.6 710.0 0.015 711.7 37 (U2, 500~ 709.4 284.6 103.3 74.5 711.2 31 0.017 FeZSM-5 (S) (vac) 284.6 103.4 74.4 709.7 0.019 711.5 43 (H2, 500~ , 709.6 103.3 74.5 711.4 33 0.022 284.6 FeZSM-5 (WS) (vac) 284.6 103.4 74.5 709.7 0.018 711.4 43 (H2, 500~ 709.7 284.6 103.4 74.4 711.6 32 0.020

From these results it can be concluded that the higher activity of FeZSM-5 catalyst, as compared to FeMCM-41 one, can be ascribed to its higher amount of iron on the surface surface However, the FeMCM-41 catalyst seems to be more resistant against deactivation by coke blockage of the pores, since it has larger pores than the other.

524 CONCLUSIONS FeMCM-41 and FeZSM-5 catalysts are both active towards ethylbenzene dehydrogenation in the presence of steam. Particularly, FeZSM-5 catalyst is more active and selective than the FeMCM-41 one. The better performance of FeZSM-5 catalyst is due to the higher amount of iron deposited on the external zeolite surface and their ability in stabilizing the active oxidation state (Fe3§ on the surface. Finally, since the FeZSM-5 catalyst is more active and selective than pure hematite and is chromium and potassium-free, it is a promising candidate to commercial applications. ACKOWLEDGEMENTS The authors thank the financial support from PADCT/FINEP. J. R. C. B. and A. C. O. acknowledge their undergraduate scholarship. REFERENCES 1. Kirk-Othmer, Encyclopedia of Chemical Technology, John Willey and Sons, New York, 1984, p. 770. 2. E.H. Lee, Catal. Rev., 8 (1973) 285. 3. B.D. Herzog and H.F. Raso, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 187. 4. S. S. E. H. Elsanashaie, B. K. Abdallah, S. S. Elshishini, S. Olkowalter, M. B. Noureldeen and T. Aboudani, Catal. Today, 64 (2001) 151. 5. Ph. Courty and J.J. Le Page, in: B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Editors), Preparation of Catalysts II (Studies in Surface Science and Catalysis), Elsevier, Amsterdam, 1979, p. 293. 6. G.H. Riesser, U.S. Patent 4,144,197 (1979). 7. A.K. Vijh, J. Chem. Phys., 72 (1975) 5. 8. J.C. Gonzalez, M.G. Gonzb]ez, M.A. Laborde and N. Moreno, Appl. Catal., 20 (1986) 3. 9. L.J. Lobree, I. Hwang, J.A,. Reimer and A.T. Bell, J. Catal., 186 (1999) 242. 10. P. Fejes, J. B. Nagy, K. L~izar and J. Halask, Appl. Catal., 190 (2000) 117. 11. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Coorporation, Eden Prairie, 1978, p. 76. 12. S. Morup and H. Topsoe, Appl. Phys. 11 (1976) 63. 13. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramian and R.C. Mercader, Hyperfine Interactions, C4 (1999) 61. 14. J.A. Morice and L.V.C. Rees, Trans. Faraday Soc. 64 (1968) 1388.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

525

Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts. N.G. GaUegos, M.V. Cagnoli, J.F. Bengoa, A.M. Alvarez, A.A. Yeramihn and S.G. Marchetti

CINDECA, Fac. Cs. Exactas, Fac. Ingenieria, U.N.L.P., CIC, CONICET. Calle 47 N ~ 257 (1900) La Plata, Argentina. Two catalyst to be used in the Fischer-Tropsch reaction, using zeolite-L in potassic form as support of iron species were prepared through to different methods of impregnation with iron salt. X-Ray Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) in controlled atmosphere, between room temperature (RT) and 15 K, H2 chemisorption and Volumetric Oxidation (VO) were used to characterise the solids. The impregnation of the zeolite L under inert gas allowed to obtain a fraction ofFe ~ in contact with Fe 2+ ions that enhanced the activity of the sites. The two catalysts presented similar selectivity towards hydrocarbons and low chain growth. 1. INTRODUCTION It is well known that in iron catalysts supported on different solids such as A1203, SiO2, MgO and zeolites, it is not possible to obtain complete reduction to Fe ~ when the iron concentration is low (approximately < 10%w/w) [1, 2]. Therefore, most of the iron catalysts used in the Fischer-Tropsch synthesis have, in addition to Fe ~ intermediate iron reduction species like Fe 2+. However, the role of Fe 2§ on the activity and selectivity in the CO hydrogenation has not been studied yet. In this paper, a commercial Zeolite L in potassic form (ZLK) was used as metal support for CO hydrogenation. The choice of this support was carried out since this reaction is sensitive to the structure. This means that the activity and selectivity of the catalyst depend on its metallic crystal size [3]. Catalysts with a narrow size distribution lead to good selectivity towards to a desirable product. Making use of the structure of channels and cages ofzeolites it is possible to reach this purpose. In order to determine the influence of the intermediate iron reduction species on the activity and selectivity of the Fischer-Tropsch reaction, two Fe/ZLK catalysts were prepared by two different methods.

2. EXPERIMENTAL SECTION Two precursors were prepared using the commercial form of the ZLK (Tosoh Corp.), with the ideal unit cell composition of dehydrated form of K9A195i27072 and 290 m~/g of specific surface area. One of them, was obtained by dry impregnation in air of the

526 zeolite with aqueous solution (pH=0.5) of a concentration to yield a solid with 5.84% w/w ofFe. Then it was calcined following the programme described in [4]. This sample was called p-Fe/ZLK(a). The other precursor was obtained outgassing the support at 773K and 0.05 torr for 1 h to eliminate the water present inside the channels and cages of the zeolite. After this time the system was filled with ultra high purity He up to 500 torr. Then, the Fe(NO3)3.9HzO aqueous volume solution equal to the pore volume of the zeolite was added to yield a solid with 4.56% w/w of iron that was calcined in the same way that p-Fe/ZLK(a).This solid was called p-Fe/ZLK(v). Both precursors were reduced in H2 stream (60 cm3/min) from 298 to 698 K at 2.66 ~ and were kept at 698 K during 26 h. The resulting solids were named c-Fe/ZLK(a) and c-Fe/ZLK(v), and characterised by XRay Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) at 298 and 15 K, 1-12chemisorption and volumetric oxidation (VO). These last two techniques were performed in a conventional static volumetric equipment with grease-free vacuum valves. The Hz uptakes at the same initial pressure, but at different temperatures between RT and 673 K were measured to determine the temperature in which the adsorption capacity is maximum. Volumetric oxidation experiments are based on the conversion of all iron species in the sample to Fe203 when it was heated in an O~ atmosphere at temperatures higher than 620 K [5]. The M6ssbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 100mCi was used. Velocity calibration was performed against a 6-1am-thick ~-Fe foil. All isomer shifts (6) mentioned in this paper are referred to this standard. The temperature between 15 and 298 K was varied using a Displex DE-202 Closed Cycle Cryogenic System. All MSssbauer spectra of the catalyst were obtained in controlled atmosphere using a cell specially built for this purpose to be used inside the cryogen [6]. The spectra were evaluated using a least-squares nonlinear computer fitting program with constraint. Lorentzian lines were considered for each spectra components. The catalytic tests were carried out in a fixed bed reactor with a H~:CO ratio of 3:1, 543 K, 1 atm of total pressure, 20 cm3/min of total volumetric flow and a space rate of 0.25 s"l. The reaction products were analyzed by gas chromatography using FID and TCD as detectors, and a GS-Alumina capilar column and Chromosorb 102 packed column respectively. 3. RESULTS AND DISCUSSION

The preservation of the crystalline structure of the samples after impregnation and calcination process was checked analysing its X-Ray difraction patterns (not shown). The same peaks as those for ZLK were obtained, although the relative intensities of these varied slightly. These results may be due to a decrease of crystallinity after impregnation and calcination process and/or a lower crystallographic planes periodicity due to the presence of Fe oxides inside the zeolite channels. The specific surface area of the precursors is of 33 m2/g for p-Fe/ZLK(a) and 45m2/g for p-Fe/ZLK(v). The very important decrease in specific surface area in comparison with ZLK would indicate that a great fraction of iron species are located inside the zeolite channels reducing the pore mouth sizes. The M6ssbauer spectra ofp-Fe/ZLK(a) and p-Fe/ZLK(v) at 298 and 15 K are shown in Figure 1.

527 'I

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,,

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.

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r C

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8

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12

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-12

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Figure 1" M6ssbauer spectra of the precursors at 298 and 15 K. At 298 K a paramagnetic doublet is observed in both precursors, while an additional magnetic sextet is also displayed by p-Fe/ZLK(a). When the temperature is lowered to 15 K, a second magnetic signal is observed in p-Fe/ZLK(v). Instead, in p-Fe/ZLK(a) only two resolved signals are still noticed, but the background is significantly curved. The spectrum at 298 K of p-Fe/ZLK(v) have hyperfme parameters (Table 1) that can be assigned to two Fe 3+ species: small particles of (x-Fe203 and/or Fe 3+ ions exchanged with the support. When the temperature decreased to 15 K, it was possible to determine the existence of two magnetic signals assignable to the "core" (sextet with higher magnetic field) and "shell" (sextet with smaller magnetic field) of (~-Fe203 "clusters" [7]. Assuming homogeneous semi-spherical particles and using the ratio of the areas of the two sextuplets, it was possible to estimate an average "cluster" diameter of 1.1 nm. Therefore, these "clusters" could be located inside the channels of the zeolite L. This result, analyzed in connection with the DRX andBET results, mentioned above, confima the existence of a great quantity of iron oxide microcrystals situated inside the zeolite structure.

528 Table 1" M/3ssbauer hyperfine parameters of the precursors. Temp. 298 K

Specie s c~-Fe203

Parameters .... p-Fe/ZLK(v) p-Fe/ZLK(a) H(T) 51.2 + 0.1 8(mm/s) 0.38 + 0.02 2e(mm/s) ~ -0.24 + 0.03 Fe 3+'' 8(mm/s) 0.34~:0.01 0'.'32 _+0.0i ....... A(mm/s). . . . . . . . . . 0.90-a:0.0! 0.87 + 0.01 15 K a-Fe203 H(T) 49.3+0.1 53.7 + 0.1 ~5(mm/s) 0.50• 0.45 + 0.02 2e(mm/s) . . . . -0.06• .... 0.36 + 0,,.03 ot-Fe203 H(T) 46.3+0.1 46.3" 5(mm/s) 0.46• 0.47* 2e (mm/s) ...... -0.03+0.02 -0.01 * Fe 3§ ~5(mm/s) 0.46• 014i~ 0.01 ................... A(mm/s) 1.03• !.0! + 0.01 *Constant used for the fit. The remaining doublet was assigned to Fe 3+ exchanged with the support, and/or superparamagnetie ct-Fe203 particles. The hyperfme parameters (Table 1) at 298 K of p-Fe/ZLK(a) can be assigned to the same iron species than in p-Fe/ZLK(v). When the temperature decreased to 15 K, the spectrum displays a curved background probably originated in a fraction of small particles undergoing an incomplete magnetic splitting. The fitting was simulated with one sextet, one doublet and a second sextet of very broad lines [3]. The relative area of the sharper sextet corresponding to ct-Fe203 (11+_2%) is the same (within experimental errors) at RT and at 15 K. To estimate roughly the average size for this fraction, we applied the Collective Magnetic Excitation Model (CMEM) [8]. A diameter of 20 nm is obtained. This value indicates that these particles must be located out of the channels of the zeolite. Although the fitting procedure is a rough approximation to the physical process actually taking place, the method yields an estimate of the fraction of the particles in the relaxing magnetic regime (55+7 %). Since these particles at 15 K have not reached the degree of magnetic order of the p-Fe/ZLK(v) particles, their size must be even smaller than 1.1 nm. Figure 2 shows the M6ssbauer spectra in controlled H2 atmosphere of both catalysts c-Fe/ZLK(v) and c-Fe/ZLK(a) at 298 and 15 K. At RT both display a magnetic sextet and several intense and highly overlapped central signals. The spectra were interpreted in terms of a superposition of one magnetic sextet, one paramagnetir doublet and one singlet. In addition to the above mentioned signals, other doublet appears in c-Fe/ZLK(v). When the temperature decreases to 15 K the spectra show the presence of two magnetic sextets, a paramagnetic doublet and a superparamagnetic singlet for both solids, and c-Fe/ZLK(v) displays an additional magnetic sextet. The values of the hyperfme parameters at 298 and 15 K are shown in Table 2. At 15 K, the values are characteristic of magnetic Fe ~ (Fe~ Fe304, Fe 2+ exchanged with the support and superparamagnetir Fe ~ (Fe~ [9, 10]. The additional sextet in c-Fe/ZLK(v) is assigned to Fe 2+ ions considering its isomer shift value. Since, we found the Fe 2+ signal magnetically splitted and its hyperfine magnetic field is very similar to the ct-Fe ~ value, we

529 think that this species is magnetically coupled with Fe~ Therefore, the Fe 2+ would be decorating the F e ~ [9]. The weak magnetic signal, assignable to Fe304, that can be seen in the MS at 15 K, can be attributed to the incomplete reduction of the oxides. Its quantity is too small to sort out one more interaction in the spectrum fitted at RT from the statistical noise. The Fe~ existence at so low temperature such as 15 K indicates the presence of very small particles of Fe*, at least smaller than ~2.9nm [11] in both catalysts. The presence of the Fe~ fraction aRer a reduction treatment suggests that these microcrystals are located inside the channels of the zeolite since this situation would avoid the sintering process. The same amount of Fe ~ inside the support (Table 3) is achieved in both catalysts, although in p-Fe/ZLK(v) all the iron oxide is inside the channels and in p-Fe/ZLK(a) there is a fraction of iron oxide out of the zeolite structure. Therefore, in the former sample a percentage of the iron crystallites have migrated to the external surface, during the reduction step.

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Figure 2: M6ssbauer spectra of the catalysts at 298 and 15 K.

530 Table 2: Hyperfine M6ssbauer parameters of c-Fe/ZLK(v) and c-Fe/ZLK(a) ~

Species Fe ~ (magnetic)

Parameters

...... c-Fe/ZLK(v) ....... c,Fe/ZLK(a ) 298 K 15 K 298 K 15 K H (T) 33.2-~0.1 34.2+0.1 33.0+0.1 34.1_+0.1 8(Fe) (mm/s) 0.00+0.01 0.12+0.01 0.01+0.01 0.11+0.01 2e ( ~ s ) -0.01+0.01 -0.01a:0.01 0.00" 0.00" H (T) 49.5+0.6 48.6+0.7 8(Fe) (mm/s) 0.58+0.08 0.77+0.09 .

FesO4 (magnetic)

(mnVs) Fe 2+ (m) (Coupled with Fe~ (m)) Fe 2+ (exchanged) Fe 2+

H (T) 8(Fe) (mm/s) 2e (mm/s) A (mm/s) 8(Fe) (mm/s) A (mm/s) 8(Fe) (mm/s) LVe0sp 8(Fe) (mm/s) *Constant used for the fit

o.oo*

1.59+0.03 1.18+0.01 0.'44+0.04 1.15+0.02 0.04+0.01

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There is a good agreement between the experimental 02 uptake for the complete reoxidation of the reduced catalysts and the consumption of 02 calculated from the percentage of each species obtained fi'om the M6ssbauer spectra at 15 K (Table 4). The cross-checking of volumetric oxidation results with the complex MOssbauer spectra of these catalysts is the only reliable method that should be used in spectra of such complexity if one has not the capability to take "in situ" spectra of samples with an external magnetic field. Other choices for assignments of the iron species, different from those of Table 2, lead to unacceptable differences between both techniques. From these results, it can be deduced that although we obtained two precursors with the same iron oxide species, the catalysts have different iron species aRer reduction. The number of Fe ~ surface atoms, was determined by 1-12chemisorption at 673 K in 10-80 Torr pressure range. The high H2 consumption observed allowed us to verify the existence of very small metallic Fe crystals inside the channels [10]. Assuming that hydrogen atoms are chemisorbed only on Fe ~ surface atoms and considering that there are two fractions of Fe ~ crystals [12], it is possible to estimate their average diameter values. Assuming a semi-spherical shape, the fraction of microcrystals located inside the zeolite structure cannot exceed 2.6 nm diameter considering the dimension of the channels. This fraction is superparamagnetic (Fe~ in the M6ssbauer spectra of both Table 3" Percentages of iron species obtained by MSssbaucr spectroscopy at 15K

c-Fe/ZLK(v) c-Fe/ZLK(a)

Fe ~ FesO4 ,,(magnetic) (magnetic) 30-A:2 5q-1 45+3 13+5

Species (%) Fe 2+(Coupled with Fe~ 13+1 ---

Fe 2+ (exchanged) 38• 27+6

Fe~ 144-1 15+4

531 Table 4: Values of O2 uptake, H2 chernisorption and Fe~ crystal diameter of catalyts.

c-Fe/ZLK(v) e-Fe/ZLK(a)

Experimental 02 uptake (~tmol O4g) 496:525 521:556

Theoretical O~ uptake (txmol O2/g) 442:514 553:542

Experimental Fe~ Diameter H2 ehemisorption (nm)

(/.tmol H2/g) 31 62

>_14.3 6.0-13.0

samples, and their percentages were obtained from these spectra. The more important structural difference between both catalysts is the presence of Fe 2§ decorating the extemal Fe ~ surface crystals in c-Fe/ZLK(v). The existence of this fraction of Fe 2§ leads to a decrease of the Fe~ and Fe304 amounts. On the other hand, the quantity of Fe ~ internal crystals (that represents the 80% of the total active sites) is the same in both catalysts. The activity and selectivity results are shown in Table 5. The tumover frequencies to total hydrocarbons of both catalysts were obtained assuming one active site per Fe~ surface atom. In the pseudo-steady state, the activity per site is about three times higher in cFe/ZLK(v) than in c-Fe/ZLK(a), and the activity per gram is twice higher in e-Fe/ZLK(v) than in c-Fe/ZLK(a). Since the inner Fe~ fraction is equal in both catalysts, the activity difference between them cannot be assigned to this fraction. In consequence, the different behaviour of the catalysts may be attributed to the presence of Fe 2+ decorating the Fe~ surface of crystals located outside the channels in fresh c-Fe/ZLK(v). Theoretical models have demonstrated that the main effect of cations in contact with a metal is an electrostatic one [ 13], which is essentially of short range. However, a long range effect is possible as a result of a cumulative electrostatic field, generating zones of minimum potential energy at the surface. Consequently, the bond between the Fe ~ and the CO adsorbed becomes stronger, while at the same time, the intra-moleeular CO bond is weakened, increasing the catalyst activity. After 48hs of reaction the Fe~ is carburized in both catalyst and in c-Fe/ZLK(v) the sextet of Fe2+ appears magnetically coupled with z-FesC2 maintaining the promoter effect of this species. This behaviour was demonstrated by MS in controlled atmosphere (not shown spectra). The olefirgparaffm ratio is similar for both catalysts. These results can be justified taking into account that the conversion values and the support basicity are practically equal in both samples Finally, similar methane production and Schulz-Flory coefficients ((z) are observed in both catalysts. Therefore, it can be deduced that Fe 2+ ions magnetically coupled with Fe~ Table 5" Activity and selectivity results

Total hydrocarbon molecules/site.see xl 04 Total hydrocarbon moleeules/g.sec xl 0"~6 CO conversion (%) Olefin/paraffln ratio(without CI-I4) cn4 (%) Schulz-Flory coefficient (a)

c-Fe/ZLK(v) 8.90 3.32 1.7 1.40 38 0.22

c-Fe/ZLK(a) 2.60 1.94 1.5 2.34 42 0.30

532 do not influence on the catalysts selectivity and chain growing. Bearing in mind that the 80% of the total active sites correspond to Fe~~, the low ot coefficient values can be justified taking into account that on very small metallic particles, the chain propagation finishes at low molecular weight hydrocarbon (up to C4) [14]. The small metallic crystal size avoids the presence of enough CHx neighbour groups to produce the propagation chain, although steric impediments inside the pores of the zeolite do not be ruled out. 4. CONCLUSIONS Through two different impregnation methods we obtained two catalysts with only one structural difference: the Fe2§ ions magnetically coupled with Fe located outside the channels of the zeolite ZLK. These ions enhanced the activity of the Fe~ sites for the total hydrocarbon production by an electrostatic effect favouring the CO dissociation. Instead, the selectivity and chain growth is not modified by the presence of these ions. REFERENCES

1. G.B. Raupp and W.N. Delgass, J.Catal., 58 (1979) 337. 2. M.V. Cagnoli, S.G. Marchetti, N.G. Gallegos, A.M. Alvarez, R.C. Mercader and A.A. Yerami~, J. Catal, 123 (1990) 21. 3. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramihn and 1LC. Mercader, Hyperfme Interactions (C), 4 (1999) 61. 4. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramiatt and M Schmal, Actas do XVII Simp6sio Ibero-americano do Cat~ilise (J.M.0rfiio, J.L. Faria, J.L. Figueiredo, Eds.), p.97, Porto, Portugal (2000). 5. M. Boudart, A. Delbouille, J.A. Dumesic, S. Khamrnouma and H. Topsoe, J.Catal. 37 (1975) 486. 6. S. G. Marchetti, J. F. Bengoa, M. V. Cagnoli, A. M. Alvarez, N. G. Gallegos, A. A. Yerami~n and R. C. Mercader, Meas. Sci. Tech. 7 (1996) 758. 7. M.Vasquez-Mansilla, R.D. Zysler, C. Arciprete, M.I. Dimitrijewits, C. Saragovi, J.M. Greneche, J. of Magnetism and Magnetic Materials, 204 (1999) 29. 8. S. Morup and H. Topsae, Appl. Phys. 11 (1976) 63. 9. M.V. Cagnoli, N.G. Gallegos, A.M. Alvarez, J.F. Bengoa, A.A. Yeramihn and S.G. Marchetti, Studies in Surface Science and Catalysis, 135 (2001) 272. 10. A.M. Alvarez, S.G. Marchetti, M.V. Cagnoli, J.F. Bengoa, R.C. Mercader and A.A. Yerami~a., Applied Surface Science, 165 (2000) 100. 11. F. Bodker, S. Morup, M.S. Pedersen, P. Svedlindh, G.T. Jonsson, J.L. Garcia-Palacios and F.J. Lazaro, J. Magn. Magn. Mater., 925 (1998) 177. 12. S.G. Marchetti, M.V. Cagnoli, A.M. Alvarez, J.F. Bengoa, R.C. Mercader and A.A. Y~eramihrg Appl. Surf. Sei., 165 (2000) 91. 13. J.W. Niemantsverdriet, "Spectroscopy in Catalysis" VCH, Weinheim, (1995), p. 219. 14. L. Guczi, in "New Trends in CO Activation", Studies in Surface Science and Catalysis, Ed. L. Guezi, Elsevier, 64 (1991) 350.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

533

On the necessity of a basic revision of the redox properties of H-zeolites Z. Sobalik, P. Kubhnek, O. Bortnovsky, A. Vondrovh, Z. Tvarfi~kovh, J.E. Sponer, and B. Wichteflov/t J. Heyrovsl@ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, CZ-182 23 Prague 8, Czech Republic. Negligible activity in NO-NO2 equilibration and SCR of NOx by propane, as well as benzene hydroxylation and N20 decomposition reactions was obtained over laboratory synthesized H-MFI, H-FER and H-BEA zeolites containing Fe with concentration below 50 ppm. On the other hand significant activity was found over H-zeolite samples of the same structural type and similar Si/Al values, but with the content of iron impurities of several hundreds of ppm. The consequence of this evidenced role of such low levels of iron content, i.e. at the level usually present in the commercial samples mostly used for the catalytic studies, and not considered in the analysis of the redox function of these systems, is discussed. INTRODUCTION There is generally accepted assumption that the protons of H-zeolites display redox properties, as H-forms of zeolites have been shown to exhibit catalytic activity in typical redox reactions. The list of such reactions includes NO-NO~ equilibration [ 1-3], selective catalytic reduction of NOx by paraffins [4], N20 decomposition, and benzene hydroxylation to phenol using nitrous oxide [5] and hydrocarbons oxidations. Reaction mechanisms of these reactions over H-zeolites are still not fully understood, nevertheless in some of them the zeolitic proton redox activity is expected to take part. Most elaborated are arguments in support of the proton redox activity in the NO-NO2 equilibration, a reaction connected with the selective catalytic reduction of NOx (SCRNOx) by paraffins. It has been recognized that the NO2 formation constitutes the prerequisite for effective reduction of NO to nitrogen by propane over H-ZSM-5 [4,6]. As no other active function was assumed in the H-form of the zeolite used for this reaction, the redox activity of the proton of bridged OH was postulated [ 1,3,4] to provide for the fairly high activity of H-ZSM-5 in the NO/O~/C3H8 reaction (see Ref. [2]). The concept of the redox function of the bridged proton was further detailed by Lukyanov et al. [3] providing direct experimental evidence of a high activity of a commercial H-ZSM5 sample in NO-NO2 equilibration, which yielded similar activity as ion exchanged CoZSM-5. Although the concept of redox activity of protonic sites has been generally accepted, there was always some base for a criticism of such view. It was indicated by

534 several experiments; e.g. Shelef [7], found the H-ZSM-5 sample nearly inactive in the NO oxidation. Less straightforward is the evidence on the active site for N20 decomposition and related benzene hydroxylation by N20. There are several controversial suggestions in the literature on the nature and structure of active sites in zeolite catalysts for hydroxylation of benzene to phenol by N20. Ono et al. [8], and Burch and Howitt [9] suggested the Broensted acid sites in H-ZSM-5 to be the active sites, and radical mechanism has been proposed to take place with participation of zeolite protons [8]. Strong acidic A1-Lewis sites, created during high-temperature dehydroxylation or under steaming of H-ZSM-5 were suggested to be the active sites for stabilization of an atomic oxygen, formed by N20 decomposition, on unsaturated aluminium, and its transfer to benzene molecule [10,11 ]. It could be summarized that in accordance with the concepts of these authors, the H-zeolites contain all necessary active functions to run the reaction. However, most of these studies were carried out with parent commercial zeolite samples with unspecified content of iron impurities. On the other hand a mechanism has been proposed, which includes redox function of Fe-connected sites and role of protons in activation of hydrocarbon. Extra-framework dinuclear iron species in ZSM-5 were suggested by Panov et al. [12,13] to be the active sites transferring so called a oxygen formed by N20 decomposition to benzene. Nott6 [14] also assumed, that the presence of Broensted acid sites is essential for benzene to phenol hydroxylation together with the a-sites, primarily linked to the presence of iron. In the presented paper we attempt to evaluate whether the occurrence of iron traces, always present in the commercial zeolites up to the range of hundreds of ppm, could have basically influenced the conclusions on the redox activity of H-zeolites, as found in the literature. For this purpose a set of test reactions, i.e. NO equilibration, SCR of NOx by propane, N20 decomposition and benzene hydroxylation was used. 1. EXPERIMENTAL SECTION

1.1 Catalyst preparation Representative collection of the frequently used zeolite system for the mentioned reactions were chosen, i.e. ZSM-5 (MFI), ferrierite (FER), and beta (BEA), see Table 1. Table 1. Commercial zeolite samples Sample

Si/A1

Fe (ppm)

Producer

LOT

ZSM-5

39 14.1 12.5 8.5 12.7

410 180 200 170 250

PQ~) Slovnaft 2) Slovnaff TOSOH 3) PQ

ZH-5

FER BEA

010812B HB-8-25

pQ Corporation, U.S.A., 2)Research Institute of Petroleum and Hydrocarbon Gases, SR; 3) TOSOH Corporation, Japan.

1)

535 Table 2. Laboratory prepared pure zeolites Sample

Si/Al

CFe, ppm ~)

MFI MFI FER BEA

28 19.5 8.2 11.9

30 50 50 26

1) The data given are based on the average of three analyses. The error of the given values for Fe analysis was estimated to be +5 ppm. Laboratory synthesized zeolites MFI, FER and BEA (see Table 2) were prepared using materials with iron content below 1 ppm of Fe. If necessary the prepared zeolites were transformed into NH4-form by 4-fold ion exchange by 1M NH4NO3 071uka, Fe contents 1 ppm) in demineralized water. MFI samples were prepared using a solution of A1 nitrate (Fluka) in deionized water, tetraethyl orthosilica (Fluka) and ethanol (laboratory grade). The mixture was stirred for 90 min, and a solution of tetrapropylammonium hydroxide in water (20% - Fluka) was added; the resulting mixture was stirred for 90 min. The reaction gel crystallized for 72 hrs at 170 ~ FER was synthesized using fumed silica (Cab-o-Sil) and pure metallic A1 dissolved in solution of sodium hydroxide (Merck) and potassium hydroxide (Fluka). No organic template was used in this synthesis. BEA was synthesized with fumed silica (Cab-o-Sil), aluminium nitrate (Fluka) and TEAOH (35 wt.%, Aldrich). XRD analysis indicated highly crystalline materials for all the samples. The elemental analysis was done by X-ray fluorescence spectroscopy and inductively coupled plasma emission spectrometry. 1.2. Structural characterization

FTIR spectra of samples in the form of self-supported pellets (- 5 mg/ cm 2) were recorded at room temperature on a Magna-IR System 550 FTIR Nicolet with a lowtemperature MCT-B detector using heatable cell connected to a vacuum/gas manifold. Analysis of the transmission window region evidencing the local perturbation of the zeolite framework due to cation presence in the cationic positions was based on the data for Fe-FER, Fe-BEA [15] and Fe-MFI [ 16]. The concentration of Broensted and Lewis sites in zeolites was determined by quantitative analysis of the spectra of adsorbed d3acetonitrile using the extinction coefficients listed in ref. [ 17]. X-band ESR spectra of Fe(III) ions were monitored at RT on an ESR spectrometer ERS-220, Germany. As an internal standard Mn(II) ions in solid matrix were used. The samples were before the spectra measurements pretreated in a stream of dry oxygen at 480 ~ for 2 hrs, then cooled to RT and evacuated for 30 minutes at 10.2 Pa, and sealed. Model calculations of simplified model of Fe cationic site were performed employing DFT/Becke3LYP level of theory using the Gaussian98 suit of programs.

536

g2.3

~

~i~

~~f''~

Figure 1 X-band ESR spectra of MFI with 30 and 410 ppm of Fe dehydrated at 450 ~ the H-ZSM-4 with 1200 ppm Fe added by ion exchange taken from Ref. [21 ]

Spectra of

1.3. Catalytic tests

The catalytic activity tests were carded out in a flow catalytic setup using a fixed-bed through-flow mieroreactor (total flow of the gas mixture 100 ml/min) with 0.4 g (NO/NO2, SCR, N20 decomposition) or 0.5 g (benzene hydroxylation) equipped with an on-line chemical analysis of the inlet and outlet streams, combining a standard GC analysis (HP 5890II series, model G1540A) (all reactions), with NOx/NO ehemiluminiseenee analysis (Analyser Vamet 138, CR) (NO-NO2, SCR and N20 decomposition). Prior to the catalytic tests of NO-NO2 equilibration, SCR-NOx, or N20 decomposition the catalysts were heated for 4 hours in the stream of He, and then the system was stabilized in a reaction mixture at 350 ~ until a stable catalytic performance was reached (usually over 3 h). Then the activity was measured between 150 and 450 ~ A stable activity was usually reached after about 1 h and was stable for several hours. Composition of the inlet stream in NO-NO2 equilibration test consists of 1000 ppm NO and 2.5 % 02, balanced by He to a total flow of 100 ml/min. The catalytic tests of SCR of NOx by propane was carried using stream consisting of 1000 ppm of NO, 1000 ppm of propane, 4% of 02, total flow of 100 ml/min in He. The gas mixture of 1000 ppm N20 in He was used for tests for N20 decomposition. The reaction mixture for benzene hydroxylation contained 20 vol. % of N20, 20 vol. % of benzene, and nitrogen as a balance. Prior to the reaction the catalyst was activated in a stream of oxygen at temperature of 480 ~ for 1.5 hr and then in a nitrogen stream at the reaction temperature

537 for 30 rain. The reaction was carried out at 350 ~ It has been checked that the activity of the empty reactor (filled by glass balls) was negligible for any of the reactions under the experimental conditions used. 2. RESULTS AND DISCUSSION

2.1.Characterization of samples The iron content in the commercial samples was between 170 ppm, and 410 ppm. By using very pure materials for zeolites synthesis was possible to suppress the content of iron to or below of 50 ppm. The total acidity of the zeolites was consistent with the Si/A1 value. Brondsted acidity prevails on both the H-ZSM-5 and H-FER samples, while higher proportion of Lewis acidity was found in the H-BEA samples (see discussion in

Ref. [18]). Structural characterization of the iron species at the level of several hundreds ppm provided only a limited information. The FTIR method was based on the approach already presented in Ref. [15]. The ,,transmission window" region between 980 and 850 cmlwas previously shown to evidence bonding of divalent metal cation to the local framework structure of the cation position (see [19]. A band at 915 cm1 was identified for both dehydrated Fe-FER and Fe-BEA samples, with intensity proportional to the iron cation content [Montp.]. For FeZSM-5 samples this band was found at 928 cmq [ 16]. Actually a very weak band at 915 cmq was found in the commercial FER sample with 170 ppm of Fe. This band was absent in the laboratory prepared FER with 50 ppm of Fe. This evidences that at least a part of the iron present in the commercial FER is placed in the regular cationic position. No band in this region was identified in either BEA or ZSM-5 samples. However, no conclusion could be drawn from this finding, as the intensities of the band due to local perturbation of the BEA and ZSM-5 samples are generally weaker than in FER, and thus the concentration of Fe in cationic sites at the commercial samples is probably well below the detection limit. ESR spectra indicate that the H-MFI zeolites with 30 ppm and 50 ppm Fe contains iron predominantly in the form of oxide-like species (broad signal at g 2.3, AH 130 mT). In samples containing 180 and 410 ppm Fe a signal at g 4.3, AH 5 mT, of Td coordinated Fe at cationic sites appeared, in addition to a broad signal (see Fig. 1). At higher Fe concentration (1200 ppm) additional signals at g 6.0 and 5.6 have been reported [21] and related to the hydroxylation of benzene. These can be hardly seen at the low Fe concentrations. It could be summarized that a direct structural analysis of such low iron content could provide only a limited evidence suggesting the presence of iron in both the ion-exchange position and as the oxide-like species. With the absence of a reliable structural data providing for identification of the active species, (i.e. single cations, defective oxides, dinuclear species ), it could be speculated that the suggested model calculations of the proposed structures could provide some insight. Actually, both the Fe (II) and Fe (III) in the cationic position displayed according to the model calculation extreme charge transfer effects between the zeolite framework and the metal, thus actually bringing the iron cation reduction. As a result, the computed NBO charges of Fe(II) and Fe(III) were 0.91 and 1.8, respectively.

538 2.2. Catalytic results

NO-N02 equilibration. The NO-NO2 equilibration of the NO-O2 mixture (the results expressed as a fraction of the NO2 content at the equilibrium composition of the gas mixture; see Tab. 3) indicated high activity of the commercial zeolites and low (FER) or negligible for pure samples (MFI, BEA). Despite the expected difference in the state of Fe impurities and difference in the zeolite structure, the catalytic performance in this reaction was roughly proportional to the amount of iron present in the zeolite. SCR of NOx by propane.As typical for SCR-NOx by paraffins, the temperature profile is characterized by a bell shape curve with maximum at about 350 ~ (not shown). Typically the declining part of the curve is connected with a complete consumption of paraffin. To eliminate the role of this parameter the data used here for evaluation of the role of iron impurities presence are those taken at lower temperature, eliminating thus the role of changing NOx/paraffin ratio. The data on the commercial and pure analogs obtained at 300 ~ are summarized in Table 3. It should be stressed that these data were obtained working with the concentration of NO2 in the inlet of the reactor below 100 ppm. Previous results have shown negligible role of high iron content for the activity of H-zeolites in SCR of NO2 rich streams [22], indicating that the reaction of propane with NO2 run exclusively on the protonic sites of a zeolite. Accordingly, the change in the performance of the two classes of samples (pure vs. commercial) in the NO/O2/propane mixtures resides exclusively in the role of iron impurities in NO to NO2 oxidation. Table 3. Summary of the data on catalytic performance of the zeolites in the NO-NO2 equilibration, SCR of NOx by propane, N20 decomposition and benzene hydroxylation. zeolite

eft, ppm

MFI

30 50 180 410

FER BEA

Reaction N20 decomposition2), %

NO-NO2 equilibration 1), %

SCR-NOx 2), %

1.5

9

0

20

19

3

50 170

11

10

2

39

32

17

26 250

2 52

19 60

Benzene hydroxylation3), mmol/g.h 0.5 0.8 6.4 7.5

1)Ratio of the equilibrium NO2 concentration at the outlet; 2)NOx or N20 conversion, %; 3)Rate of Phenol formation, T-O-S 25 rain, mmol/g.h

539

N20 decomposition. Only a limited set of experiments is presented here. Nevertheless, it is documented that by increasing iron content to the level of the commercial FER sample would provide for appreciable N20 decomposition. Benzene hydroxylation. Nearly all literature data on benzene hydroxylation have been accumulated over H-MFI type of catalysts. Accordingly, the experiments were limited also here to this material. Laboratory prepared H-ZSM-5 zeolites with very low iron content (30 and 50 ppm Fe), exhibited very low activity in benzene hydroxylation, in contrast to the commercial samples containing 180 and 410 ppm Fe. As shown previously [21], the concentration of protons in the zeolite itself does not correlate with the activity for benzene hydroxylation. It could be summarized, that in all the reaction studied, i.e. NO-NO2 equilibration, SCR-NOx in the NO/O2/propane mixture, N20 decomposition and benzene hydroxylation, a decrease of the iron content below the level typical of the standard commercial products to about 50 ppm brings about a striking suppression of the catalytic activity of the H-zeolite structures studied. Based on the results it could be speculated, that in the two categories of reactions, i.e. connected to NO or N~O species, the simpler processes, i.e. NO or N20 activation, are directly and exclusively connected with the iron active sites, while in the more complex processes, including the propane or benzene activation, i.e. SCR-NOx in NO/O2/propane mixture or benzene hydroxylation, the iron active sites cooperate with the zeolitic protons. By assuming the iron present as active site for the individual reactions would provide a very high TOF values (between 20 - 90 h-l), a value highly above the TOF value of standard catalysts for such reaction systems. Moreover, the ESR findings indicate that only small fraction of iron present bear catalytic function, thus bringing the actual value of TOF to even higher values. Based on the presented data we can speculate that the list of reactions, where the role of the iron impurities in the parent H-zeolite was overlooked in the analysis of the reaction mechanism, would go farther. Actually, we would propose the well-known positive effect of the oxygen traces in cracking over H-zeolites as a next candidate for such analysis. CONCLUSSIONS Using a set of commercial zeolite samples and laboratory prepared pure zeolites a role of iron impurities in redox activity of H-zeolites was proven. Only a limited structural analysis on this iron concentration level could be provided, and even these are based on extrapolation of the tendencies from the studies carried out at higher concentration levels. The evidence has been given suggesting, that, using the standard commercial parent samples and omitting in the data analysis the presence and contribution of the iron impurities to the catalytic reactions, could influence understanding of the redox reactions over H-form of zeolites. All this brings us to the conclusion that there exists a necessity to make a basic revision of the data on the redox properties of the H-zeolites as presented in literature. Actually, we suggest that in most cases the already published results should be re-evaluated after replacing for the new analysis the parent "H-zeolite" by a correct HFe-zeolite.

540

Acknowledgments Financial support from the Grant Agency of the Czech Academy of Sciences (Grants A 4040007 and S 4040016) is gratefully acknowledged.

REFERENCES 1. I. Halasz, A. Brenner, K.Y.S. Ng, and Y. Hou, J. Catal. 161 (1996) 359. 2. A.Y. Stakheev, C.W. Lee, S.J. Park, and P.J. Chong, Catal. Lett. 38 (1996) 271. 3. D.B. Lukyanov, G. Still, J.L. d'Itri, and W.K. Hall, J. Catal. 153 (1995) 265. 4. M. Sasaki, H. Hamada, Y. Kintaichi, and T. Ito, Catal. Lett. 15 (1992) 297. 5. V. Zholobenko, I.N. Senchenya, L.M. Kustov, and V.B. Kazansky, Kinet. Catal. Eng. Ed. 32 (1991) 132. 6. H. Hamada, Y. Kintaichi, M. Sasaki.,T. Ito, and M. Tagata, Appl. Catal. (1990) L1 64. 7. M. Shelef, C.N. Montreuil, and H.W. Jen, Catal. Lett. 26 (1994) 277. 8. E. Suzuki, K. Makasiro, and Y Ono, Chem. Soc. Jap. Chem. Commun. (1988) 953 9. R. Butch and C.Howitt, Appl. Catal A: General 103 (1993) 135. 10. J.L. Motz, H. Heinichen, W.F. Hoelderich, J. Mol. Catal. A: Chemical 136 (1998) 175. 11 L.M. Kustov A.L Tarasov., V.I Bogdan., A.A Tyrlov, and J.W. Fulmer, Catal. Today, 61 (2000) 123. 12 G.I. Panov, G.A. Sheveleva, A.S. Kharitonov, V.N. Romannikov, L.A. Vostrikova, Appl. Catal., A:Oeneral, 82 (1992) 31. 13. O.I. Panov, V.I. Sobolev, K.A, Dubkov, A.S. Kharitonov, J. Hightower, W.N. Delgass, E. Iglesia, A.T. Bell, Proc. 11th Inter. Congr. Catal., Baltimore, 1996, Stud. Surf. Sci. Catal., 101 (1996) 493. 14. P. Notte, Topics Catal., 13 (2000) 387. 15. Z. Sobalik, J.E. Sponer, and B. Wichterlovh, in A. Corma, F.V. Melo, S. Mendioros, and J.L.G. Fierro (Eds.), Studies in Surface Science and Catalysis, Vol. 130B, Elsevier, Granada, (2000) 1463 16. H.Y. Chen, X. Wang, and W.M.H. Sachtler, Phys. Chem Chem. Phys., 2 (2000) 3083. 17. B. Wichterlov/t, Z. Tvarfi~kovfi, Z. Sobalik, and P. Sarv, Microporous Mesoporous Mater., 24 (1998) 223. 18. O. Bortnovsky, Z. Sobalik, and B. Wichterlov~, Microporous. Mesoporous. Mater., 46 (2001) 265. 19. J.E. Sponer, Z. Sobalik, J. Lesczynski, and B. Wichterlovh, J. Phys. Chem. B, 105 (2001) 8285. 20. Z. Sobalik, J. E. Sponer, Z. Tvarfi~kov~i, A. Vondrovh, S. Kuryiavar, and B. Wichterlov~i, in A. Galameau, F. Di Renzo, F. Fajula, and J. Vedrine (Eds.),Studies in Surface Science and Catalysis Vol. 135 Elsevier, Montpellier, (2001) 136. 21. P. Kubhnek, B. Wichterlovfi, and Z. Sobalik, J Catal., submitted. 22. Z. Sobalik, J.E. Sponer, A. Vondrovit, Z. Tvarfi~kovfi, and B. Wichterlovb., Catal. Today, in press.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

541

The role of zeotype catalyst support in the synthesis of carbon nanotubes by CCVD K. Hernadi a*, Z. K6nya a, A. Siska a, J. Kiss b, A. Oszkd b, J. B.Nagy c and I. Kiricsi a aUniversity of Szeged, Applied and Environmental Chemistry Department, H-6720 Szeged, Rerrich Bela ter 1, Hungary bReaction Kinetics Research Group of the Hungarian Academy of Sciences, Department of Solid State and Radiochemistry, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary CLaboratoire RMN, Facult6s Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B5000 Namur, Belgium

The effect of zeotype support on the selectivity of carbon nanotube formation in the catalytic decomposition of acetylene was investigated. Catalyst supports with various pore diameters were tested. Formation and the quality of carbon deposit were followed by transmission electron microscopy (TEM) and the state of supported catalyst particles was investigated by in situ X-ray photoelectron spectroscopy (XPS) measurements. It was found that only catalyst particles deposited on the external surfaces of porous support could efficiently take part in the catalytic carbon nanotube formation.

1. INTRUDU CTI O N The catalytic chemical vapor deposition (CCVD) method for the production of carbon nanotubes is of great interest among researchers since it gives large quantity, good quality single- (SWNT) and/or multi- (MWNT) wall carbon nanotubes. In this procedure simple hydrocarbons as methane, ethylene, acetylene, or benzene, toluene

Authors thanks to the European Commission (RTN Program, NANOCOMP network, RTN 11999-00013), to the Hungarian Ministry of Education (FKFP 0643/2000) and to the National Science Foundation of Hungary (OTKA T25246).

542 were used predominantly [1]. Transition metals, most frequently Fe, Ni or Co, supported on oxides or zeolites were the catalyst precursors [2]. When bimetallic catalyst w a s used, alloy phase was formed, which was supposed to be the active component of the catalyst. The relatively high yield and excellent quality of carbon nanotubes were explained by the peculiar behavior of this alloy phase [3]. Recently, several papers dealt with the mechanism of nanotube formation [4]. Particularly, the role of the catalyst support and the particle size of the metal have been discussed [5]. The most frequently used catalyst supports were silica, zeolites and alumina. Well-crystallized carbon nanotubes were formed on catalysts supported by these materials [6]. A part of these supports are molecular sieves having sharp pore diameter distribution in molecular dimensions (0.4-1 nm), pore diameter of the others is much larger. The role of pore structure of the support in the formation of nanotubes is one of the most intriguing problems to be answered. Structural and textural properties of pyrolitic carbon formed in the inner pores of zeolitic structures have been studied [7]. In this paper we present results on the role of zeotype catalyst support and the state of the metal in the CCVD production of MWNT.

2. EXPERIMENTAL

2.1. Preparation and characterization of catalysts Zeolites (NaA: Hungalu Co., KL: Union Carbide, NaY: Union Carbide, 13X: Union Carbide) and mesoporous zeolite-like materials (MCM-41, SiMCM-48, A1MCM-48: synthesized in our laboratory [8-9]) were loaded with metal ions using ion exchange, impregnation and isomorphous substitution. After evaporating the solvent, the catalyst sample was dried at 400 K overnight. Catalyst samples prepared by the impregnation method contained 2-5 wt% of Co. Since these materials have high ion-exchange capacity, and when they are in contact with cobalt ion containing solution, ion exchange immediately starts. Upon drying the solution onto the zeolite the ion concentration in the solution increases, consequently, the ion exchange in the zeolite goes to completion. After this point, extra ion incorporation takes place if the initial Co content of the solution used for the preparation of a given amount of zeolite is larger than the ion exchange capacity of the zeolite. This is the source of Co ions on the outer surface of the zeolite catalysts. Ion exchange of A1MCM-41 in aqueous solution of Co acetate (0.1 molYdm3) was the preparation procedure for CoA1MCM-41(ex) catalyst. Ion exchange was performed twice at 343 K for 12 h each time (0.5 mmol metal ion/g silicate). CoMCM-41(iso) sample was prepared by isomorphous substitution of Si for Co

543 following the description in [10]. Co/A1MCM-41, SiMCM-48 and A1MCM-48 were prepared by impregnation. Calculated amount of Co salt was dissolved in distilled water, which was evaporated slowly under gentle heating. The composition of the catalyst was checked by X-ray fluorescence (XRF) analysis. The transition metal content was determined by classical analytical methods. The zeolite samples and the MCM materials showed the characteristic X-ray diffraction (XRD) pattern. The BET surface areas of the samples were determined by N2 adsorption isotherms measured at 77 K using a volumetric apparatus. For the MCM samples, the pore size distribution was calculated by the Barett-Joyner-Halenda method [11] from the adsorption data. 2.2. Synthesis of carbon nanotubes The catalytic reaction was carried out in a fix-bed flow reactor in the temperature range of 900-1100 K. The catalyst samples were placed in a quartz boat that was put into a horizontal tube reactor. Before introducing the reactant mixture (10% acetylene 90% N2, with a flow rate of 300 ml/min) the catalysts were purged by nitrogen stream (300 ml/min) in order to remove water and pretreat the catalyst at 999 K. The reactions were carried out for reaction time of 30 min. In situ XPS measurements were carried out to clarify the state of cobalt on the supported catalyst and the reaction was conduated in the sample prelaaration chamber of XPS instrument. [see details in ref. 12] 2.3. Characterization of the product MWNTs Since the initial weight of catalysts introduced into the reactor was known, we measured the weight increase after the reaction. From these data the total carbon production was determined. For the charaterization of catalyst activity, carbon yield (ratio of carbon deposit and catalyst) calculated as following was used: Carbon yield = (mafterreaction-- mcatalyst) / 1Tlcatalyst (g/g) For TEM and HRTEM Philips CM20 and JEOL 200CX were used, respectively. For the preparation of sample holder grids, the glue technique was used described elsewhere in detail [2]. Nominal composition, surface area, pore diameter and activity data of the catalysts are given in Table 1.

544 Table 1 Characterization of catalyst samples Sample

Metal content Surface area Pore diameter (wt%) (m2g1) (nm)

Co/A Co/L Co/Y

Activity (g/g)

2 2 2

435 216 632

0.51 0.60 0.74

0.03 0.20 0.19

Co/13X Co/A1MCM-41 CoA1MCM-4 l(ex)

2 5 0.29

615 931 931

0.74 3.4 3.4

0.21 0.73 0.82

CoMCM-41 (iso)

0.01

931

3.4

0.96

5 5

1078 994

3.1 3.0

0.67 0.71

Co/SiMCM-48 Co/A1MCM-48

3. RESULTS 3.1. Formation of M W N T s

There are obvious differences between these catalysts concerning both the quantity and the quality of MWNT formed. Using impregnated zeolite-supported catalysts of pore diameter less than 1 nm, well-graphitized carbon ~anotubes could be grown almost independently of the type of the support. Neither surface area nor pore diameter affected significantly the quality and the quantity of carbon nanotubes. For illustration, Fig. 1 shows electron microscopy images of carbon nanotubes grown over various zeolite-supported cobalt catalysts.

m

.

Figure 1. Carbon nanotubes formed on the surface of a) Co/NaY; b) Co/13X catalysts; c) high resolution image of a carbon nanotube.

545 similar dimension would be able to regulate the inner or the outer diameter of the forming carbon nanotubes. Close scrutiny of our samples (Figs. 2 and 3a) revealed, however, that the pore size of the mesoporous supports and the diameter of carbon nanotubes showed no correlation.

Figure 2. Carbon nanotubes formed on the surface of a) Co/SiMCM-48 and b) Co/A1MCM-48 catalysts. No nanotube could be detected on CoA1MCM-41 (iso) (Fig. 3b) and only a slight indication of nanotube formation is seen on CoMCM-41(ex). Numerous, wellgraphitized nanotubes formed on Co/A1MCM-41 (Fig. 3a). It is worth to emphasize here that the samples proved to be inactive in the production of MWNTs were prepared by isomorphous substitution and ion exchange, not by impregnation. 3.1. In situ XPS characterization As far as the reducibility of cobalt ions is concerned detailed in situ X P S investigations were carried out. XPS spectra of the catalyst samples were taken under vacuum at both ambient and reaction temperatures, then, measurements were performed in acetylene atmosphere. Significant changes were observed after the sample was kept at 1000 K in acetylene atmosphere for 60 min. In such a strong reducting atmosphere we could detect reduction of cobalt ions.

546

Figure 3. Electron microscopy image of samples a) Co/MCM-41 and b) CoMCM41 (is0) after CCVD.

4. DISCUSSION

Zeolites have a relatively small pore size, typically a few A. Mesoporous molecular sieves have uniform hexagonal (MCM-41) and cubic (MCM-48) pore systems ranging from 10 A to more than 100 A. This significant difference inspired us to test these materials as catalyst support in the formation of carbon nanotubes having similar dimensions. As it is known from zeolite chemistry, the ion exchange positions of zeolites are situated in their pore system, which is of molecular dimensions. In our case, zeolite NaY has a pore opening, i.e., an entrance for the ions and/or molecules, around 0.7 nm. Its ion-exchange capacity depending on the Si/A1 ratio of the framework varies, but its upper limit is around 5 mmol/g dry zeolite. This 5 mmol/g Co 2+ ion is bound to particular positions in the cage system and is accessible only for molecules of kinetic diameter less than 0.7 nm. This is true for the reverse way as well. Though, only those molecules can leave the pores whose diameter is smaller than the pore exit that is identical to the entrance. From this follows that carbon nanotube formation takes place on those metallic particles, which are generated from ions sitting on the outer surface of zeolite crystals, since the outer diameter of the thinnest MWNT is much bigger than 0.7 nm, the pore size of the zeolite. Similar consideration can be done for the other zeolitic supports applied in our system. Their pore diameter is lower than or equal to that of NaY.

547 For the MCM structures the situation is similar. Here, MWNT formation was observed neither on CoA1MCM-41 (ex) nor on CoMCM-4 l(iso). The case of the former is identical to that mentioned above for the ion-exchanged zeolites. Here, the pore opening is bigger (-3 nm), however, the Co 2§ ions are in the channels, but the pores are too small to be the nests of MWNT generation. The case of an isomorphous substituted sample is even simpler. Presumably, all Co ions are chemically bound in the wall of MCM-41 in this sample. These Co ions are immobile, almost irreducible, therefore, there is no or a very small chance to form clusters on the outer surface of the material. Therefore, they cannot act as active sites in the MWNT generation. We proved that cobalt-containing samples prepared by impregnation are good catalysts for the generation of MWNT from acetylene via CCVD. The activity differences found for the various supports can be explained by the necessary localization of the catalytically active components on the outer surface, at those places of support where a MWNT can easily accommodate, i.e., in the big pores like a silica gel has [13]. Interaction between cobalt particles and catalyst support seems to be of significant importance. Catalyst activity may slightly vary with Si/A1 ratio. Since carbon yield determined after reaction is only an approximate measure of the synthesis (MWNT content of carbon deposit varies in wide range), quantitative considerations cannot be done. Concerning the characteristics of zeotype support materials listed in Table 1, no correlation was found between these data and catalytic activity in carbon nanotube formation. Increasing amount of deposited carbon with larger pore diameter is due to stuffing the pores with non-graphitic carbon. This activity is independent of the selectivity of carbon nanotube formation for which exclusively catalyst particles on the outer surface was found to be responsible. Consequently, using mesoporous material as catalyst support, instead of presumable controlling effect, the overwhelming part of carbon deposit is composed of amorphous carbon. No indication was found suggesting that formation of MWNT starts in the pores of MCM type catalysts. Actually, their pore diameter is much smaller than that of the MWNT. From this it follows that only those catalyst particles, which are deposited on the outer surfaces can have a role in the formation of carbon nanotubes. Our in situ ESCA experiments showed that cobalt ions are reduced by the reactant acetylene and we found no indication of any kind of cobalt oxide after treatment the sample at 1000 K.

548 REFERENCES

1 L.B. Avdeeva, D.I. Kochubey, Sh.K. Shaikhutdinov, Appl. Catal. A: General 176 (1999) 135; T.E. Muller, D.G. Reid, W.K. Hsu, J.P. Hare, H.W. Kroto, D.R.M. Walton, Carbon 35 (1997) 951; A.M. Benito, Y. Maniette, E. Munoz, M.T. Martinez, Carbon 36 (1998) 681. 2. A.L Balch, M.M. Olmstead, Chem. Rev. 98 (1998) 2123; K. Hernadi, A Fonseca, J. B.Nagy, A. Fudala, D. Bemaerts, A. Lucas, Zeolites 17 (1996) 416; A. Carlsson, T. Oku, J.O. Bovin, G. Karlsson, Y. Okamoto, N. Ohnishi, O. Terasaki, Chem. Eur. J. 5 (1999) 244. 3. Z. K6nya, J. Kiss, A. Oszk6, A. Siska, I. Kiricsi, Phys. Chem. Chem. Phys. 3 (2001) 155. 4. R. Sen, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 267 (1997) 276; A. Peigney, C. Laurent, O. Dumortier, A. Rousset, J. Eur. Ceram. Soc. 18 (1998) 1995. 5. M. Terrones, W.K. Hsu, H.W. Kroto, D.R.M. Walton, Topics in Current Chemistry, Springer Verlag, Berlin Heidelberg, Vol. 199 (1999) p: 189-234. 6. K. Hemadi, A Fonseca, J. B.Nagy, D. Bemaerts, Springer Series in Materials Science: Supercarbon, Springer Berlin Heidelberg (1998) p: 81; A. Kukovecz, I. Willems, Z. Konya, A. Siska and I. Kiricsi, Phys. Chem. Chem. Phys. 2 (2000) 3071. 7. J. Rodriguez-Mirasol, T. Cordero, L.R. Radovic, J.J. Rodriguez, Chem. Mater. 10 (1998) 550. 8. J.S. Beck, J.C. Vartuli, G.J. Kennedy, C.T. Kresge, W.J. Roth and S.E. Schramm, Chem. Mater., 6 (1994) 1816. 9. R. Schmidt, D. Akporiaye, M. St6cker and O.H. Ellestad, J. Cfiem. Soc., Chem. Commun., (1994) 1493. 10. J.M. Kim, J.H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem. 99 (1995) 16742. 11. E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. 12. K. Hemadi, Z. K6nya, A. Siska, J. Kiss, A. Oszk6, J. B.Nagy and I. Kiricsi, Mater. Chem. Phys., in press 13. K. Hemadi, A Fonseca, P. Piedigrosso, J. B.Nagy, D. Bemaerts, J. Riga, A. Lucas, Catal. Lett. 48 (1997) 229.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

549

The influence o f w a t e r on the activity o f nitridated zeolites in b a s e - c a t a l y z e d reactions S. Ernst, M. Hartmann, T. Hecht, P. Cremades Ja6n and S. Sauerbeck Department of Chemistry, Chemical Technology, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany

The ammonia treatment of zeolites with FAU-topology at 700 ~ leads to materials in which Si-NH2, Si-NH-Si and Si-NHz-A1 groups are present, i.e. NaXN, NaYN and DAYN (nsi/nAl = 30). These catalysts are active in base-catalyzed reactions, such as Knoevenagel condensations. It could be shown that the catalytic activity increases with increasing nsi/nAlratio. Furthermore, TG-MS experiments revealed that the uptake of water leads to slow hydrolysis with concomitant release of ammonia. The sensitivity to water increases with increasing hydrophilicity of the catalyst, viz. increasing aluminum content of the parent zeolite. It is, however, possible to preserve the catalytic activity by keeping the freshly nitridated material in a dry nitrogen atmosphere. 1. INTRODUCTION Recently, a novel method for the preparation of zeolites and zeolite-like microporous and mesoporous materials with basic properties has been described [1,2]. Essentially, it is based on a high-temperature (i.e., 700 ~ to 850 ~ ammonia treatment of the molecular sieves. The materials obtained by such a treatment are active catalysts in typical base-catalyzed reactions like Knoevenagel condensations. By analogy with nitridated amorphous oxynitrides (e.g. [3,4]), the active basic sites in the new materials are supposed to result from a replacement of a certain amount of framework oxygen atoms by nitrogen. By substitution of bridging oxygen atoms or of oxygen in silanol groups in zeolites, Si-NH-Si and Si-NH2 groups are formed. The occurrence of both species in nitridated zeolites has been detected by Fourier-Transform-IR spectroscopy [2]. However, it has also been found in the studies conducted so far that these nitrogen-containing sites are prone to hydrolysis in the presence of water vapor with a concomitant loss of active sites. Hence, the catalytic activity of the nitridated zeolites strongly depends on their "history", viz. contact with moist air etc. Therefore, it was the aim of the present study to investigate in detail the influence of water on the catalytic activity of the novel type of catalyst. For this purpose, nitridated zeolites with FAU topology were stored for selected periods under different conditions (i.e., under nitrogen, under a controlled moisture-containing atmosphere and at elevated temperatures in air) and subsequently tested for their catalytic activities in the Knoevenagel condensation of benzaldehyde with propanedinitrile.

550 2. E X P E R I M E N T A L SECTION The zeolites NaX, NaY and dealuminated Y (DAY, nsi/nAl = 30, prepared via SiC14treatment) were used as parent materials. The nitridations were performed in an ammonia flow ( V = 60 ml- min -~ ) at 700 ~ for a period of 42 h. Afterwards, the materials were cooled down in a flow ( V = 60 ml. min -1 ) of dry nitrogen. Aliquots were then transferred as quickly as possible into the prepared reaction mixture or stored for selected periods under nitrogen, in a desiccator over a saturated aqueous solution of ammonium chloride or at 100 ~ in air. Xray powder diffraction patterns were collected on a Siemens-axs D5005 diffractometer using CuK~ radiation. TG-MS experiments were performed using a SETARAM setsys-16/MS instrument with a heating rate of 13= 5 K. min -~ in a nitrogen flow. The DRIFT spectra were collected using a Nicolet Nexus spectrometer equipped with a high-temperature in-situ chamber. The pyrrole adsorption experiment was performed in-situ in the FT-IR spectrometer at 50 ~

using a pyrrole-loaded flow of nitrogen (X7 = 40 ml. min-1,

Ppyrrole =

1 kPa ). The

catalytic experiments were conducted in the liquid phase at 8 0 ~ using 4 m m o l benzaldehyde, 4 mmol propanedinitrile and 0.2 g of catalyst in 10 cm 3 toluene as solvent. 3.

RESULTS

3.1 Characterization XRD powder patterns of the nitridated zeolites NaXN, NaYN and DAYN are shown in Figure 1. In the sample notations, the suffix "N" indicates that the zeolite has been treated with ammonia. The thermal stability of the nitridated materials after different storage times in a desiccator was studied by thermogravimetry coupled with mass spectroscopy. The maximum of the observed weight loss is around 150 ~ and fragments with m/z = 16 (NH3) and m/z = 18 (H20) are detected in the mass spectra. In Figure 2, the relative intensities of the signals of water (rn/z = 18) and ammonia (m/z = 16) are plotted as a function of the storage time for zeolite NaYN. The increase of the signal for the desorbed water is correlated to the decrease of the signal for ammonia. The nitridated materials have also been characterized by infrared spectroscopy in the diffuse reflectance mode (DRIFT). As shown earlier, the DRIFT-spectrum of the ammonia treated zeolite DAYN exhibits four additional bands, viz. at 1622, 1553, 1536 and 1450 cm -1, which were tentatively assigned according to Table 1 [2]. In particular, the bands at 1553 and 1536 cm -1 are indicative of Si-NH2 and Si-NH2-A1 groups [5]. Furthermore, several spectral changes are observed above 3000 cm ~ The intensity of the bands at 3729 cm 1 and 3616 cm -1 decreases with ammonia treatment. These bands are typically assigned to Si-OH groups and Si-OH-A1 groups [5], respectively. Simultaneously, five new bands appear at 3441, 3380, 3289, 3135 and 3035 cm 1 (cf. Figure 3). It is well known that NH3 reacts at high temperatures with dispersed SiO2 forming Si-NH2 and Si-NH-Si groups [6,7]. The Si-NH2 group is characterized by IR vibrations at 1 5 5 0 - 1550 cm ~ (SNH2), 3 4 4 0 - 3455 cm -1 (vsNH2) and 3520-3540 cm 1 (vasNH2) [8,9]. An additional band at 3390 cm -1 has been assigned to Si-NHSi groups [6]. Fink and Datka have assigned the band at 3289 cm 1 to the vsNH2 vibration of Si-NHz-A1 groups [5]. Physically adsorbed ammonia produces the band at 3378 cm 1 and the band at 3035 cm ~ is indicative for the formation of ammonium ions [10]. The bands observed in DAYN and their assignments are summarized in Table 1.

551

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

'

'

'

i

I

r--

I... v

NaYN

....... ,

,

,

I

,

,

,

,

10

I

,

,

,

,

20

I

,

,

,

_NaxN

,

30

I

I

I

............ I

I

I

40

I

,

50

I

60

Angle 2 0 / ~ Fig. 1" X-ray powder patterns of the ammonia-treated zeolites NaXN, NaYN and DAYN. "

'

'

I

"

"

'

'

I

'

"

'

'

I

,

,

,

,

I

"

'

'

'

I

'

'

O

..,,.

l-

xi

"

'

I

'

'

"

"

O

. . . .mlz ,

L_

V

0

m/z-18(H20

)

c-

20

40

60

80

100

120

140

Storage Time / h Fig. 2: Intensities of the signals for m/z = 16 and 18 in the TG/MS-experiments for zeolite NaYN, which was stored for different times in a desiccator over a saturated aqueous solution of ammonium chloride.

552 Table 1. Assignment of IR bands of nitridated DAYN zeolites. Species

Si-NH-Si

Assignment 5NH4 + v sNH4+ vasNH4+ 8NH2 vsNH2 va~NH2 8NH2 vsNH2 vasNH2 5NH3 vsNH3 v~sNH3 vNH

Si-OH Si-OH-A1

vOH vOH

NH4+

Si-NH2

Si-NHa-A1

NH3 adsorbed

....

e-

I . . . . . . . . I

I I

I I

,,

I|

|

.

9

|

1

5 4

3 2

|

9

|

9

|

a

|

*

,

'

Band in Figure 3

1553 3441 n.obs. 1536 3289 3340 1622 n.obs. 3378 3390-3400 (shoulder) 3729 3616

I .... I

,

0o

Wave number / cm "l 1450 3035

S

DAYN

I

,d r O o

I I I

i

I I

I -

I I I I

er I

3500

i

i

I

i

I I I

I

3400

I

I

I

- - + - . . . . . ~ I I I I I I I I i

3300

,

,

i

I

,

3200

i

,

,

/

iDA Y

______J-

I I I I . . . .

3100

I

. . . .

3000

I

------

I I I I I I

. . . .

2900

2800

W a v e n u m b e r / cm -~ Fig. 3" DRIFT spectra of untreated DAY and of ammonia treated DAYN. The asterisk (*) marks bands produced by the adsorption of NH3 on the ZnSe windows of the DRIFT cell.

553

After adsorption of pyrrole, an intense band at 3467 cm l with a shoulder at 3403 c m "1 is observed in the IR spectrum. These bands are assigned to the NH stretching mode of pyrrole based on a comparison with the liquid phase and the gas phase spectrum of pyrrole [ 11].

3.2 Knoevenagel condensation 3.2.1. Influence of the nsi/nAl-ratio In Figure 4, the conversions of propanedinitrile in the Knoevenagel condensation of propanedinitrile with benzaldehyde over freshly nitridated NaXN, NaYN and DAYN catalysts are shown in dependence of reaction time. The activity of the fresh materials strongly depends on the nsi/nArratio of the catalyst. With decreasing aluminum content, an increase in catalytic activity for the base-catalyzed Knoevenagel condensation is found. After two hours of reaction, the conversion of propanedinitrile over DAYN amounts to 85 %, while over NaYN and NaXN conversions of 60 % and 25 %, respectively, are observed.

100 0--9. t.. -O ~) (.{3. LO

80 60

9

X

tO t/l t_

> to o

20it"

/

9 NaXN (nsi/nAt

0

5

10

=

1.3)

NaYN (nsi / nA, = 2.5) /

o

DAYN (nsi/nAi = 30)

v

0

:

15

20

25

30

Reaction Time / h Fig. 4" Conversion of propanedinitrile in the Knoevenagel condensation over nitridated materials with different nsi/nAl ratios (reaction conditions see experimental section). 3.2.2. Influence of the storage time The results of the storage tests with nitridated zeolite NaY are shown in Figure 5. As a measure for the catalytic activity, the conversion of propanedinitrile after a reaction time of 2 h (Xzh) was used. For the fresh materials (storage time = 0 h), the catalytic activity depends on the nsi/nAl-ratio of the catalyst (cf. Figure 4). The activity of DAYN decreases very slowly with increasing storage time (Figure 5). NaYN shows a decrease to 50 % of its origin activity after ca. 50 h of storage over a saturated ammonium chloride solution. NaXN has a very small initial activity which declines only slightly with storage time.

554 100,,.

~~

.~.. "

,

. . . .

,

. . . .

,

. . . . v

_

80

-

._ C

., o

x

60

13_

I

40

.o

v

>

NaXN NaYN DAYN

9

20

0

-w-

j

0

T0

~

50

100

150

2( )0

Storage Time / h Fig. 5" Conversion of propanedinitrile after 2 h of reaction (X2h) over NaXN, NaYN and DAYN after different storage times in a desiccator over a saturated ammonium chloride solution. 70

o~ K)

60

Y

m .n t_

.-

"O

50

T

r

9 9

t~

~s 40

&.

X t-

Nitrogen Desiccator Oven

30

0 r

~ >

20

o 0

10

t-

O

A W

0

50

100

150

200

250

300

350

Storage Time / h Fig. 6" Conversion of propanedinitrile after 2 h of reaction time for different storage times of NaYN. The storage conditions are explained in the text above.

555 3.2.3. Influence of the storage conditions To evaluate the influence of water on the catalytic activity of nitridated materials, aliquots of nitridated zeolite NaYN were stored under different conditions, viz. under dry nitrogen, in an oven at 100 ~ and in a desiccator over saturated ammonium chloride solution. The initial activity of ammonia treated NaY (NaYN) was maintained by storing the material under a dry nitrogen atmosphere (cf. Figure 6). The catalytic activity is almost constant even for storage times of more than 300 h. In contrast, the activity of the NaYN catalyst stored in a desiccator decreases with storage time. While the fresh catalyst exhibits a propanedinitrile conversion of 52 % after 2 h of reaction, the conversion decreases to ca. 20 % after keeping the catalyst for 100 h in the desiccator. For still longer storage times, the catalytic activity remains almost constant. The samples stored at 100 ~ show an increased deactivation rate, which can be attributed to accelerated hydrolysis at elevated temperatures.

4. DISCUSSION The nitridation of zeolites results in the formation of Si-NH2, Si-NH-Si and Si-NH2-A1 groups, which is evident from the IR bands at 3441, 3390 and 3289 cm l (cf. Figure 3), respectively. The TG-MS experiments reveal a weight loss with a maximum around 150 ~ which is due to the simultaneous release of water and ammonia. It is, therefore, evident that the Si-NH2, Si-NH-Si and Si-NHz-A1 groups formed by nitridation are prone to hydrolysis with concomitant release of ammonia. The ammonia formed is presumably initially adsorbed on the zeolite but later replaced via adsorption of water. Therefore, after a certain storage time mainly water is desorbed from the zeolite (cf. Figure 2). The extend of hydrolysis depends on the hydrophobicity of the catalyst, which increases with increasing nsi/nAl-ratio. The nitridation of mesoporous materials such as MCM-41 and MCM-48 with different nsi/nA1ratios indeed shows the same trend [12], which supports the validity of the above made assumption. The results of the catalytic experiments also show that the activity of the nitridated zeolites is strongly affected by hydrolysis. The catalytic activity of the fresh catalyst depends on the nsi/nAl-ratio of the parent material. The initial activity (as expressed by propanedinitril conversion at a reaction temperature of 80 ~ and a reaction time of 2 h) decreases with increasing aluminum content. These differences in (initial) activity are tentatively explained by a higher number of basic sites in DaYN as a consequence of a higher concentration of silanol groups prior to the nitridation treatment. It is furthermore interesting to note that full conversion (even after 24 h of reaction) is not reached over NaYN and NaXN. The maximum conversion is 90 % and 55 %, respectively. The reason for the catalyst deactivation is still unclear, however, one possible explanation is that the pores of the less active catalysts (which are more hydrophilic) are blocked by strong adsorption of the bulky and polar condensation product and/or polar reactants. The loss of activity of the nitridated catalyst stored over a saturated NHaC1 solution is small for DAYN and marked for NaYN and NaYN, which is in line with the increasing hydrophobicity from NaX to DAY. It is, therefore, inevitable to only compare catalysts which have been stored under controlled conditions. The catalytic activity, which decreases rapidly when the catalyst is stored in a controlled humid environment (cf. Figure 6), is maintained if the catalyst is kept under dry nitrogen. Keeping the catalyst in an oven at elevated temperature (100 ~ increases the hydrolysis rate and/or the desorption rate of ammonia and results in

556 faster deactivation. From our results, it follows that hydrolysis is the major reason for deactivation of the nitridated catalysts. CONCLUSIONS In the present contribution, zeolites NaX, NaY and DAY were treated with ammonia at elevated temperatures to create basic Si-NH2, Si-NH-Si and Si-NHz-A1 groups inside their structures. The existence of these groups was proven by DRIFT spectroscopy. The novel materials exhibit catalytic activity in the base-catalyzed Knoevenagel condensation of benzaldehyde with propanedinitrile. A decrease in catalytic activity is observed upon extended exposure to moisture due to hydrolysis of the nitrogen containing groups and, hence, release of ammonia. The rate of deactivation decreases with increasing hydrophobicity in the following order: NaXN > NaYN > DAYN ( n s i / n A l = 30). The results so far indicate that the uptake of water and subsequent hydrolysis are responsible for the deactivation of the catalysts. The storage of the freshly nitridated zeolites in a dry nitrogen atmosphere preserves their catalytic activity, while storage of the fresh catalysts in air at elevated temperature accelerates the hydrolysis reaction. ACKNOWLEDGEMENTS

Financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie und Max-Buchner-Forschungsstiftung is gratefully acknowledged. REFERENCES

1. S. Ernst, M. Hartmann, S. Sauerbeck and T. Bongers, Appl. Catal. A 200 (2000), 117. 2. S. Ernst, M. Hartmann and S. Sauerbeck, in: "Zeolites and Mesoporous Materials at the Dawn of the 21 st Century", A. Galarneau, F. Di Renzo, F. Fajula and J. Vddrine (Eds.), Studies in Surface Science and Catalysis, Vol. 135, Elsevier, Amsterdam (2001), 175. 3. P. Grange, P. Bastians, R. Conanec, R. Marchand and Y. Laurent, Appl. Catal. A 114 (1994), L91. 4. M.J. Climent, A. Corma, V. Forn6s, A. Frau, R. Guil-L6pez, S. Iborra and J. Primo, J. Catal 163 (1996), 392. 5. P. Fink and J. Datka, J. Chem. Soc., Faraday Trans. 1 85 (1989) 3079. 6. B.A. Morrow and I.A. Cody, J. Phys. Chem. 80 (1976) 1998. 7. P. Fink and I. Plotzki, Wiss. Z. Friedrich-Schiller-Universit~it Jena, Math.-Nat. R. 37 (1988), 691. 8. P. Fink, I. Plotzki, G. Rudakoff, Wiss. Z. Friedrich-Schiller-Universit~it Jena, Math.-Nat. R. 37 (1988), 911. 9. P. Fink, I. Plotzki, Wiss. Z. Friedrich-Schiller-Universitgt Jena, Math.-Nat. R. 25 (1976), 853. 10. G.T. Kerr and G. F. Shipman, J. Phys. Chem. 72 (1968) 3071. 11. H. F6rster, H. Fuess, E. Geidel, B. Hunger, H. Jobic, C. Kirschhock, O. Klepel and K. Krause, Phys. Chem. Chem. Phys. 1 (1999) 593. 12. S. Sauerbeck, M. Hartmann and S. Ernst, unpublished results.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Selective catalytic reduction of N20 with decomposition over Fe-BEA zeolite catalysts

557

light

alkanes

and

N20

T. Nobukawa a, K. Kita a, S. Tanaka a, S. Ito a, T. Miyadera b, S. Kameoka a , K. Tomishige a and K. Kunimori a alnstitute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan bResearch Institute of Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569, Japan Fe ion-exchanged zeolite catalysts (Fe-BEA, Fe-MFI) were found to be active for selective catalytic reduction (SCR) of N20 with light alkanes (i.e., CH4 and C2H6) even in the presence of excess oxygen. In the NEO/CEH6/O2 system over Fe-BEA catalyst, N20 plays an important role in the oxidation of C2H6 (i.e., activation of C2H6 at an initial step). HE-TPR and catalytic measurements of Fe-BEA catalysts with different Fe contents revealed that the active sites for SCR of N20 with CH4 and N20 decomposition are Fe ion species, and Fe oxide aggregates are inactive. In O2-TPD studies, we observed new desorption peaks from Fe-BEA catalyst after N20 treatment. In the isotopic tracer study of N20 decomposition, the result shows that 02 formation on Fe-MFI catalyst proceeds via Eley-Rideal mechanism, which is different from the case of supported Rh catalysts. 1.

INTRODUCTION

Nitrous oxide (N20), which also contributes to catalytic stratospheric ozone destruction, is a strong greenhouse-effect gas with a global warming potential (GWP) per molecule of about 300 times that of carbon dioxide (CO2) [1]. From the point of view of environment, therefore, it is important to study removal of N20 in the emission gases. Catalytic decomposition of N20 [1] and SCR of N20 with reductants such as hydrocarbons [2-8] and ammonia [9] have been proposed as the effective methods of N20 abatement. Fe ion-exchanged zeolites have been investigated as interesting catalysts, which are active for SCR of N20 with C3H6 or C3H8 in the presence of excess O2, H20 and SO2 [3, 5]. Recently, we reported the simultaneous removal of N20 and CH4 as the strong greenhouse-effect gases over Fe-BEA by SCR of N20 with CH4 [6, 8]. The Fe-BEA catalyst was more active than the other Fe-zeolite catalysts (Fe-MFI etc.) [6]. In this work, the activities of SCR of N20 with CH4 and N20 decomposition have been investigated over Fe-BEA catalysts with different Fe contents, and the natures of active sites have been studied by means of HE-TPR and O2-TPD

558 techniques. Enhancement of C2H6 oxidation by the presence of N20 in SCR of N20 with C2H6 has been studied by a transient reaction technique etc.. Isotopic tracer studies using 1802 were also carried out over Fe-MFI (or Fe-BEA) to elucidate the mechanism of 02 formation for N20 decomposition. 2.

EXPERIMENTAL

Fe-zeolite catalysts have been prepared by two different ion-exchange methods: (i) wet ion-exchange (IE) with a dilute solution of FeSO4 at 50 ~ for 20 h under nitrogen atmosphere, followed by calcination in air at 500~ for 12 h [3, 6] and (ii) solid-state ion-exchange (SSI) with FeC12 and zeolite, which were mixed intensively in a ball mill under ambient conditions for 1 h, followed by calcination in air at 400 ~ for 5 h [10, 11]. The zeolite supports (H-BEA, SIO2/A1203=27.3 and Na-MFI, SIO2/A1203=23.8) were supplied by TOSOH Co. The loading weight of Fe was changed from 0 to 3.1wt% by SSI method in order to elucidate the effect of Fe contents on the catalytic activities. An Fe/MFI (IMP) catalyst (2.9wt% Fe) was also prepared by impregnation method using Fe(NO3)3 in order to compare catalytic activities and O2-TPD spectra. The reaction was carried out in a standard fixed-bed flow reactor, capable of rapid switching (ON-OFF) of the gases, using a mixture of N20 (0- 1300 ppm), HCs (CH4, C2H6:0 - 1000 ppm), 02 (0 - 10%) in He flow (SV=60,000 hl). In the TPR experiment, the sample was heated in 5% H2/Ar flow (30 ml/min) at a constant heating rate of 10 ~ and H2 consumption was monitored by TCD. The O2-TPD experiment was carried out in a microcatalytic pulse reactor [ 12]. The effluent was analyzed in an on-line gas chromatograph system equipped with a TCD and differentially pumped quadrupole mass spectrometer. The temperature was increased from room temperature to 800 ~ at a constant heating rate of 10 ~ and kept at 800 ~ for 30 min. In the isotopic study, N2160 was pulsed onto 1802-treated Fe-zeolite catalyst, and desorbed 02 molecules were monitored by means of mass spectrometry [ 12]. 3.

RESULTS AND DISCUSSION

3.1. The H2-TPR and O2-TPD studies and the activities of SCR with CH4 and decomposition of N20 Figure 1 shows TPR profiles of Fe-BEA catalysts (ion-exchanged by SSI or IE) with different Fe contents after the calcination treatment. A peak between 70 ~ and 140 ~ is an artifact, which is due to desorption of residual Ar [13]. As shown in Fig. 1 (A), the sample 1 exhibits three main reduction peaks at 470 ~ 530 ~ and 580 ~ together with the tail peaks at the lower temperatures. A peak at 530 ~ is due to the reduction of remaining C1 in the catalyst, because this peak was removed after washing by deionized water (not shown). Other two peaks were due to the reduction of Fe oxides (i.e., Fe203---~Fe0) [14, 15]. The samples 2 and 3 show one main reduction peak at 400 ~ The amount of the H2 consumption expressed

559 as the ratio H2/Fe for this peak was 0.37 and 0.43, respectively • (see also Fig. 1 (B)). In e 5 agreement with previous works 2 on TPR of Fe-BEA [14] and ._g Fe-MFI [15], this peak could correspond to the reduction of Fe 3§ to Fe2§ The lower ratio of 200 400 600 H2 consumption to Fe suggests 8 that a part of the Fe species was "1still reduced or was unreducible I in these conditions. The sample 3 200 400 600 also has a small reduction peak Temperature / ~ at 520 ~ which correspond to Fig. 1. H2-TPR of Fe-BEA catalysts:(1) 3.1wt% SSI, the reduction of Fe oxide [15]. (2) 1.5wt% SSI, (3) 0.77wt% SSI, (4) 0.30wt% SSI, The sample 4 shows very small (5) 1.6wt% IE. broad peak because of very small amount of Fe loading, but the peak may be attributed to the reduction of Fe 3+ to Fe 2+. As for the sample 1, the tail peaks at around 400 ~ may also be attributed to the reduction of Fe 3+ to Fe 2+, although the amount of the Fe ion species appears to be smaller than that of the sample 2. As shown in Fig. 1 (B), the sample 5, which was prepared by the wet ion-exchange (IE) method, exhibits almost the same TPR profile as that of the sample 2, although a small peak (530~ due to the reduction of Fe oxides was observed. These results indicate that Fe 3§ ion species are prepared by either SSI or IE method followed by the calcination treatment and that Fe oxide aggregates are formed with higher Fe loading. Figure 2 shows the results of SCR with CH4 and N20 decomposition on Fe-BEA catalysts with different Fe contents. The activities were increased with increasing the amount of Fe until 1.5wt%, beyond which the activity levelled off in both reaction systems. The results show that the active sites of these reactions are Fe ion species and that Fe oxide aggregates are inactive. The results of these catalytic activities are consistent with the results in Fig. 1. The activities of 3.1wt% Fe-BEA were a little lower than these of 1.5wt% Fe-BEA, which suggests that the amount of active Fe ions may be smaller and/or the Fe oxide aggregates may block the inlet pores of the zeolite. Figure 3 shows the activities of N20 decomposition over Fe catalysts, which were prepared by wet ion-exchange (IE) and impregnation (IMP) methods. We have already reported that Fe-BEA catalyst has higher activity in SCR of N20 with CH4 than that of Fe-MFI catalyst [6]. This tendency was also found for N20 decomposition. It is clear that Fe/MFI (IMP) has almost no activity even at 500 ~ which means that the active species for N20 decomposition are Fe ions and that Fe oxide aggregates are inactive, in agreement with the results in Fig. 2.

560 ~100 Z o

r

O

z +0

o~ 100 Z

80

O

40

9

20

y

O

..+o------_._. o

80

Z

60

N,...

O

,-- 40 O 9 tO

cO

q

(a)

,4.-J

0"

60

~

r

1

2

level / w t %

Loading

~

3

20 0

(c) 300

400 Temperature

Fig. 2. The activity of SCR with CH4 (Q" N20 950 ppm, CH4 500 ppm, 02 10% at 350 ~ and decomposition of N20 (O: N20 950 ppm at 480 ~ over of Fe-BEA (SSI) with the different ion-exchange levels.

500 / ~

Fig. 3. The activities of N20 decomposition (N20 950 ppm) over Fe catalysts; (a) 1.6wt% Fe-BEA (IE), (b) 2.9wt% Fe-MFI (IE), (c) 2.9wt% Fe/MFI (IMP).

Figure 4 shows O2-TPD spectra from the Fe catalysts after 02 or N20 pretreatment. As shown in Fig. 4 (a), Fe/MFI (IMP) does not desorb any significant amount of oxygen. For Fe-BEA (IE) after the 02 treatment, 02 starts to desorb above 520 ~ with a maximum occurring around 700 ~ (Fig. 4 (b)). After the N20 treatment, new peaks appeared at lower temperatures (300 ~ - 600 ~ The total amount of desorption was increased by the N20 treatment (see Table 1). There have been some reports that active oxygen species are formed by exposing Fe ion-exchanged zeolite catalysts to N20 [14, 16, 17], although detailed points are controversial in the 5 present stage. O

o a

'-+:9 .......... .+-.~ .......... ..,--....... I

200

,

I

400

*

I

600

Temperature

,

--I

............ (.a) -

800

"

~ . . . .

]

"*

........

[ . . . . . .

m]

800

/ ~

Fig. 4. O2-TPD profiles from Fe catalysts after 02 or N20 pretreatment: (a) 2.9wt% Fe/MFI

(IMP), N20 at 250 ~ for lh; (b) 1.6wt% Fe-BEA (IE), O2 at 500 ~ for lh; (c) 1.6wt% Fe-BEA (IE), N20 at 500 ~ for lh.

Table 1 Amount of desorbed oxygen from the O2-TPD experiments on the pretreated catalyst surface (see Fig. 4). Spectrum pretreatment 02 (lJmol)

O/Fe

(a)

N20

0.38

0.03

(b)

02

1.64

0.47

(C)

N20

2.49

0.72

561 Panov et al. [17] reported that O atom from N20 molecule (so called c~-oxygen), which cannot be produced by 02, readily reacted with benzene to produce phenol over Fe-MFI catalyst even at room temperature. However, such high reactivity with benzene was not observed on an N20-treated Fe-MFI catalyst at room temperature [16]. In addition, it should be noted that no desorption of NO was observed during O2-TPD experiments in our Fe-BEA catalyst system [ 14]. SCR of N20 with C2H6 Enhancement of C2H6 + 02 reaction by the presence of N20 Figure 5 shows N20 and C2H6 conversions in SCR of N20 with C2H6 over Fe-BEA catalyst in the presence of excess O2. For a comparison, the activities in SCR of N20 with CH4 are also shown in Fig. 5, which are taken from the Ref. [8]. The N20 conversion was increased significantly by adding reductant (CH4 and C2H6) even in the presence of excess 02 (i.e., SCR), and there was a plateau in CH4 conversion (at 300 ~ -~ 450 ~ after N20 conversion reached to 100% [8]. This is due to the fact that N20 in the mixture gas was completely consumed by the reaction with CH4, and no reaction of CH4 with 02 took place at these temperatures. On the other hand, the oxidation behavior of C2H6 in SCR of N20 was significantly different from that of CH4. No plateau in the C2H6 conversion was observed in the N20/OE/CEH6 system, but the C2H6 conversion reached to 100% at ca. 350 ~ Therefore, it was found that the oxidation of Call6 by coexistent 02 concomitantly occurred. As shown in Fig. 5, however, the oxidation of C2H6 by 02 required higher temperatures (> 350 ~ This result indicates that the oxidation of C2H6 by 02 in SCR of NaO is significantly enhanced by 3.2.

-

-

100

A

loo

V

! !

80

;

[ l i//

Z

o

O Z O

60

t~~

to 40 >

ot-- 20 O

,b q

-~0

/

O N20/O2/CH4 e N20/O2/C2H6 X N20/O 2

[-] CH,/O 2

02H6/O2

80

"~

t i l

._o ~ 40 > e0

'

0

I

esS

~r 60

o..-o ....... -6" sS s

/

r

20

:

;

/1

I

'

" 400

Temperature / ~

'

0

400

Temperature / ~

Fig. 5. Conversions of N20 and HC in various reaction systems over 1.6wt% Fe-BEA catalyst (IE).; N20 (950ppm), 02 (10%) and HC (CH4: 500ppm, C2H6: 300ppm).

562 C2Hs/O=iN20/C2Hs/O~ N=O/C2H6 the presence of N20, and that N20 plays ~--i-~ ~ 100 important roles at an initial step in the 100 ~, ~;~ oxidation of C2H6by 02 (i.e., abstraction of H o~ 80 80 atom etc.). -r"tD In order to examine in more detail the ~ 60 60 :~ oxidation behavior of C2H6 in SCR of N20, O O twe performed the transient reaction .9 40 40 ~(!) experiments in the N20/O2/C2H6 system. The O9 result is shown in Fig. 6. The C2H6 conversion => 20 0 0 was drastically increased by adding N20 to C2H6/O2 flow at 350 ~ while oxidation of 4~ 80 120 160 208 C2H6 by 02 did not occur at this temperature. T i m e / min After changing to N20/C2H6 flow on removal Fig. 6. Transient responses for C2H6 of 02 from the N20/O2/C2H6 flow at 350 ~ oxidation on addition of N20 to C2H6/O2 the C2H6 conversion decreased to ca. 18%. In flow and on removal of O2 from this case, the C2H6 conversion was small NEO/CEH6/O2 flow at 350 ~ over 1.6wt% simply due to the depletion of the oxidant (i.e., Fe-BEA catalyst (IE).; N20 (1300ppm), N20). These results directly demonstrate that C2H6:(1000ppm) and O2 (10%). the oxidation of C2H6 by 02 is significantly enhanced by the presence of N20. It should be noted that CO formation was observed only at the N20/O2/C2H6 flow. Therefore the result apparently shows that the formation of CO2 occurred in the reaction of C2H6 with N20 (in the N20/C2H6 flow) and that the formation of CO occurred in the reaction of C2H6 with 02 (in the N20/O2/C2H6 flow). The in situ DRIFT measurements [18] showed that absorption peaks of CxHy(a) and/or CxHyOz(a) species were observed in exposing the catalyst to the N20/C2H6 mixture. Therefore, we propose that N20 plays important roles in the formation of CxHy(a) and/or CxHyOz(a) species in the initial state of C2H6 oxidation, and these species can react with 02 to produce COx and H20. I -..,.

I i

3.3.

i

The isotopic study of N20 decomposition over Fe-MFI c a t a l y s t - Mechanism of 02 formation -

The reduced Fe-MFI catalyst (2.9wt% Fe, IE) was oxidized with 1802 (purity 96.5%) gas at 500 ~ and pulsed N20 decomposition was carried out at 420 ~ The result is shown in Table 2. An isotopic equilibrium constant, Ke = [180 160]2 / [1802] [1602], should be considered to judge incidental exchange reactions that would disguise the experimental results [12]. If the exchange reaction equilibrates, Ke should be close to 4 [19]. As shown in Table 2, Ke is 3.95, which suggests that the isotopic exchange of oxygen in CO2 equilibrates (Table 2, Expt. 1). Therefore, the isotopic fraction of 180 (1~ in the product CO2 should be equal to that of the surface oxygen, i.e., 0.23. After the pulsed CO2 experiment, an N2160 pulse was injected onto the catalyst at 420 ~ (Table 2, Expt. 2), and the 18f of the product 02 was 0.13, which is

563

almost the half value of that of the surface oxygen. In addition, the Ke value of oxygen produced from N20 decomposition was infinity (Table 2, Expt. 2), because 1802 was not detected. It should be noted that the exchange of oxygen in N20 with surface oxygen (eqn. (1)) could be neglected because of very low lSfvalue in the outlet N20 (Table 2, Expt. 2). As a separate experiment, we confirmed that the exchange of oxygen between the gas phase and the surface oxygen is almost negligible (Table 2, Expt. 3, 4). Therefore, we suggest that O2 formation proceeds via Eley-Rideal (ER) mechanism (eqn. (2)). N2160 + N2160 +

180(a) ~ 180(a) ~

N2180 + 160(a) N2 + 160180

(1) (2)

The ER mechanism has also been suggested for Fe-BEA, on which the l ~ o f the desorbed 02 from N20 decomposition had the half value of that of the surface oxygen. However, an isotopic mixing might have occurred, judging from the equilibrium constant (Ke = 3.4) probably because of higher activity of Fe-BEA. In addition, an isotopic study of SCR of N20 with CH4 was not possible, because produced CO2 made complete isotopic mixing (Ke - 4.0). The present result is in contrast with the mechanism of N20 decomposition over supported Rh catalysts [12], where Langmuir-Hinshelwood (LH) type desorption (20(a) 02) has been proposed. For some systems such as Fe ion-exchanged zeolite catalysts, where active sites are isolated, it may be reasonable that the ER mechanism prevails [20]. The active sites of N20 decomposition over Fe-MFI catalyst may be Fe ion species such as binuclear oxo species [13, 16, 21], which may also be supported by the fact that the O/Fe ratio from the O2-TPD experiment is 0.47 (Table 1, (b)). On the other hand, Delahay et al. [22] proposed that mononuclear Fe-oxo species are the most active sites for SCR of N20 with NH3. The isotopic study in this work clearly showed that the ER mechanism prevailed when N2160 was pulsed onto the 1802-treated Fe-zeolite catalyst. However, more detailed studies are needed to elucidate the mechanism of 02 formation during steady-state N20 decomposition reaction. The isotopic fraction of 180 (l~f) and the isotopic equilibrium constant (Ke) in the Table 2 product molecules from 1802, C1602 and N2160 pulses over Fe-MFI catalyst at 420 ~ ii

Product

18f91?~,

Kr

180

CO2

0.23

3.95

N2160

180

02

0.13

cr

2

N2160

180

N20

3

1802

160

02

0.95

4

1802

m

02

0.96 b)

Experiment No.

Pulse

1

C1602

2

Surface species

'a) The isotopic abundance of 180 is 0.002. b) The l~r the catalyst.

0.00 a)

0.18

the incident pulse measured without

564 4.

CONCLUSIONS

Studies of SCR of N20 with CH4 and N20 decomposition over Fe-BEA with different Fe contents prepared by means of SSI revealed that the most effective loading level was 1.5wt%. The active sites of N20 decomposition may be binuclear Fe-oxo species, which has also been suggested in SCR of NO with hydrocarbons [13, 16, 21]. In the N20/C2H6/O2 system over Fe-BEA catalyst, N20 plays an important role in the oxidation of C2H6 (i.e., activation of C2H6 at an initial step). In the isotopic study of N20 decomposition, 02 formation on the Fe-MFI catalyst proceeds via Eley-Rideal mechanism. REFERENCES

1. 2. 3. 4. 5. 6.

F. Kapteijn, J. Rodriguez-Mirasol and J. A. Moulijn, Appl. Catal. B, 9 (1996) 25. Y. Li and J.N. Armor, Appl. Catal. B, 3 (1993) 55. C. Pophal, T. Yogo, K. Yamada and K. Segawa, Appl. Catal. B, 16 (1998) 177. M. K6gel, R. M6nnig, W. Schwieger, A. Tissler and T. Turek, J. Catal., 182 (1999) 470. G. Centi and F. Vazzana, Catal. Today, 53 (1999) 683. S. Kameoka, T. Suzuki, K. Yuzaki, T. Takeda, S. Tanaka, S. Ito, T. Miyadera and K. Kunimori, Chem. Commun., (2000) 745. 7. S. Kameoka, K. Yuzaki, T. Takeda, S. Tanaka, S. Ito, T. Miyadera and K. Kunimori, Phys. Chem. Chem. Phys., 3 (2001) 256. 8. S. Kameoka, K. Kita, T. Takeda, S. Tanaka, S. Ito, K. Yuzaki, T. Miyadera and K. Kunimori, Catal. Lett., 69 (2000) 169. 9. M. Mauvezin, G. Delahay, F. Kil]lich, B. Coq and S. Kieger, Catal. Lett., 62 (1999) 41. 10. M. Rauscher, K. Kesore, R. M6nnig, W. Schwieger, A. Til31er and T. Turek, Appl. Catal. A: General 184 (1999) 249. 11. R. Giles, N. W. Cant, M. K6gel, T. Turek and D. L. Trimm, Appl. Catal. B: Environmental 25 (2000) 75. 12. S. Tanaka, K. Yuzaki, S. Ito, S. Kameoka and K. Kunimori, J. Catal., 200 (2001) 203. 13. H.-Y. Chen and W. M. H. Sachtler, Catal. Today, 42 (1998) 73. 14. B. Coq, M. Mauvezin, G. Delahay and S. Kieger, J. Catal., 195 (2000) 298. 15. R. Q. Long and R. T. Yang, J. Catal., 194 (2000) 80. 16. E1-M. E1-Malki, R. A. van Santen and W. M. H. Sachtler, J. Catal., 196 (2000) 212. 17. G.I. Panov, A.K. Uriarte, M.A. Rodkin and V.I. Sobolev, Catal. Today, 41 (1998) 365. 18. S. Kameoka, K. Kita, S. Tanaka, T. Nobukawa, S, Ito, K. Tomishige, T. Miyadera and K. Kunimori, Catal. Lett. in press. 19. Ozaki, A., "Isotopic Studies of Heterogeneous Catalysis." Kodansha, Tokyo, 1977. 20. A. U Yakovlev, G. M. Zhidomirov and R. A. van Santen, Catal. Lett., 75 (2001) 45. 21. P. Marturano, L. Drozdovfi, A. Kogelbauer and R. Prins, J. Catal., 192 (2000) 236. 22. G. Delahay, M. Mauvezin, B. Coq and S. Kieger, J. Catal., 202 (2001) 156.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

565

Hydroxymethylation of 2-methoxyphenol catalyzed by H-mordenite: analysis of the reaction scheme F. Cavani, L. Dal Pozzo, L. Maselli, R. Mezzogori Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy. [email protected]. Tel & fax +39-0512093680 The reaction scheme for the hydroxyalkylation of 2-methoxyphenol (guaiacol) with aqueous solutions of formaldehyde (formalin) aimed at the synthesis of 3-methoxy-4hydroxybenzyl alcohol (p-vanillol), catalyzed by an H-mordenite zeolite was analyzed. Specific attention was focused on the nature of the by-products obtained in the presence and in the absence of methanol -a component present in formalin. In the presence of methanol the main products were the vanillol isomers, and the prevailing by-product was 3-methoxy-4hydroxybenzylmethylether, obtained by reaction between p-vanillol and methanol. In the absence of methanol the prevailing by-products were diarylmethanes. Even though both byproducts can be considered secondary ones from a chemical point of view, i.e., formed starting from vanillols, they were found to form even at low guaiacol conversion. Therefore vanillols, once formed in the zeolitic pores, generate therein the corresponding benzyl carbocations and undergo nucleophilic attack by either methanol, guaiacol or another vanillol. The reactivity of vanillic alcohol isomers was also directly checked, both in the presence and in the absence of formaldehyde and of methanol. The results show the different tendencies of the isomers to give consecutive transformations. 1. INTRODUCTION The hydroxyalkylation of activated arenes (containing functional groups such as the hydroxy or methoxy groups) with aldehydes and ketones is a reaction of interest for the production of drugs, polymers, and food additives [1-3]. For instance, the hydroxymethylation of 2-methoxyphenol (guaiacol) represents one-step in the multistep synthesis of 3-methoxy-4-hydroxybenzaldehyde (vanillin, VA), an environmentally friendly process for the production of this important food additive [2]. OH ~/OCH3

OH H~C~H

H2C~ OH

Hydroxyalkylations are catalyzed by Lewis-type acids, like A1C13, and mineral Br6nsted acids. Some papers and patents have appeared in recent years, where zeolitic materials are

566 described as catalysts for this reaction [1-6]. Solid acid materials are highly desirable catalysts, since the environmental impact of the process benefits from easier separation of the catalyst, the absence of liquid wastes containing inorganic salts, and less severe corrosion problems [ 1]. Usually, the condensation between arenes and aldehydes is carried out in the liquid phase, and large-pore zeolites are necessary in order to make the reaction occur at an acceptable rate in the condensed phase. When formaldehyde is the reactant, water is the solvent, since the aqueous solution of formaldehyde (formalin) is the simplest, cheapest and most available reactant from the commercial point of view. This implies the need for hydrophobic zeolites, in order to avoid preferential filling of the pores by more polar water molecules rather than by the aromatic substrate [7]. In previous studies, we have analyzed the main reaction parameters affecting catalytic performance with zeolitic catalysts, and the effect of the methanol concentration in the formalin solution [8,9]. The objective of the present study was to analyze the nature of the by-products which are formed under different reaction conditions, with specific attention being given to the consecutive products of transformations occurring on VA isomers. 2. EXPERIMENTAL Catalytic tests were carried out in a glass, batch reactor, loading 48 ml of an aqueous solution of formaldehyde, and 1 g of a commercial zeolite (H-mordenite HM-45 supplied by Engelhard) characterized by a Si-to-A1 atomic ratio equal to 23. The mixture was then heated to 80~ and 4 ml of guaiacol were added under stirring. The reaction mixture was left at 80~ under vigorous stirring (600 rpm) for varying reaction times up to 12h. For the tests on VAs, 0.4 g of each compound was loaded in the reactor, while all the other amounts and conditions remained the same as for the tests of guaiacol hydroxymethylation. The commercial aqueous solution of formaldehyde typically contains 37 wt.% formaldehyde and 10-15% methanol (the latter inhibits the formation of higher molecular weight polyoxymethylenes, which would precipitate and separate from the aqueous solution). Some tests were carried out using non-commercial aqueous solutions of formaldehyde, containing 29-30 wt.% formaldehyde and 1.5 wt.% methanol. The products were analyzed by HPLC (TSP Spectra Series), equipped with an Alltech Hypersil ODS column, and with a UV-Vis TSP UV 150 detector (~, 280 nm). Elution was done with a mixture of acetonitrile and water. Identification of products was made by GC-MS and by comparison with the retention time of standard components (when available).

3. RESULTS AND DISCUSSION

3.1. The distribution of products in guaiacol hydroxymethylation Plotted in Figure 1 is the selectivity to products as a function of guaiacol conversion (the latter having been varied by varying the reaction time), for tests carried out using a commercial formalin solution containing approx. 15 wt. % methanol and 37% formaldehyde (Figure 1 left), and for tests carried out using a non-commercial formalin solution, containing a minimal concentration of methanol (1.5 wt.), and 29 % formaldehyde (Figure 1 right). The conversion of guaiacol had a considerable effect on the distribution of products. The main

567 products were vanillic alcohols (o-VA: 2-hydroxy-3-methoxybenzyl alcohol; m-VA: 3hydroxy-4-methoxybenzyl alcohol; p-VA: 3-methoxy-4-hydroxybenzyl alcohol), diaryl compounds (having mainly MW 260, with a minor amount of compounds having MW 290), and monoaryl compounds other than VAs. Amongst the latter, the predominant compounds were 2-methoxy-3-hydroxybenzylmethylether (MW 168), obtained by etherification between p-VA and methanol, and the compound obtained by etherification of p-VA with the hemiformal (MW 198). The main differences between the two sets of tests concern (i) the overall higher selectivity to VAs obtained in the presence of methanol, and (ii) the nature of the by-products, which were substantially different in the two cases. Summarized in Figure 2 are the details concerning the selectivities to the by-products, for the tests carried out under methanol-rich conditions and methanol-lean conditions, and in correspondence with two different guaiacol conversions. For tests carried out under methanol-rich conditions, and for low guaiacol conversion, the prevailing by-products were monoaryl compounds, while only traces of diaryl compounds (MW 260) were found. At higher conversion, the main by-products remained the monoaryl compounds (MW 168 and 198). In all cases the formation of di-hydroxyalkylated monoaryl by-products was negligible. Blank tests made in the absence of the zeolite demonstrated that a mild acidity is sufficient to protonate VAs, generate the corresponding benzyl cation and let it undergo nucleophilic attack by methanol or hemiformal. In the case of tests carried out under methanol-lean conditions, the main by-products at low guaiacol conversion were diaryl compounds having MW 260. The latter is obtained by reaction between one molecule of guaiacol and one of VA. The same compound can also be formed by reaction between two VA molecules (MW 290) followed by elimination of one formaldehyde molecule. Traces of triaryl by-products were detected. At high guaiacol conversion, diaryl and polyaryl compounds were the predominant by-products. The data indicate that the presence of methanol drastically modifies the distribution of byproducts. Methanol preferentially reacts with VAs, forming monoaryl ether, thus inhibiting the formation of diaryl by-products. This effect is observed even at low guaiacol conversion, and this indicates that the rates of the parallel reactions for the formation of the primary products are affected considerably by the presence of methanol, due to the fact that the latter is a better nucleophile than VAs.

.>

70

70

60

60

50

50

40

40

30

30

20

20

IIi1~_. i

0

20

m l

40

m

_.j

[

60

m

l

guaiacol conversion (%)

10 m i

80

0 100

0

20

40

60

guaiacol conversion (%)

80

100

Figure 1. Selectivity to the products as a function of guaiacol conversion. Left: tests carried out with 15 wt.% methanol in formalin. Right: tests carried out with 1.5 wt.% methanol in formalin. Symbols: selectivity to p-VA (A), o-VA ( . ) , m-VA (11), by-products (O).

568 20 .~ 16

._~ 12 ~

g

~

4

!iii!iiiiil !iiiiiiiiil

~ 30

ii!i!iiiiil

~

i!iiiiiiiil

iiiiiiiiiil 21% conv.

50 I

~i~i~ 49% conv.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

20 __~__

o 18% conv

67% conv

Figure 2. Selectivity to the by-products at different conversion levels. Left: tests carried out with 15 wt.% methanol in formalin. Right: tests carried out with 1.5 wt.% methanol in formalin. Dotted bars: 2-methoxy-3-hydroxybenzylmethylether (MW 168). Open bars: bisarylmethanes (MW 260). Full bars: other monoaryl by-products (mainly MW 198). Figure 1 (left) shows that in the presence of methanol the main consecutive reactions occurred on p-VA, the selectivity of which showed the steepest decrease with increasing guaiacol conversion, especially when the latter was higher than 60%. The selectivity to m-VA and o-VA initially decreased slightly, but then stabilized and finally even increased, possibly because of the non-negligible contribution of isomerization reactions for very long reaction times (up to 12 h reaction time was necessary to reach the highest guaiacol conversion). Also, it is possible that in the presence of extensive catalyst deactivation, the contribution of homogeneous acidity becomes important (the liquid bulk is always acid, due to the presence of formic acid), and finally a product distribution similar to the one achieved with homogeneous catalysis develops [8]. In the latter case, i.e., in the absence of shape-selectivity effects, the selectivity to p-VA was initially slightly higher than that to o-VA, but then for prolonged reaction times (i.e., with possible development of isomerization equilibria) the two isomers formed in comparable amounts, and both were approximately twice the amount of mVA, i.e., with a relative distribution of isomers similar to that observed in Figure 1 (left) for longer reaction times. Moreover, in heterogeneous tests and under conditions of catalyst deactivation (i.e., for long reaction times) the reaction of VA etherification with methanol is probably no longer reversible, and the selectivity to p-VA drops quickly. In the case of tests done in the absence of methanol (which were carried out for much shorter reaction times, since in the absence of methanol the conversion of guaiacol was much higher [8]), the highest contribution to consecutive reactions was again on p-VA, the selectivity of which exhibited a continuous decrease with increasing guaiacol conversion (Figure 1 right). The selectivity to o-VA and m-VA also exhibited a non-negligible decrease. In this case, the main contribution to the VAs disappearance was the irreversible formation of diarylmethanes (Figure 2 right), but also the monoaryl by-products formed in non-negligible amounts, despite the relatively low concentration of methanol. The higher reactivity of p-VA towards consecutive reactions is due to both a steric effect (p-VA and m-VA diffuse more quickly than o-VA into zeolitic cavities due to the lower steric hindrance), and to electronic effects. The carbocation which develops by protonation of the hydroxymethyl group and exit of water in p-VA is more stable (and therefore its formation is quicker) than the corresponding benzyl cations which form in o-VA and m-VA. This is due to the delocalization of the charge in those positions in the aromatic ring which feel more the

569 electron-donating mesomeric effect of the hydroxy group and less the electron-attracting inductive effect of the substituents. The benzyl cation formed in p-VA undergoes nucleophilic attack by (i) methanol or hemiformal (yielding the monoaryl by-products), (ii) guaiacol (yielding the three possible isomers of diarylmethane), and (iii) a second molecule of p-VA, o-VA or m-VA (again yielding the same three isomers of diarylmethane, after elimination of formaldehyde). The higher reactivity of p-VA as compared to the other isomers also explains why only three isomers of diarylmethane (MW 260) were found. A final consideration concerns the experimental evidence that the formation of all byproducts, at both low and high conversion, include VAs (a primary product) as the reactant, and thus correspond to compounds formed through consecutive reactions. On the contrary, Figure 1 clearly shows the important contribution of parallel reactions, with a selectivity to VAs which when extrapolated to nil conversion is not total. Therefore, even though VAs might be considered the exclusive primary products from a chemical and mechanistic point of view, they can not be considered as such from a kinetic point of view. This contradiction can be explained by making the hypothesis that a fraction of VAs generates a stable carbocation in the zeolitic pores, and is transformed into by-products before going into the bulk liquid phase. This effect should theoretically mainly involve o-VA, which is sterically more hindered than the other isomers, and the counterdiffusion of which therefore should be slower, with a corresponding higher time constant. This hypothesis might also explain why the presence of methanol had a considerable effect on the initial selectivity to o-VA (i.e., measured at low guaiacol conversion). Figure 1 shows that the overall selectivity to VAs was higher when the reaction was carried out in the presence of methanol, and the difference was mainly due to the higher selectivity to o-VA, which was systematically higher (approx. 10% more) at both low and high guaiacol conversion. Methanol may in part inhibit the interaction between o-VA and the active sites, and thus decrease the mean residence time of the molecule in the pores and accelerate its counterdiffusion, saving it from further transformations. This may have a positive effect on the initial selectivity to this compound. On the contrary, the effect of methanol on initial selectivity was practically nil for p-VA (compare Figure 1 left and right). On the other hand, p-VA in the bulk liquid phase can readily re-enter the zeolite and generate stable carbocations inside the cavities, thus undergoing consecutive transformations more quickly than the other isomers do.

3.2.The reactivity of vanUlic alcohols In order to better understand the observed phenomenology, reactivity tests were made by loading separately in the reactor each one of the three VA isomers, and by carrying out the reaction in the presence of the H-mordenite catalyst, under different reaction conditions (Figure 3): (i) in the presence of only water, (ii) with water and methanol (with an alcohol content simulating the amount which is present in the reaction medium for tests carried out with the commercial formalin solution), and (iii) with the commercial formalin solution (i.e., containing = 15 wt.% methanol). All the other reaction conditions (i.e., time, temperature and amount of catalyst) were kept the same as for the standard reactions of guaiacol hydroxymethylation. The following considerations can be drawn: 1) The tests done with VAs in water (Figure 3 top left) are indicative of the tendency of each VA isomer to react in the presence of the H-mordenite, to yield either intramolecular or

570 intermolecular transformations in acid media. The following reactivity scale was found: p-VA > m-VA > o-VA, which does not fully correspond to the trend expected on the basis of the relative stability scale of the benzyl cation generated by protonation of the hydroxymethyl group and exit of water (i.e., p-VA > o-VA > m-VA). It is useful to mention that under the same conditions guaiacol was completely unreactive. The high reactivity of p-VA derives from (i) the higher stability of the corresponding benzyl cation, which makes the latter a strong electrophilic agent, and (ii) the higher diffusivity of the molecule in the pores of the zeolite. The low reactivity of o-VA is likely due to the slower diffusion of the molecule into the zeolite, made more difficult by steric hindrance (and indeed this shape-selectivity effect is the reason why with H-mordenite a considerably higher selectivity to p-VA than to o-VA is obtained in guaiacol hydroxymethylation with respect to tests carried out in homogeneous acid media [8]). These data confirm that vanillic alcohols give consecutive reactions of transformations even in the absence of formaldehyde and methanol, and that under these conditions the most reactive VA is p-VA. The contributions to conversion were: (i) isomerization, (ii) formation of small amounts of diaryl compounds, and (iii) formation of products which were not analytically found, thus explaining the "C-unbalance" seen in Figure 3. Concerning the formation of diaryl compounds, since they are insoluble in water but soluble in water/methanol, it was possible to detect them by adding methanol to the batch after conclusion of the reaction. Surprisingly, the prevailing diaryl compounds detected were those having MW 260. This indicates that after condensation of two VA molecules, a formaldehyde molecule is soon eliminated. The low yield to diaryl compounds indicates that this reaction is not rapid under these conditions. The extent of isomerization of p-VA and o-VA was much less than that of m-VA. Under the hypothesis of kinetic control, this is explained by considering that in the case of m-VA the driving force for the intramolecular shift of the hydroxymethyl group from the meta position to the ortho or to the para position is the formation of an intermediate carbocation which is more stable than that originally formed. This reaction is analogous to the ipso-substitution in the isomerization of alkylaromatics. Since the formation of heavy, polyaryl compounds (which are not eluted in the column under our analytical conditions) was unlikely, the C-unbalance was due to the retention of VAs in the zeolite. The absolute amount of VA which was retained in the zeolite porosity, as inferred from the C-unbalance, follows the scale p-VA > m-VA ~ o-VA, that does not correspond to the stability scale of the corresponding benzyl cations. This hypothesis is also confirmed by the colour which developed in the mordenite when put in contact with VAs (absorption at 550 nm wavelength from p-VA, at 430 nm from o-VA and at 700 nm from mVA), indicating the presence of the corresponding cations adsorbed on the zeolite surface and stabilized by this interaction [3]. A strong interaction of the reactant with the catalyst has also been proposed to occur in the case of furfuryl alcohol hydroxymethylation, at low concentration of formaldehyde [ 10]. Due to the interaction between furfuryl alcohol and the zeolite, and to the competition between the two reactants, saturation of the catalyst surface occurred, with inhibition of the reaction rate. 2) In the case of tests carried out in the presence of water/methanol mixtures (Figure 3, top right), the o-VA and m-VA conversions were enhanced with respect to tests carried out in the presence of only water, while the conversion of p-VA was substantially unaffected by the presence of methanol. Also in this case, guaiacol was completely unreactive.

571 The formation of diaryl compounds was very low, and the main product obtained was that of etherification with methanol. Also in this case, however, the majority of converted VA indeed contributed to the C-unbalance, thus to the amount of vanillol retained in the catalyst pores. The absolute amount of missing VA followed the scale: p-VA > o-VA >> m-VA. The substantial absence of diaryl compounds suggests that the formation of the latter only occurs in the presence of formaldehyde and in the absence of methanol. In guaiacol hydroxymethylation and in the presence of methanol, methanol reacts quickly with p-VA, and the rate of condensation of two VA molecules becomes practically nil. An alternative explanation is a solvent effect, since the benzyl cation may be solvated by methanol molecules, thus hindering the attack on a second p-VA molecule. The effect of methanol on VAs conversion can be interpreted by considering that p-VA reacts through a SNl-type mechanism (as usually occurs for more stable carbocations), and therefore the rate of generation of the cation (the rate-determining step) is substantially unaffected by the presence of a nucleophile (while the distribution of the products obtained, instead, is obviously affected by the type of nucleophilic species present in the reaction medium). On the contrary, in the case of o-VA and m-VA, the corresponding benzyl cations are less stable, and the mechanism of transformation may reasonably involve a SN2-type mechanism, in which the concentration of methanol (the nucleophilic species) contributes positively to the reaction rate. In confirmation of this, the scale of VA reactivity for the formation of the products of etherification was m-VA > o-VA > p-VA, which corresponds to the reverse of the scale of stability for the corresponding benzyl cations. In other words, in the presence of the nucleophilic methanol, m-VA quickly reacts, and yields a large fraction of the arylmethylether (and correspondingly the fraction of vanillol retained in the zeolite is very low), while p-VA generates a stable cation, and behaves similarly in the absence and in the presence of methanol (except for the different type of product obtained), with a large fraction of the cation retained in the zeolite. Therefore, under these conditions any shape-selectivity effect of the zeolite is rendered nil by the presence of a strong chemical interaction with pVA. 3) When the reaction was carried out in the presence of the commercial formalin, further differences between VAs were observed. The conversion of all VA isomers was lower than in the previous cases; the scale of reactivity was m-VA = o-VA > p-VA. This is because formaldehyde (present in large excess) is preferentially protonated and competes with VAs for protonation on the active sites. Therefore, the nature of the rate-determining step for VA conversion is different from that occurring in the absence of formaldehyde. The products of VA transformation were mainly monoaryl compounds obtained by etherification with the hemiformal or with methanol; minor amounts of products obtained by hydroxymethylation of VAs were also found. The lower degree of interaction of VAs with the acid sites of the zeolite is also demonstrated by the decrease in the C-unbalance, which indicates a decrease in the absolute amount of species which are retained in the zeolite. This is more evident for p-VA (the corresponding C-unbalance was very low), which generates the most stable carbocation and reacts quicker than the other isomers. Furthermore, p-VA itself is a better nucleophilic agent, and reacts quicker with protonated formaldehyde than the other VA isomers do. These data also are in line with the higher contribution of consecutive reactions (i.e., etherification) occurring on p-VA rather than on other isomers in tests of guaiacol hydroxymethylation. The high yield to monoaryl compounds obtained from m-VA can be attributed to a contribution of the transformation of p-VA, the latter having been generated by isomerization of m-VA.

572 80

80 ~

60

......................

:!:! :.:.

"u "~' 40

iii ;-." !i:i

ou

~.'. !iii

20

--

*~ .~ 60

~ "~'40

1 ...~,

....

:::"- . . . . . . . . :::"

:::: ::::

i:!] iii.~ :::"

:i:i i'i" :i:i

o 20

:::: ":': ..

,

o-VA

50 40

,g~9 30 ~ 2o 8 10 0

,

m-VA

m

p-VA

iilnl

....

:;:-"

,

o-VA

~

::::

-

i:i: iiii ::::

[ -I

:':" ::::

-II

....

;';'

,

m-VA

":':

I

p-VA

Vanillols conversion Sel. to isomerization Sel. to monoaryl compounds Sel. to diaryl compounds C unbalance

iil i

o-VA

.....

i

m-VA

p-VA

Figure 3. Vanillol conversion and yield of products for tests done in only water (top left), in water/methanol (top right), and in commercial formalin (bottom). 4. REFERENCES 1. R.A. Sheldon, H. van Bekkum, in "Fine Chemicals through Heterogeneous Catalysis", R.A. Sheldon and H. van Bekkum (Eds.), Wiley-VCH, 2001, p. 1. 2. P. Metivier, in "Fine Chemicals through Heterogeneous Catalysis", R.A. Sheldon and H. van Bekkum (Eds.), Wiley-VCH, 2001, p. 173. 3. A. Corma, H. Garcia, J. Chem. Soc., Dalton Trans., (2000) 1381. 4. C. Moreau, F. Fajula, A. Finiels, S. Razigade, L. Gilbert, R. Jacquot, M. Spagnol, in "Catalysis of Organic Reactions", F.A. Herkes (Ed.), Marcel Dekker, New York, 1998, p. 51. 5. C. Moreau, S. Razigade-Trousselier, A. Finiels, F. Fazula, L. Gilbert, WO patent 96/37452 (1996), assigned to Rhone-Poulenc Chimie. 6. N. Barthel, A. Finiels, C. Moreau, R. Jacquot, M. Spagnol, J. Molec. Catal., A: Chemical, 169 (2001) 163 7. A. Finiels, P. Geneste, J. Lecomte, F. Marichez, C. Moreau, P. Moreau, J. Molec. Catal., A: Chemical, 148 (1999) 165. 8. F. Cavani, M. Corrado. R. Mezzogori, J. Molec. Catal., in press 9. F. Cavani, R. Mezzogori, Catal. Org. React., submitted 10. J. Lecomte, A. Finiels, P. Geneste, C. Moreau, J. Molec. Catal., A: Chemical, 133 (1998) 283.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

573

Unraveling the Nature and Location of the Active Sites for Butene Skeletal Isomerization over Aged H-Ferrierite Sander van Donk, Eveline Bus, Alfred Broersma, Johannes H. Bitter and Krijn P. de Jong* Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands *Corresponding author, fax: +31302511027; e-mail: [email protected]

The relation between the catalytic performance, the number and location of the accessible active sites and the nature of carbonaceous deposits was established for aged H-ferrierite during n-butene skeletal isomerization. In situ infrared spectroscopy reveals that the deposition of carbonaceous species significantly lowers the number of Bronsted sites. With short time-on-stream such deposits display large reactivity and induce by-product formation but also contribute to part of the isobutene production. With prolonged time-on-stream the deposits are converted into non-reactive carbon species and accordingly isobutene selectivity is enhanced. Probing with d3-acetonitrile does not reveal the presence of carbenium ions at this stage. Additionally, it is established that part of the Bronsted acid sites in the 10 membered-ring channels are still accessible and most likely catalyze the selective conversion of n-butene into isobutene.

1.

INTRODUCTION

Over the last decade a growing demand for isobutene has provided large industrial and scientific interest for the skeletal isomerization of linear butenes to isobutene. Isobutene is used in the petrochemical industry for the production of e.g. polyisobutene, methacrolein, synthetic rubber and MTBE. Zeolites containing 10-membered ring (MR) pores have proven to exhibit high selectivities for the butene skeletal isomerization [1-3]. Especially the twodimensional zeolite H-ferrierite in its acidic form (H-FER), for which the 10 MR main channels are interconnected by 8 MR side pores, exhibits an exceptionally high selectivity and stability [4,5]. Nevertheless initial isobutene selectivity is rather poor, but with longer timeon-stream (TOS) very high selectivities are reached which is often associated with a change in the prevailing reaction pathway [6-10]. Conclusive evidence for this was provided by Meriaudeau et al. [8], de Jong et al. [9] and Cejka et al. [10] using 13C-labelled butenes, demonstrating that over a fresh H-FER scrambling of the 13C-label occurred whereas with prolonged TOS hardly any scrambling was observed. This clearly indicates that initially nonselective dimerization-cracking reactions running over the H-FER Bronsted acid sites dominate the catalytic action. However, with prolonged TOS a highly selective reaction pathway prevails [8-10] and coincides with the presence of carbonaceous deposits that largely fill the H-FER internal pore-volume [3,7,9].

574 The nature of the selective reaction pathway over aged H-FER and the role of the carbonaceous deposits are still under debate. Several authors [8,11-14] suggested a monomolecular reaction pathway as the selective route for isobutene production. However, the direct conversion of n-butene into isobutene over a plain Bronsted acid site involves the formation of an energetically and thermodynamically highly unfavorable primary carbenium ion [15]. Guisnet et al. [6,16] therefore proposed an altemative pathway in which no primary carbenium intermediate is formed. This so-called pseudo-monomolecular reaction pathway should run over alkyl-aromatic tertiary carbenium ions prefixed in the coke instead of plain Bronsted acid sites. The aim of this study is to unravel the role of the carbonaceous deposits and the number, nature and location of the active sites during butene skeletal isomerization over aged H-FER. Therefore the following research strategy is employed: (1) The catalytic performance of H-FER as a function of the amount of carbonaceous species deposited is evaluated under differential conditions in a catalysis set-up including a tapered element oscillating microbalance (TEOM). (2) The nature of carbonaceous deposits with TOS is established by in situ infrared (IR) spectroscopy. (3) The locations of the vacant Bronsted acid sites with TOS are determined using in situ IR spectroscopy and subsequent deconvolution. Assignment of the differently located Bronsted-groups is established by taking into account previous studies [17,18] regarding the H-FER structure. (4) H-FER samples with different amounts of deposits, obtained after evaluation in the TEOM, were subsequently examined by IR spectroscopy establishing the nature and number of the accessible active sites by in situ probing with d3-acetonitrile (CD3CN). This enables discrimination between plain Bronsted acid sites and adsorbed carbenium ions [ 19,20]. Elucidation of the nature of the active sites contributes to the longstanding discussion whether a monomolecular [8,11-14] or a pseudo-monomolecular [6,16] reaction pathway dominates the catalytic action over aged H-FER. Moreover, based on the relation between the catalytic performance, the number and location of the active sites and the role of carbonaceous deposits, an overall reaction scheme for butene skeletal isomerization over aged H-FER as a function of TOS is introduced. 2.

EXPERIMENTAL

2.1. Ferrierite samples and catalysis measurements High silicon NH4-FER (Zeolyst Int. Si/A1 30) was activated in a dry N2 flOW at 823 K for 12 hours to obtain H-FER. The acid site density of H-FER is 0.53 mmol.g -1, determined by temperature programmed desorption-thermogravic analysis (TPD-TGA) of n-propylamine, and the micropore volume is 0.132 ml.g 1, established by N2 physisorption. Catalysis measurements were performed in a set-up including a tapered element oscillating microbalance (Rupprecht & Pataschnik TEOM 1500 PMA), which offers the possibility to quantitatively monitor the carbon deposition rate and amount in situ, see Hershkowitz and Madiara [21] and Chen et al. [22].

575 The TEOM is connected to a Shimadzu 17A gas chromatograph with a Chrompack PLOT capillary column (fused silica-A1203/KCL, 50m x 0.32mm) and a flame ionization detector, to analyze reaction products. For the differential catalysis measurements the TEOM reactor was loaded with 5-10 mg of H-FER particles (90-150 gin) with quartz wool on top and bottom of the bed to keep the particles firmly packed. The samples were dried in situ in N2 at 623 K and after switching to a pure n-butene gas-flow (Hoek Loos, 1-butene, > 99.5%), the catalytic performance and carbon uptake were monitored at 623 K, 1.3 bar. Conversion and selectivity were determined for H-FER samples with different amounts of deposits under differential conditions (conversion < 10 mole%) by adjusting the weight hourly space velocity (WHSV). Conversion is defined as the molar ratio of all products (4 n-butene) to all compounds detected. Selectivities are calculated as the molar ratio of a certain product to all products (;~ n-butene). All mass changes were corrected for temperature- and gas density differences by performing blank runs over inert samples. Immediately after the catalysis experiments the n-butene flow was switched off and the aged H-FER samples were studied by IR spectroscopy, probing with CD3CN (see also section 2.3.). It has been checked and confirmed for all aged H-FER samples that catalytic performance was not affected by cooling down to room temperature and contacting with air.

2.2. In situ infrared spectroscopy Spectra were recorded during the skeletal isomerization of n-butene at 623 K and 1.0 bar with an FT-IR spectrometer (Perkin Elmer Spectrum One) equipped with an in situ flow cell. The H-FER sample was pressed into a self-supporting wafer o f - 3 mg with 0.5 cm diameter, by applying a pressure of 200 MPa. The wafer was placed in a cylindrical oven and dried at 623 K under a He flow (10 ml.min-1). Next n-butene was added to the flow at 0.2 bar partial pressure. The first 4 hours of n-butene reaction, IR spectra were recorded at a time interval of 5 minutes using a MCT detector (20 scans in 22 seconds). After 4 hours spectra were recorded at a time interval of 5 hours using a MIR-TGS detector (100 scans in 9 minutes). The spectra were measured in transmission mode from 4000 to 1000 c m -1 (4 c m -1 resolution) and normalized to the overtone lattice vibration of the dried H-FER between 1880-1860 cm -1 (A = 0.3), which is a good measure of the thickness of the wafer. The v(OH) stretch region (4000-3000 cm -1) was deconvoluted using the Origin 6.1 software program, assuming Gaussian bandshapes. 2.3. Infrared spectroscopy: probing with d3-acetonitrile Probing experiments with CD3CN (Acros, 99% purity) were performed in the FT-IR instrument described above and sample preparation was identical. After drying the sample at 623 K under a He flow (10 ml.min-1), the sample was cooled to 448 K and the adsorption of CD3CN (partial pressure 5-10 mbar in 10 ml.min 1 He) was started. The applied conditions were chosen based on experimental checks revealing that 448 K is the highest temperature and 5 mbar the lowest partial pressure at which the maximum amount of Bronsted acid sites is probed. Spectra were measured in transmission mode from 4000 to 1000 cm -1 (4 cm -1 resolution) and normalized to the overtone lattice vibration of H-FER. In order to quantify the amount of CD3CN adsorbed on the sample, the v(CN) stretch region (2600-2100 cm -1) was deconvoluted using the Origin 6.1 software program.

576 Table 1. Conditions and catalytic performances of the aged H-FER samples at 623 K and 1.3 bar. Reaction conditions

WHSV (gc4=.gH.FER'I.h-1) TOS (h)

H-FER5.0C

H-FER6.6C

H-FER6.8C

169 4

142 20

21 300

5.0 9.5 35.5

6.6 8.9 38.0

6.8 8.5 91.2

16.1 5.7

12.6 4.8

1.8 1.6

T E O M - catalysis results

carbon uptake (guptake.gH_FER-1. 102) n-butene conversion (mole%) isobutene selectivity (mole%) Reaction rates

n-butene conversion (g.gH_FER'1.h"l) isobutene formation (g.gH.FER-1.h"l)

3.

RESULTS

3.1. On the catalytic performance The catalytic performance of H-FER as a function of the amount of carbonaceous deposits was evaluated in a catalysis set-up including a TEOM. Table 1 summarizes the applied conditions and the main results. Upon aging lower WHSV's are demanded to obtain similar n-butene conversions o f . . 9 mole% . This indicates that H-FER becomes less reactive with TOS, although in the end for H-FER6.8C n-butene is converted into isobutene with a selectivity o f - 91 mole% . By taking into account the applied W H S V ' s for the aged H-FER samples, reaction rates for the conversion of n-butenes and the formation of isobutene are calculated and also displayed in table 1. 3.2. On the nature of the carbonaceous deposits The in situ IR spectra for the region specific of carbonaceous species on H-FER are displayed in figure 1. The absorption at 1514 cm 1 corresponds to the C-C bond vibration of non-condensed aromatics [16,23]. This band is moderately present after 1 h TOS and increases with longer n-butene contact. Contributions around 1580 cm 1 and 1616 cm -1 reveal the formation of condensed aromatic species [16,23] and the signal around 1420 cm -1 indicates these species to be attached to cyclopentane tings. Moreover, the bending modes that are characteristic for alkyl-groups are present at 1352 c m 1 and 1438 cm -1 [16,23]. 3.3. On the location of the vacant Bronsted acid sites Figure 2 shows the decrease in intensity of the band characteristic for the Bronsted acid sites in H-FER as monitored by in situ I R spectroscopy during n-butene reaction. Initially the number of vacant Bronsted acid sites largely reduces upon n-butene contact. A f t e r - 17 hours TOS there is no further reduction and 5 % of the initial number of Bronsted sites is still vacant.

-201/ 2 :3

1.6

o

1.2

r == o

0.4 0.0

.-..100

40

0 h/ 5h 11 9

,

9

,

1600 1550 1500 14'50 14'00' 13'50 wavenumber

( c m "1)

Figure 1. In situ IR spectra of carbon deposited on H-FER during n-butene reaction at 623 K and 1.0 bar. The spectrum of the fresh H-FER is subtracted.

577

"o r

Figure 2. Intensity of the total band corresponding to Bronsted acid sites on H-FER with TOS at 623 K and 1.0 bar, normalized to the band at Oh TOS.

--&-- 10 MR

.Q "O

~ 6o c tli

-9 40

II1 ~

.

.

.

.

.

"g 20

c

A --

0

,

o

,

1o

~

,

,

20 30 TOS (hours)

,

,

40

.

Figure inset. Relative intensities of the peaks characteristic for Bronsted acid sites in the 10 MR channels (A) and in the 8 MR channels (ll) with short TOS. The values are normalised on their respective initial values at Oh TOS and obtained by deconvolution of the total Bronsted band.

50

The Bronsted band consists of a number of OH vibrations that are located at different positions in the H-FER framework. Zholobenko et al. [17] and Domokos et al. [18] demonstrated for fresh H-FER that it is possible to deconvolute this band into its separate contributions. In good agreement with the set of parameters proposed by Domokos et al. [ 18], we recently identified four types of Bronsted OH groups located in 10-, 8-, 6- and 5membered tings [24]. Moreover, it was demonstrated for the first time that it is possible to distinguish the differently located OH groups in the H-FER structure during the reaction of nbutene [24]. The inset in figure 2 displays the change in relative peak areas with short TOS of the sites in the 10 MR main channels and 8 MR side channels. 3.4. On the nature and number of the accessible active sites The H-FER samples displayed in table 1 were probed with CD3CN to establish the number and nature of the active sites. In figure 3 the difference spectra of the v(OH) and v(CN) stretch vibrations for fresh H-FER and for the aged H-FER samples are shown. Upon adsorption of CD3CN the difference band assigned to the Bronsted sites at 3580 cm -1 (left figure) is reduced in intensity for all aged H-FER samples as compared to the fresh H-FER. After CD3CN adsorption the Bronsted band does not disappear completely (not shown in figure), implying that not all sites are accessible for CD3CN. Additionally, the maximum of the remaining Brensted acid band is shifted towards slightly lower frequencies. 0.0

O c

i,., O

v(CN)

(,,)

0.2

~

E:

-0.1

V

H-FER

fresh

v(OH)

-0.2 '

36'oo ' 35'5o wavenumber (cm 4)

I,,. O ,~ .Q

H'iI~Rfresh

0.1

0.0

2300

2250

wavenumber (cm "1)

Figure 3. IR difference spectra of CD3CN (5-10 mbar in 10 ml.min -1 He-flow) adsorbed on fresh HFER, H-FERS.0C, H-FER6.6C and H-FER6.8C at 448 K. Left: v(OH)-region, negative peaks correspond to the disappearance of the OH stretch vibration. Right: v(CN)-region, positive peaks correspond to the appearance of the CN stretch vibration.

578

H-FER6.8C H-FER6.6C H-FER5.0C

0

500

1000

1500

TOF (mmol.mmol'l.h "1)

2000

Figure 4. Turn-over frequencies for the aged H-FER samples at 623 K and 1.3 bar; defined as mmol butene converted (I) or mmol isobutene formed (!"1) per mmol accessible Bronsted acid site (as probed by CD3CN) per hour.

The disappearance of the Bronsted peak correlates with the appearance of the peak at 2292 cm -1 (fight figure). This peak can be solely assigned to the stretch vibration of v(CN) coordinated to a Bronsted acid site, as has been reported in other studies [20,25-27]. The v(CN) stretching mode allows quantification of the accessible Bronsted acid sites present in the aged H-FER samples. The numbers are calculated by taking the peak areas relative to fresh H-FER, for which the number of Bronsted acid sites is 0.53 mmol.g 1 according to n-propylamine TPD-TGA. Considering the number of accessible Bronsted acid sites, the reaction rates presented in table 1 can be transformed into turn-over-frequencies (TOF). The TOF's are displayed in figure 4, giving the moles of n-butenes converted or moles of isobutenes formed per mole accessible Bronsted acid site, as probed by CD3CN, per hour. Bystrov [19] and Jolly et al. [20] reported that the adsorption of CD3CN on zeolites aged in hydrocarbon reactions may result in the appearance of a strong band between 2387 cm -1 and 2377 cm -1, characteristic for the vibration of v(CN) bound to a C + of a carbocation. However, we recently showed that no such band is present upon CD3CN probing of the aged H-FER samples as well as upon probing during butene skeletal isomerization [24]. 4.

DISCUSSION

From the results presented in table 1, figure 1 and by other groups [3,7,9], it is obvious that the formation of carbonaceous deposits accompanies the skeletal isomerization of nbutene over H-FER. Figures 2 and 3 reveal that the deposition of carbon reduces the overall number of accessible Bronsted acid sites. The inset in figure 2, obtained after deconvolution of the Bronsted band [24], indicates that already with short TOS Bronsted sites are lost in both the 8 MR side pores and 10 MR main channels. Moreover, the 8 MR-signal rapidly levels off implying that its entrances are blocked, leaving sites inaccessible for n-butene. Figure 3 proofs that with extensive aging of H-FER, Bronsted sites remain accessible for CD3CN. Given that the OH groups in the 8 MR's vibrate at lower wavenumber than those in the 10 MR's [17,18,24], the observed shift of the peak-maximum confirms that sites in the side-pores are inaccessible for CD3CN. Therefore the Bronsted sites that are still accessible will be predominantly located in the 10 MR channels; hence these sites are involved in the catalytic action. This result is in excellent agreement with Domokos et al. [ 18] who reported on the relation between acid site locations in sodium exchanged H-FER samples and their catalytic performance. In these studies a structure-activity relation was observed between the presence of Bronsted acid sites in the 10 MR channels and the selective formation of isobutene. We show that this structure-activity relation also exists under conditions where carbonaceous deposits largely fill the H-FER micropore volume [24].

579 prevailing reaction and location of the active sites

Non-selective bimolecular reactions throughout the crystals and carbon deposition Both non-selective reactions of deposits and selective isobutene formation over Bronsted sites in the 10 MR pore entrances Selective isobutene formation over Bronsted sites in the 10 MR pore entrances

N-BUTENE ~.__

$:

fast

ISOBUTENE, PROPENE, PENTENES,etc.

H-FER

fresh, without deposits

fast ; oligomerization, cyclization

L_

"~ p,

E

(~ I

H-FER

aged with aikyl-aromatic deposits

slow [H-transfer

N-BUTENE ~ I S O B U T E N E , PROPENE,

ks/~PENTENES, etc. "~ f-

H-FER

aged with condensed aromatic deposits

ISOBUTENE N-BUTENE

~l~w

ISOBUTENE

Figure 5. Schematic overview of the prevailing reactions, the nature of carbonaceous deposits and the locations of the active sites during n-butene skeletal isomerization over H-FER with TOS. Guisnet et al. [6,16] claimed that with prolonged TOS all Bronsted acid sites are inaccessible for reactants. A so-called pseudo-monomolecular reaction pathway running over alkyl-aromatic tertiary carbenium ions captured inside the pore-entrances of the zeolite should dominate the selective catalytic action. However, the CD3CN probing results [24] do not support the presence of carbenium ions and the occurrence of a pseudo-monomolecular pathway. This was further confirmed by Asensi et al. [13], who showed that on a high-silica H-FER (Si/A1 = 59, i.e. low number of Bronsted acid sites) high isobutene selectivities were reached without the substantial formation of deposits. At atmospheric or higher pressures the deposition of carbonaceous species during butene skeletal isomerization can not be avoided, see table 1 and figure 1. The role of these deposits is rather ambiguous. According to figures 2 and 3, the deposition of carbon lowers the amount of Bronsted sites, consequently suppressing non-selective bimolecular reactions. Table 1 indicates that the deposition of reactive species significantly contributes to the overall product formation, although the catalytic action is not at all selective since besides isobutene numerous by-products are formed. With extensive aging of H-FER, figure 1 reveals that the nature of deposits changes from hydrogen rich towards hydrogen-poor aromatics. These last species are not very reactive and accordingly isobutene selectivity is indirectly enhanced. Table 1 shows that for the aged and selective catalyst H-FER6.8C, these alterations result in a net decrease of both the n-butene conversion and the isobutene formation rate, indicating that indeed part of the isobutene was initially formed from these deposits. Figure 5 introduces an overview of the relations between the catalytic performance, the number, nature and location of the active sites and the role of carbonaceous deposits during butene skeletal isomerization over aged H-FER with TOS. 5.

CONCLUSIONS

In situ I R spectroscopy reveals that during butene skeletal isomerization the deposition of carbon significantly lowers the number of vacant Bronsted acid sites. Initially such deposits are involved in non-selective reactions. However, with prolonged TOS the deposits change into non-reactive species and accordingly isobutene selectivity is enhanced. Probing with CD3CN reveals that at this stage no carbenium ions are present, while Bronsted acid sites in

580 the 10 MR channels are still accessible. The latter sites most likely catalyze the selective conversion of n-butene into isobutene. ACKNOWLEDGEMENT

We thank Dr. T. Visser and Dr. F. de Groot for their contributions. This work was financially supported by the Netherlands Organization for Scientific Research (NWO/CW 700-97-019). REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

A.C. Butler and C.P. Nicolaides, Catal. Today 18 (1993) 443. P. M6riaudeau and C. Naccache, Adv. Catal. 44 (1999) 505. S. van Donk, J.H. Bitter and K.P. de Jong, Appl. Catal. A Gen. 212 (2001) 97. P. Grandvallet, K.P. de Jong, H.H. Mooiweer, A.G.T.G. Kortbeek and B. KraushaarCzarnetzki, European Patent No. 501 677 (1992), to Shell. H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork and B.C.H. Krutzen, Stud. Surf. Sci. Catal. 84 (1994) 2327. M. Guisnet, P. Andy, N.S. Gnep, C. Travers and E. Benazzi, J. Chem. Soc. Chem. Commun. (1995) 1685. W.-Q. Xu, Y.-G. Yin, S.L. Suib and C-L. O'Young, J. Phys. Chem. 99 (1995) 758. P. M6riaudeau, R. Bacaud, L.N. Hung and T.A. Vu, J. Mol. Catal. A 110 (1996) L177. K.P de Jong, H.H. Mooiweer, J.G. Buglass and P.K. Maarsen, Stud. Surf. Sci. Catal. 111 (1997) 127. J. Cejka, B. Wichterlov~ and P. Sarv, Appl. Catal. A Gen. 179 (1999). G. Seo, H.S. Jeong, D.-L. Jang, D.L. Cho and S.B. Hong, Catal. Lett. 41 (1996) 189. J. Houzvicka and V. Ponec, Ind. Eng. Chem. Res. 36 (1997) 1424 M.A. Asensi and A. Martinez, Appl. Catal. A Gen. 183 (1999) 155. G. Seo, M.-Y. Kim and J.-H. Kim, Catal. Lett. 67 (2000) 207. D. Brouwer and J. Oelderik, Rec. Trav. Chim. Pays Bas 87 (1968) 1435. P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi and C. Travers, J. Catal. 173 (1998) 322. V.L. Zholobenko, D.B. Lukyanov, J. Dwyer and W.J. Smith, J. Phys. Chem. B 102 (1998)2715. L. Domokos, L. Lefferts, K. Seshan and J.A. Lercher, J. Mol. Catal. A Chem. 162 (2000) 147. D.S. Bystrov, Zeolites 12 (1992) 328. S. Jolly, J. Saussey and J.C. Lavalley, Catal. Lett. 24 (1994) 141. F. Hershkowitz and P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969. D. Chen, A. Gronvold, H.P. Rebo, K. Moljord and A. Holmen, Appl. Catal. A Gen. 137 (1996) L1. Z.R. Finelli, C.A. Querini, N.S. Figoli and R.A. Comelli, Appl. Catal. A Gen. 187 (1999) 115. S. van Donk, E. Bus, A. Broersma, J.H. Bitter and K.P. de Jong, submitted. G. Pelmenschikov, R.A. van Santen, J. J~inchen and E. Meijer, J. Phys. Chem. 97 (1993) 11071. J. Kotrla and L. Kubelkova, Stud. Surf. Sci. Catal. 94 (1995) 509. C. Paz6, A. Zecchina, S. Spera, G. Spano and F. Rivetti, Phys. Chem. Chem. Phys. 2 (2000) 5756.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

581

H y d r o c o n v e r s i o n o f aromatics over a P t - P d ~ S Y catalyst C. Petitto a'b, G. Giordano b, F. Fajulaa and C. Moreau a aLaboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM- CNRS, Eeole Nationale Sup6rieure de Chimie de Montpellier, 8 Rue de l'Ecole Normale, 34296 MontpeUier Cedex 5, France bDipartimento di Ingegneria Chimica e dei Materiali, Universit/t della Calabria, Arcavacata di Rende (CS), Italia The hydroconversion of 1-methylnaphthalene was investigated over a 0.5 wt % Pt-0.3 wt % Pd/USY catalyst at 310 ~ and 5 MPa 1-12in the presence of different amounts of thiophene. Under those operating conditions, hydrogen transfer is not limiting and thermodynamics favours saturated products. The influence of sulphur present in the feed (200 to 1600 ppm S) was evaluated in the three major reactions involved in the hydroconversion process, i.e. hydrogenation, isomerisation and ring-opening reactions. Hydrogenation of 1methylnaphthalene into methyltetralines is always rapid whatever the sulphur amount. Hydrogenation of methyltetralines into the corresponding methyldecalines becomes lower with increased sulphur content, whereas isomerization of methyltetralines into alkylindanes as well as ring-opening of methyltetralines into alkylbenzenes becomes favoured. A mechanism involving hydrogen spillover, as recently proposed in the literature, would also account for the present results. 1. INTRODUCTION The aromatic content of diesel feedstock can vary broadly. Because of the increasing demand for cleaner distillates, de-aromatization of petroleum fractions is a basic process in the refinery. With respect to the new specifications for diesel fuel composition (decreased sulphur content, increased octane index, reduced particulate emissions, decreased aromatic content), highly active sulphur resistant catalysts were recently developed, mainly based on Pt, Pd, or mixtures of both, supported on acidic carriers [1-4]. In this paper, we present a study of the hydroconversion of 1-methylnaphthalene over a Pt-Pd/USY catalyst in the presence of thiophene. The influence of sulphur present in the feed is investigated in order to evaluate its effect on the course of the three major reactions involved, i.e. hydrogenation, isomerization and ring opening.

582 2. EXPERIMENTAL The P t - P d ~ S Y catalyst was prepared by impregnation procedures [ 1] in order to yield 0.5 wt % of Pd and 0.3 wt % of Pt. The catalyst was calcined in air for 2 h at 200 ~ and 4 h at 500 ~ and reduced in a 1-12flow for 3 h at 350 ~ Experiments were carried out in a 0.3 liter stirred autoclave working in the batch mode. 150 mg of the freshly reduced catalyst are added to 100 ml of a 0.1 M solution of 1-methylnaphthalene in cyclohexane to which is added thiophene (200 to 1600 S ppm). When the temperature reached 310 ~ hydrogen was introduced at the required pressure. Products were analysed by gas chromatography and identified by comparison with authentic samples and/or by GC/MS analyses. 3. RESULTS Preliminary experiments were performed to find experimental 1-12pressure and temperature conditions to ensure that i- hydrogen transfer is not diffusion limiting, ii- temperature is such that it is possible to measure accurately the partitioning of products resulting from the transformation of methyltetralines and iii- thermodynamics favours saturated products [5]. CH3

CH3

dimethylindanes

alkyltoluenes

l

A

4 -

i

C.H3~ Io. 3

i

5- and6-methyltetralines

1-methylnaphthalene

CH3

2_

~

--=CH3

cis + transmethyldecalines

1

1-and2-methyltetralines

CH3 ~ C H 3 dimethylindanes

3- ~ - - ~ 1 ' ~ R alkylbenzenes

Scheme 1: Simplified reaction scheme for the hydroconversion of 1-methylnaphthalene over a Pt-Pd/USY catalyst at 310 ~ and 5 MPa 1-12. In a typical run over the Pt-Pd/USY catalyst at 310 ~ in cyclohexane as the solvent, without thiophene added and 5 MPa 1-12 (Scheme 1), the main reaction intermediates in the

583 absence of sulphur are 5- and 6-methyltetralines and 1- and 2-methyltetralines (Fig. 1). 5methyltetraline results from hydrogenation of the benzene ring adjacent to the methyl group and 6-methyltetraline would result from isomerisation of 5-methyltetraline through formation of a bridged arenium ion [6]. 1-methyltetraline results from the hydrogenation of the ring bearing the methyl group and 2-methyltetraline would result from isomerisation of 1methyltetraline through formation of bridged cationic species as proposed for the cracking of lOO 8o

[]

-~ 60 "i

~o

o

2

40 20 o-

w

0

2

4

6

8

0

Time, h

2

4

6

8

Time, h

Figure l. Hydroconversion of 1-methylnaphthalene over Pt-Pd/USY catalyst, 310 ~ 5 MPa H2, cyclohexane as solvent.

Figure 2. Isomerisation and ring-opening percentages during 1-methylnaphthalene hydroconversion.

tetraline for example [7]. Hydrogenation over metal catalysts then features a behaviour similar to that observed for sulphided catalysts, i.e. one ring is less aromatic in character than the other one and, consequently, is more rapidly hydrogenated [8]. After 8 h of reaction, about 85 % of methyldecalines are formed with a mass balance close to 90-95 %. The percentage of C-C bond cleavage of methyltetralines into alkylbenzenes passes through a maximum at = 5 % alter 3 h and then decreases due to cracking of the lateral alkyl chain into C]0 and C9 hydrocarbons. In a similar manner, isomerisation of methyltetralines into alkylindanes also passes through a maximum at = 4 % after 2 h and then decreases relatively rapidly to yield both C5 ring-opening and cracking products. 100

100

8O

80

- 9

:~ ~

60

60

ppm

--D-200 --e- 400 -o--800 ~1600

ppm ppm

ppm

_

~

ppm

/

,

9 ------'

9

E 8

40

IlX L#/

0

~

- e - 400 ppm

-0-800

I!/

0

i,~

--~-200 ppm

2o I-//

~ 1

2

3

i,i,m

20

9

1600 ppm 4

Time, h

Figure 3. Influence of S content on the hydrogenation of 1-methylnaphthalene over Pt-Pd/USY catalyst, 310 ~ 5 MPa H~.

0

2

o

4

Time, h

6

8

Figure 4. Influence of S content on the formation of methyldecalines over PtPd/USY catalyst, 310 ~ 5 MPa H2.

584 In the presence of thiophene in the starting feed, significant effects are observed on the three major reactions involved for hydroconversion of 1-methylnaphthalene, i.e. hydrogenation, isomerisation and ring-opening reactions. Hydrogenation of 1-methylnaphthalene into 1- and 5- methyltetralines is always rapid whatever the amount of sulphur (Fig. 3) except at 1600 ppm S content. At the same time, hydrogenation of methyltetralines into the corresponding methyldecalines decreases with increased sulphur content (Fig. 4). Ring-opening of methyltetralines into alkylbenzenes (Fig. 5) as well as isomerisation of methyltetralines into alkylindanes (Fig. 6) then become favoured.

12

~R 10

- ' l - O ppm --g--200 ppm [ - - ~ 4 0 0 ppm / --0--800 ppm /

.,~

~ J ~ ~

~u ~ "I- -

~ /

~

~1600 ppm --o-800 ppm +400 ppm - ~ - 2 0 0 ppm

6 I

i~ ,3 4

~R 5

8 2

2

1

0

o

2

4 Time, h

6

8

Figure 5. Influence of S content on the ring-opening of methyltetralines into C11alkylbenzenes over Pt-Pd/USY catalyst, 310 ~ 5 MPa 1-/2.

0

2

4 Time, h

6

8

Figure 6. Influence of S content on the isomerisation of methyltetralines into C llalkylindanes over Pt-Pd/USY catalyst, 310 ~ 5 MPa H2.

In Table 1 are summarized the results obtained after 8 h of reaction and corresponding to a mass balance of 90-95 %. Table 1 Effect of sulphur amount on the main routes from 1-methylnaphthalene over Pt-Pd/USY at 310 ~ and PH2 = 5 MPa after 8 h of reaction time : concentrations in methyltetralines, methyldecalines, C11-alkylbenzenes and C11-alkylindanes. S content 0 ppm 200 ppm 400 ppm 800 ppm 1600 ppm

Methyltetralines (%) 0 10 40 53 61

Methyldecalines (~) 95 70 37 20 10

Cl~-alkylbenzenes (%) 2.5 8 12 14 12

C~l-alkylindanes (~) 1 1.5 3.5 6 6

A better manner to account for the importance of the effect of sulphur added in the feed is to take the partitioning of hydrogenation, ring-opening and isomerisation products from methyltetralines converted (Table 2). It can then be seen that ring-opening and isomerisation of methyltetralines represent more than 50 % of the reaction pathway at high sulphur content.

585 Table 2 Effect of sulphur amount on the partitioning of hydrogenation, ring-opening and isomerization products from methyltetralines converted on Pt-Pd/USY at 310 ~ and PH2 = 5 MPa after 8 h reaction time. S content 0 ppm 200 ppm 400 ppm 800 ppm 1600 ppm

Hydrogenation into methyldecalines (%) 96 87.5 71 50 36

Ring-opening into C11-alkylbenzenes (%) 2.5 10 23 33 43

Isomerisation into C11-alkylindanes (%) 1 2 6 17 23

A similar analysis can also be obtained from the experimental results reported at isoconversion of methyltetralines (Table 3). The influence of sulphur present in the feed is more important on C-C bond cleavage than on isomerisation reactions. Table 3 Effect of sulphur amount on ring-opening and isomerization products from methyltetralines converted at different methyltetralines conversions (10 to 30 %) over Pt-Pd/USY at 310 ~ and Pm = 5 MPa. S content 0 ppm 200 ppm 400 ppm 800 ppm 1600 ppm

C 11-alkylbenzenes %

C 11-alkylindanes %

10 %

20 %

30 %

10 %

20 %

30 %

1 3 5 9 14

3 7 6 14

4 9 12

2 3 2 3 6

3 4 3 6

3.5 3.5 3.5

4. DISCUSSION

Two additional experiments were also performed in order to have more information on the influence of hydrogen and acidity on the partitioning of reaction products and on the mechanism of hydroconversion : i- when hydrogen is removed from the autoclave after formation of methyltetralines, no further reaction occurs except partial dehydrogenation to 1methylnaphthalene. This clearly means that hydrogen must be present for hydrogenation reactions, of course, and also for cleavage of C-C bonds to yield alkylbenzenes, and ii- the addition of a stoichiometric amount of USY zeolite to the catalyst leads to an increase in the isomerisation steps rather than in hydrogenation or C-C bond cleavage reactions. Furthermore, the results obtained in the presence of Pt-Pd/USY catalysts closely parallel those obtained in the presence of conventional hydrotreatment catalysts, i.e. inhibiting effect of H2S on hydrogenation reactions and favourable effect on hydrocracking reactions through hydrogen spill-over were observed [9]. In the recent literature concerning aromatics hydroprocessing, it seems to be admitted that the modifications of the electronic properties of metal and acidic sites would be responsible for the enhancement of the thioresistance of Pt-Pd

586 catalysts supported on acidic carriers [ 1-3, 10]. Hydrogenation of aromatics would take place on acidic sites by hydrogen spilled-over from the metal sites, the participation of hydrogen spill-over to reactivity requiring close proximity of metal and acidic sites [3]. However, as already assumed by us from the comparison between hydrotreatment and metallic catalysts [8,11], protonic and hydride species resulting from the dissociation of hydrogen would be responsible for hydrogenation and hydrogenolysis reactions, respectively. In addition, the dissociation of H2S generated from the S-precursors leads to the formation of protonic and nucleophilic H S and/or S2 species [12] which should also be taken into account, as well as the possibility for tetralines and decalines intermediates to act as hydrogen donors. From the experimental results obtained, reduction of the hydrogenation route and corresponding increase of both isomerisation and C-C bond cleavage routes, we have then recently proposed that protonic species would be responsible for aromatics hydrogenation and that hydride species would be responsible for C-C bond cleavage [ 13], isomerisation reactions being more concerned by the acidity of the support as recently proposed in the literature [4].

REFERENCES

1. T. Fujikawa, K. Idei, T. Ebihara, H. Mizuguchi and K. Usui, Appl. Catal. A: Gen., 192

(2000) 253.

2. R.M. Navarro, B. Pawelec, J.M. Trejo, R. Mariscal and J.L.G. Fierro, J. Catal., 189 (2000) 184. 3. B. Pawelec, R. Mariscal, R.M. Navarro, S. van Bokhorst, S. Rojas and J.L.G. Fierro, Appl. Catal. A: Gen., 225 (2002) 223 and references therein. 4. M.A. Arribas and A. Martinez, Stud. Surf. Sci. Catal., 130 (2000) 2585. 5. B. Demirel and W.H. Wiser, Fuel Process. Technol., 55 (1998) 83. 6. Y.A. Borisov, N.I. Raevskii, E.S. Mortikov, V.A. Plakhornik and I.I. Lichehiner, Bull. Acad. Sci. USSR, Division Chemical Sciences, 3 (1992) 574. 7. A. Corma, V. Gonzalez-Alfaro and V. Orchill6s, J. Catal., 200 (2001) 34. 8. C. Moreau and P. Geneste, in "Theoretical Aspects of Heterogeneous Catalysis", J.B. Moffat, Editor, Van Nostrand Reinhold, N.Y., 1990, p. 256. 9. S. Giraldo de Le6n, P. Grange and B. Delmon, Appl. Catal. A: Gen., 107 (1993) 101, and references therein. 10. E. Guillon, J. Lynch, D. Uzio and B. Didillon, Catal. Today, 65 (2001) 201. 11. A. Finiels, P. Geneste and C. Moreau, J. Mol. Catal., A: Chem., 107 (1996), 385. 12. J. L6glise, L. Finot, J.N.M. van Gestel and J.C. Duchet, Stud. Surf. Sci. Catal., 127 (1999) 51. 13. C. Petitto, G. Cfiordano, F. Fajula and C. Moreau, Catal. Commun., 3 (2002) 15.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

587

Hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of gas oils in one step on Pt, Pd/H-USY Z. Varga a, J. Hancs6k ~, G. Tolvajb, W/thin6 I. Horvhthb, D. Kall6 c aDepartment of Hydrocarbon and Coal Processing, University of Veszpr6m, Veszpr6m, P.O. Box 158, H-8201, Hungary bDivision of Production and Trade, MOL - Hungarian Oil and Gas Co., Sz/tzhalombatta, P.O. Box 1, H-2443, Hungary r Research Center, Institute of Chemistry, Hungarian Academy Sciencies, Budapest, P.O. Box 17, H-1525, Hungary The investigation of hydrodearomatization, hydrodenitrogenation and hydrodesulfurization of gas oils (up to 188 ppm sulfur, 193 ppm nitrogen and 38.4% total aromatic content) over Pd, Pt catalysts supported on USY zeolite, whose Pd/Pt mass ratio was varied between 6:1 and 1:3, and total metal contents were between 0.90 and 0.93, are presented. The effect of change of Pd/Pt ratio on HDA, HDN and HDS activities are demonstrated. The advantageous process parameters for HDA, HDN and HDS of gas oils over a selected catalyst (e.g. Pd/Pt mass ratio 2:1) were determined. Under optimum process parameters the conversion of polyaromatics was higher than 80%, that of nitrogen compounds higher than 89% and sulfur content of the obtained products was lower than 50 ppm in case of every applied feed. 1. INTRODUCTION During the last 20 years the sulfur content of diesel fuels has been reduced stepwise from 1% to 350 ppm by January, 2000 and is to be reduced tO 50 ppm by January, 2005 in the European Union. The reason of the sulfur content reduction beside the environmental protection arises mainly because of proper working of exhaust gas affertreatment devices [ 1]. Reduction of the aromatic content of diesel fuels is relevant, too, since this contributes to increase of the cetane number and reduction of particulate and NOx emissions [2]. Hydrodearomatization (HDA) catalysts were developed recently providing hydrodesulfurization (HDS) function up to 500 ppm sulfur content beside saturation of aromatics [3-8]. Gas oil fractions having higher sulfur content are hydrodesulfurized previously on supported Co-Mo or Ni-Mo catalysts. These types of catalysts, mainly Co-Mo, do not provide, however, sufficient hydrogenating activity needed for the elimination of the most refractive sulfur and nitrogen compounds e.g. alkylated dibenzothiophenes and alkylated carbazoles, respectively [9-10]. Reduction of the nitrogen content of diesel fuels is similarly important, because NOx is favorably formed during the burning of the organonitrogen compounds resulting in air pollution (acid rains, ozone formation) and corrosion in the engine

588 and exhaust system, furthermore, they decrease the base content of the engine oil. Accordingly, search is required for catalysts having hydrodenitrogenation (HDN) activity as well as HDS and HDA activities. The objective of the authors was to find a catalyst suitable both for the saturation of aromatics of gas oils and reduction of their sulfur and nitrogen contents in one catalytic step. 2. EXPERIMENTAL HDA, HDS and HDN experiments were carried out with catalysts containing Pt and Pd in different ratios on H-USY support. The main properties of the support are the following: SIO2/A1203 ratio 33.5, total and mesopore surface areas 592.5 m2/g and 51 m2/g calculated using BET-plots and t-plots, respectively. The bimetallic Pt-Pd catalysts were prepared by incipient wetness impregnation using [Pt(NH3)4]C12 and [Pd(NH3)4]C12. The total amount of metals was 0.90-0.93% and the Pd/Pt ratio was varied between 6:1 and 1:3. Metal contents and ratios are summarized in Table 1. Pd, Pt, Si and A1 contents were measured by ICP apparatus (Jobin Yvon Ultima ICP-AES), and metal dispersion was determined from the amount of chemisorbed CO. After impregnation the catalysts were dried at 70~ calcined in oxygen stream at 210~ reduced in situ in 1-12before the catalytic tests at 400~ for 12 hours. The experiments were carried out in a high pressure flow apparatus. The 100 cm 3 tube reactor was working without back mixing. The feedstocks were gas oil fractions of different aromatic, sulfur and nitrogen contents. Their most important properties are summarized in Table 2. The catalysts were investigated between 260 and 340~ at total pressures of 30-40 bar, hydrogen to hydrocarbon ratios (in the following H2/HC) of 600-1000 Nm3/m3 and liquid hourly space velocity (in the following LHSV) of 1.0-2.0 h"1. The properties of the feeds and products were determined by test methods according to the standard EN 590:2000 regarding to commercial type diesel fuels, the sulfur content by pyro-fluorescent method (ASTM D 5453), the nitrogen content by pyro-chemiluminescent method (ASTM D 4629) and the aromatic content by high performance liquid chromatography (IP 391:1995). The percentile decrease of aromatic, sulfur and nitrogen contents were determined and defined as HDA, HDS and HDN activities, respectively. Table 1 The metal contents and Pd/Pt ratios of catalysts Catalyst Pd/Pt mass ratio Pd content, % Pt content, % Total metal content, % Dispersion

I

II

III

IV

V

6:1 0.80 0.13 0.93 055

4:1 0.72 0.18 0.90 0.51

2:1 0.60 0.31 0.91 0.48

1:1 0.45 0.46 0.91 0.43

1:3 0.23 0.69 0.92 0.41

589 Table 2 Properties of gas oil feedstocks Properties Density, 15~ kg/m 3 Sulfur, ppm Nitrogen, ppm Total aromatics, % Mono-ring aromatics, % 2+-rings aromatics, % Boiling point, ~ IBP 10% 50% 90% EP

A 832.4 118 77 25.7 21.4 4.3

B 839.2 128 85 28.9 22.3 6.6

Feed C 842.1 139 80 29.2 24.8 4.4

D 843.8 161 81 30.4 25.9 4.5

E 861.9 188 193 38.4 26.8 11.6

201 225 271 334 364

209 226 276 338 365

211 226 278 341 365

218 226 282 344 368

223 240 292 353 373

3. RESULTS AND DISCUSSION

First the influence of Pd/Pt ratio on the HI)A, HDN and HDS activity was investigated. The results obtained with extreme feeds ("A" and "E") showed that both saturation of aromatics and elimination of nitrogen and sulfur took place, but to different extent. Fig. 1 shows the change of HDA activity as function of the metal ratio of catalysts in case of feed "E" at 280~ 40 bar, H2/HC 800 m3/m3, LHSV = 1.0 h"1. The conversion of total aromatics decreased from about 39.2% to 21.1% as the Pd/Pt ratio increased. Presumably the saturation of aromatic compounds, mainly that of mono aromatics, requires higher hydrogenation activity which is provided by the platinum metal. Fig. 2 shows the HDS and HDN activities as functions of the Pd/Pt ratio. As it can be seen the conversion of sulfur compounds monotonously increased from about 51.2% to 81.2% with increasing Pd/Pt ratio. While the conversion of nitrogen compounds first increased with the Pd/Pt ratio and reaching a maximum (86.1%) at Pd/Pt ratio 2:1 began to decrease. On the base of the experimental results it could be assessed that the maximum of the HDS and HDN activities did not coincide. The explanation of the different change of I-IDS and HDN activities requires further investigations, mainly determination of the types of sulfur and nitrogen compounds being present in the feed and products. Experiments carried out with the other extreme feed "A" provided similar results. According to the results of the preliminary experiments catalyst having Pd/Pt mass ratio of 2:1 is of best performance because the obtained products have sulfur content less than 50 ppm and polyaromatic content less than 2%, satisfying the requirements of the European Union for diesel fuels coming into force by 2005. In addition, this catalyst has the highest HDN activity. The results of the experiments using this catalyst will be discussed in detail. In the next step of investigation the advantageous process parameters of I D A , HDN and HDS with the previously selected catalyst (Pd/Pt ratio is 2:1) were determined.

590 40

35

-...4 r oa

30

<

25

20 1:3

1:1

2:1

4:1

6:1

Pd/Pt mass ratio

Figure 1. Effect of the Pd/Pt mass ratio of catalysts on the HDA activity.

90 -]

~HDS,

%

--II-- HDN, %

o~

80

~

70-

-

I 50

I

1:3

I

1"1 2:1 Pd/Pt mass ratio

T

4:1

~

6:1

Figure 2. Effect of the Pd/Pt mass ratio of catalysts on the HDN and HDS activity.

591 These experiments were carried out by applying the extreme feed "E" in the temperature range between 260 and 340~ varying the HJHC ratio between 600 and 1000 Nm3/m3, while the total pressure was 40 bar and the LHSV 1.0 h"1. Fig. 3 displays the change of the HDN, HDA and HDS activities as function of temperature. The figure shows that the HDN and HDS activities increased considerably in the temperature range of 260-280~ and further increase of the temperature negligibly influenced the HDN and HDS activities. Conversion of aromatics increased in the temperature range of 260-310~ and had a maximum at 310~ (30.1%), then it decreased with increasing temperature. Perhaps the thermodynamic equilibrium was attained because of increasing reaction rates at increasing temperatures, and the thermodynamic equilibrium of the exothermic hydrogenation of aromatics is shitted to dehydrogenation at higher temperatures. Fig. 4 displays the change of the HDN, HDA and HDS activities as function of the H2/HC volume ratio. The increase of the H2/HC volume ratio showed considerable effect in the range of 600-800 Nm3/m3 on the HDN, I-IDA and HDS activities, but further increase was ineffective. Accordingly, further investigation of the selected catalyst was carried out with the following process parameters: temperature range 280-310~ pressure 40 bar, H2/CH volume ratio 800 Nm3/m3, LHSV 1.0 h"1. After every experimental run the yield of stabilized liquid products of gas oil feeds "A""E" were determined. In case of every feed the yield of liquid products were nearly 100%, which indicated that HDA, HDS, and HDN proceed selectively without cracking.

100 ~

90

~

8o

I

,ik,

m

~

70

:~

60

m

Z

9

9

~

m

m

9 mR

m

--J

_

!

~HDA --ll--HDS - HDN

50

<~ 40

t~

30 20

9

i

t

260 270 280 290 300 310 320 330 340 Temperature, ~ Figure 3. Effect of the temperature on the HDA, HDN and HDS activities.

592 100

~HDA -'-ll--HDS ..... -" HDN

90 80

"~ 70

j J

z

r

m

J

7

50 4o

30 20 600

,

!

i

700

800

900

1000

H2/HC volume ratio, Nm3/m3 Figure 4. Effect of the H2/HC volume ratio on the HDA, HDN and HDS activities.

The main properties of products obtained at 300~ are summarized in Table 3. The density of products was lower than that of their feeds in every case, because of the conversion of aromatic compounds having relatively high density.

Table 3 The main properties of products Properties A/3 A

B/3

Product C/3

828.5 14 8

834.1 19 9

Total aromatics, % Mono-ring aromatics, % 2+-rings aromatics, %

13.2 12.7 0.5

HDS, % HDN, %

88.1 89.6 48.6

Feed Density, 15~ kg/m 3 Sulfur, ppm Nitrogen, ppm

HDA, %

D/3 D

E/3

839.3 29 9

840.7 38 9

855.8 49 21

17.6 16.7 0.9

19.4 18.8 0.6

21.2 20.5 0.7

27.1 25.3 1.8

85.3 89.4 39.1

79.1 89.3 33.5

76.2 89.1 30.3

73.9 89.1 29.1

593 The data in Table 3 shows that the conversion of total aromatic content (HDA) varied between 48.6% (feed "A") and 29.1 (feed "E") depending on the composition of the feed. The reduction of the total aromatic content contributes to the decrease the NOx emission of diesel engines, because a higher aromatic content in the fuel increases the flame temperature during combustion resulting in increased NOx emissions. On the base of data summarized in the Table 3 could be assessed that the conversion of polyaromatics was higher than 80% for every investigated feed. This relatively high conversion of polyaromatics contributes the decrease of the particle emission of diesel engines, because direct correlation exits between the polyaromatic content of fuels and the particle content of exhaust gases. In contrast the conversion of polyaromatics, the conversion of mono aromatics varied within wide range, e.g. for feed "A" it was 40.5% and for feed "E" only 5.8 %. One explanation could be, that in case of feed "E" relatively high amount of mono aromatic hydrocarbons is formed by consecutive ring hydrogenation steps from polyaromatics in the feed, which is added to the mono aromatics exists originally in the feed. Further, the feed "E" contains more sulfur and nitrogen than feed "A". The sulfur and nitrogen contents of gas oils are mainly present in form of heterocyclic compounds, e.g. alkyl-substituted benzothiophenes and alkyl-substituted carbazoles, respectively. These compounds as well as polyaromatics, could exert inhibiting effects on hydrogenation of mono aromatics because of competitive adsorption. As can be seen in Table 3 the sulfur content of products was always lower than that of the feeds; these values were even lower than 50 ppm (maximum sulfur content of diesel fuels in the European Union from 2005) for each investigated feeds. The reduction of sulfur content changed between 88.1% and 73.9%. In the case of feed "A" the sulfur content was reduced from 118 ppm to 14 ppm, while in the case of feed "E" from 188 ppm to 49 ppm. The decrease of the HDS activity may be attributed to the higher content of heterocyclic and sulfur compounds, and polyaromatics evidenced by the higher end boiling point of feed "E" than that of feed "A". The inhibiting effect of single compounds could not be determined, only the cumulated effects. However, in general, the following order of inhibition has been noticed: saturated and mono-aromatic hydrocarbons < condensed aromatics ~ oxygen compounds H2S < organic sulfur compounds < basic nitrogen compounds [9, 11 ]. It is illustrated in Table 3 that the nitrogen content of products decreased also considerably in every case, even the HDN activity of the investigated catalyst was higher than its HDS activity for every feed. The nitrogen content of feed "A" decreased from 77 ppm to 8 ppm and that of feed "E" from 193 ppm to 21 ppm under identical conditions. The experimental results show that HDN activity of the catalyst, i.e. the percentile conversion is practically the same for every feed (decreases of nitrogen content of "A" and "E" are 89.6% and 89.1%, respectively) indicating a first order kinetics of HDN not influenced by the different sulfur and polyaromatic contents. The acid sites of H-USY zeolite support seem to be responsible for the high conversion of basic nitrogen compounds. The selected catalyst preserved its activity for long period of time (>160 hours) even for the feed of the highest sulfur content (188 ppm). On applying H-USY-type zeolite the supported noble metals became sulfur resistant. It seems likely that the acidic zeolite decreases the electron density of metal clusters resulting in weakening of the bond between the electron acceptor sulfur atoms and the electron deficient Pt/Pd particles.

594 4. CONCLUSIONS Hydrodearomatization, hydrodenitrogenation and hydrodesulfurization of gas oils (up to 188 ppm sulfur, 193 ppm nitrogen and 38.4% total aromatic content) were achived over bimetallic Pd,Pt catalysts supported on H-USY zeolite, whose Pd/Pt mass ratio was varied between 6:1 and 1:3. The HDA activity decreased, the HDS activity increased and the HDN activity first increased and peaked at Pd/Pt ratio 2:1 then slowly decreased as the Pd/Pt ratio increased. The advantageous process parameters applying catalyst having Pd/Pt ratio 2:1 were determined: temperature 300~ pressure 40 bar, LHSV = 1.0 h"1, H2/HC volume ratio 800 Nm3/m3. The investigation of gas oils having different composition showed that the reduction of polyaromatic content was high for every feed while the conversion of mono aromatics largely depends on the nitrogen, sulfur and polyaromatic content of the feeds. The HDS activity was high enough to produce products of sulfur content below 50 ppm (EU limit from 2005), even in case of feed having high nitrogen, sulfur and total aromatic content. The decreased HDS activity in case of feed having high nitrogen, sulfur and polyaromatic content indicated that these compounds exert negative effects on HDS reactions. The HDN activity was higher than the HDS activity in case of every applied feed, and practically independent of their composition. This result may be related to the acidity of H-USY zeolite. Summing up the results, it has been succeeded to find a zeolite based catalyst having significant HDN activity as well as considerable HDA and HDS activities. However, it should be noticed the application of the type of Pd,Pt/zeolite catalysts requires the preliminary hydrodesulfurization of the feeds up to sulfur content of about 200 ppm.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

M. Shelef, et al., Catalysis Today, 62, 35-50, 2000. ACE/L, AAMA, EMA, JAMA: ,,World-Wide Fuel Charter", 2000. T.G. Kaufmann, et al., Catalysis Today, 62, 77-90, 2000. J.P. Dath, Akzo Nobel Catalysts Symposium, 2001, Noordwijk, The Netherlands, paper No. H-9. H. Yasuda, et al., Catalysis Today, 50(1), 63-71, 1999. M. Sugioka, et al., Catalysis Today, 45, 327-334, 1998. A. Stanislaus and B.H. Cooper, Catal. Rev.- Sci. Eng., 36(1), 75-123, 1994. Y. Yoshimura, et al., Applied Catalysis A: General, 207, 303-307, 2001. P. Zeuthen, et al., Catalysis Today, 65, 307-314, 2001.. R. Shaft and G.J. Hutchings, Catalysis Today, 59, 423-442, 2000. H. Schulz, et al., Catalysis Today, 49, 87-97, 1999.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

595

Reformate upgrading to produce enriched B T X using noble metal p r o m o t e d zeolite catalyst S.H. Oha, K.H. Seonga, Y.S. Kima, S. Choia, B.S. Lima, J.H. Leea, J. ~Voltermannb, Y. F. Chub aSK R&D Center, SK Corporation, 140-1 Wonchon-dong, Yusung-gu, Taejon 305-712, South Korea bZeolyst International, PO Box 830, Valley Forge, PA 19482-0830, USA A process has been commercialized to convert lower value reformate into more valuable BTX (benzene, toluene and xylenes) and LPG using idled fixed bed units in the refinery. Aromatic compounds in the reformate feedstock are converted over a noble metal zeolite catalyst to BTX through hydrodealkylation and transalkylation, while non-aromatic hydrocarbons are hydrocracked to LPG rich gaseous products. The novel zeolite catalyst is active, stable and regenerable. The BTX produced has high purity and requires no downstream processing. The catalyst has been used in the idled reforming plant of SK in Korea successfully since June 2001. But it can also be used in any available fixed bed reactors, such as an HDS vessel, in the refinery. In this paper, novel features of the catalyst and the process are discussed in detail. I. INTRODUCTION Naphtha reformate is traditionally converted to high-octane gasoline in a fixed-bed reforming unit. However, with the advent of the modern CCR reformer and the increase of gasoline production by FCC heavy oil upgrading, the operation of conventional fixed bed reformers has declined leaving many fixed bed units idle in the refineries. BTX is a valuable feedstock for the petrochemical industry. The ample supplies of the low value naphtha reformate in the world makes its conversion to more valuable BTX attractive. Aromatics produced from the modem CCR unit or the Tatoray unit normally need to be upgraded by solvent extraction to remove non-aromatic compounds. Unfortunately, the capacity of refinery extraction units does not equal the growing demand. New environmental regulations limiting gasoline aromatic content have greatly increased the volume of aromatics available for BTX and increased demands for purification units [ 1]. Processes for aromatic purification other than solvent extraction have been used with mixed results. Catalytic reactions for separating aromatic compounds from non-aromatic compounds in the hydrocarbon stream have been suggested [2]. Non-aromatic compounds are converted to C3 - C4 hydrocarbons in the presence of catalyst by hydrocracking. Aromatic compounds then can be separated from the LPG in a complicated gas-liquid separator at the rear end of a reactor. More recently, a study [3] used a separate fixed bed reactor in sequence with the reforming unit to produce concentrated BTX in a single hydrogen circuit. A new process [4] replaces the last portion of the catalyst in the parts of a series of reforming reactors with

596 zeolite-based upgrading catalyst to increase the production of benzene and toluene. The process increased the yield of benzene and toluene, and reduced Cs aromatics yield, especially ethylbenzene (EB) compared to normal reformate. Zeolite-based catalysts have also been used to increase the yield of high purity BTX from naphtha by loading in an aromaticsisomerization reactor, which is in series with reforming reactors [5]. Although there has been many studies on reformate upgrading into concentrated BTX, no work has been done using a single reactor. This study is focused on the development of a shape selective zeolite catalyst suitable for converting naphtha reformate to produce enriched BTX and LPG using an idled fixed bed reforming unit in the refinery. It is shown that high purity BTX can be produced with the present process without a need for solvent extraction. A detailed description of the process and catalyst follows. 2. EXPERIMENTAL Zeolyst International supplied the noble metal promoted zeolite catalyst developed by SK Corporation. The catalyst was formulated to obtain both excellent selectivity and high stability in this process. The tests were conducted in a high-pressure reactor operating at a pressure of 30 kg/em2, temperature of 390 *(3,WHSV-'2.6 h"1 and H2/HC molar ratio of 3.8. Prior to use, the catalyst was reduced in a H2 flow at 430 "C for 2 h. The products were analyzed in a HP 5890 gas chromatograph equipped with FID and the HP-PONA capillary column. All mass balances were within +1% of closure.

3. PROCESS DESCRIPTION AND COMPARISON 3.1. Process description

The flow diagram of the new process is shown in Figure 1. An idled fixed bed reforming unit was employed for this process. However, the process can utilize any idled fixed bed reactors in the refinery. Reformate containing 60% to 80% aromatics and 20% to 40% paratfinic hydrocarbons along with minor amounts of naphthenic hydrocarbons was used. In this study, the reactor system comprises two reactors for the exothermic reactions. The reformate feedstock is combined with recycle hydrogen and high purity make-up (M/U) hydrogen. Hydrodealkylation and transalkylation of aromatic hydrocarbons and hydrocracking of non-aromatic hydrocarbons are performed in the presence of the catalyst within the reactor. Alter the completion of reactions, reactor effluents pass through a separator and stripper and then are separated into gas and liquid products.

597 Fuel Gas

M/UH2 Recycle Gas Reformate

C7 to I LPG Recovery Bz Extraction

/ C8 + t o

p-X Process Separator Stripper

Bz Col

Tol Col

Bz Col: benzene column Tol Col: toluene column p-X: para xylene Figure 1. Flow diagram of ART process using the ART-11 metal promoted zeolite catalyst. LPG contained in the stripper overhead stream is recovered using conventional methods. The liquid phase stripper bottom stream, which comprises aromatics and a very small amount of the remaining non-aromatics, flows into the fraetionation columns. Toluene is separated in the overhead of toluene column and the bottom stream is sent to the p-X plant for xylene isomerization and p-xylene recovery. Benzene separated in the overhead of benzene column is further processed to improve its purity but other components can be used directly.

3.2. Comparison of processes The comparison of the new process to the traditional technology is shown in Figure 2. BTX produced from reformate can be recovered by solvent extraction such as in the Sulfolane process. However, the Sulfolane process merely separates aromatics from non-aromatics using differences in polarity. The reformate is not chemically changed. Furthermore, while solvent extraction can be used to produce aromatics with high purity, the process requires extensive equipment and the solvents must be introduced continuously and properly cleaned and recycled. In contrast the new reformate upgrading technology is aimed at direct production of high purity BTX and LPG from reformate using the shape selective catalyst developed. Nonaromatic paraffins and naphthenes in the reformate are selectively converted into light gaseous non-aromatic compounds rich in LPG by ring opening and hydrocracking over the catalyst. Heavy C9+ aromatics contained in the reformate are converted to BTX through hydrodealkylation and transalkylation. Hence the BTX yield is increased compared to the conventional solvent extraction technology. The BTX produced in the novel process can be sent directly to the distillation columns for downstream processing.

598 Sulfolane Product

ART Product

Reformate

Fuel

C5+

C5+

Non Aro 4-...........................

Paraff. Novel Technology

Conventional Technology Sulfolane Process

BTX

..a

IL.

B rx

LPG

Reformate Upgrading Process BTX

C9+Aro

4--

C9+AI'O C9+ AgO

Figure 2. Concept of the advanced reformate upgrading technology (ART). Additionally, the Cs aromatics produced in the process contain very low levels of EB and are a suitable feed for xylene isomerization units [6]. The C9§ aromatics produced are also rich in trimethylbenzene which is an excellent feedstock for transalkylation processes such as the Tatoray process [7]. Integrated together, the two processes produce high concentrations of p-xylene, which is a valuable feedstock in the manufacture of polyester [8]. 4. RESULTS AND DISCUSSION

4.1. Catalytic results The catalyst testing results are shown in Table 1. Several feeds with different composition were tested over the catalyst. It is seen that most of the C5+ paraffins and naphthenes are converted to light hydrocarbons such as L~G and fuel gas. This conversion is ascribed to the strong acid function of the zeolitic component of the catalyst. Paraffins undergo hydrocracking and naphthenes ring opening and consecutive cracking over the catalyst. Most of the unconverted C5+ non-aromatics after reaction contain mainly C5 and a small amount of C6 and a trace of C7§ components. Therefore, undesired non-aromatics that would otherwise deteriorate the quality of BTX are effectively removed and toluene and xylenes can be easily produced from the existing fractionation columns in the refinery. Benzene needs to be further processed via the benzene extraction tower to improve its purity to commercial specifications. The latter will be a subject for the future work.

599 Table 1 Result of reformate upgrading reaction carried out in a high-pressure catalyst test apparatus Component, wt%

Feed

1-12 C1+C2 LPG C5+ Non-Aromatics BTX+EB EB / (BTX+EB) C9+ Aromatics

1.38 38.61 46.52 6.90 13.49

Product . . . . . . .

Feed

Produc t

-1.88

-1.35

12.95 32.87 2.29 48.45 0.17 5.32

9.79 24.86 1.73 55.42 0.18 9.55

3.55 25.46 50.74 9.14 20.25

Temperature=390 "12,Pressure=30 kg/cm2, WHSV=2.6 hl, H2/HC=3.8

It is observed that the amount of BTX in the product is greater than in the feed. The increased BTX production is attributed to the hydrodealkylation of C9§ heavy aromatics such as methylethylbenzene, (methyl) propylbenzene and dimethylethylbenzene, etc. Ethyl and propyl groups in the C9+ heavy aromatics are easily dealkylated over the catalyst. The process further offers the benefit of lowering the p-xylene production costs, because EB content is very low in the xylene feed. Separation of EB from mixed xylene streams is difficult and expensive due to their close boiling points [9]. While a typical xylene isomerization process promotes the conversion of EB to xylenes or benzene, it cannot achieve the level of EB conversion achieved with this process. The process allows nearly full conversion of EB to benzene reducing the load of EB recycle in the xylene isomerization loop. During hydrocracking and hydrodealkylation, olefins such as ethylene and propylene may be produced and these must be rapidly hydrogenated. Otherwise, aromatics alkylation may result in low conversion of C9§ heavy aromatics. In addition, the olefins themselves may be oligomerized or polymerized to form non-aromatics and facilitate the formation of coke, which causes deactivation of the catalyst. Incorporation of noble metal on the zeolite-based catalyst not only hydrogenates the olefin formed but also enhances the catalytic stability of the catalyst. However, excessive hydrogenation activity catalyzes conversion of aromatics to naphthenes. Therefore, the hydrogenation activity of the metal needs to be controlled and balanced with the acidic function of the zeolite. The metal component also suppresses the formation of coke by rapidly hydrogenating ethylene and propylene. Based on the above results, a reaction scheme for this process is proposed in Figure 3. Hydrocracking of paraffin and ring opening of naphthene produces LPG and fuel gas. Hydrodealkylation of C9§ heavy aromatics increases BTX yield. Olefin saturation is accompanied by hydrodealkylation and thus realkylation is suppressed and formation of coke is retarded. Transalkylation between BTX and C9+ heavy aromatics also occurs to some extent. Feed containing a higher concentration of aromatics and less non-aromatics is preferred since it would decrease hydrogen consumption and gas production while increasing the yield of BTX. Reducing non-aromatics concentration in the feed may also have beneficial effects on the cycle length of the catalyst.

600 Paraffin Hydrocracking

+

H2

Acid~

Naphthene Ring Opening

+

1-12

Acid

Hydrodealkylation/Olefin Saturation

[/ c~

Aci;

~ c#i 6

Metal

Acid

--

C

+

C3H6 Metal

Acid~

~

+

C2H4

I I-I~ Metal

c2n 6

Figure 3. Proposed reaction scheme for the reformate upgrading process.

4.2. Commercial experience The new catalyst and process was commercially applied in June 2001 at SK's idled reforming unit. The commercial plant has been operated with a daily production capacity of from 3000 to 6000 barrels/day. In the case of 6000 barrels/day, reactor inlet temperature adjusted to achieve the target purity of product was about 360 ~ H2/HC mole ratio was maintained in the range of 3-5 mole/mole and separator pressure was 28 kg/cm2 during normal operation. Toluene with high purity over 99.75% was produced successfully. There was also an increase of BTX production by 10-20%. The catalyst has not yet been regenerated

601 commercially but laboratory tests have shown that the catalyst can be regenerated several times maintaining more than 97% of fresh activity and selectivity. 5. CONCLUSIONS The metal promoted zeolite catalyst of this study is capable of producing BTX and LPG from the naphtha reformate. Aromatics in the reformate are converted to BTX enriched aromatics through hydrodealkylation of C9§ heavy aromatics. On the other hand, nonaromatics in the reformate are converted to LPG enriched gaseous products through hydrocracking and ring opening. Strong hydrocracking activity of this catalyst also enables production of high purity toluene and xylenes without the need for solvent extraction or added downstream reactors. The reformate upgrading technology using the catalyst was commercialized successfully. Stable production of BTX and LPG has been maintained.

REFERENCES

1. T.C. Tsai, S.B. Liu and I. Wang, Appl. Catal. A, 181 (1999) 355. 2. N.Y. Chen, Hydrocarbon Conversion, US Patent No. 3 729 409 (1973). 3. C.D. Gosling, et al., BTX from Naphtha without Extraction, US Patent No. 5 472 593 (1995). 4. J.S. Buchanan, et al., Hydrocarbon Conversion, US Patent No. 5 865 986 (1999). 5. C.D. Gosling, et al., BTX from Naphtha without Extraction, US Patent No. 6 001 241 (1999). 6. A. Smieskova, P. Hudec, M. Paciga and Z. Zidek, Appl. Catal. A, 149 (1997) 265. 7. K.J. Chao and L.J. Leu, Zeolites, 9 (1989) 193. 8. H.H. John, H.D. Neubauer and P. Birke, Catal. Today, 49 (1999) 211. 9. L.D. Femandes, J.L.F. Monteiro, E.F. Sousa-Aguiar, A. Martinez and A. Corma, J. Catal., 177 (1998) 363.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

603

Dehydroisomefization o f n-butane to isobutene over Pd/SAPO-11 The effect o f Si content o f SAPO-11, Catalyst preparation and Reaction condition Yingxu Wei, Gongwei Wang, Zhongmin Liu, Peng Xie, Lei Xu Natural Gas Utilization & Applied Catalysis Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P. O. Box 110, Dalian 116023, [email protected]., China. The SAPO-11 molecular sieves with different Si content were synthesized and modified by Pd for direct transformation of n-butane to isobutene. The effects of S i content of SAPO-11, catalyst preparation and reaction condition were studied. The catalytic properties ofPd/SAPO-11 varied with Si content of the crystal products. The impregnation time of the catalyst had effect on the product distribution and catalytic activity. The catalytic properties also changed with the reaction time. 1. INTRODUCTION Different processes for isobutene production including skeletal isomerization of n-butene received a lot of attention during recent years since the increasing demand of isobutene in industry. Direct conversion of n-butane to isobutene is a novel interesting process [1-3]. In the present work, a new series of catalysts based on the Pd modified SAPO-11 with different Si content for the direct transformation of n-butane to isobutene were prepared and studied. The effect of impregnation time of the catalyst and the effect of time on stream on the catalytic properties were also investigated in details. 2. EXPERIMENTAL Pseudoboehmite, 85% wt orthophosphoric acid and colloidal silica were used as the aluminum, phosphorus and silicon starting materials. Di(n-propyl)amine was used as the template for AIPO-11 and SAPO-11. The synthesis followed the procedures reported in the literature [4]. The products were filtrated, washed, dried and calcined at 773 K to remove the template. The elementary analysis of the synthesized samples was determined with X-ray fluorescence spectrum. The Pd modified catalysts were prepared by impregnating the calcined molecular sieves under vacuum with Pd(NH3)4CI2 solution for a certain time. The catalysts were then calcined in air at 773 K for 2 h. The BET and pore distribution were measured with physical adsorption at liquid nitrogen temperature. 1H-NMR was employed to test the hydroxyl group over the

604 surface of the synthesized SAPO-11s. NH3-TPD was measured to determine the acid amount of each sample. Pulse CO chemisorption was measured to determine the metal dispersion of the prepared catalysts. N-Butane catalytic transformations were performed in a fixed bed flow reactor at atmosphere pressure. The catalysts were reduced with hydrogen (60 cm3/min) at 773 K for one hour. The reaction parameters used were reaction temperature = 773 K, WHSV = 1.98, H2/n-butane = 2mole/mole and weight of catalyst = 0.5g. The catalytic reaction data was all obtained at TOS=60 min. The reaction products were analyzed by on-line gas chromatography. 3. RESULTS AND DISCUSSION The as-synthesized solids are all highly crystalline with AEL topology, in agreement with the literature. The Si content, BET and microporous volume of the calcined samples are listed in table 1. With the increase of Si content from 0 to 5.5-wt%, the BET surface area increases, but the microporous volume of the samples changes unmarkedly. Tablel BET and Microporous volume of the synthesized samples Sample

Si content (wt%)

BET (m2/g)

Microporous volume (cm3/g)

AIPO-11 SAPO-11(1) SAPO-11(2) SAPO-11(3) SAPO-11(4)

0 1.5 2.8 4.6 5.5

140.63 156.39 170.73 171.19 186.84

0.063 0.065 0.067 0.069 0.070

The transformations of n-butane over Pd/SAPO-1 l s with different Si content were measured. The catalytic activity and product distribution varied with Si content and extreme values of catalytic properties can be found in table 2. For the first four points of low Si content, with the increase of Si content, the conversion increased and a maximum activity was obtained around Si content 4.6 wt%. After this point, the conversion declined. Compared to the change of catalytic activity, the dehydrogenation selectivity changed in the opposite way. When Si content was 4.6 wt %, highest isobutane selectivity and lowest isobutene selectivity was obtained, but the total isomerization products (isobutane and isobutene) selectivity attained to the maximum value at the same point. These catalytic properties imply that at different silicon content, the SAPO-11 molecular sieve may have different acidity; the properties of supported palladium may also change.

605 Table 2 The influence of the Si content in SAPO-11 on the catalytic properties of Pd/SAPO- 11 s Catalystic properties Catalyst

Selectivity (%)

Conversion

(%)

iC4=

Total C4=

iC40

C1-C3 a

17.57 84.61 6.58 Pd/A1PO- 11 13.8 8.53 33.73 80.82 11.55 7.22 Pd/SAPO-11(1) 16.6 28.44 67.15 16.4~) ' li.81 Pd/SAPO-11(2) 22.01 25.63 61.12 24.86 12.81 Pd/SAPO-11(3) 24.46 31.61 77.71 9.23 8.29 Pd/SAPO- 11(4) 22.97 aCrC3 selectivity: the selectivityof (CH4+C2H6+C2H4+C3H8+C3I--I6) Figure 1 shows the 1H NMR spectra of the synthesized SAPO-11 with different Si content. For A1PO-11, very weak resonance peaks can be found, indicating the character of the nearly neutral framework. With the incorporation of silicon into the framework, the line at 3.45ppm due to surface bridge hydroxyl group Si(OH)A1 appears and with the increase of Si content, the strength increases and shows the maximum value at 4.6 wt % silicon content. It is well known that one Si substitution for one A1 will generate one Si(4A1) species and one corresponding bridge hydroxyl group. The result of 1H NMR shows this trends at low Si content -- the increase of S i incorporation will generate more bridge hydroxyl groups; But for the SAPO-11 with high Si content, Si substitution may also happen in another way -- two Si substitution for one A1 and one P. This kind of Si incorporation may generate more neutral Si (0A1) species and the bronsted acid amount associated with Si(4A1) species decreases. The catalytic property difference of Pd/SAPO-11 with different Si content should be mainly attributed to the acidity difference of SAPO-11 as the catalyst support. The catalysts Pd/SAPO-11 with different Pd impregnation time (0, 6, 12, 24h) were tested in n-butane's transformation. The result is showed in table 3. It can be found that with the impregnation time increase, the catalytic activity and isomerization selectivity increased, but the dehydrogenation selectivity decreased. The CO pulse adsorption was employed to test the Pd properties of this series of catalysts. With impregnation time increasing from 0 to 24 h, the metal dispersion increased from 32.67% to 52.53%; the catalytic properties associated with bifunctional catalyzing such as isomerization and cracking became more violent, but dehydrogenation selectivity decreased. This indicates that increasing the impregnation time will strengthen the interaction between the acid support and supported metals and weaken the metallic properties of the Pd particles.

606

: SAPO-11(4)

;.'i

SAPO-11(3)

~

~

~

.... : " ~ . . , , . . [ ' N .

.

7

0 ppm =-

SAPO-11(1) '

AIPO-11

~

....................

::'===%~;~',

...........

= = ' ~ ~ ~ . ~ - - ~ . v =

. . . .

..... I

11.4

I

7.6

I

I

I

3.8

0.0

-3.8

Chemical Shift (00m) Figure 1 IH MAS NMR patterns of AIPO- 11 and SAPO- 11 with different Si Content

607 Table 3 The catalytic properties and metal dispersion of Pd/SAPO-11 with different impregnation time Catalyst

Pd/SAPO-11 [20.1% PdD

Impregnation time (h) Conversion (%)

12

24

12.94

21.89

24.46

25.64

Selectivity (%) i-C4~

8.11

20.24

24.86

42.20

i-C4--

33.62

28.14

25.63

17.71

i-C4~ i-C4=

41.73

48.38

50.49

59.91

aTotal C4=

79.60

66.96

61.12

45.83

bC1-C3

12.21

12.11

12.10

10.10

cC5+

0.08

0.69

1.92

1.87

Metal Dispersion (%)

32.67

43.19

47.43

52.53

Total C4--:n-C4Hs+i-C4I-Is+t-Cd-Is+e-C4I-I8 bCl-C3- CH4wC2H6+C2H4+C3Hg+C3H6 cC5+"C5 and products larger than C5 The changes of the catalytic properties, acidity and metallic properties of Pd/SAPO-11 with time on stream were tested. The NH3-TPD and CO pulse adsorption results in table 4 show that when time on stream increased, the acid sites amount of the catalyst decreased at the beginning, then reached to a stable value and the Pd dispersion decreased at the same time. The catalytic properties listed in figure 3 are consistent with the characterization results of the catalyst. The acidity, isomerization selectivity and cracking selectivity decreased with reaction time but the dehydrogenation selectivity associated the metallic function increased. Table 4 The acidity and metallic properties of the Pd/SAPO-11 at different reaction time Reaction Time

(h)

Acidity Amount

(mmol/g)

Metallic Properties

0.610

CO Adsoporption Amount (ml/g STP)) 0.100

Palladium Dispersion (%) 47.43

0.5

0.559

0.076

36.17

1.5

0.535

0.066

31.21

6

0.526

0.055

25.83

608 50 45

o~ v "o ~)

>"o rm c o e-O 0

- -

40

9

35

9 m~

30 25

9

"l

n-butane

9

conversion

too_

isobutane

yield

--A~

isobutene

yield

--v--

(isobutane+isobutene)

_...,...~_ i _....~ n--------~-m

9

10 5

~V~---V-_~._V_______~___

-

O ~

9

,,~IL=~.-~jI

'

0

yield

I

'

50

I

'

100

I

150

9

"V-------'---V~v--~------__ v 9 , 9 ..... A------_& '

I

'

200

I

'

250

I

'

300

I

'

350

I

400

Reaction Time (min)

~.

"8 "o 0 L__ Q,.

70 60 50

r

40

0

30

->9

20

O9

____------q~-------~

--e~

iC40

--A--

iC4 =

--V--

gO4

9

I

X--X~X~X~X-------------- X

0

'

I

50

'

I

100

'

I

150

'

' I

200

'

X

Reaction Time

104

=

--------v~v

--4v-- total C4 = --+-CI-C3

_______._&~&

--X~

05*

./

10

04.

I

250

'

X ~ X I

300

'

b I

350

~'

I

400

'

'1

450

(min)

Figure 2 The yield and selectivity of the products at different reaction time 4. CONCLUSION

The structure of Si species in SAPO-11 changed with Si content and an extreme value for bronsted acid amount can be found. This can be used to explain the extreme value of the catalytic properties. Increasing the impregnation time of the catalyst will promote bifunctional catalyzing reaction such as isomerization and cracking, but weaken the function of supported Pd particles. The acid function and metallic function

609 of the catalyst changed with time on stream in the opposite way, and the catalytic property changes indicate the same trends. All the effect talked above, the effect of the Si content of SAPO-11, preparation condition and reaction time, show the interaction of acid support and supported metal particles of the catalysts.

REFERENCES

1. R.Byggningsbacka, N. Kumar, L.-E. Lindfors, Catal. Lett., 55 (1988) 173 2. B. Didillion, C. Travers, J. P. Burzynski, U.S.Patent 5,866,746 (1999) 3. A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mendez, C. M. Lopez, F. J. Machado, J. Goldwasser, M. M. Ramirez de Agudelo, M. Houalla, J. Molec. Catal. A: Chemical, 144 (1999) 101 4. B. M. Lok, C.A. Messina, R. L. Patton, R. T. Gajek, T. R. Canan, E. M. Flanigen, U.S. Patent 4,440,871

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Vapor phase propylene epoxidation over Au/Ti-MCM-41 graRing

611

catalyst: influence o f Ti

A.K. Sinhaa, T. Akitaa, S. Tsubotaa and M. Harutab aEnvironmental Catalysis Research Group, Special Division of Green Life Technology, AIST, 1-831, Midorigaoka, Ikeda 563-8577, Japan bResearch Institute for Green Technology, AIST, Onogawa 16-1, Tsukuba 305-8569, Japan Vapor-phase epoxidation of propylene using H2 and 02 was carried out over gold-Ti-MCM41 catalysts. Gold nanoparticles were homogeneously dispersed on the titanium incorporated MCM41 type of supports by deposition-precipitation (DP) method. Ti was incorporated into the mesoporous MCM-41 framework by (a) one-step method - hydrothermally during synthesis or by post-synthesis grafting and by (b) two-step method - hydrothermal incorporation followed by postsynthesis grafting. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. The Ti-MCM-41 supports prepared by two step titanium incorporation led to higher catalytic activity (after Au deposition) than Ti-MCM-41 prepared by one-step Ti incorporation method, at similar propylene oxide selectivities and hydrogen efficiencies. Presence of more Ti sites isolated from each other is thought to be responsible for the enhanced activity of the catalysts comprising of titanium incorporated by two-step method. 1. INTRODUCTION Propylene oxide (PO) is an industrially important chemical for the manufacture of polyurethane, unsaturated resins, surfactants and other products. Industrially PO is produced using two processes: Chlorohydrin process and Halcon (hydroperoxide) process (1). The former process produces environmentally unfriendly chlorinated organic by-products as well as calcium chloride, while the latter process produces equimolar amounts of co-products and requires heavy capital investment. Extensive efforts are being made since long to develop alternative process for direct gasphase propylene epoxidation using oxygen [2, 3] because it is a promising method of tremendous industrial significance which can replace currently used environmentally disadvantageous chlorohydrin process and hydroperoxide processes. Recently Enichem utilized TS-1 (MFI) as a catalyst for the epoxidation of propylene in the liquid phase using hydrogen peroxide [4-6]. However, due to the very high production cost of H202 and its handling problems it would be highly desirable to produce H202 in-situ. Toso Co. Ltd. [7] has developed Pd/TS-1 catalyst for the in-situ generation of of H202 from H2 and 02. Recently H61derich and coworkers [8] modified the catalyst developed by Toso with Pt and reported that the improved yield of PO could be ascribed to the maintainance of Pd in its +2 oxidation state. Our research work on the catalysis by gold [9-11] has opened a new stage for the direct epoxidation of propylene using H2 and O2. In a series of papers we have reported the vapor phase epoxidation of propylene over highly dispersed nanosize Au particles supported on TiO2, TiO2/SiO2 [ 12, 13] and titanosilicates such as TS- 1, TS-2, Ti-13, Ti-MCM-41 and Ti-MCM-48 [ 15-17]. These findings are now being followed by few other researchers [18-20] and companies [21,22]. But the problems for industrial viability still exist due to low PO yields and low H2 efficiency. Efficiency of supported metal catalysts often depends on metal dispersion and metal-support interaction. The

612 presence of well dispersed tetrahedrally coordinated Ti sites and the Au nanoparticles on the support surface is thought to be necessary for epoxidation activity [15-17] It is possible to incorporate titanium into mesoporous MCM-41 supports during hydrothermal synthesis [23, 24] or by postsynthesis grafting using titanocene [25] or titanium isopropoxide. Titanium incorporation by a twostep method, (1) during hydrothermal synthesis followed by (2) post-synthesis grafting could be a very effective method to generate more number of titanium sites in the MCM-41 structure which are highly dispersed and more accessible to reactant molecules. The present work reports the improvement in PO yield and H2 efficiency of Au catalysts supported on Ti-MCM-41 prepared by the two-step titanium incorporation method. Earlier studies on the gas phase epoxidation of propylene over Au/Ti-MCM-41 (15) have shown low propylene conversions. 2. EXPERIMENTAL Ti-MCM-41 and MCM-41 supports were prepared according to literature procedures [20, 21]. Ti grafting on the MCM-41 and Ti-MCM-41 support surfaces (dehydrated in vaccum at 300~ was performed in a glove box in an inert atmosphere to avoid TiO2 precipitation according to the method of Maschemeyer et al. [22] using (a) titanocene dichloride (TiCC) and (b) titanium isopropoxide (TilPO) corresponding to desired amount of Ti. Titanocene dichloride was dissolved in chloroform and was allowed to penetrate into dried MCM-41 powder in an inert atmosphere. The sample thus treated was exposed in-situ to triethylamine to activate the surface silanol groups of MCM-41. The color of the suspension changed from red via orange to yellow, due to substitution of the chloride with siloxide ligands. After extensive washing with chloroform, the organic components of this material were removed by clacination at 540~ under dry oxygen. For titanium grafting using titanium isopropoxide, MCM-41 sample calcined and vacuum-dehydrated was mixed with a solution of titanium isopropoxide in anhydrous hexane corresponding to the desired atomic % Ti. The mixture stood for 1 h and was then filtered and washed with anhydrous hexane. The grafted material was calcined at 400~ for 4 h in oxygen to convert unreacted alkoxide ligands into Ti-OH groups, and to remove residual isopropyl alcohol and hexane. The MCM-41 and Ti-MCM-41 materials were characterized by XRD (Rigaku R i n t - 2400, Cu-KGt radiation, 40 kV, 40 mA), UV-Vis (Photal Otsuka Electronics, MC-2530 UV/VIS light source), and nitrogen adsorption/desorption, BET surface area measurements (Micromeritics ASAP 2010 apparatus. Gold nanoparticles were deposited on the supports by deposition precipitation method [13-15] using aqueous HAuCI4 solution (corresponding to 2 wt % Au) and NaOH as precipitant followed by calcination in air at 300 ~ The catalytic tests were carried out in a vertical fixed-bed U-shaped quartz reactor (i.d. 10 mm) using a feed containing 10 vol% each C3H6, H2 and 02 diluted with Ar passed over the catalyst (0.15 g) bed at a space velocity of 4000 h-Xcm3/g.cat. The temperature was controlled and measured using a glass tube covered Cr-A1 thermocouple located in the center of the catalyst bed. Prior to testing, the catalysts were first pretreated at 250~ for 30 min. in a stream of 10 vol% H2 in Ar, followed by 10 vol% 02 in Ar streams. The feeds and products are analyzed using on-line GCs equipped with TCD (Porapak Q column) and FID (HR-20M column) detectors and auto injector. 3. RESULTS AND DISCUSSIONS

Table 1 lists the various titanium containing MCM-41 supports, their titanium content, BET surface areas, pore sizes and pore volume. The BET surface areas of the supports with up to 3.0% Ti content was similar (900-1200 m2gl). The BET surface area is found to decrease with increasing amount of Ti grafted onto Ti-MCM-41 sample (samples (3) to (8)). The BJH average pore diameter is found to decrease with increasing amounts of grafted Ti. The post-synthetically grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size with increasing amount of grafted titanium.

613 Table 1 Surface properties of titanium containing MCM-41 samples. Catalyst [Ti/Si(H)+Ti/Si(G)] Surface area

Pore size

(m2g"1)

(~)

Pore volume

(cm3g-1)

(1) Ti-MCM-41 0.015(H) 1270.4 38.4 1.80 (2) Ti-MCM-41 0.03(H) 1016.6 29.2 0.87 (3) Ti/Ti-MCM-41 0.015(H)+0.015(G) 1192.3 35.0 1.56 (4) Ti/Ti-MCM-41 0.015(H)+0.03(G) 907.3 35.0 0.77 (5) Ti/Ti-MCM-41 0.015(H)+0.04(G) 882.9 29.6 0.72 1029.9 36.0 1.09 (6) Ti-+Ti-MCM-41 0.015(H)+0.0015(a) 903.1 29.8 0.71 (7) Ti-~Ti-MCM-41 0.015(H)+0.03(G) 864.9 27.6 0.68 (8) Ti-->Ti-MCM-41 0.015(H)+0.04(G) 1205 39.4 1.60 (10) Ti-->MCM-41 0.015(G) 906.8 27.2 0.52 (11) Ti--->MCM-41 0.03(G). Ti/Si(H), ratio of hydrothermally incorporated Ti; Ti/Si(G), ratio of Ti grafted. Ti incorporation method: Samples (1), (2) hydrothermally during synthesis; samples(3)-(5) grafting using titanocene dichloride on Ti-MCM-41; samples (6)-(8) Ti grafting using titanium isopropoxide on Ti-MCM-41; samples (9), (10) by Ti grafting on MCM-41.

v

.m

i

c-

~) r-

1

;

; 2

i theta

g

;

7"

!

8

(d e g .)

Figure 1. XRD patterns for the titanium incorporated MCM-41 samples (1), (2), (3), (4), (5), (6), (9) (see Table 1 for the details of titanium composition and incorporation mode).

614 XRD spectra for the various titanium containing MCM-41 samples are shown in Figure 1. Powder XRD analysis showed that both pure MCM-41 and pure Ti-MCM-41 maintain their structure and crystallinity after Yi grafting. The samples exhibited well defined (1 0 0) reflection. There is slight decrease in the intensity of the higher order peaks which could be due to slight decrease in long range order after Ti grafting. The samples after Ti grafting also show a slight shift in the XRD peak position to higher 2-theta values.No higher order reflections could be seen, indicating the absence of bulk (> 1000 A particle size) anatase. UV-Vis spectra of the titanium containing MCM-41 samples are shown in Figure 2. The UVVis analysis of these samples show a band near 220 nm range due to tetrahedrally coordinated Ti. With increasing Ti content in the samples the UV-vis spectra was found to become broader at higher wavelength region probably due to formation of Ti-O-Ti clusters. Generally a shoulder a t - 3 3 0 nm

O

<

200

'

I

250

'

300

'

r

350

'

I

400

'

I

450

'

I

500

'

I

550

Wavelength (nm) Figure 2. UV-Vis spectra of titanium incorporated MCM-41 samples (1), (3), (5), (6), (7), (10) and (11 ) (see Table 1 for the details of titanium composition and incorporation mode).

615

appears in the spectrum if the sample contains some bulk titania, but such a shoulder could not be observed. Though the formation of Ti-O-Ti clusters with increasing titanium content can not be ruled out because the spectra become broader and show a red-shift with increasing Ti content. Absorption band at 260-270 nm has been generally attributed to the presence of Ti atoms in 5- and 6-fold coordinations, which are most likely generated through hydration of the tetrahedrally coordinated sites [23].

9

9" "

:'~';;i i

50nm

.., .~,';.,i:;,*.'.':'~

,~,...

(a)

(b)

Figure 3. TEM images of Au supported on titanium incorporated MCM-41 samples (a) catalyst # (3) Ti/Ti-MCM-41 and (b) catalyst # (4) Ti/Ti-MCM-41 (Refer Table 1 for support composition).

100-

)

80 60

60.~

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= O

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4 Diameter (nm)

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~ 9

0

2

4

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6

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Diameter (nm)

.

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10

9

i

12

,

14

Figure 4. Size distribution of Au nanoparticles supported on (a) support # (3) Ti/Ti-MCM-41 and (b) support # (4) Ti/Ti-MCM-41 (Refer Table 1 for support composition).

616 Typical TEM images of the Au nanoparticles supported on titanium containing MCM-41 samples are shown in Figures 3a and 3b. The Au particle size distributions are shown in Figures 4a and 4b. The TEM pictures for the Au deposited catalysts did not show the presence of any bulk titania phase in the samples with up to 3 % Ti grafting and the Au nanoparticles were found to be uniformly dispersed (- 3.0 nm particle size) on the surface of titanium containing MCM-41 samples with Ti incorporated either in one-step or in two-step methods. It is also noticed that at very high Ti content of the catalysts there is formation of larger Au particles. Results of the influence of the mode of Ti incorporation into the MCM-41 support framework on the propylene epoxidation activity of supported Au catalysts are presented in Table 2. Catalysts prepared by two-step Ti incorporation (catalysts # (3), (4), (6), (7)) showed higher propylene conversion and better H2 efficiency than those prepared by one-step Ti incorporation during MCM41 synthesis (catalysts # (1), (2)). Time on stream (TOS) study shows that the former catalysts also show less deactivation than the latter. The catalysts prepared by titanium grafting on pure MCM-41 (one-step Ti incorporation) showed the lowest activity (catalysts # (9) and (10)) at similar PO selectivities as other catalysts (85-90%). The catalyst prepared by two-step titanium incorporation and using titanium isopropoxide as Ti source (catalyst #7, Ti = 4.5%) was the best in terms of initial Table 2 Propylene Epoxidation activity of Au catalysts supported on Ti containing MCM-41 supports: Influence of mode of Ti incorporation [temp. 150~ (for catalyst # 2, 125~ Catalysts*

TOS

Convn. (%)

PO selectivity PO yield

H2 eff. b (%)

(h)

C3H6 H2

(%)

(1) Ti-MCM-41

1 3

4.6 2.5

20 16

89 92

3.9 2.6

23.0 15.6

(2) Ti-MCM-41

1 3

4.9 2.1

22 16

88 90

4.4 1.9

22.3 13.1

(3) Ti/Ti-MCM-41

1 3

5.1 3.5

16 12

90 91

4.6 3.2

31.9 29.2

(4) Ti/Ti-MCM-41

1 3

4.8 3.1

18 11

82 85

3.9 2.7

28.2 26.7

(5) Ti/Ti-MCM-41

1 3

5.4 3.2

23 18

75 72

4.0 2.4

17.8 24.8

(6) Ti-+Ti-MCM-41

1 3

5.2 3.2

21 15

82 84

4.2 2.9

21.3 36.7

(7) Ti---rTi-MCM-41

1 3

5.5 3.7

15 12

86 89

4.8 3.3

30.8 24.0

(8) Ti---~Ti-MCM-41

1 3

4.8 3.5

20 18

78 74

3.6 2.5

16.4 20.0

(9) Ti--+MCM-41

1 3

3.4 1.9

17 15

88 92

3.0 2.0

12.7 18.7

(10) Ti---~MCM-41

1 3

4.3 2.1

23 16

84 90

3.7 1.9

13.1 13.1

Space velocity, 4000 h-lcm3/gcat; catalyst, 0.15 g; feed, Ar/C3H6/H2/O2 = 70/10/10/10. * 1 wt % Au supported catalysts; details of titanium incorporation in Table 1. bBased on a stochiometric reaction to produce PO and water.

617 and final PO yields and its initial hydrogen efficiency was similar to that for catalyst #3 which has lower Ti content (3.0 %) and was prepared using titanocene dichloride as the titanium source. Catalyst #6 (3.0 % Ti, two-step incorporation) showed the best hydrogen efficiency. This enhanced activity and hydrogen efficiency of the Au catalysts supported on MCM-41, containing titanium incorporated by two-step method could be attributed to larger concentration of accessible, well dispersed surface Ti sites which are partly in contact with Au nanoparticles and utilize the in-situ generated hydroperoxy species for epoxidation. It has been proposed that hydroperoxy species formed on the Au surface are oxidant for the epoxidation reaction [15-20] in the reaction temperature range 373-473 K, even though it is still speculative. The results show that one step titanium grafting onto the pure MCM-41 surface does not lead to efficient formation of large amounts of isolated Ti sites whereas post-synthesis grafting of titanium on the surface of Ti-MCM-41 samples (two-step Ti incorporation) gives more isolated Ti sites. Also, titanium isopropoxide is found to be a better grafting reagent than titanocene dichloride. Higher Ti input (> 4.5 %) into the MCM-41 supports may cause the formation of Ti-O-Ti type clusters as suggested by broadening of the UV-Vis spectra (Figure 2), resulting in lower PO selectivity (catalysts # (5) and (8)) and more CO2 formation. 4. CONCLUSIONS Two-step titanium incorporation (hydrothermally, during crystallization followed by postsynthesis grafting) into mesoporous MCM-41 results in a catalyst support with more isolated/dispersed Ti sites. Gold nanoparticles deposited on such Ti containing support, by DP method, show better propylene epoxidation activity, better hydrogen efficiency and less deactivation than Ti-MCM-41 supports prepared by one-step titanium incorporation method, either hydrothermally or by post-synthesis grafting. The improved activity could be attributed to the more number of isolated Ti sites which are partly used as sites for Au nanoparticle deposition and may stabilize hydroperoxo species (formed at Au sites) that react with the propylene adsorbed on the silica matrix of the catalyst support surface. ACKNOWLEDGEMENT A. K. Sinha gratefully acknowledges the financial support in the form of STA Fellowship from Science and Technology Agency of Japan. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

S.L. Ainsworth, Chem. Eng. News 9 (1992); M. Mccoy, Chem. Eng. News 19 (2001). K. Murata and Y. Koyozumi, Chem. Commun., (2001) 1356. H. Orzesek, R. P. Schulz, U. Dingerdisses, W. E. Maier, Chem. Eng. Technol., 22 (1999) 8. M.G. Clerici, G. Bellusi and U. Romano, J. Catal., 129 (1991) 159. B. Notari, Catal. Today, 18 (1993) 163. E. Occhiello, Chem. Ind., 761 (1997). A. Sato, T. Miyake and T. Saito, Shokubai (Catalysts), 34 (1992) 132. R. Meiers, U. Dingerdissen and W. F. H61derich, J. Catal., 176 (1998) 376 M. Haruta, Catalysis Surveys of Japan, 1 (199&) 61 and references therein. M. Haruta, Catal. Today, 36 (1997) 123 and references therein. M. Haruta, Stud. Surf. Sci. Catal., 110 (1997) 123 and references therein. T. Hayashi, K. Tanaka and M. Haruta, Shokubai, 37 (1995) 72. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 178 (1998) 566. Y.A. Kalvachev, T. Hayashi, K. Tanaka and M. Haruta, Stud. Surf. Sci. Catal.,

618 110 (1997) 965. 15. M. Haruta, B. S. Uphade, S. Tsubota and A. Miyamoto, Res. Chem. Intermed. 24 (1998)329. 16. B.S. Uphade, M. Okumura, S. Tsubota and M. Haruta, Appl. Catal. A: Gen., 190 (2000) 43. 17. B.S. Uphade, Y. Yamada, T. Nakamura and M. Haruta, Appl. Catal. A: Gen., 215 (2000) 137. 18. T.A. Nijhuis, H. Huizinga, M. Makkee and J. A. Moulijn, Ind. Eng. Chem. Res. 38 (1999) 884. 19. E.E. Stangland, K. B. Stavens, R. P. Andres and W. N. Delgass, J. Catal., 191 (2000) 332. 20. G. Mul, A. Zwijnenburg, B. van der Linden, M. Makkee and J. A. Moulijn, J. Catal. 201 (1) (2001) 128. 21. R. G. Bowman, H. W. Clark, J. J. Maj, G. E. Hartwell, PCT/US97/11414, PCT Pub. No. WO 98/00413 (1998). 22. T. Hayashi, M. Wada, M. Haruta and S. Tsubota, Jpn. Pat. Pub. No. H 10-244156, PCT Pub. No. WO97/00869, U.S. Patent 5,932, 750 (1999). 23. A. Corma, M. T. Navarro and J. Perez-Parieme, J. Chem. Soc., Chem. Commun. 147 (1994). 24. P.T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 368 (1994) 321. 25. T. Maschmeyer, F. Ray, G. Sankar and J. M. Thomas, Nature 378 (1995) 159. 26. F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. Leofanti, G. Petrini, Catal. Lett. 16 (1992) 109. P. E. Sinclair, G. Sankar, C. Richard, A. Catlow, J. M. Thomas, T. Maschmeyer, J. Phys. Chem. B 101 (1997) 4232.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

619

Intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta U. Wilkenh6ner 1, D.W. Gammon:, E. van Steen 1. Catalysis Research Unit, ~Dept. Chemical Engineering, 2Dept. of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa; E-mail: [email protected] Dedicated to Professor Jens Weitkamp on the occasion of his 60th birthday The intrinsic activity of Ti-sites in the pores of TS-1 and Al-free Ti-Beta in the phenol hydroxylation with water and methanol as a solvent is estimated using the Thiele-modulus approach. The observed kinetic data were modelled using 2na order rate expressions. The intra-crystalline diffusivity of phenol was determined using Zero Length Column chromatography. The intrinsic rate constant for phenol consumption with water as a solvent over TS-1 is smaller than the one over Al-free Ti-Beta. Due to strong selective adsorption of phenol with water as a solvent, definite conclusions on the intrinsic activity of Ti-sites in these two titano-silicates with water as a solvent cannot be made. Selective adsorption is not important with methanol as a solvent. Based on the intrinsic activity with methanol as a solvent, it can be concluded that titanium sites in TS-1 are more active than those in Ti-Beta.

1. INTRODUCTION Crystalline titanium substituted silicates, such as TS-1 and Ti-Beta, are a well-known class of materials for selective oxidation using peroxides as the oxidant. Although the Ti-O bond length in both TS-1 and Ti-Beta is identical, force field calculations showed that the bond angles differ [ 1]. This can potentially lead to a different intrinsic activity of the active Ti-site. It must, however, be kept in mind that the force field calculations were performed for a titanium site in perfect tetrahedral coordination. In freshly calcined, dehydrated TS-1 titanium is in a tetrahedral coordination [2]. Under reaction conditions [3] and in the presence of water or alcohols [4] the coordination of titanium in titano-silicates such as TS-1 increases. The active sites in phenol hydroxylation are titanium peroxide species, which are formed upon hydrolysis of the Ti-O-Si bonds [5]. It has been shown that TS-1 is more active for the epoxidation of linear olefins than Al-free Ti-Beta [6-8]. However, it was recognised that diffusional constraints may exist in the epoxidation of 1-octene over Ti-Beta [9]. Diffusional limitations are well known for phenol hydroxylation [10,11] and anisole hydroxylation [12] over both TS-1 and Al-free Ti-Beta. In this study the intrinsic activity of the titanium sites in TS-1 and Al-free Ti-Beta for phenol hydroxylation are estimated taking diffusional constraints into account.

620 2. EXPERIMENTAL SECTION In the experiments a number of TS-1 samples and Ti-Beta samples of different crystallite sizes were used, viz. TS-I: 0.1 l.tm, 3x10x45 ~tm (diffusional path length ca. 31xm), Ti-Beta: 0.9 ~tm, 2-5 ~m. Small crystallites of TS-1 (Si/Ti=33) were synthesised according to the method described by Thangaraj et al. [13]. Large crystals ofTS-1 (Si/Ti=33) were synthesised according to the procedure described by Milestone et al. [14]. Large crystals of Al-free TiBeta (Si/Ti=40) were synthesised using the procedure adopted by Blasco et al. [8]. The synthesis procedure for small Al-free Ti-Beta crystals is identical to the one for the large crystals except for the addition of nano-sized, dealuminated zeolite Beta seeds (obtained by acid washing of nano-sized zeolite Beta crystals at 358K for 24hrs). All materials were shown to be crystalline (XRD). Scanning electron micrographs of the resulting crystals are shown in Figure 1. The absence of amorphous TiO2 in these materials was shown using DR-UV/VIS.

TS-1 (dcrysta~ca. 0.1 ~m)

AI-free Ti-Beta (dcrystat ca. 0.9 p,m)

TS-1 (3x10x45 ~m)

AI-free Ti-Beta

(dcrystal--

2-5 l~m)

Figure 1" Scanning electron micrographs of the titano-silicates (TS-1 and Al-free Ti-Beta) used in this study (note different magnifications)

621 In order to estimate the activity in the pores of TS-1, the external surface of the TS-1 ( d ~ l ~ 0.1 ~m) was inertised by cyclic, low temperature deposition of tetra-ethoxysilane (20 cycles) [5]. It was shown that this technique leads to the inertization of the external surface of TS-1 without significantly affecting access to the pore mouth [11 ]. The external surface of the other titano-silicates is sufficient small in comparison to the total surface area so that their contribution to the overall activity can be neglected. The diffusivity of phenol was determined using zero length column chromatography (ZLC) [15]. The transient desorption of phenol was determined at temperatures between 273K and 353K with water and methanol as a solvent using the large crystals of TS-1 and Al-free TiBeta (70 mg). Flow rates were varied between 0.5 and 3 ml/min to ensure that the response curves were not governed by interstitial fluid hold-up or axial dispersion. The independence of the transient desorption of concentration was confirmed by variation of the phenol concentration. Although the adsorption constant can principally be obtained from ZLC-experiments, the adsorption was also obtained from partition chromatographic measurements [ 16]. Phenol hydroxylation (12.8 mmol phenol) was carried out in a small batch reactor at 333K using 30% H202 in water (0.6ml; 5.3 mmol H202) and 0.2g catalyst in 5 ml solvent (water or methanol). The concentration profile of phenol, hydroquinone, catechol and H202 were fitted with second order rate expressions, from which the observed rate constant for the phenol consumption can be determined [5,11 ].

3. RESULTS AND DISCUSSION 3.1 Intra-crystalline diffusivity and adsorption of phenol in TS-I and Al-free Ti-Beta The effective diffusivity of phenol with water and methanol as a solvent was determined using the large crystals of TS-1 and Al-free Ti-Beta. Figure 2 shows the temperature dependency of the intra-crystalline diffusivity of phenol with water as a solvent in TS-1. The intra-crystalline diffusion coefficients are about a magnitude of order larger than the value reported previously (210 "18 m2/s at 333K [10]). The literature value was determined from kinetic experiments with different crystal sizes of TS-1, i.e. under reaction conditions, in the presence of the solvent, 1-120, and H202. From the Arrhenius plot, the activation energy is estimated to be of 14.8 kJ/mol. This compares well with the reported activation energy 14.7 kJ/mol for the di~sion of phenol in NaX with water as a solvent [ 14]. The diffusivity of phenol in the pores of crystalline titano-silicates is dependent on both the solvent and the pore structure (see Table 1). The effective diffusivity of phenol with water as a solvent is 4-8 times slower than with methanol as a solvent. The effective diffusivity of phenol diffiasivity in the large pore titano-silicate Ti-Beta is 3-8 times larger than in the medium pore TS-1. The Henry coefficients extracted from ZLC experiments compare well with those determined using partition chromatography. Strong selective adsorption of phenol is observed with water as a solvent. The strong adsorption of phenol in TS-1 with water as a solvent is not surprising since the TS-1 framework is known to be hydrophobic and thus less polar molecules will be preferentially adsorbed. Al-free Ti-Beta is obviously less hydrophobic due to diminished wallsorbent interaction. With methanol as a solvent the concentration in the pores is almost identical to that in the bulk liquid.

622 1E-15

E ch I-c

":- 1E-16 0 e~ e-

a 1E-17

I

2.5

I

I

3

I

I

3.5

4

10001T, K -1 Figure 2:

Temperature dependency of intra-crystalline diffusivity of phenol in TS-1 with water as a solvent

Table 1"

Effective diffusivity as determined using ZLC and the relative strength of adsorption determined using partition chromatography in TS-1 and Al-free Ti-Beta

Dr phenol, m2/s Solvent T, K 5.2 10"17 H20 303 4.4 10"16 CHaOH 333 Al-free Ti-Beta H20 303 3.9 10-16 CHaOH 333 1.5 10"15 1 Henry's constant extracted from ZLC experiments 2 Henry's constant determined using partition chromatography Catalyst TS-1

K1 77.7 0.61

K2 84.3 0.66 17.2 1.31

The much slower diffusivity of phenol with water as a solvent as compared to the diffusivity with methanol as a solvent might be attributed to the difference in phenol-solvent interaction and the phenol-phenol interaction. The interaction between phenol and water in the pores of TS-1 and Al-ffee Ti-Beta is poor, since water adsorbs much less strong in the pores. Thus, the main interaction with water as a solvent will be the phenol-phenol interaction slowing down the diffusion of phenol out of the pores of the titano-silicate. With methanol as a solvent phenol in the pores of the titano-silicate will be surrounded by methanol molecules.

3.2 Phenol hydroxylation using TS-I and AI-free Ti-Beta It is well known that phenol hydroxylation using TS-1 [5, 10-12] and Ti-Beta [11,12] is strongly mass transfer limited. Figure 3 shows the effect of the crystal size of TS-1 and AIfree Ti-Beta on the phenol conversion with water as a solvent as a function of reaction time. It can be clearly seen that the small crystals of TS-1 are less active than the larger crystal of Ti-

623

Beta. This is only observed when the external surface of TS-1 is inertised and is not contributing to the overall consumption of phenol [5]. It can further be seen that phenol hydroxylation with water as a solvent over Al-free Ti-Beta is clearly mass transfer limited.

20

20

TS-

AI-free Ti-Beta (ca. 0.9 pm) I

O

0

Al-.free Ti-Beta (2-5 pro)

.,..

....

U; L

I/1 L

>~

~o~ ~10 _.o

m

o

C

[]

a. Jr

9

0

,

9

100

,

9

200

i

300

Reaction time, min

Figure 3:

i

400

100

200

3(X)

Phenol hydroxylation with water as a solvent over TS-1 (left; O: d~stal ca. 0.1 txm; O: 3x10x45 ktm) and Al-free Ti-Beta (fight; I1: d r ca. 0.9 lxm; [-i: 2-5 ~tm). Reaction conditions: T=333K; 5.6 ml I-/20; rn~t=0.12g; Cph=o~,0=2.5mol/;%, CH2o2,0= 0.83 mol/~,. Solid lines represent fit to second order rate expressions.

The kinetic data were fitted with second order rate expressions [ 11 ]: phenol + H202 --> hydroquinone/catechol/tar + H20 r1 = k 1 -Cphenol -CH202 products + H202

--> tar + H20

r2 = k 2 -Cproducts -CH202

2H202

->

r3 - k 3 - C 2

2H20+ 89

H202

Thus, the rate of consumption and the change in the phenol and H202 concentration can be expressed as: dCphenol

-

d ~

= k 1" C phenol" C H 2O2

_ d C H 2 0 2 = k 1 9Cphenol "CH202 + k 2 "Cproducts "CH202 + k 3 .C 2 dt

4OO

Reaction time, min

H202

The observed rate constants were determined by fitting these expressions to the measured concentrations of phenol, hydroquinone, catechol and H202 (tar formation was monitored over the mass balance). The observed rate constants for the conversion of phenol (kl) using either water or methanol as a solvent over TS-1 and Al-free Ti-Beta are given in Table 2. The observed activity of both TS-1 and Al-free Ti-Beta is higher with water as a solvent than with methanol as a solvent. Phenol adsorbs preferentially when water is present (see Table 1). This would lead to a high concentration of phenol in the pores and thus enhance the rate of phenol conversion, since phenol hydroxylation is mass transfer limited.

624 Table 2:

Observed rate constants for phenol conversion (kl in L/(molhr'g)) in phenol hydroxylation at 333K TS-1

Solvent 0.1 gtm~ 1-120 2.75 CH3OH 1.26 1 External surface inertised

Al-free Ti-Beta 0.9 l.tm ca. 3 l~m 2.46 1.29 0.45 0.40

3x10x45 l.tm 0.21 0.34

Using the experimentally determined diffusion coefficients (see Table 1), the effectiveness factor can be estimated (see Figure 4). For all titano-silicates a spherical geometry was chosen, except for the large crystals of TS-1, which were approximated as slab-shaped particles. Small crystallites of TS-1 (d~,~tal ~ 0.1 lxm) have an effectiveness factor close to one for the phenol hydroxylation with both solvents. The phenol hydroxylation with methanol as a solvent approaches an effectiveness factor of equal to one for Al-free Ti-Beta crystals with a crystal size of ca. 0.9 lam. A direct comparison of the observed rate constant for phenol consumption over small crystals of TS-1 and small crystals of Al-free Ti-Beta leads to the conclusion that TS-1 is more active per gram oftitano-silicate.

1.0

1.0

A

0.8 m M

"6

o.6

=

0.6

~ 0.4

~.0.2 w 0.0 0.1

~ 0.2

1

10

Thiele m o d u l u s ,

Figure 4:

100

solvent: CH30H

0.8

9 0.4

.>

m

0.0 0.01

i

t

.,,.

0.1

1

10

100

Thiele m o d u l u s ,

Effectiveness factor of titano-silicates in phenol hydroxylation with water (left) or methanol (fight) as a solvent as a function of Thiele modulus (@: TS-1, d ~ l ca. 0.1 l.tm; O: TS-1, 3x10x45 ~tm; m: Al-free Ti-Beta, d~y~l ca. 0.9 ~tm; VI: Al-free Ti-Beta 2-5 lam). Reaction conditions: T=333K; 5 ml solvent; rn~ = 0.12g; Cph~ol,0= 2.5mol/1, CH2o2,0= 0.83 mol/1 - H202 added as 30% H202 in 1-120.

Knowing the effective diffusivities (also at reaction temperature) and the observed rate constants the effectiveness factor for the phenol hydroxylation can be determined. From these data the intrinsic rate constants can be obtained. With water as a solvent, the intrinsic 2"a order rate constant is lower with TS- 1 (2.8 L/(molhrg) corresponding to 1.6 L/(molsmol Ti-sites)) than with Al-free Ti-Beta (3.6 L/(molhrg) corresponding to 2.5 L/(mol's'mol Ti-sites)).

625 The diffiasivity used to estimate the intrinsic activity of the titanium sites was based on ZLC-experiments. This yielded a diffusivity of phenol in water, which is ca. 40 times larger than the estimate by van der Pol et al. [10]. In our experiments the diffiasivity was estimated using TS-1 pre-equilibrated in H20/phenol mixture. The diffusivity estimated by van der Pol et al. [10] was based on kinetic experiments. The coordination sphere of the titanium sites may differ in the ZLC-experiments from those under reaction conditions [2-4]. A change in the coordination of titanium is expected to affect the diffusivity in TS-1 more severely than the dif~sivity of phenol in Al-free Ti-Beta. Taking the diffusivity of phenol in TS-1 as reported by van der Pol et al. [10], the intrinsic 2"~ order rate constant with TS-1 equals 4.9 E/(molhrg) (corresponding to 2.8 E/(molsmol Ti-sites)). It should however be kept in mind that the intrinsic 2nd order rate constants are calculated using bulk concentrations. Any selective adsorption effect will falsify the possible conclusion. Phenol is selectively adsorbed with water as a solvent. Furthermore, phenol is more strongly adsorbed in TS-1 than in M-free Ti-Beta. Phenol hydroxylation requires the access of two reagents to the active site, viz. phenol and H202. Due to the selective adsorption the concentrations of phenol and H202, within the pores will differ significantly from those in the bulk of the fluid. In order to estimate the intrinsic activity of the titanium sites in TS-1 and AIfree Ti-Beta for phenol hydroxylation with water as a solvent, the adsorption of H202 needs to be taken into account, which is unknown. With methanol as a solvent effectiveness factors of approximately 1 were obtained for phenol hydroxylation over small TS-1 crystals and relatively small Al-free Ti-Beta crystals. A direct comparison of the observed activity does show that titanium sites in TS-1 are more active for phenol hydroxylation with methanol as a solvent than those in Al-free Ti-Beta. Furthermore, selective adsorption does not seem to play a significant role with methanol as a solvent. Hence, the intrinsic activity of the titanium sites can be estimated from phenol hydroxylation with methanol as a solvent. Based on the intrinsic rate constants with methanol as a solvent (TS-I: 1.27 L/(molhrg) corresponding to 0.7 E/(molsmol Ti-sites); Ti-Beta: 0.45 E/(molhrg) corresponding to 0.3 M(molsmol Ti-sites)), it can be concluded that the titanium sites in TS-1 are intrinsically more active. The intrinsic activity of titanium sites in TS-1 can even be larger than the one reported here, if the coordination of the titanium sites under reaction conditions is larger than under ZLC conditions. This would lead to a reduced diffusivity of phenol in TS-1 under reaction conditions, whereas the diffiasivity of phenol in Al-free Ti-Beta would hardly be affected. 4. CONCLUSIONS The intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta for phenol hydroxylation was estimated using the diffiasivity of phenol with water and methanol as a solvent in TS-1 and Al-free Ti-Beta as determined by ZLC and kinetic experiments. The kinetic experiments were evaluated using 2"a order rate expressions. The intrinsic activity was then estimated using a Thiele-modulus approach. The intrinsic activity of the titanium sites in TS-1 are at least twice as active as the titanium sites in Al-free Ti-Beta for the phenol hydroxylation with methanol as a solvent. With water as a solvent this approach fails due to the strong selective adsorption of phenol in TS-1 and to a lesser extent in Al-free Ti-Beta.

626 ACKNOWLEDGEMENT

Financial support for this study from Sasol, AECI, NRF, DTI (via THRIP) and UCT is gratefully acknowledged.

REFERENCES:

1. G. Sastre and A. Corma, Chem. Phys. Lett. 302 (1999), 447-453. 2. S. Pei, G.W. Zajae, J.A. Kaduk and J. Faber, Catal. Lett. 21 (1993), 333-344. 3. V. Bolis, S. Bordiga, C. Lamberti, A. Zeeehina, A. Carati, F. Rivetti, G. Spano and G. Petrini, Mieroporous and Mesoporous Mat. 30 (1999), 67-76. 4. E. Astorino, J.B. Peri, R.J. Willey and G. Busea, J. Catal. 157 (1995), 482-500. 5. U. Wilkenh6ner, G. Langhendries, F. van Laar, G.V. Baron, D.W. Gammon, P.A. Jaeobs and E. van Steen, J. Catal. 203 (2001), 201-212. 6. C.B. Dartt and M.E. Davis, Appl. Catal. A 143 (1996), 53-73. 7. J.C. van der Waal, P.J. Kooyman and J.C. Jansen, Mieroporous and Mesoporous Mat. 25 (1998), 43-57. 8. T. Blaseo, M.A. Camblor, A. Corma, P. Esteve, J.M. Guil, A. Martinez, J.A. PerdigonMelon and S. Valencia, J. Phys. Chem. B 102 (1998), 75-88. 9. J.C. van der Waal, Phi) thesis, Technical University Deltt (1998). 10. A.J.H.P. van der Pol, A.J. Verduyn and J.H.C. van Hooff, Appl. Cat. A 92 (1992), 113130. 11. U. WilkenhOner, PhD thesis, University of Cape Town (2002). 12. U. WilkenhOner, D.W. Gammon and E. van Steen, Stud. Surf. Sei. and Cat. 135 (2001), 368. 13. A. Thangaraj, M.J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites 12 (1992), 943-950. 14. N.B. Milestone and N.S. Sahasrabudhe, Proe. 12th Int. Zeolite Conf. (M.M.J. Treaey, B.K. Marcus, M.E. Bisher, J.B. Higgins, Eds.), Vol. 3, p. 1901-1908, Materials Research Society, Warrendale (1990). 15~ D.M. Ruthven and P. Stapleton, Chem. Eng. Sei. 48 (1993), 89-98. 16. G. Langhendries, D.E. De Vos, G.V. Baron and P.A. Jaeobs, J. Catal. 186 (1999), 1-11. 17. F. Awum, S. Narayan and D. Ruthven, Ind. Eng. Chem. Res. 27 (1988), 1510-1515.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

627

The effect of zeolite pore size and channel dimensionality on the selective acylation of naphthalene with acetic anhydride JiN 12ejka1, Pavla Proke~ovfi 1, Libor (;erven~ and Katefina Mikulcovfi 2 1j. Heyrovsk) Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic 2 Department of Organic Technology, Institute of Chemical Technology, Technick/t 5, CZ-166 28 Prague 6, Czech Republic Acylation of naphthalene was investigated over different large pore zeolites with the aim to synthesise selectively 2-acetylnaphthalene as the demanded product for various perfume and flavour compositions. Acetic anhydride was found to be the proper acylating agent while acetyl chloride and acetic acid exhibited very low acylation activity. The highest acetylnaphthalenes yield was achieved with zeolite Beta possessing medium concentration of acid sites and excess of naphthalene together with stepwise adding of acetic anhydride to the reaction mixture.

1. INTRODUCTION Acylation reactions represent the most important method in organic chemistry to synthesize aromatic ketones, which are at present important intermediates for the production of fine chemicals. The reaction usually proceeds via interaction of various aromatic hydrocarbons with chlorides or anhydrides of relevant carboxylic acids at the presence of proper catalyst. The conventional catalysts like Lewis acids (e.g. A1C13, BF3, ZnC12) or Broensted acids (HF, H3PO4) have been usually employed to catalyze these acylation reactions. However, the technical and environmental drawbacks connected to the utilization of these catalysts are evident. More than stoichiometric amount of Lewis acid is required to catalyze effectively acylation reactions, which leads to the formation of stable complex of product with Lewis acid. The decomposition of this complex leads to the formation of pure product, however, Lewis acid is destroyed without the possibility of the catalyst regeneration (1). This is the main reason while during the last decade a number of contributions appeared showing that various solid acids can be successfully employed to catalyze acylation reactions (2). Among them zeolites played a significant role as highly active and selective heterogeneous catalysts for acylation of different aromatic hydrocarbons. Particularly, acylation of anisole, toluene or 2-methoxynaphthalene were investigated with a significant effort over medium and large pore zeolites. Rohan et al. (3) acylated anisole to para-methoxyacetophenone and showed that the critical issue of this reaction

628 was the easy deactivation of the zeolite catalyst probably due to the formation of various bulky compounds inside the zeolite channel system. The shape-selectivity was successfully used in the case of 2-methoxynaphthalene acylation to desired 6-acetyl-2- methoxynaphthalene, which is of a particular interest for the production of the anti-inflammatory drug Naproxen. It was shown that 1-acetyl-2-methoxy naphthalene was mainly achieved in experiments carried out in the kinetic regime, while 2,6- and 2,8-isomers were favoured by thermodynamic factors (4). While the activity of zeolites in acylation of 2-methoxynaphthalene decreased in the following order Y > Beta > Mordenite > ZSM-12, zeolite Y preferred mainly the formation of 1-acetyl-2- methoxynaphthalene. The channel system of ZSM-12 was expected to be too small to accommodate acetyl-methoxy-naphthalenes (5). On the other hand zeolite Beta and mordenite exhibited high selectivity towards desired 6-acetyl isomer (6,7). Based on these numerous studies it is evident that not only the choice of the proper zeolite catalyst and reaction conditions plays very important role to achieve high conversion and selectivity but also the utilization of proper solvent and its concentration are key parameters for optimum zeolite behaviour. Recently, Rhodia company announced a new industrial process for acylation of anisole to para-acetylanisole employing zeolite catalyst (8). In this contribution the effect of zeolite pore size and dimensionality on the activity and selectivity of zeolites Y, Mordenite, ZSM-12, Beta and zeolite L is investigated in the acylation of naphthalene. Acetic anhydride, acetyl chloride and acetic acid were tested as acylating agents. In addition, the role of the type and concentration of individual acid sites on their catalytic behaviour is discussed. The interesting product, 2-acetylnaphthalene is widely used in perfume compositions, mainly in Neroli Orange Blossom, Magnolia, Honeysuckle, Jasmine and also in flavour compositions in imitation of Strawberry, Grape, various Citrus and berry-compositions (9). 2. EXPERIMENTAL S E C T I O N 2.1. Zeolites and methods used

Large pore zeolites Y (Si/A1 = 2.8), Mordenite (Si/A1 = 10.0), Beta (Si/A1 - 12.5-75), ZSM-12 (Si/A1 = 58) and zeolite L (Si/A1 = 3.2) were investigated in the naphthalene acylation with acetic anhydride. Zeolite Y was obtained from the Research Institute for Oil and Hydrocarbons, Bratislava (Slovak Republic), ultrastabilized zeolites Y, Mordenite and Beta were purchased from Zeolyst (USA). ZSM-12 and zeolite L were synthesised in our laboratory. The crystallinity and phase purity were checked by X-ray powder diffraction (Siemens D5005) with CuKa radiation in Bragg Brentano geometry and the size of the crystals by scanning electron microscopy (Jeol). The concentration and type of acid sites were determined by FTIR spectroscopy (Nicolet Protege 460) using self-supporting wafers d3-acetonitrile as probe molecule. The characteristics of zeolites used are depicted in Table 1.

629 2.2 Catalytic experiments Catalytic experiments were carried out in a 50 ml glass vessel equipped with a reflux condenser and magnetic stirrer. The reaction vessel was located in an oil bath with controlled temperature, all experiments performed in this study were carried out at 135 ~ In the typical experiments naphthalene (1.54 M), internal standard (tetradecane) and a solvent (mixture of cis- and trans-decaline) were mixed and heated in the reaction vessel to the reaction temperature. If not indicated elsewhere, the typical naphthalene to acetic anhydride molar ratio used was 2 : 1. After that an activated zeolite catalyst was added to the reaction mixture. Zeolite activation was performed at 450 ~ in a stream of air for 90 min followed by zeolite cooling to the ambient temperature in the dessicator. Finally, acetic anhydride was given to the reaction mixture and the experiment started. The tests with acetic acid or acetyl chloride were carried out in the same way. To follow the time-on-stream values of conversion and selectivity, small amounts of reaction mixture were taken away at preset time values.

Reaction products were analyzed using a gas chromatograph equipped with flame ionization and mass spectrometric detector (HP 6850 with an autosampler Agilent 7683) employing a high-resolution capillary column HP-1 (length 30 m, internal diameter 0.32 mm). In addition, reaction products were identified by gas chromatograph combined with mass spectrometer (HP 5890 Series I I - 5971A).

3. RESULTS AND DISCUSSION In our previous paper (10) we have shown that large pore zeolite catalysts can selectively catalyze naphthalene acylation with acetic anhydride to 2-acetylnaphthalene, which is a desired product for the preparation of various perfume and flavour compositions. The individual zeolite catalysts differed in acetylnaphthalenes yield (ratio of acetylnaphthalenes obtained related to the theoretical concentration of acetylnaphthalenes), selectivity and the resistance against deactivation. Therefore, now we tried to optimize the reaction conditions, type of acylating agent and the ratio between naphthalene and acylating agent with the aim to increase the long term stability of the zeolite and the selectivity to 2-acetylnaphthalene. The main characteristics of zeolites investigated, including dimensions of their pores, Si/A1 ratios and acetylnaphthalenes yields and selectivities to 2-acetylnaphthalene in naphthalene acylation with acetic anhydride are summarized in Table 1. 3.1. Effect of concentration of active sites The data in Table 1 clearly shows that there is a significant effect of the concentration of acid sites of zeolites in naphthalene acylation with acetic anhydride. It seems that this effect is reflected in the acetylnaphthalene yields despite the role of zeolite structure. The highest yields were obtained with zeolites possessing medium concentration of acid sites (e.g. for zeolite Beta/3 about 24-26 % under standard reaction conditions). The increase in the concentration of acid sites (zeolites Y and L, Mordenite, Beta/l) led to the significant decrease in the acetylnaphthalenes yield. It is seen from Table 1 that the maximum acetylnaphthalenes yield achieved with zeolites possessing the Si/A1 ratio lower than 10 was less than 3 % in maximum, in contrast to zeolites with Si/A1 ratio between 35-55. In the later case the acetylnaphthalenes yield reached more than 20 %

630 (Beta/3 and/4). On the other hand, further decrease in the concentration of acid sites for zeolite Beta/5 (Si/A1 = 75) resulted in a decrease in the acetylnaphthalenes yield to about 11-12 % (Fig. 1). Similar results were found with zeolite Y and its ultra-stabilized forms. While the acetylnaphthalenes yield for zeolite Y was about 1 % , USu and/2 exhibited significantly higher yields around 11-12 % (Fig. 2). On the other hand, the selectivity to 2-acetylnaphthalene was significantly lower in comparison to zeolite Beta (32-34 % and 72-80 %, respectively). To examine the effect of the type of acid sites on acetylnaphthalenes yield zeolites Beta/1 and/6 were compared. While Beta/1 possesses about 30 % of Broensted and 70 % of Lewis acid sites, Beta/6 (calcined in the stream of ammonia) exhibits about 70 % of Broensted and 30 % of Lewis acid sites. It is seen from Table 1 that slighly higher acetylnaphthalenes yield was observed for Beta/1 having more Lewis sites and lower total concentration of acid sites. It indicates very significant role of adsorption and desorption or even internal diffusion processes in this reaction, which overcome the possible effect of different type of acid sites. On the basis of these data it can be inferred that there exists some optimum concentration of acid sites to catalyze this acylation reaction with high acetylnaphthalenes yield. When high concentration of acid sites increases the rate of acetic anhydride decomposition and subsequent reactions, which led to the formation of various hydrocarbon deposits and in fact to the deactivation of the zeolite. The decomposition of acetic anhydride provided also the formation of acetic acid. It was

Table 1 Characteristics of zeolites used, acetylnaphthalene yields and selectivities to 2acetylnaphthalene in naphthalene acylation with acetic anhydride Zeolite Channel Channel Si/A1 ACN yield c (%) 2-ACN a system diameter (nm) ratio Selectivity (%) ZSM-12 1D 0.57 x 0.61 58 < 1.0 n.d. Zeolite L 1D 0.71 3.2 < 1.0 n.d. Zeolite Y 3D 0.74 2.8 < 1.0 n.d. USY/1 3D 0.74 15 11.4 34 USY/2 3D 0.74 40 11.5 32 Mordenite 1D a 0.67 x 0.70 10.0 2.7 73 H-beta/1 3D 13.4 7.3 79 H-beta/2 3D 17.5 9.0 77 H-beta/3 3D 0.76 x 0.64 37.5 24-26 75 H-beta/4 3D 0.55 x 0.55 55 23.3 73 H-beta/5 3D 75 11.7 74 H-beta/6 b 3D 13.3 3.8 71 a 8 membered ring of mordenite is too small to accommodate naphthalene molecules, b sample calcined in a stream of ammonia to increase the concentration of Broensted sites, c ACN yield - yield of acetylnaphthalenes to their theoretical amount, which can be formed based on naphthalene to acetic anhydride molar ratio at reaction time = 120 min, d 2-ACN selectivity- at reaction time = 120 rain (maximum acetylnaphthalenes yield).

631

30 25 "o" 20 ~,9 15

5 13,4

18,8

37,5

55,4

75

Molar ratio Si/AI

Fig. 1 Dependence of acetylnaphthalenes yield on Si/A1 ratio for zeolite Beta in acylation of naphthalene with acetic anhydride (Experimental conditions: 1.2 g of catalyst, naphthalene/acetic anhydride molar ratio=2, reaction time 120 min, temperature 135 ~

reported by Servotte et al. (11) that acid forms of zeolites catalyze easily the transformation of acetic acid into a large number of various hydrocarbons which causes the formation of bulky compounds, which cannot be easily desorbed from the zeolite channel system. This results in the fast deactivation of the respective zeolite catalyst. However, when only a small amount of acid sites is available acetylnaphthalenes yield again is rather low. In addition, it is necessary to emphasize the role of hydrophobicity-hydrophilicity of the zeolite used for adsorption/desorption and diffusion processes. We have shown (10) that the presence of more polar solvent (sulfolane) led to a lower conversion compared to decalin. This can affect also the concentration of available acid sites for the reaction. The higher is the concentration of acid sites, the slower is the desorption and transport of the reaction products. This is probably the second reason why some optimum concentration of acid sites is needed to achieve a good adsorption/desorption balance which favours high acetylnaphthalenes yield in this reaction. Fig. 3 depicts the effect of the ratio of naphthalene to acetic anhydride on the resulting acetylnaphthalenes yield and selectivity to 2-acetylnaphthalene. In agreement with results of Botella et al. (2) it is evident that this ratio influences significantly the yield of acetylnaphthalenes. This can be probably described to the inhibiting effect of acetylnaphthalenes formed during the reaction. Thus, the increase in the naphthalene to acetic anhydride molar ratio should result in the increase in acetylnaphthalenes yield. The excess of naphthalene probably facilitates easier desorption of acetylnaphthalene molecules. It is clearly seen in Fig. 3 that with increasing this ratio to 4 : 1, the acetylnaphthalene yield increases from about 6 to more than 35 %. On the other hand, no important changes in selectivities to 2-acetylnaphthalene were observed, the

632

selectivity to 2-acetylnaphthalene was about 70-80 % despite very significant differences in the reaction rate.

yield 4~ ImI~ ACN Selectivity 30 r

lff

20 <

._> o

10

t~

15

40

Molar ratio Si/AI 2 Acetylnaphthalenes yield and selectivity to 2-acetylnaphthalene over zeolites USY/1 and USY/2 in acylation of naphthalene with acetic anhydride (Experimental conditions: 1.2 g of catalyst, naphthalene/acetic anhydride molar ratio = 2, reaction time 120 min, temperature 135~ Fig.

8O

40

m ACN yield

35

-

7O

30

-

60 o

-E 25

-

50 ~

"~ 20 "~,

-

40 ~-

-

20

r

15-

~

10

Z

Selectivity

30 ._> o

10 0

1:1

2:1

4:1

Molar ratio (naphthalene" acetanhydride) Fig. 3 Dependence of acetylnaphthalenes yield on molar ratio of naphthalene to acetic anhydride for zeolite Beta (Si/A1 = 37.5); (Experimental conditions" 1.2 g of zeolite, reaction time 120 min, temperature 135 ~

633 3.2. Effect of the zeolite structure While the concentration of acid sites controls the activity of the zeolite catalysts, which is reflected in the acetylnaphthalenes yield (Fig. 1), the selectivity to individual acetylnaphthalene isomers is governed by the zeolite structure. From one side, shape selectivity influences significantly the ratio between 1- and 2-acetylnaphthalenes but also the formation of larger molecules, which are hardly to be desorbed from the zeolite channel system. In spite of the fact, that significantly different yields of acetylnaphthalenes were found over zeolite Beta possessing different concentration of acid sites (see Table 1), the selectivity to 2-acetylnaphthalene was always between 72-80 %. However, with USY/1 and USY/2 the selectivity to 2-acetylnaphthalene was only 32-34 %. It is evident that transition state selectivity strongly influences the ratio between the individual reaction products as the more open structure of USY zeolites enables the formation of sterically more demanded product (1-acetylnaphthalene). This is in contrast to zeolite Beta, the channel system of which is formed via threedimensional channels without larger cavities. In this channel system the formation of 1acetylnaphthalene is severely limited from sterical reasons and 2-acetylnaphthalene is preferentially formed. 3.3 Effect of acylating agent Three different types of acylating agents were used in this study, namely acetic anhydride, acetyl chloride and acetic acid, which in addition is also formed during the decomposition of acetic anhydride. While high yields of acetylnaphthalenes were achieved with acetic anhydride, almost no acylation activity was found for acetyl chloride and also acetic acid. During the decomposition of one molecule of acetic anhydride one molecule of acetic acid can be formed. This means that during the acylation reaction relatively high concentration of acetic acid is present in the reaction mixture. Despite this fact practically no acetylnaphthalenes were formed with acetic acid as acylating agent. Thus, it can be inferred that acetic acid probably does not represent the proper acylating agent for naphthalene acylation and naphthalene is acylated by acetyl group formed during the decomposition of acetic anhydride. In addition, as acetic acid is not the acylating agent, it means that contributes significantly to the deactivation of the catalysts due to its high reactivity on the acidic zeolites.

No higher acylated naphthalenes were obtained in the liquid phase during the reaction over any of zeolite catalysts used. This is probably due to the deactivating effect of acetyl group on aromatic hydrocarbons, which prevents the second acylation. In toluene acylation with acetic anhydride it was shown that further products (various aromatic ketones) can be formed inside of the channel system of zeolite Beta. These ketones can be removed via Soxhlet extraction with chloroform. They form only several percents of products, however, no di-acetyl products were found (2). Further experiments showed that to achieve the high acetylnaphthalenes yield over zeolite Beta/3, a combination of both the excess of naphthalene in the reaction mixture and the stepwise adding of acetic anhydride resulted in a significant improvement of the naphthalene conversion and the stability of the zeolite used (10).

634 4. CONCLUSIONS The highest acetylnaphthalenes yield in acylation of naphthalene with acetic anhydride was reached with zeolite Beta having the Si/A1 ratio around 35-40. Higher concentration of acid sites favoured rapid deactivation of the zeolite probably due to the subsequent reactions of acetic acid formed via acid catalyzed acetic anhydride decomposition and slow desorption of bulky products. Acetic anhydride was found to be the only active acylation agent while almost no acylation activity was observed with acetic acid or acetyl chloride. This indicates that acetic acid formed during the reaction can hardly be used as acylating agent. Very significant shape selective effect on the formation of individual acetylnaphthalene isomers was observed. While the selectivity to 2-acetylnaphthalene reached about 72-80 % over zeolite Beta, only 32-34 % were found with USY zeolites. It is evident that selectivity in this reaction is controlled by the restricted transitions state selectivity. Because of the higher reactivity of acetic anhydride compared to naphthalene the excess of the later one is recommended to decrease the rate of the deactivation of the catalyst. In a similar manner the stepwise addition of acetic anhydride into the reaction mixture led to higher naphthalene conversion.

REFERENCES

1.

P. Mrtivier, in Fine Chemicals though Heterogeneous Catalysis (Eds. R.A. Sheldon, H. van Bekkum), Wiley-VCH, Weinheim 2001, p. 161. 2. P. Botella, A. Corma, J.M. Lopez-Nieto, S. Valencia and R. Jacquot, J. Catal. 195 (2000) 161, and references therein. 3. D. Rohan, C. Canaff, E. Fromentin, M. Guisnet, J. Catal. 177 (1998) 296. 4. M.G. Clerici, Top. Catal. 13 (2000) 373. 5. G. Harvey, G. Maeder, Collect .Czech. Chem. Commun. 57 (1992) 863. 6. D. Das, S. Cheng, Appl. Catal. A 201 (2000) 159. 7. M. Casagrande, L. Storaro, M. Lenarda, R. Ganzerla, Appl. Catal. A 201 (2000) 263. 8. M. Spagnol, L. Gilbert, E. Benazzi and C. Marcilly, WO 96/35655 (1996). 9. S. Arctander, in Perfume and Flavour Chemicals (Aroma Chemicals), Publ. by the author, Montclair, N.J. (USA), 1969. 10. L. 12erven~, K. Mikulcovfi and J. Cejka, Appl. Catal. A, 223 (2002) 65. 11. Y. Servotte, J. Jacobs and P.A. Jacobs, Acta Phys. Chem. Szegediensis (1985) 611.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

635

A l k y l a t i o n of P h e n o l with M e t h a n o l over Zeolite H - M C M - 2 2 for the F o r m a t i o n of p - C r e s o l G. Moon, K.P. M/311er, W. B~3hringer and C.T. O'Connor Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Private Bag, Rondebosch, Cape Town, 7701, South Africa The alkylation of phenol with methanol over H-MCM-22 was investigated and the performance of this catalyst compared with that of H-ZSM-5 and amorphous silica-alumina. The reaction was studied in the liquid phase (batch reactor) at temperatures between 200 250~ and pressures between 20 - 42 bar and in the gas phase (flow reactor) at temperatures between 250 - 400~ and feed partial pressures of 0.2 bar. The C (ring) : O (side chain)alkylation ratio was found to be more dependent on reaction conditions (i.e. gas/liquid phase, pressure and temperature) than catalyst type. The highest C : O alkylation ratios were obtained using gas phase reactions and were similar for all three catalysts. The highest p/ocresol ratios were obtained using H-MCM-22 in the liquid phase at 200~ Reasons are proposed to explain the different ring vs side chain alkylation and cresol isomer distributions for the different catalysts and the different reaction conditions. 1. I N T R O D U C T I O N

Para-cresol is an important intermediate in the formation of antioxidants and preservatives for plastics, motor oil and foods, p-Cresol can be formed in the alkylation of phenol with methanol. Typical catalysts used for the alkylation of phenols are H 2 8 0 4 , BF3, MgO, A1203 and SiO2-supported Fe/V [ 1,2]. The methyl group can alkylate in two ways on the phenol; firstly, on the oxygen, which is know as O-alkylation and secondly, on the benzene ring, known as C-alkylation, as shown in Figure 1. OH -!-

CH30H

OH

OCH 3 +

O-alkylation

or

C-alkylafion

Figure 1: Schematic diagram of the alkylation of phenol with methanol.

+ OH 3

636 The primary products of this reaction are anisole, o-cresol and p-cresol, m-Cresol is the thermodynamically favoured cresol isomer but is not kinetically favoured by electrophilic substitution as the hydroxyl group on the phenol is strongly ortho- and para-directing [3]. Possible secondary products are xylenols (dimethylphenols) and methylanisoles. It is possible to produce a high selectivity to o-cresol over basic catalysts or alumina [2, 4, 5]. Highly selective synthesis of m-cresol and p-cresol respectively is desirable due to the high cost of separating these isomers but no successful process has yet been developed [6]. A mixture of o- and p-cresol without m-cresol can easily be separated by distillation. Various zeolites have been studied for this reaction. These include H-Y [6, 7, 8, 9], H-ZSM-5 [6, 10, 11, 12], H-mordenite [5] and H-beta [13]. However, these zeolites have shown poor paraselectivity with p/o- cresol ratios of 0.7 for H-ZSM-5 [6], 0.3 for H-Y [5, 11] and no p-cresol was formed over H-mordenite [5]. This paper presents results in which H-MCM-22 and H-ZSM-5 were compared for the alkylation of phenol with methanol and in which silica-alumina was used as a benchmark for a non-porous acid catalyst. MCM-22 has a pore structure that consists of two independent channel systems [14]. The first, a three-dimensional channel system, consists of supercages defined by 12-membered rings having a diameter of 7.1 A and a length of 18.2/~. These supercages are interconnected through 10-membered ring openings with dimensions of 4.0 x 5.4 A. The second channel is a two-dimensional sinusoidal channel defined by 10-membered rings (4.0 x 5.9 /~). There is no direct access between these two channel systems. The external surface of MCM-22 has 12-membered ring pockets, which have a depth of 7.1 /~. ZSM-5 has a three-dimensional pore structure consisting of two interconnecting 10membered ring channels, one of which is sinusoidal (5.3 x 5.6/~) and the other straight (5.1

x 5.5 h) [15]. The focus of the study was on the selectivity of the various catalysts to the specific cresol isomers and to the C (ring) vs O (side chain) alkylation as a function of liquid phase batch and gas phase flow reactor systems. 2. EXPERIMENTAL Liquid phase experiments were carried out in a 600 ml stirred batch reactor at temperatures between 200 and 250~ The reactor was loaded with 200 g reactants (equimolar ratio of methanol and phenol) and 5 g catalyst. The reactions were run under autogenous pressure, e.g. approximately 23 bar at 200~ Gas phase experiments were carried out in an isothermal flow reactor at temperatures between 250 and 400~ The feed mixture of equimolar amounts of methanol and phenol were pumped via an evaporator and nitrogen was added as the diluent gas. The feed partial pressure was 0.2 bar, total pressure 1 bar, and a space velocity of approximately 14 h -1. The products were analysed using gas chromatography with a Chrompack CP Cresols capillary column. Toluene was used as an internal standard and a carbon balance of between 96 and 102% was obtained.

637 H-MCM-22 (Si/Al = 12, platelet morphology, average diameter of 0.5 ktm, thickness of 0.1 - 0 . 2 l.tm) was synthesised according to the method described by Raviskankar et al. [16]. Commercial samples of H-ZSM-5 (Si/A1 = 45, spherical morphology, average diameter 0.2 0.3 l.tm) and amorphous silica-alumina (SiOx-AlxO3, Si/Al = 8, < 106 microns) were obtained from Stid-Chemie and Akzo Nobel, respectively. 3. RESULTS AND DISCUSSION

In the gas phase experiments, the catalysts initially deactivated rapidly over the first two hours and then reached essentially a steady state with respect to phenol conversion. This initial deactivation period was accompanied by some change in product selectivity. Table 1 shows the phenol conversion, C : O-alkylation ratio and p/o-cresol ratio over H-MCM-22, HZSM-5 and silica-alumina, at 300~ In terms of phenol conversion, H-ZSM-5 was the most active catalyst studied. Although this zeolite had a low aluminium content it is unlikely that this difference in conversion was due to differences in strength of acid sites. The lower decline of conversion in the case of H-ZSM-5 was probably due to it being less susceptible to coking during the initial reaction period. All the catalysts showed similar product distributions with anisole and cresol isomers dominant. Small amounts (< 4 % steady state selectivity) of secondary products, viz. methylanisoles and xylenols, were present. There is an indication of enhanced selectivity to p-cresol over the zeolites compared to silica-alumina. H-MCM-22 and H-ZSM-5 produced similar p/o-cresol ratios of ca. 0.5 whereas for silica-alumina this was ca. 0.4. The p/o-cresol was essentially unchanged with time-on-stream and conversion, m-Cresol was also formed over all the catalysts at this temperature, but consisted of less than 5% of the cresol fraction. At 300~ the C : O-alkylation ratio initially declined but in the steady state levelled out at approximately 0.6 over all the catalysts studied. Thermodynamic equilibrium calculations at 300~ predict that the products should be mainly cresols. Moreover, at this temperature the distribution of cresol isomers was close to an o : p : m = 65 : 30 : 5 which is far removed from the thermodynamically predicted distribution of 32 : 8 : 60. These product distributions indicate that the reaction is kinetically and not thermodynamically controlled at the reaction conditions used. Table 1. Phenol methylation over H-ZSM-5, H-MCM-22 and silica-alumina (gas phase, 300~ 0.2 bar feed partial pressure, weight hourly space velocity of 14 h-l). Phenol conversion (%) C : O-alkylation ratio p/o-Cresol ratio Initial sample (15 minutes time-on-stream ) H-ZSM-5 19.2 0.98 0.55 H-MCM-22 9.3 0.71 0.56 SiO2 - A 1 2 0 3 5.3 0.55 0.38 Steady state (> 5 hours time-on-stream) H-ZSM-5 11.1 0.64 0.54 H-MCM-22 3.1 0.59 0.49 SiO2 - A 1 2 0 3 2.9 0.56 0.38

638 Table 2. Phenol methylation major products converted over H-MCM-22 (gas phase, 300~ 0.2 bar feed partial pressure, weight hourly space velocity of 14 h -1) Reactants Conversion Selectivity of products formed (%) (%) Anisole Cresols Methylanisoles Xylenols Phenol 19.2 37.8 2.1 40.9 Anisole 7 Anisole/phenol 4* 68.7 12.7 1.6 17.0 (1/1 molar ratio) 0.2 68.0 0.0 15.9 15.9 o-Cresol 4 0.7 88.2 0.0 5.7 5.4 p-Cresol 7 0.0 97.7 0.0 1.2 1.1 m-Cresol 4 * Conversion of anisole Table 3. Cresol distribution formed when converting the phenol methylation major products over H-MCM-22 (gas phase, 300~ 0.2 bar feed partial pressure, weight hourly space velocity of 14 h -1) Cresols distribution (%) Reactants o-Cresol p-Cresol m-Cresol Ansiole 69 11 20 Anisole/phenol (1/1 molar ratio) o-Cresol p-Cresol m-Cresol Thermodynamic equilibrium

53

28

19

9 26 32

15 74 8

85 91 60

In order to elucidate the reaction pathways a study was made of the reactions over HMCM-22 at 300~ in the gas phase, of each of the major products formed during phenol methylation, viz. ansiole, o-cresol, p-cresol and m-cresol. These results are shown in Tables 2 and 3. Anisole formed mainly methylanisoles and cresols. Cresols are mainly formed through monomolecular methyl shift. The formation of methylanisole clearly is due to a methyl transfer mechanism which initially forms phenol and methylanisole. Further methyl transfer then may occur from anisole to the phenol product to form more cresols. This is confirmed in the reaction where an equimolar amount of phenol was co-fed with the anisole. The conversion of anisole decreased as expected from its reduced partial pressure. Cresols were the major product. The decrease in the amount of o-cresol was almost quantitatively equivalent to the increase in p-cresol in the anisole and anisole/phenol reactions. This implies that in the absence of phenol monomolecular methyl transfer dominates, producing o-cresol. In the presence of reactant phenol, however, a bimolecular transfer from the anisole to the phenol para position is now possible resulting in much higher p-cresol selectivity (Table 3). Each cresol isomer was converted over H-MCM-22. In the case of o-cresol and p-cresol, mcresol was the major product as expected. Since in the methylation of phenol over all the catalysts studied, only a small amount of methylanisoles, xylenols (< 4 % selectivity) and m-cresol (< 2 % selectivity) were formed, it can be deduced that secondary reactions of anisole and the cresols occur only to a small

639 extent at 300~ At 200~ in the liquid phase the conversion of pure cresols, anisole and the anisole/phenol mixture over H-MCM-22 was insignificantly small [17]. This indicated that the anisole, o-cresol and p-cresol formed at those conditions were all primary products. It is important to note that at 200~ at conversions of less than 15 %, selectivities were always found to be independent of conversion over all the catalysts studied [17] thus allowing the comparison of the catalysts at different conversions. The effect of temperature on the product distribution from phenol methylation over HMCM-22 was investigated, in the gas phase, in the range 250 - 400~ These results are shown in Figure 2 which summarizes all the results in the liquid and gas phase in terms of C : O-alkylation ratio and p/o-cresol ratio, respectively. Decreasing the temperature decreased the C : O-alkylation ratio from 1.36 (400~ to 0.53 (250~ (Figure 2a) but caused the p/o-cresol ratio to increase from 0.41 (400~ to 0.54 (250~ (Figure 2b). Decreasing the temperature also decreases the formation of m-cresol from 16 to 4 mol % in the cresol fraction. This indicates that at higher temperatures either the primary formation of m-cresol is enhanced or alternatively that the isomerization of the cresols to the thermodynamically most stable isomer, viz. m-cresol, is enhanced. The reaction was also studied in the liquid phase at even lower temperatures, viz. 200 250~ At these conditions H-ZSM-5 was still more active than H-MCM-22 and silicaalumina in terms of phenol conversion (Table 4). Temperature had little effect on the p/oselectivity over H-ZSM-5 and silica-alumina. The most significant effects observed under these conditions were that H-ZSM-5 showed a 3 - 4 fold greater C : O-alkylation selectivity than the other two catalysts whereas H-MCM-22 had a similarly greater extent of p/o-cresol selectivity. The fact that H-MCM-22 had a similar C : O-alkylation selectivity to silica-alumina and that H-ZSM-5 had a similar p/o-selectivity to silica-alumina may be the result of the extent to which the reactions occur on the external surface of the zeolites as opposed to inside the pores. Consequently H-MCM-22 and H-ZSM-5 were modified by carrying out a selective Na/H exchange in order to remove the internal acid sites using a method described by Chester et al. [18]. These catalysts were tested at 200~ in the liquid phase. Over the internally poisoned H/Na-MCM-22 the p/o-cresol molar ratio was only 0.63 compared to 1.30 for the HMCM-22. In the case of H/Na-ZSM-5 the p/o-cresol ratio decreased to 0.26 compared to 0.38 for the H-ZSM-5 (Table 4). These results show clearly that for both zeolites, at these reaction conditions, phenol methylation occurs to a significant extent on the internal acid sites. Table 4. Comparison of the different catalyst for the lic,tuid phase batch alkylation (200~ 21 - 23 bar, modified weight hourly space velocity = 8 h -1) . Phenol conversion (%) C : O-alkylation ratio p/o-Cresol ratio H-ZSM-5 9.5 0.3 0.4 H-MCM-22 2.4 0.08 1.3 Sit2 - A 1 2 0 3 0.4 0.07 0.4 H/Na-ZSM-5 1.2 0.2 0.6 H/Na-MCM-22 2.0 0.06 0.3 * Note: The selectivities did not change with residence time or conversion respectivity [ 17]

640 1.5

o

O (a)

o

O

9 ~,=,i

~

1.0

6 o

0

0 0.5 T' V A

0 0.0 150

O0 V

0,0

9

200

250

300

350

400

450

Temperature (~ 1.5 9

9

o

O

(b)

.~,-4

1.0o

I

0.5-

9 v 0.0

9

200

150

O0 v 9

250

~ Q

O0

,

300

O0

9

350

,

400

450

Temperature (~ 9 O V V II

H-MCM-22 liquid phase (23 - 42 bar) H-MCM-22 gas phase (0.2 bar) H-ZSM-5 liquid phase (23 - 40 bar) H-ZSM-5 gas phase (0.2 bar) SiO2-A1203liquid phase (20 bar)

I"1

SiO2-Al203gas phase (0.2 bar)

9 ik

H/Na-MCM-22 liquid phase (21 bar) H/Na-ZSM-5 liquid phase (21 bar) Thermodynamic equilibrium

Figure 2. The gas and liquid phase (a) C 9O-alkylation ratio and (b) #o-cresol ratio over HMCM-22, H-ZSM-5 and amorphous silica-alumina.

641 Table 5. The relationship between spaciousness index [ 19] and C : O-alkylation over different catalysts (liquid phase, 200~ 21 - 23 bar, modified weight hourly space velocity = 8 h-l). Spaciousness index C : O-alkylation ratio p/o-Cresol ratio H-ZSM-5 1 0.3 0.4 H-mordenite [ 17] 6 0.2 0.5 H-MCM-22 8 0.08 1.3 H-beta [ 17] 16 0.08 0.5 H-USY [ 17] 22 0.08 0.6 SiO2 - A1203 22 0.07 0.4 As mentioned above, in the liquid phase at 200~ H-ZSM-5 showed a 3 - 4 fold higher selectivity to cresols than H-MCM-22 or silica-alumina. It has been reported [19] that HZSM-5 has a lower spaciousness index (1) than H-MCM-22 (8) and silica-alumina (22), this index being a measure of internal free space inside the pore system of a zeolite. Table 5 shows the spaciousness indices reported for various catalysts and the C : O-alkylation ratios found in this study. In the more spacious catalysts the C : O-alkylation ratio was 0.08 and the p/ocresol ratio was ca. 0.5. In the medium restricted zeolites (H-MCM-22) there is no change in the C : O-alkylation ratio but there is more shape-selectivity to the p-cresol. In the highly restricted zeolite (H-ZSM-5) the selectivity to anisole has decreased and the selectivity to ocresol has increased when comparing to the medium restricted zeolites. It can be speculated that the anisole converts on site into the more stable o-cresol as also found by Parton et al. [20]. This would explain the differences in the C : O-alkylation ratios for the two zeolites and the low p/o-cresol ratio in H-ZSM-5. In the gas phase reactions, which were carried out at temperatures between 250 - 400~ a comparative experiment at 300~ showed no difference in the C : O-alkylation ratio between the zeolites and silica-alumina. This could be due to the fact that the reaction occurs very rapidly at these temperatures and predominantly on the outer shell of the zeolite. The H-ZSM5 sample used has about 2.5 times greater external surface per gram than the sample of HMCM-22. This compares approximately to the activity of these catalysts at 300~ (Table 1). This is also supported by the small difference in the p/o-cresol ratio over H-MCM-22 and HZSM-5 compared to silica-alumina at these conditions. Lastly it should be noted that at 250~ there is a large difference between the liquid phase and gas phase results with respect to both the C : O-alkylation ratio and the p/o-cresol ratio (Figure 2). The difference in the reaction pressure (ca. 42 bar vs 0.2 bar) may give rise to this difference. However, further experiments are being carried out to investigate whether there may be a hysteresis phenomenon in this temperature range due to the existence of a metastable intermediate complex. 4. CONCLUSIONS In the liquid phase at temperatures in the range 200 - 250~ H-MCM-22 showed much greater p-cresol content in the cresol fraction from the methylation of phenol than H-ZSM-5 and silica alumina but H-ZSM-5 had a much greater selectivity to ring alkylation. The low p/o-cresol ratio (0.4) in H-ZSM-5 could be due to anisole converting in its relatively more restricted pore system to the more stable o-cresol. The reaction was shown to be occurring on

642 both the internal and external acid sites and also that shape-selective formation of p-cresol was occurring in the zeolites pores. Gas phase experiments in a flow reactor at 300~ showed almost no difference in the C : O-alkylation ratio between the catalysts studied and the zeolites demonstrated only slightly higher selectivity to p-cresol than the silica-alumina. The similarity between the performance of the three catalysts at this higher temperature is probably due to the higher reaction rates, effectively resulting in the reaction occurring on the outer shell of the catalyst. The large difference between the gas phase and liquid phase C : O-alkylation ratio and the p/o-cresol ratio may be due to the difference in reaction pressure. In the liquid phase experiments, the product spectrum consists of anisole, o-cresol and p-cresol implying that the product mixture formed can easily be separated by distillation to yield high purity p-cresol. REFERENCES 1. S. Subramanian, A. Mitra, C.V.V. Satyanarayana and D.K. Chakrabarty, Appl. Catal., 159 (1997) 229 2. J.S. Beck and W.O. Haag, Handbook of Heterogeneous Catalysis, Vol 5, G. Ertl, H. Kn6zinger and J. Weitkamp (eds.), Wiley-VCH: Weinheim (1997) 2131 3. P. Sykes, A Guidebook to Mechanism in Organic Chemistry, 6th ed., Longman Scientific and Technical: Essex (1986) 153 4. S.C. Lee, S.W. Lee, K.S. Kim, T.J. Lee, D.H. Kim and J.C. Kim, Catalysis Today, 44 (1998) 253 5. M. Marczewski, G. Perot and M. Guisnet, Stud. Surf. Sci. Catal., 41 (1988) 273 6. R.F. Patton, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs, Stud. Surf. Sci. Catal., 46 (1989) 163 7. S. Namba, T. Yashima, Y. Itaba and N. Hara, Stud. Surf. Sci. Catal., 5 (1980) 105 8. L. Garcia, G. Giannetto, M.R. Goldwasser, M. Guisnet and P. Magnoux, Catalysis Lett., 37 (1996) 121 9. M. Marczewski, G. Perot and M. Guisnet, React. Kinet. Catal. Lett., 57 (1996) 21 10. M. Renaud, P.D. Chantal and S. Kaliaguine, Can. J. Chem. Eng., 64 (1986) 787 11. S. Balsama, P. Beltrame, P.L. Beltrame, P. Carniti, L. Forni and G. Zuretti, Appl. Catal., 13 (1984) 161 12. R. Pierantozzi and A.F. Nordquist, Appl. Catal., 21 (1986) 263 13. J. Xu, A-Z. Yan and Q-H. Xu, React. Kinet. Catal. Lett., 62 (1997) 71 14. S.L. Lawton, M.E. Leonowicz, R.D. Partridge, P. Chu and M.K. Rubin, Microporous Mater., 23 (1998) 109 15. Ch. Baerlocher, W.M. Meier and D.H. Olson, Atlas of Zeolite Framework Types, 5th ed., Elsevier: Amsterdam (2001) 16. R. Raviskankar, D. Bhattacharya, N.E. Jacob and S. Sivasanker, Microporous Mater., 4 (1995) 83 17. G. Moon, K.P. M/Jller, W. BOhringer and C.T. O'Connor, Stud. Surf. Sci. Catal., 135 (2001) 310 18. A.W. Chester, A.S. Fung, C.T. Kresge and W.J. Roth, US Patent 5,779,882 (1998) 19. J. Weitkamp, S. Ernst and L. Puppe, Catalysis and Zeolites. Fundamentals and Applications, J. Weitkamp and L. Puppe (eds.), Springer: Berlin (1999) 327 20. R.F. Parton, J.M. Jacobs, H. Van Ooteghem and P.A. Jacobs, Stud. Surf. Sci. Catal., 46 (1989) 211

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

643

Relative Stability of Alkoxides and Carbocations in Z e o l i t e s . Q M / M M Embedding and QM Calculations Applying Periodic Boundary Conditions Louis A. Clark, Marek Sierka t and Joachim Sauer Humboldt-Universit/it zu Berlin, Institut ffir Chemie, Unter den Linden 6, D- 10099 Berlin, Germany 1. I N T R O D U C T I O N The role of carbocations in hydrocarbon conversions over acidic zeolite catalysts is much debated. These carbocations generally take two forms. Carbenium ions have three-coordinated carbon centers and are typically formed by protolytic dehydrogenation of saturated hydrocarbons or by protonation of an unsaturated hydrocarbon. In comparison, carbonium ions are also cations, but have a five-coordinated carbon center. They are often formed by adding a proton to a saturated hydrocarbon. It was first assumed that carbenium and carbonium ions were as common in zeolites as they are in homogeneous reactions [ 1]. Later, magic angle spinning (MAS) NMR studies failed to find these intermediate species in zeolites [2, 3]. Around the same time, quantum chemistry calculations employing small cluster models also came to the conclusion that these ionic species were only present as extremely short-lived transition states [4, 5]. More recently, evidence for persistent carbocation intermediates has begun to appear. Experimental studies have succeeded in identifying bulky carbocation species in zeolites [6-8] and theoretical predictions have been made concerning when they might be stable in zeolites based on their proton affinities [9]. Our recent theoretical studies, which we outline here, have succeeded in finding stable carbocation intermediates. First, the briefest description of the methodology and contrast to other methods is given. We then illustrate the performance of the method using a case-study of isobutene conversion. Finally, evidence for carbocation intermediates during the m-xylene disproportionation reaction is presented. 2. C O M P U T A T I O N A L

METHODOLOGY

In our computational studies, we have begun to incorporate what we believe is a more realistic model of the active site environment into our calculations. This is done using a simple, but effective technique often termed mechanical embedding that enables us to handle much larger systems than is possible with pure quantum mechanical (QM) methods. In this technique, the energies, gradients and second derivatives from QM and molecular mechanical (MM) calculations are combined to yield a consistent composite description of the potential energy tPresent address: Institut ~r Physikalische Chemie, Universit~tKarlsruhe (TH), Kaiserstr. 12, D-76128 Karlsruhe, Germany

644 surface [ 10, 11 ]. (1)

E(S)QM-Pot = E(S)Pot -E(I)Pot +E(I)QM

E(S)QM-Potis the composite QM/MM energy of the system and E(S)Pot is the MM energy of the whole system. The higher accuracy description of the active site from the QM method is incorporated by subtracting out the MM results (E (I)Pot) in a small region on and around the active site and replacing them by QM results in the same region (E (I) QM). QM/MM embedding techniques in varying degrees of sophistication are gaining popularity [ 12-14] The advantage of this embedding method over full periodic QM calculations is that it is considerably faster. If the potential energy function is a good approximation to the QM method used, the QM-Pot energy converges quicklytowards the full periodic QM result when increasing the size of the QM cluster. Unfortunately, current DFT fails to give reasonable interaction energies for van der Waals systems [ 15, 16]. It seems that different functionals behave differently, PW91 as used in most plane wave codes for solid state applications such as CPMD seems to perform better than B3LYP or BLYP [17]. For studies in zeolites, this phenomenon manifests itself most strikingly by giving unreasonable adsorption energies, even in calculations where the description of the active site itself is probably reasonable [ 18, 19]. The incorporation of the MM calculations, which are capable of describing adsorption correctly [20-24], into the QM-Pot calculations allows for more realistic description of the system. The QM part (DFT) is limited to a small cluster model of the active site of the zeolite. Figure 1 gives an example of the QM/MM partitioning. The QM part consists of 3 tetrahedra only, but provides a good description of the bond breaking- bond making portion.

Here

'.

,

A

~

B

Figure 1: QM/MM system partitioning for the isobutene in FAU calculations. A) MM portion B) QM portion. The partitioning for m-xylene in FAU is identical, but a larger fraction of the QM portion is hydrocarbon. The MM interatomic potential functions describe the van der Waals interactions of the hydrocarbon molecule with the wall of the zeolite cavity and the relaxation of the framework structure. We combine the cvff force field for hydrocarbons [25] with the proven ion-pair shellmodel potential for acidic zeolites [26, 27]. Charges on the hydrocarbons are critical for the success of the simulations. Among the charge options tested is a combination of bond increments as used by existing force fields and potential derived charges for the extra-charge on carbocations. For isobutene adsorption two different models of distributing the extra charge over the hydrocarbon part of the alkoxide are used which give very similar results. Assigning

645 the full extra charge to the C atom directly bound to the zeolite framework is not recommended. Final results for m-xylene adsorption and conversion employ charges fit to electrostatics from the cluster model. The QM portion of the calculations were done with TURBOMOLE [28, 29] using the B3LYP functional [30, 31], TZP basis sets on the oxygen atoms and DZP basis sets on all other atoms. For comparison periodic DFT studies are made on m-xylene in H-FAU (cell size: 17.43A x 17.50A x 17.49A) and its reaction products using plane wave basis sets (cut-off 70 Rydberg), norm-conserving Trouiller-Martins pseudopotentials and the PBE functional [32]. The CPMD code [33] was employed. 3. R E S U L T S

3.1. Isobutene and m-xylene adsorption When unsaturated hydrocarbons such as isobutene and m-xylene interact with zeolitic hydroxyl groups an adsorption complex is formed. It is expected that the energy of the initial adsorption step suffers most from the problem of insufficient description of van der Waals interactions by DFT. We will show that our QM-Pot approach which limits the DFT description to the site of specific interactions while using forcefields for the van der Waals interactions is well suited to tackle the adsorption step and the bond breaking- bond forming steps. Our embedded cluster QM-Pot approach yields for isobutene an adsorption energy of 55 kJ/mol - a reasonable value for a C4-hydrocarbon. The DFT contribution to this value obtained for a 3T cluster model (cf. Figure 1) is 22 kJ/mol while the potential function contribution is 33 kJ/mol. This is in excellent agreement with results obtained by Sinclair et al. [34]. The embedded cluster is 55 kJ/mol and the constrained cluster result (which is comparable to our QM contribution) is 22 kJ/mol. Note that the QM method used by Sinclair et al. is MP2 for a slightly different cluster and the forcefield used is also somewhat different. The constrained cluster result is also DFT and the same functional is used. The adsorption energy for m-xylene calculated by comparing the bare acidic zeolite and isolated m-xylene species to the adsorbed state using the embedded cluster approach is approximately 62 kJ/mol, the DFT part of this result is 12 kJ/mol only. This is consistent with results of about 20 kJ/mol obtained for adsorption of toluene on small cluster models [35]. The QM contribution of the embedded cluster result is expected to be smaller than the result for a free cluster model because the QM/MM geometry optimizations may move the structure away from the pure QM minimum to a region where the MM energy is also low. Applying DFT to the periodic zeolite structure using periodic boundary conditions (CPMD) we find an adsorption energy of approximately 28 kJ/mol. Experimental values are between 60 to 85 kJ/mol on NaY and KY zeolites [36-38]. We see again that the failure of DFT to give reasonable long-range dispersion interactions results in unreasonable predictions for the adsorption energy.

3.2. Formation of t-butyl alkoxide The study of isobutene reaction with the acidic site of the faujasite zeolites illustrates our methodology. The complex formed between the hydrocarbon and the acidic hydroxyl group (~-complex), in principle, can rearrange into an alkoxide (Figure 2) or into a carbocation attached to the anionic zeolite surface. Quantum chemical calculations can provide information about the existence of alkoxide and carbocation intermediates and their stabilities relative to

646 the rt-complex. The reliability of quantum chemical calculations depends on both the choice of a reliable method and a realistic zeolite model. Small or medium size cluster models suffer from missing steric repulsion. For example, in chabazite the stability of alkoxides obtained by reaction with ethene, propene and iso-butene relative to the r~-complex is found to decrease in this sequence due to steric interactions with the zeolite surface around the active site [34]. This behavior cannot be correctly described by small cluster models. Our embedded cluster method shows that the adsorption energy for the t-butoxide structure (~-complex) is 95+5 kJ/mol, i.e. the 6-complex is by 40+5 kJ/mol more stable than the rt-complex. Boronat et al. report the alkoxide more stable by 31 kJ/mol [39]. The embedded cluster approach used by Sinclair et al. predicts the ~-complex more stable by 9 kJ/mol (MP2), while the constrained cluster result (DFT) is 18 kJ/mol. 9~ , , , ~

~

Figure 2: Isobutene in the faujasite zeolite structure. Shown here are the r~-complex (left) and the o-complex (alkoxide) structures (right).

3.3. Xylene disproportionation results In an effort to exemplify, understand and provide strategies for control of the phenomenon known as Transition State Shape Selectivity, we have studied a classic zeolite-based reaction computationally. The methyl transfer (disproportionation) reaction between two m-xylene molecules has long been assumed to be one where environmental confinement shifts reaction selectivity by directly influencing the formation of reaction transition states. As part of this study, we have located carbenium and carbonium ions that contribute to the critical shape-selectivity step in the reaction. The classical mechanism, thought to dominate in large-pore zeolites proceeds through a diphenylmethane carbonium species [35, 40--42]. After adsorption of the first m-xylene molecule (AE = -62 kJ/mol, cf. section 3.1 .) dehydrogenation at one of the methyl groups yields a surface alkoxide on the zeolite pore wall, ZOH + m-xylene (ads) --+ ZO-CH2-C6Hn-CH3 + H2

AE = + 58 kJ/mol

(2)

There is also a less stable carbenium ion intermediate shown to be a local minimum on the potential energy surface, ZO-CH2-C6H4-CH3 ~ ZO- + CH3-C6Hn-CH2+

AE = + 53 kJ/mol

(3)

A second m-xylene molecule entering the same cavity can interact with the alkoxide (figure 3A) or with the carbenium ion (figure 3B). We also consider the carbonium ion (figure 3C) obtained from the former, ZO-CH2-C6Hn-CH3 + m-xylene (ads) --+ ZO- + CH3-C6Hn-CH2 -[C6Hn(CH3)2] +

(4a,b)

647 ZO- + CH3-C6H4-CH + + m-xylene (ads) --+ ZO- + CH3-C6H4-CH2 -[C6H4(CH3)2] + which is one of the possible intermediates in the xylene disproportionation reaction. .

'

,

'

.

B

~

C

,.-

Figure 3: View of the alkoxide (A), carbenium (B) and carbonium (C) structures. This is comparable to typical energy barriers in related reactions and means that the ionic species are stable. We find that both the carbenium ion and the carbonium ion species are stationary points on the potential energy surface. This is in contrast to previous cluster-based results [35, 42] and shows that the inclusion of the pore environment stabilizes these ionic intermediates. Full periodic calculations using the PBE functional in the CPMD code also indicate that the carbenium and carbonium ions are stationary points. Preliminary calculations indicate that there is an energy barrier of approximately 45 kJ/mol between the carbenium and the alkoxide species. Note that the barrier could be different for other zeolites and if more acid sites that our in 1 A1/supercage model were included. Since this barrier is comparable to other barriers in the reaction, it seems likely that these species play an large role in the overall reaction mechanism. 5. C O N C L U S I O N S Our recent work applying the QM-Pot mechanical embedding methodology to acid catalyzed zeolite reactions has produced results that elucidate basic mechanisms. A note is also made of the synergy between the QM and MM methods. Taken together, they provide a reasonable description of bond breaking and forming from the QM as well as long-range dispersion interactions from the faster MM methodology. We also find that some aromatic carbenium and carbonium ions are stationary points on the potential energy surface and may be stable enough to observe experimentally. 6. A C K N O W L E D G E M E N T S This work has been supported by the 'Fonds der Chemischen Industrie' and the 'Deutsche Forschungsgemeinschaft'. Computer time on the T3E at Zentrum fuer Informationstechnik Berlin is acknowledged. LAC acknowledges the support of the Alexander von Humboldt Foundation.

648 REFERENCES 1. G. A. Olah, A. Molnar, Hydrocarbon Chemistry, Wiley, New York, N.Y., 1995. 2. T. Xu, J. H. Zhang, E. J. Munson, J. F. Haw, Chem. Commun. 23 (1994) 2733-2735. 3. J. F. Haw, J. B. Nicholas, T. Xu, L. W. Beck, D. B. Ferguson, Acc. Chem. Res. 29 (1996) 259-267. 4. V. B. Kazansky, M. V. Frash, R. A. van Santen, Catal. Letters 28 (1994) 211-222. 5. V. B. Kazansky, M. V. Frash, R. A. van Santen, Appl. Catalysis A 146 (1996) 225-247. 6. L. Femandez, V. Marti, H. Garcia, Phys. Chem. Chem. Phys. 1 (1999) 3689-3695. 7. W. Adam, I. Casades, V. Fornes, H. Garcia, O. Weichold, J. Org. Chem. 65 (2000) 39473951. , ,i , 8. W. G. Song, J. B. Nicholas, J. F. Haw, L Phys: Chem. B 105 (2001) 4317-4323. 9. J. B. Nicholas, J. F. Haw, J. Am. Chem. Soc. 120(1998) 11804-11805. 10. U. Eichler, C. M. Kolmel, J. Sauer, J. Comput. Chem. 18 (1997) 463-477. 11. M. Sierka, J. Sauer, J. Chem. Phys. 112 (2000) 6983-6996. 12. J. Gao, M. A. Thompson (Eds.), Combined Quantum Mechanical and Molecular Mechanical Methods, Vol. 712 of ACS symposium series, ACS, Washington, DC, 1998. 13. T. Z. Mordasini, W. Thiel, Chimia 52 (1998) 288-291. 14. J. Sauer, M. Sierka, J. Comput. Chem. 21 (2000) 1470-1493. 15. Y. K. Zhang, W. Pan, W. T. Yang, J. Chem. Phys. 107 (1997) 7921-7925. 16. T. A. Wesolowski, O. Parisel, Y. Ellinger, J. Weber, J. Phys. Chem. A 101 (1997) 78187825. 17. S. Tsuzuki, H. P. Luthi, J. Chem. Phys. 114 (2001) 3949-3957. 18. A. M. Vos, X. Rozanska, R. A. Schoonheydt, R. A. V. Santen, F. Hutschka, J. Hafner, J. Am. Chem. Soc. 123 (2001) 2799-2809. 19. T. Demuth, L. Benco, J. Hafner, H. Toulhoat, F. Hutschka, J. Chem. Phys. 114 (2001) 3703-3712. 20. R. Q. Snurr, A. T. Bell, D. N. Theod0rou, J. Phys. Chem. 97 (1993) 13742-13752. 21. T. J. H. Vlugt, R. Krishna, B. Smit, J. Phys. Chem. B 103 (1999) 1102-1118. 22. A. Gupta, L. A. Clark, R. Q. Snurr, Langmuir 16 (2000) 3910-3919. 23. M. D. Macedonia, D. D. Moore, E. J. Maginn, Langrnuir 16 (2000) 3823-3834. 24. A. H. Fuchs, A. K. Cheetham, J. Phys. Chem. B 105 (2001) 7375-7383. 25. A. T. Hagler, S. Lifson, P. Dauber, J. Am. Chem. Soc. 101 (1979) 5122, (as implemented in the Discover software of Accelrys Inc.). 26. K. P. Schr6der, J. Sauer, J. Phys. Chem. 100 (1996) 11043-11049. 27. M. Sierka, J. Sauer, Faraday Discussions 106 (1997) 41-62. 28. R. Ahlrichs, M. B~, M. H~iser, H. Horn, C. M. K61mel, Chem. Phys. Lett. 162 (1989) 165. 29. O. Treutler, R. Ahlrichs, J. Chem. Phys. 102 (1995) 346-354. 30. A. D. Becke, J. Chem. Phys. 98 (1993) 5648-5652. 31. C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 37 (1988) 785-789. 32. J. P. Perdew, K. Burke, M. Emzerhof, Phys. Rev. Lett. 77 (1996) 3865-3868. 33. J. Hutter, A. Alavi, T. Deutsch, M. Bemasconi, S. Goedecker, D. Marx, M. Tuckerman, M. Parrinello, CPMD 3.4.1, MPI ftir Festk6rperforschung and IBM Zurich Research Laboratory (1995-1999). 34. P. E. Sinclair, A. D. Vries, P. Sherwood, C. R. A. Catlow, R. A. V. Santen, J. Chem. Soc. Faraday Trans. 94 (1998) 3401-3408. 35. X. Rozanska, X. Saintigny, R. A. V. Santen, F. Hutschka, J. Catal. 202 (2001) 141-155. 36. E. Santacesaria, D. Gelosa, P. Danise, S. Carrh, Ind. Eng. Chem. Process. Des. Dev. 24 (1984) 78-83.

649 37. 38. 39. 40. 41.

D. M. Ruthven, M. Goddard, Zeolites 6 (1986) 275-282. J. Bellat, M. Simonot-Grange, Zeolites 15 (1995) 219-227. M. Boronat, P. Viruela, A. Corma, Phys. Chem. Chem. Phys. 3 (2001) 3235-3239. M. A. Lanewala, A. P. Bolton, J. Org. Chem. 34 (1969) 3107-3112. M. Guisnet, N. S. Gnep, S. Morin, Microporous and Mesoporous Mater. 35-36 (2000) 4759. 42. S. R. Blaszkowski, R. A. van Santen, in: K. Morokuma, D. G. Truhlar (Eds.), Transition State Modeling for Catalysis, ACS Symp. Series 721, ACS, Washington DC, 1999, pp. 307-320.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

651

H-Beta zeolite for acylation processes: optimization of the catalyst properties and reaction conditions P. Botella, A. Corma, F. Rey and S. Valencia Instituto de Tecnologfa Qufrnica, Av. de los Naranjos s/n, 46022 Valencia, Spain Beta zeolite has been optimized for acylation processes. In the acylation of toluene and mxylene with acetic anhydride Beta samples of low Si/A1 framework ratio and nanocrystals show improved activity and slower deactivation. In the acylation of 2-methoxynaphthalene with acetic anhydride the selective dealumination of the external surface of a nanocrystalline Beta enhances the shape selectivity of the zeolite, increasing the selectivity to the less sterically hindered 2-acetyl-6-methoxinaphthalene. Finally, the control of the process conditions is vital in the case of the acylation with carboxylic acids. Water from the liquid phase should be removed to avoid side reactions that decrease the quality of the final product. 1. INTRODUCTION The Friedel-Crafts acylation and the related Fries rearrangement are widely used in the industry for the production of arylketones as intermediates in the synthesis of fine chemicals and pharmaceuticals (1). It is known that the main drawback of these processes is the use of stoichiometric amounts of Lewis acids (i.e. A1C13, BF3, etc.) as catalysts (2), which are destroyed at the end of the reaction producing undesirable wastes. Heterogeneous catalysis with solid acids have recently introduced an alternative to leave the classical homogeneous process using less hazardous acylating agents, like carboxylic acids and their anhydrides (3). Many different solid acids have been tested in the last decade for this purpose (4-5), but zeolites have found application for the industrial production of aromatic ketones. H-Beta zeolite is one of the most widely used zeolites for acylation of arylethers (i.e.: anisole, 2-methoxynaphthalene, etc. (6-7) and substituted aromatic rings (i.e., phenol, toluene, etc.) (8-9). Moreover, recently, RHODIA has reported the first industrial application of this catalyst for the anisole acylation (10). A drawback of these catalysts is their deactivation due to the strong adsorption of the arylketone (11-12). It should be possible to improve the performance of H-Beta zeolite as an acylation catalyst through a better catalyst design and optimization of the reaction conditions. Accordingly, we have firstly studied the catalytic behavior of several Beta samples with different Si/A1 molar ratio, crystallite size and external surface acidity in the acylation of toluene (TOL), m-xylene (XYL) and 2-methoxynaphthalene (2-MN) with acetic anhydride (AA). Secondly, a particular case of acylation with carboxylic acids with application in the fragrances industry, the acylation of anisole (ANI) with propionic acid (PA) to produce the precursor of anetole, has also been studied. It will be presented that the reaction conditions and especially the presence of water strongly influences the performance of the catalyst and the quality of the f'mal product.

652

2. EXPERIMENTAL Several Beta zeolites in protonic form have been tested in the present work. A commercial Beta was provided by P.Q. Industries (CP811) and the rest were prepared in our laboratory. Samples N7.5 and N16 are nanocrystalline zeolites synthesized in an alkali-free basic medium (13) while sample L8 has been prepared by a new synthesis procedure (14). All samples were calcined at 580 ~ before using. A surface dealuminated nanocrystalline Beta, NH54, was prepared by treating sample N16 with a 1 M HC1 solution (90 ~ 5 h) in order to remove both framework and extra-framework Al from the external surface. For this purpose the zeolite was acid-treated before removing the template and then it was calcined at 580 ~ The A1 content in the samples was determined by atomic absorption spectrophotometry (Varian spectrAA-10 Plus). Crystallinity was measured by powder X-ray diffraction, using a Phillips PW1710 diffractometer with CuKa radiation and compared with a standard sample. Acidity was measured by the pyridine adsorption-desorption method, while surface area was calculated from the N2 adsorption isotherms (77 K) in a Micromeritics ASAP 2000 instrument. The crystal size was determined from the SEM images obtained in a JEOL 6300 scanning electron microscope. The most relevant physicochemical properties of these zeolites are summarized in Table 1. The acylation of the different substrates with AA was carried out in batch conditions at autogeneous pressure. Acylation of TOL was carried out in a stainless steel stirred 150-ml autoclave (Autoclave Engineerings) under N2 atmosphere, while the acylation of XYL and 2MN were carried out in a 25-ml three-neck round-bottom flask, connected to a reflux cooler system, under argon and with magnetic stirring. All reagents were supplied by Aldrich. In the TOL acylation, 1.00 g of catalyst activated in situ reacted at 150 ~ with a mixture of 400 mmol of TOL and 20 mmol of AA, and the reaction was performed for 4 h. Acylation of XYL was carried out at 110 ~ with 1.00 g of Beta activated in situ and a mixture of 100 mmol of XYL and 10 mmol of AA, and the reaction was performed for 3 h. For the acylation of 2-MN 0.20 g of catalyst were activated in situ. Then, a mixture of 4.0 mmol of 2-MN, 2.0 mmol of AA and 3.0 ml of chlorobenzene was introduced and the reaction was carried out at 132 ~ for 24 h. The propionylation of ANI with PA was carried out at 154 ~ and autogeneous pressure in a 25-ml three-neck round-bottom flask, connected to a reflux cooler system, under argon atmosphere and with magnetic stirring. 0.50 g of catalyst were activated in situ. Then a mixture of 100 mmol of ANI and 20 mmol of PA was added, and the reaction was carried out for 48 h. When necessary water was removed from the reactant mixture by a Dean-Stark or by a molecular sieve column (3/~) installed "on line". In all cases, the reaction products were analyzed by GC in a Varian 3350 Series instrument equipped with a HP-5 column and a FID detector, using nitrobenzene as internal standard. Products were also identified by mass spectrometry in a Varian Saturn II GC-MS model working with a Varian Star 3400 GC and using reference samples.

3. RESULTS AND DISCUSSION 3.1. Acylation of toluene and m-xylene with acetic anhydride Friedel-Crafis acylation of TOL and XYL with AA is a very selective process for the production of the para isomers 4-methoxyacetophenone (MAP) and 2,4-dimethylacetophe none (DMAP), respectively (Scheme 1). These reactions involve the production of one mole-

653 Table 1 Physicochemical characteristics of H-Beta zeolites. Acidity (l.tmolpy)(3) Br6nsted Sample Si/A10) AreaBET ExternalArea Crystal 250~ 350~ 400~ (m2 g-l) (m 2 g-l) (~tm)<2)

Lewis 250~

350~

400~

CP811

12.5

570

222

0.2

45

27

16

48

40

40

N7.5

7.5

722

435

0.02

34

15

6

63

51

49

L8

7.7

511

250

0.1

34

10

9

25

17

14

N16

16

641

249

0.05

66

44

30

41

39

38

NH54

54

641

249

0.05

36

21

13

44

39

37

As-mademolarratio. Averagesize determinedfromthe SEM images. o)Determinedfromthe infraredspectraof adsorbedpyridineafterevacuationat 250, 350 and 400~

(1) (2)

cule of acetic acid (HAc) for every arene molecule acylated. Isomers acylated in less favored positions of the aromatic ring appear only at very high conversion and in very small amount. In this first part of the work, we have tested a series of Beta samples with different crystal size and Si/A1 framework ratio (see Table 1). The resuks (Figure 1) show that while all samples are active, those with smaller crystal size and higher framework-A1 content (L8 and N7.5) present the highest yields in acetophenones. In fact, the catalytic activity is strongly influenced by the Br~Snsted acidity of the zeolite (15) and in this way, low framework Si/A1 ratios lead to better yields in MAP and DMAP. Moreover, as the intrapore diffusion is limited by the adsorption of reaction products (11-12), the accessibility of the acid sites to the reactant molecules is improved for the smallest crysta-llites, enhancing the catalytic activity and decreasing the rate of catalyst deactivation (9).

3.2. Acylation of 2-methoxynaphthalene with acetic anhydride The acylation of 2-MN with AA produces mainly 1-acetyl-2-methoxynaphthalene (1AMN) and 2-acetyl-6-methoxynaphthalene (2- AMN) (Figure 2). The last isomer is the desired product as an intermediate in the multi-step synthesis of the anti-inflammatory SNaproxen. As demonstrated recently, the formation of 2-AMN is favored inside the zeolite channels, where the diffusion of the 1-AMN is hindered (16-17). We have performed the acylation of 2-MN with AA over the commercial CP811 Beta and a nanocrystalline sample of similar Si/A1 framework ratio (N16) and the results are shown in Figure 3. As expected, the nano-sample is clearly more active since the beginning of the process, due to its high density of active sites, which are easily accessible to the reactant molecules. The formation of the bulky isomer 1-AMN is favored in the external surface of zeolite crystallites, and the initial rate of formation of 1-AMN is clearly higher in N16 (RI_ AMN= 31.25 mmol g-i h-1 at 18% conversion) than in CP811 (RI_AMN= 21.52 mmo1 g-1 h-1 at 11% conversion). However, at longer reaction times (24 h) the selectivity to 2-AMN can be improved by two reasons: protodeacylation of the 1-AMN to give 2-MN (which involves a decrease in conversion), and isomerization of 1-AMN to give the thermodinamically more stable 2-AMN (18). These processes are not significant for Beta CPS11 (Fig. 3a) but they take

654

Scheme 1

ci~ [ ~

100

R

R= H (toluene) R= CH3(m-xylene)

~1Yt,t~ (wt%) !-1 YDMAP(wt%)

80

COCH~ R

H-Beta + (CH3CO)20 ~

z"

60

40 20 c~

0

/

CP811

N7.5

I

L8

Figure 1. Yields of acylated products in the acylation of TOL and XYL with AA over Beta zeolites. Experimental conditions: see text. place in large extension in the case of Beta N16 (Fig. 3b). At this point, the stronger acidity showed by N16 (see pyridine data in Table 1) favors the deacylation of the sterically hindered ortho-ketone 1-AMN, leading to a sharp decrease in the conversion at long reaction times (from 68% at 1 h to 38% at 24 h). Moreover, if one takes into account that the isomerization

o/CH 3

H+--Zeol-

9

00

1-AMN

/CH3

I ~'~Z eol9

CH: 3

H+--Zeol-

(2)

2-MN

O.1"CH3 3C o

2-AMN

Figure 2. Reaction scheme of the acylation of 2-MN with AA over zeolites and possible secondary reactions over the acylation products. **, most activated position for the electhophillic substitution of 2-MN;., activated position; +, low activated position. (1) protodeacylation of 1-AMN; (2) Transacylation of 1-AMN to 2-AMN. of 1-AMN into 2-AMN is a bimolecular process involving one molecule of 2-MN and one molecule of 1-AMN (18), it is clear that the intermediate generated in this process should be too bulky to accommodate inside the zeolite channels. Therefore, the nanocrystalline sample offers fewer limitations to the molecular diffusion due to its larger outer surface, leading to an

655 .~ 80

F--

~60 -s 40 .o m L_

tO

92 0 0

I

0

240

I

480

!

I

720 960 TOS (min)

!

1200

I

I.

1440 0

240

480

720 960 TOS (min)

1200

1440

Figure 3. Acylation of 2-MN with acetic anhydride over Beta CP811 (a) and N16 (b) (see Table 1). t~, 2-MN conversion; ~,, Selectivity to 1-AMN; v, Selectivity to 2-AMN. Experimental conditions: see text. important increase of the selectivity to 2-AMN (Fig. 3b). With respect to the direct acylation of 2-MN, it has been said that the shape-selective acylation at the para-position of the 2-MN ring occurs in the interior of the crystals, while the formation of the 1-AMN takes place preferentially on the external surface (16-17). Thus, one way to improve the selectivity to 2-AMN could be the removal of the external active sites of these zeolite crystals, i.e., by a selective dealumination of the external surface. For this purpose, the N16 sample was treated with an acid solution (HC1 1M, 90 ~ 5 h) before calcination. The presence of the organic material within the zeolite channels should make more difficult the removal of the internal framework-A1 by the acid solution, while the external Br~3nsted acid sites could be more easily removed. After calcination, the acid-treated Beta A (NH54) showed a significant loss of A1 8O (Si/Al=54 by Si/AI=16 in the original sample) and a strong decrease in the Br~Snsted acidity (as measured by the pyridine method, see Table 1), indicating ~ 40 that A1 deep into the pores was also '~ 20 removed. Beta NH54 was tested in the acylation of 2-MN with AA and the 8 0 results are comparatively shown in Figure CP811 N16 NH54 4. The dealumination process involves obviously a decrease of the catalytic Figure 4. Acylation of 2-MN with AA over a activity of the zeolite. However, the selectively dealuminated on surface Beta and selective dealumination of the N16 comparison with the original zeolite, t~, 2-MN external surface minimizes the on-surface conversion; ~,, selectivity to 1-AMN; v, sereactions taking place over its crystals. lectivity to 2-AMN. TOS= 24 h. Other experiHence, the acylation of 2-MN is mental conditions: see text. conducted inside the zeolite channels, where the formation of the longish 2-AMN is clearly favored versus the bulkier isomer 1AMN (17). Accordingly, Beta NH54 shows an increase in selectivity to 2-AMN when compared to the non acid-treated catalyst (Figures 3-4).

656

i H3

u+ F H+

CH3 CH3~

ANI

PA

H

1_O( 12

l H+(4) Figure 5. Reaction scheme for the acylation of ANI with PA and possible secondary reactions. (1) Demethylation of anisole; (2) O-acylation of phenol; (3) C-acylation of phenol; (4) Fries rearrangement of phenylpropionate. Accordingly, it becomes imperative to remove water from the liquid phase during the acylation process and for this purpose two types of systems have been tested. With a DeanStark system we were able to isolate important amounts of water during the reaction,

657 100

Table 2 Acylation of ANI with PA and influence of the N 8o water concentration in the catalytic activity ~ and selectivity to 4-MPP. Experimental con- ~ so ditions: see text. Experiment XT PA Y4-MPP S4-MVP g4-MPPg-lcAT 11-1 (%) (%) (%) Standard

28

19

67

0.042

Dean-Stark

47

31

66

0.069

Mol. sieve

51

46

91

0.102

rO 40

"~ >

20

O O

0

0

6

12

18

24 30 TOS (h)

36

42

48

Figure 6. Formation of primary and secondary products in the acylation of ANI with PA. G, PA conversion; v, Selectivity to I; ~ , Selectivity to II; E, Selectivity to I l k Experimental conditions: see text.

involving a significant increase in the yield to the desired product (I, Table 2). Nevertheless, high selectivity to I with reasonable conversion was only reached by further water removal from the reaction by means of a molecular sieve column (3 A), installed between the batch reactor and the cooler system. In this case, selectivities above 90% with conversions close to 50% were obtained in 24 h (Table 2 and Figure 6). 4. CONCLUSIONS Beta zeolite is a suitable catalyst for Friedel-Crafts acylations with different substrates and acylating agents, particularly carboxylic acids and their anhydrides, and its performance can be improved through an optimization of the physicochemical characteristics of the catalyst. Hence, samples with low framework Si/A1 ratio and very small crystal size, are very active for the acylation of low activated aromatic rings (i.e., TOL, XYL), and show slower deactivation than the zeolite wkh larger crystallites. By acid treatment of the zeolite before calcination, the framework-A1 at the external surface can be preferentially removed, minimizing the reactions taking place there. In the case of 2-MN acylation wkh AA, the reaction is conducted inside the zeolite channels, where the formation of 2-AMN is clearly favored versus the bulkier isomer 1-AMN. The acylation or aromatic ethers (i.e., ANI) wkh carboxylic acids (i.e., PA) over Beta zeolite is also possible although with longer reaction times than the acylation with anhydrides. Furthermore, high selectivities to the C-acylated product only can be reached when removing absolutely water from the liquid phase, in order to avoid side reactions of the aromatic compound that decrease the performance of the final crude. REFERENCES

1. 2.

J. Marchs, "Advanced Organic Chemistry", 4th ed. Wyley, New-York, 1992. G.A. Olah, "Friedel-Crafts and Related Reactions", Vols. I-IV, Wiley, New-York, 19631964.

658 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

A. Corma, Chem. Rev., 95 (1995) 559. H.W. Kouwenhoven and H. van Bekkum, in "Handbook of Heterogeneous Catalysis" (G. Ertl, H. Kn6zinger and J. Weitkamp, Eds.), Vol. 5, p. 2358. VCH, Weinheim, 1997. M. Spagnol, L. Gilbert and D. Alby, Ind. Chem. Libr., 8 (1996) 29. a) G. Harvey and G. Mader, Collect. Czech. Chem. Commun., 57 (1992) 862; b) G. Harvey, A. Vogt, H.W. Kouwenhoven and R. Prins, Proc. 9th Int. Zeolite Conf., 2 (1993) 363. H.K. Heinichen and W.F. H/31derich, J. Catal., 185 (1999) 408. D. Rohan, C. Canaff, P. Magnoux and M. Guisnet, J. Molec. Catal. A: Chem., 129 (1998) 69. P. Botella, A. Corma, and J.M. L6pez-Nieto, J. Catal., 195 (2000) 161. M. Spagnol, L. Gilbert, E. Benazzi and C. Marcilly, Patent PCT, Int. Appl. WO 96/35655 (1996). D. Rohan, C. Canaff, E. Fromentin and M. Guisnet, J. Catal., 177 (1998) 296. E.G. Derouane, C.J. Dillon, D. Bethell and S.B. Derouane-Abdamid, J. Catal., 187 (1999) 209. M.A. Camblor, A. Corma, A. Mifsud, J. P6rez-Pariente and S. Valencia, Stud. Surf. Sci. Catal., 105 (1997) 341. A. Corma, M.L. Pefia, F. Rey and S. Valencia, Patent PCT, Int. Appl. WO 00/78677 (2000). A. Berreghis, P. Ayrault, E. Fromentin and M. Guisnet, Catal. Lett. 68 (1, 2) (2000) 121. P. Andy, J. Garcfa-Martinez, G. Lee, H. Gonz~ilez, C.W. Jones and M. E. Davis, J. Catal., 192 (2000) 215. P. Botella, A. Corma, and G. Sastre, J. Catal., 197 (2001) 81. E. Fromentin, J.-M. Coustard and M. Guisnet, J. Catal., 190 (2000) 433. a) B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch, J. Org. Chem., 51 (1986) 2128; b) Q.L. Wang, Yudao Ma, Xingdong Ji, Hao Yan and Qin Qiu, J. Chem. Soc., Chem. Commun. (1995) 2307. A. Gunnewegh, R.S. Downing and H. van Bekkum, Stud. Surf. Sci. Catal., 97 (1996) 447. A.E.W. Beers, T.A. Nijhuis, F. Kapteijn and J.A. Moulijn, Microp. Mesop. Mater., 48 (2001) 279. G.A. Olah, R. Malhotra, S.C. Narang and J.A. Olah, Synthesis (1978) 672. Yudao Ma, Q.L. Wang, Wei Jiang and Bojun Zuo, Appl. Catal A: Gen., 165 (1997) 199.

ACKNOWLEDGMENTS

The authors thank the Comision Interministerial de Ciencia y Tecnologfa (CICYT), Spain (Project MAT2000-1392) for f'mancial support.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

659

Aniline methylation on modified zeolites with acidic, basic and redox properties I.I.Ivanova a, O. A. Ponomoreva a, E.B. Pomakhina a, E. E. Knyazevaa, V. V. Yuschenko a, M. Hunger b and J. Weitkamp b a Moscow State University, Department of Chemistry, Leninskie Gory, Moscow 119899, Russia b Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany Four series of molecular sieve catalysts of different structural types, ZSM-11, L, Y and MCM-41 have been prepared. Each type of materials was modified to obtain acidic (Hforms), basic (Cs-forms impregnated with CsOH) and redox (modified with V205) catalysts. The catalysts were characterized by AES, XRD, F T I ~ 29Si, 27A1, 1H and 133Cs M A S NMR~ NH3-TPD, CO2-TPD, TPR and nitrogen adsorption. The catalytic activities of the materials in aniline methylation was evaluated in a continuous-flow microreactor system at temperatures of 523 to 723 K, a WHSV of 1.8 h~ and a molar aniline/methanol ratio of 1:5. In situ 13C MAS NMR was applied to study the mechanisms of N- and C-alkylation on acidic, basic and redox catalysts and to determine key parameters responsible for the catalyst activity, selectivity and stability. The best catalyst performance was observed on zeolite CsNaY/CsOH, which showed an aniline conversion of 99%, a selectivity to N-methylaniline of 98.7% and a high time-on, stream stability. 1. INTRODUCTION Methylation of aniline is an industrially important process aiming at the synthesis of Nmethylaniline (NMA), N,N-dimethylaniline (NNDMA) and toluidines, which are useful raw materials for organic syntheses as well as important intermediates in the dye-stuff production and in the pharmaceutical and agrochemical industries. Up till now, industrial processes leading to these products are based on the application of corrosive liquid acids as catalysts [ 1, 2] and should be replaced by environmentally more benign processes using solid catalysts such as oxides, clays and zeolites [3-6]. In this study, we investigated the main factors, which govern the activity and selectivity of molecular sieve catalysts such as zeolites and mesoporous silicas in aniline methylation. 2. EXPERIMENTAL The large-pore zeolites KL and NaY were purchased from UETIKON, Switzerland, and DEGUSSA AG, Germany, respectively. The medium-pore zeolite NaZSM-11 and the mesoporous NaMCM-41 material were synthesized as described in the literature [7, 8]. Each type of materials was further modified to obtain acidic, basic and redox catalysts. Acid catalysts were prepared by ion exchange of the parent zeolites with a 0.1 M aqueous solution

660 of NHaNO3 followed by calcination. Base materials were obtained by ion exchange with a 0.4 M aqueous solution of CsC1, subsequent impregnation with CsOH and calcination at 723 K in nitrogen. The preparation of the redox catalysts was performed in two steps. The aim of the first step was to eliminate acid and base sites by a removal of aluminum. The preparation of these siliceous materials was carried out either by a direct synthesis (zeolite ZSM-11 and MCM-41) or by post-synthesis dealumination procedures. Dealuminated zeolite Y was purchased from DEGUSSA AG, and dealuminated zeolite L was prepared according to Ref. [9]. In a second step, redox catalysts were prepared by deposition of vanadium oxide on the corresponding siliceous materials by incipient wetness impregnation with an NHaVO3 aqueous solution in oxalic acid. Table 1 gives a survey on the catalysts prepared. The chemical composition of the catalyst samples was determined by atomic emission spectroscopy (AES) using a microwave plasmatic generator Chromaton-1. X-ray diffraction (XRD) data were acquired in the range of 4 ~ < 2 0 < 40 ~ on a DRON-3M powder diffractometer using CuK~ radiation. The FTIR spectra were obtained on a Nicolet PROTEGE 460 Fourier-transform spectrometer. The of surface areas and porosities were measured using an ASAP 2010 instrument. The acidic, basic and redox properties were studied by temperature-programmed desorption of NH3 and COs and temperatureprogrammed reduction (TPR) with 1-12,respectively. The reaction mechanism was investigated using in situ 13C MAS NMR techniques both in batch (BC) and continuous-flow (CF) conditions. A detailed description of the in situ BC and CF experiments is given in Refs. [ 10,11 ] and [ 12,13], respectively. The catalytic activities in aniline methylation were evaluated in a continuous-flow microreactor system at atmospheric pressure. The reaction products were analyzed on line by GC using SE-30 (49 m, diameter 0.2 mm) and Porapak Q (3 m, diameter 3 mm) columns. The reaction temperature was varied in the range of 523 to 723 K. The weight hourly space velocity (WHSV) was 1.8 h-1, and a molar aniline/methanol ratio of 1:5 was used. Table 1. Characteristics of the catalysts under study. The nsJn~a ratio, the degree of cation exchange and the molar nM/(nsi + n~a) ratio were determined by AES. Degree of nM/ Catalysts Preparation nsi/nAI cation (nsi+nA1) a exchange acidic HZSM-11 ion exchange with 31 99 HL 0.1 M NH4NO3 3.3 75 HY 2.7 85 HMCM-41 31.5 99 CsNaZSM-11/CsOH ion exehangewith 31 54 0.052 basic CsKL/CsOH 0.4 M CsC1, 3.3 55 0.069 CsNaY/CsOH impregnation 2.7 70 0.073 CsNaMCM-41/CsOH with CsOH 90 90 0.104 ZSM- 11/V205 impregnation 9000 0.033 redox L/V205 with vanadium19 0.034 Y/V205 oxalic complex 960 0.073 MCM-41/V205 3000 0.040 . . . .

M: Cs or V

661 3. RESULTS AND DISCUSSION 3.1. Effect of the nature of the active sites: mechanistic studies using in situ MAS N M R techniques

The influence of the nature of the active sites on the mechanism of aniline methylation was studied by in situ BC and CF MAS NMR techniques. Zeolites Y modified by acidic, basic and redox components, i.e. HY, CsNaY/CsOR Y/V205 (see Tab. 1), were used as model catalysts. The results obtained by both in situ BC and CF NMR methods [ 10-13 ] point to quite different mechanisms operating on acidic, basic and redox zeolites (Fig. 1). On acidic zeolites, the first reaction step is the methanol (MeOH) dehydration [ 13,16]. At low methanol surface coverages (low methanol/aniline ratios and high contact times of the reactants on the catalyst), the main products of dehydration are methoxy groups attached to the zeolite framework. These species are shown to react rapidly with aniline leading to Nmethylanilinium cations, which can further give NMA upon deprotonation. In contrast, at high methanol coverages (high MeOH/aniline ratios and low contact times of the reactants on the catalyst), dimethyl ether (DME) is the major product of dehydration. DME is further involved in methylation along with methanol and small amounts of methoxy groups also formed under these conditions. Alkylation leads to a consecutive and reversible formation of N-methylanilinium, N,N-dimethylanilinium and N,N,N-trimethylanilinium cations attached to the surface of the acidic zeolites. The products of N-alkylation, i.e., NMA and NNDMA, are further formed via deprotonation of the corresponding N-methylanilinium and N,Ndimethylanilinium cations. The product distribution is determined by the chemical equilibrium between the different methylanilinium cations, which is in turn affected by the reaction conditions (temperature, methanol to aniline molar ratio, contact time). C-alkylated products are formed via transformation of methylanilinium cations at temperatures higher than 498 K.

~Ac ta. .catatysts ...

+NH2PL~(PhN(CH3)3)+OZ"~ -HOZ +

CH3OCH~" ' - ~ 1

(PhNH(CH3)2)OZ I

~~[

COV~+CH3OH CH3OH -H20 ~ +HOZ low \ coverages ~ +NH2~c "~ CH3OZ ~ L (PhNH2CH3)+OZJ -HOZ[ PhNHCH3 [ Base and redox catalysts

CH3OH-------~f CH20 } -H2

hi~

+NH#h

CH2=NPh------~[ PhNHCH3 [

covel'agesf low ~+OH coverages " , ~

CH3OH+HCOO

S

Figure 1. Mechanisms of aniline methylation on acidic, basic and redox catalysts.

662 On basic zeolites, the first reaction step is methanol dehydrogenation leading to the formation of very reactive formaldehyde species, which can either disproportionate into methanol and formate species on base sites or alkylate aniline to give N-methyleneaniline [ 11, 12]. The former is the major reaction pathway at low surface coverages, while the second is preferred at high coverages. It should be mentioned that formaldehyde species are neither observed under batch nor under continuous-flow conditions due to their high reactivity. However, the reaction pathway is evidenced by the observation of formate species and Nmethyleneaniline intermediates [12]~ The latter is further hydrogenated to NMA with H2 generated during methanol dehydrogenation. It is noteworthy that NMA is the only reaction product even at high tempera_hires, suggesting that base sites do not favor its secondary isomerization or disproportionation as in the case of acidic zeolites. On zeolites with redox properties, the ~3C MAS NMR spectra of reactants are broadened due to the effect of paramagnetic vanadium species_ This effect leads to an overlap of the NMR lines and makes their evaluation difficult. Further experiments are needed to draw a final conclusion on the mechanism of aniline methylation on these materials, however, preliminary data suggest that this mechanism is similar to that determined for basic zeolites. Summarizing, it can be stated that mechanistic investigations point to the following conclusions concerning the effect of the nature of active sites: 9 Base and redox sites favor selectively a mono-N-methylation of aniline. These sites do not contribute to side reactions leading to C-alkylated aniline products. The deactivation of such catalysts could be due to a polymerization of N-methyleneaniline intermediate species. To avoid polymerization, high methanol/aniline ratios of the feed are preferable. 9 Acidic zeolites also lead to N-methylaniline as a primary product, however, on these catalysts, multi-N-alkylation is preferred over mono-N-alkylation. Moreover, strong acid sites favor secondary isomerization and disproportionation leading to C-methy!ated products. Therefore, on these catalysts, the selectivity to N-methylaniline is lower. To enlaanee the selectivity to N-methylaniline, the reaction temperature and the strength of the acid sites should be moderate. 3.2. Effect of molecular sieve structure

The effect of the catalyst structure on its performance in aniline methylation was analyzed on the basis of four series of molecular sieves with different structural types: MEL (zeolite ZSM-11), LTL (zeolite L), FAU (zeolite Y) and MCM-41 (see Tab. 1). According to the XRD and FTIR data, all materials had a high phase purity of the respective framework type. 27A1 and 29Si MAS NMR spectroscopy demonstrated that the nsi/n:a ratios in their frameworks were close to those determined by the chemical analysis (AES) and suggested that no extraframework aluminum species were present in the parent materials. The pore volumes and surface areas of the parent materials increased in the following order: KL _< NaZSM-11 < NaY < Na-MCM-41. Modification by ion exchange (H- and Cs-forms) and impregnation (CsOR V205) did not lead to structural changes as evidenced by XRD and FTIR data. The dealumination procedure led to some changes of the lattice parameters, as determined by XRD, and the positions of infrared lattice vibrations. However, the structures of the samples remained intact and, a secondary mesoporosity was practically not observed. While the structure of the parent materials remained intact, the accessible pore volumes were altered significantly upon cation exchange and impregnation. For instance, for zeolite Y, the pore volume decreased by 30 to 35% upon the exchange with cesium cations and the

663 impregnation with V205 and by 55% upon the impregnation with CsOH. The same tendency was observed for the other zeolites. In some cases, the modification with V205 led to a secondary mesoporosity (zeolite L, MCM-41). It should be mentioned, however, that for the modified samples the same order of the pore volumes was observed as for the parent materials. The acidic properties of the catalysts were studied by NH3,TPD. The results are presented in Fig. 2a. According to these data, the amount of acid sites increases in the following order: HMCM-41 < HZSM-11 < HL < HY~ For the acid strength, calculated as average <E> activation energy of NH3 desorption [ 14], the following order was found: HMCM-41 < HY < HZSM-11 < HL. The amount of base sites determined by CO2-TPD was similar for all basic samples, except for CsNaZSM-11/CsOH, on which a lower concentration of base sites was observed (Fig. 2b). The strength of base sites changed in the following order: CsNaMCM-41/CsOH < CsNaZSM-11/CsOH < CsKL/CsOH = CsNaY/CsOH (Fig. 2b). The amount and strength of redox sites were characterized by TPR of H2 (Fig~ 2c).

~" 4t tm

1.2

....

b

HZSM-11

**" ~

~

0.8

/

o.a

/

\

-

HMCM-41

.~.\. 9oo

0.4 "=

HY -

.~.

0.5

~

0.4

;"

0.3

0 200

400

600

CsNaMCM-41/CsOH

-

0.035

ZSM-11/V205 L/V205

0.03

~

0.025

~

0.02

Y/V205 MCM-41/V205

o.o15 0.01

0.005 0

i

250

~--

500

,

T

,

200

400

600

_w

,

800

t, ~

it, ~

~"

-

CsNaY/CsOH '

0.2

Z

~"

CsKL/CsOH

0.6-

\ \/

- CsNaZSM- 11/CsOH 999

750

1000

t, ~

Figure 2. TPD NH3 (a), TPD CO2 (b) and TPR (c) data obtained on various catalysts.

The results indicate that the temperature of vanadium reduction by H2 on V-modified samples depends on the structure type of the material and increases in the following order: MCM41/V205 < L/V205 < Y/V205 < ZSM11/V205.. The comparison of these results with N2 adsorption data suggests that the ability of vanadium species to change the oxidation state depends on their accessibility. The amounts of reducible species were similar in all the materials under study. Only zeolite L/V205 had a slightly lower concentration of reducible species (Fig.

2c).

664 The results of catalyst evaluation in aniline methylation are presented in Table 2. These results can be summarized as follows: 9 The results of catalyst evaluation in aniline methylation are presented in Table 2. These results can be summarized as follows:The highest activity in aniline alkylation is observed on microporous materials with three-dimensional structures (zeolites Y and ZSM-11) or mesoporous materials (MCM-41)_ One-dimensional structures (zeolite L)do not favor the aniline alkylation due to a rapid pore blocking and, therefore, a rapid catalyst deactivation. Among the catalysts with the same structure, acidic materials are the most active. Among basic and redox catalysts, zeolite CsNaY/CsOH shows an extraordinarily high activity. 9 The highest selectivity to NMA was found on zeolites with strong base sites (CsOH) and redox sites (V205). The pore system of zeolite Y is optimal to accommodate alkali metal oxide guest compounds, reaction products and intermediates while for V-modified catalysts, the MEL structure is most suitable. 9 NNDMA is formed mainly on acidic zeolites. The selectivity to NNDMA is governed by the pore system of the catalyst. This product is preferentially obtained on materials with an open pore system such as on zeolite HY and on HMCM-41. 9 The increase of reaction temperature favors toluidine formation on acidic catalysts. 9 The highest stability with time-on-stream was observed on zeolite CsNaY/CsOR Analysis of these results allows for the following conclusions concerning the effect of the pore system on the catalyst_performance: 9 The geometry of the catalyst pores is responsible for the accommodation of the modifiers (CsOH, V205). It affects the formation of active guest species and their dispersion and, therefore, the catalyst reactivity. Thus, the geomet~ of zeolite Y (FAU) is found to be optimal to accommodate CsOH guest compounds, while for V-modified catalysts zeolite ZSM-11 with the MEL structure is the most suitable one. 9 The catalyst structure also influences the accessibility of active sites for the reactants. Microporous catalysts with a_three-dimensional pore system (zeolites Y and ZSM-11) or mesoporous materials (MCM-41) are required for aniline alkylation, while catalysts with one-dimensional pore systems (e.g~ zeolite L) do not favor this reaction_ 9 Besides, the catalyst pore system affects catalyst deactivation and should be optimized to prevent pore blocking. 9 Finally, the zeolite pore system also governs the shape selectivity. Thus, mono-alkylated products are preferentially obtained on zeolites with small pores (ZSM-11), while bialkylated products are favored on the catalysts w i ~ an open porosity (zeolite HY, HMCM-41). The comparative study of various micro- and mesoporous materials in aniline alkylation demonstrated that zeolite CsNaY/CsOH show the best performance due to an optimal pore system and the nature of its active sites. On this catalyst, aniline conversion was 99% at a selectivity to NMA of 98.7%. Besides, this catalyst showed the highest stability with time-onstream. 4. CONCLUSIONS The effects of chemical and geometrical factors on activity, selectivity and stability of micro- and mesoporous catalysts in aniline methylation were studied: 9 The nature of the active sites determines the catalyst activity and selectivity towards

665

Table 2 Summary of the aniline conversions and product selectivities obtained for aniline methylation on various molecular sieve catalysts Catalysts H ZSM -1 1

T,"C WHSV,g/gh anilineMeOH aniline conversion,% selectivity,mol %

acidic catalysts HL HY H MCM41

15

250 18 15

250 18 15

CsNa ZSM11/ CsOH 400 18 15

43

93 7

55 8

24,

250 18 15

250

36 5

18

basic catalysts CsKL/ CsNa CsNaZSM-11/ CSOH Y1 MCMVz05 CsOH 41ICsOH

,

redox catalysts L/ Yl MCMVZOS VZOS 411

vzo5

400 18 15

400 18 15

400 18 15

300 18 15

300 18 15

300 18 15

300 18 15

18

990

09

15 9

46

184

264

6.2

7.3

8.2 75.6

4.5 65.7

8.2 1.2 0.6

17.1 2.9 2.6

1.6 14.8 7.0 0.8 2.5 0.7 CH4 1.o czH4 1.6 CZHS 3.4 37.6 76.9 2.5 3.3 79.5 25.3 51.6 84.1 DME 1.o 91.6 45.0 16.1 44.3 15.2 98.7 60.1 19.7 10.9 37.0 NMA 0.9 Toluidine 3.3 0.9 58.2 NNDMA 55.0 1.6 35.8 0.3 0.4 0.3 0.9 NMT "DMT 0.5 3.6 4.7 DME: dimethyl ether, NMA: N-methylaniline, " D M A : N,N-dimethylaniline, NMT: N-methyltoluidine, " D M T : N,N-dimethyltoluidine

666 N- and C-alkylation. On basic and redox catalysts, the reaction proceeds via methanol dehydrogenation, followed by formation of formatde~de species, which rea~ with aniline to N-methyleneaniline. This reaction pathway leads preferentially to mon0-Nalkylation. On acidic catalysts, the reaction takes place via methanol del~dration and intermediate formation of surface methoxy groups and DME. This results in a preferred multi-N-alkylation. Moreover, strong acid sites favor the isomerization and disproportionation of N-methylanitines leading to C-methylated products. * The catalyst pore system determines the dispersion of active sites created by guest compounds (CsOH, V205) and the accessibility of these sites by the reactants and therefore affects their reactivity. In addition, it governs shape selectivity and affects catalyst deactivation. The best catalyst performance is observed on basic zeolite Y modified with CsOH. This catalyst showed an aniline conversion of 99%, a selectivity to NMA of 98.7% and high stability. ACKNOWLEDGMENTS

Financial support by the Volkswagen Foundation and RFBR is gratefully acknowledged. I.I. Ivanova thanks Russian Science Support Foundation for the grant in the frame of the program supporting talented young researches. REFERENCES

1. A.K. Battacharya and S.K. Nandi, Inch Eng. Chem__Prod_ Res. Dev. 14 (1975) 162. 2. L.K. Doraiswarny, G.R. Venkata Kirshman and S.P. Mukharjee, Chem. Eng. 88 (1981) 78. 3. S. Narayanan, and K. Deshpande, AppL Catak A: General 199 (2000) 1. 4. P.Y. Chen, M.C. Chen, H.Y. Chen, N.S. Chang and T.K. Chuang, Stud. Surf. Sei Catal., 28 (1986) 739. 5. P.R Haft Prasad Rao, P. Massiani and D. ~ m e u f , Catal. Lett. 31 (1_995) 115. 6. S. Narayanan, K. Deshpande and B.P. Prasad, J. Mol. Catal. 88 (1994) L271. 7. E. Robson, K.P. Lillerud (Eds_)~Verified Synthesis of Zeolitic Materials, 2nd Ed., Elsevier, 2001. 8. Y.Cesteros, G.L.Haller, Microporous Mesoporous Mater~ 43 (2001) 172. 9. E.E.Knyazeva, V.V.Yuschenko, F.Fajula, I.I.Ivanova. Stud. Surf. Sci. Catal. 135 (2001) 208. 10. I.I. Ivanova, E.B. Pomakhina~ A.L Rebrov, M_ Hunger~ Yu.G. Kolyagin and J. Weitkamp, J. Catal. 203 (2001) 375. 11. I.I. Ivanova, E.B. Pomakhina, A.I. Rebrov, YtLGL Kolyagin, M. Hunger and J. Weitkamp, Stud. Surf. Sci. Catal. 135 (2001) 232. 12. W_ Wang~ M. Seiler, I.I. Ivanova, J. Weitkamp and M. Hunger, Chem. Commun. (2001) 1362. 13. W. Wang, M. Seiler, I.I. Ivanova, U. Sternberg, J. Weitkamp and M. Hunger, J. Am. Chem. Sot., submitted. 14. V.V.Yuschenko, Russian J. Phys. Chem. 71 (1997)547.

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

667

Aldol condensation catalyzed by acidic zeolites T. Komatsu, M. Mitsuhashi and T. Yashima* Department of Chemistry, Tokyo Institute of Technology, 2 - 1 2 - 1 0 o k a y a m a , Meguro-ku, Tokyo 152-8550, J a p a n Aldol condensation of acetophenone, cyclohexanone and acetone was carried out on various acidic zeolites. Bimolecular condensation and the subsequent dehydration were catalyzed with high selectivity by the acid sites of H-Y, H-USY, H-beta and H-MCM-22. The role of acid sites in the catalytic activity was studied using dealuminated and Na+-exchanged H-USY and H-beta. 1. I N T R O D U C T I O N Aldol condensation is one of the most important reactions to form a carboncarbon bond in the organic syntheses. This reaction has been usually carried out in homogeneous reaction systems using liquid base or acid catalysts. In the case of heterogeneous systems, basic oxide catalysts have been mainly reported so far. For example, MgO pretreated at 600~ in vacuo catalyzes the reaction of acetone into diacetone alcohol [1]. Activity per unit surface area is in the order, BaO > SrO > CaO > MgO. Alkali containing zeolites are basic catalysts and also reported to be active for the aldol condensation: sodium clusters in X and Y zeolites for acetone to mesityl oxide (MO) and isophorone [2], Na-X and Cs-X for acetone to MO, isobutene and methylisobutylketone [3], CsNa-Y for b u t a n a l to 2-ethyl-2hexenal [ 4 ] , Cs-X, K-X, etc. for the reaction of benzaldehyde with ethylcianoacetate [5] and methyl propionate with formaldehyde [6]. However, acidic zeolites have scarcely been studied [7]. The purpose of this study is to clarify the catalytic properties of acidic zeolites for the aldol condensation of acetophenone, cyclohexanone and acetone and the effect of acid strength, acid concentration and pore dimension on the catalytic activity. 2. E X P E R I M E N T A L S E C T I O N

2.1. Catalyst preparation Zeolite beta(Si/Al=12) [8], MCM-22(13 and 23) [9], ZSM-5(17), ferrierite(10), and mordenite(10) were hydrothermally synthesized, exchanged with ammonium * Present address: Research Institute of Innovative Technologyfor the Earth, 9-2 Kizugawadai, Kizugawa-cho, Soraku-gun, Kyoto 619-0292, Japan

668 nitrate aqueous solution, and calcined in air to obtain their acidic (H-) forms. Gallosilicate and ferrisilicate with BEA and MWW structures were hydrothermally synthesized using gallium nitrate and ferric nitrate as Ga and Fe sources, respectively, and transformed into H-forms in a similar manner. H-Y (Mizusawa Ind. Chemicals, Si/AI=2.8) and H-USY (Catalysts & Chemicals Ind., Si/AI=7.5) were used as supplied. K-X and Cs-X were obtained by a usual cation exchange with Na-X (Union Carbide, Si/AI=I.2) using aqueous solutions of potassium nitrate and cesium nitrate, respectively. Dealumination of H-beta and H-USY was carried out by the acid extraction with 0.1-10 mol 11 nitric acid at 25 or 70~ Part of protons were exchanged with Na § ions using a usual ion-exchange method with sodium nitrate aqueous solution at 70~ Aluminum containing mesoporous silica, MCM-41 [10] and SBA-15 [11], were synthesized with NaA102 (MCM-41) or AI[OC(CH3)3]3 (SBA-15) as A1 sources to have Si/A1 ratios of 23 and 15, respectively.

2.2. Catalytic r e a c t i o n Aldol condensation was carried out in an autoclave with 0.2 tool of reactant (acetophenone, cyclohexanone or acetone) and 0.43 g of catalyst dehydrated in flowing helium at 400~ The reactor was sealed and heated to the reaction t e m p e r a t u r e under agitation and after the specific reaction time (usually 2 h) at the temperature, the reactor was cooled with ice bath and the reaction mixture was analyzed by a gas chromatograph. 3. R E S U L T S AND D I S C U S S I O N

3.1. Aldol c o n d e n s a t i o n on v a r i o u s solid catalysts Aldol condensation of acetophenone was carried out at 150~ on various solid catalysts. The bimolecular condensation of acetophenone produces 1,3-diphenyl3-hydroxy-l-butanone (DPHB) as a primary product (Scheme 1). The secondary dehydration of DPHB gives 1,3-diphenyl-2-butene-l-one (DPBO), which was the main product on all the catalysts tested. Table 1 shows the yield of DPBO obtained in 2 h of the reaction on various catalysts. H-USY(Si/AI=7.5), H-Y(2.8), H-beta(12) and H-MCM-22(23) gave high DPBO yields, indicating t h a t acidic zeolites would be the active catalyst for the aldol condensation of acetophenone. Turnover numbers based on the number of A1 atoms in zeolites were 65 (H-USY), 18 (H-Y), 50 (H-beta) and 120 (H-MCM-22) for 2 h of the reaction. The acid sites in these zeolites effectively catalyzed the reaction of acetophenone. However, other 0

0

aldol

OH

condensation 2

edration."

0

AJ.. 0

"

acetophenone

DPHB Scheme 1. Aldol condensation of acetophenone.

DPBO

669 Table 1. Aldol condensation of acetoacidic zeolites, H-mordenite(10), H-ZSMphenone and cyclohexanone at 150~ 5(17) and H-ferrierite(10), gave lower DPBO yields. The pore opening of these Catalyst(Si/A1) Yield / C-% zeolites is smaller t h a n t h a t of Y and beta DPBO a CHCH b but comparable to t h a t of MCM-22. Acetophenone molecules would enter the H-USY(7.5) 27.4 15.5 pores of all the zeolites used here though H-Y(2.8) 17.6 -their diffusion rate in the pore would be H-beta(12) 14.4 11.7 different with each other. Therefore, the H-MCM-22(23) 17.2 18.4 space inside the pores of mordenite, H-mordenite(10) 0.5 0.1 ZSM-5 and ferrierite m a y be too small for H-ZSM-5(17) 0.1 0.2 the bimolecular condensation of acetoH-ferrierite(10) 3.4 0.8 K-X(1.2) 0.0 -phenone. Cs-X(1.2) 0.2 -Basic zeolites, K-X and Cs-X, H-MCM-41(23) 1.5 1.6 exhibited very low activity for the H-SBA-15(15) 5.5 5.1 formation of DPBO. The concentration of MgO 0.9 -base sites in K-X and Cs-X is much BaO 0.6 -higher t h a n t h a t of acid sites in H-USY, ZrO 2 0.2 -H-Y, etc. Therefore, the 'acid sites in HTiO2(anatase ) 0.0 -form zeolites would be much effective for 7-A1203 2.2 -the aldol condensation of acetophenone. This is consistent with the previous Reaction time: 2 h. In acetophenone a report [7] t h a t H-Y catalyzes aldol and cyclohexanone b reactions. condensation of acetophenone, whereas Na-Y and Na-X scarcely produce DPBO. Al-containing mesoporous silica, H-MCM-41(23) and H-SBA-15(15), which have much larger pore opening and inner space t h a n zeolites, did not exhibit high DPBO yield probably because their acid s t r e n g t h is very weak. Low DPBO yields obtained on other basic and acidic oxides, MgO, BaO, ZrO2, TiO2 and 7-A1203 indicate the a d v a n t a g e of acidic zeolites. Figure 1 shows the change in acetophenone conversion and DPBO selectivity with reaction time on H-USY(7.5) at 150~ The observable conversion (6.8%) at 0 h indicates t h a t the reaction occurred significantly before the t e m p e r a t u r e reached 150~ After 1 h of the reaction, DPBO selectivity did not change significantly around 85 C-%, where the conversion of acetophenone was higher t h a n 30%. The other product was almost exclusively DPHB. The dehydration of DPHB into DPBO will be very fast under the reaction conditions. The conversion increased at the initial stage of reaction, but did not increase significantly (34%) after 5 h, where DPBO yield was almost constant around 30 C-%. Coke would be formed on H-USY at the initial stage. In fact, w h e n used H-USY was w a s h e d with acetone at room t e m p e r a t u r e and reused, the rate of DPBO formation was only a q u a r t e r of t h a t on the fresh H-USY. In contrast, as shown in Fig. 2, the calcination of used H-USY at 470~ in air completely regenerated its activity. Figure 3 shows the conversion and DPBO selectivity on H-beta(12) as a function of reaction time. The conversion increased gradually with reaction time to reach 41.9% after 18 h. The initial activity of H-beta will be lower t h a n t h a t of

670 100

~

80,

=~"

60

H-USY(7.5) m

r~

m

m m m m

mm

m

m

m

m

H-MCM-22(13) fresh calcined

Or) . , - i

40 ~

20 0

.......

fresh calcined

m

m

H-beta(12) !

0

,'o

,;

20

fresh calcined

Fig. 1. Aldol condensation of acetophenone on H-USY(7.5) at 150~ conversion: 9 DPBO selectivity: []

|

|

u

i

|

0 5 10 Rate of DPBO formation / 10-5mol g-l

Reaction time / h

Fig. 2. Regeneration of used catalysts by calcination at 4700(2 in air.

H-USY, w h e r e a s the final conversion was higher on H-beta. H-MCM-22(13) also gave the gradual increase in conversion with reaction time. The low initial activity m a y result from the low diffusion rate of acetophenone and DPBO in the pore of H-beta and H-MCM-22 compared with H-USY. The DPBO selectivity on H-beta was almost the same as t h a t on H-USY. On all the other solid catalysts tested, the selectivity to DPBO was always around 85 C-% w h e n the conversion was higher t h a n 10%. As shown in Fig. 2, the calcination of used H-beta did not regenerate completely the activity of fresh catalyst, whereas H-MCM-22 was almost regenerated. In IR spectra, OH absorption peaks appeared at 3610 cm 1 (H-beta), 3619 cm 1 (H-MCM-22) and 3623 cm a (H-USY). H-beta having the strongest acid sites m a y accelerate the formation of unremovable coke. As shown in Fig. 4, the conversion of acetophenone at the reaction time of 2 h increased monotonously with increasing reaction t e m p e r a t u r e on H-beta(12). The DPBO selectivity slightly increased at lower t e m p e r a t u r e s but did not decrease 100

--

100

_-

80

~

80

~

60

=~

60

"~:~

40

m

9

9

m

~m

40

20

5N 0

5

1'0

1'5

20

Reaction time / h Fig. 3. Aldol condensation of acetophenone on H-beta(12) at 150~ conversion: 9 DPBO selectivity: []

2o 0100 "-~' ' ' ' " 120 140 160 180 200 Reaction temperature / ~

Fig. 4. Effect of reaction temperature on aldol condensation of acetophenone on H-beta(12). conversion: 9 DPBO selectivity: []

671

0 2

aldol condensation ~"

O ~ oH

dehydration f

cyclohexanone

HCCH

CHCH

Scheme 2. Aldol condensation of cyclohexanone. significantly at higher temperatures, indicating that the further reaction of DPBO did not occur at higher temperatures. As a result, DPBO yield reached 37.7 C-% at 200~ Similar constancy in selectivity was observed for H-USY and HMCM-22(23); DPBO yields at 200~ were 29.2 and 34.1 C-%, respectively. Aldol condensation of cyclohexanone was also carried out on various catalysts at 150~ as shown in Table 1. The main product was 2-cyclohexylidenecyclohexanone (CHCH) formed through the secondary dehydration of condensation product, 2-(1-hydroxycyclohexyl)cyclohexanone (HCCH), as illustrated in Scheme 2. Selectivity to CHCH was higher t h a n 85 C-% when the conversion of cyclohexanone was higher t h a n 5%. H-USY, H-beta and H-MCM-22 showed high CHCH yield compared with small pore zeolites. These results were almost the same as those obtained for acetophenone reaction except for the very low yield on H-ferrierite. The molecular size of cyclohexanone would be slightly large compared with t h a t of acetophenone. Therefore, cyclohexanone would not enter the pore of H-ferrierite. On the other hand, for the reaction of acetone at 120~ (Table 2), H-ZSM-5 gave comparable conversion of acetone to H-USY, H-beta and H-MCM-22. Products were diacetone alcohol (DA) through the bimolecular condensation, mesityl oxide (MO) through the dehydration of DA, and trimers such as isophorone and trimethylbenzene through the successive condensation of MO with acetone. The molecular size of acetone will be small enough for the bimolecular condensation to form DA and MO inside the pore of H-ZSM-5. H-USY, H-MCM-22 and H-SBA-15 gave high selectivity to trimers. Supercages and mesopores would Table 2. Aldol condensation of acetone at 120~ Catalyst(Si/A1)

Conversion /%

Selectivity / C-% MO a Trimers

H-USY(7.5) H-beta(12) H-MCM-22(13) H-ZSM-5(17) H-ferrierite(10) H-MCM-41 (23) H-SBA-15(15)

2.9 8.7 6.1 5.9 0.5 1.5 4.8

24 88 28 88 48 60 28

Reaction time: 2 h.

a

Mesityl oxide

65 1 61 4 19 1 61

672 H-beta H-Ga-BEA H-Fe-BEA H-MCM-22 H-Ga-MWW H-Fe-MWW

30

9

~ "~ 20

i

. . . .

I

i

,

5 10 15 DPBO yield / C-% Fig. 5. Aldol condensation of acetophenone on metallosilicates at 150~

0

m

0

"

0.5 1 1.5 2 AI concentration / mmol g-~

Fig. 6. Effect of A1 concentration on DPBO yield for dealuminated H-USY (l) and H-beta (D) at 150~

have enough space to accelerate the formation of bulky trimers. H-SBA-15 with weak acid sites gave comparable conversion to zeolites, suggesting that acetone is more reactive t h a n acetophenone and cyclohexanone. 3.2. R o l e o f a c i d s i t e s in t h e a l d o l c o n d e n s a t i o n o f a c e t o p h e n o n e The effect of acid strength on DPBO yield in acetophenone reaction was further examined using metallosilicates. Gallosilicate and ferrisilicate with MCM-22 structure, Ga-MWW(Si/Ga=18) and Fe-MWW(Si/Fe=13), and those with beta structure, Ga-BEA(17) and Fe-BEA(13), were synthesized. Figure 5 shows DPBO yields at 150~ in 2 h of the reaction. For both structures, DPBO yield decreased in the order, aluminosilicate > gallosilicate > ferrisilicate. This order coincides with that of the acid strength, indicating that strong acid sites are more effective for the aldol condensation of acetophenone. This was suggested above by the low activity of Al-containing mesoporous silica (Table 1). The effect of acid concentration was then investigated using dealuminated HUSY and H-beta. The A1 extraction with nitric acid increased Si/A1 ratios up to 240 (H-USY) and 620 (H-beta). Figure 6 shows the effect of A1 concentration on the yield of DPBO. When the A1 concentration was low, DPBO yield increased with increasing A1 concentration for both zeolites. It is clear that the Bronsted acid sites are the active site for the reaction of acetophenone. However, DPBO yield did not increase significantly at higher A1 concentrations than 0.5 mmol g~. The bimolecular aldol condensation would need large space like supercage in USY or pore intersection in beta to take place. Therefore, two or more acid sites located in one supercage or one intersection would not work simultaneously and behave as an active site. In the case of H-USY, A1 concentration of 0.69 mmol gX (Si/Al=23) corresponds to the statistical distribution of one A1 atom per supercage though A1 could be located inside the sodalite cage. The lower turnover number (18) of H-Y(2.8) t h a n that (120) of H-MCM-22(23) would be caused by such an excess in A1 concentration in addition to the presence of inaccessible acid sites in the sodalite cages.

673

3O

30

o-e.

f

!

c,.9

0

~- 20

20

o 1....i ol,.~

o

10

o

0

20 40 60 80 Na + exchange level / %

Fig. 7. Effect of Na§ on DPBO yield for HNa-USY(7.5) (I) and HNabeta(12) (D) at 150~

10

0

0.5 1 1.5 2 A1 concentration / mmol g-1

Fig. 8. Effect of A1 concentration on DPBO yield for dealuminated (A) and Na+-exchanged (e) H-USY.

The effect of acid concentration was further investigated using HNa-USY and HNa-beta prepared by the usual Na+-exchange procedure (Fig. 7). In the case of HNa-USY, DPBO yield decreased with increasing Na § exchange level and reached 0 C-% around 40% exchange level. This indicates t h a t at the initial stage, Na § predominantly exchanges protons in accessible acid sites, which should be located in the supercage. Therefore, only 40% of A1 atoms would be located in the supercage to catalyze the reaction of acetophenone. In the case of HNa-beta, DPBO yield did not decrease with Na+-exchange at lower exchange levels t h a n 30%. This could be explained by an idea that two (or three) A1 atoms are located in a pore intersection, as in the case of those in a supercage of H-USY, to work as an acid site. DPBO yield decreased when exchange level increased from 30 to 80%. It is suggested t h a t H-beta has a significant amount of inaccessible acid sites for the acetophenone condensation. Figure 8 compares the effect of A1 concentration for the dealuminated and Na+-exchanged H-USY described above. The parent H-USY(7.5) was denoted by an open square. DPBO yields on two series of USY followed quite different curves with A1 concentration. Na § ions would first attack the acid sites in supercages to remove active protons, whereas the acid extraction might first remove acid sites inaccessible to acetophenone. The role of acid sites on the external surface of zeolite crystallites was investigated by the addition of 2,4-dimethylquinoline (2,4-DMQ) into the reactant. Figure 9 shows the DPBO yield after 2 h of the reaction for p a r e n t zeolites and poisoned ones. In the cases of H-USY and H-beta, the addition of 2,4-DMQ which amounted to the quarter of A1 atoms in zeolites did not remove their activity completely, indicating t h a t the acid sites inside the pores as well as those on the external surface worked as active sites. On the other hand, H-MCM-22 and Hferrierite lost their activity almost completely by the addition of the same fraction of 2,4-DMQ. The acetophenone reaction would occur only on the external surface of these zeolites. The relatively high DPBO yield for the p a r e n t H-MCM-22 may

674

H-USY(7.5) + 2,4-DMQ(1/4) + 2,4-DMQ(1/1) m H-beta(12) + 2,4-DMQ(1/4) + 2,4-DMQ(1/1) m H-MCM-22(13) + 2,4-DMQ(1/4) + 2,4-DMQ(1/1) H-ferrierite(10) + 2,4-DMQ(1/4)

I

0

I

10 20 DPBO yield / C-%

..... !

30

Fig. 9. Poisoning of acid sites by 2,4-DMQ in acetophenone reaction. Values after 2,4DMQ indicate molar ratio of 2,4-DMQ/A1. result from the high activity of acid site in 12-ring pockets [12] located on the external surface; each of the pocket consists of a bottom half of a supercage. REFERENCES

1. G. Zhang, H. Hattori and K. Tanabe, Appl. Catal., 36 (1988) 189. 2. L.R. Martens, W.J. Vermeiren, D.R. Huybrechts, P.J. Grobet and P.A. Jacobs, Proc. 9th Intern. Congr. Catal., (Calgary, 1988)vol. 1, p.420. 3. C.O. Veloso, J.L.F. Monteiro and E.F. Sousa-Aguiar, Stud. Surf. Sci. Catal., 84 (1994) 1913. 4. E.J. Rode, P.E. Gee, L.N. Marques, T. Uemura and M. Bazargani, Catal. Lett., 9 (1991) 103. 5. A. Corma, V. Fornes, H. Garcia and J. Primo, Appl. Catal., 59 (1990) 237. 6. P.T. Wierzchowski and L.W. Zatorski, Catal. Lett., 9 (1991) 411. 7. P.B. Venuto and P.S. Landis, J. Catal., 6 (1966) 237. 8. M.A. Camblor, A. Mifsud and J. Perez-Pariente, Zeolites, 8 (1988) 46. 9. A. Corma, C. Corell and J. Perez-Pariente, Zeolites, 15 (1995) 2. 10. J.M. Kim, J.H. Kwak, S. Jun and R. Ryoo, J. Pshys. Chem., 99 (1995) 16742. 11. Y. Yue, A. Gedeon, J. Bonardet, N. Melosh, J. Baptiste and J. Fraissard, Chem. Commun., (1999) 1967. 12. S.L. Lawton, M.E. Leonowicz, R.D. Partridge, P. Chu and M.K. Rubin, Micropor. Mesopor. Mater., 23 (1998) 109.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.

675

Role of intracrystalline tunnels of sepiolite for catalytic activity Y. Kitayama a, K. Shimizu b, T. Kodama a, S. Murai~, T. Mizusima a, M. Hayakawaa and M. Muraoka a. aDepartment of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, Ikarashi, Niigata, 950-2181, J a p a n bGraduate School of Science and Technology, Niigata University, Ikarashi, Niigata, 950-2181, J a p a n Kinetic studies of the exchange reaction of Mg 2+ in the crystal lattice of clay mineral sepiolite with various Cu 2+ salts were carried out and the reaction rate depended on anion species of the Cu 2§ salts. The substitution of Mg 2§ ions along the tunnel wall of sepiolite lattice with Cu 2§ is confirmed by unisotropic ESR spectra of Cu2§

sepiolite. Catalytic activity of sepiolite was studied on

the cyclodehydration of diethylene glycol(DEG), the conversion of ethanol to 1,3-butadiene and the biphenyl formation from aryl halide and phenylboronic acid (Suzuki reaction). In the cyclodehydration of DEG, intracrystalline tunnels of sepiolite play an important role. While, for 1,3-butadiene formation from ethanol, transition metal ions substituted with Mg 2§ along the tunnel wall of sepiolite behave as basic sites. For Suzuki reaction, the dispersion of Pd particles is an important factor for the catalytic activity. 1. Introduction

-~o

Fibrous clay mineral sepiolite

[MgsSi 12030(OH)4(H20) 4 nH20] has uniform size parallel-piped tunnels along the fiber axis and their

cross

1.13X0.39

sectional nm 2

as

area

is

shown

in

Fig. 1 [1]. Both zeolitic and crystal water molecules contain in the

~

~

4 89

~

a a~:is

O:O

|

e" Mg

~ 9Si

=I-ho

O- 2H20

Fig. 1 Structure of sepiolite tetrahydate SEP 4H20 and dihydrate SEP 2H20

676 tunnels of sepiolite crystals. The octahedral coordination of the Mg 2§ ions along the tunnel wall is completed by a coordination of two water molecules and the Mg(II) ion along the tunnel wall is exchangeable with various bivalent metal cations such as Cu 2§ Mn 2§ and Zn 2§ A half of the coordinated water molecules dehydrates reversibly at about 513K and the subsequent distortion of the coordination of Mg 2§ occurs[2]. Therefore, the tunnels of sepiolite can provide catalytic active sites for various reactions such as dehydration, polymerization[3] and acid-base bifunctional catalytic reactions[4]. In this study, metal ion exchange reaction, shape selective catalytic reaction and acid-base bifunctional catalytic bihavior of sepiolite were studied to investigate roles of the intracrystal]ine tunnels of sepiolite. Further, a property of sepiolite as a catalyst support was studied. 2.

Experimental

2.1 Cu 2§ exchange Measurements of ESR spectra were carried out by using the well oriented long fibrous bundle shape Cu2§ sepiolite. The Cu2§ sepiolite sample for ESR measurements was prepared by using the long fibrous bundle sepiolite of Chinese origin. After a thin plate of the well oriented fibrous sepiolite (3x7mm 2) was immersed in the aqueous solution of Cu(CH3COO)2 at room temperature for 2h, the sample was meticulously washed with distilled water. The sample was dried at room temperature and was set in the sample tube. Then the sample was evacuated at designated temperature. ESR measurements were performed with an X-band spectrometer (JEOL JES-FE1XG). In order to study an exchange reaction of the cation in the sepiolite lattice with Cu 2§ Chinese fibrous sepiolite was crushed and sieved to make its powder of 48-100 mesh. The powdered sepiolite (0.4g) was suspended in an aqueous solution of Cu(II)-salt. The content of Cu 2§ in the sepiolite was measured by dissolving the sample with a mixed solution HC1 and HNO3 and the resultant solution was followed to ICP emission spectroscopic measurement (Seiko ICP SPS-1500). 2.2 Catalytic reactions The natural sepiolite powder of Spanish origin (Vallecas Spain) was used for the catalytic reaction. The surface area of the sample heated at 393K was 217m2/g in particle diameter 70"83 mesh. The reaction products were analyzed by an FID gas chromathograph.

677 The catalytic cyclodehydration of diethylene glycol was carried out by using granular sepiolite catalyst of 32-60 mesh packed in a fixed bed continuous flow reactor under nitrogen flow at 573K. Metal oxides supported on sepiolite were used as catalysts for the catalytic conversion of ethanol to 1,3-butadiene. The reaction was carried out in a closed circulating reaction system. The catalyst was prepared by impregnating sepiolite with an aqueous solution of various metal acetates.

The resultant material was

calcined in air at 773K for lh to decompose acetate ion. Prior to use for the reaction, the catalyst was heated in

vacuo

at 623K for 2h.

The pd2+-complex immobilised catalyst (pd2+-sepiolite) for Suzuki reaction was prepared by exchanging the sepiolite with aqueous solution of [Pd(NH3)4] 2+ C12 at 298 K for 48 h, centrifuging and washing with deionised water, and drying in vacuo at 298 K. The catalytic reaction was carried out in batch system. The reaction mixture containing aryl halide, phenylboronic acid, potassium acetate and Pd in DMF was stirred under N2.

3. Results and Discussion 3.1 Cu(XD'ion exchange 3.1.1 Reaction kinetics Exchange reactions of Mg 2+ in sepiolite crystal lattice with Cu 2+ were carried out in an aqueous solution of Cu2+'salt at various concentrations and various temperatures. A typical reaction path using an aqueous solution of CuSO4 is shown in Fig. 2. An approximately linear relationship was observed between 1/ (raction rate) and 1/ [Cu2§ Therefore following mechanism is given for the substitution of Mg 2§ in the crystal lattice of the sepiolite with Cu2+: Sep_Mg2++

C u 2+

~ klk. 1

sepiolite

..Mg 2+ Sep-"." " ' C u 2+

k2 /xx ~

intermediate

S e p - C u 2+

cupric ion

exchanged

+- M g 2+

sepiolite

Assuming that the steady state approximation can be applied to the intermediate formation step and the release of Mg 2§ from the intermediate is a rate determining step, a rate equation for substitution reaction can be written by 1 ~--

v

1

+

K

/~[sep]o ~sep]o[C~]

,

K~ '

k 1-kk2

k~

where v, [sep]o and [Cu 2§ represent reaction rate, initial concentrations of sepiolie

678 and Cu e+ concentration respectively. Rate constant, k1, k.1 and k2, for each reaction step are shown in the scheme. Thus a linear relationship between 1/v and 1/[Cu 2§ obtained in this study can be interpreted by above reaction scheme. Rate constants (kz) calculated from experimental results at 293K and values of the activation energy in the reaction of sepiolite with various Cu2+-salt are shown in Table 1.

The rate constant decreased in an order of k2(CuC12)> kz(CuSO4)> ke(Cu(HCOO)2)>> kz(Cu(CH3COO)2).

Stokes radii of anion species are r(CH3COO)=0.22nm, r(HCOO-)=0.16nm, r(Cl)=0.12nm and r(SO42)=0.11nm. From the reaction kinetics data in the exchange reaction with various Cu(II)-salts, the exchange rate of Mg z§ with Cu z§ depended on the Stokes radii of anion species of the Cu(II) salts. This indicates that the diffusion of Cu e+ in to the intracrystalline tunnels of sepiolite contributes to the reaction rate. Table 1 Kinetic parameter of the substitution of Mg2§ in sepiolite lattice with Cu2§ rate constant

Activation

solution

k2

energy / kJ tool"1

CuC12

0.613

23.5

CuSO4

0.541

21.1

Cu(HCOO)2 Cu(CI-I~COO)2

0.535 0.085

19.2 19.4

328K

'~ 0.6

"~0.4

! 0.2

0

0

?

~t

fi

Figure 2 Exchange reaction between sepiolite and Cu(HCOO)s.solution. Sep: lOOmg; Cu(HCO0)2:50mL(lmmol/I~

Sepiolite:200mg; concentration of Cu(II)-salts: 0.1-10 mmol/L (500mL); reaction temp.: 298K. Activation energy was calculated from the data obtained in the temperature range from 288K to 318K

3.1.2 Results of ESR m e a s u r e m e n t s To determine the substituted site of Cu 2+ in sepiolite, the well orientated fiber shape bundle Cu2§

sepiolite sample was

HI/O0"

used for angular dependency measurements of ESR spectra. As shown in Fig.3, clear unisotoropic

ESR

spectra

due

to

Co)

poler

Cu 2§

(gx=2.390, gr=2.115, gz=2.045; Ax=8.5, Ay=3.0, Az=6.0 roT) were observed. These values strongly suggest t h a t the coordination

Figure 3 ESR spectra of well oriented fibrous sepiolite

679 of Cu 2+ is a distorted octahedral[5]. Further, the dissolved Mg e+ was found in the aqueous solution after the reaction. From the results, we concluded t h a t the Mg e§ along tunnel wall of sepiolite are substituted by Cu e+. 3.2 Catalytic activiW of sepiolite

3.2.1 Shape selective catalytic reaction

100%

To investigate the role of tunnels of sepiolite,

catalytic

cyclodehydration

80%

of

DEG was studied by using sepiolite alone.

60%

1,4-Dioxane

40%

Ethylene

was

glycol,

the

main

ethanol,

product.

acetic

acid,

20%

acetoaldehyde and crotonaldehyde were observed in the product. At an early stage

of

the

reaction,

the

yield

of

1,4-dioxane was 100%, the decrease of 1,4-dioxane formation and an increase of

0% 1.5

3

7.8

13.9

slmerticial velcNzitv ( 1O0 m/min~

II acetoaldehyde

[] ethanol [] acetic acid El ethylene glycol

@ 2-methyl-l,3-dioxolane I erotonaldehyde [] dioxane

by'products formation were observed with an increase of the TOS. This means t h a t conversions of 1,4-dioxane occurred on the

Figure 4 Effect of superficial Velocity on the distribution of products reaction temp: 573K;TOS:30min;WHSV:0.2h1

catalyst. To clarify the reaction path, reactions were carried out by using flow reactors of various diameters packed with the constant weight of the sepiolite at the constant WHSV. Since in each reaction, no DEG has been detected at the initial stage of the reaction, the distributions of the product at 30 min of TOS are shown in Fig.4. The superficial gas velocity is defined by vs=(flow rate of D EG)/(cross section of the reaction tube). The existence of the optimum superficial velocity for 1,4-dioxane formation indicates that a diffusion of DEG into the inner surface of

lOO 9O =

80

___- "~ ~.~.,.

~~ 7o_ 4? 60

.~ so 2 4o

-

k

_

.

o. .~ 30

"

17 3 0 m ~

,t~ 2o

"

0

~

60mh

,~.

A 90mh

lO

I

0

0.1

~

I

0.2 WHSV/h

R eactbn I m p .

,

I

0.3

~

I

0.4

,

0.5

-1 533K, s e p b l i ~ Ig

Figure 5 Relation between WHSV and selectivity for 1.3-butadiene formation.

the tunnels of sepiolite and desorption of 1,4-dioxane are important process. The selectivity of 1,4-dioxane formation depended on WHSV of the reactant. When DEG was supplied less t h a n adsorption capacity of DEG on sepiolite, the selectivity of 1,4-dioxane formation was high. On

680 the other hand, supplying an excess of DEG over the adsorption capacity of the catalyst, the selectivity for the 1,4-dioxane formation decreased as shown in Fig.5. The coordination of DEG to the Mg 2§ was observed in situ XRD measurements of the sepiolite. The results show that a shape selective 1,4-dioxane formation occurs the inner surface of sepiolite tunnels, while by-products formation occurs on the outer surface of sepiolite tunnels.

3.2.2 Acid-base bihmctional catalyst Metal oxide such as Mn, Zn, Ni and Cu supported on sepiolite (MOx-sepiolite) showed a high activity and selectivity for the catalytic conversion of ethanol to 1,3-butadiene, although ethylene and diethylether produced from ethanol by using sepiolite alone. At the early stage of the reaction acetaldehyde produced and decreased with an increase of 1,3-butadiene in the progress of the reaction. The yields of C4 on MOx-sepiolite catalysts were 50% -85% as shown in Table 2. Since 1,3-butadiene was produced via following process [6]" Aldohl Condensation 2CH3CHO -CH3CH(OH)CH2CHO I A ~ A ~ dehydration hydrogen transf~'+2C2HsOH CH3CH=CHCH2OH+CH3CHO ~ CH3CH=CHCHO A I dehydration A+B .2H 2 2C2HsOH ~ B

T

CH2=CH-CH=CH2

I A:acidir site I B:basie site

both acidic and basic sites are necessary. There are only acidic sites ((pKa=6.8 " "1.5) on sepiolite alone. An existence of basic sites on the ZnO-sepiolite catalyst (pK,=9.8) was confirmed by Benesi's titration method and the substituted Zn 2+ ions with Mg 2+ along the tunnel wall behaved as basic sites for the catalytic conversion of ethanol [7]. To clarify roles of acidic and basic sites for 1,3-butadiene formation, the catalysts were poisoned by acidic and basic materials. The results are shown in Table 3. Acidic materials such as CO2 and phenol inhibited the formation of the 1,3-butadiene formation and enhanced the ethylene formation. A small amount of NH3 and pyridine enhanced the 1,3-butadiene formation and inhibited the ethylene formation. However, when the catalyst treated with a large amount of NH3, both the 1,3-butadiene and ethylene formations were inhibited. This means that both acidic and basic sites, on the catalyst play important roles for the 1,3"butadiene formation. Especially, the basic site on the catalyst is more important than acidic site, ie. the basic site forms by the exchange of Mg 2+ along the tunnel wall with divalent transition metal ions and it plays as an important role for acetaldehyde formation step as shown in scheme.

681 Table 2 M a x i m u m yield of 1,3-butadiene on various m e t a l oxide supported on sepiohte Catalyst sepiolite Mn/sepiolite Ni/sepiolite Co/sepiolite V/sepiolite Zrgsepiolite Cu/sepiolite

1,3-butadiene

yield/% butene isomer

C4 total

0 31 33 55 43 67 58

0 20 36 6 9 18 10

0 51 69 62 52 85 68

Table 3 Effect of poisoning by acidic and basic materials on yields of 1.3-butadiene from ethanol on manganese supported on sepiolite

poison no

EtOH conversion conversion1 initial rate wt% mmol min~ 92.5 0.102

yield wt% 54.5

1.3-butadiene initial rate mmol min~ 0.023

SB2 60

ethylene yield SEa wt% 15.4 13

phenol

82.3

0.074

46.0

0.010

46

21.9

20

CO2

97.9

0.099

45.0

0.023

52

19.8

21

pyridine 89.5 0.085 62.5 0.024 65 3.4 4 NH3 (5.3x102pa) 95.2 0.213 71.0 0.037 66 6.8 5 NH3 (1.4xl04pa) 92.6 0.048 48.0 0.011 53 10.4 8 Catalyst:0.4g; Mn content is 7 mmol per lg ofsepiolite; reaction temp.:553K; 1 reaction time:24h. 21,3-butadiene formation selectivity SB=( C4H6 yield)/(EtOH conversion). 3ethylene formation selectivity SE-(C2H4 yield)/( EtOH conversion). 3.3.3 Catalytic activity for S u z u k i reaction The

paUadium-catalysed

cross'coupling

reaction

of

aryl

ha]ides

and

phenylboronic acid (Suzuki reaction) is one of the most i m p o r t a n t methods for forming sp2-sp 2 carbon-carbon bonds both in m o d e r n synthetic c h e m i s t r y and h a s great potential for i n d u s t r i a l application [8]. _~~._ ~~_ Pd'sepi~ K2CO3 ~ R X+ B(OH)2 ~R DMF As shown in Table 4, the Pd-sepiolite catalysts showed higher activities t h a n a commercial Pd supported on carbon (Pd/C) for the Suzuki reaction of various aryl h a h d derivatives a n d phenylboronic acid. The reactivity of aryl bromide is generally lower t h a n t h a t of aryl iodide. In our study, no difference in the reactivity between aryl bromide and aryl iodide could be observed. Pd-sepiolite

682 gave slightly higher yield of the substrate involving electron withdrawing substituents than electron donating substituents. Mubofu e t al. reported that electron withdrawing groups have relatively little effect on the reaction rate and selectivity in the reactions over Pd supported on modified silica catalysts [9]. The average particle size and dispersion of Pd in the Pd-sepiolite were 1.6 nm and 67% respectively. The high activity of Pd-sepiolite for Suzuki reaction is attributed to the high dispersion and micro size Pd particles in the catalyst. Table 4 Suzuki reactions of various aryl halide derivatives catalyzed by Pd-sepiolite substrate 'catalyst Aryl h a l i d e Pd/mmol C6HsI 0.02 CBHsBr 0.02 4-NHeC6HsBr 0.02 4"CH30CBHsBr 0.02 4-CHaCOC6HsBr 0.02 4-NO2C~HsBr 0.02 C6H~Br 0.001

Temp. Time Yield /~ /h /% 100 20 80 100 20 76 100 20 77 100 20 75 100 20 81 100 8 96 130 24 61

TON 40(}0 3800 3850 3750 4050 4800 61000

4-CH30C6HsBr

0.001

130

24

48

48000

4-CHaCOC6HsBr

0.001

130

20

92

92000

4-MeOC6H6Br

0.2(Pd/C)

130

24

63

315

aryl halide:2.5 mmol; phenylboronic acid :7.5 mmol; potassium acetate :7.5 retool; the catalyst; DMF: 5 mL

References 1. A. Preisinger, "Clays & Clay Minerals" Earth Sci. Series, vol.12(1963)pp365, Pergamon Oxford. 2. H. Nagata, S. Shimoda and T. Sudo, Clays Clay Miner., 22(1974) 285. 3. Y. Kitayama, H. Katoh, T. Kodama and J. Abe, Appl. Surf.Sci., 121/122(1997) 331 4. Y. Kitayama and A. Abe, J. Chem. Soc. Jpn., 1989,1824. 5. D. E. Billing, B. J. Hathway and P. Nicholis, J. Chem. Soc. (A),1969, 316. 6. H. Niiyama, S. Morii and E. Echigoya, Bull. Chem. Soc., 45(1972)655. 7. H. A. Benesi, J. Am Chem., 78(1956)5490. 8. N. Miyaura, T. Yanagi andA. Suzuki, Synth. Commun. 11(1981)513 9. E. B. Mubofu, J. H. Clark and D. J. Macquarrie, Green Chem.,3(2001)23.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

683

Catalytic wet oxidation o f reactive dyes with H 2 0 2 over m i x e d (A1-Cu) pillared clays Sung-Chul Kim, Dul-Sun Kim, Geun-Seon Lee, Ju-Ki Kang, Dong-Keun Lee a and Young Kuk yangb aDepartment of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyungnam 660-701, Korea bKukbo Ind. Co. Ltd, Sangpyung-dong 157-5, Chinju, Kyungnam 660-340, Korea AI-Cu pillared clays were prepared by direct introduction of A1-Cu pillaring solution into the dilute bentonite suspension. AI-Cu pillared clays had d001 spacing of about 18A and had surface area of about 140m2/g or higher. A1-Cu pillared clays showed excellent activity toward the catalytic wet peroxide oxidation of reactive dyes. Complete removal of reactive dyes could be achieved within 20min at atmospheric pressure and 80 ~ which are extremely milder operation condition when compared with the conventional catalytic wet oxidation process. The pillared clays were also stable against the leaching out of Cu. I. INTRODUCTION Catalytic wet oxidation has recently been the subject of numerous investigations to reduce the amount of organic pollutants in wastewaters[1-6]. Catalytic wet oxidation(CWO) is a liquid phase oxidation of organic materials in water with oxygen(the main source of oxygen is generally air) on the surface of catalyst. Although the use of catalyst makes the reaction conditions milder than uncatalyzed wet oxidation, the catalytic wet oxidation process still requires high temperature(higher than 150~ and high pressure(1-5MPa). This severe operating conditions can lead to high installation costs, and thereby practical applications of this process are limited. The catalytic wet peroxide oxidation(CWPO) is a liquid phase catalytic oxidation with hydrogen peroxide. The catalytic wet peroxide oxidation could be a more efficient process than the catalytic wet oxidation since the oxidizing properties of hydrogen peroxide are stronger than those of molecular oxygen. Moreover, the reaction conditions when hydrogen peroxide is used as oxidant can be lowered upto 80 ~ and l atm[7], allowing the possibility to treat a large amount of polluted wastewater without a too large energy consumption. In our previous paper copper was proved to act as a catalyst to accelerate the decomposition of H202 into hydroxyl radicals(HO.)[7]. Homogeneous copper nitrate, copper chloride and copper This research was supported by Korea Ministry of Industry and Resource.

684 sulfate catalysts were extremely active for the complete oxidation of organic pollutants in water into CO2 and H20. However, additional separation process was required for the removal of copper cation before the treated water was discharged. Accordingly a new heterogeneous catalyst containing copper cation needs to be developed for the practical application of the catalytic peroxide oxidation. Pillared clays are thermally stable microporous solids which are promising catalysts in numerous areas[8,9]. Copper-containing pillared clay might be a promising catalyst for the successful wet peroxide oxidation of organic pollutants. Dyehouse effluents from the textile industry impose serious environmental problems because of their color and their high chemical oxygen demand (COD). Discharge of highly colored waste is not only aesthetically displeasing, but it also interferes with the transmission of light and upsets the biological processes which may then cause the direct destruction of aquatic communities present in the receiving stream. Of all the dyestuffs used the reactive dyes present major problems because they get hydrolyzed to the extent of 20% while dyeing textile substrates and therefore are discharged into the effluents in unrecoverable form. They possess high tinctorial power and, therefore, always exist in textile dyeing effluents, though in trace amounts. The removal of color and COD from dyehouse wastewater to meet the discharge standards is currently a major problem in the textile industry. In this study catalytic wet peroxide oxidation of reactive dyes was carded out in a batch reactor using mixed (A1-Cu) pillared clays as catalysts, and the catalytic performance of the clays was investigated. 2. EXPERIMENTAL 2.1. Materials

High purity reactive black 5 and blue 19 from Aldrich Co. were employed in this study, and the chemical structure of these dyes are shown in Figure 1. Pure bentonite(DongYang Bentonite Co.) was used as the starting clay without any further purification procedure. A1 and Cu chloride(Aldrich Co.) were used as the precursors for pillaring solution.

NaO3SOCHzCHa--~~ N=N SO~4a C) ~k~/ HO 'k/-~ H2N O tt NaO3SOCH2CI'lz~ N-N/ ~O--~~ Reactive black 5

\ SO3Na

~" I

~" O

H2

SO3Na

NH---~ \~/

~/

\ SO2CHaCHaOSO3 Reactive blue 19

Figure 1. Chemical structures of reactive black 5 and reactive blue 19.

685

2.2. Catalyst preparation A1-Cu pillared clays were prepared by direct introduction of AI-Cu pillaring solution into the dilute bentonite suspension. The pillaring solutions were prepared by dissolving 0.1M A1 and Cu chlorides in 0.2M NaOH solution. The hydrolysis molar ratio OH/(AI+Cu) was kept to be 2. Solution volumes were adjusted to have a Cu/(AI+Cu) ratio between 0% and 20%. The pH of the solution was about 3.8 and the solutions remained clear. The dilute bentonite suspension(l%w/w) was prepared by adding the purified bentonite powder into the corresponding distilled deionized water. The pillaring reaction was carried out under continuous vigorous stirring at 40 ~ by adding the pillaring solution drop by drop into the bentonite suspension. The prepared samples were filtered and washed with deionized water until Cl-free samples were obtained, and then dried at 120 ~ for 6h. The dried samples were finally calcined at 400 ~ for 6h.

2.3. Reaction procedures and analysis The oxidation of two reactive dye aqueous solution was performed in a glass reactor of 1L capacity equipped with a condenser, stirrer and air flow controller. The reaction were conducted at atmospheric pressure and 80 ~ Air was bubbled into the solution during the reaction, and the flow rate of air was kept to be 200mL/min. Liquid sample were immediately filtered and analyzed for total organic carbon(TOC), hydroxyl radical(HO.), color unit and residual materials in water. TOC was measured with a Shimadzu 5000A TOC analyzer. Electron paramagnetic resonance(EPR) spin trapping of the HO" formed during the catalytic wet oxidation was performed using a Varian E-4 spectrometer. 5,5-Dimethyl-l-pyrroline N-oxide(DMPO, purchased from Aldrich Co.) was used as a trapping agent because it efficiently scavenges HO" through the following reaction to produce the DMPO/HO- adduct, which has a characteristic EPR spectrum[7,10].

MeMe~-~H + I

O-

140.

=

Me H Ma~,.N[-N--~OH t

O-

Color unit of the samples was measured by following ADMI(American Dye Manufacture Institute) tristimulus filter method[l 1], and H202 concentration was measured by a colorimetric method using a UV/Visible DMS 90 Varian spectrophotometer[12]. X-ray diffraction(XRD) patterns of the A1-Cu pillared clay were recorded at a scanning rate of 1~ 20/min with a Simens D-5000 diffractometer(CuKtx radiation). Surface area was determined by using nitrogen as the sorbate at 77K in a static volumetric apparatus(Micromeritics ASAP2010). 3. RESULTS AND DISCUSSION

3.1. Characterization of the catalyst X-ray diffraction patterns of the starting bentonite and A1-Cu pillared clays are shown in Figure 2. The prepared pillared clays will be abbreviated by the symbols of A1-PILC and Al(mole% in the pillaring solution)-Cu(mole% in the pillaring solution)-PILC. A1-PILC

686 denotes the pillared clay with alumina. AI(90)-Cu(10)-PILC is, for example, the AI-Cu pillared clay prepared by the initial pillaring (e) solution having the 90mole% AI and 10mole% eu, respectively. The 20 angle of the (001) (d) reflection of the pure bentonite was 7.5 ~ which corresponded to a d(c) spacing of 11.77 A. The corresponding 20 angles of the (hk) two-dimensional peaks were at 19.6 ~ (b) and 35.3 ~. The diffraction at 20 of 19.6 ~ was the summation of hk indices of (02) and (01), and the diffraction of 35.3 ~ was the summation of hk indices of (13) and I , I ~ I ~ I , , I (20). The peak at 20 of 27.9 ~ was a 50 0 I0 20 30 40 2O reflection of the quartz impurity[9]. Upon pillaring with AI and Cu the Figure 2. XRD patterns of bentonite(a), A1d001 peak shifted lower 20 values of PILe(b), Al(95)-Cu(5)-PILC(c), Al(90)-eu(10)PILC(d),AI(80)-Cu(20)-PILC(e). about 4.9 ~ corresponding to the increase in the d0o~ spacing, while the rest of the structure was not clearly affected. The d00~ spacing of the pillared clays was about 18.0A. In Table 1 are listed the summarized properties of the AI-Cu pillared clays. BET surface area increased significantly after intercalation.

3.2. Catalytic wet peroxide oxidation of reactive dyes To assess the extent of uncatalyzed oxidation of reactive black 5 solution, wet oxidation was performed without any catalyst and H202 at atmospheric pressure and 80"C. The initial concentration of reactive black 5 solution was 1,000mg/L. As can be seen in Figure 3, no detectable extent of the uncatalyzed oxidation of the reactive black 5 solution could be achieved. Even in the presence of 10g Ai(90)-Cu(10)-PILe the reaction did not proceed at all. The addition of H202(20mL, 0.5N), however, enhanced the efficiency of the oxidation remarkably. Most of reactive black 5 could successfully be oxidized within 20min. Table 1. Summarized prc ~erties of A1-Cu pillared clays d001(A) surface area(m2/g) Clay Bentonite AI-PILC AI(95)-Cu(5)-PILC AI(90)-Cu(10)-PILe Al(85)-Cu(15)-PILe AI(g0)-Cu(20)-PILC

11.77 17.02 18.1 18.0 18.0 18.0

9

33.2 142.3 164.5 149.4 146.8 142.9

Cu(%) 0.80 1.12 1.71 2.50

687 100

100 .=.-

80 A

(.,) 0 I..-

lOO

8o

8o

20

20

60

- i

40 20 =

5

10

15

20

25

_~ 30

Time(min)

Figure 3. Changes in TOC during the oxidation of reactive black 5 solution (O:uncatalyzed oxidation(without catalyst and H202), A:catalytic wet oxidation(with 10g AI(90)-Cu(10)-PILC in the absence of H202), ==:catalytic wet peroxide oxidation (with 10g AI(90)-Cu(10)-PILC and H202).

I

0

0

5

10

15

20

25

,0 30

Time(rain)

Figure 4. Correlation between TOC removal(O), H20~ consumption(A) and HO. formation(ll) during the catalytic wet peroxide oxidation of reactive black 5 with 10g AI(90)-Cu(10)-PILC.

The addition of H202 to wet oxidation systems has been known to enhance the reaction rate leading to high conversion in short time[13]. The fast reaction rate of the catalytic wet oxidation with H202 as opposed to the uncatalyzed oxidation and the catalytic wet oxidation with air is due to the decomposition of H202 to give two hydroxyl radicals which react with reactive black 5 in water. In Figure 4 are shown the removal of TOC together with the concentration of H202 consumed and HO. produced during the reaction with reactive black 5 in the presence of 10g AI(90)-Cu(10)-PILC. The removal of TOC was shown to be strongly related to the consumption of H202 which will be decomposed into HO.. A separate experiment of H202 decomposition in the absence of any reactive black 5 was carried out at the same reaction condition. The concentration of H202 was the same as that in the experiment of Figure 4. The measured changes in the concentration of H202 and HO. are plotted in Figure 5. As seen, in accordance with the consumption of H202 the formation of HO- occurs during the reaction. The rates of both the H202 consumption and HO. production increased greatly by the action of AI(90)-Cu(10)-PILC which must have played an important role on the activation of H202 decomposition and the subsequent HO. formation. The subtracted amount of HO., corresponding to the difference between HO- formed in Figure 5 and HO. remained in Figure 4 must have participated in the oxidation of reactive black 5 in water. As discussed from the result in Figure 5, AI(90)-Cu(10)-PILC catalyst could increase the production rate of HO" greatly. This indicates that the use of a catalyst will further enhance the rate of oxidation of reactive black 5 solution. Figure 6 shows a comparison between the results of wet peroxide oxidation with A1-Cu pillared clays having different amount of Cu.

688 lOO -

2 ~

_m

80

ttl

80

=~

60

-,-.m

0 e~ 40 "I" 2O 00

lOO

n I

-

C

m

5

10

15

20

Time(rain)

25

30

Figure 5. Time dependence of H 2 0 2 conversion and HO" formation during H 2 0 2 decomposition in t h e absence o f the catalyst(O) and in the presence of 10g Al(90)-Cu(10)-PILC(A).

2O 0

0

5

10

15

20

25

30

Time(rain)

Figure 6. Effects of Cu content in the AICu pillared clays on the removal of TOC(O:A1-PILC, A:AI(95)-Cu(5)-PILC, I:Al(90)-Cu(10)-PILC, V: AI(85)-Cu(15)PILC, . : Al(80)-Cu(20)-PILC).

There was a considerable increase in the reaction rate by using A1-Cu pillared clays instead of A1-PILC. About 14% removal of TOC was achieved in 30min with the A1-PILC, while in the presence of the A1-Cu pillared clays it took only about 20min for the complete removal of TOC. In addition the removal efficiency of TOC increased with increasing amount of Cu in the AI-Cu PILCs. Catalytic wet oxidation of reactive blue 19 was also conducted in the presence of 10g AICu pillared clay catalysts, 20mL 0.5N H202 solution. The initial concentration of reactive blue 19 solution was 1,000mg/L. As shown in 100 Figure 7, reactive blue 19 could successfully be removed by using AI-Cu pillared clay as 80 catalysts. Figure 8 represents the results of TOC and =~ 60 color unit change during catalytic wet 0 oxidation of reactive black 5 and reactive blue o k40 19 at five different initial H202 concentration in the presence of 10g AI(90)-Cu(10)-PILC. 20 When the H202 dosage was 5mL, the final TOC values were not significantly different 0 from the initial values, while the color units 0 5 10 i5 20 25 30 have dropped by about 60%. This means that Time (min) the consumption of H 2 0 2 does not Figure 7. Effects of Cu content in the A1- immediately mineralize the organics in the Cu pillared clays on the removal of reactive dye solution. Most of the organic TOC((O:A1-PILC, A:Al(95)-Cu(5)-PILC, carbons remain in the solution, but the newly ll:Al(90)-Cu(10)-PILC, V:AI(85)-Cu(15) formed organics have lower color unit per molecule. In addition both the removal of -PILC, O:AI(80)-Cu(20)-PILC). TOC and color unit was completed within A

689 100

=

=

=

._.

=

~100

100

100 It.

80

80

80

"e ";&'It

~

60

60

40

40

60

I-.

40

_o

0

0 I-

A

=~

00

"-

9 ".. 9

20

0

5

10

15

20

Time (min)

25

30

0

"'o.. " "~

: $..

20

"'.

20

""...

"-

" " " "O.

.

s

" " * .....

40

.... " . . . . .

20

-... .&.

":~

0

60

""0.

1~

i% 2~

v

2s

_o o

r

,,~0

3o

Time (min)

(A) (B) Figure 8. Effects of concentration on the removal of TOC(--) and color(---) during the catalytic wet oxidation of reactive black 5(A) and reactive blue 19(B) with 5mL(O), 10mL(A), 15mL(n), 20mL(V) and 30mL(~) of 0.5N 1-1202solution. 20min when the dosage of H202 w a s more than 20mL. These behaviors suggest that the oxidation proceeded in more than one step. The first step involves the breakdown of the large dye molecules into smaller molecules of intermediate organics. The next step will be the degradation of the smaller molecules into carbon dioxide and water. The residual organics remaining in the solution during the reaction were analyzed and identified with a HPLC-Mass spectrometer. The main organics were lower molecular weight carboxylic acids such as acetic acid, maleic acid, fumaric acid, oxalic acid and formic acid. Acetic acid and maleic acid are known to be highly refractory materials, and the oxidation of these carboxylic acids was proved to be the rate-controlling steps in the conventional catalytic wet oxidation with air[6,14,15].

3.3 Catalytic wet oxidation of a real dyehouse effluent A real effluent, produced from the washing process of a certain dyeing industry, was employed for the catalytic wet oxidation with 10g AI(80)-Cu(20)-PILC and 20mL 0.5N H202 solution. In order for dyeing textile substrates the industry had used the aqueous solutions of reactive black 5, reactive blue 19 and reactive red 198. In addition small amount of some penetrating agent together with NaOH were contained in the effluent. The dark black reddish effluent had TOC value of 6,900mg/L and its color unit was 5,200. Figure 9 shows the time dependence of the removal of TOC and color. Most of the TOC and color unit were removed after 30min reaction. The visual appearance of the effluent changed greatly during the reaction. The dark black reddish color began to disappear, and was remained strong red color which became weaker and weaker. After 30min reaction the weak red color was completely discolored. 3.4 Stability of the catalyst During the catalytic wet peroxide oxidation the active component Cu might be leached out from the A1-Cu pillared clays. To investigate the stability of the AI-Cu pillared clays with respect to metal leaching, the concentrations of dissolved Cu and A1 in the solution were analyzed using ICP. NO detectable amount of dissolved Cu and A1 could be measured.

690 7000 5OOO

60O0

4000

50OO

Accordingly at the reaction conditions employed in this research no leaching of Cu and A1 can be said to occur. 4. CONCLUSIONS

2000

Q

2000

IOO0

1000 0

0

5

10

15

20

Time/mint

25

30

0

Figure 9. Removal of TOC(--) and color(---) during the catalytic wet oxidation of a real dyehouse effluent with 10g AI(80)-Cu(20)PILC.

A1-Cu pillared clays were prepared by direct introduction of A1-Cu pillaring solution into the dilute bentonite suspension, and the prepared AI-Cu pillared clays were employed as catalysts for the wet peroxide oxidation of reactive dyes at atmospheric pressure and 80 ~ The pillaring reaction for the preparation of A1-Cu pillared clays was performed at 40~ with OH/(AI+Cu) molar ratio of 2 and at Cu/(AI+Cu) ratio

of 0, 5, 10 and 20%, respectively. Upon intercalation the d00~ spacing increased from 11.77A to 18.0A. In addition the surface area of the pillared samples also increased from 33.2m2/g of the pure bentonite upto 164.5m2/g. AI-Cu pillared clays were proved to act as excellent catalysts for the wet peroxide oxidation of reactive dyes. The initial 1,000mg/L reactive dyes could be completely removed in just 20min with 10g AI(90)-Cu(10)-PILC at atmospheric pressure and 80 ~ The catalysts were also extremely stable against the leaching out of active Cu component into the aqueous solution. REFERENCES

1. E Luck, Catal. Today, 27 (1996) 195. 2. J. Levee and A. Pintar, Catal. Today, 24 (1995) 51. 3. A. Pintar and J. Levee, Catal., 135 (1992) 345. 4. P. Gallezot, N. Laurin and P. Isnard, Appl. Catal. B, 9 (1996) L 11. 5. D. Duprez, F. Delanoe, Jr. J. Barbier and P. Isnard, G. Blanchard, Catal. Today, 29 (1996) 317. 6. D.-K. Lee and D.-S. Kim, Catal. Today, 63 (2000) 249. 7. D.-K. Lee, D.-S. Kim and S.-C. Kim, Stud. Surf. Sci. Catal., 133 (2001) 297. 8. A. Vaccari, Catal. Today, 41 (1998) 53. 9. R.T. Yang, N. Tharappiwattananon and R.Q. Long, Appl. Catal. B, 19 (1998) 289. 10. E.P. Sargent and E.M. Grady, Can. J. Chem., 54 (1976) 275. 11. W. Allen, W.B. Prescott, R.E. Derby, C.E. Garland, J.M. Peret and M. Saltzman, Proc. 28 ~ Ind. Waste. Conf., 142 (1973) 661. 12. G.M. Eisenberg, Ind. Eng. Chem., 15 (1943) 327. 13. S.H. Lin and Y.F. Wu, Environ. Technol., 17 (1996) 175. 14. H.R. Devlin and I. Harris, J. Ind. Eng. Chem. Fundam., 23 (1984) 387. 15. L. Li, P. Chem and E.F. Gloyna, AICHE J., 37 (1991) 1687.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

691

Application of zeolites as supports for catalysts of the ethylene and propylene polymerization I.N.Meshkova, T.A.Ladygina, T.M.Ushakova, N.Yu.Kovaleva, L.A.Novokshonova Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin St.4, 119 991 Moscow, [email protected]

The Na-form of zeolite ZSM-5 (SIO2/A1203=24) was studied as a support for organometallic catalysts of olefin polymerization. Zeolite-fixed aluminoxanes were prepared by partial hydr~ ,ys~s of trimethylaluminium with the zeolite's internal water.It was shown that aluminoxanes synthesized on the zeolite surface form heterogenized complexes with CpzZrC12 and Et[Ind]zZrC12 which are active for a long time in ethylene and propylene polymerization without addition of another aluminiumorganic cocatalyst. Yields were up to 1200kgPE/molZr.bar.h. and 4000kgPP/molZr.bar.h.The activation energy of ethylene and propylene polymerization in the presence of Na-ZSM-5(HzO)/A1Me3-Et[Ind]2 ZrCI2 is equal to 32 and 48,5 kJ/mol, respectively. Molecular weight and melting point of polyethylene obtained with such zeolite supported Zr-cene catalysts are higher than those of polyethylene formed with the corresponding homogeneous metallocene systems.

1. INTRODUCTION Immobilization of organometallic catalysts for olefin polymerization arose as a challenge immediately after they have been discovered and continues to be of particular significance. Most publicati, ,s in this field are focused on catalysts supported on the inorganic carrierssilica and alumina. The first works involving zeolites as supports appeared as recently as the mid-1990s [1-4]. All methods used for heterogenization of soluble metallocene catalysts on zeolite require the introduction of the external methylaluminoxane (MAO) or AIR3 to the supported catalyst for its activation in olefin polymerization [ 1,2,4]. In the present paper we report the application of zeolites as supports for metallocene catalysts for ethylene and propylene polymerization. Earlier we developed the method of metalorganic catalysts preparation on the surface of hydrated supports [5,6]. By this method fixed alkylaluminoxanes were formed on the surface of undehydrated Na-ZSM-5 zeolite by partial hydrolysis of trimethylaluminium (TMA) with intemal zeolite's water and were used for the heterogenization of Zr-cene (CpzZrC12, Et[Ind]2ZrC12) complexes. The kinetics of ethylene and propylene polymerization on Na-ZSM-5(HzO)/TMA-Zr-cenes systems were investigated and the properties of PE and PP obtained with these zeolite supported Zr-cene catalysts and appropriate homogeneous systems were compared.

692 2. EXPERIMENTAL SECTION

2.1. Materials The Na-form "f ZSM-5 had the following characteristics: SiO2/A1203 =24, total volume of pores was equal to 0,287 cm 3 / g, surface area determined with the use of benzene to 40 m 2/g, the inside water content - to 8,9wt.%. Components of catalysts: Cp2ZrC12 and Et[Ind]2 ZrC12 were used as received from Aldrich Co. TMA (from GNIIHTEOS, Moscow) and MAO (10 wt% in toluene solution, from Aldrich Co) were used without further purification. TMA contained 37,2wt% of A1 and 60,6wt% of Me. Toluene ( from Aldrich Co) was used as a solvent. Ethylene and propylene were of polymerization-grade purity. 2.2. Preparation of supported catalysts The preparation of zeolite supported catalysts involved two steps : the preparation of the zeolite-fixed methylaluminoxanes and the formation of heterogenized complexes from the zirconocene and fixed alkylaluminoxanes. The synthesis of fixed methylaluminoxanes and the preparation of zeolite supported Zr-cene catalysts were realized in a 0,4 L glass reactor equipped with stirrer and water jacket for thermostating. The reactor was evacuated and then filled with argo'-. Zeolite (1,3 g) containing 8,9 wt% of internal water was introduced into the reactor. After the removal of argon from the reactor the zeolite was suspended in 30 cm 3 of toluene. TMA (43 wt% solution in toluene) was added dropwise to the uncalcinated zeolite over one hour. The reaction of TMA with the zeolite water was carried out at 22 ~ with intensive stirring of components. The end of partial hydrolysis of TMA by the zeolite water was determined by the completion of the gaseous product (methane) evolution. After removal of methane from reaction zone 30 cm 3 of a toluene solution of the zirconocene (Cp2ZrC12 or of Et[Ind]2 ZrCI2) was added. The Zr content in catalyst was varied in the range of 7.10 .6 - 3.10 -3g/g zeolite. Molar ratio of A1/Zr was changed from 140:1 to 5000:1. 2.3. Polymerization of ethylene and propylene Slurry ethylene and propylene polymerization was carried out in the same reactor where the catalytic complex had been prepared. The ethylene was introduced into the system immediately after Zr-cene addition.The monomer concentration, temperature and rate of stirring were kept constant during a polymerization run. 2.4. Characteristics of PE and PP The molecular-weight characteristics of PE samples were measured by GPC (Waters,150C) in ortho-dichlorobenzene at 140 ~ C. The melting point (m.p.) and crystallinity(~) of polymer products were determined on a Dupont differential scanning calorimeter (DSC 910), with a heating rate of 10 K. min l from295 to 425 K. The PP isotacticity was determined by the Luongo method [8].

3. RESULTS and DISCUSSION A peculiarity of zeolites is a high mobility of the inside zeolite water. In contrast to the structure water of the other highly hydrated supports, for example, aluminium hydroxide and

693 Table 1. The consumption of AOC in dehydration of supports and the yield of gaseous products

H20 in

jg_o

3 4

Supw'rt

support, wt.%

Uncalcinated zeolite Zeolite heated at 300~ Uncalcinated kaolin Uncalcinated AI(OH)3

AOC

AOC consumption, mmol/g

RH

Yield of RH, mmol/g

8,9

A1Me3

5,4

CH4

5,8

5,4

AIMe3

2,2

CH4

2,25

13,8

A1Et2C1

0,1

C2H6

0,1

36,4

A1Et2C1

0,08

C2H6

0,08

kaolin, the zeolite water is completely removed by the heating up to 800- 1000~ without the zeolite structure destruction and can be readsorbed [7]. It was shown that in the case of zeolite aluminiumalkyls react both with water adsorbed on the external surface of zeolite and with water in its channels. As is seen ~rom Table 1 data the consumption of aluminiumorganic compound (AOC) in the dehydration process of zeolites as well as the gas (RH) evolution accompaining this process are markedly higher (Table 1, runs 1,2) than in chemical dehydration of other inorganic supports by AOC (Table 1, runs 3,4). We compared the sorption properties of zeolite ( Na-ZSM - 5, previously heated in vacuo at 300 ~ with respect to TMA and water. According to the isotherms of adsorption (Fig.l, curve l) the gaseous TMA does not penetrate into the micropores of the zeolite contrary to water (Fig. 1, curves 2,3). On the base of these sorption data we propose that the fixed methylaluminoxanes from TMA and zeolite's water and the Zr-cene complexes with fixed methylaluminoxanes can not be formed inside of the zeolite structure.In reaction with TMA the zeolite's water migrates from channels of zeolite and the formation of fixed aluminoxanes and then of complexes with Zr-cene takes place on the external zeolite surface.

o~,mmo! '~, zv

10 8 ~2 6 24~_y;_ L__~-L- ~ - ....} 2 --1 0

0

0,2

0,4

0,6 P/Ps

0,8

,

o

9

1

Fig. 1. Isotherms of adsorption at 22 ~ C of TMA (1), adsorption (2) and desorption (3) of H20 for Na-ZSM-5 zeolite heated in vacuo at 300~ C.

694

,400

............................. a;

Rp, k g P E / m o l Z r bar h

............

1000

i

6OO

-

---~--"'

--

I

200

,

---

Fig.2. Rate-time profiles for ethylene polymerization with zeolite supported Zrcene catalysts on the base of Cp2ZrC12 (a) and Et[Ind]2ZrC12 (b).

....

1

_--~__._....~

...................................................

-3- .........

0 0

20

40

60

time, min

Rp, k g P E / m o l Z r

bar h

1200

800

b) .

.

.

.

1

400 ,,

0

.

.

,

100 time,

3~~

~

~

50

b) Na-ZSM-5(H20)/TMA/Et[Ind]2ZrC12; toluene; 0,23 bar; 0,05 wt.% Zr; [A1]/[Zr]=4300; Ypo~.,~ 34 (1), 50 (2), 70(3)

-:

t 0

~o_o

a) Na-ZSM-5(H20)/TMA/Cp2ZrC12 (1,2,3); homogeneous Cp2ZrC12-MAO catalyst (4); toluene; 0,46 bar; Tpol. = 34~ Zr, wt.%: 0,024 (1), 0,028 (2), 0,16 (3); [Zr] x 105 , mol/l: 14 (4); [AOC] x 103 , mol/l" 68(1), 60 (2), 90 (3). 70 (4).

150

,

'

--~ 200

min

The high content of inside zeolite water (about 9 wt.%) and its mobility permit to form aluminoxanes on the zeolite surface in the amount enough for activation of Zr-cene component of catalyst. The systems Na-ZSM-5(HzO)/TMA/CpzZrCI2 and Na-ZSM5(HzO)/TMA-Et[Ind]2 ZrCI2 are active in the ethylene polymerization without addition of other cocatalysts for a long time (more than 100 hours). Yields were up to 1200 kgPE/molZr bar h. and 4000 kgPP/mol Zr.bar.h. (Fig.2a,b and Table 2). By this means the zeolite supported Zr-cc;,e catalysts put forward by us are significantly different from known catalysts [1,2,4,9] obtained by adsorption of zirconium compound on zeolite calcined and treated by MAO. These zeolite/MAO/Cp2ZrCl2 catalysts [2] are inactive in ethylene polymerization.They are activated only by addition of free MAO into the reaction zone. The kinetic .regularities of ethylene and propylene polymerization by our zeolite supported Zr-cene catalysts (the polymerization rate dependences of Zr-cene and monomer concentration and of temperature) are the same as for other immobilized catalysts for olefin polymerization.

Table2. Ethylene and propylene polymerization with the zeolite supported and homogeneous Zr-cene catalysts.

Run

Catalyst

i

molar

Mw

Aa

Mw/Mn

m.p.2

OC

mol/l

hb, %

Iiso c,

%

-

111

3.007 4600 1650 320 3.024 C2H4 470 3.160 0.140*

0.058 0.058 0.056 0,056

34 34 34 34

82 475 500 1500

202750 273800 52900 133700

22 10 18 4.8

133.5 135 123.5 127.5

85 85 76 75

5.

IV

0.018

0.025

75

1200

135700

30

127

81

6. 7. 8. 9. 10.

IV IV IV IV V

0.030 0.015 0.016 0.018 0.026*

30 40 62 75 75

230 48500 4200 43000 1155 3600** 3160 1500** 6600 2O0Ob*

18.6 8.8

135 123 waxes waxes waxes

63 50

1. 2. 3. 4.

I I I1

4300

I 840 4350 4100 5000 1725

C3H6

0.327 0.260 0.119 0.125 0.125

695

Clatalyst: I- ZSM-S(H20)/TMA/Cp2ZrC12 ; 11- ZSM-S(H20)/TMA/Cp2ZrClz + MAO; 111- CpzZrClz -MAO; IV- ZSM-5(H20)/TMA/Et[Ind]2ZrClz ;V- Et[Ind]zZrCl2 +MAO. 1- Activity of catalyst, kg polymer/ mol Zr.bar.hour; 2- The polymer crystallinity; c- The PP isotacticity [lo]. li- [Zr], molil; **- Mn.

-

88 82 40 10 15

696 Log Rp/IM]

Fig.3. Log(Rp / [M]) vs.1/T with Na- ZSM-5 (H20)/TMA/Et[Ind]2ZrC12 catalyst. Monomer: ethylene (1), propylene (2).

4.00 3.50 3.00 2.50 2.00 2.80

-

I

-

3.00

3.20

3.40

I/T103, K -1

The ethylene and propylene polymerization rates (Rp) with this zeolite supported catalyst increase with increasing temperature in the range of 30-75~ (Fig.2b,3). The energy of activation of ethylene polymerization is equal to 32 kJ/mol, in the case of propylene polymerization - 48,5 kJ/mol. At the same time it is known that the rate of ethylene polymerization in the presence of homogeneous Zr-cene systems reaches a maximum value in the range of 45 - 50 o C and decreases with a further increase of temperature [ 10]. The specific rate of ethylene polymerization increases with the enhancement of the Zrcene content in Na-ZSM-5(H20)/TMA/Cp2ZrC12 catalyst. It peaks at the surface Zr-cene conzentration enue i to about 3.10 .6 mol Zr/g zeolite and then Rp decreases. The complex dependences of Rp of ethylene and propylene polymerization on monomer concentration .were observed. The olefin concentration dependences of Rp have the fractional order between 1 and 2. That is two olefin molecules are involved in the insertion transition state [ 11 ]. The PE and PP properties obtained with Na-ZSM-5(HzO)/TMA/Zr-cene catalysts are presented in the Table 2. It is seen that the molecular weight, molecular weight distribution (Mw/Mn ratio); the melting point and crystallinity of polyethylene formed with zeolite supported catalyst are higher than those of polyethylene obtained with homogeneous CpzZrCI2 - MAO catalyst (Table 2, runs 1,2 and 4). The tendency for an increase of molecular weight and the melting point of PE in the case of catalyst immobilization on the support is a typical for metallocomplex catalysis.The PP properties obtained with zeolite supported and homogeneous Zr-cene catalysts are closely related (Table 2, run 9 and 10). The external MAO addition to Na-ZSM-5(H20)/TMA/Cp2ZrC12 catalyst increases its activity but decreases the molecular weight and melting point of polyethylene formed (Table2, run3). It is possible that part of active centers of the zeolite supported catalyst transfer to solution from the surface upon introduction of a free MAO into the reaction zone.

4. CONCLUSION The use of heterogenized Zr-cene catalysts based on fixed aluminoxanes as a product of partial hydrolysis of TMA with internal zeolite's water, allows to replace unstable cocatalyst

697 MAO by TMA and avoids the addition of free MAO for formation of the zeolite supported Zr-cene complexes. We suppose that the synthesis of MAO directly on the zeolite support and the absence ot free MAO may be one of the ways to the reduction of supported Zr-cene catalyst leaching. In this case the appearance of homogeneous active centres in the reaction zone is less possible. The positive temperature coefficiency of polymerization rate as well as the increase of molecular weight and melting point of PE obtained with the zeolite supported Zr-cene catalyst developed in this work, compared to PE produced by the homogeneous Zr-cene system confirms this view.

REFERENCES

1. Ciardelli F., Altomare A. Conti G.,//Macromol. Symp. 1994. 8__00.P.29. 2. Woo S.I,Ko Y.S., Han T.K//Macromol. Rapid. Commun. 1995.16. P.489. 3. MeshkovaI.N.,UshakovaT.M.,GuruliN.T.,GultsevaN.M., Novokshonova L.A.// Intern.Symp. "ZEOLITE-95"Sofia,Bulgaria. 1995.P.238 4. Marques M.F.v.,Coutinho F.M.B.//36 Intern.Syrup. Macromolecules.Seoul, Korea. 1996. 5. USSR Invei;tor's Cetificate 1066193//(1982).Byul.Izobr. 1985. P. 199. 6. Meshkova I.N.,Ushakova T.M., Dyachkovskii F.S.//31 IUPAC Macromolecular Symp. Merseburg. GDR. 1987.P. 100. 7. Dahl J.M., Jens K.J.//Catal. Today. 1992.13. P.345 8. Luongo J.P.//J. Appl. Polym. Sci. 1960.3. P.302. 9. Michelotti M., Arribas G., Bronco S., Altomare A.,//J.Mol. Catal. A: Chemical. 2000.152. P.167. 10. Giannnetti E., Nicolletti G.M. and Mazzochi R. J.Polym.Sci. Part A" Polym. Chem. 1985.23.P.2117 11. Ystenes M.//J. Catal. 1991. 129. P.383.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. GiordanoandF. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

699

Catalytic properties of Beta zeolite exchanged with Pd and Fe for toluene total oxidation J. Jacquemin, S. Siffert*, J.-F. Lamonier, E. Zhilinskaya and A. Abouka'fs Laboratoire de Catalyse et Environnement, E.A. 2598, Universit6 du Littoral - C6te d'Opale, MREID, 145, avenue M. Schumann, 59140 Dunkerque, France

Dealuminated beta zeolites exchanged with Pd and Fe were prepared to investigate the influence of iron and dealumination on the activity and selectivity of Pd/BEA zeolite for toluene total oxidation. The specific areas determined by BET method and EPR studies allowed to know that the palladium would be more easily agglomerated on the BEA than on the DBEA. Moreover, a quantification of the palladium saturation on the BEA zeolite was deduced by EPR. Effects of dynamic and static oxidation and weak and strong reduction treatments were studied by EPR. Several isolated and interacted Pd + species and hole centers were detected. The Pd was much reduced after the catalytic test in dealuminated and Fe doped samples. This result could be directly correlated to the catalytic deactivation. The deactivation could be also explain by the type of coke deposed on the catalyst and by the hydroscopic behavior of the samples. Addition of Fe or dealumination could prevent the deactivation and then lead to better catalysts for VOCs oxidation. 1. INTRODUCTION Volatile Organic Compounds (VOCs) in industrial gas represent a serious environmental problem. An effective way of removal is complete catalytic oxidation to harmless products such as H20 and CO2. In order to make the reaction economically attractive, highly active catalysts at low temperatures are required. Supported precious metals such as Pt and Pd are well established as efficient catalysts for VOC combustion [1 ] and palladium is cheaper and often more active for oxidation than platinum [2]. Moreover, iron is often used for catalytic oxidation of VOCs [3] and could imply special properties of the catalyst for deactivation [4]. However, the support is also very important for the efficiency of the catalyst. Zeolites have a good potential for VOC adsorption but the formation of water during the combustion could be a deactivation agent [5]. Therefore, in this paper, we prepared dealuminated beta zeolites exchanged with Pd and Fe for a good activity and low deactivation. Toluene, which is often found in industrial exhausts, has been chosen as probe molecule for the catalytic oxidation test. The objective of this work is to investigate the influence of iron and dealumination on the activity and selectivity of 0.5wt%Pd/BEA zeolite for toluene total oxidation. The palladium valence was also studied for this reaction.

700 2. EXPERIMENTAL 2.1. Catalyst preparation Na-Beta zeolite (Si/AI=10, from P.Q. Corporation, called BEA) was dealuminated by HC1 (0.2M, 3h, 80~ and exchanged with NaC1 (1M, 2h, 60~ 7 times) to obtain the solid called DBEA (Si/Al=66). The samples were exchanged by palladium (18h, 60~ to have 0.Swt%Pd/DBEA. The exchange by palladium was carried with two different complexes: Pd(NO3)2 for all the catalysts and Pd(NH3)4C12 only for the preparation of the solid called 0.5wt%Pd(2)/BEA to compare the influence of the both solids for the VOC oxidation. A second exchange was done by iron (Fe(NO3)2, 24h, 25~ to have 0.2wt%Fe,0.5wt%Pd/DBEA. [3-zeolite was also exchanged in the same ways without previous dealumination (samples called .../BEA). 2.2. Catalyst characterization The XRD patterns were obtained at room temperature with a Siemens D5000 diffractometer using Cu-K~ radiation. Chemical elementary analysis of the samples were obtained by the research centre of Vernaison (France, CNRS). Thermal analysis measurements were performed using a Netzsch STA 409 equipped with a microbalance. The sample (40 mg) was treated under a flow of air (75 mL.min -1) and the temperature was raised at a rate of 5~ min ~ up to 1000~ The specific areas of solids are determined by the BET method using a Quantasorb Junior apparatus, and the gas adsorbed at -196~ is pure nitrogen. The electron paramagnetic resonance (EPR) measurements are performed at 20~ on a EMX Bruker spectrometer. A cavity operating with a frequency of 9.3 GHz (X-band) is used. The magnetic field is modulated at 100 kHz. Precise g values are determined from precise frequency and magnetic field values. Reduction studies of the catalysts followed by EPR were carried on a vacuum ramp with 5 vol%H2 in N2 or pure Ha. For the FTIR studies, self-supported wafers were pressed from 25 mg of each sample at a pressure of about 1 ton/cm 2 and then placed in a sample holder inside a Pyrex cell with NaC1 windows, which allowed the pre-treatment of the samples (300~ in vaccum < 10-6 Torr for 2 h), the introduction of toluene (1 gL (9.4.10 .6 mol) to 30 gL (28.2.10 -s tool)) and the recording of the spectra. The IR spectra were recorded after adsorption-desorption equilibrium of toluene using a Fourier Transform Perkin-Elmer Spectrometer Spectrum 2000 at room temperature. 2.3. Activity tests The samples were tested in the total oxidation of toluene for 24h on stream. Before the catalytic test, the solid (200 mg) was calcined under a flow of air (4 L.h l) at 500~ (2~ 1) during 4 hours. Toluene oxidation was carried out in a flow microreactor and studied at about 250~ Temperature was controlled at the intemal oven surface and in the catalytic bed. Toluene was mixed with a flash-injector in a flow of air (99 mL/min) adjusted by a mass flow controller. The space velocity was about 50000 h 1. The analysis of combustion products was performed evaluating the toluene conversion and the CO/(CO + CO2) molar ratio from a Perkin Elmer autosystem chromatograph equipped with TCD and FID.

701 Table 1 Loss of BET specific areas after Pd and Fe exchanges on BEA and DBEA (mE/g) 0.0 lwt%Pd 0.05wt%Pd 0.Swt%Pd 0.5wt%Pd + 0.2wt%Fe BEA DBEA

52 8

64 22

112 29

128

3. RESULTS AND DISCUSSION

XRD patterns of the pure BEA and the dealuminated DBEA are similar. The cristallinity of the ]3-zeolite is then not altered by dealumination. The thermal behaviour and the decomposition of the palladium complexes (Pal(NO3)2 and Pd(NH3)4C12) of the samples 0.Swt%Pd/BEA, 0.Swt%Pd/DBEA and 0.5wt%Pd2/BEA are followed by thermal analysis and the TGA-DTA curves displayed two endothermic transitions for the all these samples. The first one corresponds to the loss of water at about 100~ The samples 0.5wt%Pd/BEA and 0.Swt%Pd(2)/BEA present a water loss of about 8 % whereas the sample 0.Swt%Pd/DBEA showed only 6 % water loss. This difference is due to dealumination which implies a lower hydrophilic character and should led to less deactivation of the catalyst during oxidation of VOCs [5]. The second endothermic transition around 230~ and 320~ for 0.5wt%Pd/BEA and 0.5wt%Pd(2)/BEA is due to the decomposition of the Pd complexes, respectively, Pd(NO3)2 and Pd(NH3)4CI2. The samples BEA and DBEA present important specific areas of, respectively, about 595 and 653 m2/g. The losses of BET specific areas after Pd and Fe exchanges are displayed in Table 1. The decrease of surface area after Pd exchange is always higher for BEA than for DBEA. More agglomeration of Pd on BEA than on DBEA could explain this tendency. This result will be correlated to EPR measurements. Fig. 1 shows the EPR spectra of 0.5%Pd/BEA recorded at T=293K for different treatment conditions : calcined at 773K under air (a); evacuated 293K (b); reduced at 523K (c) and reduced at 773K (d). All the spectra are composed of three signals A, B and S. The A signal centred at g = 4.30 could be attributed to isolated Fe 3§ ions presented as impurity in the solids [7]. The B signal has a g = 2.09+0.01 and a 8H = 1300G and is most remarkable in calcined and evacuated at 293K samples. Intensity of this signal (i) increases slightly with decreases of measurement temperature (exchange interaction with antiferromagnetic component); (ii) increases with increase the % of Pd exchanged up to 0.05w% and further stays still up to 0.5w% of Pd (saturation effect in quantity of paramagnetic Pd agglomerates); (iii) increases with increase of the reduction treatment (Fig. 2). On the base of these facts, we can suppose that some of the Pd ions are present in an oxidation state r 2 (may be in 3 or certainly 1) and are under the influence of the strong exchange interaction with ferrimagnetic or antiferromagnetic component. The interaction in Pd ions system should be antiferromagnetic because the decrease of the measurement temperature from 293K to 77K leads to a slight increase of the signal intensity (i). The same type of magnetic interaction has been observed in small Pd particles [8]. The absence of the structure peculiarities for B signal does not allow to determine unambiguously the valence state of Pd ions as it has been made in other works [9]. However, the number of this type of palladium species (B) was calculated as a function of the %Pd exchange on BEA and the saturation of this Pd species is found between 0.01 and 0.05 wt%Pd exchange (ii). Signal S with the parameters g=2,0045 and 8H = 15G and Lorentian line shape is observed in EPR spectra of all the samples after the reduction treatment at T=523K and higher. The intensity of this signal increases with increase of the exchange ratio

702 of Pd and the reduction treatment temperature (Fig.2). It should be noted that identification of the narrow symmetric line with g-=ge is difficult, but Lorentian shape of signal indicates the exchange interaction between the corresponding paramagnetic species. In our case taking into account the condition of reduction treatment and composition of studied solids, it should be the oxygen located hole for corresponding defect centres [10]. It must be noted that the increase of number of S centres is in inverse proportion to quantity of agglomerated B centres (iii), i.e. the reduction of agglomerated Pd ions from paramagnetic state to diamagnetic one leads to appearance of the defects of S type. The dealuminated sample 0.5%Pd/DBEA shows the same types of EPR spectra as the 0.5%Pd/BEA ones under vacuum and reduction treatment. The difference in quantity of signal B and signal S for these treatments are shown in fig. 2. It may be noted that the dealumination of the samples leads to a decrease of quantity of agglomerated Pd ions under all the conditions of treatment (in 1.6 time on average) and their easier reduction just up zero at 773K, an increase of S type of paramagnetic species under the same conditions (in 1.67 time on average). This result is correlated to the above BET study. After dynamic oxidation of 0.5%Pd(2)/BEA at 773K under flow of air and vacuum treatment at T=773K, only weak signal A is present. After the vacuum treatment at 673 and 77K with or without previous calcination, several EPR signals of paramagnetic defect species appears. Signal S' with g=2.0036, 5H = 15G and Lorentian line shape as the signal S may be also attributed to a oxygen located hole centre. This defect is more intense in the previously calcined samples. Under static oxidation (02, 2h, 293K), the signal S' completely disappears even without vacuum treatment (Fig.3). The static oxidation does not lead to creation of new paramagnetic species, on the contrary it eliminates the existed above defect species. Fig.3 shows the spectra EPR at T=77K of statically oxide Pd(2)/BEA under the weak (3%H2/Ar, p=200torr, T=293K) reduction conditions for different times of reduction. The large variety of Pd + centres are manifested themselves in these spectra. The impurity of Fe 3+ (signal A) and agglomerated Pd (signal B) are also present in whole EPR spectra (not shown), but the signals of defect S and S' are absent. A

60-

B a

i

50

b

2,5 2

~" 40 & 30 1

-

1

X 10

2o

d

x 0,09

1000

"

10 |

|

i

|

2000

3000

4000

5000

6000

H (Gauss)

Figure 1: EPR spectra at 293K of Pd/BEA zeolites (a) calcined at 500~ under air ; (b) treated under vacuum at 293K; reduced under H2 at (c) 523K, (d) 773K.

..................

without vaccum treatment 500~

H2, 20'

250~

0,5 0

H2, 20'

500~

Figure.2 Dependence of the quantity of agglomerated paramagnetic Pd species (NsPd) and of S species (NsS) on treatment condition for A: DBEA and [2" BEA.

703 The observed EPR signals with parameters: signals C (gl 1=2-628; g• ), signal D (g[ t=2.496), signal E (gl I=2.434) ; signal F (gl I=2-356) may be attributed to isolated Pd + [9] species in different local symmetry environment. It must be noted that the first appearance (perpendicular component of C signal) of Pd + is observed even after 5 minutes of reduction under the weak reduction condition. Further treatment during 50min at T=293K leads to increase of intensity of Pd + EPR spectra (C-F), which partially disappear (species E and F) during the longer reduction (5 h). The further increase of reduction time up to 15 h results in complete disappearance of all above observed Pd+ species and appearance of a new radical G signal (gl l=1.988; gx=2-0505) which is similar to signal observed earlier in [9] but under different (oxidation)treatment conditions and is attributed to Pd2+-Q radical. Fig.4 shows EPR spectra of the Pd(2)/BEA reduced under the strong reduction conditions (H2, 100torr) at T=293 and 393K for different times. A first 5 h reduction leads to appearance of a new Pd + centre (signal H: gl 1=2,444; gx=2.0944) or a transformation of one of precedent (C, D or E) ones and increase of intensity of earlier observed F centres. A continuous increase of reduction time up to 15 h eliminates the H signal. At the same time the beginning of the interaction between the Pd+ ions is manifested itself in EPR spectra by appearance of a signal I with isotropic giso=2.128. The increase of the reduction temperature up to T=393K leads to consecutive increase of quantity of Pd + ions which are under an influence of others (Fig.4c, d). The appearance and increase of I species proves that under the noted reduction conditions, we can continue the progressive reduction of palladium species, i.e. there is an important reserve of Pd 2+ not yet reduced. The reduction effect on Pd after the catalytic test will be discuss later. ~i H ~t F

g

x 0 25

'

g•

F

b

L'2

\ ; 2 .... glt~5 2400

I 2900

I 3400

H(gauss)

Figure 3. EPR spectra of statically oxided Pd(2)/BEA under the weak reduction during (a) 5 min ; (b) 50 min ; (c) 5 h ; (d) 15 h.

ff

2420

I

I

i

t

~

t

2620

2820

3020

3220

3420

3620

~em=SS)

Figure 4. EPR spectra at 77K under the strong reduction of sample fig.3d at (a) T=293K, 5 h; (b) T=293K, 15 h; (c) T=393K, 1 h; (d) T=393K, 3 h.

704 The IR spectra of adsorbed toluene on the samples displayed two characteristic bands at 1650 cm -1 and 1490 cm -1 corresponding to C-C ring vibrations [6] but also C-H stretching vibrations bands of the aromatic ring between 3050 and 3020 cm -~. The characteristic bands of adsorbed toluene are not shifted from one to another studied sample. The study of the intensity of toluene sorbed peaks with increasing amounts of toluene in the cell shows different behaviours of the solids. Adsorbed toluene is visible when 1gL (9.4.10 .5 moles) is injected in the cell for 0.5wt%Pd/DBEA whereas 5gL and 10gL of toluene are the minimum amounts respectively for 0.5wt%Pd/BEA and 0.2wt%Fe,0.5wt%Pd/BEA. The toluene is then more easily adsorbed on 0.5wt%Pd/DBEA and that adsorption is more difficult on the Fe exchanged catalyst. Nevertheless, the sample 0.5wt%Pd/BEA was saturated with adsorbed toluene before 0.5wt%Pd/DBEA and 0.2wt%Fe,0.5wt%Pd/BEA. The toluene total oxidation was studied by the conversion of toluene at 250~ versus time on stream for the calcined catalysts 0.5wt%Pd/BEA, 0.5wt%Pd2/BEA, 0.5wt%Pd/DBEA and 0.2wt%Fe,0.5wt%Pd/BEA (Fig.5). Although the oxidation of the toluene on these catalysts was producing only water and CO2, all the toluene was not converted at the beginning of the experiment. After 3 minutes under flow, 96% of the toluene was oxidized on 0.5wt%Pd(2)/BEA, 94% on 0.5wt%Pd/BEA, 90% on 0.5wt%Pd/DBEA and only 80% on 0.2wt%Fe,0.5wt%Pd/BEA. However, this conversion increased to 100% for all the catalysts after some time on stream : after 5 hours for 0.5wt%Pd/BEA and 0.5wt%Pd/2BEA but only after 10 h for 0.5wt%Pd/DBEA and 0.2wt%,0.5wt%Pd/BEA. The total conversion is nevertheless not stable during our experiment for the both more active catalysts at the beginning of the experiment (0.5wt%Pd/BEA and 0.5wt%Pd(2)/BEA). The deactivation could be partially explained by the DTA curves of the sample after the catalytic test (Table 2). Two exothermic transitions at about 250 and 500~ and only one at 500~ are present for, respectively, the dealuminated 0.5wt%Pd/DBEA sample and the non-dealuminated samples (0.5wt%Pd/BEA and 0.2wt%Fe,0.5wt%Pd/BEA). These transitions should correspond to the combustion of some deposed cokes on the catalysts. 100 a

95

90

"".

85I d~ / 0

8

Time (h)

16

24

Figure 5 : Toluene conversion at 250~ versus time on stream a : 0.5%Pd(2)/BEA, b : 0.5%Pd/BEA, c : 0.5%Pd/DBEA, d:O.5%Pd,O.2%Fe/BEA.

705 The losses of cokes of, respectively, 1.7% + 2.3% = 4% and 3.7% for 0.5wt%Pd/DBEA and 0.2wt%Fe,0.5wt%Pd/BEA samples are similar whereas a higher quantity is found on 0.5wt%Pd/BEA. This difference could explain the deactivation observed. But also because the first type of coke present on 0.5wt%Pd/DBEA is already decomposed at 250~ and that temperature is the oxidation temperature of toluene in the catalytic test. The second type of coke decomposed at 500~ is then more stable. Another explanation of the deactivation could be also due to the formation of water during the toluene oxidation. Non-dealuminated samples after the catalytic test present higher water losses (corresponding to an endothermic transition at about 80~ The water formed during the toluene oxidation is more sorbed on the more hydroscopic samples and then could decrease the activity of the catalysts. Moreover, the EPR spectra of these samples after the catalytic test still present another result. In Table 3, there are the number of B (NsPd) and S centres (NsS) species of the samples after the toluene catalytic test. The toluene oxidation reaction produces a reduction effect for all the studied catalysts. In Fig.2, the quantity of paramagnetic species B' (g=2.25-2.34, 6H=1100-1200 G) which have been attributed in [9] to palladium clusters, and S species after the test are presented. The catalytic effect is then similar to the reduction treatment at -573K. The value of S centres (Ns) increases after test in order successive : Pd(2)/BEA to Pd/BEA to PdFe/BEA to Pd/DBEA. For the quantity of B centres the order is inverse. So the agglomerated Pd ions are in order more reduced from Pd(2)/BEA to Pd/BEA to PdFe/BEA and to dealuminated Pd/DBEA sample. This result could be directly correlated to the catalytic deactivation observed for Pd(2)/BEA and Pd/BEA after 24 h on stream (Fig. 5). In fact, the more the palladium is reduced, the more its activity for oxidation reaction [ 11 ]. Therefore, the addition of 0.2wt% of Fe or the zeolite dealumination could prevent the deactivation and then lead to better catalysts. Table 2 : DTA results on used catalyst Water loss (80~ Pd/BEA PdFe/BEA Pd/DBEA

8.9% 8.0% 5.8%

Coke 1 loss (250~

Coke 2 loss (500~

0% 0% 1.7%

4.7% 3.7% 2.3%

Table 3 : Number of B (NsPd) and S (NsS) centres after the toluene catalytic test

Pd(2)/BEA Pd/BEA PdFe/BEA Pd/DBEA

g factor (S)

NsS (r.u.)

g factor (Pd +)

NsPd (r.u.)

2.0046 2.0045 2.0046 2.0030

2.15 1.98 2.34 3.16

2.34 2.25 2.28 2.30

0.327 0.352 0.289 0.233

706 4. CONCLUSION This study has shown that an addition of iron or the dealumination of the zeolite could prevent the deactivation of Pd/BEA for toluene total oxidation. Palladium would be more easily agglomerated on the BEA than on the dealuminated BEA (BET and EPR measurements). Moreover, the palladium saturation in function of the wt% of exchanged Pd on the BEA zeolite was deduced by EPR. Several isolated and interacted Pd + species and hole centers were also detected by EPR under dynamic and static oxidations and weak and strong reduction treatments. Moreover, the Pd was much reduced after the catalytic test in dealuminated and Fe doped samples. This result could be directly correlated to the catalytic deactivation of the other samples. The deactivation could be also explain by the type of coke deposed on the catalyst and by the hydroscopic behavior of the samples. REFERENCES

1. C.-H. Lee and Y.-W. Chen, Appl. Catal. B 17 (1998) 279 2. J.R. Gonzhlez-Velasco, A. Aranzabal, J.I. Guti6rrez, R. Lopez-Fonseca, M.A. Guti6rrezOrtiz, Appl. Catal. B 19 (1998) 189. 3. P.O. Larsson, A. Andersson, B. Svensson and L.R. Wallenberg, in 'Environmental Catalysis', G. Centi et al. (Eds), Rome, (1995) 547 4. M. Ogura, S. Kage, M. Hayashi, M. Matsukata and E. Kikuchi, Appl. Catal. B 27 (2000) L213 5. C.K.W. Meininghaus, R. Prins, Microporous. Mater. 35 (2000) 349 6. A.E. Palomares, G. Eder-Mirth and J.A. Lercher, J. Catal. 168 (1997) 442 7. J. Matta, J.-F. Lamonier, E. Abi-Aad, E.A. Zhilinskaya, A. Abouka'is, Phys. Chem. Chem. Phys. 1 (1999) 4975 8. S. Sako and K. Ohshima, J. Phys. Soc. Jap. 65(12) (1996) 4062 9. A.M. Prakash, T. Wasowicz, and L. Kevan, J.Phys.Chem. 101 (1997) 1985 10. L.D. Bogomolova, V.A. Jachkin, S.A. Prushinsky, S.A. Dmitriev, S.V. Stefanovsky, Y.G. Teplyakov, F. Caccavale, E. Cattaruzza, R. Bertoncello, F. Trivillin, J.Non-Cryst.Solides, 210 (1997) 101 11. E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

707

H y d r o i s o m e r i z a t i o n o f n - B u t a n e over P d / H Z S M - 5 and P d / H m o r d e n i t e with and w i t h o u t binder. P. Cafiizares, F. Dorado*, P. Sfinchez and R. Romero Departamento de Ingenieria Quimica, Facultad de Ciencias Quimicas, Universidad de Castilla - La Mancha, Campus Universitario s/n, 13004 - Ciudad Real, Spain.

The influence of a clay binder (bentonite) on the acid properties and performance of Pd/HZSM-5 and Pd/HM catalysts with different Si/A1 ratios for the hydroisomerization of nbutane has been studied. Temperature-programmed desorption of ammonia, atomic absorption spectroscopy, chemisorption and surface area measurements were used to characterize the catalysts. After agglomeration, some zeolite protons are neutralized by clay sodium and, consequently, a lower n-butane conversion is obtained. However, the decrease in conversion will be compensated by a much higher isobutane selectivity. The product selectivity is also strongly influenced by the binder due to the fact that zeolite hydrogen transfer activity, metal/acid site balance, and diffusion of products are modified. The isomerization activity for Pd/HZSM-5 samples were higher than for Pd/HM catalysts due to mordenite is expected to be more sensitive to the increased in the length of the effective diffusional pathway due to its one-dimensional pore structure.

1. INTRODUCTION Isomerization of n-butane to isobutane is an important commercial process. Isobutane is utilized in alkylation and production of isobutene for the synthesis of methyl-tert-butylether (MTBE), tert-butylalcohol (TBA), polyisobutene and other products. One possible route for isomerization of n-butane is the use of zeolites as catalysts, so that it is of great interest to study the influence of binder on the catalytic performance in this reaction. Most industrial zeolite catalysts require the zeolite to be pelletized with a binder to obtain larger and more resistant particles and to avoid an extremely high pressure drop in fixed bed reactors. Although binders are not active as catalysts, they can influence the catalytic performance [1 ]. This is very important for the development of industrial zeolite catalysts. The presence of binder can affect the acidic properties of a zeolite as a result of changes in the proton exchange efficiency and/or blocking of zeolite channels during the pelletization process.

*To whom correspondence should be addressed. E-mail.'[email protected] Phone: +34-926-295300. Fax: +34-926-295318

708 Bentonite is a clay mineral with wet binding properties. Sodium bentonite contains exchangeable sodium cations. When dispersed in water it breaks down into small plate-like particles negatively charged on the surface, positively charged on the edges. This unique ion exchange is responsible for the binding action. Several catalysts have been successfully used for the skeletal isomerization of n-butane, e.g. HMOR, HZSM-5 and Hbeta [2]. The use of mordenite zeolites have received much attention, mainly due to the specific pore geometry combined with the strong acidic properties. The unidirectional mordenite structure has a cross section with so-called side pockets allowing (small) molecules to pass each other. However, this zeolite is highly sensitive to coke deactivation [3]. n-Butane isomerization has been investigated over HMOR and Pt/HMOR catalysts [4]. According to most of the authors, this reaction occurs via bimolecular pathway, as the monomolecular isomerization mode would involve a primary (and hence very unstable) carbenium-ion intermediate. Metal in mordenite has also been suggested to contribute to the isomerization of n-butane via a parallel bifunctional route involving dehydrogenation of the alkane on the metal site [5]. It is interesting to rationalize the steric requirements for the bimolecular mechanism in the one-dimensional pore of mordenite. The kinetics diameters of branched alkanes (e.g. isobutane) approach the pore size of mordenite, implying that the individual molecules cannot pass each other within one channel. This type of motion is referred to as single-file diffusion [6]. Generally, concentration gradients inside the pores are the driving force for the directed motion of the molecules. However, in single-file diffusion, a displaced molecules is more likely to return to its original position than to proceed further, since the latter would stipulate a further concentration of the molecules ahead, which seems to be highly constrained because of the space limitations in one-dimensional pores. Another possibility is to use HZSM-5 zeolite, because of its acidity, shape selectivity, and temperature resistence. It can be used as catalyst to isomerize light paraffins once promoted with a metal [7]. In this work, several acid catalysts based on ZSM-5 and mordenite zeolites with different Si/A1 ratios and agglomerated with bentonite were prepared. The aim of this work is to study the influence that the pelletization process can induce on the catalyst.

2. E X P E R I M E N T A L

ZSM-5 (Si/A1 = 15 and 40) and Mordenite (Si/A1 = 10 and 45) were supplied by Zeolyst International. Bentonite was supplied by Aldrich Chemical Co. Characterization data of the raw materials (clay and zeolites) are summarized in Table 1. The method of preparation consisted of three steps: agglomeration of zeolite, incorporation of functions (acid and metallic), and activation of the metal part. The ammonium forms of zeolite were calcined at 550 ~ for 15 h to obtain the protonic form. For the binding process, the zeolite and the clay were mixed together and suspended in water at 60 ~ for 2 h, dried at 120~ overnight and finally crushed to 0.50-1.00 mm. The dried material was finally calcined in static air at 550 ~ for 15 h. In order to avoid a possible activity decrease due to partial exchange of the strongest protonic zeolite acid sites with alkaline cations from the binder during the preparation [8], the agglomerated catalysts were ion exchanged with 35 mL/g of 0.6 N HC1. Metal incorporation was carried out by an impregnation technique using an aqueous Pd(NO3)2 solution. The solvent was then removed by evaporation under vacuum. The metal concentration of the

709 impregnating solution was calculated to always yield a final Pd content of 0.82 wt%. After metal incorporation, the catalysts were air-calcined at 400 ~ for 4 h and reduced in situ under a hydrogen flow of 190 mL/(min g) at 450 ~ for 4 h. Catalysts are named as follows: firstly, the symbol of the metal is shown (Pd), followed by a character representing the zeolite (Z for ZSM-5 or M for mordenite). The subsequent number indicates the zeolite Si/A1 ratio. For non-agglomerated samples, further characters are not included. For bound catalysts, however, there is another character related to the binder name (B for bentonite), and finally a number that represents the amount of binder in the catalyst (65 wt% for all the agglomerates samples). Temperature-programmed desorption of ammonia (TPDA), surface area measurements and pore size distributions, atomic absorption spectroscopy (AA), and chemisorption measurements have been used to characterize the catalysts, as described in a previous paper [9]. The crystal size of zeolite particles was determined using a Mastersizer 2000 analyzer. Its technology is based on both Fraunhofer and Mie theories about light scattering. Prior to measurement, each sample was dispersed in water for two minutes in ultrasonic baths. The measurements of zeolite crystal sizes have an error of + 1%. The experiments were carried out in a flow-type apparatus designed for continuous operation. This apparatus consisted of a gas feed system for each component hydrogen and nbutane (+99.95% purity, significant impurities being isobutane [max. 400 ppm] and propane [max. 100 ppm]), with individual control by mass flow meters, a fixed-bed downflow reactor, and an exit gas flow meter. Experimental conditions were: weight of catalysts 5 g, reaction temperature 350-430 ~ total pressure 1.013 bar, W/Fn-c4 = 1 gzeolite/(g/h), and H2/n-C4 molar ratio 8.3:1. Reaction gas products were analyzed with a Hewlett-Packard gas chromatograph using a fused silica PLOT AlzOJKC1 column and an FID detector. Results from a reproduced experiment showed that conversion and isobutane selectivity have an error of +5%. 3. Results and discussion

Acid properties of the raw materials are given in Table 1. As it can be seen, for both zeolites (HZSM-5 and HMOR), the presence of the metal does not significantly change the zeolite acid properties. Table 1 Characterization data for the raw materials Catalyst Zeolite Sodium Acidity data Si/AI ratio content Total Acidity (wt%) (mmol NH3/g) Bentonite 0.48 0.038 HZSM-5 15 0.04 0.707 HZSM-5 40 0.04 0.467 PdZ15 15 0.04 0.712 PdZ40 40 0.04 0.469 HMOR 10 0.05 0.775 HMOR 45 0.01 0.409 PdM10 10 0.05 0.777 PdM45 45 0.01 0.411

Weak Acidity (mmol NH3/g) 0.038 0.031 0.018 0.030 0.017 0.074 0.036 0.072 0.035

Yd (~ 274 300 300 290 289 305 306 303 306

Strong Acidity (retool NH3/g) 0 0.676 0.449 0.682 0.452 0.701 0.373 0.705 0.376

Yd (~ 419 403 400 402 505 492 506 490

710 Table 2 summarizes the acidity data for the agglomerated catalysts. The experimentally measured acidity properties for all PdZ15B65, PdZ40B65, PdM10B65, and PdM45B65 catalysts show a derivation from the predicted values calculated from the contribution of the raw materials: the weak acidity of the agglomerated catalysts is higher than the predicted values, whereas the opposite effect is observed for strong acidity. First, it should be considered whether these changes could be due to total blocking of zeolite channels. To verify this possibility, the surface area of the samples was measured. Table 3 resumes these measurements together with theoretical predicted values calculated from the contribution of nonagglomerated zeolite and binder. The results show that blocking can be excluded since values of surface area are in line with those predicted. The decrease in the number of expected strong sites can be attributed to solid-state ion exchange between zeolite protons and clay sodium during the calcination that follows the ion-exchange step with HC1 [1,10]. Besides, Na + cations are also weak acid sites [ 11 ], so that the increase in weak acid sites density is due to this cation. In summary, experimental acidity values do not match the theoretically calculated values due to solid-state ion exchange and some interference from Na +. As the binder influences the strong acid site density of the zeolite, the catalyst performance will depend on the binder. This can be seen in Figure 1, where n-butane conversion for these samples is shown. Taking into account that we use a W/Fn-c4 parameter based on grams of zeolite, we should first of all refer strong acidity to grams of zeolite to relate conversion to strong acid site density (Table 3). As expected, the higher the strong acid site density per gram of zeolite, the higher the conversion.

Table 2 Characterization data for Pd/HZSM-5/bentonite and Pd/HMOR/bentonite catalysts Mechanical Dm Acidity data Catalyst Weak Aciditya Strong Acidity b resistance (%) Total Acidity (%fines) (mmol NH3/gcat) (mmol NH3/gcat) (mmol NH3/gcat) 24.7 0.273 c 0.036 c 0.237 c PdZ15B65 0.7 0.267 a 0.057 a 0.210 a +60.3 e -11.2 e 20.1 0.188 c 0.031 c 0.157 c PdZ40B65 0.7 0.185 d 0.038 a 0.147 d +22.6 e -6.5 e 22.3 0.296 ~ 0.049 c 0.247 c PdM10B65 0.7 0.295 d 0.060 d 0.235 d +22.4 e -4.8 e 25.3 0.168 c 0.037 c 0.132 ~ PdM45B65 0.7 0.165 d 0.040 d 0.125 d +8.11 e -4.6 e a Desorption temperature of ammonia ~ 290 ~ b Desorption temperature of ammonia ~ 390 ~ for ZSM-5 based catalyst, and ~ 440 ~ for mordenite-based catalysts c Predicted value proportionally calculated from the contribution of the non-agglomerated zeolite and binder. a Experimental value e Deviation from the predicted value (%)

711 Table 3 Strong acid site density per gram o f zeolite, surface area, and pore v o l u m e m e a s u r e m e n t s Pore Volume (~tL/gcat) Strong Acidity a

Surface area a Deviation from the (mmol NH3/gzeolite) (m2/g) predicted value (%)

Sample Bentonite HZSM-5 (Si/Al=l 5) HZSM-5 (Si/AI=40) PdZ15 PdZ40 HMOR (Si/AI=I 0) HMOR (Si/Al=45) PdM10 PdM45 PdZ 15B65 PdZ40B65 PdM 10B65 PdM45B65 a Experimental value

--0.676 0.449 0.682 0.452 0.701 0.373 0.705 0.376 0.600 0.420 0.671 0.357

37 441 441 440 438 535 530 537 535 181 182 221 220

------------------2.3 2.8 4.2 4.1

Micropores 4.3 140.1 140.1 149.2 137.7 195.6 196.1 190.4 188.0 47.1 46.4 78.4 73.5

Meso- and macropores 88.5 37.0 37.0 28.0 30.1 41.0 39.5 30.8 30.6 70.5 69.8 140.6 148.9

Figures 2 and 3 s h o w isobutane selectivity versus n-butane conversion for these catalysts. As it can be seen, agglomerated catalysts showed a higher selectivity to isobutane than their respective parent zeolite. This result is in good agreement with the bimolecular m e c h a n i s m proposed for n-butane isomerization [12].

45

45

(a)

40

,~,35

~35

-~ 30

30

~E2 5 • -~

(b)

40

g25

20

..~

r~ 1,1

~a15

20

~15

10

10 5

5

0

0 PdZl5

PdZ15B65 PdZ40

Catalyst

PdZ40B65

F-] PdM10

PdM10B65 PdM45

PdM45B65

Catalyst

Figure 1. Influence o f binder on n-butane conversion (Reaction Temperature = 370 °C, P = 1.013 bar, W/Fn-c4 = 1 gzeolite/(g h) and H2/n-C4 = 8.3) for (a) P d / H Z S M - 5 catalysts and (b) P d / H M O R catalysts.

712 100 90 80 ~,70 60 "~ 50 m 40 3O 2O 10 0

--m~--.PdZ 151365 9 PdZ40 - • .... PdZ40B65 20

40 X (mol %)

",

60

80

Figure 2. Isobutane selectivity as a function of n-butane conversion for Pd/HZSM-5 and Pd/HZSM-5/Bentonite.

100 90 80 ~, 70 60 50 E 40 '~' c~ 30 20 10 0

- ' ! ~ PdM10B65 --iv- PdM45 PdM45B65 20

40 X (tool %)

6O

Figure 3. Isobutane selectivity as a function of n-butane conversion for Pd/HM and Pd/HM/Bentonite.

The first step is the formation of a butylcarbenium ion through (i) protonation of butane by a Br6nsted acid site and subsequent abstraction of H2, (ii) hydride abstraction by a Lewis acid site, or (iii) protonation of trace olefins formed by thermal cracking [13]. Then, the C8+ ions can be formed via reaction of the butylcarbenium ion with an olefin forming an octylcarbenium ion or via reaction with an alkane forming an octylcarbonium ion. The C8 carbenium ions, after isomerization and p-scission, will yield n-butane and isobutane. The C8 carbonium ions, after disproportionation, will yield propane and pentane. So, the relative concentration of carbenium and carbonium ions determines the selectivity of the catalysts. A higher strong acid site density involves a higher hydrogen transfer activity, which leads to a shorter lifetime of carbenium ions and to a higher concentration of carbonium ions in the zeolite pores. Therefore, according to this mechanism, a lower isomerization and a higher disproportionation activity will be obtained if the strong acid site density is increased. Tables 4 and 5 show the hydrocarbon selectivity, at approximately 25 mol % conversion. As it can be seen easily, the highest formation of propane and pentanes (disproportionation activity) was observed for samples without binder, i.e. the catalysts with the highest zeolite strong acid site density, as one would expect. On the other hand, Tran et al. considered that a high disproportionation selectivity could be also due to a long diffusion pathway for organic molecules [14]. Thus, while the reactant molecules diffuse through the channels, they could undergo many successive intermolecular reactions with consequently a preferential formation of propane (butanes and pentanes are very reactive compared to propane and can be transformed into this product). First of all, it was measured the zeolite crystal size for these samples, as the length of the intracrystalline diffusion depends on it. However, very similar results were obtained for all samples (about 4 tam). Secondly, it was considered a partial blockage of the zeolite micropore mouths, which would lead to an increase in the length of the effective diffusional pathway. A metal dispersion of 20-25% was found for all the parent zeolites. The average diameter of the metal

713 particles, calculated from a theorical expression [9], would be then about 46.3-57.9 A, too big to think that the palladium particles could be located into the zeolite main channels. Thus, partial blocking of the zeolite micropore mouths by the metal is expected for the pure zeolites. However, for the bindered catalysts, the big metal particles are likely located not on the zeolite surface, but into the meso- and macropores provided by the binder (Table 3). Zeolite mouth pore partial blockage is then avoid, with consequently a lower diffusional constraint. Thus the diffusion pathway for the non-agglomerated samples would be clearly much longer than for the agglomerated catalysts, which strongly favors propane formation, as observed (Tables 4 and 5).

Table 4 Hydrocarbon selectivity at approx. 25 mol% conversion over Pd/HZSM-5/bentonite catalysts. Catalyst PdZl5 PdZ15B65 PdZ40 PdZ40B65 Selectivity (%) CH4

2.9

0.7

1.3

2.3

C2H6

2.6

1.1

2.1

1.3

C2H4

0

0

0

0

C3H8

11.8

9.2

11.2

4.7

C3H6

0

0

0

0

i-C4H1o

73.3

80.0

76.6

86.3

C4H8a

0.0

0.3

1.6

2.2

n-CsHlo + i-C5Hlo b

9.5

8.7

7.2

3.3

a All isomers; b Hexanes or higher were not obtained

Table 5 Hydrocarbon selectivity at approx. 25 mol% conversion over Pd/HMOR/bentonite catalysts. Catalyst PdM10 PdM10B65 PdM45 PdM45B65 Selectivity (%) CH4

0.4

0.4

0.7

0.9

C2H6

1.6

1.1

1.6

1.2

C2H4

0

0

0

0

C3H8

18.2

10.3

13.7

9.4

C3H6

0

0

0

0

i-C4Hlo

65.2

77.8

71.3

78.2

C4H8a

0.1

0.5

3.6

3.6

n-CsH10 + i-C5H10b

14.5

9.9

9.1

6.7

a

All isomers; b Hexanes or higher were not obtained

714 In agreement with this assumption, it was observed that isomerization activity for Pd/HZSM-5 catalysts was higher than for Pd/HMOR samples. Differences in isomerization selectivity for both type of samples could be explained again on the bases of differences in acidity and pore size. Mordenite is expected to be more sensitive to the increase in the length of the effective diffusional pathway due to its one-dimensional pore structure. Thus, effective diffusional pathway for Pd/HMOR samples would be clearly much longer than that for PdHZSM-5 catalysts, and consequently, a higher formation of propane and pentane will be obtained over the former, as it can be seen in Tables 4 and 5. Finally, it is interesting to compare the catalysts PdZ40 and PdM45. Strong acid site densities per gram of zeolite was higher for the former than for the latter. However, the selectivity to isobutane was higher for PdZ40, which confirms that the long diffusional pathway for Pd/MOR samples strongly favors propane formation.

CONCLUSIONS The presence of a clay binder affects the acidic properties of zeolite based catalysts, and consequently their performance in the hydroisomerization of n-butane. There is a solid ion exchange between zeolite protons and clay sodium cations. The neutralization of some zeolite protons by the clay leads to a lower n-butane conversion. However, this negative effect can be compensated by a higher selectivity to isobutane, which is due to both an easier product diffusion and a lower disproportionation activity. Catalysts based on Pd/HZSM-5 had a higher isomerization activity than those based on Pd/HMOR, as mordenite, with one-dimensional pore structure, is quite more sensitive to diffusional restrictions. REFERENCES

1. V.R. Choudhary, P. Devadas, A.K. Kinage, M Guisnet. Appl. Catal., 162 (1997) 223. 2. E. Babfirek, J. Novfikovfi. Appl. Catal. A., 185 (1999) 123. 3. M. Guisnet, P. Marnoux. Catalyst Deactivation 1994, Stu. Surf. Sci. Catal., in: B.Delmon, G. F. Froment (Eds.). 88 (1994) 53. 4. C. Bearez, F. Avendano, F. Chevalier, M. Guisnet, Bull.Soc. Chim. Fr., 3 (1985) 346. 5. R.A. Asuquo, G. Eder-Mirth, J.A.Z. Pieterse, K. Seshan, J.A. Lercher. J. Catal., 168 (1997) 292. 6. J. K/~rger, M. Petzold, H. Pfeifer, S. Ernst, J. Weitkamp. J. Catal., 136 (1992) 283. 7. W.O. Haag, R.M. Lago. U.S. Patent 4,374,296 (1983). 8. J.M. Fougerit, N.S. Gnep, M. Guisnet, P. Amigues, J.L. Duplan, F. Hughes. Stud. Surf.Sci. Catal., 84 (1994) 1723. 9. P. Cafiizares, A. de Lucas, F. Dorado, A. Dur~n, I. Asencio. Appl. Catal., 169 (1998) 137. 10. M.D. Romero, J.A. Calles, A. Rodriguez, A. de Lucas. Microporous and Mesoporous Materials, 9 (1997) 221. 11. V.R. Choudhary, V.S. Nayak. Zeolites, 5 (1985) 15. 12. R.A. Asuquo, G. Eder-Mirth, J.A. Lercher. J. Catal., 155 (1995) 376. 13. R. Shigeishi, A. Garforth, I. Harris, J. Dweyer. J. Catal., 130 (1991) 423. 14. M.-T. Tran, N.S. Gnep, G. Szabo, M. Guisnet. J. Catal., 174 (1998) 185.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

715

Butane i s o m e r i z a t i o n on several H-zeolite catalysts Sergio De Rossi*, Giuliano Moretti, Giovanni Ferraris and Delia Gazzoli Centro CNR SACSO c/o Dipartimento di Chimica, Universit/l "La Sapienza", Piazzale A. Moro 5, 00185 Roma, Italy. (e-mail: sergio.derossi@uniroma 1.it)

H-ZSM-5, H-Beta, H-Mordenite and H-Y zeolites are active catalysts for n-butane isomerization. The level of activity and resistance to poisoning are a function of the concentration of the Bronsted acid sites and framework structure. H-L and H-Ferrierite zeolites have a very low activity. Silicalite-1 (S-l) and mesoporous silica-alumina (MSA) are not active. The isobutane yield on ultrastable H-Y (Si/AI=3.15) catalyst is lower in comparison to tungsta supported on zirconia (WOx/ZrO2), H-Mordenite, H-Beta and H-ZSM-5 catalysts, however, H-Y catalysts are more resistent to poisoning and highly selective towards isobutane. In fact, no deactivation of H-Y catalysts was observed after 5 h of time on stream. On the contrary, WOx/ZrO2, H-Mordenite and H-Beta catalysts, under the same experimental conditions, are deactivated in less than 1 h of time on stream. It is suggested that the stability of the H-Y catalysts in comparison to the other acidic zeolites catalysts may be due to the 3-dimensional structure of H-Y zeolites, made of large supercages interconnected by apertures of 12 oxygen atoms. H-ZSM-5 catalysts are stable with time on stream but their selectivity to isobutane is very low. Active acidic molecular sieves with monodimensional structure (H-Mordenite, H-Beta) may favour the formation of polyenyl unsaturated chains, the precursors of the coke responsible for the catalysts deactivation. 1. INTRODUCTION The isomerization of n-butane has a specific interest because isobutane can be alkylated with butenes to produce high octane number isooctanes and is a starting material for producing isobutene via dehydrogenation. Isobutene can be reacted with methanol or ethanol to produce methyl-tert-butyl (MTBE) or ethyl-tert-butyl (ETBE) ethers, which are employed as additive of gasoline, to increase the antiknock power and improve the combustion [ 1,2]. The use of halogen-containing catalysts, to reach the acid strength necessary for skeletal isomerization, is becoming problematic for environmental reasons. Hence the search for strongly acidic catalysts which avoid the use of halogens is an important goal of the refining industry.

This paper is dedicated to Prof. Alessandro Cimino on the occasion of his 75 th birthday. Financial support from Italian Murst (Programmi di Ricerca di Rilevante Interesse Nazionale) is gratefully acknowledged.

716 Zirconia based catalysts, especially sulfated zirconia [Refs. 3,4 and references therein] and tungsta supported on zirconia [Ref. 5 and references therein], have high activity and selectivity for n-butane isomerization. On the sulfated zirconia catalysts the catalytic activity was correlated with the number of Bronsted acid sites. In particular the strongest sites, which are able to retain pyridine against evacuation at 150 ~ were suggested to be the active sites for butane isomerization [3,4]. The main problem with these catalysts is their fast deactivation due to coke deposition. On sulfated zirconia catalysts it was found that less than 0.1 wt % of carbonaceous deposits were sufficient to reduce of 90 % the initial activity in less than 1 h of time on stream [4]. H-Mordenites were tested by Asuquo et al. [2] as an interesting alternative to conventional n-butane isomerization catalysts, although also for these catalysts the main limitation was their fast deactivation due to coke deposition. The initial activity and selectivity to isobutane (n-butane 2% in He) were similar to that found on zirconia based catalysts at 250 ~ however, at higher temperature the selectivity decreases due to the increased formation of propane and pentanes. Later, the same group found that the deactivation can be minimized by using H-Mordenite containing 0.25 wt % Pt in the presence of hydrogen (Hz/n-butane ratios in the range 0-10, n-butane 20% in He) [6]. The isomerization of n-butane (10% in N2) over H-Mordenite samples with framework Si/A1 ratios from 6.6 to 80 was studied at 250 ~ by Trung Tran et al. [7]. According to these authors, the positive effect of the acid-site density on the activity is a demonstration that the reaction mechanism is bimolecular (dimerization-isomerization-cracking), as previously suggested by Asuquo et al. [2]. Babfirek and Novfikov~i investigated the role of Bronsted and Lewis acid sites in the isomerization of n-butane (Hz/n-butane ratio = 120) over HMordenite, H-ZSM-5 and H-Beta zeolites [8]. They found that the increased number of Lewis acid sites, obtained by vacuum treatment of the samples at 650 ~ results in a substantial suppression of both n-butane conversion and selectivity to isobutane. The same authors also investigated the effect of the addition of Pt to the acidic zeolites. It was found that Pt particles, with low dispersion, increase the n-butane conversion as well as the yield of isobutane [9]. The present contribution reports a study of H-zeolites as catalysts for n-butane isomerization, extending previous literature data to other framework types. The results obtained on reference zirconia based catalysts will be compared with those obtained on Hzeolites under the same experimental conditions. The catalytic properties can be related to the different framework types and acid strength. In a previous paper dealuminated H-Y zeolites with framework Si/A1 atomic ratios in the range 3-11, obtained by leaching with diluted HC1 solutions, were found very interesting catalysts for n-butane isomerization in comparison to WOx/ZrO2 catalysts [10]. 2. EXPERIMENTAL

2.1. Catalysts preparation The H-zeolites and the other catalysts studied in this work are reported in Table 1. The commercial zeolites, initially in the sodium or potassium form (L zeolite), were transformed in the ammonium form by three consecutive ion exchanges with 3 M ammonium nitrate solutions at 90~ for 4 h (1 g of zeolite per 100 mL), and finally transformed in the acid form by treatment at 500~ for 5 h in air.

717 Table 1. Chemical and textural properties of the investigated catalysts. Sample Si/A1 [A1] (a) [H +] (a) (Manufacturer and label) at./unit cell at.g-lxl0 2~ H-ZSM-5 Sfid Chemie 12.5 7.1 7.4 Eka Nobel EZ 472 16 5.7 5.9 PQ CBV 5020 25 3.7 3.9 Stid Chemie 50 1.9 2.0 Eka Nobel EZ 112 84 1.1 1.2 Sfid Chemie 120 0.79 0.82 This laboratory (S- 1) 0.00 0.00 H-Y UOP LZY 54 2.5 55 29 Tosoh US HSZ-330 HUA 3.15 46 24 H-Ferrierite Engelhard EZ 500 8.4 3.8 11 H-L Engelhard EZ 200 3.05 8.9 20 H-Beta This laboratory 12 4.9 7.7 H-Mordenite Engelhard EZ 321 8.1 5.3 11 Stid Chemie 25 1.8 3.9

MSA This laboratory

90

11

SBET

m2g-1

Sexternal m2g-1

V~t

mLg -1

400 439 443 331 346

74 29 96 54 8

309 729

9 43

0.132 0.272

378

14

0.144

414

22

0.156

0.143 0.182 0.162 0.140 0.166

0.219

555 476 554

29 35

0.175 0.208

697

522

0.080

w wt.-% [w] WOx/Zr02 at. g-1x 10.20 This laboratory 17 5.6 50 50 0.000 (a) Calculated on a dry basis and assuming only the presence of framework aluminum. The analytical content (by atomic absorption spectroscopy) of Na was < 0.2 wt.-% for all H-zeolite samples, the residual K content of the H-L zeolite was 2.7 wt.-%. Silicalite-1 (S-I) and amorphous mesoporous silica-alumina (MSA) with Si/A1 = 90 were prepared according to procedures developed by EniTecnologie SpA [Ref. 11 and references therein]. H-Beta zeolite was prepared according to the procedure reported by Kiricsi et al. [12]. The tungsta/zirconia sample was prepared by equilibrium adsorption starting from hydrous zirconia, and an ammonium metatungstate solution [5]. The final calcination temperature was 800 ~ for 5 h.

2.2. Catalysts characterization and catalytic tests The H-zeolites catalysts were characterized by X-ray diffraction (XRD) using CuK~ (Nifiltered) radiation (Philips automated PW1729 diffractometer) and by textural analysis (BET

718 specific surface area, external surface area and micropore volume by t-test using the Harkins & Jura reference isotherm equation [13]) by N2 adsorption-desorption at -196 ~ (Micromeritics ASAP 2010 analyzer). Before adsorption, the solids were preheated under vacuum in three steps: 1 h at 150 ~ 1 h at 250 ~ and finally 4h at 350 ~ The results are reported in Table 1. Catalytic isomerization of n-butane was carried out in a flow apparatus, including: (i) a feeding section equipped with independent mass flow controllers; (ii) a down flow silica reactor, containing ca. 1 g of catalyst supported on a fritted disk, and vertically positioned in an electrical heater thermoregulated to within +1 ~ (iii) a gaschromatograph equipped with a flame ionization detector connected to an integrator for peak area evaluation. Before each catalytic run the catalyst was heated in flowing oxygen at 500 ~ for 0.5 h. The reaction was run with pure n-butane at atmospheric pressure, the weight hourly space velocity (WHSV) was 0.8 h -1 and the reaction temperature 300 ~ 3. R E S U L T S AND DISCUSSION 3.1. Activity and selectivity As shown in Figure 1 H-Mordenite and H-Beta zeolites are the most active catalysts for n-butane isomerization to isobutane at 300 ~ but their activity substantially decrease with

20

~

A

1

- 12

H-Mordenite Si/A1 = N 50

. ~~~~MM-5

Si/A12 16_

.

_

H-Y Si/A1 = 3.15

5

@

~.

r

'H-

0

o

sb

16o

1go

:,6o

:,go

aoo

1.0 .._, >~

~

H-L Si/A1 = 3.05

0.5

1 MSA Si/A1 = 90 o.o

0

=

=,=

50

=

=

100

-_,

= , =

150

.

200

,

250

300

time on stream / min Figure 1. Isobutane yield for the n-butane isomerization at 300 ~ of the H-zeolites, tungsta/zirconia and MSA catalysts as a function of time on stream (WHSV=0.8 h-l).

719 time on stream. H-L and H-Ferrierite zeolites have a very low activity. S-1 and MSA are completely inactive. H-ZSM-5 and ultrastable H-Y (Si/A1 = 3.15) zeolites have a level of activity comparable to the WOx/ZrO2 catalyst and, at variance with this catalyst, are remarkably stable with time on stream. For all the active catalysts reported in Figure 1, the selectivity to isobutane as a function of time on stream is reported in Figure 2. The selectivity of H-ZSM-5 catalyst is only 20 %, the main reaction product being propane. The selectivities of H-Mordenite and H-Beta is 40-60 %. (H-Mordenite with Si/A1 = 8.1, not shown in Figures 1 and 2, underwent faster deactivation and the selectivity decreased with time on stream.) H-Y with Si/A1 = 2.5 showed very low activity (less than 2 %), whereas ultrastable H-Y with Si/A1 = 3.15 was substantially more active (about 6 %), stable with time on stream and fairly selective to isobutane (more than 80 %). Higher isobutane yields (ca. 12 %) were obtained on dealuminated H-Y zeolites with Si/A1 atomic ratios in the range 4.85 - 10.2 [ 10]. Tungsta/zirconia reference catalyst underwent fast deactivation with time on stream. The activity decreased by a factor of 5 in less than 3 hours of time on stream. The selectivity to isobutane was, however, close to 90% and constant with time on stream. 3.2. Effect of the Bronsted

acid sites concentration

and zeolite framework

The Bronsted acid sites are the active sites for the isomerization of n-butane and their strength and concentration are factors of paramount importance [2,6-10]. The effect of the concentration of the Bronsted acid sites in the MFI structure ( S-1 and H-ZSM-5 zeolites ) on the n-butane isomerization activity, measured after 2 h on stream, was studied in detail. As shown in Figure 3 H-ZSM-5 catalysts with a concentration of Bronsted acid sites less than ca. 1 x l020 sites/g, which correspond to less than 1 framework A1 atom per unit cell (uc), are not active.

100

WOx/ZrO2 = ~D

o

~9 0

80

v

w

1.

w

_

_

v

w

H-Y Si/A1 = 3.15

H-Mordenite Si/A1 = 25

60

H-Y Si/A1 = 2.5 w

~

.

-

.# .~9

40

~D

~

H-ZSM-5 Si/A1 = 16

20 0

0

5'0

160

z60

time on stream / min

300

Figure 2. Selectivity to isobutane at 300 ~ as a function of time on stream (WHSV=0.8 h-l).

720 As shown in Figure 1, amorphous MSA catalysts with ca.1.1 x l020 Bronsted acid sites/g is inactive. On the other hand Figure 3 shows that H-ZSM-5 with a similar concentration of Bronsted acid sites is active confirming that the Brensted acid sites in H-ZSM-5 are much stronger than the Si(OH)A1 groups in MSA [see Ref. 11 and references therein]. Figure 3 shows that in the range between ca. 1 and 6 x l 020 Bronsted acid sites/g the turnover frequency is constant. Between 6 and 7 xl020 Bronsted acid sites/g the activity of H-ZSM-5 levels off. Similar results were reported for H-Mordenite catalysts by Trung Tran et al. [7]: the rate of n-butane transformations is very low and not proportional to the concentration of Bronsted acid sites when they are less than ca. 1 per uc. A levelling off of the activity occurs at ca. 4.5 Bronsted acid sites/uc. The higher activity of ultrastable H-Y catalyst with Si/A1 = 3.15, in comparison to the H-Y catalyst with Si/A1 = 2.5, may be related to its higher surface area and micropore volume (see Table 1) which can contribute to an easier access to the Bronsted acid sites. We also recall that enhanced acidity of dealuminated H-Y zeolites compared to nondealuminated was observed by spectroscopic and catalytic experiment by Kotrel et al. [14]. In a previous work we found that dealuminated H-Y catalysts with framework Si/A1 atomic ratios in the range 3-11, and containing also same extra-framework aluminum species, are highly active and selective for n-butane isomerization [ 10]. Figures 4(a) and 4(b) show that in H-ZSM-5 the increase of the reaction rate, by either increasing the concentration of the Brensted acid sites or increasing the reaction temperature, leads to a decrease of isobutane yield and to an increase of the propane yield. Due to the fact that the diffusion coefficient of isobutane is much lower than that of propane in the H-ZSM-5 channels [ 15], we may tentatively suggest that at low rate the reaction is under chemical

25 o

_~ 20 "7

r.~

2 m 0

5

0

0

1

2

3

4

5

6

7

[H+] / ions g l x 10-20 Figure 3. n-butane isomerization on various H-ZSM-5 zeolites (WHSV-0.8 h-l). Correlation between isobutane production rate and the concentration of Bronsted acid sites.

721 control (main product isobutane), whereas at high rate the reaction is under diffusion control (main product propane). 3.3. Reasons for the slow deactivation of H-ZSM-5 and ultrastable H-Y catalysts H-ZSM-5 and ultrastable H-Y zeolites have a much higher stability with time on stream in comparison to both WOx/ZrO2, an acid catalyst extensively investigated for alkane isomerization [5], and the more active H-Mordenite and H-Beta zeolites. The deactivation of these catalysts is due to coke produced by entrapped unsaturated chains that cyclize to form mono- and poly-cyclic aromatics [2,6-9]. As previously reported [2,6-9], over H-Mordenite, H-Beta and H-ZSM-5 catalysts nbutane is converted to isobutene via a bimolecular mechanism at low temperature. In the first step, n-butane forms a butyl-carbenium ion by protonation at the strong Bronsted acid sites and subsequent dehydrogenation. A similar mechanism may be suggested in the case of H-Y catalysts. As shown in Figure 1, H-Ferrierite and H-L zeolites have a very low activity for n-butane isomerization, probably because their Bronsted acid sites are not strong enough to form the butyl-carbenium ion. The very weak Bronsted acid sites in MSA (A1-OH-Si) and S-1 ( internal silanols nests) [Ref. 11 and references therein] is the rationale to justify their inactivity. On the other hand, the activity for the 1-butene isomerization on H-Ferrierite is explained by Paz~ et al. [16] with a bimolecular mechanism, which would favour the formation of coke over zeolitic structures with monodimensional channels and pore apertures of molecular dimensions, like H-Mordenite, H-Ferrierite and H-Beta [17]. Moreover, note that "one coke molecule" trapped in a channel can block, in a monodimensional zeolite, the access of the reactant to all the acid sites of this channel.

80

i

i

i

(a)

i

i

i

9

80

60

60

40

40

20

20

0

0

20

40

60 80 Si/A1

100 120 140

0

'

300

(b)

350

T/~

'

400

Figure 4. (a) Conversion and selectivities for n-butane isomerization on H-ZSM-5 catalysts as a function of the Si/A1 atomic ratio at 300 ~ (WHSV=0.8 h-l). (b) Conversion and selectivities for n-butane isomerization on H-ZSM-5 catalyst with Si/Al=84 as a function of the reaction temperature. Symbols: 9 --- total conversion; 9 = selectivity to isobutane; A= selectivity to propane.

722 We suggest that the high activity and the stability with time on stream of ultrastable H-Y zeolites, in comparison to the other zeolites investigated in this work, may be tentatively explained by the 3-dimensional structure of the H-Y zeolite that favours the diffusion of reactant and product decreasing the residence time and the ensuing degradation to coke. 4. CONCLUSIONS The main result reported in this study is the remarkable activity and stability with time on stream of ultrastable H-Y catalysts for the isomerization of n-butane at 300 ~ The activity of these catalysts is lower than that reported for zirconia based catalysts and for HMordenite, and H-Beta catalysts. These more active catalysts, however, suffer a severe loss of activity with time on stream due to the poisoning of the acid sites by coke. The selectivity on H-Y catalysts is similar to that obtained on the most active tungsta/zirconia and sulfated zirconia catalysts. It seems that on ultrastable H-Y catalysts the formation of coke deposits is avoided due to the presence of strong Bronsted acid sites in the large supercages which favour the diffusion of the isomerized products as they are formed and hinder the formation of coke. Considering that in commercial practice high selectivity is usually more valuable than high activity, the relative comparison between ultrastable H-Y and H-ZSM-5, HMordenite and H-Beta catalysts is strongly in favour of the ultrastable H-Y zeolites. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

P. M~riaudeau and C. Naccache, Adv. Catal. 44 (2000) 505. R.A. Asuquo, G. Eder-Mirth and J. A. Lercher, J. Catal. 155 (1995) 376. K. Shimizu, N. Kounami, H. Wada, T. Shishido and H. Hattori, Catal. Lett. 54 (1998) 153. B. Li and R. D. Gonzalez, Catal. Lett. 54 (1998) 5. S. De Rossi, G. Ferraris, M. Valigi and D. Gazzoli, submitted for publication. R.A. Asuquo, G. Eder-Mirth, K. Seshan, J.A.Z. Pieterse and J. A. Lercher, J. Catal. 168 (1997) 292. M.- Trung Tran, N.S. Gnep, G. Szabo and M. Guisnet, Appl. Catal. A 170 (1998) 49. E. Babfirek and J. Novfikovfi, Appl. Catal. A 185 (1999) 123. E. Babfirek and J. Novfikovfi, Appl. Catal. A 190 (2000) 241. S. De Rossi, G. Moretti, G. Ferraris and D. Gazzoli, Catal. Lett., in the press. G. Moretti, C.Dossi, A. Fusi, S. Recchia, R. Psaro, Appl. Catal. B 20 (1999) 67. I. Kiricsi, C. Flego, G. Pazzuconi, W. O. Parker, Jr., R. Millini, C. Perego and G. Bellussi, J. Phys. Chem. 98 (1994) 4627. W.D. Harkins and G. Jura, J. Am. Chem. Soc. 66 (1944) 1366. S. Kotrel, J.H. Lunsford and H. Kn6zinger, J. Phys. Chem B, 105 (2001) 3917. B. Millot, A. M6thivier, H. Jobic, H. Moueddeb and M. B6e, J. Phys. Chem. B, 103 (1999) 1096. C. Pazb, B. Sazak, A. Zecchina and J. Dwyer, J. Phys. Chem B, 103 (1999) 9978. W.M. Meier, D.H. Olson and Ch.Baerlocher, Atlas of Zeolite Framework Types, Elsevier, Fifth Revised Edition 2001. World Wide Web under: http://www.izastructure.org/databases/.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

723

Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene +. J. L. Valverde*, A. de Lucas, P. S~inchez, F. Dorado and A. Romero Department of Chemical Engineering, Faculty of Chemistry. Castilla-La Mancha University. Campus Universitario s/n. 13071-Ciudad Real. Spain.

Metal loaded Ti-PILCs have been used as catalysts for the selective reduction of NO by propylene. Cu, Ni, and Fe ion exchanged Ti-PILCs were prepared. The influence of the metal loading was studied. When the metal loading increases, the catalytic activity also increases reaching a maximum of NO conversion and then decreased. Cu-TiPILs exhibited the highest NO conversion. Cu-PILCs prepared by impregnation were compared with those prepared by ion exchange. In general the ion exchange method resulted to be more adequate for the preparation of the catalyst. The presence of Cu 2+ species in the ion-exchanged samples could be the responsible of this behaviour.

1. INTRODUCTION. Pillared clays constitute one of the most studied families among the new groups of microporous materials developed by molecular engineering. These materials, also known as cross-linked clays or pillared inter-layered clays (PILCs), are synthesised by exchanging the inter-layered cations of the clay with inorganic polyoxocations, followed by calcination. The polyoxocations are then converted to metal oxide clusters by dehydration and dehydroxylation, leading to a permanently opening of the clay layers [ 1]. Properties as acidity, surface area, and pore size distribution of PILCs offer new shape selective catalysts similar to the zeolites. Nevertheless, thermal stability, lower than zeolites, limits their use as catalysts to specific reactions at relatively low temperatures. Emphasize in special titanium pillared clays (Ti-PILCs) by its catalytic activity in the selective reduction (SCR) of NOx of great importance from the environmental point of view. +Financial support from European Commission (ContractERK5-CT-1999-00001) and DGICYT (Direcci6n General de Investigaci6nCientificay T6cnica, Project 1FD97-1791, Ministryof Education, Spain) is gratefully acknowledged. *Corresponding author: Fax (+34) 926 29 53 18; e-mail:[email protected]

724 These materials showed an excellent thermal stability, high surface area and acidity, and its activity is almost unchanged in the presence of the poisons SO2 and H20, which are present in NOx containing streams [2]. Potential applications of PILCs in catalytic processes of a redox nature would require the clay structure to accommodate transition metal ions that are known to easily change their oxidation state [3]. A large number of catalyst, such as V 2 0 5 - W O 3 (or MoO3)/TiO2, other transition metals oxides (e.g., Fe, Cr, Co, Ni, Cu, Nb, etc.), and doped catalyst, as well as zeolite-type catalyst (e.g., H-ZSM-5, Fe-Y, Cu-ZSM-5), have been found active in this reaction. Despite the high activity of vanadium-based catalysts [4], major disadvantages remain, such as their toxicity and high activity for the oxidation of SO2 to SO3. In this work Ni, Fe and Cu have been used as metals for the preparation of active catalysts (metal-Ti-PILCs) in the SCR NOx reaction. The influence of the metal loading method for the Cu-Ti-PILCs catalyst preparation is also described.

2. EXPERIMENTAL.

2.1. Catalyst preparation. The starting clay was a purified grade bentonite (Fisher Company), with a particle size <2 ~tm and a cation exchange capacity of 97 meq g-1 dry clay. Ti-PILCs was prepared as follows. A pillaring solution was formed dissolving titanium metoxide in 5M HC1 until obtaining a molar relation HC1/metoxide of 2.5. This mixture was stirred at to room temperature for 3 h. The pillaring solution was then dropped to an aqueous clay suspension until obtaining 15 mmoles Ti/g clay. The mixture was kept under vigorous stirring for 12 hours at room temperature. Finally, the product was washed, dried and calcined for 2 h at 500~ Metal loaded samples (Ni, Fe and Cu) were obtained by ion exchange, using metal salts solutions. Cu was also introduced by the impregnation method. The resulting catalyst was calcined for 2 h at 400~ Ion exchanged samples were obtained adding 1g of sample to 200 mL of 0.05 M Cu acetate solution, under stirring for 15 h at room temperature. The ion-exchanged product was collected by centrifugation and washed several times with deionized water. Each sample had different metal loading, depending on how many times it was subjected to ion exchange. Impregnation samples were obtained pouring to the Ti-PILC the minimum amount of Cu(NO3)2 solution required to wet the solid. The slurry was placed in a glass vessel and kept under vacuum at room temperature until the solvent was evaporated. Each sample had different metal loading, depending on the Cu(NO3)2 solution concentration used.

2.2. Catalyst test. Activity tests of the catalysts were carried out in a fixed bed reactor. 1000 ppm NO, 1000 ppm C3H6, 5% 02 were used as flue gas component and He was used as a balance gas at a total flow-rate of 125 ml/min. The flow rates were controlled by calibrated Brooks flowmeters. The space velocity of the feed was 15000 h l (GHSV). The reaction was studied in the 200-400 ~

725 temperature interval. The outlet gases were analysed using a gas chromatograph equipped with a TCD detector and a 1010 Carboxen column (Supelco) for the separation of 02, N2, N20, C3H6, CO2 and CO2 and a chemiluminiscence NOx analyzer (Eco Physics CLD 700 EL ht) for NO and NO2. 2.3. Catalyst characterization. X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 diffractometer using Ni-filtered CuK~ radiation. To maximize the (001) reflection intensities, oriented specimens were prepared by spreading them on a glass slide. Surface area and pore size distributions were determined by using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Pillared clays were previously outgassed at 180~ for 16 h under a vacuum of 6.6 x 10 -9 bar. Specific total surface areas were calculated by using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at P/Po = 0.99. The Horvath-Kawazoe method was used to determine the micropore surface area and volume. Total acid-site density of the samples was measured by a temperature programmed desorption (TPD) of ammonia, by using a Micromeritics TPD/TPR analyzer. Samples were housed in a quartz tubular reactor and pretreated in flowing helium (99.999%) while heating at 15 ~ min 1 up to 500 ~ After 0.5 h at 500 ~ the samples were cooled to 180 ~ and saturated for 0.25 h in an ammonia (99.999%) stream. The sample was then allowed to equilibrate in a helium flow at 180 ~ for 1 h. Finally; ammonia was desorbed using a linear heating rate of 15 ~ rain 1. Temperature and detector signals were simultaneously recorded. The average relative error in the acidity determination was lower than 3%. Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described. After loading, the sample was outgassed by heating at 20 ~ min -~ in an argon flow to 500 ~ This temperature was kept constant for 30 min. Next, it was cooled to 25 ~ and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) flow. The temperature and detector signals were continuously recorded while heating at 20 ~ min 1. A cooling trap placed between the sample and the detector retained the liquids formed during the reduction process. TPR profiles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%. The metallic content (wt %) was determined by atomic absorption measurements by using a SpectrAA 220 FS analyzer. In all cases, calibrations from the corresponding patron solutions were performed.

3. RESULTS AND DISCUSSION. 3.1. Characterization of the catalysts. Figure 1 shows XRD patterns of the starting clay and the titanium pillared clay (Ti-PILC). It can be seen that the basal (001) peak around 2~=9 ~ (characteristic of the natural bentonite)

726 shift towards lower values of 28 on the pillared samples. This result clearly would indicate an enlargement of the basal spacing of the clay as a consequence of the pillaring process. A basal spacing about 24 .A was obtained in the titanium-pillared sample. It should also be noted from Figure 1 a second peak at about 7~ corresponding to an interlayer spacing ranging from 3-4 A. The presence of polymeric titanium species, smaller in size, could explain the emergence of this second peak. Moreover a homogeneous pillars distribution was reached as indicate the relative high intensity of the (001) XRD diffraction peak. Table 1 shows the metal loading, acidity and textural properties of samples. It can be observed the great increase on surface and micropore area of the Ti-PILC sample as compared with the starting clay. Obviously, this result is a consequence of the pillaring process. An increase of the metal loading led to a decrease of the surface area (mainly the micropore area) of the catalyst. This result could be explained by the partial blocking of the pillaring matrix by the metal species [5]. On the other hand, the acidity of Ti-PILC increased due to the increase on the micropore area (better accessibility of the NH3 molecules) and the presence of the pillaring titanium species with acid character. Moreover, the acidity of ion exchanged pillared clays depended on both the ion exchange transition metal and the metal loading. Cu and Fe ion exchanged samples showed an increase of the acidity with the metal loading. Nevertheless, the nickel loading reached in the preparation of Ni ion exchanged samples was lower, and accordingly the acidity kept practically constant:

I d(001)

,~

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=o

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,'2

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28 (~ Figure 1. XRD patterns of the starting clay and the titanium pillared sample (Ti-PILC).

727 60 A

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0 0

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6 8 10 Ni, Fe, Cu ( w t % )

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14

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Figure 2. Influence of the metal loading on the catalytic activity of SCR NO.

60 50-

A

04

Z o

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=

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r - - ~

Ion

exchange

- -impregnation

10-

0 z

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5

6

7

8

9

10

Cu ( w t % )

Figure 3. Influence of the preparation procedure of Cu-TiPILC samples on the SCR NO catalytic activity.

728 Table 1. Metal loading, acidity, and textural properties of samples. SAMPLE Bentonite TiPILC Fe-Ti-PILC Ion Exchange

Ni-Ti-PILC Ion Exchange

Cu-Ti-PILC Ion Exchange

Cu-Ti-PILC Impregnation

Metal Acidity Surface Area Micropore Area (wt %) (mmol NH3/g) (mZ/g) (m2/g) 0.0 0.0 5.8 8.0 12.6 15.5 1.6 2.9 3.4 3.6 4.6 7.4 9.0 9.5 4.6 8.0 8.6 9.7

0.132 0.529 0.441 0.469 0.608 0.668 0.467 0.468 0.470 0.472 0.620 0.731 0.894 0.766 0.500 0.502 0.508 0.515

35.2 273.2 244.3 217.5 201.3 197.4 260.3 247.3 236.3 222.7 241.6 234.3 201.8 198.8 259.3 228.0 226.6 225.0

15.1 224.5 195.2 154.8 143.1 149.8 195.7 180.2 173.5 160.4 202.2 189.1 153.7 137.5 227.1 202.9 198.7 197.4

Pore Volume (cm3/g) 0.069 0.266 0.234 0.234 0.241 0.200 0.269 0.278 0.265 0.257 0.226 0.236 0.219 0.233 0.229 0.207 0.198 0.180

Copper ion exchanged Ti-PILC showed the highest acidity due probably to the intrinsic acidity of the Cu 2+ species as demonstrated below by TPR analysis.

3.2. Catalytic Activity. Figure 2 shows the catalytic results achieved in the NO-SCR reaction by using Ti-PILCs ion exchanged with different amounts of Ni, Fe and Cu. Conversion of NO increased with the metal content until a maximum. On the other hand, Ni and Fe Ti-PILCs presented low conversions (under 35 %) as compared with Cu-Ti-PILC. Both the high acidity and the adequate redox characteristics of the copper species formed should explain this behaviour. It is important to note that the most effective temperature defined as the temperature of the maximum NO conversion was around 250~ over Cu ion exchanged samples, whereas Fe an Ni presented higher temperatures, 325 ~ and 425 ~ respectively. Since the metal providing the best results was the Cu, a thoroughly study comparing two ways to introduce this metal (ion exchange and impregnation) was carried out (Figure 3). When the Cu content is low the catalyst prepared by impregnation presents higher conversion than that obtained by ion exchange. Nevertheless, when the Cu content is high, similar conversion values are obtained. Sample prepared by ion exchange with 7.4 wt % of Cu presents the highest catalytic activity.

729 These results could be explained because both the preparation method and the Cu content influences the nature and the positions of the metal on the clay. TPR can be used to identify and quantify the metal species in samples. Figure 4 shows the Hz-TPR profiles of ion exchanged and impregnated Cu samples. The peak at the lowest temperature would be related with the presence of CuO aggregates [6]. The other two reduction peaks suggests a two-step reduction process of isolated Cu 2+ species [7]. The peak at the lower temperature would indicate that the Cu 2+ to Cu + reduction process occurred. The other peak at the highest temperature suggests that the produced Cu + was further reduced to Cu ~ As can be seen on Figure 4, the only peak that clearly appears on impregnated samples was the one at the lowest temperature with a small shoulder that could be related with the second reduction peak (Cu 2+ to Cu+), whereas the other two peaks are absent. On the contrary, Cu ion exchanged exhibited the three above-mentioned peaks. This result seem indicates that the increase on the catalytic activity observed in Cu ion exchanged samples as compared with the impregnated ones, is due to the presence of Cu 2+ species.

o

~

Impregnation

r/l

~ 250~ Ion Exchange

J 0

'

I

200

'

I

400

'

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600

Temperature (~ Figure 4. TPR profiles of Cu ion exchanged and impregnated samples.

730 Cu introduced by ion exchange firstly occupies very stable positions of the clay structure, practically inaccessible to the reagents or any molecule test. Once these positions are filled, the copper place in less stable but more accessible positions, which seem to be more catalytically active. The decrease on activity observed at high Cu content could indicate that the metal is deposited mainly as Cu oxide aggregates [6]. The impregnation method favours the deposition of Cu as Cu oxide on the surface, although it is accompanied of a simultaneous ion exchange process that lead the metal to accessible positions. 4. CONCLUSIONS. Metal loaded titanium pillared clays are active as catalysts in the SCR NO reaction. Cu loaded samples showed the highest activity as compared with Fe and Ni catalyst. The high acidity and mainly the redox nature of the Cu species are the responsible of this behaviour. The NO conversion increases with increasing the metal loading of samples, reaching a maximum and then, a decrease was observed. The preparation procedure of Cu loaded samples influences the catalytic activity. Cu ion exchanged samples showed the best results. This fact could be attributed to the presence of accessible Cu2§ species on the ion-exchanged samples. TPR result is in agreement with the higher acidity of theses samples.

REFERENCES

1. Gil, A.; Gandia, L.M., Catal. Rev-SCI. ENG, 42, (2000) 145. 2. A. Bahamonde, F. Mohino, M. Rd~oUar, M. Yates, P. Avila, S. Mendioroz, Catal. Today 69 (2001) 233. 3. K. Bahranowski, M. Gasior, A. Kielski, J. Podobinski, E.M. Serwicka, L.A. Vartikian, K. Wodnicka, Clays Clay Miner. 46 (1998) 98. 4. N.Y. Topsoe, H.Topsoe, J.A. Dumesic, J. Catal. 151 (1995) 241. 5. R.Q. Long, R.T. Yang, J. Catal. 196 (2000) 73. 6. R.T. Yang, N. Tharappiwattananon, R.Q. Long, App. Catal. B 19 0998) 289. 7. R. BulAnek, B. Wichtedovfi, Z. Sobalik, J. Tich:, Appl. Catal. B 31 (2001) 13.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Influence + propene.

o f cocations

on the activity o f C o - M O R

731

for N O / N 2 0

S C R by

I.Asencio*, F. Dorado, J.L. Valverde, A. De Lucas and P. Shnchez. Department of Chemical Engineering, Faculty of Chemistry. UCLM. Campus Universitario s/n. 13004-Ciudad Real. Espafia (Spain).

Zeolite based catalysts are known to be active for the selective reduction of NO by hydrocarbons. However, the activity strongly decreases with the presence of water in the feed. In this work, the effect of silver on the performance of Co-exchanged mordenite catalysts for NO and N20 reduction with propene in presence of poisons (1-120, SO2) has been investigated. Silver was chosen because it is active itself and it possesses a relativity weak af~nity for water [ 1]. Zeolite form (H vs. Na) is known to have influence on the catalytic performance as far as SCR by hydrocarbons is concerned [2], so that both acid and sodium zeolite forms were also studied.

I. I N T R O D U C T I O N

Selective catalytic reduction (SCR) of NOx by hydrocarbons is under investigation worldwide as a very interesting technique for NOx reduction to N2 in exhaust gas streams from both mobile and stationary sources. Since the 1992 disclosure by Li and Armor [3] of SCR of NOx with CH4 over cobalt zeolites, a large number of studies using cobalt and other metals and zeolite hosts have been reported. Several metals have been found to possess high selectivity for the reduction of NOx to nitrogen with hydrocarbons in the presence of excess oxygen. However, these catalysts are limited by a narrow operating temperature window and low hydrothermal stability. Most of the research was focussed on transition-metal-containing zeolites such as Cu/ZSM5 [4,5] and Co/ZSM5 [6,7,8]. The main problem of this kind of catalyst is the decline of activity in presence of poisons such water or sulphur oxide. In this work, we have studied the effect of cocations (Ag, Na) on the performance of Coexchanged mordenite catalyst, on the reduction of NOx. Nitric oxide, dinitrogen monoxide and the mixing, were used. The change in the catalytic activity upon the addition of water and + Financial support from European Commission (Contract ERK5-CT-1999-00001) and DGICYT (Direcci6n General de Investigaci6n Cientifica y Trcnica, Project 1FD97-1791,Ministry of Education, Spain) is gratefully acknowledged. * Corresponding author: Fax (+34) 926 29 53 18.

732 SO2 was also examined. Silver was chosen because it is active itself and it possesses a relativity weak ~ t y for water [ 1]. Zeolite form (H vs. Na) is known to have influence on the catalytic performance as far as SCR by hydrocarbons is concerned [2], so that both acid and sodium zeolite forms were also studied. 2. EXPERIMENTAL.

2.1. Catalyst preparation. Mordenite was supplied in the sodium form by PQ Corp., with an atomic ratio of Si/AI=7.5, crystallinity = 100% and 7.8 ~tm average particle size. The acid form of the zeolite was obtained by exchanging the Na + with 25 mL/g of 0.6 N HC1 under agitation at room temperature for 14 h. Metal incorporation was carded out by the ion exchange technique. The zeolite was added to Ag(NO3) and/or Co(CH3-COO)2 solutions (25 mL of 0.1 N solution/g of catalyst). The mixture was kept under agitation at 30 ~ for 14 h. Next, the suspension was filtered and thoroughly washed with deionised water in order to completely remove the occluded salt. The resulting solid was drying at 120 ~ for 14 h. After the last metal incorporation, the catalysts were air calcined at 550 ~ for 4 h. These catalysts were referred to as a function of the metal loading. For instance, Na-Ag(1.2%)Co(2.3%)-MOR corresponds to a sodium mordenite catalyst with a silver content of 1.2% and cobalt content of 2.3%, both by weight.

2.2. Catalyst test. Activity tests of the catalysts were carried out in a fixed bed reactor. 1000 ppm NO or 500 ppm N20 or 500 ppm NO+250 ppm N20, 1000 ppm C3I-I6, 5% 02 were used as flue gas component. 5% H20 and 50 ppm SO2 were used in some experiments. He was used as a balance gas at a total flow-rate of 125 mL/min. The flow rates were controlled by calibrated Brooks flowmeters. H20 was introduced in the reactor with a pump (SAGE Instruments, 341B). The space velocity of the feed was 15000 hq (GHSV). The reaction was studied in the 200-500 ~ temperature interval. The outlet gases were analyzed using a gas chromatograph equipped with a TCD detector and a 1010 Carboxen column (Supelco) for the separation of 02, N2, N20, C3I-I6, CO2 and CO2 and a chemiluminiscence NOx analyzer (Eco Physics CLD 700 EL ht) for NO and NO2. NO2 presence was not observed at any experiment.

2.3. Catalyst characterization. X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 di~actometer using Ni-filtered CuKa radiation. Total acid-site density of the samples was measured by a temperature programmed desorption (TPD) of ammonia, by using a Micromeritics TPD/TPR analyzer. Samples were housed in a quartz tubular reactor and pretreated in flowing helium (99.999%) while heating at 15 ~ min1 up to 500 ~ After 0.5 h at 500 ~ the samples were cooled to 180 ~ and saturated for 0.25 h in an ammonia (99.999%) stream. The sample was then allowed to equilibrate in a helium flow at 180 ~ for 1 h. Finally; ammonia was desorbed using a linear heating rate of 15 ~ minq. Temperature and detector signals were simultaneously recorded. The average relative error in the acidity determination was lower than 3%. Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described. After loading, the sample was outgassed by heating at 20 ~

733 min "1 in an argon flow to 500 ~

This temperature was kept constant for 30 min. Next, it was cooled to 25 ~ and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) flow. The temperature and detector signals were continuously recorded while heating at 20 ~ min"1. A cooling trap placed between the sample and the detector retained the liquids formed during the reduction process. TPR profiles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%. Details of TPD and TPR measurements were published in an earlier work [9]. The metallic content (wt %) was determined by atomic absorption measurements by using a SpectrAA 220 FS analyzer. In all cases, calibrations from the corresponding patron solutions were performed. Chemical compositions are listed in Table 1. 3. RESULTS AND DISCUSSION.

3.1. XRD analysis. X-ray measurements were carried out using Ni-filtered CuKcz radiation. Figure 1 shows the results of XRD measurements of all catalysts. For all the samples, only diffraction peaks of the mordenite are observed. Additional peaks indicating the presence of Co304 are not detected. The relative intensity of the strongest mordenite lines is lower in the modified catalysts than in the parent zeolite. Moreover, background of the Ag loaded samples is higher than Na-MOR and Na-Co (3.0%)-MOR ones. This fact seems to indicate that crystallinity of the zeolite was lower for the ion-exchanged samples. The relative intensity of the strongest peak for each sample is listed in Table 1. A peak at 20 = 32.8 ~ appearing in silver containing catalysts should indicates the presence of Ag20 [ 10].

Table 1. Composition and XRD analysis of mordenite catalysts. -iV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~

.....................................

Na-MOR

6.]

Na-Co(3.0%)-MOR

0.4

H - A g ( 1 . 4 % ) - C o ( 2 . 6 % ) - M O R ~ 0.1 ii

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100

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74

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Na-Ag(1.2%)-Co(2.2%)-MOR

ii

........................

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Relative intensity of Na Ag Co the strongest peaks content ~content c o n t e n t of M O R measured (wt.%) (wt.%) (wt.%) , by X R D

Catalyst

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734

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10

15

20

25

30

35

40

45

50

2 | Degree Figure 1. XRD patterns of." a) Na-MOR; b) Na-Co(3.0%)-MOR; c) Na-Ag(1.2%)-Co(2.3%)MOR; d) H-Ag(1.4%)-Co(2.6%)-MOR.

3.2. TPD analysis. Table 2 lists for all the catalysts the weak and strong acid site density and the metal content. In the same table the Co ion-exchange levels, determined taking as reference the number of aluminium atoms in the structure, are also summarized. It can be observed that the catalysts loaded with silver present more acidity than the sample without silver. This effect could be explain taking into account that silver can show strong Lewis acidity. Thus, an increase of silver amount would lead to an increase of strong acidity. For the H-Ag(1.4%)-Co(2.6%)-MOR sample an increase of strong acidity is obtained, due to the presence of protons in the zeolite.

735 Table 2. Weak and strong acid sites density ofmordenite catalysts. .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

: ................................

. ..............

=,- . . . . .

~ ............................................

,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~,-

.......

7 .....

...-,---~-

=-:=

Weak acid sites, Strong acid Co Ionsites density ~content exchange density (mmol NH3/g) (a) (mmol NH3/g) tb)~ (wt.%) level (%)

Catalyst

Na-Co(3.0%)-MOR

3.0

55

2.3

42

0.786

0.300

0.600

1.492

=

- .........................................................................................

Na-Ag(1.2%)-Co(2.3%)-MOR i

.........................................................................

? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~ H-Ag(1.4%)-Co(2.6%)-MOR i ....

2.6

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48

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:

.......................................................................................

:

0.387

1.763

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9_ .............................................

-:__- _ - -

(a) Desorption temperature = 300 ~ (b) Desorption temperature= 480 ~

3.3. TPR analysis. TPR of the catalysts provided useful information about reducibility of the cobalt and silver components of the catalysts. Na-Co(3.0%)-MOR sample shows only one reduction peak at 705 ~ (Figure 2).

/!

Na-Co(3.0%)-M OR

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,

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Na-Ag(1.2%)-eo(2.3%)-M OR

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736 According to the literature [6], this peak is assigned to the reduction of Co 2+ ions at charge compensation sites. For the samples with silver new peaks appears. The peak at 270 ~ may be assigned to the reduction of Ag20 species. The 385 ~ peak is assigned to the reduction of Co304 [6]. It is observed a shift of all the peaks to higher temperatures for the sample HAg(1.4%)-Co(2.6%)-MOR. As shown in Figure 2, the 385 ~ peak show a shift to higher temperature, over 500 ~ The corresponding peak to cobalt is not detected. The reducibility of transitions metals ions depends on the acidity of the catalyst. The ions are less reducible in the acid form. The strongly acidic protons obviously shift the equilibrium:

(1)

M 2+ + H2 ~ M ~ + 2H +

to the left, thus making more difficult the reducibility of transition metal ions. Consumption of H2 is also much lower for the acid catalyst. As observed, Co ~§ in not reduced at all. A strong interaction of Co ~+ and Ag § ions with the zeolite is now expected, which might change the redox properties of both metals.

3.4. Catalyst activity. Both conversion of NO, defined as (NOIN- NOotrr)/NOn~, and conversion of N20, defined as (N2OrN- N2Ootrr)/N2OtN, are given on Figures 3 to 8. Catalysts named Ag-Co were obtained in two ion-exchanged steps, the first one, as described above, with Ag and the second one with Co. In all the cases, feed is indicated. 100 90 80

100 90

80

~7o

"~ 50 T-4 20 10 0

200

t

~

~

!

i

250

300

350

400

450

500

Temoerature, ~ Figure 3. Effect of cocation activity of cobaltexchanged mordenite for NO-SCR by propene, [NO] = [C3H6] - 1000 ppm. [02] - 5%. (11) NaCo(3.0%)-MOR, (~) Na-Ag(1.2%)-Co(2.2%)MOR, (A)H-Ag(1.4%)-Co(2.6%)-MOR

70 60 50 40 30 20 10 0 200

250

300

350

400

450

500

Temoeratum, ~ Figure 4. Effect of cocation activity of cobaltexchanged mordenite for N20-SCR by propene. [N20] = 500 ppm, [C3I-I6] = 1000 ppm. [02] 5%. (El) Na-Co(3.0%)-MOR, (~) NaAg(1.2%)-Co(2.2%)-MOR, (~)HoAg(1.4%)Co(2.6%)-MOR

737 lOO 9o 8o

70

.~

60

40

~

50

60

20 20 10 0 200

10 250

300

350

400

450

500

0~ 200

250

Temoerature, ~

300

350

400

450

500

Temoerature, ~

Figure 5. Effect of cocation activity of cobaltexchanged mordenite for (NO+N20)-SCR by propene. [NO] = 500 ppm, [N20] = 250 ppm, [C3H6] = 1000 ppm. [02] = 5%. (ll, rt) NaCo(3.0%)-MOR, (O,~) Na-Ag(1.2%)Co(2.2%)-MOR, (A,/~)H-Ag(1.4%)-Co(2.6%)MOR. Open symbols: NO conversion. Filled symbols: N20 conversion.

Figure 6. Effect of cocation activity of cobaltexchanged mordenite for NO-SCR by propene in presence of water. [NO] = [C3I-I6] = 1000 ppm. [02] = 5% [ 1-120]= 5%. (ll) Na-Co(3.0%)MOlL (~) Na-Ag(1.2%)-Co(2.2%)-MOR, (A)H-Ag(1.4%)-Co(2.6%)-MOR

70 6O

20

1o

10 0

200

250

300

i

i

i

350

400

450

!

0

500

Temoerature, ~ Figure 7. Effect of cocation activity of cobaltexchanged mordenite for (NO+N20)-SCR by propene in presence of water. [NO] = 500 ppm, [N20] = 250 pprn, [C3H6] = 1000 ppm. [02] = 5%, [1-120]=5%. (ll, [!) Na-Co(3.0%)-MOR, ( ~ , ~ ) Na-Ag(1.2%)-Co(2.2%)-MOR, (A,/~)HAg(1.4%)-Co(2.6%)-MOR. Open symbols: NO conversion. Filled symbols: N20 conversion.

''

200

250

300

350

~

450

Temoerature. ~ Figure 8. Effect of cocation activity of cobaltexchanged mordenite for (NO+N20)-SCR by propene in presence of water and SO2. [NO] = 500 ppm, [N20] = 250 ppm, [C3H6] = 1000 ppm, [02] = 5%, [H20] = 5%, [SO2]= 50 ppm. (ll, i:1)Na-Co(3.0%)-MOR, ( ~ , ~ ) NaAg(1.2%)-Co(2.2%)-MOR, (A,A)H-Ag(1.4%)Co(2.6%)-MOR. Open symbols: NO conversion. Filled symbols: N20 conversion.

738 In the absence of water in the feed, when using NO, Na-Co(3.0%)-MOR and H-Ag(1.4%)Co(2.6%)-MOR showed high conversion levels (79.9% at 425 ~ and 72.4% at 425 ~ respectively). However, catalytic performance for Na-Ag(1.2%)-Co(2.2%)-MOR catalyst was lower (maximum conversion of 41.5%). According to literature [2], catalysts based on zeolites in sodium form are more active for NO conversion into N~ than those based on zeolites in acid form, but it is clear that this conclusion is not valid when silver is used as cocation. In the presence of water, Na-Co(2.3%)-MOR deactivated very fast, whereas HAg(1.4%)-Co(2.6%)-MOR showed a reasonable high activity (55.0% at 450 ~ for a long time on stream (12 h). According to Shichi [11], we can considered that, in presence of IT sites, water vapor suppresses the formation of carbonaceous materials and/or promotes the removal of carbonaceous materials deposited on the catalyst surface. Similar conversion levels (when compared at the same temperature) were observed for both NO and N20. However, N20 can be completely reduced at high temperature (~ 500 ~ see Figure 4). In the case of NO+N20, both in absence and presence of water, all catalysts showed lower conversion levels than that corresponding to NO or N20. In presence of H20 and SO2, the H-Ag(1.4%)-Co(2.6%)-MOR catalyst shows a low deactivation and good resistance for poisons. Higher acidity and Ag cocation can partly inhibit Co migration, avoiding deactivation. Migration to accessible sites and subsequent contribution to the reaction could be also possible. 4. CONCLUSIONS. It is possible to partly prevent the catalyst deactivation and to decrease the inhibiting water/water and sulphur effect by addition of silver as eocation to H-Co-MOR. According to the literature [12], the deactivation is mainly due to a metal migration but this migration is partly inhibited for the exchanged silver in the zeolite. A migration of silver to accessible sites and a contribution of silver to the reaction is also possible. REFERENCES

1. K. Masuda, K. Shinoda, T. Kato, T. Tsujimura, Appl. Catal. B 16 (1998) 359. 2. C. Torre-Abreu, M. Ribeiro, C. Henriques, G. Delahay, Catal. Lett. 34 (1997) 31. 3. Y. Li, J.N. Armor, US Patent 5149512 (1992). 4. Y. Li, J.N. Armor, Appl. Catal., B5 (1995) L257. 5. B.J. Adelman, T. Bentel, G.D. Lei, W.M.H. Saehtler, J. Catal., 158 (1996) 327. 6. X. Wang, H.Y. Chen, W.M.H. Saehtler, Appl. Catal. B, 26 (2000) L227. 7. A.Y. Stakheev, C.W. Lee, S.J. Park, P.J. Chong. Catal. Lett. 38 (1996) 271. 8. D.B. Lukyanov, E.A. Lombardo, G.A. Sill, J.L. d'Itri, W.K. Hall. J. Catal. 163 (1996) 447. 9. P. Cafiizares, A. de Lucas, F. Dorado, A. Durhn, I. Asencio, Appl. Catal. A 169 (1998) 137. 10. H.B. Zhang, W. Wang, Z.T. Xiong, G.D. Lin, Stud. Surf. Sci. Catal. 130 (2000) 1547. 11. A. Shiehi, A. Satsuma, T. Hattori. Chem. Letters. (2001) 44-45. 12. Z.Chajar, P. Demon, F. B. de Bernard, M. Primet, H. Praliaud, Catal. Lett. 55 (1998) 217.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

739

Catalytic p e r f o r m a n c e of m e s o p o r o u s silica S B A - 1 5 - s u p p o r t e d noble metals for thiophene h y d r o d e s u l f u r i z a t i o n M. Sugioka*, T. Aizawa, Y. Kanda, T. Kurosaka, Y. Uemichi and S. Namba* Department of Applied Chemistry, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan +Department of Materials, Teikyou University of Science and Technology, Unohara-machi, Kitatsuru-gun, Yamanashi 409-0913, Japan

The Pt/AISBA-15

catalyst showed

high

hydrodesulfurization of thiophene at 350~

and

stable

catalytic

activity for

the

and this activity was higher than those of

Pt/SBA-15 and commercial CoMo/A1203 catalysts. The Pt/A1SBA-15 catalyst has high sulfur-tolerant property toward hydrogen sulfide formed in hydrodesulfurization of thiophene. The Broensted acid sites of A1SBA-15 and the spillover hydrogen formed on Pt particle in Pt/A1SBA- 15 catalyst play an important roles for the hydrodesulfurization of thiophene.

1. I N T R O D U C T I O N Hydrodesulfurization (HDS) of petroleum feedstocks is one of the important processes in the petroleum industry to produce clean fuels. The CoMo/AI203 catalyst has been widely used in the HDS process of petroleum. However, recently, the development of highly active HDS catalysts, which exhibit higher activity than commercial CoMo/AI203 HDS catalyst, have been claimed in the petroleum industry to produce lower sulfur content fuels. It has been accepted that metal-zeolite catalysts have high possibility as a new HDS catalyst for petroleum [1, 2]. The authors have also investigated the development of highly active HDS catalysts based on zeolites [3-5]. Recently, mesoporous silicate materials such as MCM-41 and FSM-16 with large pore diameter are attracting wide attention as new materials for catalysts and catalyst supports. Some

740 attempts have been done to develop new HDS catalyst using Mo, Co(Ni)-Mo(W) and MCM-41 [6, 7]. In the previous papers [8, 9], we have reported that noble metals, especially platinum, supported on FSM-16 and MCM-41 showed high and stable activity in the HDS of thiophene. However, these mesoporous silicas have weak points with thin wall and mechanically unstable for use of industrial HDS catalyst. In the present work, we investigated the catalytic performance of noble metals supported on mesoporous silicates SBA-15 and AISBA-15, which have thicker wall than those of FSM-16 and MCM-41, for the HDS catalyst in order to develop much more highly active and mechanically stable mesoporous silicate-based HDS catalysts. 2.

EXPERIMENTAL

HDS of thiophene over noble metals supported on SBA-15 and A1SBA-15 was carried out at 350~ under 1 atm by using a conventional fixed-bed flow reactor. Thiophene was introduced into the reactor by passing hydrogen through a thiophene trap cooled at 0~

The dehydration of

2-propanol and cracking of cumeme on SBA-15 and AISBA-15 were performed at 200~ and 400~

respectively, using 30mg of catalysts by pulse reactor.

Noble metals supported on SBA-15 and A1SBA-15 (Si/AI=15) were prepared by an impregnation method using noble metal chloride aqueous solutions; the amount of metal loading was 5 wt%. All catalysts were calcined at 500~ for 4 hrs in air and were reduced at 450~ for 1 hr prior to the HDS reaction. XRD analysis of the catalyst was carried out by using Rigaku diffractmeter with CuK a radiation. Infrared spectra of pyridine adsorbed on SBA-15 and A1SBA-15 were observed by using Jasco b-T-IR spectrometer. We used SBA-15 with surface area; 847 m2/g and channel diameter; 53 A, and A1SBA-15 with surface area; 666

mZ/gand channel diameter; 55 A.

3. R E S U L T S AND D I S C U S S I O N 3.1. Catalytic activities of noble metals supported on SBA-15 and A I S B A - 1 5

The catalytic activities of transition metals supported on SBA-15 were examined at 350~ for the HDS of thiophene. It was found that the catalytic activities of transition metals/SBA-15 such as Ni(Co)/SBA-15, Mo/SBA-15 (5wt% loading), 15wt% Mo/SBA-15 and 5wt% Co(Ni) -15wt% Mo/SBA-15 showed low activity and their activity values were lower than that of commercial CoMo/AlzO3 catalyst. Thus, we examined the catalytic activities of noble metals/SBA-15 for the HDS of thiophene at 350~

It was revealed that the catalytic activities of

741 noble metals/SBA-15 varied remarkably with the kind of noble metals as shown in Figure 1. The order of the activities of these catalysts for the HDS of thiophene after 2 hrs reaction was as follows: Pt/SBA- 15 >Pd/SBA- 15 >Rh/SBA- 15 >Ru/SBA- 15.

100 ,

.

.

.

.

.

.

.

.

.

.

80

.

.

.

.

Presulfided CoMo/AI,03

60 I~ O

40

5wt%Pd/S

~ 2O t

5

%Pt/SBA- 15

5wt%Rh/SBA- 15

5wt%Ru/SBA- 15

/'~'7.~

0

~

~:z~

1

~

.

~ -

....

!

| ....

I

2 3 4 Time. on stream(hour)

W/F = 87.9 g. hr/mol,

.

I

5

I-I=/Thlophene = 30

Figure 1. Hydrodesulfurization of thiophene over noble metal/SBA-15 catalysts at 350~ The activity of Pt/SBA-15 was the highest among noble metals/SBA-15 catalysts and this activity was almost the same as that of commercial CoMo/AIzO3 catalyst. The reaction products in the HDS of thiophene over Pt/SBA-15 were mainly C4 hydrocarbons (butane 90%, butenes 9%) and small amount of CI-C 3 (1%) hydrocarbons. These results indicate that Pt/SBA-15 catalyst has high hydrogenating ability for unsaturated C 4 hydrocarbons and low hydrocracking activity for hydrocarbons in the presence of hydrogen sulfide. As the activity of Pt/SBA-15 catalyst was almost the same as that of CoMo/A1203 catalyst, we examined the catalytic activities of noble metals supported on AISBA-15 for the HDS of thiophene at 350~

in order to develop much more highly active SBA-15 based HDS catalysts.

It was found that the catalytic activities of noble metals/A1SBA-15 were higher than those of noble metals/SBA-15. Pt/A1SBA-15 showed the highest activity among noble metals/AlSBA-15 catalysts as shown in Figure 2. The Pt/A1SBA-15 catalyst showed high and stable activity and this activity was higher than that of CoMo/AI203 catalysts.

742

100 AISBA-15

80 P, 9 60 40

O

-

5wt%Pd/Ak~A-15 '1,,..~_=_..

-_ ~_ -_ _~ _= " _ - - _ ! '

\

20

5

II

m

m

_an

|

.,.i

. .....

_n

_-

V A-15

5wI~FIu/NSBA-15 -,

0

,.-,-~

:.. "

1

~.=

+

--~

2

"

I. . . .

3

4

I.

.

!

5

Time on stream(hour)

Figure 2. Hydrodesulfurization of thiophene over noble metal/AlSBA- 15 ~talysts at 350"C.

3.2.

Properties of Pt/AISBA-15 catalyst

As the Pt/A1SBA-15 catalyst showed high and stable activity for the HDS of thiophene, we studied in detail the catalytic properties of Pt/A1SBA-15 for the HDS of thiophene in order to clarify the cause of high activity of Pt/AISBA-15 for the HDS of thiophene. The effect of introduction of hydrogen sulfide on the catalytic activity of Pt/A1SBA-15 was examined in order to learn more about the origin of high and stable activity of Pt/A1SBA-15 catalyst. The introduction of hydrogen sulfide (3ml/min) was performed using a microfeeder with a glass syringe; the concentration of hydrogen sulfide in the hydrogen stream was ca.5 vol%. The catalytic activity of Pt/A1SBA-15 was remarkably decreased by the introduction of hydrogen sulfide in the course of HDS reaction. However, the decreased activity was almost restored after cutting off the introduction of hydrogen sulfide as shown in Figure 3. This shows that hydrogen sulfide is reversibly adsorbed on Pt/AISBA-15 and Pt/AISBA-15 catalyst has high sulfur-tolerant properties for the HDS of thiophene as well as Pt/FSM-16 and Pt/MCM-41 catalysts described in the previous paper [8, 9]. By this reason, Pt/AISBA-15 shows high and stable activity for the HDS of thiophene. We also examined the effect of introduction of ammonia on the catalytic activity of Pt/A1SBA-15 in the HDS of thiophene in order to clarify the role of acidic properties of Pt/A1SBA-15 in the HDS of thiophene. The introduction of ammonia (3ml/min) was carried out using microfeeder with glass syringe as well as that of hydrogen sulfide. It was revealed that

743 the catalytic activity of Pt/A1SBA-15 was decreased by the introduction of ammonia (ca.5 vol%) in the course of HDS reaction and the decreased activity was completely regenerated after cutting off the introduction of ammonia as shown in Figure 3. This result indicates that the acid site of Pt/A1SBA- 15 catalyst play an important role for the HDS of thiophene. 100

90 - ~ 80

NH~ Introduction (3 ml/min)

Introduction ~

70

v I::

o

60

(

= 50

>

)

oo 40 30

20 10 0

2

4

6

"lime on stream (hour)

8

10

Figure 3. Effect of introduction of hydrogen sulfide and ammonia on the catalytic acdvity of Pt/AISBA-15 in the hydrodesulfurization of thiophene at 350~

3.3. XRD analysis of Pt/SBA-15 and Pt/AISBA-15 catalysts Figure 4 shows the XRD analysis of A1SBA-15 and noble metal/AlSBA-15 catalysts before reduction. Almost the same XRD patterns as that of A1SBA-15 were obtained before and after loading of noble metals. This indicates that the structure of A1SBA-15 was maintained after loading of noble metals. In the case of SBA-15 before and after loading of noble metals, the situation was quite similar to that of AISBA- 15. Furthermore, almost the same XRD patterns of noble metals were observed in noble metals supported on SBA-15 and A1SBA-15 except platinum. No peaks of Pt were observed in the XRD analysis of Pt/A1SBA-15 but the sharp peaks of Pt were observed in the XRD analysis of Pt/SBA-15 as shown in Figure 4. These results indicate that Pt particles in Pt/A1SBA-15 are loaded on A1SBA-15 with high dispersion but Pt in Pt/SBA-15 is loaded on SBA-15 with relatively large particle size. Since we have reported that the acid site of HZSM-5 zeolite enhances the dispersion of Pt on HZSM-5 in our previous paper [5], high dispersion of Pt on AISBA-15 may be due to high acidity of A1SBA-15. By these results, it can be assumed that Pt/AISBA-15 has higher ability of activation of hydrogen, that is, the formation of spillover hydrogen, on highly dispersed Pt particles than Pt/SBA-15.

744

l i~ I~

a)AISEIA-- 15 b)RIVAISBA--- 15 c) Pd/AISBA-- 15

-" ~

;-;~:~

d) Ru/AISBA--- 15 e) Pt/AISBA-- 15 f)Pt/SBA-- 15

;:,,~='C'~~

9

_==

O)

o

c)

I 5

10 15 20 25 30 35 40 45 50 5 5 ' 6 0 2 0/degree

65 70 75 80 85 90

Figure 4. XRD patterns for noble metallAlSBA-15eatalystsbefore reduction. 3.4.

Mechanism of HDS of thiophene on Pt/AISBA-15 Catalyst

Pt/A1SBA-15 catalyst showed higher activity for the HDS of thiophene than Pt/SBA-15 and commercial CoMo/A1203 catalysts. We also studied the active sites and reaction mechanism in the HDS of thiophene over Pt/AISBA-15 catalyst. 100 90 8O vI:: 70 o

El 2-Propanol dehydration I= Cumene cracking

"~ 60 o

> g: 50 oo 40 30

20 10

Low activity SBA--15

AISBA-- 15

Figure 5. Catalytic activities of SBA-15 and AISBA-15 for the dehydration of 2-propanol (200~

and cracking of cumeme (400"C).

745 We evaluated the acidic properties of SBA-15 and A1SBA-15 by the reactions of 2-propanol dehydration (200~

and cumene cracking (400~

using pulse reactor. It was revealed that

SBA-15 showed very low activity for both reactions but AISBA-15 showed remarkably high activity for these reactions as shown in Figure 5. These results indicate that AISBA-15 has high acidity and there exists the Broensted acid sites on A1SBA-15. We also confirmed the existence of the Broensted acid sites at 1547 cm -1 on A1SBA-15 by the observation of b-T-IR spectra of pyridine adsorbed on AISBA-15 as shown in Figure 6. Thus, we supposed that the Broensted acid site of A1SBA- 15 acts as active site for the activation of thiophene in the HDS of thiophene.

ID

o r-co

b)AISBA-15

x~ 0

1700

1600

1500

1400

Wavenumber (cm -I)

1300

SBA- 15and AISBA- 15 were evacuated at 500~ for 2 hrs. Pyridine was

adsorbed at 150~ followed by evacuation at 150,"C for 0.5 hr. Figure 6. Infrared spectra of pyridine adsorbed on SBA-15 and A1SBA-15.

On the basis of these results, we propose a possible mechanism for the HDS of thiophene over Pt/A1SBA-15 as shown in Scheme 1. In the proposed mechanism, the Broensted acid site in the Pt/AISBA-15 acts as active site for the activation of thiophene and Pt acts as active site for the activation of hydrogen to form spillover hydrogen. The spillover hydrogen formed on Pt attacks the activated thiophene on the Broensted acid site on A1SBA-15.

746

HzS + C4Hydrocarbon

H~ H

H-~Hydmg~_

f~[

Activated thiophene ] , 4 - - - -

Pt

.

~

.

[~

.

.

.

.

H+ Br~nsted ,

0 9

.

"' '

~ ~ d , e ~ AISBA-15

. . . . . . . .

|

acid site 9

Acid" sltef " '

............

Scheme 1. The possible mechanism of hydrodesulfurization of thiophene over

Pt/AISBA-15 catalyst. 4. C O N C L U S I O N It was revealed that the Pt/A1SBA-15 catalyst showed high and stable activity for the HDS of thiophene and this activity was higher than that of commercial CoMo/AI203 HDS catalyst. Therefore, it is concluded that there is a possibility for using Pt/AISBA-15 as highly active new HDS catalyst for bulky organic sulfur compounds in the petroleum feedstocks. ACKNOWLEDGEMENT

This work was partly supported by KAWASAKI STEF.I~21 Century Foundation, Japan and Petroleum Energy Center of Japan. REFERENCES

1. M. Laniecki and W. Zmierczak, Zeolites, 11(1991)18. 2. Y. Okamoto, Catal. Today, 39(1997)45. 3. M. Sugioka, Erdol & Kohle, Erdgas, Petrochemie, 48(1995)128. 4. M. Sugioka, F. Sado, T. Kurosaka and X. Wang, Catal. Today, 45(1998)327. 5. T. Kurosaka, M. Sugioka and H. Matsuhashi, Bull. Chem. Soc. Jpn, 74(2001)747. 6. K. M. Reddy, B. Wei and C. Song, Catal. Today, 43(1998)261. 7. A. Wang, Y. Wang, T. Kabe, Y. Chen, A. Ishihara and W. Qian, J. Catal., 199(2001)19. 8. M. Sugioka, L. Andalaluna, S. Morishita and T. Kurosaka, Catal. Today, 39(1997)61. 9. M. Sugioka, S. Morishita, T. Kurosaka, A. Seino, M. Nakagawa and S. Namba, Stud. Surf. Sci. Catal., 125(1999)531.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

747

Skeletal Isomerization of 1-Hexene to Isohexenes over Zeolite Catalysts Zhihua Wu, Qingxia Wang, Longya Xu and Sujuan Xie Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, E R. China Several zeolite catalysts such as SAPO-11, ZSM-11, ZSM-12, etc. were selected to convert 1-hexene to branched hexenes in this work. Pore size of the zeolite catalyst plays an important role on the yield and the distribution of branched isohexenes. And the zeolite catalysts with the pore size of 0.6nm are optimum to produce dimethylbutenes (DMB). SAPO-11 zeolite is a suitable skeletal isomerization catalyst, especially in the production of methyl pentenes. Under the following reaction conditions: WHSV=I.0 h~, HJhexene=8, T=250 ~ P=0.2 MPa, the yield of skeletal isohexenes remains above 80% at the prolonged time-on stream of 80 h, accompanying low C5., C7+ products and low carbon deposition on the catalyst. 1. INTRODUCTION The catalytic reactions for converting unbranched olefins into branched olefins, such as the skeletal isomerization of n-butenes to isobutene, are important processes for the large-scale production of raw materials for chemical industry. To guide the screening of catalysts for the desired processes, tremendous of work has also been devoted to the mechanistic studies of these processes. To date, there are at least two proposed models for the skeletal isomerization of olefins, monomer model and dimerization model. Guisnete [1] reported that there were three steps from n-butenes to isobutene: (i) dimerization of n-butenes, (ii) skeletal isomerization of dimers, and (iii) cracking of the octene isomers. In contrast, Houzvickn [2] proposed that the dominating process for the skeletal n-butene isomerization was monomolecular and the bimolecular mechanism was mainly responsible for the formation of byproducts, such as propene and pentenes. Also, the results of Mooiweer [3] favored the mechanism of skeletal n-pentenes isomerization to isopentenes to be monomolecular. Isomerization reactions of olefins are affected by various factors. Asensi reported that the selectivity of n-butene to isobutene was greatly improved with the increased Si/A1 ratio in MCM-22. Further characterization of these catalysts revealed that the increased Si/A1 ratio led to a lower acid site density. Since these acid sites were proposed to the sites for the bimolecular side-reaction, a decreased acid site density in those catalysts was attributed to the increased isobutene selectivity [4]. Besides the acid site density, the pore size of the zeolites also affects the selectivity of the isomerization reaction. The results of Feng [5] indicated that the outcome of 2-methyl-2-pentene isomerization reaction was also greatly influenced by the pore sizes of zeolites. This was supported by the feeding experiment with several octenes over open-surface and microporous materials and it was found that the 10-membered ring (10-MR) channels were hardly accessible to double-branched hydrocarbons and the diffusion through the 10-MR by triple-branched were denied [2].

748 In the present paper, the catalytic performance of zeolites for the isomerization of 1-hexene to branched hexenes was investigated in a continuous-flow fixed bed reactor. Reported herein are the preliminary skeletal isomerization results. 2. EXPERIMENTAL

2.1. Catalyst preparation ZSM-11 (Si/AI=700), ZSM-35 (Si/AI=15) and ZSM-12 (Si/AI=50) zeolites were synthesized in our laboratory. SAPO-11 and Y-type zeolites were produced by another laboratory in our Institute of Chemical Physics. Si-ZSM11 and Si-SAPO11 were prepared by binding the zeolite and silica sol according to a definite weight ratio together, while the catalyst, A1-SAPOll, was prepared by binding A1203 and SAPO-11. The solids were calcined in air at 550 ~ for 3 h before reaction. Si-ZSM35 was prepared by binding silica sol and ZSM-35 zeolite, then was calcined in air at 550 ~ for 3 h. The catalyst was exchanged with 0.8 M ammonium nitrate solution two times (for 2 h each time), then impregnated with magnesium nitrate aqueous solution, calcined at 500 ~ for 2 h. The catalyst was about 8 wt % Mg loading. Si-ZSM12 was prepared as the catalyst Si- ZSM35, and the catalyst was about 1 wt % Mg loading. The catalyst, Si-Y, was prepared from NaY by exchanging with 0.8 M ammonium nitrate solution only one time.

2.2. Reaction performance 1-Hexene of 96.92% purity obtained from Acros Organics was used. The major impurities were 3-methyl-1-pentene (0.66%), 2- and 3-hexenes (2.41%). Olefin isomerization reaction was carried out in a microreactor (9 mm I. D., 39mm O.D.), with 3.5g catalyst (20-40 mesh). The reactor was heated from room temperature to 400 ~ at a rate of 200 ~ in a flow of hydrogen then maintained at 400 ~ for an hour. After that, it was cooled to the reaction temperature. As the desired reaction temperature was reached, the mixture feed of 1-hexene and hydrogen (1:8 molar ratio) was passed through the reactor instead of hydrogen. The tail gas was analyzed by an on-line gas chromatography equipped with a 9-m squalane column and TCD, while the liquid product was analyzed by a Varian 3800 gas chromatography with a 100-m Pond capillary column and FID. Yields to the different reaction products are calculated according to the following equation: % Yield (i) =100 •

weight of product i formed Weight of 1-hexene fed

2.3. Catalyst characterization 2.3.1. NH3 temperature programmed desorption (NH3-TPD) A catalyst sample of 140mg was first heated from room temperature to 600 ~ at a ramping rate of 25 ~ and then held at 600 ~ for 30 min under a flow of 30ml/min pure helium. The system was then cooled to 150 ~ in a He stream. At 150 ~ the adsorption of the catalyst was carried out in a He stream containing ammonia until it was saturated. Then, the sample was swept with helium. When the baseline of gas chromatography was stable, the NH3 desorption profile of the catalyst was performed from 150 ~ to 600 ~ at a heating rate of 20 ~ The amount of desorption NH3 was monitored by a thermal conductivity detector and quantified by the pulse method.

749 Table 1 Influence of temperature on the performance of Si-ZSM11 (H2/1-hexene=8, P=0.2 MPa, SV=I.0h "l) Temp. Yield of product (wt%) ~ Cs. 1-hexene hexene(-2,-3) branched hexenes 350 0.00 96.27 3.13 0.61 400 0.00 52.98 40.06 6.96 500 0.00 17.12 67.20 15.57

C7+ 0.00 0.00 0.11

2.3.2. Thermogravimetric Thermogravimetric (TG) data was acquired on a Perkin Elmer Pyrisl TGA apparatus. The used catalyst of about 10 mg was heated to 150 ~ and held at 150 ~ for 30 min under a flow of 20ml/min N2. Then N2 was switched to air and the catalyst was heated from 150 ~ to 800 ~ at a rate of 10 ~ and the weight of catalyst was monitored by the thermo-balance and recorded. 3. RESULTS AND DISCUSSION

3.1. Reaction performance of Si-ZSMll for skeletal isomerization of 1-hexene The effect of temperature on the performance of skeletal isomerization of 1-hexene to branched hexenes (BH) over Si-ZSM11 catalyst was studied. The results are shown in Table 1. The skeletal isomerizaion reaction does not occur until the reaction temperature rises up to 400 ~ And the amount of branched hexenes increases from 6.96% to 15.57% when the temperature increases from 400 ~ to 500 ~ The C7+ products appear at 500 ~ due to the polymerization of hexenes. Si/A1 ratio in ZSM-11 zeolite is 700, and the average distance of an A13§ ion in zeolite to the closest one is 4.23 nm, while the length of a 1-hexene molecule is 1.03nm. This means that the closest distance between A13+is 2 times greater than the size of a 1-hexene molecule. This excludes the possibilities of the interaction of 1-hexene absorbed on different A1> sites. Thus, the branched isohexenes in the product without C5. and Cv+ at 400 ~ might come from monomolecular hexenes adsorbed on the catalyst. In a word, the skeletal isomerization of 1-hexene to branched hexenes is monomolecular. However the farmation of C7+ at 500 ~ might come from the direct reaction between the hexene absorbed on the acid site of catalyst surface and the 1-hexene existed in the gas phase. The formation of C7+ is agreement with that of Eley-Riedeal mechanism. Thus, it can be inferred from the above results that the skeletal isomerization of 1-hexene to isohexenes over Si-ZSM-11 zeolite catalyst is monomolecular mechanism. 3.2. Reaction performance over difference zeolites Here, we investigated the relation between skeletal isomerization of 1-hexene to BH and the acid density of catalysts with similar acid strength, and table 2 shows reaction performance. The results from Figure 1 show that the acid densities of catalysts decrease as the following: Si-Y >> Si-ZSM35>Si-ZSM12>>Si-SAPOll, while the values of isohexenes over the catalysts from Table 2 are: Si-ZSM35>Si-SAPOll> Si-ZSM12 >Si-Y. The results show that the yield of branched BH over Si-ZSM35 is highest and that of Si-Y is the lowest. Since the acid site density of Si-Y is the highest among the catalysts used. The above results indicated that the acid density of a catalyst is not the sole factor directly related with the

750 Table 2 Reaction results of 1-hexene isomerization to isohexenes over catalysts (H2/1-hexene=8, P=0.2 MPa, SV=I.0h l, T=270 ~ Yield of product (wt %) Catalyst Pore diameter Acidity* (nm) (mmol/g) C5. Hexene (-1, BH (DMB)

-2,-3)

Si-Y 0.80-0.90 0.439 Si-ZSM12 0.57x0.61 0.244 Si-ZSM35 0.42x0.54 0.290 Si-SAPO11 0.39x0.63 0.035 * Values calculated from NH3-TPD

0.31 2.20 0.58 0.00

93.99 35.22 25.96 46.68

5.09 (0.13) 48.20 (3.34) 69.11 (3.95) 53.32 (1.07)

C7+ 0.61 14.38 4.35 0.00

skeletal isomerization of 1-hexene to BH if it plays an important role. Microporous materials produce less dimerization reaction than open-surface materials [2]. This implies that the pore size of the catalyst might also be crucial for the catalytic production of the desired products. And this led to our further studies on the effect of catalyst pore diameter on the selectivity of the catalyst. The results from Table 2 indicate that the catalyst pore diameter between about 0.4 nm to 0.6 nm was optical for the 1-hexene skeletal isomerization. When the catalyst pore diameter is above 0.8nm, the value of branched hexenes is very low. Our data clearly indicate that, besides the acid site density, the micropore size of the zeolite is responsible for the highest selectivity shown in the case of Si-SAPO 11. The percentage of each branched isohexene in the product mixture may be affected by the catalyst pore diameter. Figure 2 shows the ratio of the dimethylbutenes (DMB) percentage in the branched isohexene mixture over catalysts tested in our experiment to that of calculated equilibrium value (Equilibrium value refers to ref. [6]). Although the yield of branched isohexenes over Si-SAPOll is more than that over Si-ZSM12, the yield of DMB over SiSAPO 11 is less than that over Si-ZSM12. The differences in the product distribution over the catalysts used might be attributed to the pore size differences. Microporous materials such as SAPO-11 do not allow free diffusion of tribranched or even dibranched hydrocarbon [2]. It is highly possible that the wall of 0.39 nm wide pore 0.5 0.4

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Fig. 1 NH3-TPD profiles of different zeolites

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Fig.2 Ratio of DMB percentage in BH in this test to that of equilibrium.

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suppresses DMB production. Or even if the DMB were produced in the pore of SAPO-11/Si with high percentage, the very slow diffusion rate of DMB would make DMB stay in the pore. All of these might explain the low percentage of DMB in the product mixture. It is conceivable that catalyst with pore size about 0.6 nm might generate a product mixture with higher DMB percentage. 3.3 Reaction performance of Si-SAPOll under different reaction conditions Based on the discussion above, it can been seen that Si-SAPOll is a good catalyst for skeletal isomerization of 1-hexene. The effects of space velocity and reaction temperature on the performance of skeletal isomerization of 1-hexene to isohexenes over Si-SAPO 11 catalyst were investigated. The results are shown in Figure 3 and 4. The higher the space velocity is, the lower the yield of skeletal isohexenes is. The DMB percentage in the branched isohexene mixture decreases with increasing space velocity. Table 3 exhibits the production rate of methyl pentenes (MP) and DMB at various space velocities. The result shows that the production rate of MP is larger than that of DMB. If every acid site can convert 1-hexene to skeletal isohexenes, at the utmost about 180 MP and 2.3 DMB can be produced one hour at one site over Si-SAPO 11 catalyst at 250 ~ Since DMB come from the skeletal isomerazation of monomethyl pentenes, which in turn come from the skeletal isomerizatiion of 1-hexene [7]. Thus, two consecutive isomerization steps are required to produce DMB from 1-hexene. As an intermediate for the production of DMB, it is reasonable that monomethyl pentene production is higher than that of DMB.

Table 3 Influence of space velocity on the produce rate (H2/1,hexene=8, P=0.2 MPa, T=250 ~ WHSV( h 4) ............. 0,5 ....... 1.0 MP Produce rate(mmol, h 1. g-l) 3.10 5.27 DMB produce rate(mmol, h "1. g4) 0.051 0.080

1.5 6.35 0.074

2.0 6.15 0.063

752 Table 4 Influence of binder on the reaction performance (H2/1-hexene=8, WHSV=I h "l, P=0.2 MPa, T=250 ~ Catalyst Yield of produce (%) DMB/BH C 5. BH C7+ (%) Si'SAPO 11 0.00 44.97 0.00 1.5 A1-SAPO11 0.37 79.19 7.84 7.1

Acidity (~tmol/g) 350 ~ 450 ~ 33.0 1.8 62.5 16.5

600 ~ 2.2

When the reaction temperature rises from 250 ~ to 310 ~ the yield of skeletal isohexenes increases and achieves the highest value at 310 ~ The results also reveal that the C1 and C2 product are not observed in the temperature range between 250 ~ and 340 ~ The yield of Cs is larger than that of C4 in the temperature range between 280 ~ and 340 ~ The yields of C5. and C7+ products, especially that of propane, rise quickly with increased temperature. The yield of propane increases from zero at 280 ~ to 4.36% at 340 ~ while that of Cs only increases from 0.06% to 0.72%. At 340 ~ the ratio of C9/C3 is only 0.036, while the ratios of C7/Cs and C8/C4 are 2.014 and 6.653 respectively. The wide production ratio distribution indicates that there might be multiple mechanisms employed in this catalytic process. The relative low C9/C3 ratio compared to these of C7/C5 and C8/C4 implies that the production mechanism of C3 might be different from that of C4 and C5. The dimerization- cracking process produces C4 and Cs product, however maybe 13-scission of polymers produces C3 product [7]. The high reaction temperature is more benefit for 13-scission than dimerizationcracking. 3.4. Influence of binder on the reaction performance on SAPO-11

Table 4 shows the effect of the binder on the reaction performance of SAPO-11. And Figure 5 shows the NH3-TPD of A1-SAPO 11 and Si-SAPO 11. Compared to Si-SAPO 11, A1SAPOll is more acidity. And the yields of both DMB and by-product are higher. The alumina, which is impregnated in the form of an acidic aluminium nitrate, has enough acidity to efficiently convert 1-pentene to skeletal isomers [8]. In the process of binding SAPO-11 and A1203, nitric acid was added and aluminium nitrate is produced and then impregnated onto A1203,. This process is equivalent to the direct impregnation of aluminium nitrate on A1203. This is why the acidity of A1-SAPO 11 is higher than that of A1-SAPO 11. Since these acidic sites are responsible for the isomerization of 1-hexenes to BH, it is expected that the yield of BH over A1-SAPO11 is higher than that over Si-SAPO 11. The surface of alumina is opensurface, and open-surface favors the dimerization of olefins and cracking ' ' AI-'SAP() 1 1' l, Si-SAPO 1 1 ] dimerization [2]. Thus, the higher yields of both C5. and C7+ products over A1r SAPO 11 catalyst, as indicated in Table 4, Eare expected and consistent with the ...::::::::::l- .... properties of the corresponding catalyst 2;o3;o 4;o s;o 600 used. Desorl)tion Temperature(~ Neither Cl, C2, nor C3 product is observed in the product mixture in the case of A1-SAPO 11 at 250 ~ This is Fig.5 NH3-TPD profiles of different catalysts

ill

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different to that over Si-SAPO 11 at 340 ~ Since the production of C 3 products is proposed to be the result of the [3-scission mechanism, the higher yield of C3 product at higher temperature indicates that the [3-scission mechanism is favored at higher temperature. 3.5. Stability test of AI-SAPOll catalyst Figure 6 shows the stability of A1-SAPOll catalyst at WHSV = 1.0 h 1, HJhexene = 8, T-250 ~ P=0.2MPa. The yield of branched hexenes is usually 80% or above when the reaction time is less than 78 hours. After 126 hours the yield of skeletal isohexenes is still higher than 60% at the same reaction condition. The test shows that the catalyst is robust and has a relatively stable performance over long time. The C~, C2 and C3 products have not been observed from the start to the end of the stability test, while a few of C4 and Cs products are observed. The yield of Cs. is always less than 0.4%. The sum of C5 is larger than that of C4. It is also shown that the yield of C7+ is always larger than that of C5.. When the carbon number of product is larger than 6, the yield of the corresponding product decreases with the increasing chain length. Also, both C5. and C7+ products decrease with increasing time on stream. The DMB yield always decreases with increasing time on stream, even when the yield of BH keeps above 80%. The yield of DMB decreases from 5.60% at the start to 95 2.21% at 78 h, and to 1.37% at 126 h. Accordifigly, the ratio of DMB/BH ,.c:: decreases from 7.07% to 2.78%, and to 93 2.27%. These results imply that the active sites responsible for the production of the DMB lost activity at a rate faster than that 2;0 360 460 560 660 760 of MR Figure 7 shows TG of used A1-SAPO11 Temper ature (~ catalyst. The coke is about 4.3 w. %. The Fig.7 TG of used A1-SAPO11 abruptly temperature point of weight lost is "~

~JO

94'

754 about 470 ~ active sites.

This shows they might be responsible for the lost of activity of the catalytic

4. CONCLUSIONS Our data presented in this paper favors the monomolecular skeletal isomerization of 1hexene to branched isohexenes over Si-ZSMll zeolite catalyst. Also, the skeletal isomerization of 1-hexene to isohexenes is not only influenced by the acid strength and acid site density, but also by the zeolite catalyst pore size. It has also been found that the A1-SAPO 11 catalyst is an excellent catalyst for the skeletal isomerazation of 1-hexene. High yield of skeletal isohexenes with monomethyl pentenes as the major product, and high catalyst stability are obtained. Furthermore, the low yields of the side products, such as C5 and C7+ products, and low carbon deposition on the catalyst over long time make this catalyst attractive for future target for further optimization. ACKNOWLEDGEMENT We thank Dr. L. Xu for providing SAPO-11 zeolite used in this paper. REFERRENCES 1. M. Guisnet, E Andy, N. S. Gnep, E. Benazzi and C. Travers, J. Catal., 158 (1996) 551. 2. J. Houzvicka and V. Ponec, Ind. Eng. Chem. Res., 36 (1997) 1424. 3. H. H. Mooiweer, K.P. de Jong, B.Kraushaar-Czametzki, W.H.J. Stork and B.C.H. Krutzen, Stud. Surf. Sci. Catal., 84 (1994) 2327. 4. M. A. Asensi, A. Corma, and A. Martinez, J. Catal., 158 (1996) 561. 5. X. Feng, J. S. Lee, J. W. Lee, J. Y. Lee, D. Wei and G. L. Haller, Chem. Eng. J., 64 (1996) 255 6. J. E. Kilpatrick, E. J. Prosen, K. S. Pitzer and E D. Rossini, J. Res. Nati. Bur. Standarts., 36 (1946) 559. 7. W. A. Groten and B. W. Wojciechowski, J. Catal., 122 (1990) 362. 8. C. Lin, H. Yang, C. Lai, C. Chang, L. L. K. Kuo and K. Yung, Skeletal Isomerization of Olefins with an Alumina Based Catalyst, US Patent No. 5 321 193 (1991)

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

755

Preparation and catalytic characterisation o f Al-grafted M C M - 4 8 materials M. Rozwadowski, *aM. Lezanska, a J. W l o c h , a K. Erdmarln, a and J. Komatowskib aFaculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torun, Poland bLehrstuhl II fttr Technische Chemie, Technische UniversiN't Mttnchen, Lichtenbergstr. 4, 85747 Garching bei Mttnchen, Germany

Samples of A1-MCM-48 were prepared by grafting A1 onto the pure siliceous material and used as catalysts for cumene cracking and conversion of 2-propanol. The former reaction yielded mainly benzene and propene, which indicated the presence of strong Bronsted acid sites in the catalysts. The conversion of 2-propanol resulted mainly in dehydration of the substrate, yielding propene and diisopropyl ether. The catalytic activity of A1-MCM-48 grew with both the A1 content and reaction temperature. The concentrations of the Bronsted and Lewis acid sites increased with the A1 content of the material as well. 1. INTRODUCTION Mesoporous molecular sieves of the M41S family [ 1,2] have extensively been studied with respect to their unique properties [3-6]. Many efforts have been focused on silica- and alumina-based materials as potential catalysts for the reactions involving large organic molecules [7-10]. Purely siliceous M41S does not show significant catalytic activity because of its electrically neutral skeleton with no ion-exchange capability. However, substitution of silicon with various metals generates acidity in these materials and modifies their surface properties. This is a promising way to synthesise materials applicable in catalysis [11,12]; for example, introduction of boron [7,13], titanium [14-16], vanadium [17], and gallium [8] has been reported. Incorporation of aluminium is also interesting in relation to catalytic applications and has been discussed in numerous papers, especially in the case of the MCM-41 materials. Reports, although not so many, on the introduction of A1 into MCM-48, another member of the M41S family, have been published as well [18-20]. Such a modification of the M41S structure seems to be of particular importance as it can give rise to the Bronsted acid sites. These centres should primarily be responsible for the catalytic activity of the mentioned materials. Generally, the Al-containing molecular sieves can be obtained by a hydrothermal (i.e., direct) synthesis or by post-synthesis methods of impregnation or grafting. Jun and Ryoo [21 ] investigated the catalytic activity of mesoporous molecular sieves of different channel systems (MCM-41, MCM-48, and KIT-I; Si/A1 = 19 and 38) in the Friedel-Crafts alkylation reaction. They demonstrated that the materials prepared with the post-synthesis procedures were superior to those synthesised directly with respect to the structural order and

756 accessibility of the A1 centres to reactants. The authors suggest that the latter is caused by the fact that, in the case of the hydrothermal synthesis, a part of A1 becomes located inside the pore walls, especially when the A1 content is relatively low. Cheng et al. [22] showed that A1grafted MCM-41 exhibited a considerably higher acidity as compared to that of A1-MCM-41 obtained hydrothermally (both materials with Si/A1 = 20). This was reflected in the results of cumene cracking. However, when Si/A1 was in the range of 1-6 [23], the materials synthesised directly exhibited a higher acidity but their structure was not typical of MCM-41. On the other hand, the Al-grafted samples retained the MCM-41 structure. Corma et al. [24] found that the acid strength of A1-MCM-41 synthesised hydrothermally was lower than that of zeolite USY and higher than that of amorphous aluminosilicates. The aim of this work was to study the catalytic reactions of cumene cracking and conversion of 2-propanol over the Al-grafted MCM-48 samples. It was expected that the content and/or distribution of A1 might affect the strength of the Bronsted acid centres similarly as in zeolites. Therefore, we attempted to correlate the postulated reaction mechanisms with the acidic strength of these sites.

2. EXPERIMENTAL

2.1. Samples The MCM-48 material was synthesised from a mixture containing suspension of SiO2 (Ultrasil, Degussa) in water and both tetramethylammonium hydroxide and cetyltrimethylammonium chloride as templates [25]. Four different A1-MCM-48 samples were prepared by grafting aluminium onto the purely siliceous MCM-48 parent material. Aluminium isopropoxide dissolved in n-hexane was chosen as the source of aluminium for the grafting process. The resulting materials were calcined at 803 K under air for 4 h. The samples are referred to as A1-MCM-48(n) where n denotes the Si/A1 molar ratios in the reaction mixtures, equal to 32, 15, 5, and 2. The Si/A1 ratios of the calcined A1-MCM-48 samples were determined with the atomic absorption spectroscopy (AAS) (see Table 1). More details on the sample preparation can be found elsewhere [26]. 2.2. Catalysis The catalytic tests were carried out with a pulsed method using a vertical flow microreactor connected to a Shimadzu GC-14B gas chromatograph equipped with a flame ionization detector. The catalyst samples (5 mg) were placed in the reactor and treated thermally at 723 K under helium for 1 h. Cumene was injected at 25-min intervals (eight injections, 1-~tl portions) and the reaction was run at 623,673, and 723 K. The chromatographic column was packed with Carbowax 4000 and the carrier gas (helium) was flowing at a rate of 30 ml/min. In the case of 2-propanol, four injections (1-~tl portions) were applied in 15-min intervals, the reaction temperatures were 523 and 573 K, and the column was packed with Porapak N.

2.3. Acid sites For the analysis of the Bronsted and Lewis acid sites present in the studied A1-MCM-48 materials, the IR spectra were recorded with a Bruker 48 PC spectrometer equipped with a MCT detector. The samples in the form of wafers were activated in situ in the IR cell at 633 K for 1 h. Then, pyridine (POCh, Poland, dried over KOH) taken in excess of the amount necessary to neutralise all the acid sites was adsorbed at 430 K. Subsequently, the physisorbed

757 pyridine was removed under 30-min evacuation at the same temperature and then the IR spectra were recorded. Concentrations of both the Bronsted and Lewis acid sites were calculated from intensities of the IR bands assigned to pyridinium ions (HPy +) and to pyridine molecules bonded to Lewis sites (PyL) at 1545 and 1455 cm-1, respectively. The extinction coefficients used for the calculations were determined for pyridine adsorbed on both the zeolite HY containing only the Bronsted acid sites and the dehydroxylated zeolite HY containing only the Lewis acid sites. They were equal to 0.070 and 0.100 cm 2 gmo1-1 for HPy + and PyL, respectively.

3. RESULTS AND DISCUSSION

The low-angle XRD powder patterns of the studied samples demonstrated a set of peaks (including the 211 and 220 reflections), indicating a typical system of uniform cubic pores [26]. These pores are considered as the primary mesopores while void space between adjoining crystallites and large mesopores in the particles that do not form any ordered structures are referred to as the secondary mesopores [26]. The combined volume of both the primary and secondary mesopores is defined as a total pore volume. Table 1 shows some structural parameters of the studied samples. Although the values of SBET and Vt somewhat decreased with the increase in the content of A1, they were relatively high. This suggested that the materials might exhibit noteworthy catalytic properties. The reaction of the catalytic cracking of cumene results in a series of compounds with different numbers of carbon atoms in a molecule, propene and benzene being the main products [27,28]. Comparison of the level of the cumene conversion performed over different samples at a given temperature allows one to arrange these samples with respect to their acidity [29]. Here, it was found that the cumene conversion increased with the content of aluminium in the A1-MCM-48 materials (Fig. 1). For a given sample, the cumene conversion Table 1 Structural parameters of the studied MCM-48 materials [26] Parameter

Sample parent A1-MCMA1-MCMA1-MCMA1-MCMMCM-48 48(32) 48(15) 48(5) 48(2) Si/A1 (AAS) n.a. 34.5 12.7 3.8 3.5 d211 [nm] 3.71 3.40 n.d. 3.32 n.d. ao [nm] 9.10 8.33 n.d. 8.14 n.d. SBET [m2 g-~] 1315 1245 1188 1051 1030 2 -1 St [m g ] 1294 1213 1164 1010 995 Next [m2 g-l] 284 259 136 154 209 Sp [m2 g-l] 1010 954 1028 856 786 Vp [cm3g-l] 0.718 0.662 0.673 0.538 0.513 Vt [cm 3 g-l] 0.958 0.884 0.794 0.713 0.711 d211 is the (211) interplanar spacing, a 0 - unit cell parameter, aBET- the BET specific surface area, St - total surface area, Sr - external surface area, Sp - surface area of primary mesopores, Vp - volume of primary mesopores, Vt - total pore volume, n.a. - not applicable, and n.d. - not determined.

758 increased also with the reaction temperature, as seen for A1-MCM-48(5) (Fig. 2). A similar picture was observed for A1-MCM-48(2)while for A1-MCM-48(15)and A1-MCM-48(32)the increase in the conversion with the temperature was clearly lower. In general, the conversion decreased slightly with the number of injections (Figs. 1 and 2). The rate of this decrease was more pronounced for the samples with higher contents of A1 and practically independent of the reaction temperature. These observations suggest a more efficient coking of the catalysts with the higher A1 contents. 50

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Figure 1. Cumene conversion at 723 K over A1-MCM-48 with different Si/A1 ratios: 3.5 (+), 3.8 (O), 12.7 (A), and 34.5 ( 9

Figure 2. Cumene conversion at 623 ( 9 673 (A), and 723 K (F]) over A1-MCM-48 with Si/A1 = 3.8.

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Figure 4. Selectivity of cumene conversion at temperatures indicated over A1-MCM-48 with Si/A1 = 3.8; b, benzene, p, propene, m, a-methylstyrene; 1 and 4 denote selectivities after the injections no. 1 and 4, respectively.

759 In accordance with the literature findings, benzene and propene were the main products of the cumene cracking over A1-MCM-48 (Figs. 3 and 4). a-Methylstyrene was another product found in significant amounts. Some not determined compounds were also observed though they were present in trace amounts only. These products were neglected while calculating selectivity. At 723 K, the relative yield of benzene and propene slightly increased and that of a-methylstyrene slightly decreased with the growth of the aluminium content of the catalysts (Fig. 3). On the other hand, the relative yield of benzene and a-methylstyrene slightly decreased whereas that of propene slightly increased with the rising reaction temperature, as observed for A1-MCM-48(5) (Fig. 4). The selectivities of all the products did not change much with the injection number (Fig. 4). As known [27,29], benzene and propene are formed on strong Bronsted acid sites while a-methylstyrene forms at electron-acceptor centres. Thus, the presented observations (Fig. 3) suggest that the number of the electron-acceptor centres decreased while that of the Bronsted acid sites slightly increased with the A1 content of the catalysts. According to stoichiometry of the reaction, the cracking of cumene should yield equal amounts of benzene and propene. The observed lower amounts of propene (Figs. 3 and 4) result most probably from the fact that propene undergoes to a greater extent the conversion to carbonaceous deposits, especially at lower temperatures. In the case of conversion of 2-propanol, two reactions were assumed to occur: (i) dehydration, which leads to formation of propene and diisopropyl ether and (ii) dehydrogenation, which yields acetone [30,31 ]. As found here, the conversion at 523 K increased from c a . 50 to 100% with the Si/A1 of the A1-MCM-48 samples decreasing from 34.5 to 3.5 (Fig. 5). Thus, the catalytic activity of the studied materials grew clearly with the A1 content of the catalysts. At 573 K, the conversion over all the catalysts was approximately 100%. Interestingly, the conversion did not depend on the number of injections. Propene was the main 100

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Figure 5. 2-Propanol conversion at 523 K over A1-MCM-48 with different Si/A1 ratios: 3.5 (+), 3.8 ([]), 12.7 (zx), and 34.5 ( 9

760 product of this reaction. At 523 K, selectivity toward propene was c a . 98.5% for A1-MCM48(32) and it increased up to c a . 100% with the content of A1 (Fig. 6). Diisopropyl ether was the other important product while acetone was detected in trace amounts only. The selectivity toward propene decreased slightly with the number of injections of 2-propanol. At 573 K, however, the contribution of propene for all the samples was practically 100% and did not decrease with the injection number. The relation between the level of conversion of the examined compounds and the A1 content was confirmed by the IR analysis of the acid centres. As found, the studied A1-MCM48 catalysts differ in the concentrations of the Bronsted and Lewis acid centres that determine the course of the conversion of cumene and 2-propanol. The calculated concentrations of the sites in the parent MCM-48 material and selected A1-MCM-48 samples are listed in Table 2. Some amount of the Lewis sites detected in the parent material is presumably due to traces of A1 present in the reagents used for the synthesis. As seen from the table, the Bronsted acidity of A1-MCM-48(5) is only c a . 3.5 times higher than that of A1-MCM-48(32) although the A1 content is ca. 10 times higher. This implies that the Al-rich sample contains likely a relatively high amount of aluminium that is not incorporated into the structure of the material and does not give rise to the Bronsted acidity. Another reason for the observed catalytic behaviour of the studied samples may be connected with a different acid strength of the catalyst centres. The acid strength can decrease with the rising concentration of the centres that control the examined reactions. Such a dependence, although not very clear, has been found by us for the MCM-41 materials [ 10]. The analysis of the acid strength of the centres of the A1-MCM-48 samples is in progress. These results and comparison of the catalytic activity between Al-grafted MCM-48 and other molecular sieves (zeolites, amorphous aluminosilicates) are planned to be included in a next paper. 100

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94

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90

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761 Table 2 Concentrations of Bronsted and Lewis acid sites Acid sites [~tmol g-l] Bronsted Lewis

MCM-48 0 32

Sample A1-MCM-48(32) 32 150

A1-MCM-48(5) 110

430

4. CONCLUSIONS The examined A1-MCM-48 materials differ in the concentrations of the Bronsted and Lewis acid sites, which increase with the A1 content. High conversion of the reaction of cumene cracking over A1-MCM-48 indicates the presence of strongly acidic Bronsted sites. The conversions of cumene and 2-propanol grow with both the A1 content and reaction temperature. Benzene and propene are the main products of the cumene cracking, a-methylstyrene being another product found in considerable amounts. In the case of the 2-propanol conversion, dehydration is the principal reaction. It leads to formation of propene in predominating amounts and of diisopropyl ether. The concurrent reaction of dehydrogenation yields acetone in trace amounts only. In spite of large differences in the A1 contents of the catalyst, the results of the catalytic reactions do not indicate significant differences in their Bronsted acidity. An increase in the concentration of the Bronsted sites may cause some decrease in their acidic strength. Further investigations are in progress.

ACKNOWLEDGEMENT Thanks are due to Prof. J. Datka (Krakow, Poland) for the IR analysis of acid centres. The work was supported in part by the State Committee for Scientific Research (KBN).

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.W. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Monnier, F. Schtith, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, and B.F. Chmelka, Science, 261 (1993) 1299. 4. S. Biz and M.L. Occelli, Cat. Rev. Sci. Eng., 40 (1998) 329. 5. A. Corma, Chem. Rev., 97 (1997) 2373. 6. W.J. Roth and J.C. Vartuli, Stud. Surf. Sci. Catal., 135 (2001). 7. A. Sayari, C. Danumah, and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. 8. C.-F. Cheng, H. He, W. Zhou, J. Klinowski, J.A.S. Goncalves, and L.F. Gladden, J. Phys. Chem., 100 (1996) 390.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Photoreduction o f Methylviologen

Incorporated Molecules

763

in Zeolite X:

Koodali T. Ranjit and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas, 77204-5003 The photoreduction of methylviologen (MV 2+) was examined in zeolite X. A series of alkali metal ion-exchanged zeolite X materials with ion-exchanged methylviologen was photoionized with 320 nm light at room temperature in the absence of any reducing counteranion. Photoreduction of methylviologen containing alkali metal ion-exchanged zeolite X results in the formation of methylviologen cation radicals (MV+'). The radicals were identified by electron spin resonance (ESR). Upon irradiation at room temperature the samples turn light blue in color and a single line ESR spectrum characteristic of the methylviologen radical cation is observed. The photoyield depends on the nature of the alkali metal ion-exchanged into the zeolite framework. The photoyield increases in the

order

Li-X/MV 2+ < Na-X/MV 2+ < K-X/MV 2+ < Rb-X/MV 2+ <

Cs-X/MV 2+. The donor strength of the zeolite framework increases in the order Li-X
INTRODUCTION

There is considerable interest in the development of artificial photoredox systems[ 1-3]. One of the main objectives is in maintaining the integrity of the charge separated state long enough so that the free energy can be utilized in driving a chemical reaction. Current research is directed towards the design of efficient photoredox systems that can inhibit back electron-transfer. The main problem in achieving such long lived charge separation is back electron-transfer, which is thermodynamically favorable and very rapid. Back electron-transfer results in the loss of the potential converted energy into heat. Many host systems have been examined to improve the efficiency of the energy storage by preventing the back electron-transfer reaction[4-6]. Heterogeneous systems such as micelles, vesicles, silica gels and molecular sieves can provide appropriate spatial organization of both the donor and acceptor molecules to retard back electron-transfer[7-10]. Thus, appropriate tuning of the electronic and spatial properties of the host system can prevent undesirable back electron-transfer. Dimethylviologen commonly known as methylviologen is an efficient electron acceptor for artificial photoredox systems[l 1-14]. MV 2+ can be easily reduced by

764 chemical, electrochemical and photochemical methods[ 15-20]. However, most of these studies include an added molecular electron donor such as bipyridinium salts or alcohols. The photoreduction of methylviologen has been studied in vesicular and micellar suspensions[21-23]. This molecule has also been used as an electron acceptor in zeolites[24-26]. There is considerable interest in the use of microporous materials, such as zeolites, as hosts for photoinduced charge separation. Zeolites have often been the choice because their regular microporous framework provides an opportunity to organize a supramolecular assembly where the different stages of the overall photochemical process (light absorption, energy transfer and chemical reaction) can take place in a well defined arrangement[27-28]. The zeolite framework contains pores, channels and cages which possess an anionic charge. This charge is neutralized by the presence of countercations, often sodium. These cations are easily exchangeable and hence it is easy to change the donor strength or basicity of the zeolite framework. The cations in zeolites are known to play important chemical roles other than merely compensating the negative charge of the framework[29-35]. Importantly, it has been shown that the cations govern the donor strength of the zeolite framework[36-40] For example, zeolite Y normally behaves as an acid catalyst in various reactions with Na + as the countercation. However, it can be switched to a base catalyst by replacement ofNa+byCs+cation[32,34,39,41]. Thus, replacement of an alkali metal cation with another can dramatically alter the donor strength of the zeolite framework. The donor strength of the framework has been demonstrated to increase upon increasing the electropositivity of the cation[42,43]. X-ray photoelectron spectroscopy[44], infrared spectroscopy[45] and ultraviolet spectroscopy[38] have been used to establish this. The ability of zeolites to accept electrons is well established[46-48]. The structure of the sites responsible for the electron acceptor ability is not established, but it is widely accepted that this property of zeolites is related to the presence of acid centers. In contrast, the reverse situation in which zeolites act as single electron donors is far less well documented[49-51]. The photoreduction of MV 2+ in zeolites has been studied by ESR in zeolite X and Y at 77 K by McManus et a1149] However, no ESR spectrum attributable to methylviologen cation radical (MV +') was observed from samples which were irradiated at room temperature. Surprisingly, they also did not observe significant changes in the yield of the photoproduced MV +" by exchanging Na + by Li +, K + or Cs +. In contrast, Alvaro et al. have reported the photolysis of methylviologen incorporated within zeolites[50]. The yield of MV +" was influenced by the nature of the alkali metal ion-exchanged into zeolite Y. Laser flash photolysis of Li +, Na + and K + ion-exchanged into zeolite Y enabled the detection of MV +" as a long-lived transient on the microsecond time scale. For Rb + and Cs + ion-exchanged into zeolite Y, the photogenerated MV +" was found to be long-lived and could be detected by conventional diffuse reflectance

765 spectroscopy. Yoon et al[51] were able to detect MV +" from MV2+-arene donor charge transfer complexes in M-X (M = Cs +, Rb +, K +) zeolites. MV +" did not occur in the complete absence of any arene donor. In view of these conflicting reports, it is important to restudy the photoreduction of MV 2+ in alkali metal ion-exchanged zeolites. ESR provides a convenient technique to monitor the formation of any photoproduced MV +'. We have been able to detect MV +" by ESR in Li +, Na +, K +, Rb + and Cs + ion-exchanged zeolite X with incorporated MV 2+, denoted as M-X/MV 2+, at room temperature. The aim of the present study was to obtain experimental evidence that zeolites can act as single electron donors and to establish a semiquantitative relationship between the physicochemical parameters of the zeolites and their ability to act as electron donors. Relatively high photoyield and excellent stability was observed in Cs + ion-exchanged zeolite X which suggests that such systems can act as potential candidates for photochemical conversion and storage devices. The photoyield was found to increase in the order H-X/MV 2+ < Li-X/MV 2+ < Na-X/MV 2+ < K-X/MV 2+ < Rb-X/MV 2+ < Cs-X/MV 2+. The donor strength of the zeolite framework increases in the order Li-X< Na-X < K-X< Rb-X < Cs-X. Thus, we have been able to show that the basicity of the framework strongly influences the electron donor ability of zeolite X.

2.

EXPERIMENTAL SECTION

Na-X (Grace Division Chemicals, Maryland, U.S.A.) and Na-A (Union Carbide) zeolite were commercial samples and used as received. The alkali metal ions were incorporated into the zeolite materials in extraframework positions by liquid state ionexchange. Typical liquid state ion-exchange was performed by adding 20 ml of 5xl 0-1 M LiNO3, KNO3, RbNO3 or CsNO3 to 2 g of Na-X zeolite and the mixture stirred overnight at room temperature. Zeolite H-X was prepared by exchanging Na + with +

NH4 four times followed by calcination as reported in the literature[52]. For ESR measurements, 0.1 g of the sample was put into Suprasil quartz tubes and evacuated at 100•

~ for 36 h to remove traces of oxygen and water. A stock 0.1 M solution of dimethylviologen dichloride (Aldrich Chemical

Company) was exchanged into the zeolite at 70•

~ for 14 h. All the MV 2+ ion-

exchanged

80 • 6 ~

zeolite X samples were dried in air at

and are denoted as

M-X/MV 2+ where M is an alkali metal ion. X-ray diffraction patterns were recorded on a Siemens 5000 X-ray diffractometer. Chemical analysis was performed by electron microprobe analysis on a JEOL JXA-8600 spectrometer. The composition of the alkali metal ion-exchanged zeolite was determined by calibration with known standards and by averaging over several defocused areas to give

766 the bulk composition. ESR spectra were recorded at room temperature at 9.5 GHz using a Bruker ESP 300 spectrometer with 100 kHz field modulation and low microwave power to avoid power saturation. Photoproduced methylviologen radical cation (MV +') yields were determined by double integration of the ESR spectra using the ESP 300 software. Each photoyield is an average of three scans and has a precision of less than 4 %. Thermal gravimetric analysis (TGA) of the samples were performed using a TGA 2050 analyzer from TA instruments in oxygen atmosphere at a heating rate of 10 ~ The methylviologen containing zeolite materials were irradiated using a 300 W Cermax xenon lamp (ILC-LX 300 UV) at room temperature. The light was filtered to give 320 + 20 nm. The photoproduced methylviologen cation radicals were identified by ESR.

3.

RESULTS

The chemical compositions of the alkali metal (M) ion-exchanged zeolite X materials are M24Na30A154Si1380384 where M = Cs, Rb, K. Na-X contains Na54 and H-X contains H30Na24. Despite repetition of the exchange procedure four times, the elemental analyses of the final exchanged zeolites revealed incomplete exchange of the Na + ion. The Li-X sample could not be analyzed due to the instrumental limitation in detecting light elements. Preliminary photoionization experiments were carried out with Cs-X zeolitic materials. MV +" was not detected in samples that were not evacuated. ESR signals were an order of magnitude weaker in hydrated M-X zeolites compared to those that were dehydrated. The methylviologen ion-exchanged Cs-X sample prior to irradiation was colorless and ESR silent. After being photoirradiated for a few minutes, it turned blue and showed a single symmetric ESR signal at g = 2.002 with a peak-to-peak derivative linewidth of - 20 G. The spectral widths of the ESR spectra are essentially the same as that of the methylviologen cation radical in homogeneous solution at room temperature. This ESR signal and the blue color are typical of the MV +" radical cation[ 16,22,24]. After being irradiated by 320 nm light at room temperature for up to 60 min, the samples showed strong ESR signals. Thus, there is a significant increase in the production of stable methylviologen radicals. The background signals before irradiation were subtracted from the signals after irradiation to estimate the net photoyield. For H-X there is only a small increase in the intensity of the ESR signal due to MV +" After 30 min irradiation, the photoyield starts decreasing and the blue color of the sample starts fading and the sample becomes colorless after 30 min standing in the dark. This suggests that the the H-X zeolitic framework is unable to stabilize the photoproduced MV +" due to its relatively high acidity compared to other M-X samples. TGA results obtained from the incorporation of methylviologen into M-X zeolites show typically four weight losses. The first near 100 ~ desorption, the second and third peaks near 250 ~

and 350 ~

is attributed to water are attributed to

767 demethylation of MV2+[50] and the fourth peak near 400 ~ is assigned to the decomposition of methylviologen in oxygen flow within the pores. The assignment of the 400 ~ peak to decomposition of methylviologen within the pores is supported by its absence when TGA is carried out in nitrogen flow. The TGA curves for Cs-X/MV 2+, Rb-X/MV 2+, and K-X/MV 2+ are similar indicating that the amount of methylviologen incorporated inside the zeolite is similar. Thus we can conclude from the TGA experiments that methylviologen does penetrate well into the pores of zeolite X.

4.

DISCUSSION

The ESR results clearly confirm the photoreduction of methylviologen molecules into methylviologen cation radicals in alkali metal ion-exchanged zeolites at room temperature. The increase in the intensity of the ESR signal due to the MV +" radical cations with time in the case of M-X samples suggest that the alkali metal ion-exchanged zeolites assist in the formation and stabilization of the photoproduced methylviologen cation radical. However, the photoyield depends on the nature of the alkali metal ion-exchanged into the zeolite framework. Experiments with different alkali metal ions in M-X/MV 2+ samples show strong ESR signals at room temperature. An increase in the intensity of the ESR signal due to MV +" with sites.

irradiation time for M-X with alkali metal ions in ion-exchange

The presence of

other alkali

Cs + in zeolite X enhances the photoyield compared to

metal ions such as

efficiency for MV 2+ in

M-X

Rb +, K +, Na + or Li +. zeolites decreases in

The

photoionization

the order Cs-X/MV2+ >

Rb-X/MV 2+ > K-X/MV 2+ > Na-X/MV 2+ > Li-X/MV 2+ > H-X/MV 2+. Thus the photoreduction efficiency can be controlled by the nature of the metal ion in the ionexchange sites. TGA results clearly show that the amount of methylviologen incorporated in different M-X zeolites is similar, suggesting that the difference in the photoyield is due to other factors such as the electron donor strength of the zeolite framework. The cations in the zeolites, in addition to compensating the negative charges in the framework, also govern the electron donor strength of the zeolite framework. The electron donor strength or basicity of the framework has been demonstrated to increase upon increasing the electropositivity of the cation[18,30,31,43]. Thus the electron donor strength of zeolites increases in the order Li-X< Na-X < K-X < Rb-X < Cs-X. In order to directly calculate the electron donor strength of the zeolite framework, it is necessary to have the effective ionization potential of the zeolite framework Ip(Z). However, values of Ip(Z) are not known for solid zeolite X although Ip(Z) has been estimated to be 11.4 eV for Na-ZSM-5153].

768 Sanderson's electronegativity equalization principle has served as a theoretical basis to correlate the experimentally observed electron donor strength of the framework and the partial charge of the framework oxygens[48]. We use Sanderson's partial charges of the framework oxygen atoms as a measure for the framework electron donor strength since they have been shown to be linearly correlated with the experimentally observed framework electron donor strengths. Sanderson's partial charges of the framework oxygen atoms for the MV 2+ doped X zeolites were calculated on the basis of the chemical compositions.The values of Sanderson's electronegativity for each element Si, A1, O, Li, Na, K, Rb and Cs were taken from the literature[54]. The relationship between the photoyield and the calculated Sanderson's partial charge of the framework oxygens is quite linear and leads us to conclude that the yield of the photoproduced MV +" increases with an increase in the negative charge density of the framework oxygens, that is, upon increasing the framework electron donor strength or basicity. The fact that the photoyield and the stability of photoproduced MV +" decrease in the order Cs-X/MV 2+ > Rb-X/MV 2+ > K-X/MV 2+ > Na-X/MV2+ > Li-X/MV 2+ clearly indicate that not only the single electron transfer step to form MV +" but also the MV +" stability or decay dynamics are dependent on the electron donor nature of the cation. A possible explanation is that the major pathway for the decay of photoproduced MV +" cation radical is back electron transfer from MV +" to a radical center in the zeolite to regenerate MV 2+ in its ground state and an oxygen lone pair in the framework.

5.

CONCLUSIONS Microporous alkali metal ion containing zeolite X

show stable photoinduced

charge separation of methylviologen molecules. The MV +" cation radical photoyield depends on the electron donor strength or basicity of the zeolite as determined by its metal cation. The photoyields for a series of alkali metal containing zeolite X materials can be linearly correlated with Sanderson's partial charges on the framework oxygens. The electron donor sites are believed to be oxygen sites in the framework. The MV +" photoyield is dependent on the basicity and the pore size of the zeolite. The results clearly indicate that Cs-X zeolites provide the most appropriate steric and electrostatic environment to retard back electron transfer and increase the lifetime of photogenerated radical ions from methylviologen for many days at room temperature.

769 6.

ACKNOWLEDGMENT

This research was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of the Basic Energy Sciences, U.S. Department of Energy, the Texas Advanced Research Program and the Environmental Institute of Houston.

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Studies in Surface Science and Catalysis 142 R. AieUo, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

771

Effective utilization of residual type feedstock to middle distillates by hydrocracking technology and D. Biswas b. S.K. Saha ~*, G.K.Blswas, " aChemical Engineering Department, Jadavpur University, Calcutta-700032, India bChemical Technology Department, Calcutta University, Calcutta-70009, India Hydrocracking is an attractive technique among th~secondary conversion processes. The processing problem, however, goes up markedly as the crude oil .quality decreases such like ~

gravity while on the other hand increases the conradson carbon, sulfur and

metal contents are due to excessive consumption of petroleum products. Hydrocracking is the most flexible in respect to change in feed quality that handles poor quality feeds easily to produce lighter products. We studied a case using 60:40 combination of reduced crude oil and cycle oil containing 50% aromatics with 1.15% S, and 0.1% N having pour point +24~

Temperature, pressure, and residence time were studied as a process

parameters. Catalytic parameters were also studied. The maximum yield of middle distillates was found to be 49.51% under the following condition: temperature = 623 K, pressure = 7.0 MPa, initial hydrogen partial pressure = 6.0 MPa, residence time = 900 see, feed = 250 g, and catalyst = 10 g 20:80 ratio of A:Z (A- amorphous silica-alumina, Zmolecular sieve 13X). Palladium metal was chosen for hydrogenation site. 1. INTRODUCTION In the modem refinery, catalytic hydrocracking is an attractive among the secondary conversion processes to get more valuable products as well as clean atmosphere from heavier petroleum fraction. The versatility of this process makes it easy to equilibrate the supply and demand of fuels such as gasoline, diesel, and jet fuel. The main goal of hydrocraeking conversion is the reduction of the average carbon number, and the production of branched isomerization of linear paraffins is desirable to improve *Correspondence should be addressed to: S.K Saha Department of Chemistry, Faculty of Engineering, Gifu University, C_hfia501-1193, Japan. E-mail: ksshyama168 @hotmail.com

772 the quality of the different petroleum fractions. Demand patterns of petroleum products have been changed from gasoline to middle distillates and the change continues at present, all over the world [1]. In this perspective, hydrocracking is considered to be the best economic way of converting heavy ends to quality fuels, particularly to middle distillates. A recent report suggests that hydrocracking of polyaromatic compounds proceeds via initial hydrogenation of peripheral ring to naphthenic ring, [2] cleavage into aliphatic substitutes and isomerise to a branched naphthenic compound and finally undergoes into dealkylafion. Another report studied on the role of dispersed phase Mo catalyst in hydrocracking of Guado H [3] revels that cracking reaction occurs essentially through the normal cracking pathway, and that Mo catalyst can considerably inhibit coke formation and enhance desulfurisation. Evidence complemented~y the works on hydrocracking of vacuum gas oil assembled in studies with using highly dispersed metals such as W, Mo, Co and Ni [4,5,6] explored that higher temperature favours more coke whereas lower pressure gives rise to middle distillate with mild acidity. Refractory cycle oil feed could be easily hydrocracked over SiO2-A1203-Ce exchanged Y containing Ni and Mo to jet fuels [7]. Ultrastable Y zeolite catalyst has been found more active to increase middle distillates compared to commercial LZY-82 catalyst [8]. Omega zeolite containing catalyst [9] has also been reported to afford high conversion and selectivity to middle distillates. Studies conducted by Saha et al. [10] on refinery waste to middle distillates reports hydrogen partial pressure plays a vital role for the hydrocracking of refinery waste mainly refractory type of compounds. Various catalyst types viz. zeolites, amorphous SiO:z-A1203, ZrO2-SiO2, USY-zeolite, ZSM-5 etc. were tried as cracking site while Ni, Mo, W, Pt and Pd etc. were studied as hydrogenation site by a number of researchers [ 11]. Still, better catalyst is in search for economic process technology as well as quality products. In this work large pore molecular sieve 13X and SIO2-A1203 amorphous supports were chosen for cracking site and palladium metal for hydrogenation site. 13X molecular sieve adsorb critically larger diameter molecules, such as aromatics and branched chain hydrocarbon and offer very good mass transfer rate in parallel, palladium metal has higher hydrogenation capacity. Our present work designed with the mixed feed is so far the first report on catalyst support variation for middle distillate yield. The present paper deals with hydrocraeking of residual feed (mixed feed) with catalyst support variation from amorphous SiO2-A1203 to zeolite 13X and their combination at different proportion. Various parameters were also studied for maximum middle distillate yield.

773 2. EXPERIMENTAL

2.1 Feed and catalyst preparation Reduced crude oil (RCO) blended with cycle oil in the proportion of 60:40 ratio, having characteristics listed in Table 1. Feed was characterized using standard method. F.or the catalyst preparation, molecular sieve 13X support was procured from the market while the amorphous silica-alumina support was made in the laboratory. Silica-alumina ratio was maintained as to 70:30 for both catalysts. The ammonium form of molecular sieve 13X as prepared by ion-exchange of sodium form, then dried and calcined to give the protonated H-form by a treatment with a m m o n i u ~ i t r a t e solution. 0.5% palladium metal was loaded as palladous chloride in both supports by impregnation method. The detail method of preparation of the catalyst have been described elsewhere [12]. The stability of catalyst was checked by DT-TGA. The characteristics of the catalysts have been shown in Table 2.

2.2 Reactor set-up Experiments were carried out in a rocking type batch reactor of laboratory scale (1 dm3 capacity). Details of the reactor and the assembly of other parts were described elsewhere [5]. The reactor was charged with requisite amount of feed and catalyst, and closed. Purging was done with nitrogen gas to ensure an oxygen free environment inside the reactor. Initially, desired pressure was maintained with hydrogen or nitrogen or both. Purity of hydrogen and nitrogen used here was 99.6% and 99.5% respectively. The total pressure was maintained by only nitrogen. The pressure reading was obtained from the pressure gauge, and the valve was properly closed and checked with soap solution for any leakage. Heating was applied and the temperature was regulated by variac. After attaining desired temperature, rocking of the reactor was started and continued for a definite residence time. At the end of residence time, gas and vapor originated inside the reactor was allowed to pass through an ice-cooled spiral condenser. The liquid product was condensed while non-condensable gaseous product was allowed to pass through the scrubbing system for H2S absorption. The scrubber contained 10% NaOH solution. AKer H2S absorption rest of the gas was passed through a wet gas meter and escape to the atmosphere. The liquid product was analyzed by standard methods for petroleum products (IS/ASTM).

774 Table 1 Properties of feedstock at 60:40 combination of reduced crude oil and cycle oil Parameters

Values

Specific gravity, 60~176

0.8874

Viscosity at 100~ cSt

7.70

Sulfur wt., %

1.15

Nitrogen wt., %

0.10

Ramsbottom carbon residue wt., %

0.911

Carbon to hydrogen ratio

7.55

Pour point, ~

+24 240-576

Boiling range, ~

,,,

,

Table 2 Catalyst properties of palladium loaded molecular sieve 13X and amorphous silica-alumina _

|

,

Items

,

i

,,

Molecular sieve 13X

Amorphous silica-alumina

With palladium

With Palladium

Surface area (m2/g)

336.30

133.60

Total pore volume (cc/g)

0.3973

0.088

0.327 0.0870

0.3302 0.0840

81.64

16.02

Acidity (retool/g) Brrnsted acidity Lewis acidity Pore size distribution (%) > 1000A < 1000 A

18.56

,,,,,

83.98 . . . .

,

, .

.

.

.

3. RESULTS AND DISCUSSION

The process parameters studied were the temperature (573 to 683 K), partial pressure of hydrogen (2.0 to 6.0 MPa) and residence time (420 to 1800 sec). During process parameter study 250 g feed and 25 g catalyst of palladium metal loaded with at a combination of 80:20 A:Z were used. Table 3 reveals that the percentage of conversion at 573 K was only 58.60%, which increased to 93.53% at 663 K beyond which percentage conversion slowly decreased to 83.76% at 683 K. However, the yield of middle distillates was the highest, which was 33.80%, at temperature 623 K within the temperature range

775 studied. The decrease of percentage of middle distillates at higher temperature might be due to secondary cracking reactions occurred beyond temperature of 623 K, thereby augmenting yield of light distillate and gaseous product. It is, therefore, expected that the endothermic cracking reaction predominated over exothermic hydrogenation reaction, and the fact was supported by the increasing tendency of % aromatics at higher temperature. Partial pressure of hydrogen was studied with predetermined total pressure, which was 7.0 MPa at 623 K. The effect of hydrogen partial pressure has been shown in Table 4. It has been observed that effect of hydrogen partial pressure plays a significant role during hydrocracking reaction. The experimental data reveal that there is an increase in the production of middle distillates with corresponding increase of light distillate, and an increase in hydrogen partial pressure up to 6.0 M P ~ i t h correspondingly decreases in heavy distillate. It clearly indicates that initially hydrogenation of higher hydrocarbons makes cracking easier for yielding lighter products. The maximum middle distillate was found at 6.0 MPa hydrogen partial pressure. At higher hydrogen partial pressure, product quality was better and also coke deposition was minimum. Smoke point and octane index of the middle distillate cuts were higher. Table 5 shows the effect of residence time. To investigate the influence of residence time on hydro cracking of residual type feedstock, increasing reaction time from 420 see to 1800 sec resulted in conversion from 65.95 to 78.42%. However, it has been observed that percentage yield of middle distillate is increased with increment of residence time up to 900 sec reaching maximum value of 41.55% which was the summation of MDL-middle distillate light (150-250~ 17.95% and MDH- middle distillate heavy (250-320~

cut of

cut of 23.6% after which the

percentage yield of middle distillates falls. These results indicate that longer reaction time like 1800 see is not beneficial to hydrocrack, rather 900 see might be better choice. This is probably due to the fact residence time less than 900 see is not sufficient to complete the reaction while a longer residence time results in undesirable side reactions, such as partial polymerization and condensation, thus decreasing middle distillate. Catalytic parameters were studied at predetermined process condition and optimum feed to catalyst ratio. For the study of catalyst cracking site variation, 10 g of catalyst was used. Effect of cracking site variation has been shown in Table 6. The study was conducted with catalyst support varying from amorphous silica-alumina to zeolite 13X and their combination at different proportion viz. 80:20, 50:50 and 20:80. Palladium was the metallic support in all the cases for hydrogenation site. It was revealed from the study that neither amorphous silica-alumina nor zeolitel3X was suitable as cracking site when used individually for hydrocracking of residual feed to lighter products especially middle distillate. Their combination, however, was more effective for this purpose resulting in high conversion

776 and more yields of middle distillates of good quality. Again, zeolite rich A:Z of 20:80 combination was far better than amorphous rich combination. The result showed more middle distillate production having lower aromatic content, thus an improved burning characteristics (higher smoke point) and better engine performance (higher Cetane Index). Coke deposition was also minimal. 100% amorphous or zeolite-based catalyst alone was not effective. This implies that there must be some synergistic effect when amorphous-zeolite combination was used. This may be due to the fact that in one hand, amorphous catalyst has good stability against sulfur compounds present in the feed and high selectivity for middle distillate. On the other hand, zeolitic catalyst may have difficulty in converting some of the larger and higher boiling component to lighter product. Hence, presence of certain percentage of a~orphous catalyst in the zeolite matrix would be beneficial in hydrocracking of residual type feedstock. Table 3 Effect of temperature on hydrocracking of mixed feed oil (total pressure: 4.5 MPa, hydrogen partial pressure: 4.5 MPa, residence time: 900 see, feed: 250 g, catalyst: 25 g, A:Z = 80:20) ,,

Items

,

..

,,i

,.,,

i

,

,

Temperature (K) 573

623

663

683

Percentage conversion

58.60

92.44

93.53

83.76

Gas Light distillate (IBP-150~

20.00 11.50

38.75 18.55

39.68 21.43

60.32 7.14

MDL (150-250~

10.20

15.25

15.00

5.25

MDH (250-320~ Heavy distillate (320~

14.90 41.40

18.18 7.55

15.01 6.46

8.24 16.24

Coke

2.00

2.00

2.40

2.80

% Aromatics in MDL (Vol.)

35.00

25.00

26.00

27.00

Smoke point ofMDL, mm

14.00

18.00

18.00

17.00

% Aromatics in MDH (Vol.)

32.00

22.00

24.00

26.00

Cetane Index of MDH

33.00

48.00

46.00

46.00

777 Table 4 Effect of hydrogen partial pressure (temperature: 623 K, total pressure: 7.0 MPa, residence time: 900 see, feed: 250 g, catalyst: 25 g, A:Z = 80:20) ,

,,,

--

Items

=

,

,,

, , ,

,

,,

,,,

Hydrogen partial pressure (MPa) 2.0

4.5

6.0

Percentage conversion

57.20

73.20

76.61

Gas

14.03

19.64

24.00

Light distillate (mP-150~

4.00

6.82

8.26

MDL (150-250~

14.85

18.50

17.95

MDH (250-320~

19.10

22.24

23.60

Heavy distillate (320~

42.80

26.80

23.39

Coke

5.20

6.00

2.80

% Aromatics in MDL (Vol.)

28.00

26.00

22.00

Smoke point ofMDL, mm

18.00

18.00

20.00

% Aromatics in MDH (Voi.)

25.00

25.00

20.00

Cetane Index of MDH

44.00

47.00

,,

48.00 ,,,,,

.

.

.

.

.

.

.

.

Table 5 Effect of residence time (temperature: 623 K, total pressure: 7.0 MPa, hydrogen partial pressure: 6.0 MPa, feed: 250 g, catalyst: 25 g, A:Z = 80:20) .

.

.

.

,,,

,,

,

,,

,

,

,

.

.

.

.

Items

Residence time (see) 420

900

1800

Percentage conversion

65.95

76.61

78.42

Gas

11.05

24.00

26.00

Light distillate (IBP-150~

9.36

8.26

6.60

MDL (150-250~

17.75

17.95

18.52

MDH (250-320~

23.00

23.60

21.30

Heavy distillate (320~

34.04

23.39

21.58

Coke

4.80

2.80

6.00

% Aromatics in MDL (Vol.)

26.00

22.00

26.00

Smoke point ofMDL, mm

19.00

20.00

20.00

% Aromatics in MDH (Vol.)

25.00

20.00

25.00

Cetane Index of MDH

47.00

48.00

46.00

778 Table 6 Effect of catalyst cracking site variation (temperature = 623 K; total pressure = 7.0 MPa; hydrogen partial pressure = 6.0 MPa; feed = 250 g; catalyst = 10 g, all catalysts are loaded with palladium metal) ,,,,,,

,,

,,

i

J

,

Items

,

|

i,,l|l

,,

,, i

i

,

,

A

A'Z

A'Z

A'Z

Z

(100%)

(80:20)

(50:50)

20:80

(100%)

Percentage conversion

79.21

83.91

85.48

85.78

61.43

Gas

30.27

29.60

29.18

21.13

18.51

Light distillate (IBP-150 ~

4.26

7.41

10.08

12.34

2.75

MDL (150-250 ~

19.01

20.46

22.12

19.68

17.85

MDH (250-320 ~

23.67

24.04

22.74

29.83

20.31

Heavy distillate (320 ~ +)

20.79

16.10

1.4.52

14.22

38.58

Coke

2.00

2.40

2.00

2.80

2.00

% Aromatics in MDL (Vol.)

26.00

25.00

25.00

20.00

22.00

Smoke point ofMDL, mm

18.00

20.00

19.00

22.00

20.00

% Aromatics in MDH (Vol.)

20.00

20.00

21.00

16.00

16.00

47.00

47.00

50.00

Cetane index of MDH

45.00 ,

,

,

,

,

,,

49.00 ,

,

i

i

m,,

4. CONCLUSION Higher catalytic activity was observed with larger external surface area, due to the greater number of pore opening. Greater surface area, high pore volume and presence of majority of pores in the macro pore regions were the positive result for hydro cracking of residual type feedstock with zeolytic rich catalyst. The above result showed that hydrocracking reaction was not suitable at higher temperature and higher residence time but higher hydrogen partial pressure was favorable for middle distillate yield. Palladium metal based catalyst showed lower stability in presence of high sulfur containing feeds though properties of middle distillate was better.

The maximum yield of middle

distillates was found 49.51% under the following reaction condition: temperature = 623 K, Pressure - 7.0 MPa (hydrogen partial pressure 6.0 MPa), residence time = 900 sec and feed to catalyst ratio = 25:1. ACKNOWLEDGEMENTS We are greatly indebted to Prof. Y. Sugi, Department of Chemistry, Faculty of Engineering, Gifu University, Japan for helpful discussion.

779 REFERENCES

1. K.P. De Jong, Catalysis Today, 29 (1996) 171-178. 2. N. Masakatsu, A. Kenji, S. Murats, H. Matsui, Catalysis Today, 29 (1996) 235-240. 3. L. Chenguang, Q. Guohe, L. Wenjie, Z. Yajie, Shiyou Xuebao Shiyo Jiagong, 10 (2) (1994)29-37:C.A.-121 (1994) 259259y. 4. W. Kotowski, B. Heinz, B. Karsten, E Wolfgang, Chem.-Ing. -Tech., 69 (1/2) (1997) 103-107 :C.A.- 126 (1997) 279922r. 5. C.R. Lahiri and D. Biswas, Physica, 139&I40B (1986) 725-728. 6. A.Corma, & Martinez, V. Martinerz-soda and J.B. Monton, J. Catal, 153 (1995) 25-31~ 7. R.J. White, US 3,983,029 (1976) : C.A.-86 (1977) 109024n. 8. K. Nitta, S. Nakai, Japan Pat. 62,297,389 (1987) : C.A.-108 (1988) 115616w. 9. F. Raatz, C. Marcilly, E Dufresue, Fr. Pat. 214,042 (1985) : C.A.-106 (1987) 216856. 10.C.R. Lahiri, S.K. Saha, D. Biswas and G.K. Biswas, Selection of Refinery configuration by linear programming modeling in petroleum refining and petrochemical based industries in Eastern India (Eds) R.K. Saha, S. Ray, B.R. Maity. S. Ganguly, D. Bhattacharya, S.L. Chakraborty, Allied Publishers Ltd. New Delhi (2000) 99-101. l l.J.S. Bawa, N. Ray, R.E Dabral and M. Lal, Hydrocracking-A literature Review, Hydrocarbon Technology, (1991) 149-152. 12.S.K. Saha, Studies on Hydrocracking Characteristics for Middle Distillate, Ph.D (Engg.) thesis, Jadavpur University, India (2000).

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Studies in SurfaceScienceand Catalysis 142 R. Aiello,G. Giordanoand F. Testa(Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.

781

D i r e c t a n a l y s i s of d e a c t i v a t e d c a t a l y s t s i n 1 - p e n t e n e i s o m e r i z a t i o n b y High-Resolution Fast Atom Bombardment Mass Spectrometry J.M. Campelo-, F. Lafont b and J.M. Marinas a a Organic Chemistry Department, University of Cordoba, Campus de R a b a n a l e s Edificio Marie Curie (C3), E-14014 C6rdoba, Spain b Mass Spectrometry Lab-SCAI, University of Cordoba, Old Sciences Faculty, Av. San Alberto Magno s/n, E-14004 C6rdoba, Spain High-Resolution Fast-Atom Bombardment Mass Spectrometry (HR-FAB-MS) was used to study the nature of the coke formed in the isomerization of 1-pentene over silicoaluminophosphate catalysts with different pore structure (SAPO-5, SAPO-11 and SAPO-34). The formation of polyaromatic hydrocarbons was obtained in those catalysts with large pore sizes (SAPO-5) whereas in those with medium-small pore sizes (SAPO-34), polyunsaturated long chain hydrocarbons were formed. 1. INTRODUCTION In zeolite catalysts Mass Spectrometry has been applied to temperatureprogrammed desorption (TPD) of adsorbed species, such as ammonia and pyridine [1] as well as in conjunction with thermal gravimetric (TG) analysis for the study of gases from pyrolysis of tetrapropylammonium template molecules from MFI-type zeolites [2] and of occluded templates in the ALPO-11 [3], VPI-5 [3], SAPO-5, SAPO-11 and SAPO-34 [4] molecular sieves. These applications always involve EI-MS ionization technique. Fast-atom bombardment (FAB-MS) has been applied to analyse a wide variety of compounds, particularly those nonvolatile or thermally labile, and to directly analyse spots in TLC [5]. In FAB-MS, the "analyte" dispersed in a viscous liquid (called the matrix) is bombarded with neutral gas ions (typically Cs) that are accelerated through a large potential difference. The interaction of these "fast atoms", and the analyte/matrix dispersion, results in the ejection of ions and neutral molecules from the matrix surface into the vapour phase [6]. FAB-MS is classified as a soft ionization technique in that fragmentation of analyte molecules is extremely small. The resulting spectrum displays a prominent ion indicative of the sample molecular weight. Due to these characteristics FAB-MS seems to be an ideal technique to obtain structural and

782 molecular weight information from coke depositions with a multi-component nature. This work studies the nature of coke formed during 1-pentene isomerization in several silicoaluminophosphate molecular sieves by means of HR-MS employing FAB as the ionisation technique, indicating that this is a suitable and powerful technique for these purposes. 2. EXPERIMENTAL Hydrothermal synthesis of SAPO-5, SAPO-11 and SAPO-34 catalysts has been previously described [7]. Their textural and acid properties are collected in Table 1. Table 1 Textural and acid properties of SAPO-5, SAPO- 11 and SAPO-34 catalysts

SBET

S~P

dp

PY~73 a

PY67~b

NH3 c

(m2/g)

(m2/g)

(A)

(lamol/g)

(~tmol/g)

(lamol/g)

SAPO-5

183

128

7.3

259

210

686

SAPO- 11

110

65

6.3x3.9

107

83

503

SAPO-34

120

117

3.8

50

50

870

Catalyst

Pyridine adsorption at 573 K; b Pyridine adsorption at 673 K; c TPD-NH3 from 350 to 723K.

a

SAPO-5, SAPO" 11 and SAPO-34 catalysts were studied by FAB-MS after their utilization in 1-pentene isomerization during 10 h at 400~ (time after which their catalytic performance is practically negligible due to coke deactivation). The amount of coke deposited was quantified by thermogravimetric-mass spectrometry (TG/MS) experiments according to a previously described method [8], using electron impact (EI, 70 eV)) technique. Thus, portions of deactivated catalyst, were placed into a deep quartz sample capillary tube, which was inserted into the electron impact ionization source of an AutoSpec-EBE Mass Spectrometer (Micromass, Manchester, UK), and ramped from 100~ to 600~ under high vacuum (10 .6 mbar). Coke molecules are completely desorbed and this was monitored by the total ion current from scan mass experiments.

783 The same spectrometer was used for coke analysis by FAB technique. Thus, portions of 1 mg of deactivated catalysts were ultrasonically dispersed into 0.1 ml of FAB matrix (thioglycerol, glycerol, NBA and NPOE were tested; however, the best results were obtained with thioglycerol), and directly deposited on the FAB probe tip of the mass spectrometer. The optimised MS conditions were as follows: cesium was used as the bombardment ions operating at an accelerating voltage of 30 kV; spectra were acquired with a scan rate of 4 s per decade over a mass range of 150-1200 Daltons (positive ion mode) after calibrating with ICs; accurate mass measurements were obtained by scanning at 10000 resolution (10% valley definition) using an internal mass reference mixture (PEG 300, 600 and 1000 average molecular weight) added to the matrix (1 wt%) for correcting masses to an accuracy of 10 ppm. 3. RESULTS AND DISCUSSION Electron impact mass spectra (EI-MS) experiments shows that the amount of coke deposited was more important for SAPO-5 (16.3 wt%) and SAPO-34 (12.3 wt%), the most active catalysts, than for SAPO-11 (8.1 wt%) catalyst. Deactivated catalysts were directly introduced into the FAB ion source of the mass spectrometer (dispersed in thioglycerol) for characterisation of coke compounds. The high energy of cesium ions, which were accelerated with 30 kV, allows identifying not only the coke located in the pore mouths but also those molecules placed inside the pores. Figures 1, 2 and 3 show the HR-FAB mass spectra obtained from deactivated solids under vacuum (106mbar). Mass spectra of solids have always many peaks, some of those from matrix and the main corresponds to coke molecules and indicating the presence of a complex mixture of hydrocarbons. In the spectra we can observe mainly protonated molecular ions [M+H] § typically of FAB, with the absence of breakdown peaks, which facilitates their interpretation. Identification of compounds were carried out by their molecular weight as well as by their elemental composition obtained from the highresolution data. FAB mass spectra of SAPO-11 (Figure 1) and SAPO-34 (Figure 2) indicate the presence of large polyunsaturated hydrocarbon chains from approximately 200 to 400 umas, whose elemental composition (according to high-resolution mass spectra) indicates greater unsaturated levels in the molecules desorbed. These results have the same tendency observed with the same catalysts on methanol conversion where coke was studied by NMR, FT-IR and HR-EI-MS [9]. SAPO-5 (Figure 3) showed the presence of a great variety of peaks, where we can find peaks at m/z: 228, 254, 276 and 278 that correspond to C18H12, C2oH14, C22H12 and C22H14 (confirmed by high resolution) and can be attributed to polyaromatic hydrocarbon.

784

95 90

"

69

97

85

80

7

64

!II

II0 123

2b 15 i.~

222 180 I

137

h,152 l

i ~ 2 3 5 150

200

3

z|

Figure 1. HR-FAB mass spectra from deactivated SAPO- 11 catalyst.

L

I00~ 95/: 9O85: 8O-

l

252

757o_=

65 60:= 554 5o4 454

40-" 35~

170 151

3o_.=" 25~ 2o_.=

15_~ 102

8E 276

'

h 11293 331

- -

)

2

25o

3

I o

~/

Figure 2. HR-FAB mass spectra from deactivated SAPO-34 catalyst.

785

141 15

16

14

13

109

I

2

J-

219252

-,

[

153

E 76

9

,

93

331 365 3

~-' :-

4 7 485

.405

505

555

!~

150

Figure 3. HR-FAB mass spectra from deactivated SAPO-5 catalyst.

In the SAPO-5 case, the higher unsaturation level mainly corresponds to polyaromatic hydrocarbons from 200 to 600 umas with a molecular composition of CnHm (always m<
786 Acknowledgements This research was subsidized by grants from Ministerio de Ciencia y Tecnologla (Project BQU2001-2605), and from the Consejeria de Educaci6n y Ciencia (Junta de An dalucla). REFERENCES 1. C. Mirodatos, B.H. Ha, K. Otsaka and D. Barthomeuf in, Proc. 5th International Conference of Zeolites, (Naples), L.V.C. Rees (ed.), Heyden, London, 382 (1980). 2. S. Bilger, M. Souland, H. Kessler and J.L. Guth, Zeolites, 11 (1991) 784. 3. D. Young and A.B. Young, J. Mater. Chem., 3 (1993) 295. 4. J.M. Campelo, F. Lafont, J.M. Marinas and M. Ojeda, Rapid Commun. Mass Spectrom., 13 (1999) 521. 5. Y. Zhongping, W. Hanhui, Z. Qinming, H. Weide and Z. Shankai, Rapid Commun. Mass Spectrom., 8 (1994) 481. 6. E. de Pauw, Mass Spectrom Rev., 5 (1996) 191. 7. J.M. Campelo, F. Lafont and J.M. Marinas, J. Catal., 156 (1995) 11. 8. J.M. Campelo, F. Lafont, J.M. Marinas and M. Ojeda, Rapid Commun. Mass Spectrom. 13, 521 (1999) 9. J.M. Campelo, F. Lafont, M. J.M. Marinas and M. Ojeda, Appl. Catal A: General, 192 (2000) 85.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

787

Selection of an active zeolite catalyst and kinetics of vapor phase esterification of acetic acid with ethyl alcohol A.M.Aliyev, E.E.Sarijanov, O.Tun9 Sava~gi, R.Z.Mikailov, T.N.Shakhtakhtinsky, A. Sarioglan, P.F.Poladly, A.R.Kuliyev Institute of Theoretical Problems of Chemical Technology of the National Academy of Sciences of Azerbaijan, 370143, Baku, H.Javid ave., 29, Azerbaijan, E-mail: [email protected] ; Fax (99412)38-77-56 Activity and selectivity of Azerbaijan natural clinoptilolite containing different amounts of zeolite (25.0, 44.0 and 89.0%), dealuminated and hydrogen forms of clinoptilolites prepared from Azerbaijan natural clinoptilolite containing 89.0% zeolite phase with silicate modulus x=SiOJA1203=8.6; Azerbaijan natural mordenite containing 75-80% zeolite phase with X=9.6 and its hydrogen forms, synthetic zeolites: NaY with X=4.3 and its hydrogen forms - NaHY, the commercial 13 zeolite with X=25.0; the commercial zeolite HZSM-5 with X=50.0 and HZSM-5 with X=25.0 have been tested in vapor phase reaction of esterification of acetic acid with ethyl alcohol. On the basis of the kinetic investigations the efficient catalyst, 13 zeolite with X= 25.0, has been selected for this reaction. It has been developed of the theoretical grounded kinetic model of the reaction on the catalyst, 13 zeolite that satisfactorily describe of experimental data. According to this model ethyl acetate is formed on acidic sites of the catalyst at interaction of strongly absorbed molecules of acetic acid with weak absorbed molecules of ethyl alcohol. 1. INTRODUCTION Ethyl acetate is a valuable solvent, which is also used in food industry. The main Industrial method of obtaining of this product is liquid phase esterification of acetic acid with ethyl alcohol using concentrated sulphuric acid as catalyst. Use sulphuric acid creates environmental and corrosion problems; depresses product quality because of its content of sulphur-organic compounds in addition to other hazards related to the use of concentrated acid. On this view it is of interest to create of high efficient vapor phase process of esterification of acetic acid with ethyl alcohol into ethyl acetate over zeolite catalysts containing on their surface of the BrOnstead acidic sites. In our previous works [1,2] the influence of concentration and strength of the BrOnstead acidic sites of the surface of the zeolite catalyst on its activity and selectivity in the reaction of vapor phase esterification of acetic acid with benzyl alcohol into benzyl acetate have been investigated by IR-spectroscopy method and indicators. It was found that the zeolite containing on its surface relatively high concentration (CH+ ~2.4mmol/g) of the BrOnstead acidic sites of the strength, pK=4.8 shows high catalytic activity in this reaction. In the present paper are given the results of the investigation on selection of the high efficient zeolite catalyst for the vapor phase reaction of esterification of acetic acid with ethyl alcohol on the basis of above-mentioned data and on development of the kinetic model of proceeding of the reaction over the active catalyst.

788 2. EXPERIMENTAL The catalyst used were: Azerbaijan natural clinoptilolite containing different amounts of zeolite phase (25.0, 44.0, 89.0%); dealuminated and hydrogen forms of clinoptilolite prepared from Azerbaijan natural clinoptilolite containing 89.0% of zeolite phase with silicate modulus, X=8.6; dealuminated and hydrogen forms of Azerbaijan natural mordenite containing 75.0-80.0% zeolite phase with X=19.6 and 9.6 accordingly; NaY with X=4.3 and its hydrogen forms, NaHY; the commercial 13 zeolite with X=25.0; the commercial HZSM-5 with 2=50.0 modified with steam under conditions of T=535~ t--2h, PH2O=50 mm Hg, VN2 =60 ml/min and HZSM with X=25.0. Dealuminated and hydrogen forms of the natural and synthetic zeolites were obtained by the methods described in [3]. Concentration of the Br6nstead acidic sites of the strength, pK=4.8 on the surface of the zeolites were determined with indicator (methyl red, pK=4.8) by method described [ 1]. All of dealuminated and H-forms of zeolite catalysts were prepared with maximum concentration of Brrnstead acidic sites of the strength, pK=4.8 on their surface. The test of the activity of the prepared zeolite catalyst was carried out in a flow apparatus with the quarts tube reactor connected directly to the gas chromatograph. The reactor was placed inside a thermostated chamber. Small stainless-steel balls with a 0.2 cm diameter were placed before the catalytic bed in order to obtain plug flow conditions. No catalytic activity was shown by these nonporous balls. A fraction of granulated zeolites of about 0.2+0.8 cm of equivalent diameter was used as the catalyst. The analyses of the products of the reaction was performed by gas chromatography using a column filled with polysorb-1 (length, 3m), Helium as the carrier gas, hot wire detector and program control of the temperature. Runs performed at several feed rates and using granules of the catalyst of different size showed that external and internal mass transfer effects were negligible under the studied conditions. 3. RESULTS AND DISCUSSION Table 1 shows the operating conditions and the results of the activity and selectivity tests of the zeolite catalysts in the reaction of vapor phase esterification of acetic acid with ethyl alcohol. It can be seen there that the decreasing of the containing of zeolite phase in natural clinoptilolite (from 89.0 to 25.0%) leads to the increasing of the selectivity on ethyl acetate (from 77.1 to 98.5%) with maximum yield of ethyl acetate of about 75.8% over natural clinoptilolite containing 44.0% of zeolite phase (experiment No 1,2 and 3). It can be explained with relatively high concentration (1.5 mmol/g) of Brrnstead acidic sites of the strength, pK=4.8 on the surface of natural clinoptilolite containing 44.0% of zeolite phase and optimum pore structure of this zeolite for the considered reaction. Dealumination of the clinoptilolite (89.0%) by treatment with 0.01N HC1 leads to increasing of the selectivity (from 77.1 to 99.2%) and yield of ethyl acetate from 43.4 to 57.3% (experiment No 1,4 and 5) which is due to increasing of concentration of active sites. Hydrogen form of natural clinoptilolite (89.0%) obtained by ion exchange with ammonium chloride increases the conversion of ethyl alcohol (experiment No 6,7) however the yield of ethyl acetate does not increase significantly in comparison with dealuminated from of this zeolite treated with 0.01N HC1 (experiment No 4 and 5). That is the result of the high activity of A1203 in the reaction of dehydration of ethyl alcohol. The increase in the of degree of the dealumination of the natural clinoptilolite by treatment with 0. IN solution of HC1 leads to increasing of conversion of ethyl alcohol from

789

63.4 to 9 2 . 3 % (experiment N o 6 and 9) and the yield o f ethyl acetate f r o m 60.4 to 83.7 u n d e r the same operating conditions. C o m p a r i s o n o f the experiments (experiment N o 1, 2, 8, 9, 10) s h o w s that the t r e a t m e n t o f the clinoptilolite with 0 . 1 N HC1 m a k e s it possible to reduce the t e m p e r a t u r e o f the p r o c e s s f r o m 250 to 180~ w i t h o u t decreasing the yield o f ethyl acetate (experiment N o 3,6,7, 8) The increasing o f t e m p e r a t u r e o f the p r o c e s s o v e r this catalyst f r o m 180 to 250~ leads to increasing o f the yields of. ethyl acetate f r o m 77.0 to 84.9%, diethyl ether f r o m 3.0 to Table 1. The results o f the test of the activity of the zeolite catalysts in the reaction of vapor phase esterification of acetic acid with ethyl alcohol. No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Catalyst

T, ~

v, h -1

Clinoptilolite (89%) Clinoptilolite (44%) Clinoptilolite (25%) Clin. (89%,0.01N HC1) -"H-Clin. (0.01N NH4C1) -"H-Clin. (0.1N HCI) -"-"H-Clin. (0.5N HCI) -"H-Clin. (0.1N HC1) H-Clin. (0.1N HCI) H-mordenite -"H-mordenite (Z =9.6) H-mor.(z=19.6, 0.1N HC1) -"-"NaHY (Turkey) -"-"13-zeolite(Z=25) -"-"HZSM-5 (Z=50) -"-"HZSM-5 (Z=25) -"-"-

200 250 250 200 250 200 250 180 200 250 200 250 250 250 200 240 260 160 180 200 200 240 260 140 160 200 140 160 200 140 180 200

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 4.0 6.0 5.0 15.0 15.0 15.0 15.0 10.0 15.0 7.5 15.0 7.49 7.96 13.84 6.05 11.24 7.8 4.63 12.11 3.34

~ 1.0:1.25:7.75 -"-"-"-"-"-"-"-"-"-"-"-"-"1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:1 1:2 1:2

A~ 43.4 75.8 66.0 57.3 77.0 60.4 78.1 77.0 83.7 84.9 66.0 11.5 79.5 69.8 72.5 72.6 82.9 78.2 88.4 92.5 38.7 95.2 89.6 90.7 94.6 96.4 91.6 98.0 95.6 70.5 94.7 89.2

Yield, % A2 A3 1.5 1.0 2.3 3.0 3.5 4.0 6.4 2.0

11.4 1.4 1.0 0.5 1.6 2.0 5.9 1.7 5.1 7.8 21.0 88.5 1.3 0.2 0.5 0.1 0.2 2.8 -

X, %

S, %

56.3 77.2 67.0 57.8 78.6 63.4 86.3 81.7 92.3 96.7 93.4 100.0 80.8 70.0 73.0 72.7 83.1 78.2 88.4 92.5 38.7 98.0 89.6 90.7 94.6 96.4 91.6 98.0 95.6 70.5 94.7 91.2

77.1 98.1 98.5 99.2 98.0 95.4 90.4 94.2 90.7 87.8 70.6 11.5 98.4 99.7 99.3 99.9 99.8 100.0 100.0 100.0 100.0 97.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 97.8

4 . 0 % and ethylene f r o m 1.7 to 7 . 8 % (experiment N o . 8,9, and 10). F u r t h e r increase o f degree o f dealumination by t r e a t m e n t o f elinoptilolite with 0 . 5 N solution o f HC1 leads to reducing selectivity on ethyl acetate (experiment No. 11,12). It m a y be explained with increasing o f the strength o f acidic sites on the surface o f the catalyst and destruction o f crystallinity o f natural elinoptilolite. Influence o f the space velocity on the p r o c e e d i n g o f the reaction w a s investigated over the catalyst, elinoptilolite treated by 0 . 1 N solution o f HC1, at the t e m p e r a t u r e 250~ (experiment No. 13,14). It can be seen f r o m the data in Table 1 that increasing o f the space

790

velocity from 3000 to 6000h 1 at this temperature leads to decreasing o f the yield o f ethyl acetate from 84.9 to 69.8% with the selectivity increasing (from 87.8 to 99.7%). It may be explained with decreasing o f contact time. The investigation carried out with natural mordenite, natural mordenite treated by 0.1N solution ofHC1, H form o f N a Y , commercial 13 zeolite (~=25.0), H Z S M - 5 (~=50.0) and H Z S M - 5 (~=25.0) show relatively high catalytic activity and selectivity in the reaction compared to natural clinoptilolite (experiment No. 15-32). It can be seen from data in Table 1 (experiment No. 24-32) that the relatively low temperatures, the catalytic activity and selectivity o f 13 zeolite (~=25.0), H Z S M - 5 (~=50.0) and H Z S M - 5 (Z=25.0) are higher than the others. F o r selection o f the relatively high active catalyst for vapor phase esterification o f acetic acid with ethyl alcohol, the kinetic laws o f proceeding o f the reaction have been studied over these catalysts. Kinetic runs on the zeolites w e r e performed at atmospheric pressure and Table 2. The results o f kinetic runs Run T [~ No 1

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

140 140 140 140 140 140 160 160 160 160 160 180 180 180 180 200 200 200

1 2 3 4 5 6 7 8 9

140 140 140 140 140 160 160 160 160

o nl [mole/h]

o n2 [mole/h]

Space velocity, v [h-l]

Go.t Yield. % Conver- Selectivity sion ofal- on ethyl o nl A1 A2 A3 cohol, % acetate, % [g-h/mole] X S 3 4 5 6 7 8 9 10 11 13 zeolite (~=25), OH = 0.54474 g/cm 3 , V ~ = 3cm 3 , G ~ = 1.63 g 0.08 0.08 3.09 20.375 96.1 1.0 97.1 98.97 0.11 0.11 4.25 14.82 91.4 91.4 100 0.05 0.10 2.83 32.6 95.2 1.0 96.2 98.96 0.13 0.26 7.49 12.5 90.7 90.7 100 0.25 0.125 7.27 6.52 48.0 0.8 48.8 100 0.49 0.245 14.15 3.33 37.9 37.9 100 0.1 0.1 3.89 16.3 96.9 1.3 98.2 98.7 0.17 0.17 6.56 9.59 92.5 92.5 100 0.127 0.254 7.32 12.8 96.0 96.0 100 0.22 0.11 6.39 7.4 45.0 45.0 100 0.53 0.265 15.40 3.08 40.0 40.0 100 0.38 0.38 14.67 4.3 91.4 91.4 100 0.67 0.67 25.86 2.4 87.5 87.5 100 0.08 0.16 3.61 20.4 91.7 3.8 95.5 96 0.164 0.082 4.77 9.94 42.9 0.5 5.8 49.2 87.2 0.28 0.28 10.81 5.8 87.6 0.6 1.0 89.2 98 0.49 0.49 18.91 3.3 85.0 85.0 100 0.24 0.48 13.84 6.79 96.4 96.4 100 H Z S M - 5 (Z=50), On = 0.579 g/cm3 , V~at= 4cm 3 , Geat = 2.32 g 0.17 0.17 4.92 13.65 90.1 2.9 93.0 97.0 0.27 0.27 7.82 8.59 87.8 2.0 89.8 98.0 0.09 0.18 3.89 25.77 99.1 99.1 100.0 0.14 0.28 6.05 16.57 91.6 91.6 100.0 0.34 0.17 7.41 6.82 33.5 33.5 100.0 0.26 0.26 7.53 8.92 89.9 9.8 99.7 90.2 0.32 0.32 9.26 7.25 83.0 7.9 90.9 91.3 0.09 0.18 3.89 25.77 100.0 100.0 100.0 0.26 0.52 11.24 8.92 98.0 98.0 100.0

791

Continuation o f Table 2. 1 2 3 10 160 11 180 12 180 13 180 14 180 15 200 16 200 17 200 1 140 2 140 3 160 4 160 5 160 6 160 7 180 8 180 9 180 10 180 11 180 12 200 13 200

4

5

6

7

8

9

10

H Z S M - 5 (Z=25), ion = 0.66 g/cm s , Veat= 3 cm 3 , Gear = 1.98 g 0.29 0.145 6.38 8.0 49.5 49.5 0.20 0.20 5.79 11.60 89.3 5.0 94.3 0.15 0.30 6.49 15.46 95.0 1.0 96.0 0.26 0.13 5.67 8.92 40.0 10.0 50.0 0.29 0.145 6.38 8.0 38.0 9.0 47.0 0.20 0.20 5.79 11.60 96.2 1.8 98.0 0.43 0.43 12.45 5.40 94.6 1.4 96.0 0.09 0.18 3.89 25.77 98.6 1.4 100.0 0.12 0.12 4.63 16.5 70.5 70.5 0.22 0.11 6.39 12.59 27.7 27.7 0.07 0.07 2.70 28.29 75.4 75.4 0.03 0.06 1.73 66 97.8 1.8 98.8 0.21 0.42 12.11 9.43 90.9 90.9 0.36 0.18 10.46 5.5 29.8 29.8 0.19 0.19 7.33 10.42 72.6 72.6 0.069 0.138 3.97 28.7 96.7 2.8 99.5 0.375 0.75 21.62 5.28 89.6 89.6 0.316 0.158 9.18 6.27 38.8 1.8 40.6 0.42 0.21 12.21 4.7 35.2 35.2 0.047 0.094 2.71 42.13 90.8 2.5 93.3 0.058 0.116 3.34 34.14 89.2 2.0 91.2

11 100.0 94.6 98.9 80.0 80.8 98.2 98.5 98.6 100 100 100 98.19 100 100 100 97.18 100 95.56 100 97.32 97.80

in the range of. temperature, 140+200~ space velocity, 3+22h "1 while the mole ratio between ethanol and acetic acid was changed from 0.5 to 2.0. Table 2 shows the operating conditions and the results for kinetic runs obtained on these catalysts. A selection o f the relatively high active catalyst for the reaction o f esterification o f acetic acid with ethyl alcohol was carried out on the basis o f the data o f Table 2. A pseudo second order kinetics was assumed and obtained kinetic parameter was compared for these catalysts: dA~

(1)

d/Go.t ~ = kp'p2 ~n ~ ) The instantaneous partial pressures pl and p2 can be expressed in terms o f initial and instantaneous mole flow rate o f these components dA~ d ( G . , "~

= k ( n ~ 1 7 6 A1) ( n ~ 1 7 6 Zni

(2)

Zni

~,n~) E

Z n i - n~0 + n 0. 2 , P=latm;

k-k0e

RT

The results o f the calculation o f kinetic parameters o f the equation (1) for the catalysts: 13 zeolite, H Z S M - 5 (deal.) and H Z S M - 5 on the basis o f data Table 2 and equation (2) are reported in Table 3.

792 Table 3. The kinetic parameters for the catalysts 13zeolite, HZSM-5 (deal.) and HZSM-5 13zeolite (:X=25) H Z S M - 5(:~=50) HZSM-5 (:X=25) K0[mole/h.g.atm2] 8.07.1012 8.65.101~ 2.38.109 E[cal/mole] 22,000 20,000 20,000 The compared analysis of the data of Table 3 shows that the values of activation energy for all of the catalysts are almost the same but kCat-1 lrCat-2 l:at-3 0 > "'0 > "'0

Therefore the catalyst, 13 zeolite has more catalytic activity in reaction of esterification of acetic acid with ethyl alcohol. Below is given a development of the theoretical grounded kinetic model of the reaction of esterification of acetic acid with ethyl alcohol on the catalyst, 13 zeolite. On the basis of analysis of the literature material on a study of mechanism of liquid phase reactions of esterification of acetic acid with alcohols and the results of investigation of kinetic laws of the reaction have been suggested two hypothesizes of probable stage mechanism of proceeding of vapor phase reaction of esterification of acetic acid with ethyl alcohol. Hypothesis 1. Ethyl acetate is formed on acidic sites of the catalyst at interaction of strongly adsorbed molecules of acetic acid with weak adsorbed molecules of ethyl alcohol on scheme: C2HsOH + Z <

K1

CH3COOH + Z<

> ZC2HsOH

K2

> ZCH3COOH

ZC2HsOH + ZCH3COOH

k*

O II > 2Z + CH3-C-O-C2H5 + 1420

According to this hypothesis, the equation of rate of the reaction r =

k * KlK:plp 2 kK 2plp: -(l+KlP, +K2P2) 2 (l+KlP, +K2P2) 2

(3)

where k*Kl=k since K2>>K1, then r=

kK2PlP2 (l+KEP2) 2

(4)

Hypothesis 2. Ethyl acetate is formed on acidic sites of the catalyst at interaction of adsorbed molecules of acetic acid with molecules of ethyl alcohol from vapor phase on scheme: CH3COOH + Z <

K~

> ZCH3COOH

ZCH3COOH +C2HsOH

O I! > Z + CH3-C-O-C2H5 + 1-I20

793 According to this hypothesis, the equation of rate of the reaction r = kK2PlP-----------Lz l+K2p2

(5)

Q OeRT K 2 -K 2

Both of these models have been subjected to statistical analysis on the basis of the data of Table 2. The objective function was F(X)= ~--'

(6)

Where X signifies the set of parameters of the considered model. The kinetic parameters obtained for the two models (equations 4, 5) are reported in Table 4. According to these results, the equation (4) satisfactorily describes the experimental data. Table 4. Kinetic parameter of equations (4) and (5) for the 13zeolite catalyst Equation (4) 0.49.10 9 ko [mole/h.g-atm] 15.00 E [kcal/mole] 0.6-103 K~ [atm1] Q [kcal/mole] Mean of error for the runs (% on the yield of ethyl acetate)

4.00 6.48

Equation (5,,) 0.86-108 16.47 0.51-108 8.82 14.79

Thus in the studied conditions 13 zeolite with ~=25 shows relatively high catalytic activity and selectivity in the reaction of vapor phase esterification of acetic acid with ethyl alcohol. Reaction rate studies allow to be made concerning the following probable stage mechanism for the above reaction; ethyl acetate is formed on acidic sites of this catalyst at interaction of strongly adsorbed molecules of acetic acid with weak adsorbed molecules of ethyl alcohol. - mole ratio, C2HsOH:CH3COOH:He; X - conversion of ethyl alcohol; S - selectivity on ethyl acetate; T - temperature of the reaction; r - rate of the reaction; Z - silicate modulus, SiOz/A1203; p - bulk density of the catalyst; A 1 - yield of ethyl acetate, %; A 2 - yield of ethyl ether, %; A 3 - yield of ethylene, %; n i0 _ initial mole flow rate of i component; ni current mole flow rate of i component; p i - partial pressure of i component; i - component, 1 alcohol, 2 - acetic acid, 4 - water, 5 - ethyl acetate; E - activation energy ; v - space velocity on the liquid mixture of reagents; Gear - weight of catalyst; Veat- volume of the catalyst; P total pressure; k0 - preexponential factor in kinetic law; Ki - adsorption constant of i component; K~ -preexponential factor for adsorption of i component; Q - heat of adsorption. ACKNOWLEDGMENTS This research is sponsored by NATO's Scientific Affairs Division in the framework of the Science for Peace.

794 REFERENCES

1. A.M.Aliyev, A.Sh.Melikova, R.M.Babayev, A.R.Kuliyev. Azerbaijan oil economy, 1996, ~o12. 2. Russian Patent ~r 2059606 ~ 4 C07C 69/12, C07C 67/08. 3. A.s. 1549945 USSR. B.I. 1990, ~r

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

795

Hydrodesulfurization of dibenzothiophene over Mo-based catalysts supported by siliceous MCM-41 Anjie Wang a, Yao Wanga, Yongying Chen a, Xiang Li, PinNing

Yao a

and Toshiaki Kabe b

a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China b Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184, Japan

Deep hydrodesulfurization (HDS) catalysts were prepared by depositing Co-Mo or Ni-Mo species over siliceous MCM-41. The extremely high surface area of MCM-41 favors the dispersion of the active species, resulting in a very high reactivity in desulfurizing dibenzothiophenes. A maximum HDS activity was observed at Co/Mo or Ni/Mo molar ratio of 0.75 for the supported catalysts, higher than that of the conventional ]t-A1203 supported catalysts. A 35S isotope tracer technique was used to investigate the HDS reaction mechanism. It was revealed that sulfur atoms retained on the surface could be removed only by the introduction of a sulfur-containing compound, indicating that sulfur atom exchange between sulfur-containing compounds and the active sites is involved in the HDS reaction. Accordingly, a reaction mechanism for HDS is proposed. 1. INTRODUCTION Hydrodesulfurization (HDS) is a key process in producing clean engine fuels. Since cobalt or nickel promoted molybdenum/tungsten sulfides are basically established as the active species, the proper selection of a better support becomes an effective approach in developing high-performance HDS catalysts. Mesoporous MCM-41 has been the focus of much research interest since its discovery because it offers a uniform pore sizes of 15 to 100 and a very high surface area(> 1000 m 2/g). The MCM-41 supported catalysts have been developed for a variety of reactions[ 1], including hydrodesulfurization of petroleum fractions[2]. However, It is worth noting that all the researches on MCM-41 type supports in HDS catalyst preparation have focused on A1-MCM-41, probably hoping that the acidity of the support may help to crack the polyaromatic sulfur-containing compounds so as to improve the HDS activity. But no great improvement in the HDS activity has been The research was partly supported by the Natural Science Foundation of China (20003002) and by the Young Promising Teachers' Funds from the Education Ministry of China.

796 achieved. We have reported the high-performance HDS catalysts by depositing Co or Ni promoted Mo and W sulfides over siliceous MCM-41 [3,4]. 2. EXPERIMENTAL All the reagents in synthesizing siliceous MCM-41 and in preparing the catalysts were of chemical grade. Siliceous MCM-41 was synthesized according to the procedure in previous paper.[5] The catalysts were prepared by the wet impregnation method. The catalysts were presulfided prior to HDS reaction of DBT. A model fuel containing 1wt% DBT in decalin was used to investigate the HDS activities of the prepared catalysts. To elucidate the reaction mechanism of hydrodesulfurization, an isotope tracer technique was used. [3] 3. RESULTS AND DISCUSSION Figure 1 and 2 show the conversion of DBT as a function of temperature over Co-Mo/MCM-41 and Ni-Mo/MCM-41 respectively, compared with a commercial deep hydrodesulfurization catalyst (DHDS) on Japanese market. It is shown that both series of catalysts showed very high activity for converting DBT into hydrocarbons, yielding almost complete conversion of DBT at temperatures over 320 ~ A maximum HDS activity was observed at Co/No or Ni/Mo atomic ratio of 0.75 for the MCM-41 supported catalysts, different from that of conventional '/-A1203 supported catalysts. The selectivity results show that HDS over Co-Mo/MCM-41 mainly takes the route of hydrogenolysis while hydrogenation plays a more important part in HDS of DBT over Ni-Mo/MCM-41.

100

! 00

80

,

,

~

80

o= 60

~

60

40

~

40

20 0 200

,

20

240

280

320

360

400

Temperature (~

Figure 1 DBT conversion as a function of HDS reaction temperature. ( O ) MOO3, ([--]) CoO, ( ~ ) Co-No (0.25), (A)Co-Mo (0.50), (~)Co-Mo (0.75), ( I )Co-No (1.00), ( O ) Commercial DHDS catalyst

0 200

r 240

A ~ 280

320

I 360

, 400

Temperature f'C)

Figure 2 DBT conversion as a function of HDS reaction temperature. (O) MOO3, (O) Ni, ( ~ ) Ni-No (0.25), (A)Ni- Mo (0.50), (/~)Ni-Mo(0.75), ( I ) N i - M o (1.00), (f-l) Ni-Mo(1.25), (O) Commercial DHDS catalyst

797 The dynamic behavior of sulfur atom during HDS of DBT was investigated by the 35S isotope tracer technique. The HDS of DBT was conducted over sulfided Co-Mo/MCM-41 catalyst at 280 ~ and 5.0 MPa. A typical profile of radioactivity with reaction time is illustrated in Figure 3. A decalin solution of 1 wt% [32S]DBT was pumped into the reactor to start the HDS reaction. After the conversion of [32S]DBT became constant, a flow of 1 wt% [35S]DBT in decalin was introduced into the reactor to replace the [32S]DBT solution. The radioactivity of the unreacted [35S]DBT in the liquid product increased and approached a steady state immediately after the introduction of [35S]DBT, whereas there was a delay for the radioactivity of the released [35S]H2S to reach a steady state. After both the radioactivity of the unreacted [35S]DBT and that of the released [358]H28 became constant, a flow of decalin switched to replace the [35S]DBT solution. Both the radioactivity of unreacted [35S]DBT and that of released [35S]H2S decreased sharply. Little [35S]DBT was detected during the period of purging with decalin and hydrogen, suggesting that there is no physically absorbed HzS on the surface of the catalyst. After a long period of purging, a flow of 1 wt% [32S]DBT solution was introduced again to replace decalin. No noticeable radioactivity change was detected in the liquid product, but a peak of 35S radioactivity in the gas phase appeared in the form of [35S]H2S. It should be noted that a similar profile was obtained for Ni-Mo/MCM-41 catalyst at the same reaction conditions. The 35S isotope tracer investigation for both Ni-Mo/MCM-41 and Co-Mo/MCM-41 revealed that the sulfur atoms removed from DBT molecules are not released directly into gas phase as HzS but retained on the surface of the catalyst. Since purging with decalin in the presence of H2 could not remove the retained sulfur atoms on the catalyst, it is unlikely that the retained 35S exits in the form of adsorbed HzS on the surface. On the other hand, these sulfur atoms could be released by the introduction of new DBT molecules, indicating that sulfur atom exchange between DBT molecules and sulfur species on the surface of the catalyst may be involved during HDS of DBT and the sulfur atoms on the surface may serve as active sites. Accordingly, a catalytic circle 20 . . . . ' during HDS over sulfided Ni-Mo ~ [3'SIDBT Decalin [32SlDBT catalysts is proposed, as shown in ~ I ~ Scheme 1. The mechanism briefly ~ 25 o~#~o o c,,.,o Lr-~o~ o9 describes the possible reaction steps ~,.~ l0 involved on the local site. It is ~o~. ~ 9 assumed that gaseous hydrogen adsorbs .~. dissociatively on the surface of catalysts and the hydrogen species consumed in the reaction are supplemented by means 0 ___~. , , "h,-,,-~,~'~--~ . . . . ? of spillover on the surface.[6] 0 100 200 300 400 500 600 When sulfur-containing molecule Reaction time (min) absorbs on the sulfide cluster in the 111 Figure 3 Change in radioactivities of mode through hydrogen bonding, [35S]DBT(O) and [35S]H2S ( O ) with cluster reconstruction occurs to reaction time accommodate the incoming molecule,

798 "~ "H

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.-H

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Hydrocarbon

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SCHEME 1. Catalytic circle in HDS of thiophenic sulfur-containing molecules catalyzed by supported Ni-Mo sulfides three-fold bound sulfur moving to form two-fold bond. Since the two-fold bound sulfur is unstable, it is eliminated with the absorbed hydrogen species to form H2S, leaving a positive-charged "vacancy". The sulfur atom in the molecule will occupy the "vacancy", forming new sulfur bridging bond and scissoring the sulfur bonds in the molecule to form hydrocarbon. With the desorption of the hydrocarbon, the new sulfur bridging bond will move to form the three-fold bond and the structure of the cluster resumes. When a new sulfur-containing molecule absorbs on the cluster, a new catalytic circle will begin. The sulfur-containing molecule represents thiophenic molecule such as thiophene, benzothiophene, dibenzothiophene or their derivatives, in addition, it is assumed that sulfur-containing molecule could be H2S formed during HDS. H2S may be involved in the catalytic circle, as evidenced by the broadening and tailing of the output peak in the pulse tracer experiment during HDS. The competitive adsorption of H2S results in a slowdown of the overall HDS reaction rate. REFERENCES

1 2 3 4 5 6

Beck J. S., Vartuli J. C., Roth W. J. et al., J. Am. Chem. Soc. 114 (1992)10834. Corma A., Martinez A., Martinez-Soria V. et al., J. Catal., 153 (1995) 25. Wang A., Wang Y., Kabe T., et al., J. Catal., 199 (2001) 19. Wang A., Li X., Chen Y. et al., Chem. Lett., 5 (2001) 474. Wang A., Kabe T., Chem. Commun., 20 (1999) 2067. Nagai M., Koyama, H., Sakamoto, S. et al., Stud. Surf. Sci. Catal. 127 (1999) 195.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

799

Acylation of 2-methoxynaphthalene over ion-exchanged 13- zeolite i. C. Kantarh a, L. Artok a*, H. Bulut a, S. Ydmaz b, and S. Ulkijb aDepartment of Chemistry, Faculty of Science and bDepartment of Chemical Engineering, Faculty of Engineering, Izmir Institute &Technology, Urla 35437 Izmir, Turkey Friedel-Crafts acylation of 2-Methoxynaphthalene was carried out over various ionexchanged 13 zeolites (M~+I3, where M~+: Ins+, Zn 2+, A13+, Fe 3+, La 3+) with various anhydride (acetic, propionic and benzoic anhydrides), or acyl chloride (acetyl, propionyl and benzoyl chlorides) acylating reagents. The results suggested that selectivity towards the 6-substituted products was higher with the larger size anhydrides, propionic and benzoic anhydrides. The metal cation type within the zeolite significantly influenced the extent of conversion and product distribution. That La 3§ exchanged zeolite displayed higher selectivity for the 6position acylated product with anhydrides ascribed mainly to narrowing of channels by the presence of La(OH) 2§ ions that leave no room for the formation of more bulky isomeric forms and to enhanced Bronsted acidity of the zeolite. With acyl chlorides, the recovery of ketone products was found to be remarkably low. 1-Acyl-2-methoxynaphthalenes actively underwent deacylation when acyl chlorides were used as the acylation reagent.

1. I N T R O D U C T I O N

Friedel-CraRs acylation of aromatics is an important method for synthesis of aromatic ketones, used largely as intermediates in the fine chemical industry [1]. The selective acylation of 2-methoxynaphthalene (2-MN) is of particular interest. For example, 2-acetyl-6methoxynaphthalene (6-AcMN) is recognised to be an important intermediate for the production of an anti-inflammatory drug, (S)-Naproxen [2]. It was shown a decade ago that zeolites can be used as heterogeneous catalysts for the acylation of 2-MN by acetic anhydride, and in that the selective formation of 6-AcMN was achieved with HI3 catalyst [3]. [~

R

J~

~ /0CH3 (RCO)20 -...-~ x.-T orRCOCI ~ O C H ..J. ..J -. Zeofite

II O

Otcoho

...

R~c~O I

3 orRCOC1 ~ " O C H 3 Zeolite

Since then, a number of studies reporting the effect of various parameters (e. g., solvent and zeolite types, temperature, substrate-to-catalyst ratio, zeolite modification) on the Corresponding author; email: [email protected]

800 acylation of 2-MN over zeolites have appeared [4]. Yet, to our knowledge, there appears to be no existing report that investigated the catalytic effect of ion-exchanged j3- zeolites on the acylation reactions of 2-MN, though it is known that the type and amount of metal cation on zeolite may influence the strength and distribution of acid sites. Gunnewegh et al. [4b] studied the acylation of 2-MN over Zn 2+ exchanged MCM-41, in which Zn-MCM-41/acetyl chloride combination was found to be less favourable than in the case of H-MCM-41/acetic anhydride combination for both selectivity and regenerability. The leaching of zinc was another problem for the former combination as well. There are a few studies in which ion exchanged -Y, mordenite and ZSM-5 zeolites were used in acylation of various arenes other than 2-MN. Chiche and co-workers showed that rare-earth-exchanged -Y could catalyse the acylation of alkylbenzenes with carboxylic acids [5,6]. Craare and Akporiaye [7] found that LaY was very active in the acylation of anisole and the yield increased with the level of La. Laidlaw et al. [8] determined that Zn- and Feexchanged zeolites (-Y, mordenite and ZSM-5) were very active on the benzoylation of toluene. The extensive leaching of Zn was observed in the reactions, while Fe mostly remained on the zeolite. In this study, we report the catalytic activity of the cation-exchanged [3-zeolite in acylation of 2-MN, using various anhydride and acyl chloride reagents. The metal cations loaded onto zeolite by ion-exchange method were In 3+, Zn 2+, A13§ Fe 3+, and La 3+. Acylation studies were also carded out over HY and M~+Y. However, since the conversion of 2-MN and selectivity to the 6-acylated isomer was substantially low, 20-30% and <10%, respectively, the results from t h e - Y type zeolite are not presented here. 2. EXPERIMENTAL 2.1. Catalyst treatment and characterisation j3-Zeolite in protonated form (HI3) was obtained by the calcination of NH413-zeolite (Zeolyst International, CP-81gE, SiO:/A1203 mole ratio: 25) at 550~ for 10 h in air atmosphere. Metal cations were loaded onto the zeolite by stirring 5 g of NH4J3 in a 40 ml solution of 0.3 M of the proper metal chloride or metal nitrate at 80~ for 4 h. Following the ion-exchange process, the zeolite suspension was filtered, washed thoroughly by deionised water and finally dried at 120~ ovemight. BET surface areas of calcined zeolite samples were measured on a Micrometrics ASAP2010. The cation content of samples was determined using atomic absorption and ICPemission methods. The characteristics of catalysts are given in Table 1. Bronsted and Lewis acidities of samples were examined by FTIR analyses of pyridine adsorbed samples following a procedure given previously [9]. 2.2. Acylation reactions Acylation reactions were carried out in a 50 ml three necked round-bottom flask connected to a condenser in an oil bath under nitrogen atmosphere. The freshly activated (550~ 10 h) 1 g of catalyst was added quickly into the flask while still hot. 10 Mmol 2-MN, 20 mmol

801 Table 1 Physico-chemieal properties of the ion-exchanged 13-Zeolites used in the study. Mn+Metal Cation BET surface area Micropore Volume (cm3/g) Content, wt% (m2/g) H+ 584 0.192 In3+ 2.1 461 0.160 Zn2+ 1.9 452 0.148 A13+ NO a 445 0.145 Fe3+ 3.4 459 0.145 La3§ 0.7 395 0.133 aND: Not determined acylating reagent and 4.5 mmol tetradecane as an internal standard were dissolved in 20 ml nitrobenzene which was previously dried over CaC12 and distilled before use, and then introduced into the reaction flask through a dropping funnel. The reactions were performed at 130~ unless otherwise stated. Aliquots of samples were taken periodically and analysed by GC and GC/MS techniques.

2.3. Synthesis of authentic compounds 1-Acyl-2-methoxynaphthalenes were synthesised by acylation of 2-MN over a stoichiometric amount of A1C13 with the corresponding acyl chloride compound. Dry dichloromethane solution of 2-MN was cooled down to -15~ Acyl chloride was added gradually and stirred for an hour at -15~ At the end of reaction, to the mixture was added dilute HC1 solution and the product was extracted by dichloromethane and purified by recrystallization using petroleum ether and by column chromatography on silica gel using hexane-dichloromethane as eluent. 6-AcMN was commercially available. However other 6-Acyl-2-methoxynaphthalene compounds were recovered from zeolite catalysed acylation of 2-MN through column chromatography using hexane-dichloromethane eluent. 3. RESULTS AND DISCUSSION

3.1. Acylation of 2-methoxynaphthalene with anhydrides Some general remarks can be drawn on the basis of the results from acylation reactions performed with anhydrides. Regardless of the structure of anhydride and metal type on the zeolite used, 1-acyl and 6- acyl methoxynaphthalene products were the major acylation products, while selectivity of other acylated isomers was <10% overall. Except the reactions with propionic acid, 8-acyl isomer of other ketone products comprised the most (3-5%). Overall ketone selectivity was generally >95%. No indication of protiodeacylation reaction [3,10] or intermolecular trans-acylation reactions [11], which were reported to be principal reaction types responsible for deacylation of 1-acyl-2-methoxynaphthalene primary product, were observed in the presence of anhydrides.

802 6O

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. '5o

2-MN e

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,,o

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0 0

96-AcMN

i

,

,

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10

15

20

Reaction Time (h) Figure 1. Acylation of 2-MN with acetic anhydride over HI3 at 130~

25

98-AcMN

lo 0 0

+

J

i

+

5

10

15

20

25

Reaction Time (h) Figure 2. Acylation of 2-MN with acetyl chloride over HI3 at 130~

Figure 1 shows the reaction run for the acylation of 2-MN with acetic anhydride over HI3 at 130~ Apparently the conversion almost ceased after 6 h of reaction time and that there appears no indication ofinterconversion between product types. The results of the acylation reactions of 2-MN with acetic anhydride over ion exchanged 13 zeolites are presented in Table 2. Longer reaction times slightly lowered the 6-AcMN product selectivity, while the 1-AcMN selectivity increased slightly in general when reactions were performed with acetic anhydride. Table 2 Acylation of 2-methoxynaphthalene by acetic anhydride over M~+I3zeolite. Time Product Selectivity% M n+(h) Conversion% 1-AeMN 6-AcMN IT 2 40 28 65 24 48 30 62 H +a 2 47 27 68 24 47 30 63 In3+ 2 46 37 56 24 58 41 53 Zn 2+ 2 20 28 66 24 36 29 64 AI3+ 2 34 27 65 24 46 30 63 Fe 3+ 2 32 29 64 24 43 32 61 La 3+ 2 34 19 75 24 41 21 73 ant 160~

803 With the HI3 zeolite, the elevation of reaction temperature to 160~ increased the conversion rate, though the maximum 2-MN conversion was comparable to that obtained at the lower reaction temperature atter 24 h of reaction. The acylation reactions were terminated at this conversion level due to catalyst deactivation. It showed no effect on the product selectivity as well. Ion exchanging the 13 zeolite influenced its catalytic activity for acylation reactions. At 130~ although the conversion rate and maximum conversion of 2-MN were the highest with In13, this catalyst gave rise to the lowest selectivity for 6-AcMN. The conversion of 2-MN with other cation-exchanged zeolites was either comparable to or less than that observed with HI3 zeolite. The greatest selectivity for 6-AcMN product was obtained with Lal3-zeolite. However, the conversion of 2-MN was lower with this catalyst than with HI3. Zn 2+ exchanged zeolite displayed the lowest activity to the acetylation reaction. The regio-selectivity of acylation reactions should be influenced by the geometry of channels within zeolite. The introduction of larger cations reduced the effective size of these channels as can be deduced from the reduced BET surface areas of the ion-exchanged zeolites (Table 1), La loaded zeolite having the lowest BET surface area. However, intrinsic activity of cations can also play direct role in conversion of 2-MN and regio-selectivity of the products. It was reported that different metal halide compounds revealed different regioselective activity to the aeylation of 2-MN when used in catalytic amounts [ 13]. FTIR analyses of pyridine adsorbed samples revealed that I-I+-, AlS+-and La3+-13 zeolites contained only Bronsted acidity, while In3+, Fe 3+ and Zn 2+ exchanged zeolites contained both Bronsted and Lewis acid sites. In13 and Fel3 zeolites comprised similar Lewis-Bronsted ratio while the zinc exchanged zeolite possessed the highest proportion of Lewis sites. It has been reported that La exchange enhances the Bronsted acid strength of zeolites [7,14]. Bronsted acid sides appear to be more important in polarisation of anhydride into a more electrophilie species which then attacks the benzene ring resulting in the formation of ketone. Table 3 Acylation of 2-methoxynaphthalene by propionic or benzoic anhydride over M~+I3zeolite. Time Product Selectivity% Anhydride M~+(h) Conversion% 1-ACMN 6-ACMN Propionic H+ 2 36 12 75 24 50 11 75 In 3+ 2 35 13 75 24 56 13 75 La 3+ 2 28 7 81 24 43 7 79 Benzoic I-I+ 2 23 20 74 24 64 22 72 In3+ 2 29 71 29 24 45 60 38 La 3+ 2 2 12 88 24 3 9 91

804 Table 3 presents the results from the acylation of 2-MN with propionic anhydride or benzoic anhydride. The comparison of the data given in Tables 2 and 3 indicates that regioselectivity of products is dependent on the anhydride structure used. The higher ether conversion and selectivity to the acylation at 6-position were achieved with propionic and benzoic anhydrides than with acetic anhydride over HI3. Yet, benzoic anhydride afforded a slower reaction rate. This may be due to slower diffusion of bulkier benzoic anhydride. Molecular size of 1-acyl products should follow the order benzoyl>propionyl>acetyl. Hence, the formation of more bulky isomers, such as 1-acyl isomer, should be disfavoured owing to sterical constraints placed by the geometry of straight channels within the zeolite, which has 7.6 x 6.4 A opening. With propionic anhydride, the overall selectivity to the 1- and 6- position acylation was lower (85-87%), this being due to the formation of a diacylated ether product (3-4%) and increased selectivity to an acylated isomer with unknown structure (6-7%). Analogous to those with acetic anhydride, somewhat higher conversion was achieved over In13, yet product distribution was comparable. The conversion was lower and 6-position acylation was more selective over Lal3. The conversion of 2-MN with benzoic anhydride over In13 was lower compared to that achieved with HI), and the selectivity trend altered in favour of 1-benzoyl-2methoxynaphthalene product with the catalyst. In the case of Lal3, the conversion was only <3%. These results indicate that penetration of bulky anhydride thorough catalyst pores is highly restricted with the presence of cations and eventually blocked with the presence of La.

3.3. Acylation of 2-methoxynaphthalene with acyl halides Activity of catalysts was distinctly different for acylation reactions with acyl chlorides as compared to the reactions with anhydrides. Some distinct features were observed when the reactions were performed in the presence of acyl halides: the product recovery was substantially low with acyl halides and excluding the reaction performed over Fel3 with acetyl Table 4 Acylation of 2-methoxynaphthalene by acetyl.chloride over M~+I3zeolite. Time Product Product Selectivity% M ~+(h) Conversion% Recovery% a 1-AcMN 8-AcMN 6-AcMN IT 2 37 50 29 11 61 24 38 50 19 14 68 In 3§ 2 44 28 25 15 60 24 46 25 8 15 73 Zn 2§ 2 60 37 8 23 69 24 59 43 2 28 70 A13§ 2 34 59 27 9 62 24 46 47 16 12 71 Fe 3§ 2 32 59 29 4 64 24 43 44 32 11 61 La 3+ 2 37 75 39 8 53 24 39 74 26 10 64 aBased on 2-methoxynaphthalene consumed.

805 Table 5. Acylation of 2-methoxynaphthalene by acyl chlorides over M~+I3zeolite. Time Product Product Selectivity% Acyl M"+(h) Conversion Recoverya 1-ACMN 6-ACMN 8-ACMN chloride % Propionyl H+ 2 45 74 18 41 6 24 53 64 12 47 6 Zn 2+ 2 69 45 1 56 11 24 74 45 2 72 13 Benzoyl IT 2 66 59 50 38 5 24 79 50 30 56 6 Zn 2+ 2 72 77 38 46 5 24 81 51 22 61 6 aBased on 2-methoxynaphthalene consumed. chloride, 1-acyl substituted product was significantly lowered as the reaction proceeded (Figure 2 and Tables 4 and 5). The reaction mixture appeared dark brown or black, while it was yellow for the reactions with acid anhydrides. These indicate that catalytic activity was altered by the interaction of some primary or secondary reagents present within the reaction medium that favoured condensation reactions and deacylation of 1-acyl isomer. Excluding Lal3, metal exchanged 13 zeolites resulted in higher conversion than those attained with HI3 after 24 hour of acylation reaction with acetyl chloride. However product recovery determined by GC was significantly low. The results with Znl3 appear to be interesting. The higher conversion over Znl3 which has relatively higher Lewis acidity accompanied lower 1-AcMN and higher 8-AcMn (Table 4). Conversion values with Lal3 were comparable to those achieved with HI3. Nevertheless, the product recovery was the highest. With propionyl chloride a mono acylated product of 2-MN with an unknown isomeric form was determined. Its relative selectivity was 35 and 28% at the second hour of reaction and 27 and 12% after 24 h, with HI3 and Znl3, respectively. On the other hand, 1-acyl substituted product selectivity was negligible with Znl3 analogous to the case with acetyl chloride (Table 5). The 1-acyl selectivity was still high for the reaction with benzoyl chloride. In comparison, Znl3 is more active in deacetylation and selective formation of 6-benzoyl-2methoxynaphthalene directly. Probably, most of the 1-benzoyl-2-methoxynaphthalene product forms at the outer surface of the catalyst mainly over Znl3 and that it is improbable for this isomer to penetrate to active centres within the narrow sized channels to undergo deacylation which accounts for the higher recovery of 1-benzoyl-2-methoxynaphthalene. 4. CONCLUSIONS On the basis of the results summarised above, it can be stated that cation-exchanging the 13zeolite remarkably alters its activity in acylation of 2-MN and product regio-selectivity as well. Lal3 yielded higher 6-acyl-2-methoxynaphthalene selectivity with anhydrides. 2-MN

806 conversion was lower with ion-exchanged 13 and even negligible with Lal3 when benzoic anhydride used as acylating reagent probably due to size incompatibility between the cation blocked channels and transition states, and lesser diffusibility of the anhydride. Overall ketone selectivity for the acylation reactions with acyl chloride reagents was found to be significantly low due to facile formation of carbonaceous materials. 1-Acyl-2-methoxynaphthalenes were noticed to undergo deacylation in the presence of acyl chloride, while no such attitude occurred with anhydrides. ACKNOWLEDGEMENTS This work was financially supported by the Research Funding Office of Izmir Institute of Technology. Authors would like to thank Dr. A. Eroglu and Mr. S. Yilmaz for atomic absorption and ICP analyses, and Dr. H. Ozgener for his help in the purification of authentic samples.

REFERENCES

1. 2. 3. 4.

G.A. Olah, Friedel Crafts-Chemistry, Wiley, New York, 1973. P.J. Harrington, E. Lodewijk, Org. Proc. Res. Dev., 1 (1997) 72. G. Harvey, G. Mader Collect. Czech. Chem. Commurt, 57 (1992) 862. (a) H.K. Heinichen, W.F. H61drich, J. Catal., 185 (1999) 408; (b) Gunnewegh, S.S. Gopie, H.V. Bekkum, J. Mol. Catal., A: Chem., 106 (1996) 151; (c) S.D. Kim, K.H. Lee, J.S. Lee, Y.G. Kim, K.E. Yoon, J. Mol. Cat., A: Chem., 152 (2000) 33; (d) M. Casagrande, L. Storaro, M. Lenarda. R. Ganzeda, App. Catal. A General, 201 (2000) 263; (e) P. Andy, J. G.-Martinez, G. Lee, H. Gonzalez, C. W. Jones, M. E. Davis, J. Catal. 192 (2000) 215; (f) D. Das, S. Cheng, App. Catal. A General 201 (2000) 159. 5. B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Org. Chem., 51 (1986) 2128. 6. C. Gauthier, B. Chiche, A. Finiels, P. Geneste, J. Mol. Catal., 50 (1989) 219. 7. K. Craare, D. Akporiaye, J. Mol. Catal., A: Chemical, 109 (1996) 177. 8. P. Laidlaw, D. Bethell, S. M. Brown, G. J. Hutchings, J. Mol. Catal., A: Chemical, 174 (2001) 187. 9. K. Kaneda, T. Wada, S. Murata, M. Nomura, Energy Fuels, 12 (1998) 298. 10. J. AI-Ka'bi, J.A.Farooqi, P.H.Gore, A.M.G. Nassar, E.F. Saad, E.L. Short, D.N. Waters, J. Chem. Sot., Perkin Trans. 2, (1998) 943. 11. E. Fromentin, J.-M. Coustard, M. Gisnet, J. Catal., 190 (2000) 438. 12. S.P.-Art, K. Okura, M. Miura, S. Murata, M. Nomura, J. Chem. Soc., Perkin Trans. 1, (1994) 1703. 13. E.F.T. Lee, L.V.C. Rees, Zeolites, 7 (1987) 545.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

807

D e v e l o p m e n t of new ZSM-5 catalyst-additives in the Fluid Catalytic Cracking process for the maximization of gaseous alkenes yield A.A. Lappas a, C.S. Triantafillidis b, Z.A. Tsagrasouli a, V.A. Tsiatouras b, I.A. Vasalos a, and N.P. Evmiridisb aCenter for Research and Technology Hellas (CERTH), Chemical Process Engineering Research Institute (CPERI), P.O. Box 361,570 01 Thermi, Thessaloniki, Greece bDepartment of Chemistry, University of loannina, 45 110 Ioannina, Greece The catalytic activity of gallium, chromium or copper modified H-ZSM-5 zeolite as Fluid Catalytic Cracking (FCC) catalyst-additive was tested in this study. The partial ion-exchange of H-ZSM-5 with copper hindered framework dealumination upon severe steaming and resulted in significant increase of light alkenes (propylene and butylenes). The gallium and chromium impregnated H-ZSM-5 zeolites were more selective with regard to light alkenes compared to the unmodified H-ZSM-5 sample, but were not as active as the Cu-exchanged sample. An optimum balance between the number/strength of acid sites and the type/amount of metals in the modified H-ZSM-5 catalyst-additives is required for maximization of light alkenes with relatively low gasoline loss and low coke formation. 1. INTRODUCTION Higher yields of light olefins (C3 = and C4 =) in the Fluid Catalytic Cracking (FCC) process are desirable since they can be used for the production of polypropylene, high-octane gasoline additives, such as methyltertiarybutyl ether (MTBE) and alkylation gasoline. Utilization of the Ultra-Stable Y (USY) zeolite as the main active component of the FCC catalyst and addition of the shape-selective ZSM-5 zeolite enhances the production of light alkenes in the cracking of gas-oil and increases the gasoline RON [1-3]. The performance of the H-ZSM-5 additive and its effect on product yields and selectivity are mainly dependent on the number and strength of acid sites of the additive [4]. However, the modification of H-ZSM-5 with different metal ions, such as transition metal ions that exhibit high dehydrogenation activity, can promote the formation of light alkenes in the FCC process. The reaction mechanisms for the dehydrogenation/aromatization of light alkanes on metal-modified ZSM-5 and other zeolitic or porous silicate materials have been widely investigated [5-15]. The use of gallium and chromium containing catalysts in the Fluid Catalytic Cracking (FCC) process for improving the selectivity towards light alkenes is limited [16,17]. On the other hand, copper-modified ZSM-5 zeolites and other silicates have been widely tested for reducing the NOx and SO• emissions in the flue gases from the regenerator of the FCC unit [18-22]. In the present study, Cr-, Ga- and Cu-modified ZSM-5 samples were tested in a Microactivity Test (MAT) unit as additives to the main FCC catalyst in the cracking of gas-

808 oil, aiming to the maximization of light olefin production and to a better understanding of the reactions that are being catalyzed by ZSM-5 in the FCC process.

2. EXPERIMENTAL SECTION 2.1. Preparation and characterization of the ZSM-5 catalyst-additives The parent ZSM-5 zeolite samples were synthesized by typical hydrothermal procedures, based on the method introduced by Argauer and Landolt [23], and their crystallographic purity was determined by powder X-ray diffraction. The as-synthesized samples were calcined to combust the organic template and were further treated with dilute HC1 solution to produce the H-forms of the zeolites. An H-ZSM-5 sample with Si/Al = 40 was further steamed at 770~ for 6 h under the flow of a nitrogen-steam mixture at a flow rate of 30 ml/min and steam partial pressure of 97.7 kPa. The Ga-, Cr- and Cu-modified ZSM-5 zeolite samples were prepared either by impregnation (for Ga and Cr) or by ion-exchange (for Cu) of a H-ZSM-5 sample with Si/A1 = 27. The H-ZSM-5 sample was impregnated with aqueous solutions of Ga(NO3)3.xH20 or Cr(NO3)3.9H20 salts (~0,55 mmoles of metal ion per g zeolite), using vacuum rotary evaporation at 90~ The impregnated samples were further dried in air at 120~ for 12 h and were calcined at 500~ under the flow of dry air for 4h. The Cu-exchanged ZSM-5 sample was prepared by ion-exchanging the H-ZSM-5 zeolite with an Cu(NO3)3.3H20 aqueous solution, using stoichiometric amounts of Cu 2+ for a complete ion-exchange of H § at 60~ for 24 hrs; the ion-exchange process was repeated three times before final washing and drying at room temperature. All the impregnated and ion-exchanged samples were finally steamed at 770~ for 6 hrs, at a steam partial pressure of 97.7 kPa, similar to the H-ZSM-5 sample. All the steamed ZSM-5 samples prepared are listed in Table 1. The ZSM-5 zeolite samples were characterized by chemical analysis by Atomic Absorption Spectroscopy and classical analytical techniques (Si, Al, Na, Ga, Cr and Cu contents), XRD (relative crystallinity), N2 adsorption/desorption (BET surface area and pore volume), 27Al MAS-NMR (coordination of Al and determination of framework Al), ammonia-TPD (number and distribution of acid sites), SEM-EDS (particle morphology and elemental microanalysis). A detailed description of the characterization techniques has been previously reported [24]. 2.2. Evaluation of the ZSM-5 samples in a Microactivity Test (MAT) unit for gas-oil cracking The catalytic testing of the steamed ZSM-5 samples as FCC catalyst-additives in the cracking of gas-oil was performed on a Short-Contact-Time Microactivity Test (SCT-MAT) unit. This unit is a fixed bed reactor that operates at 560~ The total reaction time was 12 s and the unit severity was changed by changing the catalyst mass (the catalyst-to-oil ratio was C/O = 1-5). In this way the conversion of vacuum gas-oil (VGO) was changed in the range 55-75 wt.%. Both gaseous and liquid products were analyzed by gas chromatography (GC) while separation and identification of the individual components of the gasoline was achieved by an advanced GC technique, previously described [25]. For all tests the catalyst samples were prepared by mixing a commercial steamed FCC catalyst (base catalyst) with the steamed ZSM-5 additives at a concentration of 2 wt.%. The steam deactivation of the FCC catalyst was performed at 788~ for 9 hrs at the presence of

809 100% steam. The properties of the base catalyst were: zeolite surface area = 128 m2/g, matrix surface area = 100 m2/g, unit cell size = 24.32 A. The feedstock was a typical vacuum gas-oil from a commercial FCC unit with the following properties: specific gravity = 0.9044, sulfur content - 0.255 wt.%, mean average boiling point = 455~

3. RESULTS AND DISCUSSION 3.1. Compositional, structural and acidic characteristics of the ZSM-5 additives The physicochemical characteristics of the steamed ZSM-5 zeolite samples are listed in Table 1. The relatively severe steaming conditions of all the ZSM-5 samples reduced their framework A1 (FA1) dramatically to less than 0.1 wt. %. The FA1 content of the steamed samples was determined from the intensity of the peak at -53 ppm in the 27A1MAS NMR spectra (Fig. 3), based on an appropriate calibration curve which correlated the total A1 content of ZSM-5 samples free of extra-framework A1 (EFA1) with the intensity of the above 27A1 NMR peak [24]. The content of the chromium and gallium metals, introduced by impregnation, were -3-4 wt. %, while the ion-exchanged copper was only 0.5 wt. % (0.09 mmoles/g zeolite). This content of copper corresponds to -~20 % ion-exchange of the initial protons of the H-ZSM-5 sample, suggesting the difficulty in ion-exchanging the monovalent cations of the ZSM-5 zeolite with the relatively bulky divalent and trivalent hydrated cations of the transition metals and the rear earth metals, respectively. An interesting finding however, was that the small amount of copper ions, as hydrated Cu(I) or Cu(II) cations in ionexchange sites, hindered framework dealumination during steaming compared to the H-ZSM5 or the Ga- and Cr-modified samples (FA1 contents in Table 1).

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5

10

Figure 1.

15

20

25

30

35

40

45

20 (degrees) XRD patterns of steamed ZSM-5 samples

50 Figure 2. SEM images of (a) H/CuZSM-5 and (b) H/Ga-ZSM-5

810 Table 1 physicochemical characteristics of the steamed ZSM-5 zeolite catalyst-additives Samples Total content FAl Rel. s s m (4) Total acidity (5~ of metals (1) content (2) cryst. (3) Si Al M (+ 0.1) (+0.05) (wt. %)

(%)

(m2/g)

(mmoles NH3/g)

H-ZSM-5 H/Ga-ZSM-5

45.1 42.6

(wt. %) 1.06 1.53

3.87

0.08 0.08

91 81

363 345

0.13 0.13

H/Cr-ZSM-5

42.1

1.51

2.91

0.05

79

353

0.09

H/Cu-ZSM-5

44.5

1.60

0.58

0.30

90

359

0.28

(1) Total contents of Si, A1 and M (where M = Ga, Cr, Cu); the Na content was always < 0.08 wt.% (2) Framework A1 content determined based on 27A1MAS-NMR and chemical analysis data (3) Relative crystallinity of the steamed samples using the intensity of the XRD peaks between 23 and 25 degrees 20 and considering the fresh H-ZSM-5 samples as 100 % crystalline (4) Specific surface area (multi-point BET) (5~Total number of acid sites determined from ammonia-TPD tests

H-ZSM-5

X2 ,

140

100

60

20

-20

-60

,

,

140

100

,

,

,

,

60

,

20

. |

,

|

100

,

|

,

60

~

20

6 / ppm

Figure 3.

,

,

-20

,

l

,

-20

|

-60

H/Cr-ZSM-5

u-ZSM-5

140

,

.

.

.

.

.

.

I

-60

140

100

60

20

-20

-60

fi / p p m

27AlMAS NMR spectra of the steamed ZSM-5 samples

Although the zeolitic framework was severely dealuminated, all the zeolite samples retained their structural integrity to a high degree, as it was revealed by their high relative crystallinity (XRD data in Fig. 1 and Table 1) and their high surface area (N2 adsorption,

811 BET, Table 1). In addition, the SEM images of all the steamed modified samples showed clear phases of zeolitic particles being aggregates of smaller crystallites, with no extra-zeolitic metallic phases (Fig. 2). The EDS analysis further proved an homogeneous distribution of the metals (Si, Al, Cr, Ga, Cu) within all the zeolitic particles of the related samples. Previous studies on chromium and gallium modified zeolites or other porous silicate materials have shown that the oxidation state of the metals and their distribution in framework or extraframework positions are greatly dependent on the synthesis procedure and/or the postsynthesis modification of the samples [5,8,9,13]. High-temperature steaming (in the absence of any reducing agent) of Cr- and Ga-impregnated ZSM-5 samples usually leads to formation of metal oxides or oxy-hydroxides residing mainly on the external surface of the zeolitic particles/crystals. The total number of acid sites of the Cu-exchanged H-ZSM-5 sample was higher than the rest of the samples, in accordance with its higher FAl content (Table 1). However, the ratio of the total acid sites to the FAl content was lower for the Cu-exchanged sample compared to the rest of the samples, indicating that most of the Cu ions after steaming are still in ion-exchange sites replacing acidic protons. 3.2. Catalytic activity of the modified ZSM-5 samples as FCC catalyst-additives in the cracking of gas-oil The curves in Figure 4 correspond to the gasoline and propylene yields as a function of total conversion of gas-oil. Similar curves were produced for all the gaseous and liquid products obtained from testing the industrial, equilibrium FCC catalyst (base catalyst) and mixtures of the base catalyst with the ZSM-5 zeolitic additives (2 wt. %). All the product yields were determined at constant conversion, ca. 65 wt. % conversion, and the most important are listed in Table 2. From the short contact time (SCT)-MAT catalytic data it was shown that all the ZSM-5 additives induced higher yields for the gaseous alkenes (mainly for propylene), lower gasoline production and higher octane number, compared to the base catalyst. The higher increase in the formation of gaseous alkenes was achieved by the H/Cu-ZSM-5 additive (48 % and 20 % increase for propylene and butylenes, respectively, compared to the base catalyst). Interestingly, the significant increase in light alkenes and the decrease in gasoline were not accompanied by an increase in coke formation. In addition, the yields of hydrogen, methane and ethane are not significantly increased. Both the above results are in accord with the very low number of strong framework Bronsted acid sites in the severely steamed ZSM-5 samples [24], which can catalyze aromatization of light alkanes and/or alkenes [26,27] and enhance the protolytic cracking of small alkanes through non-classical pentacoordinated carbonium ions [4,28,29]. The gradual change in the reaction mechanisms over a ZSM-5 additive during its progressive deactivation has been previously well established [3,30]. The weak/medium acidity of the steamed ZSM-5 additives in the FCC process can be beneficial for the increase in the selectivity of light alkenes in the products, since they are not being converted to aromatics due to the absence of strong Bronsted acid sites in the ZSM-5 additive. In addition, dehydrogenation activity and selectivity to alkenes of metals like Cr-, Ga-, Cu-, Mo-, and others in zeolitic catalysts can be more easily realized in the absence of strong Bronsted acidity, as it has been clearly shown in different model reaction systems.

812

54

.i

48 9 Base x +H-ZSM-5

42

x +H/Ga-ZS M- 5 + +H/Cr-ZSM-5

36

,

|

0

6'0

7'0

9 +H/Cu-ZSM-5 , 8O

6 9

-~ a

9

4

2 50

60

70

, 80

Conversion (wt. %) Figure 4. Gasoline and propylene yields versus total conversion of gas-oil However, the complexity of the reaction network in gas-oil cracking affects product yields and properties in a different way than in a single molecule reaction. Thus, minimization of the strong acidity of the ZSM-5 additive either by severe dealumination and/or by "deactivation" of the framework Bronsted acid sites by ion-exchanging with non-acidic metal ions with dehydrogenation activity, can suppress other valuable reaction routes like the cracking of C7+ alkenes and the isomerization of gasoline range alkenes. Based on the results of this study, it appears that a partially Cu-exchanged H-ZSM-5 can provide with an optimum balance between dehydrogenation activity of the metal and acid function of the shape-selective HZSM-5 zeolite.

813 Table 2 Product yields and octane number of gasoline in the cracking of gas-oil (at 65 wt.% conversion) Base (1) +H-ZSM-5 +H/Ga-ZSM-5 +H/Cr-ZSM-5 +H/Cu-ZSM-5 Product yields C/O (2) 1.55 1.37 1.56 1.72 1.49 49.0 48.7 48.0 47.2 Gasoline 45.4 17.3 17.6 17.4 16.9 17.6 LCO 92.2 92.3 91.8 91.9 92.5 GC RON 1.65 1.57 1.72 1.62 1.58 Coke 13.9 14.5 12.9 13.3 16.5 LPG 7 20 7.25 7 90 6 60 6.95 C4 olefins 1.74 1 83 2.01 1 84 219 i-butane 0.49 047 0.54 0 5O 0 56 n-butane 3 88 3.63 4.11 515 347 Propylene 0.59 0.51 052 065 Propane 051 1.35 138 1.51 1 54 1 42 Total C1+C2 041 0.43 041 0.39 053 Ethylene O07 0.07 0 O6 0.06 008 Hydrogen (1) The base catalyst was an industrial FCC catalyst and a 2 wt.% ZSM-5 additive was used for the rest of the catalysts; (2) Catalyst-to-Oil ratio The positive effect of the impregnated Ga and Cr species on the increase of light alkenes can be more clearly realized by comparing the FA1 content and acidity of the metal-modified ZSM-5's with those of the unmodified H-ZSM-5 additive. The Ga-modified sample had similar FA1 content and acidity with the H-ZSM-5 sample, while the Cr-modified samples had even less FA1 and acidity than H-ZSM-5. This resulted in minimum gasoline loss and an appreciable increase in C3 and C4 alkenes due to the dehydrogenation activity of the extraframework oxides of gallium and chromium. However, the deposition of these oxides mainly on the external surface of the ZSM-5 crystallites may block internal acidic active sites from reacting, leading to different product yields and distribution. Further studies on the stability of the metal modified ZSM-5 additives under consecutive reaction-regeneration cycles in the FCC process and on the optimum combination of different metal ions with the zeolitic acid sites are in progress.

4. CONLUSIONS The catalytic activity of metal-modified H-ZSM-5 zeolite as FCC catalyst-additive depends both on the number and strength of its acid sites and on the nature and the amount of the metal species within the zeolite sample. Partially ion-exchanged H-ZSM-5 zeolite with copper provides an effective catalyst-additive for maximizing light alkenes yield (propylene and butylenes) without increasing coke formation. Impregnation of H-ZSM-5 with gallium and chromium metals at a level o f - 3 - 4 wt. % results in less selective catalyst-additives

814 compared to the copper-exchanged H-ZSM-5, still inducing higher light alkene yields compared to the unmodified H-ZSM-5 zeolite.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

815

C h a r a c t e r i z a t i o n o f H and C u m o r d e n i t e s with v a r y i n g SIO2/A1203 ratios, by optical s p e c t r o s c o p y , M A S N M R o f 29Si, 27A1 and ~H, t e m p e r a t u r e p r o g r a m m e d d e s o r p t i o n and catalytic activity for nitrogen oxide r e d u c t i o n v. Petranovskii a, R.F. Marzke b, G. Diaz c, A. Gomez ~, N. Bogdanchikova a, S. Fuentes ~, N. Katada d, A. Pestryakov ~ and V. Gurin f aCentro de Ciencias de la Materia Condensada, UNAM, Apdo. Postal 2681, 22800 Ensenada, B.C., Mexico bArizona State University, Tempe, AZ 85287-1504, USA r

de Fisica, UNAM, Apdo. Postal 20-364, Mexico 01000 D.F., Mexico

aDepartment of Materials Science, Tottori University, Tottori 680-8552, Japan ~Tomsk State University of Civil Engineering, Tomsk 634003, Russia fPhysico-Chemical Research Institute, BSU, Minsk, 220080 Belarus H-mordenites (HMor) with variable SIO2/A1203 molar ratio (MR) varying from 10 to 206 were incorporated with copper (CuMor). Maximum total acidity and maximum concentration of the strongest acid sites were found for MR-15 (HMorl5). This sample has the lowest micropore and mesopore content, but also show strongly disordered Si sites in the crystalline lattice. A consistent finding has been the disappearance of the NMR line from extraframework 27A1, whenever Cu is present. The reducibility of copper ions and stability of reduced species depend on MR. The MR also influences optical lineshapes from different reduced species of Cu. A plasmon resonance band at 580 nm, due to small Cu particles formed in the zeolite, occurs at low and high MR but not at MR=15. Catalytic tests of NO conversion reveal that the reduced Cu-mordenites with MR-15 are the most stable and active ones. 1. INTRODUCTION Interest in Cu-zeolite systems continues to increase, due to the capability of these compounds for catalyzing important NO-reduction processes [1-3]. This and many other useful properties of zeolites arise because these compounds can both support chemically active metal ions, such as copper, and at the same time act as catalysts themselves. Such chemical flexibility in assuming dual roles leads to an especially rich chemistry, in which metal ions interact with different Bronsted and Lewis acid sites [4]. An important factor influencing this chemistry is a zeolite's SIO2/A1203 molar ratio (MR). It has been found that the mordenite MR determines the formation and stability of different

816 reduced Ag and Cu species [5-7]. It is reasonable to expect that changing the chemical composition of the mordenite framework leads to major changes in the occupation of sites by individual non-framework atoms, as well as in the stabilization of clusters of these atoms. The framework of mordenite possesses several positions for copper localization, and there are noticeable changes in water tolerance and hydrothermal stability under heating, dehydration, reduction, etc. [8-10]. Analysis by a Monte Carlo procedure has shown that pairs of closelyspaced Bronsted acid (H) sites, which are one means of binding extra-framework Cu 2§ cations; are distributed with a probability that is highly sensitive to MR [9]. Thus in NO reduction under lean NOx wet conditions [8], it is not surprising that MR is one of the crucial characteristics determining Cu zeolites' overall effectiveness and lifetime for NO conversion. This has led us to investigate generally the effects of MR upon both framework (Si, AI) and extra-framework (H, Cu, A1) atoms of mordenites, by means of MAS NMR, thermoprogrammed desorption (TPD) of ammonia and diffuse reflectance spectroscopy (DRS). We have also correlated our findings with results of direct measurements of the catalytic properties of Cu-mordenites, e.g. NO conversion rates. 2. E X P E R I M E N T A L A series of samples was prepared from H + mordenites having MR values ranging from 10 through 206, originally fumished via the TOSOH Corporation of Japan. Cu2+exchange was carried out using aqueous 0.1 M Cu(NO3)2. After drying the samples were reduced in flowing H2 for 4 h at 350 ~ The copper content was determined by atomic absorption spectrometry, using a Varian 1475 apparatus. DRS data were recorded on a Varian Cary 300, equipped with a standard diffuse reflectance unit. High-resolution nitrogen adsorption measurements were performed on an Autosorb-1 Quantachrome unit, over the range 106< P/Po < 1. Surface area SBEr and micropore volume Vz were thereby determined for the sample set. X-ray diffraction (XRD) measurements were made using Cu Ka radiation on a Philips X'Pert diffractometer equipped with a curved graphite monochromator. TPD measurements with ammonia were performed with a Bell Japan, Inc. TPD-1-AT (NH3) apparatus, and were analyzed according to [ 11 ]. The procedure followed was first to pack a sample (0.1 g) into the quartz TPD cell and evacuate it at 773 K for 1 h, then to adsorb NH3 at 373 K and 13.3 kPa. Water vapor treatment was applied next, in order to remove weakly held ammonia [12]. Finally, the Sample was heated at 10 K rain "l under flowing helium (0.044 mol s "l, 13.3 kPa). The desorbed ammonia was detected by a mass spectrometer (ULVAC UPM-ST-200 or ANELVA M-QA 100 F). After the measurement, a known amount of ammonia was used to calibrate the peak intensity. The Br~nsted acidity of the mordenite surface was studied by non-aqueous potentiometric titration by C2HsOK in a dimethylformamide medium, using a pH-673M potentiometer with platinum and glass electrodes. Catalytic activity in the model reaction NO + CO was measured in a continuous flow microreactor (Multipulse RIG 100, In Situ Research Instruments), under atmospheric pressure and in the temperature range 673-973 K. Solid-state magic angle spinning (MAS) NMR spectra were obtained at room temperature on a Varian Infinity 300 MHz spectrometer. Acquisition parameters at 300 MHz were: 29Si - frequency 59.596 MHz, spin 6.5 kHz, r.f. pulse 6 laS, pulse angle 90~ 27AI - 78.17 MHz, spin -- 6-9 kHz, 90 ~ r.f. pulse --8 ~ts; ~H - 300 MHz, spin 2.5 kHz for unreduced and 6.5 kHz for the H2-reduced materials, 90 ~ r.f. pulse 11.75 Its, spin echo. Low Al-background spectra shown are due to Dr. L. Bull.

817 3. RESULTS AND DISCUSSION Characteristics of samples are collected in Table 1. The total volume of porosity, Vz, and other microporosity features, assume minimum values for HMorl5. An estimated value of 0.14 cm3/g micropore volume is expected for ideal mordenite crystals, arising from regular voids in the structure with cross-section 0.65x0.70 nm [4]. Additional porosity contributions can be due to irregular porosity inside the crystal volume. The mesopore volume, associated with cleaved areas of microcrystals, macro-defects, and uneven surface, is essentially independent of the amount of Si substituted by AI. Adsorption isotherms of N2 at 76 K confirm an unusual property of HMorl5, which exhibits a saturation zone in the interval 0.1 < p/p0 < 0.8. This indicates homogeneity of the micropores, and only a moderate increase in the capacity of adsorption for P/P0 > 0.9 is observed. That is to say, HMor 15 apparently is the most crystalline and defect-free. For the samples of HMor30 and HMor206 the hysteresis in adsorption cycles is better defined, indicating a higher content of mesopores. TPD patterns observed (Fig. 1) show a complicated dependence on MR, and one can observe several maxima, each contributing very differently to the total desorption profile of a sample. The peak at lowest temperature, ca. 490 K, dominates in the HMorl0 sample, but a shoulder (ca. 660 K) can also be noted. A pronounced maximum at ca. 730 K occurs for the other three samples, with little shift to the lower temperatures at increasing MR. The maxima are not symmetrical, largely because of the shoulder at 660 K. We associate the three peaks, 450-490 (Lewis); 660 and 730K (Bronsted), with three different groups of acid sites, the amounts of each being different for different MRs. Considerable extra-framework A1 exists in HMorl0, as is visible in the 27A1 MAS NMR spectra (Fig. 7). The extra-framework AI is expected to form much less acidic hydroxyls than framework aluminum atoms; and probably these are located at the acid sites that result in the 490 K TPD peak (Fig. 1). Table 1 Characteristics of mordenites used and copper content for prepared samples HMorl0

HMorl 5

HMor30

HMor206

SBET, m2/g

359

380

480

493

St, (external surface) m2/g

45

33

51

68

V~, cm3/g

0.200

0.174

0.238

0.261

Micropore volume, cm3/g (Pore radii 0.5-2 nm)

0.15

0.12

0.14

0.16

Mesopore volume, cm3/g (Pore radii 2-50 nm)

0.07

0.05

0.06

0.05

A1 concentration derived from MR, mol/kg

2.56

1.83

1.01

0.16

TPD-derived conc. of acid sites, mol/kg

0.44

1.24

0.49

0.13

Copper content after ion exchange, wt %

0.79

0.26

0.26

0.44

Theoretical ion-exchange capacity on the basis of MR , with " re sp e c t t o C u 2+, w t % o f C u

8.2

5.9

3.2

0.5

% of ion exchange

9.6

4.4

8.1

86.3

818 The distribution of pH for Br~nsted acid sites (BAS) was determined by titration (Table 2). All samples have a number of types of BAS over a wide range of pH, but the surface concentration of BAS is different for each. HMorl5 has the most acidic properties, with regard to both strength and concentration of BAS. The surface density of Bronsted sites reaches a maximum for pH -2 (Table 2). These data correlate well with the findings from TPD. 2+ UV-Vis spectra of Cu -exchanged samples show the broad band with maximum at 2+ -850 nm expected for Cu -containing silica-based materials [13]. A strong effect of MR on the lineshapes of the Cu species is only seen following reduction and heating in flowing H2 [7]. The plasmon resonance band at 580 nm, due to small Cu particles, occurs at low and high MR but not at MR=15 (Fig. 2) in the H-reduced samples. The kinetics of the reduced samples reoxidation in air depends also on MR, and we find that MR= 15 produces the most stable samples. The framework (29Si and 27A1) and extra-framework (IH, 27A1) atoms of both H- and Cumordenites were characterized by MAS NMR There are four distinct inequivalent sites for silica atoms in the mordenite crystalline structure, with relative populations as 16:16:8:8 [14]. The numbers and relative intensities of the resonances that are observed in the 29Si spectra reflect directly the numbers and occupancies of the inequivalent T-sites [15]. Only lines arising from sites with 0AI and 1A1 nearest neighbors were observed for our samples. Spectra of 27A1 show lines from 4- and 6-fold coordination sites, with possible 5-coordinated sites at some MR values. Proton NMR was also observed, for samples at MR 206 and MR 15. Spectra for MR 15 agree with observations by Baba et al. [ 16], but those at high MR differ from his. For H-mordenites at high silica-alumina ratios (128, 206) a new spectrum 29Si spectrum has been observed, as shown at the upper right of Fig. 3. The spectrum has only two prominent Si(0A1) lines, of almost equal intensities, and two much weaker lines from Si(1A1) silicon sites. Other highly siliceous mordenite 29Si NMR spectra observed to date [ 17,18] show three Si(0A1) lines, with intensities in the ratios 3:1:2 v s . increasing chemical shift (to the left). The second and third ones are attributed to the T1 and T3 sites in the 8-membered rings of the lattice, while the first arises from both T2 and T4 sites in the 12-membered rings, which have essentially identical chemical shifts. Apparently, in our highly siliceous samples of dealuminated TOSOH materials the two lines from the Si(0AI) T1 and T3 sites have merged Table 2 Specific concentration and pH of Bronsted acid sites HMor 10 BAS pH

CBAS, ktmol/m2

2.8

0.18

3.9

0.22

5.0

0.84

5.7

0.24

HMor30

HMor 15

BAS pH

CBAS, ~tmol/m2

1.5

0.32

0.50

1.8

0.34

2.9

0.92

2.4

0.36

3.4

0.52

3.3

0.24

0.18

5.0

0.20

4.0

0.06

0.28

5.6

0.10

5.4

0.06

BAS pH

CBAS,

1.5

0.92

2.0

1.00

1.8

2.3

0.76

5.5 6.1

ktmol/m2

BAS pH

CBAS, ~tmol/m2

HMor206

819 into one. This yields our MR 128 and 206 spectra, with two strong lines of approximately equal intensity. The reasons for the spectral equivalence of the T1 and T3 sites on our mordenites are not immediately obvious from XRD. As the AI content of the H mordenite increases, the two high-MR Si(0A1) lines broaden and merge into one. The increasingly intense Si(1AI) lines do the same. However, the Si(1AI) line appears anomalously large at MR 15 (Fig. 3). All other samples have Si(1AI)/Si(0AI) intensity ratios less than one, but for MR 15 the ratio is about 1.8 (Fig. 4). The reason for this is apparently a reduced Si(0A1) line intensity for this MR, rather than an increased Si(1AI) line intensity. Otherwise, the large peak at the Si(1AI) position visible in Fig. 3 for MR=15 might indicate the substantial presence of silanol (SiOH) groups, whose 29Si line has approximately the same chemical shift as the Si(1AI) line [18]. The weak Si(0AI) line probably indicates greater disorder in the non Al-containing part of the structure at MR 15, although the measured mesopore and micropore volumes are actually smallest at this ratio. These conclusions are not necessarily in contradiction with the adsorption data discussed earlier, because Si disorder represents defects in crystal structure at the atomic level, while porosity represents appearance of defects at the meso- or macro-level of sample structure. The basic pattern of 29Si spectra is remarkably stable (Fig. 5) under Cu exchange and H reduction, indicating that exchange does not disturb the underlying framework structure to a significant extent. The IH spectra, however, show marked effects of Cu 2§ exchange, as expected. In mildly dehydrated HMor206 the proton spectrum consists of two narrow, overlapping lines at 4.6 and 1.4 ppm chemical shifts (Fig. 6). After ion-exchange treatment, the peak at 4.6 ppm has disappeared, leaving the only peak at 1.4 ppm. Remarkably, after hydrogen reduction the 4.6 ~pm peak is restored, possibly according to a reaction such as CuMor + H2 --) HMor + Cu". Other proton peaks are seen as well, in the region 0 - 6 ppm, most having considerable structure and/or broadening. In addition, the very broad line of the initially Cu-exchanged material decreases in intensity. These observations are consistent with the presence of Cu 2§ accompanied by OH, in both unreduced and reduced samples. The 27A1 spectra shown for H-mordenite (Fig. 7) indicate that most of the aluminum atoms in the samples are found in tetrahedrally coordinated framework sites. However, there are also substantial amounts of extra-framework, octahedrally-coordinated aluminum atoms present, as seen in the NMR line occurring close to 0 ppm chemical shift. NMR spectra for Cu-mordenite differ from these mainly in the absence of line of 6-fold coordinated A1. This imPlies that Cu either displaces or replaces all extra-framework AI in the channels and cages of the zeolite. Catalytic tests showed that NO conversion depends strongly on the MR of samples. At temperatures > 773 K, the rate of NO conversion exhibits a pronounced maximum for MR 15 (Fig. 8). The only products of the reaction were N2 and CO2 (the absence of N20 was verified using a Porapak Q separation column). This tendency correlates with the negligible amount of reduced copper particles in the CuMorl5 sample, as indicated by DRS (Fig. 2), in contrast to other samples of Cu-mordenites [7]. The largest fraction of strong acid sites, responsible for the 730 K peak in Fig. 1, produces either few-atom copper clusters under low temperature reduction or very small Cu-nanoparticles (about 1-2 nm) at higher reduction temperatures. The high acid strength of these sites presumably prevents fast reduction of Cu 2§ to metallic copper. However, along with other reduced copper species, these sites are probably responsible for the high catalytic activity of H-reduced CuMor 15.

820 0.012

-

lSe f ' ; ' \

0.010. ,l r

0.008.

. c_

0.006.

I

~._,

._~

,

I

.

/

I" ,"

~t

,

.. , , ' "

.

.- .... ,",

400

500

600

! .

-

. . . . . . . .

",,

.g

t~

"~ \x ',,. "0~ "~ ,~ t;

700

Tempereture,

i

128 ai

".~.,,~ .. ~.,

./

:,

I

Plasmon of metal Cu

I

.IQ <

"\

", '~. -..-

"\ ,, "~,.~~

800

900

~-,.

io oooo r L)

\ "l'~"l

t i i ,,... ,, ," ..,

z "6 0"004'1

~

/ ,

........................................................................

..^^ ~ ~ _ ~ - - ~ ~ ~ . ,

--HM10 - - 9HM15 --HM20 . . . . HM30 . . . . . HM72 . . . . . HM128 . . . . HM206

p, PL, I

~;

~"=-"=-

aoo

4;0

.,,

1000

K

Fig. 1. Ammonia TPD spectra of H-mordenites with different MR

s;o

6;0

76o

Wavelength, nm

, ~

800

Fig. 2. Spectra of CuMor with different MR (indicated) reduced at 350 ~ Curves are shifted for clarity 2.0

72

r,._

30

20

i

-100

< o

1.o.

~

0.5.

1

0.0

...... w................ i ...........

-120

9 0

a, ppm

, 50

e

160

1~o

e 200

S i O 2 / A I 2 0 3 ratio

Fig. 3. 298i MAS NMR spectra of H-Mor at varying MR (indicated).

Fig. 4. Ratio of intensities of Si(1AI) to Si(0A1) 298i lines in H-mordenites as a function of MR. i

i

I C

b

b a__ -lOO

-1 lO

a, ppm

<___ -120

Fig. 5.29Si MAS NMR spectra of HMor206 (a), CuMor206 (b) and CuMor206-350 (c)

4'0

2'0

,. ~

o

a, ppm

"

-~o

a

Jo

Fig. 6. ~H MAS NMR spectra of HMor206 (a), CuMor206 (b) and CuMor206-350 (c). Intensity of spectra is not normalized.

821 ,

60

I~ A

,

---e,-- 400 ~

A

- - - 500~ -&--

40

600 ~ 700 ~

,,,

,o

._.JL___

,~

~ o r-

zo

0

0

o

B

q--

.

.

L

O

0.84

1

,,o

=za

206

ppm .... ~5' .... 5o' .... as'.... 'o....-as'.... Fig. 7.27A1 MAS NMR spectra of H-Mor at indicated MR (L. Bull).

MR

10

0.813 15

0.92

')1 31

0.96

1.00

1

206

Fig. 8. NO conversion for H-Mordenites (A) and set of reduced Cu-mordenites (B) at different reaction temperatures. Scale, showing part of Si between total population of tetrahedral atoms for convenience is duplicated lower by MR scale.

4. CONCLUSIONS Mordenites with variable SIO2/A1203 molar ratio were copper exchanged and subsequently reduced. The concentration of different acid centers and their strength vis-/t-vis copper reducibility depend strongly on molar ratio. The maximum total acidity and maximum concentration of acid sites was found for the sample with MR = 15. This sample has also the lowest amount of structural defects as revealed by gas adsorption measurements, and perhaps also the highest Si structural disorder, as seen in NMR. For highly siliceous mordenites (MR 128 and 206), an unusual MAS NMR spectrum for 29Si was observed, which indicated the near chemical equivalence of T1 and T3 silicon atoms in the highly dealuminated crystal lattice. Protons with chemical shift 4.6 ppm disappeared following copper ion exchange, and reappeared after copper reduction with H2. The reduction also resulted in nanoparticle formation, indicated by a plasmon resonance in the range of 560-600 nm. This depends upon MR, Cu mordenite at MR=15 showing essentially no plasmon absorption. This observation can be interpreted as indicating that the acidity of the zeolites strongly influences Cu particle formation. The efficiency of NO conversion depends upon MR, and H-reduced samples with MR = 15 are the most stable and active ones. ! 00 % selectivity to N2 formation was observed. From all of these observations, we conclude that control of SIO2/A1203 molar ratio of mordenites can be used as an efficient tool governing the copper reduction and catalytic activity.

822 ACKNOWLEDGMENTS Support and cooperation of Drs. L. Bull, M. Avalos, M.-A. Hemandes and J.-V. Tamariz Flores are gratefully acknowledged. The 29Si spectra were taken by E. McDaniel in the laboratory of R.F. Marzke. The authors would also like to thank J. Piwowarczyk, D. Kovalenko, I. Rodriguez-Iznaga and A. Morales for the assistance throughout several phases of this work. The authors are grateful to E. Flores, E. Aparicio, F. Ruiz, G. Vilchis, I. Gradilla, J. Peralta, M. Sainz and C. Sanchez for essential technical assistance. This research was supported by CONACYT, Mexico, through Grant # 32118-E, and by DOE, USA, through the joint Mexico-U.S. Materials Corridor Partnership Initiative "South-west Border Energy and Technology Collaboration Program", #DE-FC04-01AL67097. REFERENCES 1. P. Gilot, M. Guyon and B.R. Stanmore, Fuel, 76 (1997) 507. 2. C. Torre-Abreu, C. Henriques, F. R. Ribeiro, G. Delahay and M.F. Ribeiro, Cat. Today, 54 (1999) 407. 3. V.I. Parvulescu, P. Grange and B. Delmon, Catal. Today 46 (1998) 233. 4. D.W. Breck, Zeolite Molecular Sieves. Structure, Chemistry and Use, A WileyInterscience Publication, John Wiley & Sons: New York, 1974. 5. N.E. Bogdanchikova, M. Dulin, A.V. Toktarev, G.V. Shevnina, V.N. Kolomiichuk, V.I. Zaikovskii and V.P. Petranovskii, Stud. Surf. Sci. Catal., 84 (1994) 1067. 6. N. Bogdanchikova, V. Petranovskii, R. Machorro, Y. Sugi, V.M. Soto and S. Fuentes, Appl. Surf. Sci., 150 (1999) 58. 7. V. Petranovskii, V. Gurin, N. Bogdanchikova, A. Licea-Claverie, Y. Sugi and E. Stoyanov, Mater. Sci. Eng. A. (2002), in press. 8. S.Y. Chung, S.H. Oh, M.H. Kim, I.S. Nam, Y.G. Kim, Cat. Today, 54 (1999) 521. 9. B.R. Goodman, K.C. Hass, W.F. Schneider and J.B. Adams, Cat. Lett., 68 (2000) 85. 10. M.P. Attfield, S.J. Weigel and A.K. Cheetham, J. Catal., 170 (1997) 227. 11. N. Katada, H. Igi, J.-H. Kim and M. Niwa, J. Phys. Chem., B, 101 (1997) 5969. 12. H. Igi, N. Katada and M. Niwa, in Proc. 12th Intern. Zeolite Conf., M. Treacy, B. Marcus, M.E. Bisher, J.B. Higgins (eds.), Materials Research Society, Warrendale, 1999, p. 2643. 13. A.B.P. Lever, Inorganic electronic spectroscopy, Elsevier, 1984. 14. Atlas of Zeolite Structure Types, 5th revised ed.; Ch. Baerlocher, W.M. Meier, D.H. Olson (eds.), Elsevier: London, 2000. 15. G.A. Fyfe, K.T. Mueller and G.T. Kokotailo, In: NMR techniques in catalysis, A. Bell, A. Pines (Eds.), Marcel Dekker, Inc.; New York, 1994, p. 11. 16. T. Baba, N. Komatsu, Y. Ono, H. Sugisawa and T. Takahashi, Micropor. Mesopor. Mater., 22 (1998) 203. 17. W. Kolodziejski and J. Klinowski, In: NMR techniques in catalysis, A. Bell, A. Pines (Eds.), Marcel Dekker, Inc.; New York, 1994, p. 361. 18. P. Bodart, J. Nagy, G. Debras, Z. Gabelica and P. Jacobs, J. Phys. Chem., 90 (1986) 5183.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

823

A Comparison o f SAPO, GaPSO, M g A P O and G a P O ' s as DeNOx catalysts V. I. P~xvulescu~, M. Alifanti ~c, M. H. Zahedi-Niakib, P. Grange c and S. Kaliaguine ~ aUniversity of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: [email protected] bUniversit6 Laval, Departement de Genie Chimique, Ste-Foy, Quebec, G1K 7P4. C

~

.

Umversite Catholique de Louvain, Unite de Catalyse et Chimie des Materiaux Divises, Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium, fax.- 32 10 473649, E-mail: [email protected] A series of SAPO, GaPSO, M g ~ O and GaPO's were prepared by hydrothermal synthesis under autogeneous pressure. These catalysts were characterized by XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTA, NH3-DRIFT, XPS, and MAS NMR. They were tested under learn-burn conditions in the reduction of NO and NO/using isobutane and ethane as reductants. Under the investigated conditions, the reaction occurred selectively mainly to alkenes, namely, isobutene and ethene. The characterization of the tested catalysts indicated no changes for SAPO's and MgAPO, but only for gallium containing phosphates. However, both catalysts were stable in time. 1. INTRODUCTION The catalytic reduction of NOx under fuel-lean conditions is of major technological importance. These conditions are encountered in the exhaust streams generated by learn-bum gasoline and diesel engines, both of which offer substantial improvement in fuel economy, and hence lower CO2 emissions, as compared to conventional petrol engines. The replacement of anamonia with hydrocarbons, merely alkanes, may bring many environmental advantages. Although a large diversity of materials were tested in this reaction, most of the studies reported up to this moment in the literature focused on metal- zeolites and metal-supported catalysts [1 ]. The activation of alkanes is evidently a key factor in this process. The use of Gacontaining catalysts in the reduction of NO with hydrocarbons was reported to exhibit a positive influence. Li and Armor [2] suggested that data obtained from Ga-MFI zeolites indicated some synergetic effects between the gallium species, mainly precipitated on ZSM-5 surfaces and zeolitic H + (i.e. proton species belonging on Al framework). The role of Ga is to enhance the activation of CH4, the NO reduction occurring on H + sites. As the authors suggested, the mechanism is similar to that resulted in the aromatization of propane or butane. The importance of the H + sites was revealed by the fact that Ga-Na-ZSM-5 is inactive in the NO reduction. In contrast to these data, Satsuma et al [3] showed that for Ga-H-mordenite the

824 acid properties of zeolites did not constitute the major factors determining the selectivity of the reduction of NO with C3I-I6. For the same reaction Kikuchi et al. [4] indicated the formation of NO2 species as a determining step. These authors shown that NOx species are also formed on zeolite acid sites in the case of Ga- or In-exchanged catalysts [5]. Oxyfunctionalization of alkanes is one of the major challenges of modem catalysis [67]. Recently, Thomas et al. [8-11] have shown that microporous phosphates are versatile catalysts in aerial oxidation of alkanes. These authors indicated that on these catalysts alkane oxidation proceeds by a free-radical mechanism in an essentially shape-selective manner in the spatially restricted environment created by the active sites. The active sites are isolated M(III) ions exposed on the inner walls of the molecular sieve. This work focused on MAIPO molecular sieves (M=Fe, Co, Mn). The aim of the present contribution was to combine the effect of Ga and phosphate and to report the performances of a series of microporous phosphate catalysts in catalytic reduction of NO and NO2 in the presence of isobutane or ethane under lean-burn conditions. The use of NOx instead of oxygen or air may have some benefic environmental applications. Both total oxidation and selective oxidation to isobutene or ethene are interesting in this reaction. For such purpose a family of microporous phosphates: SAPO-5, SAPO-11, MgA1PO-5, GaPO-34, and GaPSO-11 were considered as catalysts. Catalytic oxidation of benzene to phenol on zeolites is already a good example of selective oxidation using N20 [12]. 2. EXPERIMENTAL SAPO-5, SAPO-11, MgAIPO-5, and GaPO-34 and GaPSO-11 were prepared by hydrothermal synthesis under autogeneous pressure. These syntheses were carried out using the following gel compositions: for SAPO-5:0.4 SiO2. Al203. P205. Pr3N.50 H20; for SAPO11:0.4 SiO2. Al203. P205. Pr2NH.50 H20; for MgAIPO-5:0.4 MgO. 0.8 Al203. P205. 1.2 Pr3N.50 H20; for GaPO's: Ga203. x P2Os.y HF.z Py(Pr3N).70 H20 with x= 1; y= 1 till 5, z= 1.7 till 3.4 for Py and 1.5 till 3 for Pr3N; and for GaPSO-11:0.4 SiO2. Ga203. P205. 1.5 Pr2NH. 0.5 HF. 50 H20. Fumed silica (Merck), pseudo-boehmite (VISTA Chemical Company), phosphoric acid (85% H3PO4, Aldrich), Ga203 (Alfa AESAR, 99%+), Aluminum isopropoxide (Aldrich), Magnesium nitrate (Merck), hydrofluoric acid (40%, Aldrich), pyridine (Py), dipropyl amine (Pr2NH), and tripropyl amine (Pr3N) were used as reactants. Gel compositions with indicated formulas were obtained under vigorous stirring by successive dissolution of the precursor oxides in a phosphoric acid/water solution. Depending on the composition, hydrofluoric acid and template were further added. Each dissolving step was controlled by measuring the pH value. After 24h stirring the gel was placed without stirring in a teflon lined autoclave, heated at 445 K for 24 h, and then cooled instantaneously to room temperature. After drying at 383 K, all the catalysts were calcined at 623 K for 5h. The resulted materials were analyzed using several techniques: XRD, N2 adsorptiondesorption isotherms at 77 K, TG-DTA, FTIR, NH3-DRIFT, XPS, and MAS NMR. Crystallinity of the prepared samples was evidenced by X-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer with Cu Ka radiation source and scintillation counter. N2 adsorption-desorption isotherms were recorded at 77 K, in the relative pressure range 10-6 - 0.99, with a Micromeritics ASAP 2000 equipment, after the samples were degassed at 323 K for 12 h, at 10-4 Pa. TG-DTA-DSC curves were collected using a SETARAM 92 16.18

825 equipment. In situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were collected in a Bruker IFS88 infrared spectrometer with KBr optics and a DTGS detector. Pure samples were placed inside a commercial controlled environmental chamber (Spectra-Tech 0030-103) attached to a diffuse reflectance accessory (Spectra-Tech collector). To investigate the stability of the adsorbed ammonia species during temperature elevation, the spectra were recorded under Helium (30ml/min) at room temperature, 373, 523 and, 573 K, after exposed to ammonia flow for 30 min at room temperature. Infrared Fourier Transform spectra were recorded with the same apparatus. Spectra in the lattice vibrations range were recorded for wafers of sample mixed with KBr in a Pyrex IR cell, with KBr windows. The XPS spectra were obtained by a SSI X probe FISONS spectrometer (SSX -100/ 206) with monochromated A1-Kct radiation. The spectrometer energy scale was calibrated using the Au 4f7/2 peak (binding energy 84.0 eV). For the calculation of the binding energies, the Cls peak of the C-(C,H) component at 284.8 eV was used as an internal standard. The composite peaks were decomposed by a fitting routine included in the ESCA 8,3 D software. The superficial composition of the investigated samples was determined using the same software. The Si2p, A12p,P2p, Ga2p3, Fls, Nls and O~s peaks were investigated. 3~p and ~3C MAS NMR spectra were collected at room temperature using a Bruker ASX-300 spectrometer. The chemical shifts were referenced to AI(NO3)3 solution in water, Adamantane for 31p and 13C, respectively. Spinning rate was 10 KHz and 31p MAS NMR spectra were recorded with both ~H decupling and CP MAS method. Activity measurements were performed in a continuous flow fixed bed reactor operating at atmospheric pressure. 0.13 g sample was used in each experiment. The total flow rate was 100ml/min and feed composition was: nitric oxide 0. l vol%; isobutane or ethane 0.10.5vo1%; 5-6vo1% oxygen, in helium and nitric dioxide 0.1vol%; isobutane or ethane 0.22vo1%; 5-6vo1% oxygen, in helium, respectively. The inlet and outlet gas compositions were measured using a quadrupole mass spectrometer QMC 311 Balzers coupled to the reactor. 3. RESULTS 3.1. Textural properties Table 1 compiles the textural data of the investigated catalysts. Silicon containing molecular sieves exhibit rather high surface areas, while for GaPO the surfaces were very small. SAPO's and MgAIPO also exhibit a certain external surface area. Although the surfaces of GaPO's are small, the modification of the fluorine content or template in the gel composition determined changes in these values. 3.2. XRD XRD patterns of the investigated molecular sieves confirmed the purity of the investigated structures. Typical patterns were recorded. 3.3. NrI3-DRIFT Figs. 1 shows the NH3-DRIFT spectra collected for SAPO-5 and SAPO-11 samples. These spectra contain bands due to ammonia adsorbed on both Bronsted (1470 cm"1) and Lewis (1610 cm1) acid sites. The bands at 3422, 3323 and 2994 crn-1 resulted after adsorption of NH3 at room temperature are due to Vas(N-H), vsfN-H), 28as(H-N-H) species adsorbed on Lewis acid sites of the surface [13]. The admission of ammonia and the increase of the temperature lead to a decrease of the intensity of the band located at 1642 crn-1 which

826 Table 1. Textural characteristics of the investigated molecular sieves Catalyst F/Ga ratio Surface area Pore volume m2 g-I cm3 g-1 SAPO-5 SAPO-11 MgA1PO-5 1-GaPO(Py) 0.75 2-GaPO(Py) 1.0 3-GaPO(Py) 2.5 4-GaPO(Pr3N) 0.75 GaPSO- 11 *- GaPO (1-3) were prepared using Py

t-plot surface area

m: g-1

323 0.13 281 0.12 67 0.09 4 6 11 3 148 0.10 as template, while 4-GaPO using PrsN

63 59 13

25

corresponds to water adsorbed on Lewis acid sites, and to the formation of a new band at 1610 cm1 due to ammonia adsorbed on the same sites. However, spectra collected at 573 K indicated that at this temperature Lewis acid sites are still present. More evident are the changes of the bands associated to Bronsted acid sites. A decrease of the intensity also occurs for these bands. However, as in the case of Lewis acidity, the Bronsted sites are also present at573 K. On MgA1PO-5 and GaPO samples no adsorption of ammonia under investigated conditions has been evidenced. 3.4. XPS XPS analysis of the investigated samples before and atter the catalytic tests indicated the binding energies compiled in Table 2. SAPO's samples exhibited spectra which contained well defined species in well defined coordinations. Spectra collected after the catalytic tests indicated no changes comparatively to fresh catalysts. Ga-containing microporous sieves showed a different behavior. In the fresh catalysts, Ga existed in two different coordinations, as resulted from the existence of two bands,

~.~

3500

2500

1500 Wavenumber, crn

500

3500

573 4 7

3

2500

~

~

1500

Wavenumber, cm1

Figure 1. NH3-DRIFT spectra collected for SAPO-11 (a) and SAPO-5 (b) samples

500

827 Table 2. Binding energies Of Si2p, A12p, P2p, Ga2p3,Fls, and Nls in the investigated catalysts Catalyst Binding energy, eV Si2p A12p P2p Ga2p3 Fls Nls 134.7 399, 401.3 102.8 75.0 SAPO-5 399, 401.3 SAPO-11 102.8 75.0 134.7 1118.9 688.5 395.3, 397.3, 398.7, 1-GaVO0'y) 134.4 1116.7 685.3 399.9, 401.3 1118.9 688.5 399.0 1-GaPO(Py) 134.4 685.3 tested 1118.7 688.1 399.2, 401.8 2-CraPO(Py) 134.5 11i6.5 685.0 1118.8 688.0 399.3 2-GaPO(Py) 134.5 684.9 tested 1118.8 688.5 398.7, 401.7 3-GaPO(Py) 134.1 1116.6 685.3 1118.8 688.2 399.0 3-GaVO(Py) 134.4 684.7 tested 1118.8 688.5 399.9, 401.3 4-GaPO(Pr3N) 134.5 1116.6 685.3 1118.9 688.5 399.0 4-GaPO(Pr3N) 134.5 685.3 tested GaPSO- 11 102.8 134.7 1118.8 688.5 399.9, 401.3 685.3

one located at around 1116.5 eV and another at 1118.8 eV. After the catalytic tests, only the species located at around 1118.8 eV was evidenced. Fluorine existed in all the calcined and tested catalysts, and the two bands are associated to fluorine bonded to gallium and nitrogen species. Nitrogen was found in all the investigated catalysts, but after the catalytic tests only one species was found.

3.5. Catalytic activity On the investigated catalysts NO was converted mainly to nitrogen. Only very small amounts of the compounds with M/z=44 (namely N20 and CO2) and ammonia (M/z= 17) were detected under these conditions. The total 'conversion of NO depended on both the microporous sieve nature and for a given structure on the conditions it was synthesized (Figs. 2 and 3). In the investigated series the best results were obtained using gallium phosphate catalysts. It is worth to notice that these materials exhibit very small surface areas (around 10 m 2 g-l) having a chain structure. Under these conditions conversions of about 40% are noticeable. If one express the activity in terms of converted NO moles/m 2 g'leat min, data obtained for SAPO-5 corresponds to 5.1x10 "s converted NO moles/m 2 g-lear min, while for GaPO (sample VP1.3) to 1.31x10 -6. This means that GaPO's exhibited almost two orders of magnitude higher intrinsic activity than SAPO's.

828

(30 -- .......................................................................................................................................................................................

F

50

50 45 ~40 A

0

-~30

x WI.0

X X

M~0.7

20

W0.5 M~0.4

10 5 0

10 0 SAIK)-5

SAI~ll

~

(htxJO,11 ~ 4 t ~ 5

c~ Figure 2. NO conversion on various catalysts (623 K; 0.13 g cat; flow rate 100ml/min; 0. lvol% NO; 0.4vo1% isobutane; 5vo1% 02).

0,5

0,7

0,9

1,1

1,3

1,5

1,7

1,9

%~liaixm~

Figure 3. Influence of the isobutane content for GaPO obtained for various Ga/F ratios (623 K; 0.13 g cat; flow rate 100ml/min; 0. l vol% NO; 5vo1% 02).

The NO reduction took place only in the presence of oxygen and no NO2 was detected under these conditions (Figs. 4). The presence of oxygen improves the NO conversion (Fig.4c). The main reaction product was isobutene (M/z=56) or ethene. A second species with M/z=48 was also resulted in the experiments using isobutane. Increasing the isobutane content yielded an increased conversion to isobutene. The higher conversions were obtained at 623 K. Fig. 4 show the time dependence evolution of the M/z curves corresponding to the molecules produced during NO reduction with isobutane. These dependencies indicated a very good stability of the catalysts in the investigated conditions. The use of NO2 instead of the NO led to similar results. The conversions were slightly smaller but the reaction products were similar. These data may led us to speculate a reaction mechanism in which NO is firstly oxidized to NO2 and then reduced by the hydrocarbon. No carbonaceous deposits are formed in this process because these catalysts contain very small or no acidity. 4. DISCUSSIONS The properties of the investigated phosphate molecular sieves were very different according to the chemical composition. Silicon containing phosphates exhibited large surface and t-plot surface areas, while gallium phosphates very low ones. Magnesium aluminum phosphate represented an intermediate case. Actually the textural characteristics were in direct relation with the stability of these materials. Important differences between these materials also resulted from the acid properties. SAPO's samples exhibited a quite strong acidity as resulted from the presence of the bands associated to both Bronsted and Lewis acid sites. These bands were detected even in NH3DR/FT spectra collected at 573 K.

829 (A)

-d

/

Z~

NO

(B) isobutene ! NO+ iB (0.5%) ~o

C02 + N20

NO+O2+ iB (0.5%)

J 25

50

75

100

25

125

50

75 Time, min

Time, rain

(c)

NO+Et (0.5%)+0 z (5%)

NO+O2(5%) NO+Oz(6% ) +iB (0.2%) _~n m 9o/,a _

NO

NO

C02+N20

02

130

180

(D) NO-I-O2(6%

k.....__

Z~

125

NO+iB(0.2%)

NO+Et(0.5%~ d

100

230 Time, min

280

40

90

140

190

240

290

Time, min

Figure 4. Time dependence of the M/z curves. (A) NO and CO2+N20 curves for NO reduction with isobutane on SAPO-5; (B) isobutene and M/z curves for NO reduction with isobutane (iB) on SAPO-5; (C) NO, N2 and 02 curves for NO reduction with ethane on SAPO-5; (D) NO and CO2+N20 curves for NO reduction with isobutane on (3) GaPO(Py). 0.13g catalyst, total flow rate 100ml/min, 623 K. NO conversion on these sieves depended on their chemical composition. In terms of productivity, the best results were obtained using SAPO-5. NO conversion on this catalyst was near 50%. But in terms of real intrinsic activity GAPO's were more effective. NO2 reduction with isobutane gave conversions which were very close on those obtained using NO, and almost the same selectivity. As Fig. 4 indicated the amount of released N20 and CO2 was very small for both isobutane and ethane. After the catalytic test the catalysts did not changed the color, and for the investigated time the conversion was constant. XPS analysis indicated no changes for SAPO's catalysts after these were exposed to the catalytic conditions, but changes occurred in the structure of gallium phosphates. The exposing to the

830 oxidant conditions caused the change of gallium coordination. However these changes occurred very rapidly without any modification in the catalytic performances. The comparative data obtained using NO and NO2 may suggest the reaction occurred following the same reaction pathway in which the formation of NOx is a key step. Although SAPO's samples exhibit an evident acidity under reaction conditions, we suppose that on both sieves the free-radical mechanism proposed by Thomas et al. [ 10, 11 ] is more probable. The preservation of the color after the catalytic tests may suggest that no coke was formed under these conditions. The rather high selectivity to isobutene and ethene might be an additional argument in this sense. 5. CONCLUSIONS In conclusion, phosphate catalysts are active systems for the reduction of NOx with very stable alkanes such ethane or isobutane. In terms of real activity these are among the most active catalysts. Unfortunately, the low surface areas of these materials corresponded to a small productivity. In terms of selectivity, the use of these hydrocarbons led mainly to oxydehydrogenation to alkenes, which is quite remarkably mainly for ethane. These reactions accomplish in a very effective way the DENOx process. REFERENCES

1. V.I. P~,rvulescu, P. Grange, B. Delmon, Catal. Today, 46 (1998) 233. 2. Y. Li, J. N. Armor, J. Catal., 145 (1994) 1. 3. A. Satsuma, K. Yamada, T. Mori, M. Niwa, T. Hattori, Y. Murakami, Catal. Lett., 31 (1995) 367. 4. E. Kikuchi, M. Ogura, I. Terasaki, Y. Goto, J. Catal., 161 (1996)465. 5. K. Yogo, S. Tanaka, M. Ihara, T. Hishiki, E. Kikuchi, Chem. Lett., (1992) 1025. 6. P.A. MacFaul, D. D. M. Wayner, K. U. Ingold, Ace. Chem. Res., 31 (1998) 159. 7. J.M. Thomas, Nature, 314 (1988) 669. 8. J.M. Thomas, R. Raja, G. Sankar. R. G. Bell, Nature, 398 (1999) 398. 9. R. Raja, G. Sankar, J. M. Thomas, J. Am. Chem. Soc., 121 (1999) 11926. 10. M. Dugal, G. Sankar, R. Raja and J. M. Thomas, Angew. Chem. Int. Ed., 39 (2000) 2310. 11. R. Raja, G. Sankar, J. M. Thomas, Angew. Chem. Int. Ed., 39 (2000) 2313. 12. G. I. Panov, A. S. Kharitonov, V. I. Sobolev, Appl. Catal., 98 (1993) 1. 13. M. A. Centeno, I. Carrizosa, J. A. Odriozola, Appl. Catal. B: Environmental, 29 (2001) 307.

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordano and F. Testa(Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

831

The i n f l u e n c e of t e x t u r a l p r o p e r t i e s of MFI t y p e c a t a l y s t s on d e a c t i v a t i o n p h e n o m e n a during o l i g o m e r i z a t i o n of b u t e n e s G. Giordano a, F. Cavani b a n d F. Trifir6 b aDipartimento di Ingegneria C h i m i c a e dei Materiali, Universit~ della Calabria, Via Bucci, 1-87030 Rende (CS), Italy bDipartimento di C h i m i c a I n d u s t r i a l e e dei Materiali, Viale Risorgimento 4, 4 0 1 3 6 Bologna, Italy T h e effect of s e c o n d a r y p o r o s i t y , f o r m e d b y t h e p o s t - s y n t h e s i s t r e a t m e n t of p e l l e t i z a t i o n a t h i g h p r e s s u r e of zeolite p o w d e r s on t h e d e a c t i v a t i o n p h e n o m e n a in 1 - b u t e n e oligomerization in t h e p r e s e n c e of m o l e c u l a r oxygen h a s b e e n studied. It w a s f o u n d t h a t in the p r e s e n c e of m o l e c u l a r oxygen t h e r a t e of 1-butene oligomerization quickly declined as a c o n s e q u e n c e of h e a v y c o m p o u n d s a c c u m u l a t i o n in t h e m e s o p o r o u s sites formed by a g g l o m e r a t i o n of crystals. In the a b s e n c e of m o l e c u l a r oxygen the rate of coke formation a n d catalyst deactivation w a s m u c h lower.

1. INTRODUCTION With the wide i n d u s t r i a l application of zeolitic materials, the s t u d y a n d c o m p r e h e n s i o n of the deactivation p h e n o m e n a have b e c o m e quite i m p o r t a n t [1]. M a n y s t u d i e s correlate the coke formation in zeolites, the m a i n factor for t h e d e a c t i v a t i o n p h e n o m e n a of zeolites, to t h e c r y s t a l size of zeolite crystallites, or to t h e Si/A1 r a t i o s in t h e zeolitic f r a m e w o r k s [2-4]. Less a t t e n t i o n h a s b e e n p a i d to the role p l a y e d by t h e m o r p h o l o g i c a l porosity o b t a i n e d t h r o u g h t h e pelletization or e x t r u s i o n of zeolitic p o w d e r s t h a t affects t h e c a t a l y t i c p e r f o r m a n c e a n d t h e d e a c t i v a t i o n r a t e [5]. O t h e r p a r a m e t e r s t h a t affect the catalytic b e h a v i o u r of zeolites are the diffusion p h e n o m e n a t h a t are intensively s t u d i e d by different a u t h o r s [5-81 a n d others t h a t have also t a k e n into a c c o u n t the m e s o p o r o u s volume [9]. In o u r p r e v i o u s p a p e r s [10, 1 1], we f o u n d t h a t in the r e a c t i o n of 1b u t e n e oligomerization over H-MFI zeolites the p r e s e n c e of m o l e c u l a r oxygen r e m a r k a b l y affected t h e catalytic behavior, modifying t h e activity a n d the p r o d u c t s d i s t r i b u t i o n . However, t h e m a i n effect w a s on t h e r a t e of deactivation, w h i c h w a s significantly i n c r e a s e d on i n c r e a s i n g the oxygen c o n t e n t in t h e feed. Different H - M F I - b a s e d c a t a l y s t s w e r e t e s t e d a n d

832 characterized by differences in the textural properties, i.e. the crystal size and the Si/A1 ratio, and we tried to correlate the observed p h e n o m e n a with these tex tu r al properties. However, it was not possible to find any clear correlation. In the p r es e nt work we carry out an examination of the effects induced by pelletization of zeolitic powders into particles on the material porosity, a n d on the rate of catalyst deactivation duri ng the reaction of 1-butene oligomerization. The observed effects have b e e n correlated to the coke formation, in the presence and in the absence of molecular oxygen. 2. E X P E R I M E N T A L

The MFI type zeolite was synthesized by s t a n d a r d m e t h o d s (sodiumalumina-silica hydrogel in the presence of t e t r a p r o p y l a m m o n i u m ions) [12]. MFI zeolite was calcined-exchanged-calcined in order to obtain the H form. The n a t u r e and the crystallinity of the samples was checked by X-ray powder diffraction tech n i que . S c a n n i n g electron microscope (SEM) w as u s e d to detect the m o r p h o l o g y and crystal size. Atomic a d s o r p t i o n spectrometry, EDX and CHN analyzer t e c h n i q u e s were performed to obtain the chemical composition of the s a m pl es and the carbon content of the s p e n t catalysts. Catalytic tests of 1-butene oligomerization were performed at a t m o s p h e r i c p r e s s u r e in a l a b o r a t o r y plug-flow r e a c t o r on 1 ml (0.5 g) of c a t a l y s t (residence time 0.5 s, 1-butene in feed 3.2 %, 0 2 ranging from 0 to 20 %, r e m a i n d e r He). Before the tests, the catalysts were s u b m i t t e d to a p r e s s u r e t r e a t m e n t (about 4 t o n / c m 2) in order to granulate and sieve the s a m p l e s in 30-60 m e s h sized particles. A d s o r p t i o n / d e s o r p t i o n i s o t h e r m s for nitrogen at 77 ~ were recorded u s i n g a Sorptomatic (Carlo Erba) a p p a r a t u s . For the d e t e r m i n a t i o n of the m i c r o p o r o u s volume, the t-plot m e t h o d w as used. M e r c u r y i n t r u s i o n porosimetry technique was u s e d for pore m e a s u r e m e n t s (Porosimeter 4000 Series an d Macropore Unit 120, Carlo Erba) in order to determine the pore volume d i s t r i b u t i o n in the r a d i u s range from 20 to l x l 0 6 / k . In order to extend the observation at other zeolitic materials, three kind of zeolite were examined with these technique: MFI type (see below), FAU (13 X) type and LTA (4 A) type zeolite from U.O.P. Molecular Sieves (average crystal size 1 ~m). Three kind of s a m p l e s were analyzed with this technique: s a m p l e s w i t h o u t a n y p r e s s u r e t r e a t m e n t , s a m p l e s after 1 min t r e a t m e n t at 3.8 t o n / c m 2 a n d s a m p l e s after 1 m i n of t r e a t m e n t at 7.6 t o n / c m 2. H-MFI samples were also analyzed after the catalytic tests. 3. R E S U L T S A N D D I S C U S S I O N

Table 1 reports the m a i n characteristics of the H-MFI u s e d in this work. The samples show a narrow Si/A1 ratio range (from 12 to 20), however

833 Table 1. C h a r a c t e r i s t i c s of t h e H-MFI u s e d as c a t a l y s t s . Sample

Si/A1

Morphology

1

12.2

Spheres

2

20.3

Twinned rectangular

3

12.8

Twinned rectangular

a v a r i a t i o n in t h e c r y s t a l size, t h e l e n g t h v a r y i n g from 1 to 6 ~m, a n d in t h e m o r p h o l o g y c a n be observed. T a b l e 2 s h o w s t h e c o n v e r s i o n of 1 - b u t e n e a t 5 5 3 ~ e v a l u a t e d a t different t i m e - o n - s t r e a m (TOS) a n d in a n a e r o b i c a n d a e r o b i c c o n d i t i o n s (0, 0.1 a n d 20 % of 02) for t h e H-MFI s a m p l e s . The c o n v e r s i o n of 1 - b u t e n e after a T O S of 5 m i n is a b o u t t h e s a m e for all c a t a l y s t s ; in p r e s e n c e of large a m o u n t s of o x y g e n (20 %) a s m a l l i n c r e a s e in t h e r a t e v a l u e s is observed. It w a s f o u n d [5, 10] t h a t u n d e r t h e s e c o n d i t i o n s the i n c r e a s e in c o n v e r s i o n w a s d u e to t h e f o r m a t i o n of a r o m a t i c s b e s i d e s t h e a l i p h a t i c p r o d u c t s . More d i f f e r e n c e s are o b s e r v e d w h e n t h e above r e s u l t s a r e c o m p a r e d w i t h t h e c o n v e r s i o n a f t e r 50 m i n TOS. In fact, t h e s a m p l e 3 s h o w s t h e h i g h e s t d e c r e a s e in c o n v e r s i o n , i.e. t h e h i g h e s t d e a c t i v a t i o n r a t e . For all s a m p l e s , t h e r a t e of d e a c t i v a t i o n w a s slower w h e n t h e oxygen c o n t e n t w a s Table 2. % C o n v e r s i o n a t 5 5 3 ~ as a f u n c t i o n of 0 2 c o n c e n t r a t i o n in t h e feed for different TOS, a n d a m o u n t of c a r b o n d e t e c t e d on s p e n t c a t a l y s t s for different H-MFI s a m p l e s . Sample

TOS=5 m i n

c % (wt)

TOS=50 min

A

B

C

A

B

C

62

62

62

61

60

52

3.19

64

64

69

64

63

64

5.83

63 64 66 62 60 45 6.83 A= c o n v e r s i o n % a t 0 2 = 0%. B= c o n v e r s i o n % a t 0 2 = 0. I%. C= c o n v e r s i o n % a t 0 2 = 20%.

834 lowered to the 0.1%, a n d w a s practically negligible (at least u n d e r the evaluated TOS) in the absence of molecular oxygen. The l i t e r a t u r e r e p o r t s different e x p l a n a t i o n s for the t e x t u r a l and morphological features t h a t affect the rate of catalyst deactivation. Camblor et al. [2] correlate the deactivation of zeolite Y, in the case of gasoil cracking, to the crystal size; the lower rate of deactivation being observed with the crystals of smaller sizes. Other p a r a m e t e r s affecting the deactivation rate are the zeolitic p o r o u s s y s t e m [8] a nd the acidity [4]; the coke formation is a shape-selective process which is r e m a r k a b l y affected by the architecture of the zeolite [3]. In o u r case, H-MFI s a m p l e s p o s s e s s similar crystallinity, a n d the silica-to-alumina ratio is the same for samples 1 and 3; these s a m p l e s also possess similar crystal size; sample 2 ha s a lower a l u m i n a content as well as the largest crystal size. Therefore, it is very difficult to find a correlation b e t w e e n t h e s e t e x t u r a l a n d m o r p h o l o g i c a l f e a t u r e s a n d the observed d e a c t i v a t i o n rate. It s e e m s t h a t ot her f e a t u r e s m a y affect the rate of deactivation. Last c o l u m n in Table 2 reports the a m o u n t of carbon detected in the spent catalysts after the reaction of 1-butene oligomerization in the presence of 20% oxygen. C a r b o n c o n t e n t is different for the various samples; it is interesting to observe t h a t despite the large a m o u n t of coke found, enough to block the c h a n n e l s of the zeolite, the catalytic activity r e m a i n s high (see Table 2). Table 3 reports the m i c r opor ous volume, determined by treating the a d s o r p t i o n / d e s o p r t i o n i s o t h e r m s of n i t r o g e n with the de Boer's t-plot method, an d shows t h a t there are not large differences between the values obtained for the fresh zeolite powder and for the s p e n t particles (Table 3). The small differences detected can be accounted to the pressure t r e a t m e n t to w h i c h the powder is s u b m i t t e d in order to prepare the particles in size suitable for the fixed bed application. Anyway, the u n c h a n g e d micropore volume value observed in the spent catalysts suggests t h a t the carbon found in the samples is not accomodated in the zeolitic channels. Table 3. Microporous volume, evaluated by t-plot method, for the different a s - m a d e and spent zeolitic catalyst. Sample Pore volume (ml/g) Pore volume (ml/g) as-made spent catalyst 1

2 3 3* * = after a t r e a t m e n t of I min at

0.12 0.13 0.17 0.16 3.8 t o n / c m 2.

0.11 0.12 0.15

835 Figure 1 s h o w s the pore d i s t r i b u t i o n of t h r e e different zeolites: H-MFI (sample 1) , FAU (13X) a n d LTA (4A). D a t a were r e c o r d e d on the s a m p l e s w i t h o u t p r e s s u r e t r e a t m e n t , after a t r e a t m e n t of 1 m i n at 3.8 t o n / c m 2 a n d after 1 rain t r e a t m e n t at 7.6 t o n / c m 2- W h e n t h e p r e s s u r e of the t r e a t m e n t i n c r e a s e s , the pore d i s t r i b u t i o n s h o w s a shift t o w a r d s lower v a l u e s of t h e r a d i u s . In addition, it c a n be o b s e r v e d t h a t the a m o u n t of m e s o p o r e s i n c r e a s e s d r a s t i c a l l y w h e n t h e p r e s s u r e of t h e t r e a t m e n t increases; this p h e n o m e n o n is more e m p h a s i z e d for all s a m p l e s in the range from 0 to 3.8 t o n / c m 2. This behavior, t h a t is c o m m o n for different zeolites p o s s e s s i n g various s t r u c t u r e s a n d c h a n n e l s y s t e m s , s u g g e s t s t h a t the pore d i s t r i b u t i o n of this kind of m a t e r i a l is strongly affected by the p r e s s u r e at w h i c h the m a t e r i a l is pelletized. It is possible to h y p o t h e s i z e t h a t the m e s o p o r o u s s t r u c t u r e formed by pelletization of the p o w d e r is able to a c c o m m o d a t e a large a m o u n t of coke, as d e t e r m i n e d in o u r s p e n t s a m p l e s . T h i s is in a g r e e m e n t w i t h t h e i n d i c a t i o n s given b y different a u t h o r s who s u g g e s t e d t h e p r e f e r e n t i a l formation of "external coke" [8, 13, 14]. Table 4 r e p o r t s t h e t o t a l c u m u l a t i v e v o l u m e , e v a l u a t e d b y m e r c u r y i n t r u s i o n , of s a m p l e s 1 a n d 3, as well as of the 13X a n d 4A zeolites. The total c u m u l a t i v e v o l u m e w a s d e t e r m i n e d for the a s - m a d e p r o d u c t s a n d after the p r e s s u r e t r e a t m e n t s ; for the H-MFI s a m p l e s the v a l u e s for the s p e n t c a t a l y s t s are also given. For all s a m p l e s e x a m i n e d , the t o t a l c u m u l a t i v e volume d e c r e a s e d w h e n the p r e s s u r e of t r e a t m e n t w a s i n c r e a s e d , especially w h e n the p r e s s u r e w a s c h a n g e d from 0 to 3.8 t o n / c m 2. In the s p e n t c a t a l y s t , w h i c h h a d b e e n s u b j e c t e d to t h e pelletization t r e a t m e n t before catalytic tests, t h e r e w a s no v a r i a t i o n in the total mesom a c r o p o r o u s volume. The d e c r e a s e of the total c u m u l a t i v e v o l u m e o c c u r s w i t h a r e d u c t i o n of t h e p o r e s w i t h r a d i u s g r e a t e r t h a n 1 0 0 0 A, b u t c o n s e q u e n t l y we observe a n increase of the pores in the region of mesopores. Probably, the f o r m a t i o n of pores in this r a d i u s r a n g e favours the deposition of the coke w i t h o u t occlusion of the zeolitic micropores. Table 4. Total c u m u l a t i v e v o l u m e as a f u n c t i o n of p r e s s u r e t r e a t m e n t for H-MFI ( s a m p l e s 1 a n d 3), FAU (13 X) a n d LTA (4 A) type zeolites. For H-MFI s .amples the v o l u m e w a s detected also for s p e n t catalysts. Sample Tot c u m vol Tot c u m vol Tot cure vol Tot c u m vol (ml / g) (ml / g) (ml / g) (ml / g) as-made 3.8 t o n / c m 2 7.6 t o n / c m 2 s p e n t catalyst 1 3 13 X 4A

1.17 1.42 1.35 1.04

0.47 0.63 0.27 0.27

0.33 0.16 0.14

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Figure 1. Pore distribution of three different zeolites: H-MFI (sample 1), F A U (13X) and L T A (4A). a = samples without pressure treatment, b = samples after a treatment of 1 min at 3.8 ton/cm 2 , c = samples after a treatment of 1 min at 7.6 ton/em 2

837 4. C O N C L U S I O N S

T h e r e s u l t s r e p o r t e d (see T a b l e 2) s h o w t h a t in t h e p r e s e n c e of m o l e c u l a r oxygen, even for 0.1 mol. %, a deactivation of the c a t a l y s t occurs, with a d e c r e a s e in the r a t e of 1-butene oligomerization t h a t is q u i c k e r w h e n the oxygen c o n t e n t is i n c r e a s e d . The deactivation o c c u r s t o g e t h e r w i t h the s i m u l t a n e o u s formation of coke. The r e s u l t s also indicate t h a t the deactivation p h e n o m e n o n is affected by the p r e s e n c e of a m e s o p o r o u s s t r u c t u r e . In fact, the m i c r o p o r o u s volume of t h e c a t a l y s t does n o t s h o w s u b s t a n t i a l v a r i a t i o n s after r e a c t i o n with r e s p e c t to the c a t a l y s t before reaction, a n d this i n d i c a t e s t h a t the c a r b o n c o n t e n t is n o t o c c l u d e d in the zeolitic c h a n n e l s . On the o t h e r h a n d , the activity s h o w n by the H-MFI catalysts, also in the p r e s e n c e of a large a m o u n t of coke, indicates t h a t the c a r b o n does not occlude the zeolitic acid centers. The shift of pore average d i a m e t e r s t o w a r d s lower v a l u e s w h e n the p r e s s u r e of pelletization is i n c r e a s e d , with a c o n s e q u e n t r e d u c t i o n of the total c u m u l a t i v e v o l u m e , i n d i c a t e s t h a t with this k i n d of t r e a t m e n t it is possible to i n c r e a s e the a m o u n t of m e s o p o r e s in the material. Finally, it is possible to h y p o t h e s i z e t h a t the H-MFI type zeolites are able to a c c o m m o d a t e the coke formed d u r i n g the reaction in the mesopores, a n d so therefore the p r e t r e a t m e n t of pelletization, w h i c h c a u s e s a n i n c r e a s e in the m e s o p o r o u s volume, affects the c a t a l y s t deactivation. Probably, the pelletization p r e s s u r e , d u e to the effect on the pore r a d i u s variation, is an i m p o r t a n t p a r a m e t e r t h a t m u s t be studied for a b e t t e r c o m p r e h e n s i o n of the zeolitic c a t a l y s t s deactivation processes. The fact t h a t coke f o r m a t i o n o c c u r s in m e s o p o r e s a n d in p r e s e n c e of m o l e c u l a r oxygen allows u s to h y p o t h e s i z e t h a t on the walls of mesopores, w h i c h are a c t u a l l y t h e e x t e r n a l p a r t of zeolite c r y s t a l l i t e s , are p r e s e n t defects. T h e s e d e f e c t s are able to a c t i v a t e m o l e c u l a r o x y g e n f o r m i n g s u p e r o x i d e ion 0 2- species, as i n d i c a t e d also b y C h e n a n d F r i p i a t [15]. T h e s e species m a y be t h o s e r e s p o n s i b l e for the a r o m a t i c s f o r m a t i o n w h i c h o c c u r s at lower t e m p e r a t u r e t h a n in the a b s e n c e of gas p h a s e oxygen [10, 11]. It c a n also be h y p o t h e s i z e d t h a t the p r e s e n c e of t r a c e s of oxygen in the h y d r o c a r b o n f e e d s t o c k of r e a c t i o n s typically catalyzed b y zeolites c a n be responsible of acceleration of the deactivation p h e n o m e n a . REFERENCES

1. 2. 3. 4.

S.T. Sie: A d v a n c e d Zeolite Science a n d Applications, (J.C. J a n s e n , M. Stocker, H.G. Karge, J. W e i t k a m p , Eds.), Elsevier Science, A m s t e r d a m , 1994, p. 587. M.A. C a m b l o r , A. C o r m a , A. M a r t i n e z , F.A. Mocholi a n d J. Perez Pariente, Appl. Catal., 55 (1989) 65. H.G. Karge: Introduction to Zeolite Science and Practice, (H. v a n B e k k u m , E.M. F l a n i g e n J.C. J a n s e n , Eds.), Elsevier Science, A m s t e r d a m , 1991, p. 531, a n d references therein. M. G u i s n e t a n d P. Magnoux, Appl. Catal., 54 (1989) 1.

838

.

,

,

,

,

10. 11. 12. 13. 14. 15.

L. Forni, M. Pelozzi, A. Giusti, G. Fornasari and R. Millini, J. Catal., 122 (1990) 44. R.M. Barrer,: Properties and Applications of Zeolites, (R.P. Townsend Ed.), Special Publ n~ The Chemical Society, London 1980, p. 3. D.M. Ruthven: Principles of Adsorption and Adsorption Processes, J o h n Wiley and Sons, New York, 1984. J.G. Post and J.G.H. Van Hooff, New Developments in Zeolite Science and Technology, 7th Int. Zeolite Conf., Tokyo, J a p a n , 1986, Preprints of Poster Papers 1D-20, p. 249. D. McQueen, F. Fajula, R. Dutartre, L.V.C. Rees and P. Schulz, Stud. Surf. Sci. Catal, 84 (1994) 1339. F. Cavani, F. Trifir0, G. Giordano and K.J. Waghmare, Appl. Catal. A, 94 (1993) 131. F. Cavani, G. Giordano, M. Pedatella and F. TrifirS, Stud. Surf. Sci. Catal, 84 (1994) 1425. P.A: J a c o b s and J.A. Martens, Synthesis of High-silica Aluminosilicate Zeolites, Elsevier Science, Amsterdam, 1987. D.M. Bibby, N.B. Milestone, J.E. Patterson and L.P. Aldridge, J. Catal., 97 (1986) 493. T. Behrsing, H. Jaeger and J.V. Sanders, Appl. Catal., 54 (1989) 289. F.R. Chen and J.J. Fripiat, J. Phys. Chem., 96 (1992) 819.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

839

Dehydrogenation of propane over various chromium-modified MFI-type zeolite catalysts V.A. Tsiatouras a, T.K. Katranas a, C.S. Triantafillidis a, A.G. Vlessidis a, E.G. Paulidou b and N.P. Evmiridis a* a Department of Chemistry, University of Ioannina, 451 10 Ioannina, Greece b Department of Physics, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece

The dehydrogenation of propane was studied over chromium-modified MFI-type zeolite catalysts. Three different chromium-modified ZSM-5 zeolites were synthesized following typical hydrothermal synthesis and impregnation procedures and were finally treated hydrothermally at relatively severe steaming conditions. The prepared catalyst samples were characterized by XRD, SEM-EDS, N2 adsorption/desorption, Ammonia-TPD and 27AI MAS NMR. The catalytic results showed that when chromium species are randomly dispersed within the zeolitic particles act synergistically with the remaining acidic framework A1 inducing improved dehydrogenation activity and propylene selectivity. 1.

INTRODUCTION

The increasing demand for the production of light olefins which are being used in several commercial processes, like the synthesis of high-octane gasoline additives, such as methyltertiarybutyl ether (MTBE), induced the necessity of improving the up-today processes of catalytic dehydrogenation or oxidative dehydrogenation of alkanes towards alkenes [ 1-3]. Increase of olefins yield in the gaseous products of a Fluid Catalytic Cracking (FCC) unit is an alternative way for light alkenes production, especially attractive to refiners. In addition, the dehydrogenation process constitutes the first and most significant step in the transformation of paraffins to aromatic hydrocarbons (Benzene Toluene and Xylene, BTX chemicals). In recent years, the use of aromatics as effective octane boosters lost importance on account of environmental restrictions. In the contrary, aromatics still are very important reagents for industrial reactions, e.g. polymer formation. The main route for the production of aromatics is the catalytic reforming of napthas. This process is incapable of converting light paraffins, like propane, to aromatics. This is the reason for the introduction of the M2 Forming process by Mobil [4], which converts light paraffins to aromatics over H-ZSM-5 catalyst. British Petroleum (BP) and Universal Oil Products (UOP) later developed the Cyclar process [5-6] which converts Liquid Petroleum Gas (LPG) to BTX chemicals. LPG is produced in petroleum refining processes, and mainly consists of C3 and C4 hydrocarbons. Since the development of these processes a large number of researchers reported many types of MFI-based catalysts for the conversion of light paraffins to aromatics [7-9]. Shape selective MFI-type zeolite catalysts (ZSM-5) are the most common catalyst-additives for the fluid catalytic cracking (FCC) of gas-oil on USY or RE-USY based catalytic systems. The conversion of light alkanes over H-ZSM-5 is relatively low.

840 However, modification of H-ZSM-5 with different metals, such as transition metals, can promote the dehydrogenation activity of ZSM-5 zeolite. The modification can be accomplished in many ways, e.g., direct synthesis, ion-exchange or impregnation. Platinum loaded ZSM-5 zeolites [10-11] are the most suitable catalysts for dehydrogenation of paraffins to olefins; however, these catalysts have the disadvantage of the very high hydrogenolysis activity. Gallium zeolites are less active than Pt/ZSM-5 but have a high aromatization activity with lower hydrogenolysis. The main drawback of gallium/ZSM-5 catalysts is the relatively fast deactivation which occurs due to coke formation. Gallium containing zeolites are the most referred catalysts for the aromatization of light paraffins [12-17]. Other metals which have been used for boosting dehydrogenation are zinc [ 18-19], vanadium [20], indium [21 ], and molybdenum [18]. The loading of chromium on zeolites by direct synthesis is very difficult [9]. The chromium species usually prefer octahedral coordination and it is not easy to substitute silicon or aluminum in tetrahedral framework positions. Several researchers attempted to introduce chromium into zeolites framework; however, no evidence was provided that chromium species were indeed incorporated in framework positions[22-24]. Although chromium is a very common oxidative and dehydrogenative metal which is used in commercial processes [ 1], there are limited references in the literature about the use of chromium loaded zeolite catalysts in dehydrogenation or aromatization of light paraffins. Sphiro et al [25] reported the use of intermetallic Pt-Cr clusters encapsulated in ZSM-5 in the aromatization of ethane and propane. The addition of chromium in Pt/ZSM-5 suppressed the hydrogenolysis leading to a higher selectivity to aromatics. Fu et. al. [18], reported the aromatization of propane over a chromium modified ZSM-5 prepared by solidstate reaction. This catalyst showed high selectivity to propylene. In the present work we synthesized three types of ZSM-5 zeolites with Chromium (two by direct synthesis and one by impregnation) and tested their catalytic activity in the dehydrogenation of propane. The zeolitic catalysts used were hydrothermally treated (steamed) at high temperatures in order to simulate their condition when they are being used as catalyst-additives in the Fluid Catalytic Cracking process. 2.

EXPERIMENTAL SECTION

The synthesis of the parent ZSM-5 zeolites of this work was based on a typical procedure [26,27]. A solution containing partially dissolved silica [Colloidal silica HS-40 (Du Pont), 39.6% SiO2,0.426 % Na20] in tetrapropylammonium (TPA) hydroxide reacted with a solution of sodium aluminate [(BDH), 54.2 % A1203, 38.5 % Na20] in water (Si/AI=40). The resulting gel was heated in an autoclave at 150~ for 8 days leading to crystallization of ZSM-5. The as-synthesized sample was calcined to combust the organic template and was further treated with dilute HC1 solution (pH=3) to produce the H-form of the zeolite (labeled H-[A1]ZSM-5). Silicalite samples (labeled H-ZSM-5) were synthesized following the same procedure in the absence of the sodium aluminate solution (aluminum source) from the reaction mixture. For the preparation of the chromium impregnated zeolite sample (labeled Cr203/ H-[A1]ZSM-5), the H-[A1]ZSM-5 sample was impregnated with Cr(NO3)3.9H20 aqueous solution using vacuum rotary evaporation at 90~ (-0.5 mmoles of Cr per g zeolite). The sample was then dried in air at 120 ~ for 12 h and calcined afterwards at 500~ for 4 h under the flow of dry air. The sample labeled H-[Cr/A1]ZSM-5

841 was synthesized hydrothermally by using both A13+ (sodium aluminate) and Cr 3+ (Cr(NO3)3.9H20) sources (mole ratios Cr/AI-1 and Si/M=40, where M=Cr,A1). Sample H[Cr]ZSM-5 was synthesized in a similar way with the exclusion of aluminum in the synthesis mixture and the use only of chromium (Si/Cr=40). Both the above as-synthesized products were calcined in air to remove the organic template, ion-exchanged with NH4C1 solution and then calcined at 600~ under nitrogen flow to produce the H-form of the zeolites. All the ZSM-5 samples were finally steamed at 790~ for 6 hrs, at a partial pressure of steam of 97.7 kPa, in order to simulate the deactivation state of the main FCC catalyst. All the samples were stored over a saturated MgCI2 solution to equilibrate with water vapor. Chemical analysis of the zeolite samples was carried out by means of Atomic Absorption Spectroscopy (Shimadtzu 6800 AAS) and Electron Dispersive Spectroscopy (Oxford ISIS 300 EDS). Powder X-ray Diffraction (XRD) was carried out on a Siemens D500 diffractometer with CuKa-radiation. Scanning Electron Microscopy (SEM) images were obtained with a JSM 840-A JEOL SEM. Specific Surface Area (SSA) was measured by nitrogen sorption isothermally at 77 K using a Sorptomatic 1900 instrument (multi-point BET). 27A1-MAS-NMR measurements were carried out using a Varian Infinity plus AS400 spectometer. Temperature-Programmed Desorption (TPD) of ammonia tests were performed on a conventional apparatus which consisted of a cylindrical quartz microreactor, a vertical well-controlled high-temperature fumace and a gas chromatograph equipped with a thermal conductivity detector (TCD). The mass of the catalysts sample used was 0.2 g. Sorption of dry ammonia (Merck, HzO-free) took place at 100~ in a static system for 90 rain at 1.5 bar ammonia pressure. Stripping was done afterwards for 40 rain at 100~ under He flow. In this way, the weakly and physically adsorbed ammonia was minimized in the sample. Desorption of ammonia was done at a rate of 10~ from 100 up to 700~ under He flow (50 ml/min). The desorbed ammonia was detected on a Shimadzu GC-8A gas chromatograph (with TCD), and then it was trapped in a HC1 aqueous standard solution (0.01N). The desorbed NH3 was estimated by titrimetric determination of the excess HC1 in solution, using a standard 0.01N NaOH solution. The catalytic activity of the zeolite samples in propane dehydrogenation was investigated using a cylindrical fixed-bed continuous-flow glass reactor. The reaction was carried out between 200~ and 525~ Temperature in the reactor was monitored by a thermocouple located at the center of the catalyst bed. The catalyst (mass 0.2 g) was placed into the glass reactor between two layers of glass-beads (Serva). Before reaction, the catalysts were outgassed at 500~ for 3 h under He flow. The reactant mixture had a molar composition He/C3H8 = 10/1 and space velocity of 2000-3000 h -1. Products were analyzed using a Shimatzu GC-14b gas chromatograph equipped with an Supelco SP-1700 column and a Thermal Conductivity Detector (TCD).

3.

RESULTS AND DISCUSSION

3.1. Compositional, structural and acidic characteristics of the ZSM-5 catalysts The physicochemical characterization results of the five steamed zeolites are listed in Table 1. Chemical analyses data showed that chromium was present in both the H-[Cr]ZSM5 and H-[Cr/A1]ZSM-5 samples; the molar Si/Cr ratio in the first sample was -24 while the molar ratio Si/M (where M=A1 plus Cr) in the latter sample was -39. However, aluminum

842 Table 1 Compositional, Structural and Acidic characteristics of the steamed zeolite catalysts Sample Cr A1 SSA C1) FA1 content (2) Total acidity (wt. %) (m2/g) (wt %) (mmoles NH3/g) H-ZSM-5 (3) ~ 0.04 374 ~ 0.02 H-[Cr]ZSM-5 1.88 < 0.01 343 ~0 H-[Cr/A1]ZSM-5 0.90 0.18 372 0.08 0.11 H-[A1]ZSM-5 1.06 367 0.08 0.09 Cr203/H-[A1]ZSM-5 2.90 1.51 353 0.05 0.09 (1) Specific surface area (multi-point BET) (2)Framework A1 content determined based on 27A1MAS-NMR and chemical analysis data (3)ZSM-5 sample synthesized with no A1 source (silicalite)

content of H-[Cr/A1]ZSM-5 was significantly lower than it was expected based on the aluminum content of the synthesis mixture. In contrast, the Si/A1 ratio for H-[A1]ZSM-5 was -~43, similar to that of the synthesis mixture. The chemical analyses of both Al-free synthesized samples showed traces of A1. The XRD patterns of the steamed zeolite samples (Fig. 1) revealed that they were highly crystalline. The various treatments (calcinations/combustion of the organic template, ion-exchange, impregnation, steaming) affected the crystallinity of the samples at a relatively low degree. The small loss of crystallinity (after severe steaming) was also confirmed by SSA .... measurements. The sample with the higher surface area is silicalite (HZSM-5). The lowest surface area was found for the samples H-[Cr]ZSM-5 and Cr203/H-[A1]ZSM-5. This can be attributed to the relatively big amount of chromium oxides or oxyhydroxides which are present in both steamed samples and are blocking the ZSM-5 channels. The framework aluminum (FA1) content of the steamed samples was determined from the intensity of the peak at-~53 ppm in the 27A1 MAS H-[Cr]ZSM-5 NMR spectra based on an appropriate calibration curve which correlated the total A1 content of ZSM-5 samples free of extra-framework A1 (EFA1) 15 25 35 45 with the intensity of the above 27A1 2e(degrees) NMR peak [28]. The FA1 content was Figure 1. XRD pattems of the steamed dramatically reduced by the steaming process in all the ZSM-5 samples. ZSM-5 samples

843 SEM-EDS measurements provided valuable information on the morphology (Fig 2.) and composition of the samples elucidating some of the crystal and compositional properties of the synthesized zeolites H-[Cr]ZSM-5 consists mainly of large, silicalite-type crystals of rectangular shape and of some particles of anomalous shape. Traces of chromium were detected in the zeolitic crystals whereas the nonzeolitic particles were Cr- and Si-rich. Further XRD analyses are in progress in order to elucidate the nature of these species, which could be crystalline or amorphous chromium or chromium-silicon oxides/oxy-hydroxides. On the contrary, images of H[Cr/A1]ZSM-5 showed an almost pure zeolitic phase which consisted of large, well-formed crystallites of rectangular shape, similar to those of silicalite. Both A1 and Cr were detected in these crystallites. Chromium (as oxide or oxy-hydroxide species) in this sample is most likely randomly distributed along the zeolitic crystals as it was "trapped" during the crystallization process. ESR, XPS and Diffuse Reflectance UV spectroscopy will provide additional data, mainly on the oxidation state, coordination and distribution of the chromium species. The images of Cr203/H-[A1]ZSM-5 showed that it consists mainly of typical, small-sized ZSM-5 crystals which contained both A1 and Cr; chromium in this sample is mainly distributed on the outer surface of the zeolitic particles as a result of the preparation procedure (impregnation). It is well known that the strong Figure 2. SEM images of the steamed samples" BrSnsted acidity of H-ZSM-5 zeolite is H-[Cr]ZSM-5 (top), H-[Cr/A1]ZSM-5(middle), attributed to framework aluminum and the Cr203/H-[A1]ZSM-5 (bottom) related framework hydroxyls. As it was expected, the zeolites prepared in the absence of aluminum source showed negligible acidity. On the other hand, the acidity of the rest steamed zeolites was significantly decreased upon steaming, due to framework dealumination. No significant variations in total acidity were detected between H-[Cr/A1]ZSM-5, H-[A1]ZSM-5 and CrzO3/H-[A1]ZSM-5, based on their different amounts and type of Cr species; they all had similar framework aluminum content.

844

3.2. Dehydrogenation activity of the ZSM-5 catalysts The reaction of propane over MFI-type zeolites yields cracking (e.g. methane, ethane, ethylene, coke), dehydrogenation (e.g. propylene) and cyclo-oligomerization (e.g. aromatics) products. The maximum dehydrogenating activity values of the five steamed zeolitic catalysts of this work are shown in Table 2. All the chromium modified H-ZSM-5 zeolites showed higher activity and selectivity to propylene compared to the H-[A1]ZSM-5 zeolite, with the most selective being the H[Cr]ZSM-5 and H-[Cr/A1]ZSM-5 samples. An interesting finding, revealing the diversity in the nature of the active sites in the tested catalysts was that they exhibited maximum catalytic activity at slightly different reaction temperatures. As expected the Al-free HZSM-5 sample (silicalite) was inactive in the dehydrogenation reaction. Based on the chromium content of the above Cr-modified samples it is clear that H[Cr/A1]ZSM-5 is the most active catalyst between the three catalysts since it possesses the less chromium amount (Table 1) and similar acidity. It can be suggested that the proposed random distribution of chromium species along the ZSM-5 crystals of this sample enables them to act synergistically with the internal framework or extra-framework A1 acid sites inducing an improved dehydrogenation activity. The curves showing the dependence of propane conversion and propylene yield on the reaction temperature (Fig. 3) provide with additional information on the activity of the catalysts. From the data in Fig. 3 it can be seen that H-[Cr]ZSM-5 samples activity and selectivity to propylene is very high at relatively low temperatures (maximum at -450~ compared to the rest of the catalysts. In addition, the life-time of H-[Cr]ZSM-5 at these low temperatures is much longer compared to the other catalysts, due to the lack of acid sites and the minimization of coke formation. On the contrary, sample H-[Cr/A1]ZSM-5 is very active at high temperatures, showing the highest propylene yield at 500~ However, at even higher temperatures, ca. 550~ conversion was increased to-~59% but propylene selectivity dropped significantly due to oligomerization and aromatization reactions that occured on the acid sites of the catalyst. The impregnated sample Cr2Oa/H-[A1]ZSM-5 presents two main differences from H[Cr/A1]ZSM-5. It has almost three times more chromium content and most of it resides on the external surface of the ZSM-5 crystals. As a result, the chromium oxides/oxyhydroxides might block access of the reactant to the intemal acidic sites and furthermore, Table 2. Maximum catalytic activity of the steamed catalysts (1) Sample Total Propane conversion (2)

(%)

H-ZSM-5 H-[Cr]ZSM-5 H-[Cr/A1]ZSM-5 H-[A1]ZSM-5

Selectivity to Propylene

(%)

1.1 24 32 (3) 26 29 (4) 31 28 19 Cr203/H-[A1]ZSM-5 41 20 (1) All the zeolite samples listed have been previously steamed at 790~ for 6 hrs (2) Total conversion at 525~ unless otherwise stated, reactant mixture He/Call8=10, space velocity 2500 h l (3) Total conversion at 450~ (4) Total conversion at 500~

845 | | ~-

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CONCLUSIONS

The catalytic activity of chromium modified ZSM-5 samples depends greatly on the method of chromium incorporation and the degree of framework dealumination (through steaming) which reduces the acidity of the samples. The steamed H-[Cr/A1] ZSM-5, which was prepared by incorporating both aluminum and chromium in the hydrothermal synthesis mixture, was very selective towards propylene production in the dehydrogenation of propane. The synergistic effect of the well dispersed chromium species within the zeolitic crystals with the small number of acid sites was suggested to account for the observed high dehydrogenation activity. The steamed Cr-impregnated H-ZSM-5 sample showed lower activity based on its chromium content, since most of the chromium species were on the external surface of the zeolitic crystals, isolated from the framework A1 acid sites. When the chromium species were introduced during the synthesis of an Al-free ZSM-5 sample, the final steamed catalysts dehydrogenation activity and propylene selectivity were appreciable

846 REFERENCES

1. F. Buonomo, D. Sanfilippo and F. Trifiro, Handbook of Heterogeneous Catalysis, Vol. 5, 2140. 2. F. Cavani, F. Trifiro, Catal. Today 24 (1995) 307. 3. W. Schuster, J.P.M.W.F. Hoelderich, Appl. Catal. A 209 (2001) 131. 4. N.Y. Chen, T.Y. Yan,, Ind. Eng. Chem. Process Des. Dev. 25(1986) 151. 5. E.E. Davis, A.J. Kolombos, GB Pat. 53012 (1976); AU. Pat., 509285 (1980) 6. P.C. Doolan, P.R. Pujado, Hydrocarbon Processing, (1989), 72 7. D. Seddon, Catal.Today review (1990) 351.. 8. M. Guisnet, N.S. Gnep, Appl. Catal. A: General 89 (1992) 1. 9. R. Fricke, H. Kosslick, G. Lischke, M. Richter, Chem. Rev. 100 (2000) 2303. 10. W. J. H. Dehertog, G.F. Fromen, Appl. Catal. A: General 189 (1999) 63 11. P. Meriaudeau, C. Naccache, J. Catal. 157 (1995) 283. 12. B.S. Kwak, W.M.H. Sachtler and W.O. Haag, J. Catal. 149 (1994) 465. 13. V.R. Chudhary, A. K. Kinage, C. Sivadinarayama, M. Guisnet, J. Catal. 158 (1996) 23. 14. I. Nakamura, K. Fujimoto, Catal. Today, 31 (1996) 335 15. L. Brabec, M. Jeschke, R. Klik, J. Novakova, L Kubelkova, D. Freude, V. Bosacek, J Meusinger, Appl. Catal. A: General 167 (1998) 209 16. V. Chouldary, K. Mantri, C. Sivadinarayama, Microp. & Mesop. Mater. 37 (2000) 1. 17. A. Montes, G. Gianneto, Appl. Catal. A: General 197 (2000) 31. 18. Z. Fu, D.Yin, Y.Yang, X. Guo, Appl. Catal. A: General 124 (1995) 59. 19. H. Bemdt, G. Lietz, B. LOcke and J. V61ter, Appl. Catal. A: General 146 (1996) 351. 20. G. Centi, F. Trifiro, Appl. Catal. A: General 143 (1996) 3. 21. J. Hal~isz, Z. K6nya, A. Fudala, A. B6res and I. Kiricsi, Catal. Today 31 (1996) 293 22. P. Branda ,A. Philippou, A. Valente, J. Roch, M. Anderson Phys. Chem. Chem. Phys., 3 (2001), 1773. 23. A.V. Kucherov, A.A. Slinkin, G.K. Beyer, G. Borbely, zeolites 15 (1995) 431. 24. Z. Zhu, Z. Chang, L. Kevan, J. Phys. Chem. B, 103, (1999), 2680. 25. E.S. Sphiro, R.W. Joyner, P. Johnston, G. J. Tuleuova, J. Catal 141 (1993) 266 26. Y.G.Li, W.H. Xie, S. Yong Applied Catalysis A: General 150, (1997), 231 27. R.G. Argauer and G. R. Landolt, U.S. Pat. 3702, 856, (1972) 28. C.S. Triantafillidis, A.G. Vlessidis, L. Nalbandian and N.P. Evmiridis, Micropor. Mesopor. Mater., 47 (2001) 369.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

847

E f f e c t o f P d a d d i t i o n on th e c a t a l y t i c p e r f o r m a n c e o f H - Z S M - 5

z e o l i t e in c h l o r i n a t e d VOCs c o m b u s t i o n R. L6pez-Fonseca, S. Cibrihn, J.I. Guti6rrez-Ortiz and J.R. Gonzhlez-Velasco* Departamento de Ingenierla Quimica, Facultad de Ciencias, Universidad del Pals Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-946012681; Fax: +34-944648500; E-mail: [email protected] The aim of this work was to evaluate the influence of the addition of palladium on the catalytic behaviour of H-ZSM-5 zeolite in the combustion of 1,2dichloroethane (DCE) and trichloroethylene (TCE). Both catalysts showed similar activity in the oxidation of DCE, by contrast, the metal loading led to a substantial improvement in TCE combustion. Vinyl chloride was detected as an intermediate in DCE conversion, and it was appreciably suppressed when adding Pd to the zeolite. In TCE oxidation trace amounts of tetrachloethylene were identified as a by-product, Pd/H-ZSM-5 showing larger quantities of this undesired by-product. Pd/H-ZSM-5 was more selective towards CO2 formation instead of CO, which was the major carboncontaining product formed over H-ZSM-5. However, H-ZSM-5 zeolite showed a lower selectivity to C12 generation while the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction. 1. I N T R O D U C T I O N Emissions of Volatile Organic Compounds (VOCs) into the earth's atmosphere result from naturally occurring (biogenic) as well as human-made sources. In the latter, the use of solvents is the main source of the atmospheric pollution, followed by a variety of industrial processes. One important group of VOCs consists of halogenated organic compounds, where the halogen is generally chlorine. These compounds are usually toxic, particularly through their action on the liver function, and may be carcinogenic or mutagenic. Moreover, chlorinated VOCs are implicated in the destruction of the ozone layer. Because of their harmful properties, the release of halogenated organic compounds into the environment is being controlled by increasingly stringent regulations. 1,2-Dichloroethane (DCE) is the main component of the waste stream gases from the chemical plants that produce it as an intermediate to obtain the monomer vinyl chloride, which is used for the production of polyvinylchloride (1). Trichloroethylene (TCE) is a toxic solvent widely used in dry cleaning and degreasing processes and it is also present in air stripping and soil venting remediation off-gases (2).

848 The most commonly used catalysts for the catalytic oxidation of chlorinated VOCs are alumina supported noble metals and metal oxide catalysts. Recently, zeolites have gained interest as an effective and advantageous alternative, but there are only a few investigations into these reactions over metal-modified H-zeolites (3,4). The objective of this work is to evaluate the influence of the addition of palladium to H-ZSM-5 zeolite on the catalytic behaviour for the combustion of two common chlorinated compounds, between 200-550 ~ The concentration of the chlorocarbons was set at 1000 ppm.

2. EXPERIMENTAL 2.1. Catalyst preparation The zeolite NH4-ZSM-5 (CBV 5524 G) was supplied from Zeolyst Inc. The H-ZSM-5 form was obtained by calcining the NHn-ZSM-5 zeolite in air at 550 ~ for 3 h. The preparation of Pd/H-ZSM-5 zeolite catalysts can be divided into two steps: introduction of the metal palladium and activation, the latter including pre-treatment (or the calcination in an oxygen containing atmosphere), and reduction. The preferred way of introducing a low loading of palladium into a zeolite is to exchange H § ions for positively charged palladium complexes (5). In this case the precursor salt used was [Pd(NH3)4]C12. A dilute solution of this salt, which contained an excess of 15% of the palladium, was added to a stirred HZSM-5 slurry. After stirring for 24 h at room temperature, the slurry was filtered and washed with deionized water, and the sample was then dried overnight (12 h) at 110 ~ To obtain the desired particle size of the catalyst, the sample was pelletised and sieved between 0,3-0,5 mm. The second step of the preparation was the activation of the catalyst in a glass quartz tube reactor. First, a flow of air (21% 02) of 150 ml min -1 was passed, while the temperature was increased from room temperature to 550 ~ with a heating rate of 1 ~ min -1. This final temperature was maintained for 3 h. The sample was then purged in a nitrogen flow and cooled down to 300 ~ and this temperature was maintained for 3 h in an hydrogen/nitrogen flow (1/3). Finally, the sample was cooled down to room temperature in a nitrogen flow.

2.2. Catalysts characterisation The BET surface areas of the catalysts samples were determined by nitrogen adsorption-desorption a t - 1 9 6 ~ in a Micromeritics ASAP 2010 equipment. The composition was determined using a Philips PW 1480 X-ray fluorescence (XRF) spectrometer. The X-ray powder diffraction (XRD) patterns were recorded on a Philips PW 1710 X-ray diffractometer with C u I ~ radiation. The metal content of the catalyst was measured by atomic absorption spectroscopy (AAS) in a Perkin Elmer 1100 B equipment. 2.2.a. Temperature programme desorption (TPD) of ammonia

849 T e m p e r a t u r e programme desorption (TPD) of a m m o n i a was performed on a Micromeritics AutoChem 2910 instrument. The overall process involved heating of the sample at 550 ~ in a nitrogen flow with a heating rate of 20 ~ min -1, after this the sample was cooled down to 100 ~ in a helium flow and then pulses of a m m o n i a were introduced until the sample was saturated. Finally, desorption of chemisorbed ammonia from the sample was carried out by heating from 100 ~ to 550 ~ at a rate of 10 ~ min -1. 2.2.b. T e m p e r a t u r e p r o g r a m m e d reduction (TPR) T e m p e r a t u r e p r o g r a m m e d reduction experiments were also performed in a Micromeritics AutoChem 2910 equipment. Prior to the reduction, the [Pd(NH3)4]2+/H-ZSM-5 catalyst was activated in 5% oxygen/helium flow for 1 h at 550 ~ (heating rate of 1 ~ minl), and purged in a nitrogen flow while cooling down to - 5 0 ~ TPR experiments were carried out in a 5% hydrogen/argon flow, increasing the t e m p e r a t u r e from - 5 0 ~ to 550 ~ at a rate of 10 ~ min 1.

2.3. Catalytic a c t i v i t y m e a s u r e m e n t s Oxidation reactions were carried out under atmospheric pressure in a fixed bed tubular reactor. Liquid reactants were injected into a dry, oil-free compressed air stream by a syringe pump. The flow rate through the reactor was set at 500 cm 3 min 1 and the gas hourly space velocity was maintained at 15000 h -1. Reactor effluent was analysed on line by a Hewlett Packard 5890 Series II gas chromatograph equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD). Operation conditions and reaction product analysis were described in detail elsewhere (6). 3. R E S U L T S AND D I S C U S S I O N

3.1. C h a r a c t e r i s a t i o n r e s u l t s The Si/A1 atomic ratio of the zeolite measured by XRF was found to be 27.3. The BET areas obtained from nitrogen adsorption-desorption isotherms for the H-ZSM-5 and Pd/H-ZSM-5 zeolites were 425 and 380 m 2 g-l, respectively. The content of the metal exchanged in the zeolite resulted 0,32% wt. of Pd. XRD analysis of the catalysts indicated t h a t no appreciable differences were found between Pd/H-ZSM-5 and H-ZSM-5 zeolites. TPD of a m m o n i a profiles (Figure 1) indicated t h a t the total n u m b e r of acid sites was larger for H-ZSM-5 compared to Pd/H-ZSM-5, however, the ratio of strong acid sites was higher for the containing palladium zeolite, being these 56,2% and 65,6% for H-ZSM-5 and Pd/H-ZSM-5 zeolites respectively. It is well-known t h a t calcination/reduction conditions of the catalysts have a strong influence on the final location of metal particles (7). The purpose of a slow heating ramp for the calcination step is to minimize the autoreduction of the metal and its aglomeration into larger particles (8).

850

Pd/H-ZSM-5 ;~

-

~

I~

-

.H-ZSM-5

~l

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,

,

,

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Temperature,~

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F i g u r e 1. NHa-TPD profiles from HZSM-5 and Pd/H-ZSM-5 zeolites.

-50

i

,

,

j

150

i

,

,

,

,

350

Temperature,~

,

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,

,

550

F i g u r e 2. TPR profile of the catalyst Pd/H-ZSM-5.

In the TPR profile (Figure 2) two zones could be clearly identified: the broad peak that appeared at the beginning of the temperature programme (0-150 ~ was due to the reduction of more accessible metal. The peak appearing at approximately 400-450 ~ was related to particles located in hidden positions, more difficult to be reduced, but the proportion of these particles was very low in comparison with the total metal content in the zeolite (9,10). The results from TPR experiments revealed that a reduction temperature of 300 ~ was necessary to activate the Pd/H-ZSM-5 catalyst.

3.2. A c t i v i t y results DCE and TCE were chosen due to their different H:C1 ratio and chemical structure. The light-off curves of the combustion of these CVOCs are shown in Figure 3. Both zeolite catalysts showed a noticeable activity for the destruction of both chlorinated compounds. It is reported that H-type zeolites exhibit high activity for oxidation of chlorinated VOCs such as DCE and TCE (11-13). It was noted that DCE was converted at significantly lower temperatures t h a n TCE. This behaviour can be attributed to the relatively large size and high electronegativity of the chlorine atom that can produce severe steric and electronic hindrances to the adsorption of chlorinated ethylene molecules (14,15). Windawi y cols, (16,17) also reported that catalytic oxidation of saturated hydrocarbons is easier than that of the u n s a t u r a t e d ones. The activity of Pd/H-ZSM-5 for DCE conversion was almost identical to that observed for H-ZSM-5, showing a slightly higher Ts0 (temperature at 50% conversion was attained) value of 280 ~ compared to 270 ~ over H-ZSM-5 and obtaining a complete conversion (>95%) with both catalysts at 350 ~

851

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500

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F i g u r e 3. Light-off curves of DCE and TCE. By contrast, the metal loading led to a substantial improvement in TCE catalytic activity. Hence, T~0 decreased from 440 ~ over metal-free zeolite to 360 ~ over Pd/H-ZSM-5. It was established that strong Bronsted acidity played a major role in determining the catalytic activity of H-zeolites since these sites are believed to act as chemisorption sites for chlorocarbons (18-21). The major oxidation products during chlorinated VOCs decomposition were CO, CO2, HC1 and C12. Added to this, in the case of DCE, as soon as conversion began to be noticeable vinyl chloride was detected; however, this intermediate disappeared at elevated temperatures (>450 ~ (22). The presence of vinyl chloride indicated that the abstraction of HC1 was the first step in the reaction scheme. The formation of this intermediate was appreciably suppressed when using Pd/H-ZSM-5. The maximum concentration of vinyl chloride obtained was 735 ppm with H-ZSM-5, but when adding Pd to the zeolite the peak amount was reduced to 100 ppm. As regards TCE, trace amounts of perchloroethylene were observed as a by-product. This compound was generated by chlorination of the TCE and was partially destroyed at higher temperature (23). Pd/H-ZSM-5 zeolite led to larger quantities of this undesired by-product t h a n H-ZSM-5, since this concentration increased from 120 ppm to 355 ppm. This increase is due to the known activity of noble metals in chlorination reactions (24).

852 T a b l e 1. Selectivities towards desired by-products. DCE

H-ZSM-5 Pd/H-ZSM-5

TCE

CO2,%

HCI,%

CO2, % HC1, %

54 100

96.6 91.3

63 73

57.0 42.5

As far as CO and CO2 formation was concerned, the formation of CO2 was relatively favoured as temperature increased with both catalysts, but Pd/H-ZSM5 zeolite was more selective towards CO2 formation instead of CO, which was the major product formed over H-ZSM-5. The high activity of Pd for CO oxidation is the cause of this beneficial effect (25). When decomposing DCE, the selectivity to CO2 obtained with the metal loaded zeolite at 550 ~ was 100% but this improvement in CO2 selectivity was less noticeable with TCE, as only 73% selectivity was achieved (Table 1). The H-form zeolite showed a lower selectivity to C12 generation. On the contrary, the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction (2HCl+ 89 (6,26). The selectivity towards C12 was higher for the case of TCE rather t h a n for DCE due to the H:C1 ratio greater t h a n 1. Table 1 sumarises the selectivities obtained to the desired oxidation products (CO2 and HC1) in the decomposition of the chlorinated compounds with both catalysts. It must be pointed out that the combustion of DCE over H-ZSM-5 and Pd/H-ZSM-5 was accompanied by the formation of coke due to the polymerisation of vinyl chloride (27). By contrast, no coke deposition was observed during TCE combustion with any of the catalysts. Carbon balances closed above 95-100% when decomposing TCE, but they were found to be higher than 100% at elevated temperatures in the DCE reaction due to the combustion of the coke formed during the reaction. Chlorine balance was in the range 65-85% in the oxidation of both chlorinated compounds. It is known that AI-O bonds in the zeolite framework can be easily attacked by the HC1 formed during reaction leading to the formation of volatile A1C13 which causes the partial collapse of the framework and the blockage of the porous structure (28). Furthermore, a change in colour of the Pd/H-ZSM-5 catalyst from grey to orange yellowish was observed at the end of the activity test with both compounds. This colour fitted quite well with that of metal chloride complexes (PdC12), thus the interaction of chlorine with the metal in presence of halocarbons could also explain the unfitted chlorine balance (6).

853 4. CONCLUSIONS Both catalysts, H-ZSM-5 and Pd/H-ZSM-5, studied in this work showed a high activity in the chlorinated VOCs decomposition. Pd/H-ZSM-5 zeolite was found to be most active in the oxidation of TCE, while no noticiable difference between both catalysts was noticed for DCE combustion. The main oxidation products were CO, CO2, HC1 and C12. Additionally, vinyl chloride was detected in DCE reaction, indicating that the first step in the mechanism is the dehydrochlorination of the feed. On the other hand, perchloroethylene was also observed in TCE conversion as a result of the chlorination of the feed molecule. The addition of the metal to the zeolite improved both the activity and the selectivity towards CO2, but this also involved a significant increase in undesired by-products formation such as chlorine and highly chlorinated hydrocarbons. ACKNOWLEDGEMENTS The authors whis to thank Universidad del Pals Vasco/EHU (9/UPV 0069.310-13517/2001) and Ministerio de Ciencia y Tecnologla (PPQ2001-1364) for the financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

H. Muller, K. Deller, B. Despeyroux, E. Peldszus, P. Kammerhofer, W. Kuhn, R. Spielmannleitner and M. Stoger, Catal. Today 17 (1993) 383. A.R. Gavaskar, B.C. Kim, S.H. Rosansky, S.K. Ong and E.G. Marchand, Environ. Prot. 14 (1995) 33. S. Chatterje and H.L. Greene, J. Catal. 130 (1991) 76. S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal. 138 (1992) 179. C. Gerard-Gomez, M. Dufaux, J. Morel, C. Naccache and Y.B.Taarit, Appl. Catal. A 165 (1997) 371. J.R. Gonz~lez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz and R. L6pezFonseca, Appl. Catal. B 19 (1998) 189. P. Cafiizares, A. de Lucas, F. Dorado and J. Aguirre, Microporous Mesoporous Mater. 42 (2001) 245. W.M.H. Sachtler, Catal. Today 15 (1992) 419. V.Z. Radkevich, M.F. Savchits and Y.G. Egiazarov, Russian J. Appl. Chem. 7 (1997) 759. S.T. Homeyer and W.M.H. Sachtler, J. Catal. 117 (1989) 91. M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohuchi, Appl. Catal. B 9 (1996) 167. B. Ramachandran, H.L. Greene and S. Chatterjee, Appl. Catal. B 8 (1996) 157. R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz, J.I..s and J.R. Gonz~lez-Velasco, Appl. Catal B 30 (2001) 303. G.C. Bond and N. Sadeghi, J. Appl. Chem. Biotechnol. 25 (1975) 241.

854 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

P.S. Chintawar and H.L. Greene, Appl. Catal. B 13 (1997) 81. H. Windawi and M. Wyatt, Platinum Metals Rev. 37 (1993) 186. H. Windawi and Z.C. Zhang, Catal. Today 30 (1996) 99. S. Karmakar and H.L. Greene, J. Catal. 138 (1992) 364. M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohuchi, Appl. Catal. B 9 (1996) 167. S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal. 138 (1992) 179. R. L6pez-Fonseca, A. Aranzabal, P. Steltenpohl, J.I. Guti~rrez-Ortiz and J.R. Gonz~lez-Velasco, Catal. Today 62 (2000) 367. G. Sinquin, J.P. Hinderman, C. Petit and A. Kiennemann, Catal. Today 54 (1999) 107. H. Shaw, Y. Wang, T.C. Yu and A.E. Cerkanowicz, ACS Syrup. Ser. 495 (1992) 358. J.R. Gonz~lez-Velasco, A. Aranzabal, R. L6pez-Fonseca, R. Ferret and J.A. Gonz~lez-Marcos, Appl. Catal. B 24 (2000) 33. S. Chatterjee and H. Greene, Appl. Catal. A 98 (1993) 139. J.C. Lou and S.S. Lee, Appl. Catal. B 12 (1997) 111. S. Imamura, H. Tarumoto and S. Ishida, Ind. Eng. Chem. Res. 28 (1989) 1449. Z. Konya, I. Hannus and I. Kiricsi, Appl. Catal. B 8 (1996) 391.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

855

Influence of the amount and the type of Zn species in ZSM-5 on the aromatisation of n-hexane A.Smiegkov~t, E.Rojasov~t, P.Hudec, L.~;abo, and Z.2;idek Department of Petroleum Technology and Petrochemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinsk6ho 9, 812 37 Bratislava, Slovakia The aromatisation ofn-hexane over a series of zinc modified (0.03 5-0.50 mmolZn/g)acid ZSM-5 zeolite (1.09 mmolAl/g) prepared by i) ion exchange or by ii) mechanical admixing of ZnO was investigated. The reaction of aromatisation was carried out at 420 ~ at atmospheric pressure. At this temperature thermal cracking ofn-hexane doesn't contribute to the conversion. Over ion exchanged samples with increasing amount of zinc in the zeolite the conversion of n-hexane comparing with acid ZSM-5 at first continuously decreases goes through the minimum and than begins to rise, while the selectivity to aromatics continuously raises. As was shown the differences in the acidity of samples are very low so from the results we conclude that the role of the Zn in cationic positions in n-hexane conversion depends on the zinc concentration. Zinc species at low concentration probably due to their localization are not active in n-hexane activation but only in dehydrogenation of the oligomerized products. Mechanical mixtmes of ZnO + acid ZSM-5 in contrary to the ionexchanged samples had constant activity/selectivity independently on the amount of ZnO. Their activity and selectivity are roughly on the level of ion-exchanged sample containing 0.085 mmol Zn/g. From the results follows that at 420 ~ ZnO species in mechanical mixtures are not active in n- hexane conversion and the catalytic performance of the mixtures is probably the consequence of a solid state ion exchange of Zn into zeolite from ZnO species. From the constant activity/selectivity of the mechanical mixtures containing different amount of ZnO follows that the extent of assumed solid state ion exchange is limited and is controlled probably by the surface characteristics of the zeolite and not by the amount of ZnO. 1. INTRODUCTION Effective conversion of light paraffmie hydrocarbons contained in natural gas and associated gases and of butane and light naphtha component such as n-CsH~2 and n-C6Ht4 derived from petroleum refining processes to more valuable chemicals is of the great importance. One of the important targets of the conversion would be the synthesis of important aromatics-benzene, toluene and xylenes for the petrochemical industry. It has been reported by several authors that ZSM-5 zeolites modified with gallium[ 1,2,3,4,5,6],

856 indium [6], Zn [7,8,9,10,11,12], Ni [12], Pt [7], or with the mixture of Ni and Zn[13] belong to the most convenient catalyst for conversion of this light alkanes to the aromatics. Polyfunctional Zn and Ga metalosilicates of ZSM-5 structure modified with Pt [ 14,15] were also found as selective catalysts for aromatisation of light alkanes. As is known ZSM-5 zeolites modified with zinc are a very active catalyst for this purpose. A number of works has been published from the studies of light alkanes aromatisation carded out on Zn ZSM-5 catalysts prepared by ion exchange [7,8] wet impregnation [10,13,16,17] or by mechanical mixing of ZnO and ZSM-5. Enhanced yields of aromatics obtained over these catalysts are generally attributed to the participation of the Zn in activation of alkanes and in conversion of naphtenes to aromatics. However, the influence of the type of Zn species on the activity/selectivity in aromatisation of light alkanes is still a point of discussion. The literature of Zn containing zeolites also shows that the activation of the catalysts and the experiments has been carded out at very different temperatures (420-600 ~ and that very different alkanes as model feedstocks are used - from C2- up to C7 alkanes or their mixtures. Thus, this fact can lead to the observed controversial conclusions conceming the role of Zn species in aromatics formation. Depending on the type of alkane used at high reaction temperatures thermal cracking can contribute to the alkane conversion. Moreover, at high testing or activation temperatures the loss of zinc by sublimation is pretended [ 10] from both, cationic or non-framework positions. Thus, the aim of this work was to investigate the catalytic behaviour of Zn containing ZSM-5 in dependence of Zn concentration where zinc species were incorporated by different methods, n-Hexane was used as a model feed. The reaction temperature was 420 ~ and catalysts were activated at 480 ~ Our previous experiments showed that at these temperatures neither thermal cracking of n-hexane nor zinc loss from the catalysts during activation occur. 2. EXPERIMENTAL The NaZSM-5 zeolite (Si/AI=14, 1.09 mmol A1/g) was supplied by Slovnaft Research Institute of Petroleum and Hydrocarbon Gases, Inc. Bratislava. NHa-ZSM-5 and NH4ZnZSM-5 samples (0.035-0.35 mmol Zn/g) marked ZnZSM-5 were prepared by exchanging procedures described in [ 18]. Mechanical mixtures ofZnO (p.a) and NH4ZSM-5 (0.085-0.50 mmol Zn/g) marked ZnO+ZSM-5 were prepared by thorough homogenisation of the mixture in a mill. The acidity of the catalysts was determined by temperature programmed desorption of ammonia (TPDA). Adsorption of ammonia was carried out at 220 ~ after activation of samples at 480 ~ for 1 h in a stream of helium. The quantity of ammonia desorbed in the temperature range from 220 to 550 ~ was detected by titration of the excess of 0.1 M HaSO4. Conversion of n-hexane was performed in vapour phase, in a continuous glass flow microreactor contained 100 mg of catalyst at atmospheric pressure in a stream of nitrogen (20 ml/min) saturated with n-hexane at 0~ The microreactor was on-lined with a gas chromatograph. Before the catalytic tests were performed, the catalysts were in situ activated in a stream of dry air (50 ml/min) at 480 ~ for 1 hour.

857 In the case of mechanical mixture before TPDA measurements and catalytic tests the samples were pre-activated in a stream of dry air at 480 ~ for three hours. Conversion of n-hexane was carried out at 420 ~ The reaction products were analysed each 20 minutes within 180 minutes existing run by gas chromatography (Hewlett Packard 5890, SERIE II) using a HP-1 capillary column (15 m x 0.530 mm) and FID detector. 3.RESULTS AND DISCUSSION The characteristics of the samples are show in the Table 1. Table 1 Characteristics of the investigated samples Zn content, Sample (mmol/g) NH~SM-5* 0.07ZnZSM-5 0.17ZnZSM-5 0.18ZnZSM-5 0.25ZnZSM-5 0.30ZnZSM-5 0.4ZnZSM-5 0.46ZnZSM-5 0.54ZnZSM-5 0.7 ZnZSM-5 Mechanical mixtures 0.17ZnO+ZSM-5 0.40ZnO+ZSM-5 0.46ZnO+ZSM-5 0.54ZnO+ZSM-5 0.7ZnO+ZSM-5 1.0ZnO+ZSM-5 * NHaZSM-1.09 mmol A1/g

0 0.035 0.085 0.09 0.125 0.15 0.2 0.23 0.275 0.35

Acidity, TPDA (mmol NH3 des.)/g total > 450 ~ 0.91 0.21 0.89 0.90 0.27 0.88 0.84 0.85 0.28 0.90 0.29 0.87 0.29 0.80 0.39 0.85 0.42

0.085 0.2 0.23 0.275 0.35 0.5

0.97 0.85 0.85 0.79 0.85 0.89

0.28 0.26 0.27 0.27 0.26 0.28

In our previous works [18,19,20] was reported that Zn in cationic positions in zeolite represents Lewis acid sites and causes acid sites strength redistribution in the zeolite. This conclusion came from the fact that with increasing amount of Zn in cationic position (see Tab.l). slightly increases the amount of ammonia desorbing over 450 ~ .In contrary to the ion-exchanged samples in mechanical mixtures the amount of ZnO practically has no effect on the amount of ammonia desorbing over 450~ From the data given in Table 1 can be also seen that in spite of the considerable differences in the content of Zn in samples there are very slight differences in their acidity. The conversion of n-hexane in dependence of the amount of Zn over ion exchanged samples and over the mechanical mixtures is graphically presented of Fig.1.

858 60-~

~

50-

e1! t~ 4 0 X

0 J~ I e,,

~:

,,.. 3 0 -

t

0 tO

x 9

x x 9

x

"~ 2 0 0 r,, 0

o

x ZnZSM-5 9ZnO+ZSM-5

10-

0

[

T

[

1

0,1

0,2

0,3 Zn, (mmollg)

0,4

T. . . . . . . . . . . . . . . . . . . . .

0,5

:

0,6

Fig. 1 Conversion of n-hexane over ion exchanged ZnZSM-5 and mechanical mixtures ZnO+ZSM-5, {T= 420 ~ p=0.1 MPa, WHSV = 2.6 h1 } During testing, the conversion of n-hexane in dependence on the time on both series of samples was practically constant. So the value of arithmetic average of conversion was used for evaluation of the catalysts. In our previous work [18] was shown that the conversion of n hexane at 420 ~ on sodium form of ZSM-5 and also on a pure A1203 was zero indicating that thermal cracking at this temperature doesn't contribute to the n-hexane conversion. On Fig. 1 can be seen that the conversion of n-hexane is relatively high on pure acid ZSM-5. This means that Broensted sites of NH4ZSM-5 are active in carbcation formation fi'om n-hexane via hydride abstraction. After modification of the zeolite with Zn by ion exchange with increasing amount of Zn the conversion at first decreases passes the minimum at about 0.125 mmol Zn/g zeolite and then begins to rise. At concentration 0.35 mmol Zn/g zeolite the conversion of n-hexane reaches the level of that obtained on pure NH4ZSM-5 sample. We can also see that at concentration lower than 0.27 mmol Zn/g zeolite, there are samples with different content of Zn having the similar activity in n-hexane conversion. Different situation can be observed on mechanical mixtures. Before testing mechanical mixtures were pre- activated at 480 ~ for tree hours. This procedure was chosen on the basis of our previous results published in [20]. In this work was investigated the influence of the temperature (450,500,550 ~ and the time ofpre-treatment (1,3,6, and 10.5 hours) on the activity/selectivity of the mechanical mixture ZnO+ZSM-5 containing 0.35mmol Z n / g in aromatisation of n-hexane. Results showed that at temperatures 450 and 500 ~ the activity/selectivity of the catalyst slightly changed with the time of activation but remained constant aJter pretreatment exceeding 3 hours. As can be seen on Fig. 1 mechanical mixtures

859 containing different content of ZnO have practically the constant catalytic activity. We have previously observed that on mechanical mixtures of ZnO with non-acidic carder as alumina [18] or NaZSM-5 [21] the conversion of n-hexane at 420 ~ was practically zero. This proved that ZnO as a separate phase at this reaction temperature is not active in n-hexane conversion. The constant activity of our mechanical mixtures with different content of ZnO confirms that ZnO cannot be an active component in these catalysts. On Fig.1 can be also observed that the level of n hexane conversion on our mechanical mixtures is lower comparing with that on pure acid NH4ZSM-5 and is comparable with the level of conversion reached on two ion-exchanged samples containing 0.085 or 0.123 mmolZn/g. The decrease ofn-hexane conversion over the mechanical mixtures comparing with pure acid zeolite on the level reached on ion-exchanged samples with low content of Zn indicates that the catalytic performance of mechanical mixtures is a result of some interaction between ZnO and the zeolites. Selectivity to aromatics in conversion of n-hexane in dependence of the amount of Zn in samples are presented in Fig.2 Results demonstrated on Fig.2 show that over ion exchanged samples in contrary to the conversion with increasing amount of Zn the selectivity to aromatics continuously raises and 60-

5O v

W

.2 40 t~

x x

E

x ZnZSM-5

x

9ZnO+ZSM-5

0L .

m 30 0

._> 20 X

0

9

9

9

9

,,=,=

10

0

0,1

T

1

0,2

0,3

1- ...................

0,4

T. . . . . . . . . . . . . . . . . . . . . . . .

0,5

0,6

Zn, (mmollg) Fig.2 Selectivity to aromatics in n-hexane conversion over ion exchanged ZnZSM-5 and mechanical mixtures ZnO+ZSM-5 {T= 420~ p= 1 MPa, WHSV=2.6 h ~} on Zn containing samples is evidently higher of that on pure acid ZSM-5. The selectivity to aromatics over mechanical mixtures analogous to the conversion is constant independently

860 on the content of ZnO in the mixture and reached the level of ion exchanged sample containing 0.085 mmolZn/g. The composition of aromatic fraction of the products are given in Table 2.As can be seen the prevailing aromatics are benzene toluene and xylenes. As conceming the BTX fraction composition we can see that at low Zn concentration in the zeolite in contrary to the acid ZSM-5 toluene has the highest concentration in BTX fraction. Its concentration in BTX fraction continuously decreases with increasing amount of Zn in the zeolite and simultaneously raises the concentration ofxylenes. On samples containing >0.2 mmol Zn/g the order of the concentration of the components in BTX fraction changed and is xylene> toluene > benzene and the composition of BTX fraction is very close to the thermodynamic equilibrium[22]. Table 2 C omp0sition of aromatics conv. arom. BTX*/arom. Sample .... % .... NH4ZSM-5 2.06 0.84 0.07ZnZSM-5 3.28 0.87 0.17ZnZSM-5 3.40 0.88 0.18ZnZSM-5 4.2 0.87 0.25 ZnZS M-5 4.36 0.86 0.30ZnZSM-5 6.95 0.88 0.4ZnZSM-5 10.41 0.83 0.46ZnZSM-5 12.93 0.82 0.54ZnZSM-5 17.26 0.81 0.7 ZnZSM-5 23.92 0.76 ZnO+ZSM-5 3.14 0.85 * BTX = (B)enzene, (To)luene, (X)ylenes

B/BTX

To/BTX

X/BTX

0.12 0.10 0.15 0.18 0.21 0.18 0.18 0.18 0.19 0.18 0.13

0.39 0.52 0.49 0.48 0.45 0.43 0.38 0.38 0.37 0.36 0.43

0.49 0.38 0.36 0.36 0.37 0.37 0.44 0.43 0.44 0.46 0.44

For the atypical activity/selectivity relationship in n-hexane aromatisation on ion-exchanged samples as follows from Fig.1 and Fig.2 we have not an unambiguous explanation. More detailed study can be made for elucidation of this observation. But one of the possible reasons could be a different position of the zinc cations in the zeolite framework in dependence on the concentration of zinc. At low concentration Zn cations probably don't contribute to the n-hexane activation and the conversion decreases with decreasing amount of Broensted acid sites in samples. The continuos increase of selectivity to aromatics with the increase of content of Zn in ionexchanged samples indicate that Zn at low concentration supports only the reactions of dehydrogenation of oligomerized products. Zn species probably due to their localization begin to activate n- hexane only at higher concentration and the conversion increases. Whether the activation occurred via dehydrogenation of n-hexane or hydride abstraction is still the subject of discussion. As concerning the mechanical mixture, at our reaction conditions ZnO seems to be inactive in n-hexane aromatisation. The activity/selectivity of the mechanical mixtures is comparable to the activity/selectivity of the ion exchanged sample containing 0.085 mmol Zn/g. We assume that the catalytic performance of the mechanical mixture is probably a

861 result of solid-state ion exchange of Zn from ZnO into zeolite. The fact that the conversion is constant and doesn't depend on the content of ZnO indicates that the degree of this exchange is limited. From the similar activity/selectivity of mechanical mixtures to the activity /selectivity ofionexchanged sample with low content of Zn can be deduced that the proposed SSIE proceeds to a relatively low degree Our preliminary hypothesis is that the extent of interaction of Zn species is probably not controlled by the concentration of ZnO in the mixture but by the concentration of cationic sites accessible for the migrating Zn species. They can be Zn2+(need two sites in suitable distance, ZnOH + at high temperatures hardly probable) or some clusters of the types as (ZnOZn) 2+ forming from ZnO at higher temperatures. These clusters can be too large to migrate inside the zeolite channels and accessible sites for them are only the sites on the extemal surface of the zeolite. So in this case the distribution of A1 in zeolite crystal and the crystal sizes can play an important role on the extent of expected SSIE. 4. CONCLUSIONS The presented results showed that activity/selectivity of ZnZSM samples prepared by ion exchange in n-hexane aromatisation significantly depend on the amount of Zn. While the selectivity to aromatics in comparison with pure acid form with increasing amount of zinc continuously raises the conversion at first decreases, passes the minimum at concentration roughly 0.125mmol of Zn/g and than begin to rise. This phenomenon we suppose is connected with the localization of the zinc in the zeolite. At low concentration Zn species have low activity for n -hexane activation but support reaction of dehydrogenation of oligomerized products. . The activity/selectivity of mechanical mixtures with a different content of ZnO were constant and were on the level of ion-exchanged sample containing 0.085 mmolZn/g. So we suggest that ZnO is an inactive component and in mechanical mixtures SSIE of Zn from ZnO proceeds during thermal pre-treatmen and Zn in cationic positions are responsible for the catalytic performance of mechanical mixtures. From the results followed that the extent of proposed SSIE is rather low and is not controlled by the content of ZnO in mechanical mixtures but probably by the zeolite crystals and surface characteristics. Results demonstrated that the roll of zinc in cationic position of ZSM-5 zeolite in n-hexane aromatisation at 420 ~ depends on the concentration of Zn in the zeolite and also showed that ZnO species at 420 ~ are inactive in n-hexane aromatisation.

REFERENCES

1.J.Kanai, Succesfull Design of Catalysis, Elsevier Science Publishers, Amsterdam, 1988 2. M.Guisnet, N.S.Gnep, Applied Catalysis A:General, 146 1 (1996) 33 3.K.M.Dooley, G.L.Price, V.I. Kanarizev, V.I.Hart, Catalysis Today, 31 3-4 (1996) 305 4. R.LevanMao, Y.Jianhua, L.A.Dufrense, R.Carli, Catalysis Today, 31 3-4 (1996) 293 5. M.Guisnet, N.S.Gnep, Catalysis Today, 31,3-4, (1996) 275 6. J.Hal/~sz, Z.Konya, A.Fudala, A B6res, I.Kiricsi, Catalysis Today, 31 3-4, (1996) 293 7. P.Meriandeau, G. Sapaly, C.Naccache, Proceed. ot the Inter. Symp. on Chemistry and

862 Microporous Crystals, Tokyo, June 26.-29. (1990) 267 8. J.Kanai, N.Kawata, Journal of Catalysis 114 (1988) 284 9. A,.Hagen, F. Roesner, Zeolites and Microporous Crystals, (1994) 313 10. F.Roessner, A.Hagen, M.Mrocsek, H.G.Karge, K-H Steiberg, Procc. 10thInt. Cong.on Catal. 12.-24 July, (1992), Budapest, 1707 11. J.Heemsoth, E Tegeler, F.Roessner, A.Hagen. Microporous and Mesoporous Materials,46(2001) 185 12. L.Wei, Z, Gui, H.S.Din~, X.T.Zhang, H.Y. Li, L.Song, Z.L.Sun, L.V.C. Rees, Stud.Surf.Sci. Catal.13u' Intemational Zeolite Conference, Montpellier, July 18-13, (2001)24-P-31 13. J.Z.Gui, H.S. Ding, N.N. Liu, Y.R.Gao, Z.L.Cheng, X.T.Zhang, B.Ma, L.Song, Z.L.Sun, L.V.S.Rees Stud.Surf.Sci. Catal.13th International Zeolite Conference, Montpellier, July 18-13, (2001) 28-0-02 14. A.Matsuoka, J.B.Kim, T.Inui, Microporous and Mesoporous Material, 35-36 (2000) 89 15. J.Kanai N. Kawata, Applied Catalysis 55 (1989) 115 16. N.Wiswanadhan, A.R.Pradhan, N.Ray, S.C.Vishnoi, U.Shanker, T.S.R.Prasada Rao, Applied Catalysis A: General 137(1996) 225 17.Y. Ono, H. Nakatani, H.Kitagawa, E.Suzuki, Successful Design of Catalyst. (1988) Elsewier Science Publishers, B.V.Amsterdam, 279 18. E.Rojasov~i, A.Smiegkov~i, P.Hudec,Collect. Czech.Chem.Commun 64 (1999) 168 19.E.Rojasov~, A.Smie~kov~i,P.Hudec, Zidek, React.Kinet.Catal.Lett. 66 (1999) 91 20. E.Rojasov~i, A.Smiegkov~i, P.Hudec, ~.idek Collect. Czech.Chem.Commun. 65 (2000) 1506 21. E.Rojasov~i, A.Smie~kov~, P.Hudec, ~idek, Stud. Surf. Scie. Catal.125 (1999) 441 22. Alberty R.A. Ind.Eng.Chem.Fundam. 28 (1988) 211

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

863

Simultaneous desulfurization and isomerization of sulfur containing n-pentane fractions over Pt/H-mordenite catalyst J. Hancs6k a, A. Hol16 b, I. Valkai b, Gy. Szauer b, D. Kall6 c

a Department of Hydrocarbon and Coal Processing, University ofVeszpr6m, Veszpr6m, P.O.Box 158, H-8201, Hungary b Division of Production and Trade, M O L - Hungarian Oil and Gas Co., Sz~tzhalombatta, P.O.Box 1, H-2443, Hungary c Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, Budapest, P.O.Box 17, H-1525, Hungary A new field of the catalytic application of Pt/H-mordenite catalysts is presented for production of gasoline blending components having high octane number and reduced sulfur content (<10-50 ppm) required for satisfying environmental and human health regulations. Experiments were carried out over catalysts containing 0.3-1.0% platinum on H-mordenite of various acidity and on Pt, Pd/H-Y zeolite. The n-pentane fractions containing 61-184 ppm sulfur in the form of COS, CS2, beside Me-, Et-, i-Pr-thiol were converted at 200-280~ 3040 bar, 0.8-2.0 h ~ liquid hourly space velocity and hydrogen to hydrocarbon mole ratios of 1:1-2:1. Under optimum process parameters the concentration of all sulfur compounds in the product was less than 1-2 ppm using the catalyst containing 0.6% platinum, preserving its strong metal function. During hydrodesulfurization the isomerization of n-pentane took place simultaneously with high conversion and selectivity approaching the thermodynamic equilibrium to 85-98%. 1. INTRODUCTION The environmental regulations of production, storage and use of motor gasolines are extending. Accordingly the limitation of the sulfur content of gasolines will decrease from 150 ppm to 50 ppm in the European Union from 2005, but the availability of fuels having less than 10 ppm sulfur content will also be obligatory [1-3]. The reduction of sulfur content not only contributes to the direct decrease of pollution, but also enables the application of advanced aftertreatment catalysts of low sulfur resistance [4]. In addition, reduction of the aromatic content of spark ignition engine fuels also results in octane number deficiency in the gasoline pool. Demand of high octane number and non-aromatic blending components of low sulfur content is to be satisfied. Besides alkylates (the production of which involves several ecological problems) and oxygenates (the application of which is also limited by regulations), isomerates are the most favorable blending components without the limitations mentioned above. Refiners in the European Union are allocating the greatest portion of their investment budget for the installation of new isomerization plants [2]. The sulfur concentration of fractions rich in n-paraffins used for the production of isomerates varies between 60 and 150

864 ppm, depending on their origin. In isomerization units using sulfur resistant catalysts of different compositions the isomerization of fractions rich in n-pentane proceeds without significant hydrodesulfurization. However, the isoparaffin content of the isomerates produced from such sulfur containing feeds is 1-5 absolute % lower, and the favourable reactor temperature is 3-10~ higher, than in the case of practically sulfur-free feeds. On traditional metal/zeolite isomerization catalysts the extent of desulfurization is low, tipically less than 10%, and low sulfur isomerates can not be produced this way. Therefore either desulfurization of the feedstock is needed or an isomerization catalyst should be developed which is suitable for simultaneous desulfurization and skeletal rearrangement. Some studies were reported on attempts to carry out these transformations with heavier hydrocarbons [5,6]. The objective of the authors of the present work was to select a catalyst appropriate for the hydrodesulfurization and skeletal isomerization of n-pentane fractions in one step. 2. EXPERIMENTAL 2.1 Materials Catalysts on different supports containing various amounts of metals were investigated. Two types of catalysts were found applicable for simultaneous desulfurization and isomerization of sulfur containing n-pentane fractions, namely Pt,Pd/H-Y,FAU and Pt/HMOR. This presentation mainly deals with the latter type of catalysts. The physical and chemical properties of these catalysts were partly described in [7]. The H-mordenite was prepared from the NHn-form supplied by Union Carbide Co., Linde Division. The Si/A1 ratio was adjusted to 20.5-24.2 with dealumination. Platinum was loaded on the zeolite by ion exchange with [Pt(NH3)4]C12. The H-mordenite contained Pt in concentrations between 0.3 and 1.0%. The binder was ~,-alumina. After preparation the catalysts were calcined in flowing air. Metal dispersion of the catalysts was 74-95%. The BET surface areas were between 420490 m2/g. The total acidity was 0.65-0.90 meq NH3/g(Table 1). The Pt,Pd/H-Y,FAU catalyst contained 0.2% Pt, 0.25% Pd and the Si/A1 ratio was 2.0. The feeds (Table 2) consisted of 88.6-90.1% n-pentane, 9.6-11.3% iso-pentane, 0.10.3% hydrocarbon impurities and 61-184 ppm sulfur in the form of COS, CS2, Me-, Et-, i-Prthiol. These were hydrocarbon mixtures of product streams from MOL Co. (Hungarian Oil and Gas Company) Danube Refinery and individual compounds. The amount of iso-propyl thiol was one-two orders of magnitude higher than that of other sulfur compounds (carbonylsulfide: 0.05-3 ppm; carbon-disulfide: 1-5 ppm, methyl-thiol: 10-28 ppm, ethyl-thiol: 19-71 ppm, isopropyl-thiol: 104-286 ppm). 2.2 Apparatus Experiments were carded out in an equipment comprising a flow reactor free of back mixing and practically all of the equipments and accessories like in a producing plant for light naphtha isomerization [8]. 2.3 Methods Before starting the experiments a 100 cm 3 catalyst (weighing 72.7 to 73.6 g depending on its metal content) was loaded into the isothermal section of the reactor. Each catalyst charge was dried and activated as reported earlier [7]. Thereafter, the hydrogen pressure was

865 Table 1 Main characteristics of Pt/H-MOR catalysts Properties Pt-content, % Si/A1 ratio Pt-dispersion, % Acidity, meq NH3/g

,,I" 0.3 20.5 95.0 0.90

,,II" 0.4 20.5 91.5 0.84

Catalyst ,,HI" ,,IV" 0.5 0.6 21.1 21.9 88.0 84.0 0.81 0.78

,,V" 0.75 22.4 81.0 0.74

,,VI" 1.0 24.2 74.5 0.65

Table 2 Main properties of feeds Properties Total sulfur contentl ppm Hydrocarbon composition, % n-butane iso-pentane n-pentane i-C6 Water content, ppm Research octane number(RON)

,,A" 61

,,B" 94

0.05 8.6 91.1 0.25

0.05 8.6 91.1 0.25

Feed type ,,C" ,,D" 119 151 0.1 0.1 8.7 8.8 91.0 91.0 0.2 0.1 (--15-20--)

,,E" 184

,,F" 180

0.1 8.7 91.0 0.2

0.1 11.3 88.6 0.0

increased and the hydrocarbon feed started. The experiments were carried out in continuous operating mode with each feed, over catalyst of stable activity. A small portion of the output directly after the reactor was continuously transferred into the heated sampling unit of a gas chromatograph (GC) without separation of liquid and gas phases, by means of a specially designed and thermostated control assembly. The reproducibility of experiments was better than 95% summing up the errors of catalytic measurements and GC analyses. The hydrocarbon composition of the feeds and products was determined according to ASTM D 5143-98 standard, the concentration of some sulfur compounds according to UOP791 standard using GC equipped with Sievers SCD, Sulfur Chemiluminescence Detector, and the total sulfur content according to ASTM D 4045 standard. Platinum content of the catalyst was determined according to UOP-274 standard platinum dispersion of each sample as described earlier [9]. The silicon and aluminum contents of the catalysts were determined according to UOP-303 and UOP-873 standards, respectively. The change of the lubricity of motor gasolines in consequence of the reduction of the sulfur content was measured according to the modified ISO 12156-1 standard.

3. RESULTS AND DISCUSSION On the basis of preliminary experiments the sulfur containing n-pentane fractions (Table 2) were investigated on the selected catalysts (see Table 1) at temperatures 200-280~ total pressures 30-40 bar, hydrogen to hydrocarbon mole ratios of 1:1-2:1 and liquid hourly space velocity of 0.8-2.0 h -1. Within these ranges of parameters the values ensuring highest iso-pentane content and lowest sulfur content in the products were determined for all

866

catalysts and for different sulfur contents in the feeds. From the numerous data only the results with the feedstock of lowest and highest sulfur content, at the lowest space velocity, highest pressure and hydrogen/hydrocarbon ratio, were selected for presentation, because these process parameters were found to be most favorable during the preliminary experiments for all catalysts, within the investigated temperature range. The results for the catalyst, most suitable for the one step isomerization and hydrodesulfurization of n-pentane fractions with different sulfur content will be discussed in detail. As examples, Figures l a and l b show the degree of approach to thermodynamic equilibrium concentrations of iso-pentane (hereinafter DATEC which is defined by equation 1), Figures 2a and 2b the changes of sulfur content in the products, and Figures 3a and 3b the amount of C1-C4 hydrocarbons produced through secondary side reactions like cracking as function of temperature, over catalysts containing 0.3-1.0% platinum and in case of feeds containing 61 and 184 ppm sulfur, respectively.

DATEC

C

T =

i-CsP~~T *100

(1)

Ci-CsE,T where iso-pentane concentration in the i- and n-C5 mixture of the product at T temperature C i-c5 E ,r - iso-pentane concentration in the equilibrium i- and n-C5 mixture at T temperature C i _ C s P r T --

The curves on Figures l a-2b show that both isomerization and hydrodesulfurization take place, but to different degree, depending on platinum content as well as on other characteristics of catalysts, sulfur content of feeds and temperature.

J lOO [

Pressure:40 bar; LHSV:0.8 h-l; Hz/Hydrocarbon mole ratio: 2:1 100 Sulfur in feed: 184 ppm /

Pressure:40 bar; LHSV:0.8 hl; Hz/Hydrocarbon mole ratio: 2:1 Sulfur in feed:61 ppm A ~

80] Pt-contentofcatalyst,% ~ / / ~

o~ 80

-- + 0 . 3 ....m 0.4 .&:~2 .. 0.5

- - 0 4

.......

i

60

40 Sulfur in feed <0.5ppm:---o--(catalyst Pt-content:0.6 ~)

20

o

o,

.....

, , ,

.....

Pt-content of catalyst, ~ ~

,.............. ,

,,

,

i

i

|

075 ff//~_/~/

Sulfur in feed <0.5ppm: n (catalyst Pt-content:0.6%)

-

190 200 210 220 230 240 250 260 270 280 290 TEMPERATURE, ~

x

190 200 210 220 230 240 250 260 270 280 TEMPERATURE, ~

Figure 1a Figure 1b The degree of approach to thermodynamic equilibrium concentrations of iso-pentane as a function of temperature over catalysts of various platinum contents

867

40

Pressure:40 bar; LHSV:0.8 h"; HJHydrocarbon mole ratio: 2:1 Sulfur in feed: 61 ppm

4k ~,~

35 ~ 3o

! [ [

10

......... f

Pressure:40 l~ar; LHSvi0~I8 ia:l;................. H2/Hydrocarbon mole ratio: 2:1 e ~ in ed: 184 ppm Pt-content of catalyst, %

_ 120

~_ . _ ~ C).4

-+-0.3

---II-- 0.4 ~0.5 0.6

N. o 80 ~ 60

----X-- ~.6 + D.75 --O-- 1000

"-)K"--0.7 5

~,~ 4020

Pt-content of catalyst, % !

~ 20

180 ~ 160} 140

-*-

5 0

....

I

I

1

I

|

I

1

I

I

1

1

170 180 190 200 210 220 230 240 250 260 270 280 TEMPERATURE, ~

0

I

1

.........

I .............

170 180 190 200 210 220 230 240 250 260 270 280 TEMPERATURE, ~

Figure 2a Figure 2b The changes of sulfur content in product as a function of temperature over catalysts of various platinum content It is well represented by Figures l a and l b that isomerization practically does not take place at 200-210~ on either of the catalysts, because of DATEC values equal to those calculated from i-pentane content of the feed. This conclusion is valid for all feeds and any sulfur contents. With increase of the temperature of DATEC values for both feeds are fitted to S shaped curves with maximum at high temperature. First, reaction rate increases namely with temperature resulting in higher conversion, and on approaching to the thermodynamic equilibrium the conversion decreases due to the decrease of equilibrium concentration being isomerization an exothermic reaction. The best DATEC values were obtained at 260-270~ with the catalyst containing 0.6% platinum. These DATEC values were only 1-3 absolute percent lower than the approach to the equilibrium using sulfur-free feed (Figures 1a and 1b). Since isomerization proceeds on dual function catalysts, there exists an optimum ratio of acid and metal sites, where the highest activity is exerted. Lower acidity of the catalysts with 0.75 and 1.0% platinum resulted in lower rates of the isomerization of n-pentane (feed "A"; Figure l a) and DATEC values of these are lower than those of the catalysts of 0.4 and 0.5% platinum content having higher acidity in the presence of fewer metal sites. During the isomerization of n-pentane the hydrodesulfurization of different sulfur compounds (carbonyl-sulfide, carbon-disulfide, thiols) took place simultaneously with high conversion and selectivity (see Figures 2a, 2b and 3a, 3b). The concentration of all individual sulfur compounds in the isomerates produced under favorable conditions from the lower sulfur feed was between 0.5 and 2 ppm. The presence of CS2 was clearly detectable even in the products of lowest sulfur content, while Me-SH was not present, or had the lowest concentration among the not converted sulfur compounds. Fan shaped curves were obtained when the effect of platinum content and temperature was investigated for both n-pentane fractions having both low and high sulfur contents. The projected center point of these curves was the catalyst of 0.6% platinum content. The isomerates of lowest sulfur contents were produced at the highest temperature (280~ and on the catalyst of highest platinum content. This means that strong metal function prevails in deep desulfurization. Using the low sulfur feed (61 ppm) and temperature exceeding 260~ the

868 sulfur content of the product can be reduced below 10 ppm on all catalysts, while for the high sulfur feed (184 ppm) having 0.75 and 1.0% platinum and temperatures of 260-280~ are required. The curves in Figures 3a and 3b show concentrations of C1-C4 hydrocarbons in the product as function of the temperature. The extent of cracking increased with catalyst acidity and temperature. A metal function prevails since higher sulfur content of the n-pentane fractions reduces C1-C4 hydrocarbon formation, i.e. cracking. Summarizing the previous detailed evaluation we concluded that among the tested catalysts the best results can be obtained with H-MOR of 0.6% platinum content considering isomerization, desulfurization and undesired cracking of pentane feeds both of low and high sulfur contents (feeds "A" and "E"). At the favorable combination of the process parameters (260-270~ 0,8 h -1 liquid hourly space velocity; 40 bar total pressure; 2:1 hydrogen/hydrocarbon molar ratio) the exact properties of the products obtained on this catalyst are as follows: DATEC: 95.4-98.1% and 92.3-96.1%; degree of desulfurization: 96-98% and 7388%; C~-C4 concentration: 4.9-9.3% and 5.8-11.7% for the low and high sulfur feeds, respectively. Considering the experimental results gained at 265~ on the H-mordenite catalyst containing 0.6% platinum, found to be the most active for isomerization and desulfurization in one step, we concluded that i-pentane yield increased with the reduction of space velocity, but the change was greatly effected by the sulfur content of the feed (Figure 4). Using npentane fractions with 60-90 ppm sulfur are typical in refining, and applying liquid hourly space velocity(LHSV) of 1.0-1.5 h ~, which is also acceptable in industrial practice, the achieved i-pentane yield was 61.5-63%. At these feeds, when the pressure was changed from 30 to 40 bar, there was no significant difference between the extent of desulfurization and isomerization. Lowering the space velocity, the difference between octane number of the product and feed (ARON) increased on the mentioned catalyst and applied process parameters (Figure 5). This increase, however, was lower with n-pentane fractions of higher sulfur content. Under suitable conditions the ARON was approximately 18-19 points which is a remarkable octane gain. Laboratory tests indicated the need of blending lubricity improvers into the marketed motor gasolines of low sulfur content (10-50 ppm) in order to prevent excessive fuel pump wear [ 10]. 50 ppm dosage rate of lubricity additive was enough to reduce the wear scare of commercial gasoline containing low sulfur isomerate from 900 ~tm to below 300 pm, what is acceptable. Using the low sulfur feeds ("A" and "B") any loss of activity for a period of 240 hours under favorable conditions was not observed. With high sulfur feeds (,,D" and ,,E"), however, a moderate activity loss was apparent. The concentration of iso-pentane in the product decreased by 2-4% in these cases. By treating the deactivated catalyst in hydrogen flow at 350-400~ the original isomerization and desulfurization activity could be restored. Pt, Pd/H-Y, FAU catalysts were also suitable for the simultaneous isomerization and desulfurization of sulfur containing n-pentane fractions. Experiments with these catalysts using n-pentane feeds having 61 and 180 ppm sulfur contents (feeds ,,A" and ,,F"; Table 2) under the most favorable conditions (270-280~ 40 bar, 1-2 h -1 LHSV, 2:1 hydroger~ydrocarbon molar ratio) resulted in excessive cracking (28-36%) of the hydrocarbons. The strong acidity of the catalysts is responsible for this undesired conversion. Consequently, in spite of the relatively high iso-pentane concentration (51-56%) and low sulfur content (3-9 ppm) of

869

Pressure:40 bar; LHSV:0.8 h l ' H2/Hydrocarbon mole ratio: 2:1 Sulfur in feed: 61 ppm Pt-content of catalyst, %

18 16 o "= 14 t~

o~

o~ 12 ~9 10

= 10 O o 8 O

d,

o

6

Sulfur in feed <0.5ppm:---B--

4

(catalyst Pt-content:0.6%'

6

d

d"

Pressure:40 bar; LHSV:0.8 hl; H2/Hydrocarbon mole ratio: 2:1 Sulfur in feed: 184 ppm

/~

//// ////

Pt-content of catalyst, %

-~0.3 ~0.4 ~0.5 - - x - - 0.6 ~0.75

, .........

190 200 210 220 230 240 250 260 270 280 TEMPERATURE, ~

190 200 210 220 230 240 250 260 270 280 TEMPERATURE, ~

Figure 3a Figure 3b The changes of C1-C4concentration in products as a function of temperature over catalysts of various platinum contents

the liquid products, this type of catalyst is not suitable for the economical simultaneous isomerization and desulfurization of n-pentane fractions in industrial scale.

20

65 60

~9 (D

---I,--61 ~-I~,- 94

55

---iI~- 119 --13--151

o

1~119

--,~, 184

5o

I ---t3-- 151 t ~ !.84

Sulfur content, ppm

& 45 40

<~ 5-

Temperature:265~ Pressure: 40 bar; H2/Hydrocarbon: 2:1 mole/mole

Temperature:265~ Pressure: 40 bar; H2/Hydrocarbon: 2:1 mole/mole ,,

0,5

,

|

1

. . . . . . . .

,. . . . . . . . . . .

1,5

,

,

2

2,5

.

-

0

3

LHSV, h l

Figure 4 The changes of iso-pentane yields as a function of LHSV over catalysts containing 0.6% platinum

'

,'

,

1

,

,

0,5

1

1,5

2

2,5

3

LHSV, h l

Figure 5 The changes of A RON (producti-feedi) as a function of LHSV over catalysts containing 0.6% platinum

870 4. CONCLUSIONS H-mordenite catalysts of different platinum content and acidity are suitable for the simultaneous isomerization and desulfurization of sulfur containing n-pentane fractions. An explanation of the high sulfur tolerance of the investigated Pt/H-MOR catalysts may be that the acidic sites of the zeolite (both Bronsted and Lewis sites) reduce the electron density of the supported platinum metal. Thereby the bonding strength between the platinum metal and sulfur is weakened. The ratio of metal and acid sites significantly affects the extent of isomerization and desulfurization. The HDS reaction on the investigated catalysts was relatively fast, especially on catalysts of high Pt content (0.6-1.0%). The isomerization reactions on the investigated catalysts took place at an economically acceptable rate only at 260-270~ Both isomerization and desulfurization result in a gasoline blending component having a RON increased by 17-19 points and a sulfur content between 5 and 10 ppm.

Acknowledgements The authors gratefully acknowledge the financial support from the Ministry of Education and from MOL Co.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

H.-J. Bautsch, Proc. of 2 nd European Fuel Conf., Vienna, 2001. J.-F. Lariv6, Hydrocarb. Eng., 6 (2001) 15. S. Dixson-Decleve, Proc. of 3rd Int. Coll. on Fuels, Esslingen, 2001, 33. G. Margaria, Proc. Suppl. of 2 nd European Fuel Conf., Vienna, 2001. M.A. Arribas, F. Mfirquez, A. Martinez, J. Catal., 190 (2000) 309. L.B. Galperin, Stud. Surf. Sci. Catal., 130A (2000) 257. J. Hancs6k, A. Hol16, J. Debreczeni, J. Perger, D. Kall6., Stud. Surf. Sci. Catal., 125 (1999) 417. 8. A. Hol16, J. Hancs6k, D. Kall6, Stud. Surf. Sci. Catal., 135 (2001) 26-P-19. 9. J. Hancs6k, G. Gfirdos, M. Baumann, Hung. J. Ind. Chem., 17 (1989) 131. 10. D.P. Wei, H.A. Spikes, S. Korcek., Tribology Transactions, 42 (1999) 813.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Propylene cocatalyst

polymerization

using

871

various

metal-containing

MCM-41

as

T. Miyazaki a, Y. Oumi a, T. Uozumi b , H. Nakajima c, S. Hosoda c and T. Sano a a School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan, E-mail: [email protected], Fax: +81-761-51-1625 b National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan c Sumitomo Chemical Co., Ltd., Sodegaura, Chiba 299-0295, Japan Polymerizations of propylene were conducted with rac-ethylene(bisindenyl)zirconium dichloride (rac-Et(ind)2ZrC12) with triisobutylaluminum (Al(i-CaH9)3) using various metal-containing MCM-41 (Metal-MCM-41). The catalyst system gave selectively isotactic polypropylene. The order of the cocatalytic activity of Metal-MCM-41 (Si/Metal=ca.13) was as follows: GaMCM-41>ZnMCM-41>A1MCM-41>>TiMCM-41. The linear relationship between the polymer yield and the number of strong Lewis acid sites was observed. This strongly suggests that not weak Lewis acid sites but strong Lewis acid sites generated on Metal-MCM-41 can activate the metallocene compound effectively, resulting in the formation of active sites for propylene polymerization.

1. INTRODUCTION Since the discovery of ordered mesoporous materials with large pore diameters (2-10 nm) such as MCM-41, FSM-16 and SBA-15 has opened up new possibilities in the field of heterogeneous catalysis and adsorption [ 1], a great deal of effort has been devoted to the incorporation of heteroatoms into their frameworks. The incorporation of heteroatoms such as A1, Ti and Ga is achieved by the direct synthesis or the post-synthesis [2-8]. The post-synthesis method is performed by the reacting the surface silanol groups present on the inner wall surfaces with various metal alkoxides or chlorides in nonaqueous solution followed by calcination, and does not cause the serious structural deformation of the resulting materials. We have also studied the post-synthesis alumination of MCM-41 and SBA-15 using organoaluminum compounds. It was found that the aluminum could be easily incorporated into their frameworks and that a majority of acid sites generated on the mesoporous materials are Lewis acid sites [9,10]. In general, it has been known that the active species in the metallocene catalyst system for olefin polymerization is a coordinately unsaturated transition metal cation and is stabilized by bulky counter-anions such as methylalumoxane (MAO) and triphenylcarbenium tetrakis(pentafluorophenyl) borate, which exhibit Lewis acidity [ 11,12]. More recently, we found that Lewis acid sites on the aluminum-containing MCM-41

872 prepared by a post-synthesis alumination with trimethylaluminum are able to activate the metallocene compound effectively, resulting in the formation of active species [13]. However, an influence of the Lewis acidity of MCM-41 on the polymerization behavior of metallocene catalyst is not investigated in detail. In this paper, therefore, we tried to prepare various metal-containing MCM-41 and applied to the metallocene catalyst system for propylene polymerization.

2. EXPERIMENTAL

2.1. Preparation and characterization of MetaI-MCM-41 Siliceous MCM-41 was prepared following the procedure described in the literature [ 14]. The MCM-41 prepared was calcined at 500~ for 10 h to decompose the surfactant (hexadecyltrimethylammoinium bromide) and obtained the white powder. This white powder was used as the parent MCM-41. Various metal-containing MCM-41 (Metal-MCM-41) were prepared by the post-synthesis treatment of the MCM-41 using AI(CH3)3, Zn(C2Hs)2, Ga(CH3)3 and Ti(OCaH7)4. Preparation of Metal-MCM-41 by the post-synthesis treatment was carried out as follows. 1 g of MCM-41 dried at 280~ for 24 h under vacuum was dispersed in 10 ml of dry toluene containing 1-9 mmol of organometallic compound or alkoxide. The mixture was maintained for 48 h without stirring at room temperature (AI-, Zn-, GaMCM-41) or at 115~ (TiMCM-41) [ 15]. The Metal-MCM-41 obtained was filtered, washed with dry toluene several times, dried at room temperature and then calcined at 500~ for 5 h. Identification of the products was carried out by X-ray diffraction (Rigaku, RINT2000). Elemental composition was determined by X-ray fluorescence (Philips, PW2400). Textural properties (BET surface area, pore diameter, pore volume) were evaluated by nitrogen adsorption at-196~ (Bel Japan, Belsorp 28SA). The nature of acid sites of the Metal-MCM-41 was investigated using pyridine as a probe molecule. The sample (ca. 20 mg) was pressed into self-supporting wafers and placed into the FTIR cell allowing heating under vacuum up to 900~ Prior to adsorption the sample was evacuated to ca. 10-3 Pa at 700~ for 2 h. The sample was then cooled to 150~ where nearly 1.3 x 102 Pa of pyridine was introduced into the FTIR cell. After 1 h of adsorption, the excess and weakly adsorbed pyridine was removed by evacuation at the same temperature for 30 min. FTIR spectra of chemisorbed pyridine on the Metal-MCM-41 were measured using a JEOL JIR-7000 spectrometer with a resolution of 4 cm -1.

2.2. Polymerization of propylene Polymerization of propylene was conducted in a 100 cm 3 stainless steel autoclave equipped with a magnetic stirrer. After measured amounts of toluene as a solvent, triisobutylaluminum (TIBA) as an alkylating agent, Metal-MCM-41 evacuated at 700~ for 2 h and rac-ethylene(bisindenyl)zirconium dichloride (rac-Et(Ind)2ZrC12) were added to the reactor, the reactor was evacuated at liquid nitrogen temperature, and then 7 dm 3 of propylene were introduced. Polymerization was started by quickly heating the reactor up to the polymerization temperature (40~ The polymerization reaction was terminated by adding acidified methanol (5wt% HC1). The precipitated polymers were adequately washed with methanol and dried under vacuum at 60~ for 6 h.

873 The resulting polymers were extracted with boiling o-dichlorobenzene for 8 h. The weight-average molecular weight (Mw) and molar mass distribution (Mw/Mn, Mn: number-average molecular weight) of the polymers were measured at 145~ by gel-permeation chromatography (GPC, Senshu Scientific SSC7100) using o-dichlorobenzene as a solvent. The melting points (Tm) of the polymers were measured on a Seiko DSC-220C calorimeter with a heating rate of 10~ in the temperature range of -40-200~ 13C NMR spectra of the polymers were measured in 1,2,4-trichloro benzene/benzene-d6 (9/1 v/v) at 140~ using a Varian GEM-300 spectrometer operating at 75.4 MHz. 3. RESULTS AND DISCUSSION 3.1. Characterization of MetaI-MCM-41

Fig. 1 shows X-ray diffraction (XRD) patterns and N2 adsorption isotherms of the parent siliceous MCM-41 and typical Metal-MCM-41 samples. The parent MCM-41 exhibited a typical XRD pattern with four peaks that indicates hexagonal structure. All of the Metal-MCM-41 samples gave slightly lower quality XRD patterns than that of the parent MCM-41. The (210) diffraction peak disappeared by metal incorporation. The intensities of other three diffraction peaks ((100), (110), (200)) slightly decreased. The XRD patterns were also found to be free from corresponding crystalline metal oxides. Although a slight reduction in the amounts of N2 adsorbed was observed for the Metal-MCM-41 samples, all isotherms were found to exhibit sharp inflective characteristics of capillary condensation of the relative pressure of ca. 0.3. Table 1 summarizes the BET surface areas, pore diameters and pore volumes of the parent MCM-41 and various Metal-MCM-41. These results indicate that the serious degradation of MCM-41 framework hardly takes place during 800 (B) ,~ [<>Ca) (100) (A) ~'~. 700]n (b)

1

m

(a) (b3

,_2) o)

2

:; ,

23

,

6 7 20(degree)

(e)

8

9

i0

600[zx (c) 5oo [--• (d)

~'~~ 400Ig300(e) (..q

O

'~[~

<>~0 00~O

O O O" I

XX XCKX• X ZI~K ~ ::~( ::~IK X X ~K

200

Z 10 i 0"-0.2 0

I

I

0.4 0.6 P/P0

I

0.8

1.0

Fig. 1 XRD pattems (A) and N2 adsorption isotherms (B) of the parent MCM-41 and various Metal-MCM-41 samples. (a) the parent MCM-41, (b) A1MCM-41(Si/AI=I 3.4), (c) YiMCM-41(Si/Yi=13.4), (d) ZnMCM-41 (Si/Zn=l 2.4), (e) GaMCM-41 (Si/Ga=l 2.4)

874 Table 1 Sample No. 1 2 3 4 5 6 7 8 9 10 11

Characteristics of various Metal-MCM-41 Metal-MCM-41 Parent MCM-41 A1MCM-41

TiMCM-41 ZnMCM-41

GaMCM-41

Si/Metal ratio 5.7 13.4 59.2 13.4 8.9 12.4 33.7 7.5 12.4 63.2

BET surface area Pore diameter a) (ma/g) (nm) 968 2.74 794 2.52 830 2.74 884 2.74 908 2.74 704 2.74 773 2.74 814 2.70 600 2.52 735 2.74 860 2.70

Pore volume (cm3(liquid)/g) 0.88 0.63 0.64 0.66 0.78 0.67 0.65 0.66 0.47 0.62 0.72

a) Calculated by the Dollimore-Heal method. incorporation of metal by the post-synthesis treatment. Next, the amount and type of acid sites generated on the Metal-MCM-41 were investigated using pyridine as a probe molecule. Adsorption of pyridine was carried out at 150~ for 1 h followed by evacuation at the same temperature for 30 min to remove the excess and weakly adsorbed pyridine. Fig. 2 shows FTIR spectra of pyridine adsorbed on various Metal-MCM-41. For A1MCM-41 and GaMCM-41, the strong peak at 1454 cm -1 due to pyridine bound Lewis acid sites as well as the weak one at 1540 cm -1 due to pyridine bound Bronsted acid sites were observed. On the other hand, for TiMCM-41 and ZnMCM-41, the storng peak due to Lewis acid sites was only observed. No peak was observed for the parent siliceous MCM-41. These results indicate that a majority acid sites generated on the Metal-MCM-41 are Lewis acid sites and the amount of Lewis acid

_ _ j ~

,,

L

~

(a).__..

B+L L

<

..................................

1 ...........................................................

1 ....................................................................

1600 1500 Wavenumber (cm- 1)

1400

Fig. 2 FTIR spectra of pyridine adsorbed on various Metal-MCM-41. (a) the parent MCM-41 (Sample No.1 in Table 1), (b) A1MCM-41 (No.3), (c) TiMCM-41 (No.5), (d) ZnMCM-41 (No.7), (e) ZnMCM-41 (No.8), (t) GaMCM-41 (No. 10), (g) aaMCM-41 (No. 11) B and L denote Bronsted and Lewis-bound pyridines, respectively.

875 sites is strongly dependent on the kind of metal incorporated. 3.2. Polymerization of propylene

Polymerizations of propylene were conducted at 40~ for 30 min with rac-Et(Ind)2ZrC12, which shows I isospecificity in ot-olefin [mmmm] polymerization, and Metal-MCM-41 using [l[mmmr] TIBA as an alkylating ~ J ~ m r r m ] _CH 3_ (c) agent. The yield and the analytical data of 23 22 21 20 19 18 polypropylene produced ppm are listed in Table 2. For a reference, the result using MAO as a conventional cocatalyst for the ~___~ ~ ..... (a) homogeneous catalyst (b) system was also listed. The polymer yield was ,,,,--~ , , (d) considerably smaller as 50 45 40 35 30 25 20 15 10 5 compared with the ppm homogeneous catalyst system and strongly Fig. 3 13CNMR spectra ofpolypropylenes obtained using dependent on the Si/Metal (a) A1MCM-41, (b) ZnMCM-41, (c) GaMCM-41 and (d) MAO ratio of Metal-MCM-41. as a cocatalyst. ,

Table 2 Cocatalyst

,

,

|

Results of propylene polymerization using various Metal-MCM-4 la)

Yield Tm Mw Mw/Mn [rnmmm] (g) (~ (xlO -3) (%) Parent MCM-41 trace . . . . A1MCM-41 5.7 1.5 130.5 12 3.7 81 13.4 3.5 129.8 16 3.6 75 59.2 0.8 136.7 20 4.4 80 TiMCM-41 13.4 0.1 135.1 14 4.3 79 ZnMCM-41 8.9 1.9 135.5 18 3.8 79 12.4 4.4 134.2 24 3.9 80 33.7 0.7 137.4 20 3.2 78 GaMCM-41 7.5 2.6 135.4 21 3.7 80 12.4 5.7 129.8 14 3.1 75 63.2 1.6 134.2 16 3.9 77 MAO b) 11.4 121.7 41 2.4 81 a) rac-Et(Ind)2ZrC12=O.01 mmol, Al(from TIBA)/Zr--50, Toluene-30 cm 3, Propylene-7 dm 3 (s.t.p.), Temp.=40~ Time=30 min. b) rac-Et(Ind)2ZrCl2=O.O01mmol, Al(from MAO)/Zr-1000, Toluene-30 cm 3, Propylene-7 dm 3 (s.t.p.), Temp.=40~ Time-30 min. i

Si/Metal ratio

876 Namely, the polymer yield increased and passed through a maximum value at ca. 13 of the Si/Metal ratio and then decreased again. The order of cocatalytic activity of Metal-MCM-41 was as follows when the Si/Metal ratio was ca.13: GaMCM-41>ZnMCM-41>A1MCM-41>> TiMCM-41. The use of the parent siliceous MCM-41 in place of the Metal-MCM-41 gave no polymer. Therefore, the difference in the cocatalytic activity of Metal-MCM-41 seems to be attributable to the difference in the acidity of Metal-MCM-41, probably the acidic strength. The characteristics of polypropylenes produced were hardly dependent on the kind of Metal-MCM-41 and its Si/Metal ratio. However, there were slight differences in Tm, Mw and Mw/Mn between polypropylenes produced using the Metal-MCM-41 and MAO. No difference in isotacticity [mmmm] pentad was observed. These results suggest that ZnMCM-41 and GaMCM-41 as well as A1MCM-41 can activate the metallocene compound and that the stereospecificity of rac-Et(Ind)2ZrC12 hardly changed by activation with Lewis acid sites on the Metal-MCM-41. No change in the stereospecificity of rac-Et(Ind)2ZrC12 was also confirmed from 13C NMR spectra of polypropylenes produced. As shown in Fig. 3, no difference in the microstructure of polypropylene was observed. 3.3. Relationship between cocatalytic activity and Lewis acidity To get further information concerning activation of metallocene compound by the Metal-MCM-41, the relationship between the cocatalytic activity and the Lewis acidity was investigated. Fig. 4 shows the relationship between the polymer yield and the amount of Lewis acid. The amount of Lewis acid sites is normalized based on the peak intensity at 1454 cm -1 in the FTIR spectrum of pyridine adsorbed on the A1MCM-41 (Si/Al-13.4). A clear tendency was not observed. It is recognized that the acid strength of solid acid catalysts such as zeolite and silica-alumina is not uniform, namely there exists a distribution of acid strength. Therefore, the acid strength of the Metal-MCM-41 was roughly evaluated by changing the desorption temperature of pyridine. Namely, pyridine vapor was adsorbed onto the Metal-MCM-41 at 150~ for 1 h and the FTIR spectra was recorded at ,~ 6 10 e various stages of pyridine desorption, which was e7 continued by evaporation at ~ .3 o 4 progressively highly temperatures (150-400~ As shown in Fig. 5, the intensity ~ 2 .11 o of the peak at ca. 1451 4 9 e8 cm-]due to Lewis bound CD 1 5 pyridine decreased gradually 0 ~ e,2 3 4 5 with an increase in the Normalized amount of Lewis acid sites of Metal-MCM-41 evacuation temperature. However, the degree of Fig. 4 Relationship between the polymer yield and the reduction in the peak intensity normalized amount of Lewis acid sites. was markedly dependent on Arabic numbers denote Sample No. in Table 1. the kind of metal incorporated

877

(A) 9

.

BL

I

I,

I

B B+L.~__

(D) B L

BBLj

9

k..__ (d) '~_ (c)

0

<

~__ (b)

t

1600

~(a)

,,i

1 5 0 0 1400

1600

1500 1 4 0 0 1 6 0 0 1500 ]400 Wavcnumber (cm- 1)

1600

1500

i400

Fig. 5 FTIR spectra of pyridine adsorbed on Metal-MCM-41 at various evacuation temperatures. (A) A1MCM-41 (Si/AI=I 3.4), (B) TiMCM-41 (Si/Ti=l 3.4), (C) ZnMCM-41 (Si/Zn=l 2.4), (D) GaMCM-41 (Si/Ga=12.4) Evacuation temp.: (a) 150~ (b) 200~ (c) 350~ (d) 400~ into the MCM-41 framework, suggesting that there exists a distribution of acid strength among the Metal-MCM-41. Fig. 6 shows the plot of the polymer yield against the amount of Lewis acid sites after evacuation at 400~ The amount of Lewis acid sites is normalized based on the

10T g8~ i

i "~4~i

o i

~2~ ,=-, i

..J"

e"3

7 .....------~" 9 ._.J" ..I...I

!

I0

11 I-I.----".....

4 ......~"

0 0.3 0.6 0.9 1.2 Normalized amount of Levds acid sites of MetaI-MCM-41

peak intensity at ca.1451 cm -1 in the FTIR spectrum Fig. 6 Relationship between the polymer yield and the of pyridine adsorbed at normalized amount of Lewis acid sites after evacuation at 400~ 150~ on the A1MCM-41 Arabic numbers denote Sample No. in Table 1. (Si/AI=I 3.4). A linear relationship was observed between the polymer yield and the amount of Lewis acid sites. Therefore, it became clear that not weak Lewis acids sites but strong Lewis acid sites, which adsorb pyridine even at evacuation temperature of 400~ could activate the metallocene compound effectively.

878

4. CONCLUSIONS It was found from all of the above results that the Lewis acid sites are easily generated on the MCM-41 by incorporation of various metals, especially Zn and Ga. From the results of polymerization using the Metal-MCM-41 as a cocatalyst for metallocene polymerization catalyst, it was also found that the not weak Lewis acids sites but strong Lewis acid sites can activate the metallocene compound effectively and do function as a counter-anion, resulting in the generation of the active species for olefin polymerization.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 57. Z. Luan, E. M. Maes, P. A. W. van der Heide, D. Zhao, R. S. Czernuszewicz and L. Kevan, Chem. Mater., 11 (1999) 3680. D.-H. Cho, T.-S. Chang, S.-K. Ryu and Y. K. Lee, Catal. Lett., 64 (2000) 227. L.-X. Dai, K. Tabata, E. Suzuki and T. Tatsumi, Chem. Mater., 13 (2001) 208. R. Mokoya and W. Jones, J. Mater. Chem., 9 (1999) 555. M. Cheng, Z. Wang, K. Sakurai, F. Kumata, T. Saito, T. Komatsu and T. Yashima, Chem. Lea., (1999) 131. W . S . Aim, D. H. Lee, T. J. Kim, J. H. Kim, G. Seo and R. Ryoo, Appl. Catal. A: General, 181 (1999) 39. W.-H. Zhang, J.-L. Shi, L.-Z. Wang and D.-S. Yan, Materials Lett., 46 (2000) 35. Y. Oumi, H. Takagi, S. Sumiya, R. Mizuno, T. Uozumi and T. Sano, Microporous Mesoporous Mater., 44-45 (2001) 267. S. Sumiya, Y. Oumi, T. Uozumi and T. Sano, J. Mater. Chem., 11 (2001) 1111. K. Soga and T. Shiono, Prog. Polym. Sci., 22 (1997) 1503. E.Y.-X. Chen and T. J. Marks, Chem. Rev., 100 (2000) 1391. T. Sano, T. Niimi, T. Miyazaki, S. Tsubaki, Y. Oumi and T. Uozumi, Catal. Lett., 71 (2001) 105. J.M. Kim, J. H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742. S. Zheng, L. Gao, Q. Zhang and J. Guo, J. Mater. Chem, 10 (2000) 723.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Oxidation of cyclohexene catalyzed encapsulated in two faujasites

879

by manganese(llI)

complexes

M. Silvaa, R. Ferreira b, C. Freire b, B. de Castro b and J. L. Figueiredo a aLaborat6rio de Catfilise e Materiais, Dep. t~ Engenharia Quimica, Faculdade de Engenharia 9da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, PORTUGAL e-mail: [email protected] bCEQUP/Dep, t~ Quimica, Faculdade de Ci~ncias da Universidade do Porto, Rua do Campo Alegre 687, 4169-004 Porto, PORTUGAL.

Two salen derivatives bearing a metoxi-group in the phenyl ring (positon 3) and the corresponding Mn(III) complexes free and inside zeolite cavities have been prepared. The new materials were characterized by several techniques: ICP-AES, XPS, X-ray diffraction, TG-DSC, N2 adorption and FT-IR. Both complexes catalyzed the cyclohexene oxidation with TBHP, with low conversions, primarily to allylic oxidation products. Evidence supports a radical autoxidation chain mechanism, with the complexes functioning to decompose intermediate alkylhydroperoxides.

1. INTRODUCTION Recent trends in zeolite catalyst research suggest a shift in interest from petrochemistry toward specialty chemicals. This has prompted scientists to explore the encapsulation of metal complexes and organometallic fragments within the supercage structure of synthetic zeolites X and Y, that have the potential for catalyzing important transformations [1]. Specifically, several metallophthalocyanines [2] and the Co(II) complex of N,N'-bis(salicylaldehyde)ethylenediamine [3] or SALEN, have been encapsulated. These zeolite inclusion compounds termed "ship-in-a-bottle" complexes may potentially bridge the gap between homogeneous and heterogeneous catalysis [1]. It is anticipated that the entrapped metal complex will retain much of its solution properties while constrained in the supercage, as if simply in a microreactor. However, the shape selectivity, electrostatics, and acid-base properties of the zeolite are expected to synergistically modify the reactivity of the complexes [ 1]. In this work we introduced the versatile Schiff-base ligands ([Hz(3-MeOsalen)]: N,N'-bis(3-methoxysalicylaldehyde)ethylenediamine and [H2(3-MeOsalpd)]: N,N'-bis(3methoxysalicylaldehyde)propylenediamine) into zeolites X and Y (in sodium form) by the flexible ligand method. The ligands can diffuse into the zeolite by twisting their way through the pores where upon complexation with a metal ion they become too large to exit. The resulting materials were characterized by different techniques, namelly elemental chemical

880 analysis (ICP-AES), surface analysis (XPS), X-ray diffraction, thermal analysis (TG and DSC), N2 adsorption and FTIR. Then we examined the catalytic activity of the prepared materials in the oxidation reaction of cyclohexene with TBHP (70% w/w) in dichloromethane, and compared with the homogeneous analogue. A possible mechanism is proposed on the basis of kinetic studies.

2. EXPERIMENTAL SECTION

2.1. Materials Reagent grade solvents were supplied by Merck; manganese acetate tetrahydrate was Aldrich; 3-methoxysalicylaldehyde, ethylenediamine and propylenediamine were Merck. The zeolites NaX and NaY were supplied by Grace GmbH. All the chemicals were used as received.

2.2. Ligand and complex synthesis To a solution of absolute ethanol with 2.0 mmol of dissolved 3-methoxysalicylaldehyde was added, under stirring, 1.0 mmol of ethylenediamine or propylenediamine. The mixture was elevated to reflux, during l h, and then filtered and washed with cold ethanol. A yellow cristalline solid was obtained, which was subsequently recrystallized. The ligands (2.0 mmol) dissolved in absolute ethanol were mixed with a methanolic solution of manganese acetate tetrahydrate (2.0 mmol). Then, a potassium hydroxide methanolic solution was added (4.0 mmol). The mixture was l h at room temperature, under stirring and in a nitrogen atmosphere, when a orange solid appeared. To complete the complexation, the mixture was refluxed 2h, in a nitrogen atmosphere. At the end, it was allowed to cool to room temperature, filtered and washed with methanol. After recrystallization, it was dried at 80~ under vacuum.

2.3. Encapsulation of metal complexes inside zeolites (flexible ligand method) First, an ion exchange was performed in aqueous solution, starting from a mixture of zeolite (20 g NaX and NaY calcined at 500~ under a stream of air, 6h) and manganese acetate tetrahydrate (0.7 g) dissolved in 200ml of deionised water. The mixture was stirred over 24h at room temperature. Then, it was filtered and washed with deionised water until uncoloured mother-liquors. The pale brown solid was first dryed at room temperature and then in a oven at 150~ under vacuum, during 1Oh. Then, the exchanged zeolites were mixed with the already prepared ligands, in a Schlenk tube, at 150~ under vacuum, during 3h. The resulting materials were Soxhlet extracted with a mixture CH2CI2:CH3CN (1 :l) for 8h and then with ethanol for 10h, to remove excess diamine and manganese complexes adsorbed on the external surface of zeolite crystallites. The resulting brown solids were dried at 100~ under vacuum, for 5h.

2.4. Catalyst characterization The elemental chemical analysis were done at Kingston Analytical Services, UK, by ICPAES. The surface analysis (XPS) were performed at Centro de Materiais da Universidade do Porto, CEMUP. X-ray diffraction were performed at Universidade de Aveiro. The thermal analysis were followed in a METTLER equipment. For thermogravimetry (TG) 10 mg of

881 sample were heated at 10~ to a maximum temperature of 800~ under nitrogen atmosphere (200 ml/min). All experiments were corrected with a blank. The DSC experiments were done between 25-600~ with 5 mg of sample, in a flow of helium (50 ml/min) and using perforated alumina sample holders (40 ~tl). The heating rate was 10~ and an empty sample holder was used as reference. The adsorption studies were done in a COULTER OMNISORP 100CX. All the samples were previously degassed at 150~ until a vacuum of 10.3 Pa; the adsorption of nitrogen was done at -196~ and the results were treated by the tmethod [5]. The FTIR spectra were obtained in a Nicolet 510P with a diffuse reflectance cell, working at 32 scans and a resolution of 4 cm-~; the samples were diluted by the KBr technique. The catalytic activity was tested in the epoxidation of cyclohexene with tertbutylhydroperoxide (TBHP, 70% (w/w)). A glass batch reactor of 10 ml was used, which was charged with 0.1 g of catalyst (or 0,05 g of free complexes) and a solution of dichloromethane (2 ml) with 2.5 mmol of cyclohexene, 2.0 mmol of n-decane (internal standard) and 4.0 mmol of TBHP (70% (w/w)). The reaction was run at 60~ during 6h. Sometimes, the mixture was left overnight to evaluate the equilibrium. The reaction products were analysed by GC (GC Dani 1000) with a DB Waxetr column. The quantitative results were obtained by use of calibration curves. For kinetic studies, different reactant, oxidant and catalyst concentrations were tested and the temperature effect was evaluated in the range 40~176

3. RESULTS AND DISCUSSION

3.1. Elemental chemical analysis and surface analysis The Si/A1 ratios are similar (table 1) for samples with the same zeolitic matrix, which means that the structure of the zeolites are not modified by encapsulation of the manganese complexes. As for the Mn/A1 ratios, the XPS values are higher than the elemental analysis values, implying that there are manganese species in the outer cavities of the zeolites. The XPS results also show: (a) Mn/N ratios higher than the theoretical value of 0.5, which means uncomplexed manganese ions; (b) binding energies of manganese characteristic of two types of Mn species (Mn 2+ (-~642 eV) and Mn n+ (n>2) (~645 eV)) [4]; (c) after encapsulation there are shifts in the binding energies of manganese (+0,2 eV for Mn with higher B.E. and -0,4 eV or-1,0 eV for the other Mn species encapsulated in Y or X zeolite, respectively) which implies an additional effect from the matrix. Table l Elemental composition of modified zeolites Sample

S i/Al ICP-AES

XPS

Mn/AI ICP-AES

XPS

Mn(%) ICP-AES

XPS

MnX

1,7

1,8

0,04

1,0

1,0

14

[Mn(3-MeOsalen)]X

1,7

1,7

0,03

0,50

0,80

7,0

[Mn(3-MeOsalpd)]X

1,8

1,8

0,03

0,60

0,76

7,3

MnY

3,1

2,9

0,05

0,08

0,99

4,1

882 Table 1 Elemental composition of modified zeolites [Mn(3-MeOsalen)]Y

3,1

2,8

0,04

0,06

0,76

2,8

[Mn(3-MeOsalpd)]Y

3,1

2,9

0,03

0,05

0,78

2,9

3.2. X-ray diffraction

The X-ray diffractograms of all the materials show a well crystalline FAU structure [5] of cubic symmetry. After the exchange there is a decrease in the intensities of all peaks, specially the [ 111 ] peak. 3.3. Thermal

analysis

The thermal stability of the free complexes increases in the order: sal<[Mn(3MeOsalen)]<[Mn(3-MeOsalpd)] (fig. 1). A similar behaviour was observed when they are encapsulated inside zeolites X and Y, the higher stability corresponding to those in zeolite X, which is the zeolite with lower acidity.

! / ~4o20o

.~' ~[, ~, , 3O0 6~00

,

300

i-0,3

030

T(~ Figure 1.a - (a)-H2(3-MeOsalen); (b)H2(3-MeOsalpd); (c)-Mn(3-MeOsalen); (d)-Mn(3-MeOsalpd) Figures 1.(a), (b)- Thermal analysis

~o

~o~

~

-I

600

T(~ Figure 1.b - (a')-H2(3-MeOsalen)X; (a")-H2(3MeOsalen)Y; (b')-Hz(3-MeOsalpd)X; (b")H2(3-MeOsalpd)Y

883

t

M~X I8 0-

[8 0

!

lO-

!

0

~v

300

'

I

!

6000

-1 300

-1

!

6130

0

300

6000

T(~-3

300

600

T~3

Figure 1.c - (c')-Mn(3-MeOsalen)X; (d')Mn(3-MeOsalpd)X Figures 1.(c), (d)- Thermal analysis

Figure 1.d - (c")-Mn(3-MeOsalen)Y; (d")Mn(3-MeOsalpd)Y

The values of weight loss occurring during thermal decomposition are presented in table 2. The decomposition behaviour of the free and the loaded complex was compared in order to provide evidence for the location of the complex (entrapped into the micropores or located outside). The loaded complex decomposes in several steps during heating to 600~ The first loss in weight occurs at 110~ and is endothermic (fig. 1), and results from water desorption. The second, composed by two or more steps, appears at 210~ and continues until the end of the temperature range, and is a exothermic process. After the experiment the residual sample is dark, which probably corresponds to manganese oxide. In comparison to the free complexes, these steps and their peak maxima are less well defined. Table 2 Values of weight loss in the prepared samples (25~176 Sample

Weight loss (%)

Sample

Weight loss (%)

MnX

23,1

MnY

19,3

[Mn(3-MeOsalen)]X

2 !,0

[Mn(3-MeOsalen)]Y

19,9

[Mn(3-MeOsalpd)] X

15,8

[Mn(3-MeOsalpd)] Y

19,2

[Mn(3-MeOsalen)]

37,0

[Mn(3-MeOsalpd)]

53,5

3.4. Nitrogen adsorption at-196~ The nitrogen adsorption isotherms of all samples at -196~ are typical of microporous materials. Thc values of micropore volume (Vmic) and "monolayer specific area" (Seq) [6] are within 0.28-0.24 cm3/g and 92-68 m2/g, respectively (the NaX and NaY values for that parameters are 0,28 cm3/g and 92,1 m2/g and 0,29 cm3/g and 86,8 m2/g, respectively). The

884 results show that there is a reduction of both micropore volume and specific area of the modified materials, confirming the presence of species inside the pores of the zeolites.

3.5. Fourier transform infra-red spectroscopy (FTIR) The FTIR spectra of free (fig. 2) and loaded complexes (fig. 3) were obtained in the range 4000-400 cm l using the KBr technique (20% (w/w)). Peaks due to the zeolites (NaX and NaY) dominate the spectra. These include the O-H vibrations (silanol groups, Si-OH) in the range 3700-3300 cm -~. Other bands related to the zeolite framework appear at 1250, 1140, 1040, 960 and 820 cm -~ in zeolite Y and blue shifted in about 50 cm l in ....... H ~ ( 3 - ~ ~ ) zeolite X. There are no significant shifts or broadening of the structuresensitive zeolite vibrations at 1130 ,' ~ [M~-~k<~N)] cm -~ (T-O asymmetric stretching) )',~ indicating that no significant -I" I (2),.~, ,~:i dealumination or expansion of the zeolite cavities occurred during 2 encapsulation process [7]. The assignments of the bands between 1650 and 1000 cm -! to 0,0 modes having the character of I skeletal stretching, in-plane C-H 2000 W~/fi~n'lb~g (cm-~) 1000 bending and CHz/CH3 deformation, is relatively easy to perform by Figure 2. FTIR spec~ of flee complexes reference to the comprehensive literature. #

ml

(A)NaX (B)Mg(

](Q[MI(3-NE)~en)]X /.,:\'

o, 4ooo

('q iv"

(A')b,hY (B)IVhY (C3[M1(3-~~)]Y

. . ~..~ t ~ .

.

:a~ ,,,~-,~m-h~ (~-i')

Figure 3. - FTIR spectra of modified zeolites

i~/" . i.~'~ ~: : % ' X

-

o,

~o

. . . .

20oo ~

(~')

885 The characteristic band of the manganese complexes (*) at 1570 cm -I (VC=N)is shifted to 1510 cm -I after encapsulation which confirmes the presence of zeolite in the neighbourhood of the complex.

3.6. Epoxidation of cyclohexene The catalysts were tested in the epoxidation of cyclohexene according to:

0

Cyclohexene

Mn(salen)zeolite

(1) TBHP aq., 60~

OOR

O

(c)

OH

(o)

The results, as summarized in fig. 4 (bar graph, with black bar corresponding to % in cyclohexene conversion and the others bars corresponding to % yields in B (cyclohexene oxide), in C (enone) and in D (enol)) show that in general there is an improvement in the 0 catalytic activity upon 1 2 3 4 5 6 7 8 immobilization of the complexes in zeolites. The best performance is obtained with Mn(3MeOsalen), probably as a result of the lower size of the complex and, consequently, the lower difusional constraints. The conversions were low and with <20% yield in epoxide (B). The major product was tert-butyl-cyclohexenil-peroxide (A). This specie was only detected by GC-MS. Blank experiments showed no catalytic activity of the TBHP in the absence of catalyst. =t-

t

Figure 4. - Epoxidation of cyclohexene in the presence of (l)Mn(3-MeOsalen), (2)Mn(3MeOsalpd), (3)/(6)MnX/Y, (4)/(7)[Mn(3-MeOsalen)]X/Y, (5)/(8)[Mn(3-MeOsalpd)]X/Y As shown in fig. 4, there are allylic products (2-cyclohexene-l-one (C) and 2cyclohexene-l-ol (D)) as well as additional products from the subsequent hydrolise and oxidation of epoxide (B), namely cyclohexanedione and cyclohexanediol. Kinetic studies were performed with [Mn(3-MeOsalen)]X. Based on the kinetic data a mechanism is proposed, which involves two possible pathways: a radical reaction (one electron transferred) and an oxidation reaction (two electrons transferred). By the first pathway, the tert-butyicyclohexenilperoxide (A) and the (C) and (D) species are formed; by the second, the epoxide are formed and sometimes also cyclohexanedione and

886 cyclohexanediol, depending on the cyclohexene/TBHP ratio and the catalyst used. This mechanism involves a metalloperoxy intermediate, already postulated as Mn(V)oxo complex [8]. The overall rate equation for the oxidation of cyclohexene (CHY) catalyzed by the Mn(III)L catalyst systems can be represented by the expression: Rate=ke• [MnL] [C HY] [TB HP ]/[tBuO H]

(2)

where the concentrations of the reactants are represented in square brackets and kexp is a product of constants, including equilibrium constants. The rates of oxidation of cyclohexene determined at different temperatures were plotted as -In(rate) versus 1/T (Arrhenius plot) and from the slope of the straight line, an activation energy o f - 1 4 kcal/mol was calculated.

4. CONCLUSIONS The manganese complexes are occluded in the zeolite and as consequence they are thermally stable and difficult to leach due to their size. In spite of the observed low manganese contents some activity was measured. The results obtained with the smaller complex were better and the product distribution (mainly allylic products) implies a radical autooxidation chain mechanism. From the kinetic studies, a rate dependency in cylohexene, catalyst and oxidant was determined, as well as the activation energy.

Acknowledgements M. S. and R. F. thank F.C.T. for financial support. This project was initiated with support from FCT, contract n~ PRAXISXXI/3/3.1/MMA/1780/95.

References 1. K. J. Balkus, Jr., A. A. Welch, B. E. Gnade, Zeolites 10 (1990) 722. 2. H. Diegruber et al., J. Mol. Catal. 24 (1984) 115. 3. N. Herron, J. Inorg. Chem. 25 (1986) 4714. 4. J. F. Moulder et al., Handbook of X-Ray Photoelectron Spectroscopy, J. Chastain and R. C. King, Jr. (eds.), 2 "ded. (1995), Phys. Electron., USA. 5. M. M. J. Treacy, J. B. Higgins, Collection of Simulated XRD Powder Patters for Zeolites, 4 th ed. (2001), Elsevier, Amesterdao. 6. A. Linares-Solano, Textural characterization of porous carbons by physical adsorption of gases, Carbon and Coal Gasification, J. L. Figueiredo and J. A. Moulijn (eds.), NATO ASI SERIES-E: APPLIED SCIENCES, n~ (1986). 7. C. R. Jacob et al., Appl. Catal. A: General 168 (1998) 353. 8. L. Frunza, H. Kosslick, H. Landmesser, E. H6fl, R. Fricke, J. Mol. Catal. A: Chem 123 (1997) 179.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

H e a v y aromatics upgrading using noble metal p r o m o t e d zeolite

887

catalyst

S.H. Oh a, S.I. Lee a, K.H. Seonga, Y.S. Kim a, J.H. Lee a, J. Woltermannb, W.E. Cormierb, Y. F. Chub,*

aSK R&D Center, SK Corporation, 140-1 Wonchon-dong, Yusung-gu, Taejon 305-712, South Korea bZeolyst International, PO Box 830, Valley Forge, PA 19482-0830, USA Heavy C9§ aromatics can be converted to more valuable benzene, toluene and xylene products (BTX) such as in the traditional Tatoray unit. However, existing commercial catalysts not only deactivate rapidly but also are difficult to regenerate. A precious metal promoted zeolite catalyst is developed to maximize the conversion of the heavy aromatics (HA) and to extend the life of the catalyst. The catalyst developed exhibited not only high C9+ conversion and high xylene yield over a wide range of feedstock compositions but also showed a high stability and regenerability for the transalkylation of C9§ aromatics with toluene. The catalyst has been commercialized and has been on stream for two years without yet requiring regeneration. In this study, experimental data obtained in the laboratory and kinetic explanations in accordance with catalytic results are presented. Some commercial data are also provided. 1. INTRODUCTION BTX are valuable petrochemical feedstocks that can be produced from less valuable HA generated in the refinery process. These materials include naphtha reformate and pyrolysis gasoline which have considerable of C9+ aromatics content. These heavy aromatics can be used as a gasoline blending stock but they can be more economically transalkylated with toluene to increase xylene production over zeolite catalysts [1]. Transalkylation of toluene with C9+ aromatics over large pore zeolites with 12-membered rings such as mordenite, beta and Y zeolite have been studied extensively [1-6] under laboratory conditions. Particular attention was given to the transalkylation of toluene with 1,2,4-trimethylbenzene. However, a commercial C9+ feed would typically contain other trimethylbenzene isomers (TMB), methylethylbenzenes (MEB), and propylbenzenes (PB) as well as C10+ aromatics that are very difficult to convert. Hence, the results reported do not completely represent the commercial processes. For an effective catalyst, it is also necessary to examine the maximum C9+ aromatics concentration in feed, feed impurity tolerance level, product purity, yield pattern of mixed xylenes, cycle length and regenerability of the catalyst [7]. In recent years, information on commercial transalkylation catalysts even though limited has become available [7,8] but * To whom all correspondences should be addressed.

888 few catalysts seem to meet all the criteria mentioned above. Stable, regenerable catalysts that achieve higher HA conversion activity are desired. The aim of this work is to develop an efficient transalkylation catalyst that could overcome the drawbacks of the existing catalysts. As mentioned, zeolite catalysts have been shown to be useful for this application. Studies have also shown that the performance of the catalysts are affected by the acidity and pore structure of the zeolite [4]. Large pore zeolites with 12membered rings, such as mordenite, are useful for this reaction due to their larger pore size but unfortunately they also tend to deactivate rather rapidly [1]. As constituted, the strong acidity of these materials will also result in excessive cracking [2]. A bi-functional catalyst with modified zeolite acidity and promoted with a balanced amount of noble metal to obtain high activity, selectivity and stability has been developed for the commercial transalkylation process. In this paper, we will discuss catalytic performance and the distinctive features of the new catalyst and present the commercial experience of SK Corporation in Korea. 2. EXPERIMENTAL The catalyst was manufactured by Zeolyst International based on technology developed by SK Corporation. Catalytic experiments were carried out in a high pressure catalyst testing apparatus under a pressure of 27 kg/cm 2 with WHSV=2.3 ht and H2/HC molar ratio of 3.6 and various feed compositions. All catalysts compositions were pre-reduced with 1-12flow at 400 ~ for 2 h and then cooled down to the reaction temperature of 360 ~ Feed composition was varied with C9§ aromatic contents of 0 wt%, 70 wt% and 100 wt%, balance toluene. The composition of the commercial C9§ feed used consists of TMB, MEB and PB and more than 15% of C10§ The products were analyzed with HP 5890 gas chromatography equipped with FID and the HP-PONA capillary column. All mass balances were within • of closure. The reaction results were obtained at normal operating conditions as described above. Subsequently, the catalyst was subjected to accelerated aging at high severity operating conditions of 420 ~ WHSV=2.3 h1 and H2/HC molar ratio of 1.0 for 40 h under the pressure of 10 kg/cm2. Regeneration experiments were done by calcining the accelerated aged catalyst samples in the laboratory at 450 ~ for 100 h under air and the balance Ar flow to obtain an oxygen concentration of 1-3 mol% at the pressure of 5 kg/cm2. The regenerated catalyst was subjected to a further reduction at 400 ~ for 2 h under H2 flow before testing again. 3. RESULTS AND DISCUSSION 3.1. Discussion of results and proposed reaction network

The results shown in Table 1 demonstrates that the catalyst to be very effective in the transalkylation reaction (TA) of C9§ aromatics. Xylene yield is high, particularly in the case of a toluene to C9§ aromatics weight ratio of 30/70 (case 2). The catalyst also exhibits strong hydrodealkylation activity (HDA). Ethyl and propyl groups are easily dealkylated to provide high BTX yield and a low concentration of EB and PB in the product (cases 1-3). Toluene disproportionation (TDP) has also occurred as evidenced by the high conversion of 100% toluene (case 1). As the concentration of Cg§ increases in the feed (cases 2,3) the TDP reaction is suppressed and hence the toluene conversion and benzene yield decreases

889 Table 1 Test results over the novel metal promoted zeolite catalyst with various feed compositions Reaction conditions Case Feed, wt% Toluene C9+ H A

Yield on feed, wt% Olefins Paraffins Benzene Mixed xylenea Conversion, wt% Toluene

360 ~ 27 kg/cm2, WHSV=2.3 hI, H~-IC=3.6 1

2

3

100 0

30 70

0 100

trace 1.78 21.60 24.32

trace 12.95 7.49 34.06

trace 13.50 3.10 32.97

52.73

3.84 74.20 51.75 97.38 78.45 0.91

66.00 44.80 95.89 57.40 1.30

C9+ H A

Trimethylbenzene Methylethylbenzene Clo+ HA Ethylbenzene/Mixed xylene, wt%

1.97

a Mixed xylene: p-xylene + m-xylene + o-xylene + ethylbenzene while xylene yield increases. Olefin saturation is evidenced by the trace concentration of olefins in the product. These results can be explained by a complex reaction network as shown in Figure 1. Stoichiometrically, two moles of xylene can be obtained by the transalkylation of one mole of toluene with one mole of TMB. Thus an equal molar ratio of toluene and TMB will be the ideal feed composition for xylene production. In addition to the transalkylation of toluene and TMB, other reactions such as toluene disproportionation and hydrodealkylation of ethylbenzene (EB), MEB and PB and dimethylethylbenzene (DMEB) have taken place to produce the desirable products of BTX. A sequence of reactions may also take place. For instance, MEB dealkylates into toluene and some of the toluene formed could be further reacted to form xylene through the transalkylation with TMB. This could explain the high xylene yield at low apparent toluene conversion for C9+-rich feedstocks (cases 2,3).

890 ITransalkylafiol~

~)isproportionation t

!Hydrodealkylation / 01efin saturatior~

C3H8

I

~r

C_~

IN

M e t ~ C2H6

Figure 1. Proposed reaction network of C9+HAand toluene over newly developed catalyst. 3.2. Comparison to a traditional catalyst

The newly developed catalyst has a much higher C9§ aromatic conversion compared to traditional catalysts. High conversion of C9§ aromatics implies the reduction of these compounds in the recycle. This enables refineries to save utility cost or to treat additional flesh HA. The catalyst also produces mixed xylenes with lower EB content even at 100 wt% HA feedstock. It has been reported in previous work that EB content in mixed xylenes increases as C9+ aromatics in the feedstock increases [2,7]. Low EB content in mixed xylenes (- 2.0%) is desirable since it reduces p-xylene recovery cost in the xylene isomerization loop [9]. High purity benzene (99.85%) can also be obtained in the process. The selectivity of catalyst can be attributed to the well-balanced acid/metal function of the catalyst. Saturation of ethylene or propylene that might be generated from the dealkylation of EB, MEB and PB and DMDB is rapid inhibiting secondary ethylation of benzene to EB and oligomerization of light olefins into coke precursors. Low coke formation ensures longer catalyst cycle length. Hydrogenation of aromatic tings to naphthenes is minimal It is also clear from Table 2 that

891 Table 2 Comparison between traditional catatyst and new catalyst developed ATA-11

Caalyst

Feed, wt% Toluene/C9 HA/Clo + HA Product, wt% Xylene EB C9 HA C10+ HA

Traditional

ATA- 11

66/32/2

20/66/14

28~1 2.8 12_8 4.0

34.8 0.16 22.6 4.1

Reaction conditions: Reactor inlet temperature=330~380 ~ WHSV=I.5-~2.5 hl, H~I-IC=3.0 -~7.0 mole/mole

Pressure=25-~30 kg/cm2,

there is also a net loss of C l o + aromatics in contrast to a traditional catalyst system that would produce a net increase of these compounds.

3.3. Regeneration of catalyst During normal operation, coke will gradually accumulate on the catalyst with increased Table 3 Regenerability of metal promoted zeolite catalyst developed Catalytic Performances

Feed, toluene wt~ + H A wt~ Mixed xylene yield, wt% Toluene conversion, wt% Cg+HA conversion, wt% Ethylbenzene/Mixed xylene, wt%

Fresh"

Before regenerationu

Atter regeneration"

30/70 35,11 8.90 72.52 0.40

30/70 31.66 1.85 72.07 1.26

30/70 34.93 9.10 72.23 0.40

"Reaction conditions: 360 ~ 28kg/cm2, WHSV=2.3 hq, H:/HC=3.6 mole/mole b Obtained atter high severity operation at 420 ~ 101~cm2, WHSV=2.3 h"1, H2/HC=I.0 mole/mole for 40 h

892 time on stream until it will be no longer possible to maintain desired activity. The catalyst is then regenerated. The regeneration of zeolite catalyst is generally carried out by coke combustion under air or oxygen flow [10]. An accelerated aging test was conducted to ascertain the regenerability of the catalyst. The catalyst was deactivated at high severity operating conditions, which are high temperature, low pressure and low H2/HC molar ratio. The catalyst then was regenerated by coke removal at 450 ~ in air for about 100 h. The catalyst has been regenerated successfully in the laboratory as shown in Table 3. Toluene conversion, C9+ aromatics conversion and xylene yield were almost completely recovered to their original levels over the regenerated catalyst_ More than 98% of the catalyst's activity was recovered. This regenerability has not yet been demonstrated commercially since the commercial catalyst has retained activity after more than+two years on stream. 3.4. Commercial experience The catalyst of this study has been successfully commercialized and is marketed under the tradename ATA-11. A transalkylation unit in the SK refinery at Ulsan, Korea was loaded with ATA- 11 and the run started in July 1999. The catalyst remains very active after two years TOS requiring only about half of the amount of catalyst compared to a traditional catalyst. As a result the throughput is very high even exceeding design capacity. The catalyst has been very stable for more than two years of operation even with H2/HC molar ratio as low as 2.8. The stability of the catalyst might be ascribed to the well-balanced acid/metal function of catalyst. High gas make due to overcracking of C9+ aromatics has not been observed. Aromatics ring loss through hydrogenation of BTX and consecutive hydrocraeking is less than 2%. Benzene produced is of sufficient high purity for use as chemical grade without the need of further purification by extraction. Reactor temperature is the prime, variable for control of reaction severity. During the run, an accidental sulfur poisoning of the catalyst occurred that deactivated the catalyst at the end of the year 2000. As a resulL the temperature was increased to compensate for the deactivation. Figure 2 shows the reactor inlet temperature trend during two years of commercial operation. Feed was introduced+ at low temperature to compensate for the reaction

100

500 -

o

2400

~" 300

~

80

o

60

,..

m O 41o 9

~ 4o

O

. ,...,~

+-. 100 ~

+~

r~ 20

-

0

0

Jul-99

9

. ,...~

o 200 0

9 ~

Jan-00

Jul-00 Jan-01 Jul-01 Date

Figure 2. Reactor inlet temperature changes during two years of commercial operation.

Jul-99

I

Jan-00

I

I

~

[

Jul-00 Jan-01 Jul-01 Date

Figure 3. Flexible C9 + HA concentration in feedstock during the commercial operation.

893 exotherm. After initial start-up, the temperature was adjusted to maximize xylene yield and benzene purity. On the whole, reactor inlet temperature is low and the temperature increase is also steady. An actual deactivation rate is below 20 ~ Low operating temperature has a beneficial effect on cycle length. The moderate increase of reactor inlet temperature during two years of run demonstrates the stability of this catalyst. The feed composition has deliberately been varied over the course of the commercial run as shown in Figure 3. C9§ aromatics concentration in the feed has been adjusted in response to BTX unit balance or market situation. The catalyst has been shown to be very tolerant to heavy aromatics. The feedstock is flexible for 100% toluene to 100% C9§ HA. The capability to accommodate a C9§ aromatics-rich feed is highly desirable. C9+ aromatics are converted into more valuable BTX. Increasing the HA blending ratio to toluene somewhat enhances xylene yield. However, C9+ aromatics content in the feed influences the stability of zeolite catalyst. HA, particularly C10§ can be trapped in the pore of zeolite catalyst causing catalyst deactivation involving probably relatively bulky biphenylmethane intermediate [11] in a large pore zeolites. The present catalyst is capable of handling as high as 20% concentration of C10+aromatics with proper selection of zeolite pore size and a wen balanced acidity and metal function built in the catalyst. The most desirable product of heavy aromatics up~ading is xylene. The reactor effluent is separated into gas and liquid product through separator and stripper. Figure 4 shows xylene percentage in liquid products treated by the stripper_ Some abrupt changes in the xylene content are due to variation of feedstock compositions. According to in-out mass balance, high xylene yield, which is in the range of 30-36 wt%, is achieved. Transalkylation of C9§ aromatics with toluene and hydrodealkylation of C9§ aromatics give rise to the high xylene yield. Relatively stable xylene production has been maintained throughout the two years of operation. Low EB content in mixed xylene has also resulted in increase of p-xylene productivity at the PX unit. The operating temperature and H2/HC molar ratio are low,

o•

50 45 40

A

It~

9

A

~

35 o

30 25 20 o,..~ 15 = 10 0

~A

t

r

I

I

I

I

Jul-99 Nov-99 Mar-00 Jul-00 Nov-00 Mar-01 Jtfl-01 Date Figure 4. Xylene content in the stripper bottom stream during the commercial operation.

894 resulting in energy savings. The cost savings are estimated to be more than $5MM/yr for the company from this process with a daily throughput of 6000 barrels/day. 4. CONCLUSIONS A commercial transalkylation catalyst has been developed which is capable of processing of Cg+HA at a high throughput, producing excellent xylene yield (30-36%) and very low EB yield. The catalyst exhibits strong ~ansalkylation and hydrodealkylation activity. Acid/metal function of the catalyst is controlled to enhance its activity, selectivity and stability. Well-balanced hydrogenation activity of metal retards the formation of coke via rapid saturation of olefins formed by dealkylation of heavy aromatics. Saturation of aromatics is minimal. The catalyst responds well to changes in feedstock composition ranging from concentrations of 100% toluene to 100% C9§ aromatics. The capability of processing C9§ aromatic-rich feed is a key feature of this catalyst. The catalyst is both stable and regenerable. Regenerability was confirmed by repeated accelerated aging tests in the laboratory. Heavy aromatics upgrading over ATA-11 has been operated commercially for more than two years without regeneration and its performance has exceeded expectations based on pilot tests. The operation has been smooth and high purity BTX has been produced over this catalyst.

REFERENCES

1. Y.K. Lee, S.H. Park and H.K. Rhee, Catal. Today, 44 (1998) 223. 2. J.C. Wu and L.J. Leu, Appl. Catal., 7 (1983) 283. 3. J. Das, Y.S. Bhat and A.B. Halgeri, Catal. Lett., 23 (1994) 161. 4. I. Wang, T.C. Tsai and S.T. Huang, Ind. Eng. Chem. Res., 29 (1990) 2005. 5. K.J. Chao and L.J. Leu, Zeolites, 9 (1989) 193. 6. E. Dumitriu, V. Hulea, S. Kaliaquine and M.M. Huang, Appl. Catal. A, 135 (1996) 57. 7. T.C. Tsai, S.B. Liu and I. Wang, AppL Catal. A, 181 (1999) 355. 8. C.R. Marcilly, Topics in Catal., 13 (2000) 357. 9. A. Smieskova, P. Hudec, M. Paciga andZ~ Zidek, Appk CataL A, 149 (1997) 265. 10. M. Guisnet and P. Magnoux, Catal. Today, 36 (1997) 477. 11. S.M. Csicsery, Zeolites, 4 (1984) 202.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

895

Preparation o f iron-doped titania-pillared clays and their application to selective catalytic reduction o f N O with a m m o n i a Dong-Keun Lee, Sung-Chul Kim, Seong-Ji Kim, Ju-Ki Kang, Dul-Sun Kim and Sang-Sin Oh Department of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyungnam 660-701, Korea TiO2-pillared clays were synthesized by pillaring TiO2 onto the pure bentonite. As the pillaring agent was used a solution of partially hydrolyzed Ti-polycation which had been prepared by adding TIC14 into HC1 solution. Successful intercalation of TiO2 could be achieved, and the physical properties such as d001 spacing, surface area and pore volume were influenced by the concentration of HC1, TIC14 and Ti/clay ratio. The d001 spacing and surface area of the TiO2-PILCs increased up to 29.8A and 389m2/g, respectively. The pore volume also increased up to 0.25cm3/g which was significantly higher than 0.06cm3/g of the unpillared clay. The N2 adsorption isotherms showed the presence of both the micropores and mesopores in the pillared clays. TPD and FTIR analyses showed the wide presence of the BrOnsted acid site which is essential to the SCR of NO with NH3. When Fe203was incorporated into the TiO2-PILCs, the BrOnsted acidity increased significantly, and even the complete conversion of NO could be achieved with the Fe203/TiO2-PILCs at the temperature window between 375-400 ~ 1. I N T R O D U C T I O N While clays have two-dimensional layer structures, pillared clays have three-dimensional network structures like those of zeolites. The main goal of the pillaring process has been and continues to be that of producing new and inexpensive materials having properties complementary to those of zeolite(pore size and shape, acidity, redox properties, etc.)[1-6]. Pillared clays(PILCs) are nano-composite materials with open and rigid structures obtained by linking robust, three-dimensional species to a layered host. Why PILCs are of wide interest for catalytic applications is clear when thinking about possible controlling of physical and chemical properties. The removal of NO from stack gases is an important step towards controlling air pollution. With increasing strict NOx emission regulations some form of post-combustion NOx removal is necessary. The most popular technique commercially for removing NOx is socalled selective catalytic reduction(SCR) of NO by NH3. In the SCR process NO is reduced in the presence of NH3 to N2 and H20. The catalyst typically used in the SCR process for NO removal is V205 supported on TiO2. TiO2-PILCs could also be promising catalyst supports for SCR of NO because the presence of BrOnsted acid sites for the adsorption of NH3 is

896 important to SCR reaction and intercalating YiO2 between the SiO2 tetrahedral layer is a unique way of increasing acidity of TiO2 support[7]. The outstanding features that TiOzPILCs have large pore sizes allowing further incorporation of active ingredients without hindering pore diffusion and have high thermal and hydrothermal stability among pillared clays make their catalytic applications more practical[8,9]. In this study TiOz-pillared clays were synthesized under different conditions and iron was doped onto the prepared TiOz-PILCs. The iron-doped TiO2-PILCs were used as catalysts for SCR of NO with NH3, and their catalytic performances were investigated. 2. EXPERIMENTAL 2.1. Starting Materials The starting clay for the preparation of TiO2-PILCs was a purified bentonite powder(DongYang Bentonite Co.). Only particles of clay with a size less than 2/an were used in the pillaring process. The chemical analysis of the bentonite was SiO2, 52.33%, A1203, 16.79%, Fe203, 3.84%, TiO2, 0.11%, Na20, 1.28%, MgO, 1.94%, CaO, 1.69%, K20, 0.54% by weight. The CEC(cation exchange capacity) value of the bentonite was found to be 96meq per 100g of clay. Its specific surface area and pore volume as determined from nitrogen adsorpsion isotherm were 33m2/g and 0.06cma/g, respectively. The H2 adsorption of the unpillared clay was close to type I isotherm which is generally observed for the solids containing micropores. 2.2. Synthesis of the TiO2-PILCs As the pillaring agent for the synthesis of TiO2-PILCs was used a solution of partially hydrolyzed Ti-polycation which had been prepared by adding TIC14into HC1 solution under constant stirring. The mixture was then diluted by slow addition of deionized water under stirring to reach final Ti concentrations ranging from 0.32 to 0.82M. HC1 concentration corresponding to 0.11-0.6M were used in the preparation. The prepared solutions were aged at room temperature for 12h prior to their use. A bentonite suspension in deionized water was mixed with these pillaring solutions under rigorous stirring at room temperature for 12h to have Ti/clay ratio in the range 3.9-10mmol/g clay. The mixture was filtered and washed by centrifugation with deionized water until it was chloride-free. The suspension was then dried at 120 ~ for 24hr and the resulting sample was calcined at 300 ~ for 6h. Highly dispersed Fe203 clusters on the TiOE-PILCs were prepared through carbonyl introduction method. Iron carbonyl(Fe(CO)5) dissolved in n-pentane was physically dispersed onto the TiO2-PILCs. The TiOE-PILCs was dehydrated in v a c u o at 300 ~ The impregnation took place in an evacuated sealed cell over a period of 12h at -10~ The mixture was then warmed slowly to room temperature in v a c u o over a period of 2h and was maintained under a dynamic vaccum(10 .2 Torr) for another 24h. The prepared samples were finally calcined with flowing 1% O2/He at 300 ~ for 12hr. 2.3. Characterization The X-ray diffraction(XRD) pattems were measured with a Philips model PW 1710 diffractometer using Ni-filtered CuKtx radiation. Surface areas and pore volumes were determined

897 Table. 1. Synthesis conditions of TiO2-pillared clays. HC1 conc. TIC14 conc. Symbol [M] TiO2-PILC-A TiO2-PILC-B TiOz-PILC-C TiOE-PILC-D TiOz-PILC-E

0.11 0.28 0.60 0.11 0.11

[M]

mmol Ti/g clay

0.82 0.82 0.82 0.32 0.41

10.0 10.0 10.0 3.9 5.0

by using nitrogen as the sorbate at 77K in a static volumetric apparatus(Micromeritics ASAP 2010). Temperature-programmed desorption(TPD) of ammonia was carried out in a quartz tubular reactor. 0.1g samples were loaded in the reactor and were pretreated in flowing helium while heating at 10 ~ up to 500 ~ After being maintained for 30min at this temperature, the samples were cooled to 150 ~ and saturated for 30min in an ammonia stream(1,000ppm NH3 in Ar). The samples were then allowed to equilibrate in a helium flow at 150 ~ for l h. The TPD was performed by ramping the temperature at 10~ from 25 ~ to 500 ~ The exit stream from the reactor was analyzed with a mass spectrometer. Infrared spectra were recorded with a Digilab FTS-80 FTIR spectrometer. About 50mg of the sample was pressed into a self-supporting wafer and placed inside a infrared cell similar to the one designed by Hicks et al[10]. The pretreatment and NH3 adsorption procedure were the same as TPD experiments. 2.4. S C R reaction L

.................

(D)

SCR reaction with ammonia was carried out in a quartz tubular reactor. The composition of reactants was 1,000ppm NO, 1,000ppm NH3, 2% O2, 8% H20 and balance Ar. NO concentration was continuously measured by a chemiluminescent NOx analyzer(Thermo Environmental Insmunent, Model 17C). Phosphoric acid solution ammonia trap was installed before the sample inlet to avoid the oxidation of ammonia in the converter of the NOx analyzer.

(F)

3. R E S U L T S A N D D I S C U S S I O N

(A)

= m

=I

E', m Ibm

(C) __

t---

m

= m

r

L

q

m

t

Q I=

===m

o

1'o

2'o

;o

(E)

_

so

2It

Figure 1. XRD pattems of the clay and the prepared TiO2-PILCs ((A)TiO2-PILC-A, (B)TiOz-PILC-B, (C)TiOz-PILC-C, (D)TiO2 -PILC-D, (E)TiOz-PILC-E, (F)clay).

3.1. Characterization of the TiO2-PILCs

The abbreviated symbols of the prepared TiO2-pillared clays and their synthesis conditions are listed in Table l, and Figure 1 shows the XRD patterns of the clay and the synthesized TiO2-pillared clays. The d001 peak for the unpillared clay was at 20=7.8. Upon intercalation the d001 peak

898 Table 2. Summarized pl Lysical properties of the TiO2-PILCs. do01 Surface area Symbol (A) (mZ/g) clay 11.3 33 TiO2-PILC-A 29.4 389 TiO2-PILC-B 29.2 321 TiO2-PILC-C 28.8 298 TiO2-PILC-D 24.8 169 TiO2-PILC-E 25.6 184

Pore volume (cm3/g) 0.06 0.25 0.21 0.21 0.13 0.16

of the unpillared clay was almost disappeared, which indicates that the whole clay was successfully pillared. In addition the d001 peak shifted toward lower 20 value at around 4 ~ corresponding to the increase in d001 spacing up to 29.4A as shown in Table 2. Einaga[11] suggested the existence of titanium hydroxocomplex[(TiO)8OH12] 24+ during TiCI4 hydrolysis and polymerization. Sharygin et al.[12] proposed that the titanium hydroxocomplex is the main intermediate to be transformed into hydrated titanium dioxide. The chemistry of titanium is very complex, and the pathways of the TiCI4 hydrolysis and polymerization cannot be presently elucidated. The TiO2 pillaring process is, however, believed to proceed via the formation of titanium hydroxocomplex. As shown in Figure 1 and Table 2, there was a slight decrease in the d001 spacing with decreasing concentration of TIC14 and increasing HC1 concentration. Nabivanets and Kudritskaya[13] showed that the polymerization increases with increasing concentration of titanium and chloride, and proposed that the polymeric species are probably linked by chloride. When the titanium concentration is low, the possibility of the formation of polymeric species becomes lower. The aforementioned decrease in the d001 spacing with decreasing TiCI4 concentration seems to be due to the lower possibility of formation of the polymeric species. If the polymeric species are linked by chloride as suggested by Nabivanets and Kudritskaya[ 13], the d00~ spacing will increase with increasing HC1 concentration. Contrary to the suggestion, the d001 spacing decreased slightly with increasing HC1 concentration. Accordingly the polymeric species are not believed to be linked by chloride, but HC1 seems to play a role on inhibiting the hydrolysis process. The interlayer spacings of the TiO2-PILCs ranged from 24.8 to 29.4A. These values are much higher than those of other published pillared clays. This large pore size is desirable for the application of the TiO2-PILCs as catalyst supports. All the prepared TiO2-PILCs showed typical type I7 nitrogen adsorption isotherms, which indicated the presence of both micropores and mesopores in the pillared samples. In addition the hysterisis loop of the TiO2-PILCs exhibited H3 type, revealing the slit-shaped pore structure. The incorporation of the titanium between the silicate layers increased both the surface area and pore volume significantly. The prepared TiO2-PILCs had surface areas higher than 160m2/g(the surface area of the unpillared clay was 33m2/g). The pore volumes of the pillared samples were at around 0.2cm3/g which was remarkably higher than 0.06cm3/g of the unpillared clay. This intercalation effect on surface area and pore volume was especially outstanding when the pillaring process was performed under the lowest HC1 concentration and the highest TIC14 concentration(TiO2-PILC-A sample).

899

:l

8

o e~

I

100

I

I

200 300 Temperature (~C)

I

400

500

Figure 2. TPD spectra of NH3 on the TiO2-PILCs ((A)TiO2-PILC-A, (B)TiO2PILC-B, (C)TiO2-PILC-C, (D)TiO2-PILCD, (E)TiO2-PILC-E).

1750

1650 1550 1450 Wavenurnber(cm -1)

1350

Figure 3. FTIR spectra of the adsorbed NH3 on the TiO2-PILCs ((A)TiOE-PILCA, (B)TiOE-PILC-B, (C)TiOE-PILC-C, (D)TiO2-PILC-D, (E)TiO2-PILC-E).

3.2 Acidity analysis of the TiO2-PILCs The NH3 TPD spectra of the unpillared clay and the TiO2-PILCs are shown in Figure 2. While the desorbed amount of NH3 from the unpillared clay was almost negligible, two distinct desorption peaks at around 220 ~ and 330 ~ were obtained from the TiOE-PILCs which did clearly chemisorb substantial amount of the strongly bound NH3. Moreover the intensity of the higher temperature desorption peak was much weaker than that of the lower temperature peak. FTIR spectra of the adsorbed NH3, as shown in Figure 3, could be used to investigate the nature of acid sites. The band at around 1450cm-' came from the asymmetric bending vibration of NH4§ on BrOnsted acid sites and the asymmetric bending mode of ammonia on Lewis acid sites appeared at approximately 1620cm-1. For all the TiO2-PILC samples there existed substantially more BrOnsted acid sites than Lewis site. By comparing with the FTIR spectra, the NH3 desorption peaks at around 220 ~ and 330 ~ could be deduced to be due to the NH3 adsorbed on BrOnsted and Lewis acid sites, respectively. TiO2 alone does not show strong acidity. The bulk mixed oxides, especially SiO2-TiO2 mixed oxide, developed a greater acidity than individual phases[14]. The acid sites in the TiO2-PILCs are believed to locate mainly at the interface between silicate layers and TiO2 pillars. Since the presence of acid sites for the adsorption of NH3 is important to SCR reaction with NH3, the TiO2-PILC samples were used as catalyst supports. 3.3. SCR reaction on the iron-doped TiO2- PILCs The Fe203-impregnanted TiO2-PILCs were employed as catalysts for the SCR of NO with ammonia. Figure 4 shows the plots of NO conversion versus reaction temperature. When

900 100

Fe203 was impregnated on the TiO2-PILC-A having the highest values of surface area and pore 80 size, complete conversion of NO A could be achieved in the temperature window of 375-400 ~ .9 60 At this temperature window the E TiO2-PILC-A alone showed just = 25% NO conversion. The o 40 O incorporation of Fe203 must have . o . Fe20 ,O2-P,LO-B enhanced the SCR activity O z --V- Fe203/TiO2-PILC-C remarkably. 2O --W-- Fe2Ofrio2-PlLC-D Figure 5 and 6 exhibit the results --II- Fe2Ofrio2-PlLC-E of NH3 TPD and FTIR spectra of NH3 on the iron-doped TiO2-PILCs, 0 respectively. By the presence of 200 250 300 350 400 Fe203 the intensity of BrOnsted acid sites increased significantly, Temperature(~ and there had existed a direct Figure 4. Temperature dependence of NO correlation between the SCR conversion on the iron-doped TiO2-PILCs(0.2g activity and the intensity of the catalyst, 480cm3(STP)/min flow rate). BrOnsted acid sites. Since little amount of NO could have adsorbed on the surface of the Fe203/TiO2-PILCs at temperatures ,

!

|

!

8 ~)

0

I

100

I

200

3(;0

Temperature(~

400

500

Figure 5. TPD spectra of NH3 on the irondoped TiO2-PILCs ((A)Fe203/TiO2-PILC-A, (B)Fe203/TiO2-PILC-B, (C)Fe203/TiO2-PILC -C, (D)Fe203/TiO2-PILC-D, (E)Fe203/TiO2PILC-E).

1750

I

I

I

1650 1550 1450 1350 Wavenumber(cm "1) Figure 6. FTIR spectra of the adsorbed NH3 on the iron-doped TiO2-PILCs ((A) Fe203/~O2-PILC-A, (B)Fe203/TiO2-PILCB, (C)Fe203/TiO2-PILC-C, (D)Fe203/TiO2PILC-D, (E)Fe203/TiO2-PILC-E).

901 higher than 325 ~ the SCR of NO is believed to proceed via an Ely-Rideal type mechanism which is generally accepted for the SCR with the conventional V2Os/TiO2-based catalysts[ 15-19]" Ammonia chemisorbs mainly on the BrOnsted acid sites in the Fe203/TiO2PILCs. Weakly bound NO and/or gas phase NO will react with the chemisorbed NH3 to produce N2. 4. CONCLUSION TiO2-pillared clays were synthesized under different HC1 and TIC14concentrations. Upon intercalation the d001 spacing increased up to 29.4A depending on the concentrations of HC1 and TIC14. All the prepared TiO2-PILCs had both the micropores and mesopores. The surface area, pore volume and d001 spacing increased with increasing TiCI4 and decreasing HC1 concentration. The prepared TiO2-PILCs showed the presence of both the BrOnsted and Lewis acid sites which were believed to be formed mainly at the interface between silicate layers and TiO2 pillars. Moreover there existed substantially more BrOnsted acid sites than Lewis sites. When iron was doped onto the TiO2-PILCs, the acidity, especially BrOnsted acidity, increased significantly. This wide presence of BrOnsted acid sites made the irondoped TiO2-PILCs successful catalyst for the SCR of NO with ammonia. The SCR reaction was suggested to proceed via an Ely-Rideal type mechanism 9Ammonia adsorbs mainly on the BrOnsted acid sites in the Fe203/TiO2-PILCs. Weakly bound NO and/or gas phase NO will then react with the chemisorbed NH3 to produce N2.

REFERENCES

1. P. Canizares, J.L. Valverde, M.R. Sun Kou and C.B. Molina, Microporous and Mesoporous Materials, 29 (1999) 267. 2. R.T. Yang and M.S.A. Baksh, AIChE J., 37 (1991) 679. 3. L.S. Cheng and R.T. Yang, Ind. Eng. Chem. Res., 34 (1995) 2021. 4. A. Vaccari, Catal. Today, 41 (1998) 53. 5. J. Barrault, M. Abdellaoui, C. Bouchoule, A. Majeste, J.M. Tatibouet, A. Louloud, N. Papayannakos and N.H. Gangas, Appl. Catal. B, 27 (2000) L225. 6. H.L. Delcastillo, A. Gil and P. Grange, Catal. Lett., 36 (1996) 237. 7. A. Bernier, L.F. Admaiai and P. Grange, Appl. Catal., 77 (1991) 219. 8. J. Sterte, Catal. Today, 2 (1998) 219. 9. H.L. Del Castillo and P. Grange, Appl. Catal. A, 103 (1993) 23. 10. R.F. Hicks, C.S. Kellner, B.J. Savatsky, W.C. Hecker and A.T. Bell, J. Catal., 71 (1981) 216. 11. H. Einaga, J. Chem. Soc. Dalton Trans., (1974) 1917. 12. L.M. Sharygin, S.M. Vovk and V.F. Gonchar, Russ. J. Inorg. Chem., 33 (1988) 970. 13. B.I. Nabinavets and L.N. Kudritskaya, Russ. J. Inorg. Chem., 12 (1969) 611. 14. K. Shibata, T. Kiyoura, J. Kitagawa, T. Sumiyoshi and K. Tamabe, Bull. Chem. Soc. Jpn., 46 (1973) 2985. 15. A. Miyamoto, K. Kobayashi, M. Inomata and Y. Murakami, J. Phys. Chem., 86 (1982) 2945. 16. J.A. Dumesic, N.Y. Topsoe, H. Topsoe, Y. Chem and T. Slabiak, J. Catal., 163 (1996) 409.

902 17. G. Busca, L. Lietti, G. Ramis and E Berti, Appl. Catal. B, 18 (1998) 1. 18. L. Lietti, G. Ramis, E Berti, G. Toledo, D. Robba, G.Busca and P. Forzatti, Catal. Today, 42(1998) 101. 19. N.Y. Topsoe, J.A. Dumesic and H. Topsoe, J. Catal., 151 (1995) 241.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

903

Sulfated Zr-pillared saponite : preparation, properties and thermal stability L. Bergaoui a, A. Ghorbel a and J.-F. Lambert b aLaboratoire de Chimie des MatEriaux et Catalyse, Facult6 des Sciences de Tunis, Universit6 E1 Manar, Campus Universitaire, 1060 Le B61ved~re, Tunis, Tunisie bLaboratoire de R6activit6 de Surface (URA 1106), Tour 54-55, 2~me 6tage Universit6 Pierre et Marie Curie, 4, Place Jussieu, 75252 - Paris cedex 05, France The intercalation of a saponite with zirconium oligomers containing variable amounts of sulfate has been studied (SO4:Zr molar ratio between 0 and 0.3). Well-ordered intercalated clays with basal spacing between 18 and 20 A were obtained. For the higher SO4:Zr ratio, a highly polymerized species is intercalated, giving a nanocomposite material with a low surface area (50 m2/g). For the lower ratios, higher surface areas are obtained (160 m2/g). After calcination of those clays, the presence of sulfate induces a loss of cristallinity but surface areas remain important. Thermal evolution of the intercalated compound was followed by mass spectroscopy showing that sulfur is not eliminated from the solids before 450~ (under helium). Above this temperature, two distinct thermal events were observed, suggesting two different modes of linking of sulfates with the polycation, in agreement with Raman data on the SO4-Zr intercalating solution. 1. INTRODUCTION The modification of swelling clays by "pillaring" is a simple way to prepare materials with new physical and chemical properties. The intercalation of large size inorganic cations (pillars precursors) between the clay layers, followed by calcination, allows the preparation of thermally stable microporous solids [1-4]. The chemical properties of the pillared clays obtained in this way depend on the constitution of the clay layers [5] and the nature of the intercalating agent [6]. The polycations intercalated during the first step of the synthesis are obtained by partial hydrolysis of aqueous solution of multivalent cations. In particular, to prepare zirconium-pillared clays, zirconium oxychloride solutions are commonly used as the pillaring precursors. It has been shown that the predominent polycations in these solutions have a structure built up from [Zr4(OH)8(H20)16] 8+ tetramers [7]. Since the promotion of zirconium oxide by sulfates is widely used to prepare strong acidic solids [8], it was tempting to try similar modifications on zirconium-pillared clays. Indeed, the introduction of sulfate ions into a zirconium pillared clay has been shown to enhance the acidity of this solid but it also decreases its thermal stability and its porosity [9].

904 In the present work, we have tested a new preparation method to optimise the structural properties of sulfate-promoted zirconium-pillared clays. In particular, the influence of the amount of sulfate on the structure and the thermal stability of Zr-pillared clays has been studied. 2. E X P E R I M E N T A L SECTION

2.1. Materials A sample of saponite from Ballarat (USA) was obtained from the Source Clays Minerals Repository and its < 2 [tm fraction was separated by gravity sedimentation. It was then exchanged three times with a 1 mol/L NaC1 solution and washed thoroughly. Chemical analysis yielded the following formula for the Na-exchanged saponite : Nao.600Cao.028K0.006(Si7.262A10.738) (Mg5.920Fe2+0.104Tio.006Mn0.002)O20(OH)4 There are only minor differences with the formula proposed by Prost [ 10] for the Ballarat saponite. 2.2. Intercalation and pillaring procedures Ammonium sulfate was added to freshly prepared 0.05 mol/L ZrOC12 solutions, with SO4/Zr molar ratios varying from 0 to 0.3. Intercalated saponites were then prepared by adding the SO4/Zr solution dropwise to 10 g/L clay suspension until an Zr/Clay ratio of 5 mmol/g was reached. The slurry was stirred overnight at 50~ washed by successive dialyses, and dried in air at room temperature, giving SO4/Zr-intercalated clays. The samples were then calcined in flowing oxygen. The temperature was raised at 60~ to 400~ and the final temperature was maintained for 4 h. The term of " pillared " clays will here be reserved to samples stabilised by calcination. The intercalated (uncalcined) saponites will be called ZrI-r, and the corresponding pillared saponites will be called ZrP-r (where r is the SO4/Zr molar ratio in the intercalation solution). 2.3. Samples characterisation A SIEMENS X-Ray D 500 diffractometer using Cu Kc~ radiation was used to record powder diffractograms. Oriented ZrI-r samples were prepared by air drying of a clay slurry on a glass plate. The ZrP-r powder samples were pressed onto a glass holder. Surface area measurements were performed by nitrogen physisorption at 77 K using a static volumetric apparatus (Micromeritic ASAP 2000 adsorption analyzer) ; the BET equation was applied to the adsorption isotherm. All samples pretreated in vacuum at 110~ prior to nitrogen physisorption. Elemental analyses for zirconium and sulfur were performed at the Centre d'Analyse du CNRS (Vernaison, France). Thermal treatments of the intercalated solids from room temperature to 1000 ~ were carried out in quartz reactors under helium flow, with a flow rate of 3 cc/mn. A Hiden Analytical HPR 20 Mass spectrometer (MS) was used to conduct evolved gas analysis upon thermal treatment, in the mass range between 1 and 200 a.m.u. A capillary leak maintained at 170 ~ was used to divert a fraction of the gas flow to the analysis chamber. 2.4. Raman Raman measurements were performed using the 514.5 nm line of a coherent argon ion laser. The laser beam power was 80-100 mW and the resolution was set at 1 cm -1. The spectra were recorded using a Jobin-Yvon U1000 spectrometer over two frequency ranges 850-1200 cm-1 for sulfate vibrations and 300-600 cm-1 for Zr-O vibrations.

905 3. R E S U L T S AND D I S C U S S I O N

3.1. Solution Raman study Figure 1 shows the Raman spectrum of the intercalation solution with a SO4:Zr ratio = 0.2 in the 300-600 cm -1 region. The low zirconium concentration results in a poor resolution because of interference with the bands of water [ 11 ]. Nevertheless, the band at 430 cm -1 can be attributed to the presence of the tetrameric form [Zr4(OH)8(H20)16] 8+ and/or of a more hydrolyzed species in solution [ 11 ]. In the higher wavelength region a sharp band is apparent at 983 cm-1 for the 10-2 mol/L (NH4)2SO4 solution (figure 2-a). The 5.10 -2 mol/L ZrOC12 solution with a SO4:Zr ratio = 0.2 shows a non symmetric band at 1012 cm -1 (figure 2-b). In spite of the intensity of the noise, we can see two shoulders at 986 cm -1 and at 999 cm -1. The band at 983-986 cm -1 for both solutions can be assigned to a free sulfate in solution. It seems reasonable to attribute the bands at 1012 cm -1 and the shoulder at 999 cm -1 for the SO4-Zr solution to two types of interaction of (SO4) 2- with the zirconium polycations. 3.2. Amount of fixed zirconium and sulfate As seen in table 1, the amount of zirconium fixed by ZrI-0 is 20.7 wt % (or 28 wt % of ZrO2). This amount corresponds to one pillar per 2.3 unit cells. If we suppose that all the sodium ions have been exchanged with hydroxy-zirconium cations, the positive charge per intercalated Zr4-polycation should be close to 1.4 to compensate the negative layer charge. Former studies of Zr-pillared clays have reported very different amounts of intercalated Zr, depending on the host clay and the pillaring method used. In the classical work of Yamanaka and Brindly, the amount of zirconium fixed was about 13.9 wt % of ZrO2 [12]. The intercalation reaction was carried out at room temperature in this case. The higher amount of fixed zirconium in our work is probably do to the temperature of intercalation ; in fact, an amount of fixed Zr closer to our value has been reported by Farfan-Torres et al., where the suspension was stirred at 40 ~ during the intercalation [ 13]. When the SO4:Zr ratio increases, the amount of fixed zirconium increases (table 1). This trend is not surprising if sulfate anions are cointercalated, since the Zr-containing polycations would have to compensate for their negative charge as well. Table 1 also shows the S:Zr molar ratios in the intercalated clays as a function of the SO4:Zr ratio in solution. For ZrI-0.1 and ZrI-0.15 we find the same ratios in the intercalated solids as in the intercalating solutions : apparently, most of the sulfate is bonded to the Zr polycations in the solution. On the other hand, for the higher SO4:Zr ratio, part of the sulfate seems to remain free in solution, in agreement with the Raman data. Table 1. Quantities of zirconium and sulfur fixed on intercalation of saponite for different SO4:Zr ratio. SO4:Zr molar ratio in solution wc % Zr wt. % S S:Zr molar ratio in solid 0

20.7

< 0.1

0

0.1

21.2

0.65

0.09

0.15

22.3

1.15

0.15

0.2

22.5

1.25

0.16

0.3

26.8

2.27

0.24

906

1012 0

9 9 9 ~d

430

I

987'

. ,...q r~

~ _ _ . . . . . ~ ' "

300

'I " '

'I "

"l

"

"

I"

"

I' " '

I' "

400 500 600 Wavelenght, crn1

Figure 1. Raman spectrum of ZrOC12 solution with SO4:Zr ratio = 0.2.

'''

850

'1''''1

l

b

~_.

''''l''

.....

''l''"'

.

._

.a

I''''1'"''

950 1050 1150 Wavelength, cni s

Figure 2. Raman spectra of (NH4)2SO4 solution (a) and ZrOC12 solution with SO4:Zr ratio = 0.2 (b).

3.3. X-Ray diffraction and N2 physisorption The XRD pattems of the intercalated samples are shown in figure 3. In the case of ZrI-0, two main peaks are observed at 20.2 A and 11.4 A. Those peaks could correspond to the first and second order, respectively, of the (001) family of planes ; the difference between the apparent d001 and 2d002 could be explained by interstratification of intercalated and unintercalated interlayers. Similar results are reported by Yamanaka and Brindly [12] who suggest that the intercalated species is [Zr4(OH)16_n(H20)8+n] n+. When (SO4)2" is added to the solution of intercalation, the d001 and d002 peaks shift to higher theta values, their intensity decreases and their width increases. Thus, the higher the amount of fixed sulfate, the lower the cristallinity. After calcination at 400~ the ZrP-0 shows a d001 basal spacing at 19.4 A (figure 4-a), slightly lower than after intercalation. This is probably due to the departure of the hydration water of polycations. This could be sufficient to induce some loss of crystallinity, but it is likely that the pillars mostly retain their structure upon moderate heating, as happen for aluminum intercalated clays [14]. For those Al-clays, [All3] polycations release proton from some of terminal (H20) ligands but the rest of the polycation structure is not much affected [15]. Indeed, Mieh6-Brendl6 et al. [ 16] have shown by EXAFS that the nearly square frame of Zr4 zirconyl units is preserved after calcination at moderate temperatures. For samples prepared with sulfates (figure 4-b to 4-e) the dool peaks are broad after calcination, indicative of a loss of structural organization.

907

20.2/k

11.4.& .9]k .;.%:..----;

a :

--.,

_

---

.

L _ / ~ .gA f4i" 10.7A L % ~ " : - : -

I'"l'"l'"t"'l"

2

6

2O

10

:"- cbl

';'i'

14

18

Figure 3. XRD patterns of ZrI-r: (a) r = 0, (b) r = 0.1, (c) r = 0.15, (d) r = 0.2 and (e) r = 0.3.

2

6

10

14

18

20 Figure 4. XRD patterns of ZrP-r: (a) r = 0, (b) r = 0.1, (c) r = 0.15, (d) r = 0.2 and (e) r = 0.3.

Table 2. BET surface areas of pillared saponite for different SO4:Zr ratio. SO4:Zr molar ratio in solutioz Surface area (m2/g) Before calcination After calcination 0 178 191 0.1 160 154 0.15 155 157 0.2 139 153 0.3 59 70

Table 2 shows the BET surface areas of intercalated and pillared saponites. When the quantity of sulfate increases, the surface area decreases but remains rather high except for the sample prepared with the highest SO4:Zr ratio where the decrease is more drastic. The surface area does not change much upon calcination. All of these data indicate successful intercalation of Zr polycations, at least for SO4:Zr ratios lower than 0.3 which have both high interlayer distances (9 to 10 ]k) and high surface areas (between 160 and 139 m2/g). The state of the intercalated sulfate cannot be ascertained from textural data alone, but a preliminary comment can be made. No significant difference in d001 is apparent as a function of the SO4:Zr ratio in the solution, suggesting that the intercalated species all have approximately the same gyration radius. This is not unlikely since the solid contain less than one sulfate ion per Zr4 polycation on average 9therefore, the most likely intercalated species should consist in sulfate-free polycations, and polycations with at least one sulfate attached. The calcination affect differently those species that induces

908 a deterioration of the XRD pattern but surface areas remain high: the layers do not collapse at 400 ~ For a SO4:Zr ratio = 0.3, on the other hand, the low surface area together with a high interlayer distance (about 9 A before calcination) could be explained by extensive intercalation of bulkier species, clogging up the interlayer space. 3.4. Thermal stability of sulfate Thermal treatment of the ZrI-0.1 sample under an inert gas (helium) between 50 and 1000 ~ resulted in MS peaks corresponding to the masses : 16 (NH2 +, O+), 17 (OH+), 18 (H2 O+ and NH4+), 32 (O2+), 48 (SO +) and 64 (SO2+). Figure 5 shows the evolution of the peaks at 16, 17 and 18 a.m.u., which exhibit similar patterns for all pillared clays samples. The departure of a large quantity of physisorbed water is observed at 100 ~ as expected. These peaks show a secondary maximum at 800~ corresponding to the condensation of hydroxy groups of the clay octahedral sheets. Water evolution only stops at very high temperatures (> 900~ Featureless H20 loss between 300 and 700~ can arise from dehydration and dehydroxylation processes of the zirconiumcontaining pillars. The evolution with temperature of MS peaks at 48 a.m.u. (SO +) and 64 a.m.u. (SO:z+) is shown in figure 6 for the ZrI-0.1 sample. The elimination of sulfur-containing compounds only occurs above 450~ proving the high thermal stability of sulfate species. Two separate thermal events can be observed : the first one has a maximum at 550~ and the second one at 750 ~ This profile could be explained by the existence of two interaction modes between (SO4) 2- ions and the zirconium tetramers. Figure 7 compares the profiles of the signal at 64 a.m.u. (SO2 +) for the samples prepared with solutions with different SO4:Zr ratios (0.1, 0.15, 0.2 and 0.3). As the amount of sulfate increases, the relative intensity of the second peak (at 750 ~ with respect to the first one also increases. Sample ZrI-0.3 shows a more complex phenomenon (figure 7-d) where the two peaks are larger and hardly distinguishable.

_t ~'''l'''l'''l"'l'

100 300 500 700 900 Temperature (~ Figure 5. Profile of the MS signals at 18 a.m.u. (a), 17 a.m.u (b) and 16 a.m.u. (c) upon heating the ZrI-0.1 sample.

200

400 600 800 Temperature (~

1000

Figure 6. Profile of the MS signals at 64 a.m.u. (a) and 48 a.m.u (b) upon heating the ZrI-0.1 sample.

909

400

500

600 700 Temperature (~

800

Figure 7 : Profile of the MS signals at 64 a.m.u, for ZrI-r: (a) r = 0.1, (b) r = 0.15, (c) r = 0.2, and (d) r = 0.3. At this point of the discussion, we may try to put forward some hypotheses on the nature of the intercalated species, although they will remain temptative due to the lack of quantitative data and the complexity of these systems [17]. In solution, the zirconium tetrameric polycation (figure 8, species a) seems to exist in equilibrium with a sulfated tetramer. Raman spectroscopy and mass spectroscopy reveal the presence of two modes of interaction between sulfate ions and zirconium tetramers. It is possible to present some simple ideas based on our observations. These two modes of interaction may correspond to (SO4) 2- linked to one tetramer (figure 8, species b) and (SO4) 2- bridging two tetramers (figure 8, species c), respectively. Zr~

Zr~ j Zr ~ , . ,

j Zr

,.,/Zr~

j Zr

/ / Zr-v / / ~ ZJ~ . .Zr Z. .r.-[- 0\0/S~ 0~ r/Z ~~ / / ~Zr\ 0 U / S \ / t o O ~ ! ~ r ~ / / ~ ~ Zr

~~.~ Zr Zr4 tetramer

zU

Zr.

0 /~Z~~

Oz Z .zr

j

z

o,,

Zr

o

II

Z

r- O - S - 0G Z

r-O--

O--

r

0 Z (a)

(b)

(c)

Figure 8. Schematic view of different species existing in a zirconyl solution with a low SO4:Zr ratio.

910 4. CONCLUSION The intercalation of saponite clays with sulfated zirconium tetramer has been clearly evidenced, even though the chemical nature of the intercalating species remains somewhat hypothetical. Adding mixed SO4:Zr solutions to a saponite suspension gives an intercalated clay with a surface area of about 150 m2/g and a d001 peak around 18 A. At this point, the interlayer contains heterogeneous species : non-sulfated Zr tetramers, and sulfated polymers with two different modes of sulfate/polycation binding. After calcination at 400~ the surface area is still important but a loss of cristallinity is observed, probably because the sulfated and the non-sulfated zirconium species react differently up calcination. The sulfated species in these solids are not eliminated before at least 450~ ; the two modes of sulfate/polycation binding have significantly different stabilities. High SO4:Zr ratios should be avoided, because they result in the intercalation of bulky species, giving solids with a low surface area. In this paper, we have established a correlation between the properties of the intercalant solution and the textural properties of the resultant solids. The description of the acid sites needs more techniques (FTIR, model catalytic reaction). After calcination of the solid, the interaction between sulfate ions and zirconium polycations changes and the correlation with study of the solution will not be perhaps evident. References

1. D.E.W. Vaughan, Catal.Today, 2 (1988) 187. 2. F. Figueras, Catal. Rev. Sci. Eng., 30 (1988) 457. 3. M. L. Occelli, Physicochemical proporties of pillared clay catalysts. In: Keynotes in Energy-Related Catalysis. S. Kaliaguine (eds.), Elsevier, Amesterdam. Stud. Surf. Sci. Catal., 35 (1988) 101. 4. I. V. Michell, Pillared Layered Structure: Current Trends and Applications. Elsevier, London, (1990). 5. L. Bergaoui, I. Mrad, J.-F. Lambert and A. Ghorbel, J. phys. Chem. B, 103 (1999) 2897. 6. J. -F. Lambert and G. Poncelet, Top. Catal. 4 (1997) 43. 7. A. Clearfield and P. Vaughan, Acta Crystallogr. 9 (1956) 555. 8. G. D. Yadave and J. J. Nair, Microporous Mesoporous Materials, 33 (1999) 1. 9. E. M. Farfan-Torres, E. Sham and P. Grange, Catal.Today, 15 (1992) 515. 10. J. L. Post, Clays Clay Miner., 32 (1984) 147. 11. S. Hannane, F. Bertin and J. Bouix, Bull. Soc. Chim. Fr., 127 (1990) 43. 12. S. Yamanaka and G. W. Brindly, Clays Clay Miner, 27 (1979) 119. 13. E. M. Farfan-Torres, O. Dedeykcker and P. Grange, Preparation Of Catalysts V, G.Poncelet, P. A. Jacobs, P. Grange and B. Delmon, (eds.), Elsevier, Amsterdam, (1990) 337. 14. J.-F. Lambert, S Chevalier, R. Franck, H. Suquet and D. Barthomeuf, J. Chem. Soc., Faraday Trans. 90 (1994) 675. 15. L. Bergaoui, J.-F. Lambert, R. Franck and H. Suquet, J. Chem. Soc., Faraday Trans. 91 (1995) 229. 16. J. Mieh6-Brendl6, L. Khouchaf, J. Baron, R. Le Dred and M. -H. Tuilier, Microporous Materials, 11 (1997) 171. 17. A. Clearfield, Rev. Pur. App. Chem, 14 (1964) 91.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

911

I s o m e r i z a t i o n and h y d r o c r a c k i n g o f n-decane over P t - P d / A 1 M C M - 4 1 catalysts S.P. Elangovan, Christian Bischof and Martin Hartmann* Department of Chemistry, Chemical Technology, University of Kaiserslautern, P. O. Box 3049, D-67653 Kaiserslautern, Germany

Bimetallic Pt-Pd clusters supported on an A1MCM-41 ( n s i / n A 1 - 23) mesoporous molecular sieve have been tested in the isomerization and hydrocracking of n-decane. It has been found that the catalytic activity of the bimetallic Pt-Pd catalysts is higher than that of the monometallic Pt and Pd catalysts. Moreover, cracking on the metal sites (hydrogenolysis) is largely suppressed over certain bimetallic catalysts. These results are ascribed to a better balance between acid and the metal function of bifunctional Pt-Pd/A1MCM-41 (23) catalysts, which results in a higher isomer yield at a substantially lower reaction temperature.

1. INTRODUCTION Bifunctional metal/acid zeolite catalysts are used in various industrial processes, viz. isomerization of CJC6 alkanes, hydrocracking, dewaxing, and isomerization of C8 aromatics. The hydroconversion of n-alkanes is achieved over bifunctional catalysts containing fine dispersed noble metal clusters on a matrix which contains Bronsted acid sites. During the reaction, the noble metal catalyzes hydrogen transfer reactions (hydrogenationdehydrogenation), while isomerization and hydrocracking occur on the Bronsted acid sites [1]. For a catalyst where the metal function and the acid function are well balanced, isomerization and hydrocracking are consecutive reactions and the rate-limiting step is the skeletal rearrangement of the alkenes obtained via dehydrogenation over the metal sites, which takes place on the Bronsted acid sites [2]. Platinum- and palladium-containing zeolites are known to give high isomerization yields at medium conversion levels [3], whereas at high conversion level hydrocracking becomes dominant due to faster cracking of the branched isomers. The exact value of the isomerization maximum is expected to be independent on the balance between the two catalytic functions, viz. the density and the strength of Bronsted acid sites and the nature, amount and dispersion of the metal. Only little attention has been paid to study the influence of bimetallic clusters. Zeolite Beta and Y containing Pt-Pd clusters were tested in the isomerization of n-heptane [4], while Meriaudeau et al. investigated the isomerization of n-octane over Pt-Pd/SAPO-11 and Pt-Pd/SAPO-41 catalysts [5]. Since the discovery of the new class of mesoporous materials [6], much research work has been devoted to the evaluation of their catalytic potential. Because of their medium acidity and the possibility to vary the nsi/nA1-ratio in a wide range (~10- oo) without significant changes in pore structure, these materials are very attractive model catalysts for the hydroconversion of normal paraffins. Del Rossi et al. [7] reported that the selectivity for nhexane isomers is higher on Pt/MCM-41 as compared to amorphous silica-alumina. At

912 equivalent conversion, the yield of cracked products is significant lower on Pt/MCM-41. The hydroconversion of n-hexane was also studied over a series of aluminum-containing MCM-41 materials with different nsi/nAl-ratios and varying platinum content by Chaudhari et al. [8]. High selectivities for feed isomers were reported for an optimized metal/acid-site ratio. Mokaya et al. [2] studied the hydroconversion of n-heptane over a series of platinumcontaining mesoporous molecular sieves prepared with dodecylamine and found remarkable selectivities for aromatics at higher conversion levels in particular when Pt/A1MCM-41(20) was used as a catalyst. Klemt et al. [9] reported recently that NiMo/A1MCM-41/ZSM-5 composite catalysts exhibit higher activity in n-decane cracking and higher selectivity for monobranched isomers than a NiMo/A1MCM-41 catalyst. In the present study, Pt and Pd containing monometallic and bimetallic clusters supported on aluminum-containing mesoporous molecular sieves (A1MCM-41) were tested in the hydroconversion of n-decane. It was found that catalysts containing both platinum and palladium lead to superior catalytic performances with respect to activity and isomerization yield. A close inspection of the cracked products revealed that hydrogenolysis is suppressed to a large extend over the bimetallic systems. 2. EXPERIMENTAL SECTION A1MCM-41(nsi/nAl=23) was synthesized using tetradecyltrimethylammoniumbromide (C14TMABr), sodium water glass, sodium aluminate and diluted sulfuric acid according to reference [ 10]. After synthesis, the sample was calcined at 540 ~ for 10 h and was repeatedly ion-exchanged with ammonium nitrate at 40 ~ The metal incorporation was carried out by ion exchange with Pt(NH3)4CI2 and Pd(NH3)4CI2, respectively. To obtain catalysts with similar molar metal loadings, the molecular sieve was loaded with 0.27 wt.-% Pd, 0.5 wt.-% Pt and different mixtures of the two metals. The metal precursor was diluted in 50 ml of distilled water and the solution was added drop wise to 1.5 g of the molecular sieve slurred in 50 ml of distilled water. After stirring the solution for 6 h at 40 ~ the water was removed in a rotary evaporator at 70 ~ under vacuum. Thereafter the samples were dried at 110~ for 12 h. The resulting materials were pressed without binder at moderate pelletizing pressure, crushed and sieved to obtain catalyst particles with a diameter of 0.25 to 0.355 mm. The catalytic experiments were carried out in a fixed bed flow-type apparatus with on-line gas chromatographic analysis of the reaction products. The experiments were conducted at a hydrogen pressure of 1 MPa (10 bar) and the pressure of n-decane was adjusted to 10 KPa to obtain a nHz/n,-aecane ratio of 100. The dry mass of the catalysts and the hydrogen gas flow were adjusted to achieve a modified residence time W c a t . / F n - d e c a n e - 400 g.h.mol 1. The conversion of n-decane was varied by varying the reaction temperature. Prior to the catalytic experiments, the catalysts were dehydrated at 260 ~ for 2 h in a flow of argon, activated in oxygen at 310 ~ for 4 h and flushed with argon at 410 ~ for 4 h. Finally the metal clusters were prepared by hydrogen reduction at 310 ~ for 4 h. 3. RESULTS AND DISCUSSION Specific surface Area (ABET = 1100 m2/g), specific pore volume (Vpore = 0.67 cm3/g) and the pore diameter (dpore = 2.4 nm) of the A1MCM-41(23) sample used in this investigation corresponds well to those data reported in the literature [1 0]. 27A1 MAS NMR spectra (not shown) reveal that aluminum is exclusively tetrahedrally coordinated in the A1MCM-41 (23) material.

913 oo 90 W/F = 400 g.h.mol1 Pn-d. . . . " 10 kPa C ~ 80 PH2 -1 MPa ./~ =~

/ #

~ ~/-

50

70

c o

30

(')

20

~ H~ / //~

'

j \ |

~ 30

~

40

>

0.375Pt-0.068Pd _A 0.25Pt- 0.135Pd I

"O

50

~

~

~ 40

~' 60

"1o

Xc.. .o_

,I

' .........

o

20 10

10 o~

200

250

300

350

.....

o ,'o'g~

400

~or176

200

Reaction T e m p e r a t u r e / ~

' . . . . . . . . . .

250

300

~1

350

400

Reaction Temperature / ~

Fig. 1" Conversion of n-decane (left) and isomer yields (right) as a function of the reaction temperature over different Pt-Pd/A1MCM-41 catalysts.

100 9O end ....

o-e.

8O

PH2

kPa = 1M

--10

~(("

~= 70 8

~

3o

0

20

50

'"

.

.

.

,

.

.

.

.

.

.

|

.

.

.

.

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.

.

.

.

|

.

.

.

.

|

i 0.25Pt- 0.135Pd 0.125Pt - 0.068Pd

"o

~ 3o

x~ 5o 4O

~.

~o 40

60

"~

60

~ 2o ]1~'? _,~

I-~-- 0"5Pt- 0"27Pd_ _ I~ 0.25Pt- 0.135Pd ]

10

10 O~ 200

250 3o0 350 Reaction Temperature / ~

400

0~ j 200

.

.

250

300

I

350

400

Reaction Temperature / ~

Fig. 2: Conversion of n-decane (left) and isomer yields (right) over Pt-Pd/A1MCM-41 catalysts with different metal content. In Figure 1, the results of n-decane conversion over bimetallic Pt-Pd catalysts with different npt/npd ratios are depicted in comparison to the monometallic catalysts. Conversion of n-decane over the monometallic catalysts 0.27 Pd/HA1MCM-41 and 0.5 Pt/A1MCM-41 starts at reaction temperatures around 250 ~ (Figure 1 (left)). For conversions up to 30 %,

914 isomerization is the sole reaction. With increasing conversion, hydrocracking of C~0 isomers occurs as a consecutive reaction. The yield of feed isomers passes through a maximum around 55 % and then declines again (Figure 1(right)). These results are in complete agreement with the well known reaction mechanism for isomerization and hydrocracking of alkanes. It is noteworthy that due to the high reaction temperature needed to achieve full conversion over the monometallic clusters containing catalysts aromatization also occurs to a significant extent. The yield for substituted benzene isomers, viz. 1-ethyl-3,5-dimethyl benzene, 1-ethyl2,4-dimethyl benzene, 1,2,3,4-tetramethyl benzene and 1,2,4,5 tetramethyl benzene, at a reaction temperature of 410 ~ amounts to 8.3 % over 0.27 Pd/HA1MCM-41 (Xn-De = 91%) and to 14 % over 0.5 Pt/HA1MCM-41 (Xn-De = 99 %). Aromatization (on the metal function) is favored at high reaction temperatures and competes with hydrocracking (on the acid sites). The bimetallic systems are significantly more active compared to the monometallic catalysts, n-Decane conversion already starts at 200 ~ and the maximum yield of feed isomers is already reached at reaction temperatures around 300 ~ which is 50 to 80 ~ below the maximum for the monometallic systems. However, the maximum isomer yield reaches only 45 % for the 0.375Pt-0.068Pd/A1MCM-41 catalyst and 38 % for 0.25Pt0.135Pd/A1MCM-41 (Figure l(right)). In Figure 2, variation of the n-decane conversion (left) and the isomer yield (right) with the reaction temperature as function of the metal content for the bimetallic Pt-Pd systems is depicted. The activity and the isomer yield increases in the order 0.125 Pt-0.068Pd/A1MCM-41 < 0.25Pd-0.135Pt/A1MCM-41 < 0.5 Pt-0.27Pd/A1MCM41. Due to the lower reaction temperature, no aromatization products are detected over these catalysts. In Table 1, the isomerization, hydrocracking and aromatization yields at the temperature of maximum isomer yield are compared. A lower temperature at maximum isomerization indicates higher activity of the catalyst. All catalysts containing bimetallic clusters exhibit a higher catalytic activity as compared to the monometallic catalyst. In particular, the catalyst 0.25Pt-0.135Pd/A1MCM-41 exhibits superior activity, while the isomerization selectivity is still high. The higher activity of the bimetallic systems is tentatively attributed to a higher hydrogenation/dehydrogenation activity of the bimetallic Pt-Pd clusters and, hence, a higher concentration of alkenes, which are the intermediates in the hydroconversion of alkanes [3]. Table 1 Yields of isomers (Yiso.), cracked products (Ycr.) and aromatization products (Yarom.) at maximum isomer yield. Metal compound Tmax / ~ Xn-decane/ % Yiso./% Ycr./% Yarom./% S iso./% 0.5 wt.-% Pt

355

81.0

53.5

24.6

2.9

66.0

0.27 wt.-% Pd

380

74.3

54.5

17.1

1.4

73.4

0.375 Pt-0.068 Pd

320

77.4

44.0

33.4

0.0

56.8

0.25 Pt- 0.135 Pd

300

58.6

38.2

20.4

0.0

65.2

0.125 Pt- 0.203 Pd

310

69.6

37.3

32.3

0.0

53.6

0.125Pt- 0.068 Pd

330

82.4

32.4

49.9

0.1

39.3

0.5 Pt - 0.27 Pd

310

73.3

48.9

24.4

0.0

66.7

915 The variations of the monobranched and dibranched isomers with the conversion are the same irrespective of the sample considered and are in close agreement with our data on the monometallic catalysts [11 ]. This illustrates the consecutive nature of the transformation of the monobranched into dibranched and tribranched isomers which is favored at high temperature. The formation of tribranched isomers is detected only at high temperature and to a small extent. Moreover cracking of dibranched and tribranched isomers is fast compared to cracking of monobranched isomers. Therefore, the selectivity for monobranched isomers is high over the whole conversion range. The distribution of the monobranched isomers changes with reaction temperature and approaches thermodynamic equilibrium at high conversion levels. Detailed analysis of the cracked products (Table 2) shows that methane and ethane formation is significantly reduced for all catalysts containing bimetallic Pt-Pd clusters compared to 0.5Pt/A1MCM-41. Therefore, hydrogenolysis (cracking on the metal function) does contribute to a smaller extend to the yield of the cracked products, which is probably a consequence of the increased activity at lower reaction temperatures. 0.25Pt0.135Pd/A1MCM-41 and 0.5Pt-027Pd/A1MCM-41 exhibit product distributions which are characteristic for ideal hydrocracking, which is further supported by the high selectivity for the iso-isomers in the C4 and C5 fraction, respectively. It is assumed that these catalysts possess a better balance between the two catalytic functions. Furthermore aromatization is almost completely suppressed, which is a consequence of the reduced reaction temperature and, hence, the improved balance between the acid and metal sites. Table 2. Distribution of the cracked products at a cracking yield YCr. of 15 %. Metal

0.5 Pt

0.27 Pd

0.375 Pt-

0.25 P t -

0.125 Pt-

0.125Pt-

0.5 P t -

0.068 Pd

0.135 Pd

0.203 Pd

0.068 Pd

0.27 Pd

Number of moles/100 moles cracked C1

37

7

10

1

9

10

5

C2

9

2

8

0.5

8

10

5

C3+C7

45

37

50

26

69

63

43

C4+C6

80

105

95

103

80

73

96

C5

43

60

48

71

45

39

53

C8

6

0

5

0

6

8

3

C9

7

0

4

3

4

5

1

227

211

220

204.5

221

208

206

C4

42.3

55.5

40.0

68.9

24.0

12.6

55.0

C5

48.5

57.3

45.0

70.5

30.3

15.8

60.0

Total % iso in

916

100 (a) C~ fraction

90 L_

80

E

70

0 o ~

60

-o r

o

50

.Q

40

c t~ L_ 0

c

30

--i--0---0---[3--

0

20

o L_

w

10 I

10

20

,

I

30

,

I

40

,

I

50

Conversion

,

0.25Pt - O. 135Pd 0.375Pt- O.068Pd O. 125Pt - 0.203Pd 0.27 Pd O.5Pt

I

60

,

70

I

,

80

I

90

100

X n _ d e c a n e ] 0~

100 o-?,

90

(t) L_

80

E

70

0 (/)

, m

-o

60

o

50

_Q

40

o

30

o

20

(I) (-. c

E 0

LL

(b) C 4 fraction

,r

--0---I-

10 0

,

0

I

10

,

I

20

,

I

30

,

I

40

,

I

50

Conversion

,

I

60

,

0.125Pt- 0.203Pd 0.375Pt - O.068Pd 0.25Pt - O. 135Pd --i0.27 Pd - - - 0 - O.5Pt , , , , , ,

70

80

90

X n . d e c a n e / 0~

Fig. 3" Hydrocracking of n-decane over Pt-Pd/A1MCM-41 catalysts, percentage of branched isomers in the (a) C5 and (b) C4 fraction.

I

I II

I

I

100

917 In Figure 3a and Figure 3b, the amount of branched isomers in the C5 and C4 fraction, respectively, are plotted as a function of n-decane conversion. For 0.27Pd/A1MCM-41 and 0.25Pt-0.135Pd/A1MCM-41, the C4 and C5 products are mainly branched and their relative amount changes only slightly with the degree of conversion. In contrast, over all other catalysts mainly n-alkanes are formed at low conversion level, while the formation of branched isomers dominates at high conversion. The iso-selectivity for the 0.25Pt-0.135Pd catalyst is 68 % and 70 % in the C4 fraction and the C5 fraction, respectively. Comparable results are obtained for a 0.5Pt/CaY zeolite, where the selectivities for branched C4 and C5 isomers amount to 66 % and 73 %, respectively [3]. The observed high selectivities for linear cracking products (in particular at low conversion levels) can only be partially explained by the occurrence of hydrogenolysis (cracking at the metal function), which would result in the formation of n-isomers. The spectrum of cracked products (Table 2) reveals that hydrogenolysis indeed occurs over 0.5Pt/A1MCM-41, 0.375Pt-0.068Pd/A1MCM-41 and 0.125Pt-0.203Pd/A1MCM-41, but not to a sufficient extend to explain the high selectivities for n-butane and n-pentane at low conversion. Consequently, the product spectrum is presumably also influenced by several types of [3-scission [12]. Cracking of monobranched alkenes proceeds through a type C mechanism (which involves two secondary carbenium ion intermediates) and results in the formation of two linear alkanes. Dibranched alkenes undergo cracking through type B 1 or B2 mechanisms (one secondary plus one tertiary carbenium ion) and equimolar amounts of branched and linear products are formed. Finally, tribranched alkenes are cracked through type A [~-scission (two tertiary carbenium ions) and result in the formation of branched isomers. Type D ~-scissions via primary carbenium ions are typically not observed. Over zeolite catalyst, type A cracking is much faster than type B which is again faster than type C cracking. Therefore, over zeolite catalysts ca. 70 % of the cracked products are formed via type A [~-scission, which accounts for the high selectivities for branched isomers [13]. To explain the lower selectivities for branched products over our catalysts, the occurrence of type B or type C hydrocracking has also to be considered. The relative high reaction temperatures (as compared to zeolite-based catalysts) might result in a sufficient reaction rate for these mechanisms to proceed. The substitution of part of the platinum by palladium results in bimetallic catalysts, which exhibit an increased catalytic activity in n-decane conversion. We have found that the bimetallic catalyst exhibits distinctly different properties compared to the monometallic catalysts. Similar results were obtained for Pt-Pd clusters supported on LaNaY [ 14] and on HBeta [4]. In the latter case, it was found that the dispersion of bimetallic clusters is significantly higher compared to the monometallic systems, which results in a better proximity of the metal and the acid sites. It is generally accepted that a good "balance" between the acid sites and the metal sites is important for a high activity of the catalyst and a high isomerization yield. The observed higher catalytic activity and decreased hydrogenolysis activity for our Pt-Pd/A1MCM-41 catalysts, in particular for 0.25Pt-0.135Pd/A1MCM-41, is therefore presumably a consequence of a higher dispersion of both platinum and palladium. Hydrogen adsorption and spectroscopic studies aiming at the clarification of this point are currently underway.

918 4. C O N C L U S I O N S In comparison to monometallic Pd/A1MCM-41 and Pt/A1MCM-41 catalysts, bimetallic PtPd/A1MCM-41 materials are superior bifunctional catalysts for n-decane isomerization. The use of bimetallic Pt-Pd clusters results in higher catalytic activity and a higher yield of C10 isomers at a substantially lower reaction temperature. This improvement is due to a better balance between the two catalytic functions. Compared to palladium, platinum is a more active hydrogenation/dehydrogenation function, but the realization of sufficient dispersions with current catalysts preparation techniques is difficult. Large platinum clusters catalyze ndecane hydrogenolysis, which is significantly reduced for the bimetallic Pt-Pd/A1MCM41(23) catalysts. It is therefore assumed that the dispersion of the Pt-Pd clusters is enhanced in comparison to the monometallic Pd and Pt clusters, respectively.

ACKNOWLEDGEMENTS Financial support by Deutsche Forschungsgemeinschaft (DFG) and Fonds der Chemischen Industrie is gratefully acknowledged.

REFERENCES 1. 2. 3. 4. 5.

M.L. Coonradt and W.E. Garwood, Ind. Eng. Chem. Prod. Res. Dev. 3 (1964) 38. R. Mokaya, W. Jones, S. Moreno and G. Poncelet, Catal. Lett. 49 (1997) 87. J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 550. E. Blomsma, J.A. Martens and P.A. Jacobs, J. Catal. 165 (1997) 241. P.Meriaudeau, Vu.A. Tuan, G. Sapaly, Vu.T. Nghiem and C. Naccache, Catal. Today 49 (1999) 285. 6. J.S. Beck, J.C. Vartuli, W. J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. 7. K.J. Del Rossi, G.H. Hatzikos, A. Huss, US Patent 5,256.277 assigned to Mobil Oil Corp. (1993). 8. K. Chaudhari, T.K. Das, A.J. Chandwadkar and S. Sivasanker, J. Catal. 186 (1999) 81. 9. A. Klemt and W. Reschetilowski, Chem. Ing. Tech. 73 (2001) 872. 10. M. Hartmann, S. Racouchot and C. Bischof, Microporous and Mesoporous Mater. 27 (1999) 309. 11. C. Bischof and M. Hartmann, in "Zeolites and Mesoporous Materials at the Dawn of the 21 st Century", A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (eds.), Studies in Surface Science and Catalysis, Vol. 135, 26-P-18, Elsevier: Amsterdam (2001). 12. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal. 8 (1983) 123. 13. J.A. Martens, P.A. Jacobs and J. Weitkamp, Appl. Catal. 20 (1986) 283. 14. S.P. Elangovan and M. Hartmann, unpublished results.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

919

Influence o f nickel m e t a l distribution in N i / Y - z e o l i t e on the reactivity t o w a r d CO hydrogenation Dul-Sun Kim, Sung-Chul Kim, Seong-Ji Kim and Dong-Keun Lee Department of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyungnam 660-701, Korea The influence of nickel metal distribution on the properties of Ni/Y-zeolite catalysts for CO hydrogenation was investigated. The studies using TPR/TPO, TEM, XRD and FMR have shown the existence of a bidispersion of nickel metal particles; i.e., small particles were restricted inside the zeolite pores, and large particles were formed outside the zeolite crystal. CO hydrogenation reaction was performed in a fixed bed reactor operated at 1.5Mpa, 270 ~ and H2/CO ratio of 3. Nickel metal distribution affected the activity and hydrocarbon product distribution significantly. With increasing fraction of nickel metal inside the zeolite pores the activity continued to decrease, but the production of the long chain and olefinic hydrocarbons was enhanced remarkably. Especially when all the nickel metals were restricted within the pores of zeolite, a bimodal product distribution of CI and C4 was observed. This distribution was proved to be due to the rapid dimerization of the primarily formed ethylene to butene probably on the Ni + ion within the zeolite pores. 1. INTRODUTION Recent researches in CO hydrogenation aim mainly at improving the hydrocarbon product selectivity, and zeolites came to appear to be a promising support because they can be utilized to prepare catalysts containing highly dispersed metals, to show molecular seiving effect and to induce polyfunctional activity[ 1-5]. Metals of ruthenium[6-9], iron[7,8,10] and cobalt[2-5, 7,8,10], which have been shown to be acitve for CO hydrogenation, were employed for the preparation of metal-zeolite catalysts. Chain-length limitation in hydrocarbon product distribution occurred on the small metallic aggregates restricted into the zeolite pores[6-8], and anomalously high selectivities for certain hydrocarbons were also observed for iron- [ 10] and cobalt-zeolite[ 10,11 ] catalysts. Nickel metal is also known to be active for CO hydrogenation[ 12,13], but its main product is methane. Accordingly nickel metal has generally been excluded from the candidates of controlling the hydrocarbon product distribution. Our investigations on the reduced nickel-zeolite catalysts have shown the existence of a bidispersion of nickel particles; i.e., small particles were restricted inside the zeolite pores, and large particles were formed outside the zeolite crystal. Although methane is believed to be produced as a main product on the large nickel particles, a different pattern of hydrocarbon product distribution can be inferred to occur on small nickel particles restricted into pores of

920 zeolite. In this research Ni/Y-zeolite catalysts having different nickel metal distribution were prepared, and the influence of nickel metal distribution on the properties of Ni/Y-zeolite catalyst toward CO hydrogenation was investigated.

2. EXPERIMENTAL 2.1. Materials and catalysts NaY zeolite with a Si/A1 molar ratio of 2.5 was provided from Strem Chemicals. Ni(NO3)2-6H20, supplied from Alpha Division, was used as a precursor for the catalyst preparation. Hydrogen(Matheson, 99.999%) was further purified by passing it through an oxy- and a molecular sieve trap. Carbon monoxide(Takachiho, 99.95%) was passed through a molecular sieve trap. Oxygen(Matheson, 99.99%) was used without further purification procedure. The catalyst samples were prepared by a conventional ion exchange method. After suspending 10g zeolite in 1000mL of 0.04N aqueous solution of Ni(NO3)2"6H20, the mixture were stirred continuously at 85~ for 48h. After being ion exchanged, the samples were filtered and washed sufficiently to remove remaining nickel salt solution which was not yet ion exchanged. By repeating the ion exchange four times Ni/Y-zeolite catalyst having about 10wt% nickel loading could be obtained. The prepared Ni/Y-zeolite was dried in air at 80 ~ for 12h, and was then reduced in hydrogen stream to the desired extent of reduction. All the catalysts used in the present study will be abbreviated by the symbols of (reduction temperature, ~ time, h) in the rest of the text. The catalyst of 500-8, for example, denotes the 10wt% Ni/Y-zeolite catalyst reduced at 500~ for 8h. All the catalysts were heated to the reduction temperature at a linear rate of 2 ~ 2.2. Characterization Temperature programmed reduction(TPR) and oxidation(TPO) experiments were performed in a closed system with gas circulating unit to determine the amount of nickel metal inside and outside the zeolite by following the procedure of Jacobs et al.[ 14,15]. Ferromagnetic resonance(FMR) spectra of nickel metal were recorded at X-band frequencies on a Varian E-4 spectrometer working at a microwave power of 13mW and field modulation of 100KHz. DPPH was used as a standard to determine the g-factors. The quartz sample tube was designed for the in situ operation. X-ray diffraction line broadening was measured on a diffractometer(JEOL, JDX-1193A) using CuKot radiation. Nickel particle sizes were calculated from the line broadening data for Ni(111) using Scherrer equation. Transmission electron microscopy was done with a JEOL 200 CX microscope using 160KeV electrons. About 40 measured points were used to calculate the average value of nickel crystallite size. The adsorption isotherms of H2 were measured at 25~ in a conventional Pyrex glass volumetric adsorption apparatus. The gas uptakes were obtained by extrapolating the straight portion of the isotherms to zero pressure, and the irreversible uptakes were determined from the difference between the total and reversible ones. The dispersion of nickel particles were calculated from the irreversible uptakes of H2[ 16].

921 2.8

,_, x

2.3. C O h y d r o g e n a t i o n

2.1

[

U ~

. \r

~

/

E

!!

0

t

'

i

I \

"

/'",

"

"

iii

",,,

CO hydrogenation reaction was performed in a fixed bed reactor operating at 1.5Mpa, 270~ and HJCO ratio of 3. The steady state rates were measured after 1Oh from the introduction of the reactants to the catalysts.

3. RESULTS AND DISCUSSION 200

300

400

TEMPERATURE

500

600

(~

3.1. C h a r a c t e r i z a t i o n

A typical result of TPR/TPO consecutive experiments for a 10wt% Ni/Y-zeolite catalyst is shown in Figure 1. Curve A denotes the rate of nickel ion reduction to nickel metal which may be formed inside or outside the zeolite. The degree of nickel ion reduction can be calculated from the ratio of the amount of the hydrogen consumed and the total amount of the nickel ion in zeolite by considering the reaction of Ni 2++ H2 ~ Ni ~ + 2H § Upon oxidation each metal phase inside or outside the zeolite may be oxidized at different routes as suggested by Jacobs et al.[15]. Curve B represents the oxidation rate, but no discrimination could be achieved. During a second reductive treatment of this oxidized sample (curve C) a new low temperature maximum together with a new high temperature maximum in the reduction rate is observed. At the end of the first and second reduction nearly the same amounts of hydrogen were consumed. From the peaks of the low- and hightemperature maxima the amounts of nickel metal phase inside and outside the zeolite can be deduced, respectively. In Table 1 are listed the degree of reduction and the fraction of the amounts of nickel metal inside and outside the zeolite. Both the degree of reduction and the fraction of nickel metal outside the zeolite are known to increase with increasing reduction temperature and time. Figure 1. Typical curves of TPR/TPO experiments.

Table 1. Degree of reduction and nickel metal distribution in the Ni/Y-zeolite catalysts reduced at different conditions Catalyst

Degree of reduction(%)

500-12 500-4 500-1 400-12 400-4 450-1 400-1 300-1

60.0 59.8 59.7 44.3 40.1 37.5 35.0 8.5

Nickel metal distribution(%) Inside Outside 64.0 36.0 60.0 40.0 58.5 41.5 32.2 67.8 28.4 71.6 26.1 73.9 25.0 75.0 0.0 100.0

922

Figure 2. Four representative TEM photographs of Ni/Y-zeolite catalysts (a:500-8, b: 400-8, c:350-8, d:300-1). Figure 2 shows the four representative photographs obtained by TEM on the 10wt% Ni/Yzeolite catalysts reduced at different conditions. Photographs a, b and c are representative for the samples of 500-8, 400-8 and 350-8, respectively. The size of nickel crystallites existing at the exterior surface of the zeolite is shown to increase with increasing reduction temperature indicating that more and more metal migrates out of the zeolite pores and agglomerates as bulky crystallites. When the sample was reduced at 300 ~ for l h(photograph d), no nickel crystallites of detectable size are found. In this case most of the nickel crystallites are believed to exist within the pores of the zeolite and the size will be less than that of faujasite supercage(_=_l.3nm) FMR spectra for the 10wt% Ni/Y-zeolite catalysts do change significantly with reduction conditions as shown in Figure 3. When the catalyst was reduced at 300~ for l h, nearly symmetric and narrow peak(line width=ll00G) appears at the g-value of 2.22. As the reduction temperature increases, the peak becomes asymmetric and its line width broadens to have 2300 gauss for the 600-1 catalyst. Following the suggestions by Jacobs et al.[17], the symmetric and narrow peak is representative of the small nickel metal within the pores of Yzeolite, while the highly asymmetric broad peaks are mainly due to the large nickel metal at the exterior surface of the zeolite. Aforementioned results of TPR/TPO, TEM and FMR show the existence of the bidispersion of nickel metal inside and outside the zeolite.

923 The sizes of nickel crystallites obtained from X-ray line broadening, TEM and H2 chemisorption are listed 600-1 in Table 2. Between the sizeg from XX 1/25 ray line broadening and those from H2 chemisorption incompatible results Z are obtained. The size from X-ray line broadening increases with increasing reduction temperature, while the size calculated from H2 chemisorption E3 does not show a distinct tendency and rr is always larger than that from X-ray X 1/4 line broadening. In particular the size of 45.0nm for the 300-1 catalyst from u3 H2 chemisorption is unreasonably too z iii large when compared with the TEM FZ X1 00-1 photograph(Figure 2-d) and X-ray line broadening which showed no nickel crystallites of detectable size. 1000 3000 5000 Kubo et al.[18] reported that H (gauss) atomically dispersed platinum was inactive to H2 chemisorption. Therefore, if the zeolite includes a Figure 3. FMR spectra of Ni/Y-zeolite catalysts number of nickel species inactive to (detection temperature = 20 ~ H2 chemisorption, the size from H2 chemisorption must have been overestimated. The size of nickel crystallites was calculated again from H2 chemisorption by assuming that the nickel metal phase within the zeolite pores is inactive to H2 chemisorption. The sizes from the modified hydrogen chemisorption(listed in the fifth column in Table 2) agree well with those from X-ray line broadening. Therefore H2 chemisorption is believed to be suppressed on the small nickel metal particles within the zeolite pores.

E

Table 2. Nickel crystallite size in the Ni/Y-zeolite catalysts Nickel crystallite size (nm) Catalyst XRD TEM H2 Chem.

H2 Chem.*

500-12 20.4 31.0 30.6 19.6 500-8 21.8 30.9 30.4 18.8 500-1 16.3 29.9 17.5 450-1 15.2 43.0 11.2 400-8 13.6 28.9 36.0 10.9 400-1 12.4 30.4 7.6 350-8 12.2 21.7 58.1 7.0 300-1 + + 45.0 * Nickel crystallite size was calculated by following the assumption that the nickel metals inside the zeolite pores are inactive to H2 chemisorption. + No nickel crystallites of detectable size were observed.

924 Table 3. Activity and hydrocarbon product distribution Catalyst

Turnover frequency Product distribution (wt%) Nco x 103 (see-1) C1 C2 C3 C4 C5 C6 500-12 231.1 80.3 10.9 5.9 1.8 0.7 0.4 500-8 295.4 88.1 7.8 3.6 0.5 500-1 140.9 75.4 12.6 9.3 1.5 1.2 400-12 86.4 81.0 10.5 5.0 2.1 1.4 450-1 50.2 55.6 16.7 20.6 6.0 1.1 400-1 26.1 60.4 13.6 16.3 7.9 2.0 350-1 20.3 56.0 13.0 9.3 11.0 0.7 300-1 1.8" 40.4 11.7 7.0 40.9 * Turnover frequency of the 300-1 catalyst was calculated under the assumption that the size of nickel crystallites is the same as that of faujasite supercage(=1.3nm).

3.2. CO hydrogenation In Table 3 are listed the summarized data for CO hydrogenation. The activity was expressed in terms of turnover frequency for CO conversion, i.e., the number of CO molecules converted per catalytic site per second. The hydrocarbon products are represented as the number of carbon atoms per molecule, and their respective concentrations are listed as weight percentage. The activity continues to decrease with increasing fraction of nickel metal within the pores of the zeolite (or decreasing size of nickel crystallites). Van Hardeveld and Hartog[19] have shown that the decrease in nickel crystallite size from 21 to 4nm favored the formation of a more strongly bound CO species. As CO adsorbs less strongly on the metal, hydrogen competes more easily with CO for the adsorption sites thereby enhancing the rate of hydrogenation. This is consistent with the suggestion of Vannice[20] that the most active metal surface is that which adsorbs CO the least strongly. The increasing adsorption strength of CO with decreasing size of nickel crystallites seems also to be concerned with the hydrocarbon product distribution in Table 3. Fu and Bartholomew[21 ] showed that longer chain hydrocarbons were produced on the surface with the more strongly bound CO owing to the longer residence time of carbon containing reaction intermediates on the surface. Therefore the production of long chain hydrocarbons seems to be favored with increasing fraction of nickel crystallites inside the zeolite. Especially when all the nickel metals are restricted within the pores of the zeolite(in this case 300-1 catalyst), a bimodal product distribution of C~ and C4 was observed. Several papers concerning the dimerization of ethylene over transition metal exchanged zeolite catalysts have been published. Riekert[22] found that NiY was an active catalyst for ethylene dimerization. Ni + in zeolite framework was proved to be active site for ethylene dimerization[23,24]. The bimodal distribution in the 300-1 catalyst might be resulted from the dimerization of the primarily formed ethylene to butene on the active species of Ni + ion which may be present within the zeolite. To verify this ethylene was fed in a closed system where 0.5g sample of 300-1 catalyst was loaded at 270 ~ The dimerization of ethylene to butene did occur at very fast rate. Moreover the addition of small amount of ethylene to the H2/CO feed(CO:H2:C2H4 = 1:3:0.15 in molar ratio) resulted in the significant increase in the formation of butene. These two results confirm the dimerization of the primarily formed ethylene to butene on the 300-1

925

u+

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

o 0.2 0.0 300

~

o

+

,~--

o

o~

0.2

350

' 400

' 450

catalyst. Figure 4 shows the changes in the formation of olefinic hydrocarbons with reduction temperature. With increasing reduction temperature(or decreasing fraction of nickel metal inside the zeolite pores) more olefins are formed. This seems to be due to the suppression of hydrogen chemisorption on the small nickel metal restricted within the pores of the zeolite as discussed previously.

0.0 500

Reduction temp. ( ~

4. C O N C L U S I O N S

Figure 4. Changes in the fraction of olefinic hydrocarbons over the Ni/Y-zeolite catalysts with reduction temperature,

TPR/TPO, TEM and FMR investigations on the reduced Ni/Yzeolite catalysts have shown the existence of a bidispersion of nickel metal particles. Small nickel metal particles were restricted inside the zeolite, while large particles were formed outside the zeolite crystal. CO hydrogenation reactivities of the reduced Ni/Y-zeolite catalysts were significantly affected by the distribution of nickel metals in the catalyst. With increasing fraction of nickel metal inside the zeolite the activity continued to decrease, but the production of the long chain and olefinic hydrocarbons was enhanced significantly. Especially when all the nickel metals were restricted within the zeolite, a bimodal hydrocarbon product distribution of C~ and C4 were observed. This was ascribed to the dimerization of the primarily formed ethylene to butene probably on the active species of Ni § ion. REFERENCES 1. K.M. Minachev and Y.I. Isakov, Zeolite Chemistry and Catalysis(J.A. Rabo, Eds.), American Chemical Society, Washington, 1976. 2. D.-K. Lee and S.-K. Ihm, Appl. Catal., 32 (1987) 85. 3. D.-K. Lee and S.-K. Ihm, J. Catal., 106 (1987) 386. 4. J.H. Lee, S.-K. Ihm and D.-K. Lee, React. Kinet. Catal. Letts., 57 (1996) 301. 5. J.H. Lee, D.-K. Lee and S.-K. Ihm, Stud. Surf. Sci. Catal., 83 (1994) 355. 6. H.H. Nijs, P.A. Jacobs and J.V. Uytterhoeven, J. Chem. Soc. Chem. Comm., 180 (1979) 1095. 7. D.B. Tratchenko and I. Tratchenko, J. Mol. Catal., 13 (1981) 1. 8. D.B. Tratchenko, G. Coudurier and N.D. Chau, Stud. Surf. Sci. Catal., 12 (1982) 123. 9. Y.W. Chew, H.T. Wag and J.G. Goodwin Jr., J. Catal., 83 (1983) 415. 10. L.F. Nazar, G.A. Ozin, F. Hugues and J. Godber, J. Mol. Catal., 21 (1983) 313. 11. D. Fraenkel and B.C. Gates, J. Amer. Chem. Soc., 102 (1980) 2478. 12. M.A. Vannice, J. Catal., 37 (1975) 449.

926 13. M. A. Vannice, J. Catal., 44 (1976) 152. 14. P.A. Jacobs, M. Tielen, J.P. Linart, J.B. Uytterhoeven and H.K. Beyer, J. Chem. Soc. Faraday I, 72 (1976) 2793. 15. P.A. Jaeobs, J.P. Linart, H. Nijs and J.B. Uytterhoeven, J. Chem. Soc. Faraday I, 73 (1977) 1745. 16. C.H. Bartholomew and R.B. Pannell, J. Catal., 65 (1980) 390. 17. P.A. Jacobs, H. Nijs, J. Verdonck, E.G. Derouane, J.P. Gilson and A. Simoens, J. Chem. Soe. Faraday Trans. I, 75 (1979) 1196. 18. T. Kubo, H. Arai, H. Tominaga and T. Kunugi, Bull. Chem. Soc. Japan, 45 (1972) 607. 19. R. Van Hardeveld and F. Hartog, Advances in the Catalysis, Vol. 22, p. 75, Academic Press, New York, 1972. 20. M.A. Vannice, J. Catal., 44 (1976) 152. 21. L. Fu and C.H. Bartholomew, J. Catal., 92 (1985) 376. 22. L. Liekert, J. Catal., 19 (1970) 8. 23. I.V. Elev, B.N. Shelimov and V.B. Kazansky, J. Catal., 89 (1984) 470. 24. L. Zheng, G. Wang and X. Bai, Proe. 7th Intern. Cone On Zeolite, Tokyo, 965 (1986).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

927

Hydrodechlorination of chlorinated compounds on different zeolites B. Imre a, Z. K6nya a, I. Hannus a*, J. Halfisz a, J. B.Nagyband I. Kiricsi a aDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich t6r 1, H-6720 Szeged, Hungary bLaboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium

Adsorption and transformation of chlorinated hydrocarbons on zeolite Y-FAU modified by different metals such as Co, Pt or both were studied. The combined instrumental methods implied revealed that trichloro-ethylene reacts immediately after adsorption giving ethane and HC1 as main products appearing in the gas phase, tetrachloroethane gives similar product distribution. CHC1F2 transforms on the surface resulting in adsorbed CO and complex surface intermediates involving the acidic OH groups of the zeolite.

1. INTRODUCTION Chlorinated compounds emitted into the atmosphere are responsible for diminishing the ozone layer in the stratosphere. Therefore, their environmentally safe and economically valuable transformation is very important and a big challenge as well. Two main potential ways have been suggested: their (oxidative) destruction to form environmentally safe products or their transformation to potentially valuable chemical compounds by hydrodechlorination [ 1]. It has been assumed that zeolites are appropriate catalysts both in the oxidative decomposition [2,3] and in the hydrodechlorination of chlorinated hydrocarbons [4]. It was shown that different transition metals (for example Co), nobel metals (Pt), and their bimetallic form on zeolite support are promising catalysts for hydrodechlorination [5,6]. We report here on the preparation of Co-, Pt- and Co,Pt ion-exchanged and impregnated Y-type zeolites, their characterization and investigation in reductive hydrodechlorination reactions of different chlorinated compounds.

2. EXPERIMENTAL NaY-FAU (Union Carbide product) was the parent zeolite with Na58A158Si1340384 This work was performed with the help of grants FKFP 400/2000 and OTKA T 025248, Hungary, and with financial help from CGRI, Belgium.

928 unit cell composition. NaY zeolite was impregnated with Pt(NH3)4C12 solution. After drying the Pt content of this P t ~ a Y sample was 1.2 %. Co,NaY- and Pt,NaY-FAU samples were obtained after ion exchange of NaY-FAU in C0(NO3)2 and Pt(NH3)4C12 solution followed by filtering, washing and drying. Unit cell composition of ion-exchanged samples are Co14Na30A158Si1340384 and Pt21Na16A158Si1340384,respectively. Pt/Co,NaY-FAU sample was obtained from Co,NaY-FAU impregnated by 1.2 % Pt. The used reactants (trichloro-ethylene, TCE, 1,1,2,2 tetrachloro-ethane, TCA, and CHC1F2, HCFC-22) were commercial products. Self-supporting wafer technique was employed for acidity and adsorption measurements. The wafers (10 mg/cm2) were prepared from the powdered zeolites and placed into the sample holder of the in situ IR cell. The pretreatment of the samples was as follows: the temperature of the wafer was slowly increased to 723 K under continuous evacuation of the cell and it was held at 723 K for 2 h. Hydrogen was used for the reduction, 26.6 kPa (200 Torr) Ha was introduced in to the cell and reacted with the wafer at 573 K, for 1 h, followed by degassing at the same temperature for an additional hour. After this treatment the sample was cooled to room temperature and the background spectrum of the zeolite was recorded. 16 scans were accumulated for a spectrum. For the acidity measurements 1.33 kPa (10 Torr) of pyridine was introduced into the cell containing the pretreated, reduced self-supporting wafer and heated to 473 K. After 1 h adsorption, the cell was evacuated for 1 h at the same temperature. After cooling the sample to room temperature spectra of the adsorbed pyridine were taken. The peak areas of the bands around 1450 and 1540 cm ~, characteristic of pyridine bonded to Lewis and Brtinsted acid sites [7], were integrated and were divided by the mass of the wafer. These values (summarised in Table 1) were used to characterize the acidity of the samples.

Table 1 Data on surface acidity measured by pyridine adsorption Zeolites NaY-FAU Pt~aY-FAU Co,NaY-FAU Pt/CoY-FAU Pt,NaY-FAU HY-FAU

Surface acidity (cml/mg) * Br6nsted Lewis 0.00 Traces 0.05 0.59 0.11 0.67 0.14 0.47 0.34 0.03 0.78 0.09

*Integrated absorbances from the IR spectra of adsorbed pyridine divided by the mass of the self-supported wafer For in situ IR spectroscopic adsorption and reaction measurements the self-supported wafers were outgassed and reduced with hydrogen. After 1.33 kPa (10 Torr) vapour of the reactant and 26.6 kPa (200 Torr) of H2 were introduced into the cell and heated up to preselected temperatures. The spectra were recorded at beam temperature in each case. Structural changes occurring in the zeolite framework upon reaction with chlorinated compounds were investigated by KBr pellet technique of IR spectroscopy.

929 Spectra were run on a Mattson Genesis 1 FTIR spectrometer (lithium tanthalate detector), with a resolution of 2 cm "x. In situ MAS 13C NMR measurements were performed on an MSL-400 BRUKER spectrometer operating at 100.6 MHz (4.0 ~ts (| pulse). The samples were packed into the NMR tubes and evacuated at 723 K for 2 h. The activated zeolite samples were loaded with the reactant, then the tubes were carefully sealed to achieve proper balance and a high spinning rate (3.8 kHz). Spectra were recorded after heating the tube at various preselected temperatures. The catalysts placed in a glass reactor were pretreated in vacuo and ,finally, at 723 K in 100 cm3/min H2 flow. After adjusting the desired reaction temperature the reactant was fed using hydrogen flow with a flow rate around 50 cm3/min. Hydrodechlorination reactions of different reactants were carried out in a flow system applying GC product analysis (Shimadzu apparatus with capillar column). 3. RESULTS AND DISCUSSION 3.1. Acidity measurements Table 1 summarizes the data of BrOnsted and Lewis acidity of our metal-containing zeolites and that of a neutral NaY-FAU and a mostly BrOnsted acidic HY-FAU for comparison. (The latter sample was obtained by ion exchange of NaY-FAU in NH4C1 solution followed by filtering, washing, drying and deammonization at 723 K in vacuum.) Figure 1 shows the OH-region of the IR spectra of the catalysts used. The Pt/NaY-FAU, impregnated sample has no acidic OH groups in the 3500-3700 cm l range (spectrum a). In good agreement with this, it has practically no Br6nsted acidity : -,, i : .... determined by pyridine adsorption (see ! C / "+"-,, Table 1). It means that during the impregnation only negligible extent of ion-exchange took place, and b ,^ "/'~"..................... therefore,Br6nsted acidity was not p+~.+ formed during the reduction of Pt 2+ ions. fail ....... ..+,~.~ The Co,NaY-FAU zeolite has both Br6nsted and Lewis acidity. It is ........................... well-known that the reduction of Co 2+i ions is difficult in exchanged position 3800 3600 3400 3200 3000 of zeolites. This, and the low ionWevenumberO/can) exchange degree are responsible for the relatively low Br6nsted acidity. Figure 1. Infrared spectra of the OH region of The reduction of PtZ+-ions our catalysts; (a) Pt~aY, (b) Co,NaY, (c) takes place completely under the Pt/Co,NaY, (d) Pt,NaY. .............................................................................................................

930 experimental conditions applied. In this process acidic OH groups are formed as spectrum d in Figure 1 shows. Three absorptions were obtained at 3740, 3644 and 3548 cm "l. These are assigned, as a non acidic terminal SiOH, and two acidic, bridging OH groups sited in superand sodalite cages, respectively. The acidity of the Pt/Co,NaY-FAU bimetallic zeolite sample is similar to the Co,NaY-FAU (see data in Table 1), because the impregnated Pt-salt did not create too many new acidic OH-group.

3.2. Infrared spectroscopy

Trichloro-ethylene, (TCE) The gas phase spectra of TCE reacted over Pt~aY-FAU (impregnated) are depicted in Figure 2. The characteristic absorptions of the starting material are as follows: different C-C1 vibrations below 1000 cm 1, C-H deformation at 1250, C-H stretching at 3080 cm 1, and the band due to the C=C bond appeared at 1580 cm ]. It is clearly seen that hydrodechlorination reaction takes place with increasing temperature. In the spectrum recorded after heat treatment at 473 K characteristic vibration of HC1 centered at 2880 cm 1 appeared. At higher temperature the intensity of different C-C1 vibrations, C-H stretching and deformation vibrations, C=C vibration of TCE decreased and simultaneously, the intensity of a broad band at 3000 cm 1 and a smaller at 1500 cm 1 due to the C-H stretching and deformation vibration of ethane [8] increased. Formation of HC1 could also be followed.

I

,

i

'

'

i

L

i

/ h/

i!;

'

c

,,

~!/!,,~,...........

!,!.i ~j ........

b i . . . . . . . . r , " .........

ii

'

t

i ,J ~,

, ! ,I

h !i f~,,j i'

#

1

3600

3400

3200

3000

~umberO,~rn)

2800

2600

I t

........ "Ls ........:............ :..j:......... :, 1600

1400

1200

10O0

[

'

,~.

,:,.t,..': ;L..L-4 800

600

WIl~l:)l~l/c~nn)

Figure 2. Infrared gas phase spectra of trichloro-ethylene reacted over Pt/NaY-FAU; A) C-H stretching region, B) C=C, C-H deformation and C-C1 vibration region; (a) starting material, (b) after reaction at 473 K, (c) at 573 K and (d) at 673 K.

931 The increasing signal of HC1 on the high frequency side of the ethane band reveals the enhanced formation of HC1 with increasing temperature.

Tetrachloro-ethane, (TCA) The gas phase spectrum of TCA is similar to the spectum of TCE, however, the absorption of C=C bond is lacking. The hydrodechlorination takes place with increasing temperature over Pt~aY-FAU catalyst and the main products are ethane and HC1 as well. Figure 3 shows the spectra of TCA adsorbed and reacted on the self-supported wafer of Pt~aY-FAU. A broad band developed in the OH region probably as a result of the reaction of HC1 with the Na+-ion of the zeolite. A doublet with broad bands centered at around 2700 and 2400 cm l became clearly visible with increasing temperature. This is characteristic of the H-bonded interaction between the HC1 and acidic OH group on zeolite. There are several explanations for this phenomenon. According to Ozin et al. the band at 2700 cm l belongs to the X)OHstretching and the band at 2400 cm "l to the ~Ci-H stretching vibration in the O-H'"(C1-H)n structure [9]. By another explanation the reason of this phenomenon is a special Fermi resonance between a combination vibration (~oH + ngOH--El ) and an overtone (26on) [ 10,11 ].

~.~

: / :

/ i /'

i /

"-.,

",..,. "'~

W <'''~"

.-

:/t't.,J"

;

//I

.....

'-,

"II

LTL-"

: !

<,

i

",

\../

i

--

I

........ ,...--~-~-~.

3700

",,.

3500

-~*;,~,~'-.~

3300

.........

e ................

3 I00

!

- ........ ~ .......... ~ ......

2900

2'700

2500

2300

Wavenumber(l/cm)

Figure 3. Infrared spectra of tetrachloro-ethane adsorbed and reacted on Pt~aY-FAU; (a) activated catalyst, (b) after reaction at 473 K, (c) at 573 K and (d) at 673 K. A very similar feature was observed in the case of TCE as well. The intensity of C-H stretching of TCA at 3000 cm 1 decreases with increasing temperature, because the reaction product ethane adsorbs weakly compared to the chlorinated starting hydrocarbons.

932

CHCIF2, (HCFC-22) The characteristic absorptions of CHC1F2 are as follows: C-C1 stretching at 809, CF2 asymmetric at 1116 and symmetric at 1178, C-H deformation at 1311, C-H stretching at 3023 cm "1 [12]. In the case of CHC1F2 no hydrodechlorination reaction was observed at all under the experimental conditions used. This can be explained by the stronger C-C1 bond in CHC1F2 compared to the other model compounds, TCE and TCA. However, we observed some decomposition instead of hydrodechlorination over the most Br6nsted acidic sample, Pt,NaY-FAU. Upon adsorption of CHC1F2 on reduced Pt,NaY-FAU the bands of OH groups of the zeolite (spectrum a in Figure 4) shifted and a rather complex spectrum appeared in this region as can be seen in spectrum c. As a result of this complex surface transformation a new intense band appeared at 2080 cm "~. We assigned this band to the vibration of CO molecules bonded linearly to the Pt clusters. The fact that CO does not adsorb at room temperature on zeolites having no transition or noble metals or metal ions, supports this assigment. However, a separated experiment was performed in which CO was adsorbed on Pt,NaY-FAU. The obtained spectrum (spectrum b in Figure 4) is very similar in the CO vibration range %z~,.A to that discussed above. Another x\ f remarkable feature of this spectrum is [ \ that the OH groups are intact, i.e. they \,. change neither their position nor their ,,., ,~ ~, ..~. intensity. From this follows that substantial transformation should have b ! ~,'~",,2..\ occurred upon adsorption and surface ~ r r reaction of CHC1F2. In these ~,... , transformations both the OH groups and the Pt particles are involved. As the conversion of CHC1F2 took place in hydrogen flow, the question arises, where the oxygen to the CO formation comes from. 4000 3500 3000 2500 2000 To our knowledge it should Wavenumber(l/cm) come from the framework of the Figure 4. Infrared spectra of CHC1F2 reacted on zeolite. NaY-FAU has low Si/A1 ratio Pt,NaY-FAU; (a) activated catalyst, (b) after CO meaning that the structural stability of adsorption, (c)after reaction at 673 K. this type is moderate. Note that HYFAU zeolite may loose some of its aluminium content under mild treatment, as some sort of aluminium hydroxo complex. The dealumination procedure is always accompanied by the removal of framework oxygens as well. We assume that the oxygen appearing in the CO originates from the framework. If it comes from the framework, vacancies must be generated in the ~,

j

933 zeolite skeleton. These vacancies can be detected by IR spectroscopy by the band appearing generally around 930 cm l [13]. Taking the IR spectra of the zeolite samples treated with CHC1F2 using KBr matrix technique, we observed the band near 930 cm "l, indeed. This finding supports the assigment of CO band and proves the occurrence of surface transformations after adsorption of CHC1F2.

3.3. NMR spectroscopy 13C NMR spectroscopy measurement proved that during the decomposition of CHC1F2 the intermediate product is CHC13 and the final product of the carbon is CO. The fate of the fluorine content of the molecule was investigated by 19F NMR technique and a signal due to the aluminium fluoride, as the reaction product of fluorine was assigned. 3.4. Reactor experiment The reactor experiments supplemented the product distribution analysed by IR and NMR techniques. It was found that besides the main product ethane - that formed with ~- 97 ~ selectivity at high ( 93 %) conversion level- ethyl-chloride, dichloro-ethane, trichloroethane were formed with rather low selectivity. Table 2 Product selectivities in the reaction of tetrachloro-ethane and trichloro-ethylene at 673 K reaction temperature

Reactant

tetrachloroethane

trichloroethylene

Catalyst Co,NaY Pt,NaY Pt/NaY Pt/Co,NaY Co,NaY Pt,NaY Pt~aY Pt/Co,NaY

ethane 18.9 98.3 97.5 95.1 15.3 97.1 96.8 95.1

Selectivity~ % ethyldichlorochloride ethane 8.7 13.0 0.1 0.2 0.4 0.4 0.4 1.3 4.7 5.3 0.2 0.2 0.4 0.6 0.1 0.7

Trichloroethane 57.3 0.2 0.5 1.2 72.5 0.4 1.7 1.1

The selectivity data summarised in Table 2 show that on each Pt-containing catalyst ethane is the main product. The Co,NaY-FAU behaves differently. On this catalyst formation of saturated, chlorine-containing compounds predominanted. These results evidence that the hydrodechlorination reaction is accompanied by intensive hydrogenation under the experimental condition of the reactor investigations.

934 4. CONCLUSIONS In the absence of transition or noble metal, no hydrodechlorination took place. On Na or HY-FAU the reactions occurring gave similar products as the oxidative decomposition, CO and small amount of HC1. In this case the zeolite framework is the oxygen source of the surface reactions. Hence, the zeolite is the reactant in this transformation its crystallinity continuously decreases in the reaction [ 14]. On Co-, Pt- and Pt,CoY-FAU the product distribution corresponded to hydrodechlorination reaction in which the main products were HC1 and ethane. At elevated temperatures (above 623 K) the total hydrodechlorinated products (ethane in the case of TCA and TCE) were observed predominantly. With the help of IR and 13C NMR spectroscopics we identified the intermediate products of the reactions. We found that in the simple decomposition reaction of chlorinated compounds the acidity of zeolite played predominant role. Here the activation of both the chlorinated compound and the hydrogen proved to be the main reaction.

REFERENCES

1. A. Viersma, E.J.A.X. van de Sandt, M. Makkee, C.P. Luteijn, H. van Bekkum and J.A.Moulijn, Catal. Today, 27 (1996) 257. 2. S. Karmakar and H.L. Greene, J. Catal., 138 (1992) 364; 148 (1994) 524. 3. J. Halfisz, M. Hegedfis, I~. Kun, D. M6hn and I. Kiricsi, Stud. Surf. Sci. Catal., 125 (1999) 793. 4. I. Hannus, A. Tamfisi, Z. K6nya, S-I. Niwa, F. Mizukami, J. B.Nagy and I. Kiricsi, Stud. Surf. Sci. Catal., 130 (2000) 1235. 5. L. Guczi, Z. K6nya, Zs. Koppfiny, G. Stefler and I. Kiricsi, Catal. Lett., 44 (1997) 7. 6. A. Tamfisi, I. Kiricsi, Z. K6nya, J. Halfisz and L. Guczi, J. Mol. Struct., 482-83 (1999) 1. 7. J. Take, T. Yamaguchi, K. Miyamoto, H. Ohyama and M. Misono, Stud. Surf. Sci. Catal., 28 (1986) 495. 8. B. Schrader, Raman/Infrared Atlas of Organic Compounds, VCH Publisher, New York, USA, 1989 9. G.A. Ozin, S. Ozkar and D. Stucky, J. Phys. Chem., 94 (1990) 7562. 10. M. F. Claydon and N. Sheppard, Chem. Commun., (1969) 1431. 11 A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Spano and F. Geobaldo, J. Chem. Soc. Faraday Trans., 92 (1996) 4863. 12. E. K. Plyler and W. S. Benedict, J. Res. NBS., 47 (1951) 202. 13. P. Fejes, I. Hannus and I. Kiricsi, Zeolites, 4, (1984) 73. 14. I. Hannus, Z. K6nya, T. Kollfir, Y. Kiyozumi, F. Mizukami, P. Lentz, J. B.Nagy and I Kiricsi, Stud. Surf. Sci. Catal., 125, (1999) 245.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

935

Ammoxidation of ethylene into acetonitrile over Co-Zeolites catalysts Mourad Mhamdi a, Sihem Khaddar-

Zinc a'b, Abdelhamid Ghorbel a

aLaboratoire de Chimie des Mat6riaux et Catalyse, D6partement de Chimie, Facult6 des Sciences de Tunis, 1060 Tunis, Tunisia. blnstitut Pr6paratoire aux Etudes d'Ing6nieur de Nabeul, E1 Merazka 8000 Nabeul, Tunisia. Acetonitrile can be produced from alkanes by ammoxidation over a variety of multidimensional 10 or 12 rings zeolites exchanged with cobalt. So as to find a suitable catalyst for ethylene ammoxidation into acetonitrile, Y and ZSM-5 zeolites were exchanged with cobalt, through aqueous ion exchange and solid-state ion exchange. The catalyst prepared from pentasil zeolites and cobalt acetate displayed high catalytic activity and selectivity to produce acetonitrile. 1. INTRODUCTION Because the low cost and the abundance of the light alkanes in natural gas, there is a considerable interest in the conversion of relatively inert alkanes into value-added chemicals, such as olefins (via dehydration); nitrile (via ammoxidation or nitroxidation) and other functional oxygenate groups (via oxidation). Acetonitrile can be produced by catalytic dehydration of acetamide, or dehydrogenation of ethylamine but there is a little effective and economic process for its synthesis. A few attempts have been made to obtain acetonitrile from ethane and ethylene ammoxidation by means of metal-based oxide catalysts. In a preliminary communication [1], it was shown that acetonitrile can be synthesized from ethane on alumina supported Nb-Sb oxide at 480-540~ with selectivity equal to 50%. Earlier, a USSP patent [2] disclosed that ethane can be converted to acetonitrile over Cr-Nb-Mo oxide at 350-500~ with a yield of 10%. In addition, Armor and Li have recently observed that cobalt exchanged zeolites are active and selective for ethane and ethylene ammoxidation into acetonitrile with a comparable behaviour [3-5]. In fact, acetonitrile can be transformed over Br6nsted acid catalysts into corresponding amide, earlier transformed to acetic acid. It can also be oxidized to glycolonitrile or glycolamide. In this paper, we report the preliminary results of the ammoxidation of ethylene with Co-Y and Co-ZSM 5 as catalysts. Y and ZSM 5 exchanged with cobalt have been synthesized following two methods of exchange. The first is the classical way, using cobalt salt solutions and the second one based on the solid-state exchange. The obtained catalysts have been characterized by X-ray diffraction, UV-Vis spectroscopy, BET surface area and porosity measurements, TPD of ammonia and n-hexane reaction. Then, they were studied in the ethylene ammoxidation.

936 2. EXPERIMENTAL

2.1 Catalyst preparation -By aqueous ionic exchange: The catalyst was prepared as described in literature [4]. The protonic forms of Y and ZSM 5 (Si/AI = 26) were, separately, exchanged with cobalt acetate solution 0.1 M at 80~ during 24 hours. After three identical exchanges, the zeolites slurry was filtered, washed with de-ionized water and dried at 110~ Finally, the catalyst was pre-treated with flowing oxygen at 500~ for 1 hour. All the catalysts prepared by aqueous ionic exchange will be referred to as follows Co-Y, x and Co- ZSM 5,x; that's to say: Cozeolite topology- number of exchange. -By solid-state exchange (SSE): A physical mixture of zeolites H-ZSM 5 and precursor cobalt acetate or cobalt chloride with a desired molar ratio (Co/Al = 3/2) was pretreated with flowing helium at 500~ for 12 hours. Then, the catalyst was washed with deionized water, filtered and dried. Finally, it was pre-treated with flowing oxygen at 500~ for 1 hour. Zeolites exchanged in this way will be designated by Co-Zb, in reference to precursor b used; b corresponds to "A" and "CI" respectively for cobalt acetate or cobalt chloride. The catalyst activation and regeneration following each experiment were carried out in situ. Activation was performed at pre-treatment temperature, while regeneration was done at 500~ for I hour under a flow of oxygen and helium.

2.2 Catalyst Characterization XRD spectra were obtained on a difl~actometer with a copper anode. The K~ radiation was selected with a diffracted beam monochromator. Analysis by UV-Visible was performed under ambient conditions, on a Perkin Elmer Lambda 9 spectrometer operating in the diffuse reflectance mode and using BaSO4 as the reference. Measurements on solid were obtained by pressing the powder into pellet form in a special cell possessing spectrasil windows. The spectral data were acquired in the range 200-1200nm at a scan rate of 120 nm/min. N2-BET analysis and porosity measurements were done on a Micrometrics ASAP 2000 apparatus. The temperature programmed desorption of ammonia was made as follows: First, the catalyst was exposed to a helium flow at 500~ for 1 hour, then saturated with ammonia at 100~ and flushed with helium at the same temperature. Finally, the temperature was ramped to 550~ at a rate of 5~ This analysis was done by means of a catharometer, n-hexane tests were carried out on 100 mg catalytic charge. The solid was placed in the micro reactor and activated under a flow of oxygen and helium at 500~ during 1 hour. The reacting mixture consisted of 45 tort of n-hexane and 715 tort of hydrogen. The global flow was fixed at 30 cm3/min. The study interval of the reaction temperatures ranged from 250~ to 400~ Passage of one temperature to another, occurred as soon as the stationary state was reached. In these experimental conditions, the transformation of n-hexane provided isomers in C6 (2 pentane methyl, 2-2 butane dimethyl and 2-3 butane dimethyl) and cracking products in C1 to C5, independently of the type of the sample. Ammoxidation of ethylene was studied between 450 and 500~ temperature range by using a dynamic micro reactor operating at atmospheric pressure. A total flow rate equal to 100 cm3/min and a catalyst weight of 0.05 g and 0.15 g for Co-ZSM 5 and Co-H-Y were respectively used. In all cases, the inlet reagent composition was 6.5% 02, 10% C2H4, 10% NH3 and the rest helium. Preliminary tests and calculations were made in order to prove that the reaction rate was not affected by physical phenomena such as Inter/Intra panicle mass and heat diffusion. The

937 analysis of the reaction partners was recorded on line by two chromatography units, one operated with a flame ionization detector while the other is equipped with a thermal conductivity detector. 3. RESULTS AND DISCUSSION 3.1 Structural and textural properties X-ray diffraction patterns were obtained from a catalyst of ZSM 5 and Y zeolite supports exchanged with cobalt and compared with that of H-ZSM 5 and H-Y. All diffraction peaks obtained were only attributed to zeolite structure, in particular no cobalt-species could be identified. This result suggests that cobalt particles deposited during the preparation measured less than 4 nm and were well dispersed over the zeolite. Table 1 gives BET surface areas and porosity characteristics of H-ZSM-5, Na-Y, H-Y zeolites and their corresponding ones exchanged with cobalt. Table.1 BET surface area and porosity measurements (m2/g)

SBET

Micro porous volume (cm3/g)

Porous volume (cm3/g)

Average pore diameter (A)

H-ZSM 5

545

0.15

0.81

36.22

Co-ZSM 5,1 (500)

288

0.08

0.52

43.77

Co-ZSM 5,3 (500)

346

0.05

0.41

28.90

Co-ZC1

342

0.09

0.50

36.06

Co-ZA

377

0.09

0.54

34.53

Na-Y

627

0.33

0.38

15.17

H-Y

550

0.29

0.33

15.21

Co-Y ,1

515

0.26

0.32

15.62

Co-Y, 3

436

0.15

0.39

22.1

Catalysts

The adsorption isotherms of the different catalysts are of type IV by reference to IUPAC classification. However, Hysterisis loops of adsorption-desorption isotherms present different shapes depending on the ionic exchange method and on the nature of the zeolitic support. The significant changes in the measured surface area and porous volume may be interpreted as a consequence of structural alteration. So, some pore blocking might have occurred due to cobalt species either dispersed in the channels or deposited in the outer surface of the zeolite, leading to the observed surface area decrease. 3.2 Optic Spectroscopy Figure 1 shows the uv-vis spectra of the samples. The spectrum (a) referred to the hydrated samples obtained by aqueous ion exchange shows one band around 510 nm. This absorption corresponds to the 4Tlg(F) --~ 4Tlg(P) transition, assigned to transition of octahedral [Co(H20)6] 2§ complex [6,7]. Whereas, uv-vis spectrum (b) of the sample prepared

938 by solid state exchange shows two bands at 570 and 610 nm assigned to high spin (d 7) ions in a tetrahedral symmetry and coordinated to three oxygen atoms of the environment [6] and one absorption at 520-530 nm [7] attributed to Co s+ zeolite framework. All the samples present also a band centred around 230 nm, which has been attributed to a charge transfer.

032

i

0,27

J

~ O,22

b

i

0,17 0,12

, ....,.......... , ..... ~......... k........ 450

550

650

,.....

,

,

750

, 850

, 1200

Wave length (nm) Figure 1 9UV-Visible spectra of (a) CoZSM-5,3 ; (b) CoZA

3.3 T P D of a m m o n i a Typical temperature programmed desorption spectra of ammonia on parent and exchanged zeolites are shown in figures 2 and 3. The amount of ammonia desorbed from the catalysts and the maximum peak temperature (TM) are summarized in table2.

Table.2 Amount of desorbed aman0nia and temperature of maximum Peak (TM) Catalysts

1-peak TM

h-peak TM

Amount of desorbed ammonia

(K)

(K)

m.mole/g

10 3 m.mole/m 2

H-ZSM 5

428

612

0.71

1.3

Co-ZSM 5,1

446

-

0.19

0.66

Co-ZSM 5,3

463

-

0.75

2.16

Co-ZA

462

-

0.72

1.91

Co-ZC1

480

-

0.60

1.75

H-Y

435

-

1.12

2.18

Co-Y, 1

463

-

2.00

3.88

Co-Y ,3

455

-

2.30

5.27

939 H-ZSM-5 TPD spectrum (Figure 2d) exhibits a fairly good resolution of two peaks, one in the low temperature zone and the other in the high temperature zone respectively named 1 and h peaks. The Temperature of the maximum peaks (TM) on H-ZSM 5 are about 428 K and 612 K. Spectrum is basically similar to those reported in literature [8]. The TPD profile over CoZA, Co-ZC1 and Co-ZSM-5,3 catalysts (Figure 2 spectra a, b and c) show that the cationic exchange of H-ZSM-5 resulted in the preferential decrease of h-peak rather than 1-peak for which TM increase as the percentage of the introduced cobalt does. In a similar study coupled with an IR analysis, J.N. Armor and coil [ 1] have shown the absence of OH (3610 cm"~) acid groups in the catalysts exchanged with cobalt. However, not only the amount of acidity but also its distribution was modified by cobalt exchange because it seems that cationic ,,,,h~,,,~ poisoned the strong acid sites preferentially. Besides, the catalysts prepared from Y type zeolite show (Figure 3b) broad unresolved spectra [9]. The preferential elimination of strong acid sites by cationic exchange has thus been confirmed as on Co-ZSM-5. All these results seem to indicate that the introduction of cobalt is accompanied by the creation of a new acidity depending on the nature of the zeolitic support and the amount of cobalt loading.

%

70 ~

~$O'r~ ~

"

. . . . . . .

'

3~)S~

T"C

Figure 2:NH3 TPD profiles of: (a) Co-ZCI, (b) Co-ZA, (c) Co-ZSM-5,3 (d) H-ZSM-5

\

T~

7

Figure 3: NHsTPD profiles of: (a) Co-Y, 1 (b) Co-Y ,3

(c) H-v

3.4 lsomerisation of n-Hexane As it can be deduced from table 3, the product distribution in the n-hexane transformation [ 10-14] depends on the nature of zeolite and is also strongly influenced by the cobalt exchange. Comparison of the catalytic activity of the zeolites with and without cobalt in the n-hexane reaction shows that the activity decreases when the amount of cobalt increases. However, the selectivity of Co-ZSM 5 towards the n-hexane cracking is extremely higher than that of Co-Y and the low isomefization selectivity of the protonic zeolites

940 indicates that the n-hexane cracking occurs essentially on the strong protonic acid sites. We also notice that Co-Y catalysts show a higher selectivity toward the isomerization product than Co-ZSM 5. So, the activity drop interprets the effect of cobalt on H-ZSM-5 acidity whereas the activity of H-Y is marked by a progressive and less sharp reduction together with a definitely higher improvement of the isomerization selectivity. These results can be correlated to those of NH3 desorption which proved that the introduction of cobalt shifts the strong protonic acid sites and introduces a new acidity depending on the nature of the zeolitic support. The latter seems to be responsible for the isomerization activity at 250~ whereas the cracking would be due to the strong acid sites inhibited by cobalt. We can conclude that the acid strength corresponds to the order Co-ZSM 5 > Co-Y. Table.3 Isomerization of n-hexane at 250~ Catalysts

Activity 106 mole gls-~

TTG (%)

H-ZSM 5

3.03

Co-ZSM 5,1

Selectivity (%) Isomerization

Cracking

0.28

1.34

98.66

0.47

0.05

2.33

97.67

Co-ZSM 5,3

0.26

0.03

7.36

92.64

Co-ZA

0.38

0.05

2.77

97.23

Co-ZC1

0.18

0.02

5.33

94.67

H-Y

3.47

0.39

37.14

62.86

Co-Y, 1

3.67

0.37

60.77

39.23

Co-Y,3

2.37

0.29

67.24

32.76

3.5 Ethylene Ammoxidation Table 4 summarizes the catalytic performances in ethylene ammoxidation of Co-Y and Co-ZSM 5 zeolites as a function of the different exchange procedures, while figures 4 and 5 show the catalytic performance during the first 180 min run at 500~ The evolution of the catalytic properties of Co-Y, 3 as a function of time on stream is characterized by an increase in the specific activity and the selectivity toward acetonitrile product, to reach stationary state after 150 minutes. Meanwhile, the activity and the selectivity in CO2 undergo a decrease before becoming steady after the same time. The weak activity of zeolite Co-Y in the ammoxidation reaction [5-8] seems to be caused by the migration of active sites Co 2+ from the supercages to the sodalites cages leaving no active sites available to the reactants and/or to its relative weak acidity. Compared to zeolite Co-Y, zeolite ZSM-5 exchanged with cobalt (Co-ZSM 5,3) exhibits better activity and selectivity in the ammoxidation reaction probably due to the more open structure and to the nature of its pores [ 1-2]. As for ethylene ammoxidation reaction, both the activity and the selectivity towards acetonitrile increase with time to reach a constant level after 150 minutes. In opposition, the

941 activity and the selectivity in carbon dioxide undergo a progressive reduction to reach a stationary state after the same time.

..

16

10o

14

90

12 ~

80 70

8

~ 60

~ 4 <

50

2 0

~

3

33

~

63

~

93

~

123

40

i

153

3

183

33

63

93

123

153

183

t(min)

t(min)

--u-- Co-ZA

--o-- Co-ZC1

]

---o- Co-Y,3

- - ~ Co-ZSM- 523

Figure 4: Activity to acetonitrile versus time during the first 180 min at 450~

---0- Co-ZA

--.o- Co-ZC1

---o- Co-Y,3

---n- Co-ZSM-5,3

Figure 5" Selectivity to acetonitrile versus time during the first 180 min at 450~

Table.4 Ammoxidation of ethylene on Co-ZSM 5 and Co-H-Y Catalysts TTG (%) S(CH3CN) in % 450

475

500

450

Ac(CH3CN) 104 mole.gl.s 1

475

500 450

475

500

Energy of Activation (KJ mo1-1)

CoZSM-5,3

2.17 5.27 11.80 97

98

99 0.87

2.08

4.88

69.0

Co-ZA

3.90 9.50 24.20

97

98

99 1.39

5.20

9.80

94.2

Co-ZC1

1.01 5.33 21.83

92

97

99 0.56

2.47

9.

Co-Y,3

1.29 1.87

90

94

95 0.19

0.29

0.47

,

3.11

104.5 35.7

i

The most important parameter controlling the ionic solid-state exchange is the nature of cobalt precursor (b) to be introduced. At low temperature (450-475) CoZC1 reacts in the same way as CoZSM-5,3. However, at 500~ it exhibits an activity in acetonitrile and in CO2 almost twice as important at the steady state whereas the selectivity toward the main product is similar to that of CoZSM-5,3. This behaviour can be explained by the inhibiting effect of chloride for the lower temperatures. As for Co-ZA, a steady state is reached after a transitory period of 180 minutes, characterized by an increase of the catalytic activity towards acetonitrile and CO2. At the same time, the selectivity into nitrile product grows while that relative to the deep oxidation (CO2) decreases remarkably. It must be noted that the activity of the catalysts prepared by the solid-state exchange with different precursors is higher than that

942 of the catalysts obtained from aqueous exchange. Furthermore, in terms of product selectivity, the two catalysts prepared by solid-state exchange are similar. However, Co-ZA catalyst exhibits a better activity. 3.6 Characterization of the catalysts after reaction

UV-visible study of the samples after reaction run indicate that Co 2§ coordination undergoes a change from octahedral to tetrahedral for the catalyst prepared by aqueous ion exchange, while, the tetrahedrally coordination of Co 2+ is maintained for the sample issued from solid-state exchange. Furthermore, the characterization of the catalysts by different techniques (XRD, FTIR, NH3 TPD, N2BET measurements) shows that physicochemical properties of the catalysts are maintained. 4. CONCLUSION The results obtained in the presence of different catalysts tested at various temperature shows that the catalytic properties of the zeolites exchanged with cobalt depend on the nature of the zeolitic support and the way of exchange as well as the nature of cobalt precursor. As a matter of fact, the sample with cobalt in zeolite Y is less active and selective than the catalysts with cobalt in zeolite ZSM-5 which exhibits comparable selectivity despite a notable difference in the activity, depending on the method of exchange and on the nature of the cobalt salts. These differences can be explained by acidity and textural properties of the catalysts studied above and discussed in terms of cobalt location in the two types of zeolites and the modification caused by the insertion of cobalt to the acidic properties. Among the tested samples the catalyst which results from solid-state exchange using cobalt acetate develops the best catalytic performances in ethylene ammoxidation. Indeed, it is the most active and selective towards acetonitrile and shows a great stability in time. REFERENCES

1. R. Cavani and G. Centi, J. Chem. Soc., Chem. Commu., 1081 (1991). 2. S.M. Aliev, V.D. Sokolovskii and G.B. Horoskov, USSR Patent SU-738657 (1988). 3. Y. Li and J.N. Armor, J. Chem. Commu., 2013 (1997). 4. Y. Li and J.N. Armor, J. Catal., 173, 511 (1998). 5. Y. Li and J.N. Armor, J. Catal., 176, 495 (1998). 6. M.G. Uytterboven, R. A. Schoonhetd. Microporous. Material., 3 (1994) 265. 7. H. Praliaud, G. Coudurier, J. Chem. Soc., Faraday Trans. I, 75, 2601 (1979). 8. N.Y. Topsoe, K. Pedersen and E.G. Derouane, J. Catal., 70, 41 (1981). 9. J. Cattanach and P.B. Venuto, J. Catal., 11,342 (1968). 10. M. Guisnet, "Catalysis by acids and bases", (B. Imelik and al Ed)., Stu. Surf. Sci. Catal., 20 283 (1985). 11. G. Bourdillon., Th~se ; Universit~ de Poitiers (1985). 12. F. Alvarez., G. Gianetto and G. Perot, Appl. Catal., 34 353 (1987). 13. M. Belloum, Th~se ; Universit~ Pierre et Marie Curie, Paris VI (1990). 14. F.R. Riberto, Th~se ; Universit6 de Poitiers (1980).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

943

Physicochemical characterization of vanadium-containing K10 epoxidation catalyst LKhedher", A. Ghorbela and A. Tuelb aLaboratoire de chimie des matrriaux et catalyse. Drpartement de Chimie. Facult6 des Sciences de Tunis 1060 TUNIS. TUNISIE blnstitut de Recherche sur la Catalyse, CNRS, 2, Avenue Albert Einstein 69626 Villeurbanne Cedex- France. Vanadium-containing K10 was synthesized and characterized by electron paramagnetic resonance, nuclear magnetic resonance, N2 absorption at 77K, diffuse reflectance spectroscopy and chemical analysis. Two different vanadium species have been identified in dried V-K10, the first one is V 4+ in square pyramidal environment and the second is V 5+ in tetrahedral environment as isolated monomeric state (VO43) and dimeric state [O3V-O-VO3] 2 9V4+ transforms into V 5+ species with a tetrahedral environment during calcinations in the presence of air. It can be suggested that, V 5+ in calcined V-K10 is easily reduced by organic molecule adsorption with a change in coordination from tetrahedral to square pyramidal. From 27A1 and 29SiNMR data, it can be assumed that, the vanadium species are more likely in interaction with silicon environment. Only vanadium, in highest oxidation state, is active in the epoxidation of allylic alcohol.

1. INTRODUCTION Transition metals modified montmorillonite, particularly those containing titanium and vanadium cations, were found to exhibit excellent catalytic properties in redox processes such as the oxidation of a variety of organic substrates. Nevertheless few reports have dealt with the characterization of vanadium containing montmorillonite. On the other hand, a large number of techniques including FTIR, EPR, UV-vis and 51V solid state NMR provided strong evidence that the vanadium is located in monmorillonite framework probably as a defect site [1,2]. The present paper reports new observations about the coordination and oxidation state of vanadium dispersed on K10 montmorillonite and its interaction with the support, whereas the catalytic performances are tested in epoxidation of allylic alcohols using Tert-butyl hydroperoxide (TBHP) as oxidant. 2. EXPERIMENTAL Vanadium catalyst was obtained by refluxing VCls (5mmol) in dry tBuOH (20 ml) with HK10 (lg) under helium atmosphere until the solid clay suspension turned to a deep green colour (within approximatively 6 h). The solid was filtered and washed many times with dry

944 tBuOH to remove the excess of VCI3 and then oven dried at 383K and calcined in dry air at different temperatures. K10 and K10 containing vanadium "V-K10" calcined at different temperatures were characterized by BET surface area and porosity measurements. To get further insight about the location and the nature of vanadium species present in these catalysts, ESR, UV-vis, NMR and IR investigations were performed on all samples. Chemical analysis was carded out by atomic absorption spectrometry on a Perkin-Elmer 3100 apparatus, after sample dissolution through acid attack. BET surface areas and pore volumes of the samples were measured on a Micrometrics ASAP 2000 apparatus. Infrared study was realized with Perkin-Elmer FTIR paragon 1000PC. EPR spectra were recorded with a spectrometer Bruker ER 200tt at 77K. UV-visible diffuse reflectance spectra were measured at room tenaperature with Perkin-Elmer lamda 9 with the use of BaSO4 as reference. A spectrometer Bruker MSL 400 was used for NMR spectra registration. Catalytic properties of V-KI 0 were tested in epoxidation of Trans-2-hexen-l-ol (7 mmol) to 2,3-epoxihexan-l-ol, performed by stirring 25mg of V-K10 in presence of tertbutylhydroperoxide (4 mmol) in dry toluene under an inert atmosphere followed by the addition of the allylic alcohol.

3. RESULTS AND DISCUSSION 3.1. Characterization

Table.l Results of chemical analysis (Vanadium weight-%) and textural properties of the samples. Samples

Natural montmorillonite KI0 V-K10 a V-K10 b V-K10 r V-K10 d

V%

..... ..... 1.72 1.72 1.92 1.92

SBEX(m2/g)

Total pore volume (cm3/g)

40.8 233.3 133.3 161.9 155.3 198.2

0.10 0.31 0.22 0.25 0.25 0.28

a dried at 383K, b calcined at 473K, c calcined at 573K and d calcined at 673K. KlO-montmorillonite is obtained from the natural montmorillonite by treatment with mineral acids at high temperature. The natural montmorillonite structure is progressively destroyed, which results in a loss of crystallinity. Dealumination by acid treatment leads to the creation of a large surface areas compared to the natural montmorillonite. The treatment of K10 with vanadium solution decreases the surface area (table.I). This can be attributed to

945 the filling of pore volume with vanadium species. Furthermore, Na adsorption isotherm of K10 and V-K10 show that their large surface areas result mainly from mesopores with a pore size distribution and an average pore diameter of 54 - 65A whereas microporosity remains negligible.

g• 1.997 Al = 75 .

.

/ , ~ ~

.

.

.

~

, . . ,

.

g//=1.935 A//--(la~7

~=

(b) -3

(c) ~_~

Gain 5x103

(d) Gain 3.2x103

Fig.1 EPR spectra of V-K10 calcined at 383K (a), at 473K (b), at 573K (c) and after alcohol adsorption on calcined sample at 573K (d).

In order to gain some insight into the location and the nature of the vanadium centres, EPR investigations, described in detail elsewhere [3], have been undertaken. The samples show signal (fig.l.a) typical of V (IV) 3d ~ centres with eight-line hyperfine patterns deriving from the interaction of free unpaired electron of V 4+ with the magnetic nuclear moment of 51V (1=7/2). The spectrum is very similar to those obtained by Montes and al. [4] at 77K, showing a monomeric vanadyl (IV) species, with g and A anisotropic tensor values, deduced From the signal, characteristic of a square pyramidal structure with approximate axial symmetry. The spectra have a well-resolved hyperfine structure but the baseline is not horizontal. A superimposed broad singlet (marked with a dashed line), is observed, which may be attributed to V 4+ in dipolar interaction with other V 4§ ions. Similar spectra have been obtained by Taouk and al. [5]. It has been shown that, when paramagnetic centers are numerous and closes, an electronic exchange between several centers may be resulted and only one line was observed. It may be concluded that, the broading of the signal indicates an increase _.in the integral of exchange and the dipolar interaction between paramagnetic centers [6]. The spectra of tetrahedral coordinate V 4+, exhibit different parameters (V 4§ in ThGeO4 exhibits [7] g/i=1.831, g.L = 1.980 and A//= 191G, Aa. = 35G), with smaller coupling constants and are detected only at 77K or at lower temperature [8]. As our spectra recorded at 77K and 298K are identical, with a lower intensity of signal at 298K, tetrahedral symmetry of the V 4+ environment should be discarded.

946 The signal intensity decreases with increasing calcination, temperature in air, which could be explained by a change of the oxidation state from V 4§ to V 5+ (dO). Furthermore, the EPR study of V-K10, shows that, after adsorption of organic molecules, such as alcohol, V 5+ can be reduced easily to V 4§ (fig.2.e) with increase of signal intensity compared to the one obtained in dried V-K10. This effect can be explained by the reduction of V 5+ in calcined V-K10. These results indicate that V-K10 have redox properties and readily change oxidation state between V 4§ and V 5§ 5~VNMR studies on V-containing catalysts -,1, I have shown that it is possible to obtain I information on the symmetry environment of vanadium by comparison with model compounds. The spectra obtained for dried V-K10 samples can be interpreted as due to vanadium present in the lattice with tetrahedral oxygen coordination (Fig.2.a). The occurrence of two peaks in the spectrum may be indicative of two vanadium species with different local coordination. The line at around-722ppm, can be attributed likely to an isolated vanadium ions with tetrahedral oxygen coordination (VO43"), while the peak at -616ppm can be characteristic of tetrahedric coordination in dimeric state [OaV-O-VO3] 2", as the same peak was detected with pyrovanadates cases, like e.g., MzV207 (M: Pd, Cd, Zn, Mg) [9,10]. On calcination sample in air at 573K (fig.2.b), Fig.2 51V RMN spectra of V-K10 the intensity of the signal a r o u n d - 7 2 2 p p m calcined at 383K (a) and at 573K (b). decreases, while the -616ppm signal appears more clearly. This modification is certainly a result of the increase of the interaction between the vanadium species in the lattice, which is probably favoured by calcination. According to NMR data, no distinct resonance, attributed to an octahedral vanadium species at around -300pprn, was detected. -"72:2

J

I'

_

2f~

~

9

""

9

'1 ' " l

9 'i

"'i

I

m

9

9

w

I

27O i

(b)

! 200

i ~00

!

! 400

!

! ~

i

a 600

i

! 700

!

i

200

|

300

'!

|

400

!

!

500

'

J

1

600

!

!

700

Fig.6 UV-visible RD spectra of V-K10, (a) dried at 383K and (b) calcined at 573K.

!

947 Diffuse reflectance spectra in the UV-VIS region were obtained to characterize the structure of vanadium in V-K10 (Fig.6). It is known that the electron-charge transfer energy is strongly influenced by the number of ligands on the central ion and gives information on the coordination of the vanadium in the lattice [11,12]. It has been reported that as the number of ligands on vanadium decreases from 6 to 4, the absorptions shift to shorter wavelength [12]. In accord with previous studies [13,14], V 5+ in octahedral environment, such as V205, shows three main absorption bands at --~454 ran, at --~322nm and at---238nm, while for tetrahedral V 5+ compounds [NH4VO3, Na3VO4, Mg3(VO4)2], in contrast, one absorption band is found at-~333 nm and a second at ~277 nm. The UV-visible DR spectra of the dried V-K10 (Fig.6.a) can be analysed on the basis of the above information. The dried V-K10 exhibits two charge transfer bands at 260 and 360nm, indicating the presence of V 5+ in tetrahedral environment, with an absorption band at 550-850 nm due to d-d transitions of VO 2+ ions [11]. The intensities of these d-d transitions, in fact, are generally 10-30 times lower than those of charge transfer transitions. On cacination V-K10 (fig.6.b), the same charge transfer bands are observed with increase of their intensities and the d-d transition band has disappeared. This result may be explained by the oxidation of V 4+ to V 5+, as it was yet shown by EPR data, with more likely, a change in coordination from square pyramidal to a tetrahedral. It may be concluded that, V 5+ ions extu'bit tetrahedral coordination in both dried and calcined V-K10. All those findings, show that framework vanadium ions can occur in dried V-K10 simultaneously in two valence state and at two different coordination: V 4§ square pyramidal indicated by EPR and V 5+ tetrahedral indicated by 5~VNMR and UV-vis DR. When V-K10 was calcined in air, V 4+ was oxidized to V 5+ and its square pyramidal structure, transformed into tetrahedral one. This interpretation is supported by previous studies, in vanadium containing silicate [15,16] and in A1PO4 [17], where the same phenomena have been assumed. The 298i and 27A1MAS NMR spectra of K10 have been reported previously [18]. These spectra relative to K10 are reported also in fig.3 and fig.4, which show the appearance of two resonances, compared to the original montmorillonite, at -109 and -102 ppm in the 29Si spectra and an increase of the intensity of the peak at 73 ppm in the 27A1 spectra. In their study of aluminosilictaes, Magi and al [19], assigned the signal a t - 1 0 9 p p m to siliceous impurities associated with the clay and the peak at -91ppm to Si atoms in the tetrahedral layer of the clay linked to three Si atoms and either to an A1. The peak a t - 1 0 0 ppm may be explained by the presence of 4Q(A1) structure formed by Si-O-A1t~t linkages. This interpretation is supported by the observation of the 27A1NMR signal at 73ppm that is typical oftetrahedral A1 in aluminosilicates. Figure.3 shows ~7A1 MAS NMR spectra of vanadium containing K10, which is similar to that of K10. The chemical shifts corresponding to the different A1 species are not affected by the presence of vanadium species in the lattice. Thus the interaction of the vanadium species with the A1 species is completely excluded. On the other hand, the 27Si NMR studies of the original K10 and V-K10, showed that the vanadium is more likely in interaction with the silicon environment. According to other studies [20], the broadening and the displacement of the lines in the spectrum, notably the resonance around -91ppm, can be an indirect indication of the distribution of the vanadium in the lattice. This effect is more pronounced in the sample calcined at 573K, which can be

948 due to the increasing interaction between the vanadium and silicon species with increasing calcination temperature. No distinct line attr~utable to V-O-Si sites was observed. c:)

i

73.79

-

I I

"i

I

I

i

!

9 '

-91.54

I

' '{d;,'"6,

c~.

: ) c=, ~.., c--;,, ...~.,

3..91

I

.... ;"

':6,

-100

plpmm

9

9

9 ..

| -..qD

Fig.3 27A1RMN spectra of K10 (a) and V-K10 calcined at 573K (b).

u

i

,

-)LEO

9

9 |

"

-)1.~

"

"

'

"

lt~pmm.

Fig.4 298i RMN spectra of K10 (a), VK10 calcined at 383K (b) and at 573 (c).

From previous studies in V-silicates [21, 22], The location of vanadium in the lattice could be suggested directly from IR study. On the other hand, in alumino-silicates case, V-O vibrations are generally masked by more intense bands vibration of the lattice. For this reason, we have studied the subtraction of K10 spectrum to V-K10 one. In fact, the spectrum obtained show a band at around 950cmq, which has been attributed to V-O-Si vibration due to the presence of vanadium in the lattice [11 ]. This attribution seems to be supported by the present 298i NMR study. 3.2. Catalytic properties Table.2 Catalytic activity of V-K10 as function of calcination temperature Samples V-K10 b V-K10 c V-K10 d

V0.103mol.lq.min"1 2.45 2.68 2.17

epoxide yield (%) 31.15 32.50 30.10

Conditions: the epoxidation reaction was performed on 7mmol allylic alcohol in dry toluene using 25mg of catalyst (V-K10) and 4mmol of azeotropically dried TBHP in DCM at 65~

949 The results summarized in table.II show that the catalytic activity depends essentially on the calcination temperature. In fact the reaction leads to epoxide with high yield and initial rate in the presence of the catalyst calcined at 573K, while the initial rate of the catalyst calcined at 673K decreases, which may be caused by the agglomeration of vanadium species. This result indicates a good dispersion of active vanadium species, in V-K10 calcined at 573K, accessibles for the complexation with the reactant and oxidant. Taking in account the results of previous studies of the epoxidation reaction [23] and TBHP and alcohol adsorption on the catalyst surface followed by NMR study [17], it is assumed that the activity of V-K10 in this reaction result from the site isolation of vanadia center, which may be vanadyl specie in its highest oxidation state (V=O) 3+. In fact, the vanadium in low oxidation state, are rapidly oxidized by tert-butyl hydroperoxide to their highest oxidation state. The mechanism retained to explain the vanadium epoxidation reactions of allylic alcohols, suggests that TBHP and alcohol adsorption on vanadium lead to a complex intermediate which renders the peroxidic oxygens more electrophilic and, hence, more liable to attack by allylic double bond [23]. 4. CONCLUSION The spectroscopic techniques adopted in this study indicate the presence of vanadiuln, in dried V-K10, simultaneously in two different states, tetrahedral coordinated V 5+ and V 4+ square pyramidal. In case of the sample calcined at 573K, the vanadium is present mainly in V 5+ state with a tetrahedral coordination as monomeric and dimeric species and could be reversibly transformed between V 4+ and V s+ states. The vanadium species located in the lattice are more likely in interaction with silicon environment. V-K10 is efficient for the epoxidation reaction with the advantage of high activity in particular with V-K10 calcined at 573K, which is due probably to the well-dispersed state of the active species (V=O)3tet.

REFERENCES

1. I. Khedher and A. Ghorbel, Studies Surface Science and Catalysis 130 (2000) 1649. 2. I. Khedher and A. Ghorbel, Mechanistic study of allylic alcohols epoxidation over vanadium-montmorillonite as a heterogeneous catalyst, published in 4th World Congress on Oxidation Catalysis, September 16-21,2001 Berlin/Postdam-Germany. 3. J. W. johnson, D. Johnston. A. Jacobson and J. F. Brody, J. Am. Chem. Soc., 106 (1984) 8123. 4. C. Montes, M. E. Davis, B. Murray and M. Narayana, J. Phys. Chem., 94 (1990) 61. 5. Bechara TAOUK, Th6se de Doctorat, Universit6 Pierre et Made Curie-Paris VI (France), (1988). 6. O Reilley, D. E., Maclver, D. S., J. Phys. Chem. 1962, 66, 276. 7. C. Montes, M. E. Davis, B. Murray and M. Narayana, J. Phys. Chem., 94 (1990) 61. 8. E. Fritsh, F. Babonneau, C. Sanchez, G. Galas, J. Non-Cryst. Sol., 92 (1987)282.

950 9. O. B. Lapina, A. V. Simakov, V. M. Mastikhin, S. A. Veniaminov, A. A. Shibin, J. Mol. Cata., 50, 55(1989). 10. M. L. Occelli, R. S. Maxwell and H. Eckert, J. Catal. 137, 36(1990). 11. Camblor, M. A., Conna, A., and Perez-pariente, J., J. Chem. Soc., Chem. Commun. 557(1993). 12. G. Lische, W. Hanke, H. G. Jerschkewitz and G. Ohlmann, J. Catal., 91 (1985) 54. 13. G. Centi, S. Paratthoner, F. Trifiro, A. Aboukais, C. F. Aissi et M. Guelton, J. Phys. Chem., 96, 2617(1992). 14. So, H., Pope, M. T. Inorg. Chem. 1972. 11, 1441. 15. M. S. Rigutto, H. van Bekkum, Appl. Catal., 68 (1991) L 1. 16. T. Sen, V. Ramaswamy, S. Ganapothy, P. R. Rajamohanan and S. Sivasanker, J. Phys. Chem., 100, 3809(1996). 17. M. S. Rigutto, H. van Bekkum, J. Mol. Catal. 81 (1993) 77-98. 18. C. Cativiela, F. Figueras, J. M. Fraiele, J. I. Garcia, L. C. deMenorval and E. Pires, Appl. Catal. A, 101(1993) 253. 19. M. Magi, E. Lippmaa, A. Samonson, G. Engelhardt and T. Cnfimmer, J. Phys. Chem., 88 (1984) 1518. 20. Moudrakovski, I. L., Sayari, A., Ratcliffe, C. L., Ripmeester, J. A., Preston, K. F., J. Phys. Chem., 98, 10895(1994). 21. Sen, T., Chatterjee, M., and Sivasanker, S., J. Chem. Soc., Chem. Commun. 207(1995). 22. T. Sen, P. R. Rajamohanan, S. Ganapathy, and S. Sivasanker, J. Catal., 163 (1996) 354. 23. Sheldon, R. A., Van Doorn, J. A., J. Catal. 31 (1973) 427-437.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

951

Conversion of aromatic hydrocarbons over M C M - 2 2 and M C M - 3 6 catalysts E. Dumitriu a, I. Fechete a, P. Caullet b, H. Kessler b, V. Hulea a, C. Chelaru c, T. Hulea a and X. Bourdon b aLaboratory of Catalysis, Technical University of Iasi, 71 D. Mangeron, Iasi-6600, Romania bLaboratoire de Mat6riaux Min6raux, ENSCM-UHA, Mulhouse-68093, France Clnstitute of Macromolecular Chemistry, 41 Gr. Ghica-Voda, Iasi-6600, Romania Swelling the layered MCM-22 precursors with large organic molecules and then pillaring the resulting material with polymeric silica MCM-36 materials have been prepared. The BET surface area of MCM-36 was 2 times higher than that of MCM-22. The catalytic activity and selectivity of both MCM-36 and MCM-22 were investigated in the gas-phase alkylation of toluene with methanol at reaction temperatures ranging from 498 to 673K. The methylation of toluene produces a mixture of xylenes as main products. The influence of reaction temperature, pulse number, and toluene/methanol ratio upon the conversion of toluene and selectivities of the products were investigated. The superior catalytic performance of MCM-36 compared with MCM-22 for this reaction indicates that the open mesoporous structure can be successfully utilized to make acid sites of the layers accessible to large molecules. 1. I N T R O D U C T I O N Various zeolites, such as ZSM-5, Y, SAPOs [1-3] etc., were tested as catalysts in the alkylation of toluene with methanol. This reaction could be of great interest to industry as a potential source of p- and o-xylene [4]. It now accepted that the alkylation reaction proceeds through a Rideal-mechanism [5], where the Br~Snsted acid sites are the active centers and the reaction intermediates are carbenium ions [6]. Also, the previous studies showed that the selectivity in this reaction is strongly influenced by the acidity strength and the pore size of the catalyst. Thus, medium pore zeolites such as M-ZSM-5 (M = B, Cr, Fe), which possess only weak and medium acid sites, are very selective towards p-xylene in the alkylation of toluene with methanol [7]. In our study two relatively new zeolites, MCM-22 and MCM-36, have been tested as catalysts in gas phase alkylation of toluene with methanol. Zeolite MCM-22 combines the properties of 10 MR and 12 MR porosity [8]. However, the narrow access to the 12 MR channels through the 10 MR openings, seriously hampering the diffusion of bulky molecules. To avoid this limitation, the pillared zeolite MCM-36 has been developed [9,10]. This material combines the benefits of microporous crystalline zeolite layers with those of the pillared mesoporous structures, i.e. due to this doubly porous structure, an increased accessibility of parts of the crystalline surface, a high thermal stability, and a large sorption capacity are achieved [11]. In this paper, alkylation of toluene with methanol was used to probe the influence of the

952 mesoporous structure in MCM-36 on the catalytic activity and to compare with those of the microporous material MCM-22. 2. EXPERIMENTAL MCM-22 and MCM-36 were synthesized according to the method reported elsewhere [12]. The materials were characterized by various methods: XRD (Philips PW 1800 diffractometer and STOE STADI-P, CuK~), nitrogen adsorption (Micromeritics ASAP 2100), scanning electron microscopy (PHILIPS XL 30). The catalytic reactions using toluene and methanol (high purity reagents) were run in a pulse type microreactor containing 30 mg of catalyst with particle size 0.25-0.43 mm. The microreactor consisted in a stainless steel tube (o.d. 6 mm, i.d. 3.5 mm and length 80 mm) with catalyst particles packed between quartz plugs. Prior to reaction, the catalyst was activated under airflow at 500~ for 3h, followed by cooling to the reaction temperature under nitrogen flow (26 ml/min, 140 kPa). Samples of 1.0 lal of the reagents were injected at constant temperature and the reaction products were analyzed using an on-line GC equipped with FI detector. 3. RESULTS AND DISCUSSION 3.1. Characterization of the catalysts The first data concerning the synthesis of MCM-22 have been reported by the scientists from Mobil in 1990 [13]. The structure of MCM-22 has been shown to consist of layers linked together along the c-axis by oxygen bridges and contains two independent pore systems [14,15]. Within the layers are two-dimensional sinusoidal 10-M ring channels, and between two adjacent layers are 12-M ring supercages (- 0.71 x 0.71 x 1.82 nm) communicating with each other through 10-M ring apertures. The unusual structure of zeolite MCM-22 is formed from a layered precursor designated as an MCM-22(P) [13], which is able to condensate the silanol groups present on the layer surfaces by calcination, and leading to the formation of the 3D structure shown in Figure 1. The pillared zeolite MCM-36 can be prepared from the same MCM-22(P) starting materials, using large molecules, such as cethyltrimethylammonium chloride (CTMAC) and polymeric silica (Figure 1). We have synthesized a MCM-22(P) samples with SIO2/A1203 ratio of 100. The diffraction pattern of this material agrees well with those previously reported [ 16] (Figure 2a). The calcined sample gives sharper reflections than as-synthesized samples, as shown in Figure 2b, and the (001) reflection characteristic to the layered structure of MCM-22(P) disappears. The high crystallinity and phase purity of the MCM-22 zeolite could be considered as prove for the quality of its precursor. MCM-36 material was prepared by swelling the layered MCM-22 precursor with large molecules of CTMAC and then pillaring the resulting material with polymeric silica. As known, in the MCM-36 phase, the polymeric silica as pillars is formed during the hydrolysis and condensation of silicates from tetraethylorthosilicate. The hydrolysis reaction replaces the ethoxy groups with hydroxyl groups. In the next stage, the condensation reactions involving the silanol groups produce siloxane bonds, leading initially to oligomeric and polymeric structures. Depending on the conditions, the final structures of the polymeric SiO2 can be formed as nearly linear polymeric structures or three-dimensional branched structures. The XRD pattern of the MCM-36 sample is shown in Figure 3. All peaks observed correspond perfectly to those of the MCM-36 material reported previously [ 12].

953 Compared with the pattern of MCM-22(P) in Figure 2, the characteristic 002 plane reflection at 2 - 6.6 ~ disappears upon pillaring. On the other hand, an intense low-angle reflection appears at 2 between 1 and 2 ~ which corresponds to a d-spacing of 5.9 nm. This represents the new cparameter of the unit cell. The d-value includes both the c-parameter of the unit cell of MCM-22 and the spacing distance between the layers of MCM-36. 10~ MR

~

~---~l~--~.~J HMI

J~,~

...ca&!nati ..... on:.=.~

__ intralayer IB MR pores

cages

~ - ~ - ~ J ~ . 10 MR window 'r-~.,L~._.... ! connecting

-Y

l

I--t

MCM-22

MCM-22(P)

I

,~,~2

-i_J

micropores

calc#' tion .

.

.

.

.

meeopores

" -

i swollen MCM - 22 (P)

MCM-36

Fig. 1. Schematic representation of MCM-22(P), MCM-22 and MCM-36 structures

954 Therefore, the distance between two layers in MCM-36 can be calculated by subtracting the thickness of the layer (c-parameter of MCM-22 is equal to 2.51 nm [14]). The values for the sample obtained suggest an average interlayer distance of 3.4 nm. The morphologies of crystals, determined by SEM, are mostly platelets of approximately 2gin diameter and 0.1-0.2 thickness bunched into 4-5 gm particles.

(a)

• ~i 3.002.50'-' 2.00c e 1.50-

1.00 0.50

0.0 100-

'

10'.0

'

26.0

'

30.0

'

40'.0

two theta

(b)

908070605040-

30200 0

,

5.0

10'.0

15'.0 21~.0 25.0

313.0 35.0

413.0 45.0 I:wo theta

Fig. 2. Powder XRD patterns of the (a) MCM-22(P) sample and (b) the calcined sample (MCM22) synthesized from the mixture with SIO2/A1203molar ratio of 100. The textural properties of the calcined MCM-22 and MCM-36 samples were measured by 2 1 nitrogen adsorption. The pure MCM-22 sample gives a specific surface area (St) of 465 m g , 2 1 which are typical for this type of zeolite [ 17]. The higher specific area of MCM-36 (807 m g ) sample than that of MCM-22 (465 mZgl), approximately twice, without doubt demonstrates that MCM-36 does not have pore system as MCM-22. The external surface areas were estimated by using t-plots. Therefore, for MCM-22 zeolite resulted that the micropores are dominant ( V m i c r o = 0.12 cm 3 g-1), while for the pillared structure of MCM-36 the mesopores are dominant (Vmeso = 0.497 cm3g-1). The external surface area, Sext, in the larger mesopores was estimated at about 76

955 m2g-1. From a tubular model as 4Vmeso/(St-Sext), the average mesopores size of approximately 3nm was estimated, and this value satisfactorily agrees with XRD data. 300025002000-

~

1500-

C

1000 500

s'.oo

10'.00

IS'.oo

2o1oo

2sioo

two theta degrees

30:00

3s:oo

40.00

Fig. 3. Powder X-ray diffraction pattem of MCM-36. 3.2. Alkylation of toluene with methanol During the alkylation reaction of toluene with methanol on acid catalysts, para- and orthoxylene are mainly formed, especially at low levels of conversion. Moreover, besides the primary alkylation, consecutive reactions, such as xylenes isomerization and toluene polyalkylation giving trimethylbenzenes (TMB) and tetramethylbenzenes (TeMB) can also occur. In the case of zeolites, the competition between these reactions and the selectivity of the alkylation process depends of both reaction conditions (temperature, contact time, pressure, etc) and textural properties of catalyst (shape and size of pores, external surface area, etc).

Table 1. The main results of the alkylation of toluene with methanol MCM-22 Reaction temperature, K 498 548 673 Toluene conversion, %mol 22.4 32.5 46.5 Alkylated products, %mol Xylenes 18.13 22.49 30.3 Trimethylbenzenes 4.24 9.67 14.57 Tetramethylbenzenes 1.03 5.23 8.56 Xylenes selectivity, % p-xylene 72.15 52.69 30.39 m-xylene 16.77 26.76 50.52 o-xylene 11.08 20.55 19.09 TMBs selectivity, % 1,3,5-TMB 0.00 6.11 20.59 1,2,4-TMB 97.40 86.24 66.98 1,2,3-TMB 2.60 7.65 12.43

498 7.9

MCM-36 548 19.1

673 35.7

7.35 0.58 0.43

13.42 3.87 1.87

24.06 9.36 6.45

28.29 17.56 54.15

29.95 22.21 47.84

29.44 43.54 27.02

0.00 55.17 44.83

0.00 59.68 40.32

19.02 69.02 11.96

956

Effect of reaction temperature. Table 1 summarizes the main results of the alkylation reaction between toluene and methanol over MCM-22 and MCM-36, obtained in the range temperature of 498-673K, with an equimolar toluene to methanol ratio. These data lead to the following remarks: (i)

As expected, the conversion of toluene increases with the reaction temperature. Compared to MCM-22, MCM-36 showed a lower toluene conversion for all temperatures.

(ii)

A high selectivity for the ring alkylation of toluene towards xylenes was obtained under our reaction conditions. However, over both zeolites, the amount of polyalkylated products, such as TMB and TeMB, increases when the reaction temperature increases.

(iii)

The reaction temperature affects the distribution of xylene isomers. But, this distribution also depends on the catalyst type. Thus, at low temperature, MCM-22 catalyst showed a high para-selectivity (more than 70% p-xylene in the mixture of xylenes), while MCM-36 leads to o-xylene as major isomer (about 55%). An important amount of p-xylene (amount 30%) was also obtained over MCM-36. It is known that the para- and ortho- isomers are the primary products of the electrophile substitution reactions.

The difference in behavior of MCM-22 and MCM-36 can be explained taking into account their porosity. The selectivity seems to be a consequence of product shape selectivity. Thus, in the case of MCM-22, the 10 MR windows of about 0.52 nm in diameter cause great hindrances in the diffusion of o-xylene isomer, while p-xylene, a "linear" molecule, can easily passed through these windows. On the contrary, the diffusion of products is not influenced by the mesopores of MCM-36, and the xylene isomer composition is similar to those of non-shape-selective catalysts. The ortho-rich product distribution is obtained as required by the principles of aromatic electrophilic substitution [ 18]. As can be seen from table 1 data, the increase in the reaction temperature leads to a decrease of both o- and p-xylene, while m-xylene increases and at 673K the xylene distribution becomes close to the thermodynamic equilibrium (50% m-xylene, 25% p-xylene and 25% o-xylene). At a high reaction temperature, the primary products p- and o-xylene are involved into the rearrangement reaction towards m-xylene, the highest thermodynamic stable dimethylbenzene isomer.

Effect of the toluene~methanol ratio. Some catalytic tests were performed over MCM-22 and MCM-36 at 498K, using three toluene/methanol initial ratios. It can be seen from Table 2 that with increase in toluene/methanol ratio, the selectivity to xylene increases too. An equimolar reagent ratio leads to the highest conversion of toluene. Table 2. The effect of toluene/methanol ratio on the catalysts behavior Toluene/methanol, molar ratio Toluene conversion, %mol Xylene selectivity, % TMB selectivity, %

0.5 22.6 67.4 32.6

MCM-22 1 30.2 77.5 22.5

2 13.7 81.1 18.9

0.5 10.7 86.7 13.3

MCM-36 1 12.1 90.0 10.0

2 7.3 90.3 9.7

957 Effect o f time on stream. Generally, the activity and selectivity of zeolites change during the reactions of organic compounds, because of progressive blocking of acid sites and pores by coke. The rate of the coke formation strongly depends on the strength of acid sites and the pore size of catalyst. The effect of the pulse number on the catalytic activity and selectivity over MCM-22 MCM-36 is shown in Figures 4 and 5, respectively. 80 ~ 60o

E 40-.13-9 X sel.

20-

---.IX--- T M B sel. I

I

1

5

I

I

I

I

I

10 15 20 25 30 number of pulses

I

35

40

Fig. 4. Variation of toluene conversion (T con.), xylene selectivity (X sel.) and TMB selectivity (TMB sel.) as function of the number of pulses for MCM-22 at 673K. As can be seen from Figure 4, the catalytic activity of medium pore MCM-22, expressed in term of toluene conversion progressively decreases when the number of pulses increases. The selectivity to xylene decreases too. 80

P.

."1

r

r

~

13----[3

~

60 o o

E40 20

A-------A

I

1

e

r

A

I

5

r

A

r

A

II

r

A

I

r

A

A

A

---<>-- T conv. - - - B - - X sel. ~TMB sel. I

10 15 20 25 30 number of pulses

I

I

35

40

Fig. 5. Variation of toluene conversion (T cov.), xylene selectivity (X sel.) and TMB selectivity (TMB sel.) as function of the number of pulses for MCM-36 at 673K. On the contrary, large pore MCM-36 shows a very high stability to coking. Thus, toluene conversion, xylenes and TMBs selectivities remain almost unchanged after 40 pulses (Figure 5). 4. CONCLUSION As stated before, both MCM-22 and MCM-36 zeolites are active catalysts in the gas phase alkylation of toluene with methanol. At a low reaction temperature, the selectivity to p- and oxylene, the primary alkylation products, was very high over tested catalysts. Large pore MCM-36 is more stable to coking that medium pore MCM-22.

958 REFERENCES

1. N.Y. Chen, J. Catal., 114 (1988) 17. 2. E. Dumitriu, S. Oprea and K. Yousef, Rev. Chim. (Bucharest), 35 (1984) 906. 3. A.M. Prakash, S.V.V. Chilukuri, R.P. Bagwe, S. Ashtekar and D.K. Chakrabarty, Microp. Mater., 6 (1996) 89. 4. W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein and S.A. Butter, J.Catal., 67 (1981) 159. 5. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M . Marina and A.A. Romero, React. Kinet. Catal. Lett., 57 (1996) 61. 6. P.B. Venuto, L.A. Hamilton, P.S. Landis and J.J. Wise, J. Catal., 5 (1966) 484. 7. R.B. Borade, A.B. Halgeri and T.S.P. Prasada Rao, New Developments in Zeolite Science and Technology, Proceedings of the 7-th International Zeolite Conference, Y. Murakami, A. Iijima and J.W. Ward (eds), Elsevier Sci. Publishers B.V., Amsterdam, 1986, p.851. 8. M. Leonowicz, J.A. Lawton, S.L. Lawton and M.K. Rubin, Science, 264A (1994) 1910. 9. C.T.-W. Chu, C.T. Kresge, W.J. Roth, K.G. Simmons and J.C. Vartuli, US Patent No. 5 292 698 (1994). 10. C.T. Kresge, W.J. Roth, K.G. Simmons and J.C. Vartuli, US Patent No. 5 229 341 (1993). 11. C.T. Chu, H. Altaf, A.M.J. Huss, C.T. Kresge and W.J. Roth, US Patent No. 5 258 569 (1993). 12 Y.J. He, G.S. Nivarthy, F. Eder, K. Seshan and J.A. Lercher, Microp. Mesopor. Mater., 25 (1998) 207. 13. M.K. Rubin and P. Chu, US Patent No. 4 954 325 (1990). 14. M.E. Leonowicz, J.A. Lawton, S.L. Lawton and M.K. Rubin, Science, 264 (1994) 19 ! 0. 15. S.L. Lawton, M.E. Leonowicz, R.D. Partidge, P. Chu and M.K. Rubin, Microp. Mesopor. Mater., 23 (1998) 109. 16. S.L. Lawton, A.S. Fung, G.J. Kennedy, L.B. Alemany, C.D. Chang, G. Hatzikos, D.N. Lissy, M.K. Rubin, H.K.C. Timken, S. Steurnagel and D.E. Wossner, J. Phys. Chem., 100 (1996) 3788. 17. A. Corma, V. Fornes, J. Martinez-Trifuero and S.B. Pergher, J. Catal., 186 (1999) 57. 18. J. Kaspi, D. D. Montogmeri and G.A. Olah, J. Org. Chem., 43 (1978) 3137.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

959

H2-D2 exchange and migration of Ga in H - Z S M 5 and H - M O R zeolites M. Garcia-Sanchez, P. Magusin, E. J. M. Hensen and R. A. van Santen Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. Trimethylgallium (TMGa) was loaded by chemical vapor deposition (CVD) on two different zeolites, H-ZSM5 and H-MOR. The interaction of TMGa on zeolite acid sites and the effect of reduction and oxidation treatments were characterized by ICP, XPS, FT-IR and 71Ga NMR. The capacity of dissociatively adsorption of 1"t2 and D2 and the resultant HD desorption was analysed by H2-D2 exchange reaction. The oxidation and specially the reduction treatment provoke the migration and redistribution thought zeolite crystals. FT-IR results show that TMGa reacts with both silanol and stericaly accessible Bronsted acid sites. 7~Ga NMR spectra show that Ga is presents in two different geometries: octahedral and Ga compensating the negative charged AI from the framework zeolite structure. Hydrogen pretreatment of both zeolites has a positive effect on the distribution of Ga. However, Ga species obtained were less active for H2-D2 exchange than the Ga species obtained after oxidation. I. INTRODUCTION The aromatisation of alkanes over bifunctional catalyst like Ga/H-ZSM5 involves different reactions: dehydrogenation, oligomerization, cyclization and aromatisation. These types of material are presently used in the Cyclar process developed by British Petroleum and UOP [ 1]. Several authors assume that dehydrogenation of alkanes is the rate-determining step during the aromatisation [2]. Aromatisation of alkanes occurs from the combination of zeolitic Bronsted sites with dehydrogenation sites provided by Ga components [3]. However, the structure of gallium species and the function of Bronsted acidity on the dehydrogenation is not yet fully understood. There are four common methods by which extra-framework gallium can be incorporated into the micropores of H-ZSM5: ion-exchange, impregnation, chemical vapor deposition and physical mixing. As indicated by Nash et al. [4], the preparation method of catalysts has no significant influence on the aromatisation selectivities as long as Ga is welldispersed. Highly dispersed Ga located inside the zeolite crystallites has been obtained by chemical vapor deposition (CVD) of trimethylgallium (TMGa) [5]. The adsortion of Ga(CH3)3 on H-ZSM5 is probably accompanies by the formation of methane, Ga(CH3)2§ and Ga(CH3) § ions. These ions are attached to the negatively charged oxygen atoms, which have lost their proton during the formation of the methane molecules [6].

960 Biscardi et al. [7] concluded that dehydrogenation of propane on H-ZSM5 requires the activation of C-H bonds and the removal of the hydrogen atoms produced during the reaction. The sequential release of several H-atoms during the propane aromatization turnover limits the rate and selectivirty of this reaction on H-ZSM5. The presence of Ga increases the rate of re-combination and desorption of H-atoms as 1-12. The H2-D2 exchange reaction has been used to gain insight into the dissociatively adsorption of 1-12and D2 and the resultant HD desorption over zeolites modified with Ga. In the present work, we introduced the Ga component in H-ZSM5 and H-MOR zeolites using the CVD method of TMGa. The synthesized materials were extensively characterized after oxidation or reduction treatments. Finally, we studied the 1-12 activation over these materials by H2-D2 exchange reaction. 2. EXPERIMENTAL SECTION

2.1 Samples preparation TMGa~-ZSM5 (Si/Al=19.4) and TMGa/H-MOR (Si/AI=10) were prepared by chemical vapor deposition of trimethylgallium. An amount of dried zeolites was loaded in a reactor with excess amount of TMGa in inert atmosphere. After 24h the solids were maintained under vacuum for 2h to remove unreacted TMGa and gaseous reaction products such as methane. Subsequently, the material was either calcined (100 ml N2/20%O2) or reduced (100 ml N2/20%H2) for 2h at 523 K and heated further to 773 K for 4h with the same flow and at a heating rate of 7.5 ~ 2.2 Characterization The Ga and A1 contents were determined by ICP-OES technique using a SPECTRO CIROS ccD spectrometer. The XPS measurements were done with a VG Escalab MKII spectrometer, equipped whit a dual A1/Mg Kcz X-ray source. Spectra were obtained using the aluminium anode (A1 Kcz=1486.6eV) operating at 480W and constant pass energy of 20eV with a background pressure of 2xl 0-9 mbar. FTIR spectra were obtained in a Bruker IFS 113V FT absorbance spectrometer with a spectral resolution of 4 cm-1 and 125 scans. In a typical experiment, the samples were pretreated at 823 K under vacuum for 60 min and the IR spectra were recorded at room temperature. Magic-angle spinning (MAS) 71 Ga NMR spectra were recorded on a Bruker DMX 500 NMR spectrometer at 152.5 MHz using a 2.5 mm rotor and spinning speed of 35 kHz.

2.3 H2-D2 equilibration The H2-D2 exchange was measured using a recirculation reactor set-up (Figure 1). A membrane pump was used to recirculate gases in the system formed by three loops. Initially, the primary and secondary loops were filled with dried N2 and pressurised at 1.082 bar. Samples of 10 mL D2 (Hoekloos, purity 99.98%) and 10 mL 1-12(Hoekloos, purity 99.95%) were introduced into the primary loop. During the sampling, the catalyst was reduced or oxidized in the pretreatment loop at 673 K for 4 h, under N2 flow. When the pretreatment was finished, the temperature of the reactor was decreased to the reaction temperature of 473 or 523 K. The reaction was started by connecting the primary and secondary loops with the

961 pretreatment loop. After connecting the three loops, gases were quickly distributed in all the volume. Continuous on-line analysis of all gas-phase components was performed by a quadropole mass spectrometer (Balzers QMG 200M system) equipped with secondary electron multiplier. During a typical experiment the gas flow rate into the mass spectrometer was so low that the decrease in system pressure was less than 1%. Mass spectra were collected every 0.5 min. Gas-phase concentration of 1-12, D2 and HD were calculated after calibration with the individual pure gases.

ir

@ 2nd loop

1st loop

I ,~,

! ~

' retreatment loop

Figure 1. Schematic representation of H2-D2 exchange apparatus. 3. RESULTS AND DISCUSSION The total amount of gallium loaded by CVD of TMGa on H-ZSM5 and H-MOR zeolites is shown in Table 1. These results demonstrate that oxidation or reduction treatments of TMGa/ZSM5 and TMCm/MOR did not modify the total amount of Ga in the sample. During the deposition, TMGa reacts with both silanols groups and stericaly accessible Bronsted acid sites. Although the parent H-MOR zeolite has a lower Si/Al ratio, it contains less Ga than H-ZSM5 zeolite. Tentatively, we attribute this fact to pore blockage by adsorbed dimethylgallium in the one-dimensional micropore structure of H-MOR, whereas the threedimensional accessibility of H-ZSM5 results in a higher Ga loading. The oxygen and especially the hydrogen treatment of the catalysts provokes the migration of Ga to deeper positions within the zeolite crystals. This migration is underpinned by the decrease of surface Ga/Al ratio obtained by XPS (Table 1), while no loss of Ga is

962 detected by ICP. Initially, TMGa/MOR catalyst has an external surface enriched in Ga, but oxidation and especially reduction treatments lead to migration of Ga species and redistribution thought zeolite crystals. Table 1 Gallium content and Ga/AI ratio from ICP and XPS. mol Ga/AI

w t % Ga Sample ~

bulk b

Average b

TMGa/ZSM5

6.8

1.48

TMGa/ZSM5oxd

6.4

1.49

8.82

TMGa/ZSM5red

6.3

1.46

6.85

TMGa/MOR

3.42

0.44

1.74

TMGa/MORoxd

3.43

0.44

0.88

TMGa/MORred

3.11

0.43

0.22

XPS

a oxd=oxidized and red = reduced catalyst. b Determined with chemical analysis (ICP)

The FT-IR spectra of H-ZSM5 and TMGa-ZSM5 after oxygen or hydrogen treatment are shown in Figure 2. Three bands due to -OH stretching vibrations were identified for the parent H-ZSM5 zeolite (Figure 2a). These bands are assi.g~ed as follows: 3613 cm "1, Bronsted acid sites; 3664 cm1, extralattic A1OH ions; and 3744 c m , terminal silanol groups [8]. The signal of both silanol and Bronsted acid sites decreases after treatment with H2 or 02 (Figure 2b-c). Remarkably, the spectrum of TMGa/ZSM5 after reduction does not show the acid Bronsted signal anymore. This result indicates that oxygen and especially hydrogen treatments promote the reaction between Ga and Bronsted acid sites, located into the zeolite micro pores. IR results confirm the picture already obtained from XPS: oxidation and reduction treatments induce the Ga migration from the outer surface into zeolite micro pores and the interaction with Bronsted acid sites. H-ZSM5 zeolite present a band at 3664 cm 1, while the TMGa-ZSM5 catalyst after oxidation shows a signal at 3672cm -~. This shift could indicate the formation of gallium species similar to the extralattice AIOH species present in the starting zeolite. Figure 3a shows the 71 Ga MAS NMR spectra of the Ga2C14 used as model compound. The signals at 209 and-608 ppm can be ascribed to Ga 3§ and Ga 1§ respectively. 71 Ga MAS NMR spectra of TMGa/H-ZSM5 catalyst after oxidation (Figure 3b) and reduction (Figure 3c) indicate that only Ga(III) is present. The two resonances are related to Ga(III) in octahedral coordination (-1 ppm) and Ga coordinated to the zeolite framework (-140 ppm) [9]. We surmise that the first species is Ga-oxide located on the external zeolite surface and/or in the micropores, while the second species are Ga ions that counterbalance the zeolite negative

963 change. Note that the samples were loaded into the NMR rotor in contact with air; hence, we cannot exclude that reduced Ga species are oxidized and not observed by NMR.

0.1

Silanol 3744

Bronsted

3613

b) T

M

G

a

/

Z

3672

S

M

5

r

e

~

~

a) H-ZSM 3500

35150

36})0

36'50

37})0

37u50

3800

Wavenumber (em ) - 1

Figure 2. FTIR spectra of H-ZSM5 and TMGa/H-ZSM5 after oxygen and hydrogen treatmem.

-1

55. 9

_

'

.

I

.

1500

.

.

.

.

.

.

.

.

.

I

.

.

1000

.

.

.................

.

.

.

.

I

500

.

.

.

.

I

9

0 (ppm)

9

'

'

i. I

-500

.

.

,

.

.

I

.

-1000

.

.

.

.

.

.

I

.

'

-1500

Figure 3.71Ga NMR spectra of (a) Ga2C14, (b) TMGa-ZSM5 after oxidation and (c) TMGaZSM5 after reduction.

964 Figure 4 shows the results of the H2-D2 equilibration experiments over TMGa/ZSM5 after oxidation at two different temperatures. The gas-phase fraction of 1-12, D2 and HD are plotted against the reaction time. The reaction performed at 473K presents an increase of HD fraction at the expense of H2 and D2 (Figure 4a). However, after a reaction time of 3 h an increase in the H2 fraction is observed, while the HD equilibrium state is reached after approximately 5 h. The H2-D2 reaction at 523 K (Figure 4b) shows a similar behavior, with the difference that the final 1-12 fraction is much higher. This fact indicates that initially exchangeable hydrogen species were present in the closed system. Apparently, these hydrogen species are present on the catalyst and can be exchanged into the gas phase especially at 523 K. When the levels of 1-/2, D2 and HD reach equilibrium, the total amount of exchangeable hydrogen species initially present on the catalysts (Ho) can be calculated from the final hydrogen to deuterium ratio in the gas phase using the following expression [ 10].

t=0

equilibrium

where the subscript g indicate the molar amount in the gas phase and the subscript 0 indicate the molar amount on the catalyst. Table 2 shows that the number of rio exchangeable per gram of catalyst increases with increasing reaction temperature. The source of exchangeable hydrogen is probably the hydrogen atoms present at the catalyst surface. (a) 473 K

(b) 523 K

0.5.

HD

0.5-

0.4.

0.4

.2 0.3

.2 0.3. O t~

HD

O

D~

0.2. 0.1. 0.0

~ 0.2, 0.1 ~

0

9.

i

., . . . . 6 Time[h]

, 9

0.0 1'2

0

'

~

~

~

1~

1'5

Time[~

Figure 4. H2-D2 equilibration over TMGa/ZSM5oxd catalyst at (a) 473 K and (b) 523 K.

965 Table 2 Number of exchangeable catalyst H atoms (H0) for oxidized TMCra/ZSM5 Temperature K

Ho (mol H / g cat)

473

3.05.10 -3

523

7.41.10 -3

Figure 5 presents the initial HD formation rate as a function of Ga/A1 ratio. These results show that modified H-MOR zeolite with a lower Ga/A1 ratio gives a higher H2-D2 exchange activity than modified H-ZSM5 zeolite. In both zeolites, the oxygen treatment leads to a higher H2-D2 exchange activity than the hydrogen treatment. As shown in Figure 4, calcined TMGa/MOR has the highest initial H2-D2 exchange activity. xd 12. T M G / M O R o O TMG/MORred~~ 8 o E

6

SM5oxd

_~ 4 o E

2 T M G / Z S M 5red 0.0 ' 012 ' 0'.4 ' 0'.6 ' 0'.8 ' 110 ' 1'.2 ' 1'.4 ' 1'.6 ' 118

Ga/A1 (ICP) Figure 5. H2-D2 initial rate exchange at 473 K. 4. CONCLUSION The chemical vapour deposition (CVD) of trimethylgallium (TMGa) on H-ZSM5 and H-MOR zeolites occurs on both silanol and Bronsted acid sites. The results shown suggest that content of Ga depends on the size and dimensionality of the channel system. H-MOR zeolite, with three-dimensional pore system, leaded to higher content of Ga than H-MOR zeolite with one-dimensional pore system. Presumably, pores of H-MOR are more susceptible to blockade by dimethylgallium than pores of H-ZSM5. The dispersion of Ga on zeolites modified with TMGa depends of the 1-12 or 02 treatments. During the oxidation and especially reduction the extracrystalline Ga species migrate from the outer surface into the zeolite channels and react with Bronsted acid sites.

966

71Ga NMR results show the presence of only Ga(III) after oxidation and reduction treatments. However, if reduced Ga species arise after reduction, they could quickly oxidized to Ga(III) in comact with air. M R results show the presence of both Ga-oxide and Ga on extra-framework position. H2-D2 exchange shows that TMGaJzeolite materials can activate hydrogen at 473 K. The exchange reaction is higher for the H-MOR zeolite and the oxidation pretreatment is found to be more effective. The number of hydrogen atoms released from Bronsted and silanol sites increases directly with the reaction temperature. Based on XPS, FTIR and H2-D2 exchange results, we conclude that the hydrogen pretreatment of both zeolites has a positive effect on the distribution of Ga. However, Ga species obtained were less active for H2-D2 exchange reaction than the Ga species obtained after oxidation. REFERENCES

1. P.C. Doolan and P.R. Pujado, Hydrocarbon Process., 68-9 (1989) 72-74, 76. 2. A. Montes and G. Giannetto, Appl. Catal. A., 197 (2000) 31-39. 3. B.S Kwak, W.M.H. Sachtler and W.O. Haag, J. Catal., 149 (1994) 465-473. 4. R.J Nash, M.E. Dry and C.T. O'Connor, App. Catal., A, 134 (1996) 285-297. 5. C.R. Bayense, J.H.C. van Hooff, J.W. de Haan, L.J.M. vd Ven and A.P.M Kentgens, Cat. Lett., 17 (1993) 349-361. 6. U. Seidel, M. Koch, E. Brunner, B. Staudte and H. Pfeifer, Micropor. Mesopor. Mater., 3536 (2000) 341-347. 7. J.A. Biscardi and E. Iglesia, Catal. Today, 31 (1996) 207-231. 8. S. Kumar, A.K. Sinha, S.G. Hegde and S. Sivasanker, J. Mol. Catal. A: Chem., 154 (2000) 115-120. 9. A. Wei, P. Liu, K. Chao, E. Yang and H. Cheng, Micropor. Mesopor. Mater., 47 (2001) 147-156. 10. E.J.M. Hensen, G.M.H.J. Lardinois, V.H.J. de Beer, J.A.R. van Veen and R.A. van Santen, J. of Catal., 187 (1999) 95-108.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

967

Catalytic conversion of trichloroethylene over HY-zeolite E.Finocchioa, C.Pistarino", P.Comite", E. Mazzei Justin", M. Baldib and G.Busca" "Dipartimento di Ingegneria Chimica e di Processo (DICheP), Facolt/t di Ingegneria, Universit/l di Genova, P.le Kennedy 1, I-16129 Genova, Italy b Dipartimento di Chimica, Universit/t di Pavia, v. Taramelli 12, 1-27100 Pavia (Italy) The conversion of tricloroethylene (TCE) in the presence of water (i.e. in the conditions of steam reforming) has been investigated in a flow reactor. The catalyst was an H-Y zeolite (SiO2/AI~O3 = 5,9). Some experiments with ferrierite have also been done for comparison. IR experiments have been performed in order to have information on the reaction mechanism and on the catalyst stability. With 1000 ppm feed TCE concentration, stoichiometrie water and oxygen excess, TCE can be totally converted to COx and HCI at 850 K or above. In these conditions the reaction kinetics is governed by the chemical step and deactivation phenomena are slow. With 10000 ppm TCE the reaction can be complete near 700 K. However, the reaction rate is in these conditions affected by pore diffusion, which becomes determinant at contact times higher than 0.15 s. In the absence of oxygen fast deactivation phenomena occur by coking and catalyst dealumination. 1. INTRODUCTION. The destruction of chlorinated volatile organic compounds (CVOC) is needed to clean waste gases from industry and also to treat waste liquids such as used solvents. This is the case of TCE (trichloroethylene) which is largely used for cleaning processes. The catalytic abatement of these compounds such as deep catalytic oxidation (over noble metal based or chromia-alumina catalysts [1]) or hydrodechlorination (at high hydrogen pressures and over noble metal catalysts [2]) are possible choices. However, the use of very expensive and quite toxic noble metal catalysts and the need of hydrogen high pressures, as well as the easy deactivation of noble metals by chlorine are clear drawbacks of these processes. In previous papers we proposed the use of acidic catalysts for the conversion at reduced pressure and temperature of weakly chlorinated compounds via dehydrochlorination [3,4]. However, this process cannot be applied to heavily chlorinated compounds. Gonzalez Velasco and his co-workers [5,6] proposed the use of different protonic zeolites as catalysts for the oxidation in the presence and in the absence of water of chlorinated organics including TCE. In the present communication we report on our work concerning the steam reforming of TCE in the presence of a wide pore zeolite i.e. H-Y. The aim is to check the possibility of a low pressure-low temperature process for TCE destruction with noble-metal free catalysts.

968

2. EXPERIMENTAL. The catalyst studied is HY zeolite from Grace, surface area of 827 m2/g, pore volume of 0.37 cm3/g, molecular ratio SiO2/A1203 = 5,9, Na20 0.87 % w/w. Ferriefite from Zeolyst has also been used for comparison (surface area of 480 m2/g, molecular ratio SiO2/A1203 = 55, Na20 0.05 % w/w). 0.1 up to 1.5 g of catalysts were loaded in a conventional fixed bed reactor, with or without mixing with quartz. Trichloroethylene (TCE) has been fed as gaseous mixture with helium (from SIAD) and after evaporation of the liquid (from Aldrich) in a saturator, in amount varying from 0.1 to 1% of the total flow which is usually 350 ml/min. Large excess oxygen has been fed in order to approach the condition of a practical process aimed at the treatment of waste gases with air. Catalytic runs with low oxygen content in the feed, or no oxygen at all, were also performed. The effect of water addition was tested by introducing vapour from a second saturator. Most of experiments were carried out with a total GHSV ranging from 37000 h-1 to 12400 h-1 (calculated on the total feed flow). The reactants and the reaction products were analysed on line using a gas chromatograph connected to a TCD and a FID detector. GC-MS analysis of the products has also been performed. To analyse HC1 and C12, the effluents were contacted with NaOH water solution. Chloride and hypochlorite anions have been analyzed and quantified by means of ionic chromatography. HC1 only is produced in humid conditions and always nearly fulfilled the chlorine balance, if organic chlorided compounds analysed by GC and GC-MS are taken into account. In catalytic runs without fed water, molecular chlorine has been detected using DPD (diethyl-pphenylendiammine) reagent. Only conversion and selectivities based on carbon-containing compounds are considered here. Infrared spectra have been recorded by a Nicolet Magna 750 instrument connected to a conventional gas manipulation apparatus, using self-supporting pure powder disks. Catalyst powders have been outgassed at 673 or 873 K prior to every adsorption experiment. 3. RESULTS AND DISCUSSION.

3.1 Catalytic results. Steam reforming of TCE corresponds to the following reaction: C2HC13 + 2H20 --> 2CO + H2 + 3HC1 This reaction is essentially athermic at any temperature (M~298 = -8.4 kJ/mol, becoming slightly endothermic above 700 K) while it is thermodinamically favoured the more the higher is the temperature due to the increasing in entropy. Successive reaction of CO with water can produce CO2 and further hydrogen (water gas shift reaction). Also CO oxidation to CO2 can occur in the presence of oxygen. The alternative reaction that can occur in the presence of oxygen is the total combustion of TCE, with production of COx, C12 and water, which is much more exothermic. As shown in Fig. 1, the conversion of TCE (1000 ppm) on a 0.5 g H-Y catalytic bed in the presence of stoichiometric water and excess oxygen starts to be significant above 650 K and is complete near 900 K. The only significant C-containing products are COx, while chlorine is recovered as HCI. By increasing the water content the conversion is decreased without changing the selectivity profile. In this TCE concentration range the presence of oxygen seems to not influence the conversion. However, without fed oxygen the catalyst darkens a

969 little in colour, possibly due to incipient coking. CO/CO2 ratio decreases in the presence of oxygen, possibly due to CO oxidation. The conversion increases by increasing the contact time (see Fig. 2), while the activation energies evaluated in these low reactant partial pressure conditions are of the order of 105 kJ/mol (see Fig. 3), and this shows that in these conditions the kinetic regime is dominated by the chemical step.

100 -

i I

80 C O

60-

HVOC:H20

= 1"6

Standard conditions. No oxvaen

i

40

I i

20

o

0-

450

total flowrate

=

175 ml/min

ii

t

I

I

650

850

1050

T/K

Figure 1. TCE conversion over HY zeolite (0.1% TCE) with excess oxygen, stoichiometric water, total flow 350 ml/min (standard conditions), except where specified otherwise.

80

60

It,,,"

... ~--~.

,....

"" ....

,.""

....... A . . . . . . . . 0.1% TCE

20 0

r

0

,

, ......

, ...... ,

]

,

~

,

0.5

,

~

1

,

~

i

1.5

[s]

Figure 2. TCE conversion over HY zeolite at 823 K at several contact times, (TCE:water = 1"2, oxygen excess, feed TCE concentration 1% or 0,1%) At higher TCE concentrations (TCE:10000 ppm/water:20000 ppm, excess oxygen) total conversion is approached at significantly lower temperatures (e.g. 700 K with x ~ 0.2 s).

970 Selectivity to COx approaches 100% and the CO/COs ratio is strongly in favour of CO, at least in the lower temperatures range. The formation of reaction by-products (tetrachloroethylene) is significant (although well below 1 % in selectivity based on C) in the temperature range 673-723 K. With these high reactant concentrations, two well defined regimes can be observed. In fact at low contact times (0.02-0.2 s) the conversion increases with contact time (see Fig. 2 for T = 823 K) as expected for a regime where the ratedetermining step is chemical. However, at higher contact times, the conversion decreases by increasing contact times. This suggests that conditions where pore diffusion is kinetically determinant are reached at x > 0.2 s. With high-concentration feeds we can evaluate (at lower temperatures when conversion is small and the assumption for a differential reactor can be applied) activation energy of the order of 55 kJ/mol, i.e. definitely smaller than that measured for low-concentration feeds. This shows that, with these concentrated feeds, the pore diffusion step is kinetically not negligible at low temperature and can become determinant at higher temperatures. As a result of this, the best conversion results were in fact obtained in the 0.05-0.2 s contact time range with high concentration feed. We further investigated the contact time range above 0.2 s with high concentration TCE stream (10000 ppm), stoichiometric water vapour (1:2) and in the absence of oxygen (Fig. 4). Also in this case we observe a decrease of the conversion by increasing contact times in agreement with a dit~sion-limited regime, but we observe later a plateau in the conversion versus contact time curve. . L

v\

3 =

2

"-

1

o

A TCE 0.1%, tau

-,%

=1.2s X TCE1%,tau= 0.02 s

m

0 0.00

0.0012

0.0014

1/T r l i K 1

0.0016

Figure 3. Arrhenius plot of activation energies, TCE conversion over HY zeolite (stoichiometric water, excess oxygen). On the other hand, the comparison of Fig. 2 and 4 indicates that, in the range near x -~ 0.2 s, the presence of oxygen has a positive effect on the conversion, measured after 30 min on stream, i.e. when apparent steady state conditions are reached. Actually, in the absence of oxygen and 1% TCE concentration the reaction rate (conversion) declines with time, so that the points reported in Fig. 4 are to be taken with precaution. Experiments of conversion of 10000 ppm TCE with stoichiometric water but in the absence of oxygen and x = 0.13 s show that after 3 h the conversion declined down to 5 % but it stays later constant for 20 h. Accordingly, after even short runs in the absence of oxygen the catalyst is completely black while after runs in the presence of oxygen the catalyst is white or pale grey. This again suggests that coking is significant in particular in the absence of oxygen.

971 0 ........................................................................................................................................................................................................................... ~ 50 [] HY (TCE, noO2)

40

o FER (TeE + 02) xFER (TCE, noO2)

0

~

20

[]

[]

10

0.1

I

I

I

I

0.15

0.2

0.25

0.3

-clsl

0.35

Figure 4. TCE conversion over HY and FER zeolite at 823 K (stoichiometric water). Chlorinated by-products (i.e. tetrachloroethylene) have also been found in trace amounts at temperatures where conversion is not complete. Selectivity to CO is above 90%, the rest being CO2, and this could indicate that the water gas shift or the CO oxidation reactions occur at a low extent. To have a confirmation of the likely role of pore-diffusion in the behaviour of zeolite HY, we also performed some runs with the smaller pore zeolite H-FER. Note that TCE should not enter the cavities of the FER structure, so that we can suppose that in this case the pore diffusion should not affect conversion. In fact, we can see that in the same contact time range and in the same conditions, the conversion on H-FER increases by increasing contact time. So, at x ~ 0.13 s H-FER is much less active than HY, just because the external sites of the zeolite H-FER crystals are weaker as acids than the typical internal zeolite sites [7] and the available catalytic area is much lower if the internal zeolite surface cannot work. On the other hand, when the HY zeolite catalyst enters in the diffusion-limited region, the H-FER catalyst becomes more active than HY. So this behaviour strongly supports the conclusion that, with > 0.2 s and 10000 ppm TCE, pore-diffusion limited conditions are reached on the HY catalyst. 3.2. F T - I R studies.

FT-IR spectroscopy has been applied to have some characterisation of the catalyst and information on the reaction mechanism. In Fig. 5 the skeletal IR spectra of the fresh catalyst, of the catalyst alter catalytic runs with 10000 ppm of TCE in the reactor in excess oxygen and after the prolonged experiments performed in the absence of oxygen are reported. The spectrum of the fresh catalyst fully agrees with that of HY zeolite [8], and is dominated by the Si-O-Si asymmetric stretching modes (1056 cm1 and 1170 cm1, shoulder), the Si-O-Si symmetric stretching / bending modes (818 cm-1) and the modes due to Si-O-Si rocking (596, 513 and 456 cm-1). After runs in oxygen the spectrum presents only a slight modification with the appearance of a more pronounced band near 925 cm1. In the case of zeolites a band in this region can be assigned to Si-(OH) stretching, but, on the other hand, an assignment to C-CI stretching mode of transformation products of TCE with sp3 carbon atoms is also possible.

972 On the contrary, the spectrum recorded after prolonged runs performed in the absence of oxygen is deeply modified, in particular in the region of Si-O-Si stretchings where a sharp component at 1102 cm"1 appears together with a new shoulder near 1220 cm1. These new features can be due to Al-free silica-like particles that are in fact characterized by higher Si-OSi stretchings than Al-containing silicas or zeolites [8]. This is likely an evident effect of dealumination. It is consequently concluded that the deactivation at concentrated feeds is not only due to coking but also by simultaneous dealumination.

1.0i ro

0-8i

o co

< 0.4 1000 Wavenumbers (cm-t) Fig. 5. FT-IR spectra of flesh catalyst (a), after catalytic runs with 10000 ppm of TCE in the reactor in excess oxygen (b) and after the prolonged experiments performed in the absence of oxygen (c), all in KBr pressed disks. The FT-IR spectra of the pure catalyst powder after high temperature activation and TCE interaction at room temperature are reported in figure 6. After activation at 673 K, a main band is evident in the spectrum at 3741 cm1, due to the terminal silanol groups while at lower frequencies a complex absorption shows three maxima at 3688, 3624 and 3550 cm-1. The band at 3688 cm1 is generally assigned to OH stretching of extraframework groups, while the 3624 and 3550 cmq are due to the high frequency (HF) and low frequency (LF) bridging SiOH-A1 groups typical of the HY structure. This spectrum is typical for a high-Al content HY zeolite with some extraframework alumina. After trichloroethylene adsorption at room temperature bands due to physisorbed TCE are detected at 3085 cm1 (stretching CH) and 1583 cm1 (stretching C=C). The perturbation of these bands with respect to gas-phase IR bands (detected at 3098 cm~ and 1580 cm1, respectively) is not very significant and this indicates that the molecular interaction at room temperature is medium-weak. Outgassing at room temperature allows a progressive weakening of IR peaks, which disappear after 30 min in vacuum, thus indicating the disappearance of the molecularly adsorbed TCE. The interaction at room temperature is reversible and involves the OH groups, which are restored after evacuation at room temperature. All the OH's groups are involved, but those due to HF OH's and silanols are involved to a bigger extent, as evidenced in the subtraction spectra. The data show that a

973 medium strong interaction occurs with the OH stretching modes at the main HF zeolite cavity but also on the external crystal surface where terminal silanols are located.

0 25-

9 ~.. f .~,~k~

o.=. T J

"

Evacuatedat r.t.

~

~

1.01

Evacuatedat r.t.

oo,

o 00

4173O

ActivatedHY

~

~~-~

~--/~ ~)x -

-0.05 2OOO VVavenumbers (cm-1)

Fig. 5. FT-IR spectra of activated HY, and after adsorption of TCE and successive evacuation, in the enlargement: FT-IR subtraction spectra [surface + TCE] - [activated surface], OH stretching region After heating the catalyst in the presence of TCE gas at increasing temperatures new broad bands are detected at 1580 cm1 (with a shoulder at 1635 crn1) and at 1470 cm~. These bands are characteristic of carboxylate species formation. A likely assignment is to dichloroacetate species. By analysis the gas phase we can detect the typical sharp rotovibrational features of HC1 starting from 773 K. In these conditions the above cited carboxylate bands raise their maximum intensity, and decrease at immediately higher temperatures. 0.6 o tO ($)

<:

0.4 0.2

2000

1500 Wavenumbers (cm-1)

Fig. 6. FT-IR spectra spectra of surface species arising from TCE adsorption over HY zeolite (no water added); from the bottom: after heating at 573 K, 673 K, 773 K. The spectrum of the activated surface has been subtracted.

974 In these conditions, in the gas phase, we can also observe the presence of weak, new bands at 916 and 783 cm1, which can be due to the asymmetric and symmetric CCI stretchings of tetrachloroethylene [9]). Another new band is detectable at 1361 cm~, likely due to CH2 deformation mode, as in the vinylchloride molecule [10]. This attribution should be confirmed by the detection of the C=C stretching mode around 1605 crn"1, almost impossible to identify in our spectrum. In this region however we detect two bands at 1629 and 1688 cm~ which point out the formation of other reaction products, possibly dichloroethene (C=C stretching reported at 1622 cm~ [ 11]) and a carbonylic compound likely dichloroacetaldehyde which as been observed upon TCE reaction over chromium exchanged zeolites [9]. 4. CONCLUSIONS The present study shows that HY zeolite is an active catalyst for the destruction of TCE vapor with steam. This reaction can be performed with low reactant partial pressure (1000 ppm) without the appearance of important deactivation phenomena at the laboratory scale-time (20 h). The reaction can be complete at temperatures of the order of 850 K and likely involves the formation of dichloroacetates as strongly adsorbed intermediates and dichloroacetaldehyde, tetrachloroethylene and dichloroethylene as gas-phase by products. The reaction can be performed at lower tenmperature when working with higher TCE concentration like 10000 ppm. In these conditions, however, pore diffusion limitation become determinant at contact times higher than 0.2 s. The reaction occurs also in the absence of oxygen but in these conditions coking causes fast deactivation. Deep dealumination of the zeolite is also evident after 20 h. REFERENCES

1. S.K.Argaval, J.J.Spivey, G.B. Howe, J.B.Butt and E.Marchand, Catalysts Deactivation 1991, C.H.Bartholomew and J.B.Butt (eds.), Elsevier, Amsterdam, 1991 2. L.Prati and M.Rossi, Appl. Catal. B: Environ. 23 (1999) 135 3. C.Pistarino, E.Finocchio, M.A.Larrubia, B.Serra, S.Braggio, G.Busca and M.Baldi, Ind. Eng. Chem. Res. 40 (2001) 3263 4. C.Pistarino, E.Finocchio, G.Romezzano, F.Brichese, R.Di Felice, G.Busca and M.Baldi, Ind. Eng. Chem. Res. 39 (2000) 2752 5. R.Lopez-Fonseca, A.Aranzabal, J.I. Gutierrez Ortiz, J.I.Alvarez-Uriarte, J.R. GonzalezVelasco, Appl. Catal. B:Environ. 30 (2001) 303 6. R.Lopez-Fonseca, A.Aranzabal, P.Steltenpohl, J.I. Gutierrez-Ortiz, J.R.Gonzalez-Velasco, Catal. Today 62 (2000) 367 7. M. Trombetta, G. Busca, M.Lenarda, L. Storaro and M. Pavan, Appl. Catal. A General, 182, 225-235 (1999) 8. G. Busca and C. Resini in "Encyclopedia of Analytical Chemistry", Robert A. Meyers ed., Wiley, Chichester, 2000, pp. 10984-11020 9. P.S.Chintawar and H.L.Greene, J. Catal. 165 (1997) 12 10. M.R.Fejien-Jeurissen, J.J.Jorna, B.E.Nieuwenhuys, G.Sinquin, C.Petit, J.P.Hindermann, Catal. Today 54 (1999) 65 / 11. T.H.Ballinger and J.T.Yeates Jr., J.Phys.Chem. 96 (1992) 1417

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

975

FT-IR studies of internal, external and extraframework sites of FER, MFI, BEA and M O R type protonic zeolite materials G. Busca a'x, M. Bevilacqua a, T. Armaroli a and M. Trombetta a'b'2 a Dipartimento di Ingegneria Chimica e di Processo, Universitg, P.le J.F.Kennedy, I16149, Genova, Italy. b Fac. Ingegneria, Universit~t Campus BioMedico, Via E. Longoni 83, 00155 Roma, Italy.

FT-IR studies of the adsorption of nitriles show that the external surface of H-FER, HMFI, and H-MOR carries terminal silanols and strong Lewis sites. Terminal silanols present wide heterogeneity with respect their Bronsted acidity also in relation to the zeolite structure. Bridging Si-(OH)-A1 exist at the internal channel surface only of H-FER and HMFI and are stronger Bronsted acids than the external terminal silanols. The distinction of the bridging Si-(OH)-A1 sites located in the different channels of H-BEA and H-MOR can be obtained. Extraframework material can be located both at the internal channel surface and at the external surtface of H-MFI and H-BEA.

1.

INTRODUCTION

Protonic zeolites such as H-FER, H-ZSM5, H-BEA and H-MOR find industrial application as acid catalysts in several hydrocarbon conversion reactions (see table I). The activity of these materials is associated to the strong Bronsted acidity of the bridging Si(OH)-A1 sites. Additional shape selectivity effects are due to the molecular sieving associated to the limited size of the cavities where at least part of the sites are located. The main factor allowing molecular sieving and, consequently, shape selectivity is generally considered to be exclusively steric, i.e. only the molecules having a critical diameter lower than the channel diameter are allowed to enter the cavity and to react or, in case, to exit it and be recovered as a product. However, active sites also exist at the external surface of zeolite crystals. These sites are sometimes considered to be responsible for unwanted non-selective catalysis. On the other hand real zeolite catalysts are frequently pretreated in various ways such as steaming, and are not "perfect" structures: extraframework sites are frequently present and can also have a role either as active site or as material hindering the molecular diffusion into the cavities. In some cases the role of shape selectivity and of pretreatments such as dealumination are still umperfectly known or under debate. This is the case of the H-FER n-butene skeletal isomerization catalysts,

1 e-mail: [email protected] 2 e-mail: [email protected]

976 where different mecahnisms seem to be active on the extemal and on the intemal surfaces and coking and/or aging improve the performances [1 ]. Similarly, the details relative to the behaviour of H-MOR n-alkane skeletal isomerization catalysts are still under study [2]. In this communication we will summarize recent data obtained in our laboratory on the characterization and the distinction of intemal, external and extraframework sites in H-MFI type zeolite structures [3,4,5,6,7] as well as on other zeolites such as H-FER [3,8,9] and HBEA [10] and H-MOR also in comparison with amorphous [9] and mesoporous [8] materials of similar compositions. 2.

EXPERIMENTAL

In Table I the families of zeolites under study are summarized. For every zeolite we worked with several preparations from different commercial origins and with different A1 content. The Ft-IR spectra were recorded with a Nicolet Magna 750 instrument with a resolution of 4 cm -1 using pressed disks of pure zeolite powders, activated by outgassing at 773 K into the IR cell. A conventional gas manipulation / outgassing ramp connected to the IR cell was used. The adsorption procedure involves contact of the activated sample disk with gases and vapors at increasing pressures and outgassing in steps at r.t. or increasing temperatures. Adsorbants were purchased from Aldrich. Table 1. Channel structure and application of the protonic zeolites investigated here. Symbol

Channel structures

Application

H-FER

10 ring channel [001] 4.2/~ x 5.4 ~ 8 ring channel [010] 3.5 ~ x 4.8/~

n-butene skeletal isomerization

H-MFI

10-ring channel [010] 5.3/~ x 5.6/~ (straight) 10-ring channel [100] 5.1 ~ x 5.5 ~ (sinusoidal)

xylenes isomerization

H-BEA

12 ring channels [100] 6.6 ~ x 6.7/l. 12 ring channels [001] 5.6/~ x 5.6

cumene synthesis

H-MOR

12-ring main channels [001] 6.5 ~ x 7.0 ~ 8-ring compressed channels [001] 2.6 ~ x 5.7 ~ 8-ring side pockets [010] 3.4/~ x 4.8

n-alkane skeletal isomerization

3. RESULTS AND DISCUSSION In Fig. 1 the spectra of a H-FER sample after activation (a), after contact with acetonitrile gas (b) and after outgassing at r.t. (c) are reported. The spectrum of the activated sample shows the two typical bands of the hydroxy groups of the zeolite samples, at 3746 cm -1, assigned to terminal silanol groups, and at 3598 cm -1, due to the bridging SiOH-A1 groups. The ratio between these bands is definitely different with respect to those measured for different samples we investigated previously [3,8,9]. This is mainly due to the higher Si/A1 atomic ratio of this sample with respect to those we investigated previously (27,5 vs 8-20) with a consequent slower intensity of the band due to the bridging Si-OH-A1 groups. It is evident that acetonitrile causes the complete (and immediate) disappearance of both these bands although a broad weak feature is still present in contact with acetonitrile at 3650 cm -1. This suggests that few hydroxy groups

977 which are probably H-bonded silanol groups, do not interact actually with acetonitrile. In 1 the presence of the gas a band is found at 3440 cm-, but it disappears by outgassing at r.t. However, the careful inspection of Fig. 1 shows that also a broad absorption in the region 3200-2800 cm -1 disappears by outgassing at r.t.. This treatment results also in the restoration of the terminal silanol band at 3746 cm -~. This shows that the terminal silanols undergo different shifts when interacting reversibly with acetonitrile. Part of silanols undergo a shift Av - 300 cm -1 (as typical silanols) while part are more acidic and undergo a much stronger shift Av - 750 cm l. This has already been observed with higher-Al-content samples [9]. Outgassing at r.t. does not restore at all the band of bridging silanols, and also leaves intact the so-called A,B,C components at 2800, 2400 and 1600 cm -1. These features, which have been the object of detailed studies [ 11 ], are evidence of strong quasisymmetrical H-bondings between acetonitrile and the bridging OHs ofFER.

2

'i

30 3746

3598

0,

Fi~. 1. FT-IR spectra of H-FER (Si/A1 = 27,5) after activation (a), after contact with acetonitrile ~as (b) and after out~zassin~z at r.t. (c)

Fi~. 2. FT-IR spectra of H-FER (Si/A1 - 27,5) after activation (a) and after contact with pivalonitrile vapour (b).

In Fig. 2 the spectra relative to the experiment of the adsorption of 2,2-dimethylpropionitrile (pivalonitrile) with the same H-FER sample are reported. Also in this case the band of terminal silanol groups is totally disappeared in contact with the nitrile while it is evident that the band due to bridging OHs is fully unperturbed. Interestingly, its maximum -I is now observed at 3603 cm versus 3598 cm -~ in the activated sample. Components with maxima near 3450 and 3000 cm 1 (the last more evident in the subtraction spectra) are -I 9 again disappeared after outgassing at r.t. when the band at 3746 cm is restored. These data show that although all terminal silanol groups are accessible to pivalonitrile, the bridging hydroxy groups are entirely not accessible to pivalonitrile in these conditions. In fact, the A,B,C spectrum is not observed at all. This contrasts what happens with acetonitrile that perturbs all these groups. The straightforward explaination for this behavior is that the terminal silanols are entirely located at the external surface of the zeolite crystals, where molecular sieving effects are not working, while the bridging OHs are entirely located at the internal surface and are consequently accessible only to small

978 molecules such as acetonitrile. Actually, pivalonitrile, due to the presence of the terz-butyl group, is reported to have a critical diameter of near 6 A and cannot consequently enter the cavities of FER (see table 1). This result has been reported previously for samples with higher A1 content (Si/A1 ratio 8-20) some of which contained evident extraframework material. This is confirmed here also for a sample with smaller A1 content and that does not present any evidence of extraframework material. The shift towards higher frequencies of the band of bridging hydroxy groups is interpreted as due to a framework effect, being associated to a small but significant shift also of the skeletal vibrations where the external crystal surface is covered by adsorbates. Note that neither the position of the band in the activated samples nor those of perturbed silanol groups seem to be significantly affected by the A1 content at least in the Si/A1 ratio range 8 to 23 in the case of H-FER samples.

0.06 a.u.

3900

3800

3700

3600

3500

3400

3300

Fig. 3. FT-IR spectra of H-MFI (Si/A1 = 23) after activation (a), and after contact with pivalonitrile vapour (b), and after outgassing at r.t.. The results obtained with H-FER parallels those obtained with H-MFI (H-ZSM5) samples whose Si/A1 ratio ranges from oo (pure silicalite) up to 23. For the last sample, the spectra are reported in Fig. 3. Also in this case the band, very intense in this sample, -1 observed at 3618 cm -~ shifts of few cm to higher frequencies upon adsorption of pivalonitrile, while it is fully disappeared giving rise to the A,B,C spectrum when it is in contact with acetonitrile vapour. On the contrary, the band at 3746 cm ~ (very small in this sample) is fully shifted to near 3400 cm by adsorption of plvalonltrlle (in the subtraction spectrum the maximum is observed at 3395 cm ). Additionally, a band is also evident near 3730 cm 1 and a continuous absorption in the region 3730-3650 cm -1, due to hydroxy groups possibly on extraframework material. These features are not perturbed upon pivalonitrile adsorption, and are consequently thought to be due to species which are located in the interior of the cavities. Experiments show that in H-MFI samples containing evident extraframework species, at least part of it is located in the internal cavities. After outgassing pivalonitrile adsorbed on the external surface of both H-FER and HMFI the OH stretcing bands are essentially restored. However, few pivalonitrile species -1

.

-1

.

.

.

.

.

.

.

.

.

979 still resist adsorbed. These species are characterized by a significant shift upwards of the CN stretching band from 2235 cm l (the value for the liquid) to 2305 cm -1, which is the value observed when this molecule is coordinated on the strong Lewis sites of alumina and of A1F3 [12]. Note that this occurs also with poor-Al-content samples where extraframework material is observed neither by IR nor by 27A1 NMR. This shows that strong Lewis acid sites are located at the external surface of virtually "perfect" samples of both H-FER and H-MFI. It is interesting to remark that a common opinion is that Lewis acidity in zeolites is associated to extraframework materials or to defects produced by dehydroxylation [13] while framework tetrahedral A1 cations should not act as Lewis sites at the internal cavity of zeolites. In contrast, our data suggest that the strong Lewis acidity is generated at the external crystal surface of zeolites and can compete with the strong Bronsted acidity which is, on the contrary, located at the internal cavity surface.

~ || ,

b

3380

1,6

i "2t e

lOJ

I

I n

I/

2

3742 It 3737

|

][

3747

I1~~11

~[1!_iI ll,i

\tll

\',

22551

,~ o

0:4 4000

3800

3600

3400

3200

3000

2800

2600

2400

2200

Fig. 4. FT-IR spectra of H-BEA (Si/A1 = 50) after activation (a), and after contact with pivalonitrile vapour (b), and after outgassing at r.t. 5 rain (c) and 30 min (d). The spectra of H-BEA samples after activation differ from those typical of H-FER and H-MFI with similar Si/A1 ratios because the bands in the region near 3600 cm l (3608 cm ~ in this case) are definitely smaller whereas the band near 3740 cm is much stronger, in part due to the small crystal size typical of the samples, but it also appears to be multiple. The spectrum of a sample whose Si/A1 ratio is 50 (Fig. 5) does not present evident bands due to extraframework material, in contrast to samples previously investigated [ 10]. The adsorption of pivalonitrile causes the complete disappearance of the band of terminal silanols which is however clearly composed of at least two bands, the main maximum being observed at 3742 cm but with an evident shoulder at 3737 cm -1. After brief outgassing at r.t. the silanol band is partly restored but its maximum is now at 3747 -1 cm . The perturbation undergone by silanols upon interaction with pivalonitrile is different -I

9

-I

9

9

9

980 here than in the case of the H-FER and H-MFI samples. Here the predominant band of -1 perturbed silanols is observed at 3380 cm . In the case of H-BEA the small band of bridging OHs is mostly disappeared upon pivalonitrile adsorption, showing that most of them can be accessible to such a molecule. This agrees with the diameter of the main 12-ring channels of BEA which are sufficiently big to be entered by molecules containing the terz-butyl group. On the other hand, a component can be seen even in presence of pivalonitrile near 3605 cm 4. This confirms the previous data obtained on a sample with Si/A1 atomic ratio of 25 [ 10] and suggests that the residual unperturbed OHs are located in the smaller 10-ring channels that actually should not allow the entrance of pivalonitrile. 0,7 5 0,7 0

i -i

0,e 5

3743 3610 I1' . A t ~'i3604 I~ I ~

3425

0,6 0 A b

so r

b n

c

0,5 5

o" 0 0,4 5 0,4 0

%

~,

~

|

,

............................................................................

iI

38OO

34OO

0,3 5 0,3 0 0,~ 5 0,~ 0 0,1 5 4000

3800

3600

3400

3200

3000

2800

2600

2400

2200

Fig. 5. FT-IR spectra of H-MOR (Si/A1 = 20) after activation (a), and after contact with pivalonitrile vapour (b), and after outgassing at r.t. 5 min 30 min (c). Analogous studies have been also performed on H-MOR samples with Si/A1 ratios in the range 10-45. When the A1 content is relatively high the band due to bridging Si-OH-A1 in the case of H-MOR is very strong and located near 3605 cm 1. Several authors (such as Maache et al. [14] and Datka et al. [15]) reported that the OHs in the side pockets and smaller channels are associated to a band which is located at distinctly lower frequencies (near 3580 cm 1) with respect to those located in the main channels. In fact we are not able to separate two components in the spectra of our samples. The spectra we recorded seem to be more consistent with an higher multiplicity of the components of this band. In the case of H-MOR, the band near 3600 cm is very slightly eroded upon adsorption of pivalonitrile. This is evident in Fig. 5 where the spectra relative to a sample with Si/A1 1 ratio of 20 are reported. The band, which is observed 3604 c m in this sample, after activation, is still mostly unperturbed in contact with pivalonitrile vapour, but again shifted a little bit to higher frequencies (i.e. at 3610 cm-]). On the other hand, a small part of the band (no more than 15 %) is actually eroded, and A,B,C components are actually observed near 2850, 2400 and 1600 cm ]. The OH groups which give rise to the strong quasi symmetric H-bonding interaction giving rise to the A,B,C spectrum are expected to be located in the main 12-ring mordenite channels. These channels in fact have a sufficiently ol

9

981 wide diameter (see table 1) to allow the diffusion of this molecule. We can note that the pivalonitrile molecule is quite a rigid and asymmetric one, due to the sp hybridization of the carbon atom involved in the nitrile group. Due to this, only protons which are standing near the centre of the cylindric main channel of MOR should be allowed to interact with pivalonitrile. However, quantitative studies indicate that pyridine (which allows less sterically hindered adsorption) interact with the same amount of protons than pivalonitrile. The bridging OHs which are not perturbed by pivalonitrile and pyridine should actually be located in the smaller cavities, i.e. in the 8-ring channels and in the side pockets, that (for their dimensions, see Table 1) cannot allow the diffusion of these molecules In our spectra both the band due to the fraction of OHs which interact with pivalonitrile and the residual band after pivalonitrile adsorption are located in the range 3600-3610 cm -1. Studies with less hindered molecules have been undertaken to distinguish the sites located in the 8-ring channels and in the side pockets. We can note that also on H-MOR Lewis sites are observed, responsible for the adsorption of pivalonitrile with a shift of the CN stretching up to 2305 cm -~. These sites should also be located (as for H-FER and H-MFI) on the external crystal surface, due to unlikely ability of pivalonitrile to interact (for steric reasons) with Lewis sites on the channel surface. Preliminary experiments show that the situation with dealuminated H-MOR samples is quite different. In this case in fact also sterically hindered adsorbates interact with all bridging OHs. This could be associated to the preferential damage and dealumination of the smaller cavities. 4. CONCLUSIONS The results reported in this communication allow to have some new comparative information on the nature and location of the acid sites of protonic zeolites. In particular: 1. At the external crystal surface of H-FER, H-MFI and H-MOR strong Lewis acid sites and terminal silanol groups are observed. Only in the case of H-BEA heterogeneous terminal silanols are observed, due to clear splittings of the OH-stretching band in the range 3750-3730 cm -1. 2. The Bronsted acidity of the terminal silanols is heterogeneous, as measured by the shift the OH stretching undergoes upon interaction with nitriles. The most intense and evident band of silanol interacting with nitriles actually is located at decreasing frequencies in the trend H-FER (3440 cm -1) < H-MOR (3420 cm -1) < H-MFI (3395 cm -1) < H-BEA (3380 cm-1). This indicates that the most abundant silanols increase in acidity in this sense. However additional much broader components can be seen as in the case of H-FER where a broad absorption is observed centered near 3000 cm -1, providing evidence for a small fraction of very acidic terminal silanols. 3. Lewis sites are apparently characteristic of the external crystal surface of "virtually perfect" protonic zeolites. This suggests that a competition exists between strong Lewis sites and medium-acidity Bronsted sites at the external surface, and strong Bronsted sites located only in the internal cavity surface, for samples which do not contain extraframework material. 4. It is possible to distinguish the sites located in the wider and smaller channels of H-BEA and H-MOR. Preliminary studies indicate that a small fraction of bridging OHs of highaluminum H-MOR are actually located in the main 12-ring channels and can interact with pivalonitrile and pyridine.

982 ACKNOWLEDGEMENT The collaborations with Dr. A. Gutierrez Alejandre and Prof. J. Ramirez (UNAM, Mexico City), and with Proff. M. Lenarda and L. Storaro (University of Venice) are gratefully acknowledged. This work has been funded by MURST. REFERENCES

1 B. Witcherlowfi, N. 2;ilkova, E. Uvarova, J. 12ejka, P. Sarv, C. Paganini and J.A. Lercher, Appl. Catal. A General 182 (1999) 297. 2 J.A. van Bekhoven, M. Tromp, D.C. Konigsberger, J.T. Miller, J.A.Z. Pieterse, J.A. Lercher, B.A. Williams and H.H. Kung, J. Catal. 202 (2001) 129. 3. M. Trombetta and G. Busca, J. Catal., 187 (1999) 521. 4. M. Trombetta, T.Armaroli, A.Guti6rrez-Alejandre, J.Ramirez and G.Busca, Appl. Catal. A General, 192 (2000) 125. 5. M. Trombetta, A. Guti6rrez-Alejandre, J. Ramirez and G. Busca, Appl. Cat. A General, 198 (2000) 81. 6. T. Armaroli, M. Trombetta, A. Guti6rrez Alejandre, J. Ramirez Solis and G. Busca, Phys. Chem. Chem. Phys., 2 (2000) 3341. 7. T. Armaroli, M. Bevilacqua, M. Trombetta, F. Milella, A. Guti6rrez Alejandre, J. Ramirez, B. Notari, R.J. Willey and G. Busca, Appl. Catal. A General, 216, (2001) 59. 8. M. Trombetta, G. Busca, M.Lenarda, L. Storaro and M. Pavan, Appl. Catal. A General, 182 (1999) 225. 9. M. Trombetta, G. Busca, S. Rossini, V. Piccoli, U. Cornaro, A. Guercio, R. Catani and R. J. Willey, J.Catal., 179 (1998) 581. 10. M. Trombetta, G. Busca, L. Storaro, M. Lenarda, M. Casagrande, V. Lucchini, Phys. Chem. Chem. Phys., 2, (2000).3529 11 A.G. Pelkmenshikov, G.H.M.C. vanWolput, J. J~inchen and R.A. van Santen, J. Phys. Chem. 99 (1995) 3612. 12 C.U.I. Odenbrand, J.G.M. Brandin and G. Busca, J. Catal. 135 (1992) 505. 13 B. Witcherlowfi, Z. Tvarfl~kovfi, Z. Sobalik and P. Sarv, Micropor. Mesopor. Mater. 24 (1998) 223. 14 M. Maache, A. Janin, J.C. Lavalley and E. Benazzi, Zeolites, 15 (1995) 507. 15 J. Datka, B. Gil and A. Kubacka, Zeolites, 17 (1996) 428.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

983

NO reduction with isobutane on Fe/ZSM-5 catalysts prepared by different procedures M. S. Batista and E. A. Urquieta-Gonz/dez Department of Chemical Engineering, Federal University of S~.o Carlos, Caixa Postal 676, CEP 13565-905 - S~.o Carlos- SP, Brazil. e-mail: [email protected] Fe/ZSM-5 catalysts were prepared from Na/ZSM-5 zeolite by ion exchange with Fe +2 in solution or in the solid state and characterized by XRD, 27A1 MAS-NMR, EPR, XANES and Mrssbauer spectroscopy (MOS-S). The catalysts were evaluated by the reduction of NO with isobutane. EPR, XANES and MOS-S data of all thermally treated Fe/ZSM-5 showed the Fe atoms as Fe 3§ The EPR signals with g=4.29 or 4.23 were attributed to a mononuclear Fe 3§ in tetrahedral coordination. The MOS-S data revealed the coexistence of Fe 3§ cationic species in charge-compensation sites and extra-framework precipitated Fe species, such as a-goethite or hematite; the different IS and QS MOS-S values of these cationic species were attributed to Fe 3+ in tetrahedral or octahedral coordination. It was proposed that the active sites for the NO reduction with isobutane were the Fe cationic species balancing the charge of the structure. In this sense, the catalytic activity was correlated with the Fe 3+ cations with tetrahedral coordination. As these cationic species were identified at lower temperatures than were used for catalysis, they may have suffered modifications in their nature. The difficulty in identifying Fe species at higher temperatures impeded determination of the exact relation between the nature and activity of those species.

1. INTRODUCTION Nitrogen oxides (NO, NO2 and N20) emitted during combustion of fossil fuels are one of the major sources of air pollution (involved in smog, acid rain, ozone depletion and the greenhouse effect). The selective catalytic reduction of these oxides to N2 by hydrocarbons (HC-SCR) in the presence of oxygen on metal/zeolites, is one of the most attractive methods of removing these environmental toxicants. For this reaction, iron-exchanged ZSM-5 zeolites have proved highly active and stable under hydrothermal conditions and in the presence of SO2 [ 1]. The nature of the active sites responsible for the reduction of NO has been discussed by several researchers. Chen et al. [2] proposed a binuclear oxygen-bridged complex, [HOFe-O-Fe-OH] 2+, as charge compensation species, in which the Fe/A1 ratio could attain the value 1 for the maximum Fe loading. Depending on the preparation method used, other cationic Fe species, such as isolated Fe 3§ FeC12§ Fe(OH)x § and FeO § have also been proposed [2,3,4]. On the other hand, in thermally treated Fe/ZSM-5 catalysts, the presence of iron oxide nanoclusters which are not active in NO reduction or in hydrocarbon oxidation [5] were also observed. Various preparation methods have been recommended to optimize the performance of Fe/ZSM-5: classic and solid-state ion exchange and vapor deposition with volatile iron compounds. The effects of the preparation procedure and the structure of the iron

984 species present in the final material on its catalytic behavior is not exactly known and a great deal of work will be necessary to understand and describe them. Thus, the aim of this study was to examine the Fe species formed during the preparation of the catalysts by ionic exchange, in an aqueous medium or in the solid state and to test their catalytic activity in the reduction of NO with isobutane.

2. EXPERIMENTAL SECTION The Fe/ZSM-5 catalysts were prepared by ion exchange in an aqueous solution or in the solid-state. In the first method the precursor zeolite was a Na/ZSM-5 (Si/AI=I 1), which was synthes!zed hydrothermically using n-butylamine as organic template, and then calcined in air at 520 C for 12 h. Ion exchange was carried out with a 0.033 mol/L solution of FeC12 (Merck 99 %), pH=5.5, in a N2 atmosphere, and the samples were then filtered, washed and finally dried at 110 C overnight. In the solid state, the precursor zeolite was an H/ZSM-5 (Si/AI=13), prepared from the above-specified Na/ZSM-5 zeolite by ion exchange in a 0.1-mol/L solution of HC1. The H/ZSM-5 was mixed with solid FeC12 and further treated at 520 ~ for 6 h, 2 h beinog under flowing N2 and 4 h in a flow of air. Finally, the samples were washed and dried at 110 C overnight. A physical mixture of Na/ZSM-5 and Fe203 was prepared as reference. The samples were denominated Fe(X)Z(Si/A1), where X represents the Fe content (w/w). The sample prepared in the solid state was identified by the letter S (solid) and the sample prepared by physical mixture as Fe203/NaZ(11). The catalysts were characterized by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (27A1 MAS NMR), 57FeM6ssbauer spectroscopy (MOS-S) and X-ray absorption spectroscopy (XANES). The XRD patterns were obtained on a Siemens DS00 diffractometer using monochromatic CuKct radiation in the angular range from 3 to 40~ with a scanning speed of 2~ The 27A1 MAS NMR spectra were obtained using a Varian spectrometer at a proton frequency of 400 MHz. The pulse width used was 0.4 las (rt/16 pulse for 1H) with recycles of 2 s. The EPR spectra were recorded in a Bruker ESP 300 E spectrometer at 77 K. The 57Fe M0ssbauer spectroscopy measurements were performed in the transmission geometry at 4.2 K, using a 25 mCi 57Co:Rh source moving in sinusoidal mode. The zero velocity was defined with respect to the centroid of the metallic iron spectrum, the source and the absorber being kept at the same temperature throughout the experiments. The data obtained were analyzed using a least-squares fitting program (Normos 95), assuming Lorentzian line shapes [6]. The X-ray absorption spectroscopy (XANES) was performed in the National Laboratory of Synchrotron Light (LNLS/Brazil). The data were collected at room temperature in the fluorescence mode and processed using the FEFFIT program. The reduction of NO with isobutane was carded out in a fixed bed reactor using a GHSV=42,000 h~ and a feed of 0.30 % NO, 0.24 % iso-C4H10 and 2.2 % 02, in He (v/v). The reaction temperature was varied in the range from 100 to 500 ~ and before the catalyst tests the samples were activated for 1 h at 520 ~ under flowing air. The product stream was analyzed on line by GC using an AI203/KC1 capillary column and two packed columns: a 5 A molecular sieve and a Hayesep D.

985 3. RESULTS AND DISCUSSION

The Fe content and the Fe/A1 ratio of the Fe/ZSM-5 catalysts obtained are shown in Table 1, together with the time and the number of ion-exchange steps. For the ion exchange in aqueous medium a decrease in the pH value was observed, indicating a consumption of OH ions during the formation of iron species. Table 1 - Fe content and the Fe/A1 ratio of the prepared Fe/ZSM-5 catalysts. Sample

Exchanges / Time (h)

Fe/A1

Fe (w/w %)

2/6 3/24 1/6 -

0.15 0.90 0.37 -

1.1 5.2 2.8 1.6

Fe( 1.1)Z(11) Fe(5.2)Z(11) Fe(2.8)Z(13)S Fe203/NaZ(11)

The XRD pattem of the precursor NaZSM-5 shows the typical reflections of the MFI structure (Figure 1, curve d), with practically all the aluminum atoms within the structure in tetrahedral coordination, as is revealed by NMR spectroscopy (resonance peak at 55 ppm in Figure 2). In Figure 1, it can also be observed that a decrease in peak intensities occurs with the incorporation of Fe atoms in the solids. This behavior is attributed to the X-ray absorption coefficient of Fe compounds being higher than that of Na compounds [7]. In the diffractograms of the catalysts or precursor (Figure 1), there is no evidence for the presence of Fe203 (intense peak lines at 20=33.15 ~ and 35.65 ~ curve a) or any other phase beside ZSM-5. The EPR spectra of all the analyzed Fe/ZSM-5 samples (Figure 3) show the presence of Fe in the oxidation state +3, irrespective of the method of preparation used. It is known that Fe 2+ in aqueous solution is readily oxidized to Fe 3§ by traces of 02 unavoidably present. Therefore, even using a N2 atmosphere during the ion exchange, Fe 2+ was oxidized to F-3 e +, some of which at pH-5.5 precipitated as a-FeOOH. |

i

i

,

i

i

,

i

d i

"i

5

,""

i

10

,

i

15

.

i

20

.

25

'.

30

~ c

35

.

'

40

2O

Figure 1 - X-ray diffraction patterns of: (a) hematite (Fe203); (b) and (c) Fe/ZSM-5 catalysts prepared in aqueous medium and (d) the precursor Na/ZSM-5.

'

I

1oo

i

t

50

~

I

o

i

(ppm)

t

-80

~

I

i

-1oo

Figure 2 - 27A1MAS NMR spectrum of Na/ZSM-5.

986 During the catalyst activation at 520 ~ the precipitated ot-FeOOH is dehydrated to form hematite (ct-Fe203) (equation 1). In ion exchange in the solid state, the Fe 2+ oxidation may occur during the physical mixing of reactants and specially during the thermal treatment. 2 (x-FeOOH

~-Fe203

520~

+

H20

(1)

The EPR spectra of Fe/ZSM-5 samples shown in Figure 3 are similar to those reported by Sachtler's group [2,8]. They are interpreted in terms of g values, which were 4.29 for the activated Fe(1.1)Z(11) and 4.23 for the Fe(2.8)Z(13)S sample, produced in aqueous solution or in the solid state respectively. These signals, in the low magnetic field region (g>3), are usually attributed to the presence of mononuclear Fe +3 ions in tetrahedral coordination. The g signal between 5 to 6, observed in all the samples, is also attributed to mononuclear Fe § ions, but in this case with a distorted tetrahedral coordination. Lee and Rhee [3] have reported that these iron species, in tetrahedral or distorted tetrahedral coordination and located in charge compensation sites in ZSM-5 zeolites, are the active sites in the reduction of NO with hydrocarbons [3]. In Figure 3, no g signals between 1.6 to 2.0 were observed, indicating the absence of Fe clusters with mixed valences (Fe II and Fe nI) [3]. The XANES spectra of the iron reference compounds and the prepared Fe/ZSM-5 catalysts are shown in Figure 4. It can be seen, that the spectra of Fe(II) and Fe(III) compounds (Figure 4a) differ in the intensity of the pre-edge peak near 7.12 keV, which is attributed to Is --~ 3d transitions of iron, in the oxidation state of Fe3§ The spectra of all the Fe/ZSM-5 catalysts also show this small pre-edge peak near 7.12 keV (Figure 4b), which is similar to that reported by Chen et al. [2], who performed m situ EPR analysis and verified that the intensity of this pre-edge peak is directly related to the content of Fe 3§ ions in tetrahedral coordination. In Table 2 are presented the data derived from the 4.2 K MOssbauer spectra shown in Figure 5. Analysis of the samples at such a low temperature was necessary, since at room temperature the paramagnetic hematite (in activated Fe(5.2)Z(11) and Fe(2.8)Z(13)S) and the paramagnetic ct-goethite (in non-activated Fe(5.2)Z(11)) exhibit only doublets, which overlap with the paramagnetic doublets belonging to Fe species in charge-compensation sites [9]. mg--=4.29

__g=4.28

mg=4.23

g =5-6

0

1000

2000

(a)

(~)

3(X)O

0

1000

~)00

(b)

(~)

3030

0

1000

2000

(~)

3030

(c)

Figure 3 - EPR spectra of: (a) Fe(1.1)Z(11) atter ion exchange; (b) Fe(1.1)Z(11) aider activation at 520 ~ and (c) Fe(2.8)Z(13)S.

987

!

_•

__J FeO

. . . .

7.o8"7.~o

,

,

7.~2 7.~4 7.~6 7.~8 7.~,o 7.22 ,.

,

,

,

E nergy

(a)

.,,

(keY)

,

.,

7.08 7.~0 r.~2 r.~, 7.~6 '7.~8 7.50'7= ...

.

.

Energy

,

,

, , ,

,,

. . . . . . . . . . .

(keV)

(b)

Figure 4 - XANES spectra of: (a) iron reference compounds and (b) Fe/ZSM-5 catalysts.

In Table 2, values of the isomer shift (IS), below 1 mm/s, corresponding to activated Fe(5.2)Z(11) and Fe(2.8)Z(13)S samples, are attributed to Fe atoms in the oxidation state 3+, confirming the results obtained by EPR and XANES. It can be seen from the data in Table 2 that Fe 2+ and Fe 3+ ions coexist only in the as-prepared Fe(5.2)Z(11) sample, Fe 2+ being transformed into Fe3+ during the thermal activation (activated Fe(5.2)Z(ll) sample). The different observed IS and QS (quadrupole splitting) values for the Fe 3+ ions can be attributed to different chemical environments. This is in agreement with the idea that in the Fe/ZSM-5 catalysts coexist both Fe 3+ cationic species located in charge-compensation sites and extraframework precipitated Fe species, such as c~-goethite (before thermal activation in the Fe(5.2)Z(ll) sample) or hematite (in the same catalyst after activation and in the catalyst prepared by solid-state ion exchange). The analysis of Fe(1.1)Z(11) was not possible due to its low iron content (Table 1). Long and Yang [ 10] reported that QS values can also provide important information about the coordination symmetry of Fe cationic species located in charge-compensation sites. The authors suggest that dehydrated Fe 3+ sites, in which the ferric ions are presumably in tetrahedral coordination, are indicated by QS>lmm/s. On the other hand, in hydrated Fe 3+ ions the QS decreases to values below 1 mm/s, indicating an increase in the coordination number, probably by direct addition of H20 or OH ligands. However, the identification of the Fe 3+ ligand species by MOssbauer spectroscopy is limited, as it is difficult to prepare standards with the predicted iron species in charge-compensation sites. The relative areas of the M6ssbauer subspectra (Table 2) show that 70 % of the Fe atoms in the as prepared Fe(5.2)Z(11) sample are precipitated as ot-FeOOH (goethite) and 75 % as Fe203 (hematite) in the thermally-treated catalyst. However, the sample prepared in the solid state has around 50 % of hematite and 50 % of Fe species in charge-compensation sites.

988 1 .000 0 .995 F e ( 5 .2 ) Z (1 1 )

0 .990 1 .000 0 .995 0.990

~Fe(5.2;Z(11)

activated

1 .000 9

0 .995 0 .990

V e (2 .8 ) Z (1 3 ) S

0 .985 -10

-8

-6

V

X

,

-4

e

I o

-2

c

14

i t y

0

(m

2

m

4

Is

)

6

8

1 0

Figure 5 - MOssbauer spectra obtained at 4.2 K from Fe/ZSM-5 catalysts.

Considering the reduction of NO with isobutane (Figure 6a), the sample Fe203/NaZ(11), prepared by physical mixture, has no significant activity in this reaction, indicating that other iron species present in Fe/ZSM-5 are responsible for the catalytic activity. As discussed above, the catalysts have various cationic species at temperatures lower than those used in the catalyst tests, and will be taken as the active catalytic species in what follows. However, it should be kept in mind that those species may suffer chemical and/or physical modifications at higher temperatures and/or by interaction with NO, isobutane or 02. Table 2 - M6ssbauer spectroscopy data obtained at 4.2 K from Fe/ZSM-5 catalysts. Sample Fe(5.2)Z(11) As prepared

Fe(5.2)Z(11) Activated

Fe(2.8)Z(13)S

IS(mm/s)

QS(mm/s)

BHF(T)

Area (%)

0.30

0.29

---

11

Fe+3ocT

0.33

1.35

---

6

Fe+3

1.45 <0.37> 0.25

3.01 -0.25 0.30

0.34

1.20

0.35 0.26

-0.20 0.30

--<49.6> ---

13 70 6

--"

19

53.0 ---

75 31

Fe

specie

TET

Fe+2 /)-GO ETHITE FeB+ OCT

Fe+3

TET

MAG-HEMATITE Fe+3ocw

0.34 1.40 19 Fe+3 0.38 0.38 53.5 50 MAG-HEMATITE IS: isomer shift; QS: quadrupole splitting; p-GOETHITE: para-magnetic goethite; MAGHEMATITE= magnetic hematite; BHF: magnetic field; OCT and TET: octahedral and tetrahedral coordination. "'-

TET

989 100

100

,

9ot --,-- Fe(1.1)Z(11) 8ol - ' - F~OJN~I 1) ._. ] --'=-- Fe(5.2)Z(11) o'- ~l--v--Fe(2.8)Z(13)S

=~80-

~ 0 t-

~>5or o 4o-

90-

r-

70-

0

------e---A---v--

Fe(1.1)Z(11) Fe2Os/NaZ(11) Fe(5.2)Z(11) Fe(2.8)Z(13)S

/-///

9

A~

60-

o Z

2o-

2 O

10O-

150

2(}0

250

I &50

300

'

1 400

Temperature (%)

'

i 450

..

500

,

,

,

,.

..

,.

Temperature (~

(a)

(b)

Figure 6 - Conversion on Fe/ZSM-5 catalysts: (a) NO to N2 and (b) isobutane.

As can be seen in Figure 6a, the conversion of NO to N2 increases with firing temperature at a different rate for each Fe catalyst up to 350 - 400~ decreasing at higher temperatures, due principally to the increasing velocity of oxidation reaction. This behavior shows that specific activity (TOF) and/or the distribution of Fe species in the catalysts vary in different ways with the temperature. This can be observed best in the activity changes in the various samples studied, between 350 and 400~ In Figure 6a, it may also be noted that the Fe(5.2)Z(11) catalyst has a higher NO reduction activity of than Fe(2.8)Z(13)S. MOS-S showed that while these samples have a very similar contents of Fe 3+ cationic species, Fe(5.2)Z(11) has a higher number of Fe 3+ ions in tetrahedral coordination (Table 3), which are, as mentioned above, thought to be the active sites for the reduction of NO with isobutane. Recently, Long and Yang [12] also reported that Fe 3+ ions with tetrahedral coordination were the active sites in the reduction of NO with ammonia. It can be observed in Figure 6b that all the catalysts show an increase in the conversion of isobutane conversion with increasing reaction temperature. It is interesting to note that, in this case, the Fe 3+ species in charge-compensation sites are again responsible for the catalytic activity. These catalytic data also show that the Fe atoms in hematite have low activity in the isobutane oxidation. As discussed in the case of NO reduction, the lower activity of Fe(1.1)Z(ll) than Fe(5.2)Z(ll), both samples prepared in aqueous medium, must also be attributed to a lower number of Fe 3+ species in charge-compensation sites. Table 3 - Fe distribution in Fe/ZSM-5 catalysts. % Fe species in charge-compensation sites Sample

. Fe3+

Fe(5.2)Z(11) 1.3 Fe(2.8)Z(13)S 1.4 (a) Total (b) Tetrahedral

b Fe3+ 1.0 0.5 (c) Octahedral

% Fe3+in hematite

c Fe3+

Fe3+

0.3 0.9

3.9 1.4

990 4. CONCLUSION EPR, XANES and MOS-S data of the thermally-treated Fe/ZSM-5 catalysts show the presence of Fe atoms in the oxidation state 3+, irrespective of the method used in the catalyst preparation. The most important EPR signal, with g value of 4.29 for the activated Fe(1.1)Z(ll) and 4.23 for the Fe(2.8)Z(13)S sample, produced respectively in an aqueous solution and in the solid state, was attributed to the presence of mononuclear Fe3§ ions in tetrahedral coordination. The MOS-S data also showed that in the catalysts coexist both Fe 3§ cationic species located in charge-compensation sites and extra-framework precipitated Fe species, such as a-goethite or hematite. The different IS and QS MOS-S values of the Fe cationic species were attributed to their different chemical environments, as the Fe3+ ions can have tetrahedral or octahedral coordination. The Fe atoms in Fe203 are not active in the reduction of NO or in the oxidation of isobutane, confirming that the active sites are iron cationic species balancing the negative charge of the zeolite structure. The catalytic activity was correlated with the Fe3§ cations with tetrahedral coordination. As described, these cationic species were identified in the catalysts at temperatures lower than those used in the catalyst tests. It must be taken into consideration that these species may suffer chemical and/or physical modification at higher temperatures and/or by interaction with NO, isobutane or 02, modifying the nature of the Fe cationic species. The difficulty in identifying Fe species at higher temperatures prevented determination of the exact relation between the nature and activity of those species. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support for this study provided by CNPq, Brazil (grant 641444/00-3) and for the Doctor's scholarship to Marcelo S. Batista provided by FAPESP (Sao Paulo State Research Aid Foundation, Brazil: grant n~ 1998/02495-5). We thank Dr Fabio B. Noronha for the XANES experiments and valuable discussions. REFERENCES

1. X. Feng and W. K Hall, J. Catal. 166 (1997) 368. 2. H.-Y. Chen, E1-M. E1-Malki, X. Wang, R. A. Van Santen and W. M. H. Sachtler, J. Molec. Catal. A 162 (2000) 159. 3. H.-T. Lee and H.-K. Rhee, Catal. Letter 61 (1999) 71. 4. L. J. Lobree, I.-C. Hwang, J. A. Reimer and A. T. Bell, J. Catal. 186 (1999) 242. 5. M. KOgel, R. Monning, W. Schwieger, A. Tissler and T. Turek, J. Catal. 182 (1999) 470. 6. N. N. Greenwood and T. C. Gibb, Mrssbauer Spectroscopy, Chapman and Hall Ltd., London, 1971. 7. B. D. Cullity, Elements of X-Ray Diffraction, 3th Ed., Addison-Wesley, USA, 1967. 8. E1-M. E1-Malki, X. Wang, R. A. V. Santen, W. M. H. Sachtler, J. Catal. 196 (2000) 212. 9. K. D. Abhaya, J. Yarning, M. Linda, M. R. Thato, D. T. Humphrey and J. C Neil, Stud. Surf. Sci. Catal. 130, Elsevier, Amsterdam, (2000) 1139. 10. W. N. Delgass, G. L. Hailer, R. Kellerman and J. H. Lunsford, Spectroscopy in Heterogeneous Catalysis, Academic Press, New York, 1979. 11. H. Y. Chen and W. M. H..Sachtler, Catal. Today 42 (1998) 73. 12. Long R. Q. and Yang R. T., Catal. Letter 74 (2001) 201.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

991

Catalytic and infrared spectroscopic study of N O + C O reaction over ironcontaining pillared montmorillonite F. L6nyi, a J. Valyon a and I. Kiricsi b aInstitute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P. O. Box 17, H- 1525 Budapest, Hungary bDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich B61a t6r 1, H-6720 Szeged, Hungary Iron-containing montmorillonites, prepared by A1/Fe mixed-metal pillaring (PILC) and by Fe 3+ ion exchange were studied in the catalytic reduction of NO by CO. The Fe,A1-P!LC sample was significantly more active than the Fe-montmorillonite, having the same iron content. The surface species obtained from the adsorption of the reactants were examined using transmission IR and DR/FT spectroscopy. Results suggested that redox type catalytic cycle prevailed, wherein Fe atoms were reduced by CO and reoxidized by NO. Under comparable reaction conditions a larger number of iron site was active in the more easily reducible PILC than in the Fe-montmorillonite catalysts. 1. INTRODUCTION The catalytic abatement of NO emission is in the forefront of industrial and academic research [1, 2]. The pillared-layer clays (PILCs) appeared as possible NO conversion catalysts only recently [3-5]. The iron-containing PILCs were found most promising [3, 4]. The catalysts studied in the present work were montmorillonites, ion-exchanged by Fe 3+ ions (Fe,Na-Mont) and pillared montmorillonites, containing mixed oxide pillars of A1 and Fe metals (Fe, A1-PILCs). Previous results suggested that in the Fe, A1-PILC, alumina pillars are decorated by oligomeric hydrous iron oxide species [5-8]. Similar iron oxide species were found in the Fe,Na-Mont samples [4, 5]. M6ssbauer spectroscopic examination [5-7] has shown that at moderate temperatures, for instance at 473 K, the H2 reduction of the Fe ions was deeper in the Fe, A1-PILC than in the Fe,Na-Mont samples, suggesting that different chemical environments are affecting the reducibility of the Fe ions in the different preparations. However, at temperatures as high as 673 K the average degrees of Fe reduction were found to be about the same regardless of the structure of the sample. Because of the easily reducible Fe ions the Fe, A1-PILC samples were presumed to be active NO conversion catalysts. In the present study the activity of the Fe, A1-PILC and the Fe,Na-Mont samples were tested in the NO conversion with a reducing agent, namely with

992 CO. The pillared samples were found to be regularly more active than the non-pillared clays having the same iron content. The vibration spectra of the surface species, obtained from NO adsorption were studied and the catalytic activities were interpreted in relation with the results of the spectroscopic examinations.

2. EXPERIMENTAL The iron content of the Fe, A1-PILC and the Fe,Na-Mont samples, expressed as Fe203, was in the range of 0.6 - 1.7 wt %. The method of sample preparation and a thorough characterization of the samples was given in our previous papers [4-8]. In the procedure of alumina pillaring polyhydroxy Al ions, namely All3-Keggin ions, were exchanged in the preswollen clay. The sample obtained is designated as All3-PILC. The designation of the samples, obtained via pillaring with Fe, A1 mixed-metal polyoxometalate solution, reflects the composition of hypothetical mixed-metal Keggin-ions, such as Fe4A19-PILC. The iron exchanged Na-montmorillonite sample is designated similarly to that PILC sample, having about the same iron content, such as, Fe4Na-Mont. The catalytic examinations were carried out using a conventional, atmospheric, flowthrough U-tube quartz reactor. The inner diameter of the tube was 6 mm. About 50 mg of the catalyst particles from the 0.25-0.50-mm size sieve fraction was placed into the reactor tube and pretreated in flowing He at 773 K for 1 h. The reaction was started by switching the He flow to a flow of 1.5 % NO/1.5 % CO/He mixture. The total flow rate was 40 ml/min. The reaction temperature was varied between 573 K and 923 K. An on-line GC, equipped with TC detector and a column filled with molecular sieve 5A, was used to analyze the reactor effluent. The apparent turnover frequency (TOF) of the reaction was calculated by relating the reaction rate to the total iron content of the sample. The species, formed from adsorption of NO over the catalysts at room temperature, were studied by transmission infrared (IR) spectroscopy using a conventional all-glass IR cell, equipped with KBr windows and a built-on furnace for in-situ treatments. Self-supporting pellets of 5-8 mg/cm 2 thickness were pressed from the samples and fixed to the quartz sample holder of the cell. Oxidized and reduced forms of the samples were prepared by heating a pellet either in 02 or in H2 flow at 773 K for 1 h. The pellet was flushed then by a He flow at 773 K for 1 h, cooled down to room temperature and its spectrum was recorded. The He flow was switched then to a flow of 3 % NO/He mixture. The spectrum of the surface species was obtained by subtracting the spectrum of the pellet from that of the pellet, in contact with NO. A Nicolet 5PC type FTIR spectrometer was used. Using the same spectrometer the surface species formed under reaction conditions were studied by diffuse reflectance Fourier transform infrared spectroscopy (DRIFT). Finely powdered sample was pretreated in the sample cup of the DRIFT cell (COLLECTOR and High Temperature Chamber, Products of Nicolet/Spectra-Tech) in flowing He at 773 K for 1 h. The sample was cooled then to room temperature and the He flow was switched to the 1.5 % NO/1.5 % CO/He reactant flow. The flow rate was 40 ml/min. Temperature was raised in steps up to 773 K and cooled back in steps to 298 K. A single beam spectrum was recorded at each selected temperature, whereon sample was held for about 10 min. The absorbance

993 spectrum was obtained by relating this single beam spectrum to the single beam spectrum of the sample in He at the same temperature. At each temperature a spectrum was also determined with the reactant mixture and the sample cup filled with KBr powder. These spectra were used to correct the corresponding absorbance spectra for the spectrum of the gas phase.

3. RESULTS The NO conversion data over the pillared and ion-exchanged iron-containing montmorillonite catalysts are shown as a function of the reaction temperature in Figure 1. The activity of the Fe, A1-PILC catalysts was regularly higher than that of the Fe,Na-Mont catalysts with about the same Fe content. Obviously, the Fe, A1-PILC samples contain either a larger number of active sites or sites of significantly higher activity. I

0

. 60

.

I

I

I

I

0

9F e 4 A I e - P I L C

A

9F e z A I I I - P I L C

n

9A I I 3 - P I L C

I

I

A -

I

I

I

o

" Fe4Na-Mont

A

9F e 3 N a - M o n t

n

9N a - M o n t

I

I

B

Z

0

r =

0

40

/

m

it_

(D

tO

/

2O

/

I

673

773

873

I

673

Temperature, K

773

873

Figure 1. Conversion of NO as a function of the reaction temperature over (A) Fe, A1 copillared, and (B) iron ion-exchanged Na-montmorillonite samples. Catalysts were pretreated in flowing He at 773 K for 1 h. A gas mixture of 1.5 % NO/1.5 % CO/He was passed through the catalyst bed (50 mg) at a total flow rate of 40 ml/min.

994 For all the examined catalyst samples the apparent TOF values are given in Table 1. The TOF is higher for the PILC sample containing more iron. However this difference is not as significant at 873 K than at 773 K. It is to be noticed that the parent Na-montmorillonite (NaMont) contains a significant amount of iron. The Na-Mont sample was inactive at 773 K, but showed catalytic activity at 873 K. Interestingly, pillaring with alumina alone also increased the activity of the montmorillonite. At 873 K the TOF over the AI13-PILC and the FeaA19PILC was about the same indicating that at this temperature the specific activity of iron atoms is the same regardless whether the iron was in the parent montmorillonite or was introduced in the pillaring procedure.

A

0

.

i

G;

0

00

0

o~

].H

h-

0

t

oo

'

.12

aO ~--

00~ OJoO

9r - -

x--

i

0

c-

oo

._:~~:- . . . . . . . .

.

.

.-

i

0

....... :,"5 , , , , I

red.

.... ..:?

reox

<eL-

2000

9

"'

\,'.

i

,

'r--

fi

.:,:

L_

_.,,,=_.red

'

'

'

'

'

'

'

I

l

j

p

I

9

I

1500

2000 -1

Wavenumbers, cm

1500

Figure 2. Infrared spectra of the species obtained from adsorption of NO at 298 K over (A) pillared montmorillonites, Fe4A19-PILC (solid lines) and All3-PILC (dashed lines), and (B) ion-exchanged montmorillonite, Fe3Na-Mont (solid lines) and parent Na-montmorillonite, Na-Mont (dashed lines). Samples were treated in 02 or 1-12flow at 773 K for 1 h and flushed in He at 773 K for 1 h to get the oxidized (ox.), reduced (red.) and reoxidized (reox.) samples. Spectra were recorded alter contacting the sample with 3 % NO/He at 298 K for 15 min.

995 Infrared spectra of the adsorbed species formed from NO on the pillared and on the ionexchanged samples are shown in Figure 2A and 2B, respectively. Two characteristic regions were distinguished: the bands appearing below 1700 c m "1 that can be assigned to different nitrate, nitro and nitrito species, and the bands in the 2 0 0 0 - 1700 c m 1 region, which stem from surface-bound nitrosyl species [9]. The assignment of the additional band at 2230 cm-1 is quite uncertain yet. Similar bands were usually assigned to NO + or NO2 + bound to basic surface sites [9]. The spectra in Figure 2 show that the binding of NO to the Fe4AIg-PILC and to the Fe3NaMont samples is distinctly different (cf. Figure. 2A and 2B, solid lines). In the spectrum

d

co cq

d

cO

oo ,.r-

,r--

673 K

8

4

473 K

1_o

3

373 K

773 K

eL.

0

673 K

6

573 K

5

473 K ,.

- -

_

298 K

//

298 K 10..._ rain. _ I

I

I

I

\\

/ I

=

1900

\ I

I

I

I

_ I

1800

I

1 I

I

298 K _ H e purge ~ . , - ' ' ~ ' ~ ,

~

,

,

I

,

1900

Wavenumbers, cm

-1

,

,

13

,

I

,

,

,

,

1800

Figure 3. DR/FT spectra of surface species from adsorption of NO under reaction conditions over Fe4A19-PILC catalyst (A) at room temperature (spectra 1 and 2) and at increasing temperatures (spectra 3 to 7), then (B) at decreasing temperatures (spectra 8 to 13). A gas mixture of 1.5 % NO/1.5 % CO/He was passed through the catalyst powder, placed in sample cup and spectra were recorded at the indicated temperatures.

996 obtained for the PILC sample characteristic band appeared at 1829 cm -1 when sample was pretreated in oxygen. At the same frequency a substantially more intense band appeared, if sample was reduced in H2 prior to contacting it with NO (Figure 2A). We assign the band at about 1830 cm -1 to a nitrosyl group bound to Fe-ions in oxidation state lower than 3+. The spectra obtained on Al13-PILC sample are also shown for comparison (Figure. 1A, dashed lines). The absence of the 1830-cm 1 band suggests that, unlike to the iron in the Fe4A19-PILC sample, the native iron present in the All3-PILC sample was not reduced in H2 at 773 K. The original oxidized sample could not be recovered from the H2-reduced sample by hightemperature 02 treatment. The band of the nitrosyl group formed over the reoxidized sample appeared at 1852 cm -1, i.e., at wavenumber 23-cm -1 higher than found for the original oxidized and for the reduced Fe4A19-PILC catalyst. In the spectrum of the Na-Mont and Fe3Na-Mont samples nitrosyl bands appeared in the 1860- 1900 cm ~ region (Figure 2B). Regardless of the applied pre-treating gas, the nitrosyl bands were very weak for the parent Na-Mont sample (Figure 2B, dotted lines). The NO bands in the spectra obtained with the Fe3Na-Mont sample were more intense than those of the parent Na-montmofillonite, however, they were of similar intensity following the 02 and the H2 treatments. Furthermore, the characteristic mononitrosyl bands appeared at higher wavenumbers, suggesting that the sorption site iron species were in higher oxidation state in the Fe3Na-Mont sample than in the corresponding Fe4A19-PILC sample. The most active Fe4A19-PILC catalyst was contacted with the NO/CO/He reaction mixture and examined by DRIFT spectroscopy. Spectra of the surface species from adsorbed NO were obtained at higher and higher temperatures up to 773 K (Figure 3A, spectra 1 to 7). The spectra obtained at a following stepwise temperature decrease are shown in Figure 3B, spectra 8 to 13. From adsorption of CO no band could be observed. In the presence of CO the nitrosyl band appeared at 1838 cm 1 at room temperature, i.e., at somewhat higher wavenumber than that obtained with the H2-reduced sample in absence of CO (1829 cm~). The intensity of the band decreased with time on stream (Figure 3, spectra 1 and 2), while the bands below 1700 cm 1 were growing (not shown). The intensity of the nitrosyl band further decreased as temperature was raised (Figure 3A, spectra 2 to 7). The band practically disappeared above 473 K. Surprisingly, when the reaction temperature was dropped from 773 K to 673 K, a nitrosyl band appeared again at 1825 cm~, i.e., at a slightly lower frequency than before. As temperature was further decreased the band shifted to higher wavenumbers. At room temperature the band was at 1855 cm 1 (Figure 3B, spectra 8 to 13). The frequency of this band is similar to that obtained before (Figure 2) with the reduced and reoxidized sample. Results suggest that in contact with the reaction mixture at high temperature reduced iron sites were generated in significant concentration. These sites were adsorbing NO and became only slowly reoxidized by the reactant at the lower temperatures. 4. DISCUSSION In the catalytic reduction of NO with CO the supported Fe203 catalysts show usually higher activity than the various other supported metal oxides [2, 10, 11]. The reaction is generally believed to demand reducible metal atoms as active sites. The reduced metal sites

997 are oxidized by NO, while N20 or, favorably, N2 is formed. A redox type catalytic cycle is established if a process is available that maintains a steady state concentration of the active sites by reducing the sites at the same rate as they are oxidized by the NO [2]. At elevated temperature some oxides can release 02 and undergo autoreduction, such as Cu-ZSM-5. Over such catalysts the decomposition of NO to N2 and 02 can proceed in absence of any reducing agent [12]. The iron containing clays, examined in this study, proved to be inactive as NO decomposition catalysts. They were found, however, active in the reduction of NO by CO. The activity of the Na-montmorillonite was found to be enhanced by alumina pillaring and especially by introducing iron oxide together with the alumina in the pillaring procedure. The IR spectroscopic examination of the samples has shown the presence of reduced iron atom NO adsorption sites in the most active preparations. The iron was reducible in the Fe, AIPILC at temperatures whereon it remained unreduced in the Fe,Na-Mont. The specific activity, calculated for one iron atom, was shown to parallel the reducibility of the iron (Table 1). Previous studies has unambiguously revealed that in the pillars of the Fe, A1-PILC preparations, formed from Keggin-ion like structures, no tetrahedral A1 was substituted by iron [4-8]. Instead, iron oxide/alumina co-pillaring was substantiated. Upon heat treatment at higher temperatures bulky alumina pillars are formed, which pillars can be decorated by iron oxide species [4-8]. The iron in the catalytically active Fe, A1-PILC sample was reducible at temperatures, whereon the native iron content of the inactive A1-PILC and the iron in the Fe,Na-Mont samples could not been reduced. However, when the reaction temperature was as high as 873 K, the difference in the apparent specific activities of the different PILC samples disappeared. At this temperature both the easily reducible and the hard-to-reduce iron became activated, resulting in similar apparent turnover frequencies for the samples, containing the two kinds of iron in different proportions (Table 1). Table 1. Apparent turnover frequencies (TOF) a for NO conversion in the NO+CO reaction as a function of the reaction temperature Catalyst ID Fe203 content, TOF x 103 sl wt%

673 K

773 K

873K

Fe4A19-PILC

1 69

1.22

7.29

15.69

Fe2A111-PILC

079

2.52

19.65

All3-PILC

0 64

1.05

15.25

Fe4Na-Mont

157

1.84

5.89

Fe3Na-Mont

141

Na-Mont

0.70

4.64 -

-

5.87

a Calculated by relating the reaction rate to the total iron content of the sample. It is to be noted, however, that even at 873 K lower apparent TOF values were obtained for the ion-exchanged than for the pillared clay samples. Surprisingly, the specific activity of the

998 iron atoms in the Al-pillared sample corresponds to that of the Fe, A1 co-pillared sample. This finding suggests that, as a result of A1 pillaring, the native Fe content of the clay became more active. Accordingly, previous temperature-programmed reduction examinations showed that the iron in the All3-PILC sample is more easily reducible than in the parent Na-Mont sample

[4].

The results presented suggest that the kinetics of the iron reduction depends on the chemical environment of the iron atoms. The iron is more reducible in the pillared samples than in the original layer structure of the clay. CONCLUSIONS The alumina-pillared montmorillonite (PILC) samples contain iron species that are active sites in the catalytic reduction of NO by CO. The iron in the PILC is more accessible for the reactants and more easily reducible and, therefore, more active than in the parent montmorillonite. The Fe, A1-PILC preparations obtained by introducing Fe and A1 together into the clay during the pill~iring procedure are the most active. ACKNOWLEDGMENTS This work was supported by the Hungarian Scientific Research Fund (OTKA, Contract Nos. T 016761 and T 029717).

REFERENCES

1. 2. 3. 4. 5.

M. Shelef, Chem. Rev., 95 (1995) 209. V.I., Parvulescu, P. Grange and B. Delmon, Catalysis Today, 46 (1998) 233. J.P. Chen, M.C. Hausladen and R.T. Yang, J. Catal., 151 (1995) 135. J. Valyon, I. Phlink6, and I. Kiricsi, React. Kinet. Catal. Lett., 58(2) (1996) 249. I. Phlink6, A. Molnhr, J.B. Nagy, J.-C. Bertrand, K. Lhzhr, J. Valyon and I. Kiricsi, J. Chem. Soc., Faraday Trans., 93(8) (1997) 1591. 6. I. Palink6, K. Lhzhr and I. Kiricsi, J. Mol. Struct., 410 (1997) 547. 7. I. Kiricsi, A. Molnhr, I. Phlink6 and K. Lhzhr, in: Catalysis by Microporous Materials (H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy, eds.) Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 1995, Vol.94, p. 63. 8. J.B. Nagy, J.C. Bertrand, I. P~link6 and I. Kiricsi, in: Progress in Zeolite and Microporous Materials (H. Chon, S.-K. Ihm and Y.S. Uh, eds.) Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 1997, Vol. 105, p. 1957. 9. K.I. Hadjiivanov, Catal. Rev.-Sci. Eng., 42 (2000) 71. 10. M. Shelef, K. Otto and H. Ghandi, J. Catal., 12 (1968) 361. 11. T.P. Kobylinski and B.W. Taylor, J. Catal., 31 (1973) 450. 12. J. Valyon and W.K. Hall,. J. Catal., 143 (1993)520.

Studies in Surface Science and Catalysis 142 - Part B IMPACT OF ZEOLITES AND OTHER POROUS MATERIALS ON THE NEW TECHNOLOGIES AT THE BEGINNING OF THE NEW MILLENNIUM

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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates

Vol. 142

IMPACT OF ZEOLITES AND OTHER POROUS MATERIALS ON THE NEW TECHNOLOGIES AT THE BEGINNING OF THE NEW MILLENNIUM PART B P r o c e e d i n g s of the 2 nd International F E Z A (Federation of the E u r o p e a n Zeolite Associations) Conference T a o r m i n a , Italy, S e p t e m b e r 1-5, 2002

Organized by the ITALIAN ZEOLITE ASSOCIATION under the auspices of the Federation of the European Zeolite Associations Edited by

R. Aiello, G. Giordano and F. Testa

Dipartimento di Ingegneria Chimica e dei Materia/i, Universit& della Calabria Arcavacata di Rende, Italy

2002 ELSEVIER Amsterdam - Boston - London - New Y o r k - Oxford - Paris - San Diego San Francisco - Singapore - Sydney - Tokyo

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PREFACE

It is a pleasure to present the Proceedings of the Second Conference of the Federation of the European Zeolite Associations where are collected the contributions of internationally renowned researchers in the field of the Science and Technology of micro and mesoporous materials. Aim of the Conference, organized by the Italian Zeolite Association, is to create an international forum where researchers from academia as well as from industry can bring and discuss ideas finalized to evaluate the impact of zeolites and other porous materials on the new technologies at the beginning of the new millennium. Among the others, in fact, the technologies for the production of chemicals, which will become always more important for maintaining our standard of life and our environment safe, will need substantial innovation and we hope that this book will be a source of new ideas for further fundamental and applied research work not only for the participants of the Conference but also for the whole scientific community. These proceedings report the oral and poster communications presented during the FEZA Conference, subdivided into 8 thematic sessions. The volume contains also the full text of the three plenary and two keynote lectures. The scientific contributions, coming from 35 countries both European and extra European, testify of the great vitality of the zeolite science in its various branches, from those always represented at the zeolite conferences (synthesis, catalysis, ion exchange and modification, natural zeolites... ) to the new emerging areas (mesoporous materials, environmental sciences, computational chemistry, advanced materials... ) and, at the same time, of the blend of multidisciplinary knowledge involved in this science in continuous evolution. The editors would like to acknowledge the dedication of the members of the Paper Selection Committee: A. Alberti, G. Centi, M. Derewinski, F. Fajula and J. B.Nagy, and express their gratitude to all the referees who contributed to the selection of the Conference papers. A special and grateful acknowledgment has to be addressed to Dr. A. Katovic (Treasurer) for her great involvement all along the Conference organization. Rosario Aiello Girolamo Giordano Flaviano Testa Editors

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vii The Federation of the European Zeolite Associations (FEZA) was constituted in Szombathely (Hungary) on 12 July 1995 by the representatives of the National Zeolite Associations from France, Germany, Hungary, Italy, the Netherlands and UK, plus Bulgaria and Spain, which were going at that time to constitute the respective national associations. The Constitution of the FEZA was approved on 25 January 1996. At the same date and in successive meetings of the FEZA Committee, other national associations were accepted, i.e., the Romanian Zeolite Association, the Georgian Association of Zeolites, the Polish Zeolite Association, the Czech Zeolite Group, and finally, in the course of last meeting in Montpellier, on 9 July 2001, the admission of Portugal and Slovakia was decided. Among the objects of the Federation, there is the task to arrange Specialist Workshops, Euroconferences or Meetings of an educational character. Accordingly, in the three-year period from 1996 to 1998, a series of six Euroworkshops on Zeolites have been organized, with the financial support of the European Union, on synthesis; ordered mesoporous materials; sorption, diffusion and separation; natural zeolites; application in catalysis, and modification and characterization. In the same frame, the FEZA originated the proposal of a cycle of Euresco Conferences on Zeolite Molecular Sieves. The first Euroconference of this cycle has been held in Obernai (France) during the last March on the "Isomorphous Substitution by Transition Metals". The proposal to organize an International Thematic Conference trader the auspices of the FEZA was made by the leading members of the Hungarian Zeolite Association during the FEZA Committee Meeting, held in Budapest in 1998. Although this type of Conference was not expressly considered in the FEZA Constitution, the proposal was accepted with enthusiasm by the members of the Committee. The 1st International FEZA Conference was therefore held in Eger (Hungary) on 1-4 September 1999, on the theme "Porous Materials in Environmentally Friendly Processes". The Eger Conference was a very successful Conference and this encouraged the FEZA Committee to continue on the same way. Now I have the particular pleasure and pride to present the volume constituting the Proceedings of the 2nd International FEZA Conference, which will be held in Taormina (Italy) on 1-5 September 2002 on the theme "Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium". The reading of the contents and the information directly gathered from the organizers makes me convinced that this will be a very successful Conference either for the richness of themes or for the quality of the contributions. In addition, I am sure that these Proceedings will be prepared by the Editors and printed by the Publisher with the usual care and attention to the printing quality. One last information for the reader. The next FEZA Conference, the 3rd of the series, will be held in Prague in August-September 2005, under the auspices of the Czech Zeolite Group on the theme of "Molecular Sieves from Basic Research to Industrial Applications". Carmine Colella Chairman of the FEZA Committee

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SUPPORT AND SPONSORING (as of May 30, 2002) The Organizing Committee wishes to thank various Institutions and Companies for their financial support to FEZA 2002.Their contributions allowed a reduced registration fee for students and a bursary program. INSTITUTIONS Universit~ della Calabria Dipartimento di Ingegneria Chimica e dei Materiali- Universith della Calabria Universit~ di Messina Universit~ di Catania Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM) COMPANIES EniTecnologie Sasol Italy Philips Netzsch Jeol Micromeritics UOP M.S. COECO Pirossigeno

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xi

ORGANIZING COMMITTEE

Chairman R. Aiello

University of Calabria, Italy

Vice-Chairman G. Giordano

University of Calabria, Italy

Secretary E Testa

University of Calabria, Italy

Treasurer A. Katovic

University of Calabria, Italy

Members E Crea S. Crisafulli S. Galvagno A. Parmaliana

University University University University

of of of of

Calabria, Italy Catania, Italy Messina, Italy Messina, Italy

INTERNATIONAL SCIENTIFIC ADVISORY BOARD (Council of FEZA)

C. Colella (President) H. Van Bekkum (Former President) E Di Renzo (Secretary) M.W. Anderson P. Behrens J. Ceika P. Ciambelli E Hudec I. Kiricsi D.C. Koningsberger S. Kowalak C. Minchev J. Perez-Pariente E Ramoa Ribeiro R. Russu G. Tsitsishvili

Italy The Netherlands France United Kingdom Germany Czech Republic Italy Slovakia Hungary The Netherlands Poland Bulgaria Spain Portugal Romania Georgia

xii PAPER SELECTION COMMITTEE R. Aiello A. Alberti G. Centi M. Derewinski E Fajula J.B. Nagy

University of Calabria, Italy University of Ferrara, Italy University of Messina, Italy Polish Academy of Science, Krakov, Poland CNRS-ENSCM, Montpellier, France University of Namur, Belgium

xiii

CONTENTS

ZEOLITE SYNTHESIS AND CHARACTERIZATION Zeolite characterization with spectroscopic methods A. Zecchina, G. Spoto, G. Ricchiardi, S. Bordiga, E Bonino, C. Prestipino and C. Lamberti (PLENARY LECTURE) Synthesis of alumino, boro, and gallosilicate zeolites by steam-assisted conversion method and their characterization R. Bandyopadhyay, Y. Kubota and Y. Sugi Aluminium distribution in MCM-22. The effect of framework aluminium content and synthesis procedure J. D~dedek, J. Cejka, M. Oberlinger and S. Ernst Grafting of aluminium on dealuminated H-BEA using alkoxides A. Omegna, M. Haouas, G. Pirngruber and R. Prins Influence of various synthesis parameters on the morphology and crystal size of zeolite Zn-MFI A. Katovic, G. Giordano and S. Kowalak In situ dynamic light scattering and synchrotron X-Ray powder diffraction study of the early stages of zeolite growth G. Artioli, R. Grizzetti, L. Carotenuto, C. Piccolo, C. Colella, B. Liguori, R. Aiello and P Frontera Synthesis of MCM-22 zeolite by the vapor-phase transport method S. Inagaki, M. Hoshino, E. Kikuchi and M. Matsukata Defect-flee MEL-type zeolites synthesized in the presence of an azoniaspiro-compound R. Millini, D. Berti, D. Ghisletti, W.O. Parker, Jr., L.C. Carluccio and G. Bellussi Chemical and structural aspects of the transformation of MCM-22 precursor into ITQ-2 R. Schenkel, J.-O. Barth, J. Kornatowski and J.A. Lercher Nanocrystalline ZSM-5: a highly active catalyst for polyolefin feedstock recycling D.P Serrano, J. Aguado, J.M. Escola and J.M. Rodriguez Modeling superoxide dismutase: immobilizing a Cu-Zn complex in porous matrices and activity testing in H202 decomposition K. Hernadi, D. M~hn, I. Labddi, I. Pdlink6, E. Sitkei and I. Kiricsi Crystal growth of zeolite Y studied by computer modelling and atomic force microscopy JR. Agger and M. W. Anderson Interaction of small molecules with transition metal ions in zeolites: the effect of the local environment P Nachtigall, M. Davidovd, M. Silhan and D. Nachtigallovd

3

15

23 31

39

45

53 61 69 77

85 93

101

xiv Preparation and characterization of mesoporous TS-1 catalyst K. Johannsen, A. Boisen, M. Brorson, 1. Schmidt and C.J.H. Jacobsen Observations of layer growth in synthetic zeolites by field emission scanning electron microscopy S. Bazzana, S. Dumrul, J. Warzywoda, L. Hsiao, L. Klass, M. Knapp, J.A. Rains, E.M. Stein, M.J Sullivan, C.M. West, J Y. Woo and A. Sacco, Jr. XANES and XPS studies of titanium aluminophosphate molecular sieves M.H. Zahedi-Niaki, E Beland, L. Bonneviot and S. Kaliaguine An investigation of the intermediate gel phases of A1PO4-11 synthesis by solid state NMR spectroscopy Y. Huang, R. Richer and C. Kirby The benzene molecule as a probe for steric hindrance at proton sites in zeolites: an IR study B. Onida, B. Bonelli, L. Borello, S. Fiorilli, E Geobaldo and E. Garrone Structural characterization of Co- and Si-substituted A1PO-34 synthesized in the presence of morpholine A. MartuccL A. AlbertL G. CrucianL A. Frache and L. Marchese Chemical linking of MFI-type colloidal zeolite crystals P. Agren, S. Thomson, Y. Ilhan, B. Zibrowius, W. Schmidt and E Schfith Synthesis and characterization of MCM-22 zeolites for the N20 oxidation of benzene to phenol D. MelonL R. MonacL E. RombL C. Guimon, H. Martinez, 1. Fechete and E. Dumitriu Novel solid strong base derived from zeolite supported CaO X. W. Han, G. Xie, Y. Chun, X.. W. Yan, Y. Wang, J Xue and JH. Zhu ZSM-5 spheres prepared by resin templating L. Tosheva and J Sterte Novel Nanocomposite Material A. Carati, C. Rizzo, L. Dalloro, B. Stocchi, R. Millini and C. Perego Vibrational and optical spectroscopic studies on copper-exchanged ferrierite G. Turnes Palomino, S. Bordiga, C. Lamberti, A. Zecchina and C. Otero Aredm Variable temperature FTIR spectroscopy of carbon monoxide adsorbed on protonic and rubidium-exchanged ZSM-5 zeolites C. Otero Are6n, M. Pe~arroya Mentruit, M. Rodriguez Delgado, G. Turnes Palomino, O. V. Manoilova, A.A. Tsyganenko and E. Garrone Preparation and characterization of Zn-MFI zeolites using short chain alkylamines as mineralizing agents S. Valange, B. Onida, E Geobaldo, E. Garrone and Z. Gabelica

109

117

125

135

143

151 159

167 175 183 191 199

207

215

Crystal growth of nanosized LTA zeolite from precursor colloids S. Mintova, B. Fieres and T. Bein

223

Synthesis of hybrid zeolite disc from layered silicate Y. Kiyozumi, M. Salou and E Mizukami

231

XV

Effect of alkali metal ions on synthesis of zeolites and layered compounds by solid-state transformation T. Nishide, H. Nakajima, Y. Kiyozumi and E Mizukami (A1)-ZSM-12: syhnthesis and modification of acid sites J. Cejka, G. Ko~ovd, N. Zilkov6 and I. Hrub6 Formation of new microporous silica phase in protonated kanemite-TMAOHwater system E KoolL Y. Kiyozumi, M. Salou and E Mizukami Raman spectroscopic studies of the templated synthesis of zeolites P.P.H.JM. Knops-Gerrits and M. Cuypers Preparation, characterization and catalytic activity of non-hydrothermally synthesized saponite-like materials R. Prihod'ko, M. Sychev, E.J.M. Hensen, J.A.R. van Veen and R.A. van Santen Self-bonded A1, B-ZSM-5 pellets C. Perri, P. De Luca, D. Vuono, M. Bruno, J. B.Nagy and A. Nastro Syntheses and characterization of A1, B-LEV type zeolite from systems containing methyl-quinuclidinium ions D. Violante, P. De Luca, C.V. Tuoto, L. Catanzaro, M. Bruno, J. B.Nagy and A. Nastro Synthesis and ion exchange properties of the ETS-4 and ETS-10 microporous crystalline titanosilicates C.C. Pavel, D. Vuono, A. Nastro, J. B.Nagy and N. Bilba Quasiisothermal degradation kinetics of tetrapropylammonium cations in silicalite-1 matrices O. Pachtova, M. Kodigik, B. Bernauer and E Bauer Cationic silver clusters in zeolite rho and sodalite J. Michalik, J. Sadlo, M. Danilczuk, J. Perlinska and H. Yamada The first example of a small-pore framework hafnium silicate Z. Lin and J. Rocha Synthesis, characterization and catalytic activity of vanadium-containing ETS-10 P. Brand, o, A.A. Valente, J. Rocha and M. W. Anderson Infrared evidence for the reversible protonation of acetonitrile at high temperature in mordenite J. Czyzniewska, S. Chenevarin and E Thibault-Starzyk Spectroscopic and catalytic studies on Cu-MCM-22: effect of copper loading A.J.S. Mascarenhas, H.O. Pastore, H.M.C. Andrade, A. Frache, M. Cadoni and L. Marchese Preparation and properties of MFI zincosilicate S. Kowalak, E. Szymkowiak, M. Gierczyfiska and G. Giordano Influence of Cs loading and carbonates on TPR profiles of PtCsBEA L. Stievano, C. Caldeira, M.E Ribeiro and P. Massiani New evidences for the fluoride contribution in synthesis of gallium phosphates V.1. Pdrvulescu, C.M. Visinescu, M.H. Zahedi-Niaki and S. Kaliaguine

239 247

255 263

271 279

287

295

303 311 319 327

335 343

351 359 367

xvi High-field ESR spectroscopy of Cu(I)-NO complexes in zeolite CuZSM-5 A. Pb'ppl and M. Hartmann

375

Characterization of acid sites in dehydrated H-Beta zeolite by solid state NMR E'. Montouillout, S. Aiello, E Fayon and C. Fernandez

383

Characterization and quantification of aluminum species in zeolites using high-resolution 27A1 solid state NMR A.A. Quoineaud, E Montouillout, S. Gautier, S. Lacombe and C. Fernandez Control of AFI type crystal synthesis with additional gel components J Kornatowski, G. Zadrozna and JA. Lercher

391 399

Synthesis and characterization of mordenite (MOR) zeolite derived from a layered silicate, Na-magadiite T. Selvam and W. Schwieger

407

Hydrothermal synthesis and characterization of new phosphate-based materials prepared in the presence of 1,4-dimethylpiperazine L. Josien, A. Simon-Masseron, S. Fleith, E Gramlich and J Patarin

415

Modeling of crystal growth at early stages of analcime synthesis from clear solutions B. Suboti~, R. Aiello, J. Bronik and E Testa

423

Synthesis of zincosilicate molecular sieve VPI-7 using vapor phase transport J Dong, C.E Xue and G. Liu

431

Combined IR and catalytic studies of the role of Lewis acid sites in creating acid sites of enhanced catalytic activity in steamed HZSM-5 J Datka, B. Gil, P. Baran and B. Staudte Heterogeneity of Cu + in CuZSM-5, TPD-IR studies of CO desorption J Datka and P. Kozyra

439 445

Speciation and structure of cobalt carbonyl and nitrosyl adducts in ZSM-5 zeolite investigated by EPR, IR and DFT techniques P. Pietrzyk, Z. Sojka, B. Gil, J Datka and E. Broctawik

453

Spectroscopic and catalytic behaviour of [015-CsHs)Rh(TI4-1,5-CsH12)] in Mt56Y and Hs6Y (M ' = Li, Na, K, Rb and Cs) E.C. de Oliveira, R.G. da Rosa, H. 0. Pastore

461

Improved synthesis procedure for Fe-BEA zeolite D. Aloi, E Testa, L. Pasqua, R. Aiello and J B.Nagy One-step benzene oxidation to phenol. Part I: preparation and characterization of Fe-(A1)MFI type catalysts G. Giordano, A. Katovic, S. Perathoner, E Pino, G. Centi, J B.Nagy, K. Lazar and P. Fejes

469

477

xvii CATALYSIS

From micro to mesoporous molecular sieves: adapting composition and structure for catalysis 487 A. Corma and M.T. Navarro (PLENARY LECTURE) One step benzene oxidation to phenol. Part II: catalytic behavior of Fe-(A1)MFI zeolites 503 S. Perathoner, F. Pino, G. CentL G. Giordano, A. Katovic, J. B.Nagy, K. Lazar and P. Fejes Synthesis, structure, and reactivity of iron-sulfur species in zeolite ZSM-5 511 R. W. Joyner, M. Stockenhuber and O.P. Tkachenko Characterization of FeMCM-41 and FeZSM-5 catalysts to styrene production 517 J.R.C. Bispo, A.C. Oliveira, M.L.S. Corr~a, J.L.G. Fierro, S.G. Marchetti and M. C. Rangel Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts 525 N.G. Gallegos, M.V. CagnolL J.E Bengoa, A.M. Aloarez, A.A. Yeramidm and S. G. Marchetti On the necessity of a basic revision of the redox properties of H-Zeolites 533 Z. Sobalik, P. Kubdmek, O. Bortnovsky, A. Vondrovdt, Z. Tva~Skovr, JE. Sponer and B. Wichterlovdt The role of zeotype catalyst support in the synthesis of carbon nanotubes by CCVD 541 K. Hernadi, Z. Krnya, A. Siska, J Kiss, A. Oszkr, J. B.Nagy and I. Kiricsi The influence of water on the activity of nitridated zeolites in base-catalyzed reactions 549 S. Ernst, M. Hartmann, T. Hecht, P. Cremades Ja~,n and S. Sauerbeck Selective catalytic reduction of N20 with light alkanes and N20 decomposition over Fe-BEA zeolite catalysts 557 T. Nobukawa, K. Kita, S. Tanaka, S. Ito, T. Miyadera, S. Kameoka, K. Tomishige and K. Kunimori Hydroxymethylation of 2-methoxyphenol catalyzed by H-mordenite: analysis of the reaction scheme 565 E Caoani, L. Dal Pozzo, L. Maselli and R. Mezzogori Unraveling the nature and location of the active sites for butene skeletal isomerization over aged H-Ferrierite 573 S. van Donk, E. Bus, A. Broersma, J.H. Bitter and K.P. de Jong Hydroconversion of aromatics over a Pt-Pd/USY catalyst 581 C. Petitto, G. Giordano, E Fajula and C. Moreau Hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of gas oils in one step on Pt,Pd/H-USY 587 Z Varga, J. Hancsrk, G. Tolvaj, W.I. Horvrth and D. Kall6 Reformate upgrading to produce enriched BTX using noble metal promoted zeolite catalyst 595 S.H. Oh, K.H. Seong, Y.S. Kim, S. ChoL B.S. Lim, J.H. Lee, J. Woltermann and Y.E Chu

xviii Dehydroisomerization of n-butane to isobutene over Pd/SAPO-11. The effect of Si content of SAPO-11, catalyst preparation and reaction condition Y. Wei, G. Wang, Z Liu, P Xie and L. Xu Vapor phase propylene epoxidation over Au/Ti-MCM-41 catalyst: influence of Ti grafting A.K. Sinha, T. Akita, S. Tsubota and M. Haruta Intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta U Wilkenh6ner, D.W. Gammon and E. van Steen The effect of zeolite pore size and channel dimensionality on the selective acylation of naphtalene with acetic anhydride J (~ejka, P Prokegovd, L. Ceroenfi and K. Mikulcovd Alkylation of phenol with methanol over zeolite H-MCM-22 for the formation of p-cresol G. Moon, K.P M6ller, W. B6hringer and C.T. O'Connor Relative stability of alkoxides and carbocations in zeolites. QM/MM embedding and QM calculations applying periodic boundary conditions L.A. Clark, M. Sierka and J. Sauer H-Beta zeolite for acylation processes: optimization of the catalyst properties and reaction conditions P Botella, A. Corma, E Rey and S. Valencia Aniline methylation on modified zeolites with acidic, basic and redox properties I.L Ioanova, O.A. Ponomoreva, E.B. Pomakhina, E.E. Knyazeva, V.V. Yuschenko, M. Hunger and J. Weitkamp Aldol condensation catalyzed by acidic zeolites T. Komatsu, M.Mitsuhashi and T. Yashima Role of intracrystalline tunnels of sepiolite for catalytic activity Y. Kitayama, K. Shimizu, T. Kodama, S. Murai, T. Mizusima, M. Hayakawa and M. Muraoka Catalytic wet oxidation of reactive dyes with H202 over mixed (A1-Cu) pillared clays S.-C. Kim, D.-S. Kim, G.-S. Lee, J.-K. Kang, D.-K. Lee and Y.K. Yang Application of zeolites as supports for catalysts of the ethylene and propylene polymerization I.N. Meshkova, T.A. Ladygina, T.M. Ushakova, N. Yu. Kovaleva and L.A. Novokshonova Catalytic properties of beta zeolite exchanged with Pd and Fe for toluene total oxidation J. Jacquemin, S. Siffert, J.-E Lamonier, E. Zhilinskaya, A.Aboukai's Hydroisomerization of n-Butane over Pd/HZSM-5 and Pd/Hmordenite with and without binder P Ca~izares, E Dorado, P Shnchez and R. Romero Butane isomerization on several H-zeolite catalysts S. De RossL G. Moretti, G. Ferraris and D. Gazzoli Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene JL. Valverde, A. de Lucas, P Sdnchez, E Dorado and A. Romero

603

611 619

627

635

643

651 659

667 675

683

691 699

707 715 723

xix Influence of cocations on the activity of Co-MOR for NO/N20 SCR by propene I. Asencio, E Dorado, JL. Valverde, A. De Lucas and P. Sdnchez Catalytic performance of mesoporous silica SBA-15-supported noble metals for thiopene hydrodesulfurization M. Sugioka, T. Aizawa, Y. Kanda, T. Kurosaka, Y. Uemichi and S. Namba

731

739

Skeletal isomerization of 1-hexene to isohexenes over zeolite catalysts Z. Wu, Q. Wang, L. Xu and S. Xie

747

Preparation and catalytic characterisation of Al-grafted MCM-48 materials M. Rozwadowsla', M. Lezanska, J Wloch, K. Erdmann and J Kornatowski

755

Photoreduction of incorporated molecules in zeolite X: methylviologen K.T. Ranjit and L. Kevan

763

Effective utilization of residual type feedstock to middle distillates by hydrocracking technology S.K. Saha, G.K. Biswas and D. Biswas

771

Direct analysis of deactivated catalysts in 1-pentene isomerization by high-resolution fast atom bombardment mass spectrometry J.M. Campelo, E Lafont and J.M. Marinas

781

Selection of an active zeolite catalyst and kinetics of vapor phase esterification of acetic acid with ethyl alcohol A.M. Aliyev, E.E. Sarijanoo, O. Tun 9 Sava~gi, R.Z. Mikailov, T.N. Shakhtakhtinsky, A. Sario~lan, P.E Poladly and A.R. Kuliyeo Hydrodesulfurization of dibenzothiophene over Mo-based catalysts supported by siliceous MCM-41 A. Wang, Y. Wang, Y. Chen, X. Li, P. Yao and T. Kabe Acylation of 2-methoxynaphtalene over ion-exchanged ~-Zeolite ]. C. Kantarh, L. Artok, H. Bulut, S. Ydmaz and S. Olkii Development of new ZSM-5 catalyst-additives in the fluid catalytic cracking process for the maximization of gaseous alkenes yield A.A. Lappas, C.S. Triantafillidis, Z.A. Tsagrasouli, V.A. Tsiatouras, I.A. Vasalos and N.P. Evmiridis Characterization of H and Cu mordenites with varying SIO2/A1203 ratios, by optical spectroscopy, MAS NMR of 29Si, 27A1 and 1H, temperature programmed desorption and catalytic activity for nitrogen oxide reduction V. Petranovskii, R.E Marzke, G. Diaz, A. Gomez, N. Bogdanchikova, S. Fuentes, N. Katada, A. Pestryakov and V. Gurin A comparison of SAPO, GaPSO, MgAPO and GaPO's as DeNOx catalysts V.I. Pdrvulescu, M. Alifanti, M.H. Zahedi-Niaki, P. Grange and S. Kaliaguine The influence of textural properties of MFI type catalysts on deactivation phenomena during oligomerization of butenes G. Giordano, E Cavani and E Trifir6

787

795 799

807

815

823

831

XX

Dehydrogenation of propane over various chromium-modified MFI-type zeolite catalysts V.A. Tsiatouras, T.K. Katranas, C.S. Triantafillidis, A.G. Vlessidis, E.G. Paulidou and N.P Evmiridis Effect of Pd addition on the catalytic performance of H-ZSM-5 zeolite in chlorinated VOCs combustion R. Lrpez-Fonseca, S. Cibridm, J.1. Guti~rrez-Ortiz and J.R. Gonzdtlez-Velasco Influence of the amount and the type of Zn species in ZSM-5 on the aromatisation of n-hexane A. Smie~kovLt, E. Rojasovr, P Hudec, L. Sabo and Z. Zidek Simultaneous desulfurization and isomerization of sulfur containing n-pentane fractions over Pt/H-mordenite catalyst J. Hancsrk, A. Holl6, I. Valkai, Gy. Szauer and D. Kall6 Propylene polymerization using various metal-containing MCM-41 as cocatalyst T. Miyazaki, Y. Oumi, T. Uozumi, H. Nakajima, S. Hosoda and T. Sano Oxidation of cyclohexene catalyzed by manganese(III) complexes encapsulated in two faujasites M. Silva, R. Ferreira, C. Freire, B. de Castro and JL. Figueiredo Heavy aromatics upgrading using noble metal promoted zeolite catalyst S.H. Oh, S.I. Lee, K.H. Seong, Y.S. Kim, JH. Lee, J Woltermann, WE. Cormier and Y.E Chu Preparation of iron-doped titania-pillared clays and their application to selective catalytic reduction of NO with ammonia D.-K. Lee, S.-C Kim, S.-J. Kim, J.-K. Kang, D.-S. Kim and S.-S. Oh Sulfated Zr-pillared saponite: preparation, properties and thermal stability L. Bergaoui, A. Ghorbel and J.-E Lambert Isomerization and hydrocracking of n-decane over Pt-Pd/A1MCM-41 catalysts S.P. Elangovan, C Bischof and M. Hartmann Influence of nickel metal distribution in Ni/Y-zeolite on the reactivity toward CO hydrogenation D.-S. Kim, S.-C. Kim, S.-J Kim and D.-K. Lee Hydrodechlorination of chlorinated compounds on different zeolites B. Imre, Z. Krnya, I. Hannus, J. Halrsz, J B.Nagy and I. Kiricsi Ammoxidation of ethylene into acetonitrile over Co-zeolites catalysts M. Mhamdi, S. Khaddar-Zine and A. Ghorbel Physicochemical characterization of vanadium-containing K10 epoxidation catalyst I. Khedher, A. Ghorbel and A. Tuel Conversion of aromatic hydrocarbons over MCM-22 and MCM-36 catalysts E. Dumitriu, I. Fechete, P. Caullet, H. Kessler, V. Hulea, C. Chelaru, T. Hulea and X. Bourdon HE-DE exchange and migration of Ga in H-ZSM5 and H-MOR zeolites M. Garcia-Sanchez, P. Magusin, E.JM. Hensen and R.A. van Santen

839

847

855

863 871

879 887

895 903 911

919 927 935 943 951

959

xxi Catalytic conversion of trichloroethylene over HY-zeolite E. Finocchio, C. Pistarino, P. Comite, E. Mazzei Justin, M. Baldi and G. Busca FT-IR studies of internal, external and extraframework sites of FER, MFI, BEA and MOR type protonic zeolite materials G. Busca, M. Beoilacqua, T. Armaroli and M. Trombetta NO reduction with isobutane on Fe/ZSM-5 catalysts prepared by different procedures M.S. Batista and E.A. Urquieta-Gonzdlez

967

975 983

Catalytic and infrared spectroscopic study of NO+CO reaction over iron-containing pillared montmorillonite 991 E L6nyi, J. Valyon and I. Kiricsi A study on alkylation of naphtalene with long chain olefins over zeolite catalyst 999 H. Guo, Y. Liang, W. Qiao, G. Wang and Z. Li Synthesis of anthraquinone from Phthalic Anhydride with Benzene over Zeolite Catalyst 1007 Y. Wang, W.-R. Miao, Q. Liu, L.-B. Cheng and G.-R. Wang Simultaneous hydrogenation and ring opening of aromatics for diesel upgrading on Pt/zeolite catalysts. The influence of zeolite pore topology and reactant on catalyst performance M.A. Arribas, A. Martinez and G. Sastre Catalytic combustion of chlorobenzene over Pt/zeolite catalysts S. Scird, S. Minicd, C. CrisafullL G. Burgio and V. Giuffrida Ag and Co exchanged ferrierite in lean NOx abatement with CH4 P. Ciambelli, D. Sannino, M.C. Gaudino and M. Flytzani-Stephanopoulos The effect of sulfate ion on the synthesis and stability of mesoporous materials M.L. Guzmdn-Castillo, H. Armend6riz-Herrera, A. Tob6n-Ceroantes, D.R. Acosta, P. Salas-Castillo, A. Montoya de la F. and A. Vfzquez-Rodriguez Catalytic behavior of Cd-clinoptilolite prepared by introduction of cadmium metal onto cationic sites G. Onyestydtk and D. Kall6

1015 1023 1031 1039

1047

MESOPOROUS M O L E C U L A R SIEVES

Confinement at nanometer scale: why and how? E Di Renzo, A. Galarneau, P. Trens, N. Tanchoux and E Fajula (PLENARY LECTURE)

1057

Anchorage of dye molecules and organic moieties to the inner surface of Si-MCM-41 Y. Rohlfing, D. W6hrle, J. Rathousk~, A. Zukal and M. Wark Mesocellular aluminosilicate foams (MSU-S/F) and large pore hexagonal mesostructures (MSU-S/H) assembled from zeolite seeds: hydrothermal stability and properties as cumene cracking catalysts Y. Liu and T.J Pinnaoaia

1067

1075

xxii Fabrication of large secondary mesopores in MCM-41 particles assisted by aminoacids and hydrophobic functional groups 1083 I. Diaz and J P~rez-Pariente

Hexagonal and cubic thermally stable mesoporous Tin(IV) phosphates with acidic, basic, and catalytic properties C. Serre, A. Auroux, A. Geruasini, M. Hervieu, G. Ferey

Characterization of [Cu]-MCM-41 by XPS and CO or NO adsorption heat measurements

1091 1101

M. Broyer, J.P Bellat, O. Heintz, C. Paulin, S. Valange and Z Gabelica

Synthesis and characterisation of iron-containing SBA-15 mesoporous silica

1109

E Martinez, Y.-J Han, G. Stueky, JL. Sotelo, G. Ouejero and JA. Melero

Synthesis and characterization of mesoscopically ordered surfactant/co-surfactant templated metal oxides

1117

T. Czuryszla'ewicz, J. Rosenholm, E Kleitz and M. Linden

Preparation of novel organic-inorganic hybrid micelle templated silicas. Comparison of different routes for materials preparation D.J. Maequarrie, D.B. Jackson, B.L. King and A. Watson

Structure and catalytic performance of cobalt Fischer Tropsch catalysts supported by periodic mesoporous silicas

1125 1133

A. E Khodakou, R. Bechara and A. Gribooal-Constant

Highly dispersed VOx species on mesoporous supports: promising catalysts for the oxidative dehydrogenation (ODH) of propane

1141

A. Briickner, P Rybarczyk, H. Kosslick, G.-U. Wolf and M. Baerns

Modelling mesoporous materials

1149

M. W. Anderson, C.C. Egger, G.JT. Tiddy and J.L. Casei

Acidity and thermal stability of mesoporous aluminosilicates synthesized by cationic surfactant route

1157

M. Derewinski, M. Machowska and P Sarv

Mesoporous silicate as matrix for drug delivery systems of non-steroidal antinflammatory drugs

1165

R. Aiello, G. Cauallaro, G. Giammona, L. Pasqua, P Pierro and E Testa

Aluminum incorporation and interracial structures in A1SBA-15 mesoporous solids: double resonance and optically pumped hyperpolarized 129XeNMR Studies

1173

E. Haddad, J.-B. d'Espinose, A. Nossov, E Guenneau and A. G~d~on

Tailoring the pore size of hexagonally ordered mesoporous materials containing acid sulfonic groups

1181

R. van Grieken, J.A. Melero and G. Morales

Novel vesicular mesoporous material templated by catanionic surfactant self-assembly 1189 X. W. Yan and J.H. Zhu

Preparation and characterization of Co-Fe-Cu mixed oxides via hydrotalcite-like precursors for toluene catalytic oxidation J. Carpentier, J.-E Lamonier, S. Siffert, H. Laversin, and A. Aboukai's

1197

xxiii Catalytic oxidation over transition metal doped MCM-48 molecular sieves C. WeL Q. Cai, X. Yang, 14(.Pang, Y. Bi and K. Zhen Highly selective oxidation of aromatic hydrocarbons (styrene, benzene and toluene) with H202 over Ni, Ni-Cr and Ni-Ru modified MCM-41 catalysts V..Parvulescu, C. Anastasescu, C. Constantin and B.L. Su Mesoporous materials as supports for heteropolyacid based catalysts M. Gulbihska, M. wrjtowski and M. Laniecki Synthesis and characterization of A1-MCM-48 type materials using coal fly ash P. Kumar, N.K. Mal, Y. OumL T Sano and K. Yamana Synthesis of well-aligned carbon nanotubes on MCM-41 W. Chen, A.M. Zhang, X. Yan and D. Han Synthesis and characterization of CuO and Fe203 nanoparticles within mesoporous MCM-41/-48 silica C. Minchev, R. Krhn, T. Tsoncheva, M. Dimitrov, 1. Mitov, D. Paneva, H. Huwe and M. Frrba Study of the porosity of montmorillonite pillared with aluminum/cerium M.J. Hernando, C. Blanco, C. Pesquera and E Gonzdlez X-ray absorption fine structure investigation of MCM-41 materials containing Pt and PtSn nanoparticles prepared via direct hydrothermal synthesis C. Pak, N. Yao and G.L. Hailer Ordered assembling of precursors of colloidal faujasite mediated by a cationic surfactant J Agfindez, 1. Diaz, C. Mdrquez-Alvarez, E. Sastre and J P~rez-Pariente Synthesis, characterisation and catalytic activity of SO3H-phenyl-MCM-41 materials F. Mohino, I. Diaz, J. POrez-Pariente and E. Sastre Synthesis of ordered mesoporous and microporous aluminas: strategies for tailoring texture and aluminum coordination V. Gonzdlez-Pe~a, C. Mdrquez-Alvarez, E. Sastre and J P~rez-Pariente Characterization of a heteropolyacid supported on mesoporous silica and its application in the aromatization of a-pinene H. Jaramillo, L.A. Palacio and L. Sierra Catalytic activity, deactivation and re-use of A1-MCM-41 for N-methylation of aniline J.M. Campelo, R.M. Leon, D. Luna, J.M. Marinas and A.A. Romero Restructured V-MCM-41 with non-leaching vanadium and improved hydrothermal stability prepared by secondary synthesis N.K. Mal, P. Kumar, M. Fujiwara and K. Kuraoka Comparative study of MCM-41 acidity by using the integrated molar extinction coefficients for infrared absorption bands of adsorbed ammonia A. Taouli and W. Reschetilowski Confinement of nematic liquid crystals in SBA mesoporous materials L. Frunza, S. Frunza, A. Schrnhals, U. Bentrup, R. Fricke, 1. Pitsch and H. Kosslick Synthesis and characterization of bimetallic Ga,A1-MCM-41 and Fe,A1-MCM-41 R. Bfrjega, C. Nenu, R. Ganea, Gr. Pop, S. "-erban and T. Blasco

1205

1213 1221 1229 1237

1245

1253

1261 1267 1275

1283

1291 1299

1307

1315 1323 1331

xxiv Fischer-Tropsch synthesis on iron catalysts supported on MCM-41 and MCM-41 modified with Cs A.M. Alvarez, J E Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramidn and S. G. Marchetti Coordination and oxidation states of iron incorporated into MCM-41 K. Ldzdr, G. Pdl-Borb~ly, A. Szegedi and H.K. Beyer Synthesis and characterization of In-MCM-41 mesoporous molecular sieves with different Si/In ratios W. Brhlmann, O. Klepel, D. Michel and H. Papp The effect of niobium source used in the synthesis on the properties of NbMCM-41 materials 1. Nowak Inclusion of europium(III) ~-diketonates in mesoporous MCM-41 silica A. Fernandes, J. Dexpert-Ghys, C. Brouca-Cabarrecq, E. Philippot, A. Gleizes, A. Galarneau and D. Brunel Synthesis, characterization and catalytic properties of mesoporous titanostanno silicate, Ti-Sn-MCM-41 N.K. Mal, P. Kumar, M. Fujiwara and K. Kuraoka Alternative synthetic routes for NiA1 layered double hydroxides with alkyl and alkylbenzene sulfonates R. Trujillano, M.J. Holgado and V. Rives Spectroscopic studies on aminopropyl-containing micelle templated silicas. Comparison of grafted and co-condensation routes D. Brunel, A.C. Blanc, E. Garrone, B. Onida, M. Rocchia, JB.Nagy and D. J Macquarrie Preparation, characterization, stability and catalytic reactivity of the 3d transition metals incorporated MCM-41 molecular sieves V. Pdrvulescu and B.L. Su Amine-functionalized SiMCM-41 as carrier for heteropolyacid structures L. Pizzio, P. Vdzquez, A.Kikot and E.Basaldella Acidity of mesoporous aluminophosphates and silicas MCM-41. A combined FTIR and UV-Vis-NIR study E. GianottL V. Dellarocca, E.C. Oliveira, S. Coluccia, H.O. Pastore and L. Marchese Modification of silica walls of mesoporous silicate and alumino-silicate by reaction with benzoyl chloride L. Pasqua, E Testa, R. Aiello, G. Madeo and J. B.Nagy

1339

1347

1355

1363 1371

1379

1387

1395

1403 1411

1419

1427

ADVANCED MATERIALS AND APPLICATIONS Options for the design of structured molecular sieve materials J. Sterte , J. Hedlund and L. Tosheoa (KEYNOTE)

1437

XXV

Chromium containing zeolite beta macrostructures V. Naydenov, L. Tosheva and J Sterte Semiconductor nanoparticles in the channels of mesoporous silica and titania thin films M. Wark, H. Wellmann, J Rathousk~ and A. Zukal Spin-coating induced self-assembly of pure silica and Fe-containing mesoporous films N. Petkov, S. Mintooa and T. Bein Guanidine catalysts supported on micelle templated silicas. New basic catalysts for organic chemistry D.J. Macquarrie, K.A. Utting, D. Brunel, G. Renard and A. Blanc Attempts on generating basic sites on mesoporous materials X.W. Yan, X. W. Han, W.Y. Huang, J.H. Zhu and K. Min Application of zeolite in the health science: novel additive for cigarette to remove N-nitrosamines in smoke Z Xu, Y Wang, JH. Zhu, L.L. Ma, L. Liu and J Xue Direct synthesis of ZSM-5 crystals on gold modified by zirconiumphosphonate multilayers S. Dumrul, J Warzywoda and A. Sacco, Jr. Square root relationship in growth kinetics of silicalite-1 membranes P. Nov6k, L. Brabec, O. Solcov6, O. Bortnovsky, A. Zik[mov6 and M. Ko6i~ik Transport characteristics of zeolite membrane from dynamic experiments A. Zikfnov6, B. Bernauer, V. Fila, P. Hrab6nek, J Hradil, V. Krystl and M. Ko6ifik Incorporation of zeolites in polyimide matrices P. Sysel, M. Fry6ov6, R. Hobzov6, V. Krystl, P. Hrab6nek, B. Bernauer, L. Brabec and M. Ko6i~ik The formation mechanism of ZSM-5 zeolite membranes Y LL J. ShL J Wang and D. Yan Mesoporous molecular sieves for albumin A.Y. Eltekov and N.A. Eltekova Characterizing the novel porous superbase K+/ZrO2 by probe adsorption: a Raman study W.Y. Huang, Y Wang, Q. Wang and Q. Yu The synthesis and characterization of zeolite ZSM-5 and ZSM-35 films by self-transformation of glass J. Dong, W. Fan, G. Liu and J. Li Preparation of mesoporous materials as a support for the immobilization of lipase A. Macario, V. Calabr~, S. Curcio, M. De Paola, G. Giordano, G. lorio and A. Katovic

1449 1457 1465

1473 1481

1489

1497 1505 1513 1521

1529 1537 1545

1553 1561

ADSORPTION, DIFFUSION, SEPARATION AND PERMEATION Adsorption and diffusion of linear and dibranched C6 paraffins in a ZSM-5 zeolite E. Lemaire, A. Decrette, JP. Bellat, JM. Simon, A. M~thioier and E. Jolimaftre

1571

xxvi Adsorption of indole and benzothiophene over zeolites with faujasite structure J.L. Sotelo, M.A. Uguina and V.1. Agueda

1579

Determination of microporous structure of zeolites by t-plot method - State-of-the-art 1587 P. Hudec, A. Smiegkovd, Z. Zidek, P. Schneider and O. Solcov6 Binary mixture adsorption of water and ethanol on silicalite Y. OumL A. Miyajima, J. Miyamoto and T. Sano

1595

Influence of water adsorption on zeolite Beta C. Flego, G. Pazzuconi and C. Perego

1603

Diffusion and adsorption of hydrocarbons from automotive engine exhaust in zeolitic adsorbents D. Caputo, M. Eik and C. Colella

1611

Kinetic processes during sorption and diffusion of aromatic molecules on medium pore zeolites studied by time resolved IR-spectroscopy H. Tanaka, S. Zheng, A. Jentys and J.A. Lercher

1619

Calorimetric study of C2H4 adsorption on synthetic zeolites with Na § and Ca 2§ cations 1627 1. V. Karetina, G.Ju. Zemljanova and S.S. Khvoshchev A1-MCM-48: synthesis and adsorption properties for water, benzene, and nitrogen M. Rozwadowski, M. Lezanska, R. Golembiewski, K. Erdmann and J. Kornatowsla"

1631

A frequency-response study of the kinetics of ammonia sorption in zeolite particles Gy. Onyestydk, J. Valyon and L. V.C. Rees

1639

An attempt to correlate the non-isothermal desorption behavior of heterocyclic compounds on a NaY zeolite B. Hunger, 1.A. Beta, C. Engler, E. Geidel, O. Klepel and H. B6hlig

1647

Determination of diffusion coefficient for Cu(II) retention on chemically activated clinoptilolite R. Pode, T. Todinca, A. lovL R. Radovet and G. Burticd

1655

Dynamics of sorption columns in dewatering of bioethanol using zeolites M. Boldi~, K. Melzoch, J. Pokorny and M. Ko~i~ik

1663

Adsorption properties of MCM-41 materials for the VOCs abatement G. Calleja, D.P. Serrano, J.A. Botas and EJ. Guti~rrez

1671

Adsorption of linear and branched paraffms in silicalite: thermodynamic and kinetic study I. Gener, J. Rigoreau, G. Joly, A. Renaud and S. Mignard

1679

Location and transport properties of ammonia molecules in a series of faujasite zeolite structures as studied by FT-IR and 2H-NMR spectroscopies 1687 E Gilles, J.-L. Blin, H. Toufar and B.L. Su Characterization of mesoporous solids: pore condensation and sorption hysteresis phenomena in mesoporous molecular sieves M. Thommes, R. K6hn and M. Fr6ba

1695

xxvii NATURAL ZEOLITES Ion exchange selectivity of phillipsite A.E GualtierL E. Passaglia and E. Galli Sorption of ammonia from gas streams on clinoptilolite impregnated with inorganic acids K. Ciahotn~, L. Melenov6, H. Jirglov6, M. Boldi~ and M. Ko6iHk Microtopographic features and dissolution behavior of natural zeolite surfaces studied by Atomic Force Microscopy (AFM) M. VoltolinL G. Artioli and M. Moret Occurrence and crystal structure of magnesian chabazite E. Passaglia and O. Ferro Treatment of urban dump leachates with natural zeolite packed bed column T. Rodriguez F., E. Acevedo del Monte, G. Mori and B. Rafuzzi Phosphorous removal from wastewater by bioaugmented activated sludge with different amounts of natural zeolite addition J. Hrenovic and D. Tibljas Zeolitized tufts as pedogenic substrate for soil re-building. Early evolution of zeolite/organic matter proto-horizons A. Buondonno, E. Coppola, M. BuccL G. Battaglia, A. Colella, A. Langella and C. Co#ella Neapolitan yellow tuff for the recovery of soils polluted by potential toxic elements in illegal dumps of Campania Region E. Coppola, G. Battaglia, M. BuccL D. Ceglie, A. Colella, A. Langella, A. Buondonno and C. Colella Application of Jordanian faujasite-phillipsite tuff in ammonium removal K.M. Ibrahim Evidence of the relationship occurring between zeolitization and lithification in the yellow facies of Campanian Ignimbrite (southern Italy) A. Langella, P. De Simone, D. Calcaterra, P. Cappelletti and M. de' Gennaro

1705

1713

1721 1729 1737

1743

1751

1759

1767

1775

ION E X C H A N G E AND MODIFICATION

Characterisation of iron containing molecular sieves - the effect of T-element on Fe species 1785 P. Decyk, M. Trejda, M. Ziolek and A. Lewandowska Thermal decomposition of sodium azide in various microporous materials 1793 Gy. Onyesty6k Modifying the acidic properties of PtZSM-5 and PtY zeolites by appropriately varying reduction methods 1801 A. Tam6sL K. Niesz, 1. P6link6, L. Guczi and 1. Kiricsi

xxviii Vibrational studies of iron phthalocyanines in zeolites P.P.H.JM. Knops-Gerrits, E Thibault-Starzyk and R. Parton

1809

The effect of dealumination on the A1 distribution in pentasil ring zeolites J Dgdedek, V. Grbov6 and B. Wichterlov6

1817

Pb(II) ion exchange on zeolite-supported magnetite. Characterization of process by effective diffusivity coefficient V. Pode, T. Todinca, R. Pode, V. Dalea and E. Popovici

1825

Galliation of beta zeolite by the pH control method Y. Oumi, S. KikuchL S. Nawata, T. Fukushima and T. Sano

1833

Ion exchange behaviour of two synthetic phillipsite-like phases C. Colella, B. de' Gennaro, B. Liguori and E. Torracca

1841

Competitive exchange of lead(II) and cadmium(II) from aqueous solution on clinoptilolite S. Berber-Mendoza, R. Leyva-Ramos, J. Mendoza-Barron and R.M. GuerreroCoronado Competitive ion exchange of transition metals in low silica zeolites C. Weidenthaler, Y Mao and W. Schmidt

1849

1857

STRUCTURE ANALYSIS AND MODELLING Computational methods for the design of zeolitic materials M. Elanany, K. Sasata, T. Yokosuka, S. Takami, M. Kubo and A. Miyamoto

1867

(KEYNOTE)

A theoretical investigation on pressure-induced changes in the vibrational spectrum of 1877 zeolite bikitaite E. Fois, A. Gamba, G. TabacchL O. Ferro, S. Quartieri and G. Vezzalini Flexible aluminium coordination of zeolites as function of temperature and water content, an in-situ method to determine aluminium coordinations J.A. van Bokhoven, A.M.J. van der Eerden and D.C. Koningsberger

1885

Structure analysis of boron-silicalite and of a "defect-free" MFI-silicalite by synchrotron radiation single crystal X-ray diffraction M. Milanesio, D. Viterbo, L. Palin, G.L. Marra, C. Lamberti, R. Aiello and E Testa

1891

Density functional theory modelling EPR spectra of Cu(II) in Y zeolite D. Berthomieu, J.M. Duc~r~ and A. Goursot

1899

Molecular modeling: a complement to experiment for designing porous materials used 1907 in separation technologies by adsorption S. Girard, C. Mellot-Draznieks, G. FOrey and P. Pullumbi NMR-crystallographic studies of aluminophosphate A1PO4-40 C.M. Morais, C. Fernandez, V. Montouillout, E Taulelle and J. Rocha

1915

xxix Structural characterization of borosilicates synthesized in the presence of ethylenediamine S. Zanardi, A. Alberti, R. Millini, G. Bellussi and G. Perego Molecular dynamics simulations of water confined in zeolites P. Demontis, G. Stara and G.B. Suffritti EXAFS and optical spectroscopy characterisation of silver within zeolite matrices S.G. Fiddy, N.E. Bogdanchikova, V.P. Petranooskii, J.S. Ogden and M.Avalos-Borja Molecular dynamics simulations of static and dynamic properties of water adsorbed in chabazite S. Jost, S. Fritzsche and R. Haberlandt Correlations in anisotropic diffusion of guest molecules in silicalite-1 S. Fritzsche and J. Kiirger A combined anomalous XRPD, EXAFS, IR, UV-Vis and photoluminescence study on isolated and clustered silver species in Y zeolite C. Prestipino, C. Lamberti, A. Zecchina, S. Cresi, S. Bordiga, L. Palin, A.N. Fitch, P. Perlo and G.L. Marra DFT and IR studies on copper sites in CuZSM-5: structure-redox conditionsdenox activity relationship E. Broctawik, J. Datka, B. Gil and P. Kozyra Diffusion of water in silicalite by molecular dynamics simulations: ab initio based interactions C. Bussai, S. Hannongbua, S.Fritzsche and R. Haberlandt Comparison of small size alumino- and borosilicates optimised by periodic Hartree-Fock A. V. Larin and D.P. Vercauteren Monte Carlo simulation of the temperature dependence of adsorption of nitrogen and oxygen by LiLSX zeolite S.R. Jale, D. Shen, M. Biilow and ER. Fitch Density functional theory calculations of Henry's constant for N2, 02 and Ar molecules in Ca-A and Ca-LSX zeolites G. De Luca, P. Pullumbi and N. Russo Author index Subject index

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

999

A Study on Alkylation of Naphthalene with Long Chain Olefins over Zeolite

Catalyst Haitao GUO, Yan LIANG, Weihong QIAO, Guiru WANG and Zongshi LI State Key Laboratory of Fine Chemicals, Dalian University of Technology No. 158 Zhong Shan Road, Dalian 116012, China

The liquid-phase alkylation reactions of naphthalene with long chain olefins(Cll-C12) over two different zeolite catalysts, HY and HI3, were investigated. It was found that zeolite HY showed higher activity and mono-alkylnaphthalene selectivity than that of HI3. Zeolite HY being modified by La 3+ and alkaline earth ions, Mg 2+, Ca2+, Sr2+, Ba 2+, were studied. Reaction results showed that zeolite HY modified by La 3+ and Mg 2+ would provide better activity, selectivity and catalyst stability than HY. The optimum loading amount was 1% for MgO and 7% for La203 respectively and the conversion of olefins was over 90% as well as the mono-alkylnaphthalene selectivity is 100% at 403K, 1.0Mpa, VHSV=10ml/g-cat and naphthalene/olefin/cyclohexane (molar ratio)=6/1/60. Key words: naphthalene, long chain olefins, zeolite, alkylation. 1. INTRODUCTION The long chain (C10-C14)alkylnaphthalenes are important intermediates in the synthesis of sulphonated alkylnaphthalenes with long chain olefins which are effective surfactants to be used in the field of the enhanced oil recovery, dyeing, weaveng and spinning. Alkylnaphthalenes currently are synthesized via the alkylation reaction of naphthalene with long chain(C10-C14) a-olefins using conventional Friedel-Craits catalysts such as HF, A1C13, which are highly corrosive and could not be easily recovered. In order to overcome these disadvantages and environmental limitations, therefore, it has to find some suitable, recyclable and environment-friendly solid acid catalysts [ 1]. Zeolites, proved to be promising solids for achieving highly shape-selective catalysis, have been extensively studied for the alkylation of mononuclear aromatic hydrocarbons [2-4]. Fraenkel et al. first investigated the gas phase alkylation of naphthalene[5-7] with methanol over H-ZSM-5, H-Mordenites and H-Y zeolites. Medium pore H-ZSM-5 showed a high 13-selectivity, but only of moderate activity, while the large pore zeolites, HY and H-M,

1000 owing to the existence of supercages providing enough space for multiple reactions would lead to lower 15-selectivity, and higher activity. Moreau et al. recently reported that HY, in terms of activity or [3-[3' selectivity, is a better catalyst than H-M or H-I3 for the selective synthesis of 2,6-dialkylnaphthalenes in the liquid phase isopropylation, cyclohexylation or tert-butylation [8-13]. The alkylation of benzene with 1-dodecene over a variety of catalysts was studied by Sivasakar and Thangaraj[14].They compared their selectivity at total convention of the olefin and observed that the conversion was incomplete over mordenite that gave rise to the highest selectivity in the 2-phenyl dodecane ; HI5 samples gave only slightly higher selectivity than A1C13 or silica-alumina. They also investigated the alkylation of benzene with a mixture of C10-C13Q-olefins over H-Mordenite, HIS, HY, Rare-earth Y and SIO2-A1203. Araujo et al.[15] studied the reaction about the alkylation of benzene with 1-dodecene over Rare-earth Y in a batch reactor. The main product is 2-LAB without the polymer and cracking products of olefins. The activity of the catalysts increased as the following sequence: LaCa~aY> CeCafNaY = NdCa/NaY> GdCa/NaY and the selectivity of 2-LAB is that: CeCa~aY > NdCa~aY-- GdCa~aY > LaCa/NaY. Though there are some papers about the alkylation of naphthalene with isopropene and the alkylation of benzene with 1-dodecene, the reaction about the alkylation of naphthalene with long chain (C10-C12) olefins has little been reported. In our study using for suitable catalysts for the synthesis of long chain alkylnaphthalene, the large pore, high activity zeolites HY and HI3 have been investigated. Our studies focused on the modification of the MgO and La203, and the mechanism of modification by NH3-TPD and IR were elucidated. 2. EXPERIMENAL

2.1 Preparation of Catalyst and Reactants The HY and HI5 were prepared by the conventional liquid-state ion-exchange method. The starting material was NaY(SiO2/A1203=5) and NalS(SiO2/A1203=27). NaY and Nal3 zeolite are respectively soaked in 0.6M aqueous solution of NH4NO3 with a ratio of 4ml/g and are stirred at 90D for lh. Aiter being repeated for four times, the zeolite was washed thoroughly with deionized water to get ride of any residual ions which may have been occluded in zeolite pores. The washed sample was then dried at 120~ and calcined in a flow of dried air at 540~ for 5 hours. The prepared HY zeolite was pressed, crushed and then sieved to 20-40 meshes. The modification of HY zeolite was carried out by equal-volume impregnation method by infusing with a certain concentration aqueous solution of alkaline-earth and rare-earth nitrite forl2 h. After this, the sample was dried at 120~ and then calcined at 540~ for 5h. 2.2 Catalysts Characterization Infrared spectra were recorded at room temperature on a Fourier transform infrared spectrometer (Nicolet Impact 410) with a resolution of 4cm "1 and 64 scans in the region from

1001 4000 to 400cm 1. The HY, HI3 and HY modified by Mg 2§ and La 3§ catalysts were pressed into a self-supporting wafer (ca.15mg-cm-2), and introduced into a quartz IR cell with CaF2 windows. The samples were pretreated in-situ in a stream of 30 ml/min He from RT to 773K and 5• 103pa for 90 minutes. The cell was subsequently cooled to room temperature and pyridine vapor was passed into the cell and adsorbed onto the zeolite for a period of 30min. After removal of the excess pyridine, the spectrum was recorded. Then the sample was evacuated at 473K for 30min and a corresponding spectrum was recorded to distinguish the acid site. The relative intensities of vibration bands at 1540cm 1 and 1450cm "1 were ascribed to the Br~nsted and Lewis acid site respectively. NH3 temperature-programmed desorption (NH3-TPD) was performed on a convention set-up equipped with a thermal conductivity detector (TCD). The catalyst charge was 0.2g with particle size of 20-60 meshes. The sample was first flushed with He (30ml/min) at 873K for 30min, then cooled to 423K and saturated with NH3 until equilibrium. It was then flushed with He (30ml/min) again until the integrator baseline was stable. NH3-TPD was then promptly started at a heating rate of 15K/min from 423 to 873K. All NH3-TPD profiled were deconvoluted into three peaks using a Gaussian and Lorentzian curve-fitting method.

2.3 Catalyst Evaluation and Reactants Catalyst evaluation was performed in 20mm I.D. stainless steal tube reactor fixed-bed continuous-flow reactor. The catalytic reaction was adopted liquid-solid phase's catalytic reaction and the cyclohexane taken as a solvent to solute naphthalene and olefin. The reaction mixture was fed into reactor by a quantity pump and increase the pressure by N2. The reaction conditions were T=403K, P=l.0Mpa, VHSV=I 0ml/h-gcat. The products were quantitatively analyzed by GC(HP6890) using FID furnished with HP-5 30m capillary column and the composition was qualitatively confirmed by GC-MS(HP6890/5973). The conversion of olefins was defined as CL%, which is the wt% of olefins consumed in the reaction. The selectivity of mono-alkylnaphthalene was calculated by: SAN----MAN/MN,where MAN is the amount of mono-alkylnaphthalene and MN is the total amount of products. Analytical grade naphthalene, cyclohexane, NH4NO3, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2 and La(NO3)3 were used without purification. Industrial grade of Cll-C12(wt%=45/55) a-olefins mixture were used. 3. RESULTS AND DISCUSSION

3.1 The comparison of the catalytic activity of HY and HI3 in aikylation reaction The reaction results over HY and HI3 zeolite catalysts were listed in Table 1. It is obvious to observe that HY showed higher activity and selectivity than that of HI3. This result can be explained by the TPD profiles of the two samples presented in Fig. 1. The higher Si/A1 ratio HI3 sample exhibited three NH3 desorption maxima at ca.270~ 400~ and 600~ and they could be assigned to the site of weak, moderate-strong and strong acidity respectively. The HY sample also showed site of weak acidity at ca.270~ but a relatively broader range of moderate-strong acidity site between 350~ and 480~ Moreover, the amount of acid site

1002 determined by the ammonia desorption was much higher for HY zeolite than that for HI}, which was in agreement with their activity and A1 content. This suggested that the catalyst's activity was proportionally increasing with the amount of acid site, especially with the amount of moderate-strong acidity site. The in-situ IR spectra of pyridine adsorbed on different zeolite catalysts were showed in Fig.2, from which one could readily find that HY contained more Brq~nsted acidity site(1540cm -1) than that of HI3, as the amount of Lewis acidity site (1450cm -1) almost remained the same. So the Brqmsted acidic site was of advantage to this reaction. It also can be seen in Table 1 that in the reaction catalyzed by HI3, there might be room considerable amounts of oligo-alkylnaphthalene(11.99%) and olefins polymer(22.96%) in the products in the reaction catalyzed by HY, they were 0 and 2.3% respectively. As observed in the TPD profiles of HY and HI3 samples in Fig. 1, there existed a strong acidity site at 600D in HI3 sample but not in HY, which might result in the low mono-alkylnaphthalene selectivity of HI3 since the strong acidity site could lead to the cracking and polymeric reactions. For HY, 2.3% olefins polymer may be produced over the relatively strong acidity site along with the moderate-strong acidity site, which was a part of strong Brq~nsted acidic site in HY. Additionally, the pore size of HY was about 0.8-0.9nm, which was proper for the products to diffuse out of the pre channel, while that of HI3 was about 0.6-0.75nm, which was small for alkylnaphthalene and long chain olefins. The long staying time in the pore passage of HI3 may cause the products to crack to oligo-alkylnaphthalene or cause the olefins to polymerize, that is why the mono-alkylnaphthalene selectivity of HI3 was lower than that of HY. Table 1. The initial activity of the catalysts OligoOlefins MonoConversion of Catalysts o o ..................................A!~!naphth~.ene .. ..................P o ! ~ e r ...............~!naphtha!ene(S~..%.) ...............O!efions_(CL.%)........ HY 0 2.3 97.70 85.22 HI3 11.99 22.96 65.05 71.21 L

7

0

I

I

I

I

t

I

100

200

303

430

500

6130

700

t enlDerat ure( i )E

Figure 1. NH3-TPD profiles of HY and HI3 samples

1003

a-HI3 b-HY c-MgHY(7%) d-LaHY(7%)

lsoo'

' ]7oo

.... Wavenumbers(cm

16oo ' ~

i5oo

,4oo

"1)

Figure 2. In-situ IR spectra of pyridine adsorbed on different zeolite catalysts 3.2 The effect of the modification by alkaline-earth ions to the alkylation Alkylation reaction results over the modified HY with different alkaline-earth ions were given in Table 2. It can be found that as the alkaline-earth base property increased, the conversion of olefins decreased and the selectivity of the mono-alkylnaphthalene intially increased and then decreased slightly. NH3-TPD of HY before and after modification by the different alkaline-earth ions were given in Fig.3. It can be seen that the amount of medium-strong acidity site of HY were decreased, at the same time that amount of weaker acidity site increased aiter modification except MgHY(I%). The total amount of HY zeolite acidity site was not obviously changed. Therefore, a reasonable explanation for the decrease of the activity with base property in Table 2 was that the medium-strong acidity site were covered by alkaline-earth ions and turned to weaker acid centers, which were not favorable to the alkylation reaction.

Table2. The effect of the modification by alkaline-earth ions SAN% Catalysts . CL% 97.70 85.22 HY 100 85.O9 Mg-HY (1%) 100 84.86 Ca-HY (1%) 97.86 83.06 Sr-HY (1%) 97.27 82.45 Ba-HY (1%) i , , ,

1004

Figure 3. NH3-TPD profiles of HY with different alkaline-earth ions The effect of the MgO content on the alkylation reaction was further investigated as shown in Fig.4 and Fig.5. The conversion of olefins decreased gradually with the exception by an additive amount 1% MgO, this was in agreement with the decrease of amount of moderate-strong acidity site as shown in the TPD profiles. Therefore a certain amount of moderate-strong acid sites present is necessary in this reaction. It was worthwhile to notice that after modification the selectivity of mono-alkylnaphthalene all increased to 100%. This might be due to that some moderate-strong acidity site on HY, especially the relatively strong acidity site that resulted in the polymeric reaction, was masked by MgO. We can deduce from the IR spectra (Fig.2) of MgHY(7%) that these suppressed acidic sites were to be Brqmsted acidic sites. 103

8O

hY

i

"'-"

--'~HY(I~

I 6O o~

cr

40

--4k--~

20

- - e - ~H~( 1~ ~H~( 7o~ - - 0 - - I~l-ff( 129~

0 2

r

,

,

,

I

4

6

8

10

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Figure 4. The effect of MgO content on the alkylation reaction

0

tO3

I

i

t

2130 3130 430

t

i

500

600

703

803

t e n ~ r at ur e( i ~E

Figure 5. NH3-TPD profiles of MgHY with different MgO content

1005 3.3 The effect of the modification by LazO3 The effect of the loading amount of La203 to the alkylation reaction can be seen from Fig.6. After modification by La203, the activity of the catalyst increased except for LaHY(1%) and the stability of the catalyst also enhanced greatly. During a continuous reaction time of 10h, the activities of the modified HY persistently increased and the conversion of olefins were over 90% however, the conversion of olefins began to fall after 6h for unmodified HY. As can be seen from the IR spectra of LaHY(7%) in Fig.2, it was quite similar to that of HY only with the decrease of Brqmsted acidic intensity. But the TPD profile of LaHY was quite different from that of unmodified HY (as shown in Fig7). It can be readily seen that after the modification by La203(7%) the amount of weak acidity site decreased as the amount of some moderate-strong acidity site increased, especially at ca.400~ and 460~ If modified by La203(12%), owing to the large amount of acidity sites were masked by La203, the amount of weak and moderate-strong acidity sites were sharply decreased, but the same two obvious maxiuma at ca.400~ and 460~ still existed. These new acidity sites may have originated from the hydrolyzed La 3§ in the presence of trace quantity of water: La3++n20 ~ La2+(OH) + H + Obviously it was quite different from the modification by MgO, the modification of" HY by La203 not only masked the active Brqmsted acidity sites but also produced new acidity sites both at ca.400~ and 460~ From the reaction result showed in Fig6, we can draw a conclusion that these new acidity sites were of great advantage to this alkylation reaction. Though the two modification methods have different effect on the surface acidity, the concentration of moderate-strong acid found to decrease in both methods. Satsuma et al. [ 16] thought that the mainly effective factor to deactivate the zeolite is not the acid intensity but the concentration of the acid site. Compact acid centers are more liable to carbonation than lOO "

-..

^,--

'

H~

- LaHr

- L a ~ 12o~

~ so

0

2

i

i

4

6

,

react i on t i ne(h)

i

8

t

10

Figure 6. The effect of La203 content on the alkylation reaction

0

100 200 300 400 500 600 700 t enlt~r at ur e( i ~E

Figure 7. NH3-TPD profiles of LaHY with different La203 content

1006 isolated ones since the olefins tend to polymerize in the adjacent acidity site. So the modification by MgO and La203 is favourable to reducing the carbonation side reaction and increasing the catalysts stability. 4. CONCLUSION The liquid-phase alkylatin of naphthalene with long chain olefins was carried out over HY and HI3 zeolites. HY showed higher activity and selectivity than HI3. The optimum loading amount is 1% for MgO and 7% for La203 respectively and the conversion of olefins is over 90% with the mono-alkyinaphthalene selectivity 100%. The activity of the catalyst remains almost unchanged after a continuous reaction time of 10h. Modification of HY by MgO would mask some active Br q0 nsted acidic sites to increase the selectivity of mono-alkylnaphthalene. Modification of HY by La203 covers the original acid sites and would produce new ones at ca.400~ and 450~ which might be the active sites to the reaction. The modification by MgO and La203 would reduce the carbonation side reaction and increase the catalysts stability. REFERENCES

1. J.H.Clark, Green Chem. (1999) 1. 2. P.B. Venuto, Microporous Mater. 2 (1994) 297, and references therein. 3. W.W. Keading, C. Chu, L.B. Young, B. Weinstein, S.A. Butter, J. Catal. 67 (1981) 159. 4. N.Y. Chen, W.E. Garwood, Catal. Rev. Sci. Eng. 28 (1986) 185. 5. D. Fraenkel, M. Cherniavsky, B. Ittah, M. Levy, J. Catal. 101 (1986) 273. 6. M. Neuber, H.G. Karge, J. Weitkamp, Catal. Today 3 (1988) 11. 7. J. Weitkamp, M. Neuber, Stud. Surf. Sci. Catal. 60 (1991) 291. 8. P. Moreau, A. Finiels, P. Geneste, F. Moreau, J. Solofo, J. Org. Chem. 57 (1992) 5040. 114 P. Moreau et al. /Journal o f Molecular Catalysis A: Chemical 168 (2001) 105-114

9. P. Moreau, A. Finiels, P. Geneste, F. Moreau, J. Solofo, Stud. Surf. Sci. Catal. 83 (1993) 575. 10. P. Moreau, A. Finiels, P. Geneste, J. Joffre, F. Moreau, J. Solofo, Catal. Today 31 (1996) 11. 11. D. Mravec, M. Michvocik, M. Hronec, P. Moreau, A. Finiels, P. Geneste, Catal. Lett. 38 (1996) 267. 12. Z. Liu, P. Moreau, F. Fajula, Chem. Commun. (Cambridge) 23 (1996) 2653. 13. Z. Liu, P. Moreau, F. Fajula, Appl. Catal. A: Gen. 159 (1997) 305. 14. S. Sivasankar, A. Thangaraj, J. Catal. 138 (1992)386. 15. Araujo, S.A., Thesis, Alkylation of benzene with 1-Dodecene in presence of Zeolite, Universidede de Sao Paulo I.Q. 1992 16. A.Satsuma and T.Ishihura, Symposium on General papers and poster session, presented before the Division of Petroleum Chemistry, Inc.210th National Meeting, American Chemical Society, Chicago.IL, August 20-25(1995)

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1007

Synthesis o f Anthraquinone from Phthalic Anhydride with Benzene over Zeolite Catalyst Y. Wang, W.-R. Miao, Q. Liu, L.-B. Cheng and G.-R. Wang* State Key Laboratory of Fine Chemicals, Dalian University of Technology No.158 Zhong Shan Road, Dalian, 116012 China *To whom correspondence should be addressed The synthesis of anthraquinone(AQ) from phthalic anhydride(PhA) with benzene over acidic type of zeolite. The protonic form of 13zeolite modified CeO2(0.6g/g zeolite) catalyst was the best catalyst: at 523K WHSV=5h 1, and benzene/phthalic anhydride(mol)=25. The reaction was proved to be highly selective for anthraquinone(94.25%) over catalyst of Hl3modified by CeO2, with small amouts of orthobenzoyl benzoic acid(BBA)(5.75%), which dehydrated to produce anthraquinone. The reaction result suggested that the reaction for the formation anthraqinone from phthalic anhydride with benzene required the catalyst having more medium stronger Br6nsted acidic sites in[3zeolite catalyst channels. Key word: 13zeolitecatalyst, synthesis anthraquinone, ortho-benzoyl benaoic acid

1. I N T R O D U T I O N Anthraquinone is main material to synthesize anthraquinone dye and intermediate. Anthraquinone and tetrahydro-anthraquinone have found broad application as pulp assistant in paper-industry since 1980s. Therefore, anthraquinone is in great demand. Nowadays the advantages of Friedel-Crafi (F-C) reaction, in which anthraquinone is synthesized by phthalic anhydride with benzene, are of enough materials and low cost, but the process consumed large quantities of AIC13 and H2SO4, which would cause serious environment pollution and equipment corrosion. For this reason, it is necessary to develop a new sort of pollution-free solid acid catalyst. There are many references in literature concerning use of solid acid catalysts in this reaction. Kokai [1 ] disclosed that the magnesium and silicon oxides or sulfate with many other metal oxides suggested additives as well as the combination of silica, alumina, titania and boria [2-3] used as catalysts in this reaction. Hino et al. [4] disclosed that the super acid catalysts could catalyze the acylation of toluene and benzoic anhydride to produce methylzenzophenone, and also can catalyze this reaction [5-7]. Michnel O.Natt. et al. [8] provided a cation exchange resin catalysts for making anthraquinone. Z e o l i t e s w e r e u s e d as

1008 catalysts in this reaction [9-11]. However, a detailed study on modified HI3zeolite for use of this reaction is not known until now. In this paper, the catalytic performance of synthesis of anthraquinone from phthalic anhydride with benzene over modified Hi3zeolite catalysts is reported. Moreover, the characterization of these catalysts by pyridin-IR and NH3-TPD is conducted and the correlation of catalytic performances of modified Hl3zeolite with their acidic properties is discussed.

2. E X P E R I M E N T A L

2.1 Catalyst preparation: HY and Hl3zeolite catalysts are prepared by ion-exchange ofNa-form of Y (SIO2/A1203--5) and of 13(SiO2/A1203=27) zeolite with aqueous solution of NHaNO3; Ion-exchanged zeolites were dried at 393K for 12h and then calcined in the fimaace at 773K for 5h; the powder of HY and Hl3zeolite were pressed, crushed and sieved into 0.2-0.4mm granules for further use. Modified HI3zeolites are prepared by impregnation HI3zeolite granule with an amount of solutions of various materials at 298K for 12h. They were dried at 373K and calcined at 773K for 5h. 2.2 Catalyst evaluation: Catalytic reaction was carried out in a continuous-flow fixed-bed micro-reactor. The amount of catalyst charged in the reactor was 2.0g. The loaded catalyst was pre-treated with air stream at 773K for lh and then with nitrogen stream at 773K for 30min. Benzene (Merck>99%) was fed by a metering pump. Vapor of PhA (Merck; distilled) was fed into the reactor by means of gas-carrier (nitrogen) passed through melt PhA in a stainless steel melt tank, placed in an oven with controlled temperature (433-453K). In upper part of the reactor the vapor of PhA was mixed with benzene and the reaction mixture was passed through the catalyst layer and reacted over catalysts. Reaction products were collected in a glass cooler-collector, soluted in dioxane (Merck>99%) and analyzed by ultraviolet-liquid chromatograph (HP 1050, HPLC) 2.3 Characterization of acidic properties of catalyst NH3-TPD was performed on a conventioned set-up equipped with a thermal conductivity detector (TCD). The catalyst charge was 0.2g (20-60mesh). The sample was first flushed with He (30ml/min) at 873K for 3h, then cooled to 423K and saturated with NH3 until equilibrium. It was then flushed with He again until the integrator baseline was stable. NH3-TPD was then promptly started at a heating rate of 15K/rain from 423K to 873K. All NH3-TPD profiles were deconvoluted into three peaks using a Gaussian and Larentzian curve-fitting method. Pyridine-FT-IR was recorded on a Fourier transform infrared spectrometer (Nicolet Impact 410) with a resolution of4cm ~ and 64 scans in the region from 4000 to 400 cm1. The catalyst was pressed into a self-supporting wafer (ca. 15mg/cm2), and introduced into a quartz IR cell

1009 with CaF2 windows, and pretreated in-situ in a stream of 30ml/min He from RT to 773K at a heating rate of 10K/min, and then evacuated at 773K and 5x 103Pa for 90 minutes. The cell was cooled to room temperature and saturated with pyridine. After removal of the excess pyridine, the spectrtma was recorded. Then the sample was evacuated at different temperatures (473,573 and 673K, respectively) from 30 min and a corresponding spectrum was recorded to distinguish the acid sites with different strength. The relative intensities of vibration band of 1540cm 1 and 1450crn "1 were ascribed to the relative concentration of Br6nsted and Lewis acid site respectively.

3. RESULTS AND DISCUSSION Catalytic performances of HY and HI3catalysts were examined in the reaction in the range of temperature 473K-570K. Obtained experimental data are presented in Table 1. It is seen (Table 1) that the conversion of PhA is observed very high over HY catalyst at these temperatures, the selectivities toward AQ and BBA are observed little. The conversion of PhA and the selectivities toward AQ and BBA over HI3 catalyst at these temperatures are observed better. Increase of reaction temperature causes increase of conversion of PhA over H 13catalyst; however, selectivity towards AQ and BBA decrease; the highest selectivity towards AQ over HI3catalyst appears at reaction temperature 525K. This phenomenon clearly shows that HY catalyst can not be used to catalyze this reaction because produced large organic molecules (AQ and BB) and these products on the acid sites are difficult to move away from super cage of HY zeolite, so in the products, AQ and BBA can not be measure off. Catalytic performance of catalyst of HI3 zeolite modified by H3PO4, HSO4, (NH4)2SO4, Ce203, CeO2 and ThO2 were compared with HI3 catalyst. Obtain experimental data are presented in Table 2. It is seen (Table 2) that conversion of PhA and selectivies toward AQ and BBA over catalyst of HI3 modified by H3PO4 and H2SO4 are even lower than that over HI3catalyst and other HI3 modified catalysts, selectivity towards AQ of Hl30ver catalyst of Hl3modified by HaPO4 is higher, however, selectivity toward BBA over catalyst of Hl3modified by H2SO4 is higher. Table 1. Catalytic performances of HY and HI3zeolte at various temperatures HY Catalyst n~ Reaction temperature K 473 523 573 473 523 573 Conversion of PhA % 100 100 100 53.17 65.92 88.11 Selectivity of AQ % 0 0 0 76.69 83.41 53.03 0 0 0 23.31 13.29 6.47 Selectivity of BBA % 0 0 0 100.00 96.70 59.50 Selectivity of AQ and BBA % Reaction condition: PhA:Bz=1:25(mol) WHSV of reaction mixturc=5h~, reaction time 5h.

1010 Table 2. Conversion and selectivity of Hl~ and modified HI3 catalysts. Catalyst CphA~ SAQ~ SBBAO~ SAQ+BBAO~ HI3 65.92 83.41 13.29 96.70 H3PO4 26.71 78.81 0 78.81 H2SO4 32.86 21.57 59.13 80.70 ('N-I-I4)2SO4 82.62 74.78 18.74 93.52 Ce203 56.62 88.54 3.92 92.46 CeO2 59.13 94.25 5.75 100 ThO2 65.36 74.35 19.33 93.68 Reaction condition: PhA:Bz=1:25(mol.). WHSV of reaction mixture =5h1, Tre,~on=523K,reaction time 5h The conversion of PhA over catalysts of HI3modified by (NH4)2SO4 and ThO2 is higher than over other catalysts, but selectivities toward AQ and BBA over HI3modified by (NH4)2SO4 and ThO2 is lower than over catalysts selectivities of HI3modified by Ce203 and CeO2. The conversion of PhA over catalysts of HI3modified by Ce203 and CeO2 is slightly lower than HI3zeolite, however, the selectivities towards AQ and BBA over catalysts of HI3 modified by Ce203 and CeO2 have been increased obviously, in particular, the selectivity towards AQ over catalyst of Hl3modified by CeO2 is up to 94.20% and the selectivity of BBA is 5.75%, which dehydrated to produce AQ. Acidic properties of catalysts of HI3and HI3modified were measured by NHa-TPD and pyridine-FT-IR (Fig. 1-4) respectively. The characterized results of catalysts of the HI3and HI3 modified are presented in Table 3, Table 4 and Fig. 1-4.

Blank 200~

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~. 1300

1400

1500

I

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1700

1800

1900

2000

Wavenumber(cm "1) Fig. 1 Pyridine-IR of HI3zoelite catalyst

2100

2200

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Wavenumber(cm-1) Fig.2 Pyridine-IR of Hl3zoelite catalyst modified by Ce203

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Wavenumber(cm"1) Fig.3 Pyridine-IR of H[3zoelite catalyst modified by CeO2

9

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Fig.4 Pyridine-IR of H[3zoelite catalyst modified by ThO2 Table 3. NH3-TPD characterization results of catalyst surface acidic property Catalyst

HI3 H3PO4 H2504

(NH4)2504

Ce203

CeO2 ThO2

NH3Desorption Amount mmol/g.cat % mmol/g.cat % mmol/g-cat % mmol/g-cat % mmol/g.cat % mmol/g-cat % mmol/g-cat %

Weak acid site (423-623K) 3.35 53.01 1.31 59.55 1.29 50.79 1.25 59.52 1.58 55.44 2.09 56.03 2.12 54.64

Middle strong acid site (623-723K) 1.42

22.47 0.55 25.00 0.65 25 59 0.56 26.67 0.81 28.42 1.05 28.15 1.14 29.38

Strong acid site (723-873K)

Total

1.55 24.52 0.34 15.45 O.6O 23.62 0.29 13.81

6.32 100 2.20 100 2.54 100 2.10 100

0.46 16.14

2.85 100

0.59 15.82 0.62 15.98

3.73 100 3.88 100

1013 Table 4.IR characterization results of catalysts surface acidic property

HIB

Catalyst

Ce203-H~ CeO2-H~

ThO2-H~

B/L

. . . .

Desorption Temperature

473K 573K 673K

2.7 2.3 2.5

1.5 2.2 2.5

1.3 3.0 4.0

1.1 2.3 2.2

The NH3-TPD results show that the amount of acid site of HI3catalyst is the highest and haft of it is weak acid site and quarter of it is middle strong acid site and the rest is strong acid site. The amount of acid site of catalysts of HI3modified by various materials decreases obviously, however the distribution of the intensity of acid site of catalysts of HI3modified is different from HI3catalyst. The ratio of weak acid and ratio of middle strong acid site of catalyst of modified Hl3zeolites, except modified Hl3by H2804 catalysts, is higher than HI3catalysts, however, ratio of strong acid site of catalysts of modified Hl3is lower than Hl3catalysts. We can suggest that conversion of PhA and selectivity towards AQ and BBA over these catalysts were affected by the amount and intensity of middle strong acid site of catalysts as acidic properties of catalysts are correlated with catalytic performance of catalysts. The FT-IR spectroscopy results show that catalysts of HI3zeolite and modified HI3zeolites have Br6nsted acid site and Lewis acid site. Br6nsted acid site/Lewis acid site radio order is ThO2-HI3
4. C O N C L U S I O N S Experimental data presented that synthesis anthraquinone from phthalic anhydride with benzene over catalysts of Hl3modified by CeO2 is possible. The reaction results proved to be highly selective towards Anthraquinone (94.25%) over Hl3modified CeO2 catalyst, with small amounts of orthobenzoylbenzoic acid (5.75%), which dehydrated to produce anthraquinone. The conversion of phthalic Anhydride and the selectivity toward anthraquinone are effected by amount of middle strong acid site and radio of Br6nsted acid site with Lewis acid site.

REFERENCES 1. S. Aoura and Y. Kokura, Patent of Japan No. 82-31637 (1982) 2. S. Aoura and Y. Kokura, Patent of Japan No. 82-70832 (1982) 3. S. Aoura and Y. Kokura, Patent of Japan No. 82-70833 (1982) 4. Makoto Hino and Kazushi Arata, J. ChelrL Soc. Chem. Commun. (1985) 112

1014 5. Motoo Kawamata and Shiro Fujikake, U.S. Patent 4,459,234 (1984) 6. Alan E. Goliaszewski and Richard F. Salinaro, U.S. Patent 4,666,632 (1987) 7. Alan E. Goliaszewski, U.S. Patent 4,781,862 (1988) 8. Michaei O.Nutt, U.S. Patent 4,304,724 (1981) 9. J.M. Newsarn, M.M.Treacy, W.T.Koetsier and C.B.de Greye, Proc. R. Soc. Land. A, 420(1988) 375 10. H. Kawamata and S. Fujikake, Patent of Japan No. 81-142233 (1981) 11. O.V. Kikhtyanin, K.G.Ione, G.P.Snytnikova, L.V.Malysheva, A.V.Toktarev, E.A.Paukshtis, R.Spichtinger, F.SchOth and K.K Unger, Studies in Surface Science and Catalysis Vol. 84(1994): 1905

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1015

Simultaneous hydrogenation and ring opening of aromatics for diesel upgrading on Pt/zeolite catalysts. The influence of zeolite pore topology and reactant on catalyst performance M.A. Arribas, A. Martinez and G. Sastre Instituto de Tecnologia Quimica, UPV-CSIC, Avda. de los Naranjos s/n, 46022 Valencia, Spain, e-mail: [email protected] The influence of zeolite pore size and dimensionality in bifunctional Pt/zeolite catalysts has been studied for the combined hydrogenation and ring opening of aromatics in diesel fuels. The formation of high cetane ring opening products (ROP) from tetralin (C10) was seen to be favored on large-pore zeolites (beta, USY, mordenite) as compared to medium-pore zeolites (ZSM-5, MCM-22). The latter zeolites imposed serious steric restrictions for the formation and/or diffusion of ROP as supported from molecular docking simulation results. Among the large-pore zeolites, beta and USY having a tri-dimensional pore structure produced higher yields of ROP than mordenite possessing a one-dimensional pore system. The maximum yield of ROP from tetralin was obtained for the Pt/~ta catalyst. However, Pt/USY was more effective than Pt/beta for the ring opening of bulkier aromatic reactants that could be present in real diesel feeds (e.g. 1-methylnaphthalene). In this case, the molecular docking results suggested that the diffusion of the desired ROP was restricted in the pores of zeolite beta, while they could freely diffuse in the faujasite structure having internal cages of larger diameter (ca. 1.2 nm) than the pore openings (ca. 0.74 nm). 1. INTRODUCTION Hydrogenation of aromatics is a key process in modem refineries for increasing the octane of low quality diesel fuels (1). Moreover, a further increase of cetane could be achieved if the hydrogenation reaction is coupled with the selective ring opening of the naphthenic rings giving 1-ring naphthenes or even paraffins with higher cetane values (2). Numerous studies were aimed at studying the hydrogenolytic ring opening of methylcyclopentane (3, 4) or even cyclohexane (5) as model reactants on supported noble metal catalysts (Ir, Ru, Re, Rh), but few works were devoted to the ring opening of larger naphthenes that would be more representative of the type of molecules present in real diesel feeds (6). Moreover, it would be more attractive to perform the hydrogenation of the aromatics and the ring opening of the naphthenes produced in a single catalytic step. Following this approach, we started to study the combined hydrogenation and ring opening of tetralin (7) or 1-methylnaphthalene (8, 9) as representative molecules of the mono and diaromatics present in diesel fractions. For that purpose, we proposed the use ofbifunctional metal/acid catalysts, in where the hydrogenation reactions will take place predominantly on the metal (typically Pt) sites and the C-C bond rupture in the naphthenic ring will occur on the acid sites of the support. Our previous studies

1016 showed that Pt/zeolite catalysts produced higher yields to the high cetane ring opening products than Pt/SiO2-A1203 and Pt/A1-MCM-41 (8). Moreover, it was shown that in the case of Pt/zeolite catalysts the acidity of the zeolite needs to be finely tuned in order to reduce as much as possible the extent of undesired cracking and dealkylation reactions leading to lighter products (9). Besides acidity, the crystal size of the zeolite was also shown to affect significatively the product distribution (7), thus evidencing the importance of diffusional problems in this reaction. This is not surprising considering that most of the desired ring opening products (e.g. alkylbenzenes and alkylcycloalkanes) have a kinetic diameter larger than the starting aromatic or naphthenic molecules. Therefore, the particular topology of the zeolite used should have an effect on the product distribution, and thus on the quality of the product obtained. This would be even more so if one takes into account that a wide spectrum of aromatic compounds with different sizes can be present in real distillate feeds. Based on the above premises, in this work we have studied the influence of both the zeolite structure and aromatic reactant size on the activity and selectivity of Pt/zeolite catalysts for the combined hydrogenation and ring opening reactions. The catalytic results were supported by molecular docking simulation of selected product molecules in the different zeolite structures.

2. EXPERIMENTAL 2.1 Preparation of catalysts Five zeolites with different pore topology and similar Si/A1 ratio were used in this study: ZSM-5 (CBV3020, H + form), mordenite (CBV20A, NH4 + form) and beta (CP811, H + form) samples were obtained from Zeolyst International (previously PQ Corp.) The NH4+-mordenite sample was calcined at 500~ for 3 h to produce the protonic form. MCM-22 zeolite was synthesized using hexamethylenimine (HMI, from Aldrich) as template using the procedure reported in (10). An ultrastable Y zeolite (USY) was prepared from a commercial NaY sample (CBV100, Zeolyst Int.) as follows: first the parent NaY sample was submitted to an NH4 + exchange with a 2.5N aqueous solution of NH4C1 at 80~ for 2 h, then it was steamed (100% steam atmosphere) at 600~ for 3 h, NH4+ exchanged again as described above, and steamed at 750~ for 5 h. All zeolites in the protonic form were then impregnated with a 0.2N HC1 solution containing the required amount of HEPtC16 to obtain a nominal concentration of l wt% Pt in the final catalysts. After Pt impregnation the samples were kept in a disecator overnight, dried at 100~ for 3 h and finally calcined in a muffle at 500~ for 3 h. 2.2 Characterization techniques Powder X-ray diffraction (XRD) was performed in a Philips PW 1830 apparatus using CuK~ radiation. XRD was used to estimate the crystallinity of the samples and to obtain the unit cell parameter (a0) of the USY zeolite as described in the ASTM D-3942-80 method. The framework Si/A1 ratio of the USY sample was obtained from a0 using the Fichtner-Schmitler equation. Textural properties were determined by N2 adsorption at-196~ on a ASAP-2000 (Micromeritics) apparatus after pretreating the samples at 400~ under vacuum overnight. Acidity of the samples was measured by infrared spectroscopy combined with adsorption of pyridine and desorption at different temperatures using a Nicolet 710 FTIR apparatus. The amount of Br6nsted and Lewis acid sites was calculated from the intensities of the bands at

1017 ca. 1450 and 1545 c m -1, respectively, and using the extinction coefficients given by Emeis (11). The chemical composition of the zeolites (bulk Si/A1 ratio) was determined by atomic absorption spectrophotometry in a Spectra A-plus (Varian) apparatus. Molecular docking simulation studies were carried out using the Cerius 2 software, version 3.8 in order to estimate the Van der Waals interactions between the atoms at the zeolite pores and selected reactant/product molecules located inside the channels.

2.3 Catalytic experiments The conversion of tetralin (T) or 1-methylnaphthalene (1-MN) (both reactants diluted in cyclohexane in a proportion of 50:50 wt/wt) was carried out in a continuous fixed-bed stainless steel reactor. Before starting the reaction, the catalysts were reduced in situ with 300 cma/min of HE at 450~ for 2 h at atmospheric pressure. The reaction of T was performed at 3.0 MPa total pressure, HE/T ratio molar of 10, WHSV = 2 h-1 (referred to the aromatic) and temperature in the range of 225-325~ Reaction conditions used for 1-MN conversion were: 4.0 MPa total pressure, H2/1-MN molar ratio of 30, WHSV- 2.5 h "1 and T = 250-400~ Liquid products were condensed after depressurisation and analysed at regular intervals in a GC (Varian 3800) equipped with a capillary column (Petrocol DHS0.2, 50mx0.2mm) and a FID. Reaction gases were analysed on-line using the same GC. Identification of reaction products was done by mass spectrometry and by comparing the retention times with those of available standard mixtures. The data reported here correspond to the analysis of products in the pseudo steady-state period, which was usually achieved after 6 hours of reaction. 3. RESULTS AND DISCUSSION 3.1 Characterization of the catalysts The main physicochemical properties of the different zeolites are given in Table 1. As observed in this table, all zeolites have similar chemical composition (Si/A1 ratio of ca. 15). In the case of USY, prepared by steam dealumination, we selected a sample with a framework Si/A1 ratio of ca. 15 (bulk Si/A1 ratio of 2.6). From the structural point of view, ZSM-5 and MCM-22 are medium pore zeolites (10 MR channels), but the latter contains large 12MR cavities of ca. 1.82 x 0.71 nm size which can be accessed only through 10MR windows. On the other hand, MOR, beta, and USY are all three large pore zeolites (12MR) but with different structural characteristics. Although MOR has also an 8MR system of channels perpendicular to the 12MR ones, its narrow size (0.57 x 0.26 nm) does not allow the diffusion of most of hydrocarbon molecules and thus will be considered here as formed by a monodimensional system of 12MR channels. Beta and USY have tridimensional structures, but in the case of USY large internal cavities (supercages) of ca. 1.2 nm diameter are formed at the channels intersections, whereas internal cages of significantly larger diameter than the pore apertures are absent in beta. As observed in Table l, large pore zeolites typically present higher surface areas and micropore volumes than the medium pore ones. The main structural characteristics of the zeolites have also been included in Table 1. The amount of Br6nsted and Lewis acid sites of the zeolite samples measured at different desorption temperatures of pyridine is presented in Table 2.

1018 Table 1 Physicochemical and structural characteristics of the different zeolites Beta Zeolite ZSM-5 MCM-22 MOR

USY

Si/A1 ratio (bulk) Area BET (mE/g)

15 383

15 453

10 463

13 598

2.6 (17) a 466

MPV b (cm3/g)

0.11

0.16

0.19

0.17

0.20

Crystal size (~tm) Pore diameters

1-3 10MR (0.55 x 0.51 nm) 10MR (0.56 x 0.53 nm)

0.5 10MR (0.54 x 0.40 nm) 12MR (1.82 x 0.71 nm)

0.2 12MR (0.72 x 0.65 nm)

0.15 12MR (0.76 x 0.64 nm) 12MR (0.55 x 0.55 nm)

0.4-0.6 12MR (0.74 nm) surpercage (1.2 nm)

Framework Si/A1 ratio given in parenthesis. b MPV= micropore volume.

a

Table 2 Acidity ofzeolites (~tmol Py/8 zeol) determined from IR-pyridine Lewis Br6nsted Zeolite

250~

350~

400~

250~

350~

400~

ZSM-5

43.8

25.1

12.4

9.2

5.7

4.3

MCM22

54.3

47.7

34.1

23.1

21.0

20.2

MOR

65.1

36.7

15.4

20.9

24.0

21.0

Beta-1

46.0

27.1

15.3

49.0

41.0

42.0

USY

14.2

8.0

3.0

9.2

4.9

3.4

As expected from their similar Si/A1 ratio, all zeolites (with the exception of USY) contained similar amount of Br6nsted acid sites measured at 250~ pyridine desorption temperature. The lower density of Br6nsted acid sites in USY can be ascribed to the presence of large amounts of EFAL formed during the dealumination process. EFAL species can reduce the zeolite Br6nsted acidity either by compensating part of the negative framework charge or by partially blocking the access of the pyridine molecules to the acid sites.

3.2. Catalytic experiments 3.2.1. Influence of zeolite structure In order to study the influence of zeolite structure on activity and product yields, we first carried out the hydrogenation-ring opening of tetralin as model reactant. All Pt/zeolite catalysts displayed very high tetralin conversions (above 92%) in the 225-250~ temperature range. At high temperatures (300-325~ a decrease of conversion with time on stream was observed for the medium-pore ZSM-5 and MCM-22 samples and for the 1-dimensional largepore MOR zeolite. This behavior is in agreement with the general trends typically observed in acid-catalyzed reactions, in where medium-pore and 1-dimensional large-pore zeolites are more susceptible to pore plugging by accumulation of carbonaceous deposits than 3dimensional large-pore zeolites.

1019 The conversion of tetralin on bifunctional Pt/zeolite catalysts leads to a wide spectrum of products with less (C3-C6 alkanes and C7-C9 naphthenes), equal (decalins, decalin isomers, alkylcyclohexanes, alkylcyclopentanes, and naphthalene), and more (alkyltetralins and alkyldecalins) carbon atoms than the starting aromatic feed (C~0). C10-alkylcyclopentanes and C10-alkylcyclohexanes are most probably formed by opening of the Cs-ring of decalin isomers (7), and have been grouped together as ring opening products (ROP). The yield to the high cetane ROP passes a maximum with the reaction temperature, and the temperature at which the maximum occurs depends on both the acidity and structural characteristics of the zeolite. At low temperatures, decalins formed by tetralin hydrogenation on the Pt sites are the predominant products. As the temperature increases, decalins isomerize on the zeolite acid sites to give decalin isomers having one or two Cs-rings. Then, C10alkylcycloalkanes start to be formed by selective opening of the Cs-ring of isodecalins. At high temperatures the yield of ROP decreases as they are further converted into lower molecular weight products (C3-C6 alkanes and C7-C9 naphthenes) through cracking and dealkylation reactions. The tetralin conversions and the yields to the different fractions obtained at the reaction temperature at which the maximum ROP yield is attained are given in Table 3. Results in Table 3 show that the formation of ring opening products is favored on large-pore (12MR) zeolites, and in particular on the three-dimensional 12MR beta and USY zeolites. The observed product distribution could be explained, at least to a certain extent, by considering the structural differences between the zeolites studied. Thus, the formation and/or diffusion of ROP should be more impeded in the 10MR pores of ZSM-5 and MCM-22 as compared to the 12MR zeolites. The existence of diffusional limitations in the 10MR zeolites was supported by the results of molecular docking simulation using butyl-cyclohexane (BCH) and diethyl-cyclohexane (DECH) as representative molecules for the ROP (C10-alkylcycloalkanes) formed from tetralin. The results showed the existence of serious steric restrictions for the formation and/or diffusion of the C~0-alkylcyclohexanes in the 10MR channels of ZSM-5 and MCM-22, as deduced from the large number of interactions between the Van der Waals radii of the guest molecules and the internal zeolite wall (Fig. 1). Table 3 Tetralin conversions and product yields obtained at the maximum ROP yield for the different Pt/zeolite catalysts Pt/ZSM-5 Pt/MCM-22 Pt/MOR Pt/beta Pt/USY Temperature (~ 300 300 275 250 275 Tetralin convers. (wt%) 90.78 97.13 83.75 99.84 99.99 Product yields (wt%): C3-C6alkanes 7.69 2.99 2.68 16.88 3.40 C7-C9naphthenes 4.83 6.28 5.01 13.84 2.81 C~0 fraction: Decalins 65.95 44.43 28.43 5.14 24.93 Decalin isomers 11.13 36.74 37.33 40.25 50.81 ROP

Naphthalene Others Total C10 products C1~+products

O.86

4. 64

6. 83

22. 44

16.12

0.09 0.11 78.14 0.12

0.00 1.75 87.56 0.30

0.21 0.79 73.59 2.47

0.07 0.92 68.82 0.30

0.03 1.85 93.74 0.13

1020

Fig. 1. Molecular docking simulation ofbutyl-cyclohexane (BCH) and diethyl-cyclohexane (DECH) molecules located in the 10MR channels of ZSM-5 and MCM-22 zeolites. In the case of ZSM-5, the relatively large zeolite crystallites (Table 1) can also contribute to reduce the diffusivity of the alkylcycloalkanes through the 10MR pores. Moreover, although Cl0-cyclohexanes might be formed in the large 12MR cages of MCM-22, the simulation results predicted that their diffusion through the 10MR windows accessing the 12MR cavities is highly impeded. On the other hand, the simulation results did not show any interaction between the guest molecules and the 12MR pores of the large-pore zeolites. However, a relatively low yield of ROP was experimentally obtained for MOR as compared to USY and beta (Table 3). As it has been commented before, a certain deactivation with time on stream was observed for MOR, particularly at reaction temperatures above 275~ Therefore, the accumulation of carbonaceous deposits responsible for the observed deactivation would hinder the diffusion of the ROP through the one-dimensional pores of MOR. As shown in Table 3, the maximum yield of ROP was obtained for the Pt/beta catalysts (ca. 22 wt%) followed by Pt/USY (ca. 16 wt%) under the reaction conditions used. However, Pt/beta also produced higher yields of light products (C3-C6 alkanes and C7-C9 naphthenes), despite the maximum ROP yield occurred at a lower reaction temperature (250~ These products are most probably formed from ROP by consecutive cracking and dealkylation reactions, suggesting that the potential yield of ROP should be even higher than that experimentally obtained on Pt/beta. The relatively low yields of light products obtained on ZSM-5, MCM-22 and MOR is probably a consequence of the low formation of ROP, as

1021 discussed before, but the low cracking-dealkylation activity of USY is ascribed to its lower Br6nsted acidity as compared to the rest of zeolites (Table 2). 3.2.1. Influence of reactant size It could be apparent from the above results that Pt/beta is a more adequate catalyst than Pt/USY for improving the cetane of diesel fuels through the combined hydrogenation and ring opening of aromatics. However, one has to take into account that the presence of bulkier aromatic molecules in real diesel feeds (2- and even 3-ring aromatic structures with alkyl groups) may accentuate the diffusional limitations in zeolite-based catalysts. Thus, in this part of the work we used 1-methylnaphthalene (1-MN) as a representative molecule of alkyldiaromatics in real distillate streams. In this case we have only compared the results obtained for the two large-pore zeolites that produced the highest ROP yields from tetralin, i.e., USY and beta. As it was observed with tetralin, both Pt/USY and Pt/beta catalysts showed very high conversion (above 97%) and a high stability with time on stream under the whole range of temperatures studied (275-400~ The conversion of 1-MN on Pt/zeolite catalysts also lead to a large variety of products with less (C3=C10), equal (Cll), and more (C12+) carbon atoms than the reactant molecule. A more detailed description of the different products formed from 1MN can be found in (8). For simplicity we will focus our discussion in the C~l fraction containing the desired ring opening products (ROP- Cl~-alkylbenzenes + Cllalkylcycloalkanes). For both catalysts the yield to Cll products (including methyltetralins, dimethylindans, methyldecalins, methyldecalin isomers, and ROP) decreases with increasing the reaction temperature as cracking and dealkylation processes are favored at high temperatures. At constant temperature, however, the C~l yield is higher for Pt/USY, which can be ascribed to its lower Br6nsted acidity as compared to Pt/beta, as previously discussed. As it can be seen in Fig. 2, the yield of ROP went through a maximum with reaction temperature, but contrarily to what was observed for tetralin, the maximum yield of ROP produced from 1-MN was obtained for Pt/USY (ca. 15 wt%) at a temperature of 350~ Obviously, the lower Br6nsted acidity of the USY sample with respect to the beta zeolite would contribute to the higher ROP yield by preventing consecutive reactions leading to lighter products. However, Pt/USY still produced a higher ROP yield when it was compared with a dealuminated beta zeolite (Si/A1 ratio of 93) having a similar density of Br6nsted acid sites measured by IR-pyridine at 250~ desorption temperature (sample Pt/beta-d in Fig. 2). Therefore, there would be other reasons, besides acidity, that determine a better performance of the USY zeolite for the ring opening of bulky aromatics, such as 1-MN, and these could be related with the differences in pore structure. In this case, we performed the molecular docking simulation of a representative CI~ ring opening product formed from 1-MN, e.g. propyl-ethyl-cyclohexane (PECH) located in the 12MR pores of beta and USY structures. As observed in Fig. 3, a large amount of van der Waals interactions appeared between this compound and the atoms in the walls of the beta structure, suggesting that the diffusion of these type of products may be hindered in the 12MR channels (0.76-0.64 nm) of this zeolite. By contrast, no interactions were observed when this molecule was located inside the supercages of Y zeolite nor during its diffusion through the 12MR windows (0.74 nm) accessing the supercage. Therefore, it could be expected that the ROP formed from 1-MN would undergo in a larger extent consecutive cracking and dealkylation reactions during their diffusion in the pores of beta zeolite with respect to USY. This could explain the higher yields of light products and the lower ROP yields obtained on

1022 Pt/beta catalysts as compared with Pt/USY when bulky aromatics typically present in real diesel feeds have to be reacted.

14

- V - Beta I

_._usu i

/

/41,-~

\

ii

% _~,~,..~,,S:;,% /

it

~ ~~ .~_

~vL.

o. 6

o .

0

260

.

~7

. 280

.

/

~ . 300

~'v

, 320

340

*

"

~'

360

'

I 380

'

y ......

%

.,,.

f

......:,,.

I 400

Temperature (oC)

Fig. 2. Yield of ROP produced from 1-MN as a function of reaction temperature for Pt/USY and Pt/beta catalysts.

Fig. 3. Molecular docking simulation of propyl-ethyl-cyclohexane (PECH) in the beta structure.

Acknowledgments

Financial support by the Comisi6n Interministerial de Ciencia y Tecnologia of Spain (Project MAT99-0689) is gratefully acknowledged. M.A.A. thanks the Generalitat Valenciana for a postgraduate scholarship. REFERENCES

1. A. Stanislaus and B.H. Cooper, Catal. Rev.-Sci. Eng., 36 (1994) 75. 2. J.P. Van der Berg, J.P. Lucien, G. Germaine, and G.L.B. Thielemans, Fuel Process. and Technol., 35 (1993) 119. 3. F.G.J. Gault, Adv. Catal., 30 (1981) 1. 4. Z. Pa~l, P. T6t6nyi, Nature 267 (1977) 234. 5. L.M. Kustov, T.V. Vasina, O.V. Masloboishchikova, E.G. Khelkovskaya-Sergeeva, P. Zeuthen, Stud. Surf. Sci. Catal., 130 (2000) 227. 6. L.M. Kustov, A. Yu. Stakheev, T.V. Vasina, O.V. Masloboishchikova, E.G. KhelkovslayaSergeeva, P. Zeuthen, Stud. Surf. Sci. Catal., 138 (2001) 307. 7. M.A. Arribas, J.J. Mahiques, A. Martinez, Stud. Surf. Sci. Catal., 135 (2001) 303. 8. M.A. Arribas and A. Martinez, Stud. Surf. Sci. Catal., 130 (2000) 2585. 9. M.A. Arribas and A. Martinez, Appl. Catal. A: General (2002) in press. 10. A. Corma, C. Corell, F. Llopis, A. Martinez, J. P6rez-Pariente, Appl. Catal. A: General, 115 (1994) 121. 11. C.A. Emeis, J. Catal., 141 (1993) 347.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1023

Catalytic c o m b u s t i o n o f chlorobenzene over Pt/zeolite catalysts S. Scir6, S. Minic6, C. Crisafulli, G. Burgio and V. Giuffrida Dipartimento di Scienze Chimiche, Universit~ di Catania, Viale Andrea Doria, 6 - 95125 Catania (Italy). Fax: +39/095/580138 E-mail: [email protected]

This paper investigates the effect of different zeolites on the performance of zeolite supported Pt samples towards the catalytic combustion of chlorobenzene. Pt/zeolite samples showed a higher activity compared to a Pt/A1203 sample used as reference. Pt catalysts supported on H-Y and H-beta zeolites was found to be more active than Pt on H-ZSM5 and H-ferrierite. The yield to polychlorinated benzenes (PhClx) was in the order Pt/A1203 > Pt/H-beta ~ Pt/H-Y > Pt/H-ZSM5 > Pt/H-ferrierite. This trend has been explained on the basis of a pore size effect induced by the zeolite which restrains the chlorination of chlorobenzene to PhClx

1. INTRODUCTION Volatile organic compounds (VOCs) are an important class of air pollutants, emitted from many industrial processes and transportation activities [1]. Among all VOCs, chlorinecontaining organic compounds (C1-VOCs) require a special attention due to their toxicity, high stability and widespread application in industry. At present the main industrial process for C1-VOCs destruction involves thermal incineration which requires temperatures near 1000~ to achieve a complete combustion. This is a rather expensive process, which can also lead to the formation of highly toxic by-products such as dioxins and dibenzofurans [2]. Catalytic combustion is an emerging technology for the removal of C1-VOCs from waste gases [2]. The major advantages of this approach are that the combustion can be carried out at lower temperatures (< 500~ and lower concentration of pollutants (<1% ) than thermal oxidation. Metal oxides or supported noble metals (Pt and Pd) are the most studied catalysts for the combustion of C1-VOCs [2-8]. Generally noble metals exhibit a higher activity compared to metal oxides [2-7]. However on noble metals considerable amounts of polychlorinated compounds are formed, which are more toxic and recalcitrant than the starting material. In the case of metal oxides the formation of volatile metaloxychlorides is the main problem, leading to a relevant catalyst deactivation [3-4]. U308 has been reported as the only exception, but an industrial application of this catalyst requires special procedures for its safe handling due to chemical toxicity considerations [2]. More recently zeolites has been considered as potential catalysts for the oxidation of halogenated hydrocarbons, because of their pore structures, acid properties and thermal stability [7].

1024 On this basis the present paper aims to investigate the effect of different zeolites on the catalytic activity and products selectivity of Pt/zeolite samples towards the catalytic combustion of chlorobenzene (PhC1). It must be underlined that PhC1 has been chosen as the reactant considering that it is not only a pollutant itself but it can be also considered as a suitable probe for the destruction of polychlorinated biphenyls [2-5]. Moreover PhC1 is a particularly stable VOC that is difficult to oxidize.

2. EXPERIMENTAL Pt catalysts were prepared by incipient wetness impregnation of supports with appropriate amounts of aqueous solutions of H2PtCI6 in order to obtain samples with 0.5 wt % of Pt. One ~/-A1203, supplied by Harshaw, and four different classes of zeolites (H-Y, H-beta, HZSM5, NH4-ferrierite), each of them with two SIO2/A1203 ratios, all supplied by Zeolyst, were used as supports. It must be noted that, before the impregnation, the NH4-ferrierite was transformed in the H-form by calcination in air at 450~ H2 chemisorption was measured in a static system operating at room temperature. Before the measurements all samples were pretreated as follows: reduction in Hz at 400~ for lh, outgassing at 400~ for lh and cooling at room temperature. For IR studies the powdered samples were compressed into thin self-supporting discs of about 25 mg crn2 and 0.1 mm thick. The disc was placed in an IR cell which allows thermal treatments in vacuum or in a controlled atmosphere. In the cell all samples were evacuated slowly increasing temperature up to 450~ and then reduced in pure H2 at this temperature for 30 min. The sample was then evacuated a 450~ for 1 h and finally cooled at room temperature. Pyridine was then admitted by opening for some seconds the valve of the vessel containing the substance at 10~ Subsequent evacuations were then performed at room temperature or higher temperatures. Spectra were recorded with a Perkin Elmer System 2000 FT-IR spectrophotometer with a resolution of 2 cm"l. Data are reported as difference spectra obtained by subtracting the spectrum of the sample before the admission of the adsorbate and are normalized to the same amount of catalysts per cm1. TPD experiments of ammonia were carried out in a quartz U-shape reactor in a flow of He with a constant heating rate of 10~ min"l. The desorbed products were detected by a quadrupole mass spectrometer (Sensorlab VG Quadrupoles). Before TPD all samples were reduced in flowing H2 for 1 h at 450~ calcined in air at 450~ for 3 h, heated and maintained in flowing He for 30 min at 600~ and then cooled to 30~ in a flow of He. Catalytic activity tests were carried out in a fixed-bed reactor at atmospheric pressure in the 200-550~ range, using 0.1 g of catalyst. PhC1 was fed to the reactor by a carrying gas of He flow through a saturator maintained at 2.8~ and then mixed with 02 and He before reaching the catalyst. The reactant mixture was 10% 02 and 2000 ppm PhC1 diluted with He. The total gas flow was 44.3 cc/min with a GHSV of 18600 h1. The reaction products were analysed using two on-line gas chromatographs, one equipped with FID detector and HP-INNOWax column for the analysis of PhC1 and polychlorinated benzenes (PhClx) and the other with TCD detector and Octoil S at 3% on silica gel column for the CO/CO2 analysis. Before catalytic runs, all samples were reduced at 450~ in Hz for l h and then calcined in air at the same temperature for 3h. For all experiments COz was the main carbon-containing product, only very small amounts of CO were found at low conversions.

1025

Table 1 Code and phy,sico-chemical properties,. of . . . .sup,. . . ported. Pt samp!es.' ' ' 8iO21A1203 Surface Area Code Support ratio (m2/g) PtA1 q/-A1203 " 100 PtFAU5 H-Y 5 660 PtFAU80 H-Y 80 780 PtBEA75 H-beta 75 650 PtBEA300 H-beta 300 620 PtMFI30 H-ZSM5 30 400 PtMFI280 H-ZSM5 280 420 PtFER20 H-ferrierite 20 400 PtFER55 H-ferrierite 55 400 .

.

.

..

.

.

H/Pt ratio 0.90 0.60 0.25 0.09 0.03 0.02 0.01 0.09 0.05

"' NH3 adsorbed (mmol/g) -

0.73 0.02 0.85 0.30 1.10 0.16 1.73 1.01

Code of all samples, together with support used, SIO2/A1203 ratios of zeolites, BET surface area, H/Pt ratio and total amount of adsorbed ammonia are reported in Table 1. 3. RESULTS 3.1. Catalytic activity measurements Fig. 1 shows the conversion of chlorobenzene and the yield to PhClx as a function of the reaction temperature for the A1203 supported Pt sample (PtA1). It can be observed that the reaction starts at about 300~ reaching 50% PhC1 conversion at ca. 340~ The curve of PhClx formation presents a maximum (2.0% yield) at 450~ temperature at which the chlorobenzene reaches complete conversion. Among all PhCI~ p- and m-dichlorobenzene isomers were the most abundant (ca. 85% of total PhClx) whereas o-PhCh was formed in a much smaller quantity (ca. 10%). PhC13 isomers were also detected, however always in a considerably lower amount (<5%).

l.oy

,oOi~p~,(c) /e..__4__ ~4

._80 o~

3o~"

60

,o, "~>~ 40 o

""

2 ~-o

"

20

"o

"-

0 250

1 ~-

0 300 350 400 Temperature (~

450

500

Fig. 1. PhC1 conversion (C) and PhClx yield (Y) vs. reaction temperatures over A1203 supported Pt sample.

1026

The curves of PhC1 conversion and PhClx yield as a function of the reaction temperature for all zeolite based catalysts are shown in Fig. 2. It is possible to note that on zeolite supported Pt samples the light-off temperature ranges from 250~ (H-Y and H-beta based Pt catalysts) to 300~ (H-ZSM5 and H-ferrierite based Pt catalysts). On all samples PhC1 conversion approaches 100% at ca. 350~ apart for PtMFI280 and PtFER55 where total combustion was reached at 400~ Fig. 2 shows also that the curves of PhClx formation of all Pt/zeolite samples present a maximum in the 300-350~ temperature range, i.e. about 100~ lower than that observed in the case of PtA1. Moreover the yields to PhClx are sensibly lower compared to the A1203 supported sample, varying between 0.1% and 1.6% respectively on PtFER20 and PtBEA75. The much lower formation of PhClx observed on the PtFER20 sample is confirmed by results reported in Fig. 3 where the selectivity to PhClx as a function of the conversion level over Pt samples supported on the four zeolites with the lower SIO2/A1203 ratio is shown. With regard to the PhClx distribution, it is noteworthy that in the case of Pt on H-ZSM5 and H-ferrierite samples no PhC13 or higher polychlorinated benzenes were detected, whereas with Pt on H-Y and H-beta the PhClx distribution is similar to that previously observed on the PtA1 catalyst. .....

80 o~"

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.....

----i----- PtFAUS0(C) -- O - - PtFAU5(Y)

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.

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.

.

.

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.

.

.

.

.

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.

.

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---O---- PtMFI30(C) -= PtMFI280(C) - - O--PtMFI30(Y) -- - i ' - - PtMFI280(Y)

/ /

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~ - - P t

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./"

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_x

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~ / /" / A/r

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A

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0 250 300 350 Temperature (~

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v

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3~

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/

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0 200

--

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.

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._o >

4

v

> 40

100

8O

3~

PtBEA75(C) ~ : : ~ :-" PtBEA300(C) / / - - O - -PtBEA75(Y) / /

- - "A" - -

.9

200

~

400

0 200

250 300 350 Temperature (~

400

Fig. 2. PhC1 conversions (C) and PhClx yields (Y) vs. reaction temperatures over zeolite supported Pt samples.

1027

0.8 v

x r ,13_

~X~

0.6

.>_ oo

PtFAU5 PtBEA75

O -t-,

PtMFI30

0.4

PtFER20 0.2 0 60

70

80

90

100

Conversion (%)

Fig. 3. PhClx selectivity vs. PhC1 conversion levels over zeolite supported Pt samples (reaction temperature: 350~ It is important to underline that stability tests carried out in the 300-340~ range showed that under the experimental conditions used, all samples presented no measurable decrease in the activity during 10 hours of operations. It must be also reminded that on all supports alone the oxidation of PhC1 occurs at much higher temperatures (light-offs higher than 400~ than on the corresponding supported Pt sample.

3.2. Characterization of acid sites FT-IR of adsorbed pyridine and TPD of ammonia experiments were performed in order to study the acidic properties (nature and strength of acid sites) of zeolite supported Pt samples. Pyridine is widely used as a probe molecule for Lewis or Bronsted acidities [9]. The ring stretching vibrations are the most sensitive modes of pyridine with regard to the nature and strength of the adsorptive interaction. Bands at ca. 1640 cm1 and 1540 cm-1 are assumed to be characteristics for pyridinium ions (adsorption of pyridine on Bronsted sites), whereas bands in the region 1600-1630 cm-1 and 1440-1455 cm-1 are attributed to coordinatively adsorbed pyridine on Lewis sites. These latter bands increase in wavenumber as the strength of interaction increases and therefore gives an indication of the strength of Lewis acid sites [9]. Fig. 4 shows the FT-IR spectra in the region 1700-1400 crn-~ of zeolite supported Pt samples after admission of pyridine and subsequent evacuation at 250~ to eliminate the physisorbed pyridine. All samples exhibited bands characteristics of both Bronsted and Lewis acidity. However it appears clearly that H-ferrierite supported samples contain predominantly Bronsted acid sites and rather few Lewis acid sites. The ratio of Lewis to Bronsted sites increases moving from Pt/H-ZSM5 to Pt/H-beta to Pt/H-Y catalysts. From the figure it can be also noted that, according to the literature [10] on each type of zeolite the number of both Bronsted and Lewis sites strongly decreases on increasing the SiO2/Al203 ratio of the zeolite.

1028

PtFER20

_~.~L,.~

._....__..F~.~ _

,,-:,.

PtMFI30

v

O t-

PtMFI280

t-

,-, - PtMFI280

t/) (/) r t~

0 .Q

<

PtBEA300

1

PtFAU5 ............. rtr/~uon4"trAJ-l~

--

'

-

-- .

.

.

.

.

1700 1650 1600 1550 1500 1450 1400 Wavenumber (cm -1)

Fig. 4. FT-IR spectra after admission of pyridine and subsequent evacuation at 250~ over zeolite supported Pt samples.

0

PtFAU80

i

,

i

,

i

100

200

300

400

500

600

Temperature (~

Fig. 5. TPD of ammonia profiles (m/z=16 fragment) of zeolite supported Pt samples.

Information on the acidity can be also obtained by TPD of ammonia, which allows to measure the total number of acid sites, giving also an indication on their strength. NH3-TPD profiles for all zeolite based catalysts, normalized to the mass of the catalyst, are reported in Fig. 5. The total amounts of desorbed ammonia, expressed as mmol of adsorbed NH3 per gram of catalyst and estimated by integrating the area of desorption peaks, are listed in Table 1. On all samples two broad desorption peaks can be observed respectively in the 100-350~ and 350-600~ range (Fig. 5). The low temperature (L-T) peak, which appears generally to include two different components, can be ascribed to weak (maximum at ca. 150~ and medium strength acid sites (maximum at ca. 250~ whereas the peak above 350~ (H-T) is typical of strong acidity [10]. It is clear that the area and the position of these peaks depend on the type of zeolite used as support. Within the same class of zeolite, the area of both L-T and H-T peaks decreases on increasing the SIO2/A1203 ratio of the zeolite (Fig.5 and Table 1). On H-ferrierite and H-ZSM5 zeolite based catalysts the intensity of the H-T peak is higher compared to the other zeolite based catalysts with similar SIO2/A1203 ratios. Moreover it can be observed that the maximum of the H-T peak, and therefore the strength of strong acid sites, is in the order Pt/H-ferrierite > Pt/H-ZSM5 > Pt/H-beta > Pt/H-Y. In the case of Pt/H-ferrierite samples the high value of number of acid sites calculated by NH3 desorption appears to be not in agreement with the results of FT-IR of adsorbed pyridine (Fig. 4), which showed, on these samples, pyridine bands of rather low

1029 intensity. This can be explained considering that not all acid sites of ferrierite are easily accessible to piridine molecules, due to channel size constrains [11 ]. 4. DISCUSSION Table 2 summarizes the main catalytic results for all the investigated samples. It must be reminded that all supported Pt samples exhibited a much higher activity than the corresponding unsupported ones, clearly indicating that the presence of platinum strongly improves the oxidation activity of catalysts [3-4]. By comparing data reported in Table 2 it is possible to note that all zeolite based Pt samples (apart from PtMFI280) exhibit a higher activity compared to the PtA1 sample. In particular Pt catalysts supported on H-Y and Hbeta are more active than Pt on H-ZSM5 and H-ferrierite. Moreover it can be observed that on each type of zeolite the activity of Pt/zeolite catalysts is higher on the sample with lower SIO2/A1203 ratio. Catalysts characterization has shown that the number of both Bronsted and Lewis acid sites decreases on increasing the Si02/A1203 ratio. This could suggest that the acidity of the support is, to some extent, involved in the PhC1 oxidation over Pt/zeolite catalysts. A relationship between activity and strong Bronsted acidity has been reported in the oxidation of aliphatic chlorinated hydrocarbons over H-zeolites [7]. Nevertheless it must be noted that, for example, the activity of PtFAU80, which is the sample with the lowest total acidity, is higher than that of PtMFI30 which has a number of acid sites sensibly (ca. 50 times) higher than PtFAU80. This latter sample shows, however, a higher H/Pt ratio, and therefore a higher Pt dispersion, compared to PtMFI30. These considerations suggest that acidity has a role in controlling the oxidation activity of supported Pt samples which is less important than that played by the active metal dispersion. With regard to the reaction products distribution it must be reminded that, in all catalytic tests, CO2 was the main carbon containing product, with very small amounts of CO formed only at low conversion levels. In the case of supported Pt catalysts PhClx were also formed, Table 2 CatalYtic activity data of supported Pt samples . Code

Ts0% (-) (~

.

.

.

.

Vx 104 (b) (moles gca( 1 h "1)

. PhClx (c)

(%)

PtA1 340 1.8 2.0 (450 ~ PtFAU5 290 12.7 1.2 (300~ PtFAU80 320 7.5 1.5 (300~ PtBEA75 290 13.3 1.6 (300~ PtBEA300 320 5.0 1.4 (300~ PtMFI30 320 5.6 0.6 (300~ PtMFI280 340 1.5 0.5 (350~ PtFER20 320 4.2 0.1 (300~ PtFER55 330 3.5 0.4 (350~ (a)' Temperature at which 50% PhCI Conversion was reached; co) calculated at 300~ ~c)maximum production of total polychlorinated benzenes expressed as % yield (in parentheses the temperature to match).

1030 in accordance to literature data [3-4]. From Table 2 it is evident that Pt/zeolite samples produced lower amounts of PhClx compared to the Pt/AbO3 sample. In particular the formation of PhClx is in the order Pt/A1203 > Pt/H-beta _= Pt/H-Y > Pt/H-ZSM5 > Pt/Hferrierite. It is noteworthy that on each zeolite the formation of PhClx is roughly independent of the SiO2/A1203 ratio. This suggests that acidity is not directly involved in directing the selectivity to PhClx of Pt/zeolite samples. In order to explain the above trend of PhClx formation it could be considered that zeolites used as support have different structures and pore sizes, the latter progressively increasing from ferrierite to ZSM5 to beta and Y [12]. Therefore it is likely that the zeolite pore dimension can be the main responsible for the selectivity to PhClx. This is quite reasonable considering that a lower size of the zeolite channels should hinder the chlorination of PhC1 to PhClx. A further confirm of this hypothesis derives from the observation that in the case of Pt/H-ZSM5 and Pt/H-ferrierite samples the further chlorination of PhC1 is limited to the formation of PhC12 whereas on HY and H-beta supported Pt samples PhC13 isomers were also detected. It has been also reported that dispersion can have an influence on the formation of PhClx, with smaller particles producing more PhClx [4]. In our case, however, considering that Pt/H-beta and Pt/H-ferrierite samples show similar dispersions but different PhClx selectivities, it can be reasonably supposed that PhClx formation on Pt/zeolite catalysts is mainly controlled by a size selectivity effect induced by the zeolite. 5. CONCLUSIONS On the basis of the results here reported it can be concluded that zeolite supported Pt samples can be suitable catalysts for the combustion of chlorobenzene. Among all systems studied in this paper, Pt supported on H-ferrierite appears to be the best catalyst considering that on this system the formation of polychlorinated benzenes is very low. REFERENCES

1. 2. 3. 4. 5. 6. 7.

J. Spivey, Ind. Eng. CherrL Res., 26 (1987) 2165. G.H. Hutchings, C.S. Heneghan, I.D. Hudson, S.H. Taylor, Nature, 384 (1996) 341. R.W. van den Brink, R. Louw, P. Mulder, Appl. Catal. B, 16 (1998) 219. R.W. van den Brink, R. Louw, P. Mulder, Appl. Catal. B, 24 (2000) 255. Y. Liu, M. Luo, Z. Wei, Q. Xin, P. Ying, C.Li, Appl. Catal. B, 29 (2001) 61. G. Sinquirg J.P. Hindermmm, C. Petit, A.Kiennemmm, Catal. Today, 54 (1999) 107. J.R. Gonzalez-Velasco, R. Lopez-Fonseca, A. Aranzabal, J.I. Gutierrez-Ortiz, P. Steltenpohl, Appl. Catal. B, 24 (2000) 233. 8. L. Becker, H. Forster, J. Catal., 170 (1997) 200. 9. J.A. Lercher, C. Grtmdiling, G. Eder-Mirth, Catal. Today, 27 (1996) 353. 10. S. Narayanan, A. Sultana, Q.T. Le, A. Auroux, Appl. Catal. A, 168 (1998) 373. 11. B. Wichterlovh, Z. Tvruzukovh, Z. Sobalik, P. Sarv, Microporous and Mesoporous Mater., 24 (1998) 223. 12. D.W. Meier, D.H. Olson, Atlas of Zeolites Structure Types, Butterworth-Heinemann, London, 1992.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1031

Ag a n d C o e x c h a n g e d f e r r i e r i t e in lean N O x a b a t e m e n t w i t h CH4

P. Ciambelli, D. Sannino, M.C. Gaudino and M. Flytzani-Stephanopoulos* Department of Chemical and Food Engineering, University of Salerno, 84084 Fisciano, Italy. * Department of Chemical and Biological Engineering, Tufts University, 4 Colby St., Medford MA 2155, USA. Ag-Co/FER catalyst in lean NOx-SCR with CH4 has been investigated. Preliminary results show that bimetallic Ag-Co/FER gives higher performances in NOx-SCR with CH4 than the relevant monometallic systems in terms of both NOx reduction and CO2 selectivity. At low temperature the ability to convert NO to NO2 is associated to the presence of Ag. 1. INTRODUCTION The interest towards natural gas as fuel for automotive applications is based on ecological characteristics such as reduced photochemical reactivity and toxicity, low CO2 emissions, and high efficiency in lean engines. However, CH4 and NOx exhaust emissions must be reduced in order to match the future regulated limits. Selective catalytic reduction (SCR) of NOx by hydrocarbons (HC) in oxygen rich atmosphere on Me-exchanged zeolites was deeply investigated [1]. Particularly, Cu/ZSM5 with alkenes and Co-exchanged ZSM5, FER and BEA with light alkanes are active for SCR-NOx in the absence of water and sulphur dioxide. Co/FER catalysts showed good performances for lean NOx reduction with metane [2-4], but the activity was depressed in the presence of water. In very few papers zeolites containing only silver were usually reported to be not very active in HC-SCR. Halasz et al. [5] reported that Ag-H/ZSM5 showed negligible activity in the reduction of NO by propane, due to the inability of Ag to promote NO2 production, with NO2 probably being an initial reaction intermediate. Good performances of Ag-containing zeolites in lean NO reduction with methane were achieved by promoting the catalysts with a metal active in NO2 formation, such as cerium in Ce-Ag/Na-ZSM5 [6, 7]. Recently it has been reported that silver-proton-exchanged zeolites are highly active in NO2 abatement with propane in the presence of water [8]. High SCR activity of Ce-Ag-Na/ZSN5 [9] and Ag/A1203 [10] in the presence of water was also found. However very few data on CoAg bimetallic zeolite catalysts have been reported in some patents [11-13], dealing also with the use as adsorbents for exhaust gas purification. In this work the catalytic activity of Ag/FER, Co/FER, and Co-Ag/FER in lean NOx-SCR with CH4 has been investigated.

1032 2. EXPERIMENTAL

2.1 Catalyst preparation Synthetic Na,K-ferrierite (FER) with Si/A1 ratio of 8.4 (Engelhard) was used as parent zeolite. Ammonium ferrierite (AFER) was obtained by ion exchange at room temperature with 1 M aqueous solution of NH4NO3. The ion exchange was carded out for 48 hours and three times to obtain an exhaustive exchange. AFER was washed with bidistiUed water and dried at 120~ overnight. AFER was successively calcined at 550 ~ for 2 hours to obtain hydrogen ferrierite, HFER. Ag-ferrierite (AgFER) samples were prepared by exchanging either HFER or AFER with AgNO3 aqueous solutions (AgI~ and A ~ respectively) at different temperatures and different concentrations of the solution, renewing the solution many times. By varying the exchange time, different Ag contents were obtained. Since Ag + materials may be sensitive to light, all the above procedures were carried out in the dark. Co-ferrierite (CoFER) samples were prepared by ion exchange of AFER at 80 ~ with a Co(CH3COO)2 aqueous solution at a concentration of 1.6"10 .2 mol/1 [4]. Ag, Co containing samples were prepared by ion exchange of A ~ R at 80 ~ with a Co(CH3COO)2 aqueous solution at a concentration of 1.6" 10-2 mol/l. After drying at 120~ overnight, the samples were calcined at 550 ~ for 2 h. Table 1 contains the list of the samples and the procedures for the ion exchange. Briefly, all the samples are indicated with the symbol of the metal, the symbol for the starting zeolite (F, HF, AF), a number that indicates the weight percentage of the metal in the sample, and the symbol HT (high temperature) or LT (low temperature) to specify if the ion exchange was carried out at 80~ or at room temperature. For example, Ag(2.2)AFLT is the sample obtained starting from AFER by ion exchange at room temperature, and with an amount of cobalt of 2.2 wt %. 2.2 Catalysts characterisation An ICP-AES Varian Liberty II instrument was used for the analysis of Ag, Co, Si, A1, Na and K. Samples were solubilised with a mixture of hydrofluoric and perchloric acid at high temperature, then dissolved with hydrochloric acid in bidistilled water, before analysis. The thermal behavior of the samples as a function of the temperature was determined by air flow thermal analysis (TG-DTG) using a NETZSCH STA 209 thermobalance. Measurements were carried out with 15 mg of sample in chromatographic air flow (20 Nml/min) with heating rate of 5 ~ in the temperature range 20- 800 ~ In order to evidence any microporosity modification occurring after the introduction of the metal in the zeolite, N2 adsorption and desorption isotherms were obtained with a Sorptometer Kelvin 1040 instrument (Costech Instruments), after pretreatment at 350 ~ C for 1 hour in He flow. The Dubinin model was used for micropore volume evaluation. 2.3 Catalytic test The catalytic activity for the reduction of NO with CH4 was determined with a laboratory apparatus consisting of a flow-rate measuring and control system (mass flow controllers HITECH, AS A), and a fixed bed flow microreactor electrically heated and equipped with a temperature programmer-controller (Yokogawa P27). Two on-line IR analysers for NO, NOx, CH4, 02, (HB URAS 10E with NO2-NO converter CGO-K) and CO, CO2 (HB URAS 10E), and one on-line gas chromatograph (Dani 86.10 HT) for the analysis of 02, N2 and N20 were used. An analog-digital board (NI, AT MIO16E) was employed for PC acquisition of

1033 concentration data from the analyzers. The catalytic tests were carried out in the temperature range of 300 -700 ~ The heating rate was 5 ~ [gausing at 50~ intervals so as to reach a steady state condition. Space velocity was 30,000 h". The reactor was loaded with 300 mg of catalyst (180-355 Bm particle size), fed with 1000 ppm of NO, 100 ppm of NO2, 1000 ppm of CI-I4, 2.5 vol % of 02, balance helium. 1 vol % of water was added to the feed stream for catalytic experiments under wet conditions. All the results of the catalytic tests will be reported in terms of reactants conversion and products yield. For each reactant the percentage conversion, Xi, is defined as: Xi =100*(Ci~ ~ where index i indicates the ith reactant, Ci~ and Ci are, respectively, the feed concentration and actual concentration.The percentage yield (Yi) for each product is defined as: YNO2=100*CNo2/CNox~ , YN2=100*2*CN2/CNox~ Yco2=100*Cco~JCcn4~ Yco=100*Cco/CcH4~ where NOx is the sum of NO and NO2. As previously [3], the reaction system is more complex with respect to that generally assumed in the literature: it contains both heterogeneous reactions and homogeneous NOx catalysed CH4 combustion. NO + 1/2 02 <=:>NO2 2NO + CH4 + 02 -~ N2+ C02 +2H20 2CH4 + 202 --> CO2 +CO+ 2H20 CO+ 1/202 ---) CO2

Heterogeneous reactions

Homogeneous reaction

CH4+NOx+7/202-~CO2+CO+4H20 +NOx

Table 1. List of the catalysts and preparation conditions Samples

Number of Time, h exchanges

Starting cation

Solution concentration, mol/l Temperature

Ag(1.1)AFLT

1

20

NH4§

0.01 M AgN03,RT

Ag(2.2)AFLT

1

40

NH4§

0.01M AgN03, RT

Ag(3.7)AFLT

2

186

NH4§

0.005M AgNO3, RT

Ag(6.8)AFLTHT

3

186/115

NH4§

0.005M AgNO3, RT, 80~

Ag(2.1)HFLT

1

20

IT

0.01 M AgNO3, RT

Ag(1.7)HFLT

1

94

IT

0.005 M AgNO3, RT

Ag(6.7)HFLT

2

186

IT

0.005 M AgNO3, RT

Ag(3.8)HFHT

1

72

IT

0.005 M AgNO3, 80~

Co(1.3)AFHT

1

6

NHa§

0.016 M Co(CHaCOO)2, 80 ~

Co(2.0)AFHT

1

18

NH4§

0.016 M Co(CH3COO)2, 80 ~

Co(1.7)Ag(2.5)AFLTHT

2

301/67

NH4§

Ag(4.0)Co(1.3)AFHTLT

2

6/24

NH4

0.005M AgNO3, RT, 0.016 M Co(CH3COO):, 80 ~ 0.016 M Co(CH3COO)2, 80 ~ 0.002 M AgNO3, RT, dark

1034

3. RESULTS AND DISCUSSION In Table 2 the Ag and or Co wt % contents of samples are reported. By comparison with Table 1, Ag loading increases with exchange time for both AgAF and A g I ~ catalysts, as found for Co based catalysts [4]. Starting the preparation from HFER results in higher metal loading with respect to A g ~ , at comparable time of ion exchange. The temperature of ion exchange does not influence substantially the final metal loading as the initial concentration used. The degree of exchange is strongly lower than the theoretical one, also for AgI-I catalysts (max 0.40). Also bimetallic catalysts were well below the theoretical exchange capacity (<55%). In the case of Co(1.7)Ag(2.5)AFLTHT, prepared from Ag(6.8)AFLTHT, the final Ag content decreased, indicating that the successive Co-exchange removes part of exchanged Ag. Starting from Co(1.3)AFHT, the following exchange with AgNO3 performed at room temperature does not modify the Co content ( Ag(4.0)Co(1.3)AFHTLT sample), leading to higher Ag content in a shorter time. The micropore volume (Table 2) is mostly comparable to that measured on the parent zeolite [4], 130 mm3/g, while in highly rich Ag catalysts decreases slightly. Therefore neither ion exchange neither calcination conditions lead to pore occlusion. Figure 1 shows TG and DTG curves for Ag-ferrierite samples, obtained by AFER and HFER, without calcination. All the samples showed an initial weight loss at temperatures below 120 ~ due to the adsorbed water loss. The samples prepared from AFER showed a second weight loss in the 200-500 ~ range, due to ammonium decomposition, absent in the AgI-IF samples.

Table 2. Results of ICP analyses and N2 adsorption-desorption tests at 77K. Ag, wt%

Ag/AI

Co, wt%

Ag(1.1)AFLT

1.09

0.06

-

Micropore volume, mm3/g 115

Ag(2.2)AFLT

2.16

0.12

-

114

Ag(3.7)AFLT

3.66

0.21

-

Ag(6.8)AFLTHT

6.82

0.40

-

99

Samples

Co/Al

Ag(2.1)HFLT

2.12

0.12

-

101

Ag(1.7)HFLT

1.69

0.09

-

105

Ag(6.7)HFLT

6.66

0.39

-

102

Ag(3.8)HFHT

3.84

0.22

-

127

Co(1.3)AFHT

-

-

1.3

0.16

116

Co(2.0)AFHT

-

-

2.0

0.21

127

Co(1.7)Ag(2.5)AFLTHT

2.49

0.15

1.66

0.18

98

Ag(4.0)Co(1.3)AFHTLT

4.03

0.25

1.3

0.16

110

~ _ _ _

,

,

1035

o~176 i -0.1

~

-0.2 ~-0.3 -0.4 --

100

A

,

96 ~: 92 88

'

0 Figure

150

300

"

450

600

'

750

'

900

I

'

150

I

'

300

I

450

'

I

'

I

600

'

750

900

1. TG and DTG curves ofAg(1.1)AFLT ( e ) ; Ag(1.1)AFLT after calcination (o);

Ag(1.7)HFLT ( e ) ; Ag(2.1)HFLT (A); Ag(3.7)AFLT (11). After calcination the weight loss in the 200-500 ~ range disappeared, so confirming that the calcination procedure can remove all the ammonium from the zeolite. In Figure 2 NOx conversions in the reduction of NOx with CH4 on AgAF samples are reported. The activity of all the samples was low (NOx conversion in the 5-10 % range) and poorly dependent of the Ag content. However, AgAF catalysts were active in the oxidation of NO to NO2 in the range 300-500~ (maximum yield of 15 % at 350~ Higher yield, close to the equilibrium values (27% at 450~ was achieved with Ag(2.2)AFLT. 30

CH 4

-d

g

CO

l 20

100

so

9

~

"~ 10

40 ~j

"

o

j 0

, 3~

4~

, 5~

6~

'::T-T-; I ' I ' 7~

4~

5~

6~

7~

4~

5~

6~

~::1-- ~ I 7~

4~

5~

' 6~

7~

Figura 2. CH4-SCR results for Ag(1.1)AFLT (o), Ag(2.2)AFLT (11), Ag(3.7)AFLT (A), Ag(6.8)AFLT (+). Equilibrium NO2 yield ( 9

r

1036 Methane conversion curves (Figure 2) show similar onset temperatures of oxidation (around 400~ on all samples, but higher conversion was observed for Ag(2.2)AFLT in the range 400-600~ Then methane conversion becomes similar on all samples, reaching 100% at 700~ Significant selectivities to CO were shown in the range 450-700~ showing a maximum at 650~ As described previously, CO yield is to ascribe to the homogeneous, NOx catalysed, combustion of methane, active in this temperature range [4]. Catalytic tests performed on the AgI~ samples (not reported) showed similar performances (10% NOx conversion at 550~ despite the higher Ag content than in the AgAF samples. Low NOx conversion and high activity in NO oxidation to NO2 were also obtained. Selectivity to CO2 was the same as for the AgAF samples. The comparison of reaction rates indicate that A g ~ R catalyts are less active with respect to Ce-Ag/ZSM5 and Ag/A1203 [6, 10]. Bimetallic Co(1. 7)Ag(2.5) AFHT (Figure 3) was active in the range 300-650 ~ At low temperatures, NO was converted to NO2 (25 % maximum conversion at 400 ~ then NO2 yield decreased, according to the reaction equilibrium curve. NOx reduction began after 370 ~ and increased with the temperature, showing a maximum of 20 % at 550 ~ CH4 oxidation starts at 370~ with 100 % selectivity to CO2 up to 650~ The comparison of the performance of Co containing ferrierite and Ag containing ferrierite with Co(1.7)Ag(2.5)AFHT, reported in Figure 4, shows that the presence of both metals in the zeolite resulted in increased conversion of both NOx (20% at 550~ and CH4 (100% at 650 ~ The effect is especially marked for the conversion of methane (100 % at 650 ~

O

90-

~ ~

A

50-

so

v

70-

~

O

d. 40 --

o

60-

o .

Oo

"" 3o-q

*v

~

-t

.Jilt

A rt

9

g~

A

o O

20

f: ~

-

~ 30 -,~, _

o v

o 40--

~

,v

o

~0

9

r~

..-~

300

9

400

o oo

500 Temperature,

600

700

~

300

400

500

600

tOO

Temperature, ~

Figure 3. CI-I4-SCR results for

Figure 4. CH4-SCR results for Ag(2.2)AFLT

Co(1.7)Ag(2.5)AFLTHT: YNO2 (+); XNO (A);

(I1,[3), Co(1.7)Ag(2.5)AFHTLT (0, 9

XNOx (ll); XCH4 (V); VC02 (O); YCO ( , ) ;

Co(2.0)AFHT (A,A): XNOx (solid symbols), XCH4 (void symbols).

Equilibrium YNO2 ( 9

1037 60

100 0

g

o

g d ~

0

~0~

g

o

40--

o

o

-

~ ~176 ~ 400

500 Temperature, ~

600

d

20

oo

300

40-

oQ

10-

~p co

,im

~m

20--

O 0

o @o

-

o

9

o~

60--

o

0

9

o

30--

0

80--

' ' 700

Figure 5. CH4-SCR results for Ag(3.7)AFLT

(11,13); Ag(4.0)Co(1.3)AFHTLT(A, A). Co( 1.3)AFHT(~, <>): XNOx (solid symbols), XCH4 (void symbols).

~o

300

400

500

600

700

Temperature, ~ Figure 6. CH4-SCR for Co(1.7)Ag(2.5)AFLTHT

conditions (solid symbols) and with 1 vol. % of water (void symbols): XCH4 ( 9149 XNOx

(re,n); YNO~ (A,A).

Analogous results were observed on Ag(4.0)Co(1.3)AFHTLT, reported in Figure 5. By comparing the deNOx activity of Co-ferrierite and Ag-ferrierite at similar metal loading, it is evident that the presence of Ag enhanced the catalytic activity at low temperature. At high temperature the deNOx activity, ascribable to Co species, is depressed. Moreover, the conversion of methane was promoted at low temperature. Despite the higher metals content, activity of Ag(4.0)Co( 1.3)AFHTLT was lower than Co(1.7)Ag(2.5)AFLTHT. This effect, not attributable to a decrease in micropore volume, could be due to the preparation conditions. Both the sequence of metals introduction or a changing in the kind of exchanged sites in the double ion exchange could cause modification in the distribution of active species. Moreover the influence of metal loading could be considered, as on Ag catalysts best performances were shown by samples at low Ag content [ 10]. This phenomenon will be studied in the future work. NO2 formation, observed on all the Ag containing samples, was slightly dependent of the Ag loading, whereas Co-ferrierite catalysts did not have any NO oxidation activity [4]. Thus NO2 formation could be attributed to Ag+ species, in contrast with the literature reports [ 1, 5] that attribute low NOx reduction activity to the inability of Ag to act for NO2 production. On Ag(2.5)Co(1.7)AFHT a slight decrease of NOx conversion was found in the presence of 1% of water (Figure 6), evidencing higher stability than Co-exchanged catalysts [4]. Catalytic selectivity didn't seem to be affected by the presence of water. Less NO oxidation to NO2 was also found.

1038 4. CONCLUSIONS Ag-ferrierite catalysts showed low DeNOx activity in the range 300-700~ without substantial differences with respect to the preparation conditions. Significant NO2 production is observed in the range 300-450~ little depending on Ag content. Bimetallic Ag, Co-ferrierite catalysts give higher performances in NOx-SCR with CH4 than monometallic systems in terms of NOx reduction and CO2 selectivity. The presence of Ag seems to have a promotional effect on the catalytic activity, at low temperature favouring the conversion of NO to NO2. At high temperature the deNOx activity, ascribable to Co species is depressed. Under wet feed Ag,Co-FER catalysts showed higher stability with respect to Co catalysts. 5. REFERENCES

1. Y. Traa, B. Burger, J. Weitkamp, Microporous Mesoporous Mat. 30 (1999) 3. 2. Y. Li, J.N. Armor, J. Catal. 150 (1994) 376. 3. P. Ciambelli, P. Corbo, M.C. Gaudino, F. Migliardini, D. Sannino, Top. Catal. 16/17 (2001) 413. 4. D. Sannino, M. C. Gaudino, P. Ciambelli, Stud. Surf. Sci. Catal. 135 (2001) 329 5. Halasz, A. Brenner, Catal. Lett. 51 (1998) 195 6. Z. Li, M. Flytzani- Stephanopoulos, Appl. Catal. A, 165 (1997) 15. 7. Z. Li, M. Flytzani- Stephanopoulos, J. Catal. 182 (1999) 313. 8. J. A. Martens, A. Cauvel, A. Francis, C. Hermans, F. Jayat, M. Remy, M. Keung, J. Lievens, P.A. Jacobs, Angew. Chem., Int. Ed. Engl. 37 (1998) 1901. 9. Z. Li, M. Flytzani- Stephanopoulos Appl. Catal. B, 22 (1999) 35. 10. Keshavaraja, X. She, M. Flytzani-Stephanopoulos, Appl. Catal. B, 27 (2000) L1. 11. Y. Li, J.N. Armor, US Patent 5 260 043, 9 November, 1993. 12. Y. Li, J.N. Armor, US Patent 5 149 512, 22 September 1992. 13. G. Bellusi, L.M.F. Sabatino, T. Tabata, M. Kokitsu, O. Okada, H. Ohtsuka, European Patent Application 739 651, 30 October 1996 14. C. Shi, M. Cheng, Z. Qu, X. Yang, X. Bao, Appl. Catal. B, 36 (2002) 173.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1039

The effect of sulfate ion on the synthesis and stability of mesoporous materials M. L. Guzmb.n-Castilloa, H. Armendhriz-Herreraa, A. Tobrn-Cervantes a, D. R. Acostab, P. Salas-Castillo a, A. Montoya de la F a. and A. Vb.zquez-Rodriguez a alnstituto Mexicano del Petr61eo. Eje Central Lhzaro Chrdenas 152, Col. San Bartolo Atepehuacan, C.P. 07730, Mdxico, D. F. E-mail: [email protected] blnstituto de Fisica, Universidad Nacional Aut6noma de Mrxico. A.P. 20-364, Mrxico, D. F. The formation of hexagonal MCM-41 structures was studied using cetyltrimethyl ammonium bromide as the surfactant template and tetraethyl-orthosilicate as the silica source under hydrothermal conditions in the presence of sulfate ions. For the as-synthesized samples sulfate ions improved the hydrothermal stability of the MCM-41 material. Increasing sulfate concentration in the synthesis gel the amount of water lost by the sample decreases, indicating the formation of a solid with a more hydrophobic character. Since sulfate groups also favors of condensation of silicate units, which increase of wall thickness. 1. INTRODUCTION The molecular sieves with ordered microporous such as zeolites have been used widely as catalysts, adsorbents and ion exchange media [ 1]. Recently, there have been growing interests in the synthesis of new molecular sieves extending the pore diameter to the mesoporous region [2, 3]. Ordered forms of mesoporous materials may be synthesized in a variety of hexagonal, cubic and lamellar phases [2, 4-6]. A self-assemble process has been used for obtaining silicate and alumino-silicate forms of these materials in either acidic or basic solution. This process is driven by charge matching considerations between a surfactant assembly and the polymerizing inorganic framework [6, 7]. The electrostatic assembly approach has been extended to other mesostructured compositions by implementation of other complementary synthesis routes [8], which differ from the original one by the nature of the electrostatic interactions between the organic and inorganic phases. The synthesis of the HMS (hexagonal mesoporous silica) afforded an important subset of hexagonal mesoporous molecular sieves. They differ significantly from molecular sieves obtained by the electrostatic assembly pathways. They usually possess thicker framework walls (2-3 nm compared with ca. 1 nm for MCM-41), small X-ray scattering domain size and textural mesoporosity. The small X-ray scattering domain sizes, evidenced by the absence of narrow dll0 and d200 reflections in the X-ray pattern, reflect a quite short range of hexagonal order compared with MCM-41. It has been found that the mesoporous structure MCM-41 purely siliceous collapsed in hot water and aqueous solution due to silicate hydrolysis [9, 10] limiting its applications associated with aqueous solutions. Research efforts have thus been made to improve the hydrothermal stability of MCM-41 by either changing the synthesis procedure

1040 [11] and composition of initial gel [12, 13] or post-synthesis modification [14, 15]. Recently, new varieties of silica mesostructures that exhibit greater hydrothermal stability have been synthesized [3, 9, 16]. The addition of various sodium salts into the gel mixtures was reported to yield Si-MCM-41 stable in boiling water [ 17]. It has also been stabilized by post-synthesis modification with trimethylsilylation [18] and phosphoric acid [19], fluorinated MCM-41 material [20], etc. The aim of this work is to report a synthesis procedure for the direct obtaining of a hydrothermally stable mesoporous silica material with a large wall thickness using sulfate as additive. 2. EXPERIMENTAL SECTION

2.1. Synthesis The Si-MCM-41 samples were prepared by hydrothermal conditions with the following molar ratios: OH-/SiO2 = 10.99, H20/SiO2 = 67.8, CTMA/SiO2 = 0.304, SO4/SIO2 = 0-1. The gel mixture was prepared using tetraethyl orthosilicate, TEOS (Aldrich), NH4OH (Baker), (NH4)2SO4, (Baker), Cethyltrimethylammonium bromide, CTMABr (Sigma) and distilled water. The CTMABr was dissolved in distillated water and mixed with NH4OH solution. TEOS was mixed with (NH4)2SO4 solution and then added slowly to the CTMABr solution maintaining stirring until a white gel was obtained. The gel was heated at 120~ during 24 h. The solid phases were recovered by filtration, washed to neutrality and dried at 100~ The organic material was eliminated by calcination at 550~ for 6 h.

2.2. Characterization Thermogravimetdc analysis (TGA) of the samples was performed using a TG-7 PerkinElmer apparatus with a heating rate of 10~ under dry air flow (50cc/min). The powder xray diffraction patterns were measured in a D-500 SIEMENS diffractometer with a graphite secondary beam monochromator to obtain a monochromatic CuKal radiation and the evaluation of the diffractograms was made by DIFFRAC/AT software. The scanning was made from 1.5 to 10, 20 degrees with a 20 step size of 0.02 and step time of 2 s. In order to obtain comparable x-ray intensities of the MCM type materials, the same illuminated sample area was carefully controlled. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2405 apparatus. Before analysis the samples were evacuated at 350~ under vacuum (10 .4 torr). The surface area was calculated using the BET method based on adsorption date in the partial pressure P/Po range from 0.01 to 1. The mesopore volume was determined from the amount of N2 adsorbed at a P/Po = 0.4 High Resolution Transmission Electron Microscopy was carried out in a JEOL 4000 EX electron microscope at a relative low magnification. The images were studied directly by conventional methods and also using the CRISP compute program. Powder samples of mesoporous-like materials were softly ground in a mortar and then deposited on 200-mesh copper covered with holey carbon films. 3. RESULTS AND DISCUSSION

3.1. X-Ray Diffraction (XRD) Analysis The X-ray powder diffraction pattems of as-synthesized and calcined samples obtained at different sulfate concentrations are shown in Figures 1A and 1B respectively.

1041 B

2

4

6 20

8

S04/SiO a o 0.01 0.1 0.5 10

SO4/SiO2 0 oo

4

6 2O

0.01 0.1 0.5 1'0

Figure 1. X-ray powder diffraction patterns of as-synthesized (A) and calcined (B) MCM samples prepared using different SOa/SiO2 ratios. The XRD spectra of the as-synthesized samples showed the characteristic pattern of the hexagonal array of typical MCM-41 materials. However, the intensities of XRD peaks and the resolution of higher order peaks decreases as the sulfate content increases. After calcination at 550~ the hexagonal structure of the as-synthesized samples is retained. Moreover, at lower sulfate content (SO~-/SiO 2 -0.01 and 0.1 samples) the calcination step brought about also an increase in the order of the structure. The XRD patterns showed a more intense and sharper peak corresponding to the dl00 reflection. The d110 and d200 reflections were also clearly identified, indicating the long-range order of MCM-41 hexagonal framework. The development of X-ray intensity as well as the increase of the cell parameters with the temperature suggests that the hexagonal MCM-41 structure can continue its formation in the calcination step. These results show that the incorporation of a small amount of sulfur to silica MCM-41 synthesis gel brings about an improvement in structural ordering and the thermal stability of mesoporous samples. However, the structural ordering decreases progressively as the content of sulfate is increased. The sample prepared with SO42-/SiO2 - 0 . 5 ratio exhibited only the single dl00 reflection peak, which became broader and less intense. Efforts to prepare sulfated-promoted MCM-41 with higher sulfate contents under hydrothermal conditions (SO42-/SIO2 - 0 . 7 5 and 1 ratios) led to amorphous products. Probably at these high sulfate contents, the silica ionization is accelerated and a severely disorder of SO42- interactions at the micelle-solution interface is present, which does not allow the hexagonal liquid crystal mesophase formation. Although a preformed surfactant mesophase is by no means a prerequisite [21, 22] for the formation of MCM-41 structure, the silica-surfactant cooperatively seems to be essential in the formation of composite mesostructures [23]. Table 1 XRD dl00 peak, Hexagonal Unit Cell Parameter (ao), and Wall thickness (t) of the calcined Sulfate-Promoted MCM-type samples. SO24-/8iO2 dloo (A) ao (/~.) t (/~) 0 40.1839 46.4017 11.6417 0.01 40.1830 46.4007 12.2924 0.1 41.1460 47.5127 14.7172 0.5 52.6160 60.7575 29.5417 t parameter was calculated as reported by Di Renzo et al [24, 25]

1042

A

3"

:g

SO4/SIO2 = 0.01

.01 SO/SiO = 0

SO4/SIO2 = 0 2

4

6

8

10

20

zl

6

8

1'0

20

Figure 2. XRD pattems of S024-/SIO2 = 0 and 0.01 samples treated hydrothermaly in boiliing water at 120~ for 24 h. (A) as-synthesized samples and (B) calcined samples. Parameters as pH, temperature, the ionic strength, counterions presence and other additives were found to exert a key influence on both the surfactant behavior and the distribution of silicate species. In Table 1, it can be observed that as the sulfate content is increasing the growing of the wall thickness was favored (from 11.6417 A of SO2-/SIO2 = 0 sample to 29.5417 A of SO2-/SIO2 - 0 . 5 sample). In these same samples, the interplanar spacing dll0 peak increases from 40.1839 A to 52.6160/~. Considering that the type of surfactant was not changed and co-solvents were not used, the observed increase in the interplanar spacing is explained by the increase in wall thickness as showed in Table 1. The addition of sulfate ions to the synthesis gel results in the improvement of the hydrothermal stability of mesoporous material. In Figure 2 are shown the XRD patterns of SO4z-/SiO2 - 0 and SO4z-/SiO2 -0.01 samples treated hydrothermaly in boiling water at 120 ~ for 24 h. Interestingly both assynthesized samples retained the MCM hexagonal structure after hydrothermal treatment. Moreover, in the sulfate-promoted sample an increasing in the long-range order was observed. By contrast, in Figure 2B, it can be seen that calcined samples presented a more great damage. However, although broader and less intense the sharper peak corresponding to the dl00 reflection was identified in the SO4z-/SiOz -0.01 sample and no in the non-promoted sulfate sample. It is worth to mention that the sulfate content in the final solids was independent of the sulfated added in the synthesis gel. For all sulfate-promoted samples the chemical analyses showed about 1 wt. % of sulfate. Since, the sulfate content was the same in all sulfate-promoted samples the distortion of the long-range ordering due to the gradual incorporation of sulfate into framework structure, like in Si-A1-MCM-41 type structures [26] can be discarded. Then, although the alkalinity in the synthesis medium remained constant, it seems that the sulfate groups promoted the condensation of silicate units. Finally, the incorporation of condensed silica units accounts for the increased wall thickness.

3.2. Thermogravimetric analysis (TGA)

Thermogravimetric analysis of the sulfated-promoted mesoporous samples showed distinct weight losses stages, which depended on the SO]-/SiO 2 synthesis ratio used. In a general way, it was the material temperature SO24-/SIO2

observed that as the SO]-/SiO 2 ratio increases the total weight loss showed by decreases. The quantitative analysis of the different weight loss stages, wt % and range, are summarized in Table 2. The first weight stage (I), proportional to ratio, occurs below 145~ this weight loss shows a systematic decreasing. It has

1043 Table 2 Weight Loss Stage of Sulfated-Promoted Mesoporous Samples. Stage I Stage II Stage III Stage IV Stage V SO 24-/SiO 2 Lost Lost Lost Lost Lost (wt.%) ....... (wt.%) (wt.%) (wt.~ (wt.%) 0 5.19 8.83 12.75 2.51 2.72 0.01 4.77 9.09 11.79 2.36 2.47 0.1 3.94 5.98 9.36 2.46 2.50 0.5 3.27 0 5.51 1.25 3.43

Stage VI Lost

Total Lost

(wt.%)

(wt.%)

3.46

32 30.48 24.24 16.95

been normally associated to desorption and removal of water and/or ethanol molecules physisorbed on the external surface area or occluded in the mesoporous present between the crystallite aggregates. Considering that both 1-120 and C2HsOH are polar molecules a more hydrophobic mesoporous material is being formed by the sulfate presence. In a general way, it was observed that as the SO42-/SIO2 ratio increases the total weight loss in the material decreases. In SO42-/SiO2 - 0.5 sample a reduction of about 37 % in the first weight loss stage respect to SO42-/SIO2 = 0 sample was measured. For the SO42-/SIO2-0, 0.01 and 0.1 samples after removal of physisorbed ethanol and water molecules, four weight loss stages were identified, in the following temperature ranges: 125-220~ (II), 220-290~ (III), 280390~ (IV) and 390-800~ (V). In accord to literature [1-3] the three first weight loss stages are associated to removal of organic compound. The last one, which extended up to practically 800~ is related to water losses from the condensation of adjacent silanol groups to form siloxane bonds (dehydroxylation). For SO24-/SIO2 - 0 . 5 sample an additional weight loss stage was detected in the 510-800~ temperature range (stage VI). It can be associated to sulfate decomposition. At lower sulfate content the lost of sulfur probably is incorporated in the stage V. As was described above only a little fraction of sulfate used in the synthesis was incorporated to the final material. Moreover, if sulfate was not clearly detected in TGA study, it means that sulfate was eliminated in the filtration step after the synthesis. On this sample the weight loss stage II was not detected. From Table 2, it can be seen that the weight loss stages II and III continuously decreases as the SO42-/SiO2 ratio increases. In addition, with the increasing of the sulfate concentration in the material, the amount of water lost decreases, indicating that the solid with high sulfate concentration is more hydrophobic. 3.3. Textural Properties Figure 3A shows the N2 adsorption-desorption isotherms for calcined products. The isotherms for SO24-/SiO2 = 0 and 0.01 samples were very similar and showed two types of mesoporous. According to Tanev et al [27] the presence of framework-confined mesoporous Table 3 Textural Properties of Sulfated-Promoted MCM samples Determined by N2 Physisorption SO~-/SiO 2 SBET(m2/g)) Dp (A) Vmes (cc/g) 0 700.782 36.590 0.5879 0.01 681.348 35.903 0.5402 0.1 547.680 34.522 0.4176 0.5 229.860 32.859 0.1641 Dp and Vmes were calculated as reported by Di Renzo et al [24,25]

1044

so/s~%

800I

4

.-. 7~176

600t

B

o

A

0.01

500

SOJSiO~

0.1 3oo t

0 0.5

9 200]

oo

&'of4 RELATIVE

0.'6 o18 1.o

PRESSURE

(P/Po)

,~ "7--- . . . . 10

,,~%.,..E. ........

............... , ........

|

100 1000 Porous Diameter (A)

0.01 0.1 .0.5 9

Figure 3. Nitrogen adsorption-desorption isotherms (A) for the materials after calcination. Differential pore size distribution (B) calculated from nitrogen adsorption isotherms. is indicated by the adsorption step centered in the relative pressure (P/Po) region from 0.1 to 0.5, in our results this relative pressure is defined from 0.1 to 0.4. The height and steepness of the adsorption step indicate that the uniformity of the framework mesoporosity decrease with the sulfate concentration. The lost of long-order of the samples as sulfate increases is indirectly supported by the wide interval of relative pressure A(P/Po), in which the nitrogen capillary condensation occurs during the low-temperature adsorption. Table 3 lists the textural properties of the samples prepared with different sulfate content. It is clear that both the specific surface area and the pore volume of sulfate-promoted samples decrease with increasing sulfate content. For the most concentrated sample (SO]-/SiO 2 - 0.5 ) a decreasing in the specific surface area of about 67 % was observed. Although the long-order of the MCM-41 type samples was lost with increase sulfate content, the growth of the MCM aggregates was favored over those presents in non sulfated-promoted sample. On the basis of both the XRD results and the N2 adsorption data, the increasing of sulfate concentrations changes the uniformity of the framework mesoporosity of the MCM materials. On the other hand, the appearance of a well-defined hysteresis loop on the isotherms in the P/Po region from 0.4 to 1.0 indicates the arising of porosity from non-crystalline intra-aggregate voids [ 16]. From Figure 3A it can be observed that as the sulfate content increases this space formed by inter-particle contacts decreases (the hysteresis loop of the SO]-/SiO 2 = 0.5 sample is greatly reduced). The pore-size distribution of the MCM materials calculated from the desorption branch of the isotherm by the BJH method is shown in Figure 3B. All the samples showed a bimodal porous diameter distribution. It is worth to noting that both pore size distributions fall in a very narrow range. The main pore size distribution, whose pore diameter is centered at about 25-27 A, is related to intra-aggregate voids observed in N2 isotherms between 0.4-1.0. This pore-size distribution is shifted to lower values as the sulfate content increases. The pore-size distribution observed around 37-38 A corresponds to the frameworkconfined mesoporous. It is in accord to the length of the micelle used, so that its diameter is not affected by the sulfate content. The diminution of the intensity of both peaks in SO42-/SiO2 =0.5 sample is indicative of the lost of the long-range order of MCM-41 hexagonal framework at this high sulfate content, as observed in XRD results.

3.4. High resolution transmission electron microscopy (HRTEM) analysis In all the samples MCM-41 was the majority phase detected either in isolated laminates or in different laminates. However, MCM-48 material was also detected in some laminates.

1045 A

B

Figure 4. HRTEM micrographs of (A) SO]-/SiO 2 =0 sample and (B) SO]-/SiO 2 - 0 . 5 sample. In non sulfate-promoted sample

(SO]-/SiO 2 - 0

sample,

Figure

4A)

and

SO24-/SIO2 > 0.01 sample (not showed), in accord with X R results, an extended zone of a laminate presented the MCM-41 hexagonal profile of pores with uniform wall thickness. By contrast, in the sample prepared with the higher sulfate content (Figure 4B, SO]-/SiO 2 -0.5 sample) an extended zone of amorphous ring-like configuration was observed. Diffraction spots in some parts of micrograph also indicate short-range periodicity, which is in accord with XRD and textural results. 4. CONCLUSIONS The incorporation of sulfate to the synthesis gel (SO]-/SiO2 <0.3) favors the hydrothermal resistance of the as-synthesized samples. For the calcined samples this behavior was not clearly evident. The coexistence of MCM-41 and MCM-48 mesoporous material phases were observed in HRTEM micrographs. At high sulfate concentration (SO42-/SIO2 > 0.3 ) the material presents a short range of hexagonal order and the formation of HMS material beginnings to be detected, this is in according with the increasing of the wall thickness (t) measured around 30 A, characteristic of these materials. Just for SO24-/SIO2-0.5 sample a predominant disordered phase is detected. Crystallographic parameters derived from HRTEM observations are in close agreement with those derived from X-ray diffraction measurements. Increasing sulfate concentration in the synthesis gel the amount of water retained by the mesoporous material decreases, generating a material with a more hydrophobic character. ACKNOWLEDGEMENTS We thank D.00817 and D.01234 IMP Projects by financial support and Dr. J.P.P. for Sulfate Chemical Analysis.

1046 REFERENCES

1. Corma, A. Chem. Rev. 1997, 97, 2373 2. Kresge, C. T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C., Beck, J.S., Nature 1992, 359, 710 3. Kim, S.S., Zhang, W., Pinnavaia, T. J., Science 1998, 282, 1302 4. Beck, J., Vartulli, J., Roth, W., Leonowicz, M., Kresge, C., Schmitt, K., Chu, C:T.-W, Olson, D:, Sheppard, E:, McCullen, S. Higgins, J., Schlenker, J., J.A.C.S. 1992, 114, 10834. 5. Zhao, D., Feng, P., Huo, Q., Frederickson, G., Chmelka, B., Stucky, G., Science 1998, 279, 548. 6. Huo, Q., Margolese, D. I.:, Stucky, G.D. Chem.Mater. 1996, 8, 1147. 7. Firouzi, A., Kumar, D., Bull, L.M., Besier, T., Sieger, P., Huo, Q., Walker, S.A. Zasagzinski, J.A, Glinka, C., Nicol, J., Margolese, D. I.:, Stucky, G.D., Chmelka, B.F., Science. 1995, 267, 1138. 8. Huo, Q., Margolese, D.I., Ciesla, U., Feng, P., Gier, T:E:, Sieger, P., Leon, R., Petroff, P.M:, Schtith, F., Stucky, G.D., Nature 1994, 368, 317. 9. Ryoo, R., Kim, J.M., Ko, C.H., Shin, C.H., J. Phys. Chem. 1996, 100, 17718 10. Chen, L. Y., Jaenicke, S., Chuah, G. K., Microporous Mater. 1997, 12, 323 11. Robert Mokaya, J. Phys. Chem. B, 1999, 103, 10204 12. Bharat, L.N. and Ssridhar, K., Chem. Mater. 2001, 13, 4573 13. Robert Mokaya. J. Phys. Chem. B, 2000, 104, 8279 14. Vasant R. Ch., and Kshudiram, M., J. Catalysis, 2002, 205, 221 15. Laiyuan, Ch., Tatsubbro, H., Toshiaki, M., and Kazuyuki, M., J. Phys. Chem. B, 1999, 103, 1216 16. Jian, Y., Jian-Lin, S., Lian-Zhou,W., Mei-Lyn,T., Dong-Sheng,Y., Mat. Lett., 2001, 48, 112 17. Ryoo, R., Jun, S., J. Phys. Chem. B, 1997, 101, 317 18. Koyano, K., Tatsumi, T., Tanaka, Y., Nakata, S., J. Phys. Chem. B, 1997, 101,943 19. L. Huang, Q.Li, Chem. Lett. 1999, 829 20. Q-H.Xia, K.Hedajat, S. Kawi, Mat. Lett. 2000, 42, 102 21 Meyers, D., Surfactant Science and Technology, VCH New York, 1992. 22. McCormick, A. V.; Bell, A.T., Catal. Rev. Sci. Eng., 1989, 31, 97 23. M. Lindrn, S. Schacht, F. Schuth, A. Steel, and K. K. Unger. J. Porous Mat. 1998, 5, 177 24. Di Renzo, F. Testa, J.Chen, H.Cambon, A.Galarneau, D. Plee, F. Fajula. Microporous and Mesopsorous Mater., 1999, 28, 437 25. A. Galarneau, D. Desplantier, R. Dutartre, F. Di Renzo, Microporous and Mesopsorous Mater., 1999, 27, 297 26. Y.Cesteros, G.L. Haller, Microporous and Mesoporous Materials, 2001, 43, 171 27. Tanev, P.T., Pinnavaia, T.J., Chem. Mater., 1996, 2068

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1047

Catalytic b e h a v i o r o f Cd-clinoptilolite prepared b y introduction o f c a d m i u m metal onto cationic sites G. Onyesty~ik and D. Kall6 Chemical Research Center, Institute of Chemistry, Hung. Acad. Sci., P.O. Box 17, H-1525 Budapest, Hungary Cd-clinoptilolites were prepared using the conventional aqueous phase ion exchange procedure and by reacting a mixture of cadmium dust and NH4-clinoptilolite powder around 310 ~ i.e., near to the melting point of the metal. The solid phase reaction was indicated by H2 evolution and the decrease of XRD line intensities of cadmium metal. The NH3, leaving the cationic sites was retarded to some extent by cadmium species. Higher degree of ion exchange was achieved using the solid phase reaction than by the conventional aqueous phase procedure. The catalytic activities of the preparations were compared in hydration of acetylene. The activities were tested in IR cell by reacting gas phase acetylene at 100 ~ with water adsorbed on the catalyst wafer resulting in formation of adsorbed acetaldehyde. The catalyst prepared by the aqueous phase Cd-exchange was more active, however, after treatment at 300 ~ i.e., at temperature close to that of the solid phase reaction, its activity was essentially lower than that of the solid state preparation. 1. INTRODUCTION Late transition metal forms of zeolites catalyze the hydration of acetylene to acetaldehyde [1]. Among them Cd-clinoptilolite proved to be the only stable and selective catalyst having the highest activity under steady state conditions, i.e., at 180 ~ under atmospheric pressure [2]. At 25-150 ~ the adsorption of acetylene and water [3], and the formation of adsorbed acetaldehyde [4] on catalyst wafers were detected by IR spectroscopy. Both acetylene and water were found to adsorb on Cd2§ sites (Z-: zeolite lattice anion). The surface reaction between adsorbed reactants has been confirmed by the kinetics determined under steady state reaction conditions [5]. In flow reactor the reaction rates are well measurable above 160 ~ At 220 ~ the crystal lattice of Cd-clinoptilolite prepared by aqueous phase ion exchange started to damage [6]. Cd-derivative ofheulandite having the same crystal structure as clinoptilolite but of lower Si/A1 ratio can not be prepared at all in this way because of structure collapse [6]. Hydrated bivalent cations are responsible for the low thermal stability of these crystal structures [7]. Solid state ion exchange seems rather promising since metal cations are substituted in absence of water when NH4-zeolites are mixed and heated with a salt of desired metal. The removal of volatile ammonium salt completes the ion exchange [8]. The efficiency of exchange has been shown to depend on the anion of the salt [9]. The reaction of metal dust and the acid sites of

1048 zeolites seems even more suitable for stoichiometric substitution of protons by metal cations. This solid state reaction resembles the dissolution of metals in protic acids when metals are oxidized to cations and hydrogen evolves. NH4-form may similarly be reacted with the metal dust near to deammoniation temperature. The preparation of Zn-zeolites in these ways was published lately [10]. Recently, the Zn-ZSM-5 was reported to be active in the aromatization of ethane [ 11 ]. This is the only known reaction, wherein a zeolite prepared by the mentioned solid state reaction, gained significance as catalyst. The introduction of metal cations in solid state reactions seems favorable because metal salts have not to be dissolved, the pH dependence of dissolution does not play any role, aqueous effluent containing the excess salt after ion exchange is not produced, hydrated cations do not participate in the process, by removal of volatile products, mainly hydrogen, the ion exchange equilibria are shifted to completness. The preparation procedure of the Znand the Cd-zeolites are distinctly different due to the different melting points of the metals, such as 419 ~ for Zn and 321 ~ for Cd. When the metal ion form is prepared from NH4form zeolite the deammoniation temperature is a key parameter. For instance, the deammoniation temperatures are 350 ~ for Y-zeolites and 450 ~ for clinoptilolite. We prepared Cd-clinoptilolite by reacting Cd dust with the H- or NH4-forms of the zeolite and tested the activities of preparations in acetylene hydration.

2. EXPERIMENTAL

Rhyolite tuff from Tokaj Hills/Hungary, with 53 % clinoptilolite content (Cp) was used. After exhaustive ion exchange (50 g Cp under refluxing for 7 h with 5x 1 L 1 N NHnC1) the zeolitic rock contained 1.358 meq NH4+/g (NHn-Cp, sample (1)). Deammoniation was carried out in air raising the temperature from 25 ~ by 10 ~ and keeping the sample at 450 ~ for 4 h (H-Cp). 30 g of Cp in its native form was exchanged for cadmium with 5x2 L 0.1 N Cd(NO3)2 solution under refluxing for 8 h (sample (2)). Sample (2) contained 0.5 meq Cd2+/g. Solid state reactions were carried out with 20 % excess of cadmium in well ground mixtures of Cd(NO3)2/NH4-Cp, Cd dust/NH4-Cp, and Cd dust/H-Cp (samples after thermal treatment are denoted by (3), (4), and (5), respectively). The Cd dust of < 60 ~tm particle size was a product of Merck, Germany. The temperature of treatments was raised stepwise in vacuum keeping the samples at 25, 100, 200, 300, 350 ~ for 20, 10, 30, 10, 30 min, respectively, in order to minimize the damage of crystal lattice by adsorbed water. High purity acetylene was produced by Messer Hungarog~WHungary. The reaction was followed by monitoring the hydrogen evolution while the Cd dust/zeolite mixture was heated up at a rate of 10 ~ in N2 stream (details are given in ref. [10]). The concentration of H2 was recorded using a thermal conductivity cell as a function of temperature. The amount of H2 was determined from the integrated area under the hydrogen evolution curve. Deammoniation was followed in N2 stream by TPD. Temperature was increased by 10 ~ The amount of desorbed N/-I3 was determined by acidimetric titration as a function of temperature. The cation content of samples (1)-(5) were related to 1 g Cp calcined at 600 ~ for 30 min. XRD patterns were measured with Philips P W 1810 X-ray diffractometer equipped with graphite monochromator using CuKct = 0.154018 nm radiation. IR spectra of self-supporting wafers were recorded with a Nicolet 5PC FTIR spectrometer. Absorbances were related to 5 mg/cm2 film thickness. Before IR measurements water was

1049 adsorbed on (3)-(5) heat treated samples at 0.026 bar, 25 ~ for 10 min, followed by treatment in vacuum at 100 ~ for 1 h. Only the latter treatment was used for samples (1) and (2). After recording IR spectra of pretreated samples, they were contacted with 0.03 bar acetylene at 100 ~ Spectra were recorded thereafter from time to time for 3 h in order to follow the formation of adsorbed acetaldehyde. 3. RESULTS XRD pattern of the Cd dust/sample (1) mixture was measured at 25 ~ before and after heat treatment in N2 at 350 ~ for 1 h. On effect of treatment, the intensity of Cd reflections decreased by 50-70 % while that of Cp reflections remained unchanged (patterns are not attached). The curves of temperature programmed H2 evolution of Cd dust/sample (1) and Cd dust/H-Cp mixtures of the same compositions as samples (4) and (5), are plotted in Figure 1.

1000

I

Cd+H-Cp I Cd+NH -Cp I . . . .

4

800

5

t~ tO 0

#%

i'

600

t_

E 0 E 0 to "1-

400

200

0 200 I,

,

,

,

,

,

9

,

9

I

,

300

,

,

,

,

,

,

,

,

I

,

400

,

t

|

|

|

|

|

.

I

i

|

500

Temperature,

|

,

,

,

,

|

,

I

,

600

,

,

,

,

,

,

,

,

700

~

Figure 1. Temperature-programmed H2 evolution from well ground mixtures of cadmium dust and NH4-Cp, of cadmium dust and H-Cp. The evolution of H2 starts at 255 ~ and ends at 410 ~ The peak positions are at 310 ~ and 304 ~ the amounts of evolved hydrogen are 0.695 and 0.515 mmol/g corresponding to

1050 oxidation of 1.390 and 1.030 meq Cd/g for mixtures (4) and (5), respectively. Because of about 20 % excess Cd the mixture contained 1.644 meq Cd/g since the ion exchange capacity of Cp 1.358 meq/g. All the cationic sites could have been occupied by Cd 2§ in the case of NH4-Cp according to the reaction Cd ~ + 2 NH4+Z---~ Cd2+Z-2 + 2 NI-I3 + H2 Some surplus H2 evolved: 2 x 0.695 - 1.358 = 0.032 meq/g ~ 0.016 mrnol H2/g and 1.644 1.390 = 0.254 meq Cd/g did not react at all. TPD curves of NU3 for sample (1) and Cd/sample (1) are shown in Figure 2. The two curves overlap up to 400 ~ when 0.38 mmol NH3/g has been desorbed. The desorption of NH3 attains 1.36 mmol/g at 600 ~ for (1) while in the presence of Cd dust this desorbed amount could be attained at higher temperature, only.

-Cd+NH4-C p m

0

E E tO

E E

_

"o 0 t~

a

0

0

200

400 Temperature,

600

~

Figure 2. Temperature-programmed NH3 desorption from NHa-Cp in absence and in presence of cadmium dust. IR spectra of samples (1)-(5) (Figure 3) display the absorbances of 6NH4 band at 1440 cm ~ and 6H20 band at 1630 cm l characterizing the ammonium content and water adsorption. Sample (2) after heat treatment at 350 ~ as samples (3)-(5) is denoted by (2/a).

1051

JE~

(2/a)

/'~

L--

o (/)

< (4) (5)

2000

1900

1800

1700

Wavenumbers,

1600

1500

1400

1300

cm 1

Figure 3. IR spectra of samples (1)-(5); (1): NH4-Cp alter treatment in vacuum at 100 ~ for 1 h; (2): Cp ion exchanged in aqueous phase for Cd 2+, treated in vacuum at 100 ~ for 1 h; (2/a): sample (2) aiter heat treatment at 350 ~ following samples were heated stepwise to and treated at 350 ~ (see text), cooled, water adsorbed at 0.026 bar and 25 ~ for 10 min, then treated in vacuum at 100 ~ for 1 h: (3): solid state ion exchange with Cd(NO3)2/NH4-Cp mixture; (4): mixture of cadmium dust and NH4-Cp; (5): mixture of cadmium dust and HCp. Aiter establishing the water contents of (2)-(5) the samples were contacted with acetylene and the formation of adsorbed acetaldehyde (AAa) was detected by IR spectroscopy [6]. First spectra were recorded for each sample at 25 ~ aiter contacting 0.03 bar acetylene for 10 min then these spectra were subtracted from the spectra determined afLer contacting 0.03 bar acetylene at 100 ~ for 3 h in order to eliminate disturbing bands such as that of 8H20 at 1630 cm -~ (Figure 4). The absorbances of 5Cn,sym band at 1355 crn~ are characteristic for AA~ since this band is well separated and is of sufficient intensity. However, the band of vco at 1680-1710 cm 1 contributed to AA~ is more intense, comprises different bands making uncertain the evaluation. The following assignments can be distinguished [12]: at 1681 crn~ bonding to Bronsted acid site, at 1699 c m "I t o C d 2+ and at 1709 crff I to Ca 2+, Na + ions remaining after cadmium ion exchanges. For samples (3) and (5) characteristic bands of adsorbed crotonic aldehyde (CA~) also appears at 1630 and 1658 crn~; CAa is the aldol condensation product of acetaldehyde [2]. The absorbances of Gcn,sym band were determined at 100~ and 0.03 bar acetylene pressure for (2)-(5) samples of established water content as a function of time (Figure 5).

1052

'1--0.2 O O

E ,, (2)

.O O t/) .t)

<

-L3)--

.

..

(4} ......

E 2000

,

, 1900

- _

_,,J...%%-

iL

I

_

i

1800 W

I

1700 avenum

i

,

I

,

1600 bers,

I

1500

i

I

1400

i

1300

cm

Figure 4. IR spectra of samples (2)-(5) after contacting with 0.03 bar acetylene for 3 h at 100 ~ The spectra recorded at 25 ~ contacting with 0.03 bar acetylene for 10 min are subtracted (see text). The symbol of samples is the same as in Figure 3.

4. DISCUSSION Introduction of cadmium to the cationic sites of Cp is indicated by Ha evolution (Figure 1). For NH4-Cp the evolved H2 is a bit higher (by 0.016 mmol/g) than it would correspond to the cation exchange capacity of Cp. Presumably silanol groups of Cp reacted, too, as suggested in ref. [10]. The incorporation of Cd dust is confirmed by XRD measurement, however, the amount of remaining Cd metal (about 30-50 %) is higher than expected from the excess of Cd in the reacting mixture (100 x 0.254/1.644 = 15 %). The difference may be attributed to the different pretreatments: The temperature was slowly increased up to 400 ~ in the case of temperature programmed hydrogen evolution and it was kept at 350 ~ for 1 h in the case ofXRD measuremem. For H-Cp the HE evolution corresponding to 1.029 meq Cda+/g introduction is lower than for NH4-Cp since the heating of H-form holding adsorbed water because of grinding under ambient conditions results in partial hydrolytic destruction of the crystal lattice [ 13]. The desorption of NH3 from NH4-Cp up to 400 ~ is the same in presence and in absence of Cd dust (Figure 2). It amounts to 0.38 mmol/g whereas under the same condition, from H2 evolution, 1.39 meq Cd2+/g substitutes the NH4 § 1.39 - 0.38 = 1.01 mmol NH3/g should to be shiited to and retarded by any cadmium species, because it desorbs at higher temperatures than from NH4-Cp alone. Cd 2+ may act as a Lewis acid site for bonding NH3 [3].

1053 8 (2)

"7

E6

0 14'3 I..(3

5

II

E

J

"4 0 o

3

E:

0

2

5) (3),

or)

<

4

01

o

-

0

,

,

50

,

,

100

,

i

150

'

,

200

'

I

250

'

300

Time, min Figure 5. Formation of adsorbed acetaldehyde on samples (2)-(5) at 100 ~ 0.03 bar acetylene as a function of time. The intensities of ~q~IH4 bands at 1440 cm ~ relating to that of sample (1) (Figure 3) indicates that Cd 2+ exchange was 76 % for sample (3) and 78 % for sample (4), more than in sample (2). Solid state exchange seems to be more effective than conventional aqueous phase procedure. Cadmium content of sample (5) can not be estimated in this way. The water content of similarly conditioned samples related to sample (2) from intensities of 6H20 band at 1630 cm ~ are 66 %, 30 %, 70 %, and 40 % for samples (2/a), (3), (4), and (5), respectively. The formation of AAa as a function of time (Figure 5) shows that (i) the highest conversions can be attained with sample (2), but after heating it at 350 ~ (sample (2/a)) the lowest activity is observed which can not be explained by the decrease of water content; probably structural changes are responsible for the low activity [6,7]; (ii) the formation of AAa on sample (3) after 30 min is around 30 % of that of sample (2) in agreement with their relative water contents; thereafter AAa decreases due to the appearance of CA. (see in Figure 4); (iii) in spite of the larger ion exchange for Cd (78 %) in (4) than in (2) the initial formation rate of AAa after 140 min is less than 40 % of that on (2), however, the water content is only by 30 % lower and inhibiting CA, [2] is not formed;

1054

(iv) similar initial formation rates of AA~ are observable on (5) as on (4), but AA~ decreases after 30 min because of crotonic condensation (see in Figure 4) catalyzed by the rest of protonic sites [2]; (v) these observations reveal that catalytically active Cd2+-ions occupy less accessible or more hidden positions at high temperature. 5. CONCLUSIONS 1. Cd dust can be reacted with NH4-Cp resulting in active Cd-Cp catalyst for acetylene hydration to acetaldehyde. 2. However, the introduction of transition metal to the cationic sites results in less active catalyst than the conventional aqueous phase ion exchange, the former method may be sometimes more desirable because brine effluent is not produced, the cation loading is higher and introduction of additional metal(s) seems possible. 3. The results confirmed that transition metals react with NH4-zeolites near to their melting point. 4. The zeolite structure is not damaged when the metal is introduced at temperature as high as 350 ~ A C K N O WLIDG EM ENTS

The authors thank Dr. G. P~l-Borb61y for temperature programmed hydrogen evolution experiments and Mrs. Agnes Wellisch for valuable technical assistance. REFERENCES

1. 2. 3. 4. 5. 6.

G. Gut, K. Aufdereggen, Helv. Chim. Acta 57 (1974) 441. D. Kall6, G. Onyestyfik, Stud. Surf. Sci. Catal. 34 (1987) 605. D. Kall6, G. Onyesty/tk, Zeolites 17 (1996)489. G. Onyestyfik, D. Kall6, J. Molec. Catal. A, 106 (1996) 103. D. Kall6, G. Onyestyfik, Helv. Chim. Acta 84 (2001) 1157. G. Onyestyfik, D. Kall6, in Natural Zeolites '93 (eds.: D. W. Ming, F. A. Mumpton). Int. Comm. Natural Zeolites, Brockport, New York, 1995, pp. 437-445. 7. G. Gottardi. E. Galli: Natural Zeolites. Springer Verlag, Berlin, 1985, pp.256-284. 8. H.G. Karge, Stud. Surf. Sci. Catal. 105 (1997) 1901. 9. G. Onyestyhk, D. Kall6, J.Papp, Jr., Stud. Surf. Sci. Catal. 69 (1991) 287. 10. H. K. Beyer, G. P~il-Borb61y, M. Keindl, Micropor. Mesopor. Mater. 31 (1999) 333. 11. J. Heemsoth, E. Tegeler, F. Roessner, A. Hagen, Micropor. Mesopor. Mater. 46 (2001) 185. 12. J. Howard, J. M. Nicol, J. Chem. Soc. Faraday Trans. 86 (1990) 205. 13. H. Beyer, J. Papp, D. Kall6, Acta Chim. Hung. 84 (1975) 7.

MESOPOROUS MOLECULAR SIEVES

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1057

Confinement at n a n o m e t e r scale: w h y and h o w ? Francesco Di Renzo, Anne Galarneau, Philippe Trens, Nathalie Tanchoux and Francois Fajula Laboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRSENSCM, 8 rue Ecole Normale, 34296 MontpeUier, FRANCE direnzo@c i t. enscm, f r 1. INTRODUCTION

Is the research on new materials driven by the demand of the market or are unexpected applications made possible by the availability of materials discovered by serendipity? This debate about the most efficient strategy of research is proposed again at each stalemate of the invention-application cycle, when a dreamt application is looking for the proper material to become realistic or when a new class of materials is looking for practical applications. The breakthroughs on micelle-templated synthesis in the nineties have made available a whole panoply of ordered mesoporous materials with extremely narrow pore size distribution and high surface area [ 1-3]. The mechanisms of self-assembly of surfactant molecules and inorganic species have been studied in depth, thanks to the available knowledge on the physical chemistry of colloid phases [4-8]. Tailor-made materials can be prepared at any scale between 2 and 10 nm, and the available palette is broadening at every other issue of the scientific journals. Which customer will be able to profit of the original properties of these materials? A better knowledge of the phases confined in nanometer-scale matrices is urgently needed, to provide a link between the synthesis of the host solid and the processing of guest molecules. In this communication, we will present some data about the physics of molecules adsorbed in pores at the nanometer scale and provide some hints about the assessment of the properties of mesoporous adsorbents. 2. PHYSICAL CHEMISTRY OF ADSORBED PHASES

The peculiar properties of phases confined inside a mesoporous host have been largely studied as far as solid state physics is involved [9]. The preparation of quantum dots and quantum wires has allowed to tailor electronic and magnetic properties of semiconductors [10]. Embedding inside a mesoporous matrix has been used to modulate conductive properties of unsaturated polymers [11] and non-linear optical properties of dyes [12]. We are especially interested here to the properties of confined fluid phases. The most obvious effect of the presence of a porous matrix is a concentration effect. A dense phase is formed at a much lower partial pressure inside the porosity than in an open system. In a large field of partial pressure, the concentration of a molecular component passes from the range 102 g m 3 in the gas phase to the range 10 6 g m -3 inside the pore system. This concentration effect is at the basis of the use of adsorbents like zeolites, silica, or carbons in many separation and catalysis processes. In which way a confined phase differs from a bulk liquid phase? We will not deal here with the properties of fluid phases in zeolites and other microporous hosts, in which most adsorbed molecules are in direct interaction with the pore walls. However,

1058

confmement effects seem to modify the energetics of adsorbed fluids also in larger mesopores, with a pore size one order of magnitude larger than the size of the adsorbed molecules. Calorimetric and isosteric measurements of various adsorbates have been carried out on MCM-41 samples with pore diameter 3-4 nm. In the table 1, measured enthalpies of the pore falling step are compared with the condensation enthalpies in the absence of a confining system. The heat released by pore falling is nearly 20 % larger than the enthalpy of bulk condensation, indicating that the thermodynamics of the confined phase are affected by the walls of the host well beyond the first adsorbed monolayer. Table 1: Differential molar enthalpies of the pore filling step on MCM-41 compared with enthalpies of condensation. Temperature of the measurement and relative pressure of the step are provided. adsorbate T/K p/Po.................. A H p f AI-Ic AHpf/AI-~ ref. (kJ/mol) (kJ/mol) cyclopentane 293 0.25 ............ -35 -29 1.21 13 n-hexane 303 0.23 -38 -31 1.23 14 acetonitrile 303 0.41 -37 -32 1.16 14 water 292 0.52 -57 -44 1.30 15 tert-butanol 303 0.21 -52 -44 1.18 16 Adsorption-desorption cycles in small mesopores follow a peculiar non-hysteretical pattern. In Figure 1, the adsorption-desorption isotherms of N2 at 77 K are reported for mesoporous silicas of various pore size. While mesopores larger than nearly 4 nm present the usual type IV isotherm with the adsorption branch at higher relative pressure than the desorption branch, smaller mesopores present a type IV isotherm without any hysteresis. Intermediate mesopores present a shortened hysteresis loop, interrupted by a sudden desorption at a relative pressure of p/p0 0.42. The lowest closure point of the hysteresis loop depends on the nature of the adsorbate and the temperature of the measurement. The existence of this lowest limit of the hysteresis loop was early attributed to a tensional instability of the meniscus, unable to sustain the high curvature corresponding to small mesopores [ 17].

6o0i

700

500

,

'

,,,

I-

,

'

I

'

,

,

I

,

'

,

i

~

,

,

,

,~

400

~300 >. 200

100

0 ~ 0

0.2

0.4

0.6

0.8

1

P/P0

Figure 1. Adsorption-desorption isotherms of N2 at 77K on MCM-41 (left), trimethylbenzeneswelled MCM-41 (middle), and SBA-15 (fight). Lowest closure point of the hysteresis loop at p/po 0.42.

1059

The passage from a dense phase to a low-density gas in conditions in which the liquid-gas interface is unstable is strongly reminiscent of the definition of supercritical phenomena. The shift of the critical point in capillary conditions was calculated in the early stages of the density functional theory (DFT) methods [18, 19]. Adsorption phenomena on MCM-41 have been recently investigated to ascertain at which extent the lowest limit of the hysteresis loop can be identified with the capillary critical point [20-22]. The lowest closure points of the hysteresis loop in adsorbents of different mesopore size at several temperature levels can be plotted by using reduced coordinates Pr = P/Pc and Tr = T/To. In a log Pr VS. 1/Tr graph, the points for each adsorbate are aligned on straight lines passing through the critical point T = Tc and P = Pc. This behaviour indicates that the lowest limit of the hysteresis loop can be described by a law of corresponding states [ 16].

13.. O

1.0

1.5

2.0

2.5

1/Tr Figure 2. Position of the lowest closure point of the hysteresis loop in reduced coordinates for several adsorbates and pores of various size (Data from [ 17]). From a practical point of view, these linear correlations allow to predict the lowest closure point of hysteresis for a given adsorbate at any temperature once a single lowest closure point has been determined. The lowest limit of hysteresis seems not to depend on the nature of the adsorbent, the data of Figure 2 having been obtained on adsorbents as different as silica, titania, and carbon. Very likely, the presence of one or more adsorbed layers between the surface of the adsorbent and the condensing phase smooths down the potential differences between the different solids. 3. METHODS OF ASSESSMENT OF THE POROSITY

Adsorption methods are the main tools to characterize the porosity of any kind of adsorbents. In the case of the ordered mesoporous adsorbents prepared by micelle-templated synthesis, other techniques can usefully integrate the evidences from adsorption methods. High-resolution transmission electron microscopy (TEM) has provided some spectacular breakthroughs in the knowledge of micelle-templated materials. For instance, the section of the pores of MCM-41 has been recognized to be hexagonal by TEM [23], the structure of SBA- 1 and SBA-6 have been solved by TEM and microdiffraction [24], and the connections between the mesopores of SBA-15 have been identified by TEM of platinum replicas [25].

1060 Notwithstanding these outstanding results on carefully selected samples, TEM is not a userfriendly technique to provide routine quantitative data. Beyond the inevitable problems of homogeneity of the sample, it has been shown that any evaluation of wall thickness by TEM methods can be severely affected by the focusing procedure [26]. X-ray diffraction can provide relevant information on the periodical properties of micelletemplated materials. In the case of well-ordered solids, several diffraction lines can be observed and allow to identify the symmetry group. Once the symmetry (usually hexagonal or cubic) is known, the cell parameter a can be easily determined from the diffraction angles. The cell parameter is correlated to the pore size. In the case of MCM-41, a = d + t, where d is the diameter of the hexagonal pore and t is the thickness of the wall between the pores. In the case of MCM-48, a = 2(d + t). If the diffraction angles provide useful information on the cell parameter, any quantitative exploitation of the intensity of the diffraction lines is much more difficult. The diffraction pattern of the ordered mesoporous materials do not allow to define the position and occupancy of individual atomic sites, but depends on the average contrast between empty and filled parts of the solid. The presence of residual template or any adsorbate inside the porosity decreases the contrast level and strongly affects the intensity of the diffraction lines [27]. The periodicity of micelle-templated silicas allow to use the results of X-ray diffraction to better understand the adsorption data. The main information on the pore size is provided by the pressure of the adsorption and desorption steps of the type IV isotherm. How to calculate a pore size from the experimental isotherm? Several models allow to calculate the adsorptiondesorption isotherm and X-ray diffraction data allow to independently evaluate the reliability of these methods. The plain geometry of the pores of MCM-41 allows to calculate the pore size from the cell parameter a and the mesopore volume Vp, independently on the pressure of the isotherm step [28]. These values of diameter Dvmes+x~ are reported in Figure 3 for several samples of MCM-41. 100

-

~ 80 ~< 60 .,,.~

o

,J,

/

_! / D B d B

#r-/

"x

DDvr 3"

~ o 40

f

DBJH

J

20

I

/

S

s

9 D Vmes+XRD t:l 4V/S*

0(}.0. . . .0.2. . . . 0.4 . . . . 0.6 . . . . . 0.8 . .

p/p0

1.0

Figure 3. Comparison among several methods to evaluate the pore diameter of MCM-41 as a function of the relative pressure of the desorption step of N2 at 77 K [28]. The widely used BJH (Barrett, Joyner and Halenda) method, which applies the Kelvin equation to the calculation of the pore size, patently underestimates the pore diameter. Better results are obtained by the method of Broekhoff and de Boer [29], which modifies the Kelvin equation to

1061 take into account the interaction with the curved surface of the adsorbent. The DFF methods also provide a good evaluation of the pore diameter. It can be observed that the region of the smallest mesopores should be outside the limits of validity of the Kelvin equation: the instability of the meniscus for pressure below the lowest limit of the hysteresis seems to render meaningless any calculation based on the curvature of the meniscus. However, the method of Broekhoff and de Boer provides also in this pressure range a fair evaluation of the pore diameter at which desorption takes place. The usual Gurvitch method, in which the equivalent hydraulic diameter of the pore is calculated from the ratio between pore volume and surface area, severely underevaluates the pore diameter. The drawback of the method does not come from the geometry of the system but from the use of an inadequate value for the area corresponding to an adsorbed N2 molecule. The routine value of 16.2 ~2 for N2 molecule is calculated from the density of liquid nitrogen and has been shown to be valid for the adsorption on an hydrocarbon-lined surface. In the case of the adsorption on a silica surface, a more appropriate value is 13.5 ]k2 per molecule [30]. If this value is used in the calculation of a surface area S* from the volume of the BET monolayer, the formula D = 4V/S* provides a correct estimate of the pore diameter. 4. INFLUENCE OF THE PORE GEOMETRY ON THE PROPERTIES OF THE ADSORBENT

The model of the hexagonal honeycomb of MCM-41 can be used to study the correlations among geometry-dependent properties of the solid [31]. The surface area Sg (m2 g-l) of a perfect honeycomb is a function of the cell parameter a (nm) and the wall thickness t (nm): Sg = 4103 (a-t) / psit (2a-t) 2000 1800 [ 1600 1400 ~

1200

~ 1000 -% 800 600 400 200 0

2

4

6

8

10

a(nm)

Figure 4. Surface area of a hexagonal silica honeycomb as a function of cell parameter and wall thickness [31].

1062 In Figure 4 are reported the surface areas calculated from the geometry of the honeycomb taking into account the volumic mass of amorphous silica psi 2.2 g cm "3. For each value of wall thickness, a very low surface area is expected when the cell parameter is only slightly higher than the wall thickness. For more realistic honeycomb with a >> t, the surface area only depends on t and goes towards an asymptotic value Sg = 2103 / psit The pore volume Vf of the hexagonal honeycomb can be calculated by the same model as (a-t) 2 Vf-PsiXtx(2a-t ) 4.5 4 3.5 3 "7

2.5 o

x.j

2 1.5 1 0.5 0 2

4

6

8

10

a(nm) Figure 5. Pore volume of a hexagonal silica honeycomb as a function of cell parameter and wall thickness. The calculated volumes are reported in Figure 5 as a function of cell parameter a and wall thickness t. Starting from a = t, the pore volume increases about linearly with the cell parameter by a slope which increases when the wall thickness becomes smaller. These estimations of the properties of perfect silica honeycombs allow to evaluate at which point experimental solids correspond to the ideal geometry. Differences between calculated and experimental porosity have been at the basis of the characterization of the microporosity of SBA- 15 [32]. An abacus of the properties of MCM-41 as a function of the cell parameters can also orient the choice of the proper material for a given application. Such a choice is always the result of a compromise between conflicting properties. For instance, thinner walls allow to encapsulate a larger volume of fluid phase by using a smaller amount of confining solid. As a consequence, it should be desirable to use an adsorbent with the thinnest possible walls. A lowest limit to wall thickness is imposed by the stability of the solid.

1063 As an example, mechanical stability can be calculated for cellular solids. The crushing strength ar for the brittle failure of a hexagonal honeycomb upon in-plane uniaxial loading is ar162 = 4/9 (t/L) 2 where ae is the crushing strength of the bulk material (7.2 GPa for vitreous silica), t is the wall thickness and L is the side of the hexagonal cell [33]. This correlation, which becomes a_~=~x t2~ a c ~ (a-t) ~ if the cell parameter a is used instead of the cell side L, has been compared with experimental data on the stress-strain relationship of MCM-41, and has be found to slowly underestimate the actual strength [34]. The difference between experiment and calculation was probably due to a significant component of axial loading in the powder bed used for testing. It seems reasonable to assume that the calculated strength of the honeycomb provides a conservative evaluation of the strength of ordered MCM-41.

2.5

0nm

2 -~ 1.5 1 0.5

t-05 nm-.___ 0

2

4

-----___ 6

8

10

a (nm) Figure 6. Crushing strength of a hexagonal silica honeycomb upon in-plane uniaxial loading as a function of cell parameter and wall thickness. In Figure 6, the calculated strength of hexagonal silica honeycombs are reported. The crushing strength decreases when the unit cell becomes larger, as faster as the walls are thinner. It is clear that, for a given unit cell, any decrease of the wall thickness brings about a decrease of mechanical stability. Thermal stability has also been shown to be significantly worsened by a decrease of wall thickness [34]. The tailoring of the properties of the solid and their relevance to the catalytic behaviour have been recently reviewed [35, 36].

1064 5. T E M P L A T E EFFECT IN THE SYNTHESIS

The template effect in zeolite synthesis has often been described as the organisation of silica units around an organic molecule to form a structure which can retain the memory of the shape of the template after its extraction. The generality of this model, developed on a limited number of successful examples, has been questioned by the synthesis of very large-pore zeolites, VPI-5 and cloverite, with no direct correlation between the size and shape of the small template molecules and the large voids of the pore system. The need for a simultaneous organisation of the template and the silicate units was already clear in the formation of inorganic-templated zeolites, in which silicate units enter the coordination sphere of hydrated cations. It seems more and more clear that only in a limited number of instances templated synthesis corresponds to the condensation of network-forming units around a pre-existing template. In most cases, inorganic units assemble together with the template molecules in aggregates quite different from the state of the template alone. This effect of self-assembly was early shown for materials at the borderline between micropores and mesopores, formed by aggregation of small organic molecules [37], and confirmed by the formation of ordered silica-alkyltrimethylammonium mesophases well below the concentration threshold for the formation of surfactant mesophases in the absence of silica [38]. The mechanism of self-assembly of ordered mesoporous materials has been largely elucidated by the charge matching effect in the case of cationic surfactants [4, 7]. When nonionic surfactants are used as templates, the formation of ordered materials at low charge concentration is more difficult to model. A good example is provided by the complex synthesis of SBA-15, a well-ordered hexagonal mesoporous silica formed in the presence of polyethylene oxide (PEO)-polypropylene oxide (PPO) triblock copolymers [8]. The micelles of nonionic surfactant in solution are surrounded by a corona of hydrated PEO chains protruding from the micelle surface. The repulsion between these organic brushes keeps the micelles at a minimum distance of 3-4 nm [39]. PPO

Figure 7. Schematic representation of micelles of polyethylene oxide (PEO)-polypropylene oxide (PPO) triblock copolymers. A rise in temperature decreases the hydration of the PEO chains and the repulsion forces between micelles. SBA-15 is formed by a long low-temperature reaction of the surfactant with silica issued from the hydrolysis of tetraethyl orthosilicate. The solid formed presents a well-ordered mesoporosity between thick walls containing a disordered microporosity [32]. When this solid

1065 is treated at higher temperature, the mesopore size increases and the microporosity disappears. This behaviour is only partially comparable with the behaviour of the surfactant in the absence of silica. At high temperature, protruding PEO chains are indeed replied upon micelles also in the absence of silica, and the decreased repulsion between micelles is at the basis of the cloud point phenomena, but the increase of pore size with temperature seems much larger than the micelle swelling observed in the absence of silica [40]. It seems that, in the synthesis of mesoporous materials by using nonionic surfactants as templates, a step of impregnation by silica of a preexisting mesophase can compete with cooperative changes of the silica-surfactant system when temperature conditions are changed. The complexity of the formation mechanism hinders the prediction of the properties of the final material. ACKNOWLEDGMENT

The authors gladly acknowledge Edoardo Garrone for useful discussions on the physical chemistry of adsorbed phases. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147. F. Schiith, Chem. Mater. 13 (2001) 3184. A. Monnier, F. Schtith, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P.Petroff, A. Firouzi, M. Janicke, B.F. Chmelka, Science 261 (1993) 1299. A. Firouzi, D. Kumar, L.M. Bull, T. Besier, P. Sieger, Q. Huo, S.A. Walker, J.A. Zasadzinski, C. Glinka, J. Nicol, D. Margolese, G.D. Stucky, B.F. Chmelka, Science 267 (1995) 1138. A. Galarneau, F. Di Renzo, F. Fajula, L. Mollo, B. Fubini, M.F. Ottaviani, J. Colloid Interface Sci. 201 (1998) 105. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schiith, G.D. Stucky, Nature 368 (1994) 317. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. G.A. Ozin, A. Kuperman, A. Stein, Angew. Chem. Int. Ed. Engl. 28 (1989) 359. L. Chen, P.J. Klar, W. Heimbrodt, F. Brieler, M. Fr/3ba, H.A. Krug von Nidda, A. Loidl, Physica E 10 (2001) 368. C.G. Wu, T. Bein, Science 264 (1994) 1757. F. Marlow, M.D. McGehee, D. Zhao, B.F. Chmelka, G.D. Stucky, Adv. Mater. 11 (1999) 632. J. Rathousky, A. Zukal, O. Franke and G. Schulz-Ekloff, J. Chem. Soc. Faraday Trans. 91 (1995) 937. J. Janchen, H. Stach, M. Busio, J.H.M.C. van Wolput, Thermochim. Acta 312 (1998) 33. P.L. Llewellyn, F. Schiith, Y. Grillet, F. Rouquerol, J. Rouquerol and K.K. Unger, Langmuir 11 (1995) 574. F. Di Renzo, E. Garrone, A. Galarneau, P. Trens, N. Tanchoux, D. Brunel, B. Fubini, F. Fajula, submitted.

1066 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

C.G.V. Burgess, D.H. Everett, J. Colloid Interface Sci. 33 (1970) 611. H. Nakanishi, M.E. Fisher, J. Chem. Phys. 78 (1983) 3279. R. Evans, U. Marini Bettolo Marconi and P. Tarazona, J. Chem. Phys. 84 (1986) 2376. P.I. Ravikovitch, S.C. O' Domhnaill, A.V. Neimark, F. Schiith, K.K. Unger, Langmuir 11 (1995) 4765. K. Morishige, M. Shikimi, J. Chem. Phys. 108 (1998) 7821. S. Gross, G.H. Findenegg, Ber. Bunsenges. Phys. Chem. 101 (1997) 1726. V. Alfredsson, M. Keung, A. Monnier, G.D. Stucky, K.K. Unger, F. Schtith, J. Chem. Soc. Chem. CommurL 1994, 921. Y. Sakamoto, M. Kaneda, O. Terasaki, D.Y. Zhao, J.M. Kim, G. Stucky, H.J. Shin, R. Ryoo, Nature 408 (2000) 449. Z. Liu, O. Terasaki, T. Ohsuna, K. Hiraga, H.J. Shin, R. Ryoo, ChemPhysChem (2001) 229. S. Schacht, M. Janicke, F. Schtith, Microporous Mesoporous Mater. 22 (1998) 485. B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Microporous Materials 6 (1996) 375. A. Galarneau, D. Desplantier, R. Dutartre, F. Di Renzo, Microporous Mesoporous Mater. 27 (1999) 297. J.C.P. Broekhoff, J.H. de Boer, J. Catal. 10 (1968) 377. L. Jelinek, E. s. Kov~its, Langmuir 10 (1994) 4225. F. Di Renzo, D. Desplantier, A. Galarneau, F. Fajula, Catal. Today 66 (2001) 75. A. Galarneau, H. Cambon, F. Di Renzo, F. Fajula, Langmuir 17 (2001) 8328. L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, 2nd ed., Cambridge 1997. A. Galarneau, D. Desplantier-Giscard, F. Di Renzo, F. Fajula, Catal. Today 68 (2001) 191. F. Di Renzo, A. Galarneau, P. Trens, F. Fajula, in Handbook of Porous Materials, F. Schiith, K. Sing, J. Weitkamp (Eds.), Wiley-VCH, 2002, 1311. D. Trong On, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Appl. Catal. A 222 (2001) 299. G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde-Massara, G. Perego, Stud. Surface Science Catal. 84 (1994) 85. C.F. Cheng, Z. Luan, J. Klinowski, Langmuir 11 (1995) 2815 J.N. Israelachvili, H. Wennerstrtim, J. Phys. Chem. 96 (1992) 520 C. Booth, D. Attwood, Macromol. Rapid Commun. 21 (2000) 511.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1067

Anchorage of dye molecules and organic moieties to the inner surface of SiMCM-41 Yven Rohlfing a, Dieter W0hrle a, Ji[i Rathousl@ b, Arno~t Zukalb and Michael Wark c a Institute of Organic and Macromolecular Chemistry, University Bremen, D-28334 Bremen, Germany. b j. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, CZ-182 23 Prague 8, Czech Republic. c Institute &Physical and Electrochemistry, Universitfit Hannover, D-30167 Hannover, Germany. Based on siliceous MCM-41 material obtained according to a pH dependent method of homogeneous precipitation host-guest-systems with covalently gra~ed organic moieties, especially chromophores, were prepared. The highly ordered mesoporous material does not suffer from the multi-step procedure including passivation of surface silanol groups, functionalization with 3-aminopropyltriethoxysilane and subsequent anchorage of dye molecules and anhydrides via peptide, sulfonamide and anhydride imide bonds. The present work gives a survey on the incorporation of anhydrides and chromophores of four different types and proves their uniform distribution in the host pore system as well as their diffusion stability by different optical spectroscopy methods and physisorption studies. 1. INTRODUCTION Incorporation of organic moieties into porous inorganic solids results frequently in strong changes of the physicochemical properties of both, host and guest material. The embedding of larger organic molecules into intact zeolite structures by diffusion is restricted by the small pore diameters of the host; crystallization inclusion of guest species may conduct to lattice defects [ 1]. Since BECK et al. closed the gap between microporous and macroporous materials by introducing the mesoporous molecular sieves of the M41S family in 1992 [2], these limitations in the accessibility could be overcome. The wide pore opening and narrow pore size distribution of the channels open the way for manifold inclusion chemistry [3], e.g. covalent grafting of silane precursors followed by covalent or ionic anchorage of organic moieties. The tunable pore size and the opportunities for modification of interior channel walls of the highly ordered Si-MCM-41 lead to tailor-made materials with higher mechanical stability and hydrophobicity for catalysis and adsorption [4]. In the present communication a multi-step procedure for covalent anchorage of dye molecules is described. The definitive covalent graffing was lead out via peptide, sulfonamide or anhydride imide bonding.

1068 The properties of structure and surface of the organically modified materials were investigated by different analytic methods like N2 adsorption measurements or UV/VIS and IR spectroscopy in diffuse reflectance. The homogeneous distribution of the chromophore molecules was also proved by confocal fluorescence microscopy.

2. EXPERIMENTAL SECTION 2.1 Synthesis of siliceous MCM-41 and pre-silylation of its external surface The Si-MCM-41 host was synthesized by the homogeneous precipitation method using sodium metasilicate as silica source and cetyltrimethylammonium bromide as structuredirecting agent [5]. All the silylation reactions, described in the following, were carried out with freshly dried Si-MCM-41 material under nitrogen atmosphere. The solvents were distilled over desiccants under inert gas. In order to passivate the external surface 1 g of Si-MCM-41 was given in a flask and evacuated for 2 h at 10.3 mbar. Subsequently, the flask was filled with nitrogen. The sample was suspended in 30 mL dry tetrahydrofurane (THF) and 300 ~L of diphenyldichlorosilane (Ph2SiC12) were added under stirring. After 45 min the solid was filtered and extensively washed with THF and dichloromethane. Afterwards it was dried in a heating box and evacuated (10 .3 mbar) for 2 h. 2.2 Functionalization of the inner surface of Si-MCM-41 The functionalization was performed by anchoring varying amounts of 3-aminopropyltriethoxysilane (APTES) at the silanol groups of the inner pore walls. As solvent either dichloromethane or toluene were used depending on the desired reaction temperature. A detailed description of the reaction conditions has been given previously [6]. 2.3 Covalent bonding of dyes and anhydride functions The dyes 4'-dimethylaminoazobenzene-4-carbonic acid 1, Zn phthalocyanine tetrasulfonylchloride acid 3a and Si phthalocyanine tetrasulfonylchloride 3b (ZnPcTSCI) were prepared in our laboratory. RhodamineB sulfonylchloride 2 (FLU~) and 1-ethyl-l'-[hexanoicacid N-succinimide ester] indodicarbocyanine 6,6'-disulfonic acid 4 (AMERSHAM PHARMACL~) are commercial available. The inserted amounts of dyes are related to the amounts of parent Si-MCM-41 used. For the anchoring the dyes 1 - 3b the procedure was as follows. A distinct amount of 3-aminopropylsilyl-Si-MCM-41 (usually 0 . 5 - 0 . 7 g) was mixed in a flask with a desired quantity of the dye (0.001 - 1 mmol), dried under N2 atmosphere and suspended in 30 mL dichloromethane. The suspensions were pre-cooled with an ice/NaC1 mixture and stirred for 1.5 h. For the activation of the dyes either dicyclohexylcarbodiimide (DCC), dissolved in dichloromethane, or pyridine were added as reaction promoters. The reaction mixture was allowed to reach ambient temperature. After 20 h of stirring the recovered solid was extensively washed, subjected to a Soxhlet treatment for 3 d and dried in a heating box (see chapter 2.1).

1069 In case of 1-ethyl-l'-[hexanoic acid N-succinimide ester] indodicarbocyanine 6,6'-disulfonic acid (Cy5) 4 series of Si-MCM-41 samples containing extremely low dye concentrations were synthesized in order to perform single molecule detection (see below). To 1 g Si-MCM-41, silylated with 5 mmol of APTES, alcoholic solutions containing 4.7-10 -3- 4.7.10 -9 mmol of 4 were added. Then the reaction mixtures were stirred for 20 h in the absence of light. The obtained solids were vigorously washed with ethanol and subsequently treated in a Soxhlet apparatus. For the anchoring of the maleic anhydride 5 and phthalic anhydride 6, aiming the application of the anhydride group as potential coupling function for the attachment of diarylethene dyes, each 5 mmol were added to suspensions of aminopropylsilyl-functionalized Si-MCM-41 in ethanol. The reactions were stopped after heating overnight under reflux and the obtained functionalized solids were manipulated as described before. 2.4 Characterization Measurements of nitrogen isotherms at -196 ~ C on reference silica gels and on Si-MCM-41 samples were performed on a MICROMERITICS ASAP 2010 volumetric adsorption instrument. Before the measurements purely siliceous samples were degassed at 300~ for 24 h; the chemically modified samples were degassed at 100 ~ C for 48 h. XRD patterns were recorded on a PHILn,S X'pert Alpha 1 diffractometer. Diffuse reflectance UV/VIS spectra were obtained with a PERKIN-ELMER Lambda 9 spectrometer. The samples were also examined by DRIFT (diffuse reflectance infrared fourier transform) spectroscopy carried out with an BIO-RAD FTS-60A instrument equipped with a praying mantis (HARmCK) and a sealed sample holder with a vacuum system giving a base pressure of 10-8 bar. Single molecule detections via confocal fluorescence micrographs were carried out with a CARL ZEISS UMSP 80 microscope spectral photometer.

3. RESULTS AND DISCUSSION The surface silanol groups of the Si-MCM-41 parent material were functionalized with 3-aminopropyltriethoxysilane precursors. For grafting dyes and organic moieties to the amino groups peptide (a), sulfonamide (b) or anhydride imide bonding (c) were employed (Scheme 1). Activation is necessary in case of carbonic acid groups, e.g. by addition of dicyclohexylcarbodiimide (DCC) or by preparation of the anhydride, respectively. Sulfonic acid groups were activated by chlorination and further reaction in presence of pyridine. The outer surface of Si-MCM-41 particles and the most reactive silanol sites were blocked in a pre-silylation procedure with diphenyldichlorosilane under mild conditions in absence of promotors [7] to ensure the exclusive functionalization of the inner surface in the third step.

o -~- ~ O ~ s i ~

o ~N/x~r~

----~0 ~--O~si~Ss~ -~/

b

O~k//O ~

k"-~

--

0

~ ~ ~ 0/

o c o/~~

Scheme 1. Different methods for covalent grafting on amino-functionalized silica supports.

1070 Recent investigations designate that post-synthetic grafting also allows the embedding of single molecules in homogeneous distribution in lowest concentrations without any preliminary treatment (see chapter 3.2). It could be demonstrated that every synthesis step can be detected by DRIFT spectroscopy and XRD patterns [6]. The parent silica and pre-silylated materials are showing the characteristic reflections (100, 110, 200, 210) in X-ray diffraction patterns. The peak intensities of the diphenyldichlorosilane treated samples are slightly decreased. Due to the modification of the inner pore walls the intensity of reflections, especially of the 110-, 200- and 210-peaks, considerably decreases and the relative intensity of the 200-peak increases. The negligible changed d spacings confirm the intact structure. The chemical modification of silica surfaces results in several changes in the DRIFT spectra. The sharp absorption band at 3745 cm-1 and the broad band down to 2500 crn1 are attributed to the free silanol groups and hydrogen-bonded silanol groups, respectively [8]. The pre-silylation results in a slight increased intensity of the band of free silanol groups. Effected by further functionalization of the pore walls the band at 3745 crn-~ disappeared while the broad band ascribed to residual silanol groups shiRed to lower wavenumbers. The occurrence of characteristic peaks is indicating the organic modification, e.g. 3365 cm1, 3305 cm-~ (N-H), 2975 cm~, 2935 cm1, 2895 cm1 (C-H) for aminopropylsilane moieties [9]. Peaks in the fingerprint region are signifying the presence of amine, amide and imide functions.

i

CH3--....~

S02CI

/

ClO2S

\ ~

-

.~3

o

~

,~SO3

~=

1

0

4

!

k~'CH3

Scheme 2. Multi-step procedure for anchoring dye molecules: pre-silylation, functionalization of inner surface, grafting of 4'-dimethylaminoazobenzene-4-carbonic acid 1, rhodamine B sulfonylchloride 2, Si/Zn phthalocyanine tetrasulfonylchloride 3 and 1-ethyl-l'[hexanoic acid N-succinimide ester] indodicarbocyanine 6,6'-disulfonic acid (Cy5) 4.

1071 Scheme 2 shows the incorporated dyes coupled with the amino-functionalized interior surface of Si-MCM-41. Resulting from host-guest interactions and confining pores the inserted dyes exhibit a higher organization of molecular dipoles (see chapter 3.2) and alteration of UV/VIS spectra in comparison to measurements in solution.

3.1 Anchorage of azo 1_, rhodamine dye 2 and phthalocyanine dyes 3

~l~' " ii

!

I a

I -" I '~ I o" .,/,,"

,

I

, :

I t

~ . " ",

t

I

I

1

"

:'

/

500

',b ~

600

700

800

Wavelength /nm Figure 1. Diffuse reflectance UV/VIS spectra of covalently grafted SiPcPTSC1 (a, solid), covalently grafted ZnPTSC1 (b, dashed) and ionically anchored ZnPTS (c, dotted), 0.005 mmol dye offered/g.

The incorporation of dye molecules into the channel system of Si-MCM-41 conducts to a shift of the absorption bands in UV/VIS reflectance spectroscopy. The main absorption bands of azo dye 1 and rhodamine dye 2 are blue-shifted from 468 nm, measured in transmittance in aqueous solution (pH 7), to 441nm and from 565nm to 553 nm, respectively. It could be shown previously [6] that this strongly depends on the changed chemical environment in the modified pores containing residual basic amino-functions and silanol groups. Reference experiments with adsorbed azo dyes in modified and parent material indicate a strong red-shift in case of the pure Si-MCM-41 and only negligible differences of absorbanee for amino-modified materials. For providing strong fluorescence the presence of individual chromophore molecules is necessary. In case of anchored rhodamine dye the fluorescence maximum was found at 0.0075 mmol 2/g Si-MCM-41. It is claimed out that the aggregation of dyes is mostly inhibited at this low concentration.

Figure 1 represents spectra of anchored phthalocyanines in the Q-band region. The main absorption bands at higher wavelengths are attributed to monomers whereas the shoulder peaks at ~ ~ 600 nm are caused by excitation of dimers. In opposite to SiPc, with silicone coordinated by two OH-groups, ZnPc derivates tend to aggregate. Thus, spectra of SiPcTSC1 3b (a, main absorption ~ ~ 664 nm) indicate a mostly monomeric incorporation and a blue-shift in comparison to spectra from organic solution in DMSO (~ ~ 676 nm). Covalent anchoring of ZnPcTSC1 3a enhances the aggregation, confirmed by a broadening of the band at ~, ~ 677 nm (b). Recently, it was demonstrated that the increased aggregation in the constrained space of mesopores can be avoided by ionic anchorage of ZnPTS in the pores of Si-MCM-41 functionalized with trimethylammonium silane precursors (c, ~ ~ 667 nm) [ 10].

1072

3.2 Anchorage of dicarbocyanine dye 4_ Dicarbocyanine dyes are mainly used as slow-response potentiometric fluorescence dyes and fluorescent labels in life science. Cy5 dye 4 exhibits a maximum of absorption at ~, = 650 nm while the fluorescence maximum is found at ~, = 670 nm. Due to its 5-carbon alkyl chain between the indolenine units it is highly fluorescent and shows a satisfying photostability. Thus the dye is applicable for the detection in confocal fluorescence microscopy at low excitation energy. For linkage onto amino-functionalized supports 4 is activated by a N-succinimide ester. Figure 2. x-y-Scan of Cy5 covalently grafted to Si-MCM-41. (a) 4.7.10 -6 mol, (b) 9.4-10 .8 mol and (c) 4.7-10 -1~mol Cy5 4 offered per 1 g siliceous parent material. Si-MCM-41 particles fixed by embedding in poly(methyl methacrylate) (Figure 2) were scanned with laser light (~,ox= 633 nm, 1.2 ~tW). The excitation laser beam reflected by a dichroic mirror was focused by a lens (microscope objective) to a diffraction-limited spot in the sample. The emitted fluorescence light passes through the same optical dements and a pinhole [ 11 ]. The detected volume element has a height of 0 . 6 - 0.8 ~tm and 0.3 ~tm diameter waist. Figure 2 shows x-y-scans of particles with different dye concentrations. The necessary high dilution of chromophores can be obtained by photobleaching of particles with higher dye concentrations (a, b) or direct offer of very small amounts of activated dye (c). Thus, the method proves the graining of single chromophores onto the internal pore walls of the host material. Almost every detected single molecule followed a preferential orientation along the z-axes of the Si-MCM-41 particles indicating a strict homogeneity of the incorporation and hexagonal texture [ 12].

3.3 Anchorage of maleic anhydride 5 and phthalic anhydride 6 Aiming the prospective anchoring of ethene moieties of optical switchable diarylethene dyes also the reactions of maleic anhydride and phthalic anhydride with amino-functionalized Si-MCM-41 were carried out. Figure 3 presents the nitrogen adsorption isotherms on samples obtained by two-step grafting procedures. The isotherms on modified samples functionalized with APTES and subsequently gratted with anhydrides manifest the influence of the chemical modification causing decreases in the surface areas of mesopores and mesopore volumes (Table 1). The bigger organic moiety (phthalic imide) conducts to a stronger decrease of the pore diameter. This signifies that a high degree of the anhydride is anchored by imide bonding. The isotherms on all samples were processed by the method of comparison plots up to relative pressure p/po = 0.8 (Figure 4) [ 13]. As reference data the isotherms on unmodified and APTES-modified macroporous silica DAVISIL were used. All the comparison plots are

1073 30

30

25 :0

25 etl) ~z 20

.~~ 15

.~~ 15

10

10

5

5

,.e~ i

"~

0 0,0

012

014

016

0

0,8

p/po Figure 3. N2 adsorption isotherms on Si-MCM-41 (o), APTES-functionalized ([], +2), maleic anhydride-grat'ted (A, +4) and phthalic anhydride-grained (V, +6) Si-MCM-41.

0

5

10

1'5

20

25

are f / p,m O1 m-2 Figure 4. Comparison plot for Si-MCM-41 (o), APTES-functionalized (D, +2), maleic anhydride-gra~ed (A, +4) and phthalic anhydride-gra~ed (V, +6) Si-MCM-41.

characterized by two linear parts. The first one corresponds to the formation of a monolayer and the beginning of multilayer adsorption. The linear fit goes through the origin and its slope gives the total surface area Stot. The subsequent steep increase is caused by the capillary condensation of nitrogen in the mesopores. The decrease of external surface area S~t of chemically modified samples, determined from the second linear part (plateau), is probably caused by a loss of the smallest particles in the course of preparation (Table 1). The geometric diameter of mesopores calculated by the equation Dme = 4 Vme)/Smemarkedly decreases with increasing amount of organic species on the mesopore surface. It is obvious that the coveting of the mesopore surface with organic compounds causes a certain narrowing of the pores. However, the calculated values of Dmo of samples with imide bonding seem to be unrealistically small. Since the decrease in mesopore volume is more pronounced than the decrease in mesopore surface, additional phenomena must be taken into account. Organic species can cause an enhanced roughness of the host surface, which manifests itself in some increase in the mesopore surface and in a decrease of geometric diameter Dm,, consequently. Table 1. Texture parameters from comparison plots of adsorption isotherms, i.e. total (Stot), external (S~) and mesopore surface area (Sine), mesopore volume (Vmo)and diameter (Dine). Stot [me8"1] Soxt[m2g"1] Si-MCM-41, parent APTES-functionalized Maleic imide Phthalic imide

1082 819 770 716

228 111 94 92

Smo [m~g -1]

Vine[cm38"1]

Dme [nm]

854 708 676 624

0.682 0.451 0.357 0.310

3.19 2.55 2.11 1.99

1074 4. CONCLUSIONS Mesoporous Si-MCM-41 was utilized as a host for monomeric grafted chromophores. Covalent anchorage of the organic guests was obtained in a multi-step post-synthetic treatment of the silica. It was proved, especially by XRD measurements and the construction of comparison plots of N2 adsorption isotherms, that there is no damage of the pore texture due to steps of surface modification. Optical methods for solid-state measurements like DR/FT spectroscopy, diffuse reflectance UV/VIS and fluorescence spectroscopy signify the strong interaction of grafted dye and the host material. Especially confocal fluorescence microscopy is a powerful tool not only for detecting homogenous distribution and orientation of chromophores but also proving the high order of the molecular sieve.

5. A C K N O W L E D G E M E N T

The authors gratefully acknowledge funding from Deutsche Forschungsgemeinschaft (DFG) (WO 237/16-3). We thank C. Br~iuchle, E. Kneuper and C. Seebacher (Department of Chemistry and Pharmacy, LMU Munich) for the confocal fluorescence microscope measurements.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

D. Wrhrle, G. Schulz-Ekloff, Adv. Mater. 6 (1994) 875. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. K. Mrller, T. Bein, Chem. Mater. 10 (1998) 2950. X.S. Zhao, G.Q. Lu, X. Hu, Micropor. Mesopor. Mater. 41 (2000) 37. (a) G. Schulz-Ekloff, J. Rathousk~, A. Zukal, Microporous Mesoporous Mater. 27 (1999) 273; (b) J. Rathousk~, M. Zukalova, A. Zukal, J. Had, Collect. Czech. Chem. Commun. 63 (1998) 1893. Y. Rohlfing, D. Wrhrle, M. Wark, G. Schulz-Ekloff, J. Rathousl~, A. Zukal, Stud. Surf. Sci. Catal. 129 (2000) 295. D.S. Shephard, W. Zhou, T. Mashmeyer, J.M. Matters, C. L. Roper, S. Parsons, B.F.G. Johnson, M. J. Duer, Angew. Chem. 110 (1998) 2847. X. S. Zhao, G. Q. Lu, A. K. Whittaker, G. J. Millar, H. Y. Zhu, J. Phys. Chem. B 101 (1997) 6525. X. S. Zhao, G. Q. Lu, J. Phys. Chem. B 102 (1998) 1556. Y. Rohlfing, O. Barrels, D. WOhrle, M. Wark, 14. Deutsche Zeolith-Tagung, Book of Abstracts, 2002, Frankfiart. W.P. Ambrose, P.M. Goodwin, J.H. Jett, A. Van Orden, J.H. Werner, R.A. Keller, Chem. Rev. 10 (1999) 2947. C. Br~iuchle, Y. Rohlfing, C. Seebacher, D. Wrhrle, paper in preperation. M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir 15 (1999) 5410.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Mesocellular

aluminosilicate

foams

1075

(MSU-S/F)

and

large

pore

hexagonal

m e s o s t r u c t u r e s ( M S U - S / H ) a s s e m b l e d from zeolite seeds: h y d r o t h e r m a l stability and properties as c u m e n e cracking catalysts Yu Liu and Thomas J. Pinnavaia* Departmem of Chemistry, Michigan State University, East Lansing, MI 48824-1322

Mesostructured aluminosilicate foams (pore size > 20 nm) and very large pore hexagonal SBA-15 analogs (pore size >8 nm), denoted MSU-S/F and -S/H, respectively, were assembled from aluminosilicate nanoclusters that seed the crystallization of zeolites Y, ZSM-5 and Beta. Although the zeolite seeds are nucleated under basic pH conditions and the mesostuctures are assemble at acidic pH, the results of hydrothermal stability tests in steam at 800~ and in boiling water for 250 h reveal that MSU-S/F and MSU-S/H aluminosilicates are much more hydrolytically stable than their A1-MCF and A1-SBA-15 analogues. 27A1 MAS NMR spectra indicate that calcined MSU-S/F and MSU-S/H mesostructures retained between 80 and 95% of the aluminum centers in tetrahedral framework sites at an overall Si/A1 ratio of 50. Both mesostructures showed high activity for acid - catalyzed cumene cracking at 300 ~ 1. INTRODUCTION Significant advances have been made in improving the hydrothermal and steaming stability as well as acidity of A1-MCM-41 by using "zeolite seeds" or "aluminosilicate nanoclusters" as precursors [ 1-4]. These zeolite seeds promote zeolite nucleation by adopting A104 and SiO4 connectivities and consequentially resemble the primary and secondary building units of crystalline zeolite on the wall of the final mesostructured materials. The first demonstration of this approach to stable aluminosilicate mesostructures utilized faujasitic zeolite (FAU) seeds to construct the walls of a hexagonal MCM-41 structure [ 1]. Shortly after, we [1,2] and others [3,4] have used ZSM-5 (MFI) and zeolite Beta (BEA) seeds to assemble steam-stable A1-MCM-41 derivatives containing the 5-ring subunits of these pentasil zeolites. Unlike MCM-41 mesostructures, which are assembled under basic pH conditions compatible with protozeolitic seeds formation, mesostructured cellular foams (MCF) [5,6] and Corresponding author, email: [email protected]. This research is supported by the National Science Foundation through CRG grant 99-03706.

1076 distributions were calculated from the N2 adsorption branch using the BJH model. TEM images were taken on a JEOL 100CX with a CeB6 gun that was operated at an acceleration voltage of 100 kV. 27A1 MAS NMR spectrum were recorded on a Varian VXR-400S spectrometer with 7 mm zirconia rotor, a spinning frequency of 4 kHz. External Al(H20)63+ with a chemical shift of 0 ppm was used as a reference. To test the hydrothermal stability ofmesoporous MSU-S/F aluminosilicate materials, 0.15 g calcined samples were put into 20 ml H20 and boiling for 250 h. The steam stability was tested by exposure 0.2 g samples in 20 ml/min N2 flow saturated with 20% water vapor at 800~ for 2 h. The nitrogen stream was bubbled through a water bath at a controlled temperature to achieve the desired partial pressure of water vapor. Cumene cracking experiments at 300 ~ were performed according to methods described in our previous report [ 1,2]. 3. RESULTS AND DISCUSSIONS 3.1 Mesocellular MSU-S/F Aluminosilicate Foams The pure silica nanoclusters that nucleate a pentasil MFI structure have been extensively studied by Martens et al. [9-11 ]. The procedure used for the extraction and subsequent stable in acid media and suitable for assembling MCF and SBA-15 analogs at under strong acid conditions. In agreement with our previous results, the 27A1MAS NMR spectra showed only one peak

300(

After steaming

Before steaming 250( "0

200(

o

A

A

150( O>

100(

50C Z

o'.~ o'.~ o'.~ 0'.~ ,

0.2 0.4 0.6 0.8

1

P/Po

Figure 1. N2 isotherms for me so structured aluminosilicate foams before and after exposure to 20% steam in nitrogen at 800~ for 2 h: (A) MSU-S/FFAu, (B) MSU-S/FMFI, (C) MSU-SFBEA, (D) MCE Each isotherm is offset by 500 cm3g-1.

200

i

I

I

l

I

150

100

50

0

-50

-100

ppm

Figure 2. 27A1 MAS NMR spectra of A: MSU-SFFAu, B: MSU-SFMFI, C: MSU-SFBEA and D: A1-MCF

1077 very large pore hexagonal (SBA-15) [5,7] mesostructures require acidic reaction conditions for assembly. Such conditions may not be favorable for the incorporation of zeolitic subunits into the framework walls. As we show here, however, zeolite seeds can indeed be used under the acid conditions needed to assemble mesostructures that are structurally analogous to MCF and SBA-15. In addition to describing the physical properties of these mesostructures, we show that the resulting mesostructures are effective acid catalysts for cumene cracking.

2. EXPERIMENTAL 2.1 Synthesis Faujasitic zeolite Y (FAU), zeolite ZSM-5 (MFI), and zeolite Beta (BEA) seeds (Si/A1 = 50) were prepared using procedures analogous to those described previously [8-13]. FAU seeds (Si/A1- 5.6) were prepared by reacting sodium silicate and sodium aluminate at 100 ~ for 12 h, diluting the mixture with sodium silicate solution to obtain a Si/A1 ratio of 50, and then digesting the mixture an additional 12 h at 100 ~ before use. MFI and BEA seeds were prepared using aluminum sec-butoxide and tetraethylorthosilicate as precursors and tetrapropyl- and tetraethyl ammonium ions as structure directors, respectively. These solutions were digested at 100 ~ for 3-6 h before use in constructing mesostructures. A mesostructured aluminosilicate cellular foam (Si/A1 = 50) was prepared from FAU seeds by adding the seeds to a microemulsion containing Pluronic 123 surfactant, (EO)20(PO)70(EO)20, and 1,3,5-trimethylbenzene (TMB) as a co-surfactant to provide a mixture with the molar composition 1.00 SiO2:0.010 A1203:0.013 P123:0.51 TMB: 70.0 H20. The pH of the mixture was adjusted to 4.5-6.5 by the addition of 1.7 M H2SO4, aged at 25-60~ for 20-40 h and finally heated at 100~ under static conditions for 24 h. Analogous foam compositions were prepared from MFI and BEA seeds by adding the seeds to an acidic emulsion of P123, TMB and HC1 to obtain a reaction mixture with the composition 1.00 SiO2:0.010 A1203:0.017 P123:0.79 TMB: 4.95 HCI: 158 H20. The strongly acidic mixture (pH < 2) was stirred at 35 ~ for 20 h, and then the mixture was allowed to digest under static conditions at 100 ~ for 24 h. The as-made products were washed, air-dried, and calcined at 600~ for 4 h to remove the surfactant. Extending the use of zeolite seeds to the assembly of large pore hexagonal structures, we prepared SBA-15 analogs using the same FAU, MFI, and BEA seeds precursors, reaction conditions, and procedures that were used to prepare the above MSU-S/F mesostructures, except that the TMB co-surfactant was eliminated from the reaction mixtures. Two comparison samples of 2 mol% Al-mesostructured cellular foam (denoted as A1-MCF) and 2 mol% A1-SBA-15 (denoted as A1-SBA-15) were prepared exactly as above one formed by MFI and BEA seeds except that a same stoichiometric ratios of Al(i-BuO)3 and TEOS were added to the microemulsion and surfactant solution simultaneously. 2.2 Characterization Powder X-ray diffraction patterns were measured using Cu-Ka radiation (~=1.542 A) and a Rigaku Rotaflex. N2 adsorption and desorption isotherms were obtained at-196~ on a Micromeritics ASAP 2010 Sorptometer using static adsorption procedures. Pore size

1078 Table 1 Textural properties of calcined mesostructured aluminosilicate foams (Si/A1 = 50) before and after hydrothermal stability tests. Window size Cell size Surface area Pore vol. Cumene conv. Sample (%) (rim) (rim) (m2/g) (cc/g)

MSU-SF~ Before steaming After steaming Boiling H20 MSU-SFMFI Before steaming After steaming Boiling H20 MSU-SFBEA Before steaming After steaming Boiling H20 MCF Before steaming After steaming Boiling H20

132 126 118

208 204 196

570 462 273

1.79 1.46 0.78

33

102 95 90

201 195 187

888 748 463

1.95 1.68 0.96

35

128 124 117

220 215 210

861 737 647

2.18 1.86 1.87

36

110 105 -

228 220 -

715 147 103

1.79 0.44

<2%

at chemical shift of 62, 54 and 53 ppm for FAU, MFI and BEA seeds, respectively. This indicates that all of the A1 centers are tetrahedrally coordinated as A104 units and crosslinked to SiO4 units, as in the corresponding zeolite Y, ZSM-5 and Beta structures. Lippmaa et al [14] reported that the chemical shift oftetrahedrally coordinated A104 centers depended on the mean Si-O-A1 bond angles of the zeolite framework. The relationship between the A1 chemical 8 and the mean Si-O-A1 bond angle 0 followed the linear equation: 8(A1) = 0.500 - 132 (ppm). Generally, faujasitic zeolites with single and double six membered ring subunits exhibit a chemical shift higher than 60 ppm because of a lower mean Si-O-A1 bond angle in the framework in comparison to pentasil zeolites. Shown in Figure 1 are the nitrogen adsorption isotherms for the calcined mesocellular aluminosilicate foams assembled from FAU, MFI, BEA seeds and conventional precursors before and after steaming. Table 1 provides the BJH window and cell sizes deduced from the desorption and adsorption branches of the isotherms, respectively, along with the BET surface areas and pore volumes. Included in Table 1 are the textural properties and the physical properties of the samples before and after steaming. All the isotherms are of type IV and show steep hysteresis of type H1 at high relative pressures, which is typical for mesoporous materials that exhibit capillary condensationand evaporation with large pore sizes and narrow pore size distribution. The isotherms for MSU-SF assembled from FAU, MFI and BEA seeds are very similar to the ones reported by the group of Stucky [7, 15] and our group [ 16]. As shown in Figure 1, there were no significant changes in the cell size, window sizes, pore volume, and surface areas of

1079 our MSU-S/F derivatives after steaming at 800~ for 2 h. The framework of MSU-S~ remains intact during the steaming treatment at high temperature. At least 80% of the surface area and pore volume was retained for MSU-S~ after steaming (see Table 1). In contrast, AI-MCF assembled from AI(i-BuO)3 and TEOS retained only 20% of its surface area and 24% of its pore volume upon steaming at 800~ for 2 h, although the window and cell size show little contraction. The hydrothermal stability of MSU-S/F also was tested in boiling water. As we mentioned earlier, MCF was reported to be stable in boiling conditions. In agreement with earlier reports [7,15], we found that A1-MCF retained 96% (687 m2/g) of its surface area and 95% (1.70 cc/g) of its pore volume without window and cell size contraction upon exposure to boiling water for 70 h. Similarly, MSU-S/F retained at least 98% of the surface areas and pore volume without window and pore size contraction after boiling 70 h. However, as listed in Table 1, after 250 h exposure to boiling water, A1-MCF completely collapsed without mesopore retention. In contrast, MSU-S/FFAu, MSU-S/FMFI and MSU-S/FBEAremained 48% (270 m2/g), 52% (463 m2/g) and 75% (647 m2/g) of their surface areas and 44% (0.78 cc/g), 49% (0.96 cc/g) and 86% (1.87 cc/g), respectively. In addition, the window sizes and cell sizes of MSU-S/FFAu, MSU-S/FMF~ and MSU-S/FBEA remained largely unchanged upon boiling for 250 h. Further evidence for a foam structure was provided by the TEM images. As shown in Figure 3 our MSU-S/F foams are made of large spherical cells interconnected by narrow windows. Similarly, MSU-S/F derivatives possess a disordered array of aluminosilicate struts, which is considered to be a characteristic structural feature of mesocellular foams. Moreover, the wall thickness of our MSU-S/F is estimated by the TEM images to be 4~6 nm, which is lager than the anticipated size of the nanoclustered zeolite seeds we used for the synthesis. To further elucidate the difference of A1 environment between MSU-S/F and A1-MCF, 27A1 MAS NMR experiments were performed on these materials. As depicted in Figure 2, the calcined MSU-S/FMFIand MSU-S/FBEAfoams exhibited the same 27A1NMRchemical shifts as the starting seeds solutions. The calcined samples retained between 80 and 90% of the aluminum centers in tetrahedral sites, indicating that most of the aluminum adopted tetrahedral positions in the framework walls. For the MSU-S/FFAufoam, more than 95% of the aluminum centers are in tetrahedrally coordinated sites, which correspond to Bransted acidic sites. Moreover, the chemical shift of the tetrahedrally coordinated A1 sites is around 58 ppm, which is still higher than the chemical shift of normal tetrahedrally coordinated A1 sites in mesoporous materials, but lower than that of the original zeolite Y seeds which were used to assemble the

~'."~

70 n m

~"

:":

" .... :

'

Figure 3. TEM images of A: MSU-S/FFAu,B" MSU-S/FMFI,and C: MSU-S/FBEA

1080 fmal MSU-S/FFAu. The change of chemical shift may mean that the zeolite Y seeds are less stable than MFI and BEA seeds under acid conditions. Nevertheless, these nanoclusters are far more stable than conventional aluminosilicate precursors. Although the chemical shift of tetrahedrally coordinated A1 in final MSU-S/FFAuwas changed at acid conditions, around 95% of the A1 nuclei are tetrahedrally coordinated in the framework and less than 5% of the A1 nuclei are in octahedrally coordinated sites. At this point, more than 95% of the A1 is still incorporated into the framework, which consequently should provide enhanced cumene cracking activity (Table 1). In contrast to MSU-S[FFAu, A1-MCF has 100% of the aluminum centers in octahedrally coordinated sites, which means there is essentially no aluminum incorporated in the framework walls of A1-MCE This is consistent with the poorer cumene cracking activity of A1-MCF (see below). Obviously, in our cases, aluminum atoms can be easily incorporated the framework wall of the final foam structures under strong acid conditions, because 100% of the aluminum atoms were pre-incorporated into the nanoclustered zeolite seeds, as confirmed by 27A1-NMR (not shown). Moreover, these protozeolitic nanoclusters are far more stable than conventional aluminosilicate precursors under strong acid conditions. Thus, the zeolite seeds can be assembled to form foam structures with a high percentage of aluminum in the framework and a correspondingly strong acidity for acid catalysis (Table 1).

3.2 Hexagonal Large Pore MSU-S/H Mesostructures The XRD patterns and N2 sorption isotherms of MSU-S/H and A1-SBA-15 before and after steaming (not shown) reveal that the freshly calcined mesostructures have well ordered hexagonal arrays of very large pores of uniform size. The nitrogen isotherms for MSU-S/H exhibit steep hysteresis oftype H1 at high relative pressure from 0.60-0.80, which is typical for mesoporous materials that exhibit capillary condensation and evaporation with large pore sizes and narrow pore size distribution. Figure 4 shows the TEM images of MSU-S/I-IFAu, MSU-S/HMFI and MSU-S/HBEA. The well-ordered hexagonal array of 75-90 A channels and walls -~40 A thick are clearly visible. These images are consistent with the results from XRD and N2 adsorption experiments. The textural properties of the MSU-S/H mesostructures (Si/A1 = 50) before and after

Figure 4. TEM images of A: MSU-S/HFAu, B: MSU-S/HMFI and C: MSU-S/HBEA

1081 Table 2 Textural properties of calcined large pore hexagonal mesostructured aluminosilicates (Si/A1 = 50) before and after hydrothermal stability tests. Sample Surface Area d-Spacing Unit Cell Pore Size Pore Volume Cumene (m2/g) (A) Size (A) a (A) (cm3/g) Conv.(%) MSU-S/HMFI Before Steaming After Steaming MSU-S/HBEA Before Steaming After Steaming MSU-S/HFAu Before Steaming After Steaming SBA-15 Before Steaming After Steaming a Unit size was calculate

886 701

102 95

118 110

77 69

0.93 0.78

37

849 687

101 96

117 111

76 70

0.90 0.77

34

653 421

110 95

127 110

90 80

0.85 0.57

32

823 101 117 77 305 86 96 60 from the relationship a = 2dl00/~f3.

0.89 0.31

2%

steaming are provide in Table 2. After exposure to steam, MSU-S/HFAu retains > 65% of its initial surface area and pore volume. MSU-S/H~n~ and MSU-S/HBEAare even more stable to steam, retaining > 80% of their surface areas and pore volumes with little pore contraction. In contrast, a conventional A1-SBA-15 retains o n l y - 35% of its surface area and pore volume and undergoes substantial pore contraction upon steaming. The hydrotherrnal stability of MSU-S/H is similar to that of the MSU-S/F foam mesostructures described above. The 27A1 MAS NMR spectra of MSU-S/H and A1-SBA-15 are identical to those of MSU-S/F and A1-MCF, which indicates the most of the A1 centers in MSU-S/H are tetrahedrally coordinated in the framework.

3.3 Cumene Cracking Properties Cumene cracking is a wildly used probe reaction for testing the acidity of aluminosilicate materials, because this reaction requires medium to strong acid sites. As shown in Table 1, MSU-S/FFAu, M S U - S / F ~ and MSU-S/FBEA showed 33%, 35% and 36% of conversion cumene at 300 ~ respectively. Similar results were obtained for MSU-S/H materials, as shown in Table 2. Only a trace amount of a-methylstyrene was detected in the products, which indicates that most of the acid sites are of the Bronsted acid type in both MSU-S/F and MSU-S/H mesostructures. In contrast,-~2% cumene conversion was observed over A1-MCF and A1-SBA-15 under equivalent conditions. Furthermore, only a-methylstyrene was detected in the products, which signifies that only weak Lewis acid sites existed in A1-MCF and A1-SBA-15. These results reveal that it is difficult to incorporate A1 into the framework of conventional MCF and SBA-15 materials under strong acid condition by means of direct alumination.

1082 4. CONCLUSIONS Hydrothermally stable MSU-S~ aluminosilicate foam mesostructures and very large pore hexagonal MSU-S/H mesostructures were synthesized under strongly acidic conditions using zeolite Y, ZSM-5 and Beta seeds as the aluminosilicate precursors. Aside from the hydrothermal stability, MSU-S/F and MSU-S/H exhibited stronger acidity than structurally analogous A1-MCF and A1-SBA-15, respectively, as evidenced by a comparison of cumene cracking conversions at 300 ~ Although the construction of MSU-S/F and MSU-S/H mesostructures from zeolite seeds required the pH of the initial seeds mixtures to be lowed from initially basic values to acid values in the range from <2.5 to 6.5, the protozeolitic nanoclusters clearly persist under acid conditions and can be incorporated into the framework walls of the f'mal mesostructures. Consequently, the concept of incorporating nanosized zeolite seeds into the walls of a meso-structure as an efficient means of improving hydrothermal stability and acidity is quite general and applies even to mesostructures that require strongly acidic conditions for assembly. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Y. Liu, W. Zhang and T.J. Pinnavaia, J. Am. Chem. Soc., 122 (2000) 222. Y. Liu, W. Zhang and T.J. Pinnavaia, Angew. Chem., Int. Ed., 40 (2001) 1255. Z.T. Zhang, Y. Han, F. S. Xiao, S.L. Qiu, L. Zhu, R.W. Wang, Y. Yu, Z. Zhang, B.S. Zou, Y.Q. Wang, H.P. Sun, D.Y. Zhao, Y. Wei, J. Am. Chem. Soc., 123 (2001) 5014. Z.T. Zhang, Y. Han, L. Zhu, R. W. Wang, Y. Yu, S.L. Qiu, D.Y. Zhao, F.S. Xiao, Angew. Chem. Int. Edit., 40 (2001)1258. D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (1998) 548. D.Y. Zhao, Q.S. Huo, J.L. Feng, B.E Chmelka, G.D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. P. Schmidt-Wmkel, C.J. Glinka, G.D. Stucky, Langmuir, 16 (2000) 356. L.Lechert, P. Staelin, M. Wrobel, U. Schimmel, Stud. Surf. Sci. Catal., 84 (1994)147. C.E.A. Kirschhock, R. Ravishankar, F. Verspeurt, P.J. Grobet, P.A. Jacobs, J. A. Martens, J. Phys. Chem. B, 103 (1999) 4965. C.E.A. Kirschhock, R. Ravishankar, L. Van Looveren, P. A. Jacobs, J. A. Martens, J. Phys. Chem. B, 103 (1999) 4972 C.E.A. Kirschhock, R. Ravishankar, P.A. Jacobs, J.A. Martens, J. Phys. Chem. B, 103 (1999) 11021. J. Perezpariente, J.A. Martens, P.A. Jacobs, Appl. Catal., 31 (1987) 35. J. Perezpariente, J.A. Martens, P.A. Jacobs, Zeolites, 8 (1988) 46. E. Lippmaa, A. Samoson, M. Magi, J. Am. Chem. Soc., 108 (1986) 1730. P. Schmidt-Winkel, W.W. Lukens, D.Y. Zhao, P.D. Yang, G.D. Stucky, J. Am. Chem. Soc., 121 (1999)254. S. S.Kim,.T.R. Pauly, T.J. Pinnavaia, Chem. Commun. (2000) 1661.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1083

Fabrication of large secondary mesopores in MCM-41 particles assisted by aminoacids and hydrophobic functional groups Isabel Diaz and Joaquin P6rez-Pariente* Instituto de Cat/disis y Petroleoquimica (CSIC), Campus Cantoblanco, 28049 Madrid, Spain.

The synthesis of MCM-41 type of materials functionalised with (methylmercaptopropyl) and/or methyl groups have been performed by co-condensation of the corresponding alcoxysilanes and tetramethoxysilane, in the presence of hexadecyltrimethylammonium and the aminoacid leucine. The presence of the organic moieties in the resulting material has been assessed by 29Si MAS NMR. Nitrogen adsorption and Transmission electron microscopy (TEM) studies of the samples synthesised from gels containing leucine reveal the presence of large secondary mesopores that permeate the entire bulk of the particles, in addition to the conventional mesopores in hexagonal arrangement. The simultaneous presence of methyl functional groups and leucine is required for the formation of such secondary mesoporosity.

1. INTRODUCTION The synthesis of hybrid organic/inorganic mesoporous materials is a field of expanding interest, due to the potential applications of these materials in a variety of process, covering catalysis, adsorption and nanotechnology. For these purposes, the incorporation of appropriate active functional groups into the mesoporous silica matrix and the presence of a regular array of tailored pore sizes are required. Several single functional groups have been incorporated into the silica framework of mesoporous materials, particularly in MCM-41, by using one pot synthesis procedure [1-11 ]. However investigations aiming to combine two or more different functional groups, in order to meet specific applications requirements, are scarce. The recently reported improvement of the catalyst activity and selectivity in the esterification of glycerol with fatty acids by a combination of methyl and sulfonic groups anchored onto MCM-41 is one example of such a dual funcionalisation [12]. Regarding sulfonic acid groups, most of the studies restrict themselves to the use of trialkoxy-mercaptopropyl as precursor, which yield the corresponding sulfonic acid after mild oxidation. However, precursors of sulfonic acid other than trialkoxide might also be used. Indeed, we believe that the use of a molecule as mercaptopropyl methyl dimethoxysilane, which contains both functionalities attached to the same silicon atom, could eventually offer some advantages over the conventional synthesis Author for correspondence. Phone: 34 91 585 4784. Fax: 34 91 585 4760. E-mail: [email protected]. Http ://www.icp.csic.es/gtm.

1084 pathway involving the use of two separate reagents, methyl and mercaptopropyltrimethoxysilane. Additionally, it has been found that the use of a long chain amine, as dodecylamine, as co-surfactant together with the hexadecyltrimethylammonium cation influences the catalytic properties of the resulting sulfonic acid material [13]. Based on these previous grounds, in this work we report the formation of a large amount of what could be describe as large secondary mesopores in MCM-41 materials obtained by adding an aminoacid as leucine, which replace the dodecylamine used previously, to a synthesis gel containing mercaptopropylmethyl and methyl precursors.

2. EXPERIMENTAL SECTION

2.1. Sample Preparation. The synthesis of the hybrid materials is based on the hydrolysis and co-condensation of tetramethylorthosilicate (TMOS, Aldrich), 3-mercaptopropyl methyldimethoxysilane (MPMDS, Sigma) and methyltrimethoxysilane (MTMS, Aldrich) in presence of a surfactant/aminoacid solution in basic medium. Three gels with the following molar composition have been prepared: [1-(x+y)] TMOS: x MPMDS: y MTMS: 0.12 Leucine: 0.12 CTAB: 0.27 TMAOH: 18.8 CH3OH: 77.7 H20, where CTAB is Cetyltrimethylammonium Bromide (Aldrich) and TMAOH is Tetramethylammonium Hydroxide (25 wt % in water, Aldrich). Chemical composition of the gels is given in Table 1. In a typical synthesis, first CTAB and leucine were dissolved in H20: CH3OH and stirred for 30 min at room temperature. Then the mixture of silicon sources, previously homogenised during 10 min, was added drop by drop. Finally, after the slow addition of the TMAOH, the gels were stirred at room temperature for 16 h to evaporate the entire methanol, and then treated at 368 K for 48 h into 60-ml Teflon-lined stainless steel autoclaves. The solid products were recovered by filtration, washed with water and dried at 333 K. The surfactant was removed by treating 1.5 g of dried solid two times with 225 ml of an acid solution EtOH: HC1 (35wt%) (10:1 v/v) at 343 K for 8 h.

2.2. Sample Characterization. Analyses of the organic material present in the solids were done in a Perkin-Elmer 2400 CHN-analyzer. X-ray powder diffraction patterns were collected using CuK~ radiation, on a Seifert XRD 3000P diffractometer operating at low angle (20 from 1 to 10~ For the TEM studies the samples were dispersed in acetone and dropped on a holey carbon copper microgrid. Micrographs and selected area electron diffraction patterns (SAED) were recorded in a JEOL JEM 3010 transmission electron microscope operating at 300 kV. Adsorption of nitrogen was carried out at 77 K in a Micromeritics ASAP 2000 apparatus. Thermogravimetric analysis (TG) was performed on a Perkin Elmer TGA7 from 30 to 900~ with a heating rate of 10~ under air flow. Solid-state 29Si MAS NMR spectra were recorded on a Varian VXR-400S WB spectrometer at 79.5 MHz, by using a pulse length of 4.0 gs and a recycle delay of 60 s.

1085 3. RESULTS AND DISCUSSION The X ray diffraction patterns of the as-synthesised materials (Figure 1) show the presence of an intense low angle reflection and at least two other of lower intensity, corresponding to the 100, 110 and 200 reflections of the hexagonal MCM-41 (p6mm) [14]. A unit cell size of 39.5 +0.5 A is obtained for all the three samples, and it remains practically unaffected by removing the occluded surfactant and the leucine. The chemical analysis shows the presence of sulphur, 1.3 meq/g, in the two samples synthesised from gels containing MPMDS.

c

_=

I

'

2

I

'

4

I

2O

6

'

I

'

8

10

Figure 1. XRD patterns of samples 1 (x - 0.116, y = 0); sample 2 (x = 0.116, y = 0.174) and sample 3 (x = 0, y = 0.29).

The presence of leucine in the as-made materials has been assessed by the following procedure: the sample is extracted with ethanol; after solvent evaporation, the residual solid is treated with deuterated chloroform and the resulting solution is analysed by 1H NMR. Both leucine and CTA + are present in the solution. Table 1 Composition and properties of the samples prepared from gels: 1-(x+y) TMOS" x MPMDS" y MTMS" 0.12 Leucine" 0.12 CTAB" 0.27 TMAOH" 18.8 MeOH: 77.7 H20. SAMPLE

1 2 3

gel x

y

ao (A.)

extracted SBrT (mZ/g)

0.116

-

0.116 -

0.174 0.29

40 39 39

644 888 756

Vp (cm3/g) 0.35 0.53 0.71

1086 100

'

i

'

i

i

i

i

'

i

90

o 80

zo

I"12

~o 9

300

400

500

600

700

Temperatu re (~

800

,

0

.

,

-20

.

,

.

-40

,

.

-60

,

.

-80

;

-100

. ....

,

-120

.

,

-140

9

,

-160

ppm

Figure 2. TG/DTG of sample 2.

Figure 3. 298i-RMN spectra of samples 2 and 3.

By thermogravimetric (TG) and chemical analyses, it is possible to identify and quantify the organic material in the extracted solids. In Figure 2 the TG/DTG curves of sample 2 as made and extracted are shown. After desorbing water at temperature below 120~ surfactant decomposition takes place at 250~ in the as made sample. The peak at higher temperature (350~ corresponds to the decomposition of propylthiol groups as it remains unaltered after acidic extraction. From this curves we can assume that the functional groups are stable until 250~ what is of special interest for catalytic applications. At T > 450~ the weight loss due to the isolated methyl groups is added to the desorption of propylthiol fragments and the water resulting from the condensation of silanol groups. Figure 3 depicts 298i MAS NMR spectra of the samples functionalised with methyl groups and with a mixture of methyl and methylproylthiol groups. Resonances at -101 ppm and -111 ppm correspond to the Q3 ((SiO)3SiOH) and Q4 ((SiO)4Si) silicon species, respectively. The signal centred at-65 ppm has been assigned to silicon atoms attached to the methyl groups in T 3 configuration, CH3 Si(OSi)3, whereas the shoulder a t - 5 6 ppm is attributed to T 2 centres, i.e., Si atoms attached to one residual OH group, CH3 (SiO)2SiOH. The new signal observed at-19.6 ppm in the spectrum of the sample 2 has been assigned to =Si(CH3)(CHzCHzCHzSH) organic moieties. It can be observed in Table 2 that the total functionalisation degree of the two samples is practically the same, - 30%, according to the equal content of functional groups of the two gels. Table 2

29Si M_AS M R data for extracted samples functionalised with methyl-propilthiol (sample 2) and methyl groups (sample 3). Normalised peak area is in brackets.

Q4

Q3

T~

T2

7,12

Si-(OSi)4

-Si-OH

=Si-CH3

=Si(OH)CH3

=Si-[CH3,SH]

2

-110,7 (50%)

-101,1 (19%)

-65,1 (14%)

-56,3 (3%)

-19,7 (14%)

3

-110,2 (53%)

-101,1 (16%)

-65,2 (27%)

-55,1 (3%)

-

Sample

1087 The N2 isotherm of the extracted sample 1 obtained in the absence of 400 MTMS (Figure 4) shows a smooth adsorption of nitrogen at low p/p0 2 values, characteristic of the pore E ~ 300 filling in the mesopore region [15]. E A very small hysteresis loop is observed, which is usually .~ 200 associated to the interparticular ,.D porosity in MCM-41 [ 16, 17]. ~ 100 However, if MTMS is added to the synthesis gel, the N2 isotherm ' I ' i ' I ' of the resulting extracted solid 0,0 0,2 0,4 0,6 0,8 shows the presence of a clear P/Po hysteresis loop (sample 2 in Figure Figure 4. N2 isotherms of the samples. 4), which is abruptly closed at p/p0 0.47. At the same time, a strong increase of the pore volume from 0.07 cm3/g to 0.53 cm3/g is found. It is remarkable that this effect is not observed if pure amines are used as co-surfactants [ 13]. This result suggests that the presence of methyl groups decorating the pore walls of MCM-41 would induce the formation of such a large secondary mesopores in the presence of leucine. Therefore, a sample that contains only methyl groups but no sulphur was prepared. The N2 isotherm of this sample shows a strong enhancement of the secondary mesoporosity at roughly the same relative pressures as in the sulphur-containing sample (sample 3 in Figure 4). According to the shape of the isotherm, such porosity would be associated to pores with "ink bottle" morphology having a very large inner diameter and an homogeneous size of the pore mouth, as suggested by the sharpness of the desorption branch. The pore size distribution obtained from the adsorption branch of the isotherm indicates the presence of pores having 13~ average diameter in the pure thiol-containing material, corresponding to the conventional functionalised structured mesopores. The maximum of the distribution shifts to 15 A in the sample that do not contain thiol groups. Indeed, the low pore size is similar to that previously found for thiol or methyl/thiol-containing samples prepared from CTA + or mixtures of this cation with dodecylamine [ 13, 18-19]. The good pore ordering of these functionalised MCM-41 type structures is confirmed by transmission electron microscopy (TEM). The strong interaction between the electrons and the sample results in high symmetry order selected area electron diffraction (SAED) patterns along both parallel and perpendicular to the c axis (Figure 5). There is a good agreement in the cell parameter, 37 and 39 A, calculated by Figure 5. ED patterns of sample 2 along directions perpendicular (A) and parallel (B) electron diffraction (ED) and XRD to the channels axis. respectively. In the TEM images it can be also clearly seen that the high ordered pore 3

1088

arrangement of the MCM-41 is randomly interrupted by regions with low contrast (Figure 6), which would correspond to the large mesopores detected by nitrogen adsorption in samples 2 and 3. Indeed, these regions are much more abundant in the pure methyl sample (sample 3, Figure 6C), which possesses higher secondary mesoporosity. Nevertheless, these mesopores seem to extend several unit cells across the particles, permeating their entire bulk. Regarding the cause that produces the formation of such a large volume of secondary mesopores, it has to be considered first that they are not observed if pure amines are used as co-surfactants in the synthesis gels [ 13]. Therefore, it seems likely that the acid group present in the leucine molecule should be involve in the process leading to such large cavities. Some other interesting features can also be observed by TEM in these materials. It is remarkable the peculiar morphology of the particles (Figure 7), having in general "peanut" shape, which has been already observed when an amine is used as co-surfactant [13]. Second, what it seems to be a thick amorphous layer surrounding the particles is clearly observed in the samples having secondary mesopores (Figure 6C). This layer is not present in the pure thiol-sample, where the hexagonal packing of the pores is observed even at the edge of the particles (Figure 6A), and it could be related eventually with the mechanism of the selfassembly process leading to the mesoestructured material. Indeed, particles with a distinct morphology are detected in samples 2 and 3 (Figure 8). They resemble vesicles surrounded by a thick edge that reminds the thick amorphous surface of the "peanut" particles previously mentioned. This fact, together with the similar thickness of the amorphous layer around the particles, could lead us to think that they are first stages in the growing process of the material in such a way that the mesoporous structure should grow up inside these particles. These vesicles have not been observed in other functionalised materials. The interest of these functionalised materials having high porosity relies upon the possibility to control the population and size of the big mesopores. The presence of these large cavities might have catalytic implications, as they would probably affect the diffusion properties of the material, and hence the final catalytic performance. This aspect is being actually explored. '# ~"~'" " "

"

~7~~~

"

"~k~!"4t

"4

C 20 nm

Figure 6. TEM images along the channel direction of A) sample 1 with MCM-41 type structure, B) some weak contrasts in sample 2, and C) high presence of secondary mesoporosity in sample 3.

1089

.ks

,. ,', ",, f,," .i~ ~?"

100 nm Figure 7. Image of a typical "peanut" shape particle (sample 3). Elongation takes place following the direction perpendicular to the c axe.

:(i!

Figure 8. Particles with vesicle like morphology present in sample 3.

1090 Acknowledgement The authors acknowledge the CICYT (Spain) for financial support within the Project MAT2000-1167-C02-02 and O. Terasaki for the TEM facilities and his helpful discussion. The help of T. Blasco and C. Marquez in collecting and analyzing the 298i MAS NMR is greatly appreciated. I. Diaz acknowledges the Spanish Ministry of Education for a Ph.D. grant.

4. REFERENCES 1. 2. 3. 4.

S.L. Burkett, S. D. Sims, S. Mann, Chem. Commun. (1996) 1367. C.E. Fowler, S. L. Burkett, S. Mann, Chem. Commun. (1997) 1769. M.H. Lim, C. F. Blanford, A. Stein, J. Am. Chem. Soc. 119 (1997) 4090. W.M. Van Rhijn, D. E. De Vos, B. F. Sels, W. D.; Bossaert, P. A. Jacobs, Chem. Commun. (1998)317. 5. M.H. Lim, C. F. Blanford, A. Stein, Chem. Mater. 10 (1998) 467. 6. C.E. Fowler, B. Lebeau, S. Mann, Chem. Commun. (1998) 1825. 7. W.M. Van Rhijn, D. E. De Vos, W. Bossaert, J. Bullen, B. Wouters, P. J. Grobet, P. A. Jacobs, Stud. Surf. Sci. Catal. 117 (1998) 183. 8. M.H. Lira, A. Stein, Chem. Mater. 11 (1999) 3285. 9. F. Babonneau, L. Leite, S. J. Fontlupt, Mater. Chem. 9 (1999) 175. 10. R. J. P. Corriu, C. Hoarau, A. Mehdi, C. Reye, Chem. Commun. (2000) 71. 11. V. Goletto, M. Imperor, F. Babonneau, Stud. Surf. Sci. Catal. 129 (2000) 287 (several papers about co-condensation in that issue) 12. I. Diaz, C. Mfirquez-Alvarez, F. Mohino, J. P6rez-Pariente, E. Sastre, J. Catal. 193 (2000) 295. 13. I. Diaz, C. Mfirquez-Alvarez, F. Mohino, J. P6rez-Pariente, E. Sastre, Micropor. Mesopor. Mater. 44-45 (2001) 203. 14. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 15. A. Sayari, M. Kruk,. M. Jaroniec, Cat. Lett. 49 (1997) 147. 16. M. Kruk, M. Jaroniec, R. Ryoo, J. M. Kim, Chem. Mater. 11 (1999) 2568. 17. M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, C. H. Ko, J. Phys. Chem. B, 104 (2000) 292. 18. I. Diaz, C. Mfirquez-Alvarez, F. Mohino, J. P6rez-Pariente, E. Sastre, J. Catal. 193 (2000) 283. 19. I. Diaz, F. Mohino, J. P6rez-Pariente, E. Sastre, Appl. Catal. A: Gen. 19 (2001) 205.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1091

Hexagonal and Cubic Thermally Stable Mesoporous Tin(IV) Phosphates with Acidic, Basic and Catalytic Properties (b) Christian SERRE (a)*, Aline AUROUx , Antonella GERVAS1NI (c), Maryvonne HERVIEU (d) and Gdrard FEREY (a)

(a)Institut Lavoisier, UMR CNRS 8637, Universitd de Versailles St-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France; Fax:(33)l 39 25 43 58; e-mail: [email protected] (b)Institut de Recherches sur la Catalyse, CNRS, 2 avenue Einstein, 69626 VILLEURBANNE Cedex, France, Fax:(33)-(0)472-44-53-99; e-mail: [email protected] (c)Dipartimento di Chimica Fisica de Electrochimica, Universit~ degli Studi di Milano, via C. Golgi 19; 1-20133 Milano Italy; fax. 0039 02 70638129; e-mail: [email protected] (d)CRISMAT, ISMRA, UMR CNRS, 6 Boulevard du Marrchal Juin, 14050 Caen, France; Fax:(33) 2 31 95 16 00; Email: [email protected] Thermally stable hexagonal and cubic mesoporous tin(IV) phosphates have been synthesised via a fluoride route using alkyl di- or tri-methylammonium bromide surfactants. X-ray powder diffraction and HREM show that the hexagonal phase exhibits a MCM-41 organisation while the structure of cubic phase is similar to those of the micellar cubic solid SBA-1. Removal of the surfactant by calcination under air atmosphere of both phases does not destabilise the framework and leads to mesoporous solids exhibiting surface area within the 200-630 m2/g range and pore sizes between 12 and 25 A. Finally, microcalorimetric studies of the adsorption of NH3 and SO2, indicated that both solids exhibit both a strong acidic and a weak basic character. First de-NOx catalytic tests have also shown positive results for both solids, which indicates that these porous solids represent an interesting family of materials with good catalytic properties. 1. INTRODUCTION. Since the initial work of Mobil researchers on the synthesis of mesoporous silica,[ 1] extending the composition of mesoporous materials to metal oxides other than silica for applications in acid, redox catalysis or photocatalytic processes, is a great challenge.[ 2-4]

*author for correspondence

1092 Up to now, numerous non-silica based mesostructured solids based on metal oxides of A1, Ti, , Nb, V, W,[ 5-9] or metal phosphates of A1, Zr, or V,[ 10-12] have been reported. However, cubic mesoporous solids are still scarce despite results concerning oxides of Nb,[13] Sb,[14] Ti,[6b] and Zr[15] reported previously. The first mesotextured metallophosphate with a cubic structure was described recently by Mizuno et aL[ 16] However, its framework collapses after removal of the surfactant. Tin oxide is widely used as a semi-conductor or as a catalyst for oxidation of organic compounds. Synthesis of tin-based mesoporous solids is therefore of a great interest. To date, several mesotextured tin oxides have been reported.[ 17] However, they exhibit either a low thermal stability, a lack of long-range order or low surface areas. We recently reported new hexagonal or lamellar titanium(IV) fluorophosphates synthesised via the fluoride route.[ 18] By extending this pathway to the tin(IV) system, we report here the first hexagonal and cubic porous tin(IV) fluorophosphates which exhibit both a high thermal stability, a long-range order and high surface areas.[ 19] For catalysis application, the determination of the acid-base or redox properties of the samples is of great importance. The acidic and basic character determination as well as first de-NOx catalytic tests are reported here.

2. EXPERIMENTAL SECTION. These solids are first prepared by mixing SRF4 with a aqueous solution of phosphoric acid with a final P/Sn ratio between 4 and 16. Then, the surfactant (Cetyl or tetradecyl trimethylammoniumbromide (CTAB or TTAB)) solution is poured into the tin solution under stirring (S/Sn ratio: 1:2). The final tin concentration is 0.1 Mol/1. The suspensions are aged at 90~ ovemight in a Teflon-lined PARR bomb and cooled down to room temperature. The solids, filtered, washed and dried at room temperature, are finally calcined under air 8 hours at 400~ with a 2~ heating slope. X-ray powder diffraction patterns were conducted on a conventional high resolution (0-20) Siemens DS000 Diffractometer using ~,Cu Ka in steps of 0.02 ~ for 6 s per step with 1/01 mm slits. Tin, phosphorus, fluorine, carbon, nitrogen and hydrogen contents were determined at the C.N.R.S. Central Laboratory of Analysis of Vemaison (69, France). Ratios of P/Sn, F/Sn and S/Sn (S=Surfactant) equal respectively to 1.35, 1 and 0.65 for the TTAB-hexagonal and 1.55, 0.65 and 0.45 for the TTAB-cubic as-synthesised solids were measured. Only traces of fluorine were reported on the calcined solids. The BET surface area measurements were measured with a Micromeretics ASAP 2010 apparatus using nitrogen (N2) as the adsorbed gas. The High Resolution Electron Microscopy (HREM) was performed with a TOPCON 002B microscope (point resolution of 1.8A) equipped with an Energy Dispersive Spectroscopy (EDS) analyser. Samples were prepared by dispersing the powder in alcohol without grinding. Ammonia and sulfur dioxide were used as probe molecules to probe the acidity and basicity of the samples respectively.

1093 The microcalorimetric studies of ammonia and SO2 adsorption were performed at 353K in a heat flow calorimeter of the Tian-Calvet type ( C80 from Setaram) linked to a conventional volumetric line. Before each experiment the samples were outgassed overnight at 673K. The differential heats of adsorption were measured as a function of coverage by repeated addition of amounts of gas until an equilibrium pressure of about 66pa was reached. Then, the samples were evacuated for 1 hour at the adsorption temperature and a second adsorption was done in order to allow the determination of chemisorption uptakes [a]. The catalytic tests were carded out with samples of mass ~ 0.15 g contained in a quartz tubular microreactor (5 mm ID). The reactant stream was provided from a set of mass flow controllers (Bronkhorst, Hi-Tec) supplying 1000 ppmV NO and 1000 ppmV C2H4, and 20,000 ppmV 02 in helium at a total flow rate of 50 cm3(STP)/min, with the reactor at close to atmospheric pressure. Contact time was maintained constant at 0.168 kg*s*l -~. The interval from 200 up to 750~ of reaction temperature was investigated. The exit gas stream from the reactor flowed through a gas cell (pathlength 2.4 m multiple reflection gas cell) in the beam of an FTIR spectrometer (Bio-Rad with DTGS detector). The spectrometer gave analyses for NO, N20, and NO2 for N-containing species, and C2H4, CO and CO2 for C-containing species. The measurements were carried out at 0.50 cm -~ resolution with an accuracy of + 10 ppmV for NO, and 4 for N20 and NO2 using lines at 1876, 2225, and 1619, respectively, respectively. The samples were contained in the reactor between plugs of quartz wool and initially pretreated in flowing 20% O2/He while raising the temperature in stages up to 350~ and maintained it for 4 h. The tests were repeated three times using fresh portions of catalyst and working in low (200-500~ high (450-750~ and medium (350-600~ zones of reaction temperature, respectively.

3. SYNTHESIS AND CHARACTERISATION. The existence of either hexagonal or cubic phases is strongly dependent on syntheses parameters such as concentration, P/Sn, S/Sn ratios and the alkyl chain lengths (S for surfactant). Using alkyl tdmethylammonium surfactants, hexagonal solids are obtained for long alkyl chains surfactants (n>14) while cubic phases appear for small chain surfactants (n
1094 solid SBA-I,[ 14], respectively. Both structures are, retained upon calcination at 400~ under air. Their thermal stabilities, up to 600~ are almost identical. C

200

210 211

321

Calcined solids still exhibit d(A) 43.8 39.6 36.3 24.1 a long range organisation since secondary order diffraction peaks H 100 110 200 210 ((110) and (200) for H and (321) v d(A) 35.1 20.2 17.5 13.3 for C) are still visible on X-ray patterns, despite a decrease of their 200~ intensity. The pore contraction is important, within the 8-10 .A range in both cases. BET experiments, performed on the TTAB hexagonal (H) and cubic (C) solids calcined under air at 400~ indicate specific surface area respectively of 350(H) and 425(C) mZ/g. These values are 40 30 20 noticeable for tin phosphates. Both d - Scale (N) N2 adsorption-desorption isotherms do not show any hysteresis loop Figure 1: X-Ray powder diffraction patterns (fig. 2). Schtith et al. obtained of the as-synthesised and calcined forms of the similar isotherms with mesoporous hexagonal (H) and cubic (C) tin(IV) phosphates titanium oxophosphates and obtained using TTAB surfactant. explained this through the presence of"supermicropores" within the 15-20 A range.[6e].

a.U,oo

p.l,.,J~,,l~,,,

I

,

,

,

,

I

,

,

,

_

TEM experiments (see next paragraph) indicate that the pore wall thickness is about 15 A in both cases; considering the important pore contraction occurring upon calcination, it gives for each solid an estimated pore size below 20 A. This is in agreement with results obtained previously with SBA-1 where the pore size was close to 20 A with a very narrow hysteresis loop. Finally, by changing the nature of the surfactant (alkyl chain length, polar head), cubic or hexagonal solids, with surface areas between 200 and 630 m2/g with pore sizes within the 12-25 A range, are obtained.

160 H "E 80

> 40 0

0

0,2

0,4

0,6

0,8

1

PIPo F i g u r e 2: N 2 adsorption-desorption isotherm of the TTAB-hexagonal (H) and cubic (C) tin(IV) phosphate solid calcined at 400~ under air.

1095 Electron microscopy was performed on two solids: a H-sample (CTAB-hexagonal) and a C-sample (TTAB-cubic). The electron microscopy images show that the two samples, exhibit similar morphologies. The particles consist of a three dimensional skeleton, built up from the interlacing of pasted grains. This results in building up "spongelike" structures with large tunnels whose diameter is commonly a few tenths of micrometers. Besides, one can observe minor differences: the C-type grains are clearly round shaped, with an average size of the order of the micrometer whereas the H-type grains are slightly smaller and nearly faceted (fig. 3). Each of the grains is made of one or several randomly oriented small domains with a periodicity of the order of several nanometers. Then, high resolution TEM was performed on the same solids. Images show that the two materials are not amorphous but clearly organised. For the hexagonal solid (H), the high resolution electron microscopy images exhibit a contrast which can be described as an unperfected honeycomb-like arrangement of six-fold faceted spheroids (fig.4). The average periodicity of the close packed spheroids is close to 45 .A, which is consistent with XRD results: 2 a = d(100) * - ~ ~ 45.5 A. Concerning the C system, globular cages such as those observed previously with SBA-1 are present (fig.5).[ 14] The cages are surrounded by dark rings due to the higher electron density of the inorganic framework. Within each organised arrangement of particles, we can distinguish the bright core of the particles, correlated with a zone of low electron density corresponding to the surfactant moities.

l-I

,,.~,,,,,

1 ktm w

w

Figure 3- Scanning electron microscopy micrograph showing particles of the as-synthesised hexagonal tin(IV) phosphate solid.

Figure 4: High resolution electron microscopy micrograph domains of the as-synthesised CTAB-hexagonal tin(IV) phosphate solid.

1096 High resolution images indicate no preferential orientation of the grains and images suggest that the symmetry is of a P-type (hkO: no condition, hOl: l=2n and hhl .'l=2n). This is consistent with a pseudo-cubic (or tetragonal) lattice of a P-type symmetry with a~c~80 A. This is smaller than the parameter reported for the assynthesised SBA-I" a~89 A,[ 20] but in agreement with the use of a smaller alkyl chain surfactant (n=14) compared with SBA-1 (CTAB (n = 16)). Electron microscopy images also show that, in both cases, there is no system of sharp reflections detectable by working in selected area electron diffraction (SAED) mode.

~8nm ~" H

Figure 5: High resolution electron microscopy

micrograph of the organised domains of the assynthesised TTAB-cubic tin(IV) phosphate solid.

Thus, pore walls of these solids exhibit no crystallinity unlike in the case of the hexagonal titanium fluorophosphates reported previously.[ 18] EDS indicates for both solids (H and C) that pore walls exhibit a stoechiometry of three Sn per four phosphorus (Sn~=0.75). This is in good agreement with quantitative analysis results (Sn/P~0.73 and 0.65, respectively).

4. ACIDIC A N D BASIC P R O P E R T I E S .

Tin(IV) is well-known for its acidic properties [b]. A microcalorimetric study of the adsorption of NH3 was performed on these compounds to evaluate their acidic strength. In a second step, SO2 adsorption experiments were also realised to determine their basic character. First, the adsorption of NH3 was performed on the calcined forms of Hand C-TTAB solids. The proportion of acidic sites as a function of the binding energy was determined for each solid. The curves of differential heat of NH3 adsorption vs. coverage indicate (fig. 6) that both solids are strongly acidic with a large proportion of sites presenting a heat evolved above 100 kJ/mol. These solids and especially the hexagonal phase, are thus more acidic than MCM41 and on the whole as acidic as some H-ZSM-5.[ 21] The hexagonal form is more acidic than the cubic phase. The number of acidic sites whose strength is between 100 and 150 kJ/mol is twice

250 JU

200

'

[] H

B 150

0'

100 50 200 400 600 800 1000 1200 Vmid(~tmol/g) Figure 6: Differential Heat as a function

of the adsorbed amount of NH 3 for the H: hexagonal or C: Cubic calcined solids.

1097 higher for the hexagonal phase than for the cubic phase. However, the number of weak sites is similar for the two samples. Secondly, the adsorption of SO2 was realised on the same solids. The distribution of basic sites strength as a function of the coverage was determined for each solid. Basic sites are observed for both phases (fig.7); their quantitative amount is however very small, representing only 5 % of the number of the acid sites observed previously. The adsorption is totally reversible on the cubic phase at the temperature of 353 K, which correspond to the presence of weak sites only. For the hexagonal phase the basicity is also weak, mainly reversible. However for this sample, 10 % of the total basic sites can be classified as chemisomtion sites.

160 [] H

~ 120 --

* C -tB

~

40 0

10

20

30

40

50

60

Vmid(~mol/g) F i g u r e 7: Differential Heat as a function

of the adsorbed amount of SO 2 for the H: hexagonal or C: Cubic calcined solids.

Finally, these mesoporous tin(IV) phosphates are mainly acidic solids with a strong acidic character and a very weak basic character.

5. CATALYTIC BEHAVIOUR. As Tin(IV)-containing amorphous oxides are well known for their activity in the reduction of NO by hydrocarbon,[ 22-23] de-NOx catalytic tests were performed on the hexagonal and cubic calcined solids. These compounds were ~, evaluated in the NO-C2H4-O2 "~ reaction performed in lean conditions. Significant conversion of NO to N2 as well as of C2H4 to CO2 Z 1,6 '~-H O were detected between 400 and 1,2 600~ Plotting N2 yield versus temperature led to a Volano-shaped .-~ 0,8 curve, as usually observed for most -~ de-NOx catalysts.[ 23] The maximum < 0,4 amount of N2 produced was < observed around 475~ for both the " 0,0 100 200 300 400 500 600 700 800 samples, hexagonal tin phosphate being more active than the cubic one Temperature (~ (28 and 23% of N2 formation, Figure 8: Intrinsic activity ofN 2 formation from NO respectively). Figure 8 shows the reduction by C2H4 in oxygen atmosphere on TTAB higher for the hexagonal phase than hexagonal (H) and cubic(C) tin phosphate calcined solids. for the cubic phase. However, the number of weak sites is similar for the two samples. The better activity

1098 of the hexagonal form of tin(IV) phosphate could derive from a higher amount of tin(IV) species exposed at the surface with respect to the cubic phase. Finally, the de-NOx reactivity of the two solids, used for a previous reaction at temperatures up to 850~ decrease by 50%; this is in agreement with a structural destabilisation of the solids at high temperatures (T>600~ Besides nitrogen, by-products such as nitrous oxide and nitrogen dioxide were present in the reactor outlet at very low concentration, around 10-15 pmV of NO2 and 40-50 ppmV for N20. This behaviour indicates a good selectivity of the samples. 6. CONCLUSION Finally, thermally stable hexagonal and cubic mesoporous tin(IV) phosphates have been synthesised using the fluoride route. These solids are amphoteric compounds which exhibit both strong acidic properties and a weak basic character. The first catalytic tests have shown positive results and comparing their catalytic performances with those of known amorphous catalysts.[ 22] It seems that these new series of solids could represent an interesting family of materials with good catalytic properties. Other catalytic experiments are currently in progress to evaluate the catalytic behaviour of these new mesoporous solids.

ACKNOWLEDGELENTS. Dr F. Taulelle and C. Lorentz (Laboratoire Chimie et RMN du solide, Universit6 Louis Pasteur, Strasbourg, France) and Prof. J.C. Dumas (LAMMI, Universit6 Montpellier II, Montpellier, France) are greatfully acknowledged for their collaboration in NMR and Mossbatier experiments.

REFERENCES.

1 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. 2 A. Corma, Chem. Rev., 97 (1997) 2373. 3 J. Y. Ying, C. P. Mehnert, M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 4 F. SchtRh, Chem. Mater., 13 (2001) 3184 5 (a) F. Vaudry, S. Khodabandeh, M. E. Davis, Chem. Mater., 8 (1996) 1451; (b) B.T. Holland, P. K. Isbester, C. F. Blanford, E. J. Munson, A. Stein, J. Am. Chem. Soc., 119 (1997) 6796. 6 (a) D. M. Antonelli, J. Y. Ying., Angew. Chem. Int. Ed. Engl., 34 (1995) 2014; (b) P. D. Yang., D. Y. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature, 396 (1998) 513; (c) M. Thieme, F. Schtith, Microporous and Mesoporous Mater, 27 (1999) 193; (d) D. J. Jones, G. Aptel, M. Brandhorst, M. Jacquin, J. Jimrnez-Jimrnez, A. Jimrnez-Lopez, P. MairelesTorres, I. Piwonski, E. Rodriguez-Castellon, J. Zajac, J. Roziere, J. Mater. Chem., 10 (2000)

1099 1957; (e) J. Blanchard, F. Schtith, P. Trens, M. Hudson, Microporous and Mesoporous Mater., 39 (2000) 163. 7 (a) D. Antonelli, J. Y. Ying, Angew. Chem. Int. Ed. Engl., 35 (1996) 426; (b) P. Yang, D. Zhao, D. I. Margolese B. F. Chmelka, G. D. Stucky, Chem. Mater., 111 (1999) 2813. 8 V. Luca, J. M. Hook, Chem. Mater., 9 (1997) 2731. 9 A. Stein, M. Fendrof, T. P. Jarvie, K. T. Mueller, A. Benesi, T. E. Mallouk, Chem. Mater., 7 (1995) 304. 10 J. O. Perez, R. B. Borade, A. Clearfield, J. Mol. Struct., 470 (1998) 221. 11 U. Ciesla, S. Schacht, G. D. Stucky, K. K. Unger, F. Schtith, Angew. Chem. Int. Ed. Engl., 35 (1996) 541. 12 J. E1 Haskouri, Roca, S. Cabrera, M. Bertran-Polter, D. Beltran-Porter, M. D. Marcos, P. Amoros, Chem. Mater., 11 (1999) 1446. 13 D. M. Antonelli, A. Nakahira, J. Y. Ying, Inorg. Chem., 35 (1996) 3126. 14 Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schtith, G. D. Stucky, Chem. Mater., 6 (1994) 1176. 15 S. Neeraj, C. N. R. Rao, J. Mater. Chem., 8 (1998) 1631. 16 N. Mizuno, H. Hatayama, S. Uchida, A. Tagushi, Chem. Mater., 13 (2001) 179. 17 (a) N. Ulagappan, C. N. R. Rao, Chem. Comm., (1996) 1685; (b) L. Qi, J. Ma, M. Cheng, Z. Zhao, Langmuir, 14 (1998) 2579; (c) K. G. Severin, T.M. Abdel-Fattah, T.J. Pinnavia, Chem. Comm., (1998) 1471; (d) F. Chen and M. Liu, Chem. Comm., (1999) 1829; (e) Y. Wang, C. Ma, X. Sun, H. Li, Microporous and Mesoporous Mater., 49 (2001) 171. 18 C. Serre, C. Magnier, M. Hervieu, F. Taulelle, G. F6rey, Chem. Mater.,14 (2002), 180. 19 C. Serre, A. Auroux, A. Gervasini, M. Hervieu, G. F6rey, Angew. Chem. Int. Ed. Engl., in press (2002). 20 Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater., 8 (1996) 1147. 21 C. S. Triantafillidis, A. G. Vlessidis, L. Nalbandian, N. P. Evmiridis, Microporous and Mesoporous Mater., 47 (2001) 369. 22 M.C. Kung, P.W. Park, D.-W. Kim, H.H. Kung, J. Catal., 181 (1999) 1. 23 (a) A. Auroux, D. Sprinceana, A. Gervasini, J. Catal., 195 (2000) 140; (b) A. Auroux, Topics Catal., 4 (1997) 71; (c) C. Guimon, A. Gervasini, A. Auroux, J. Phys. Chem.B, 105 (2001) 10316.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1101

Characterization of [Cu]-MCM-41 by XPS and CO or NO adsorption heat measurements M. Broyer a, J.P. Bellat a*, O. Heintz a, C. Paulin a, S. Valange b and Z. Gabelica c a LRRS, UMR CNRS 5613, Universit6 de Bourgogne, 9 Av. A. Savary, BP 47870, F-21078 Dijon, France.

b LACCO, UMR CNRS 6503, ESIP, 40 Av. du Recteur Pineau, F-86022 Poitiers, France. c Universit6 de Haute Alsace, ENSCMu, GSEC, 3 rue A. Werner, F-68093 Mulhouse, France.

We report the characterization of copper doped MCM-41 prepared by original direct synthesis by XPS and adsorption calorimetry of CO and NO, which are selective molecular probes for Cu(I) and Cu(II) respectively. Investigation of the nature of the copper ions in this particular calcined Cu-MCM-41 by NO adsorption calorimetry shows that two types of energetically distinct adsorption sites exist, meaning the presence of two populations of Cu(II) species differently coordinated to the silica surface in quasi-equimolar concentration, as ascertained by XPS data. A small amount of Cu(I) was also detected, probably stemming from a partial reduction upon the successive vacuum treatments. The respective proportion of Cu(II) and Cu(I) were estimated from the amounts of NO and CO chemisorbed on the surface and by deconvolution of the XPS Cuzp3/2 peak. Despite the same Cu(II)/Cu(I) ratios are obtained by both XPS and calorimetry, the major part of copper is inaccessible to the molecular probes, probably because copper is well dispersed within the siliceous pore walls. But, even if the percentage of copper detected is quite low, the adsorption calorimetry of NO and CO proved a very powerful technique for the characterization of the accessible catalytically copper species. 1. INTRODUCTION Copper ion-exchanged ZSM-5 zeolites are considered among the most efficient catalysts for the selective catalytic reduction of NOx with hydrocarbons 1' 2, 3. For a given substrate, the catalytic activity depends in large part on the oxidation state of copper, but also on its location within the substrate framework. However ZSM-5 based copper catalysts show a relatively low stability especially in the presence of water vapor. The incorporation of copper inside the MCM-41 mesoporous materials which exhibit large pores, high surface areas and good thermal stabilities seems to be a logical alternative to explore better catalysts for NOx reduction. Cu-MCM-41 materials are currently prepared by classical ionic exchange of either the pure silica form or its A1-MCM-41 aluminosilicate analog, with Cu 2+ ions 4. So far, such catalysts, and, in particular, copper exchanged Si-MCM-41 (where Cu 2+ ions replace protons of the * To whom correspondence should be addressed. Email: [email protected]

1102 silanols), proved less efficient in NOx reduction than Cu-ZSM-5, probably because of the low amount of copper incorporated through such a procedure 5 but possibly also because of their weak interaction with the substrate 4' 6. Impregnation of A1-MCM-41 by various Cu 2+ salts 7 or direct synthesis of Cu-MCM-418' 9 yield various materials in which the structure and Cu 2+ ions and the strength of their interaction with the silica surface very much depend on synthesis parameters and/or post-synthesis treatments. This work is devoted to the characterization of copper doped MCM-41 prepared by original direct synthesis procedures using various additives able to adequately pre-structurate Cu 2+ ions and facilitate their steady incorporation in the silica mesophase 9. The techniques used were XPS, which is a well adapted surface technique to investigate the oxidation state of copper ~~ and adsorption calorimetry of CO and NO, which are selective molecular probes for Cu(I) and Cu(II), respectively. Indeed, it is reported in the literature TM 12 that CO is strongly adsorbed on Cu + ions and weakly on Cu 2+ ions. No stable carbonyls are formed with Cu 2+ cations at room temperature. NO is a somewhat stronger Lewis base than CO as it involves one more electron on its outer shell. It therefore readily forms stable nitrosyl complexes with Cu 2+ cations but does not interact with Cu + ions, provided care is taken to avoid oxidation of particular Cu(I) sites by NO, even at room temperature, as documented by Lamberti et al. 13.

2. E X P E R I M E N T A L

Two mesophases, namely Al-free Cu-MCM-41 and its pure silica analog, here called Si-MCM-4 l, were synthesized following an original procedure 9. Copper was added as Cu(II) nitrate to the synthesis gel along with various complexing agents that allow a more efficient Cu E+ framework incorporation and stabilization, in a cetyltrimethylammonium bromidecontaining solution prior to the addition of tetramethylorthosolicate as the Si s o u r c e 14. The gel was stirred for 1 h at ambient temperature prior to a further hydrothermal heating in autoclave at 373K for 24h. After synthesis, the catalyst was calcined to 873 K at a heating rate of 1 KJmin in an air flow. The chemical compositions of [Cu]-MCM-41 before and after calcination are given in table 1. These bulk chemical analyses were achieved at the Service Central d'Analyses du CNRS, Vernaison, France. XPS spectra were recorded at 298 K with a Ribber SIA 100 instrument using a non-monochromatic A1-Kal,2 X-ray source at a power that does not exceed 300 W in order to minimize possible reduction of Cu E+ cations induced by X-ray irradiation. The adsorption heats were measured at 298 K for pressures ranging from l0 -2 to 1000 hPa, by means of a heat flow calorimeter (C80 Setaram) coupled with ca manometric glass apparatus. The catalyst mass was about 300 mg. Before adsorption of CO or NO, the sample was evacuated in-situ under vacuum at 523 K and 363 K for the pure siliceous and copper containing forms of MCM-41, respectively. Once the first adsorption isotherm was obtained, Cu-MCM-41 was submitted to vacuum at 298 K for one night in order to evacuate all the species physisorbed at the surface. The sample was then submitted to a second isothermal adsorption. The difference in amounts adsorbed between the first and second isotherms corresponds to the amount of chemisorbed species on copper cations 15. Table 1 Chemical composition of [Cu]-MCM-41 before and after calcination. Cu (wt %) Si (wt %) Cu / Si (wt ratios)

As-synthesized [Cu]-MCM-41 2.73 23.30 0.117

[Cu]-MCM-41 after calcination 4.39 40.93 0.107

1103 3. RESULTS ,AND DISCUSSION At first, the adsorption of CO and NO was performed on the pure siliceous MCM-41. The slope of the adsorption isotherm in the low pressure range shows a very low adsorption affinity of the silica surface for these molecules (Figure 1). This result is confirmed by the low values measured for the corresponding adsorption heats, which account for very weak interactions between CO or NO and the silica surface (Figure 2). Indeed, for both gases, the adsorption heats are lower than the vaporization heats of CO (6.75 kJ.mo1-1) or NO (13.8 kJ.moll). The physical state of CO and NO adsorbed in the mesopores is more similar to a gas than a liquid. The adsorption heats measured by differential calorimetry are in very good agreement with the isosteric adsorption heats determined from adsorption isotherms performed at different temperatures (Qiso(CO) = 12 kJ.mol 1 and Qiso(NO) = 10 kJ.mo1-1 at zero filling). As the filling increases, the adsorption heats of CO and NO decrease down to about 2 - 3 kJ.mo1-1 for both molecules. This corresponds to the adsorption heat of the monolayer of CO and NO on the silica surface. The first and second adsorptions are the same, meaning that no chemisorption occurs. Besides, a simple pumping under vacuum at room temperature for a few hours was sufficient to completely evacuate all the species adsorbed in the mesopores. It is noteworthy that the adsorption heats at zero filling are slightly higher for CO than for NO. This could be attributed to a stronger interaction of CO with specific adsorption sites like, for example, silanol groups that are abundant on the pore walls under such conditions (about 5 SiOH/nm2) 16. NO, exhibiting a less basic character than CO, is not an efficient probe of the hydroxyl groups that would not readily polarize in their presence. Regarding the [Cu]-MCM-41 material, the adsorption isotherms and adsorption heats of carbon monoxide are given in figures 3 and 4. The adsorption isotherms have the same shape as for MCM-41, indicating a weak adsorption affinity of the solid for CO. However, the amounts of CO adsorbed are higher when copper is present in the material. On the other hand, the adsorption of CO is not reversible. After pumping at room temperature, the CO molecules are not all evacuated, indicating that some of them stay chemisorbed, probably on Cu + sites. The adsorption heats at zero filling, which can exceed 50 kJ.mo1-1, suggest that strong interactions between adsorbate and adsorbent occur. These values are lower than those reported by Kuroda et al. 17 (90 - 70 kJ.mol -~) and by Bolis et al. ~8 (130-100 kJ.mol ~) on Cu +exchanged ZSM-5 zeolites. This difference could be explained by assuming that the 0.8

20

1

~o 0.6 ""

-

,,..~

o

10

~

0

O

0.4

(Y

~Z 0.2 0 ~, 0

--

200

400

600

800

1000

p/hPa Figure 1" Adsorption isotherms of CO (open circles) and NO (full circles) on Si-MCM-41 at 298 K.

N O

~ o ~

~ Oe o eo

9 CO

10 0

0.1

,0,,

0.2 0.3 N a / mmol.g -1

0.4

Figure 2: Adsorption heats of CO and NO on Si-MCM-41 at 298 K. [open circles: first adsorption; full circles: second adsorption].

1104 Table 2: Amount of CO and NO chemisorbed respectively on Cu + and Cu 2+ sites and weight percent fraction of such copper species evaluated by using a simple copper-probe interaction model. Nairrev(CO)

Nairrev(NO)

Cu +

Cu 2+

mmol.g -~

mmol.g ~

wt %

wt %

0.0007

0.011

6

94

interaction between CO and the specific adsorption sites is less influenced by the surface curvature of the mesoporous [Cu]-MCM-41 material than of the microporous ZSM-5 zeolite. Moreover, the coordination of Cu § and, consequently, its interaction with the host molecules is probably different when the silica surface is crystallized (ZSM-5) rather than amorphous (MCM-41). Finally, the whole structural state of Cu(I) in ZSM-5 is definitely different, as these ions are basically counterions to the zeolite negative charges induced by the presence of framework aluminum 9' 19, z0, while the close neighbors of Cu(I) in Cu-MCM-41 are the oxygens linked to the adjacent Si atoms 4, 6, 9. As the filling increases, the adsorption heats sharply decrease suggesting that these strong specific adsorption Cu + sites are not numerous. If we assume the simplest Cu(I)-CO interaction, namely that only one CO is coordinated to one Cu § the amount of Cu(I) sites can be estimated by calculating the amount of CO chemisorbed after a first adsorption. This amount is given by the difference between the amounts of CO adsorbed on the first and second adsorption isotherms at the equilibrium pressure at which the adsorption heat reaches 3 kJ.mol ~ i.e. when the completion of the monolayer on the silica surface is achieved, thus when the specific adsorption sites are saturated. The amount of CO chemisorbed on copper(l) sites calculated by estimating this equilibrium pressure at about 20 hPa from figures 3 and 4, are given in Table 2. The amount of copper (I) detected by the molecular probe CO is actually very low and represents only a very small part (about 0.1%) of the total copper present in the solid. We believe that these Cu(I) species probably originate from a partial reduction upon the successive vacuum treatments. The adsorption isotherm of NO in the low pressure region shows that the adsorption affinity of [Cu]-MCM-41 is stronger for NO than for CO (Figure 5). The adsorption is not reversible, indicating that some of NO molecules are chemisorbed on Cu 2+. As shown on figure 6, the 0.8

50 -

exl)

CO

CO

'~ 40

..: 0.6 O

30

E

E 0.4

~z

~ 2o

0.2

lO

, • • J P ~

0

200

400 600 p/hPa

800

1000

Figure 3" Adsorption isotherms of CO on [Cu]-MCM-41 at 298 K [open circles" first adsorption, full circles" second adsorption].

0

0.2

~O,~Ke,ce

,Oe

0.4

0.6

,

~

,

0.8

-1

N" / mmol. g

Figure 4: Adsorption heats of CO on [Cu]-MCM-41 at 298 K [open circles: first adsorption, full circles: second adsorption].

1105 adsorption of NO is very exothermic with adsorption heats at zero filling being close to 110 kJ.mol -]. This value is of the same order of magnitude as those found by Gervasini et al. 12 on ETS-10 and ZSM-5 exchanged with Cu 2+ cations. This high energy of interaction is characteristic of formation of nitrosyl-type complexes with copper(II). It is worth noting on the calorimetric curve that the adsorption process of NO is very heterogeneous. Two types of adsorption sites were indeed evidenced at low filling ( N a < 0.04 mol.g-l). The first kind of sites identified in the coverage range 0 - 0.02 mmol.g ] have an adsorption energy of about 111 kJ.mol ]. The second set of sites have an adsorption energy two times lower and correspond to fillings ranging from 0.02 to 0.04 mmol.g -~. After pumping under vacuum at room temperature, the calorimetric measurements indicated that the NO molecules are adsorbed with an energy that does not exceed 80 kJ.mo1-1. This suggests that only the sites of second type are evacuated upon pumping. However a partial evacuation of sites of the first type is not excluded. This energetic heterogeneity of the adsorption process can be attributed either to the presence of two types of copper (II) differently coordinated to the siliceous framework or to the formation of dinitrosyl complexes on Cu 2+. The fact that the amount of NO adsorbed on the second type of sites is twice as much as the one on the first type, is in favor of the second hypothesis. However, the formation of dinitrosyl species on Cu z+ has never been observed so far by other techniques such as, for example, FTIR, even under a pressure of NO close to 100 hPa 21. Thus the formation of dinitrosyl complex does not appear very probable in our case where the equilibrium pressure is lower than 20 hPa, although this possibility should not be completely ruled out. If we retain the first hypothesis, namely that the energetic heterogeneity results from the adsorption of NO on two different Cu 2+ cations, the amount of copper (II) probed by NO can be evaluated from the adsorption isotherms in the same way as for CO. The values given in Table 2 show that the amount of NO coordinated with copper is far larger than the amount of CO. However, the quantity of copper (II) detected by NO does not exceed 1.6 wt % of the total copper content. This result suggests that the major part of copper is inaccessible to the molecular probes. From this relatively surprising but important finding, it can be concluded that copper is probably well inserted and dispersed within the siliceous pore walls of [Cu]-MCM-41. Only a few copper cations are located at the surface of the cylindrical mesopores or at the external surface of the mesophase particles and can interact with the NO or CO molecular probes. Assuming that only one CO and one NO are essentially coordinated with one Cu § and one Cu 2+ respectively, the proportion of copper (II) can be estimated from the irreversible amounts of CO and NO adsorbed, by using the following relation: 0.8 =,...

"~ 0.6

120

o

E

,...j o

80

E 0.4 ~

~Z 0.2

4O I

0

200

400 600 p/hPa

800

1000

Figure 5: Adsorption isotherms of NO on [Cu]-MCM-41 at 298 K [open circles: first adsorption, full circles" second adsorption].

0.01

J

I

0.02

,

I

,

0.03

N a / mmol.

g

I

0.04

,

0.05

-1

Figure 6" Adsorption heats of NO on [Cu]-MCM-41 at 298 K [open circles: first adsorption, full circles" second adsorption].

1106

%Cu 2+ =

Niaev (NO) xlO0 N airrev(CO)+ Nirrev a (NO)

In [Cu]-MCM-41 calcined under air flow, 94 wt % of copper accessible to the probe molecules is in the oxidation state (+II) (Table 2). This proportion is possibly underestimated because during the pumping under vacuum at room temperature following the first adsorption, some NO molecules can be desorbed in spite of their strong interaction with Cu 2+. Moreover it is well known that NO can oxidize the Cu + cations to Cu 2+as shown by Lamberti et al. 13, and Hadjiivanov et al. ~1. In our case, however, the content in Cu + is low enough so as to neglect this phenomenon. As shown on figure 7, the XPS spectrum of [Cu]-MCM-41 is quasi the same as for CuO. The presence of an important satellite peak indicates without ambiguity that most of the copper is at the oxidation state (+II). For both [Cu]-MCM-41 and copper oxide, the relative intensities between the Cuzp3/2photoelectron peak and its satellite peak are quite the same. The values of binding energy of the peaks observed in the Cu2p spectral range are in good agreement with those obtained by Griinert et al. ~~ and Bolis et al. 18 on [Cu]-ZSM-5 zeolites. The Cu/Si ratio evaluated from the integrated intensity of Cu2p3/2and Sizp peaks ~5 is of 0.404 while the ratio given by the chemical analysis is 0.107 (Table 1). This indicates that the copper implanted in the material is essentially located next to the external surface of the [Cu]-MCM-41 particles. It is worth pointing out the presence of a shoulder on the Cu2p3/2peak for [Cu]-MCM-41 at the binding energy of 934 eV (Figure 7). This shoulder is interpreted, in view of calorimetric data, as the signature of the presence of Cu + and two types of Cu 2+ in different coordination. The spectral deconvolution of the Cuzp3/2peak actually leads to the identification of three peaks at 935.25, 932.65 and 929.75 eV (Figure 8). Their contributions in the Cuzp3/2photoelectron peak are of 50%, 45% and 5% respectively. The first two peaks are attributed to two types of Cu 2+ and the third one to the presence of Cu +. The fact that the contributions of the first two peaks are similar suggests the presence of two populations of Cu(II) species, both quasi equally responsible of the energetic heterogeneity of NO adsorption. As a conclusion, the presence of two steps on the calorimetric curve should not be considered as the result of the formation of dinitrosyl species but is due to these two types of

Cu-M CM-41

.,..~ r/k

CuO

~

o ~

1/

I

I

Cu 2P3/2~_,.,~ I

I

I

[

I

980 970 960 950 940 930 920 910 Binding energy / eV Figure 7" XPS spectra in the Cu2p spectral range of [Cu]-MCM-41 calcined in air.

945

.

.

.

.

940 935 930 Binding energy / eV

925

Figure 8: Deconvolution of the Cu2p3/2 XPS peak assuming two different coordinations for Cu 2+ and the presence of small amount of Cu +.

1107 Cu 2+ cations. The Cu 2+ and Cu + contents extracted from the spectral deconvolution of the Cuzp3/2 photoelectron peaks are of 95 wt % and 5 wt % respectively, values that are in

excellent agreement with those deduced from the calorimetric data (Table 2). However, one should never exclude the possibility for part of copper (I) detected by XPS to stem from the reduction of Cu 2+ cations induced by X-ray irradiation under ultra high vacuum during the measurements. To confirm this possibility, several photoelectron spectra have been recorded on CuO and [Cu]-MCM-41 for different irradiation times. The results obtained clearly show that some Cu 2+ cations are indeed reduced under irradiation but in neither case the amount of Cu + so formed oversteps 5 wt %. Finally, the fact that the same CuZ+/Cu+ ratios are obtained by both XPS and calorimetry, indicates that both techniques probe the very surface of the material, where only a minor percentage of the total copper is located. EPR, the ideal technique to detect all the Cu 2+ species in the bulk framework of MCM-41, seems to confirm that the amount of Cu + is negligible in the pore walls 22, thereby confirming that Cu + is probably generated on the surface upon vacuum evacuation.

4. CONCLUSION Adsorption calorimetry of CO and NO and X-ray photoelectron spectroscopy proved powerful techniques for the characterization of copper essentially located on the surface of [Cu]-MCM-41 prepared by direct synthesis. These two complementary techniques provided valuable information about the oxidation state of copper and its location. They also allowed to check whether several types of copper can exist in different coordination states in the silica framework. The incorporation of copper during synthesis achieved following our original recipe leads to a mesoporous material that contains essentially Cu 2+ cations after calcination under air at 873 K. Two types of copper differently coordinated to the silica surface and in quasi-equimolar concentration were evidenced. The major amount of copper is located inside the pore walls. Less than 2 wt % of the total copper content is located on the mesopore surface or at the external surface of the particles. This is probably the mean reason explaining why such materials showed a very reduced catalytic activity in the case of NOx decomposition 23. Finally, although the material was calcined under air flow, the formation of a small amount of Cu + (about wt 0.1% with respect to the total copper content), could not been avoided. The presence of copper at the oxidation state (+I) is probably the result of a reductive phenomenon, that may occur either during the calcination of the template or during the further activation of the calcined material under vacuum. A study of the influence of the temperature of calcination under different atmospheres on the structure and oxidation state of copper in this particular [Cu]-MCM-41 material is in progress.

REFERENCES 1. A. Zecchina, S. Bordiga, G. Turnes Palomino, D. Scarano, C. Lamberti and M. Salvalaggio, J. Phys. Chem. B, 103 (1999) 3833. 2. J.L. D'Itri and W.M.H. Sachtler, Applied Catalysis B: Environmental, 2 (1993) L7. 3. V.I. Pftrvulescu, P. Grange and B. Delmon, Catalysis Today 46 (1998) 233. 4. A. P6ppl, M. Hartmann, L. Kevan, J. Phys. Chem. 99 (1995) 17251. 5. Z. Luan, P. Xu, L. Kevan, Nukleonika, 42 (1997) 493. 6. A. P6ppl, M. Newhouse, L. Kevan, J. Phys. Chem. 99 (1995) 10019.

1108 7. A. Zecchina, D. Scarano, G. Spoto, S. Bordiga, C. Lamberti, G. Bellussi, Stud. Surf. Sci. Catal., 117 (1998) 343. 8. M. Hartmann, S. Rachouchot, C. Bischof, Microporous and Mesoporous Mater., 27 (1999) 309. 9. S. Valange, PhD Thesis, (2000), Universit6 de Haute Alsace, Mulhouse, France. 10. W. Grtinert, N.W. Hayes, R. W. Joyner, E. S. Shpiro, M. R. H. Siddiqui and G.N. Baeva, J. Phys. Chem. 98 (1994) 10832. 11. K. Hadjiivanov and L. Dimitrov, Microporous and Mesoporous Materials 27 (199) 49. 12. A. Gervasini, C. Picciau and A. Auroux, Microporous and Mesoporous Materials 35-36 (2000) 457. 13. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo, G. Vlaic and M. Bellatreccia, J. Phys. Chem. 101 (1997) 344. 14. S. Valange, Z. Gabelica, in preparation. 15. M. Broyer, PhD Thesis, (2001), Universit6 de Bourgogne, Dijon, France 16. A. Galarneau, G. Desplantier-Giscard, F. Di Renzo, F. Fajula, Catal. Today 68 (2001) 191. 17. Y. Kuroda, Y. Yoshikawa, R. Kumashiro and M. Nagao, J. Phys. Chem. B 101 (1997) 6497. 18. V. Bolis, S. Maggiorini, L. Meda, F. D'Acapito, G. Turnes Palomino and S. Bordiga, J. Chem. Phys. 113 20 (2000) 9248. 19. G. Spoto, S. Bordiga, G. Ricchiardi, D. Scarano, A. Zecchina, F. Geobaldo J. Chem. Soc. Faraday Trans. 91 (1995) 3285. 20. S. Valange, F. Di Renzo, E. Garrone, F. Geobaldo, B. Onida, Z. Gabelica, Stud. Surf. Sci. Catal. 135 (2001), 175; 13th IZC Montpellier, France, 2001, Publication #04-O-01 (CDROM, ISBN 0-444-50238-8, 8 pp). 21. M. Iwamoto, H. Yahiro, N. Mizuno, W.X. Zhang, Y. Mine, H. Furukawa and S. Kagawa, J. Phys. Chem. 96 (1992) 9360. 22. S. Valange, L. Bonneviot, B. Echchahad, Z. Gabelica, in preparation. 23. M.F. Ribeiro, S. Valange, Z. Gabelica, unpublished results.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1109

Synthesis and characterisation of iron-containing SBA-15 mesoporous silica F. Martinez a, Yong-Jin Han a, Galen Stucky a, J.L Sotelo b, G. Ovejero b and J.A. Melero c Department of Chemistry and Biochemistry. University of California. Santa Barbara. CA 93106. a

b Department of Chemical Engineering. Complutense University. 28040 Madrid. Spain. c Environmental and Chemical Engineering Group. Rey Juan Carlos University. 28933 M6stoles (Madrid). Spain.

Mesoporous silica SBA-15 molecular sieve has been synthesised and modified with Fe via a post-synthesis procedure by reacting as-made SBA-15 with different iron precursors in dry ethanol and followed by calcination. The influence of variables such as the nature of the starting silica support and the iron source (FeC13 6H20 and Fe (O-CHzCH3)3 in absolute anhydrous conditions) on the grafting of metallic species into SBA-15 materials has been studied. Likewise, direct synthesis by co-condensation of Fe chloride with silica species templated with Pluronic 123 has been also checked. Different strategies of synthesis lead to significant changes in the bonding and environment of iron species within the silica materials. These differences have been monitored by means of spectroscopic techniques such as: PASFTIR, DRUV-VIS, 29Si NMR and EPR.

1. INTRODUCTION Nonionic poly(ethylene oxide)-poly-(propylene oxide)-poly(ethylene oxide) triblock copolymer (PEO-PPO-PEO) as structure-directing agent have led to the preparation of a new family of highly ordered mesoporous silica structures. Hexagonally ordered SBA-15 silica [1] due to its outstanding thermal stability, pore size adjustment and tailored particle morphology is highly desirable for catalytic applications, sorption processes and the preparation of advanced optical materials [2-3]. Several strategies can be employed for immobilisation of metallic active species over silica mesoporous materials either as extraframework nanoscale oxide clusters and chelated complexes or in their appropriate valence state as tetrahedral framework species [3,4]. In particular, SBA-15 has been functionalised by incorporation of different heteroatoms such as Ti [5] and A1 [6,7] as well as by chemical bonding of organosilanes through silylation and direct synthesis procedures.

1110 Post-synthetic grafting routes are based on the presence of surface sylanol groups as anchoring sites. In order to achieve a high surface coverage with functional groups or metallic species, it is important to remain a large number of surface sylanol groups after removal of the surfactant. On the basis of avoiding the condensation of many surface groups in a typical template removal by calcination, appropriate extraction methods appeared as an interesting alternative. Likewise, several works have described a template displacement method for surface modification of uncalcined MCM-41 materials in which the chemical functionality and surfactant removal occur simultaneously [8,9]. This research prompted us to develop a novel method for the preparation of iron modified SBA-15 materials by a direct displacement of the template with ethanolic solutions containing Fe species. Under acid conditions, hydrophilic EO moiety blocks of the copolymer are expected to interact with the protonated silica by a S+X-I+ mechanism based on hydrogen bondings [1]. The weak S+X-I+ interactions between surfactant and silica species make the template replacing easier than that of strong electrostatic ones (S+I-) [8]. Therefore, mild reaction conditions and short times were enough for removal of surfactant and incorporation of iron species. Finally, direct synthesis of iron containing SBA-15 materials was also carried out by co-condensation of iron and silica species during the preparation of the mesostructured molecular sieves. The incorporation of iron in the SBA-15 support as well as the interaction between metal species and silicon oxide network has been discussed for both strategies of synthesis by means of different spectroscopic techniques. 2. E X P E R I M E N T A L SECTION

2.1. Chemical reagents The silica source was tetraethylorthosilicate (TEOS, 98%) supplied by Aldrich. Amphiphilic triblock copolymer poly(ethylene oxide)-poly-(propylene oxide)-poly(ethylene oxide) (Pluronic P123, MW=5800) was obtained from BASF. Iron chloride (FeC13 6H20) and iron ethoxide (20-22% in ethanol) were purchased from Aldrich and Gelest and used as iron sources, respectively.

2.2. Synthesis of Fe containing SBA-15 mesoporous materials Parent SBA-15 mesoporous silica was synthesised according to the procedure reported in the literature [1]. Iron species were incorporated in the silica support using two different methodologies: Grafting on as-made SBA-15 materials. As-synthesised SBA-15 silica material was dispersed in 0.1M ethanolic solution containing iron species and stirred at 60~ or 100~ for 3 hours. Thereafter, grafted sample was recovered from the alcoholic solution by centrifugation and washed with water and ethanol until supernatant liquid was colourless. The washed sample was dried at 100~ overnight and calcined in air for 6 h. at 500~ The resultant Fe-SBA-15 materials are denoted as IC and IE when FeC13.6H20 and Fe(O-CHz-CH3)3 were used as iron sources, respectively. In order to check the reactive nature of the mesostructured surface, calcined (500~ in air) and solvent extracted (1.8 vol. % HC1 12N at room temperature for 8h) SBA-15 materials were also used as supports. Direct synthesis. 1 g of Pluronic 123 was firstly dissolved at 38~ in a HC1 aqueous solution with different acid concentrations. Following dissolution of block copolymer, 0.8 g of FeC13 6H20 was added and left under stirring during 30 minutes. Thereafter, 2.15 g of TEOS was added and the resultant solution was stirred overnight at 38~ after which the mixture was

1111 aged for 24h at 100~ under static conditions. The solid product was recovered by filtration and air dried at 100~ Finally, the resulting powder was calcined at 500~ for 6 hours. These Fe-SBA-15 samples are denoted as DS.

2.3. Samples characterisation X-ray powder diffraction (XRD) data were acquired on a S C I N T A G PADX diffractometer using Cu K ~ radiation. The data were collected from 0.6 to 4 ~ (20) with a resolution of 0.02 ~ Photoacoustic infrared spectra were collected with a Nicolet 850 IR spectrometer and a photoacoustic cell, MTEC Model 300. For each sample, 150 scans were added to achieve acceptable signal-to-noise levels. Nitrogen adsorption and desorption isotherms at 77 K were measured using a Micromeritics ASAP 2000 system. Transmission electron microscopy (TEM) studies were carried out on a JEOL 2000 electron microscope operating at 200 keV. Energy-dispersive X-Ray (EDX) spectra were taken on a Gatan detector connected to the electron microscope (electron beam size 10-100nm). The UV/visible spectra were measured with a CARY5 UV/VIS-NIR spectrophotometer equipped with the Varian diffuse reflectance accessory for solids. Solid-state 29Si NMR experiments were performed on a CMX-500 spectrometer with the following conditions: magic-angle spinning at 5 kHz; n/2 pulse, 3 las; a repetition delay of 300 s; and 200 scans. EPR spectra were recorded at 77 K with a Bruker ER-200D spectrometer, operating in X-band and calibrated with a DPPH standard (g=2.0036). Before recording the EPR spectrum, calcined samples were outgassed using a conventional high-vacuum line. Iron content of the samples was determined by means of ICP-AES analysis collected in a VARIAN VISTA apparatus. 3. R E S U L T S AND D I S C U S S I O N Table 1 lists physicochemical and textural properties as well as iron degree incorporation of the Fe-SBA-15 materials. The difference between both strategies of synthesis has been discussed in terms of iron incorporation and bonding of iron species in the silica network. Table 1. Synthesis conditions and physicochemical properties of Fe-SBA-15 materials Starting SBA-15

Fe Source

Fe Content (wt %)

SBET (m2/g)

Pore sizeb (/~,)

aoc (,~)

Wall Thicknessd (/~)

60 100 60 60

0.80 1.31 0.86 3.34

805.6 768.3 631.7 692.9

56.2 61.0 60.9 60.2

106.2 108.4 97.9 108.8

50 47 37 49

60

3.11

739.7

57.2

100.7

43

T (~

.....Post-synthetic grafiin8 method IC- 1 IC-2 IC-3 IE-1 IE-2

As-made Calcined As-made HCl/Ethanol extracted

FeCI3 6H20 Fe (O-CHzCH3)3

Direct Synthesis H+/TEOS (molar ratio) DS-1 9.6 0.88 . . . . DS-2 5.8 FeC13 6H20 100a 1.06 735.3 62.8 101.9 39 DS-3 1.9 1.18 . . . . a Ageing temperature; b Pore size was estimated by BJH method; c Defined as ao= 2 x d(100)/x/3 and d(100) was determined from XRD spectra; d Wall thickness was calculated as" ao-pore size

1112

3.1. Iron incorporation degree and textural properties Fe-SBA-15 materials prepared from a post-synthetic grafting method using FeC136H20 as iron source (samples IC 1-3; Table 1) showed a lower iron content than those synthesised with iron ethoxide (samples IE 1-2; Table 1). Additionally, an increase of exchange temperature using FeC13.6H20 as iron source promoted a significant enhancement of the iron loading (sample IC-2; Table 1). The surface properties of silica support should influence significantly on the incorporation degree of iron species by means of grafting procedures. In this way, extracted silica based materials with a high concentration of silanol groups (Q3 = 41%; Figure 1), not thermally reacted by condensation, might achieve a more efficient incorporation of iron species than that using calcined supports (Q3 = 28%). However, the iron loadings obtained for IC-1 and IC-3 samples (Table 1) show that this effect is almost negligible using silica SBA-15 materials as support. This result suggests that although the amount of anchoring sites is different, the reactive nature of silanol groups is dramatically influenced by the extraction treatment. Likewise, the anchoring of iron species in the SBA-15 support (IC-1 and IE-1) yielded a significant decrease of Q3 and Q2 peaks (Figure 1) which demonstrates the incorporation of iron species into the silica matrix. Thereby, the proposed one step treatment for incorporation of iron species in SBA-15 materials allows removal of surfactant, avoiding high temperature calcination, accompanied with the incorporation of metallic species. Physicochemical properties of iron containing SBA-15 materials synthesised through cocondensation of iron chloride and silica species under different acidic conditions and templated by P123 are also shown in Table 1. An increase in the incorporation of iron species into the structure (from 0.8 to 1.18 Fe wt. %) was readily observed with the decreasing of H+/TEOS molar ratio. A feasible explanation regards to the use of milder acid media, in which the preparation of mesoporous molecular sieves silica might approach to (S~ ~ assembly pathways, promoting the incorporation of iron species. Indeed, incorporation of metallic species such as Fe, Cr and Rh through electrically neutral assembly pathways has been successfully demonstrated [ 10].

/

L_ L___

L___

|

CS IE-1 IC-1 ES

i

60

i

I

"

]

H I I

/

~[I

]

IC-1

,

/

II1

-~ I ' - - -

'/I/1\

91.9

IE-,

DS-2'

t

~ '

93.8

88.3

C a l c i n eed d S - 1 5 (CS) SEB A,-is (CS)

"---Ds--2

L__ i

94.2

IC-2

141;

i

80 100 120 140 ppm Q4 Q3 Q~_

(-110.5 ppm) 67.5 64.2 61.0 53.8

(-,101.3 ppm) 28.1 28.9 31.7 40.9

(-91.5 ppm) 4.4 6.8 7.2 5.2

Figure 1. 295i MAS-NMR spectra

0.5

1.0

1.5 20

2.0

2.5

Figure 2. X-Ray Diffraction patterns of ironcontaining SBA-15 silica materials

1113 Textural properties of the Fe-SBA-15 materials are showed in Table 1. All the materials reported in this work displayed typical type IV adsorption isotherms with relative high surface areas (700-800 mZ/g) and hydrothermally thick walls. It is clearly evidenced that the incorporation of iron species using both strategies of synthesis maintains the mesoscopic structure of SBA-15 materials.

3.2. Mesoscopic ordering of Fe-SBA-15 materials Low angle XRD patterns of as-made and calcined SBA-15 silica as well as different calcined Fe containing SBA-15 materials are depicted in Figure 2. All the samples, except asmade, showed a well resolved pattern with three clear reflections evidencing the high ordering of the synthesised materials. It is well known that as-made pure silica SBA-15 suffers significant structure shrinkage after calcination [1] as it is readily evidenced in Figure 2. Otherwise, it can be seen that calcined iron-containing materials prepared through postsynthetic grafting methods did not suffer a decreasing of unit cell parameter with dl00 spacing values similar to those shown by as-made silica materials. This fact can be attributed to the isomorphous substitution of trivalent iron for the tetravalent silicon in the framework (rsi4+ = 0.40 and r~e3+= 0.63) [ 11] and the lower concentration of hydroxyl groups after anchoring of iron species, which are responsible of thermal shrinking after calcination. TEM images shown in Figure 3 for a Fe-SBA-15 sample (IE-1) confirm the good mesoscopic ordering of these materials. However, it must be noted the presence of darker areas in some regions of the samples, probably associated to amorphous iron species not detected by high-angle XRD. Elemental analysis using EDX of these dark spots yielded iron contents higher than those obtained by ICP over the bulk powder samples. However, random analyses of well ordered regions showed similar iron concentrations to those initially calculated by ICP. These results confirm the presence of extraframework species as reported in the synthesis of A1-SBA-15 materials [6,7]. Our research efforts are currently addressed to the study of the washing conditions of the final materials in order to remove the presence of extraframework iron species.

~,~

Figure 3. Transmission electron micrographs of the Fe-containing SBA-15 prepared by surfactant exchange with Fe-ethoxide in ethanolic solutions (IE-1 sample; Table 1)

1114 3.3. E n v i r o n m e n t a n d location of Fe species

Different spectroscopic techniques including infrared, DR UV-VIS and EPR have been used to elucidate the environment of Fe species supported in silica based SBA-15 materials. Photoacustic FTIR. Infrared spectroscopy yields valuable information concerning the framework of the support and the local silicon-bonding environment in metallic containing mesoporous silicates. Infrared spectra of as-made and calcined iron SBA-15 materials prepared by different procedures are compared in Figure 4. The peak assignments of silica mesoporous materials have been made on the basis of studies developed on several silica matrixes in presence of metallic species [ 12,13]. Spectra of uncalcined Fe-SBA-15 materials prepared by the grafting procedure (samples IE-1 and IC-1) show that practically all the template was removed, demonstrating a significant exchange of iron species. In contrast, Fe-SBA-15 materials prepared by direct synthesis (DS-2 sample) show a higher content of surfactant molecules. A significant feature of the PAS-FTIR spectrum of calcined Fe-SBA-15 samples is present within the region 990-930 cm -~ in comparison with their uncalcined homologous materials. Absorptions in this region are source of ample controversy, since multiple metallic species are believed to absorb in this frequency region, and the absorption is also present in pure silica samples [13]. Absorption bands at 958 cm -~ have been attributed to either the perturbation of the Si-OH vibration by a neighbouring metallic centres or the stretching of a newly formed asymmetric Si-O-metal bond. This assumption can be considered to justify the presence of iron species in Fe-SBA-15 samples. Likewise, it is noteworthy that this peak was just remained for the IC-1 sample after calcination, whereas for IE-1 and more significant for DS-2, this band shifted to a slight shoulder located at 980 cm -~, indicating a change of environment of iron species upon calcination. 0

I I As-made Surfactant Fe-SB/X,-1 5 ~ ',,_

/ ~

/

>,

/

t-" (D .i-,

.=_ .o_ .i-, t./) 0 0 0

jrx,

: Calcined

Fe-SBh-

_ ~

,6

958" cm 1 i

, ~

d

0 r 13..

980 cm 1 35100 2800

I

::

1400 1200 10'00 W avenumbers (cm 1)

i i I

800

6(;0

Figure 4. PAS-FTIR spectra of Fe-SBA-15 materials

'

400

F

.

/ .

.

I

.

. I

I

'

I

'

200 250 300 350 400 450 5 C)0 550 600 Wavelength (nm)

Figure 5. DRUV-VIS spectra of calcined FeSBA- 15 materials

1115

Diffuse reflectance in UV-VIS. DR UV-VIS spectra of calcined Fe-SBA-15 materials are depicted in Figure 5. Spectrum of Fe-SBA-15 material, denoted as DS-2, is dominated by a strong absorption in the range of 200-300 nm, due to metal-oxygen charge transfer. The bands centred at 218 and 250 nm, present in all the Fe-SBA-15 materials, indicate the presence of iron in tetrahedral coordination at framework positions [14]. Unlike Fe-SBA-15 materials prepared through direct synthesis (DS samples) and post synthetically routes (IE samples), materials synthesised with iron chloride as metal source (IC samples) showed a clear modification of UV-VIS spectra: (i) broadening of charge transfer bands, (ii) a shift towards higher wavelengths and (iii) an increase in absorbance in the high-wavelength tail. These features observed for IC samples evidence the presence of Fe in extra-framework positions as iron oxide clusters located onto the walls of mesostructured materials. Finally, it must be remarkable that the disappearance of 958 cm -1 IR band and the subsequent appearance of 980 cm -1 band upon calcination for DS-2 and IE-1 samples seem to be related with the low presence of extra-framework iron species confirmed by DR UV-VIS spectroscopic results. EPR spectroscopy. EPR spectra of calcined Fe containing SBA-15 materials measured at 77K are shown in Figure 6. Three different signals appeared at geff values = 4.3, 2.4 and 2.0, which are usually observed in iron containing silica matrix [15]. The assignments of these signals are ambiguous and different works are described in literature [9,11,14,15]. According to these studies, transitions at 4.3 and 2.0 seem to be attributed to trivalent (paramagnetic) iron in the distorted and symmetrical tetrahedral framework sites whereas the signal centred at 2.4, which resulted especially visible at 77K, might be assigned to nanosized (superparamagnetic) clusters located within the mesopores of SBA-15. From these results, it is inferred that during the process of loading and subsequent calcination treatment, trivalent iron is partially substituted in the silicate framework and part is present as extraframework nanoparticles located in the channels. However, the proportion of both iron species is clearly -4.3 -2.0 Samples I IC-1~ IC-2 lIE-1 IDS-; dependent on the strategy of synthesis. -2.4 g ~ 72"~++ 4.3) EPR results correlates fairly well with those obtained by PAS-FTIR and DR IC-1 UV-VIS. IC samples showed a low geff [(2+4.3)/2.4] ratio suggesting a higher presence of extraframework Fe species and confirming DR UV-VIS results. On the other hand, post-synthetic route using Fe ethoxide in anhydrous conditions (sample IE-1) as well as the direct synthesis procedure (sample DS-2) led to a higher tetrahedral substitution of Fe species in the silica framework after calcination treatment. Likewise, iron-containing materials are usually unstable upon calcination, 0 2000 4000 6000 8000 10000 showing a high amount of Magnetic field (G) extraframework iron species after Figure 6. EPR spectra at 77K of calcined Fe-SBA-15 thermal treatments. In this work, it must materials

1116 be noteworthy that calcined DS and IE SBA-15 materials present a high thermal stability of framework iron species as it is evidenced by EPR and DRUV-VIS spectra. 4. CONCLUSIONS Iron-containing ordered SBA-15 materials have been synthesised by grafting routes (IC and IE samples) and through direct synthesis procedures (DS samples). Iron contents around 1 wt. % were achieved for IC and DS samples, whereas values up to 3.3 wt % have been obtained for IE samples using iron ethoxide in absolute anhydrous conditions. Spectroscopic techniques demonstrate that Fe species are partially isomorphously substituted or grafted in the silica framework accompanied by the presence of extraframewok iron oxide clusters located within the channels. The degree of iron loadings as well as the environment of these metallic species in the silica network is strongly dependent on the synthesis strategy. Further efforts are currently carried out in order to minimize the presence of unstable iron species, which are responsible of leaching effects in heterogeneous catalytic systems. These Femodified mesostructured materials display high surface areas (650-800 A0, defined pore size (55-60 A~) and remarkable hydrothermal stability which are appealing features for oxidation catalytic processes.

ACKNOWLEDGEMENTS We wish to thank Dr. J.Soria et al. from Instituto de Catfilisis y Petroqufmica (CSIC) for their help with EPR measurements.

REFERENCES 1. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky. J. Am. Chem. Soc., 120 (1998) 6024. 2. J.Y. Ying, C.P. Mehnert and M.S. Wong. Angew. Chem. Int. Ed., 38 (1999) 56. 3. A. Stein, B.J. Melde and R.C. Schroden. Adv. Mater., 12(19) (2000) 1403. 4. K. Moller and T. Bein, Chem. Mater., 10 (1998) 2950. 5. Z. Luan, E. M. Maes, P. A.W. van der Heide, D. Zhao, R.S. Czernuszewicz and L. Kevan, Chem. Mater., 11 (1999) 3680. 6. Y. Yue, A. Gdd6on, J. Bonardet, N. Melosh, J. D'Espinose and J. Fraissard, Chem. Commun., (1999) 1967. 7. Z. Luan, M. Hartmann, D. Zhao, W. Zhou and L. Kevan, Chem. Mater., 11 (1999) 1621. 8. V. Antochshuk and M. Jaroniec, Chem. Commun., (1999) 2373. 9. A.B. Bourlinos, M.A. Karakasides and D. Petridis, J. Phys. Chem., 104 (2000) 4375. 10. W. Zhang, B. Glomski, T.R. Pauly and T.J. Pinnavaia, Chem. Commun., (1999) 1803. 11. P. Selvam, S.E. Dapurkar, S.K. Badamali, M Murugasan and H. Kuwano, Catal. Today, 68 (2001) 69. 12. M.S. Morey, S. O'Brien, S. Schwarz and G.D. Stucky, Chem. Mater., 12 (2000) 898. 13. M.S. Morey, S. O'Brien, S. Schwarz and G.D. Stucky, Chem. Mater., 12 (2000) 3435. 14. S.K. Badamali, A. Sakthivel and P. Selvam, Catal. Lett., 65 (2000) 153. 15. A. Tuel and S. Gontier, Chem Mater., 8 (1996) 114.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1117

Synthesis and characterization of mesoscopically ordered surfactant/cosurfactant templated metal oxides T. Czuryszkiewicz a, J. Rosenholm a, F. Kleitz b, M. Lindena'* aDepartment of Physical Chemistry, Abo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland bDepartment of Heterogeneous Catalysis, Max-Planck-lnstitut fer Kohlenforschung, Kaiser-Wilhelm-Platz 1,45470 MQIheim, Germany * To whom correspondence should be addressed. The objective of this study was to synthesize mesoscopically ordered surfactant/cosurfactant templated metal oxides (silica, titania and zirconia). Hexadecyltrimethylammonium bromide was used as structure directing agent. 1-octanol was used as co-surfactant and trimethylbenzene was added to achieve a more swollen structure. Hexagonall~/ordered titanium- and zirconium- oxo phosphates with dl00-spacings of around 80 A were successfully derived. The silica materials reached dlo0-spacings of 50 A. The surfactant removal of the silica and titanium oxo phosphate materials succeeded with preservation of the hexagonal structure upon calcination, while the removal of the surfactants from zirconium oxo phosphate by thermal treatment was unsuccessful, and resulted in a collapsed structure. The materials were characterized by XRD and N2-sorption. 1. I N T R O D U C T I O N

The synthesis of porous silica [1-3], titania [4], and zirconia [5-8], with a twodimensional hexagonal arrangement of pores have been reported by several authors. Usually quaternary alkylammonium-based surfactants have been used as structure directing agents. The inorganic-surfactant interaction, in the titania and zirconia synthesis referred to, is electrostatic in nature and occurs between negatively charged sulfate groups of the transition metal poly-ions and the positively charged surfactant. The synthesis pH is very low due to the strongly acidic nature of the transition metal ions. Although the dl0o-spacing of these materials is about 40 - 45 ~,, microporous materials are often obtained after removal of the surfactant. However, truly mesoporous transition metal oxides can also be prepared using block-copolymers as structure directing agents [9,10], due mainly to the larger size of the block-co-polymer supramolecular aggregates compared to those of standard surfactants. This report deals with another approach to enhance the swelling of the

1118

composite mesophase by the addition of substituted, aromatic swelling agent with or without the use of a non-ionic co-surfactant under acidic conditions. This approach has proved very useful in the synthesis of large pore silica materials under basic conditions [11]. The experimental procedure follows those previously described in the literature for mesoscopic titania [4], zirconia [12], and silica [13], respectively, with the exception that 1-octanol has been used as a co-surfactant and 1,2,3trimethylbenzene as a swelling agent. The as-synthesized and calcined materials have been characterized by SAXS/XRD and N2-sorption, respectively. 2. EXPERIMENTAL 2.1. Chemicals Hexadecyltrimethylammonium bromide, CTAB, (Aldrich); 1,2,3-trimethylbenzene, TMB, (Merck); 1-octanol (Fluka); titanium isopropoxide (Merck); zirconium sulfate (Alfa); tetraethylorthosilicate, TEOS, (Aldrich), ethanol 99.9% (Primalco), hydrochloric acid fuming 37% (Merck), phosphoric acid 85% (Merck) and sulfuric acid 96% (J.T. Baker) were used as received without further purification. The water was purified by distillation and de-ionization. The syntheses of the transition metal oxides and the silica were performed at 30~ and room temperature, respectively, with a stirring rate of 500 rpm. 2.2. Titanium-Oxo Phosphate The synthesis was performed according to the procedure described by Blanchard et al. [4], with the addition of an organic swelling agent and a co-surfactant. A total of 6.6xl 0 .3 mol CTAB was used in the following molar ratios, CTAB/H20/H2SO4/Ti(iOPr)4=0.3/399.625/1.59/1. TMB and 1-octanol were used as swelling agent and co-surfactant, respectively, and they were added to the CTAB solution in molar ratios of TMB to CTAB=I, 2, 3 and 4 and 1-octanol to CTAB=0.048, 0.096 and 0.144. The samples were typically calcined 2h at 250~ 350~ and 450~ respectively, with a heating rate of 1 K/2min. 2.3. Zirconium-Oxo Phosphate The swollen zirconium-oxo phosphates were synthesized according to the report of Ciesla et al. [12], modified with the addition of TMB and 1-octanol. A total of 6.87x10 .3 mol CTAB, was used in the following molar ratios, CTAB/H20/Zr(SO4)2.4H20=0.54/476.9/1. TMB and 1-octanol were added to the CTAB solution in molar ratios of TMB to CTAB=I, 2, 3, and 4 and 1-octanol to CTAB=0.046, 0.092, and 0.138, respectively. The samples were typically calcined 2h at 250~ 350~ and 450~ respectively, with a heating rate of 1 K/2min. 2.4. Silica The synthesis of SBA-3 type mesoporous silica, with a synthesis composition of CTAB/HCI/H20/Si=O.12/9.2/130/1 in molar ratios, proposed by Babonneu et al. [13] was modified by the addition of TMB and 1-octanol. The molar ratio of TMB to CTAB was 1, 2.5, 4 and 5, and the molar ratio of 1-octanol to CTAB was 0.1, 0.2 and 0.3, respectively. The reactant mixture was stirred for 3 h, filtered, rinsed and dried at

1119

90~ The dried samples were calcined for 4 h at 600~ K/min.

with a heating rate of 1

2.5. Analysis The Small-Angle X-ray Scattering (SAXS) measurements were performed on a Kratky compact small-angle system. A Seifert ID-3003 X-ray generator operating at a maximum intensity of 50 kV and 40 mA, provided the Cu Ks radiation of wavelength 1.542 ,&,. A Ni filter was used to remove the KI3 radiation, and a W filter was used to protect the detector from the primary beam. The system was equipped with a position-sensitive detector consisting of 1024 channels of 55.4 I~m each. The sample to-detector-distance was 277 mm. In order to minimize the background scattering from air, the camera volume was kept under vacuum during the measurements. The measurements were performed on wet and calcined samples. The N2-sorption isotherms were determined at 77 K using an ASAP 2010 sorptometer (Micromeritics). The calcined samples were outgassed at 423 K prior to the measurements. 3. RESULTS

The hexagonal structure of the as-synthesized titanium- and zirconium-oxo phosphates showed to be very well ordered, as seen from the XRD-diffractograms presented in Figures l a and lb. The (100) reflections are very narrow and the (110) and (200) reflections are also clearly visible, even though more markedly for the titanium oxide materials. When TMB was added to the systems there was a distinct increase of the dlo0-spacing.

5

n(TMB)/n(CTAB)=4

>.,

(1)

~,J ~

n(TMB)/n(CTAB)=2 ~

v

~J n(TMB)/n(CTAB)=4

>,, 03 c" (1)

n(TMB)/n(CTAB)=~

-4-'-'

n(TMB)/n(CTAB)=2

n(TMB)/n(CTAB)=~

n(TMB)/n(CTAB)=0

~B)/n(CTAB)=0 !

1

1

2

I

3

I

I

4 5 2O (deg)

I

6

I

7

Figure la. XRD pattern of titanium oxo phosphates, with n(TMB)/n(CTAB) = 0, 1,2, and 4.

1

I

2

I

3

I

I

4 5 20 (deg)

I

6

I

7

8

Figure lb. XRD pattern of zirconium oxo phosphates with n(TMB)/n(CTAB) = 0, 2, 3, and 4.

1120

The very well ordered hexagonal structure of the titanium oxo phosphates was maintained with the TMB addition, while the zirconium oxo phosphate showed a slight broadening of the (100) reflection, indicating some loss of order. The addition of TMB using the titanium oxo-phosphate synthesis resulted in a material with a maximum dloo-spacing of 68 ,&,, which should be compared to the d~oo-spacing of 42 A of the material containing nor co-surfactant neither swelling agent. Figure 2 presents the dloo-spacing versus added amount TMB for the three systems investigated. The zirconium oxo phosphates reached dloo-spacings of 79 A, also hexagonal in structure, at a TMB to CTAB molar ratio of 4. The d~o0-spacing was enlarged almost twice in this case, compared to the original material. Addition of TMB to the silica synthesis resulted in an increase of the d~oo-spacing from 38 ,&, to 47 ,~,, at a TMB to CTAB molar ratio of 5.

Zirconia Titania

80 o~" 70

Silica

O3

~9 60

A

,~0 5O

9

0

9

40 I

0

a

I

1

,

I

m

I

,

I

2 3 4 n(TU B)/n(CTAB)

,

I

5

Figure 2. dloo-spacing versus added amount TMB in molar ratio to CTAB, for the zirconia, titania and silica systems. At 1-octanol to CTAB molar ratios up to 0.144 a hexagonally ordered titanium oxo phopshate phase was formed at TMB/CTAB=3 and 4. A larger amount of 1-octanol at TMB/CTAB=4 resulted in the formation of a lamellar phase. With the addition of 1octanol to the zirconium oxide synthesis, a mixture of a hexagonal and a lamellar phase were always formed, except at a very high TMB amount and a low 1-octanol addition (1-octanol/TMB/CTAB=0.048/4/1). Figure 3 clearly demonstrates the linear increase in dloo-spacing with increasing 1-octanol to CTAB molar ratio, for the titanium oxide and silica systems. The titania oxo phosphate reached dlo0-spacings of 78 A when 1-octanol and TMB were added in the molar amounts of 0.144 and 4 to CTAB. As shown in Figure 3 the dl0o-spacing of silica was increased by 13 A, when TMB and 1-octanol were added. Larger ratios of 1-octanol to CTAB resulted in a disordered silica structure.

1121

80

o<

70

Ti: ~~____.._~

O3 E

Ti: TMB/CTAB=3

60 i

0

Si" TMB/CTAB=4

2 50

_---------I

1

J

0.00

I

I

I

~

1

i

I

0.05 0.10 0.15 n(1 -octanol)/n(CTAB)

0.20

Figure 3. dl0o-spacing versus added amount 1-octanol in molar ratio to CTAB, for the systems with n(TMB)/n(CTAB)=3 and 4 (titania) and 4 (silica). Syntheses, where the 1-octanol content was replaced by the equal molar amount of CTAB, did not result in an increase of the dl00-spacings of the titania and silica materials. An attempt was made to get thermostable titanium and zirconium oxide materials. This was made by aging the samples for 7h in a 0.5 M aqueous solution of phosphoric acid. In this aging process the sulfate groups were exchanged for phosphate groups [4,12]. The removal of the surfactants by thermal treatment from the phosphated titanium oxide was successful, while the removal of the template from zirconium oxo phosphates resulted in a collapse of the mesostructure, although successful surfactant removal has been demonstrated for both the materials without co-surfactant and swelling agent [14]. Some coking was evident after calcination for titanium and zirconium oxide materials.

"-7.

v

I

I

n(1 -octanol)/n (TM B)/n (CTAB)=0.2/4/1

n(1-octanol)/n(TM B)/n(CTAB)=0.2/4/1

5 >.,

~9

>,, .

E (1)

_

as-synthesized

E calcined at 600~

I

2

I

3

1

4

I

5

1

6

I

7

2e (deg) Figure 4a. XRD pattern of SBA-3 and swollen SBA-3.

I

1

.

L

2

1

3

I

4

!

5

I

6

1

7

1

8

9

2e (deg) Figure 4b. XRD pattern of swollen SBA3, as-synthesized and calcined at 600~

1122

The as synthesized swollen silica material, possessed a hexagonal order with dlo0spacings reaching 50 ,& when TMB was added. Two intense low-angle reflections were observed upon addition of 1-octanol, as shown in Figure 4a. The reflection at lower d-spacing clearly corresponds to a hexagonal phase, while the reflection at higher d-spacing probably originates from a distorted hexagonal structure, as previously observed for S+I materials synthesized in presence of hexane [15]. Figure 4b demonstrates the maintained structure of the mesoscopically ordered swollen silica upon calcination. The calcined silica synthesized with addition of TMB and 1-octanol had BET surface areas around 1000 m2g1 and pores in the mesoporous range of 3-6 nm. Figure 5a shows nitrogen sorption isotherms for a silica material containing 1-octanol and TMB in the molar ratios of 0.2 and 4 to CTAB, and a corresponding material with no added swelling agent or co-surfactant. The BET surface area of the material with no added organics was 1400 m2g1 and the nitrogen sorption isotherm indicated a mesoporous structure with pore sizes in the range of 2-3 nm. The nitrogen sorption isotherm of the swollen SBA-3 showed a hysteresis with a clear indication of a bimodal pore structure in the adsorption branch, in agreement with the XRD results. The TMB containing titanium oxo phosphates with n(TMB)/n(CTAB)=3 showed a typical type IV isotherm indicative of mesoporosity, see Figure 5b.

80O o 600

,~.

rim--nm-nmn----umn-mm-~

-~ 500

~ ~

300

0

200~!

- - . - - Ti n(TMB)/n(CTAB)=3 Ti: n(TMB)/n(CTAB)=2 - - " - - Ti: no TMB

90

m 60

"0

100 - - o - - n(1-octanol)/n(TMS)/n(CTAB)=0.2/4/1 0 - - ' - - ~BA-3 . . . . . . 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (p/po)

Figure 5a.

150

~ 120

.~ 400

~

180

N2-sorption

isotherm

of

SBA-3 and swollen SBA-3, calcined at 600~

-0 >

30 0 --~

0.0

"nnl---tdll~

0.2 0.4 0.6 0.8 Relative pressure (p/p0)

1.0

Figure 5b. N2-sorption isotherms of titanium oxo phopsphate materials with n(TMB)/n(CTAB) = 0, 2 and 3, calcined stepwise to 450~

The stability of the material upon heating was dramatically increased when TMB was used. A substantial contraction of about 1.5 nm was observed during calcination, but the mesoporous structure was still retained. The BET-surface areas and the total pore volumes decreased with increasing calcination temperature.

1123

4. DISCUSSION

The d~00-spacing of titania and zirconia composite mesostructures could controllably be increased up to 70 ,&, and 80 ~,, respectively, (TMB to CTAB molar ratio of 4) without the loss of the long-range order of the hexagonal mesophase. The swelling of the hexagonal mesophase of titania could further be increased to 78 ,& through the use of 1-octanol as a co-surfactant, the 1-octanol to TMB molar ratio being the key parameter for determining the transition from a hexagonal to lamellar phase in the material. For zirconia, the addition of 1-octanol generally led to the formation of a mixed hexagonal-lamellar mesophase. Surfactant removal by calcination was successful for titania. However, loss of order for zirconia was usually observed upon removal of the template. In contrast, the d~oo-spacing of SBA-3 type 2-D hexagonally ordered silica could controllably be increased to 50 ,&, using the same approach, clearly indicating the potential of the use of mixed surfactant templates together with a swelling agent for the synthesis of large pore mesoscopic materials. Replacing the 1-octanol by an equal molar amount of CTAB did not provide any further swelling of the titania or silica materials, why it is clear that 1-octanol facilitates the formation of a supra-molecular assembly with a lower interfacial curvature needed for an enhanced solubilization of TMB. The formation of the inorganic-surfactant mesophase occurs immediately upon mixing of the reagents for both titania and zirconia due to the presence of large transition metal poly-ions in solution. The large size of the inorganic poly-ions decreases the interfacial flexibility of the composite mesophase, limiting the solubilization capacity of the composite mesophase [15]. Therefore the locus of solubilization of the swelling agent in the initially formed surfactant micelles is crucial for the performance of the swelling agent. Here, kinetic stabilization of the swollen micelles is of utmost importance. The addition of a co-surfactant aids in stabilizing surfactant aggregates with a lower interfacial curvature. Largely swollen, ordered inorganic-surfactant mesophases can therefore be easily synthesized by the right choice of swelling agent and co-surfactant. 5. CONCLUSIONS

The synthesis of large pore mesoscopic titania, zirconia and silica materials has successfully been performed. The dloo-spacings of the hexagonal structures increased with the addition of TMB and 1-octanol, used as swelling agent and cosurfactant, respectively. The dloo-spacings of titania and zirconia were almost doubled in size (40 ,&, - 80 ,&,) with the addition of organics, while the d~0o-spacing of silica increased with 13 ,&,. The calcined silica and titanium oxo phosphate materials showed a hexagonal structure while the thermal instability of swollen zirconia oxo phosphate still remains a challenge for future work.

1124

REFERENCES 1. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. TW. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. J. Frasch, B. Lebeau, M. Soulard, J. Patarin, Langmuir, 16 (2000) 9049. 3. R. Richer, L. Mercier, Chem. Mater., 13 (2001) 2999. 4. J. Blanchard, F. Schath, P. Trens, M. Hudson, Microporous Mesoporous Mater., 39 (2000) 163. 5. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schtith, Angew. Chem. Int. Ed., 35 (1996) 541. 6. J. Reddy, A. Sayari, Catal. Lett., 38 (1996) 219. 7. D.J. McIntosh, R.A. Kydd, Microporous Mesoporous Mater., 37 (2000) 281. 8. M. Linddn, J. Blanchard, S. Schacht, S.A. Schunk, F. Schtith, Chem. Mater., 11 (1999) 3002. 9. Q. Huo, D.I. Margolese, U. Ciesla, D. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schtith, G. D. Stucky, Chem. Mater., 6 (1994) 1176. 10. P. Yang, D. Xhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater., 11 (1999) 2813. 11. A. Lind, B. Spliethoff, M. Lind6n, submitted. 12. U. Ciesla, M. Fr6ba, G. Stucky, F. Scht~th, Chem. Mater. 11 (1999) 227. 13. F. Babonneau, L. Leite, S. Fontlupt, J. Mater. Chem., 9 (1999) 175. 14. Kleitz, W. Schmidt, F. Schtith, Microporous Mesoporous Mater., 44-45 (2001) 95. 15. M. Linddn, P. Agren, S. Karlsson, P. Bussian, H. Amenitsch, Langmuir, 16 (2000) 5831.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1125

Preparation o f novel organic-inorganic hybrid Micelle Comparison of different routes for materials preparation

Templated

Silicas.

Duncan J Macquarrie*, Dominic B Jackson, Bethan L King and Andrea Watson Department of Chemistry, University of York, Heslington, YORK, YO 10 5DD, England A series of different routes to organic-inorganic hybrids, based on the (co) condensation of silica (and a functional silane) around an amine template, followed by different template removal methodologies, is described. High loadings and bifunctional materials can be readily achieved with the direct template displacement method, while different alcohols can be grafted onto the surface during conventional template removal techniques. 1. INTRODUCTION The development of efficient routes to organic-inorganic hybrid materials, such as organically modified silicas has led to a range of important materials, of use in catalysis, adsorbency and a range of other applications. Of the many routes available for their synthesis, one of the most promising involves the direct co-condensation of two (or more) silanes in the presence of non-

~

___.~

(RO)4Si~

reTo~

RS, R +(RO)45i

_ i~~--..~

Scheme 1. Different routes to organically modified Micelle Templated Silicas ionic interactions[I,2]. While these routes lead to materials with slightly lower long range order than the ionically templated routes[3,4], they allow easy extraction of the template, meaning that a wide range of organic functionality can be incorporated directly in the sol-gel process, and that the template can be readily recovered and reused.[5] The latter is a major

1126

advantage both in terms of cost and environmental impact, both of which are important in the acceptance of such new materials in areas such as the development of cleaner chemical processes. In order to fully exploit the products of this technology, it is critical to investigate the synthetic methodology in order to understand the processes occurring. Here we present results of investigations into the extraction of neutral amine template using a variety of methods. The results indicate that the standard extraction methodology (involving ethanol) is a non-passive method which results in surface functionalisation. We also present results demonstrating that direct removal of template with a functional silane is possible in non-ionic systems, as well as in the ionic-template materials, as reported by Jaroniec and Antochshuk[6-8]. We also show that the latter process can be extended to remove template from functional materials, leading to higher loaded materials and bifunctional materials. 2. EXPERIMENTAL All chemicals were purchased from Lab suppliers and were used without further purification. Distilled water was used throughout. Preparation of MTS materials was carried out using literature procedures[ 1,3] Porosity and surface area measurements were carried out using dinitrogen on a Micromeritics ASAP2100 instrument. Thermal analysis was carried out on a Stanton-Redcroft 625 instrument; elemental analysis was carried out at the University of Manchester. Extractions in solvents were carried out at the reflux temperature of the solvent, unless stated otherwise. 2.1. Removal of OEt groups by HCI treatment This was accomplished by stirring the materials (lg) in 1.2M HC1 (50ml) containing a few drops ethanol as "wetting agent" at room temperature for 24 hours, or at 55~ for 6 hours. Filtration and washing with water and ethanol was followed by drying at 100~ Elemental analysis indicated that no chlorine was detected. 2.2. Grafting of functional silanes to extracted materials (acid treated or not) The material (lg) to be grafted was suspended in the solvent (120ml), and the silane (15retool) was added. The suspension was heated at the desired temperature (reflux unless stated otherwise) for 24 hours. 2.3. Knoevenagel reaction The catalyst under test (0.5g) was suspended in cyclohexane (25ml) and 0.25g n-dodecane internal standard was added. The suspension was brought to reflux and 20mmol of ethyl cyanoacetate and 20mmol of cyclohexanone were added. The reaction was monitored by GC. 2.4. Reactive extractions The as-synthesised material (i.e as filtered from the reaction mixture) was dried at 100~ for 16h before reactive extractions were carried out. l g of the material to be extracted was suspended in the appropriate solvent (ethanol or toluene, 25ml), 5.1mmol silane was added, and the reaction mixture stirred at the appropriate temperature for 24h. The solid was then filtered and washed with the solvent (3x25ml) and dried. In the case of neat extractions, 50mmol, of silane was used and the mixture treated at 55~ for 24h.

1127 3. RESULTS AND DISCUSSION The preparation of a range of organic-inorganic hybrid materials, using aqueous ethanol solvent and n-dodecylamine has already been described.[1,9] Template extraction from these materials is achieved through the use of ethanol, either at reflux or using continuous extraction such as a Soxhlet apparatus. The resultant materials are template free (as evidenced by Thermal Analysis, NMR, IR and elemental analysis) but contain residual OEt groups, purported to be from incomplete hydrolysis of the tetraethoxysilane precursor. However, it is possible that some of these groups are formed during the ethanol extraction stage. It has been suggested that a high concentration of such groups may impede attachment of functional silanes,[10] and thus it is important to investigate this step of the process to understand the source of these groups, and optimise the incorporation of functional silanes. 3.1. Template extraction and reaction of the surface with alcohols Template extraction from an as-synthesised all silica MTS was thus attempted with a range of solvents (dichloroethane, ethyl acetate, THF, toluene, methanol, ethanol i-propanol), after drying of the as-synthesised material at 110~ for 24h. It was found that only alcohols and refluxing toluene could remove template. While the alcohols would remove template even from wet material, toluene required the material to be dried. Extraction with ethanol caused the template to be completely removed and resulted in a material with 4.2mmol g-Z of OEt groups. Toluene extraction gave a similar material, but with only 3.2mmol g-10Et loading. In order to investigate the role that OEt groups play in the grafting of aminopropyl silane, we have undertaken a series of grafting experiments onto MTS materials which have been pretreated in different ways. The results are summarised in Table 1.

Table 1 Summary of results from grafting of ar ainopropyl groups under various conditions Extractio HC1 treatment Loading AMP Loading OEt before Loading OEt after before grafting (mmol g-l) grafting (mmol grafting (mmol g-l) EtOH No 0.45 4.2 2.3 Toluene No 1.37 3.3 0.85 EtOH Yes 2.64 1.71 0.08 Toluene Yes 1.41 1.39 0.52

g-l)

.......

Thus it can be seen that, in ethanol, the high loading of OEt groups substantially reduces the amount of aminopropyl groups which are attached to the MTS, whereas in toluene, very little influence is seen. Remarkably, the loss of OEt groups is almost complete in the ethanolgrafted material which has undergone acid treatment. Thus, while acid treatment removes a significant amount of OEt groups, those remaining are almost completely removed by the grafting process. This may indicate that amine catalysis is more effective than acid catalysis, or that there are two subsets of OEt groups whose immediate environment requires different conditions for hydrolysis (it should be noted that amines are generally considered as catalysts for hydrolysis / condensation reactions. Losses of OEt during the toluene experiments is thought to be due to hydrolysis from residual water on the surface of the materials. In no case is there evidence that the losses of OEt groups results exclusively from a concerted

1128 grafting/displacement of OEt, although such a pathway cannot be ruled out entirely. Thus the grafting and loss of OEt processes are thought to be predominantly unrelated. The reactivity of alcohols towards the surface of these materials can also be exploited to attach larger alcohols, which may serve as surface modifiers, reducing build-up of polar impurities during catalytic reactions. We have thus treated extracted materials with 1-hexanol and 2-phenylethanol in order to examine this possibility. The Cg-alkoxy materials were then grafted with aminopropyl trimethoxysilane to give basic catalysts, active in the Knoevenagel reaction [ 11 ]. 80 7o 60 5040 30

Pheny lethy Ioxy @0.86m mol/g -" [email protected]/g - - x - untreated

10 0

0

i

1

r

q

10

20

30

4O

time (minutes)

Figure 1. Reaction rates in the reaction of ethyl cyanoacetate and cyclohexanone using three differently alkoxy-functyionalised aminopropyl MTS materials. Rates are normalised for amine content. These catalysts are known to be poisoned by the adsorption of a polar impurity, and the incorporation of such non-polar groups onto the surface was thought to be a route to reducing their deactivation. Figure 1 indicates that this was successful, with the most highly functionalised material having the longest lifetime. 3.2. Reactive extraction with organosilanes. Recently, Antochshuk and Jaroniec reported on the surprising simplicity with which simple silanes such as trimethylchlorosilane and RSi(OMe)3[6-8] could enter the as-synthesised MCM material, and displace the template. Similarly, Onida et al. have also published work indicating the diffusion through the template-filled material is simple, even for large dye molecules.[12] The potential to remove template from the as-synthesised neutrally templated materials has not been explored, but would be of value as an alternative route to functional materials using the neutral amine method. Furthermore, the extension to extraction of organically modified materials to increase the loading of organic functionality, as well as to extract with a second organosilane to give bifunctional materials would be extremely interesting, and would provide access to a range of materials with interesting properties.

1129 3.2.1. Extraction with trimethyl chlorosilane. An as-synthesised material prepared from the condensation of TEOS in the presence of ndodecylamine was used to investigate the removal of template in the initial phase of the investigation. Initially, the template was removed under conventional conditions (extraction with ethanol in a Soxhlet extractor. The resultant material was compared to two other materials prepared by subsequent grafting of trimethylchlorosilane at room temperature either neat or as a solution in toluene. The results of this preliminary work are summarised in Table 2. As can be seen, the functionalisation of the materials with TMS gives TMS loadings of 2.3mmol g-1 and 1.95mmol g-1 for the neat and toluene samples respectively. This probably reflects the larger quantities of silane used in the neat experiments. Surface areas, pore volumes and pore diameters all drop upon functionalisation, by similar amounts in each of the two experiments, as would be expected for the incorporation of the silane groups in the pores of the material. Direct reaction of trimethylchlorosilane with the as-synthesised material (after drying at 100~ for 16h) was found to successfully remove the template whether the silane was used neat or as a toluene solution. After reaction, the material was filtered and washed with toluene. Elemental analysis and porosimetry indicated that the template had been successfully removed in both cases, and that incorporation of silane had taken place. It should be noted that no removal of template occurs at room temperature with toluene under the conditions of the experiment. The quantities of silane incorporated were 2.5mmol g-1 and 2.3mmol g-1 for the neat and the toluene experiment respectively. These figures indicate that the grafting is approximately as effective whether the template is removed or not, and the overall loading is similar to the loadings of silanes onto similar mesoporous systems.J13] Perhaps surprisingly, the surface areas are only slightly lower than the parent material, although pore volume and pore diameter are reduced somewhat more than in the two-step process described earlier.

Table 2 Grafting of trimethylsilyl roups onto Micelle Tem 91ated Silica Loading SSA Pore volume Sample (cm 3 g-l) (mmol gq) (m2g "l) 1084 0.96 Control (EtOH extraction only) Extracted then grafted neat 2.3 517 0.63 Extracted then grafted in PhMe 2.0 580 0.69 Direct extraction neat 2.5 906 0.54 Direct extraction in PhMe 2.3 963 0.56

Pore diameter

(rim) 2.4 2.1 2.2 1.8 1.8

3.2.2. Extraction with functional silanes. Having established that trimethylchlorosilane can extract template under mild conditions, and concomitantly functionalise the surface of the material to a significant extent, we then attempted to remove the template in this way, but with functional silanes. Again, this was successful, with a range of silanes being successfully used to replace the template in a mild and simple process. As was found with the direct template replacement with trimethylchlorosilane, the pore diameter is relatively small, with reductions observed in the surface area and pore volume as compared to the control experiment (as shown in Table 3).

1130 Table 3. Template qtisplacement with functional silanes

Silane

Conditions

Loading (mmol g")

SSA (m2g-1)

ore volume Pore diameter

(cm3g"1) (nm) Control 1.6 759 Aminopropyl 0.31 1.6 Aminopropyl Ethanol, reflux 2.3 759 0.63 2.2 Aminopropyl Ethanol 20~ 2.3 618 0.51 2.3 Aminopropyl Toluene, 20~ 1.9 561 0.19 1.8 Control 1.0 722 0.57 1.9 Ch!oropropyl Chloropropyl Ethanol, reflux 1.3 861 0.39 1.8 Chloropropyl Ethanol, 20~ 1.5 905 0.46 1.7 Chloropropyl Toluene, reflux 1.9 593 0.39 1.7 ercaptopropyl Control 1.9 816 0.45 1.9 ercaptopropyl Ethanol, reflux 1.2 868 0.51 18 ercaptopropyl Ethanol 20~ 1.4 840 0.47 1.9 ercaptopropy! Toluene, reflux 2.3 391 0.27 2.0 Iodopropyl Control 0.3 643 0.76 2.2 Iodopropyl Ethanol, reflux 0.8 527 0.23 1.9 Iodopropyl Toluene, reflux 1.2 14 0]02 6.2 Allyl Toluene, reflux (**) 937 0.66 1.8 Cyanoethyl Toluene, reflux 2.0 810 0'32 117 (*) Phenyl Toluene, reflux 788 0.46 1.7 vinyl Toluene, reflux ....(*) ... 777 0.39 1.8 Loadings determined by elemental analysis, based on heteroatom. For those materials(*) with hydrocarbon chains, loadings have not been determined, but presence confirmed by FTIR spectroscopy. .

.

.

.

.

.

.

.

.

.

.

.

The loadings are again good, and the quantity loaded is independent of whether triethoxy or trimethoxy silanes are used. It can be seen from the above that all the silanes shown above can be used to effect direct template removal and grafting. Only in the case of the iodopropyl material under conditions of refluxing toluene was structural collapse evident. For all others, loadings were comparable to, or often higher than those achieved by conventional solvent extraction and subsequent grafting. It should be noted that some template removal would be expected with ethanol, either hot or cold. However, in comparison with the control experiments, the loadings and final physical parameters of the materials indicate that the process here is not a simple extraction followed by grafting. While there is no obvious pattem regarding the relative effects of the different reaction conditions, grafting in toluene, whether at reflux or at room temperature is often the most effective in terms of achieving a high loading, although this often gives a lower surface area and pore volume, partly at least due to the increased amount of organics in the pore. Thus it can be seen that the method is general, and usually gives materials of equivalent or superior physical properties to the conventional route.

3.2.3. Template displacement with functional silanes in functional silicas A further extension of this work demonstrates that it is equally straightforward to displace template from an as-synthesised material which has organofunctional silanes present in the

1131 preparation step. Here, co-condensation of TEOS with MeO)3SiR (or Me3SiC1) was carried out, and the template removed by displacement with a second silane. The second silane can be the same or different to the original, leading to increased loadings, or to bifunctional materials. Examples of this approach are given in Table 4, using aminopropyl modified assynthesised material, although the approach appears to be more general: Table 4. Templatt displacement in aminopropylsilicas Loading AMP Loading 2 nd SSA Second Pore volume Pore diameter cm 3 g-1 (nm) (mmol g-l). Silane (mmol g-l) (m 2 g-l) silane Chloropropyl 1.2 1.8 304 0.19 2.0 1.1 1.9 229 Mercaptopropyl 0.14 1.9 Aminopropyl 2.25 359 0.23 1.9 Yrimethylsilyl'~ 1.2 1.6 562 0.30 1.8 * Loading theoretically identical for all four batches. Therefore the amount of aminopropyl grafted in the third example is expected to be ca. 1. l mmol g~. t Amine is protonated by the HCI released Here again, it seems that the procedure succeeds in attaching functional groups and simultaneously removing template. The loading of the second silane is typically higher than the first, and the overall amount attached reached 3mmol g-1 in two cases, very high values for such materials. Initial results indicate that the procedure can be extended to other silanes, both in the sol-gel stage, and in the template exchange step. Such a method holds promise for extending the loadings achievable in such material, without the structural collapse which is evident at or below 2.4mmol g~ in the direct co-condensation route[9] or the similar levels which can be directly grafted.[13] 4. CONCLUSIONS Organically functionalised micelle templated silicas can be prepared in several ways in order to control the degree of surface coverage with OR groups. Such groups can be important modifiers of the activity of catalysts, and additional control can be exercised by choice of solvent, giving further flexibility to the method. Additionally, reactive displacement of template, by the use of silanes, has been shown to be a general and valuable method for the preparation of high loading materials and bifunctional materials in a simple and direct route. ACKNOWLEDGEMENTS DJM thanks the Royal Society for a University Research Fellowship, DBJ and AW thank the EPSRC for funding, BLK the University of York. REFERENCES 1. 2. 3. 4. 5.

D J Macquarrie, Chem. Commun (1996) 1961 R Richer and L Mercier, Chem. Commun (1998) 1775 P T Tanev and T J Pinnavaia, Science, (1995) 267 865 S A Bagshaw; E Prouzet, T J Pinnavaia, Science (1995) 267 1242 D J Macquarrie, Green Chem (1999) 1 195

1132 6. V Antochshuk, M Jaroniec, Chem. Mater., (2000) 12 2496 7. V Antochshuk, M Jaroniec., Stud. Surf. Sci., Catal., (2000) 129 265 8. V Antochshuk, M Jaroniec., Chem. Commun., (1999) 2373 9. D J Macquarrie, D B Jackson, J E G Mdoe, J H Clark, New J Chem., (1999) 23 539 10. K Cassier, P van der Voort, EF Vansant, Chem. Commun., (2000) 2489 11. D J Macquarrie, D B Jackson, Chem. Commun., (1997) 1781 12. B Onida, B Bonelli, L Flora F Geobaldo, CO Arean, E Garrone, Chem. Commun., (2001) 2216 13. D Brunel, Microp. Mesop. Mater., (1999) 27 329

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Structure and catalytic p e r f o r m a n c e of cobalt Fischer T r o p s c h supported by periodic m e s o p o r o u s silicas

1133

catalysts

A.Y. Khodakov, R. Bechara and A. Griboval-Constant Laboratoire de Catalyse de Lille, USTL, Cit6 Scientifique, Brit. C3, 59655 Villeneuve d'Ascq, France The structure of cobalt catalysts supported by periodic mesoporous silicas at different stages of preparation was characterized by XRD, N2 adsorption, XPS, in situ Xray absorption and TGA. It was shown that the size and reducibility of supported cobalt particles were strongly affected by porous structure; larger and more easily reducible particles being detected in wider pore silicas. Cobalt dispersion was found to be controlled by silica pore sizes even at high cobalt contents (up to 30 wt.%) It was shown that catalytic behavior of cobalt supported mesoporous silicas in Fischer Tropsch synthesis strongly depended on cobalt dispersion and catalyst porous structure. Wide pore SBA-15 supported Co catalysts were found to be much (about 5-10 times) more active than narrow pore MCM-41 supported catalysts with the same cobalt content. Product distribution was found to be a function of cobalt particle sizes and cobalt reducibility. Fischer Tropsch reaction rates increased monotonically with increase in cobalt content up to 30 wt %, whereas product distributions for completely reduced wide pore catalysts were nearly the same at high and low cobalt loadings. 1. INTRODUCTION Concerns about environment and rational management of natural resources are major reasons for renewed interest in Fischer-Tropsch (FT) synthesis [1]. FT synthesis produces valuable hydrocarbons from relatively cheap synthesis gas. Synthetic liquid fuels prepared using FT technology contain negligible concentrations of sulfur and heavy metals relative to crude oil. Availability of large reserves of natural gas in different countries is another motivation for further development of FT technology. The efficiency of FT technology can be improved by using catalysts with higher hydrocarbon productivities and lower methane selectivities [2]. The reaction proceeds on supported metal particles. The catalytic performance is a function of metal dispersion, reducibility and volumetric density of active metal sites. Cobalt catalysts have been found to be most suitable for synthesis of higher hydrocarbons [3]. Most of FT publications have addressed optimization of metal function in catalysts supported by amorphous oxides. Our approach suggests that porosity of the oxide support could play a significant, often decisive role in the performance of FT supported catalysts. Broad pore size distributions in common catalytic supports make it difficult to draw unambiguous conclusions about influence of catalyst pore sizes on FT reaction rates and selectivities. Tailored pore size distribution in recently discovered periodic mesoporous silicas [4, 5] makes these materials model catalytic supports for FT catalysts. In the present work the effects of porosity on the

1134

structure of supported cobalt species, on FT reaction rates and selectivities were studied over wide ranges of silica mesopore diameters and cobalt contents. MCM-41 and SBA-15 periodic mesoporous silicas were used as catalyst supports. A commercial mesoporous silica was also studied for comparison. 2.

EXPERIMENTAL

MCM-41 and SBA-15 type periodic mesoporous silicas (PMS) were ~;ynthesized using halide cetyltrimethyl and dodecyl ammonium compounds and polyethylene glycols as templates. BET surface areas, total pore volumes, pore diameters calculated from nitrogen isotherms and details of synthesis procedure are presented in Table 1 and references therein. Low angle XRD patterns (~,=1.668 /k) showed an intense peak at 20-2.3 degrees and low intensity peaks at 3-6 degrees for MCM-41 and an intense peak at 20-0.9 degrees and low intensity peaks at 1.6-1.9 degrees for SBA-15 materials. The observed XRD patterns were characteristic of the hexagonal structure [4, 5]. A commercial silica (Cab-o-sil M-5) was agglomerated by wetting and dried in an oven at 393 K overnight. Cobalt was introduced by incipient wetness impregnation using solutions of cobalt nitrate. The samples were dried overnight at 373 K and calcined at 773 K for 5 h. Cobalt contents were varied between 5 and 30 wt %. The nomenclature for the catalysts (xPMSn) consists of two parts; the first part (x) indicates calculated cobalt weight content; the second part (PMSn) refers to a periodic mesoporous silica used as a support. The catalysts were characterized by XRD, XPS, adsorption techniques, in situ X-ray absorption and TGA. In situ X- ray absorption and low angle XRD measurements were Table 1. Adsorption properties and synthesis of mesoporous silicas. Silica Type Adsorption properties Synthesis .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

=_

_

SBET, TPV, Pore Template mZ/g cm3/g diameter, A PMS1 MCM-41 1481 0.75 =20 dodecyltrimethylammonium bromide PMS2 MCM-41 742 0.59 =20 cetyltrimethyl ammonium chloride PMS3 SBA-15 679 0.78 42 poly (ethylene glycol)block- poly (propylene glycol)- block- poly (ethylene glycol) PMS4 SBA-15 887 1.91 91 poly (ethylene glycol)block- poly (propylene glycol)- block- poly (ethylene glycol) SiO2 213 0.84 330 (Cab)

,,,,

,,

Source Ref. of SiO2 Fumed [21] silica Fumed silica TEOS

[22]

TEOS

[26]

-

Cabo-sil M5

[25]

1135 2

1600

O.O6

1400

Boo

o

004

d

0.03

3

/ /2

0.02

~ i

4O0

001

2OO 0

1

0,05

0

02

0.4 0.6 Relative Pressure, P/P.

0

08

10

4 '

loo

,,,I lOOO

Pore Diameter, A

Figure 1. Isotherms of nitrogen adsorptiondesorption on narrow MCM-41 and wide pore SBA-15 periodic mesoporous silicas 9 1- PMS1 (flpore=20 A) and 2- PMS4 (dpore-91 A). The isotherm of PMS4 is offset for clarity.

Figure 2 Pore size distribution curves calculated from nitrogen desorption isotherms: 1- PMS1, 2- PMS2, 3-PMS3, 4-PMS4, 5 - SiO2(Cab).

carried out in L.U.R.E., Orsay (France) using synchrotron radiation from the DCI storage ring. The extent of overall cobalt reduction was calculated from TGA weight loss in the atmosphere of hydrogen at 753 K assuming stoichiometric reduction of Co304 to metallic cobalt. The data were corrected by subtracting weight losses of silica supports treated under the same conditions. Co content in the samples was measured by atomic absorption. Prior to catalytic measurements, the catalysts were reduced at 753 K for 5 h in a flow of hydrogen. FT catalytic rates and selectivities were measured at atmospheric pressure in a fixed bed flow reactor (H2/CO=2) with on-line analysis of products by gas chromatography. The reaction rates and hydrocarbon selectivities were measured at quasi-state 2 conditions generally attained after 7 hours on-stream. FT reaction 1.5 rates were normalized by the o ~ 400 number of cobalt atoms. The hydrocarbon 0.5 selectivities were calculated on carbon basis. The chain growth probabilities, a, were calculated Cobalt content, from the slope of the curve ln(Sn/n) Figure 3. Surface areas and total pore volume versus n, where n is the carbon periodic silica as a function of cobalt content in number of the hydrocaroon and S, PMS4 silica. the selectivity to corresponding hydrocarbon. 1000

2.5

800

600

v

0

9

.

9

5

10

15

0

20

wt.%

25

30

1136 4

b 0.5

!2: t-

2

0.4 0.3, 0.2.

0.5

2

0.1,

~

'

,

r ,,

77

Photon Energy, keV

,81

0 0

1

2

3

4

5

6

7

R,A

Figure 4. XANES spectra (a) and moduli of Fourier transform of EXAFS (b) for oxidized catalysts: 1- 5CoPMS2, 2- 5CoPMS3, 3- 5CoPMS4, 4- 5CoSiO2(Cab) and 5bulk C o 3 0 4 . 3. RESULTS AND DISCUSSION 3.1. Structure of mesoporous silicas and supported cobalt species Figure 1 shows isotherms of nitrogen adsorption-desorption on mesoporous silicas" MCM-41 and SBA-15. Isotherms without hysteresis loop are observed for MCM-41 type silicas (Figure 1, curve 1). This type of isotherms is typical for mesoporous silicas with pore diameters smaller than 30 A [4]. Nitrogen adsorption isotherms of SBA-15 silicas (Figure 1, curve 2) belong to type IV in accordance with classification of Brunauer [6]. All SBA-15 isotherms show a reversible part and a type A hysteresis loop at higher pressures. The P/Po position of the inflection points is related to a diameter of mesopores. The pore sizes in SBA-15 silicas varied in the range from 42 to 91 A as a function of synthesis procedure (Figure 2). Pore size distributions calculated from the desorption branches of the isotherms using BJH method [7], were much more narrow in periodic mesoporous silicas than in the commercial SiO2(Cab). Cobalt impregnation led to a decrease in specific surface areas and total pore volumes for both MCM-41 and SBA-15 type silicas (Figure 3), but did not affect the shape of pore distribution curves. The surface areas however, remain high even at higher cobalt loadings (30 wt. %). Small C O 3 0 4 crystallites in oxidized catalysts were observed by XRD, X ray 6OO

a

b

5OO

5O0

5

z;~o

3

3

2 1

100

lOO

1

0

0 55

6O

65 2 " l ' h ~ , degree

70

s5

60

65

70

2 Theta

Figure 5. XRD patterns of oxidized catalysts: a- 5 wt% Co catalysts supported by PMS1 (1), PMS2 (2), PMS3 (3), PMS4 (4), SiO2(Cab) (5) silicas; b- catalysts supported by PMS4 silica containing 5 wt% (1), 10 wt%(2), 20 wt%(3) and 30 wt% (4) Co.

1137 Table 2. Characterization of Co catalysts Co catalyst Co Pore C0304'"'crystallite content diameter, diameter from, A wt % ,& XRD XPS

TGA extent of overall reduction at753 K, %

5CoPMS1 5CoPMS2 5CoPMS3 5CoPMS4 30CoPMS4 5CoSiO2(Cab) 30CoSiO2 (Cab)

43.2 62.9 72.8 94.8 94.4 -

5.47 5.67 6.95 5.39 27.3 4.75 26.0

20 20 43 75 75 200-300 200-300

57 43 92 121 125 230 286

8 16 70 67 74 76 103

absorption and XPS. The X-ray absorption near edge structure (XANES) and the moduli of Fourier transforms of EXAFS of 5 wt. %Co/SiO2 oxidized catalysts and bulk Co304 are presented in Figure 4. The XANES spectra of oxidized Co supported silicas were practically identical; they resemble the spectrum of bulk Co304. Similarity of both XANES spectra and of moduli of Fourier transform shows similar local structure of Co species in bulk Co304 and oxidized Co supported catalysts. XRD patterns of Co supported catalysts are presented in Figure 5. Only Co304 crystalline phase was detected. Very broad Co304 X-ray diffraction patterns were observed for supports with pore sizes smaller than 30 ,~, (PMS1 and PMS2). Table 2 shows that the size of Co304 crystallites estimated from the width of XRD profiles using Scherrer equation depends on the pore diameters in mesoporous silicas; larger Co304 crystallites are found in large pore periodic (PMS3, PMS4) and commercial (Cab-o-sil) silicas. An increase in cobalt loading up to 30-wt% in periodic mesoporous silicas did not results in any noticeable modifications of the width of XRD patterns and therefore Co304 particle sizes (Figure 5b, Table 2). This suggests that most of cobalt particles introduced by 3.5 impregnation are located in 3 catalyst pores where their size is g 2.5 limited by diameters of silica ~ 2 pores even at high cobalt contents. z 1 XPS measurements of 0.5 particle sizes using Kerkhof and Moulijn model [8] showed 0 7.69 7.71 7.73 7.75 7.77 7.79 7.81 similar dependence of CoaO4 Photon Energy, keV particle sizes on silica pore Figure 6. XANES spectra of CoO (1), 5CoPMS2 (2), diameter (Table 2). The 5CoPMS3 (3), 5CoPMS4 (4) and 5CoSiO2 (Cab) (5) differences in absolute values of catalysts reduced in situ at 773 K and Co foil (6). Co304 particle sizes evaluated The spectra were measured in hydrogen at room from XRD and XPS data seem to temperature. be related to the limitations of these techniques. The sizes of

1138 Co304 crystallites measured by XRD were found slightly larger 120 than silica pore diameters. It is 200 known that the limitations and 100 approximations of analysis of E 150 _~ XRD profiles using Scherrer mm 100 equation could overestimate the actual particle sizes [9]. It should 5O be also noted that particles supported on periodic 0 10 20 30 40 50 60 70 80 90 1 O0 mesoporous silicas co, rid adopt a Pore Diameter, A slightly elongated shape in the mesopores. Kerkhof and Moulijn Figure 7. Diameters of C0304 crystallites measured model assumes uniform by XRD and extent of overall cobalt reduction as distribution of the supported functions of silica pore sizes (5 wt.%Co/SiO2 phase between the bulk and outer catalysts). surface of catalyst grains. Previous works [10] however, showed that impregnation of silicas followed by calcinations could lead to enrichment of CO304 particles on the external surface of the SiOe grains. Higher concentration of CO304 near the outer surface of catalyst grains could lead to higher intensity of Co2p XPS signal and therefore, to underestimating CO304 particle sizes. In agreement with previous reports [11, 12] in situ X-ray absorption and TGA showed that reduction of catalysts proceeds as C0304--->C00-->C0. The reduced catalysts were characterized by in situ X-ray absorption. The XANES spectra and moduli of Fourier transform of EXAFS of the catalysts and of Co foil are shown in Figure 6. Comparison of X ray absorption data of the reduced Co catalysts with those of Co foil indicates the presence of Co metal species in all samples. At the same time, the resemblance of the XANES spectrum of 5CoPMS2 and CoO suggests considerable concentrations of unreduced CoO phase in that sample. As the diameter of pores in supported Co catalysts increases from 5CoPMS2 to 5CoPMS4 , the near-edge spectrum of cobalt shifts from one resembling that of CoO to one resembling Co foil. XANES also showed high extent of cobalt reduction in commercial wide pore 5CoSiOe(Cab). The extent of reduction measured by TGA and X-ray absorption was found to depend on pore sizes (Table 2, Figure 6). Larger cobalt particle in wide pore SBA-15 silicas were significantly easier to reduce than small particles in narrow pore catalysts (Figure 7). The effect of particle sizes on the reducibility of Co species was attributed to the interaction between metal and support. As shown previously [12], in smaller particles this interaction could be much stronger than in larger ones and this interaction was likely to stabilize small oxidized particles and clusters in silica. 8o

60

o~

'~ -o

40

20

.

.

.

.

.

.

.

.

,

0

3.2. Catalytic behavior of cobalt supported mesoporous silicas. Catalytic results are presented in Table 3 and Figure 8. Wide pore SBA-15 supported Co catalysts were found to be much (about 5-10 times) more active than MCM-41 supported Co catalysts with the same cobalt content (Table 3). Higher methane (25%) and lower C5+ selectivities were observed on narrow pore silicas, whereas the chain growth probabilities (c~) were in the range of 0.66-0.78 for both narrow and wide pore catalysts.

1139 Table 3. Catalytic performance of mesoporous silicas in FT synthesis (H2/CO=2, P= 1 bar, Tr=463 K, 7 h on-stream, conversion < 5%). Co catalyst Reaction rates, 10-~ CH4 selectivity, C- C5+ selectivity, Ci

5%CoPMS1 5%CoPMS2 5 %CoPMS3 5 %CoPMS4 30%CoPMS4 5%CoSiO2(Cab) 30%CoSiO2(Cab)

S-1

%

O-~

0.5 0.1 1.55 1.38 2.55 2.68 1.88

24.9 22.7 15.2 15.3 15.2 16.9 16.5

50.1 57.0 65.0 68.4 68.0 60.7 62.4

0.66 0.78 0.74 0.77 0.77 0.70 0.70

Characterization results show that narrow pore silica supported catalysts contain smaller cobalt particles. These smaller particles were found more difficult to reduce than larger one. It can be suggested therefore, that FT activity could be a function of cobalt particle size and their reducibility. This suggestion is consistent with previous reports. Reuel and Bartholomew found [13] that activity of Co supported catalysts prepared by impregnation increased with decreasing metal dispersion. This effect was assigned to changes in surface structure with decreasing particle size and to electronic modifications due to interaction of small crystallites with the support. Higher methane selectivity was observed on partially reduced cobalt species [13]. Iglesia et al [2, 14] showed that FT is structure-insensitive reaction by definition of Boudart; lower activity of small cobalt particles observed in previous reports was explained in terms of lower reducibility of smaller cobalt particles and their possible reoxidation by water or by other reaction products at FT reaction condition. In line with these observations, our results also display lower activity of smaller cobalt particles in FT synthesis. Lower reducibility of smaller particles of cobalt oxide in narrow pores of mesoporous silicas (Table 2) seems to be one of the major reasons responsible for their lower activity and higher methane selectivity. FT reaction rates increase with increase in cobalt content in periodic mesoporous silicas (Figure 8). This finding is consistent with relatively high cobalt dispersion observed by both XRD and ,40 XPS in periodic silicas with high ,35~ ~2 cobalt loadings. For commercial ~o ~ silicas with wide pore size distribution r" =o curves the activity per cobalt atom decreases at higher cobalt loadings in ,t;agreement with a decrease in cobalt dispersion (Table 2, 3). Narrow pore 0 ,0 0 5 10 15 20 25 30 size distributions and high specific Co content, % surface areas (>700-1000 mZ/g) of Figure 8. FT reaction rate and methane periodic mesoporous silicas are likely selectivity as function of Co content in MSS4 to facilitate stabilization of relatively peridic mesoporous silicas. small cobalt particles (~100 ~,) and to prevent them from sintering even at higher cobalt loadings. Therefore, in m

2o

lO

1140 periodic mesoporous silicas with higher metal contents, high local concentrations of cobalt sites could be suggested. High volumetric densities of cobalt sites in periodic mesoporous silica with high cobalt loadings seems to be one of the reasons [14] responsible for their enhanced activity. 4. CONLUSIONS The results showed that the structure of supported cobalt species, their reducibility and catalytic behavior in FT synthesis were strongly affected by pore sizes and porous structure of periodic mesoporous silicas. The sizes of cobalt particles were found to depend on pore sizes in silicas even at high cobalt contents (30 wt.%); smaller and hardly reducible particles being observed in narrow pore supports. Catalytic measurements reveled strong impact of pore sizes on catalytic behavior: FT reaction rates increased more than 5-10 times as pore sizes increased from 20 to 100 A. A larger diameter of catalyst pores also led to significantly lower methane selectivities. Relatively high cobalt dispersion stabilized by porous structure of periodic mesoporous silicas in catalysts with high cobalt contents led to high activity of these materials in FT synthesis. ACKNOLEDGEMENTS

The authors thank C. Guelton for TGA measurements and Dr. V.L. Zholobenko for providing PMS1 silica. The authors are grateful to F. Villain (D42) and D. Durand (D43) for help during synchrotron experiments. The Laboratoire pour l'Utilisation du Rayonnement Electromagndtique (L.U.R.E.), Orsay, France is acknowledged for the use of beamline. REFERENCES

1. 2. 3. 4.

A.M. Thayer, Chem. & Eng. News, March 13 (2000) 20. E. Iglesia, S.C. Reyes, R.J. Madon, and S.L. Soled, Adv. Catal., 39(1993) 221. P. Chaumette, Revue IFP 51(1996) 711. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, CT. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114(1992) 10834. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (1998) 548. 6. S. Brunauer, L.S. Deming, W.S. Deming and E., Teller, J. Am. Chem. Soc. 62(1940), 1723. 7. E.P. Barrett, L.G. Joyner, and P.P. Halenda, J.Am.Chem.Soc. 73(1951) 373. 8. F.P.J. Kerkhof and J.A. Moulijn, J. Phys. Chem., 83(1979) 1612. 9. P. Ganesan, H.K. Kuo, A. Saaverda, and R.J. DeAngelis, J.Catal. 52(1978) 319. 10. D.G. Castner, P.R.Watson and I.Y. Chan, J.Phys. Chem. 93(1989) 3188 11. D.G. Castner, P.R.Watson and I.Y. Chan, J.Phys.Chem., 94(1990) 819. 12. A.Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, and P. Chaumette, J.Catal., 168(1997) 16. 13. R.C. Reuel, and C.H. Bartholomew, J.Catal., 85(1984) 78. 14. E.Ig.'_esia, Applied Catalysis A:General, 161 (1997) 59.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1141

Highly dispersed VOx species on mesoporous supports: Promising catalysts for the oxidative dehydrogenation (ODH) of propane A. Brtickner, P. Rybarczyk, H. Kosslick, G.-U. Wolf and M. Baerns Institute for Applied Chemistry Berlin-Adlershof Richard-Willstiitter-Str. 12, D-12489 Berlin, Germany

Dedicated to Prof. Jens Weitkamp on the occasion of his 60 th birthday

VOx species supported on mesoporous A1203, SBA-15 and MCM-48 materials have been studied by simultaneous in situ-EPR/UV-vis/on line-GC measurements and tested as catalysts for the ODH of propane. Highest propene yields can be obtained with catalysts of very high surface area in which active V sites have a mean valence state close to +4, are highly dispersed and preferably tetrahedrally coordinated by oxygen. Coke deposits formed at high reaction temperature and propane concentrations do not deactivate the catalysts but enhance the propene selectivity by covering acidic sites of the support.

1. INTRODUCTION Vanadia-based catalysts are widely used in a number of industrial oxidation processes. Particularly, oxidative dehydrogenation (ODH) of light alkanes to the corresponding alkenes would be an attractive subject since cheap and environmentally friendly starting materials can be converted to valuable olef'ms used as feedstock for other processes. The ODH of propane has been extensively studied since the 1970's and discussed in several review papers [1-3]. Unfortunately, maximum propene yields obtained so far hardly exceed 20 % since total combustion of both propane and propene leads to low selectivities, in particular at higher degrees of conversion. Due to these limitations, the ODH of propane is still far from being attractive for industrial application and a major goal of research is to develop highly selective catalysts for this process. Recemly we have shown by comparative studies of a variety of vanadia-comaining mixed metal oxides that selectivity and activity strongly depend on the valence state, coordination and dispersion of the V ions [4]. Highest selectivities could be obtained with catalysts exposing preferably isolated and/or low oligomeric tetrahedral VOx species with a mean surface V valence close to +4 although their intrinsic activity was lower than that of V sites in octahedral symmetry and/or within VOx clusters. Furthermore we have seen that, under reaction conditions, coordination and valence state of the V species equilibrate and differ from those of flesh and used catalysts in ambiem atmosphere. This clearly indicates the need of using in situ-techrfiques for deriving reliable structure-reactivity relationships.

1142 In this work, we tried to achieve the desired high dispersion of vanadium sites in preferably tetrahedral coordination by depositing VOx species on mesoporous support materials (A1203 and SiO2). This implies low surface concentration of active sites which in turn might lower the overall activity of the catalysts. To compensate for this undesired consequence, supports with very high surface area were used. Changes of the structure and valence state of the active V sites under reaction conditions have been studied by a novel simultaneous coupling of in situ-EPR/on line-GC/UV-vis-DRS [5].

2. EXPERIMENTAL Catalysts with 2.8 and 5.8 wt.-% V were prepared by wet impregnation of mesoporous A1203 [6], MCM-48 [7], and SBA-15 [8] materials with aqueous solutions of NHaVO3. The suspensions were treated at 70 ~ and ambient pressure for 1 h in a rotary evaporator d h reactor t u b e = =~ 1 before water was removed at the same temperature in vacuum. The obtained powders were dried at 150 ~ in vacuum and finally calcined quartz wool at 600 ~ for 6 h in air. Structure, redox behaviour and valence state of the VOx sites as well as surface ~products acidity have been studied by TPR, - ~ conr'~-'tion to FTIR, potentiometric titration, 51Von t i n e - G C NMR, EPR and UV-vis-DRS. ~ p # e ....................... =i UV-vis-DRS spectra were recorded by a Cary 400 UV-vis Figure 1. Experimental set-up for simultaneous in situspectrometer (Varian) equipped EPR/on line-GC/UV-vis coupling with a diffuse reflectance accessory including a heatable reaction chamber (praying mantis, Harrick). To reduce light absorption, the catalysts were diluted with ~-A1203 (calcined at 1473 K for 4h). Spectra deconvolution was performed by the program GRAMS32 (Galactic). EPR spectra were recorded by the c.w. spectrometer ELEXSYS 500-10/12 (Bruker) in Xband. For in situ-studies a homemade flow reactor equipped with a temperature programmer and connected to a gas dosing apparatus was used [9, 10]. For on-line product analysis the reactor outlet was connected to a GC 17AAF capillary gas chromatograph (Shimadzu) equipped with a 30 m x 0.32 mm Silicaplot column (Chrompack) and a FID. For simultaneous EPR/on line-GC/UV-vis coupling, a fibre optic quartz sensor (Optran WF, 200 x 1.5 mm) was directly implemented in the EPR flow reactor through a Teflon gasket (Fig. 1) [5]. The sensor is connected to an AVS-PC-2000 plug-in spectrometer (Avantes) by fibre optic cables (2000 x 0.4 mm). In a typical run, 100 mg catalyst particles (0.3 - 0.6 mm) were treated with

1143 mixtures of C3H8 and 02 in a molar ratio of 2 diluted by different amounts of N2 to adjust W/F values between 0.6 and 2.7 g h mo1-1. Acidic surface sites were determined by pyridine adsorption using a FTIR spectrometer (Bruker IFS 66) equipped with a heatable adsorption cell. Self-supporting wafers were pretreated in vacuum at 400 ~ FTIR spectra were recorded after pyridine adsorption at room temperature and subsequent evacuation. BET surface areas and mean pore diameters were determined by N2 adsorption a t - 1 9 6 ~ using a Gemini III 2375 surface area analyzer (Micromeritics). The mean vanadium valence state was determined by potentiometric titration using a variant of the method developed by Niwa and Murakami [ 11 ]. Catalytic tests were performed in a fixed-bed U-type quartz reactor at 500 ~ (educt mixture: 40% C3H8, 20% 02, balance N2; W/F = 0.6-0.9 goat h mol'lprooane).

2. RESULTS AND DISCUSSION

2. 1. Structural properties of the catalysts Pentavalent vanadium sites in supported VOx catalysts give rise to intense charge-transfer (CT) bands in the respective UV-vis-DRS spectra (Fig. 2). From the position of the lowenergy CT band conclusions on the coordination number and the degree of V site agglomeration can be derived. In the spectra of hydrated VOx/MCM-48 and VOx/SBA-15 samples 0,5

o,2oJ

o.~5~f~

A

0.5

o,4

A

0,3

0.3

0,2

0.2

0,1

0.1

0,0

0.0

LL

0,080,04 0,00

0.8

0,4

032-

360

d o - 5bo - 6bo - 7bo-

nm

0.4

0,3

0.6

0,2

0.4

0,1

0.2

O,Oi

936o

4oo

~~bo?6osoo

nm

0.0

3oo 46o 560 6ob ~o6 8~

nm

Figure 2. Room temperature UV/VIS-DRS spectra of samples 2.8 % V/A1203(left), 2.8 % V/SBA15-200 (middle) and 2.8 % V/MCM-48 before (A) and after dehydration in air at 773 K (B) which are very similar these bands occur above 450 nm suggesting octahedral coordination (Fig. 2A [12]). This is also supported by a SlV-NMR signal at -270 ppm for V s§ in octahedral symmetry [12]. After heating in air to 500 ~ these signals disappear since the octahedral V

1144 sites loose coordinated water ligands and become tetrahedral. Accordingly, the most intense low-energy CT band occurs at 320 nm being characteristic of mainly isolated tetrahedral VO4 sites while the small band around 380 nm indicates the presence of some VO4 sites connected via V-O-V bridges, too (Fig. 2B [13]). The latter band is not visible in VOx/MCM-48, probably due to its very high surface area (Table 1) which facilitates high V dispersion. In agreement with the UV-vis-DRS results, a single line at -575 ppm is observed in the 5~V-NMR spectra a~er dehydration which is assigned to tetrahedrally coordinated V 5+ [12]. These spectral changes are completely reversible by rehydrating the samples in ambient atmosphere suggesting that all V sites are exposed on the surface and accessible to water ligands and, thus, also to potential reactant molecules. Thus, it is justified to calculate VOx surface densities and turnover frequencies (TOF) assuming that all V sites are exposed (Table 1). In contrast to VO• and VOx/MCM-48, the majority of V sites in as-synthesized VOx/ml203, is in tetrahedral coordination which is confirmed, too, by an intense 5 1 V - N M R signal at -560 ppm [12]. The V coordination number does almost not change upon heating (Fig. 2A and B). However, a weak band at 455 nm points to the presence of a small amount of octahedrally coordinated, rather oligomeric vanadium sites which do not become tetrahedral upon heating (Fig. 2B). They are evidenced, too, by a very weak 5~V-NMR peak at -330 ppm. Table 1. Structural properties and catalytic results at 500 ~ of catalysts with 2.8 wt.-% of V Sample

Surface densitya

[V/nm~] VOx/A1203 VOx/SBA50 VOx/SBA200 VOx/MCM48

1.0 0.43 0.7 0.37

SBET Mean pore [ m 2 / g ] diameter

[A]

273 645 421 889

48.2 52.6 190.5 26.2

Mean V valence 4.81 4.81 4.83 4.86

TOF a, b S(C3H6)b Ymax(C3H6) [s"l] [%] [%] 0.44 0.18 0.26 0.21

73.3 83.3 82.3 80.1

12.3 14.5 12.4 18.0

aapparent values, calculated assuming exposure of all V sites, b for Xpropane--2.5 - 3.9 %

Table 2. Acidic surface sites determined by FTIR of adsorbed pyridine Relative band area b Sample a

VOx/A1203 VOx/SBA50 VOx/SBA200 VOx/MCM48

1445 crn-~

1540 c m -1

2.3 1.3 2.0 1.6

0.2 0.3 0.1

a V content 2.8 wt.-%, b normalized on the specific surface area

As shown in Table 1, all samples contain a small amount of tetravalent V species. They are also detected by a characteristic EPR signal with hyperfine structure arising from VO 2+ sites in hydrated sampies. This line is, however, not observed in dehydrated VOx/SBA-15 and VOx/MCM48 samples due to the change to tetrahedral V coordination which shortens the relaxation times. Acidic surface sites in the catalysts were determined by FTIR spectroscopy of adsorbed pyridine using the band area at 1445 cm"1 (Lewis sites) and 1540 cm 1 (Bronsted sites) (Table 2). All catalysts

1145 contain Lewis sites, their concentration being highest for the alumina-supported sample. Bronsted sites are almost negligible in silica-supported catalysts and are not detectable at all in VOx/A1203.

2.2. Behaviour of V sites under reaction conditions In situ-EPR/on line-GC/UV-vis experiments have been performed with all samples during heating under ODH conditions. For example, the results obtained with VOx/A1203 are shown in Fig. 3. UV-vis spectra of the fresh catalyst at 20 ~ are dominated by CT bands of V 5+ as observed accordingly also in Fig. 2A. By raising the temperature stepwise to 400 ~ light absorption increases gradually above 500 nm due to partial reduction of V 5+ to V 4+, the d-d transitions of which fall in the higher wavelength range of the spectrum [ 14] (Fig. 3A, left). It is interesting that this reduction, which is also confirmed by the growing EPR signal of interacting and isolated VO 2§ species (Fig. 3A, middle), starts already at temperatures well below the onset ofpropene formation.

7 6 ~,

60 % C3H8/30 %02 / N2 W/F = 1.25 g h mol -~

T! ~

5TF~.,.

\

vz2o,

700

;00

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--.

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,,

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B

0

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1

o

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,

~

~

,

-

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,

/z 500~

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900

I'

-

100 mT

-~

b

20 4.0 60 ~}0 "100 X, S / %

'1

_,--2~ ~

............

I'

600 mT

Figure 3. A) In situ-EPR/UV-vis/on line-GC measurement of sample VOx/A1203 (5.8 wt.-% V)

during ODH of propane and B) in situ-EPR/UV-vis measurement of the same sample during heating in H2 flow. Further heating above 400 ~ gives rise to a strong increase of absorbance in the whole visible range of the spectrum (Fig. 3A, left). This is caused by the formation of carbon depos-

1146 its which have been detected, too, by FTIR spectroscopy. The contributions of carbon deposits and reduced V species to the overall absorbance can be distinguished by comparing the in situ-UV-vis spectra of Fig. 3A with those of Fig. 3B obtained at similar temperatures in 1-12 flow in which only reduction of V s+ but no carbon deposition is possible. It is interesting to note that the catalyst is not deactivated by carbon deposits (Fig. 3A, right). Obviously, those carbonaceous residues are mainly deposited on the support material while the active VOx species remain flee. This could reduce the surface acidity of the support under reaction conditions and, thus, be the reason for the strongly increasing propene selectivity in the initial period of the reaction (Fig. 3A, right). By using a VOx/ml203 catalyst with a lower V loading (2.8 wt.-%) and under less severe reaction conditions coke formation can be at least partly suppressed (Fig. 4, left). In this case, only a slight increase of propene selectivity occurs in the initial reaction period and the final value after 45 min time on stream is less than 40 % although the degree of conversion is markedly lower in comparison to the experiment shown in Fig. 3A. g'~ 2 55-

T/*C 4

50-

/30"

45-

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~

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propene pro

ne

151005

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Figure 4. In situ-EPR/UV-vis/on line-GC measurement of sample VOx/A1203(2.8 wt.-% V) during heating in a flow of 28 % C3H8, 14 % O2/N2 (W/F = 2.7 g h mo14).

The in situ-UV-vis spectra of VOx/MCM-48 and VO• catalysts do also indicate that V 5+ is partly reduced under ODH conditions. However, in contrast to VOx/A1203, the intensity of the VO 2+ signal observed in the in situ-EPR spectra of VO,/MCM-48 and VOx/SBA-15 is negligible. As shown by UV-vis measurements, the VOx species in these materials are essentially in tetrahedral coordination (Fig. 2B). When this coordination symmetry persists during reduction, the respective V 4+ species remain EPR-silent at ambient and elevated temperatures. However, when a flow of wet nitrogen is passed through the catalyst bed after cooling to room temperature, the typical EPR signal of VO 2+ species in octahedral and/or square-pyramidal coordination appears since the tetrahedral V 4+ species formed under reaction conditions adsorb additional water ligands. In agreement with the in situ-UV-vis spectra and the mean V valence state of the used catalysts determined by potentiometric titration this indicates clearly, that VOx species on silica supports are also reduced to a certain degree un-

1147 der ODH conditions. As for VOx/A1203, coke formation does also take place on these catalysts depending on the reaction conditions.

2.3. Catalytic tests Catalytic tests of samples with the same V content under similar reaction conditions revealed that the intrinsic activity of the VOx sites reflected by TOF values as well as the propene selectivities do not differ much for the three silica-supported VOx catalysts (Table 1). This agrees well with the fact that their local structure and valence state under reaction conditions is also very similar. Moreover, the different pore diameters seem to be of minor influence (Table 1). Due to the much higher surface area of sample VOx/MCM-48, the maximum propene yield achieved with this catalyst is higher in comparison to the VOx/SBA-15 samples. The intrinsic activity of VOx/A1203 (TOF values, Table 1) is higher in comparison to that of VOx/SBA-15 and VOx/MCM-48. As shown by the in situ-studies described above, a certain amount of octahedral vanadium sites is present under reaction conditions in VOx/A1203. These species and the higher number of V-O-V bonds might be the reason for the higher intrinsic activity of the VOx sites in comparison to VOx/SBA-15 and VOx/MCM-48. However, propene selectivities over VOx/A1203 are lower than over the silica-supported samples. This might be due to the higher concentration and strength of Lewis acidic sites that have been detected on VOx/A1203 by FTIR spectroscopy of pyridine adsorption. 3. CONCLUSIONS By comparing the properties of VOx species deposited on mesoporous supports with their catalytic performance in the ODH of propane some general relationships can be derived: 9 Vanadium catalyzes this reaction in both oxidation states +5 and +4. However, under reaction conditions, initial V 5§ is partly reduced whereby a mean equilibrium valence state well below +5 is established depending on the feed composition. This process lowers the vanadium redox potential and could be one reason for enhanced propene selectivities. 9 High specific surface areas and low surface acidity of the support material (especially valid for VOx/MCM-48) favour the formation of isolated, completely accessible V sites being beneficial for high propene selectivities. Their low intrinsic activity (TOF values) can be compensated for by high specific surface areas of the support, thus, leading to maximum propene yields. 9 V sites tetrahedrally coordinated by oxygen seem to be less active but more selective than higher coordinated ones. This is evident by comparing the results of VOx/AI203 containing besides VO4 also VO6 units with those of VOx/MCM-48 in which vanadium is essentially tetrahedrally coordinated under reaction conditions. 9 Feed compositions containing propane in excess can give rise to coke deposits. However, these species do not deactivate the V sites but cover preferably acidic sites of the support which in tuna enhances the propene selectivity. These results suggest that good catalysts should contain highly dispersed, preferably tetrahedrally coordinated VOx species on non- or low-acidic support surfaces. The catalytic data obtained in this work with VOx supported on mesoporous A1203 and SiO2 promise that further

1148 improvement could still be achieved by optimizing these materials on the basis of the knowledge described above. ACKNOWLEDGEMENT

The authors thank Dr. U. Bentrup, Dr. D. MtiHer and Mrs. R. Jentzsch for experimental support and the German Federal Ministry of Education and Research for financial support (grant no. 03C0280). REFERENCES

1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14.

T. Blasco and J. M. L6pez-Nieto, Applied Catalysis A: General, 157 (1997) 117. M.A. Banares, Catalysis Today, 51 (1999) 319. E.A. Mamedov and V. Cort6s Cober~_n, Applied Catalysis A: General, 127 (1995) 1. P. Rybarczyk, H. Berndt, J. Radnik, M.-M. Pohl, O. Buyevskaya, M. Baems and A. Brtickner, J. Catal., 202 (2001) 45. A. Briickner, Chem. Commun., (2001) 2122. H. Kosslick, R. Eckelt, D. Mtiller, M.-M. Pohl, M. Richter, R. Fricke, Proccedings of the International Conference on Advanced Materials ,,Materials Week", Mttnchen, September 25th-28th, 2000. V. Alfredsson and M. W. Andersson, Chem. Mater., 8 (1996) 1141. Z. Luan, M. Hartmann, D. Zhao, W. Zhou and L. Kevan, Chem. Mater., 11 (1999) 1621. H.G. Karge, J.-P. Lange, A. Gutsze and M. Laniecki, J. Catal., 114 (1988) 144. A. Brtickner, B. Kubias and B. Lticke, Catal. Today, 32 (1996) 215. M. Niwa, and Y. Murakami, J. Catal., 76 (1982) 9. G. Centi, S. Perathoner, F. Trifir6, A. Aboukais, C. F. Aissi and M. Guelton, J. Phys. Chem., 96 (1992) 2617. X. Gao and I. E. Wachs,, J. Phys. Chem., 104 (2000) 1261. C.J. Ballhausen and H. B. Gray, Inorg. Chem., 1 (1962) 111.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1149

M o d e l l i n g M e s o p o r o u s Materials M.W. Anderson a, C.C. Egger a, G.J.T. Tiddy b, J.L. Casci c* aUMIST Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK bDepartment of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, UK CSynetix, P.O. Box 1, Billingham. Cleveland TS23 1LB UK

1. I N T R O D U C T I O N Mesoporous materials of the MCM or SBA variety are by their nature organised amorphous material. Consequently, in order to describe their structure, it is necessary to utilise a model which is able to accommodate both the organisation and the disorder. Such models are useful for a number of reasons. First, as a method of characterisation, if a model can be generated then a variety of experimental data can be simulated e.g. x-ray diffraction, electron microscopy, gas adsorption etc. Second, a structural model allows further properties of a phase to be anticipated. Third, a model allows a visualistion of a structure which aids our understanding of these novel complex materials. Fourth, the details of the structure yield clues to the synthetic mechanism thereby aiding strategies to design and control new structures. Recently it has been shown [1 ] that an electron density map of a mesoporous structure can be directly determined from electron crystallography. That is, a very well ordered sample was examined by electron microscopy, both electron diffraction patterns and images were collected providing, after indexing, both intensity and phase information resulting in a low resolution electron density map (low resoltuion means that only the wall structure is located, as precise atomic coordinates are random for an amorphous structure). This is the ultimate method to directly determine the structure (wall curvature and thickness) of such a material. However, the technique is laborious and is not suitable for screening materials. Furthermore, subtle structural features are not readily extracted from the resulting three-dimensional electron density map. Our approach is to build structures using a certain amount of previous knowledge in a manner which is then easily manipulated to reflect different synthetic conditions and qualities of material. In this paper we concentrate on the structure of SBA-1 [2] a material first synthesised by the Santa Barbara group, Stucky et al. 2. BUILDING S T R U C T U R E S BY HAND SBA-1 is a hydroxylated silica mesoporous material which is synthesised using a suffactant template cetyl-triethylammonium bromide in a highly acidic silica solution. Under the conditions of synthesis the surfactant forms globular micelles which pack together to give a cubic unit cell. The space group of the resulting material appears to be Pm3n and is related to the suffactant mesophase known as the I1 phase[3]. Working on the basis that the resulting This work was funded by Synetix.

1150 silica structure will in some manner wrap around the globular micellar water structure, in order to describe the silica walls in an analytical mathematical form a type of mathematics is required which will easily describe surfaces wrapped around spheres and distorted spheres. Such a mathematics exists based upon the Gauss distribution function: e

_x 2

--C

and is described in detail by Jacob and Andersson[4]. For our purposes we will require spheres for which the x, y, and z coordinates are given by the equation:

e -(x~§247 = C The radius of the sphere is determined by the constant C and the centre of the sphere can be moved to any coordinate h, k, I by the following transformation: e-[(x-hi +(y-kr162 ] = C The sphere can be elongated or squashed in any dimension to produce for instance an oblate ellipsoid by the following transformation: e-[a (~:-hr +b~O,-er +b3(z-l)Z].__C Finally, an object with a different radius can be formed not only by changing C but also by adding a constant, a, within the exponential thus: e "-[~ (~-h)~+b~('v-e)~+b~(z- t)~] = C This provides the tools to build a mesoporous material synthesised from globular micelles by now adding these functions in the exponential scale. Figure 1 shows what happens when two exponential functions are added together, one representing a sphere, and a second oblate ellipsoid displaced to a different coordinate. When the objects are far apart they form perfect spheres or ellipsoids. However, as they approach the surface begins to form a continuous wrapping. Such a construction should be ideal for the description of mesoporous materials based on globular micelles as the silica surface should indeed wrap around the micellar body forming a continuous surface. Pores

G

a

C

b d

Figure 1. a) and b) are oblique and top views of a small sphere and oblate ellipsoid generated with Gauss distribution functions. c) and d) show how the surfaces wrap as the object become larger and approach one another.

1151 will be generated by the excluded zone between the objects. The arrangement of micelles in the Pm3n, 11, structure is given by adding 21 objects together according to the following equation:

s {O'-h)~+Cv-k)~+~:z-')}+ ~ s h,k,t h',~',I' Table 1. h,k,l fractional coordinates of 9 spheres

Table 2. h ' k' /' coordinates for 12 ellipsoids with values of bl, b2 and b3 in terms o f f r~ = radius o f sphere

rz

f = radius o f long axis o f ellipsoid radius o f short axis o f ellipsoid

h 0

k 0

1

0 0

= radius o f short axis o f ellipsoid

C = e -~

a = r~ + In(C)

1 0

h' 1

k' 0.25

1' 0.5

bl 1/f2

be 1

0

0

1

0.75

0.5

1/f2

1

1 0

0 1

1

1

0

1

0

1

0

1

1

1

1

1

0.5

0.5

0.5

0 0 0.5 0.5 0.5 0.5 0.25

0.25 0.75 1 1 0 0 0.5

0.5 0.5 0.25 0.75 0.25 0.75 1

1/f2 1/ f 2 l/ f 2 1/ f 2 1/f2 1If 2 1

1 1 l/ f 2 l/ f 2 1/f2 l/ f 2 1/f2

b3 1/f2 1 / f2 1/f2 1/f2 1 1 1 1 1/f2

0.75 0.25 0.75

0.5 0.5 0.5

1 0 0

1 1 1

1/ f 2 1/f2 1 / f2

1/ f 2 1/f2 1 / f2

Figure 2: Pm3n arrangement of 9 spherical micelles, one marked $5, (on a body centre) and 12 oblate ellipsoids, marked El-E4, two on each face o f t h e unit cell. Based on a unit cell of 85A the sphere radius and the radius of the short axis of the ellipsoid are both 10A resulting in the constant, a, equal zero.

1152 The first summation represents 9 spheres with a body centred arrangement and the second summation 12 oblate ellipsoids. The coordinates for these objects are given in tables 1 and 2 as well as the derivation of the constants. The result is the arrangement of micelles shown in figure 2. This figure is just a schematic representation of the relative positions of the spherical and oblate micelles and the sizes are scaled in order to aid the reader to understand this arrangement. In this figure the extension of the oblate ellipsoid is given by the factor f, which in this case has been chosen as 1.3. Although the final structure can be calculated for any unit cell in order for the mathematics to remain robust it is important that the constant C does not become excessively large or small. In order to prevent this from occurring all surface calculations have been based on a unit cell of 6A which is then scaled accordingly. '~

:':~,i ~ , ~ . , r

................ : : , ; G ' ~ : . r

~.!-',~_.!~,:'... " ",,,: :t-.tl ~ . ~ g , _~ "- 'r 9" p " r .

....t

' ~~ ~ ~t ~"

~~

t

~

,

~

~

9 . . , t l - ;o .. -.

"...

.~:~: ':

: ....:.,,,-t.~.'~..

".

t -' ~ ; ; ~ " . . ~". ~ : - ,,t ' t

, ~,

.r . ~ . -

..

.

~'~*~m~I~["~3.:--.-"" ~7",':'" " r,'.-v;...,'-. " "~,. ,,'." ".'_,,.2 II'

~ J ~ ' ~ m ~ i I I ~ l ~ '

r ~ ~ , , ~ M ~ m m ~ I

""

9

9 ,-

~

~

~

~ :

~"

. .. ,~ = e,,~.. ~ 9 t- ~

.~,r . ! -

9:.~ r

" .,~".'t . . . . ~'..~

" - : --I',

.m

~.--

9 " 9 ".

Figure 3: left picture shows the surface generated with an 85A unit cell, radii rl and r2 equal 19.5A and an oblateness given by f=l.18A; right picture shows the atomistic model of SBA-1 by placing a random army of silicon atoms on the outer side of the surface. The structure ofSBA-1 is then built by increasing the size of the spheres and ellipsoids until they merge and the surface becomes continuously wrapped. However, as the surface will wrap into adjacent unit cells it is important to include in the calculation a further 12 virtual ellipsoids. The coordinates of these twelve ellipsoids can be derived from those in Table 2 by replacing the coordinate 0.25 by -0.25 and 0.75 by 1.25. Consequently a total of 33 objects are required to describe the whole structure ofSBA-1 (9 spheres, 12 ellipsoids and 12 virtual ellipsoids). When this is done the result is the surface shown in figure 3. One unit cell is shown and windows can be seen which are generated as the surface wraps from one object to the next. This surface will represent the periphery of the edge of the wall of SBA-1 which will presumably be in contact with a water sheath around the micelles. In order to generate an atomistic model of SBA-1 it is the necessary to fill the space on the outer side of the surface, not occupied by the surfactant molecules and water layer with a random array of silicon atoms. This method was successfully used before in an atomistic description of

1153 MCM-4815]. The density of silicon atoms is maintained by keeping an average Si...Si separation of 3.5/!t and the atomistic model so formed is shown in the lower part of figure 3. In order to optimise the parameters, radius of the sphere rs, radius of the short axis of the ellipsoid re and degree of oblatenessfa large number of structures were generated in this manner from which the x-ray diffraction pattern and electron micrographs could be generated for comparison between the our model and experimental data. The results of some of these calculations are shown in figure 4.

'

!i

.............

~"

t

. . . . .

t

~

t

.

123456123456123456 "20

.

.

.

.

.

.

234567

. ............. .

.

.J ' , . . . . .

.~,,s,~

.....

; _ : ~ _ _ ~

Figure 4: top 16 x-ray diffraction patterns calculated with sphere radius rl equal to the radius of the short axis of the oblate ellipsoid r2. The value off, the measure of oblateness is given inset in each figure. The 16 atomistic models generated are layed out below in the same order. The x-ray diffractions pattems agree very well with those reported in the literature and with those that we measure. The best agreement between is found for rs=rz=l 9 to 19.5/!t and f=l.18. As soon as rs deviated from r2 extra reflections appeared, most significantly the [ 110] reflection, which rapidly became very strong and is not observed experimentally. The fact that the radius of the sphere and that of the short axis of the oblate ellipsoid are similar is not surprising as both will be governed by the length of the surfactant chain. The models which best fit the x-ray diffractions patterns also show a strong correspondence between the projected electron potential maps (not shown) and the electron micrographs described in the literature[ 1]. As the ellipsoids tend to spheres (when f tends to 1) the size of the two types of micelles has to be significantly different (e.g. 23A and 19A radii to be able to explain the electron micrographs. This was the description for the structure given previously[1 ], however, the x-ray diffraction pattems of such a structure are vastly incorrect, including a large [110] reflection (see figure 5).

1154

I. -,',:L:e~,-,.

...i,..

.......... -:~-~s_-7..r

:<-.

-.~

; ~.~

I

2

3

4

5

6

+

...., ~_.-~,-,.-I ........

3

~'~ : ~ , ~ . ~

" ~ : . . - - . i ..

. .::-~,

.%.

-

. ?_

"I

Figure 5: X-ray diffraction patterns and atomistic models illustrating presence of low angle [110] reflection, *, when the radii rs and r2 deviate from each other. Top model for r~=18.5A, re=21A, f=l.13; bottom for spherical micelles similar to description in reference 1, rr r2=23.5A, f=l.0. The optimum model for SBA-1 synthesised in our laboratory is with rs=re=19 to 19.5A andf=l.18 for an 85/~ unit cell. Such a surface generates three types of windows between pores all of which can be seen in the [100] projection by transmission electron microscopy[1 ]. The three window types are illustrated in figure 6 and are generated when the micelles are closest, separated by a water layer. The largest windows are created by two oblate ellipsoid micelles with the flatter sides close together. The medium pores are created by an oblate ellipsoid and a spherical micelle. In this case the oblate micelle nearly has the thin end towards the spherical micelle. The third, and smallest window is generated by two oblate ellipsoid micelles, orthogonal to each other, where the thin edges are close together.

1155 It is interesting to conjecture why the windows are of such different sizes. There appears to be a relationship between the window size and the contact angle between the micelles. For instance the largest window is between the two oblate micelles which approach on the flatter sides giving a small contact angle between the micelles. The water layer between the micelles will probably exclude the silicate thereby generating the pore. This is illustrated in figure 7. When the contact angle is small, E2 approaching E4, then a water layer of a given thickness will exclude a relatively large window. When the contact angle is larger, E1 approaching $5, then the same water thickness layer will exclude a smaller window. When E1 approaches E2 at the largest contact angle only a very small region is excluded by the water layer.

Figure 6: four unit cells of the optimum surface of SBA-1 in the [100] projection. Three windows are apparent. The A window were micelles E2 and E4

.

.

.

.

.

.

.

.

.

.

~ m

I~

,

-.

I

approach; the B window where micelles E1 and $5 approach; the narrow window where micelles E2 and E1 approach. Nomenclature from figure 2. 3. CONCLUSIONS The wall structure of mesoporous materials can in general be describe via an analytical expression. Where the mesoporous material is synthesised from a surfactant mesophase based upon a three-dimensional packing of globular micelles then the mathematics based on the exponential scale of a Gaussian distribution works very well. We have successfully described the structure of SBA-1 in this manner and revealed that the details of the micellar structure, including oblate distortions of globular micelles, are retained in the final inorganic structure. A preliminary mechanism for window size in mesoporous materials is discussed.

1156

Figure 7: micelles surrounded by a water layer and then the silica wall. The water layer excludes the silica wall from a region between the micelles thereby generating a window. The window size is governed by the contact angle between the micelles. 4. R E F E R E N C E S

1. Sakamoto, Y; Kaneda, M; Terasaki, O.; Zhao, D.; Kim, J.M.; Stucky, G.; Shin, H.J.; Ryoo, R. Nature 2000, 408, 449. 2. Huo, G; Margolese, D.I.; Ciesla, U.; Demuth, D.G.; Feng, P.; Gier, T.E.; Siegel P.; Firouzi, A.; Chmelka, B.F.; Schtith, F.; Stucky, G.D. Chem. Mater., 1994, 6, 1176. 3. Luzzati, V." Delacroix, H.; Gulik, A. J. Phys. II, 1996, 6, 405. 4. "The Nature of Mathematics and the Mathematics of Nature" by Jacob, M. and Andersson, S., Elsevier 1998. 5. Alfredsson, V; Anderson, M.W. Chem. Mater., 1996, 8, 1141.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1157

Acidity and thermal stability of mesoporous aluminosilicates synthesized by cationic surfactant route. M. Derewinski, a * M. Machowska, a R Sarv b a

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-238 Cracow, Poland

b Institute of Chemical Physics and Biophysics, Akadeemia Tee 23, EE0026 Tallinn, Estonia

The thermal stability and acidity of a series of mesoporous aluminosilicates obtained by the cationic surfactant route were investigated by means of 27A1 and 29Si MAS NMR and IR spectroscopy as a function of Si/A1 ratio (Si/A1 = 0 . 2 - 5). High temperature treatment applied to remove the occluded molecules of the surfactant results in a partial dealumination of the mesostructured solids. The extent of the dealumination depends on the chemical composition of the calcined material and is considerably higher for A1 rich samples. Sorption of pyridine shows that both Br6nsted- and Lewis acid sites are present in all mesoporous aluminosilicates under study. The number of the Br6nsted sites depends on the Si content and increases together with the increase of the silicon concentration. The nature of the Lewis sites depends on the composition of the starting mesostructured material. For samples with high A1 content the Lewis sites are mainly the extraframework octahedral aluminium species. For the more thermally stable silicon rich aluminosilicates, the trigonally coordinated A1 atoms, formed during the dehydroxylation of the neighbouring Br6nsted sites, mainly account for the high aprotonic acidity. 1. INTRODUCTION The discovery of M41S family of mesoporous silicas [1], obtained in the presence of micellar aggregates, has promoted considerable effort on the generation of the acidic function in these materials, necessary for their catalytic use. The incorporation of aluminium into the framework of mesoporous silica can be achieved either by a direct surfactant-assisted synthesis [2-5] or a post-synthesis modification [6]. Unfortunately, high-temperature surfactant removal results very often in a collapse of the porous structure and conversion of the tetrahedral A1 into octahedral ones. Moreover, it is difficult to obtain mesoporous material with a considerable amount of tetrahedral A1 in the framework. A new method of the synthesis of thermally stable, alumina rich aluminosilicates using cetyltrimethylammonium bromide (CTABr) as a surfactant-directing agent was recently reported [7,8]. The important feature of this "cationic surfactant route" is use of a "hydrolysis retarding agent" i.e. triethanolamine (TEA) as one of the reactants. In the present study the results of the synthesis of mesoporous aluminosilicates with varying A1 content are reported. Acidity of these materials as well as changes in the state of the tetrahedral, framework aluminium upon thermal treatment were studied as a function of the A1 content.

1158 2. EXPERIMENTAL 2.1. Mesoporous materials preparation The mesostructured alumina was synthesized according to [9], using cetyltrimethylammonium bromide as surfactant agent in a water/triethanolamine medium. The molar composition of the starting gel was: (A1203)l(CTABr)~(TEA)~5(Na20)0.25(H20)~10. The synthesis was carried out in hydrothermal conditions at 120~ for 72 h. The syntheses of mesoporous aluminosilicates were carried out at room temperature for 48 h, according to the recipe [7], in the system: (SiO2)0.5_2.5(A1203)0.5_zs(fTABr)~.~(TEA) ~3.7(Na20)~(H20)605 using tetraethylorthosilicate (TEOS), aluminium sec-butoxide, sodium hydroxide, CTABr, TEA and water. The syntheses were performed for several different Si/A1 ratios and obtained samples are designed as SA(X), where X = 0.2, 0.5, 1, 2 and 5 is the Si/A1 ratio in the synthesis gel. The molecules of the surfactant trapped in the pores of mesostructured materials were removed by careful thermal procedure involving low heating rate (l~ until 520~ is reached, and calcination in argon and subsequently in dry air at that temperature for 8h in each atmosphere. To transform Na +- into the NHa+-form, calcined mesoporous aluminosilicates were ionexchanged at 60~ using 1M aqueous solution of NHaNO 3. After two-fold ammonium exchange the sodium content in the samples was below 0.3 wt.%. 2.2. Characterization The obtained preparations were characterized by XRD and their BET surface area and mesoporosity were checked by the N 2 adsorption/desorption isotherms. 27A1and 29Si MAS NMR was applied to study the state of A1 and Si atoms in as-synthesized, calcined and ion-exchanged materials. The 29Si MAS NMR single pulse spectra were measured in a 360 MHz ~H frequency magnet with a 90 ~ pulse and a 60 s relaxation delay. 29Si MAS CP NMR spectra were measured in a 200 MHz 1H frequency magnet. The mixing pulse was 5 ms and the relaxation delay was 10 s. 27A1 MAS NMR spectra were measured in a 500 MHz 1H frequency magnet with a 10~ pulse, a 0.2 s relaxation delay and a 15 kHz rotation speed. All samples were kept at 75% relative humidity for at least 48 hours prior to the experiment. The nature and number of the acid sites were determined by pyridine adsorption after activation of the samples in vacuum at 450~ for 2 h (heating rate 10~ IR spectra were recorded on a Nicolet 800 spectrometer equipped with MCT A detector at a resolution of 4 cm-~. 3. RESULTS AND DISCUSSION The mesoporous products were obtained for all systems i.e. from the aluminium rich gels (Si/A1 = 0, 0.2, 0.5 and 1) and from gels containing silica as the main component (Si/A1 = 2 and 5). The morphology and approximate crystal size of meso-AlzO 3 and aluminosilicates, determined from SEM micrographs indicated in all samples irregular blocks, which consist of small (about 1 ~t in diameter) and similar in shape particles. EDX analysis showed the chemical homogeneity and regular distribution of silicon and aluminium atoms in the solids. The Si/A1 molar ratio of as-made mesoporous materials is closed to that of the synthesis mixtures. The XRD pattems of all as-synthesized mesostructured materials exhibit a single and relatively broad peak with a maximum at about 2 degrees (2 theta scale), which evidences the presence of mesopores in the structure. Transmission electron microscope (TEM) analysis of the obtained

1159 Table 1. Composition data and selected physical properties of the mesoporous alumina and aluminosilicates Sample

Si/A1 ratio gel

SA(5) SA(2) SA(1) SA(0.5) SA(0.2) A1203

5 2 1 0.5 0.2 -

solid EDX a

bulk b

4.7 2.8 2.1 1.0 0.6 -

3.6 1.9 1.6 -

BET surface area (m 2 g-l)

BJH pore size (A)

746 557 412 357 412 370

37 38 38 37 37 40

a Analysis of as-synthesized materials Analysis of calcined and ion-exchanged materials

b

samples confirmed the XRD data and showed no order in the pore arrangement and presence of so-called "sponge-like" pore distribution. Some ordering i.e. presence in the structure of patches containing the parallel mesopores resembling a disordered hexagonal packing motif was observed in the silica rich (Si/A1 = 5) material. The high-temperature calcination (520~ does not result in amorphisation of the preparations. The BET surface area of the calcined samples increases from 412 mE/g (for the silica poor sample i.e. Si/A1 = 0.2) to 746 m2/g for the material with Si/A1 = 5. The average pore diameter of all samples is 37 - 38A. The elemental analysis revealed the presence of considerable amount of sodium in mesoporous products. The Na + cations present in the gels when complexes of aluminium with triethanolamine are formed, remain in the synthesized solids balancing the charge introduced into the framework by the Si-O--A1 species and have to be removed by ion-exchange. The physical properties of the materials under study are summarized in Table 1. 3.1.27A1 and 29Si MAS N M R - thermal stability

The 27A1MAS NMR analysis shows (Figure 1) that the incorporation of Si into the flamework results in the formation of tetrahedrally coordinated, framework aluminium (A1TM)(peak at ~ 58 ppm), whereas in the non-calcined mesoporous alumina almost all A1 atoms are in the octahedral coordination (A1vI) (peak at 8.7 ppm). The number of the tetrahedral, framework A1 atoms increases rapidly with amount of silicon introduced into the mesoporous material. Simultaneously the maximum of A1TMline is shifted from 61.1 ppm for SA(0.2) to 55,5 ppm for SA(5). Starting from the sample with Si/A1 = 1, only A1TMpeak is detected in 27A1 MAS NMR spectra. Thus, for the mesoporous aluminosilicates with Si/A1 = 1, most of A1 atoms could be involved in the generation of acid sites. 29Si MAS NMR single pulse and CP spectra from the mesoporous aluminosilicates exhibit one featureless hump (Figure 2). Both CP and single pulse spectra are practically the same indicating that observed chemical shift distribution stems not only from the aluminium atoms as nextnearest tetrahedral neighbours, but also from the presence of terminal hydroxyls Si(OH) units [ 10].

1160 AlW

AlvI

SA (5) SA (2) SA(1)

CP MAS

/

sA (0.5) CP

alumina 55

3'5

i5

-'5-2'5

o

-86

Ill

-90 -92 -94

~

-96 -98 .... , 0

Il 9

,

1

"''"

i"'

9

89 3 Si/Al(gel)

!

' 1 -

-80

-100

-120

Figure 2. 295i MAS N]VIR and ~H - 298i CP/ MAS NMR spectra of the sample SA(2)" a) as-synthesized, b) calcined and 520~

-88

~

'

[ppm from TMS]

Figure 1.27A1 MAS NMR spectra of as-synthesized mesostructured alumina and aluminosilicates.

f/3

'1

-60

[ppm from Al(H20)63+]'

-84

-

i

4

9

9

5

---'--

Figure 3. Plot of the average 298i MAS NMR chemical shift of mesoporous aluminosilicates against the composition of the starting gel.

l

6

Nevertheless, there is a clear correlation between the composition of the starting gel and the average 29Si MAS NMR chemical shift of the spectrum. It has been found that more aluminium in the starting gel shifts the line of the as-synthesized materials towards positive chemical shifts (Figure 3). Similar effect can be observed for the amorphous silica-alumina with different Si/A1 ratio, which indicates an amorphous character of the walls of synthesized aluminosilicates. The high temperature calcination results in a partial dealumination of mesoordered materials. The extent of dealumination depends on the chemical composition of starting mesoporous aluminosilicates and can be determined with 27A1 MAS NMR. The

1161

lIV

/1\

AlvI

AlVI

Alv

A

1~

a

/ --

i

I

-20

80 60 40 20

80

[ppm from Al(H20)63+]

/

/

I

' -60

'

- 8 "0

I

-100

30

6

-3'0

[ppm from Al(H20)63+]

Figure 4. 27A1 MAS NMR spectra of aluminosilicate SA(2): a) as-synthesized, b) calcined at 520~ c) ion-exchanged.

'

60

I

'

-120

[ppm from TMS] Figure 6. 29Si MAS NMR spectra of aluminosilicate SA(2): a) as-synthesized, b) calcined at 520~ c) ion-exchanged.

Figure 5. 27A1 MAS NMR spectra of aluminosilicate SA(0.5): a) as-synthesized, b) calcined at 520~ c) ion-exchanged. important feature of silicon rich materials is their high thermal stability. The extent of conversion from tetrahedral to octahedral aluminium, which occurs as a consequence of the surfactant removal and ion-exchange, is very limited (below 10%) (Figure 4). On the other hand, for the aluminium rich samples i.e. SA (0.2) and SA (0.5), which have considerable amount of octahedral sites already in the as-synthesized form, dealumination is more significant. After calcination at 520~ the five coordinated (A1v) and distorted tetrahedral sites are formed, which during ion-exchange are converted to octahedral sites (Figure 5). The dealumination process is confirmed by the 29Si MAS NMR spectra. They show that the 29Si line of the calcined sample is shifted towards the negative edge of the spectrum (Figure 6). The ionexchange probably also reduces the number of Q2 and Q3 groups, which contributes to the 29SiNMR line shift towards more negative chemical shifts.

1162

3.2. Pyridine adsorption- acidic properties The acidity of mesoporous aluminosilicates arises from the presence of intrachannel hydroxyl groups associated with the tetracoordinated framework A1 atoms as well as from the presence of extraframework species containing octahedral aluminium. The nature, density and strength of the acid sites present in the solids under study were determined by means of the pyridine adsorption monitored with IR spectroscopy. IR spectra of the H-form of the samples in the hydroxyl groups vibration region contain a single band at 3746 cm -~ assigned to non-acidic Si-OH groups [4,11]. Sorption of pyridine resulted in a significant decrease in intensity of that band. Simultaneously the band at 1547 cm- ' characteristic of pyridinium ions (HPy+) and of pyridine coordinatively bonded on Lewis acidsites (LPy) (band at ~1455 cm -~) appeared in the IR spectra. The concentration and acid strength of the Brrnsted (B) and Lewis (L) acid sites present in mesoporous aluminosilicates were estimated from the IR spectra recorded after outgassing the samples with preadsorbed pyridine, at elevated temperatures. An example of the difference spectra of pyridine recorded for one of the sample under study (SA (5)) after outgassing at 150~ 250 ~ 350~ and 450~ is presented in Figure 7. The spectra recorded after the desorption of pyridine at 150~ and 250~ contain bands characteristic both of LPy HPy + LPy and HPy § The desorption at 350~ eliminates completely band at 1547 cm-~ from the spectrum, whereas that of pyridine bonded to Lewis sites (band at 1457 cm -~) is present even after outgassing at 450~ This indicates the considerably higher acidic strength of the Lewis sites in comparison to that of the Brrnsted type. The concentration of B and L sites in the mesoporous aluminosilicates activated at 450~ were calculated from the adsorption bands at 1547 cm -1 and 1457 cm -~ respectively [13], recorded after the a desorption of weakly bonded pyridine at 150~ for 1 h (Table 2). The analysis of the IR spectra of pyridine adsorbed on different mesoporous aluminosilicates indicated 0 that the number of Brrnsted acid sites depends on the amount of Si atoms incorporated into the structure ofmeso0 9l s ~ 9 ~9 ls39 l~s9 1579 1599 porous materials. The sample with the ' W~VENUMBEB highest silicon concentration (SA(5)) contains the highest number of the Brrnsted acid sites (55 ~tmol/g). The Fig. 7. Difference IR spectra of pyridine on mesonumber of these sites decreases as porous aluminisilicate SA(5) after outgassing at: amount of Si atoms in the solid a) 150~ b) 250~ c) 350~ d) 450~

1163 Table 2. Concentration of acid sites in mesoporous aluminosilicates Sample

SA(5) SA(2) SA(1) SA(0.5) SA(0.2)

Number of acid sites (~tmol g_~)a Br6nsted (B) b

Lewis (L) c

55 d 31 17 16 16

146 92 78 80 136

B/L

0.35 0.34 0.22 0.20 0.12

a Concentraction of the Lewis and Br6nsted acid sites were calculated from the infrared adsorption bands of pyridine (HPy+ and LPy) according to [13]. bIntegration region: 1565-1528 cm-'. c Integration region: 1475-1435 cm". dThe systematic errors were estimated at + 15% decreases. For the whole range of samples SA(1) - SA(0.2) the number of Br6nsted sites is practically the same (about 15 gmol/g). A high number of Lewis acid centres found for the aluminum rich preparations (136 gmol/g for SA(0.2)) first decreases with increase of the Si content and subsequently increases up to 146 gmol/g for sample SA(5). The B/L acid site ratio for SA(5) and SA(2) samples (at 150~ is 0.36, which is in a good agreement with the literature data reported for A1-MCM-41 [4,12]. The B/L value for the preparations with high A1 content is significantly lower, which results from the decrease of the Br6nsted acid sites concentration and the increase of the number of the Lewis centres (Table 2). It has been shown that the calcination of A1 rich aluminosilicates followed by the ionexchange results in the significant dealumination of mesoporous material (Figure 5). The extralattice species formed during these processes, which contain octahedrally coordinated aluminium, are responsible for the high number of Lewis acid sites. On the other hand, the mesoporous aluminosilicates with higher Si content are characterized by high thermal stability and limited extent of dealumination (below 10% of octahedral aluminium found for the samples SA(5) and SA(2)) (Figure 4). Thus, the Lewis acidity found for these samples has to be of a different nature. The preparations with high Si content show high concentrations of silanols and Br6nsted acid sites. The partial dehydroxylation, which occurs during the activation at temperatures above 400~ transforms the Br6nsted sites into Lewis ones (one Lewis site per each two Br6nsted). High population of Br6nsted sites and shorter distance between Si-OH-A1 groups in silicon rich preparations facilitate such process. The formation of the Lewis centres of the AI(OSi)3 type, which contain trigonal aluminium atoms in the structure, was already postulated for mesoporous A1-MCM-41 aluminosilicates. Their acidic strength may vary depending on the local geometry of such centres [ 14]. The presence of at least two different type of Lewis acid sites was confirmed by the results of pyridine adsorption.

1164 4. CONCLUSIONS It has been shown that using CTABr as surfactant in the triethanolamine/water medium it is possible to synthesize mesostructured materials with the Si/A1 ratio between 0.2 - 5. The obtained materials are thermally stable and do not loose their mesoporosity during the calcination at 520~ On the other hand the high-temperature removal of organic surfactant causes the dealumination of mesoporous aluminosilicates. The extent of the A1 removal from the structure depends on the Si/A1 ratio. Whereas, for aluminium rich samples the number of the extraframework species containing octahedrally coordinated A1 is high, the aluminosilicates with Si/A1 > 1 show low extent of the dealumination and hence reasonable thermal stability. The pyridine adsorption showed, that both the Br6nsted and Lewis acid centres are present in the mesoporous materials. The Lewis acidity is predominating in all the samples under study. The number of the Br6nsted sites depends on the Si content and is the highest for sample SA(5). It has been also shown that nature of the Lewis sites present depends on the mesoporous aluminosilicate composition. For the A1 rich samples, the Lewis centres are mainly the extraframework species containing octahedral A1. In the case of more stable, silicon rich preparations (Si/A1 > 1), trigonally coordinated A1 atoms i.e. AI(OSi)3 formed as a result of dehydroxylation of the neighbouring Br6nsted sites account for a high number of Lewis acid sites. 5. ACKNOWLEDGEMENTS This work was supported by the NATO Science for Peace Programme under project NATO SfP 974217. REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature (London) 359 (1992) 710. 2. Z. Luan, C-F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 3. Z. Luan, C-F. Cheng, H. He, J. Klinowski, J. Phys. Chem., 99 (1995) 10590. 4. A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal., 148 (1994) 569. 5. R.B. Borade, A. Clearfield, Catal. Lett., 31 (1995) 267. 6. R. Mokaya, W. Jones, Chem. Commun., (1998) 1839. 7. S. Cabrera, J. E1 Haskouri, S. Mendioroz, C. Guillem, J. Latorre, A. Beltr/m-Porter, D. Beltr~in-Porter, M.D. Marcos, E Amor6s, Chem. Commun., (1999) 1679. 8. S. Cabrera, J. E1 Haskouri, C. Guillem, J. Latorre, A. Beltr/m-Porter, D. Beltr~in -Porter, M.D. Marcos, P. Amor6s, Solid State Sci., 2 (2000) 405. 9. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltr~in, D. Beltr/m, S. Mendioroz, M.D. Marcos, P. Amor6s, Adv. Mater., 11 (1999) 379. 10. G. Engelhardt, D. Michel, "High-Resolution Solid State NMR of Silicates and Zeolites ", John Viley & Sons, Chichester, 1987. 11. J. Weglarski, J. Datka, H. He, J. Klinowski, J. Chem. Soc, Faraday Trans., 92 (24) (1996) 5161. 12. R.B. Borade, A. Clearfield, Synthesis of Porous Materials: Zeolites, Clays and Nanostructures, M.L. Occelli, H. Kessler, eds. Marcel Dekker, Inc., New York, 1997. 13. C.A. Emeis, J. Catal., 141 (1993) 347. 14. F. Di Renzo, B. Chiche, E Fajula, S. Viale, E. Garrone, Stud. Surf. Sci. Catal., 101 (1996) 851.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

M e s o p o r o u s silicate as matrix for drug delivery a n t i n f l a m m a t o r y drugs

1165

s y s t e m s of non-steroidal

R. Aiello ~ G. Cavallaro*, G. Giammona*, L. Pasqua ~ P. Pierro*, F. Testa ~ ~ di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, Via Pietro Bucci 87030 Rende (CS) ITALIA *Dipartimento di Chimica e Tecnologie Farmaceutiche, Universit~ di Palermo, Via Archirafi 32 90123 Palermo ITALIA 1. I N T R O D U C T I O N

During the last two decades significant advances have been made in the area of controlled release technology in the attempt to overcome drawbacks due to the use of the conventional dosage forms such as frequent administrations, toxic and side effects, high doses; to this aim Drug Delivery Systems (DDSs) seem to have many interesting properties being able to modify the absorption and/or the distribution and/or the elimination of a drug and, so, to modify its pharmacokinetic profile [1,2]. The advantages due to the use of a DDSs are considerable; in fact, among other things, these systems are potentially able to maintain therapeutic drug levels for large periods of time preventing the onset of potentially toxic peaks in drug concentration, to reduce the amount of drug and the number of administrations and to protect the drug against chemical and enzymatic degradations. Therefore with a DDS is possible not only to improve the therapeutic efficacy of a known drug but also to allow the administration of drugs characterised by a short plasma half-life and/or by remarkable collateral and toxic effects. Furthermore, another advantage in the development of a known drug as DDS is in relatively low cost compared to that of the discovery of a new drug. Both natural and synthetic materials have been tested and proposed as component of DDSs [3] and many efforts have been doing in many fields to synthesise materials with the "ad hoc" biological, technological and mechanical properties for each application in drug delivery. The aim of this investigation has been to test the possibility of employing a particular inorganic mesoporous matrix based on siliceous material as drug controlled release device. The introduction of the M41S family of mesoporous materials by Mobil Corporation scientists in 1992 created a new field in research on advanced materials. MCM-41 is the member of the M41S family that has been more extensively studied [4]. It shows hexagonal arrays of cylindrical mesopores. Pore size ranges between 15 and 100 ]k depending basically on the surfactant employed for the synthesis process. Pore walls present free silanol groups that could be reactive toward appropriate guest molecules. These properties allow MCM-41 to work as a matrix for adsorption and release of suitable organic molecules such as drugs without any chemical modification of pore walls. Recently disks of MCM-41 charged with Ibuprofen, an anti-inflammatory drug, were prepared and characterised [5].

1166 In this contest, we have evaluated the capacity of a mesoporous silicate matrix to entrap drugs and, subsequently, to release them under different experimental conditions mimicking some biological compartments. Non-steroidal anti-inflammatory agents such as Diflunisal (Dr), Naproxen (Np), Ibuprofen (Ib) and his sodium salt (IbNa) have been employed in this study. The four drug impregnated matrices have been characterised with regard to drug content and swelling measurements in aqueous media which simulate some biological fluids such as gastric and intestinal liquids. Moreover the capacity of the mesoporous matrix to act as delivery system by carrying out in vitro studies under experimental conditions mimicking gastrointestinal fluids has been evaluated.

~H3

OOH

~H3

CHCOOE

OH

(CH3)2CHC2 h H

CH30~ DIFLUNISAL

NAPROXEN

HCOOH

IBUPROFEN

Fig. 1. Chemical Structure of Diflunisal, Naproxen and Ibuprofen 2. EXPERIMENTAL SECTON

2.1. MCM-41 synthesis MCM-41 was obtained from gel with the following molar composition: 1SiO2-0.2NaOH0.04AI(OH)3-0.2CTABr-40H20. The initial gel was prepared according to the following procedure: 24.3 g of cetyltrimethylammonium bromide (Aldrich), 1.04 g of Ai(OH)3 (Pfaltz & Bauer), 2.7 g of NaOH (Carlo Erba) and 20 g of fumed SiO2 (Sigma) were added in this order in 240 g of bi-distilled water. The resulting gel was aged for 2 hours at room temperature and then transferred to a Teflon-lined autoclave in a thermostated oven at 150 ~ for 24 hours. The synthesis product was filtered, washed with distilled water and then dried at 80 ~ The synthesized sample was calcined in air at the temperature of 550 ~ for 8 hours with a heating rate of 1 ~ The N2 adsorption-desorption volumetric isotherm was measured on a Micromeritics Asap 2010 instrument. Sample was pre-treated under vacuum at 300~ to a residual pressure of 2 ktmHg. Surface area was obtained by BET linearization in the pressure range 0.05 to 0.2 P/Po. Lattice pore volume was obtained from the amount of nitrogen gas adsorbed at the top of the rising section of the of type IV isotherm. The pore size distribution was calculated on the basis of desorption data by employing the Barrett-JoynerHalenda (BJH) method [6].

2.2. Drug loading by soaking procedure As an example, drug loading into mesoporous matrix was achieved by impregnation, soaking for three days at room temperature, under continuos stirring, 600 mg of the matrix in a concentrated solution (200 mg of Diflunisal in 10 ml of methanol) acting as a swelling agent. Methanol was also used for Np and Ib, while water was used for IbNa. The solvent was then removed by filtration and the samples dried under vacuum.

1167

2.2.1 Determination of drug amount entrapped in the matrix In order to determine the amounts of loaded drug into the mesoporous matrix, different methods have been employed depending on the drug: 1. sample impregnated with Df was dispersed in methanol at room temperature and kept under constant stirring for 4 days; 2. samples impregnated with Np and Ib were extensively extracted with methanol at 60~ for 4 hours; 3. sample impregnated with Ib-Na was dispersed in water at 60~ and kept under constant stirring for 3 days. The solvents after extraction were collected by filtration under vacuum and assayed by HPLC analysis for the quantitative determination of the drug loaded in the matrices. HPLC analyses were carried out on a system consisting of a Varian 9012 Liquid Chromatography equipped with a Rhedyne Injector 7125 (fitted with a 10 B1 loop), a Kontron HPLC Detector 432 on line with a computerized HP workstation. In the method HPLC, a reversed phase C18 column (BBondapack; 10 Bm of 250 mm x 4.6 mm i.d., obtained from Waters) equipped with a direct-connect guard column C18 (Waters) was used; mobile phase (flow 1 mL/min) was methanol:phosphoric acid (0.1% v/v) in the ratio 70:30 (v/v) for Df analyses and acetonitrile:acetic acid (5 g/l) in the ratio 50:50 (v/v) for Np, Ib and IbNa. Eluate was monitored at 254 nm for all drugs.

2.2.2. Swelling studies 25 mg of the impregnated matrices were kept in contact with 20 ml of double-distilled water, HC1 0.1N at pH 1 or 20 ml of a phosphate buffer (NaC1, Na2HPO4, KH2PO4) at pH 6.8 until the swelling equilibrium was reached, then each swollen sample was filtered, blotted with paper and weighed. Water content (WC %) was calculated as follows: WC (%) -- (Ws-Wd)/Ws X 100 where Ws and Wd are the weights of the swollen and dry matrix, respectively.

2.2.3. Drug release studies In vitro release studies of drug from mesoporous silicate matrix were carried out by keeping

25 mg of each sample of matrix loaded with drug in 20 ml of a 0.1N HC1 solution (pH 1.0 simulated gastric juice), at 37_+0.1~ in a water bath with magnetic stirring (100 r.p.m.), for 2 hours. As, in all cases, the releases were not complete after this period of incubation time, a mixture of Na3PO4 0.2M and NaOH 0.1N was added in order to adjust the pH to 6.8 (simulated intestinal fluid). Sink conditions were maintained throughout the experiment. Then, at suitable time intervals, samples were filtered and the aqueous solutions analysed by HPLC according to the conditions previously reported.

3. RESULTS AND DISCUSSION The reaction of cetyltrimethylammonium bromide, aluminium hydroxide, sodium hydroxide and fumed silica in water at 150 ~ for 24 hours in autoclave gave rise to a solid material that after proper filtration and calcination gave mesoporous material belonging to MCM-41 family. The obtained sample was characterized by elemental analysis, X-ray diffraction, thermogravimetric analysis and N2 adsorption- desorption isotherm at 77K.

1168 The amount of aluminium in 100 g of MCM-41 sample, calculated by elemental analysis, is equal to 1.04 g, confirming the complete incorporation of aluminium into the mesoporous material. X-ray powder diffraction pattern of calcined MCM-41 shows a single reflection (Fig. 2) that indicates the small size of the crystals [7]. The unit cell parameter a~ (distance from centres of two adjacent pores) is 49.2 A. The N2 adsorption-desorption isotherm at 77K (Fig. 3) shows the main nitrogen uptake typical of mesopore filling in the range 0.3-0.4 P/Po. Pore volume at P/Po=0.8 is 0.92 cm3/g while specific surface area calculated according BET method is 1124 m2/g. Average pore diameter (4V/A by BET) is 43]k. The BJH pore size distribution obtained from desorption isotherm of mesoporous sample is shown in Fig. 4. Two main regions of nitrogen consumption can be noted on the logaritmic plot of pore volume versus pore diameter. The sharper one is assigned to pores whose diameters ranges between 26 and 32 A, the regular micelles-produced porosity typical of MCM-41 materials. BJH method was shown to underestimate the size of pores [8-10] and therefore it can be used just for a qualitative evaluation of the actual pore size distribution. The broader one is assigned to pores whose diameter ranges between 300 and 1000 A. It derives from the amount of nitrogen desorbed in the high pressures region on the isotherm (relative pressures comprised between 0.9 and 1) and is assigned to very large mesopores or macropores generated from further silica condensation on the composite particles. Molecular size of active agents investigated (Diflunisal, 10.3x6.0/~ Ibuprofen 11.5x6.0/~ Naproxen 12.6x7.7 A) are widely compatible with calculated pore diameter so that the influence of different size on dynamic behaviour in mesoporous channels is negligible. The composite sample obtained from synthesis has been calcined at 550~ Mesoporous matrices used for drug loading have been stored at room temperature and atmospheric pressures so that it is reasonable to suppose that silica surface is in a maximum state of hydroxylation and hydration. Progressive heating of sample first removes mukiple layers of physically adsorbed water on silica surface. Dry silica surface is in a maximum state of hydroxylation and isolated singles, geminals and vicinals hydroxyl groups are present. At this stage hydration is totally reversible. Successively the concentration of isolated single OH groups increases and concentration of vicinal OH groups decreases. Water desorption is fast, rehydroxylation is reversible. Further heating completely removes vicinal OH groups while free single and free geminal OH groups still remain on the silica surface. Rehydroxylation is reversible for weakened and strained siloxane bridges (Si-O-Si). Overall amount of OH groups on the surface progressively decreases and concentration of siloxane bridges increases. Siloxane surface is hydrophobic and the reydroxylation rate is very low at room temperature. Hydroxyl population is now not reversibly decreased. This last phenomenon is probably not important for samples calcined at 550~ Thermogravimetric analysis of calcined sample (Figure 5, curve a) shows a weight loss starting at 300 ~ assigned to the condensation of two SiOH groups with loss of a molecule of water. Starting from this assumption a concentration of 1 SiOH group every ca. 4 atoms of Si can be calculated. The surface of silicoaluminate MCM-41 sample is characterized by hydroxyl groups with different acid strength: 1) SiOH groups showing increasing acidity with the increase of their hydrogen bonding interaction; these sites have weak acidity; 2) aluminium atoms form Br6nsted acid sites AI(OH)Si that are due to the presence of hydroxyl groups on aluminium oxide species and Lewis acid sites generated from an electron vacancy on a aluminium atom in a cluster of aluminium oxide. Figure 5 (curve b) shows thermogravimetric analysis of Df-loaded matrix. Three main regions are found: the first, below 200~ is assigned to removal of muki-

1169

600

~

~Adsorpt!on

l

500

~

400 "6 ;> 300

t"

_.=

"~ 200 r~

100

.f ,, ,

4

6 8 10 2 Theta Degrees

1'2

14

Fig. 2. X-ray pattern of calcined MCM- 41

0.0

o12 " 0 1 4 .... 0 ' . 6 " 0 ' . 8 Relative Pressure P/Po

1.0

Fig. 3. N2 adsorption-desorption isotherm at 77 K of calcined MCM-41

8 7-

6-

~32" 1 100 Pore diaraXer (Angstrom)

1000

Fig. 4. BJH desorption pore size distribution. Table 1. Drug 10adingvalues-

Diflunisal Naproxen ~6uprofen.... Ibuprofen sodium salt Drug Loading % (a) 8.7 7.3 6.4 6.9 (a) Amount of drug impregnated (%W/w) with respect to the impregnateci mesoporous matrix. and mono-layer of adsorbed water; a region, between 200 and 620~ assigned to the combined effect of condensation of vicinal silanols, degradation of drug molecules and removal of drug fragments adsorbed on the surface; the last region corresponding to temperatures higher than 620~ corresponds to the dehydroxylation on silica surfaces. Simple soaking procedures were used for the loading of each drug into the matrix using a proper solvent as swelling agent. Therefore the mechanism of the loading of the drug into the matrix involves the occurrence of physic interaction between drug molecule groups and matrix residues. In Table 1 drug loading values for Df, Np, Ib and IbNa are reported.

1170 In order to evaluate the affmity of prepared matrices towards aqueous medium, the value of water content percentage (WC%) was determined in aqueous media which simulate some biological fluids such as gastric (pH 1) and intestinal (pH 6.8) as well as in bidistilled water. In order to understand how the drug presence influences the swelling behaviour, the results obtained from matrices containing Df, Np, Ib and IbNa have been compared with those related to the matrix without any drug. The results of these experiments are reported in Table 2. Table 2. Percentage of water content of free and drug loaded MS matrices Experimental conditions

MS

H20 65.5 pH=l 77.9 pH=6.8 74.7 (") Each measurement is the

% WATER CONTENT (,o Df loaded-MS Ib loaded-MS IbNa loadedMS 59.7 73.7 66.8 68.9 73.9 70.8 64.0 75.4 68.1 average of three experiments.

100

Np loadedMS 74.4 73.0 76.7

(a)

98

(b)

96 94" 92 ~

~

90

88 86 84 82 80

0

100

200

300

400

500

Temperature(~

6()0

700

8000"

' 100 ' 200 ' 300

400 ' 500

600 ' 700

800

Temperature(~

Fig. 5. Thermogravimetric curves of" (a) mesoporous matrix and (b) Df-loaded matrix These results show that the mesoporous matrix has a high water affinity and this makes it potentially biocompatible; furthermore they show that the drugs do not affect the value of WC %. With reference to the release of the drug from the matrix, it is important to outline that many variables, such as physicochemical properties of the drug, hydrophilicity of the mesoporous silica and loading method of the drug, control release kinetics. Figures 6, 7, 8 and 9 show drug release rates expressed as percent of drug delivered, related to the drug loading value, as a function of time (see paragraph 2.2.3). In particular Figure 6 shows that drug release of Df is quite low at pH 1.0, whereas it increases quickly when pH jumps (after two hours) to a value near to the neutrality. On the contrary, drug release of Ib occurs already at pH 1.0 and it seems to be complete within 2 hours; for Np about 75% of the drug is released within 1 hour and the release becomes complete after pH change, while for IbNa drug release starts quickly at pH 1.0, but it is not complete also after pH change and until about 12 hours.

1171 100

i

i

I

t

6

1oo

75

75

so

},0

25

25

0

.

.

.

.

2

4

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.

.

10

12

0 0

.

.

.

.

.

.

2

4

6

8

10

12

Time (hours)

Time (hours)

Fig. 6. Diflunisal release at pH 1 and 6.8 as a function of time 100"

Fig. 7. Ibuprofen release at pH 1 and 6.8 as a function of time

o'|

es

P

a-50.

'l

25"

0 0

.

.

.

.

2

4

6

8

1h~ 0m~)

.

10

.

12

0 0

,

,

,

,

2

4

6

8

,

10

,

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lh~(lms)

Fig. 8. Naproxen release at pH 1 and 6.8 as Fig. 9. Sodium Ibuprofen release at pH 1 and a function of time 6.8 as a function of time Being the average pore size quite greater than the major diameter of all examined drug molecules, these quite different release behaviours seem not to be correlated to the drug ability to go across the pores of the matrix, but different phenomena have to be evoked. In the case of Ib and Np, the strong burst effect should be reasonable explained mainly considering the occurring of surface pore adsorption phenomena; besides this interaction seems not affected by pH changes. On the contrary, in the case of Df the change of pH value markedly increases the amount of dissociated drug molecules whose affinity for the solvent seems to be greater then for matrix groups. It seems reasonable to prevent the release of a drug not soluble in water or acid at the gastric level by means of an inorganic matrix and allowing it in the intestinal treat at neutral pH where the drug solubility increases. Electrostatic interaction between IbNa and matrix groups are probably the responsible for the incomplete release of this drug molecules. 4. CONCLUSIONS The suitability of the mesoporous silicate matrix as drug delivery systems has been evaluated by using different nonsteroid anti-inflammatory agents as model drugs. In particular, it has been shown that this type of matrix is able to trap the bioactive agents by a soaking procedure and, then, to release them in conditions mimicking the biological fluids. Besides the high affinity of these matrices for water make them potentially biocompatible.

1172 Release data suggest that the matrix impregnated with Diflunisal can offer a good potential as system for the controlled drug release. In fact, only 20% of drug is released at the gastric level allowing, in this way, the reduction of side effects related with oral administration of nonsteroidal anti-inflammatory agents and the release of the most part of drug in the intestinal duct. In our opinion that the activation of composite samples drastically influences the adsorption properties of the final porous matrix. In this regard, hydroxyl population, which is the main responsible for the interaction between drug and silicate surface, depends on the activation method. Chemical methods, other than calcination, can be, in fact, used for the removal of organic micelles without any thermal exposure of the sample. Further work is needed to verify if these methods can improve the DDSs properties of the MS samples. ACKNOWLEDGMENTS

The authors thank Prof. Nevio Picci (Department of Chemistry, University of Calabria) for his useful suggestions during the development of this research. REFERENCES

1. 2. 3. 4.

R. Langer; Nature,392 (1998) 5. J.R.B. Brouwers; J. Pharm. World Sci., 18 (1996) 158. E. Methiowitz, Enciclopedia of controlled drug delivery, Wiley, New York, 1999. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck; Nature, 359 (1992) 710. 5. M. Vallet-Regi, A. Ramila, R.P. del Real, J. Perez-Pariente; Chem. Mater.,13 (2001) 308. 6. E.P. Barrer, L. Joyner, P.P. Halenda, J.Am.Chem. Soc.,73 (1951) 373 7. A.Corma, Chem. Rev., 97 (1997) 2373 8. P.I. Ravikovitch, S.C.O. Domhnaill, A.V. Neimark, F.Scuth and K.K. Unger, Langmuir, 11 (1995) 4765 9. M.W. Maddox, K.E.Gubbins, Int. J. Thermophys.,15 (1994) 1115 10. C. Lastoskie, K.E. Gubbins and N.Quirke, J.Phys. Chem., 97 (1993) 4786

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1173

Aluminum Incorporation and Interfacial Structures in AISBA-15 mesoporous solids- double resonance and Optically Pumped Hyperpolarized 129Xe N M R Studies. Elias Haddad a, Jean-Baptiste d'Espinose b, Andrei Nossov a, Flavien Guenneau a, Antoine G6d6ona* Laboratoire Syst6mes Interfaciaux h l'Echelle Nanom6trique (SIEN). CNRS-FRE 2312 aUniversit6 Pierre et Marie Curie, case courrier 196, 4 place Jussieu, 75252 Paris Cedex 05, France. Email : [email protected] b Physique Quantique, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France Aluminum-incorporated SBA-15 mesoporous materials have been obtained by direct synthesis. The surfactant- aluminosilicate interaction during synthesis was studied by double resonance NMR and confronted with the structural properties of the materials obtained after calcinations. Continuous-flow laser-polarized ~29Xe NMR spectroscopy was applied for the first time to explore the porosity of the A1SBA-15 mesoporous molecular sieves. TRAPDOR experiments firmly established a strong interaction between segments of the PEO block of the surfactant with the silica-alumina framework. ~H Dipolar Dephasing revealed that the amount of segments rigidified by this interaction increased with the maturation time. The increased rigidity of the surfactant is to be linked with the increased mesoscopic ordering during maturation, resulting in the higher mesoporous surface obtained after calcinations. The invariability of the TRAPDOR effect proved that the strength of the interaction, that is the degree of interpenetration of the organic/inorganic phases remained the same irrespective of maturation time. Together with the dramatic decrease of the microporous volume with maturation time, this established that the origin of the microporosity of A1SBA-15 is to be found in the incomplete hydrolysis of the TEOS precursor itself rather than in the incomplete PEOaluminosilicate phase separation. 1. INTRODUCTION We have synthesized acid A1SBA-15 mesoporous solids with regular channels and very high thermal and hydrothermal stability [ 1]. Incorporation of A1 was established by HETCOR double resonance l H - 27A1 NMR [2]. A1SBA-15 materials retain the hexagonal order and physical properties of purely siliceous SBA-15. They present higher thermal stability and catalytic activity in cumene cracking reaction than A1MCM-41 solids. To better understand the origin of these improved properties, textural results from N2 porosity measurements are confronted with molecular scale double resonance MAS

1174 NMR results in order to discuss the incorporation of A1 and the interpenetration of the organic/inorganic phases during synthesis. Indeed, recent publications have evidenced the significant occurrence of a microporous "corona" around the internal surface of the mesopores in SBA-15 [3,4]. Considering that micropores result from the calcination of an incompletely hydrolyzed silicate precursor, it is of primary importance, if one wants to be able to control the extent of the microporosity, to understand at the molecular scale why the silicate network did not fully condense: Is it because of the interpenetration of the hydrophilic part of the surfactant with the forming inorganic phase [3]? Or is it because the organometallic TEOS precursor was not fully mineralized prior to calcination? To address this question, samples of different hydrolysis levels were prepared by varying the maturation time. It was then possible to investigate phase separation and ordering in the parent material by NMR double resonance between the organic protons and the aluminum of the solid, the results were then related to the structural properties of the calcined final mesoporous A1SBA-15. 2. EXPERIMENTAL 2.1. Materials and synthesis Al-containing SBA-15 mesoporous solids were synthesized by using tetraethyl orthosilicate (TEOS), aluminum tri-tert-butoxide, and triblock poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (EOEoPO70EO20) Pluronic 123 copolymers. The synthesis conditions were described elsewhere [ 1]. After being stirred for 3 hours the gel solution was transferred into a Teflon bottle and heated at 100 ~ for different reaction or maturation times 0, 16 and 48 h. The solid products were filtered (parent composites) and finally calcined (calcined samples) in air flow (9 L h-1) at 823 K for 4 h with a heating rate of 24 K h ~. In what follows, the samples are denoted A1SBA-15. 2.2. Hyperpolarized 129XeNMR 129Xe NMR spectra were collected on a Bruker AMX 300 spectrometer operating at 83.03 MHz. Hyperpolarized (HP) xenon was produced in the optical pumping cell in the fringe field of the spectrometer magnet. The gas mixture containing 800 torr of He and 40 torr of Xe polarized to ca. 1% was delivered at 70 cc/min flow rate to the sample via plastic tubing. 256 FIDs were accumulated with 10~ts (n/2) pulses and 5s delays. 2.3. MAS-NMR Magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a Bruker ASX500 spectrometer at 11.7 T. 27A1one-pulse experiments were performed at 14 kHz with a selective pulse (<x/6) duration of 0.5 ps, recycle time 1 s, and 5000 acquisitions. ~H one-pulse experiments were performed at 13 kHz with a n/2 pulse duration of 5.5 ~ts, recycle time 20 s, and 8 acquisitions. The 1H to 27A1 Cross-Polarization (CP) pulse scheme (Fig. la) used to establish 2D heteronuclear correlation (HETCOR or WISE) [5] was the usual one. The experimental conditions for CP to an abundant quadrupolar nuclei were determined by using the same considerations developed in our study of alumina surfaces by 1n to 27A1 CP [6]. The

1175 spectra were obtained with radio frequency magnetic field strengths (,Q/2rc) of approximately 44 kHz for 1H and 90 kHz for 27A1, and a spinning frequency (vr) of 4 kHz. The JH{a7A1} TRAPDOR pulse scheme is given in Fig. lb [7]. The dipolar dephased lH spin echo is recorded after evolution at different integer multiples of the rotation period (So) and compared to the spin echo obtained under the same conditions but with additional dephasing due to irradiation of Z7A1during half of the evolution period (S). The 27A1 TRAPDOR dephasing of the ~H signal (So-S)/So is then plotted as a function of the evolution time. The radio frequency fields were 45 kHz and 140 kHz for 1H and 27A1. n/2

1H

~1

n/2

rt

1H Aq

27A1

]

CP

I t~ V

27A1

Aq "-

Fig.la. Scheme of the HETCOR pulse Fig. lb. Scheme of the TRAPDOR sequence, pulse sequence.

2.4. Structural characterization N2 adsorption/desorption isotherms were measured on a Micromeritics ASAP-2000 instrument at liquid N2 temperature. Specific surface areas were calculated from the adsorption isotherms by the BET method, and pore size distributions from the desorption isotherms by the BJH method. To assess the presence of micropores, we used the modified c~s-plot analysis [8]. 3. RESULTS AND DISCUSSION

3.1. Structural properties The structural properties of A1SBA-15 samples have been analyzed in earlier studies by Synchrotron XRD and transmission electron microscopy [9,10]. The five wellresolved XRD peaks associated with 2D p6mm hexagonal symmetry and the well ordered hexagonal arrays of mesoporous channels from TEM images indicate the high structural quality of these materials. The nitrogen adsorption-desorption isotherms of the A1SBA-15 studied samples have been used to obtain information about their mesoporosity. These isotherms, illustrated in figure 2, are similar to those reported earlier [2]. As the maturation time decreased, the N2 adsorption isotherms preserved the same shape with a significant decrease of the hysteresis loop towards lower P/P0 values. This led to smaller surface area and mean pore diameter. The pore size distributions (PSD) for the studied samples are shown in figure 3. They indicated that the mesoporous channels were very regular with a narrow gaussian distribution centered at 4.5, 7.0, and 8.2 nm respectively for samples at 0, 16 and 48 h. of maturation time. The textural characteristics of the A1SBA-15 samples are listed in table1.

1176 The adsorption-desorption isotherms have been ft~her used to evidence an additional microporosity. The specific mesoporous volume and the micropore volume were calculated using modified as-plots. The micropore volume evolved inversely to the maturation time. 48 h

0,10

48 h

1000

0,08 800

16h ~

00

O >

0,06

3

9

rabon

400

"~

"E

o,o4

no maturation

G) O

i5

<

0,02 200

0,00

0 I

o,o

.

oi~

.

.

oi,

.

o',~

oi~

relative pressure (P/Po)

~io

9

!

20

9

!

9

i

9

i

9

!

9

!

,

40 6o 80 loo 12o 14o pore diameter (A)

,

Fig. 2. N2 adsorption-desorption isotherms Fig. 3. BJH pore size distribution of for A1SBA-15 samples for different A1SBA-15 samples for different maturation maturation times, times. Table 1 Structural-textural parameters of the A1SBA- 15 samples studied. Maturat ion SBeT P ore P ore micro pore time (m2/g) a volume diameter volume b (cm3/g)

(nm)

Vmi ( c m 3 / g )

Mesopore volume b

Vme(Cm3/g)

none

525

0.60

4.5

0.045

0.536

16 h

648

0.96

7.0

0.010

0.696

48 h

908

1.41

8.3

0.002

0.972

a) total pore volume estimated from the amount adsorbed at P/P0 = 0.99 b) micropore and mesopore volumes evaluated from the modified tx -plot method.

1177 Figure 4 shows X-ray diffraction patterns of samples prepared with different maturation times. While the samples maturated at 16 and 48 h show well-resolved XRD peaks associated with 2D p6mm hexagonal symmetry, a significant decrease of the intensities of the higher order (110) and (200) reflections is observed for the non maturated sample. This shows the influence of maturation time on the structuring order of these materials as established also by nitrogen measurements of pore size distribution and hyperpolarized 129Xe N I V I R

3.2. Probing the evolution of poroisity during maturation time by hyperpolarized (Hp)129Xe NMR Figure 5 shows the t29Xe NMR spectrtra of HP xenon adsorbed on the A1SBA-15 samples. Both spectra exhibit the lines at 0 ppm from xenon gas in the voids between the particles of the solids and the lines shifted to lower field due to xenon, adsorbed in the pores. In the case of the nonmatured sample the latter part of the spectrum consists of two lines: at 86.6 ppm and a broader signal at ca. 65 ppm. Maturation during 48 hours leads to the spectrum, comprised of two lines: at 65.6 ppm and a low intensity broad signal, centered at ca. 60 ppm. Chemical shift of xenon in mesoporous materials depends on the pore size, the smaller value of the chemical shift corresponding to the bigger pore diameter [11]. Taking this into account, the ~29Xe M R data are in a good qualitative agreement with the pore size distribution observed for the samples during maturation. The higher value of the xenon chemical shift and broader lines observed in the spectrum of the nonmatured sample corresponds well to its smaller pore diameter and broader size distribution (Fig.3). The results of more detailed study of xenon adsorption in this system will be published elsewhere.

-

•1

/

48 h

tion

,,,,,,, 0

1

3

2

Oh 16h ...............~ 48 h

4

l

,

!

120

,

I

,

I

80

,I

I

!

I

'

40

I

,

!

0

,

!

'

I

-40

(ppm)

2O Fig. 4. XRD patterns of A1SBA-15 A1SBA-15 samples for different maturation times.

Fig. 5. 129Xe ~ spectra of HP Xe adsorbed on the nonmatured (lower trace) and matured for 48h (upper trace) A1SBA- 15 samples

1178 3.3. Speciation of Aluminum: one-pulse and 27AI{IH} HETCOR of AISBA-15

In one-pulse 27A1 NMR, three resonances appeared: IVA1 at 52 ppm, VA1 at 38 ppm, and VIA1 at 0 ppm (not shown). However, only the tetrahedral sites were part of the aluminosilicate framework as the other resonances disappeared upon mild washing by NH4C1 solutions after calcination (Fig. 6, above the F2 axis). On the same NH4+ exchanged sample, two correlations were established by HETCOR between the IrA1 resonance and F1 resonances at 4-5 ppm and 7 ppm. The former corresponded again to the protons of physisorbed water and possibly of unresolved hydrogen bound silanols, but the latter corresponded to a correlation with the newly introduced protons of the NH4 surface moiety. The proximity of tetrahedral alumina to the surface charge balancing ammonium cations proved that IrA1 occurred as A13§ substitutions of Si4§ in the oxide framework. Furthermore, it confirmed that at least part of this IrA1 is located near the surface. Incidentally, ammonium exchange also allowed for the identification of the octahedral aluminum species. The octahedral resonance almost totally disappeared from the quantitative one-pulse spectra. This signified that it belonged to an extra-framework charge balancing cationic species and, therefore, that no alumina (with IVA1 as well as WA1) phase was formed apart from the (A1)TM containing aluminosilicate. t~v

............

...........

~

~

.

.................

I

t

0

I

-I-

5 o

tt

10

0

o

0

100

E

O.

15

~

50 0 ppm 2rAI

-50

Fig. 6. 27Al{1H } WISE and one-pulse spectra of A1SBA-15 matured 48 h, calcined, and washed by NH4C1. Contour lines are drawn at 15, 30, 45, 60, 75, and 90% of the maximum. Contact time (tcp)" 1.5 ms; number of acquisitions in t:: 1352; tz increments: 61 times 15 ~ts.

1179 3.4. Molecular scale interaction with the template: 1H{27Al} TRAPDOR of parent AISBA-15 While the 27A1 spectra were identical irrespective of maturation time, the 1H resonances broadened during maturation resonances. As the template rigidified, the mesoporous surface area of the forming aluminosilicate increased. The rigidification therefore reflected the larger interaction of the PEO blocks with the larger aluminosilicate mesoporous surface. However, this could also be due to an increased interpenetration between the organic polymer and the aluminosilicate network as the I + cations condense around the S+ chains interacting through the S+CI-I+ mechanism [12]. This would translate eventually also in a stronger interaction as the PEO fragments become imbedded within the forming aluminosilicate. To evidence such an increase in the strength of the interaction, the TRAPDOR effect on the CH and CH2 protons of the surfactant due to dephasing by the 27A1 of the aluminosilicate was quantitatively followed for samples at different maturation time. It appeared that the TRAPDOR effect and thus the strength of the organic-inorganic interaction remained the same irrespective of maturation time and of the microporous volume of the final A1SBA-15 material. (Fig. 7). 0,7-

0,6

~"

~o v r

.O

0,5

0,4

0 a

o,3

I--

0,2

=:

9

: no m a t u r a t i o n

9

:6h

9

:16h

9

:48h

212

0,0

0,0

,

!

0,5

,

i

1,o

'

!

1,s

,

(ms)

!

2,o

,

i

2,2

,

i

3,0

Fig.7.1H{27AI} TRAPDOR dephasing (see Fig. lb) as a function of the 27A1irradiation time of the CH and CH 2 protons A1SBA-15 PEO-PPO-PEO parent composites synthesized with different maturation times. Amplitudes were obtained from integration o f the two resolved lines o f the ~H spectra. 4. CONCLUSION The increased rigidity of the PPO-PEO-PPO backbone evidenced by the increasing fraction of protons in strong homonuclear interaction must be attributed to the forming mesoporous surface and thus to a larger amount of PPO fragments interacting with it.

1180 Actually, the microporous volume decreased drastically with maturation time as hydrolysis proceeded therefore suggesting that it resulted instead from incomplete condensation due to the TEOS not being fully hydrolyzed REFERENCES

Y. Yue, A. G6d6on, J.-L. Bonardet, N. Melosh, J.-B. d'Espinose, J. Fraissard, Chem. Commun. (1999) 1967. J. B. d'Espinose de la Caillerie, Y.-H. Yue, E. Haddad, A. G6d6on, Stud. Surf. Sci. Catal. 135 (2001) 1321. M. Kruk, M. Jaroniec, C. H. Ko, R. Ryoo, Chem. Mater. 12 (2000) 1961. 4. M. Imp6ror-Clerc, P. Davidson, A. Davidson, J. Am. Chem. Soc. 122 (2000) 11925. 5. K. Schmidt-Rohr, H. W. Spiess, Multidimensional Solid-State NMR and Polymers, Academic Press, San Diego, 1994, p. 213. D. Mertens de Wilmar, O. Clause, J.-B. d'Espinose de la Caillerie, J. Phys. Chem. B 102 (1998) 7023. C. Grey, A. J. Vega, J. Am. Chem. Soc. 117 (1995) 8232. 8. V. B. Fenelonov, V. N. Romannikov, A. Y. Derevyankin, Microp. Mesop. Mater. 28 (1999) 57. Y.-H. Yue, A. G6d6on, J.-L. Bonardet, J. B. d'Espinose, J. Fraissard, Stud. Surf. Sci. Catal. 130 (2000) 3035. 10. Y.-H. Yue, A. G6d6on, J.-L. Bonardet, J. B. d'Espinose, N. Melosh, J. Fraissard, Stud. Surf. Sci. Catal. 129 (2000) 209. 11. V.V. Terskikh, I.L. Mudrakovskii and V.M Mastikhin., J.Chem.Soc. Faraday Trans. 89, (1993), 4239 12. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. .

,

.

.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1181

Tailoring the pore size of hexagonally ordered mesoporous materials containing acid sulfonic groups R. van Grieken, J.A. Melero and G. Morales Environmental and Chemical Engineering Department. ESCET. Rey Juan Carlos University. 28933 Mostoles. Madrid. Spain. [email protected] The pore size of sulfonic modified mesoporous materials has been tailored through a direct synthesis procedure under a wide range of reaction conditions and using different nonionic surfactants. The resultant materials displayed pore sizes ranging from 30/k to 110A and acid exchange capacities up to 1,2 meq H+/g SiO2. 1. INTRODUCTION The heterogenisation of catalytic homogeneous systems is currently considered as a priority research field. Serious research efforts are addressed to the development of heterogeneous solid acid catalyst in order to avoid the use of traditional homogeneous acid catalytic systems (H2SO4, HF, A1C13, BF3 .... ) which present serious drawbacks including hazards in handling, corrosiveness, production of toxic waste and difficulties in separation. Zeolites have been widely used in acid catalysed processses 1 but unfortunately their limited pore size (< 8 /k) hinders the access of bulky substrates. In this context novel silica-based ordered mesoporous materials such as MCM-41 and SBA-15 are being widely used as inorganic supports of acid active species 2. Sulfonic-acid functionalised mesoporous silica 3-4, which are solid Br6nsted acids, have been recently synthesised and successfully used in different acid-catalysed reactions, including esterifications and condensations 5-6. In contrast with postsynthetical methods 3, a direct synthesis method has been recently developed to create periodic ordered sulfonic-functionalized mesostructures using Pluronic 123 as the templating surfactant in acid medium 4. This new procedure involves an one-step synthetic strategy based on the co-condensation of tetraethoxysilane (TEOS) and MPTMS, in the presence of Pluronic 123 species and H202 in HC1 aqueous solutions. This approach allows the in-situ oxidation of thiol groups and consequent acid exchange of the formed sulfonic groups. Since steric constraints imposed by the pore size of the acid solid influence the reaction pathway resulting in "shape-selective catalysis", in this contribution we have conveniently modified the synthesis conditions in order to tailor the pore size of these sulfonic modified mesoporous materials using this novel strategy of synthesis. Likewise, this one-step procedure has been generalised for the synthesis of sulfonic-functionalised mesoporous silica using nonionic surfactants other than Pluronic 123. The financial support of the CAM, through the Program contract "Grupos Estrat6gicos de Investigaci6n" (Project 07M/0050/1998) is gratefully acknowledged.

1182

2. EXPERIMENTAL SECTION 2.1 Sample Preparation. Sulfonic modified mesoporous materials using P123 as polymeric template. Materials were synthesised as follows: Pluronic 123 (Aldrich) was dissolved with stirring in 125 g of 1.9 M HC1 at room temperature. The solution was heated up to 40~ before adding TEOS (Aldrich). Prior to the addition of the thiol precursor (MPTMS, Aldrich) and the oxidising agent (aqueous solution of H202, 30 wt. %, Merck), a prehydrolysis time was fixed. The resulting mixture was stirred at 40~ for 20 h and aged at 100~ for additional 24 h under static conditions. The molar composition of the mixture was: 0.0368TEOS: 0.0041MPTMS: 0.0368H202: 0.24HCl:=6.67H20. After synthesis, the solid product was recovered by filtration and air-dried overnight. The template was removed from the as-synthesised material by washing with ethanol under reflux for 24 h (2 g of as-synthesised material per 200 ml of ethanol).

Sulfonic modified mesoporous materials using non-ionic surfactants other than P123.

Other non-ionic surfactants were used (PL64, EO13PO30EO13; Brij56, C16EO10; Brij76, C18EO~0, all of them supplied from Aldrich). The molar composition of the mixture was similar to that used for the Pluronic 123. However, the dissolving conditions were changed in order to obtain a clear micellar solution before adding the silicon source: Brij56 was stirred for 30min at 50~ Brij76 was stirred 3 h at 50~ and the PL64 was dissolved almost instantaneously at room temperature. Tailoring of the pore size. Different experimental conditions were tested with the purpose of modifying the pore size of the sulfonic modified materials including: different TEOS hydrolysis times, TEOS/surfactant molar ratios and ageing conditions. Additionally, swelling agents such as TMB (1,3,5-trimethylbenzene) and n-decane have also been added to the mixture in different concentrations.

2.2 Sample Characterization. Nitrogen adsorption and desorption isotherms at 77 K were measured using a Micromeritics TRISTAR 3000 system. The data were analysed using the BJH model and the pore volume (Vp) was taken at P/Po= 0.989 single point. X-ray powder diffraction (XRD) data were acquired on a PHILIPS X'PERT diffractometer using Cu K~ radiation. The data were collected from 0.6 to 4 ~ (20) with a resolution of 0.02 ~ Ion-exchange capacities of the sulfonic mesoporous materials were determined using aqueous solutions of sodium chloride (NaC1, 2M) as exchange agent. In a typical experiment, 0.05g of solid was added to 10g of aqueous solution containing the salt. The resulting suspension was allowed to equilibrate and thereafter titrated potentiometrically by dropwise addition of 0.01 M NaOH (aq). Sulphur content was determined by means of Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) analysis collected in a VARIAN VISTA apparatus. Transmission electron microscopy (TEM) microphotographs were carried out on a JEOL 2000 electron microscope operating at 200 kV.

1183 3. RESULTS AND DISCUSSION

3.1 Sulfonic-Functionalised Mesoporous Silica Using Pluronic 123. Tables 1-4 summarise the preparation conditions as well as the physicochemical and textural properties of the sulfonic modified mesoporous materials using Pluronic 123 as surfactant. Thickness of the silica walls was calculated by a o - pore size (ao = 2dloo~/3). Acid capacity and sulphur content are defined per g of dried sample. Firstly, it is important to note that most of the samples synthesised show a close agreement between the ion-exchange capacities measured by acid titration and sulphur loading determined by ICP-AES independently of the synthesis conditions. This is a clear evidence that most of the sulfonic groups are located on the pore wall and are accessible and useful for adsorption and catalytic reaction processes. The influence of the different synthesis conditions on the pore size was as follows: Influence of swelling agent. TMB and n-decane have been used to increase the pore diameter of the sulfonic modified mesoporous material. The swelling effect of the n-decane is slightly higher than that observed using TMB. In contrast with the results reported by other authors 7 for silica based mesoporous materials the presence of cosolvent organic molecules expands slightly the pore size. The in-situ formation of sulfonic moieties apparently produces hydrophilic/hydrophobic interracial conditions that do not promote expanding of the hydrophobic core of the PEO-PPO-PEO copolymer blocks in presence of swelling agents. Moreover, the presence of TMB seems to have a negligible influence on the thickness of mesoporous walls, whereas the increase of n-decane yields significant lowering of the wall thickness. Influence of ageing conditions. Enlargement of the hydrothermal treatment promotes higher pore sizes for the sulfonic modified materials. Alternatively, increasing of temperature leads to an increase of unit cell size and the pore diameter, but the wall thickness decreases with the temperature. Likewise, sulfonic modified mesoporous materials after hydrothermal treatment at 170 ~ for 24 h become slightly amorphous showing a poorly ordered XRD patterns. Table 1. Influence of swellin~ a~;ent on the physicochemical and textural properties. Textural properties Sample

Parameter

dlo0 (,&)

Dp (A)

SBET V Wall t--g)lm21"" ~ (cm3P/g), Thickness

Acidity mmol H+/g

S content mmol S/g

(A) 1 0 97 82 666 1.23 30 1.16 1.20 2 0.25 108 91 663 1.20 33 1.13 1.08 3 0.5 107 89 708 1.27 35 1.17 1.12 4 1 109 90 621 1.01 36 1.23 1.08 5 0.5" 100 95 691 0.64 21 1.02 1.02 6 0.05* 95 95 726 1.27 15 1.07 1.16 Tlae parameter is the swelling agent/surfactant mass ratio.* indicates that the swelling agent is n-decane, otherwise is TMB. The synthesis hydrolysis conditions were 40~ for 20 h and the ageing conditions were 100~ for 24 h. The TEOS/surfactant molar ratio was 55.

1184 Table 2. Influence of a~ein~; conditions on the ph~,sicochemical and textural properties. Textural properties Wall Acidity S content Sample Parameter dl00 (~p) SBET V~ Thickness mmol I-F/g mmol S/g (/~) (mE/g) (cm/g) (~) 7 24/80 90 70 772 0.96 34 1.05 0.97 8 72/80 88 81 836 1.26 21 1.10 1.03 9 24/60 79 37 484 0.43 54 1.07 1.04 10 24/170 101 110 441 1.35 7 0.84 0.78 The parameter indicates ageing conditions: time in hours / temperature in ~ The hydrolysis conditions were 40~ for 20 h and the TEOS/surfactant molar ratio was 55. No swelling agent was used. Table 3. Influence of TEOS/surfactant ratio on the physicochemical and textural properties. Textural properties Wall Acidity S content Sample Parameter dl00 (Dkp) SBET V3P Thickness mmol H+/g mmol S/g (X) (rnE/g) (cm/g) (A) 11 90 94 68 719 0.87 40 1.13 1.03 12 120 96 54 647 0.63 57 0.92 0.98 The parameter is TEOS/surfactant molar ratio. The synthesis hydrolysis conditions were 40~ for 20 h and the ageing conditions were 100~ for 24 h. No swelling agent was used. Table 4. Influence of h~,drol~,sis time on the ph~,sicochemical and textural properties. Textural properties S content Sample Parameter dl00 Do SBET V Wall Acidity (~) (A) (rn2/g) (cm~/g) Thickness mmol H+/g mmol S/g

(A)

13 2 92 71 764 1.04 35 0.42 0.72 14 4 95 83 742 1.31 27 1.20 1.13 15 10 96 77 745 1.16 34 1.13 1.09 The parameter is the hydrolysis time in hours. The TEOS/surfactant molar ratio was 55 and the ageing conditions were 100~ for 24 h. No swelling agent was used. Influence of TEOS/surfactant molar ratio. The increase of TEOS species promotes more silica condensation and a decrease of pore size with and enlargement of wall thickness as shown in Table 3. Influence of hydrolysis time. XRD and adsorption results demonstrate that the mesostructure of the triblock copolymer-silica complex is just assembled within 2 h. However, this time is low for the complete condensation of sulphur species as evidenced by the low acidity and sulphur content of this sample (Table 4; sample 13). After four hours of hydrolysis, the sulphur species are almost completely incorporated into the structure.

Figure 1 (A) and (B) depict the nitrogen adsorption/desorption isotherms and pore size distributions of samples synthesised using Pluronic 123 as polymeric template under different conditions. Under the particular conditions checked in this work, we have expanded the pore size of this sulfonic modified materials from 40 tol00 A obtaining relative high surface areas (600-750 m2/g; Tables 1-4). The combination of high surface areas and the presence of accessible Br6nsted acid sites provide these materials with good potential catalytic properties.

1185 1000

A

+-

800,

Sample 1

14

/

^.::~_~#~

....~ .... Sample 10

IX. I-- 600, 09

.

200,

-

-

.fa'"

Sample 12 Sample 10

o

9@ %,an~ ,~

L ~iA~ ~-id~~

E

o

10

--<~-- Sample 12

400

B

12

~

6

~ "o

4

Sample 1

2 0

0

,

0.0

,

0:2

,

0'.4

P/Po

0:6

0:8

1.0

10

O0

Pore Diameter (A)

1000

Figure 1. (A) N2 adsorption/desorption isotherms and (B) Pore size distributions of sulfonicftmctionalised mesoporous silica using Pluronic 123. Figure 2 shows TEM images of two samples synthesised under different conditions (Samples 1, Table 1; and Sample 11, Table 3). These images confirm the mesoscopic order of the materials where it is evident the hexagonal array of uniform channels with the typical honeycomb appearance of SBA-15 materials 7.

3.2 Sulfonic-Functionalised Mesoporous Silica using non-ionic templates other than P123. Table 5 summarises the preparation conditions as well as the physicochemical and textural properties of the sulfonic modified mesoporous materials prepared with Brij56, Brij76 and PL64 as surfactants.

Figure 2. Transmission Electron Microscopy images for samples 1 (A) and 11 (B), in the direction parallel and perpendicular to the pore axis.

1186 Table 5. Physicochemical and textural properties of sulfonic modified materials prepared usin~ Pluronic PL64, Bri)56 and Bri)76 as surfactants Synthesis conditions Textural properties Ageing TEOS/ TMBa d D b SBET V Wall c Acidityd S contente Sample t T Surf. / Surf. (~) (~) mE/g cmJ'/g Thickness mmolH§ mmolS/g (h) (~ (A) Pluronic L64 (EO13/PO30/EO13) 16 24 100 27 0 74 48 725 0.68 33 0.43 0.82 Brij 56 (C16EOlo) 17 24 100 6 0 57 34 763 0.83 31 0.93 0.99 Brij 76 (C18EOlo) , 18 0 7 0 53 30 718 0.51 31 1.06 1.10 19 24 80 7 0 56 38 710 0.87 27 1.15 1.22 20 24 100 7 0 60 38 678 0.81 31 1.23 1.28 21 24 100 7 0.5 60 38 713 0.85 31 1.16 1.09 22 24 100 7 1.0 58 47 745 1.11 20 1.17 1.09 23 24 100 12 0 58 37 764 0.83 31 0.92 1.04 24 24 100 20 0 60 34 779 0.76 35 1.24 1.04 TMB/Surfactant mass ratio, b'Calculated from the adsorption branch, c Calculated by a o - pore size (ao 2dwov/3). d Acidity obtained from ion exchange and titration data and defined per g of dried sample, Sulphur content obtained from ICP-AES measurementsand defined per g of dried sample. Hydrolysis conditions were 40 o C during 20 hours for sample 18 (PL64) and 50 ~ during 20 hours for the other samples (Brij 56 and 76). a

e

Pluronic L64 as surfactant. Figure 3 (A) depicts the pore size distribution whereas Figure 3 (B) illustrates the XRD pattern corresponding to the sulfonic modified material after surfactant removal and its comparison with sample 1 synthesised in presence of Pluronic 123. As expected, a lower molecular-weight block copolymer gives a smaller pore system. Nevertheless, a clear broad distribution of pore sizes is evidenced, which reveals a low ordered material. This lack of mesoscopic ordering is also evidenced from XRD diffractogram where a unique peak is observed with complete absence of high ordering reflections typical of materials synthesised using Pluronic 123. The incorporation of the sulphur species is lower to that obtained using Pluronic 123 and, more important, the acid capacity disagrees with this content evidencing limited accessibility of sulfonic sites which might be occluded within the walls. Oiigomeric alkyl-ethylene oxide surfactants: Brij 56 and 76. All the synthesis conditions tested using Brij56 or Brij76 as surfactants yielded similar organic degree incorporation than that achieved using Pluronic 123. Likewise, these materials show a close agreement between sulphur content and acid capacity indicating the high accessibility of the acid groups. Figure 4 (A) shows the pore size distribution whereas Figure 4 (B) shows X-ray pattern corresponding to sample 17, and its comparison with sample 1 synthesised in presence of Pluronic 123. This organically modified material displays a well-defined pore size distribution with a mean pore size of 34/~, which is significantly lower than the pore size obtained using Pluronic 123. This is consistent with the size of the hydrophobic block of the template: the smaller the hydrophobic block, the lower the size of the micelle core and the lower the resultant pore size. X-ray pattern of this sample prepared with Brij56 evidenced a clear signal at 20 = 1.51 and two weak long ordering reflections confirming the good mesoscopic ordering of the material.

1187

4

A

~97 ,~,

P123

il-

"-" >,,

PL64o ~

I

"~ o

B

xl

P123

E

l 0

o

~oo

Pore D Jam eter (,~)

~ooo

1:o

lls

21o

2:s 20

a:o

ai~

4.0

Figure 3. (A) Pore size distribution and (B) X-ray diffraction pattern of sulfonic-functionalised mesoporous silica using Pluronic L64.

A 4

E3 o 'o

>

"O

[]

B

]~ d=57A

5

P 123

d=97/~/~

3

X5

2

010

E

Brij56

X5

L

r'l100, _

P123 , ._.~ ~.Z,^.. ,~-

Pore D i a m e t e r (,~)

1000

1:0

1:5

2:0

2:5

20

3:0

3:5

4.0

Figure 4. (A) Pore size distribution and (B) X-ray diffraction pattern of sulfonic-functionalised mesoporous silica using Brij56. The pore size of the sulfonic modified materials synthesised with Brij 76 has been modified using analogous strategies of synthesis than those described before for Pluronic 123. The enlargement of ageing time as well as an increase of temperature yielded an increase of pore size. However, the effect was less pronounced than that monitored using Pluronic 123 as surfactant. Likewise, the addition of a swelling agent such as TMB, leads to a moderate increase of the mean pore size up to 47/k for a TMB/surfactant mass ratio of 1. The increase of TEOS/surfactant ratio promoted more silica condensation and a decrease of pore size with and enlargement of wall thickness. Figure 5 (A) shows the tailoring of the pore size of these materials under different synthetical conditions. These materials display well-defined poresize distributions with mean pore sizes ranging from 30 to 47/k and high surface areas (600800 mZ/g). Finally, the good mesoscopic ordering of these materials is evidenced in the XRD diffractograms depicted in Figure 5 (B) and confirmed by the TEM images showed in Figure 6.

1188 8

o zx n

7

6"6 v

Sample 18 Sample 20 Sample 22

A

I

o~ 5

Sample 18

>,

"~

"O 4

"t~ 3

~=58 ~,

r-

2

Sample 22 d 60 A

1 0

,

o

,

Sample 20

,

Pore Diameter (,~,)

loo

1.5

2.0

2.5

3.0

315

4.0

20 Figure 5. (A) Pore size distributions and (B) X-ray diffraction patterns of sulfonic-functionalised mesoporous silica using Brij76.

!iii!i,li!i i Figure 6. Transmission Electron Microscopy images for sample 22, in the direction parallel and perpendicular to the pore axis. 4. CONCLUSIONS An one-step procedure has been generalised for the synthesis of sulfonic-functionalised mesoporous silica using non-ionic surfactants other than Pluronic 123. Pore size of sulfonic modified mesoporous materials prepared using non-ionic templates has been tailored using a judicious choice of TEOS/surfactant molar ratio, prehydrolysis time, duration and temperature of ageing process and the use of swelling agents. Under the particular conditions tested in this work, the pore sizes of the resultant sulfonic modified mesoporous materials were tailored from 30/k to l l0A which provides them with an enlargement of the catalytic potential applications. REFERENCES 1. 2. 3. 4. 5. 6. 7.

A. Corma, Chem. Rev., 95 (1995) 559. K. Wilson and J.H. Clark, Pure Appl. Chem., 72 (2000) 1313. M.H. Lim et al., Chem. Mater., 10 (1998) 467. D.Margolese et al., Chem. Mater., 12 (2000) 2448. C.E. Fowler et al., J. Chem. Soc. Chem. Commun., (1998) 1825. M.H. Lim et al., Chem. Mater., 11 (1999) 3285. D. Zhao et al., J. Am. Chem. Soc., 120 (1998) 6024.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1189

Novel Vesicular Mesoporous Material Templated by Catanionic Surfactant Self-assembly* X.W. Yan and J.H. Zhu** Department of Chemistry, Nanjing University, Nanjing, 210093, China

A novel templating approach to the synthesis of vesicular mesostructured silica is demonstrated. Based on vesicular self-assembly formed in dilute cationic and anionic surfactants solution, and the hydrolysis and condensation of silicon alkoxide precursor on the interface of inorganic and organic species, this method is a simple and economic way to obtain vesicle-like mesostructure. The resulting vesicular mesoporous material is a good candidate in catalysis and separation as well as nanoscale devices.

1. INTRODUCTION On the beginning of 1990's, a series of ordered mesoporous materials denoted as M41S and constituted usually by siliceous framework, was developed by the scientists of Mobil Company [1,2]. The principles of the synthesis strategies are interfacial interaction between inorganic (compose framework) and organic (template) species, and wet chemistry of mesostructured inorganic-organic composite. Emergence of M41S not only realized the desire to ordered mesoporous materials with tunable mesopore, but also gave rise to worldwide interests in obtaining mesoporous inorganic materials by templating of amphiphilic molecule assembly [3,4]. Among the great efforts to develop inorganic-organic composites with new mesophases since then, three processes might be classified, i.e., electrostatic attraction between ionic inorganic and organic species [1,2], hydrogen bond between both neutral inorganic precursors and surfactants (e.g. primary amine [5], alkyl polyether (PEO) [6]), and template of polymer surfactant to ultra large mesoporous (5-30 nm pores) materials [7]. In these processes, the templates would spontaneously form liquid crystal phases in aqueous solutions, which were the counterparts of the corresponding *This work was funded by key laboratory of chemical engineering and technology of Jiangsu province. ** Corresponding author, E-mail: [email protected], FAX: 0086-25-3317761.

1190

mesostructured inorganic-organic composites consequently. Vesicular mesophase is one of the important supramolecular self-assemblies of surfactants. The study on vesicles has attracted more and more attention recently [8,9], since vesicles can be used as drug delivery and gene therapy vehicles [ 10], even for humans as well as for a surprisingly wide range of other applications [11]. In particular, vesicle is a good candidate acting as template to mesoporous inorganic functional materials because it compartmentalizes the aqueous domain on submicron length scales. In fact, vesicular surfactant self-assemblies have already been exploited in several cases. Pinnavaia and his colleagues firstly reported that porous lamellar silica could be biomimetic templated by neutral diamine bola-surfactant or Gemini surfactants of C,H2n+~NH(CH2)2NHCmH2m+~ [12,13]. Brinker and his cooperators used Brij-56 or P123 as templates and generated aerosol dispersion with a heater to collect vesicular nanoparticles [ 14]. However, it should be pointed out that these processes either used noxious amine or needed inconvenient and uneconomical heating step. A faciler and more economical method to produce vesicles is by mixing cationic and anionic surfactants, resulting in "catanionic" vesicles spontaneously, in which, the obtained bilayers are the equilibrium state of aggregation [ 15]. In this paper, some efforts have been made into a novel pathway for the synthesis of vesicular mesoporous materials with catanionic surfactant as template. 2. EXPERIMENTAL 2.1 Synthesis Cetyltrimethylammonium bromide (CTAB, with the purity higher than 98%) and sodium dodecylsulfonate (SDS) were used without further purification and acted as cationic and anionic surfactants respectively. Silica source was tetraethyl orthosilicate (TEOS). The pH value was adjusted with hydrochloric acid and the distilled water with an electrical conductivity larger than 2 Mf~ was used as the solvent. In a typical synthetic procedure, catanionic surfactant was made of CTAB and SDS with a proper ratio, then this surfactant was dissolved in distilled water with vigorously stirring in a 308 K water bath. When the solution is clear, hydrochloric acid was added to adjust the value of pH from about 7 to below 2. Once the solution maintained its pH value in about 5 minutes, TEOS was added dropwise and the reaction mixture had the molar composition: TEOS: 0.1 Catanionic: 4.5 HCI: 1892 H20. The obtained mixture kept stirring for a whole week, then it was recovered, washed thoroughly with distilled water and finally dried in ambient condition to yield as-made sample. The sample was calcined in N2 with a rate of 2 K/min to 823 K and kept for 1 h followed by calcination at 823 K in air for 5 h to remove the template. 2.2 Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 ADVANCE employing CuKc~ ()~=0.15418 nm) radiation (40 kV, 20 mA) with a 0.02 deg. step size and 0.5 s step time over the range 1.2<20<10. The samples were prepared as thin layers on plastic sample holder.

1191 Nitrogen adsorption-desorption isotherms of the sample were measured at 77 K using a MICROMERITICS ASAP 2000 analyzer and the volume of adsorbed N 2 w a s normalized to the standard temperature and pressure. Samples were evacuated at 573 K prior to the adsorption experiments, and their specific surface area was determined from the linear part of the Brunaer-Emmett-Teller (BET) equation (P/Po = 0.06-0.21). The calculation of the pore size distribution was performed using the adsorption branch of the N 2 isotherm. Morphology and size of the particles were determined by scanning electron microscopy (SEM) with a HITACHI H-800 microscope (20kV). In transmission electron microscopes (TEM) measurements, the specimens were dispersed in ethanol and placed on holey copper grids. Two TEM instruments were used. Those with magnification of below 100000 times were conducted on a JOEL-JEM-100X with operation voltage of 100 kV, while the others magnified above 200000 times were gotten from a JEOL-2010 HREM with accelerating voltage of 200kV. Its resolving power on point is 0.194 nm and on line is 0.14 nm. The maximum magnifying power is 400k. 3. RESULTS AND DISCUSSION Silica source TEOS has to be hydrolyzed in the synthesis of inorganic-organic mesostructured composite, and such hydrolysis could be performed in either acidic catalysis or basic catalysis procedures. Unfortunately, two reactions would occur on the silica precursor in the condition of basic catalyzed hydrolysis: silica species react with cationic surfactant by electrostatic interaction, but they also condense rapidly at the same time. Therefore the vesicle formation, acting as template in this synthesis, should be disrupted by these two aspects. In acidic condition, however, the interaction of inorganic and organic species was mediated by X- (in here, X- is CI), and this mediated electrostatic pathway was supposed not to disrupt the catanionic vesicle. Before the synthesis, blank test has been conducted to investigate whether the disruption to catanionic vesicle would take place, and the equivalent amount of hydrochloric acid was added to the clear solution of catanionic surfactant (CTAB/SDS=4:1). If dodecylsulfonate was protonated at pH below 1, it would be educed from solution and the solution would thus appear cloudy. In fact, however, the solution kept clear and transparent during the course of continuously stirring for more than one week. Clearly the anion-cation surfactant pairs acted as double tailed zwitterionic surfactants [ 15], more stable than single anionic surfactant in acidic aqueous solution. In the synthesis presented here, the catanionic vesicle was stable to such a extent that the precipitation came forth after stirring with the addition of silica source for one whole day. The XRD patterns of as-synthesized and calcined samples, prepared at 308 K and named tentatively as NJU-V, are shown in Fig.1. The as-synthesized sample exhibits only one diffraction peak (001), corresponding to a basal spacing of 4.09 nm. There seemed a "wave" in the region with 2-theta degree from 3~ to 6~ but no peak could be definitely resolved. The absence of additional reflections of the lamellar phase could be attributed to limited order in the vesicle framework of this silicate. Similar phenomena of limited reflections were also

1192 6000

hkl 001 ] Period(days) d(nm)J

5000 o~ 4ooo o

7

If'

3oo0

4.05J

4

IJ

Q.

3.871

2000 1000 1

2

3

4

5

6

7

8

9 10

2 theta (degree)

Figure 1. XRD patterns of (a) as-made and (b) calcined samples synthesized in the mixture of TEOS:0.5 Surf: 1850 H20 at pH=0.68

2

4

6

8

2 theta (degree)

10

Figure 2. XRD pattems of the samples synthesized in the mixture of TEOS: 0.25 Surf: 500 H20 at pH=0.30 with different synthesis times. (a) 2d, (b) 4d and (c) 7d.

reported in the multi-lamellar silica materials such as MSU-V and MSU-G [13]. Different from parent material, the calcined sample presented a much higher main refraction, d spacing of 3.89nm, accompanied with little shrinkage in its crystal lattice. This indicated the stability of the sample framework in the calcination at 823 K. Consulting the d spacing in the XRD patterns, NJU-V was assumed to own more commodious inter-spaces than the other multilamellar mesoporous material such as MSU-V (2.3 nm) [12] and MCM-50 (2.0 nm) [2]. Figure 2 shows the XRD patterns of three samples with different hydrothermal crystallization time at the same starting gel composition. When the mixing time in the synthesis process was prolonged, the silica species were allowed to condense more and more extensively. As a result, a higher diffraction peak and greater d spacing were observed on the XRD patterns, which meant the existence of much well ordered framework in this vesicular mesoporous material. In coincidence with the result of XRD, the images from TEM measurement gave an intuitionistic insight into the hierarchical multi-lamellar mesostruture of NJU-V silica. Fractured vesicular silica was shown in Fig.3A, whose parallel silica sheets inflect with their interspacial distance of about 3.9 nm. Figure 3C exhibits the undulated silica sheets of an integrated multilamellar vesicle. The shell of this vesicular silica was staved in the shape of a bow tie, which meant that the mesostrustured inorganic-organic composite was not rigid during the synthetic procedure and it could be transformed according to its local circumstance. The tubular morphology with thick walls is demonstrated in Fig.3D in which the silica sheets grew in the same direction of the tube. The arrow pointed out the opening of the tube. Figure 3B exhibits a close insight of the opening of a tubular silica vesicle. The silica sheets curled into close circles and became concentric multilayers, different form the report of Lin and

1193 Mou that it was silica channels in hexagonal array existing in the wall [ 16]. The phenomenon observed in Fig.3B was also different from the oil-water interface templating result that it was hexagonal mesoporous silica synthesized at the crust of an emulsion [17]. The tubular silica wall shows the presence of wormholelike framework pores that run orthogonal to the undulated silica sheets, creating the three-dimensional pore network. The picture of ED in Fig. 4 shows a typical result of lamellar phase. Because of the limited repeating periodicity in vesicular mesostructured material, more than the secondary electron

!g

. ...-~J~"

50 nm-

.

'

. . . . 9

A

,

B

C D Figure 3. Transmission Electron Microscopes of NJU-V samples. The bars in C and D are 20 nm and 50 nm respectively

Figure 4. Selected area electron diffraction patterns (ED) of NJU-V sample.

1194

diffraction points cannot be investigated. Furthermore, this result could only be taken at the straight part of multilamellar silica vesicle whit the multiple of about more then 20. The N2 adsorption-desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore size distribution for the calcined NJU-V sample is shown in Fig.5. The adsorption isotherm below the relative pressure of 0.15 may be classified as Type I with microporosity. However, it does not level off below the relative pressure of 0.1, indicating that the sample was likely to be exclusively mesoporous [18,19]. When the relative pressure is above 1.5, the adsorption isotherm may be classified as Type IV. There is a step on the adsorption isotherm at the relative pressure of about 0.30 resulting from the capillary condensation in mesopores. After these pores are filled, the adsorption isotherm levels off. The capillary condensation and the capillary evaporation (desorption isotherm) occur at the almost same pressure but not change the shape of hysteresis loop, indicating that a kinetic equilibrium of capillary condensationevaporation exist in mesoporous. Based on these discussions it is very likely that the pore size of this sample is relatively small, close to the micropore range [20]. A proof on this inference came from the BJH pore size distribution in Fig.5, a maximum appeared in the spectrum indicted the mesopores with a diameter of 2.6 nm existing in this vesicular silica. Due to the broad distribution of mesopore size, the capillary condensation rises diagonally on this sample. The hysteresis loop at higher relative pressures is a consequence of N2 filling the textural pores that are associated with particle morphology. In addition, the framework thickness might be obtained by subtracting the pore size (BJH) from the d spacing (XRD), and it is 1.3 nm for this sample. The Brunaer-Emmett-Teller (BET) surface area of the NJUV sample is 1536 m2/g, larger than that of MSU-V (984 m2/g) [12], MSU-G (523 m2/g) [13] and MCM-41 (900-1200 m2/g). Morphology of NJU-V silica samples can be tunable to some extent by changing the parameters in hydrothermal synthesis. There are products synthesized with the morphologies

7 co 800 ~.~ --+--des ..... ~ ...~ 600 >je~d,,.e..c>~-~-K>~-o

~ 6 ~IE 5 4"9~ .~

-~ 400

~ 3 ~- 2

200

~

1

o12

o14

o16

o18

Relative pressure P/Po

1.o

0

100 1000 Porediameter(0.1 nm)

Figure 5. Nitrogen adsorption-desorption isotherm plots (the left) and pore size distribution curve (the right) using the data from the adsorption branch of the calcined sample from TEOS: 0.5 Catanionic: 1850 H20 at pH=0.73.

1195

i ............................

Figure 6. SEM of the sample synthesized from TEOS" 0.5 Catanionic" 1850 H20 at pH = 0.68 and ambient temperature.

Figure 7. TEM of NJU-V sample from the synthetic mixture stirred at 308 K after the template aged for a night.

of long fibers, solid spherical particles (Fig 6 and 7) after varying the mixture temperature, template aging time and degree of acidity in solution. This diversity is actualized depending on the catanionic vesicular template sensitive to its circumstance. SEM picture of the sample shows a fiber with a diameter of about 10 gm and a length of about 200 ~tm. On the outer surface, there are gyroidal lines going along the fiber. It seems as if there were three or four wires twisted together into a single rope. Scheme SDS Silica species

CTAB

o

Fiber Sphere

O"

Loop

Cross section Slice ,..

of bilayer ,~

In the research field of micelle chemistry, it is one of the focused topics that catanionic surfactant self-assembly forms vesicle in dilute solution. The abundant works on this topic are very valuable to the biomimetic synthesis of inorganic-organic mesostructured composite. In this paper, vesicular mesostructured materials have been synthesized by templating of catanionic surfactant vesicle. Flexural silica sheets in the pattern of concentric circles were

1196 observed with the characteristic of vesicle. In view of syntheses, these silica sheets were constituted with inorganic-organic bilayers. (see Scheme) 4. CONCLUSION A novel vesicular mesostructured material named NJU-V has been successfully synthesized templating by catanionic surfactant vesicle. This material processes a considerable large surface area and mesopore of 2.6 nm, and is characterized by its circled multilamellar silica in architecture. With different morphologies of fiber, solid sphere and close loop, these samples could be obtained according to the tunable synthesis conditions. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H.Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Stephen, Bashaw and T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 35 (1996) 1102 4. Q-S. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P-Y. Feng, T.E. Gier, R Sieger, A. Firouzi, B.F. Chmelka, F. Schuth and G.D. Stucky, Chem. Mater., 6 (1994) 1176. 5. ET. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865 6. A.S. Bashaw, E. Prouzet, and T.J. Pinnavaia, Science, 269 (1995) 1242 7. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 279 (1998) 548. 8. A. Khan and E.F. Marques, Current Opinion in Colloid & Interface Science, 4 (2000) 402; 9. T. Zemb, M. Dubois, B. Deme and T. Gulk-Krzywicki, Science, 283(1999) 816 10. S. S. Chrai, R. Murari, and I. Ahmad, BioPharm, Jan (2002), 40 11. T.M. Allen, Curr. Opin. Colloid Interface Sci. 1 (1996) 645. 12. P.T. Tanev and T.J. Pinnavaia, Science, 271 (1996) 1267 13. S.S. Kim, W. Zhang and T.J. Pinnavaia, Science, 282 (1998) 1302 14. Y. Lu, H. Fan, A. Stump, T.L. Ward, T.Rieker and C.J. Brinker, Nature, 398 (1999) 223 15. E. W. Kaler, A K. Murthy, B.E. Rodriguez and J.A.N. Zasadzinski, Science, 245 (1989) 1371. 16. H. P. Lin and C. Y. Mou, Science, 273 (1996) 765 17. S. Schacht, Q. Huo, I. G. Voigt-Martin, G. D. Stucky, F. Schtith, Science, 273 (1996) 768 18. R. Ryoo, I. Park, S. Jun, C. W. Lee, M. Kruk, and M. Jaroniec, J. Am. Chem. Soc. 123 (2001) 1650 19. M. Kruk and M. Jaroniec, Chem. Mater., 12 (2000) 222 20. M. Kruk and M. Jaroniec, Chem. Mater., 13 (2001) 3169

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1197

Preparation and characterization of Co-Fe-Cu mixed oxides via hydrotalcite-like precursors for toluene catalytic oxidation J. Carpentier, J.-F. Lamonier*, S. Siffert, H. Laversin and A. Aboukais Laboratoire de Catalyse et Environnement, EA 2598, MREID, Universitd du Littoral-C6te d'Opale, 145, Avenue Maurice Schumann, 59140 DUNKERQUE Cedex, FRANCE fax : 03 28 65 82 39 ; e-mail : [email protected] Different Co2_xCuxFel hydrotalcites with x = 0, 0.5, 1, 1.5 and 2 have been synthesized in order to be used as precursors of mixed oxides for total toluene oxidation. The formation of hydrotalcite phase has been evidenced for x < 1 and its decomposition at 500~ led to Co304 and/or CoFe204 spinels. At 500~ besides the Co-Fe spinels, CuO formation has been related for x >_ 1. The presence of CuO phase was not the determining parameter since a catalytic synergetic effect has been obtained for Co~.sCu0.sFel sample. 1. I N T R O D U C T I O N Volatile Organic Compounds (VOCs) in industrial gas represent a serious environmental problem. An effective way of removal is complete catalytic oxidation to harmless products such as H20 and CO2. Among group VII! metal catalyst systems supported on alumina, Pt, Pd and Co were found to be the most active for benzene oxidation [1 ] and Co is cheaper than precious metals. Moreover, iron oxide was often used for catalytic oxidation of VOCs [2]. The support is also very important for the efficiency of the catalyst and the surface and catalytic properties of oxide materials depend strongly on the preparation method and on the nature of the precursor [3]. The use of hydrotalcite (HT) precursor to prepare mixed oxides can be interesting way to improve the catalytic performance of oxides [4]. The thermal decomposition of hydrotalcite leads to mixed oxides having high metal dispersion and large surface area [4]. A hydrotalcite is composed of positively charged metal hydroxide layers which are compensated by interstitial layers built of anions (often CO32) and water molecules. The general formula to describe the chemical composition of HT is : [Ma+1_xM3+• z" mH20. The formation of Co-Fe hydrotalcite has already been reported [5]. Its destruction led to cobalt-substituted Fe304 phase (CoxFe3.xO4). Moreover CuO in interaction with CuCr204 derived from the hydrotalcite precursor seemed to induce a beneficial catalytic effect in the oxydation reaction [6]. In this context, starting from Co-Fe HT, the partial and the total substitution of Co 2+ by Cu 2+ has been studied. Different hydrotalcite samples with (Co+Cu)/Fe = 2 with various Co/Cu ratio have been synthesized, characterised and used as precursors of mixed oxides. Toluene, which is often found in industrial exhaust, has been chosen as probe molecule for the oxidation test and the deactivation study of calcined hydrotalcites.

1198 2. E X P E R I M E N T A L 2.1. Preparation of hydrotalcites A solution containing appropriated quantities of Co(NO3)2.6H20, Cu(NO3)2.3H20 and Fe(NO3)s.9H20 was added slowly under vigorous stirring into NaOH and Na2CO3 solution. The final pH was 8 and the resulting slurry was heated at 55~ for 24 hours. Then, the precipitate was filtered, washed several times with deionized water and dried at 50~ for 48 hours. Five samples have been synthesized with different Cu and Co contents : Co>,Cu,Fel (HT) with x = 0, 0.5, 1, 1.5 and 2. Co2Fe~(OH) sample has also been prepared by coprecipitation of nitrate elements with NaOH. The final pH was 8 and the precipitate was immediately filtered, washed several times with deionized water and dried at 100~ for 24 hours. In order to study the catalytic behaviour of these samples, a calcination treatment has been performed under flow of air (4 L.h 1) at 500~ (2~ 1 and 4 hours at desired temperature) ; the solids obtained were named Co2.• (HT) and CozFel500 (OH). 2.2. Characterization The specific areas of solids were determined by BET method using a Quantasorb Junior apparatus and the gas adsorbed at -196~ is pure nitrogen. The structures of solids were analysed by X-ray diffraction (XRD) technique in a Siemens D5000 diffractometer equipped with a copper anode. The XRD patterns were recorded at room temperature. All the XRD patterns were assigned using the JCPDS data-base. Differential thermal analysis (DTA) (Netzsch STA 409 equipped with a microbalance (TG)) was conducted in flow air (75 mL.min 1) at a heating rate of 5~ 1 from room temperature to 1000~ with around 50 mg of sample. The IR spectrum of each sample was recorded by accumulating 16 scans at 2 cm ~ resolution using a Perkin Elmer System 2000 Fourier transform infrared spectrometer.

2.3. Catalytic tests Before the catalytic test, the solid (200 mg) was calcined under a flow of air (4 L.h ~) at 500~ during 4 hours. Toluene oxidation was carried out in a flow microreactor and studied up to the reaction starts (ignition temperature). Then, the catalyst deactivation was studied for 48 hours. The reactive flow was composed of 99 mL.min ~ of air and 2.6 mL.min t of gaseous toluene. A high quantity of toluene was used in order to observe the catalytic behaviour in severe conditions. The analysis of combustion products was performed using a Varian 3600 chromatography equipped with TCD and FID. 3. RESULTS AND DISCUSSION 3.1. XRD

3.1.1. Uncalcined samples Figure l a shows the XRD pattern of C02Fe~(OH) sample. The comparison of the XRD pattern with the JCPDS data base indicates the presence of Fe3Oa, Co304 or CoFe204 phases. Ifideed, the XRD lines of these three oxide phases are very closed. However, the formation of Fe304 phase can be eliminated since the presence of Fe 2+ ions in the solution is necessary to form the Fe(n)Fe~m)204 spinel. Moreover, the reduction of Fe 3+ into Fe 2+ by Co 2+ in the solution

1199

is not possible taking into consideration the oxydo-reduction potential values of Co~+/Co 2+ (1.9V) and Fe3+/Fe2+ (0.77V). Using Co 2+ and Fe 2+ nitrates, Christoskova et al. [7] showed that C02Fel freshly prepared and calcined at 200~ are amorphous. So, it seems that the oxidation of Co 2+ into Co 3" to lead to C0304 spinel oxide does not take place at low temperature. Moreover, the synthesis of MnFe204 ferrites by coprecipitation of Fe 3+ and Co 2+ ions is possible in the temperature range of 50-100~ [8]. Therefore, the XRD pattern of our sample can be attributed to the CoFe204 phase : the simultaneous presence of Fe 3§ and Co 2§ into the solution allows to form this oxide at low temperature, the excess of cobalt in the solid (Co/Fe = 2) is probably in amorphous phase.

d "2".

[]

C

,.a

d c

.,..,

e~

nzx

~D

[]

[]

ix

zx

b

[] []

'

5

o

a

25

45

65

20 (o) Figure 1 9XRD patterns of (a) Co2Fe~(OH), (b) Co2Fel(HT), (c) Co2FelS00(OH) and (d) C02Fel500(HT) samples.

5

[]

25

o

45

oo

a 65

20 (o) Figure 2 9 XRD patterns of uncalcined Co2_xCuxFel (HT) samples with (a) x = 0.5, (b) x = 1, (c) x = 1.5, (d) x = 2.

The XRD pattern of Co2FeI(HT) is shown in Figure lb. It revealed narrow, symmetric, strong lines at low 20 values and weaker and less symmetric lines at high 20 values (see D on Figure 1) characteristics of layered materials. From the position of the two strongest lines of crystallographic indices (003) and (006), the lattice distances, d003 and d006, were calculated and used to determine the lattice parameter " c' " representing the thickness of a hydroxide layer and an interlayer. The "c" parameter is equal to " 3c' ". This parameter depends on the anion size (component in the interlayer), the value of MU/M m ratio (M = Co 2+, Cu 2" and Fe 3+) and the degree of hydration. The lattice parameter " a " was determined from the (110) reflection line. This parameter depends on the nature of the cations (in the bmcite-like sheet) and the value of Mn/M m ratio [4]. The crystallographic parameters obtained for Co2Fe~(HT) sample (a = 3.1267 A and c = 22.8001 A) are closed to those found in the literature for a Co/Fe ratio of 3 [5]. The XRD patterns of Co2..~Cu.~Fe~ (HT) with x = 0.5, 1, 1.5 and 2 are shown in Figure 2. The hydrotalcite phase was formed for x = 0.5 (Figure 2a). The crystallographic parameters of Co~.sCu0.sFe~ (HT) sample (a = 3.1257 .~ and c = 22.7759 A) are lower than those of Co2Fel (HT) sample. This result can be explained by the substitution of Co 2+ by Cu 2., the ionic radius

1200 of Cu 2§ (0.69 ,a,) being lower than this of Co 2+ (0.745 A) [4]. For ColCulFel (HT) sample (Figure 2b), a low crystallised HT phase (see [] in Figure 2b) was observed but it was mixed with another non identified phase (see A in Figure 2b). With increasing copper content, an amorphous phase was obtained. Then, for the sample without cobalt (x = 2), the monoclinic malachite phase (Cu2(OH)2CO3) was observed. Therefore, when the Cu quantity is superior to 1, the HT phase is not pure or not formed. Even though Cu 2+ ions have a suitable ionic radius to form anionic clays, they give rise preferentially to the precipitation of malachite-like phases, because of the Jahn-Teller effect, which for d 9 ions favours the formation of distorted octahedral structures [9]. But associated cations (M 2+ = M g 2+, Co 2+ or Zn 2+) favour the entrance of Cu 2+ ions into aluminium (A13+) based hydrotalcite phases, which are obtained without side phases for Cu2+/m 2+ ratios < 1 [4]. According to this result, when copper is associated with Co 2+, iron (Fe 3+) based HT phase is also formed for Cu/C0=0.33 and Cu/Co=l. The surface area values of CoiFed(OH) and C02..~CuxFe~ (HT) are reported in Table 1. For C02Fel(OH) sample, the formation of amorphous phase with CoFe204 is confirmed by the high surface area value. When the HT phase is formed, the BET value is three times lower. The surface areas of others untreated samples increased with the Cu content. The rise of Cu quantity (up to x = 1.5) probably leads to an amorphous phase having a high specific area. The surface area of Cu2Fel(HT) is lower according to the malachite phase crystallisation. Table 1 Surface areas (m2.g 1) of CozFel(OH) and Co2.xCuxFel (HT) with x = 0, 0.5, 1, 1.5 and 2 Samples x = 0 (OH) x = 0 (HT) x = 0.5 x= 1 x = 1.5 x=2 Uncalcined 152 55 86 168 213 109 Calcined 66 74 78 70 58 26 After test 22 30 31 31 18 10

3.1.2. Calcined samples The XRD patterns of Co2.xCuxFel (HT) with x = 0.5, 1, 1.5 and 2 calcined at 500~ are presented in Figure 3. The X R profile of C02Fel500(OH) was similar to that of Co2Fe~(OH). However, the lines were more narrow, indicating a higher crystallisation of the solid. This result was confirmed by the ~ decrease of the surface area value. The XRD 2 pattern of C02Fel500(HT) sample revealed the destruction of the HT phase to form a similar phase to that of C02Fe~500(OH). But, for Co2Fe~500(HT), the diffraction peaks were broader according to the higher BET value of 5 25 45 65 C02Fel500(HT). A high surface area and a 20 (o) better metal dispersion were often observed when oxides were derived from HT phases [4]. Figure 3 9XRD patterns of Co>xCu.~Fe~(HT) The broadness of diffraction peaks can be also samples calcined at 500~ with (a) x = 0.5, explained by the presence of a mixture of two (b) x - 1, (c) x = 1.5, (d) x = 2. oxide phases 9CoFe204 and Co304.

1201 Indeed, the oxidation of Co 2+ into Co 3§ to form the Co304 spinel could occur during the calcination at 500~ For x = 0.5, the destruction of the hydrotalcite phase into spinels is also observed (Figure 3a). For x = 1 and 1.5 (Figure 3b and 3c), in addition to the oxide spinel phases, monoclinic CuO phase (tenorite) is produced (see * in Figure 3). The CuO quantity increases with the rise of copper content in the solid. For x = 2 (Figure 3d), the presence of CuO and ct-Fe203 (haematite) phases is detected. The haematite phase seems not to be formed when cobalt is present in the solid. Uzunova et al. [6] shown that for Co-Fe oxide derived from HT phase, ot-Fe203 phase could be formed if cobalt content in the solid is low (Fe/Co>l), the cobalt-rich samples leading to a cobalt-substituted Fe304 phase (CoxFe3.xO4). 3.2. T G - DTA The TG-DTA curves of CozFel(OH) and CozFel(HT) are shown in Figure 4. A broad endothermic peak in the DTA profile of Co2Fe~(OH), accompanied with a slow mass loss (TG) was observed. The shoulder of the DTA peak can be attributed to the presence of cobalt (II) and/or iron (III) hydroxides, the decomposition of such amorphous compounds taking place respectively at 266~ and 238~ (broad peak). On the contrary, for Co?Fel(HT) sample, a narrow endothermic signal registered at 170~ indicated the collapse of the HT structure, to form metal oxides as Co304 and/or CoFe204. This decomposition temperature value is in good agreement with data reported in the literature. In fact, Uzunova et al. [5] observed the destruction of Co3Fel(HT) at 170~ The endothermic signal was accompanied with a quick mass loss (28.01%) which corresponds to the removal in a single step of hydroxide groups, carbonates anions and interlayer water in Co?Fe~(OH)6(co3Z)vz, 1.25 HzO hydrotalcite, yielding Co304 and CoFezO4 spinels. In addition, CozFe~(OH) and CozFe~(HT) samples presented a second endothermic signal at respectively 970~ and 965~ which can be attributed to the thermal decomposition of cobalt and/or iron spinels. Indeed, starting from CoCo204, an endothermic DTA signal accompanied with a mass loss value of 5.49% was observed at 933~ The experimental mass loss value corresponds to the theoretical one obtained with the reaction : Co304 --+ 3 CoO + '/2 Oz. The DTA signals obtained during the calcination of Co2.xCu.~Fe~(HT) samples with x = 0.5, 1, 1.5 and 2 are presented in Figure 5. For x = 0.5, the narrow endothermic peak was always present indicating the destruction of the HT structure. For x _> 1, the HT phase has not been formed and the endothermic signal around 170~ disappeared. However, an exothermic signal (without weight mass loss) around 510-530~ appeared. The DTA signal intensity increased with copper content. It can be attributed to the crystallisation of CuO phase. Indeed, after calcination at 500~ for 4 hours, the presence of CuO phase (tenorite) has been detected by XRD analysis for x > 1, the quantity of CuO increasing with the copper content in the solid. The signal at around 900~ corresponding to the spinel destruction, was present in the samples containing cobalt and disappeared for CuzFe1(HT) sample. The DTA signal intensity decreased and the maximum was shifted to lower temperatures when cobalt content decreased. This result confirms that the endothermic peak corresponds to the decomposition of a cobalt based spinel. For the CuzFel (HT) sample, the DTA curve revealed a single endothermic peak at 333~ which corresponds to the destruction of malachite structure. Indeed, the experimental mass loss value (21.5%) is in agreement with the theoretical one (20.07%) and corresponds to the destruction of malachite into c~-Fe203 (haematite) and CuO, oxides detected by XRD analysis after calcination at 500~ during 4 hours.

1202

531~

::i ~

/

~

!28.01% d

.~o "E- |

~o 170~

~

//

25

9

I

225

]

3

3

3

~

1

I I

xq73~

--N'I

425 625 825 Temperature (~C)

Figure 4 : TG-DTA curves of (a) CozFel(OH) and (b) CozFel(HT) samples. 3.3.

~

970oct

13.85%[

I

_

b .,..~

D

b[~

./v_...~ I

_

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20

220

I

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420 620 Temperature (~

t

820

Figure 5 : DTA curves of Co2,~CuxFel(HT) (a) x = 0.5, (b) x = 1, (c) x = 1.5, (d) x = 2.

FTIR

The IR spectrum of Co2Fel(OH) sample in the range of 450-4450 cm 1 (not presented) showed two bands characteristics of the stretching vibrations of metal-oxygen bonds. The first band at 559 cm 1 could be associated with the BOB3 vibrations in the spinel lattice and the second band at 656 cm "1 could be attributed to the ABO3 vibrations (B = metal ions in an octahedral position and A - metal ions in a tetrahedral position). In comparison with C o 3 0 4 spinel, a slight difference in wavenumbers was however observed : 571 and 664 cm 1 for respectively the both bands characteristics of the stretching vibrations of Co-oxygen bonds [5, 10]. In fact, the lower wavenumbers associated with the decrease of the bond strength of cation-oxygen could be explained by the substitution of Co ions with Fe ions leading a bathochromic shift of the two bands. This IR result confirmed the formation at low temperature of CoFe204 spinel assumed from XRD analysis. FTIR spectra in the range of 1200-1600 cm l of Co2Fel(OH) and COz..~CuxFel(HT) samples with 0 < x _< 2 are presented in Figure 6. Figure 6a showed a prominent band at 1384 cm ~ which can be attributed to free nitrate vibrations (nitrates not eliminated during washing of the precipitate) [11]. In the other hand, Co2Fe~(HT) and Co~.sCu0.sFel(HT) samples (Figure 6b and 6c) showed a strong band at 1353 cm ~ which can be attributed to u(CO3) of interlayer carbonates [5] in accordance with the formation of the hydrotalcite phase. When the HT phase has not been formed (x >_ 1), this band disappeared and a double band detected at 1397-1530 cm ~ (Figure 6d and 6e) was getting more significant with increasing copper concentration in the sample. This doublet can be assigned to carbonates in interaction with Cu 2+ ions [12] in a bidentate complex [13]. For the CuzFe~(HT) sample (Figure 60, shoulders at 1423 cm ~ and 1498 cm ~ appeared and can be explained by the presence of carbonates in a monodentate complex [13]. The rise of the Cu content increases the amount of carbonate anions and in the case of the malachite structure leads to different types of carbonates which have different interactions with copper. Alejandre et al. [12] n o t i c e d t h e same trends studying Cu-AI hydrotalcite.

1203 The calcination at 500~ of Co2Fel(OH) sample leaded to the elimination of the nitrates anions since the strong band at 1384 c m "1 disappeared. No significant change (shift in wave numbers) was observed for the bands characteristics to the spinel lattice vibrations. IR analysis of Co2FeI500(HT) revealed the disappearance of the bands related to HT structure and the appearance of the bands at 545 and 640 cm -1 characteristics to the spinel structure vibrations. 1522 1498

II Ignition Temperature

1,397

[] Catalyst Temperature 450 400 e

1353

d

o

350 300 250 200

r

150 100 50 1600

1500 1400 1300 Wavenumbers (cm l )

1200

Figure 6 : IR profiles of (a) Co2Fel (OH) and Co2.xCuxFel (HT) samples with (b) x = 0, (c) x = 0.5, (d) x = 1, (e) x = 1.5 and (f) x = 2.

0

I

a

b

c

i

1

1

d

e

f

Figure 7 : Ignition and catalyst temperatures of (a) Co2Fe1500 (OH) and Co2,,Cu.,Fe1500 (HT) with (b) x = 0, (c) x = 0.5, (d) x = 1, (e) x = 1.5 and (f) x = 2.

3.4. C A T A L Y T I C A C T I V I T Y The catalytic activity of CO2_xCuxFe1500(HT) for the total oxidation of toluene has been evaluating by comparing the ignition temperature (Figure 7). In order to check the role of the catalyst preparation method, the same experiment with Co2Fe1500(OH) was performed. The result included in Figure 7, showed clearly that the use of the HT phase precursor of the oxide phase was beneficial since the ignition temperature was 25~ lower for Co2Fet500(HT). The exothermicity of the reaction could be estimated by comparing the ignition and the catalyst temperatures. Regarding the oxides derived from the HT phase, it is obvious to deduce that higher cobalt content induces higher exothermicity. The difference observed between Co2Fel500(OH) and Co2Fel500(HT) in terms of reactivity and exothermicity can be explained by different dispersion of" cobalt and iron coming from different preparation methods for precursors [4]. Besides, higher specific area was obtained for Co2Fe1500(HT) (Table 1). The copper addition in the solid produced a beneficial effect on the ignition temperature. But the rise of copper content in the sample increased the ignition temperature. The formation of well developed CuO crystallites (evidenced by XRD analysis for x ___ 1) probably lowers the number of available active sites at the surface. Bahranowski et al. [6] shown the role of interface boundaries between CuO and CuCr204 since the combustion of toluene over CuO + CuCr204 mixture was less effective than Cu-Cr mixed oxide including CuO. However the lower ignition temperature was obtained for COl.sCu0.sFe~500(HT) in which CuO phase has

1204 not been produced. So the best reactivity of this sample can be explained by the formation of another spinel phase including copper as CuyMzO4 (M = Fe or Co). The only carbonaceous product for the toluene oxidation was CO2 except for Co2Fe1500(OH) where benzene appeared in trace amounts. The formation of benzene can be explained by the mechanism of combustion in which the first step is the cracking of the C-C bonds before oxidation to CO2 and H20. For the toluene, the cracking of C-C bond of the exocyclic methyl is the easier rupture. The formed benzene is very stable and then much more difficult to crack. The presence of benzene can therefore be explained for the less active catalysts as CozFe~500(OH). No deactivation for Co containing samples was observed after 48 hours on stream despite of the strong decrease of specific area (Table 1). The lack of dependence between catalytic deactivation and BET values can be due to the complex phase composition of mixed oxides. 4. CONCLUSION This study reports an investigation in the total toluene oxidation on Co2.xCu.~Fel mixed oxides synthesized by hydrotalcite method. For x > 1, the HT structure was not formed because of a strong interaction of Cu 2+ and CO32. However a layered material was obtained for x < 1. For x < 2, the thermal decomposition at 500~ of the different hydroxides led to cobalt and/or ferrite spinels. These mixed oxides were more active than Fe203 obtained for x = 2. A catalytic synergetic effect was observed for copper containing samples. But the ignition temperature increased with the Cu content. The disappearance of the synergetic effect could be related to the presence of CuO, which probably lowers the number of available active sites at the surface. Better performance of Co~.sCu0.sFel derived from a HT precursor could be explained by CuyMzO4 (M - Fe or Co) oxide formation besides ferrite spinels. REFERENCES

[ 1] P. Papaeffhimiou, T. Ioannides, X.E. Verykios, Appl. Catal. B 13 (1997) 175 [2] P.O. Larsson, A. Andersson, B. Svensson, L.R. Wallenberg, in "Environmental Catalysis", G. Centi et al. (Eds), Rome, (1995) 547 [3] S. Scire, S. Minico, C. Crisafulli, S. Galvagnio, Catalysis Communications, 2 (2001) 229 [4] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today, 11 (1991) 173 [5] E. Uzunova, D. Klissurski, I. Mitov, P. Stefanov, Chem. Mater., 5 (1993) 576 [6] K. Bahranowski, E. Bielanska, R. Janik, T. Machej, E.M. Serwicka, Clay Minerals, 34 (1999) 67 [7] St.G. Christoskova, M. Stoyanova, M. Georgieva, Appl. Catal. A, 208 (2001) 235 [8] R.M. Cornell, Clay Minerals, 23 (1988) 329 [9] A. Vaccari, Catalysis Today, 41 (1998) 53 [10] St.G. Christoskova, M. Stoyanova, M. Georgieva, D. Mehandjiev, Materials Chemistry and Physics, 60 (1999) 39 [11] J.T. Kloprogge, L. Hickey, R.L. Frost, Applied Clay Science, 18 (2001) 37 [12] A. Alejandre, F. Medina, X. Rodriguez, P. Salagre, J.E. Sueiras, J. Catal., 188 (1999) 311 [13] K. Nakamoto, Infrared spectra of inorganic and coordination compounds, second edition, Willey Interscience, New-York, 1970, p. 169

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1205

Catalytic oxidation over transition metal doped MCM-48 molecular sieves Changping Wei a*, Qiang Cai b, Xuwei yange, Wenqin pangb, Yingli Bi ~ and Kaiji Zhen~ aDepartment of Chemistry Engineering, Jilin Institute of Technology, Changchun 130012, P.R.China bKey Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P.R.China CDepartment of Chemistry, Jilin University, Changchun 130023, P.R.China A series of MCM-48 mesoporous molecular sieves doped with Ti, Cr, Mo, Zr, and Cu were synthesized by hydrothermal crystallization and characterized by XRD, UV, HRTEM and N2 adsorption. Selectively catalytic oxidation of a-long chain eicosanol to the corresponding t~eicosanoic acid over this series of catalysts has been tested. The optimum reaction temperature and reaction time were given. Experimental results show that MCM-48 molecular sieves doped a suitable amount of transition metal may enhance the yield and the selectivity of a-eicosanoic acid. The sort and the amount of the doped transitional metal have important effect on the catalytic activity. 1. INTRODUCTION Much attention has been paid to a new family of mesoporous molecular sieves denoted as M41S [1-2]. Due to their regular pore arrangement and narrow pore size distribution, they extended the range of ordered microporous molecular sieves. Ti- and V-substituted MCM41 and Ti-substituted hexagonal mesoporous silica such as Ti-HMS have been synthesized [37]. The mesoporous M41S materials have expanded significantly the possibilities for processing bulky molecules for catalytic and adsorption purposes. But as a catalyst, MCM-48 characterized by a three-dimensional channel system has several advantages over MCM-41 which has a one-dimensional channel system. For instance, the three-dimensional pore system is more resistant to blockage by extraneous ions than the one-dimensional pore system. Thus, MCM-48 may be applied to chemical industry and biochemistry [8-11 ]. In this work, we synthesized MCM-48 mesoporous molecular sieves doped with Ti, Cr, Mo, Zr, and Cu by hydrothermal crystallization, and characterized them by XRD, UV, HRTEM and N2 adsorption. The catalytic performance of M (Ti, Cr, Mo, Zr, and Cu)-MCM-

*Corresponding author, E-mail: [email protected]; Fax: 86-0431-5952413.

1206 48 for the oxidation of a-eicosanol to a-eicosanoic acid has been tested. 2. E X P E R I M E N T A L

2.1. Synthesis of M-MCM-48 molecular sieves The M-MCM-48 (M = Ti, Cr, Mo, Zr, and Cu) molecular sieves were synthesized [12] hydrothermally using TEOS, transition metal salts, CTAB, NaOH and distilled water. The procedure is described below: NaOH was dissolved in distilled water, then transition metal salts and the CTAB were added. When the solution became homogeneous, TEOS was added and the resulting solution was transferred to an autoclave and heated at 373 K for three days. The products were washed with distilled water, dried at ambient temperature and calcined at 823 K for 4h. The obtained catalyst samples are as M-MCM-48, where M denotes the transition metal. 2.2. XRD measurement The X-ray diffraction patterns of the M-MCM-48 were recorded on a SCINAG XDS-2000 Diffractometer with Cu-I~ radiation.

2.3. UV and HRTEM measurement The UV diffusion reflection spectra were recorded on a UV-3100 (HITACHI). HRTEM profiles were obtained on a HIACHI-8100 transmission electron microscope operated at 200 KV within a thin section prepared by ultramicroscope. 2.4. Adsorption / desorption measurement Nitrogen adsorption and desorption isotherms at 77K were measured using a Micromeritics ASAP 2400 Instrument. The data were analyzed by the BJH (Barrett-Joyner-Halenda) method using the Halsey equation for multilayer thickness. The pore-size distribution was obtained from the analysis of adsorption branch of the isotherm. 2.5. Test of the catalytic oxidation The catalytic oxidation of a-eicosanol was carried out in a 4-neck flask equipped with a stirring rod, a thermometer, an oxygen inlet and a condenser. Reactions were carried out at 413 K for 5h. 0.1-0.2 g catalyst (100 mesh) was used. The a-eicosanol was purified before used. Conversion of a-eicosanol and the yield of a-eicosanoic acid were calculated according to a stearic acidity of the product which was determined as following: 1.0 g product was dissolved in 70 ml hot ethanol. To the solution 6 drops of phenol phthalein and excessive amount of 0.2 M KOH were added, followed by titration with 0.2 M HC1. The stearic acidity was calculated based upon the titer.

3. RESULTS AND DISCUSSION

3.1. X-ray diffraction The X-ray diffraction pattems of M-MCM-48 (M = Ti, Cr, Mo, Zr, and Cu) (Figure 1) are

1207 in agreement with those of typical MCM-48 materials [13]. All as-synthesized samples exhibited a very strong diffraction peak at around 2.30 ~ two weak peaks at 2.70 ~ and 4.40 ~ corresponding to diffraction planes of (211), (220) and (332), respectively. The XRD patterns of calcined M-MCM-48 looked similar to those of the as-synthesized samples except that the diffraction peaks shifted slightly to the higher 20 angle. 3.2. U V spectra and H R T E M profiles

The existence of metal atoms in MCM-48 framework were confirmed by UV and HRTEM analysis. The UV spectra of the Si-MCM-48 and the Ti-MCM-48 are shown in Figure 2. The band at 210 nm was assigned to isolated framework titanium in tetrahedral coordination, and

The as-synthesized

The calcined

d

g~

Zr-MCM-48 Ti-MCM.-48

Si-MCM-48 1

3

5

7

9

2

4

6

8

10

20, ~

Figure 1. XRD patterns of Si-MCM-48 and 2% M-MCM-48 catalysts

1208 the band at 230 nm was assigned to framework titanium in octahedral coordination[4]. A band at ca. 270 nm was attributed to extraframe titanium [ 14]. HRTEM images of Si-MCM-48 and Ti-MCM-48 were shown in Figure 3 indicating HRTEM images along (111) direction. The data obtained from HRTEM are well consistent with those obtain from XRD [4]. Both UV and HRTEM results indicated that the titanium atoms exist in the MCM-48 framework. 3.3. Pore size distribution The pore-size distribution of the pure silicon and M-MCM-48 were measured. Si-MCM-48, Zr-MCM-48, and Cr-MCM-48 all have a narrow pore diameter distributions at around 2.6 nm. But the pore radius of Zr-MCM-48 is smaller, which is caused by residual of small amount of heteratom oxides inside the channels. Since the atomic radius of chromium is smaller than that of zirconium, the pore size of Cr-MCM-48 is larger than that of Zr-MCM48. 3.4. Reaction conditions We first carried out the gas phase (non-catalytic) oxidation of a-eicosanol as a blank test and the results indicated that the yield of t~-eicosanoic acid is low (14.6 %). There are brokenoff chain materials in products measured by GC-MS, which can not be separated easily. Over

0.6

I

d

O t o

-o- Ti-MCM-48

-o-

Si-MCM-48

0.4

r~

o.2

0

200

250

300 350 400 WAVELENGTHS, nm

450

Figure 2. UV spectra of Si-MCM-48 and Ti-MCM/-48 catalysts

1209

Figure 3a. HRTEM profiles of Si-MCM-48 catalyst

Figure 3b. HRTEM profiles of Ti-MCM-48 catalyst

other catalysts such as simple metal oxides, the highest yield of ~t-eicosanoic acid was 26 %. However, the selectivity of a-eicosanoic acid was greatly enhanced when M-MCM-48 were used. GC-MS measurements indicate no cracking and decarboxylation occurring during the catalytic reaction. The effect of temperature on catalytic activity over Ti-MCM-48 was studied and experimental results are given in Table 1. The optimum reaction temperature was 413K. In this work unless particular state, in most run reaction temperature was 413K. The same conclusion can be drawn for M (Cr, Mo, Zr, and Cu)-MCM-48. The product of the oxidation of a-eicosanol over Ti-MCM-48 at 413K was extracted from the reaction system for composition analysis. The results show that after 5h of reaction, the highest yield of a-eicosanoic acid was obtained (Table 1). Further increasing reaction time did not result in a higher yield. This probably is due to decarbonation of the acid caused by heating for a longer time.

Table 1 Effect of reaction temperature and reaction time on yield (CH3(CH/)IsCOOH) over 1% ( n (Ti) / n (Si) = 0.01) Ti-MCM-48 catalysts Reaction temperature (K)

Yieldaof a-eicosanic acid (%)

393 403 413 423 433

18.4 40.2 54.4 38.8 21.4

aReaction time: 5h. bReaction temperature: 413K.

Reaction time (h) 3.0 5.0 7.5 10.0

Yield bof a-eicosanic acid (%) 27.0 54.4 54.0 46.4

1210 Table 2 Effect of M content on catalytic oxidation activity over M-MCM-48 catalysts The catalyst Ma-MCM-48

Selectivityb of ct-eicosanic acid (%)

Yield b of a-eicosanic acid (%)

31.9 44.9

14.9 47.8 51.6 54.4 30.0 41.8

n (Ti) / n (Si) = 0 n n n n n

(Ti) / n (Ti) / n (Ti) / n (Cu)/n (Cu)/n

(Si) (Si) (Si) (Si) (Si)

= 0.001 = 0.005 = 0.01 -- 0.01 = 0.02

aM = Ti, Cr, Mo, Zr, and Cu. bReaction temperature: 413K; Reaction time: 5h.

3.5. Influence of M content on catalytic oxidation activity The effect of M content on catalytic activities was examined. As shown in table 2, the yield of the desired product, a-eicosanoic acid, increases gradually with increasing Ti content and reaches a maximum over 1% Ti-MCM-48. However further increase in the Ti content results in a decrease in the yield. The effect of Cu content on catalytic activities over Cu-MCM-48 was also shown in Table 2. Figure 4 gave the effect of M content on catalytic activities over Cr-MCM-48, Mo-MCM-48 and Zr-MCM-48. In Table 2 and Figure 4, the yield and the selectivity of t~-eicosanic acid both increase with increasing M content. Influence of different transition metals on the catalytic properties for this reaction is seen from Table 2 and Figure 4.

70

Yield

/

6s

< 65-

~

60-

60~

~

r~

O 55 ~509 r 45-

40

4s ~ 0

i

4

;

Cr DOPED AMOUNTS, %

....

10

40

"~

Figure 4a. Effect of Cr content on the Yield and selectivity of a-eicosanoic acid over x%CrMCM-48 catalysts

1211 65:

---,

~<- 60"

,

--a---

{0 O 55.

: :: ~

,

",

,

.

. . . .

-

7o

Yield 9S e l e c t i v i t y

-6o

O ~

~50"

O r,.) ~

45,

50

O 40.

~

~< r~

4Q ~

35" .

O"

|

i.

i

'

-

l

-'

l

1 " 2 "'3' 4 5 6 7 M o DOPED AMOUNTS, %

-

i

8

'

-

y

Figure 4b. Effect of Mo content on the Yield and selectivity of a-eicosanoic acid over x%MoMCM-48 catalysts As displayed doping all the 5 transition metals in the MCM-48 sample can cause the increase in the yield and the seletivity of a-eicosanoic acid for the above mentioned reaction. Among them, Ti, Cr, Mo, and Zr show better modification function giving better yield and seletivity of a-eicosanoic acid. 8% Zr-MCM-48 gives the highest yield of 84.3 %, and a selectivity of 87.7 %. This probably is caused by the difference of valence and radium of the doped transition metal ions.

90 .... .

i

80

, ~

,

,

......

,

.

.

.

.

.

,L.

j

Yield

90~

70

~

70~i 60

i

<

60

s

so o

i

2

~

~

~

~

~

~

9

.~ ~

Zr DOPED AMOUNTS, % Figure 4c. Effect of Zr content on the Yield and selectivity of a-eicosanoic acid over x%ZrMCM-48 catalysts

1212 To sum up, the M contents of MCM-48 doped with transitional metal can influence the catalytic activities for the selectively oxidation of a-eicosanol to tx-eicosanoic acid. The catalytic activity strongly depends on the amount of the doped transition metal ions. The catalytic properties of MCM-48 catalysts doped with different sort transitional metal varied following this regular. To learn more about the catalytic properties of transition metals doped MCM-48 for the selectively catalytic oxidation of tx-eicosanol to a-eicosanoic acid, more detailed study have to be carried out, which are planned for the future. 4. CONCLUSION MCM-48 molecular sieves doped with Ti, Cr, Mo, Zr, and Cu synthesized by hydrothermal crystallization can be used as catalyst for the oxidation of a-eicosanol to its corresponding acid. M-MCM-48 exhibits higher catalytic activity for the eicosanol conversion than pure MCM-48. The sort and the amount of the doped transitional metal have important effect on the catalytic activity. The 8% Zr-MCM-48 gave the highest yield and selectivity of aeicosanic acid. ACKNOWLEDGEMENT This work was financially supported by the Committee of Science and Technology of Jilin Province, China, National Nature Science Foundation (29571011, 29873018) of China, and China Scholarship Council. REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth et al., Nature 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth et al., J. Chem. Soc., Chem. Commun. (1994) 147. 3. K.M.Reddy, I.Mondrakovski et al., J.Chem. Sot., Chem. Commun. (1994) 1059. 4. A. Corma, M.T. Navarro and J.P. Pariente, J. Chem. Soc., Chem. Commun.(1994)147. 5. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. 6. T. Blasco, A. Corma, M.T. Navarro and J.P. Pariente, J.Catal. 156 (1995) 65. 7. N. Vlagappan, C.N.R.Rao, J.Chem. Soc., Chem. Commun. (1996) 1064. 8. V. Alfredsson, M.W. Anderson, Chem. Mater. 8 (1996) 1141. 9. S. Anderson, S.T. Hyde, K. Larsson, Chem Rev. 88 (1988) 221. 10. M. Morey, A. Davidson, H. Eckert, Chem. Mater. 8 (1996) 486. 11. M.J.Hudson, J.Knowles, Chem. Mater. 6 (1) (1996) 89. 12 W. Changping, C. Qiang et al., Chem. J. Chinese Universities, 19(7) (1998) 1154 13. S. Kawi, M. te, Catalysis Today 44 (1998) 101-109. 14. K. A. Koyano, T. Tatsumi, Chem. Commun. (1996) 145.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1213

Highly selective oxidation of aromatic hydrocarbons (Styrene, Benzene and Toluene) with H202 over Ni, Ni-Cr and Ni-Ru modified MCM-41 catalysts V. Parvulescu 1, C. Anastasescu l, C. Constantin 2 and B. L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium. Ni, Cr, Ru ions incorporated mono- and bimetallic MCM-41 catalysts were investigated in the selective oxidation of styrene, benzene and toluene to benzaldehyde, phenol and cresol, respectively, with diluted hydrogen peroxide between 293-343K. Two kinds of catalysts, dried as-synthesized samples without elimination of surfactants and calcined samples where surfactants were removed, have been tested. The fresh catalysts and used ones in the oxidation reaction after several cycles were intensively characterised by XRD, N2 adsorptiondesorption, SEM, TEM, FTIR and TGA. Under the same reaction conditions, Ni-Cr-MCM-41 catalyst is much more active compared with Ni and Ni-Ru-MCM-41 catalysts in oxidation of styrene. However, all these three catalysts showed relatively low activity in the oxidation of benzene. It is shown that the activity and selectivity of Ni-MCM-41 catalysts and the efficiency of H202 depend on the presence of the second metal, amount of nickel, calcination conditions of the catalyst, temperature and time of reaction and utilization of acetonitrile as solvent. It reveals that only dried samples containing surfactants give a much higher activity in oxidation of styrene and toluene than calcined catalysts. A benefit effect of surfactant in the phase transfer of H202 has been evidenced. Active species leaching is also studied. 1. INTRODUCTION Liquid-phase oxidation of hydrocarbons with hydrogen peroxide has been studied intensively during the last years [1-4]. Considerable attention has been focused on the development of heterogeneous catalyst with transition metals (Ti, V, Fe, Mn, Co, Cr) incorporated in mesoporous materials such as MCM-41 and MCM-48 [5-13]. These materials containing transition metal ions highly dispersed in a regular heterogeneous matrix combine the activity and selectivity perfomances of homogeneous catalysts with the advantages of heterogeneous catalysts in oxidation of a wide variety of organic substrates as hydrocarbons, alcohols, thioethers, phenols [11-14]. The metal ions can be introduced by direct hydrothermal synthesis [12, 13], ion exchange and impregnation [15, 16] methods. Recently, we have reported the preparation, characterization and the interesting catalytic results of CoMCM-41, Fe-MCM-41, Ni-MCM-41, V-MCM-41, La-MCM-41, V-Co-MCM-41, La-CoMCM-41 catalysts for the oxidation of styrene, benzene, 1-hexane and alcohols [12, 13]. In this paper, we present the preparation, characterization and catalytic behavior in the oxidation of benzene, toluene and styrene using H202 (30%) ofmono- and bimetallic (Ni, Ni-Cr and NiRu) incorporated MCM-41 silica molecular sieves in order to understand the effect of the incorporation of second metal in Ni-MCM-41 catalyst. 1,2On leave from: 1Institute of Physical Chemistry" I.G. Murgulescu", Spl. Independentei202 and 2CCMMM, Spl. Independentei 204, Bucharest, Romania * Corresponding author

1214

2. EXPERIMENTAL 2.1. Synthesis The ordered mono- and bimetallic substituted MCM-41 catalysts were synthesized from a mixture with following composition: 1 SiO2: x Mn+: 0.96 Na20:0.48 (CTMABr): 3.70 TMAOH: 222 H20 (where M = Ni or Ni-Cr, Ni-Ru; x=0.4 for Ni-Cr, Ni-Ru-MCM-41 and x = 0.02-0.1 for Ni-MCM-41). Ni/Cr and Ni/Ru molar ratio was fixed at 1.0. The gel obtained was sealed into Teflon-lined steel autoclaves and heated 5 days at 373K. The as-synthesized samples were calcined at 773K. Ni-Ru/MCM-41 sample was calcined at 723K. The reagents used were sodium silicate (25.5-28.5% silica), cethyltrimethylammonium bromid (CTMABr), tetramethylammonium hydroxide (25 wt% TMAOH in water), Ni(CH3COO)2-4H20, RuC13nH20, Cr(NO3)2-9H:O, NaOH and H2SO4.

2.2. Characterization The fresh and used catalysts were characterized by XRD (Philips PW 170 diffractometer), N2 adsorption/desorption (Micromeritics, Tristar), scanning electron microscopy (SEM) with a Philips XL-20 microscope and transmission electron microscopy (TEM) with Philips Tecnai microscope, FTIR (Spectrum 2000, Perkin) and TG-DSC analysis. Concentration of the metals into the MCM-41 was obtained by atomic adsoption.

2.3. Catalytic experiments Oxidation of aromatic hydrocarbons was carried out in the thermostated glass reactor or Teflon lined autoclave with magnetic stirring in the presence or absence of the solvent (acetonitrile). The reaction temperature and time varied from 293 to 343K and from 12 to 48h, respectively. A total amount of 12 g of reagent, solvent and oxidant with a molar ratio of hydrocarbon/solvent/hydrogen peroxide of 1/-/3 for benzene and toluene or 1/3.6/3, 1/1.8/3 and 1/0/3, corresponding to 47.3, 31.0 and 0.0, molar percent of the solvent in reaction solution, for styrene and 70 mg of the catalyst were used. After reaction, the catalyst was separated by centrifugation and the oxidation products were analyzed using a GC coupled with a FID detector (Carlo Erba) with a column containing OV-101. The catalysts were reutilized in the oxidation reactions and characterized after each utilization. Hydrogen peroxide consummation was determined by iodometric titration and metal ions leaching were also verified. Two kinds of catalysts modified with metal ions have been used. One kind concerns the as-synthesized catalysts without elimination of surfactants. After synthesis, these samples were only dried. The second kind of catalysts is those after a calcination in N2 and then in air, i.e. surfactant molecules were completely eliminated. In most of cases, the calcined catalysts were used except it is stated. 3. RESULTS AND DISCUSSION

3.1. Characterization Ni incorporated meosporous sieves show 4 diffraction lines at low angle region, characteristic of mesoporous materials with very regular hexagonal arrangement of their cylindrical channels. The diffraction signal of the Ni-MCM-41 powder material (Fig.lA) is affected by incorporation of the second metal. The unit cell variation was accompanied with a loss of structural ordering. In Figure 1A it can be seen the broadening and disappearance of the (110) and (200) reflections and

1215 Table 1. Concentration of the metal and characteristics of the mono- and bimetallic mesoporous sieves M cont. a0 SBET OBJH Catalyst M cont. ao SBET OBJH nm m2/g nm nm m2/g nm wt.% wt.% Ni-N (1) 1.19 3.42 4.6 945 2.85 Ni-N (2) 4.7 828 2.63 Ni-Cr-N 1.665.34 4.1 914 2.74 Ni-N (3) 4.6 777 2.58 1.36 Ni-Ru-N 1.467.12 4.9 805 2.49 Ni-N (4) 4.3 654 2.68 1.61 Ni-N (1)s 1.19 Ni-N (5) 9.04 4.7 4.2 568 2.72 N: MCM-41, 1-5 are Ni-MCM-41 samples with x=0.02, 0.04, 0.06, 0.08 and 0.1, s: dried sample Catalyst

2

4

6

8

10

Ni-MCM-4I

~

20

~

~

~o

Fig. 1. XRD patterns of the calcined samples

Fig. 2. TEM images of the Ni (a) and Ni-Ru (b) -MCM-41 materials

weakening of the (100) peak. The increase in the amount of the Ni in the MCM-41 support (Fig. 1B) induces also the reduction of structural ordering. As can be seen that the intensity of (100) plane and the resulotion of secondary reflections decrease with increasing the amount of supported Ni (5-9 % w.t.). All the prepared samples were analyzed by TEM and show the highly ordered hexagonal arrangement of cylindrical channels (Fig. 2) although the XRD patterns of some samples show less ordering and less resolved secondary reflections. This phenomenon was previously observed for the highly ordered metal ions incorporated mesoporous materials. The spherical morphology visualized by SEM (Fig. 3) is Fig.3. SEM image of typical of transition metal ions modified mesoporous samples the Ni-Cr-MCM-41 [12,13]. catalyst after The concentration of the metals incorporated, the calcination changes in the porous structure and textural characteristics of the mono and bimetallic molecular sieves are listed in Table 1. All catalysts obtained have a very high surface area which, however, decreases by incorporation of the second metal and with increasing the Ni content into the MCM-41 molecular sieve. The pore size determined by BJH method shows a very narrow monomodal pore size distribution centered at about 2.6 + 0.2 nm (Table 1).

1216

3.2. Catalytic activity 3.2.L Effect of second metal inorporation, metal content, types of reactors and presence of surfactant in the mesopores The activity and selectivity of the catalysts was varied with the incorporated metal. The association of the nickel with other trivalent cations modifies activity and selectivity. Under all experimental conditions investigated for oxidation of styrene, toluene and benzene the principal reaction products are benzaldehyde, benzyl alcohol and phenol, respectively. Table 2 compares the activities of fresh and reused catalysts under identical reaction conditions. NiCr- MCM-41 catalyst has shown to have the highest activity and selectivity in oxidation reaction of styrene and relatively low activity has been observed for Ni-MCM-41 and Ni-RuMCM-41 samples. The introduction of Cr in Ni-MCM-41 gives a benefit effect in aromatics conversion and in the production of desired product (benzaldehyde) and the efficiency of the H202 while the incorporation of Ru in Ni-MCM-41 reduces the styrene conversion and the efficiency of H202 but the selectivity seems to be improved (Table 2). The highest catalytic activity and selectivity of Ni-Cr-MCM-41 catalyst are attributed to Cr 3+, more instable, compared to Ru 3§ and their possibility to form Cr 6§ Table 2. Results of styrene selective oxidation on fresh (I) or reused (II, III.) catalysts

I.Ca,

Catalyst

Ceff.H202,

%

Said.,

%

II.Ca, %

%

CeffH202, %

Said., %

III.Cst., CefrI-I2O2, Saict, % % %

12.4 48.8 8.6 22.1 36.2 Ni-N a 12.8 5.5 62.8 16.7 . . . . . Ni-N b 15.2 24.6 40.0 NiCr-N a 79.5 31.2 92.7 84.2 38.4 90.2 64.2 40.1 79.8 . . . . . NiCr-N b 80.1 42.6 80.1 5.8 3.2 22.6 69.1 NiRu-N a 8.1 2.7 84.5 12.2 , 86.4 . . . . . NiRu-N b 10.2 3.4 38.4 N: MCM-41, Reaction conditions: meat: 70g, temperature: 343K; time" 48h and molar ratio" 1/1.8/3 (a: in glass reactor and b: in autoclave reactor) 00~

60

o~

601

,.o

40 I

50 40

(d) -" St(c) ~ T (d)

// //

30

...............................................................................

.2

20 ~r-~/

---r

Bz (d)

/

r

_~ 20 cD

0

0,01

0,06

0,11

Ni/Si molar ratio

Fig. 4. The effect of the nickel concentration on the conversion (343K, 48h,1/1.8/3)

0,02

0,04

0,06

0,08

0,1

Ni/Si molar ratio

Fig.5. Effect of the nickel content on the conversion of styrene

1217 In autoclaves, the conversion, the selectivity to benzaldehyde and the efficiency of the H202 (H202 quantity used for oxidation/H202 quantity transformed) are only slightly higher than those obtained in the glass reactor. Figures 4 and 5 show the variation of the conversion of styrene, toluene and benzene with increasing nickel content for nickel catalysts (dried-d or calcined-c). It can be seen that both dried and calcined Ni-MCM-41 catalysts have a very low activity in the oxidation of benzene. A much higher activity of the dried catalysts compared to that of calcined ones in oxidation reaction of styrene and toluene was evidenced. For dried catalysts, the conversion of styrene increases first with increasing metal content and then reaches a plateau from a Ni/Si molar ratio of 0.07 while that of toluene increases sharply first and then decreases from a Ni/Si molar ratio of 0.06. However, it has been shown that the Ni content in the range of 0.020.04 gives much higher selectivity to benzaldehyde in the oxidation reaction of styrene while the higher Ni content, the high activity. It has to find a compromise for a catalyst with good activity and selecivity. This indicate that only the Ni species incorporated in the framework will be active, at high loading of Ni, all the Ni species can not be incorporated in the framework. The extraframework Ni species will block the pores an active species and reduce the surface area, in consequence, the activity will be reduced. The presence of the surfactants in the mesopores which will be favorable for the liquid phase transfer of H202 can explain the high activity of the dried catalysts in oxidation of styrene and toluene since in liquid phase, the surfactant molecules in the mesopores can be extracted by solvent and leave the places to reactants. These surfactant molecules in the liquid phase can be considered as solvent which will reduce the possible decomposition of H202 and facilitate the transport of H202 into mesopores for the oxidation. However, more studies have to be performed to better understand why the presence of surfacants can play this very favorable role in the oxidation of styrene and toluene. 3. 2. 2. Effect of reaction time and temperature Figures 6 and 7 depicts the variation of styrene conversion of Ni-, Ni-Cr- and Ni-RuMCM-41 calcined catalysts as a function of reaction temperature (Fig. 6) and time (Fig. 7). It is evident that the higher reaction temeprature and longer reaction time, the higher catalytic

8oi/

21

[g$ 293 K

90 ~-...................................................................

]

80

i t

]

l i 323 K

,

sot

i

40 4

I= o "~,~

~., 30

-

30 -]

]

t

r

-

-

-

---I---Ni-Cr-MCM-41

10-

0,/

Ni-MCM41

NiCr-MCM41 NiRu-MCM41

Fig. 6. Effect of temperature on the activity of the catalysts in oxidation of styrene

0

10

20

30

40 Tnaae,h

50

60

Fig.7. Effect of the reaction time on them conversion of styrene over nickel catalysts

1218 activity. At any moment and any reaction temperature, Ni-Cr-MCM-41 catalyst gives the highest conversion. It has to stress that after around 20 hours for Ni-MCM-41 and Ni-RuMCM-41 catalysts and 36 hours reaction for Ni-Cr-MCM-41 catalyst, the reaction reaches the maximum conversion. (Figure 7). The polycondesation products are observed after 28 h reaction. It is observed that the increase in the reaction temperature and concentration of the metal and the decrease in the acetonitrile molar percent can accelerate the rate of secondary reactions (polymerization of styrene, polycondesation of the oxidation products, decomposition of hydrogen peroxide).

3.2.3, Catalyst leaching study 90-

For this study, we have carried out the first experiment using styrene, 80solvent and H202 without catalysts. No conversion of styrene was observed. 70Then we performed the experiments as with cat, follow. Alter first cycle reaction, the Ni-Cr-MCM-41 catalyst was separated 60i ~ cat, removed by centrifugation. The liquid phase was subjected to ftn'ther reaction with new styrene reactant and H202 oxidant 20 30 40 50 without catalyst under identical reaction Time, h conditions. Although the reaction with fresh catalyst can achieve a higher conversion value, the filtrate can still convert around 80% of styrene after 30 Fig. 8. Effect of Ni-Cr-MCM-41 catalyst hours reaction. This means that some separation on the conversion of styrene at 343K active species remain in the filtrate, indicative of leaching of metal ions. The fmal reaction solution was analyzed using atomic adsorption spectroscopy, proving the presence of metal ion species in the filtrate. This demonstrates also that the hydrocarbons conversion can proceed both through heterogeneous and homogeneous catalysis on Ni, Ni-Cr and Ni-Ru-MCM-41 catalysts. Presumably, a minor amount of Cr leached out from the catalyst may be able to catalyze the reaction. .......

50

~

It

,

3. 2. 4. Recycling of the catalyst In order to check the stability of the catalysts, three reaction cycles were carried out using the same catalyst in the oxidation reaction of styrene. The catalytic results on the reaction, presented in Table 2, evidence an increase of the conversion in the second reaction cycle with similar selectivity and a decrease in activity and selectivity in the third cycle reaction. Chromium and ruthenium leached was evidenced only after first reaction cycle. No leaching was observed after second utilization of catalysts. After each reaction the catalyst was removed by centrifugation from reaction mixture, washed with acetonitrile, dried at 373K and characterized by XRD, TEM, SEM, IR spectroscopy. The increase in the catalytic activity after first utilisation of catalyst and the presence of the aromatic species, evidenced by IR spectra (Fig.9) confirm the radical mechanism of the oxidation reaction. The strong adsorption of the aromatic species on the surface catalyst is a limiting state for the oxidation of styrene. We will not go further here to discuss the reaction mechanism, a detailed study is carrying out to better understand the reaction pathway.

1219

3.2.3. Characterization of used catalysts

~,,~_~\~~'~_

The IR spectra (Fig.9) ofNi/MCM-41 catalyst after the first reaction and after desorption of all the samples at a series of temperatures (293,373, 623 and 723K) evidenced a strong adsorption of the aromatic species (styrene, benzaldehyde and condensation products). Complete desorption of these species after 623K, evidenced by IR spectra (Fig. 9) was confirmed by thermal analysis, too. After oxidation of benzene and toluene a weak adsorption of the aromatic species on the catalysts surface was also evidenced by IR spectroscopy (not shown here).

after reaction

350 ~ 450 ~ 1400

16'00

18'00

2000

Wavenumber,c m "1

Fig. 9. FTIR spectra of used Ni-RuMCM-41 after desorption at different temperatures

:N

. ..- ,~

i?: ~-~

Fig.10. TEM images of the Ni-Ru MCM-41 catalysts after oxidation of benzene (a) and oxidation of styrene (b) After reactions, the TEM and SEM micro-graphs (Fig. 10 and 11) show that the hexagonal structure and spherical morphology ofMCM-41 were conserved. 4. CONCLUSIONS Nickel, chromium and ruthenium species are able to incorporate into the framework of MCM-41 and act as active and selective sites for the oxidation of styrene to benzaldehyde, of benzene to phenol and toluene to benzaldehide. The catalysts with an ordered hexagonal structure are stable and active in three cycle of reaction. The introduction of Cr and Ru into Ni-MCM41 material gives a beneficial and negative effect on the catalytic activity in the oxidation reaction. The increase in Ni content can increase the activity, but too high Ni content, no further

Fig.11. SEM images of the Ni-Cr-MCM41 catalysts after first reaction

1220 beneficial effect on the catalytic activity. This is probably due to the dispersion of Ni species. Only the Ni species incorporated in the framework will be active. ACKNOWLEDGMENTS

This work was performed within the framework of PAI-IUAP 4/10. VP thanks the SSTC (Federal scientific, technological and cultural office of Premier Minister, Belgium) for a scholarship and a research grant from The University of Namur and Direction G6n6rale des Relations Ext6rieures du Gouvernement de la R6gion Wallonne, Belgique for CC and CA, respectively is also acknowledged. REFERENCES

1. 2. 3. 4. 5. 6.

M. Dusi, T. Mallat and A. Baiker, Catal. Rev.Sci. Eng., 42 (2000) 213. S. Biz and M.L. Occelli, Catal. Rev.Sci. Eng., 40 (1998) 329. A.P. Singh and T. Selvan, J. Mol. Catal. A, 113 (1996) 489. J. Okamura, S. Nishiyama, S. Tsuruya and M. Masai, J. Mol. Catal. A, 135 (1998) 133. D. Wei, W.T. Chueh and G. Haller, Catal. Today, 5 (1999) 501. F. Di Rezo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater., 28 (1999) 437. 7. C. M. Pradier, F. Rodrigues, P. Marcus, M.V. Landau, M.L. Kaliya, A. Gutman, and Herskowitz, M., Appl. Catal. B., 27 (2000) 73. 8. W.A. Carvalho, M. Wallau and U. Schuchadt, J. Mol. Catal. A., 144 (1999) 91. 9. T. Blasco, A. Corma, M.T. Navarro, J.P. Pariente, J. Catal. 156 (1995) 65. 10. Z. Y. Yuan, S. Q. Liu, T.H. Chen, J.Z. Wang, H.X. Li, J. Chem. Soc., Chem. Commun. (1995) 973. 11. D. Zhao, D. Goldfarb, J.Chem.Soc., Chem. Commun (1995) 875. 12. V. Pgtrvulescu, C. Dascalescu and B.L. Su, Stud. Surf. Sci. Catal., 135 52001) 4772 13. V. Parvulescu and B.L. Su, Catal. Today, 69 (2001) 315 14. N. N. Trukhan, A. Yu, Derevyankin, A.N. Shmakov, E.A. Pankshtis, O.A. Kholdeeva and V.N. Romannikov, Microporous Mesoporous Mat., 44-45 (2001) 603. 15. G. Grubert, J. Rathousky, G. Schulz-Ekloff, M. Wark and A. Zukal, Microporous and mesoporous Mater., 22 (1998) 225 16. D. Brae and K. Serf, Zeolites, 17 (1996) 444

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1221

Mesoporous materials as supports for heteropolyacid based catalysts 9 r

*

M. Gulblnska, M. W6jtowski, M. Laniecki Faculty of Chemistry, A Mickiewicz University, ul. Grunwaldzka 6, 60-780 Poznafi, Poland Siliceous mesoporous materials of the MCM-41 type, as well as those containing aluminum or both aluminum and boron, were applied as supports for phosphomolybdic acid and its nickel and cobalt salts. Three series of the catalysts: reduced, sulfided and nitrided were obtained respectively during reduction in hydrogen, sulfidation with H2S and NH3 nitridation of the impregnated mesoporous materials at elevated temperatures. These catalysts were tested in hydrogenolysis of tripropylamine. The best performance was found for catalysts impregnated with nickel salt ofheteropolyacid and nitrided at 875 K. This effect was assigned to the presence of medium strength Lewis acid sites at the initial stage of the reaction and high activity of molybdenum nitride. The XRD, temperature programmed reduction (TPR), FTIR with pyridine and BET surface area measurements were applied to characterize Mo, Ni, or Co containing catalysts. 1. INTRODUCTION The discovery of the siliceous mesoporous structures of M41S type by the researchers of Mobil [1] and Toyota [2] triggered a new era of studies on these materials. Although, pure siliceous mesoporous materials still attract the great attention in many laboratories, the new classes of mesoporous inorganic materials including oxides [3-5], sulfides [6], or even metals [7,8] have been already reported. Simultaneously with the discoveries of new types of mesoporous structures different groups tried to modify the basic structures by post-synthesis procedures which included grafting, anchoring, or encapsulation of different organic and inorganic species. For example, Ernst and Selle [9] introduced Ru2+-tetrafluorophtalocyanine into the channels of MCM-41, Armengol et al.[ 10] synthesized Cu 2§ and Co2+-phtalocyanines inside mesoporous silica-alumina materials, whereas Diaz and Balkus [ 11 ] applied MCM-41 as supports for differing in size enzymes. Salen complex of chromium(III) anchored inside MCM-41 channels indicated much higher catalytic activity than homogeneous system [ 12]. An application of MCM-41 mesoporous materials as the supports for hetropolyacids has been first reported by the group of Kozhevnikov and van Bekkum [13,14]. Although proposed systems indicated relatively high catalytic activity, the active phase of heteropolyacid was easily washed out in contact with polar solvents. Works by Nowifiska and Kaleta [15,16] showed the possibility of application of encapsulated heteropolycompounds in MCM-41 and MCM-48 as good catalysts for synthesis of bisphenol-A. An application of heteropolyacids supported over MCM-41 materials as catalysts in cracking or hydrocracking processes of heavy fractions of petroleum can significantly influence a yield *Present address: Department of Chemistry, Univ. Connecticut, Storrs, USA

1222 of light hydrocarbons. Moreover, due to the uniform porous structure of mesoporous materials they can be used as excellent supports for Ni, Co and Mo ions (usually supported on alumina) and used in sulfided form in hydrocracking, hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) reactions. Because the existing catalysts which have been developed largely for petroleum HDS reactions are clearly not optimum for the HDN there is still a need to develop better catalysts for this process. Hydrodenitrogenation involves both hydrogenation and carbon-nitrogen bond scission therefore HDN catalysts must be bifunctional, having both hydrogenation and hydrogenolysis sites. From the reasons described above it seemed to us that properly modified mesoporous materials with supported heteropolycompounds can fulfill the requirements of good catalyst for HDN. This paper presents results of hydrogenolysis of spacey tripropylamine molecule over MCM-41 supported, pretreated (1-12,H2S, NH3) phosphomolybdic heteropolycompounds.

2. EXPERIMENTAL Mesoporous supports of the MCM - 41 type were synthesized according to the procedures by Beck et al. [17] and Schmidt et al. [18]. Cetyl- or myristylammonium bromides from Aldrich, containing respectively 16 or 14 carbon atoms in the alkyl chain of surfactant, were applied as templates both in the synthesis of the silicious materials as well as those containing aluminum or boron atoms. Sodium aluminate (p.a. P O C h - Poland) or aluminum borate (Int. Enzyme Ltd., Windsor- England) were used respectively as the sources of A1 or B. Supports after removal of unreacted compounds were thoroughly washed with distilled water and dried in air. After removal of the surfactant by 1 hour heating at 815 K in Ar stream followed by calcination for 6 hours at the same temperature, the mesoporous supports were next impregnated at room temperature with 5, 10 or 20 % ethanol solutions of phosphomolybdic heteropolyacid ( H P A - from Aldrich) and its nickel (NiHPA) or cobalt (CoHPA) salts by the incipient wetness technique. All impregnated samples were calcined for 2 hours at 675 K. Nickel or cobalt salts of phosphomolybdic acid were prepared from fleshly precipitated respective carbonates (anhydrous NiC12 or COC12 from Merck reacted with boiling solution of Na2CO3) and HPA according to the method described by Tsigdinos [ 19]. Supports and catalysts were characterized by XRD (modified T U R - 21 spectrometer), measurements of surface area and porosity (ASAP - 2010 from Micrometrics) and temperature programmed reduction (TPR) (ASAP - 2007 - Micrometrics). Acidity of the studied samples in the oxidized form were measured with FTIR spectroscopy applying pyridine as probe molecule. Catalytic hydrogenolysis of tripropylamine (Merck-Schuchardt, p.a.) was performed in fixed-bed reactor under atmospheric pressure in stream of hydrogen flowing with the rate of 3 dm 3. h -1. Catalyst weighing 0,45 g (grains 0,5 - 1 mm) before the reaction were either reduced in 1-12or presulfided in catalyst reactor in a stream of 10 vol. % of H2S in 1-I2 for 2 hours at 675 K. Another series of catalysts was nitrided in quartz catalytic reactor with dried ammonia and heating rate of 15 K. min 1 up to 875 K. Pure, siliceous supports are designated as A14 or A16, whereas those containing aluminum from NaA102 depending from the A1 content are designated as e.g. A16.2, A16.6 etc. Supports containing boron are described as B family. Other rules are the same as for those with aluminum. Concentration of the solution used to support HPA, NiHPA or CoHPA is expressed in parentheses after the symbol of the applied support.

1223 3. RESULTS AND DISCUSSION X-ray diffraction patterns of the siliceous materials applied in this study as supports for Mo, Ni-Mo or Co-Mo loaded catalysts, exhibited characteristic reflexes of the hexagonal structure (dl00, da]0, d2o0 and d210) [ 17 ] of the MCM-41, independently of template applied. Our attempts with isomorphous substitution with aluminum atoms at low concentrations always resulted in the loss of hexagonal symmetry. This was demonstrated by the decrease in the intensities of the XRD low-angle reflexes. Similar effects were observed while aluminum borate was applied during synthesis of the mesoporous supports containing boron. The effect of decreased crystallinity upon incorporation of aluminum or boron into the MCM-41 structures finds reflection in a loss of specific surface area of the studied supports as well. Data presented in Table 1 clearly indicate that increased concentration of A1 in the crystalization liquor can reduce BET surface area of the final product of about 50-70 %. Simultaneously, the pore size distribution can serve as a good indicator of the support amorphization. An average pore radius calculated by the BJH method from the desorption curve of adsorbed nitrogen shows 4-fold increase in the case of A14 samples and 3-fold increase for A16 samples containing large amounts (Si/A1 = 4.8) of aluminum. An application of the aluminum borate for synthesis of mesoporus materials resulted in the similar decrease of specific surface area with increasing boron content. However, all materials containing boron were more stable than those with aluminum. Table 1 Surface properties of mesoporous materials impregnated with phosphomolybdic heteropolyacid.

Sample A 14 A 14.2 A 14.8 A 14(5)* A14(10)* A 14(20)* A14NiHPA* A14CoHPA*

Si/A1

BET surface area [m:.g -1] Before After

Average pore radius [A] Before After

oo 17.5 4.3

Impregnation 1215 895 1055 188 377 141

Impregnation 13 26 20 29 50 65

oo oo oo oo ~

-

1175 1146 895 1032 1050

-

27 27 26 25 26

Pore volume [cm3.g "]] before After Impregnation 1.03 0.45 0.91 0.44 0.72 0.40 -

0.97 0.76 0.45 0.62 0.60

A 16 oo 1070 707 18 16 1.22 0.51 A 16.2 17.5 1029 487 25 27 1.65 0.71 A 16.4 8.7 846 270 30 38 1.13 0.45 A16.6 5.7 497 175 46 61 0.96 0.45 A 16.8 4.3 542 196 48 60 1.09 0.49 A16 AHM** oo 171 30 0.30 * impregnated with HPA, NiHPA or CoHPA. Numbers in parentheses indicate initial concentrationof HPA ** impregnated with ammoniumheptamolybdate (AHM).

1224 Fig. 1. Pore size distribution of the samples containing boron. Filled circles - B16.1 (Si/B = 35) Open circles - B 16.2 (Si/B = 17) Triangle - B 16.4 (Si/B = 9) Square - B 16.8 (Si/B = 4)

0,4

~<

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f |I

I t

I ,, ,

5

10

15

20

25

30

35

4O

Avarage radius [A] 0,35

Fig. 2. Pore size distribution and N2 isotherms of the siliceous sample A16 (filled circles) and A16 loaded with phospho molybdic acid (open circles). Mo content = 6.8 wt.%

0,30 ,--.-, 0,25 <

"7

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.

.

.

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All materials with well defined mesoporous structure shows on the respective adsorptiondesorption isotherms the characteristic step at p/po - 0.3 (see the example on Fig.2). On the other hand an appearance of large, irregular hysteresis loop at p/po-~0.7-0.9 indicated that regular mesoporosity for such samples does not exist any more. This was reflected in pore size distribution. Figure 1 exemplifies the typical pore size distribution for the samples containing boron, however, similar shapes were obtained for those with different content of AI. Increasing boron content gives less uniform porous structure, what is demonstrated by the shift of the average pore radius towards higher values. Impregnation of the mesoporous materials of well defined structure, as well as those with less uniform porosity, with phosphomolybdic heteroploacid (I~A) or its nickel (NiHPA) or cobalt (CoHPA) salts resulted both in the decrease of total volume of nitrogen adsorbed and in consequence decrease in BET surface area of all samples. After impregnation with solutions containing 20% of HPA, N H P A or CoHPA a pore volume in majority of samples decreased of about 50%. Curves presented on Fig.2 and Fig.3 shows that incorporation of heteropolyacid and its salts inside regular mesopores do not cause changes in the geometry of channels. The shape of nitrogen adsorption-desorption isotherms is well preserved indpendently of the

1225 700

,

600 I 500

- 0 - A14 .-o-9 A14(5)HPA A14(10)HPA ---O-- A14(20)HPA

-

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i

0,2

i

0,4

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,

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Fig. 3. Nitrogen isotherms of the A14 samples loaded with different amount of HPA the template applied and concentration of molybdenum containing compound. In contrast, an application of ammonium heptamolybdate (AHM) as a source of Mo for impregnation resulted in almost complete clogging of the mesoporous structure. No maximum characteristic for mesopores after impregnation with AHM (Mo concentration the same as for HPA) was observed. Samples containing boron showed very similar effects upon incorporation of HPA, NiHPA or CoHPA inside the mesoporous structure. Infrared studies indicated that upon calcination of supported heteropolycompounds at 675 K the Keggin structure is still well preserved what was demonstrated by the presence in the infrared spectrum characteristic bands at 958 (M=O2) and 880 cm "1 (M-O3-M) [20]. The measurements of acidity with pyridine as a probe molecule, confirmed the absence of any acidity for siliceous materials. Presence of A1 or B atoms in the mesoporous materials generate relatively strong acidity, of the Lewis type (presence of bands at 1444 and 1600 cml). The strongest Lewis type acidity was observed for samples with Si/A1 or Si/B higher or equal 17. For such samples bands at 1444 and 1600 cmldisappeared only after desorption at 523 K. The appearance of the bands at 1540 and 1452 crn] for mesoporous materials upon impregnation with HPA and calcination at 675 K indicates that generation both Brrnsted and Lewis acidity inside the channels of MCM-41 occurs. These acidic centers are relatively strong because the characteristic bands disappears only after desorption at temperatures higher than 473 K. According to the works of Kozhevnikov [14] it is expected that proton a~er dehydration can be localized on the terminal oxygen atoms of Keggin units. An impregnation of MCM-41 materials with nickel or cobalt salts of HPA in oxidizing atmosphere results in complete elimination of the Brrnsted acidity detectable via infrared spectroscopy. The only acidity detected was of the Lewis type and proportional to the concentration of the impregnating salt. According to the suggestion of Misono [21 ] the source of proton acidity in heteropolysalts can be related with the reduction of cations as well. This reason prompted us

1226 400 350

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0 400

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5 w t % Co-HPA 10 wt % Co-HPA 20 wt % Co-HPA

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100

200

400

600

800

1000

Temperature [ K ]

Fig. 4. TPR profiles of M C M - 41 impregnated with HPA, NiHPA and CoHPA.

1200

to perform a series of experiments with temperature programmed reduction (TPR) of the studied systems. Fig. 4. presents the TPR profiles of A14 mesoporous material supported with different amounts of phosphomolybdic hetropolycompounds. Similar shapes and intensities of the TPR profiles were always obtained for samples containing A1 or B (Si/A1 or Si/B close to 17), however, those maxima we re always shifted of about 50 K towards lower temperature. The assignment of the characteristic reduction maxima to the specific oxidation number using TPR technique is very difficult, especially that there are not too many literature data concerning this specific topic. Misono [20] proposed for the reduction of the solid H3[PMo12040] multistep mechanism involving reduction to the Mo 5+ ions without removal of oxygen, followed by formation of Mo 4+ species and water evolution. In our case with supported NiHPA and CoHPA mainly two reduction maxima were always observed. Both for low and high loadings samples containing NiHPA showed better reduction abilities (temperatures lower of about 50 K) than for those with CoHPA. Comparison of these results with pure supported HPA suggests that the first maximum (- 750-820 K) is related with the reduction of M06+ probably to Mo4+.Simultaneusly the reduction of nickel or cobalt ions can occur at the same range of temperatures. This can find a confirmation in the inten-

1227 Table 2 Hydrogenolysis o f tripropylamine over A16 and B 16 supports impregnated with HPA and NiHPA. T e m p . r e a c t i o n - 525 K, Mo c o n t e n t - 6 . 8 wt.%.

Conversion after 1 hour [%]

Si/AI or B/A1

H2-675K* HPA Ni-HPA

H2S/H2-675K* HPA Ni-HPA

NH3-875K* HPA Ni-HPA

A16 A16.2 A16.4 A16.6 A16.8

oo 17.5 8.7 5.7 4.3

12.4 35.3 19.4 10.0 13.8

18.2 53.0 21.0 18.4 20.0

46.7 28.2 23.3 22.4 20.8

66.5 43.2 41.8 40.0 35.3

60.4 93.2 30.6 18.3 9.2

B16 B16.1 B16.2 B16.4 B16.8

oo 35.0 17.1 9.3 4.0

4.2 7.1 13.1 17.8 13.7

-

8.4 10.2 24.2 28.2 13.9

-

4.8 12.0 21.0 20.3 14.9

Support

88.3 100.0 60.8 -

* pretreatment in different atmospheres Table 3 Activity and selectivity during hydrogenolysis of tripropylamine over MCM-41 materials loaded with molybdic heterpolycompounds. Temp. reaction- 575 K, Mo c o n t e n t - 6.8 wt.%

Catalyst

Reduction H 2 - 675 K Activity[%]

A14(5) H P A A14(10) H P A A14(20) H P A A14(5) N i H P A A14(10) N i H P A A14(20) N i H P A A14(5) C o H P A A14(10) C o H P A A14(20) C o H P A

43.7 75.4 80.3 70.1 95 2 100.0 59.4 778 86.0

Selectivity[%] 16.7 20.3 26.8 20.8 50.8 37.0 17.7 45.3 41.2

Sulfidation H2S/H2 - 675 K Activity[%] 69.2 84 3 100.0 100.0 100.0 100.0 88.6 93.3 90.0

Selectivity[%] 243 26 5 308 42.0 63.3 48.6 24.0 49.7 43.3

sities of the maxima at 750 and 770 K respectively for Ni 2§ and Co2+.These maxima shows much higher intensities than those related with supported HPA and same concentrations. Second maximum at 920 K can be linked with more deep reduction of molybdenum ions and at this point a total decomposition and reduction even to Mo o can not be excluded. Still remain unresolved the degree of reduction of nickel and cobalt ions in supported NiI-IPA or CoHPA. The detailed study concerning this topic is currently under way. The intention of TPR measurements was to find out any effects which can be related to the

1228 catalytic activity of the studied materials in the tripropylamine hydrogenolysis reaction. Table 2 and 3 presents part of the catalytic tests results. It was found that among the products mainly propane and small amounts of propylene were present. All catalysts deactivate in time, however, alter one hour of reaction significant stabilization in conversion was observed. In all studied cases the best catalytic performance was found for catalysts impregnated with NiHPA. These catalysts independently of the pretreatment conditions, temperature of reaction or concentration of the heteroplycompound showed the best catalytic activity. Moreover, also selectivity towards propane was the highest. High activity of the NiHPA based catalysts can be related to the low reduction temperature of nickel ions and consequently lower temperature of Ni-Mo-S or Ni-Mo-N species formation. The absence of strong BrOnsted acid sites for NiHPA or CoHPA based catalysts decrease the ability of carbocations formations and finally results in better selectivity. Proposed synthesis and test reaction allowed us to establish the first rules governing the preparation future good catalysts for HDN reaction based on MCM41 materials and reacting with large organic molecules.

Acknowledgement This work was supported by the Polish State Committee for Scientific Research (KBN) within the project : 7 T09B 027 21. REFERENCES 1. 2. 3. 4.

C.T. Kresge, M. Leonowicz. W. Roth, J.S. Beck, Nature, 359(1992)710 T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull.Chem.Soc.Japan, 63(1990)988 S. Bagshaw, E. Prouzet, T. Pinnavaia, Science 269(1995)1242 Q. Huo, D. I. Margolese, U. Ciesla, P.Feng, T. E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth, G. D. Stucky, Nature, 368(1994)317 5. J.Kim, C. Shin, R. Ryoo, Catalysis Today, 38(1997)221 6. M.T. Anderson, P. Newcomer, Mater. Res. Soc. Symp. Proc. 3 71 (1995) 117 7. G.S. Attard, C.G. Goltner, J.M. Corker, S. Henke, R.H. Templer, Angew. Chem. Intl. Ed. Engl. 36(1997) 1315 8. H. Kang, Y.-W. Jun, J.-I. Park, K.-B. Lee, J. Cheon, J. Chem. Mater. 12(2000)3530 9. S. Ernst, M. Selle, Microporous and Mesoporous Mater. 27 (1999) 355 10. E. Armengol, A. Corma, V. Fornes, H. Garcia, J. Primo, Appl. Catal.A., 181 (1999)305 11. J. F. Diaz, K.J. Balkus Jr., J. Mol. Catal. B. Enzymatic, 2 (1996) 115 12. S. Koner, K. Chaudhari, T. K. Das, S. Sivasanker, J. Mol. Catal.A. Chem. 150 (1999)295 13. I. V. Kozhevnikov, A. Sinnema, R.J.J. Jansen, K. Pamin, H. van Bekkum, Catal. Lett., 30 (1995) 241 14. I. V. Kozhevnikov, K. R. Kloestra, A. Sinnema, H.-W. Zandbergen, H. van Bekkum, J. Mol. Catal.A. Chem., 114 (1996) 287 15. K. Nowiflska, W. Kaleta, Appl. Catal. A., 203 (2000) 91 16. W. Kaleta, K. Nowifiska, Chem. Comm. 2001,535 17. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C. T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J.Am. Chem. Soc., 114 (1992) 10834. 18. R. Schmidt, D. Akporiaye, M.Stocker, O.H.Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61 19. G.A. Tsigdinos, Ind. Eng. Chem., Prod. Res. Develop., 13 (1974) 267 20. M. Misono, Catal.Rev.-Sci. Eng, 29 (1987) 269 21. T. Okuhara, N. Mizuro, M. Misono, Adv. Catal., 41 (1996) 113

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1229

Synthesis and characterization of A 1 - M C M - 4 8 type materials using coal fly ash P. Kumar*, N.K. Mal, Y. Oumi 1, T. Sano I and K. Yamana Ceramic Section of Chemistry & Food Department, Industrial Research Institute of Ishikawa Kanazawa, Ishikawa 920-0223, Japan. 1School of Materials Science, Japan Advanced Institute of Science & Technology, Tatsunokuchi, Ishikawa 923-1292, Japan.

Supernatant of the coal fly ash solution was used to prepare aluminum containing MCM48 (A1-MCM-48). It was found that most of the Si and A1 components in the fly ash could be effectively transformed into MCM-48 when a surfactant mixture containing cationic cetyltetramethylammonium bromide, CTMABr and tetraoxyethylene dodecyl ether, C12(EO)4 were used as templates. Alkali fusion was found to be necessary as it improves the hydrothermal condition for synthesis of the mesoporous materials. High degree of aluminum incorporation into the tetrahedral positions was revealed when A1-MCM-48 was prepared under controlled pH condition. 1. INTRODUCTION MCM-48 (cubic, space group Ia3d) with its highly branched and interwoven threedimensional networks of the mesopore channels is one of the most interesting mesoporous materials among many mesoporous silica molecular sieves [1]. It is believed that MCM-48 is much more resistant to pore blockage than one-dimensional channel system with a hexagonal MCM-41 while being used as absorbents and catalyst supports [1-6]. Although the discovery of the MCM-48 materials was reported simultaneously with the hexagonal MCM-41 in 1992 [1-2], research reports until now on synthesis and application of these materials have been severely biased to MCM-41 [3-4]. The bias may be attributed largely to the fact that the synthesis of MCM-48 required very specific synthesis conditions [5]. Presently however, both the economic and environmental costs for large-scale manufacture of these materials are high due to the cost and toxicity of both templates and preferred silica source. A variety of silica sources are generally used to prepare these materials including fumed silica and silicon tetraethoxide. The industrial manufacture of mesoporous materials is likely to be economically prohibitive if silicon alkoxides and fumed silica in particular are selected. * Corresponding author. Tel: + 49-241/80-20115; Fax: + 49-241/8022-291 E-mail: pnt67 @hotmail.com Present address: Chemical Technology and Heterogeneous Catalysis University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany.

1230 Since the synthesis of MCM-48 requires some very specific condition, a variety of synthesis routes have been developed in order to overcome the synthesis shortcomings [7]. These synthesis results demonstrated that the crystallinity of the MCM-48 pass through an optimum as a function of time. The MCM-48 products were obtained as an intermediate between a hexagonal or disordered surfactant-silica mesophase and a more stable lamellar mesophase [8]. Similarly, one report suggested that the transformation of the MCM-48 mesophase to lamellar can be quenched by adjusting the pH of the reaction mixture [9]. Another report indicated that the mixed surfactant approach resulted into high quality MCM48 as an energetically favored mesophase [10]. Very recently, it was reported that the use of gemini surfactants induce the formation of cubic structure even using fumed silica as silicon source [11]. All these studies indicate that the formation of MCM-48 type materials is possible under certain synthesis conditions. Coal combustion, which accounts for about 37% of the world's electricity production generates, about 600 million Tons, coal fly ash per year as a by-product [12]. Current applications of this vast amount of coal fly ash (only 15%) is not enough and requires further attention to utilize this waste material [13-16]. Since fly ash contains mainly amorphous aluminosilicates (glassy phase) and some crystalline minerals (quartz, mullite, etc.), it can be used as a raw material for the synthesis of porous materials. Very recently we have reported our studies on the synthesis of aluminum containing MCM-41 (A1-MCM-41) and SBA-15 type of materials and their characterization as well as the catalytic properties [17-18]. To further extend this synthesis regime, we have carried out the studies on the preparation condition of A1-MCM-48 type materials using coal fly ash as the silicon and aluminum source [19]. In this report various characterization techniques such as 27A1MAS NMR, FF-IR, TEM, N2 adsorption and cumene cracking reaction are used to further evaluate the materials obtained.

2.

EXPERIMENTAL

2.1.

Materials Coal fly ash used in this study was obtained from Nanao-Ota power plant, Hokuriku and used as obtained. The chemical composition of fly ash revealed apart from the main constituents such as silica (67.5%) and alumina (18.7%), the other impurities such as Fe203, CaO, MgO, K20, TiO2, Cr203, P205 Na20, K20 and SO3 with 3.6%, 2.0%, 0.7%, 0.9%, 0.8%, 0.9%, 0.3%, 0.2%, 0.4%, 0.7%, respectively. The specific surface area (BET) and cation exchange capacity (CEC) of the coal fly ash were found to be 4.5 mE/g and 0.8 meq/100g, respectively. 2.2. Synthesis of AI-MCM-48 The supernatant obtained from fused fly ash powder was used as the silica and aluminum source [17]. The concentrations of Si, A1 and Na measured in supernatant were 11,000, 380 and 35,000 ppm, respectively. The detail synthesis procedure for MCM-41 was followed from our previous study [ 18]. Different samples of MCM-48 type materials with varying Si/A1 ratio were prepared using both single surfactant and a surfactant mixture of CTMABr and C12(EO)4 (Aldrich) [19]. In brief all batches were prepared using a synthesis gel with the following molar composition: CTMABr/C12(EO)4]I-I20/Si = 0.35-0.55/0.15-0.25/100/1. The Si/A1 ratio

1231 was also varied from 60 to 14. To remove the surfactant in the mesoporous materials, the assynthesized sample was calcined in air under static conditions at 813 K for 6 hours, with a linear temperature ramp of 0.5K / min and two plateaus of 60 minutes each at 423 and 623 K.

2.3. Analysis and characterization Powder X-ray diffraction (XRD) patterns obtained from CuK~ radiation were measured by using MAX18X. cE The chemical composition was analyzed by the LilEB404 method using the X-ray fluorescence (XRF) technique (Philips PW2400). BET specific surface area was determined from NE-adsorption at liquid nitrogen temperature (Belsorp 28SA). Transmission electron microscope (TEM) image was obtained by using JEOL 2010. FI'-IR spectra of the self supporting wafers were measured by JEOL JIR-7000. 27A1 MAS NMR spectra were obtained on a Varian VXP-400.

2.4.

Catalytic activity

The cumene cracking was performed in an atmospheric pressure flow system. The sample placed in the quartz tube reactor of a 10mm inner diameter was dehydrated at 673 K for 1 h in a nitrogen stream. The temperature was then brought into a reaction temperature (623 K). The reactant was fed into the catalyst bed with micro-feeder. Nitrogen was used as a carrier gas (40 ml/min), the contact time (W/F) was 0.20 h, and the partial pressure of the cumene was 7.9 kPa. On line product analysis was done on a Shimadzu GC-17A gas chromatograph (FID) with a GL-Science TC-1 capillary column (30 m).

3. RESULTS AND DISCUSSION 9

|

9

|

9

3.1. AI-MCM-48 prepared by direct thermal synthesis from supernatant Figure 1 shows the XRD patterns of different MCM phases of calcined samples prepared under different surfactant/silica ratio. It can be seen that the low concentration of surfactant (CTMABr) results into MCM-41 type materials as suggested from the XRD pattern (Fig. la) with four peaks that are consistent with indexing to a hexagonal cell, typical of an MCM-41 type product. The observation of three higher angle reflections other than the dl00 indicates that the product is likely to possess the symmetrical hexagonal pore structure of MCM41. A further increase in surfactant concentration resulted into mesophases, poor in hexagonal structural order as indicated from the gradual disappearance of diffraction peaks assigned to (110), (200) and (210) reflections (Figure lb, lc). By increasing the concentration of CTMABr in the synthesis gel, a phase transitions from hexagonal to lamellar passing

211

4

-_~" --=

110 2

a-~ 4

6

8

20/degree Figure 1. XRD profiles of the different calcined MCM type materials. CTMABr/SiO2 9a = 0.22, b = 0.35, c, d and e = 0.55; C12(EO)4/SIO2: d = 0.15 and e = 0.18

1232 Table 1 Physical properties of the raw material and the calcined mesoporous Sample

/SiO2

/SiO2

SBET/ m 2 g-1

Fly ash A1-MCM-41 (a)

0.20

-

4.5 761

Si/A1 Pore d 100 volume /nm / c m 3 g-1 2.9 . . . 14.0 0.57 4.24

A1-MCM-41 (b)

0.35

-

738

18.5

0.57

A1-MCM-41 (c)

0.55

-

731

65.0

A1-MCM-48 (d)

0.55

0.15

639

A1-MCM-48 (e)

0.55

0.18

848

A1-MCM-48 (f)

0.55

0.18

A1-MCM-48 (g)

0.55

0.18

1 2 3 4

Surf 1 Surf 2

d 211 /nm .

ao 3

/nm

Pore size4 /nm

. -

4.9

2.8

3.56

-

4.1

2.9

0.57

3.56

-

4.1

2.7

62.3

0.55

-

3.17

7.8

2.5

59.4

0.82

-

3.04

7.4

3.0

760

18.2

0.76

-

2.98

7.3

3.0

756

14.0

0.74

-

2.98

7.3

3.0

cetyltrimethyl ammonium bromide tetraoxyethylene dodecyl ether unit cell parameter, using 2d100/~/3 for MCM-41 and d211~/6 for MCM-48 Dollimore-Heal method

through an intermediate state of cubic structure is reported [ 1-4]. But using the supernatant as a silica source it was not observed, in other words MCM-48 formation was not facilitated under the synthesis condition using CTMABr alone. Figure l d and l e shows the XRD patterns of materials the surfactant-silica mesophase obtained from the starting mixtures of CTMABr/C12(EO)4 = 0.55/0.15 and 0.55/0.18, respectively. It can be seen that the presence of neutral surfactant has resulted into mesophase, identical to the cubic MCM-48. We observed that the optimum condition for MCM-48 using the supernatant as a silica source was CTMABr/ClE(EO)4 = 0.55/0.18 as it showed the sharpest XRD pattern. From the XRD pattern in Figure le, a highly ordered MCM-48, without any trace of lamellar phase peaks was obtained. The high ordered array of these materials could be inferred from the presence of a well defined set of diffraction peaks between 3 ~ and 6 ~ in the XRD patterns assigned to the (211), (220), (321), (420), (422) and (431). Two more samples A1-MCM-48 (f) and A1MCM-48 (g) (XRD not shown) with high aluminum concentration was then prepared using the similar composition. Table 1 summarizes characteristics of the calcined mesoporous materials obtained. The gel representing higher than 0.18 of C 1 2 ( E O ) 4 resulted either into unidentified mesophase or didn't show any XRD pattern. The (211) reflection is found at approximately 3.6 nm for all the as-synthesized samples. This correspond to a unit cell size of -- 8.7 nm. For the calcined samples the same reflection occurs at 3.1 nm, a unit cell length of --7.5 nm. This shrinkage of the unit cell (--13%) during calcinations probably is due to silanol condensation. This magnitude of unit cell shrinkage was in the range of values normally reported in the literature using other silicon source, approximately in the 5-15% range [14-17]. The same tendency is observed for the (220) reflection, suggesting that the supernatant of coal fly ash containing dissolved silica species could be used as the source materials for the preparation of such kind of materials.

1233 The N~ adsorption-desorption isotherms of 600 different samples (c, d and e) are shown in ,,.-:,. Figure 2. It belongs to a reversible type IV n 500 isotherm, characteristic for mesoporous o~~:)o c} materials. An inflection point is observed at g o relative pressures between 0.25 and 0.3. din400 (5) This corresponds to the filling of the mesopores and the sharp increase in the 300 ~=,~'~" [] " adsorbed volume indicates a uniform poresize distribution. It can be seen (Table 1) "~ 200 @ that the presence of neutral surfactant 0 M C M 4 8 (mixed,e) facilitates the formation of MCM-48. The ::3 [] MCM-48 (mi~Ex:l, d ) presence of a small hysteresis loop in sample c, indicates the formation of lamellar A M C M 4 8 (single. c ) phase which is very similar to the studies 0,,. I that has been reported at the high surfactant/ 0 0.5 1 silica ratio [20]. TEM image of microRelative pressure (P/Po) sectioned sample (Figure 3) also showed well developed pores arranged on the cubic Figure 2. N2 isotherms of different samples. plane (sample g), confirming that the materials possess the pore system symmetries that are inferred from XRD and N2 isotherms. Another factor that affected the formation of cubic phase was the pH of the supematant-surfactant mesostructure. Generally, a high pH condition is a major driving force for the transformation to lamellar [21]. In our case the pH adjustment to 10.2 during the synthesis arrested this transformation and also helped to improve the product yields. This is in agreement with the report where the pH adjustment was mentioned as a means for quenching the transformation of the MCM-48 mesophase to lamellar [ 10]. A mixed surfactant approach has been reported in the literature for the preparation of mesoporous materials [20,22]. In many cases, two different surfactants are completely miscible and form liquid crystalline misceller mesophase cooperatively. This phase behavior becomes more complicated when silica and alumina sources are present in the form of supernatant of coal fly ash. Supematant is a highly alkaline solution of silicate and aluminate (anions) and are strongly attracted by electrostatic interaction surrounding the head groups of the CTMABr, which may lead to the high concentration of the anions on the surface of the surfactant micelles. The neutral surfactant has no strong interaction with the ~;~:i? " 50nm , ,~,f~.i anions, and consequently its incorporation to the micelles will bring a dilution of the anions at the surface. This low surface Figure 3. TEM image of sample g. concentration may further lead to a certain

1234 contraction of the micelles surface, resulting in a phase transition from hexagonal to cubic. At this stage we are not advancing any explanation about the complexities of phase behavior of the supernatant-surfactant mesostructures in the aqueous solution, however we believe that C12(EO)4 acts more as a diluents and based on our observation facilitated the formation of MCM-48 structure.

3.2. Acidity of various AI-MCM-48 samples One of the most important features of our study using coal fly ash is the aluminum incorporation into the framework of the synthesized materials [17-18]. We found in the previous study on A1-MCM-41 that although there is no clear explanation for a large amount of tetrahedrally (Ta) coordinated framework aluminum in A1-MCM-41 derived from the supernatant, the supernatant is very effective for preparation of A1-MCM-41 without any Oh nonframework (0 ppm) aluminum. Very similar results we have also observed for the different MCM-48 samples. In Figure 4 the 27A1MAS NMR spectra of A1-MCM-48 (samples e, f and 100 50 0 -50 g) are presented. Chemical shift is referenced to 1 M Al(NO3)3 aqueous solution and the peak 27A1MAS-PPN~ spectra for Alintensity was normalized based on 1 g of Figure4. material. A single peak at ca 54 ppm, without MCM-48 prepared from supernatant of any evidence of any Oh aluminum can be seen fused fly ash powder. Si/A1 ratio; e=59.4, f=18.2, g=14.0 in all three samples, the intensity for which increased with low Si/A1 ratio. This is H H interesting and suggests the formation of acid sites in the mesoporous system. To further authenticate this, the samples were tested for pyridine adsorption using FT-IR. Aluminum in tetrahedral position creates ion exchange site associated with the charge compensating Na § ions. Figure 5 shows IR spectra of pyridine < adsorbed on the samples (Si/A1 = 59.4, 18.2 and 14.0 for e, f and g, respectively) after degassing at 423 K for 30 min. The samples did not show any acidity as expected, the weak bands at 1446 and 1598 cm -1 are probably due to pyridine 1600 1500 1400 adsorbed via H-bond interaction. When the samples were ion-exchanged twice with the Wave number (cm-1) NH4+ salt and calcined (protonation), a clear Figure 5. Fr-IR spectra of adsorbed pattern of acidity generated on the samples can be seen in Figure 6. Intense bands were pyridineonA1-MCM-48 samples before measured around 1456 cm 1 and 1623 cm 1 protonation. !

i .

.

.

.

1235 (Lewis acid sites), 1556 c m "1 (BrCnsted acid sites) and 1494 cm -1 (overlapping BrCnsted and Lewis L B+L acid sites). The intensity of these bands increases with the A1 content of the samples, showing a corresponding increase in the number of acidic sites. However, majority of acid sites generated on the samples were found to be Lewis acid sites (Figure 6) and the peaks arising from BrCnsted acid sites disappeared after evacuation at 523 K for 1 h, suggesting that the acidic strength of the BrCnsted acid sites in the A1-MCM-48 synthesized is very weak. Nevertheless, it is I I 1600 1500 1400 interesting to observe the acidity in the A1-MCM48 derived from coal fly ash, which confirms the Wave number (cm-1) aluminum incorporation suggested by the 27A1 MAS NMR measurement. Figure 6. FF-IR spectra of adsorbed pyridine on protonated A1-MCM-48 Catalytic activity of the A1-MCM-48 samples. prepared was further evaluated using the cumene cracking reaction at different time on stream and compared with the A1-MCM-48 prepared from 25 pure chemicals. The initial activity of coal fly ash derived materials (Si/AI= 18.2) was lower compared to the initial activity of A1-MCM-48 9 (Si/AI= 22.0) prepared from pure chemicals. Taking into account the fact that the cracking reaction require medium to strong BrCnsted acid r sites and the peaks derived from BrCnsted acid o [] a sites disappeared after evacuation at 523 K, it is suggested that the acidity of A1-MCM-48 prepared from the supernatant of coal fly ash is = "0" b o o o very weak. In other words, all the aluminums I I present in the A1-MCM-48 prepared from fly ash are catalytically not active. A number of reports 0 1 2 3 Time on stream (h) provides sufficient evidence for the partial inaccessibility of aluminum due to its Figure 7. Conversion profile of cumene incorporation in separate aluminum phases or on protonated A1-MCM-48 samples deeply imbedded in the porous walls if an prepared from (a) pure chemicals and (b) coal fly ash. aluminum source is added to the initial synthesis gel [26-27]. Our observation in this study provides further support for this point that the catalytic active sites are not connected with the total aluminum concentration but linked only to the amount of accessible aluminum, preferably on the surface.

12

4. CONCLUSIONS Supernatant of coal fly ash can be used as a raw material for the synthesis of aluminum containing MCM-48. The use of surfactant mixture has greatly facilitated the synthesis of

1236 MCM-48 performed under controlled pH condition. A high aluminum incorporation in tetrahedral position is revealed in the mesoporous materials which in turn generate ionexchange sites as well as acid sites when measured by pyridine adsorption using FF-IR. The experimental data produced here suggest that the coal fly ash could be a suitable source of silicon/aluminum with a low economy and environmentally friendly reagent for the preparation of well ordered mesoporous materials. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680. 3. A. Corma, Chem. Rev., 97 (1997) 2373. 4. A. Sayari, Y. Yang, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 103 (1999) 3651. 5. J..M. Kim, S. K. Kim and R. Ryoo, J. Chem. Soc., Chem. Commun., (1998) 259. 6. C.L. Landry, S. H. Tolbert, K. W. Gallis, A. M. Monnier, G, D. Stucky, P. Norby and J. C. Hanson, Chem. Mater., 12 (2001) 1600. 7. M.L. Pena, Q. Kan, A. Corma and F. Rey, Microporous Mesoporous Mater., 44-45 (2001) 267. 8. A. Corma, Q. Kan, and F. Rey, J. Chem. Soc., Chem. Commun., (1998) 579. 9. J. Xu, Z. Luan, H. He, W. Zhou and L. Kevan, Chem. Mater., 10 (1998) 3690. 10. R. Ryoo, S.H. Joo and J.M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 11. P. Van Der Voort, M. Mathieu, F. Mees and E. F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. 12. C. Zevenbergen, J.P. Bradley, L.P.V. Reeuwijk, A.K. Shyam, O. Hjelmar and R.N.J. Comans, Environ. Sci. Technol., 33 (1999) 3405. 13. G. Belardi, S. Massimilla and L. Piga, Resource, Conservation and Recycling, 24 (1998) 167. 14. A. Singer and V. Berkgaut, Environ. Sci. Technol., 29 (1995) 1748. 15. S. Rayalu, N. K. Labhasetwar and P. Khanna, U.S. Patent No. 6027708 (22 February 2000). 16. N. Shigemoto, S. Sugiyama, H. Hayashi and K. Miyaura, J. Mater. Sci., 30 (1995) 5777. 17. P. Kumar, Y.Oumi, K. Yamana and T. Sano, J. Ceram. Soc. Japan, 109 (2001) 968. 18. P. Kumar, N. K. Mal, Y.Oumi, K. Yamana and T. Sano, J. Mater. Chem., 11 (2001) 3279. 19. P. Kumar, Y. Oumi, K. Yamana and T. Sano, accepted to Nanoporous Materials III, June 12-15 th 2002, Canada. 20. G. Oye, J. Sjoblom and M. Stocker, Microporous Mesoporous Mater., 27 (1999) 171. 21. R. Ryoo and J.M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 22. J. L. Palous, M. Turmine and P. Letellier, J. Phys. Chem. B, 102 (1998) 5886. 23. K.R. Kloetstra, H.W. Zandergen and H. van Bekkum, Catal. Lett., 33 (1995) 157. 24. A. Jentys, K. Kleestofer and H. Vinek, Microporous Mesoporous Mater., 27 (1999) 321.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1237

Synthesis of w e l l - a l i g n e d carbon nanotubes on M C M - 4 1 Wei Chen, Ai Min Zhang*, Xuewu Yan, Dongcheng Han Department of chemistry, Nanjing University, Nanjing, 210093, P.R. China Fax: +86-25-3317761, E-mail: [email protected] Well-Aligned carbon nanotubes (CNTs) have been fabricated on mesoporous molecular sieves (MCM-41) embedded with iron oxide nanoparticles by chemical vapor deposition (CVD). Benzene with 1% thiophene was used as the carbon source. And large pore size MCM-41 was obtained by using 1,3,5-trimethyl benzene (TMB) as swelling agent. It has been found the mesoporous MCM-41 is an ideal substrate for growing well-aligned carbon nanotubes.

1. I N T R O D U C T I O N Since the discovery of carbon nanotubs, both theoretical models and experimental measurement have demonstrated their remarkable mechanical as well as novel electrical and magnetic properties. Growing Well-Aligned CNTs is important for obtaining functional devices for use as scanning probes [1] and sensors, as new field emitters in panel displays [2], and single-molecular transistors in microelectronics [3]. Aligned carbon nanotubes have been prepared either by postsynthesis fabrication [4] or by synthesis-induced alighment [5]. Recently, Jung Sang Suh [6] fabricated highly ordered two-dimensional CNTs on porous anodic alumina templates; Ren [7] used plasma-enhanced CVD and synthesized self-aligned CNTs on glass substrates. Previous studies show that the template plays an important role in the procedure of CNTs growth. Since the appearance of mesoporous molecular sieves [8,9], such as MCM-41, it has been found that mesoporous molecular sieve (MCM-41) is an ideal substrate for encapsulating catalyst [10,11]. Here we report the well-aligned CNTs have been obtained by using CVD over iron oxide nanoparticles embedded in MCM-41. It is known that the size of formed micelles determines the pore size of final mesoporous materials [12]. Some researchers have already used post-synthesis treatments [13,14], surfactants of different chain lengths [15] and polymers such as triblock-copolymers [16] as templates or incorporation of swelling agent to form large pore mesoporous materials. In the previous studies, 1,3,5-trimethylbenzene (TMB)[17,18] and decane [19] have been used as expanders, and materials with pore size superior to 80 A were obtained. In order to synthesize * Corresponding author.

1238 CNTs with uniform diameters through controlling the size distribution of active iron particles, we want to synthesis MCM-41 with large pore diameter. Here, we used TMB as swelling agent to expand the pore diameter of mesoporous materials. 2. E X P E R I M E N T A L 2.1. Preparation of catalyst Cetyltrimethylammonium bromide (CTAB) was first dissolved in water with stirring at room temperature to obtain a clean colloidal solution. 1,3,5-trimetyl benzene (TMB), tetraethyl orthosilicate (TEOS) and NaOH were then separately added drop by drop to the solution. After being stirred at room temperature for 1 hour, the homogenous gel with the molar composition of 1.0 cetyltrimethylammonium bromide (CTAB)" x TMB' 20.0 tetraethyl orthosilicate (TEOS): 10.0 NaOH" 1500.0 H20 (0~< x ~< 2.5) was sealed in Teflon autoclaves and statically heated at 373K for 72 hours. Resultant white product was filtered and washed several times with hot deionized water. After drying it was calcined at 773 K in air for 6 hours. The loading of iron oxides onto MCM-41 was carried out by the wet impregnation technique with a 1.6 M aqueous solution of Fe(NO3)3 9H20 for certain time. Then the resulting product was washed with deionized water and dried at room temperature under vacuum for several hours. Afterward the material was calcined at 673K under N2 atmosphere for 6 hours, which led to a transformation of iron nitrate to ferric oxide indicated by the disappearance of the IR band of the NO3-at 1380 cm -~. 2.2. Growth of carbon nanotubes CNTs were prepared in a conventional CVD equipment consisted of a horizontal tubular furnace and gas flow controlling units. A typical growth experiment, about 50 mg catalysts was put into ceramic boat inside a quartz tube. The catalysts were first actived at 500~

for

1.5 h in N2 with flow rate of 60 ml/min, and subsequently reduced by H2 (60 ml/min) at 500~ for l h, then rise to reaction temperature, at 8 0 0 - 9 5 0 ~ maintained for 1 h with the N2 flowing rate of 60 ml/min. Finally the benzene vapor with 1% thiophene was draw into the reaction system by hydrogen gas at certain flowing rate for 30 rain. Carbon nanotubes formed over catalyst were weighed at room temperature. 2.3. Characterization of the carbon nanotubes and catalysts The morphology and diameter of carbon nanotubes were observed by the JEM-200CX type transmission electron microscope (TEM). The crystallogram was determined with Japan

1239 D/max-Y RA X-ray diffractometer using CuK~, radiation (X = 1.54178 ). Pore diameter distribution and specific surface area were performed on an ASAP 2000 adsorption apparatus made by Micromeritics Corporation. The chemical compositions of catalysts were analysed with atom scan 2500 ICP emission spectrometer. 3. R E S U L T S A N D D I S C U S S I O N

3.1. Synthesis of carbon nanotubes The key result we reported in this research work is the synthesis of Well-Aligned CNTs using the new catalyst, MCM-41 embedded with iron nanoparticles. The TEM image of the as-synthesized material (see Figure 1) shows the well-aligned carbon nanotubes with diameter from 10 to 15 nanometers. For a typical 30 rain growth experiment at 900~

the average

weight increase percent using the catalyst contained 2.0 wt.% of ferric oxide is about 22 wt.%, which is relative to the total weight of the catalyst. It is obvious from Fig.1 that nanotubes self-assemble into aligned structures. We have predicted the possible aligned mechanism in our current work. As the nanotubes growth, their outmost walls interact with those of neighboring nanotubes via van der Waals force to form a large bundle with sufficient rigidity. This rigidity enables nanotubes to keep growing along the original direction. Even the outmost nanotubes are held by the inner nanotubes without branching away.

-~.,

,~

,

~..,.~ "

~,,

, ~,

]

!

Figure 1. Well-aligned carbon nanotubes on MCM-41 embedded with iron oxide nanoparticles We have found that the catalyst preparing process is a crucial step in obtaining the high performance carbon nanotubes. Impregnation with aqueous solution of ferric nitrate for more

1240 than one hour will cause the collapse of the mesoporous structures due to the poor hydrothermal stability of MCM-41 in acidic solutions (pH < 1.0 ), which was indicated by the disappearance of the typical XRD reflection peaks of MCM-41 after impregnation. This collapse would significantly reduce the total surface areas and pore volume. As a result, iron oxide nanoparticles, could not be well dispersed in such template. But, the well-dispersed nanoparticles are very essential to CNTs growth as indicated in other people's work [20-22]. So, in order to avoid this limitation, we adopted different ways: 1. the impregnation time was reduced; 2. impregnation carried out in methanol solution of ferric nitrate; and 3. ultrasonic disperse was adopted. In our experiment, all the typical hkl reflections of the MCM-41 XRD pattern were well maintained after impregnation or ultrasonic loading compared with the assynthesis materials, showing that the loading process in such condition has little influence on the mesoporous phase of MCM-41. The details of the XRD results are shown in figure 2 and figure 3 respectively. Besides the typical MCM-41 reflections, no additional peaks are observed, indicating that no crystalline iron oxide phase has been formed outside the pore structure.

Figure 2. XRD pattern after impregnation with ferric nitrate aqueous solution for l0 rain

Figure 3. XRD pattern atker impregnation with ferric nitrate methanol solution for 1 hour

1241 We also found that the pore diameter, surface area and pore volume of MCM-41 were changed little after impregnation, which was indicated by BET experiment. The results of BET experiment are shown in Table 1. So, with an average MCM-41 pore diameter of 2.9nm, the iron oxide nanoparticles should be dispersed well, which is the vital factor to synthesis Well-Aligned CNTs. Table 1 BET results of as synthesised materials and the materials after impregnation. BJH surface area (mZigi................i;0reV0iume .. ....................P0re::~ciiameter .. ................. (cc/g) (nm) As-synthesised materials

1313.35

0.83

3.40

After impregnation

1297.32

0.78

2.75

3.2. Synthesis of MCM-41 with large pore diameter by using TMB as the swelling agent The pore structure of the mesoporous MCM-41, as the substrate of catalyst, influences immediately on the states of loading iron nanoparticles. Beck et al. [8,9] have demonstrated that the pore size of MCM-41 can be varied as a function of the concentration of expander molecules such as TMB. According to the methodology introduced by Beck et al, we obtained enlarged pore size materials only at the molar ratio of 1.0 CTAB: 2.5 TMB. The experiment results also indicated that the quantity of smeller (TMB) is an important factor on the phase and pore diameter of final mesoporous materials. In our experiment, the molar composition of mixture may be described as: 1.0 CTAB: x TMB: 20.0 TEOS : 10.0 NaOH: 1500.0 H20 ( 0 ~ x ~ 2.5) As a result, the mixture of MCM-41 and MCM-50 or pure MCM-50 were obtained when the CTAB / TMB molar ratio is between the range of 1.0 to 2.0. The lamellar MCM-50 occurred when the molar ratio of TMB/CTAB reached 1.5. However. However when the molar ratio further increasing, the hexagonal MCM-41 was restored again and the pore diameter was enlarged. The chemical composition of mixture and products of synthesis materials for expanding procedure are presented in table 2. We predicted the possible phase transformation mechanism of MCM-41 pore size expanding procedure by using TMB as the swelling agent. As Kunieda et a1.[23] said in his paper, the penetrate tendency was very large for alcohol and aromatic hydrocarbons such as m-xylene. In this case, there will be no significant change in the micelle size by using 1,3,5trimethyl benzene (TMB) as the swelling agent at the lower TMB/CTAB ratio (less than 1.5). But this penetration would destroy the structure of hexagonal MCM-41, and result in the

1242 formation of lamellar MCM-50. While increasing the amount of TMB, TMB molecules would congregate to form "big oil particles", and " dissolve" in the organic hydrophobic tail of the surfactant (CTAB). The hydrophobic solvate interaction of the aromatic molecule with the hydrocarbon tails is analogous to the hydrophilic solvate interaction of water with the charged head groups of surfactants (CTAB). In this sense the inorganic/organic molecular ion pair species are organized with the organic TMB molecules as a co-solvent for the hydrophobic portion of the bi-phase synthesis mixture. As a result, the pore diameter of MCM-41 would be enlarged, which was checked by the increased dl00 value of MCM-41 reflection peaks. Table 2 Chemical compositions of mixture and products of synthesis mesoporous material CTAM

TMB

H20

0.05

0.0

75.0

0

MCM-41 (fine)

0.5

0.05

0.05

75.0

1.0

MCM-41 and MCM-50

0.5

0.05

0.075

75.0

1.5

Disordered MCM-50

1.0

0.5

0.05

0.100

75.0

2.0

MCM-50 and MCM-41

1.0

0.5

0.05

0.125

75.0

2.5

MCM-41

TEOS

NaOH

1.0

0.5

1.0 1.0

TMB :CTAB

Product

Above procedure was indicated by the XRD patterns (see figure 4-8). When the molar ratio of TMB/CTAB was 1.0, the reflection peaks of lamellar MCM-50 were occurred around 2 0 = 3.4 (figure 5), and the dl00Value (44.125 A ) of the 100 reflection peaks of MCM-41 phase was changed little compared with the pure MCM-41 (41.925 A) (figure 4). When the ratio reached 1.5, no reflection peaks of MCM-41 phase were detected and only MCM-50 reflection peaks could be observed (figure 6), which means that MCM-41 phase was completely transformed to MCM-50 phase. At the molar ratio of 2.0, the reflection peaks of MCM-41 phase occurred again, and the dl00 value was increased to 56.718 A (figure 7), indicated that the pore of MCM-41 was enlarged by the expander molecule (TMB). But the MCM-50 phase still existed at this condition. Finally, when the ratio reached 2.5, the reflection peaks of MCM-50 phase were disappeared in the XRD patterns, only the pure MCM-41 with enlarged pore diameter was found, and value of dl00 was 69.531 A (figure 8). But the 110 and 200 reflection peaks of hexagonal MCM-41 couldn't be observed due to the broadening the 100 reflection peak. We also found that the 100 reflection peak shifted toward smaller angel region when the pore was enlarged, which was as the same as the previous work [8,9,19].

1243

2-11o

~

t

20o

1

L,

I

I

-

Figure

4.

TMB

9C T A B

,~

Figure

= 0.0

~,

5.

r

o,

TMB

,=

r

9C T A B

e

,,,

o

= 1.0

\ I;

Figure

6.

TMB

9C T A B

Figure

= 1.5

Figure

8.

TMB

"CTAB

7.

TMB

' CTAB

= 2.0

= 2.5

4. C O N C L U S I O N We have synthesized Well-Aligned CNTS on MCM-41 with diameter from 10 to 15 nanometers. Our synthetic approach involves prepare of mesoporous molecular sieves (MCM-41), impregnation with ferric nitrate aqueous solution, and chemical vapor deposition. All of these allow the production of the Well-Aligned carbon nanotubes. And we have synthesized the MCM-41 with large pore diameter, which will be used to fabricate different

1244 carbon nanotubes grown from the pores of the template in our future work.

REFERENCE 1. J.H.Hanfer, C.L.Cheung, A.T.Woolley, C.M.Lieber, Progress in Biophysics & Molecular Biology, 77 (2001) 73. 2. De Heer, W.A. Bonard, J.M. Fauth, et al., Adv. Mater., 9 (1997 ) 87. 3. S.Frank, P.Poncharal, Z.L Wang, W.A.De Heer, Science, 280 (1998) 1744. 4. W.A. De Heer, W.S.Bacsa, C.A. Telain, T. Gerfin, R.Humphreybaker, L.Forro, D. Ugarte, Science, 268 (1995) 845. 5. S.Huang, L. Dai, A.W.H Mau, J. Mater. Chem, 9 (1999) 1221. 6. J.S. Sub and J.S. Lee, Applied Physics Letters, 75 (1999) 2047. 7. Z. F. Ren, Z. P Huang, et.al science, 282 (1998) 1105. 8. C.T. Kresge, M.E. Leonowicz, W.J.Roth, J.C.Vartuli, J.S.Beck, Nature, 359 (1992) 710. 9. J. S. Beck, et al., J. Am. Chem. Soc., 114 (1992) 10834. 10. F.Michael, et al., Chem. Mater., 274 (1999) 1701. 11. T.Abe, Y.Tachibana, T.Uemastsu, M.Iwamoto, J. Chem. Soc. Chem.Commun. (1995) 1617. 12. A.Corma, Chem. Rev., 97 (1997) 2373. 13. Q.Huo, D.I. Margolez, G.D. Stucky, Chem. Mater., 8 (1996) 1147. 14. A. Sayari, P. Liu, M.Kruk, M. Jaroniec, Chem. Mater., 9 (1997) 2499. 15. A. Sayari,V.R. Karra, R.J. Sudhakar, Presented at the Symposium on Synthesis of Zeolites, Layered compounds and other Microporous Solids, 209th National Meeting of the American Chemical Society, Anaheim, CA, 1995. 16. D. Zhao, J.Feng, Q. Huo, N.Melosh, G.H.Fredrickson, B.F.Chmelka, G.D.Stucky, Science 279 (1995) 548. 17. J. S. Beck, U.S. Patent, 5,057,57296 (1991). 18 P.J. Branton, J.Dougherty, G.Lockhart, J.W.White, Charact. Porous Solids IV(1997) 668. 19 J.L. Blin, C. Otjacques, G. Herrier, and Bao-Lian Su. Langmuir, 16 (2000) 4229. 20 W.W. Li, S.S. Xie, et al., Science, 274 (1996) 1701. 21 J. Kong, A. Cassell, H.Dai, Chem.Phys.Lett., 292 (1998) 4. 22 E. Flahaut et al., Chem.Phys.Lett., 300 (1999) 236. 23 H. Kunieda, K. Ozawa, K.L.Huang, J. Phys. Chem. B, 102 (1998) 831.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1245

Synthesis and characterization of CuO and Fe203 nanoparticles within mesoporous MCM-41/-48 silica C. Minchev~, R. K6hnb, T. Tsoncheva~, M. Dimitrov~, I. Mitov ~, D. Paneva r H. Huwe d and M. Fr6ba d* aInstitute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria bInstitute of Inorganic and Applied Chemistry, University of Hamburg, Germany ~Institute o f Catalysis' Bulgarian Academy o f Sciences, 1113 Sofia, Bulgaria d Institute of Inorganic and Analytical Chemistry, Justus-Liebig-University Giessen, Germany

Two simple methods for the synthesis of pure siliceous MCM-41 and MCM-48 silica materials, modified with CuO or Fe203 nanoparticles, located almost exclusively within the mesopores are presented. The modified samples were characterized by powder X-ray diffraction, nitrogen physisorption, temperature progranamed reduction, X-ray absorption spectroscopy (XANES/EXAFS) or M6ssbauer spectroscopy and methanol decomposition as a catalytic test reaction. The existence of small, slightly disordered metal oxide nanoparticles was proved. The redox and catalytic behavior of the modified samples depending on the metal oxide, the preparation method used and the type of the mesoporous support are studied and compared to the corresponding bulk oxide phases. 1. INTRODUCTION In the last years the mesoporous MCM-41 and MCM-48 molecular sieves have gained large interest as catalytic supports for metals, metal oxides and organometallic compounds [1-4]. In case of Cu and Fe containing mesoporous materials different methods of preparation: direct synthesis or post-synthetic modification have been described [5-10]. Depending on the preparation method used, the presence of metal ions and / or highly dispersed metal oxide species has been proved. In the present work the preparation of CuO and Fe203 modified MCM-41 and MCM-48 silica materials by means of different impregnation techniques in aqueous or organic media has been investigated. The aim of this study is to examine the influence of the different methods used on the dispersion and redox behavior of the generated CuO or Fe203 species and their catalytic activity in methanol decomposition as a test reaction.

2. EXPERIMENTAL The parent MCM-41 and MCM-48 silica materials with specific surface areas of 1000-1300 m2/g were synthesized by standard procedures described elsewhere [8,11 ]. After *Corresponding author: FAX: **49-641-9934109. E-mail: Michael'Fr~176

1246 drying, the as-synthesized silica samples were calcined in air for 6 h at 823 K. The loading of copper or iron oxide was carried out by two different impregnation techniques: Method A: The parent materials were stirred for 1 h first at room temperature and then for 1 h at 323 K with 0.023 M solution of Cu(II) or Fe(III) acetylacetonate in chloroform. After that the chloroform was evaporated. The sample was dried at room temperature. Method N: The parent materials were stirred at room temperature with 0.5 M aqueous solutions of Cu(II) or Fe(III) nitrates. The obtained product was dried at room temperature and then under vacuum for several hours. After drying all modified materials were calcined in air at 770 K (CuO/MCM-41/-48) or 670 K (Fe203/MCM-41/-48). Reference samples of bulk copper oxide (CuO) and iron oxide (Fe203) were prepared by mechanical mixing of amorphous silica and 4% wt CuO and 7% wt Fe203. The samples were characterized by powder X-ray diffraction (P-XRD), nitrogen physisorption, temperature programmed reduction combined with thermogravimetric analysis (TPR-TGA) and M6ssbauer spectroscopy (MS) as described elsewhere [ 11,12]. The methanol decomposition to H2, CO and/or methane was conducted at 450-700 K at atmospheric pressure in a flow type apparatus and Ar as carrier gas. Product analysis were carried out by online gas chromatography [ 13]. Before the catalytic experiments the samples were pretreated in situ in air at 773 K for 2 h. 3. R E S U L T S A N D D I S C U S S I O N 3.1. Textural characterization Representative examples of the nitrogen adsorption/desorption isotherms and pore diameter distributions of the samples are shown in Figure 1 and 2. The isotherms show type IV profiles (IUPAC classification) for the parent and modified materials as expected for mesoporous systems. The BET surface area varies in the range of 870-1280 m2/g (Tablel). After the modification reductions in the pore diameter, pore volume and BET surface area are observed. These changes are more pronounced in case of MCM-48 silica in comparison to MCM-41 samples (Figure 1a and b). The observed effects are substantially influenced by the preparation method used for the copper containing samples. However no essential differences have been observed in case of iron modified samples as shown in Figure 2a and b.

Table 1: Characteristics of copper and iron oxide modified samples. Sample Metal content Precursor: Host structure* (% wt) metal Cu-M 1-A 3.7 acetonate MCM-41 Cu-M8-A 3.7 acetonate MCM-48 Cu-M 1-N 3.3 nitrate MCM-41 Cu-M8-N 3.8 nitrate MCM-48 Fe-M 1-A 6.8 acetonate MCM-41 F e-M 8-A 6.8 acetonate MCM-48 Fe-M 1-N 6.8 nitrate MCM-41 Fe-M8-N 6.3 nitrate MCM-48

BET surface (mZ/g) 1277 1119 876 1075 873 1117 980 1018

* MCM-41 (1000-1300 m2/g) and MCM-48 (1200 m2/g) silica were used as parent materials.

1247 ads

600

des

pristine MCM-41 -

J ads des 600~ p~is.neMCM~8 -

~a.a/,

~J .

a4o0t=~f

,-.-, 500 E ~'o 400

300

E _= o 200-

. . . .

200

~:~5<

pore diameter [nm] 100 ' O' 0.0 . 2 ' 0'.4' 0 : 6 ' 0'.8'

100 0.0

ore diameter nm

0.2

0.4

0.6

0.8

1.0

relative pressure P/Po relative pressure P/Po Fig. 1" N2 physisorption isotherms (77 K) for pristine MCM-41/-48 silica in comparison to CuO modified Cu-M1-N(a) and Cu-M8-N(b). Insets depict pore diameter distributions (BJH).

600 ~

ads

des

Pr's~i:.M8.MCNM "48 .__~

@

E

%

E = 200 100-

0.0

@/.

F

e

~

500

% 400

'-' 300 0>

des

pristine MCMJ,8 -

600

,...-, 500 E 400

ads

700

,,"

.

.

,

012 0.4

1 2 3 4 p.ore diameter [nm],

0.6

0'.8 ' 1.0

E 300 _= 0 > 200 100

~.. 0.0

0

'.2

p.ore 2iamet3r. [nm:

014 0.6

O'.8

1.0

relative pressure P/Po relative pressure P/Po Fig. 2:N2 physisorption isotherms (77 K) for pristine MCM-48 silica in comparison to iron oxide modified Fe-M8-N(a) and Fe-M8-A(b). Insets depict pore diameter distributions (BJH).

Some powder X-ray diffraction patterns of the investigated samples are presented in Figure 3 a and b. The modified molecular sieves still show all reflections typical of the corresponding parent material, but with lower intensity. A comparable decrease in the signal intensity due to the filling of the mesopores for the MCM-48 and MCM-41 silica systems is found for both modification methods A and N. So, according to the textural characterization by physisorption and P-XRD neither pore blocking nor structural collapse for both copper and iron modified mesoporous M41S phases is observed. Except for the sample Cu-M1-N (Figure 3 a, enlargement) no additional reflections typical of CuO (35.7 ~ and 38.55 ~ 2 0) or Fe203 (24.1 ~ 33.0 ~ and 35.6 ~ 2 0) are observed in the P-XRD patterns of the modified mesoporous materials as shown exemplary in the enlargement of Figure 3 b. This indicates that practically no crystalline metal oxide phase has been formed in all cases. The very high dispersion of iron oxide species is also confirmed by the M6ssbauer

1248

|

8000 7000 ,-, 6 0 0 0

"~ 5000

o 5000

.ca. 4000

>, 4 0 0 0

"~ 3 0 0 0 e,,, 2000

.,..,

e-

@

7000 6000

3000

"c 2 0 0 0

, m ,

._=

Cu-M1-N

1000

10

2'0

30 2 0 [ ~]

1000 0

40

10

20

30

40

20[3

Fig. 3" P-XRD for pristine MCM-41/-48 silica in comparison to CuO modified Cu-M1-N(a) and Cu-MS-N(b). Enlargement shows 2 0 region (300-45 ~ for the strongest CuO reflections. Table 2: M6ssbauer parameters of the iron oxide modified samples Sample Components IS QS Heff [mm/s] [mm/s] [kOe] Fe-M1-A Dbl - S P M - Fe3+octa 0.34 0.99 -

FWHM [mm/s] 0.55

G [%] 100

Fe-M8-A

Dbl - S P M - Fe3+octa Sxt - c~-Fe203

0.33 0.36

1.02 - 0.12

514

0.61 0.54

93 7

Fe-M1-N

Sxt- c~-Fe203 3+ Dbl - S P M - Fe octa

0.37 0.36

- 0.10 0.70

513 -

0.38 0.53

32 68

Fe-M8-N

Sxt - c~-Fe203 Dbl - SPM - F-3+ e octa

0.37 0.35

- 0.11 0.72

512 -

0.31 0.52

22 78

IS: isomer shift related to (x-iron; QS" quadrupole splitting; He~r: internal magnetic field; FWHM: line width; G: relative weight of the components; SPM: superparamagnetic. data (Table 2). The M6ssbauer spectrum of Fe-M1-A sample represents quadruple doublet, while those of Fe-MS-A, Fe-M1-N, and Fe-M8-N samples are a superposition of the lines of sextet (Sxt) and doublet (Dbl) components. The determined parameters for all spectral components show that they belong to high spin Fe3+-ions. The parameters of the sextet part are close to those for c~-Fe203 (IS = 0.37 mm/s, QS = - 0.11 mm/s, Herr = 513 kOe). The main component of the doublet part in the spectra belongs to Fe3+-ions in octahedral coordination (IS = 0.33-36 mm/s, QS = 0.7-1.1 mm/s). This component could be assigned to nanosized particles of iron oxide with superparamagnetic behavior (SPM). It should be noted that this component is smallest for the sample Fe-M1-N. Obviously the metal oxides are spread as very small species within the mesoporous host material. This is confirmed by the combined results obtained from the physisorption, P-XRD, M6ssbauer and X-ray absorption spectroscopy. The X-ray absorption spectroscopy (XAS) data [8,14] show that these metal oxides are slightly disordered and do not have the same structure as their corresponding bulk oxide phases. Consequently, both impregnation techniques (Table 1) allow the formation of very highly dispersed CuO or Fe203 nanoparticles located almost exclusively within the mesopores of the parent silica materials. More over, by method N haematite nanoparticles located within the

1249 mesopores even at very high Fe203 concentrations (up to 42.5 wt %) could be obtained [15]. At the same time the increase of the copper concentration above 6-7 wt% for CuO/M41S samples leads to the formation of large CuO particles on the outer surface of the support [ 16]. 3.2. TPR-TGA measurements TPR-TGA measurements on copper oxide modified MCM-41 and MCM-48 silica showed that the reduction to copper metal is achieved in a temperature range of 473-750 K [4,11]. Bulk copper oxide particles are reduced to the copper metal at ca. 633 K while the highly dispersed nanostructures within the mesoporous host structure (Cu-M1-N and Cu-M8-N) are reduced in a temperature range of 473-590 K with TPR peaks between 523-573 K [4,11]. More complicated is the case of the samples Cu-M1-A and Cu-MS-A where due to the reduction process two temperature ranges are found. A first reduction step is observed between 473-590 K ascribed to highly dispersed material. However, the main reduction takes part at relatively high temperatures 600-750 K which is assigned to the reduction of large copper oxide particles of various size [11 ] and isolated copper entities stabilized by the host structure. On the contrary the reduction of the iron oxide nanoparticles to stable products failed for iron contents below 20 %wt (5% H2 in Ar, 300-873 K). No differences in the reduction behavior due to the preparation method were found. Under these conditions the reduction of bulk haematite to magnetite is observed at 573 K and further reduction at temperatures above 673 K leads to metal iron. Even at higher H2 concentrations the reduction of the host/guest compounds leads only to a partial reduction of Fe(III) to Fe(II) as indicated by XAS data [15]. This mixed iron oxide exhibits a ferromagnetic behavior like magnetite. 3.3. Catalytic studies Low catalytic activity in methanol decomposition to CO and hydrogen, not exceeding 10 % even at 750 K, is observed on bulk CuO. Quite different is the case when CuO is supported on mesoporous silica materials (Figure 4 a and b). All catalysts show a good activity above 500 K (Figure 4 a). CO is the main product in all cases, but at lower temperatures methylformiate (up to 20 %) is also registered. The samples exhibit comparable catalytic properties in the range of 550-600 K, but a lower activity for Cu-M1-N above 600 K is observed. This effect seems to be due to the faster deactivation of Cu-M1-N sample, which is confirmed by the experiments under isothermal conditions at 650 K (Figure 5 a). The activity of the latter decreases more than 3 times for 2 hours, while it is preserved almost unchanged when acetylacetonate was used as a precursor. At the same time all copper modified MCM-48 silica materials exhibit stable catalytic activity despite of the used copper precursor. The iron analogues are less active in methanol decomposition and they show catalytic activity just above 600 K (Figure 6 a and b). Methane, as well as CO are the main products registered in the whole investigated temperature interval. Small amounts of CO2, dimethyl ether (DME), and C2-C4 hydrocarbons are also observed. Despite the close conversion of the samples under thermo-programmed regime, some differences in their products distribution are registered (Figure 6 b). Higher CO yield for Fe-M1-N is observed. It decreases essentially when acetylacetonate was used as a precursor. At the same time all samples obtained on MCM-48 silica show a good selectivity to methane independent of the preparation method

1250 applied. On contrary to the corresponding copper samples, all iron modified catalysts exhibit fast deactivation at 650 K (Figure 5 b). Their activity decreases about 2.5 times in the first 30 minutes time on stream and remains almost unchanged within the next two hours. The bulk Fe203 reference material exhibits also high catalytic activity above 600 K (not depicted). However in comparison with the M41S supported iron oxide catalysts, it is characterized with high selectivity to CO (about 100%) and low degree of deactivation. The parent MCM-41 and MCM-48 silica materials do not exhibit catalytic activity in the whole temperature interval (not depicted). So the observed effects could be considered as an

~oo~

o.
100

80

80

C

t,.=,=z

o

60

"~

60

t=..a

cO

o

40 > e,o 20

~

Cu-M1-N

mo~

Cu-M8-N

~=~

Cu-M1-A

~o~

Cu-M8-A

r 40 "ID

20

0 -~D----=Q"7=f

.so

--D--

,..

Cu-M8-N

~=~

C u - M 1 -A

~o~

Cu-M8-A

D ~ n - - - ' -

0

5oo 510 60o 650 70o 750

Cu-M1-N

--o--

,

4so

t

500

temperature [K]

,

1

sso

,

i

61o To0 ~so

~

600

temperature [K]

Figure 4. Total conversion and yield of CO vs. temperature in methanol decomposition on CuO supported on MCM-41/-48 samples obtained from nitrate (N) and acetylacetonate (A). 100

100

|

9 80

8O =-~

O

O .~.

60

uo--

F e - M 1 -N

60~

.._._,

e-

c o

.o

.--.

=> cO

4o

40

20

roD--

Cu-M1-N

>

---o--

Cu-M8-N

~

--=--

Cu-M1-A

--o--

Cu-M8-A

e,-

20

0

0

"

2'5

5'0

'

75

time [min]

100

1:25

o

,

i

25

10

,

7'S

,

i00

T - -

115

~--

time [min]

Figure 5. Methanol decomposition at 650 K vs. time on stream on copper or iron oxide supported on MCM-41/-48 samples obtained from nitrate (N) and acetylacetonate (A).

1251

,00!i O

100 -~

80~

80-

60

60-

e--

o ,--

i-

o

40

> C

i I

o

/,/~i~

S o--~>~, 9

0

5S0

9

o - - Fe-M1-N - - o - - Fe-M8-N

.

--*--

. . . . . 600 650

40

o..%.,6 t

@

ii:,.

-0

L

"'. d"..

q..5;~.

--o-- Fe-M1-N --o-- Fe-M8-N --.--

'.. '~.; ...... 'q "'i="/.. -.

40 N"

"'"'/"'"'O

F e-M8-A

~..

}-60 '~__ "'..

'..

20

20

9 '.

o~

...@

Fe-M8-A 700

750

0

550

o

~ 600

650

700

750

100

temperature [K] temperature [K] Figure 6. Total conversion (a) and yield (b) of CO and CH4 (dotted) vs. temperature in methanol decomposition on various iron oxide containing samples.

evidence for some differences in the state of the nanostructured metal/metal oxide species depending on the preparation method and the host structure. Rapid reduction of the active phase to metal copper and following agglomeration of the latter seems to be most probably the reason for the conversion decrease with time on stream for the Cu-M1-N sample (cf. Figure 5 a). This processes seems to be favored by the one-dimensional channel system of MCM-41 silica, which is more susceptible to pore blocking effect in comparison to the MCM-48 silica. However, the slower deactivation of all acetylacetonate samples could be an indication of different mode of active phase deposition. We assume that this is caused by a higher dispersion of copper precursor species homogeneously spread within the whole mesoporous system. This is due to the preparation method A with chloroform which allows an increased wetting of the whole hydrophobic channel system during impregnation in comparison to the water/nitrate system that does not achieve the inner parts of the long mesoporous channels of the MCM-41 host structure. In case of the three-dimensional mesoporous channel system of the MCM-48 silica this effect is not observed due to the better accessibility. Comparable effects were found for the iron oxide system with respect to the selectivity of sample Fe-M1-N. In this case the blocking effect of the active sites within the mesoporous host system leads probably to the reduced selectivity for methane (cf. Figure 6 b). For all iron containing samples the reduction changes the iron oxide species (see also part 3.2) and that could be the reason for the observed fast decrease in the catalytic activity (cf. Figure 5 b) as it was shown recently [ 12]. 4. CONCLUSIONS Two simple methods for the preparation of highly dispersed CuO or Fe203 nanoparticles loaded within mesoporous silica by impregnation with metal acetylacetonates or nitrates are presented. The results from the complex investigation as P-XRD, physisorption, TPR-TGA, MS and the catalytic methanol decomposition show that these particles are located almost

1252 exclusively within the mesopores while the host structure is preserved. Some unusual redox and catalytic properties of these materials have been registered. The catalytic properties differ for the hexagonal MCM-41 silica material depending on the preparation method. The threedimensional MCM-48 silica material shows no differences in the catalytic properties for both preparation methods. The advantage of the metal nitrate impregnation is the possibility to achieve much higher metal contents in one single impregnation/calcination procedure. ACKNOWLEDGEMENTS Financial support from the National Science Fund at the Ministry of Education and Science of Bulgaria, Bulgarian Academy of Sciences and the Deutsche Forschungsgemeinschaft (Fr1372/2-1,2-2) and the Fonds der Chemischen Industrie is gratefully acknowledged. Ch. M. also wishes to thank to the Deutscher Akademischer Austauschdienst for the permanent support. The very helpful assistance of Uta Sazama is gratefully acknowledged. REFERENCES [ 1] [2] [3] [4] [5] [6] [7]

A. Corma, Chem. Rev., 97 (1997) 2373. U. Ciesla and F. Scht~th, Microporous Mesoporous Mater., 27 (1999) 131. F. Scht~th, A. Wingen, J. Sauer, Microporous Mesoporous Mater., 44-45 (2001) 465. R. KOhn and M. FrOba, Catal. Today, 68 (2001) 227. M. Hartmann, Stud. Surf. Sci. Catal., 128 (2000) 215. M. Ziolek, I. Sobczak, P. Decyk, I. Nowak, Stud. Surf. Sci. Catal., 125 (1999) 633. A. Zecchina, D. Scarano, G. Spoto, S. Bordiga, C. Lamberti, G. Bellussi, Stud. Surf. Sci. Catal., 117 (1998) 343. [8] M. Fr6ba, R. K6hn, G. Bouffaud, O. Richard, G. v. Tendeloo, Chem.Mater., 11 (1999) 2858. [9] M. Stockenhuber, R. W. Joyner, J. M. Dixon, M. J. Hudson, G. Grubert, Microporous Mesoporous Mater., 44-45 (2001) 367. [ 10] A. Wingen, W. Schmidt, F. Scht~th, A. C. Wie, C. N. Liao, K. J. Chao, Stud. Surf. Sci. Catal., 135 (2001) 317. [11 ] C. Minchev, R. KOhn, T. Tsoncheva, M. Dimitrov, M. Fr6ba, Stud. Surf. Sci. Catal., 135 (2001) 235. [12]T. Tsoncheva, M. Dimitrtov, D. Paneva, I. Mitov, R. KOhn, M. Fr6ba, C. Minchev, React. Kinet. Catal. Lett., 74 (2001) 385. [13] T. Tsoncheva, R. Dimitrova, C. Minchev, Appl. Catal. A: General, 171 (1998) 241. [ 14] R. K6hn, C. Minchev, M. Fr6ba, Annual Report, Hamburger Synchrotronstrahlungslabor, (2001) 759. [15] R. K6hn, PhD Thesis, University of Hamburg, 2001. [ 16] R. K6hn, C. Minchev, and M. Fr/Sba, unpublished results.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1253

Study o f the p o r o s i t y o f m o n t m o r i l l o n i t e pillared with a l u m i n u m / c e r i u m M.J. Hernando, C. Blanco, C. Pesquera, F. Gonzfilez Inorganic Group. Department of Engineering Chemistry and Inorganic Chemistry. University of Cantabria. Avda de los Castros, s/n 39005-Santander. SPAIN This study compares the porosity created in montmorillonites pillared with aluminum/cerium pillars with that of samples pillared only with aluminum. Studies of the pore size distribution indicate that the A1Ce-pillared samples have a new porous system, with a pore size at the limit between microporosity and mesoporosity. The presence of larger pores in the A1Ce-pillared samples is due to the inorganic polyoxycations intercalated between the clay layers. These polyoxycationes are larger than the Keggin ion intercalated in the samples pillared only with aluminum. After successive thermal treatments the micropore volume is still high in the A1Ce-pillared samples, whereas the thermal stability of the micropore volume, developed after the pillared process, is lower in the Al-pillared samples, being reduced practically to zero after thermal treatment at the same temperature. Moreover, the two different types of pores generated in the A1Ce-pillared samples show different thermal evolution and this compares them with the thermal evolution of the one single type of pores generated in the Al-pillared samples.

1. INTRODUCTION Pillared clays are smectite clay minerals that have been modified through the introduction of large inorganic polyoxycations into their interlayer regions, followed by calcination. The intercalated polycations increase the basal spacing of the clay and, upon heating, the resultant materials contain metal oxide pillars capable of preventing the collapse of the interlayer spaces. As a result, an interlayer space of molecular dimensions, a two-dimensional porous network, is generated. The microporous structure created, the high specific surface area and the presence of acid centers both on the surface of the layers and on their pillars make these materials suitable for use as adsorbents and catalysts (1-2). To prevent the clay layers from sintering, the thermal resistance of the pillars must be increased. One well-tested way of achieving this is to use mixed pillars in the materials (3-4), and the most widely used methods have been doping of Al-pillaring solutions with lanthanide cations. Shabtai et al were the first to prepare pillared solids containing Ce and A1 (5). Later, Sterte (6) and McCauley (7) found that the incorporation of lanthanide elements in the preparation of the pillaring agent resulted in material whose basal spacing was greater than that in conventional materials. In this study, we have prepared and characterized montmorillonite pillared with A1 and Ce under different synthesis conditions. The textural and structural parameters of the materials were compared with those of montmorillonite pillared only with A1. We have applied low pressure nitrogen adsorption data to obtain a quantitative evaluation of the microporosity of the synthesized materials and their evolution with thermal treatment.

1254 2. EXPERIMENTAL SECTION

2.1. Starting material The starting material used in this work was a montmorillonite from Wyoming, supplied by Missouri University and denominated here as Wy. The fraction <2~tm of this sample had a surface area of 33 mZ/g and a pore volume of 0.042 cm3/g at P/Po -- 0.98. Its exchange capacity was 106 mequiv/1 O0 g of clay. 2.2.Preparation of the pillaring agent The solutions of the pillaring agent had different A1/Ce ratios (25/1; 50/1; 75/1 and 100/1). They were prepared by adding suitable amounts of CeC13.7H20 to 2.5M A1 solutions obtained from Locron [A12(OH)5C1. 2-3H20]. These solutions were heated in a reactor autoclave for 72 hours at 130~ The reactor reaches pressures of 3 bar. After returning to room temperature and atmospheric pressure, the reaction mixture was diluted with the quantity of water necessary to yield an A1 concentration of 0.1M. The pillaring agent with aluminum, the Keggin ion, [A104Allz(OH)24(H20)12] 7+, was obtained from a 0.1M A1 solution from Locron at room temperature. These different conditions of the synthesis for each pillaring agent were the optimum after several experiments in the laboratory. 2.3. Pillaring Process The solutions of pillaring agent were added with vigorous stirring to a clay slurry of 2.5 g/100 mL. The final proportion in all cases was 20 mequiv of A1/g of clay, with a solid/liquid ratio of 0.5%. The reaction mixture was stirred continuously for 24 h at room temperature. It was then washed by means of dialysis with distilled water, using 1 L of water/g of clay. Dialysis was continued with water being renewed every 24 h until the C1- ion concentrations decreased to the point where the conductivity of the water was < 30 ~tS. Finally, the samples were freeze-dried. The materials obtained are denominated A1Ce-Wy-25, A1Ce-Wy-50, and A1Ce-Wy-75 according to the proportion of cations used in the pillaring agent. The sample denominated A1-Wy was obtained in the same conditions from a pillaring agent prepared with solutions containing only aluminum. Samples were then calcined for 2 hours at 400~ to obtain the pillars. The samples are denominated A1-Wy-400, A1Ce-Wy-25-400, A1Ce-Wy-50400, and A1Ce-Wy-75-400. In order to study the thermal stability of the samples, they were later subjected to successive thermal treatments at 500~ 600~ 700~ 800~ for two hours at each temperature. Equipment and Methods. The following equipment and techniques were used for the physicochemical characterization of the materials: X-ray diffraction. The X-ray diffraction diagrams were obtained on powder with the particles oriented so as to increase the intensity of the (001) reflection. The apparatus used was a Philips PW- 1710 diffractometer with CuK~ radiation. Textural Parameters. The adsorption isotherm of N2 at 77K was determined in a Micromeritics ASAP 2010 with a micropore system. Specific surface area was determined by applying the BET equation to the isotherm (8). The total volume was considered to be the volume of liquid N2 adsorbed at a relative pressure of 0.98. Microporosity was deduced by the t-plot method (9) using the Harkins and Jura (10) thickness. We used model isotherms calculated from density functional theory (DFT) to determine the pore size distributions of the pillared samples by taking the adsorption branch of the experimental nitrogen isotherm, assuming slit-like pores (11).

1255 3. RESULTS AND DISCUSSION The structural characterization of the materials (12) showed an incorporation of 12.1 mequiv/g and 0.1 mequiv/g of A1 and Ce for the A1Ce-Wy-50 sample, of 15.6 mequiv/g and 0.3 mequiv/g of A1 and the Ce for the A1Ce-Wy-25 sample, and of 7.1 and 6.8 mequiv/g of A1 for the A1Ce-Wy-75 and A1-Wy samples, respectively. From now on we denominated A1Ce-pillared samples those that incorporate aluminum and cerium and Al-pillared those samples that only incorporate aluminum. Figure 1 shows the X-ray diffraction diagrams between 3 and 12~ (20) of samples A1CeWy-25 and A1-Wy. The Al-pillared sample displays a d(001) peak at 20.0,~, associated with the basal spacing between the clay layers, and is characteristic of clays pillared with aluminum polyoxycations, which is the Keggin ion (13-15). In contrast, sample A1Ce-Wy-25, pillared with A1-Ce displayed peaks at 27.2/~ and 13.6~, which correspond to inorganic polyoxycations with sizes different from that of the Keggin ion. In the samples treated at 400~ the basal spacing was reduced to 24.7~ for the A1Ce-Wy-25 sample and 18.8~ for the A1-Wy sample. Figures 2 and 3 show the nitrogen isotherms at 77 K for A1-Wy and A1Ce-Wy-25 respectively, using one A1Ce-pillared sample and one Al-pillared sample, after successive thermal treatments at temperatures between 400 and 800~ with a linear relative pressure axis (right) and with a logarithmic relative pressure axis (left). The isotherms are type IV, corresponding to mesoporous solids; however, in the zone of low values, both pillared samples present Langmuir adsorption isotherms (type I), which indicate the presence of micropores, generated in the pillaring process to intercalate inorganic polyoxycations between the clay layers. In the A1Ce-pillared sample (Figure 3), an increase in the volume of adsorbed

220-0/~

:5 m

i I

4

I

6

28

I

I

8

10

12

Figure 1. X-ray diffraction patterns of samples A1Ce-Wy-25 and Al-Wy.

1256 250

-----"

250-

2oo1

2oo-

9A,

"~ O 1501 13

o

~ Al-Wy-500 ~ Al-Wy-600 -I- Al-Wy-700 _ _ _ _

"-" "~ ~~ 150-

~[

.

50

0-

o

0.00001

0.0001

0.001

0.01

Relative Pressure (P/Po)

0.1

1

"----

0

T

-

0.2

,

,

0.4

0.6

,

0.8 Relative Pressure ( P / P o )

Figure 2. N2 adsorption isotherms at 77K of the A1-Wy sample after thermal treatments, between 400 and 700~ Relative pressure axis: left, logarithmic and right, linear. nitrogen is observed at approximately P/Po>0.1 This new step in the level of nitrogen adsorption is seen more clearly when the isotherms of nitrow adsorption are represented in semilogarithmic scale (left of figure 3). Between P/Po=5.10- and P/Po=2.10 -2, the A1Ce-Wy25-400 sample displays a notable increase in nitrogen adsorption. This step in nitrogen adsorption, in a certain range of relative pressures, must be due to the insertion of larger polyoxycations between the clay sheets during the pillaring process (greater basal spacing), thus this generated a greater space interlayer and, consequently, pores of larger diameter. This results in nitrogen adsorption up to high relative pressures. This adsorption is at the limit between microporous and mesoporous size, corresponding to a diameter of 20A in the classification of Dubinin (8). However, a single level of adsorption at low pressures is observed in the Al-pillared sample, in figure 2 (left) the step around P/Po=0.1 does not appear, which indicates a single type of size of micropores.

250 --*- AICe-Wy-25-400 4 - AICe-Wy-25-500 AICe-Wy-25-600 --e-AICe-Wy-25-700 AICe-Wy-25-800

AE~ ~o ~ -lo ,.Q '-- 150 o oo -lo

(D

E 100 o > 50 0 0.00001

o.oool

o.ool

o o~

o~

Relative Pressure (P/Po)

1

0

0.2

0.4

0.6

0.8

Relative Pressure (P/Po)

Figure 3. N2 adsorption isotherms at 77 K of the A1Ce-Wy-25 sample after thermal treatments, between 400 and 800~ Relative pressure axis: left, logarithmic and right, linear.

1257 Table 1 Surface area and pore volumes calculated by the BET equation and the t-plot Samples

SBET(m2/g)

A1Ce-Wy-25-400 A1Ce-Wy-50-400 A1Ce-Wy-75-400 A1-Wy-400 A1-Wy

Vad(cm3/g)

373 411 277 346 33

0.312 0.337 0.189 0.226 0.042

Vmp/Wad (%)

Vmp(Cm3/g) 0.196 0.223 0.100 0.131 0

62.8 66.1 52.9 57.9 0

In figure 3, it is possible to observe how the new step, which is present in the pillared samples with A1Ce, remains when materials are treated at temperatures of up to 700~ As temperature is increased, a decrease in intensity of the step is observed with inferior nitrogen adsorptions when the samples are treated at higher temperatures. This relationship between the level of nitrogen adsorption and the increase in thermal treatment, is also noted in the A1Wy sample (figure 2). In both samples the adsorption branches of the isotherms (figures 2 and 3), at high pressures P/Po>0.2, remain parallel to each other after the consecutive thermal treatments, which indicates that the mesoporosity of the materials is not modified, affecting the decrease in the adsorption of nitrogen at low pressure values and therefore to the microporous zone. In addition, the adsorption branches of the Al-pillared material (A1-Wy) are seen to display lower values that decrease more quickly with the thermal treatment. Table 1 shows the values for specific surface area, SBET and volume of N2 adsorbed at P/Po=0.98, Vad, of pillared samples and the raw material. All the pillared samples show a more developed porosity than the raw material (montmorillonite). The specific surface area and pore volume increase in both types of pillared samples, but the increase is greater in the A1Ce-pillared samples (A1Ce-Wy-25 and A1Ce-Wy-50) than in the Al-pillared samples (A1Ce-Wy-75 and A1-Wy). The increase in specific surface area was consistent with the expansion of the structure observed by XRD. The A1-Wy-400 sample had a SBETof 346 mZ/g while the A1Ce-Wy-50-400 had a value of 41 lmZ/g. 200 "~5o -o

~loo..p E 50 >o 0

0

i

2

3 Thic~nessS(A), H6arkins7and j8ura

9

Figure 4. t-plot of pillared samples: e, Al-Wy-400 and a, A1Ce-Wy-50-400.

1258 Figure 4 shows the t-plot for two of the pillared samples. From this, the micropore volume,

gmp, was calculated (Table 1). The micropore volume is seen to increase in the pillared samples, and again the increase is greater in the A1Ce-pillared samples. This increase in micropore volume, which is greater than that in specific surface area shows a two-fold increase in the A1Ce-Wy-50-400 sample over the A1-Wy-400 sample (0.223 and 0.131 cm3/g respectively). The former sample presents a second step in the adsorbed volume at greater thickness, t, than in the A1-Wy-400 sample, which presents only one step. This second step is related to relative pressure of adsorption around P/Po = 0.1, which corresponds to the abovementioned increase in the adsorption isotherm. The ratio of micropore volume to total pore volume, Vmp/gad, is given in Table 1. There is a higher ratio of micropore volume to total pore volume for the A1Ce-pillared samples than for the Al-pillared samples. This indicates greater development of microporosity and an increase in pore size in the former. The micropore size distribution and cumulative pore volume of the pillared samples was obtained by the DFT method, using the slit-like pore model, given the laminar structure of the pillared montmorillonite samples. Figure 5 show the micropore distribution and cumulative pore volume by the DFT method for samples A1-Wy-400 (Figure 5, left) and A1Ce-Wy-50400 (Figure 5, right). For the latter, the micropore volume is seen to accumulate approximately two type of pores. The cumulative pore volume curve shows two steps, each associated with one of the two pore distribution maxima. In contrast, for the A1-Wy-400 sample, the pore volume distribution presents a single type pore, which corresponds to the smallest diameter pore for the A1Ce-Wy-50-400 sample.

o.14

0.07 0.06~

o.1s

0.05~

m

>~

0.0.2

~o.0~

0.030<

~-

0.02~

~_~0.o~

0.01~

(.,~0.o~ 5

10

15

20

25

Pore Width

30 35 (,~,)

40

45

0 50

0.06

0.25

c~).1,~

0.05

~~176

0.04~

~o15

0.03 9 ~

~>o.1

0.02 9

-5 E 0.050

0

001 ~-~10

15

20

25

30

Pore Width

35 (A)

40

45

0 50

Figure 5. Distribution and cumulative pore volume by the DFT method of the pillared samples: left, A1-Wy-400 and right, A1Ce-Wy-50-400.

1259 0.25

400 350

,0.2

300

~o.~5

"~

2~o~ 200 't?:

~0.1 o "-

150 o 100

~0.05

5O 0 400

500

600

0 800

700

Temperature (~

Figure 6. Thermal evolution of the specific surface area of the samples: , , A1-Wy and II, A1Ce-Wy-25 and the micropore volume of the samples: A, A1-Wy and O, A1Ce-Wy-25. The hydrothermal stability of the pillars was also studied by means of the N2 isotherms of the samples subjected to hydrothermal treatment. Figure 6 presents the thermal evolution of specific surface area, SBET, and micropore volume, Vmp, of the A1Ce-Wy-25 sample from 400~ until 800~ and the A1-Wy sample from 400~ to 700~ It can be seen that with the successive thermal treatments up to 700~ low values of specific surface area and micropore volume are reached in the A1-Wy sample. While, for the A1Ce-Wy-25 sample these textural parameters are still high at the same temperature. After treatment at 700~ the SBETof the A1Ce-Wy-25 and A1Ce-Wy-50 samples was 304 and 270 m2/g, respectively. In contrast, the A1Ce-Wy-75 and A1-Wy samples treated at 700~ had a SBETof 43 and 50 m2/g, indicating a total collapse of the pillars, with microporosity falling sharply to values similar to those of the initial sample. This can be attributed to the presence of cerium in the pillars which gives an increase in the basal spacing of these samples and seems to display greater thermal stability than in the samples that only incorporate A1 in this process. E~ E >o

g

0.25

0.045

0.2 E0.03 0.15

0.025

J~ ~1~

L / r~ /

Ill

/*, ~1

-l- AICe-Wv-25-5o0 -i- AICe-Wy-25-600 -e- AICe~Wy~2~700

~ 0.02

"-= 0.1

~

E

0.015

-- 0.01

0 0.05

0.005 10

20

30

Pore Width (/~)

40

50

0

0

10

20

30

40

50

Pore Width (,~)

Figure 7. Distribution (right) and cumulative pore volume (left) by the DFT method of the pillared sample A1Ce-Wy-25 after thermal treatments between 400 and 800~

1260 In order to study the evolution of the microporosity generated in these materials with thermal treatments, the micropore size distribution and cumulative pore volume of the pillared samples have been analyzed. Figure 7 shows the micropore distribution (right) and cumulative pore volume (left) for the A1Ce-Wy-25 sample at different temperatures. This figure shows that when the sample was subjected to thermal treatment, the behaviour of the two type of pores was totally different. As seen the smaller pores remained nearly constant, while the larger pores decreased. This may be because the larger pores protect the smaller pores from thermic damage in the A1Ce-Wy-25 sample. 4. CONCLUSIONS The montmorillonite pillared with aluminum and cerium has inorganic polyoxycations incorporated between the clay sheets. This modifies the textural characteristics of the raw material. The specific surface area is increased and the porous structure, particularly the micropore volume, with generation of pores at the limit between microporosity and mesoporosity. The A1Ce-pillared samples have larger pores than those generated in the A1pillared materials, whose pores clearly belong to the micropore region. In addition to the fact that the textural parameters show higher values, they are thermally more stable, maintaining high values of specific surface area and micropore volume up to 700~ Thanks to this greater thermal stability in the A1Ce-pillared samples, which show two types of pores, the largersized pores prevent the collapse of the smaller-sized pores, as the collapse of the larger-sized pores when subjected to thermal treatments simultaneously protects those of smaller diameter against deterioration. ACKNOWLEDGMENT: We acknowledgment to CICYT for financial support of this work under Project MAT 99/1093-CO2-O2. REFERENCES 1. J. T. Kloprogge, J. Porous Mater. 5 (1998) 5. 2. A. Gil, L.M. Gandia, M.A. Vicente, Catal. Rev.- Sci. Eng. 42 (2000) 145. 3. X. Tang, W. Q. Shu, Y.F. Shen, S.L. Suib, Chem. Mater. 7 (1995) 102. 4. M.J. Hernando, C. Pesquera, C. Blanco, I. Benito, F. Gonz~.lez, Chem. Mater. 8 (1995) 76. 5. J. Shabtai, M. Rossell, M. Tokarz, Clays Clay Miner. 32 (1984) 99. 6. J. Sterte, Clays Clay Miner. 39 (1991) 167. 7. J.R. McCauley, U.S. Patent No. 4,818,737 (1988). 8. S.J.Gregg, K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London, 1982. 9. J. H. de Boer, J. Catal. 3 (1964) 268. 10. W. D. Harkins, G.J. Jura, J. Chem. Phys. 11 (1943) 431. 11. J. P. Olivier, J. Porous Mater. 2 (1995) 9. 12. M.J. Hernando, Thesis, University of Cantabria, 2000. 13. S. M. Bradley, R.A. Kydd, J. Yamdagni, J. Chem. Soc., Dalton Trans. 7 (1990) 102. 14. C. Pesquera, F. GonzS.lez, I. Benito, S. Mendioroz, J.A. Pajares, Appl. Catal. 8 (1991) 587. 15. F. Gonz~lez, C. Pesquera, C. Blanco, I. Benito, S. Mendioroz, Inorg. Chem. 31 (1992) 727.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1261

X-ray absorption fine structure investigation o f M C M - 4 1 materials containing Pt and PtSn nanoparticles prepared via direct hydrothermal synthesis Chanho Pak a, Naisheng Yaob and Gary L. Hallerb aMaterials and Devices Laboratory, Samsung Advanced Institute of Technology, EO. Box 111, Suwon, 440-600, Korea bDepartment of Chemical Engineering, Yale University, EO. Box 208286, New Haven, CT 06520-8286, USA

Pt- and PtSn- containing MCM-41 materials were prepared by direct introduction of metal precursors with fumed silica and cetyltrimethylammonium surfactant. The precursor of Pt, with or without Sn precursor, was introduced in the initial synthesis mixture by dissolving both Pt(NHa)4(NO3)and K2SnO3. The highly ordered MCM-41 mesoporous structure of two samples was revealed by a four-peaked XRD patterns, sharp capillary condensation in nitrogen isotherms, and a high surface area. Alter reduction, the coordination number of Pt particles in the MCM-41 samples decreased from 8.7 to 6.4 with simultaneous incorporation of Sn. The nearest Pt-Pt distance is almost the same, 0.276 ran, in both samples. The whiteline intensity of X-ray absorption at the Pt LIn edge was decreased to that of metal foil during the reduction. It is suggested from the coordination number of the first shell that Pt particles are generated in the MCM-41 mesoporous molecular sieves by direct introduction and that simultaneous addition of Sn leads to a decrease of particle size. 1. INTRODUCTION Mesoporous materials such as the M41S family and SBA-n have stimulated interest as sorbents, catalysts and supports of high surface area and uniform pore size. Pt-containing mesoporous materials prepared by direct incorporation [1-3] or post impregnation [4-7] with several precursors have been studied for the conversion of carbon monoxide [1 ], aromatics [4], and hexane [5,6]. Recently, we investigated three preparation methods for the preparation of Pt particles in MCM-41 material and obtained highly dispersed Pt particles in MCM-41 [7]. In this study, the Pt- and PtSn-containing MCM-41 samples were prepared by direct introduction of the Pt precursor without or simultaneously with the Sn precursor. The structure of MCM-41 demonstrated by X-ray diffraction is slightly perturbed by introduction of metal atoms, but retains a highly ordered structure. The particle size estimated by the coordination number of Pt particle was decreased by simultaneous introduction of Sn.

1262 2. EXPERIMENTAL

It-containing MCM-41 (It-MCM-41) material was synthesized by a modified method used for pure MCM-41 [7,8]. Ten g of tetramethylammonium silicate (SACHEM, 10-wt% SiO2) was mixed with 50g of doubly deionized water (DW) and then 2.5g of HiSil-233 (Pittsburgh Plate Glass) was added. Pt(NH3)4NO3 (0.0968g, Aldrich) was added to the above solution. Then, cetyltrimethylammonium (CTA) hydroxide solution (29.0g), obtained from ion exchange of the 20 wt% of CTABr aqueous solution with Amberjet 4400 OH (Sigma) resin by batch mixing, was poured into the silica solution with vigorous stirring. After stirring for 10rain, the solution pH was corrected to 11 with dilute H2SO4 solution. The synthetic gel, having molar ratios of 3.67 SiO2:0.30 (TMA)20:0.02 It: 1 CTAOH: 255 H20, was transferred to a polypropylene (PP) bottle and placed in an autoclave at 383 K for 96 h. For the ItSn-containing MCM-41 (ItSn-MCM-41) was prepared by the same procedure as above except the simultaneous addition of 0.2989 g of K2SnO3 (STREM) with Pt(NH3)4NO3. The molar ratios in the mixture at this time was 3.67 SiO2:0.30 (TMA)20:0.02 It: 0.08 Sn: 1 CTAOH: 255 H20. The resulting product was recovered by filtration on a Btichner funnel, washed with DW, and dried in air at room temperature (RT). Calcination of this product was conducted by heating from RT to 823 K at 1K/min and holding for 6 h in flowing air. Reduction of the calcined sample was carried out with 5% H2/He flow at 623 K for 2 h. The XRD patterns were recorded on a SCINTAG X-ray diffractometer (Cu I~, k= 0.154 ran, 40 kV, 45mA). The scanning range of 20 was between 1.6 ~ and 7 ~ with a step increase of 0.02. The sample powder was pressed into a depressed square area of a plastic slide to obtain a smooth plane of sample with regard to the slide surface. Nitrogen adsorption-desorption isotherms were measured at 77 K with a static volumetric instrument (Autosorb-lC, Quantachrome) after outgassing the sample at 473 K to a vacuum of ca. 10"7 bar. A Baratron (10 .6 - 10.2 bar) pressure transducer was used for low-pressure measurement. At each isotherm point, the saturated vapor pressure of liquid nitrogen was measured in a reference cell. Thermal analysis was performed by simultaneous TG-DTA measurements in flowing air using the STA 449C of NETZSCH Co. Samples were heated in the temperature range 3 0 0 1073K at a heating rate 10 K/min. X-Ray absorption measurement was performed at the I t Lm edge (11564 eV), using Si(lll) as the monochromator crystal at station X18B in NSLS, 2.5 GeV storage ring, Brookhaven National Laboratory. Samples were pressed into self-supporting wafers and placed into an in-situ cell, equipped with Kapton windows, gas inlet, outlet and heating unit around the sample, which allowed gas treatment and measurement. In the XANES analysis the first inflection points of a metal foil reference and of all samples were adjusted and aligned to the edge energy of the metal foil. The X-ray absorption fine structure (XAFS) spectra were analyzed relative to the reference XAFS from the I t foil by using UWXAFS 2.01 from the Department of Physics at the University of Washington.

1263 3. RESULTS AND DISCUSSION The color of Pt and PtSn containing MCM-41 samples changed from light gray to dark gray after calcination, which is evidence of the presence of metal particles in these materials. The loading of each metal was analyzed by elemental analysis as listed in Table 1. The amount of Pt in the two samples is very similar at about 1.3 wt%, which indicates the loading of Pt is not effected by the simultaneous addition of Sn with this synthesis method. All samples displayed at least four well-resolved peaks corresponding for p6m symmetry in the XRD patterns below 2 0 - 7 ~ as shown in Fig. 1 [1,6-8]. The direct introduction of Pt and Sn precursors into the synthesis mixture for MCM-41 materials did not affect significantly the formation of the hexagonal mesoporous structure. A This result agreed with the 5 previous observation [1]. The intensities of the most intense peaks of Pt-MCM-41 and PtSnMCM-41 samples were decreased from that of pure MCM-41, which suggests that the hexagonal structure was perturbed a little by introduction of Pt and Sn metal atoms. The four peaks indexed as (1 00), (1 1 0), ( 2 0 0 ) and ( 2 2 0 ) 2 3 4 5 6 planes. The lattice constant of the 2 Theta (degree) structure was calculated from the Figure 1. Powder XRD patterns of (a) pure MCM-41, (b) PtSn-MCMequation a0 = 2d100A/3. The unit 41 and (c) Pt-MCM-41. cell parameters are given in Table 1. The lattice constant is increased by 0.3 nm for Pt-MCM-41 and is the same for PtSn-MCM-41 when compared with the pure MCM-41. This is additional evidence of a small perturbation of the hexagonal structure of MCM-41 by transition metal atoms.

Table 1. Physical properties ofPt-MCM-41 and PtSn-MCM-41 samples. Sample MCM-41 Pt-MCM-41

Matal loading

(wt%)

Surface area (m//g)

Lattice Constant

Wall thickness a

3.7 3.8

4.6 4.9

0.9 1.1

3.6

4.6

1.0

Pore size (nm)

1261 Pt: 1.3 780 Pt: 1.2 PtSn-MCM-41 893 Sn: 3.1 aobtained from the difference between lattice constant

and pore size.

(nm)

(nm)

1264

100

*-

b

80

k

,,m

o 6O I

400

600

800

t

I

1000

Temperature (K)

400

600

800

1000

Temperature (K)

Figure 2. (a) Weight change curves for (--) Pt-MCM-41 and ( - - - ) PtSnMCM-41 and (b) weight change derivatives Figure 2 shows the TGA plots of weight loss observed for the Pt- and PtSn-MCM-41 samples. Both samples exhibited typical patterns in the plots as reported in the literature for M41S materials [9-13]. The final residue is 64 wt% for Pt-MCM-41 and 60 wt% for PtSnMCM-41, respectively. In the case of PtSn-MCM-41 sample, the weight loss is larger than that of Pt-MCM-41. This is attributed to the additional weight loss of the Sn component. In Fig. 2 b, three regions of 600 weight loss were observed in both samples. The first weight loss between RT and 390 K is due to adsorbed water (-~5 wt%). Most of weight loss (20-23 A wt%) was observed in the second region b ( 3 9 0 - 563 K), which is caused by the 400 E decomposition of surfaetants [9-13]. A difference of both samples was obvious Q} in the third weight loss region (563-773 K). It appears that this region is related E to the decomposition of the metal 200 complexes. The increased portion of weight loss in this region supports this interpretation because the PtSn-MCM41 sample had additional SnO32 complex in the as-made sample. 0 N I I I I Fig. 3 shows the nitrogen adsorption0.0 0.2 0.4 0.6 0.8 1.0 desorption isotherms of Pt-MCM-41 Relative Pressure (pip.) and PtSn-MCM-41 attar calcination. A Figure 3. Nitrogen adsorption-desorption capillary condensation of a typical Type isotherms of (a) Pt-MCM-41 and VI isotherm was exhibited in the range (b) PtSn-MCM-41

~

I

1265 of 0.2-0.4 relative pressure (P/P0) without hysteresis for all samples. This is a characteristic of highly ordered mesoporous structures such as MCM-41 and MCM-48 [8,14,15]. The surface area of samples was decreased to 780 m2/g for Pt-MCM-41 and 893 m2/g for PtSn-MCM-41 from that (1261 m2/g) of MCM-41 material. Although the introduction of metal atoms caused a decrease of the surface area, the absolute value of the materials is still high so that the mesoporous structure is slightly disturbed. The pore size was calculated using the BJH method with the corrected form of the Kelvin equation and the statistical film thickness curve for porous silica, both derived from the Kruk, Jaroniec and Sayari (KJS) approach [16]. The pore size for both Pt-MCM-41 and PtSnMCM-41 samples is almost identical (3.7 + 0.1nm) to the pure MCM-41 materials. The suggests that the pore structure of MCM-41 is retained, even when transition metal atoms were directly introduced in the initial synthesis mixture, as was the case for Cr and V [8,17]. The X-ray absorption near edge structure (XANES) was recorded during the in situ reduction of the calcined sample from RT to 623K (not shown). The whiteline area at the Lin edge of Pt was gradually decreased with increased temperature, which indicates that the Pt was reduced to the zero valent state. The temperature of maximum reduction rate, estimated from the rate of change ofthe white line area [7], is 423K for Pt-MCM-41 and 388K for PtSnMCM-41, respectively. For XAFS analysis, after Fourier transforming, the spectra in R space of the isolated Pt-Pt coordination shell were obtained using UWXAFS 2.01 [18] as shown in Fig. 4. The height of the peaks in R space is indicative of the number of Pt atoms that are present at the corresponding distance between two atoms. The fitted spectra in R space are also illustrated in Fig. 4. The structural parameters are calculated from the fitting with Pt foil as reference and listed in Table 2. The average distance between Pt-Pt atoms of the first shell was 0.276 nm for Pt- MCM-41 and 0.277 nm for PtSn-MCM-41, respectively, which is almost identical to the value of the bulk Pt foil. The coordination number of the first shell is decreased from the 8.7 for Pt-MCM-41 to 6.4 for PtSn-MCM-41 by simultaneous introduction of Sn. For the

b

_= I

1 2 3 4 Distance (A)

5

0

1

2 3 4 Distance (A)

Figure 4. Fourier transfoms magnitudes of (a) Pt-MCM-41 and (b) PtSn-MCM-41 ( ( - - ) experimental and (ooo) fitted).

9 . . . .

1266 Table 2. XAFS fitting results for the Pt-MCM-41 and PtSn-MCM-41 samples. Sample Distance ( n m ) Coordination number Debye-Wallerfactor Pt-MCM-41 0.276 8.7 0.0038 PtSn-MCM-41 0.277 6.4 0.0031 determination of particle size, the high-angle XRD and transmission electron microscopy is now in progress. Highly ordered mesoporous Pt and PtSn containing MCM-41 materials were prepared by direct hydrothermal synthesis. The mesoporous structure of these materials is suggested from the XRD, TGA and nitrogen isotherms. It was hypothesized from the XAFS analysis that the simultaneous introduction of Sn with Pt precursor caused the decrease of the coordination number at the first shell of Pt particles. ACKNOWLEDGEMENTS

We acknowledge financial support from DOE, Office of Basic Energy Science. C. Pak also thanks the Korea Science and Engineering Foundation (KOSEF) for partial financial support for post-doctoral fellowship. REFERENCES

1. U. Junges, W. Jacobs, I. Voigt-Martin, B. Krutzsch, F. Schuth, Chem. Commurt, (1995) 2283. 2. M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, F.J. Romero, Chem. Commun., (1999) 873. 3. M. Chatterjee, T. Iwasaki, Y. Onodera, T. Nagase, Catal. Lett., 61 (1999) 199. 4. A. Corma, A. Martinez, V. Martinez-Soria, J. CataL, 169 (1997) 480. 5. T. Takeguchi, J.-B. Kim, M. Kang, T. Inui, W.-T. Cheuh, G.L. Haller, J. CataL, 175 (1998) 1. 6. K. Chaudhari, T.K. Das, A.J. Chandwadkar, S. Sivasanker, J. CataL, 186 (1999) 81. 7. N. Yao, C. Pinckey, S. Lim, C. Pak, G.L. Haller, Micro. Mesopo. Mater., 44-45 (2001) 377. 8. C. Pak, G.L. Haller, Micro. Mesopo. Mater., 48 (2001) 165. 9. Y. Cesteros, G.L. Haller, Micro. Mesopo. Mater., 43 (2001) 171. 10. M. Kn~, M. Jaroniec, R. Ryoo, S.H. Joo, Chem. Mater., 12 (2000) 1414. 11. A.A. Romero, M.D. Alba, J. Klinowski, J. Phys. Chem. B, 102 (1998) 123. 12. S. Kawi, M. Te, Catal. Today, 44 (1998) 101. 13. M. Busio, J. Janchen, J.H.C. van Hooff, Micro. Mater., 5 (1995) 211 14. M. Jaroniec, M. Kn~, H.J. Shin, R. Ryoo, Y. Sakamoto, O. Terasaki, Micro. Mesopo. Mater., 48 (2001) 127. 15. F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by powders and porous solids: Principles, Methodology and Applications, Academic Press, San Diego, 1999. 16. M. Knfl(, M. Jaroniec, A. Sayari, Langmuir, 13 (1997) 6267. 17. S. Lim, G.L. HaUer, Appl. Catal. A: Gen., 188 (1999) 277. 18. M. Newville, P. Livins, Y. Yacoby, J.J. Rehr, E.A. Stern, Phys. Rev. B, 47 (1993) 14126.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1267

Ordered assembling of precursors of colloidal faujasite mediated by a cationic surfactant. J. Agfindez, I. Diaz, C. M/trquez-Alvarez, E. Sastre and J. P6rez Pariente*. Instituto de Cat/disis y Petroleoquimica, C.S.I.C. Campus Cantoblanco, 28049 Madrid (Spain)

The synthesis of ordered microporous materials have been carried out by adding a cationic surfactant, cetyltrimethylammonium bromide, to TMA-containing solutions precursors of colloidal faujasite. The influence of the synthesis temperature on the physicochemical properties and catalytic activity in m-xylene conversion of the calcined samples has been studied. Calcination of the as-made, MCM-41-1ike samples produces a strong unit cell contraction, and microporosity develops. No FAU structural elements are detected by IR spectroscopy. The reaction rate for m-xylene transformation is much higher than that of conventional A1-MCM-41, but lower than that of USY, whereas the isomerization/disproportionation ratio is close to 10.

1. INTRODUCTION The discovery of the Ordered Mesoporous Materials (OMM) opened the possibility to process organic molecules bulkier than those currently converted by conventional zeolites. The isomorphous substitution of part of the silicon atoms of the framework by aluminum imparts structural acidity, but it was soon realised that the acidity and stability of the aluminum-containing materials is below that required for most applications in the refining industry (1). The mild acidity has been attributed to the absence of structural ordering within the silica-alumina inorganic framework of the material. On the other hand, it is well known since long the strong acidity and good stability (generally speaking) of many zeolite structures, although obviously these properties vary from one structure to another, as well as a function of the A1 content. Therefore, it has been argued that the eventual ordering of the framework of mesoporous materials would increase their acidity and thermal stability. This goal, if reached, would have important consequences for the catalytic application of these materials in many chemical processes, especially those dealing with molecules excessively large to be activated by microporous structures. Fuelled by these potential benefits, a number of researches have been pursued, aiming to confer some structural order to the framework of OMM. Basically, three different approaches have been followed: A) on-site structuring of the framework during the self-assembling process leading to the mesostructured material. B) *Author for correspondence. Phone: 34 91 585 4784. Fax: 34 91 585 4760. E-mail: [email protected]. Http://www.icp.csic.es/gtm

1268 post-synthesis recrystallization of MCM-41 by treatment with a template-containing solution. C) the use of pre-ordered blocks synthesised e x s i t u and their further assembly by addition of surfactants. Following route A, mixtures of ZSM-5 and MCM-41 are obtained by using tetrapropylammonium (TPA) and a cationic surfactant (2-4). Composite materials containing zeolite and MCM-41 are obtained by using procedure B. For instance, mixtures of ZSM-5 and MCM-41 are obtained by hydrothermal treatment of MCM-41 exchanged with TPA (5) or by impregnation of amorphous mesoporous aluminium-containing silica (Si/A1 = 100) with a TPAOH solution (6). Aluminosilicate mesostructures stable under steaming have been obtained by using precursors of the zeolites faujasite (7), ZSM-5 (8) and Beta (8-10), i.e., route C. However, although the benefits of the assembling of the so-called nanoclustered aluminosilicate precursors on the steam stability of the OMM is well documented, the increase in stability is not accompanied by a simultaneous enhancement of the acidity. An exception to this behaviour has been reported recently by Zhang and col. (10), who claimed the synthesis of a mesoporous material that possesses an acid strength similar to that of zeolite H-Beta. However, if precursors of zeolite Y are used to built up the mesoporous material, only minor differences between this material and conventional A1-MCM-41 in the cracking of cumene are observed before steaming (7). In contrast to this, the use of ZSM-5 seeds leads to a three-fold increase of activity for the same reaction (8). The synthesis of the precursors of both ZSM-5 and Beta requires the use of an organic cation, TPA and TEA (tetraethylammonium), respectively, whereas the reported synthesis of the faujasite precursor made use of a pure inorganic seeding gel (7). Therefore, it would be possible that the presence of organic cations in the zeolite gel precursor be required to obtain materials possessing high acid strength. Accordingly, we have carried out the synthesis of mesoporous materials by using as building blocks precursors of zeolite faujasite obtained in the presence of tetramethylammonium (TMA) cations. The procedure reported in ref. (11), which allows the synthesis of colloidal crystals of faujasite, has been followed.

2. E X P E R I M E N T A L 2.1. Materials

Synthesis gels precursors of faujasite with a molar composition: A1203 : 1.53 (TMA)20 : 0.088 Na20 : 3.62 SiO2 : 246 H20 were prepared as follows: freshly precipitated AI(OH)3 was added under stirring to a solution containing TMAOH (25 wt%, Aldrich) and NaOH (Prolabo), until complete dissolution of the AI(OH)3. Then, Ludox SM30 (Aldrich, previously exchanged with Dowex HCR-S ion exchange resin) was added and the mixture stirred for 45 minutes. The resulting opalescent solution was poured into polypropylene bottles and kept at 100~ for 24 h. After this, a hexadecyltrimethylammonium (CTA) bromide aqueous solution (20 wt%) was added at room temperature, and the bottles were heated again at selected temperatures for 3 h. For T > 100~ 60ml, teflon-lined, stainless-steel autoclaves were used. The resulting solid was filtered, washed with deionized water and dried at 60~ overnight. The total yield of oxides averaged 60%. The samples were calcined at 550~ under continuous flow of N2 (130 cm3-min-1) for lh, followed by air (130 cm3.min-1) for 6 h. Reference A1-MCM-41 (Si/A1 = 15) was prepared following ref. (12). A commercial ultrastable Y zeolite (USY CBV-720,

1269 unit cell parameter a0 = 24.28 A), kindly provided by Zeolyst International, was also used as reference in the m-xylene isomerization experiments.

2.2. Characterization X-ray powder diffraction patterns were collected using CuKc~ radiation, on a Seiffert XRD 3000P diffractometer operating at low angle (1-10 ~ using a primary automatic divergence slit and a 0.2 mm detector slit. Analysis of the organic material present in the solid was carried out using a Perkin-Elmer 2400 CHN analyser. Thermogravimetric analyses (TGA) were performed using a Perkin-Elmer TGA7 instrument, from 30 to 900~ at a heating rate of 10~ -1 under air flow. Adsorption of nitrogen was carried out at-196~ using a Micromeritics ASAP 2000 apparatus. Specific surface areas were calculated following the BET procedure. The Si/A1 ratio of calcined samples, finely grounded and dispersed in water, was measured using a Rich & Seifert EXTRA-II total reflectance X-ray fluorescence (TXRF) spectrometer. For the transmission electron microscopy (TEM) experiments, the samples were crushed, dispersed in acetone and dropped on a holey carbon grid. Micrographs were recorded using a JEOL JEM 2000FX microscope operating at 200 kV equipped with an X-ray detector (XEDS) and a Philips 120 Biotwin microscope operating at 120 kV equipped with a Gatan CCD camera. Infrared (IR) spectra in the range 400-4000 cm -1 were recorded at 4 cm -1 resolution, in the transmission mode, using a Nicolet 5ZDX FTIR spectrometer. IR spectra of the solid samples diluted in KBr were recorded at room temperature. For acid sites -2 characterisation, the samples were pressed into self-supporting wafers of 6 to 12 mg.cm thickness, placed into a glass cell provided with CaF2 windows and greaseless stopcocks, and heated under vacuum (10 .3 Pa) at 350~ for 8 h; pyridine (8 Torr) was dosed at room temperature, the sample subsequently evacuated at selected temperatures for 1 h, and IR spectra recorded at room temperature. The amount of pyridine adsorbed on acid sites were estimated from the integrated absorbance of the IR bands centred at ca. 1547 (Br6nsted sites) and 1456 cm -1 (Lewis sites), assuming for the integrated molar extinction coefficients the values 1.67 and 2.22 cm.gmo1-1, respectively (13). 2.3. Catalytic Activity Isomerization of m-xylene was carried out in a fixed bed reactor at atmospheric pressure, working at a temperature of 400~ and a molar ratio N2/m-xylene - 4. The contact time was varied accordingly to obtain conversions below 10%. The reaction products were analysed by gas chromatography in a Hewlett-Packard GC 5710A provided with a thermal conductivity detector and equipped with a column filled with DC-200 methylsilicone (16%) and Bentone 34 (3%) on Chromosorb W (80-100 mesh). In order to compare the activity of the catalysts in the absence of deactivation, the initial activity (V0) of all products were calculated by extrapolating the conversion at time zero of reaction (14).

3. RESULTS AND DISCUSSION The X-ray diffraction patterns of the samples obtained from gels heated at different temperatures are given in Figure 1. Reflections corresponding to faujasite were not observed. Patterns characteristics of MCM-41 were obtained, the ordering of which increases with the synthesis temperature, whereas the unit cell dimensions follow the opposite trend (Table 1).

1270

[s2:il

[, $2-1 cal. ] 5 20 25 30 35 40

"-2',

d

s 2, A ~

r~

3

5

40

]s2-3ca,I J 5 10 15 20 25 30 35 40 I x 1.7 2

3

4

5

6

1

z0

2

3

4

5

6

Figure 1. XRD patterns of as-made and calcined samples. Upon calcination, only one intense peak at 20 - 3.7 ~ is observed in the XRD patterns (Figure 1), which intensity also increases with the synthesis temperature. The TEM image of sample S1-3 calcined, synthesised at 80~ shows that this is a heterogeneous material exhibiting some MCM-41 orientations (Figure 2A). On the other hand, the calcined sample synthesised at 150~ (sample $2-3) possesses large domains exhibiting well ordered hexagonal pore arrangement characteristic of MCM-41 (Figure 2B). From the TEM image, a unit cell dimension (a0) of 29 A is calculated for sample $2-3 by Fast Fourier Transform (FFT). This value compares well with the a0 value determined from XRD (27.7 A), assuming that the intense peak at 3.7 ~ corresponds to the [100] reflection of the hexagonal p6mm symmetry. For sample $2-3, the Si/A1 ratio obtained by TXRF is 2.7, close to the value of 3.0 determined by XEDS. Table 1. Synthesis conditions and properties.

Sample

S1-3 $2-1 $2-2 $2-3

as-made Synthesis d temperature spacing (~ (A) 80 100 120 150

. . 38.55 38.45 37.49

,

ao

d spacing (A)

(A) .

. . 44.6 44.4 43.3

.

. . 23.56 23.39 23.26

Calcined ao Surface area @) (m2.~-') 27.2 27.2 27.7

544 617 593 642

Micropore volume (cm3-~-~) 0 0.12 0.15 0.24

1271

~flnm_v_...

imilllmllmm

." ,,

B

~'.

Figure 2. TEM images of calcined samples S 1-3 (A) and $2-3 (B). It is noteworthy the large unit cell contraction brought about by calcination, --- 17 A, and particularly the fact that despite of this severe shrinkage, the hexagonal arrangement is nevertheless maintained in the samples synthesised at T > 100~ (Table 1). This remarkable cell contraction might be due to the presence of a large number of structural defects, i.e., SiOH groups. From the chemical analysis and the TG data, the total content of silanol groups has been estimated to be 0.95 mol per mol of SiO2, i.e., a very high population of structural defects. Indeed, the large amount of organic material contained in the solid, as evidenced by TG (Figure 3), points to a thin pore wall. The reference A1-MCM-41 material contains 35wt%

,

i

i

i

i

220 t 200-[" ~

a.<"

o~

.z" .=

I

'~i

400 T (~

600

t/ / ,6Ol/

~ ,4Ol/ / !~ l~176 60

$2-3] i

200

$2-3 __---

800

40 0.0

0.2

0.4

0.6

0.8

P/P0

Figure 3. TG curves and derivatives of as-

Figure 4. N2 isotherms of calcined S1-3

made $2-1 and $2-3 samples.

and $2-3 samples.

1272 of organic, whereas sample $2-3 contains 49 wt%. The C/N ratio in the as-made $2-3 sample is 19 (as in CTA), which excludes the presence of a significant amount of TMA. The IR spectrum of this sample (self supporting wafer, 4 mg.cm -2 thickness) also supports this conclusion. Only IR bands corresponding to CTA are identified, while the characteristic methyl asymmetric bending band of TMA (ca. 1488 cm -1) is not observed. N2 adsorption isotherms of the samples synthesised at 80~ and 150~ are given in Figure 4. The shape of the isotherm changes from one characteristic of a poorly ordered mesoporous material, for a synthesis temperature of 80~ (S1-3), to one associated to the presence of micropores, for samples synthesised at T > 100~ This result is also consistent with the cell parameter of the calcined samples, 27 A, which suggests that the pore size could not be larger than - 20 A, in order to avoid an unrealistic wall thickness value. As shown in Table 1, the solids possess a high surface area, and the micropore volume determined by the tplot method increases with the synthesis temperature. The IR spectrum of the sample $2-3 (calcined) in the framework vibration region is very similar to that of the reference A1-MCM-41, and no IR bands characteristic of faujausite are present (Figure 5). This result suggests that building units resembling those present in the FAU structure would not be forming part of the $2-3 framework. However, this does not exclude the eventual presence of FAU fragments formed by a small number of tings. The IR spectrum of the sample $2-3 (calcined) after pyridine adsorption is presented in the Figure 6. It can be seen that the intensity of the 1547 cm ~ band corresponding to protonated pyridine decreases with the evacuation temperature, but it is still observed at

1456

0.2

90.2

1

r

O ra0

20'00

'

15'00 '

10'00 '

500

Wavenumber (cm-1) Figure 5. IR spectra of samples $2-3 calcined (a) and A1-MCM-41 (b), diluted in KBr.

165o

15'5o

14'5o

Wavenumber (cm-1) Figure 6. IR spectra of pyridine adsorbed on sample $2-3 calcined, evacuated at 200 (a), 250 (b) and 300~ (c). The spectrum of the sample before pyridine adsorption has been subtracted.

1273 50

'

i

'

i

'

I

'

i

'

i

'

i

'

i

9

o

- 10

40 30 ZJ

o c,.) 20

4

10 3

[]

[]

o

'

6'0

A v

A w

[]

[]

1

=

o

I

v

,qw

[]

[].

180 ' 240 ' 3;0 ' 3;0 ' 420

TOS (s.)

Figure 7. Conversion (filled symbols) and I/D ratio (open) in m-xylene isomerization. USY (squares) and $2-3 (circles). 300~ although with small intensity. Indeed, the acidity of the samples increases with the synthesis temperature (Table 2), and it is higher for sample $2-3 than for the reference A1MCM-41. Catalytic activity of the calcined samples in m-xylene conversion is collected in Table 3. It can be seen that the activity not only increases with the synthesis temperature by two orders of magnitude, but it is also much higher than that of A1-MCM-41. Moreover, the activity of the $2-3 sample is intermediate between that of A1-MCM-41 and USY (Figure 7) in agreement with the higher acid strength of the latter. The para- to orto-xylene ratio (P/O) is close to 1 in all the experiments, which is the expected value for large pore catalysts without any shape selectivity effect (14). It is also noteworthy the higher isomerization to disproportionation (I/D) ratio of sample $2-3, close to 10, while the I/D ratio for USY is always below 2 (15). This effect should be related to the lower acidity of the $2-3 sample and the higher acid demand of the disproportionation reaction.

Table 2. Concentration of pyridine (gmol-g -1) adsorbed on Br6nsted (B) and Lewis (L) sites as determined by IR, at given desorption temperatures. Sample i i i

A1-MCM-41 $2-2 $2-3

200~

250~

350~

Table 3. Catalytic activity and isomerization/disproportionation ratio in the m-xylene conversion.

B

L

B

L

B

L

Sample

37.3 12.8 68.9

116 57 151

12.6 3.4 21.7

107 50 130

1.4 0.2 2.9

88 41 120

USY A1-MCM-41 $2-2 $2-3

I/D Initial rate ratio ( m o l - f l ' h -1) 0.7 18.7 1.5 9.4

1.4 7.5 10.3 1.9 10 .3 3.2 10 1

1274 4. CONCLUSIONS To summarise, the synthesis procedure disclosed in this paper leads to materials having hexagonal pore arrangement in the micropore region, which exhibit activity in acidcatalysed reactions intermediate between those of conventional A1-MCM-41 and zeolites. It could be anticipated that by proper choice of the synthesis conditions, it would be possible to tune the acidity to that required to catalyse selectively specific reactions. Acknowledgements The authors acknowledge the CICYT (Spain) for financial support within the Project MAT2000-1167-C02-0. I. Diaz and J. Agfindez acknowledge the MEC and the ITQ (UPVCSIC) for Ph.D. grants, respectively. 5. REFERENCES

1. A. Corma, V. Fom6s, M. T. Navarro, J. P6rez-Pariente, J. Catal., 148 (1994) 569. 2. A. Karlsson, M. St6cker, R. Schmidt, Microporous.and Mesoporous. Mater., 27 (1999), 181. 3. A. Karlsson, M. St6cker, K. Sch~ifer, Stud. Surf. Sci. Catal., 125 (1999), 61. 4. A. Karlsson, M. St6cker, K. Sch~ifer, Stud. Surf. Sci. Catal., 129 (2000), 99. 5. M.J. Ver hoef, P. J. Kooyman, J.C. van der Waal, M. S. Rigutto, J.A. Peters, H.van Bekkum. Chem. Mater., 13 (2001) 683. 6. D.T. On, S. Kaliaguine, Angew. Chem. Int. Ed., 40 (2001) 3248. 7. Y. Liu, W. Zang and T.J. Pinnavaia, J. Am. Chem. Soc., 122 (2000) 8791. 8. Y. Liu, W. Zang and T.J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 9. Y. Liu, W. Zang and T.J. Pinnavaia, Stud. Surf. Sci. Catal., 135 (2001). 10. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao, F-S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 11. B.J. Schoeman, J. Sterte, and J.E. Otterstedt, Zeolites, 14, (1994) 110. 12. I. Diaz, J P6rez-Pariente, E Sastre, Stud. Surf. Sci. Catal., 125 (1999) 53. 13. C.A. Emeis, J. Catal. 141 (1993) 347. 14. J.A. Martens, J. P6rez-Pariente, E. Sastre, A. Corma, and P.A. Jacobs, P. A., Appl. Catal. 45 (1988) 85. 15. A. Corma, V. Fom6s, J. P6rez-Pariente, E. Sastre, J.A. Martens, P.A. Jacobs, ACS Symposium Series, 368 (1988) 555.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1275

Synthesis, characterisation and catalytic activity of SO3H-phenyl-MCM-41 materials F. Mohino, I. Diaz, J. P6rez-Pariente and E. Sastre*. Instituto de Cat/tlisis y Petroleoquimica, C.S.I.C. Campus Cantoblanco, 28049 Madrid (Spain)

Ordered Mesoporous Materials (OMS) functionalised with alkyl sulfonic acid groups have been proved to be efficient esterification catalysts. Among them, materials having MCM-41 type structure exhibit the best performance. In this paper we report the synthesis and characterisation of SO3H-phenyl-MCM-41 materials and their catalytic activity in the esterification of glycerol with lauric and oleic acids. We have studied several synthesis parameters: the amount of phenyl groups, the presence of additional methyl groups and the sulfonation procedure of the phenyl groups. The materials obtained have been characterised by XRD, Elemental Analysis, Nitrogen adsorption, TGA and TEM. Finally, it has been studied the influence of the different synthesis parameters on the behaviour of the catalysts in the mentioned reaction of esterification.

1. INTRODUCTION The discovery and study of Ordered Mesoporous Materials [1,2] has opened interesting perspectives in their potential use as catalysts in chemical processes involving bulky molecules [3]. One of these reactions is the direct esterification of glycerol with fatty acids in order to obtain selectively the corresponding monoesters, which are valuable chemical products widely used as emulsifiers in the food, pharmaceutical and cosmetics industries [4,5]. The two main routes to obtain monoglycerides are the transesterification of the triglycerides with glycerol at high temperature in the presence of a basic catalyst, and the direct esterification of fatty acids [6,7]. In the later case, the catalysts used are strong mineral or organic acids, like sulfonic acid and a high glycerol/oil molar ratio (- 12) is required to achieve good selectivity to monoesters [8]. Different solid acids have been tested as catalysts in this reaction trying to improve the actual process. By using large pore zeolites some authors have obtained good results in the esterification of glycerol with short chain fatty acids (< C10) [9] but the results are less interesting when larger fatty acids are used, such as lauric [10,11] or oleic [12] acids. Recently, some papers have reported the good catalytic performance of mesoporous SO3H-propyl-MCM-41 catalysts in the esterification of lauric and oleic acids with glycerol [13-17]. These results are justified on base of their physicochemical properties, mainly the high acidity and the adequate porosity. Based on these antecedents we report in Author for correspondence. Phone: (+34) 915854795. Fax: (+34) 915854760. E-mail: [email protected]. http://www.icp.csic.es/gtm

1276 this paper the synthesis and characterisation of phenyl and methyl/phenyl functionalised MCM-41 materials and the sulfonation procedure to obtain the corresponding SO3H-phenylMCM-41 catalysts. The influence of the different acidity, porosity and hydrophobic/hydrophilic character of the catalysts on their performance in the esterification of glycerol with lauric and oleic acids is discussed.

2. E X P E R I M E N T A L 2.1. Synthesis Procedure The direct synthesis of phenyl and combined methyl/phenyl-functionalised MCM-41 materials, has been carried out by co-condensation of tetraethoxysilane (TEOS, Aldrich), phenyl-triethoxysilane (PTES, Sigma) and methyltriethoxysilane (MTES, Aldrich), in the presence of hexadecyltrimethylammonium bromide (CTAB, Aldrich) and tetramethylammonium hydroxide, following a procedure similar to that described in previous papers [18,19]. By using the same approach, synthesis gels of the general molar composition [1-(x+y)] TEOS: x PTES: y MTES: 0.12 CTAB: 0.27 TMAOH: 18.8 MeOH: 77.7 H20 were prepared. The surfactant was removed by stirring 1.5 g of the dried sample with a solution of 20 ml of HC1 (35wt%) in 205 ml of ethanol at 70~ for 24h. The chemical compositions for the synthesis gels are given in Table 1. The extracted phenyl and combined methyl/phenylMCM-41 materials were then sulfonated to the corresponding SO3H- catalysts according to two different methods described in the literature, which are detailed below. 2.1.1. Sulfonation with chlorosulfonic acid [20] In a typical synthesis procedure, the sulfonation of the solids bearing phenyl groups was carried out by dehydration of 1.0 g of the extracted solid, by passing a N2 steady flow of 10 ml/min while keeping the flask at 120~ for 4h. Then the solid was dispersed, under nitrogen atmosphere, in 10 ml of dry CH2C12 and kept at - 4~ while 0.4 ml of chlorosulfonic acid diluted in 8 ml of dry CH2C12 were added slowly. The suspension was stirred at 25~ during 4h under nitrogen atmosphere and was poured onto a water/ice mixture. The solid was filtered, washed with water and dried at 65~ during 24h. These catalysts will be noted as "F..-C". 2.1.2. Sulfonation with 30 % oleum [21] In this case the sulfonation of the extracted solid was carried out after dehydration of-~l.0 g under vacuum at 130~ for 12 h. The solid was then contacted with a SO3 atmosphere, obtained from 6.25 g of 30 % oleum (Alfa Caesar), during 60h at 35~ The solid was poured onto a water/ice mixture and then recovered by filtration, washed with water and dried at 65~ during 24h. These catalysts will be noted as "F..-S". 2.2. Characterisation X-ray powder diffraction patterns were collected using CuKa radiation, on a Seifert XRD 3000 P diffractometer operating at low angle (20 from 1 to 6~ Analysis of the organic material present in the solid was carried out using a Perking-Elmer 2400 CHN analyser. Thermogravimetric analyses (TGA) was performed using a Perkin-Elmer TGA7 instrument, from 30 to 900~ at a heating rate of 10~ under air flow. Adsorption of nitrogen was carried out a t - 1 9 6 ~ using a Micromeritics ASAP 2000 apparatus. Specific surface areas

1277 Table 1. Molar ratio of the synthesis gel and properties of the extracted and sulfonated samples. [1-(x+y)] TEOS: x PTES: y MTES: 0.12 CTAB: 0.27 TMAOH: 18.8 MeOH: 77.7 H20 BET Pore Unit cell Pore volume Sample* x Y surface area t g / ' 3''x cm diameter parameter (m2/g) (BJH)/(A~) (XRD)/(A) F20ex F20H-C F10ex F10H-C F10H-S F10Mex F10MH-C F10MH-S

0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1

0.19 0.19 0.19

632 n.d. 974 866 880 916 796 779

0.53 n.d. 0.67 0.59 0.58 0.58 0.54 0.52

<10 n.d. 18 19 19 12 13 12

35 n.d. 42 41 41 36 34 34

F..ex: Samples extracted. F..-C: Samples sulfonated with chlorosulfonic acid. F..-S: Samples sulfonated with SO3 were calculated following the BET procedure. Pore size distribution was obtained using the BJH pore analysis applied to the adsorption branch of the nitrogen adsorption/desorption isotherm. The samples for the TEM experiments were prepared by crushing the particles, dispersing in acetone and spreading them on a holey carbon grid. The images and selected area electron diffraction patterns were recorded using a JEOL JEM-200CX operating at 200 kV. 2.3. Catalytic Tests The sulfonated SO3H-Ph- and SO3H-Ph-(Me)-MCM-41 samples were used as catalysts in the esterification of glycerol with oleic and lauric acid. Reactions were accomplished directly in the liquid phase (without solvent), in a stirred four-opening flask heated in an oil bath under atmospheric pressure. An N2 steady flow of 10 ml/min was passed over the reaction mixture through one flask opening, in order to eliminate the water formed during the esterification, which was adsorbed by means of a glass elbow-shaped tube packaged with zeolite A. Reaction temperatures were 100 and 120~ for the esterification of lauric and oleic acids, respectively. The amount of catalyst used was 5 wt %, previously dried at 100~ overnight. A molar ratio of glycerol to fatty acid was 1 in both cases. Full details of reaction and analytical procedures are given elsewhere [ 17].

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterisation of the materials The main physical-chemical and textural properties of the different samples prepared are presented in Table 1. The X-ray diffraction patterns and N2 isotherms of the F10 samples extracted and sulfonated are shown in Figures 1 and 2 respectively. In Figure 1 it can be observed that the extracted sample shows an intense peak at -2.2 ~ which indicates mesoporous arrangement.

1278

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20 Figure 1. XRD patterns of the F10 and F20

extracted and F10 sulfonated samples.

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Figure 2.

The broadness of the band at -4-5 ~ does not allow to index a defined symmetry. This broad band disappears in the sulfonated samples and the intensity of the peak at low angle decreases simultaneously, indicating probably a less ordered pore arrangement in these materials. Nevertheless, the structural and textural properties-i.e, pore volume, pore diameter, surface area and cell parameter- remain almost unaltered independently of the method of sulfonation (Table 1). All the samples, even when SO3H is incorporated, possess the conventional N2 isotherm characteristic of MCM-41 containing functional groups covering the pore walls [17,18] (F i gure 2). In the case of sample F20ex, the structure could be assessed by TEM. Hexagonal MCM41 type structure can be assigned as it is observed in the images of the parallel and normal directions to the channel axis in Figure 3. In Table 1 it can be observed that by increasing the amount of PTES in the synthesis gel, both the surface area and pore volume decreases appreciably, samples F10ex and F20ex. This must be due to the incorporation of a higher amount of phenyl groups to the solid, which can be confirmed from the data of the organic and thermogravimetric analyses presented in Table 2. From these data, it can be calculated the degree of incorporation of the PTES species from the synthesis gel,-84% in sample F20ex a n d - 9 9 % in sample F10ex, which corresponds to 3.68 and 2.03 mmol of phenyl per gram of SiO2, respectively. In the case of sample F10Mex, which contains also methyl groups, the quantification of the functional groups will be described later with the help of the 29Si MAS-NMR results. The incorporation of such amount of functional groups limits the access to the pores strongly, decreasing the por.e diameter of the F20ex sample. As it will be shown later this fact will affect to the catalytic activity of the corresponding sulfonated samples. A similar effect, although not so marked, is observed when methyl groups are added to the synthesis gel, samples F10ex and F10Mex. In this case the decrease in the surface area and the pore volume is less important, probably due to the incorporation of an important number of methyl groups, less bulky than the phenyl groups.

1279

A

Figure 3. TEM images of F20ex in a direction parallel (A) and normal (B) to the channel axis. The incorporation of the functional groups to the solid can also be demonstrated by 298i MAS-NMR. Figure 4 depicts 298i MAS NMR spectrum of the sample F 10Mex, functionalised with a mixture of methyl and phenyl groups. Resonances a t - 9 0 , - 1 0 0 a n d - 1 1 0 ppm correspond to the Q2 ((SiO)3Si(OH)2), Q3 ((SiO)3SiOH) and Q4 ((SiO)4Si) silicon species, respectively. The signal centred at-64 ppm has been assigned to silicon atoms attached to the methyl groups in 74 configuration, (CH3Si(OSi)3), whereas the shoulder a t - 5 5 ppm is attributed to T2 species, i.e., Si atoms attached to one residual OH group, (CH3(SiO)zSiOH). The last signal observed at -79 ppm has been assigned to -Si-phenyl (T13) groups. After deconvolution of the spectrum it has been possible to calculate the degree of functionalisation of this sample (Table 3), which is 27%, and it can be concluded that almost all the methyl and about a 70% of the phenyl groups from the synthesis gel are incorporated to the solid. Table 2.

Elemental analyses, and termogravimetric analysis of the extracted and sulfonated samples. Elemental analysis

TGA

Sample 1

F20ex 2 F10ex F10H-C F10H-S F10Mex F10MH-C F10MH-S

C (wt. %)

S (wt. %)

C/S

mmols/gcat

Weight losses (%) (200_800oc)

17.01 11.66 6.29 3.91 13.91 8.17 8.12

0 0 1.32 1.64 0 1.57 2.08

12.6 6.4 13.6 10.4

0.41 0.51 0.50 0.65

23.6 13.7 13.2 11.0 14.8 14.0 14.6

1F..ex" Samples extracted. F..-C: Samples sulfonated with chlorosulfonic acid. F..-S" Samples sulfonated with SO3.2Sample F20H-C contains 1.15 mmols/gcat determined by base titration.

1280 The sulfonation process, using SO3 or chlorosulfonic acid, produces a notable decreases in the organic content of the sample (Table 2). This decrease is specially marked in the case of the F10 sample, while the F10M sample, also functionalised with methyl groups, retains a higher amount of organic groups. The amount of sulphur is also slightly higher in this latter case, indicating that the number of phenyl groups that have been sulfonated .Jq is also higher. Moreover, r the amount of sulphur determined in the samples sulfonated with SO3 is higher than the present in the samples sulfonated with -150 -200 0 -50 -100 chlorosulfonic acid, indicating that the former 5 (ppm) method looks to be more Figure 4. 29Si MAS-NMR spectrum of sample F10Mex adequate. In sample F10ex the sulfonation with SO3 eliminates a Table 3. Assignment and normalised peak area (%) of 29Si MAS-NMR in higher amount of phenyl groups, but the C/S close Figure 4. Q4 Q3 Q2 T13 T~ T2 to 6 indicates that all the phenyl groups remaining Si(OSi)4 =Si-OH =Si(OHh =Si-Ph =Si-CH~ =Si(On)CH~ have been sulfonated, while in the case of -110,6 -101,4 -92,2 -79,0 -64,1 -55,1 F10H-C, the total amount (48%) (22%) (3%) (7%) (13%) (7%) of organic content is higher but also the high C/S ratio leads to think that the sulfonation of the phenyl groups has not been complete. A similar effect is observed in the sample F10M but, in this case, the presence of additional methyl groups should be considered in order to estimate the sulfonation degree. i

,

i

,

i

3.2. Catalytic activity The catalytic activity of the samples F10H and F10MH sulfonated with chlorosulfonic acid or SO3 in the esterification of glycerol with lauric acid is presented in Figure 5. It can be observed that independent of the sulfonation method, samples F10MH are more active than the catalysts without methyl groups, F1 OH. The effect on the catalytic activity produced by the incorporation of methyl groups to functionalised SO3H-Propyl-MCM-41 has been described previously, and it has been attributed to the depletion of the hydrophilic character of the catalysts [17]. The higher hydrophobic surface of the F10MH catalysts not only improves the adequate adsorption of both reactants but also favours the desorption of the water formed in the reaction. Additionally, in both cases, samples sulfonated with chlorosulfonic acid

1281 present higher activities despite they have a lower amount of sulphur (Table 2), which means that the turnover of these catalysts is slightly higher. The selectivity to monolaurine is similar with these catalysts (-72-74 wt. % for a conversion of acid of 50 tool %) because their pore sizes are large enough to allow the diffusion of the products [ 17]. The catalytic activity and selectivity obtained with the different samples sulfonated with chlorosulfonic acid in the esterification of glycerol with oleic acid is presented in Figure 6. It can be observed that catalysts F10H-C and F20H-C presents similar activity and selectivity to monoolein, in spite of the number of acid centers is much higher in the latter case (Table 2). The low activity of the F20H-C sample can be attributed to its low average pore diameter, <10 A, which difficulties the diffusion of the bulky molecules of the acid to the centres situated into the channels. The F10MH-C presents a higher activity and selectivity to monoolein, which can be attributed, as it has been demonstrated previously, to the effect of the hydrophobic methyl groups present in the surface of the catalyst [ 17].

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symbols) in the esterification of glycerol with oleic acid at 120~ with the samples: F10HC (circles), F20H-C (triangles) and F10MH-C (squares).

1282 4. CONCLUSIONS MCM-41 materials bearing methyl and phenyl groups have been prepared by one-step hydrothermal synthesis. The amount of phenyl groups incorporated has a substantial influence on the physical-chemical properties of the final material. The incorporation of the phenyl and methyl groups to the MCM-41 structure has been demonstrated by 298i MAS-NMR. The sulfonation of the phenyl groups, by using clorosulfonic acid in liquid phase or with SO3 gas, lead to SO3-Ph-(Me)-MCM-41 catalysts which are active and selective in the esterification of glycerol with lauric and oleic acids. Acknowledgements The authors acknowledge the CICYT (Spain) for financial support within the Project MAT2000-1167-C02-0. The help of Dr. T. Blasco and C. Mfirquez-Alvarez in collecting and analysing the 298i MAS NMR is greatly appreciated. 5. REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Corma, Chem. Rev., 97 (1997) 2373 4. J.B. Lauridsen, J. Am. Oil Chem. Soc., 53 (1976) 400. 5. E. Jungermann, Cosmet. Sci. Technol. Serv., 11 (1991) 97. 6. A. Corma, S. Iborra, S. Miquel and J. Primo, J. Catal, 173 (1998) 315. 7. N.O.V. Sonntag, J. Am. Oil Chem.Soc. 59 (1982) 795A. 8. G. Devinat and J.L. Coustille, Rev. Fran. Corps Gras, 30 (1983) 463. 9. Corma, H. Garcia, S. Iborra and J. Primo, J. Catal, 120 (1989) 78. 10. E. Heykants, W.H. Verrelst, R.F. Parton and P.A. Jacobs, Stud. Surf. Sci. Catal., 105 (1996) 1277. 11. M. da S. Machado, D. Cardoso. E. Sastre and J. P6rez-Pariente, Appl. Catal. A General, 203 (2000) 321 12. S. Abro, Y. Pouilloux and J. Barrault, Stud. Surf. Sci. Catal., 108 (1997) 539. 13. L. Mercier and T.J. Pinnavaia, Adv. Mater., 9 (1997) 500. 14. W.M. Van Rhijn, D.E. De Vos, B.F. Sels, W.D. Bossaert and P.A. Jacobs, Chem. Commun., 317 (1998). 15. M.H. Lira and C.F. Blanford, A. Stein, Chem. Mater., 10 (1998) 467. 16. W.M. Van Rhijn, D. De Vos, W. Bossaert, J. Bullen, B. Wouters, P.J. Grobet and P.A. Jacobs, Stud. Surf. Sci. Catal., 117 (1998) 183. 17. I. Diaz, C. Mfirquez, F. Mohino, J. P6rez-Pariente and E. Sastre, J. Catal., 193 (2000) 295. 18. I. Diaz, C. Mfirquez, F. Mohino, J. P6rez-Pariente and E. Sastre, J. Catal., 193 (2000) 283. 19. I. Diaz, F. Mohino, J. P6rez-Pariente and E. Sastre, Appl. Catal., 205 (2001) 19. 20. A.J. Aznar and E. Ruiz-Hitzky, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt., 161 (1988) 459. 21. C.W. Jones, K. Tsuji and M.E. Davis, 33 (1999) 223. 22. I. Diaz, C. Mfirquez-Alvfirez, F. Mohino, J. P6rez-Pariente and E. Sastre, Microp. Mesop. Mat., 44-45 (2001) 295.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1283

Synthesis of ordered mesoporous and microporous aluminas: strategies for tailoring texture and aluminum coordination V. Gonzfilez-Pefia, C. Mfirquez-Alvarez, E. Sastre and J. P6rez-Pariente* Instituto de Catfilisis y Petroleoquimica, CSIC. Campus Cantoblanco. 28049-Madrid (Spain)

Organised mesoporous and microporous aluminas have been synthesised by a sol-gel route in non-aqueous media using polyethylene oxide surfactants as directing agents. The use of 1,4-dioxane as solvent enables the control of pore size in mesoporous aluminas. Aluminum coordination is modified by addition of ammonium fluoride, ethyl acetoacetate or triethanolamine to the synthesis gel. The chelating ligands modify the alumina porous structure and generate microporosity.

1. I N T R O D U C T I O N High surface area aluminas are commonly used in industry as catalysts, catalyst supports and adsorbents [1]. The production of aluminas with a narrow pore size distribution has a great industrial interest, as the performance of classical aluminas is limited by their uncontrolled porosity. Besides pore structure, aluminum coordination, which determines surface acidity, and thermal stability strongly influence the performance of alumina-based catalysts. The successful synthesis of silicas and aluminosilicates of the M41S family has derived a number of studies on the synthesis of mesostructured aluminas, in aqueous [2-9] and nonaqueous media [i 0-13]. Among the synthesis procedures leading to thermally stable aluminas, the hydrolysis of an aluminum alkoxide in an organic solvent, in the presence of non-ionic polyalkylene oxide surfactants [11 ], led to the highest surface areas reported for aluminas calcined at temperatures above 500~ The addition of small amounts of Ce 3+ or La 3§ has shown to improve the thermal stability of these mesostructured aluminas [13]. We have also proved that the addition of amines results in an increased thermal stability of aluminas synthesised by hydrolysis of aluminum sec-butoxide in 2-butanoI, in the presence of a polyethylene oxide surfactant [ 14,15]. We report here several strategies, based on this synthesis approach, aiming at tailoring both aluminum coordination and pore structure of the alumina. We have explored the effect that the type of solvent, ageing temperature and the presence of modifiers in the synthesis gel have on these parameters. Among the modifiers tested, chelating ligands like ethyl acetoacetate have been introduced in order to control the hydrolysis and condensation rates of the aluminum alkoxide precursor. Author for correspondence. Phone: +34 915 854 784. Fax: +34 915 854 760. E-mail:[email protected], http://www.icp.csic.es/gtm

1284 2. EXPERIMENTAL

The aluminas were synthesised from gels with the following molar composition: 1.0 A1 tri-sec-butoxide :0.1 Surfactant : 2.0 H20 : X Modifier: Y Solvent Aluminum tri-sec-butoxide, 97% (Acros), was used as the aluminum source. The surfactants used were polyoxyethylene isooctylphenyl ether, Triton X-114, with 8 ethylene oxide (EO) units, and two polyoxyethylene alkyl(Cll_lS) ethers, Tergitol 15-S-9 and Tergitol 15-S-15, with 9 and 15 EO units, respectively, provided by Sigma. The modifiers were ammonium fluoride (X=0.25), 98+% (Aldrich), triethanolamine (X=0.2), 98% (Aldrich) and ethyl acetoacetate (X-0.5), 99% (Aldrich). The solvents used were 2-butanol (Y-18.20), 99.5% (Sigma) and 1,4-dioxane (Y-19.46), stabilised with ca. 25 ppm o f B H T purissimum. The synthesis procedure was as follows: the surfactant, the aluminum tri-sec-butoxide and triethanolamine or ethyl acetoacetate (when used) were mixed with part of the solvent (solvent to aluminum alkoxide mole ratio of 13.0 for 2-butanol and 13.9 for 1,4-dioxane) and the solution stirred for 30 min. at room temperature. A solution containing the water, the rest of the solvent and the ammonium fluoride (when used) was then added slowly, and the stirring maintained for 3 more hours. The obtained sols were transferred to polypropylene bottles, aged at 25, 55 or 95~ for 24 h. The product was recovered by filtration, washed with ethanol and dried at room temperature. This product was introduced in a Soxhlet apparatus for removal of surfactant with ethanol for 15 h. The solids were finally dried at room temperature, heated in nitrogen at 550~ for 1 h (heating rate of 2~ and then calcined in air at 550~ A portion of the sample was further calcined in air at 600~ for 3 h. Nitrogen adsorption/desorption isotherms were obtained in an ASAP 2000 Micromeritics apparatus. The samples were evacuated at 350~ for 24 h before analysis. X-ray powder diffraction patterns were collected using CuKc~ radiation, on a Seiffert XRD 3000P diffractometer with a curved graphite secondary monochromator, operating at low angle (1-10 ~ using a primary automatic divergence slit and a 0.2 mm detector slit. The diffractograms of all the calcined samples showed a single reflection in this region. Transmission electron microscopy images were recorded in a JEOL JEM 2000 FX microscope operating at 200 kV. For the TEM experiments the samples were dispersed in acetone and dropped on a holey carbon copper microgfid. -1 Infrared spectra in the diffuse reflectance mode were recorded in the range 400-4000 cm , at 4 cm -1 resolution, using a Nicolet 5ZDX FTIR spectrometer. Solid state 27A1 MAS NMR spectra were recorded on a Varian Unity VXR-400 WB spectrometer at 104.2 MHz, a rr/18 rad pulse length, a recycle delay of 0.5 s and spinning rate of 7 kHz.

3. RESULTS AND DISCUSSION 3.1. Mesoporous aluminas" pore size Table 1 summarises the pore structure data of mesoporous aluminas prepared from sols obtained using 2-butanol as solvent and aged at 25 and 95~ As shown in Table 1, the type and head volume of the non-ionic surfactant used has little effect on both pore diameter and volume. Nearly the same pore size is obtained for all samples, independent of the surfactant and ageing temperature chosen. Samples calcined at 550~ show a maximum around 3.8-4.0 nm in the BJH pore size distribution calculated from the desorption branch of the nitrogen

1285 Table 1 Pore structure of mesoporous aluminas prepared from sols obtained in 2-butanol Calcined at 550~ Ageing temperature /~

Surfactant (number of EO units)

25 25 25 95 95 95

X-114 15-S-9 15-S-15 X-114 15-S-9 15-S-15

(8) (9) (15) (8) (9) (15)

BET surface area /m2.g-1 555 485 520 510 515 540

Pore diameter /nm

4.0 3.8 3.9 3.8 3.9 3.8

Calcined at 600~ Pore volume /cm3-g-1

0.71 0.56 0.62 0.71 0.64 0.64

BET surface area /m2.g-1 345 335 380 350 380 330

Pore diameter /nm

5.7 5.8 5.2 5.9 5.2 6.0

Pore volume /cm3.g-l

0.68 0.64 0.65 0.76 0.68 0.67

adsorption-desorption isotherm. After calcination at 600~ the pore size increases, but yet little differences are observed among the six samples. No correlation is therefore observed between pore diameter and surfactant head volume when the synthesis is carried out in 2-butanol. One possible explanation for this result is that the surfactant molecules do not form micelles in 2-butanol under the conditions used in these experiments. In fact, Ray results [16] suggest that polyethylene oxide surfactants would not form micelles when the solvent is an alcohol. Therefore, aiming to control the alumina pore size by means of the adequate selection of surfactant size, we have carried out the synthesis in 1,4-dioxane, as polyethylene oxide surfactants are expected to form micelles in this solvent. The N2 adsorption-desorption isotherms (Figure 1) and TEM images (Figure 2) of the samples synthesised in 1,4-dioxane and calcined at 550~ show that the porous structure of these samples is similar to that of the aluminas synthesised in 2-butanol [ 14]. The mesoporous aluminas prepared using either solvent exhibit an intricate crosslinking of corrugated oxidehydroxide platelets, giving rise to pores with a relatively narrow pore size distribution. The aluminas synthesised in 1,4-dioxane possess high BET surface area (Table 2), comparable to that of samples prepared from sols obtained in 2-butanol. Surface areas around 500 m2"g-1 are obtained for samples calcined at 550~ and 350 m2-g-1 and higher for samples calcined at 600~ At the ageing and calcination temperatures tested, an increase in pore size and volume is observed for the samples synthesised in 1,4-dioxane as the surfactant head volume increases (Table 2), in contrast to samples synthesised in 2-butanol. This result would support the proposed formation of micelles in 1,4-dioxane. This variation of pore size is more pronounced for the aluminas prepared from sols obtained at lower ageing temperature. 3.2. Mesoporous aluminas: aluminum coordination Figure 3 shows the 27A1 MAS NMR spectrum of a sample synthesised in 1,4-dioxane, using Tergitol 15-S-15, aged at 95~ and calcined at 550~ (spectrum a). As for samples

1286

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8

10

Figure 1. Nitrogen adsorption-desorption isotherms at 77 K and BJH pore size distributions (desorption branch) of aluminas synthesised in 1,4-dioxane with surfactants Triton X-114 (a), Tergitol 15-S-9 (b) and Tergitol 15-S-15 (c), and aged at 25~ Samples calcined at 550~ prepared from sols obtained in 2-butanol [ 14], the most intense resonance is observed at ca. 5 ppm, which corresponds to hexacoordinated aluminum species (AlW). A second, less intense resonance is observed at ca. 70 ppm, attributed to tetracoordinated aluminum (AlW). Finally, the weak resonance at ca. 35 ppm indicates the presence of a small fraction of aluminum atoms in 5-fold coordination. In order to obtain more acidic aluminas, it would be necessary to increase the number of coordinatively unsaturated A1. For this reason, we have tested the effect of fluoride ions added to the synthesis gel as ammonium fluoride. Figure 3 shows the 27A1 MAS NMR spectrum of one selected sample synthesised in 2-butanol in the presence of ammonimn fluoride (spectrum d) and that of a sample prepared using the same synthesis conditions but without modifier (spectrum c), both calcined at 550~ It can be observed that the use of ammonium fluoride produces a significant increase in the intensity of the resonances assigned to tetra- and

50nm

30nm

Figure 2. TEM images of alumina samples synthesised in 1,4-dioxane, in the presence of Tergitol 15-S-15, using no modifiers (left) and using ethyl acetoacetate as modifier (right). The gels were aged at 95~ and the obtained sols, calcined at 550~

1287 Table 2 Pore structure of mesoporous aluminas prepared from sols obtained in 1,4-dioxane Calcined at 6000C

Calcined at 550~ Ageing temperature /oc

Surfactant (number of EO units)

25 25 25 95 95 95

X-114 15-S-9 15-S-15 X-114 15-S-9 15-S-15

BET surface area /m2.g-1

(8) (9) (15) (8) (9) (15)

Pore diameter /nm

550 500 445 490 525 510

3.5 4.8 5.2 3.5 4.0 4.5

Pore volume /cm3"g-1

0.44 0.75 0.78 0.51 0.71 0.71

BET surface area /m2.g-1

Pore diameter /nm

4.1 6.5 7.1 4.2 5.8 6.0

370 350 355 350 355 375

Pore volume /cm3-g-1

0.50 0.76 0.85 0.60 0.72 0.74

pentacoordinated aluminum, particularly the later one. The N2 adsorption-desorption isotherms of calcined aluminas synthesised in the presence of ammonium fluoride (Figure 4,a) are characteristic of mesoporous solids. These isotherms are similar to those of samples prepared using identical synthesis conditions but without ammonium fluoride. However, the hysteresis loop is shifted to higher relative pressure. The nitrogen adsorption data reported in Tables 1 and 3 indicate that the addition of ammonium fluoride to the synthesis gel leads to aluminas with higher pore size and lower BET surface area. The TEM images of aluminas synthesised in the presence of fluoride ions (not shown for brevity) reveal that this porosity is textural, as in the case of aluminas synthesised with no modifiers.

d

r/3 ~D

,

|

200

100

()

-100

Chemical shift (ppm)

-200

200

100

0

-100

-200

Chemical shift (ppm)

Figure 3.27A1 MAS NMR spectra of aluminas synthesised in 1,4-dioxane using Tergitol 15-S9 and aged at 95~ (left), and in 2-butanol using Triton X-114 and aged at 25~ (right). The modifiers added to the synthesis gel were: none (a,c), ethyl acetoacetate (b) or ammonium fluoride (d). All the samples were calcined at 550~

1288 400

b 100

300 [--, c,r

>

o,.......................................................... C

9

75

200 0

,,'~

~0 50

100

;> 25 f

0.0

012

0'.4

016

0'.8

1.0

0

0.0

012

P/P0

0'.4

016

0'.8

" ....

1.0

P/P0

Figure 4. Nitrogen adsorption-desorption isotherms of aluminas synthesised using modifiers: ammonium fluoride (a), ethyl acetoacetate (b,c) and triethanolamine (d). The surfactant used was Triton X-114 and the solvents, 2-butanol (a,b,d) and 1,4-dioxane (c). The sols were aged at 95~ (except sample a, aged at 25~ and calcined at 550~

3.3. Microporous aluminas A second strategy that we have explored is the chemical modification of the aluminum alkoxide precursor with chelating ligands, in order to retard the hydrolysis and condensation reaction rates [17]. As a result of such a modification, the number of coordinatively unsaturated aluminum atoms, and therefore the acidity of the alumina, is increased. For this purpose, we have carried out the hydrolysis of aluminum sec-butoxide modified with ethyl acetoacetate and triethanolamine. Figure 3 shows the 27A1 NMR spectrum of an alumina sample synthesised using ethyl acetoacetate as modifier and calcined at 550~ (spectrum b). A strong increase is observed in the intensity of the resonances assigned to tetra- and pentacoordinated aluminum as compared

Table 3 Pore structure of mesoporous aluminas prepared from sols obtained in 2-butanol, using ammonium fluoride as modifier. Calcined at 600~

Calcined at 550~ Ageing temperature /oC

Surfactant (number of EO units)

BET surface area

25 25 55

X-114 (8) 15-S-9 (9) 15-S-9 (9)

260 325 335

/m2.g -I

Pore diameter /nm

6.8 5.8 5.7

Pore volume Icm3.g -I

0.56 0.62 0.67

BET surface area

/m2.g -I

115 180 180

Pore diameter /nm

12.0 8.6 11.0

Pore volume /cm3"g-I

0.50 0.54 0.66

1289 Table 4 Pore structure of microporous aluminas prepared from aluminum sec-butoxide modified with chelating ligands, and calcined at 550~ Solvent

Modifier

2-butanol 2-butanol 2-butanol 2-butanol 1,4-dioxane 1,4-dioxane 1,4-dioxane

triethanolamine ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate

Ageing Surfactant temperat (number of ure EO units) /oC 25 95 95 95 95 95 95

X-114 (8) X-114 (8) 15-S-9 (9) 15-S- 15 (15) X-114 (8) 15-S-9 (9) 15-S- 15 (15)

BET surface area /m2.g -1 290 325 305 310 290 360 400

Micropore Total pore volume volume /cm3.g -1 /cm3"g-I

0.10 0.09 0.10 0.11 0.09 0.12 0.12

0.13 0.15 0.14 0.14 0.13 0.16 0.19

to a sample obtained from the non-modified alkoxide precursor (spectrum a). The actual modification of the aluminum alkoxide by ethyl acetoacetate and the preservation of the bidentate complex during hydrolysis and condensation of the chelated aluminate precursor were confirmed by infrared spectroscopy. The spectrum of the gel showed the characteristic carbonyl stretching bands at ca. 1636 and 1613 cm -I indicative of the bidentate complex [18], which is preserved until calcination at temperatures above 300~ The TEM image of a sample synthesised from ethyl acetoacetate-modified aluminum secbutoxide and calcined at 550~ (Figure 2, right) shows the presence of irregularly shaped pore channels, in marked contrast to samples synthesised with the non-modified alkoxlde. Therefore, besides the effect on aluminum coordination, the chemical modification of the precursor with ethyl acetoacetate also produces a dramatic change in the porous structure of the a!umina. Thic suggests teat the stabi!ity of the chelate with regard to hydrolysis and condensation reactions prevents the formation of the octahedral hydroxyaquo AI(III) complexes that account for the tendency to form the pseudoboehmite platelets in the absence of modifiers. The nitrogen adsorption-desorption isotherms of aluminas synthesised with ethyl acetoacetate- or triethanolamine-modified aluminum sec-butoxide and calcined at 550~ (Figure 4, right) show that these samples are microporous. The BET surface area, the micropore volume and the total pore volume of aluminas synthesised using different solvents and surfactants are reported in Table 4. The micropore volume has been calculated as the intercept of the straight line that fits the low relative pressure region of the adsorption isotherm in a Dubinin-Radushkevitch plot [19], and the density of tile adsorbed nitrogen has been taken as 0.81 cm3.g-~. As shown in Table 4, after calcination at 550~ aluminas with BET surface area in the range 300 to 400 m2.g -1, and micropore volume around 0.1 cm3.g -I are obtained. The effect of surfactant on porosity is much more pronounced for samples synthesised in 1,4-dioxane respect to those obtained in 2-butanol.

1290 4. CONCLUSION In the absence of modifiers, organised mesoporous aluminas are obtained by hydrolysis of aluminum tri-sec-butoxide, in the presence of polyethylene oxide surfactants, either in 2butanol or 1,4-dioxane solution. In 1,4-dioxane, the alumina pore size can be tuned by modifying the head volume of the surfactant, in contrast to 2-butanol. It is proposed that this effect be related to the surfactant aggregation into micelles in the former solvent. The addition of fluoride does not modify the mesoporous structure of the alumina but causes a decrease in surface area and an increase in the amount of tetra- and pentacoordinated aluminum. Microporous aluminas with high concentration of tetra- and pentacoordinated aluminum are obtained when the aluminum alkoxide precursor is modified by chelating agents such as ethyl acetoacetate or triethanolamine.

Acknowledgements The authors thank the CICYT (Spain) project MAT2000-1167-C02-02 for financial support, Dr. I. Diaz for collecting the TEM images, and Dr. T. Blasco for collecting the NMR spectra. V.G.P. acknowledges the Conacyt (M6xico) for a Ph.D. grant.

REFERENCES 1. C. Misra, Industrial Alumina Chemicals, ACS monograph 184, ACS, Washington, 1986 2. Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E, Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schfith and G.L. Stucky, Chem. Mater. 6 (1994) 1176 3. M. Yada, M. Machida and T. Kijima, Chem. Commun. (1996) 769 4. M. Yada, H. Hiyoshi, K. Ohe, M. Machida and T. Kijima, Inorg. Chem. 36 (1997) 5565 5. M. Yada, H. Kitamura, M. Machida and T. Kijima, Langmuir 13 (1997) 5252 6. A. Stein and B. Holland, J. Porous Mater. 3 (1996) 83 7. N.R.E. Radwan, A.A. Atia and A.M. Youssef, Ads. Sci. Technol. 17 (1999) 17 8. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltrfin, D. Beltrfin, S. Mendioroz, M.D. Marcos and P. Amords, Adv. Mater. 11 (1999) 379 9. S. Valange, J.-L. Guth, F. Kolenda, S. Lacombe and Z. Gabelica, Micropor. Mesopor. Mater. 35-36 (2000) 597 10. F. Vaudry, S. Khodabandeh and M.E. Davis, Chem. Mater. 8 (1996) 1451 11. S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 35 (1996) 1102 12. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature 396 (1998) 152 13. W. Zhang and T.J. Pinnavaia, Chem. Commun. (1998) 1185 14. V. Gonz/dez-Pefia, I. Diaz, C. Mfirquez-Alvarez, E. Sastre and J. P6rez-Pariente, Micropor. Mesopor. Mater. 44-45 (2001) 203 15. V. Gonzfilez-Pefia, C. Mfirquez-Alvarez, E. Sastre and J. P6rez-Pariente, Stud. Surf. Sci. Catal. 135 (2001) 1072 16. A. Ray, Nature, 231 (1971) 313 17. C.J. Brinker, G.W. Scherer, Sol-gel science, Academic Press, San Diego, 1990 18. F. Babonneau, L. Coury and J. Livage, J. Non-Cryst. Sol., 121 (1990) 153 19. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1291

Characterization of a heteropolyacid supported on mesoporous silica and its application in the aromatization of ~-pinene H. Jaramillo a, L. A. Palacio b and L. Sierra b aDepartment of Chemistry. bGroup of Material Science University of Antioquia, A. A. 1226 - Medellin, Colombia

H3PW12040.xH20 was loaded (33 wt%) by impregnation into a calcined mesoporous silica. The Keggin structure was characterized by XRD, TGA, N2 adsorption and 31p MAS NMR. The bi-functionality (acid/redox) of this catalyst was then studied using the transformation of tx-pinene between 40 and 160 ~ in a batch reactor. The substrate is transformed into camphene and terpinene by isomerization on the acid sites. The latter is transformed in a second step at higher temperature into p-cymene by a dehydrogenation on the acid and redox sites. 3-p-menthene and carvomenthene are produced, in low yield, together with p-cymene, by a disproportionation reaction of the terpinenes. 1. INTRODUCTION Catalytic properties of the heteropolyacids (HPAs) have attracted the attention in the two precedent decades due to the versatility of these compounds, since they possess acid and redox properties [1,2,3]. Mineral acids as catalysts cause a reduced life of the reactors and equipment used in many important processes. The HPAs could replace them giving good catalytic efficiency with less difficulties in their handling. To carry out heterogeneous catalysis in liquid phase, HPAs are usually supported on a carrier such as activated carbon, titania, alumina or silica [4,5]. The present work studies the characterization of the 12-Tungnstophosphoric acid: H3PW12040 (HPW) supported on a mesoporous silica and its use in the heterogeneous catalytic conversion of tx-pinene. A reasonable amount of research work has been done on apinene reactions for their applications in fine chemistry [5,6,7,8]. Among such reactions, the isomerization of ct-pinene by heterogeneous acid catalysis produces mono-, bi- and tricyclic terpenes. With catalysts offering also redox sites, the isomerization of tx-pinene can be followed by dehydrogenation to give aromatic compounds such as cymenes. 2. EXPERIMENTAL SECTION The Keggin form of HPW was prepared following the method of Grutther [9] and Misono [10], with a modification of the extraction and purification. 75 g of Na2WO4.2H20 and 37.5 g of Na2HPO4.12H20 were dissolved in 100 ml of hot water; then 90 ml of concentrated HC1 were added. The extraction was made with 120 ml of ether. In order to favor the formation of the Keggin anion, the part with the ether was subjected to acid hydrolysis by means of the addition of concentrated HC1 (20 ml). 40 ml of H20 and more ether were added to the acidic ether solution to form three layers from which the most dense layer containing the HPA species in ether was evaporated by air bubbling and the recovered solid was dried at 40~

1292 The mesoporous silica was prepared by using a sodium silicate solution and the neutral surfactant Triton X-100 as template, with a method developed in our laboratory in which the mechanism S~ ~ is partly replaced by the mechanism S~ [ 11 ]. After calcination to remove the surfactant, the mesoporous silica was impregnated with an aqueous solution of HPW in order to obtain 33% of load. Before using, the catalyst was activated at 130~ under vacuum. The as-synthesized mesoporous silica, and the supported HPW were characterized by XRD with a Phillips PW 1130 equipment, by nitrogen adsorption in a Micromeritics ASAP2010, by TGA in a TA Instruments Hi-Res TGA 2950, by FTIR in a Mattson 5000 FT-IR Spectrometer and by 31p MAS NMR in a Bruker MSL 300 instrument. A preliminary evaluation of the acidity of the supported HPW was carried out based upon the catalytic dehydration of 2-propanol in a continuous reactor at 100~ with a space velocity of 248 cm3/g.min (catalyst weight of 125 mg). For the catalytic transformation of ot-pinene a static batch reactor was used, in which the temperature was set between 40 and 160~ The reaction was followed by gas chromatography (Perkin Elmer Gas Chromatograph equipped with a capillar Carbowax 20 M column and Q-Mass 910 Mass Spectrometer).

3. RESULTS AND DISCUSSION 3. 1. Characterization 3.1.1. Characterization of the mesoporous silica The XRD pattem for this material shows a peak with a d spacing value o f - 3 . 8 nm corresponding to the d~00 reflexion for the mesoporous material. The N2 adsorption results afford a BET area of 783 m2/g, a pore volume of 0.76 cm3/g and an average pore diameter of 2.0 nm (BJH method), which are characteristic of mesoporous phases.

3.1.2. Characterization of HPW: as-synthesized and supported on the mesoporous silica. XRD analysis The XRD pattem for the as-synthesized HPW compound shown in figure 1, corresponds to that assigned to the Keggin species H3PW12Oa0.xH20 by Chen et al [12]. Since the crystallographic array varies with the number of hydration water molecules, it is possible from the information of the unit cell and space group to determine this number. By using the software Checkcell [ 13], it was determined that the HPW XRD pattem corresponds to a cubic unit cell with ao = 1.2113(3) nm and to the space group Pn3m, from which it can be concluded that HPW crystallizes with 6 water molecules [ 14]. r

0.7

~-

0.3

t

~=~

~"

"~"

0

;

I

(.q

~

1 l

r,~

I

i

o

oo

o, 0.0

10.0

20.0

30.0

40.0

2O

Figure 1. XRD pattern for the as-synthesized HPW (H3PWlzO4o.6H20)

1293 100

0.05

100

0.05

-TGA

. . . . DTGA 0.04

;r

0.03 ~

96.

o o~

94,

92

0.01

o

1~o

'

~;o Temperature,

~;o *C

,;o

'

0.00 500

0.04

98

0.03 .~

:~ 96

~ ~

0.02 94

92

0.01

0

.

. 100

. 200

.

300

Temperature,

*C

400

,

500

,

0.00 600

Figure 2. TGA thermographs. (a) as-synthesized HPW, (b) supported HPW In order to check if the crystalline structure of HPW still remains in the supported HPW, a XRD pattern was performed for HPW supported on the mesoporous silica (33 wt %). The lines of the crystalline HPW are not seen as it is the case when the HPW load on mesoporous silicas is lower than 50 wt % [15,16]. The strong peak observed at a d spacing value of 3.8 nm, corresponds to the dl00 reflexion of the mesoporous silica.

TGA analysis The TGA thermogram for the as-synthesized HPW (figure 2a) shows a first weight loss of 3.4 % until 140~ corresponding to non-coordinated water and a second weight loss of 3.1% between 140 and 320~ due to hydration water, which corresponds to 5.3 water molecules for each Keggin anion. This value is close to the one found by XRD. There is a third weight loss of 0.5% between 320 and 500 ~ which is produced by the condensation of hydroxyl groups formed by the bonding of terminal oxygen of the Keggin anion with the acidic protons. The TGA thermogram for the supported HPW (figure 2b) shows an important and steep weight loss between 25 and 140 ~ followed by a gradual loss between 140 and 500~ This means that the loss of hydration water in the supported HPW occurs at lower rate than for the as-synthesized HPW. This indicates that the removal of this kind of water is more difficult probably due to the inclusion of the hydrated HPW inside of the mesopores. This effect could stabilize the HPW on the support.

31p MAS NMR analysis

The 31p MAS NMR spectra for the as-synthesized HPW and for the supported HPW are shown in figure 3a and 3b respectively. The as-synthesized HPW exhibits a signal at -15.89 ppm, which can be assigned to the Keggin species with 6 hydration water molecules [8]. The signal at-13.47 ppm can correspond to lacunary species formed during the preparation of the HPW [11]. The resonance signals between-0.05 y - 5 . 1 6 ppm correspond to HxPO4(3"x)" species, where 0 < x <3 [ 17]. A simple inspection of the intensities of the signals at -0.05 ppm and-15.89 ppm due to phosphates and to the HPW species respectively, shows that both have approximately the same intensity. Since the phosphates have a phosphorous composition from 21.8% for

1294

Co)

1'6....... 6...... -ifi ...... _sfi...... -~'o 1'6........6....... -'i~ ...... _'5_'6...... -'~'o ppm

ppm

Figure 3. 3Sp MAS NMR spectra. (a) as-synthesized HPW (b) supported HPW Na2HPO4 to 31.6% for H3PO4 (in average 26.7%), and the hexahydrated HPW contains only about 1.1% of phosphorous, it can be figured out that the as-synthesized HPW contains about 4 wt % of phosphate impurities. The supported HPW silica exhibits a signal at -15.31 ppm, which corresponds to the Keggin structure of HPW, without a significant shift with respect to the as-synthesized HPW. This indicates that there is not appreciable interaction of the HPW species with the support. The broad resonance signals at 0.18 and -12.90 ppm appear with low intensity, suggesting that the corresponding species can be dispersed in different chemical environments on the surface of the support.

FTIR analysis The IR spectrum for the as-synthesized HPW (figure 4a) corresponds to the one for the primary Keggin "cx type" structure [1,18], which is therefore present in the secondary structure corroborated by XRD. The bands at 1620 cm -1 and 1710 cm -1 are assigned respectively to bending vibrations for H20 and H30 + bound to the Keggin species in the secondary structure [ 12]. The infrared spectrum for the supported HPW (figure 4b) shows a broad band at 1088 cm l, which is due to the Si---O bond stretching in the silica, that overlaps with the band at 1080 cm -1 (Vas P---O) of the HPW. The absorption bands at 982 cm 1 (Va~ W=O), 895 cm 1 and 808 cm 1 (Va~ W--O---W) correspond to characteristic absorption bands of the Keggin anion, indicating that the support does not affect the HPW structure. The small shift - 6 cm -~ for the V~s of the W - - O - - W group presented in the supported HPW with respect to the assynthesized HPW can be attributed to a weak interaction of the heteropoly anion with the internal surface of the mesopores.

1295

1630

=:

.=43(a)

1079.9

fo)

705.8

23983. 80.

2000

1500

i000

W a v e n u m b e r , cm -1

500

2000

1500

I000

W a v e n u m b e r , c l n -1

Figure 4. Infrared spectra. (a) as-synthesized HPW (b) supported HPW 3. 2. Reactions catalyzed by mesoporous silica - supported HPW

3.2.1. Dehydration of 2-propanol The dehydration of 2-propanol was chosen to evaluate the acidic properties of the catalyst. Propene was identified as the main reaction product and isopropyl ether as a secondary one. This means that a unimolecular mechanism, producing propene is favored compared to the bimolecular mechanism, which produces isopropyl ether. For the experimental conditions already described above, there was 35.7% conversion of 2-propanol with selectivity of 72% and 28% towards propene and ether respectively. These values are obtained in a relative short time and at low temperature. This means a good activity of this material as an acid catalyst, which could reach conversions comparable to an A12Oa catalyst [19] if the temperature of reaction is increased.

3.2.2. Reaction of a-pinene An experimental design was made with three reaction temperatures (40, 100 and 160 ~ five cx-pinene/catalyst mass ratios (1.7, 3.3, 7, 10.7 and 16.7) and four reaction times (1, 3.5, 6 and 15 minutes). Additional experiments were run without catalyst but in presence of the mesoporous silica, at temperatures of 160 and 40~ with times of 6 and 1 hour. In these cases were no appreciable changes, which indicate that the transformation of cx-pinene is due mainly to the presence of the catalyst. It could be observed through the different experiments that the conversion degree of cxpinene is a linear function of the reaction time and the cx-pinene/eatalyst mass ratio. But these two parameters do not change the selectivity towards the different products. For this reason the reaction was studied taking into account only the variation with temperature.

1296 100- I *~

80-

"~ 60 tlf ,~

4020-

40

100

160

Temperature,~

Figure 5. Variation of the conversion of a-pinene with temperature. Reaction time: 1 hour, ~-pinene/catalyst mass ratio" 7.0 Figure 5 shows the conversion of a-pinene as a function of reaction temperature for a c~-pinene/catalyst mass ratio of 7.0 and a reaction time of 1 hour. The conversion of ot-pinene increases with temperature. At 100 ~ the conversion is almost complete. The evolution of the products of the transformation of et-pinene with temperature can be observed in figure 6. The appearance and disappearance of several compounds can be explained from the results of studies realized for a-pinene transformations using other catalysts [20,7,8]. It is possible to interpret the results as follows: 70 60-

~ 50 ~ 4o 2,5

E

~

10 20

7,5

10 ~ 0

2,5 0

40

60

80

100

Temperature, ~

120

140

160

40

100

160

Temperature, ~

Figure 6. (a) Evolution with temperature of the reaction products of of the ~-pinene transformation. (b) Enlarged area. Reaction time: 1 h, ~-pinene/catalyst mass ratio = 7.0. 9 camphene, 9 tricyclene, [] limonene, 9 terpinolene, x (~+7) terpinene, [] p-cimene, A 3-pmenthene, o carvomenthene.

1297 Camphene and tricyclene are present at low and intermediate temperature but at high temperature they disappear. This shows that they correspond to primary isomerization products, which transform to other type of compounds as the temperature increases (figure 7). Dipentene (limonene) and terpinolene are also primary isomerization products, which disappear more rapidly as the temperature increases. They are transformed into (ct+T) terpinenes by isomerization of the double bonds. At the same time the concentrations of (ix+7) terpinenes increase with temperature until an important production of p-cymenes starts. This suggests that the terpinenes are secondary isomerization products that undergo dehydrogenation, favored at high temperature, to form mainly the aromatic ring of the pcimene, according to reported results for the pinene transformation on a chromia-alumina catalyst [7]. A less important competitive transformation of terpinenes into 3-p-menthene, carvomenthene (1-p-menthene) and p-cymene occurs by a hydrogen transfer mechanism involving disproportionation. These are considered as tertiary reaction products. The scheme in figure 7 showing the transformations of ct-pinene, includes the isomerization to 13-pinene. This isomer is observed in small concentrations since it is less stable thermally and more reactive than the tx isomer.

<

>

a-pinene

1

13-pinene

camphene

M.

Y

tricyclen.~ Y

I,

+

>

+

dipentene

<

2

terpinolene

~.~-terpinene

T-terpinen~, I

+

+

carvomenthene

3-p-menthene

p-cymene

p-cymene

Figure 7. Scheme for the transformation of ct-pinene catalyzed by supported HPW. 1: Primary isomerization 2:Secondary isomerization 3:Dehydrogenation 4" Disproportionation.

1298 At 160~ high conversions to cymene (more than 60%) were obtained, in 1 hour of reaction, with a ct-pinene/catalyst mass ratio of 3.3. With longer reaction times, higher conversions can be obtained even with lower relative amounts of the catalyst. For instance a conversion of 76% was obtained for a mass ratio a-pinene/catalyst of 10.7 in 6 hours of reaction. 4. CONCLUSIONS The qualitative and quantitative analyses of the reaction products of ~-pinene show that the supported HPW catalyst has bi-functional characteristics, presenting both acidic and redox sites, tx-pinene is transformed primarily into camphene and t~-terpinene, by means of an isomerization catalyzed by the acid sites, and into p-cymene by mainly a dehydrogenation where the acid sites as well the redox ones are involved. 4. ACKNOWLODGEMENTS We thank Dr. Jean Louis Guth, at Laboratoire de Matriaux Mineraux, Mulhouse, France, for the 31p MAS NMR and XRD characterization. REFERENCES

T. Okuhara, N. Mizuno and M. Misono, Advances in Catalysis, 41 (1996) 113 C. Hill, C. Prosser-Mc-Cartha, Coord. Chem. Rev., 143 (1995) 407 A. Corma, A. Martinez and C. Martinez, J. of Catalysis, 166 (1996) 422 M. Kim, W. Kim, J. Kim, Y. Sugi and G. Seo, Studies in surface science and catalysis, 135, 2001 5. J. Vital, H. Thomas, et al, Studies in surface science and catalysis, 135 (2001), 234 6. Allahverdier, S. Irandoust and D. Murzin, J. of Catalysis, 185 (1999) 352 7. A. Stanislaus and L. Yeddanapalli, Can. J. Chem., 50 (1972) 113 8. A. Stanislaus and L. Yeddanapalli, Can. J. Chem., 50 (1972) 61 9. B. Gruttner, G. Janderen and G. Brauer, Hanbook of Preparative Inorganic Chemistry, Vol. 2, McGraw Hill, New York, 1986 10. M. Misono, et al, Bulletin of the Chemical Society of Japan, 55 (1982) 400 11. L. Sierra, B. L6pez, H. Gil. and J.L.Guth, Advanced Materials, 11, 1999, 307 12. N. Chen and R. Yang, J. Catalysis, 157 (1995) 76 13. J. Laugier and B. Bochu, Laboratoire des Mat6riaux et du G6nie Physique, Ecole Nationale Sup6rieure de Physique de Grenoble, 2000 14. A. Corma, Chem. Reviews, 95 (1995) 559 15. I. Kozhevnikov, et al., J. Molecular Catalysis, 114 (1996) 287 16. I. Kozhevnikov, et al., Catalysis Letters, 30 (1995) 241 17. V. Odyakov, Kinetics and Catalysis, 36 (1995) 733 18. H. Changwen, et al., J. Catalysis, 143 (1993) 437 19. E. Doskocil, et al, J. Catalysis, 169 (1997) 327 20. V. Wystrach, L. L. Barnum and M Garber, J. Am. Chem. Soc., 79 (1957) 578

1. 2. 3. 4.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1299

Catalytic activity, deactivation and re-use of AI-MCM-41 for N-methylation of aniline J.M. Campelo, R.M. Leon, D. Luna, J.M. Marinas and A.A. Romero. Departamento de Quimica Orgfinica, Facultad de Ciencias, Universidad de C6rdoba, Campus Universitario de Rabanales, Edificio Marie Curie (C3), E-14014 C6rdoba, Spain.

A1-MCM mesoporous molecular sieves with Si/A1 ratio in the range 10-40 were characterized by XRD, N2 adsorption, MAS NMR, and DRIFT, and their acid properties were determined by pyridine (PY) adsorption. Aniline methylation was a pseudo-first-order process with respect to aniline concentration. N-methylation products were predominant with a selectivity of the 100 mol% at 573 K after two hours on stream. The aniline conversion and selectivity to NNDMA decreased whereas selectivity to NMA continuously increased with time on stream. Besides, the selectivity to NMA remained almost the same, irrespective of the aniline conversion, aluminum content, and acidity, this fact was strictly observed for A1-B-X catalysts. On the other hand, the re-used A1-MCM-41 catalysts would be described as amorphous aluminosilicates because of its structure deteriorate with reaction water. 1. INTRODUCTION Acid M41S materials have potential applications in the field of organic synthesis and production of fine chemical [1]. Alkylation of aniline is industrially important owing to the nomerous uses of various substituted anilines as raw materials for the synthesis of organic chemicals and chemical intermediates or additives in dyes, synthetic rubbers, explosives, herbicides and pharmaceuticals. The traditional route presents some disadvantages like high capital cost, reactor corrosion and formation of by-products that cannot be recycled. With the increasing awareness of environmental issues, various solid acid catalysts and alkylating reagents have been used for the reaction. Thus, vapor-phase aniline alkylation over environmentally safe solid acid catalysts is an answer to the conventional method of producing alkylanilines using mineral acids and Friedel-Crafts type catalysts. Recently, progress in the application of solid acid catalysts for aniline alkylation had been reviewed [2]. The main factors influencing activity and selectivity (N- and/or C-alkylation) are acid-base properties (number and strength) and shape-selectivity, in the solid acid catalyst, as well as reaction conditions (temperature, composition and feed rate). Moreover, the activity of acid catalysts for aniline alkylation might be suppressed by the adsorption of aniline, since it is a strong base. It therefore seems that the use of a strong acid catalyst is not suitable for this reaction. In the present work, we have carried out the vapor-phase N-methylation of aniline over A1-MCM-41 catalysts. We will focus on the effect of surface acidity upon activity and reaction selectivity, and on the deactivation process.

1300 2. EXPERIMENTAL SECTION 2.1. Catalysts A1-MCM-41 samples were prepared by two procedures. (i) Synthesis at room temperature (for catalytic comparison, gel A), following the procedure described by Griin and col. [3]. (ii) Synthesis gel A was filtered and the product was suspended in 100mL of filtration solution and the resultant suspensions were then hydrotermally treated, in a static teflon bottle at 383 K for 24 hours. The product thus obtained was filtered, dried at 298 K and calcined at 823 K in air for 24 hours. TEOS and A1C13.6H20 were used as Si and A1 sources, respectively, and cetyl-trimethyl-ammonium bromide as template. Samples are denoted A1-A-X (at room temperature) or A1-B-X (hydrothermally treated), where X = 10, 20, 30 and 40 are the Si/A1 ratios in the synthesis gel.

2.2. Characterization XRD patterns were carried out using a Siemens D-5000 diffractometer with CuK~ (~,=1.5418 A), a step size of 0.02 ~ and counting time per step of 1.2 s. Thermal analysis was performed by simultaneous TG-DTA measurement using the Setaram thermobalance Setsys 12. Samples were heated in the temperature range 293-1173 K at a heating rate of 10 K min 1. 27A1 (pulse: 1 ~s; recycle delay: 0.3 s) and 29Si (pulse: 6 ~ts; recycle delay: 600 s) MAS NMR spectra were recorded on a Bruker ACP-400 multinuclear spectrometer at 104.26 and 79.45 MHz, respectively. Nitrogen physisorption was measured with a Micromeritics instrument model ASAP 2000 at 77 K.

2.3. Surface Acidity 2.3.1. Pulse method The surface acidity (sum of Br6nsted and Lewis sites) was measured in a dynamic mode by means of the gas-phase adsorption of pyridine (PY) as probe molecule by using a pulse chromatographic technique [4]. Very small volumes of solutes were injected so as to approach conditions of gas-chromatographic linearity. The acidity measurements were repeated several times and good reproducibility of the results was obtained.

2.3.2. Temperature programmed desorption of pyridine Before adsorption experiments were started, the catalysts were pretreated in situ by passing nitrogen, at a flow rate of 50 mL min -1, and heating from 323 to 723 K at 10 K min-1; the temperature was maintained at 723 K for 10 min. After the activation treatment, the samples were cooled down to 373 K at which the adsorption experiment was carried out according to a chromatographic method described elsewhere [5]. Repeated adsorption/TPD experiments using the fresh sample did not show any change in the adsorption curve.

2.4. Catalytic Activity Measurements The reactions were conducted at 573 K in a vapor-phase continuous stainless-steel downflow fixed-bed reactor (6 mm ID) surrounded by an electric heater. An iron-constantan thermocouple was placed in the middle of the catalyst bed and the unit operated at atmospheric pressure. The substrate was delivered at a set flow rate using a liquid syringe pump (Harward Md. 44) and was vaporized prior to passing it through the catalyst bed in the presence of a flow of nitrogen carrier gas (3 L hl). The catalyst charges (W) were small,

1301 usually 0.03-0.06 g, retained by quartz wool at almost the center of the reactor. Standard catalyst pretreatment was carried out in situ at 573 K for 1 h under a stream of high purity nitrogen. In order to prevent any condensation of reactant and products all connections were heated at 490 K. Blank runs at 573 K showed that under the experimental conditions used in this work, the thermal effects could be neglected. The reaction products were on-line sampled every 15 min and analyzed by GC (FISONS Md. 8000) by using a stainless steel column (2 m x 3 mm) of 10% Carbowax 20 M/2% KOH on Chromosorb W-AW 80/100. Product characterization was performed by GC-MS (HP 5800 gas-chromatographic coupled with a VG AutoSpec high-resolution mass spectrometer) using products condensed in a cold trap. Reaction products were: N-methyl (NMA) and N,N-dimethylaniline (NNDMA). N,N-dimethyltoluidines (NNDMT, p - > o - ) were only present in very small amounts. Product such as diphenylamine was never found by highresolution mass spectrometry. Response factors of the reaction products were determined with respect to aniline from GC analysis using known compounds in calibration mixtures of specified compositions. The conversions reported here are on a methanol-free aniline basis and the selectivities are expressed as the ratio of moles produced (mol%). The process for reactivation of partially deactivated catalyst comprised the successive steps of: (a) A1-X catalysts (after 12 h on stream) were quickly heated from 573 to 823 K and then purged during 30 min under nitrogen flow (50 mL min-1); (b) the thermal reactivation was carried out under inert (N2, 50 mL minl), reductive (H2, 50 mL minl), or oxidative (02, 50 or 120 mL min ~) atmosphere during 1 h; and (c) then the catalysts were quickly cooled to reaction temperature under nitrogen flow. 2.5. DRIFT Measurements

DRIFT spectra were recorded on an FTIR instrument (Bomen MB-100) equipped with an "environmental chamber" (Spectra Tech, P/N0030-100) placed in a diffuse reflectance attachment (Spectra Tech, Collector). A resolution of 8cm -1 was used with 256 scans averaged to obtain a spectrum. Samples were equilibrated for at least 1 h at 473 K in flowing nitrogen (50 mL min -1) prior to collection of spectra. 3. RESULTS AND DISCUSSION The Si/A1 molar ratios of calcined A1-X samples (determined by EDX, not shown) were in close agreement with the composition of the gel mixtures. The quality of the XRD pattern and the pore wall thickness increased for A1-B-X with respect to A1-A-X samples. As can be expected, except for A1-B-10 sample, the BET surface area (A~ET) decreased (until ca. 30%) for hydrothermally treated samples. In all cases, the BJH plot for the physisorption of N2 on the aluminosilicate MCM-41 gave a remarkably narrow pore size distribution with a pore size of ca. 23 A. The sharp pore size distribution, with a ca. 6 and 3 ~, width at half-height for A1-A-X and A1-B-X samples, respectively, shows that the mesopores are exceptionally uniform (Table 1). The results of thermogravimetric (TG) and differential thermal analysis (DTA) of A1-X samples (not shown) were similar to that of Klinowski et al. [6] for M41S materials. The 295i MAS NMR spectra of aluminosilicate A1-X (not shown) were very broad showing that the silicon in A1-MCM-41 was therefore in a highly disordered environment. On the orther hand, the 27A1 MAS NMR spectra of A1-X samples (not shown) were similar, exhibiting an intense line at c.a. 54 ppm from 4-coordinate aluminum (Alt) and a low-intensity

1302 Table 1 Hexagonal unit cell parameter (ao = 2dl00/vt3), textural properties, wall thickness (ao-DBjH, e), surface acidity (vs PY/~tmol g-l, pulse method) and contributions to the total area for PY-TPD profiles of A1-MCM-41 catalysts a ao ABET DBJH E PY PY-TPD (Area %) Sample (~) (m 2 g-i) (~) (~) 573 K ~450 K ~600 K -700 K ,-850 K AI-A-40

39

1232

22(6)*

17

130

15

38

30

16

A1-A-30

39

1179

22(6)

17

131

19

36

31

14

A1-A-20

41

1250

23(6)

18

177

20

27

36

17

A1-A-10

43

1237

23(6)

20

251

24

29

30

17

A1-B-40

42

979

23(3)

19

130

34

32

24

10

A1-B-30

44

891

23(3)

21

141

28

36

24

12

A1-B-20

43

877

23(3)

20

187

31

37

24

8

A1-B-10

44

1173

22(7)

22

255

18

34

28

20

* DBjH is followed (in parentheses) by the width at half-height (in * ) of PSD curve. line at c.a. 0 ppm from 6-coordinated aluminum (Alo). The Alt/A1o ratios were c.a. 7 and 10, for all A1-A-X and A1-B-X samples, respectively. These results showed that the incorporation of the aluminum in the silicate network was improved for hydrothermally treated samples. The surface acidity of catalysts is given in Table 1 as the amount of pyridine adsorbed at saturation at 573 K temperature. The acidity measurements showed that the number and density of acid sites on MCM-41 catalysts was increased with the aluminum content and did not depend on the synthesis procedure. A representative pyridine-TPD curve of A1-MCM-41 samples is shown in Figure 1. Moreover, the TPD spectra were deconvolved assuming four independent types of parallel desorption processes, and thus, the theoretical desorption curves of the individual peak components summed to an overall contour, that was well-correlated to the experimental data. Thus, Figure 1 presents for sample A1-B-30 the experimental data, the individual components as the results of deconvolution, and the theoretical spectrum obtained by summing the individual peaks (standard deviations <3%). Analogous spectra were obtained for all A1-X samples. The PY-TPD data are fitted assuming that there are four Gaussian peaks. So, the low temperature pyridine peak at around 450 K is attributable to weak acid sites, pyridine peak at around 600 K is attributable to medium acid sites, whereas the third and fourth peaks (around 700 and 850, respectively) are assumed to be due to strong acid sites (Br6nsted and Lewis). The experimental data obtained from the TPD of pyridine for A1-X catalysts are given in Table 2. Acidity of A1-MCM-41 samples increased on increasing the A1 content (pulse method, see before), however this increase was not accompanied by any change in acid strength distribution. Thus, the PY-TPD profile was almost the same for these materials. Analogous acid properties were obtained for the A1-HMS materials in our laboratories [7]. In the absence of diffusion effects aniline conversion data (XAN) are fitted in a first-order rate equation: ln[1/(1-XAN)]=k(W/F)

(1)

1303

|

Ai-B-30 d

i

p~

.i

o

I

I

I

I

I

I

373

473

573

673

773

873

973

Temperature / K Figure 1. TPD spectrum of pyridine desorbed from Ai-B-30 catalyst: Experimental data (solid line); calculated curves for desorption from four different types of sites and theoretical curve of overall desorption (dotted lines). where W is the catalyst weight and F the feed rate. Nevertheless, calculations are performed only in order to compare the reactivities of the different catalysts and are not aimed at finding the detailed rate equations. All values are reproducible to within about 8%. The initial reaction rate constants (k), XAN, and reaction selectivity to N-methylaniline (SNMA) are collected in Table 2. According to acidity data (Table 1) and the corresponding aniline methylation data (Table 2), we can conclude that on increasing the aluminum content of the catalysts, the acidity as well as the aniline conversion were increased. However, this relationship between surface acidity and catalytic activity was not strictly linear. Table 2 Aniline conversion (XAN, mol%), reaction rate constant (k, mol g'ls-1) and product selectivities (S, mol%) in aniline alkylation with methanol over AI-MCM-41 catalysts a 4 h on stream 8 h on stream 12 h on stream Catalyst XAN k x 10 6 SNMA XAN k x 10 6 SNMA XAN k x 10 6 SNMA AI-A-40

25.1

12.7

71.9

23.4

11.8

78.1

-

-

-

AI-A-30

27.9

14.7

72.8

23.8

12.2

76.1

23.5

12.0

76.8

AI-A-20

28.6

15.0

69.5

25.6

13.1

72.0

25.4

13.1

72.3

AI-A-10

41.3

23.4

64.9

37.7

20.8

67.0

35.5

19.3

69.9

AI-B-40

21.1

10.5

69.1

19.7

9.7

70.7

-

-

-

AI-B-30

28.2

14.6

69.3

25.5

13.0

71.0

23.7

11.9

72.5

AI-B-20

30.0

15.7

69.5

27.6

14.2

72.4

25.5

13.0

74.4

AI-B-10

35.0

18.9

70.4

30.5

16.0

72.7

28.5

14.7

74.2

a T=573 K; F=1.33 x 10 -6 mol s-l; WHSV= 14.8 h-l; 2 M aniline in methanol.

1304 Product selectivities were of the 100mo1% to the N-alkylation of aniline. NNDT (<2 mol%) were only presents at short time on stream (<2 h). The effect of time on stream is shown in Table 2. Here we can see that the aniline conversion and selectivity to NNDMA decreased whereas selectivity to NMA continuously increased. Besides, the selectivity to NMA remained almost the same, irrespective of the aniline conversion, aluminum content, and acidity, this fact was strictly observed for A1-B-X catalysts. These results are contrary to the expectation for aniline alkylation being a consecutive reaction and, also, as compared to previously studied A1PO4, A1POa-metal oxide, CrPO4-A1PO4 and A1-HMS catalysts [7-10]. Nevertheless, similar relation between Si/A1 ratio, total acidity and aniline conversion against selectivity to N,N-diethylaniline had been described for HZSM-5 samples, and it was suggested that, rather than the total acidity, probably acidity of particular strength and type were responsible for the conversion of aniline and selectivity of alkylanilines [2]. Another sets of experiment were carried out to establish the stability of the systems. TG, DTG and DTA thermal analysis curves showed that the decomposition process undergone by the organic on deactivated catalysts could be divided into three steps (Figure 2). (i) The first one, characterized in DTA by a broad endothermic zone, which did not generally extend far away than 413 K, could correspond to the removal of physically adsorbed water [10], arising from alkylation reaction. Weight losses at this stage were under 6%. (ii) Second step corresponds to the progressive removal of water and/or organic compound. TG curves showed a regular decrease in the 413-643 K region, constituting about 1,6% weight loss. (iii)In the third stage, a broad exothermal zone was displayed in DTA curves in the range 643-923 K in oxidative atmosphere and 643-1023 K for inert atmosphere, with weight losses being associated about 4%, and may be attributed to the removal of the carbonaceous compounds (coke and/or basic organic). These features were confirmed by DRIFT spectra of deactivated and reactivated catalysts (see below). On the other hand, XRD peak of the fresh A1-MCM-41 samples decreased in intensity after catalytic reaction, irrespective to the regeneration process (not shown). Thus, A1-MCM-41 structures are deteriorated as a consequence of the water produced in the alkylation of aniline with methanol, which is known well in the literature. TG (mg, ~ )

DTG (mg min-1,...)

ATD (~V . . . .

0.0 -0.5

,,

.

/-

-3 1 A1-B-10, in air (50 mL min ) _ -6

/i ~ ~ f// ~ -1.0 - \. "~ ;.~" -0.09 ~

-9

-1.5 373

473

573

673

773

873

973

1073

Temperature / K Figure 2. TG, DTG and ATD obtained for the deactivated A1-B-10 sample.

-12 1173

)

1305

3739

A

B

"~ / N ~ 9

4000

3500

3000

1800

Wavenumber / cm -1

1700

1600

~

,,,

"~"

i

,4j

1500

1400

Wavenumber / cm -1

Figure 3. DRIFT spectra of A1-B-10 sample: a, a') after the reaction at 573 K for 12 h; b, b') reactivated at 823 K under oxygen flow (50 mL min-1); and c, c') reactivated at 873 K under oxygen flow (120 mL mina). Figure 3 shows the DRIFT spectra of the deactivated A1-B-10 catalyst as well as the reactivated one, obtained under thermal conditions and oxidative atmosphere. As M. Rozwadowski et al. [10], we found two main frequency regions in which the detected band occur (A:4000-2800 and B:1800-1350 cma). These bands could be attributed to paraffinic, olefinic and/or aromatic compounds, and coke (Figure 3Aa and 3Ba'). After the reactivation of the A1-B-10 catalyst at 823 K in flowing 02 (120 mL mina), it could be observed a broad band occurring at ca. 1625 cm -a (Figure 3.Bc') that is know as the coke band. Consequently, our reactivation procedure was not enough for the total removal of the coke, as could be expected from TG-DTA results. 60 50 ~" 40 o

g

X

.

-

~

.

AI-A- 10

~

zx

30

20 10 ! 0

I

I

I

I

I

I

2

4

6

8

10

12

Time on stream (h) Figure 4. Time-on-stream dependence of aniline conversion at 573 K over A1-A-10 catalyst: (o) fresh catalyst, (0) reactivated at 823 K under nitrogen flow (50 ml minl), (V) reactivated at 823 K under oxygen flow (50 ml min-1), and (A) reactivated at 823 K under oxygen flow ( 120 ml min-1).

1306 Furthermore, the thermal reactivation at 823 K of deactivated A1-X catalysts (12 h on stream) strongly depends on the inert, reductive or oxidative atmosphere (N2, H2 or 02, respectively) and on its flow rate during the reactivation treatment. Thus, inert and reductive treatment did not improve the catalytic activities, whereas oxidative treatment restores up to ca. 80% of the AI-X catalysts activities in flowing 02 at 120 mL min 1 (Figure 4). However, additional thermal reactivation studies were not required because of A1-MCM-41 structure deteriorate with reaction water (see above) and, thus, the re-used catalysts would be described as amorphous aluminosilicates. 4. CONCLUSSION The number of active sites responsible for the mono-methylation of aniline did not increase with aluminum content and, besides, its decreased with time on stream on A1-MCM-41 catalysts. Thus, the deactivation of aluminosilicate MCM-41 catalysts can be explained in two ways: 1. MCM-41 structure deteriorates as a consequence of the reaction water in the alkylation of aniline with methanol. 2. The adsorption of aniline and/or its derivatives as well as coke formation on aluminosilicate MCM-41 is confirmed by DRIFT bands in 3000-2800cm -1 and 1700-1400 cm -1 regions. This fact can produce the pores occlusion and the lost of catalytic active sites. This research was subsidized by grants from Direcci6n General de Investigaci6n (Project BQU2001-2605), Ministerio de Ciencia y Tecnologia, and from the Consejeria de Educaci6n y Ciencia (Junta de Andalucia). References 1. G. 0ye, J. Sj6blom and M. St6cker, Adv. Colloid Interface Sci., 439 (2001) 89/90. 2. S. Narayanan and K. Deshpande, Appl. Catal. A, 199 (2000) 1. 3. M. Gr/in, K.K. Unger, A. Matsumoto and K. Tsutsumi, Microporous Mesoporous Mater., 27 (1999) 207. 4. J.M. Campelo, A. Garcia, D. Luna and J.M. Marinas, J. Mater. Sci., 25 (1990) 2513. 5. A.A. Romero, M.D. Alba and J. Klinowski, J. Phys. Chem. B, 102 (1998) 123. 6. J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Thermochim. Acta, 265 (1995) 103. 7. J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and J.J. Toledano, Stud. Surf. Sci. Cat., 135 (2001) 281. 8. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Stud. Surf. Sci. Cat., 108 (1997) 123. 9. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.R. Urbano, J. Catal., 172 (1997) 103. 10. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Appl. Catal. A, 166 (1998) 39. 11. M. Rozwadowski, M. Lezanka, J. Wloch, K. Erdmann, R. Golembiewski and J. Kornatowski, Chem. Mater., 13 (2001) 1609.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Restructured V-MCM-41 with non-leaching vanadium hydrothermal stability prepared by secondary synthesis

1307

and

improved

Nawal Kishor Mal,a'* Prashant Kumar,b Masahiro Fujiwara a and K. Kuraokaa aAIST

Kansai, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

bCeramic Section, Industrial Research Institute of Ishikawa, Kanazawa 920-0223, Japan

Restructured V-MCM-41 prepared using parent V-MCM-41 (calcined form) via secondary synthesis possessing non-leaching vanadium in framework shows remarkable hydrothermal stability. Samples were characterized using various techniques such as XRD, N2-sorption, UV-visible, FT-IR, TG-DTA and 51V MAS M R . Activity of parent and restructured V-MCM-41 were compare in the oxidation of toluene and 1-naphthol in the presence of aqueous HzO2. Conversion of toluene and 1-naphthol over reused restructured V-MCM-41 are 2 fold more than that of parent V-MCM-41 with similar Si/V ratio. 1. INTRODUCTION Microporous vanadium silicate is known to be an excellent hydrocarbon oxidation catalyst utilizing hydrogen peroxide [ 1,2]. However, these microporous molecular sieves with channel less than 1 nm restrict the diffusion of relatively larger organic molecules. Following the discovery of mesoporous silicate designated as M41S by Beck and Coworkers [3] many successful attempts have been made to prepare vanadium substituted MCM-41 by direct hydrothermal synthesis either in presence or absence of sodium [4-8]. However, leaching of vanadium [9] and structural collapse of vanadium containing mesoporous molecular sieves in the presence of aqueous H202 limits their practical applications in the liquid phase oxidation reactions. The objective of this paper is to report the preparation of V-MCM-41 by secondary synthesis using calcined form of parent V-MCM-41, characterization, improvement in hydrothermal stability and stability &vanadium towards leaching during oxidation of toluene and 1-naphthol in the presence of aqueous HzO2. +NKM is grateful to AIST, Tokyo for STA fellowship.

1308

2. MATERIALS AND METHODS 2.1. Materials

The reactam

used

in this

study were

fumed

silica

(99.9%,

Sigma

Chem.),

cetyltrimethylammonium bromide (96%, Kanto Chem.) (CTMABr), tetramethylammonium hydroxide (25% aqueous, Aldrich Chem.) (TMAOH), VOSO4-3H20 (Aldrich Chem.) and H2SO4 (96%, Wako Chem.). Parent V-MCM-41 was prepared in absence of sodium using following molar composition: 1.0 SiO2 : 0.35 CTMABr : 0.40 TMAOH : (0.0 - 0.0125) VO2 : 0.14 H2SO4 : 60 H20. In a typical synthesis, 14.58 g of TMAOH and 13.29 g of CTMABr were dissolved in 50 g of water by stirring at 308 K

to give a clear solution. 6.01 g of

fumed silica was added and stirred for 1 h. 0.655 g of VOSO4"3H20 (Si/V = 25) in 25 g of H20 was then added under stirring for 1 h. Finally, 1.43 g ofa2so4 in 22 g of H20 was added and stirred for 2 h. The resultant gel was aged at room temperature for 1 day, and then heated statically at 413 K for 2 days under autogenous pressure. The products were filtered, washed, dried at 378 K and calcined at 823 K for 6 h to yield parent V-MCM-41. Pure Si-MCM-41 was prepared using the same procedure without addition of VOSO4"3H20. For restructuring (secondary synthesis), a synthesis gel of molar ratio as above was prepared except that the parent V-MCM-41 was used as silica and vanadium sources instead of fumed silica and VOSO4"3H20. Three different Si/V molar ratios, 25, 60 and 80 (in gel) were prepared by primary synthesis method denoted as sample 1, 2 and 3, respectively, and the corresponding restructured samples were denoted as 1R, 2R and 3R, respectively. 2.2. Hydrothermal treatment and catalytic reactions 0.1 g of calcined MCM-41 in 100 g of water was heated in a propylene bottle to 373 K for

4 days. The Samples was then filtered, dried at 378 K and calcined at 773 K for 90 m. Liquid phase oxidation reactions of toluene and 1-naphthol were carried out batch wise in two-necked round bottom flask fitted with a condenser and placed in oil bath at 353 K for 24 h under the reaction conditions: 0.20 g catalyst, 1 g substrate, 10 ml acetonitrile (solvent), 3 mole ratio of reactant to H202 (30 wt% aqueous). The reaction products were analyzed in a capillary GC (HP 5880) using 50 m long silicon gum column and identified by known standards and GC-MS. 2.3. Characterization Elemental analyses of the samples were carried out using ICP (Shimadzu ICPV-1017). Characterization of the samples were carried out using X R

(Shimadzu XRD-6000), N2

sorption at 77 K (Bellsorp 28 instrument), FT-IR (JASCO FT/IR-230, UV-visible (JASCO V-560) and Thermogravimetric analyses (Seiko, SSC/5200). 51V MAS NMR spectra were

1309 recorded at 11.7 T on an Am-500 Bruker spectrometer at 131.375 MHz with a multinuclear MAS probe using 5 mm standard zirconia rotor. Data were acquired with 5.0 gs recycle delays at 3kHz. The chemical shift is referenced against VOC13. 2.4. Methods The BET surface area [10] was calculated in the relative pressure range between 0.04 and 0.2. The average pore size (WKJs) was calculated using adsorption branch of isotherms according to method describe elsewhere [11] that is; W~s = cd(pVp)l/2/(1 + pVp) 1/2, where C = 1.213, 9 = 2.2 gcm 3, d is the lattice spacing of dl00, and Vp is the primary mesopore volume. Total pore volume was determined from the amount adsorbed at relative pressure of 0.99 [ 10]. The pore size distributions were calculated from adsorption branches of the nitrogen adsorption isotherms using Barrett-Joyner-Halenda (BJH) method [12]. For comparison, average pore size was also calculated using BJH method [ 12].

3. RESULTS AND DISCUSSION

3.1. Synthesis, structure and adsorption properties Si/V molar ratio in the product of parent samples 1, 2, and 3 are 48, 138 and 206, respectively, and their corresponding restructured samples 1R, 2R and 3R are 208, 386 and 448, respectively. XRD patterns of parent and restructured samples show the intense dloo peak and other three higher order peaks, characteristics of long range ordering of a typical MCM-41 materials, as shown in Fig. 1. The intensity of dxoo peak of all restructured samples are higher than that of parent samples. Textural properties of all vanadium f containing MCM-41 are shown in Table 1. e

Difference in dl00 spacing between parent and corresponding

c

-

2

3

4

samples

non-changing dl00 spacing observed here is

a

5 6 7 2e (degree)

restructured

were very small (0.01 to 0.03 nm). The

,.

8

9

10

probably due to the structural backbone of parent

V-MCM-41

maintained

during

recrystallization. The surface area and pore Figure 1. XRD profiles of parent samples (a) volume of restructured materials gradually sample 1, (b) sample 2, (c) sample 3, and decreases after restructuring. It is probably restructured samples (d) sample 1R, (e) sample 2R due to the formation of thicker pore walls and (f) sample 3R.

within the existing pore of the parent

1310 Table 1. Textural properties of V-MCM-41 a Sample

dloo (nm)

8.0 (nm)

aBET (m2g1)

SBJH Vp (m2g"1) (mlg4)

WKJS (nm)

bins (nm)

WBJH (nm)

bBJH (nm)

Si-MCM-41

3.95

4.56

1040

1246

0.92

2.95

1.61

3.92

0.64

1

4.12

4.76

993

1209

0.86

2.85

1.91

4.04

0.72

2

4.08

4.71

997

1221

0.89

2.92

1.79

4.03

0.68

3

4.01

4.63

1006

1234

0.90

2.92

1.71

3.96

0.67

Restructured samples 1R

4.09

4.72

960

1154

0.76

2.63

2.09

3.92

0.80

2R

4.06

4.69

923

1136

0.77

2.71

1.98

3.91

0.78

3R 4.00 4.62 910 1130 0.75 2.65 1.97 3.83 0.79 adloo: X-ray diffraction (100) interplanar spacing; ao: unit cell parameter = 2dloo/31/2;SBET:BET specific surface area; SBra:BJH specific surface area; Vp: Primary.meesopore volume; Wins: Average pore size (Ref 11); bus: wall thickness = ao- Wins; WBm:Averagepore size (Ref. 12);bBra:wall thickness = ao- WBra. V-MCM-41. This therefore causes decrease in pore size (Table 1). Wall thickness (unit cell parameter- average pore size) of restructured samples is 10 to 15 % higher than that of parent samples. Among the restructured samples specific surface area and pore volume of high vanadium content sample (sample 1R) are higher than low vanadium content samples (sample 2R and 3R). N2 sorption isotherms and pore size distribution of samples 1 and 1R are shown in Fig. 2. N2 sorption isotherms show a feature of a type IV isotherm with sharp capillary condensation at p/p0 ca. 0.3. Pore size distribution curve shows narrow pore size distribution with peak pore diameter at 2.80 nm. 3.2. UV-visible, FT-IR and 51V MAN NMR UV-visible spectra of calcined samples 1, 2, 3 and 1R are shown in Fig. 3. All the samples were analyzed in dehydrated state (white color) just after calcinations. Restructured sample

600] ,001

""9 400

,;,~

a

,..,.-,

f . . . . . . . . .

1

E ,., "~

3oo~1

>

200

( ]

0.0

,z-,,

"r

2.8

B

E 2.0

b

~ 1.6

..-, 1.2 -o ~" 0.8 1~ 0.4 -6 0.0

~ ~,~.~.~'~" -~--~"'012 014 016 018 Relative pressure (P/Po)

110

<

0

a

5

10 15 20 25 Pore diameter (nm)

3'0

Figure 2. (A) N2 adsorption-desorption isotherms and (B) pore size distribution curve of (a) sample 1 and (b) sample 1R.

1311 1R shows band at 275 nm due to V 5§ in tetrahedral environment

a

5 d

[5-8,13].

This

sample

probably

contains a weak shoulder at ca. 320 nm. Parent

o9 8

\\

t-~

8

<

300

46o

Wave length (nm)

samples show main band at 275 nm and second band at 333 nm (samples 2 and 3) is due to V 5§ ions soo

with a short V - O double bond and three longer V-O bond, where as 390 nm band (sample 1) is due to presence of octahedral V 5§ species [5,6,13].

Figure 3. UV-visible spectra of (a) When all the calcined samples exposed to air, white sample 1, (b) sample 2, (c) sample 3 color of the samples changed to pale yellow and a and (d) sample 1R.

weak band was developed at ca. 390 nm (sample 2, 3 and 1R). However, in the case of sample 1 broad

band was observed at ca. 415 nm. Other authors also found that freshly calcined V-MCM-41 shows only two bands at ca. 275 and 365 nm [5,6,8]. Arnold et al. observed a single band at 245 nm in calcined V-MCM-41, which is assigned to distorted tetrahedral coordination of V 5§ ions [7]. In our calcined samples a~er exposed to air, very small decrease in absorption at 275 nm was observed. In contrast, absorption near 275 nm does not decrease [5]. This suggests that some of the tetrahedral V 5§ ions are inaccessible to water or at least it does not affect the symmetry, probably located inside the walls of the hexagonal tubular MCM-41 structure. FT-IR spectra of samples 3 and sample 1R are shown in Fig. 4. Presence of vibration band at 1078 cm 1 in the vanadium containing samples, which is somewhat lower than compared to pure Si-MCM-41 (1098 cm 1) is an indicative of substitution of vanadium in the frame work ofV-MCM-41 [5,6]. Vanadium containing samples show vibration band at 960 cm 1 assigned to the framework vibration of Si-O-V bond. However, this vibration band is also exit in pure Si-MCM-41 due to presence of excess silanol groups. Therefore, origin and interpretation of this bond is controversial [ 14]. 51V MAS NMR of calcined hydrated ,,,?.

v O t-

b

.*2_ E 09 t-

O L..

10'00 " Wave number (cm "1)

20'00

__J

Figure 4. FT-IR spectra of (a) sample 3 and (b) Figure 5.51V MAS NMR spectra of sample 1R. sample 1R

1312 restructured sample 1R is shown in Fig. 5. It shows that strong signal at 5 = -532 ppm is an indication of tetrahedral coordination of V 5+ ions [8,18]. Absence of NMR signal near-310 ppm suggest that V205 phase is not present in our samples.

3.3. Thermogravimetric analysis (TGA) and hydrothermal stability TG analyses of all as-synthesized parent and restructured samples are shown in Table 2 and Fig. 6. All samples show four distinct weight losses in TG diagram [15-17]. Weight loss below 403 K corresponds to desorption of physisorbed water and ethanol, between 403 and 623 K correspond to breakage, decomposition and combustion of residual organic. Weight loss above 623 K is attributed to water losses resulting from dehydroxylation reaction [ 15-17]. TG analyses indicate that no weight loss occurred above 873 K (Fig.6). Total weight loss between 293 and 1073 K is 48.2 to 51.5% Table 2.

for parent samples and 40.1 to 42.2% for

TG analyses of as-synthesized samples. Weight loss (%) Sample 2 9 3 - 1 0 7 3 K 4 0 3 - 6 2 3 K 1 48.2 36.6 1R 41.0 31.3 2 49.9 37.6 2R 42.2 33.0 3 51.5 38.7 3R 40.1 30.3

restructured samples (Table 2). TG analysis

,.-., c- ~

shows that the amount of surfactant in all restructured materials is 30 to 33% (weight loss between 403 to 623 K), which are in the range of 14 to 22% less than that of parent samples (37 to 39%). XRD patterns of all parent and restructured samples after 1oo

1oo

"" |

r

80

60

4O

a

ed (sample 1 R)

"-" ._~

parent (sam pie 1 ) 400 600 800 10'00 Tern perature (K)

80

ed ( s a m p l e 2R)

60 40

40o

660

Parent (sample

Temperature (K)

8oo

lo'oo

2)

100 f

G

80

red (sample 3R)

c-

"~

40

.g c

60

4oo

660

parent (sample 3)

Temperature (K)

8oo

~o'oo

g 2

3

4

5 6 7 20 (degree)

8

9

10

a

Figure 6. TG analyses of parent and restructured Figure 7. XRD patterns of samples after samples (a) sample 1 and 1R, (b) sample 2 and hydrothermal treatment at 373 K for 4 days (a-f) 2R, and (c) Sample 3 and 3R. sample 1, 2, 3, 1R, 2R and 3R, respectively.

1313 hydrothermal treatment at 373 K for 4 days are shown in Figure 7. Parent samples 1, 2 and 3 are severely degraded, where as all the restructured samples 1R, 2R and 3R show an intense dl00 peak and two higher order peaks. These results suggest that hydrothermal stability of the V-MCM-41 improved after secondary synthesis. 3.4. Catalytic reactions

Catalytic activity of parent sample 3 (Si/V - 206) and restructured sample 1R (Si/V = 208) with nearly the similar Si/V molar ratio were compared in the oxidation of toluene and 1-naphthol in presence of aqueous H202 (30 wt%) as shown in Table 3 and 4, respectively. Before reuse catalysts were calcined at 823 K for 4 h. In the oxidation of toluene, when sample was reused, conversion of toluene decreased from 13.2 (fresh) to 5.0% (reused) and H202 selectivity decreased from 70 to 25.5% (Table 3). Where as when sample 1R was reused the decreases in toluene conversion (12.3 to 12.1%) and H202 selectivity (60.9 to 59.9) are negligible. Major product is benzaldehyde over both parent (sel. 76.7 to 70.3%) and restructured samples (sel. 65.1 to 65%). A similar phenomenon was also observed in the oxidation of 1-naphthol, the 1-naphthol conversion and H202 selectivity over reused sample 1R are two fold higher than reused parent sample 3, despite fact that fresh sample 3 shows slightly higher 1-naphthol conversion and H20/ selectivity than that of fresh sample 1R (Table 4). 1-4-naphthaquinone is major product over parent and restructured samples. This Table 3. Catalytic activity in the oxidation of toluene Sample Conversion H;O2 efficien, a Product distribution (%) (%) (mole%) PhCHO PhCH2OH o-cresol p-cresol 9.1 6.9 3 13.2 70.0 76.7 7.3 reused 5.0 25.5 70.3 25.2 1.5 3.0 12.8 15.7 1R 12.3 60.9 65.1 6.4 12.7 15.7 reused 12.1 59.9 65.0 6.6 aH202 efficiency = mole% of H202 consumed in the formation of benzaldehyde (PhCHO), benzylalcohol (PhCHzOH), ortho cresol (o-cresol) and para cresol (o-cresol). Table 4. Catalytic activity in the oxidation of 1-naphthol Sample Conversion H202 efficiency a Product distribution (%) (%) (mole%) 1,2-one 1,4-one 1,2-diol 1,4-diol 3 13.8 78.3 4.4 84.8 5.0 5.8 reused 6.4 33.9 12.6 64.1 8.2 15.1 1R 12.8 69.0 4.5 75.3 2.2 18.0 reused 12.7 68.5 4.6 75.1 2.2 18.1 all202 efficiency = mole% of H202 consumed in the formation of 1,2- naphthaquinone (1,2-one), 1,4naphthaquinone (1,4-one), 1,2- dihydroxynaphthalene (1,2-diol) and 1,4-dihydroxynaphthalene (1,4-diol).

1314 result clearly indicates that during the reaction leaching of vanadium took place from parent sample 3, where as did not over restructured sample 1R. This result was further confirmed, according to procedure described by Neumann et al. [19]. Mixture of acetonitrile, H202 and fresh catalyst were heated at 3 53 K for 2 h and then filtered. Substrate was then added to the filtrate and heated overnight to check the leaching activity. Conversion of substrate was observed over sample 3 and no conversion of substrate was observed over sample 1R. When 5 ppm of vanadium as VOSO4"3H20 was used instead of catalyst under similar reaction conditions, the toluene and 1-naphthol conversion were 0.6 and 0.8%, respectively. In conclusion, restructured V-MCM-41 prepared using calcined form of parent V-MCM-41 via secondary synthesis shows remarkable hydrothermal stability. Substrate (toluene or 1-naphthaol) conversion over regenerated and reused parent (Si/V = 206) and restructured V-MCM-41 (Si/V = 208) in the oxidation of toluene are 5.0 and 12.1%, respectively, 6.4 and 12.7%, respectively, in the oxidation of 1-naphthol. Homogeneous catalysis takes place over parent samples where as heterogeneous catalysis occurs over restructured samples. REFERENCES 1. A. Miyamoto, D. Medhanavyn and T. Inui, Appl. Catal., 28 (1986) 89. 2. M.S. Rigutto and H. Van Bekkum,, Appl. Catal., 68 (1991), L 1. 3. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 4. K.M. Reddy, I. Moudrakovski and A. Syari, J. chem. Soc., Chem. Commun., (1994) 1059. 5. Z. Luan, J. Xu, H. He, J. Kilnowski and L. Kevan. J. Phys. Chem., 100 (1996) 19595. 6. Y.-W.Chen and Y.-H Lu, Ind. Eng. Chem. Res., 38 (1999) 1893. 7. A.B.J. Arnold, J. P. M. Niederer, T. E. W. Niel3en and W. F. H61derich, Micropor. Mesopor. Mater., 28 (1999) 353. 8. D. Wei, H. Wang, X. Feng, W.-T. Chueh, P. Ravikovitch, M. Lyubovsky, C. Li, T. Takeguchi and G. L. Haller, J. Phys. Chem. B, 103 (1999) 2113. 9. J.S. Reddy and A. Sayari, J. Chem. Soc., Chem. Commun., (1995) 2231. 10. S. BrunaueL P. H. Emmett and E. Teller. J. Am. Chem. Soc., 60 (1938) 309. 11. M. Kruk, M. Jaroniec and A. Sayari, (a) Langmuir, 13 (1997) 6267, (b) J. Phys. Chem., 101 (1997) 583, (c) Chem. Mater., 9 (1997) 2499. 12. E.P. Barrett, L. G. Joyer and P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 13. G. Centi, S. Perathoner, F. Trifiro, A. Aboukais, C. E Aissi and M. Guelton, J. phys. Chem., 96 (1992)2617. 14. J. Weitkemp, H. G. Karge, H. Preifer and W. F. H61derich, Eds.; Studies in Surface Science and Catalysis 84; Elsevier: Amsterdam, 1994, 69. 15. C.- Y. then, H.- X Li and M. E. Davis, Micropor. Mater., 2 (1993) 17. 16. R. Schmidt, D. Akporiaye, M. Stocker and O. Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61. 17. R T. Tanev and T. J. Pinnavaia, Chem. Mater., 8 (1996) 2068. 18. P.T. Tanev and T. J. Pinnavaia, Science, 267 (1995) 865. 19. R. Neumann and M. L.-Elad, Appl. Catal., 122 (1995) 85.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1315

C o m p a r a t i v e study o f M C M - 4 1 acidity b y using the integrated m o l a r extinction coefficients for infrared a b s o r p t i o n b a n d s o f a d s o r b e d a m m o n i a A. Taouli and W. Reschetilowski Institute for Industrial Chemistry, University of Technology Dresden, D-01062 Dresden, Germany. The acidity of modified MCM-41 samples was characterised by Temperature-Programmed Ammonia-Desorption (TPAD) measurements with an UHV system by using QMS for monitoring the desorbed ammonia and in situ FTIR spectrometer for monitoring the surface species during the adsorption and desorption process. The Integrated Molar Extinction Coefficients (IMEC) for the infrared absorption bands of ammonia on the different samples were determined. The resulting IMEC values were 3.03 cm/lamole for the band 1450 cm 1 assigned to ammonia on the Bronsted acid site, 1.24 cm/lamole for the band 1620 cm ~ assigned to ammonia on the Lewis acid site and 0.176 cm/gmole for the band 1560cm ~, attributed to Si-NH 2. The observed IMEC values are clearly independent of the nature of the samples and their acidic properties. 1. INTRODUCTION The discovery of MCM-41 has stimulated considerable interest in this material because of its large surface and pore size [1,2]. Such properties make it attractive for many uses in the field of adsorption and catalysis involving large organic molecules [3,4]. Incorporating aluminium or other elements into the framework positions of the siliceous MCM-41 gives acidic properties. TPAD measurements reveal the existence of Bronsted and Lewis acid sites [5,6]. However, the quantitative determination of B and L acid sites of MCM-41 probed by basic molecules requires a knowledge of IMEC(B) and IMEC(L). Recently the IMEC of ammonia adsorbed on different MCM-41 samples and of pyridine adsorbed on zeolite and amorphous silica-alumina samples were reported [7,8]. The objective of this paper is to continue the examination of the IMEC values of three characteristic bands of NH 3 adsorbed on various modified MCM-41 samples. An attempt was made to compare the acidity of different MCM-41 materials obtained by using QMS and FTIR spectrometer. 2. EXPERIMENTAL SECTION MCM-41 samples with varying contents of aluminium (Si/A1 ratio of 3 and 19) and pure Si-MCM-41 (for comparison) were synthesised according to procedures described by Schmidt et al. [9] and by Genske et al. [10], respectively. The modification of materials with zirconium or aluminium was carried out by incorporation of a hydroxy-zirconium or a hydroxy-

1316 aluminium complex in the MCM-41 structure respectively, following a similar procedure as described in the literature [11-13]. The amount of Zr and A1 used in the reaction was equivalent to 4 and 2 mmole for each g of MCM-41, respectively. The modified samples (TMCM-41...) indicate that the MCM-41 used in the modification is a material-containing template. The protonic form of the investigated samples was obtained according to an earlier reported procedure [7]. The characterisation of acidity was carried out by TPAD combined with in situ FTIR measurements (150-550~ the practical procedure of which is described by Liepold et al. [5] and Taouli et al. [7]. To obtain the Integrated Absorbance (IA) values of the Bronsted (B), Lewis (L) and amine (Si-NH2) bands (integration regions approximately 1503-1375cm -1, 1655-1575cm -1 and 1530-1570 cm -1 respectively), the difference FTIR-spectra were used. The IMEC values of these three characteristic bands of ammonia adsorbed on MCM-41 samples were determined according to the procedure described by Emeis [8]. 3. R E S U L T S A N D D I S C U S S I O N 3.1. Structure and textural parameters The results of the physicochemical characterisation of the investigated samples are shown in Table 1. XRD measurements and the nitrogen adsorption isotherms performed still verified the typical mesoporous MCM-41 structure. Those results show that the modification of MCM-41 materials partly reduces the specific surface area, the pore volume and the pore size as compared to the standard samples, Si-MCM-41, TMCM-41-19-H, MCM-41-19-H and MCM-41-3-H. Table 1 shows that the Si/A1 ratio of the samples modified with the hydroxy-zirconium increases, which means a dealumination during the modification. The modification with the hydroxy-aluminium complex decreases the Si/A1 ratio. The structure of modified samples is significantly destroyed, due to the incorporation of polycations of aluminium and zirconium in the pores.

Table 1 Physicochemical properties of the investigated samples Samples BET surface Pore volume area (m 2 g-~) (cm 3 g-l) Si-MCM-41 1300 0.94 TMCM-41-19-H 1000 0.88 MCM-41-19-H 770 0.46 MCM-41-3-H 710 0.55 MCM-41-3-A1-H 350 0.26 MCM-41-19-A1-H 650 0.42 TMCM-41-19-A1-H 880 0.54 MCM-41-3-Zr-H 550 0.34 TMCM-41-19-Zr-H (1) 900 0.67 TMCM-41-19-Zr-H (2) 940 0.63 TMCM-41-19-Zr-H (3) 880 0.64

BJH pore diameter (nm) 2.2 2.1 1.8 1.9 1.4 1.6 1.9 1.4 1.9 1.9 2.0

Si/A1 ratio co 18.6 18.6 3.1 10 14 24 22 22

1317 3.2. Determination of IMEC(B), IMEC(L) and IMEC(amine) Figure 1 shows an example of the difference FTIR-spectra of Si-MCM-41and MCM-4119-H samples obtained after the addition of ammonia and of the last one in the NH 4 form. The stretching vibration bands of the adsorbed ammonia, which are to be observed between 3500 and 2800 cm 1, were not generally used for the determination of the acidic centres, because they are broadly overlapping and are not specific for the nature of acidic centres. In contrast, absorption bands in the domain of N-H deformation vibration under 1700 cm -1 are well resolved. The FTIR spectra of used MCM-41 materials show a very intensive band at 3750 cm 1 in the hydroxyl range, corresponding to the terminal silanol groups found on silica and Si-MCM-41. No bands of bridging Si-OH-A1 groups were observed, which normally appear at about 3500-3700 cm -1 in zeolite Y and ZSM-5. On adding ammonia, three bands appear in the region of N-H bending vibrations: a band at 1620 cm -~ commonly related to ammonia coordinatively bonded to Lewis acid sites, a band at 1450 cm -~ generally assigned to ammonium ions (MCM-41-19-NH4) [5-7,14,15] and the last one at 1560 cm -I assigned to the group Si-NH 2, according to the reaction of ammonia with a Si-O-Si bridge [16,17]. This was confirmed by the obtained result with pure Si-MCM-41 material, where only this band is present in N-H bending vibrations (Figure 1). The adsorbed amount of NH 3 on this sample is not negligible. Therefore, this Si-NH 2 band can not remain unconsidered in the determination of IMEC values. For this reason, the earlier results [7] must be expanded to this Band. However, this expansion leads inevitably to other IMEC values for B and L bands, which F--

-

Bronsted MCM-41-19-H

:

,,-~-

:

Si-MCM-41 .....

'.

,i

,,

MCM-41-19-NH4 ',

!

A

Si-NH 2

~9 O

!

9

o

o

!

!

i o

'

r/l ! :

i

J

MCM-41-19-H

/i

__

Si-MCM-41

.....

MCM-41-19-NH4

\

/., T

3700

l

3450

l

i

3200

r

E

I

2950

Wavenumbers (cm-1)

2700

1650

i

,

,

1550

1450

1350

Wavenumbers (cm "l)

Figure 1. Difference FTIR-spectra of various samples obtained after addition of ammonia.

1318 present now a correct solution for the quantification of the infrared absorption bands of adsorbed ammonia. Figure 2 shows an example of the difference FTIR-spectra obtained after various additions of ammonia on MCM-41-19-A1-H. The increase of added amounts of ammonia increases the absorbance of the three characteristic bands of ammonia adsorbed on the MCM-41 sample. The integrated absorbance (IA) values of B, L and amine bands were plotted against the amount of added ammonia. Figure 3 shows the result for MCM-41-19-A1-H. The amount of ammonia adsorbed on the walls of the ultra-high vacuum system was determined by a control measurement and included in the evaluation of the residual chemisorbed ammonia. For this reason, the amount of added ammonia was set equal to the amount adsorbed by the sample, which gives: slope (X) = AIA(X)/A (amount of added ammonia), cm-1/gmole where X = B, L and amine bands. The slopes of B, L and amine bands are given in Table 2. For simplicity, the IMEC(B), IMEC(L) and IMEC(amine) were calculated assuming that they do not depend on the strength of the acidic sites and they are the same for all the investigated samples. Beer's law gives for each sample only one equation for IMEC(B), IMEC(L) and IMEC(amine) [8]: 3.14"R 2 [slope(B)/IMEC(B) + slope(L)/IMEC(L) + slope(amine)/IMEC(amine)] = 1 where R represents the pellet radius. The ratio of the amounts of ammonia adsorbed on B, L acid sites and amine sites are unknown. Therefore, the resulting set of equations of the investigated samples was solved by the least-squares procedure, which gives the following IMEC values: 3.03 and 1.24 cm/gmole for the absorption band of ammonia on BrBnsted and Lewis acid sites respectively, and

~, I

- - 1st adsorption (2 gmole) BrBnsted Lewis r, 2nd adsorption (4 gmole) / " N / ".. - - - 3rd adsorption (12 gmole~//

~=

/

\

.o9 Bransted -~- Lewis

/ /

Si-NH2

,.Q

<

T

1700

1650

1600

p

i

~

r

1550

1500

1450

1400

Wavenumbers (cm "l)

Figure 2. Difference FTIR-spectra obtained after addition of varying amount of ammonia on MCM-41-19-A1-H sample.

j 1350

0 5 10 15 Amount of added ammonia (gmole)

Figure 3. Integrated absorbances of ammonia adsorbed on MCM-41-19A1-H.

1319 Table 2 Infrared spectroscopic data for ammonia adsorption at 150 ~ The slope represents the integrated absorbance of IR band by gmole of added ammonia. Samples Slope (Lewis) Slope (Bronsted) Slope (amine) (10 -2 cm-1/pmole) (10 .2 cm-1/lamole) (10 .2 cm-1/lamole) TMCM-41-19-H 12.5 20.5 1.2 MCM-41-19-H 12.1 36.7 1.9 MCM-41-3-H 20.3 24.3 1.9 MCM-41-3-A1-H 17.6 37.5 0.9 MCM-41-19-A1-H 17.1 32.9 1.0 TMCM-41-19-A1-H 19.2 44.7 0.5 MCM-41-3-Zr-H 17.2 22.8 2.0 TMCM-41-19-Zr-H (1) 14.8 36.2 1.4 TMCM-41-19-Zr-H (2) 15.3 37.9 1.4 TMCM-41-19-Zr-H (3) 15.8 38.0 1.3 0.176 cm/gmole for the absorption band of S i - N H 2. This last one is in good agreement with the IMEC value (0.177 cm/gmole) obtained with pure Si-MCM-41, where there is only this band. The standard deviation of the total calculated amount of ammonia desorbed from different sites of the samples by using the obtained IMEC values from the expected total quantity, which was measured, is estimated to be smaller than 10%. 3.3. T P A D and FTIR-TPAD profiles The TPAD profile of some MCM-41 samples obtained by QMS are presented in Figure 4. The obtained IMEC values are used to quantify the absorbance bands assigned to ammonia on B, L acid sites and amine sites in the difference FTIR-spectra (obtained during the TPAD procedure) of the investigated samples. This leads to the calculation of FTIR-TPAD profiles, showing the ammonia desorption from B, L and amine sites versus temperature. The FTIRTPAD profile of the total desorbed ammonia of some samples and the sum of Bronsted and Lewis acid sites of the investigated samples are presented in Figures 5 and 6 respectively. Figure 6 represents the acid strength distribution of the examined samples. Figures 4 and 5 show that the FTIR-TPAD profile of the total desorbed ammonia is comparable to the TPAD profile obtained with QMS. The ammonia desorption generally shows a gentle rise to a weak maximum around 300 ~ The calculated amount of ammonia desorbed from different sites of the investigated samples is presented in Table 3. These results show that the adsorption of ammonia on MCM41 samples are distributed on three sites, and that the distribution amount of ammonia desorbed from these sites is approximately comparable. The values reported in the residual column represent the amount of ammonia remaining adsorbed after 550 ~ These are determined from the difference FTIR-spectra. The amount of ammonia attributed to amine sites represents around 30 % of the total adsorbed ammonia showing that the TPAD profile obtained with QMS is not representing the desorption of ammonia from only acidic sites but from amine sites too. This should be taken into consideration when interpreting these TPAD profiles.

1320 600

,. J

i

! !

6_1/ I\

--

//

1'9oZr|

/~

.

400

13oo i 200

~

2E- 11

100

~

0

0 20

,'-',

"'m"r't"r'' L

T~I2~M

4E- 11

0

MCM-41-3-H

.3

40

60

80

Time (rain) Figure 4: QMS desorption curves (m/z = 15) of adsorbed ammonia with a heating rate of 10 ~

-

~

MCM-41-19-H

"x.x. -.-. MCM-41-3-AI-H x'\ -+~ TMCM-41-19-Zr-H (2) t\

'I / \\.\. \~.

il ~

II

//

\ \",

\),

1

0 150

250

350

450

550

Temperature (~ Figure 5" FTIR-TPAD profile of the total desorbed ammonia.

Table 3 and Figure 6 show that the amount of ammonia desorbed from acidic sites of MCM-41-3-H is larger than that of MCM-41-19-H, but the latter sample contains stronger acidic sites than the first. This is in direct relation with the amount of aluminium contained in the sample, indicating that the aluminium content increases the amount of acidic sites but decreases the amount of strong acidic sites. The similar effect is observed by the incorporation of aluminium in the structure of MCM-41-3-H. This can be explained by the ultrastabilisation phenomenon as it is known in zeolite Y and ZSM-5. The comparison of modified MCM-41 materials to MCM-41-3-H and MCM41-19-H samples shows an increase of the amount of acidic sites. The modification of samples with aluminium generates great amounts of both Bronsted and Lewis acid sites, but the modification with zirconium generates more Lewis than Bronsted acid sites. This is due to the fact that the formation of Bronsted and Lewis acid sites is possible by incorporation of aluminium in the framework structure, but the incorporation of zirconium normally forms only the Lewis acid sites. The acid strength distribution shows that the modification of aluminium-rich samples increases the amount of moderate acidic sites more than the medium acidic sites. Contrary to this, the modification of silica-rich samples increases the moderate and significantly- the medium acidic sites as well.

1321 The comparison of acidity of MCM-41-19-A1-H and TMCM-41-19-A1-H samples shows the influence of the nature of the sample used in the modification procedure. TMCM-41-19A1-H obtained from the sample with template contains a larger amount of acidic sites. Generally, the large increase of acidity is obtained with the samples modified by the hydroxy-aluminium complex and the large increase of medium acidic sites is obtained with TMCM-41-19 sample modified by the hydroxy-zirconium complex.

,, ,,'" ""'/

i

" " " ',,

./"

MCM-41-3-H N

".

.

/

~z~

/

.

o

.

MCM-41-3-A1-H

\,\

/

'

~

...... MCM-41-3-Zr-H "

\,\

i J/ 1"'.,[JJ / /../"

+

.

"\,\ X

%-.~'~'~'\\

.-:If/

MCM-41- 19-A1-H

i

TMCM-41-19-Zr-H(2)

"\

/~

1 ~0

~

o

, 01

0

MCM-41-19-H

o TMCM-41-19-A1-H

1" 7 ~

,

--

,~

r

?

T

~

200

250

300

350

400

450

500

550

Temperature (~ Figure 6: FTIR-TPAD profile of the sum [Bronsted + Lewis] acid sites of the investigated samples.

Table 3 Calculated amount of ammonia in mmole/g desorbed from different sites of investigated samples Samples Bronsted Lewis Amine [B + L] Total Residual sites sites sites sites MCM-41-19-H 0.13 0.09 0.09 0.22 0.31 0.12 MCM-41-3-H 0.15 0.16 0.17 0.30 0.47 0.04 MCM-41-3-A1-H 0.26 0.25 0.10 0.51 0.61 0.06 MCM-41-3-Zr-H 0.13 0.24 0.21 0.37 0.58 0.03 MCM-41-19-A1-H 0.17 0.19 0.15 0.36 0.51 0.09 TMCM-41-19-A1-H 0.21 0.19 0.19 0.41 0.60 0.07 TMCM-41-19-Zr-H (2) 0.16 0.19 0.17 0.35 0.52 0.10 TMCM-41-19-Zr-H (3) 0.17 0.20 0.14 0.37 0.51 0.07

1322 4. CONCLUSION The IMEC values obtained in this investigation are 3.03, 1.24 and 0.176 cm/gmole for the absorption bands of ammonia adsorbed on Bronsted and Lewis acid sites and the S i - N H 2 sites, respectively. The IMEC value of S i - N H 2 sites agrees with that obtained with pure Si-MCM-41 (0.177 cm/gmole). These IMEC values permit the determination of the amount, the nature and the acid strength distribution of acidic sites. The part of ammonia desorbed from Si-NH 2 sites in the investigated samples represents around 30 % of the total adsorbed NH3. The desorption curve of ammonia obtained with QMS in the TPAD measurements should not be used as a total acidity distribution. The results obtained from the TPAD measurements are in accordance with those obtained from FTIR spectra. They indicate that the incorporation of aluminium in the structure of MCM-41 samples increases the amount of both Bronsted and Lewis acid sites, but the incorporation of zirconium increases the Lewis acid sites more. The large amount and stronger acidic sites are found in the modified sample obtained by using a silica-rich sample with template in the modification procedure with the zirconium complex. REFERENCES

1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. K. Roos, A. Liepold, W. Reschetilowski, R. Schmidt, A. Karlsson and M. St6cker, Stud. Surf. Sci. Catal., 94 (1995) 389. A. Corma, A. Martinez, V. Martinez-Soria and J.B. Monton, J. Catal. 153 (1995) 25. A. Liepold, K. Roos, R. Reschetilowski, A.P. Esculcas, J. Rocha, A. Philippou and M.W. Anderson, J. Chem. Soc., Faraday Trans., 92 (1996) 4623. H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Fricke, Appl. Catal. A: General, 184 (1999) 49. A. Taouli, A. Klemt, M. Breede, W. Reschetilowski, Stud. Surf. Sci. Catal., 125 (1999) 307. C.A. Emeis, J. Catal., 141 (1993) 347. R. Schmidt, D. Akporiaye, M. St6cker and O.E. Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61. D. Genske, K. Bornholdt, H. Lechert, Stud. Surf. Sci. Catal. 117 (1998) 421. D.E.W. Vaughan, R.J. Lussier, J.S. Magee Jr., U. S. Patent, 4 176 090 (1979). S. Yamanaka and G.W. Brindley, Clays and Clay Minerals, 27 (1979) 119. D.E.W. Vaughan, U. S. Patent, 4 666 877 (1987). M. Busio, J. J~inchen and J.C.H. van Hooff, Microporous Mater., 5 (1995) 211. A. Corma, V. Fornes, M.T. Navarro and J. Perez-Pariente, J. Catal., 148 (1994) 569. B.A. Morrow, I.A. Cody, J. Phys. Chem. 80 (1976) 1998. E.F. Vansant, P. Van der Voort, K.C. Vrancken, Stud. Surf. Sci. Catal., 93 (1995) 383.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1323

C o n f i n e m e n t o f n e m a t i c liquid crystals in S B A m e s o p o r o u s materials L. Frunza a'b, S. Frunza a, A. Sch6nhals c, U. Bentrup b, R. Fricke b, I. Pitsch b and H. Kosslick b %Iational Institute of Materials Physics, PO Box Mg07, R-76900 Bucharest-Magurele, Romania bInstitute of Applied Chemistry, R. Willst~itterstr. 12, D-12489 Berlin, Germany ~ Institute of Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin, Germany This work presents some results obtained by confining octylcyanobiphenyl (8CB) to the pores of two SBA-15 type host materials: a silica SBA-15 (pores of 10.2 nm) and an aluminum containing A1SBA-15 (pores of 7.5 nm). The LC molecules were loaded inside the pores from a solution in acetone, in a percentage higher than 70% from the maximum possible loading. It was observed that the 8CB molecules confined to molecular sieves may preserve the phase transitions characteristic for the bulk LC provided that the confining pores are sufficiently large, as offered by SBA materials. However, present guest-host interactions play an important role in these phase transitions. Such a behavior is at variance with the 8CB confined to Controlled Porous Glass with the same pore dimensions, for which the bulk-like phase transitions can be always observed.

1. INTRODUCTION Since their discovery in 1992 [1], the mesoporous molecular sieves MCM-41 have attracted much interest because of the high surface area, the large pore volume and the welldefined pore size. Potential applications of these materials have been suggested in catalytic reactions involving bulky molecules, such as those encountered in refining industry of heavy fractions, producing fine chemicals and pharmaceuticals [2] as well as in heterogenizing the homogeneous catalysts [3]. MCM-41 molecular sieves [4] (and some microporous zeolites as well [5]) have been also used as host materials for the confinement of liquid crystals (LCs) of the cyanobiphenyl class, because it was expected that the confinement leads to a higher stability of the LC molecules, to a change in their phase behavior and to the generation of new properties compared to the bulk. For pores with diameters of (or less than) 2 nm, the confined LC molecules do not undergo any phase transition known for the bulk LC. Moreover, a much slower dynamics with a temperature dependence characteristic for a glass forming liquid was observed for confined LC [4]. Recently the synthesis of novel mesoporous molecular sieves of SBA-15 type was reported [6,7]. These have larger pores, thicker walls and consequently, higher stability than MCM-41.

1324 Thus these materials become important for other applications as in chemical sensors, water separation processes etc. This work presents some results obtained by confining 8-octylcyanobiphenyl (8CB) to the pores of some SBA-15 and A1SBA-15 materials. A surface layer dynamics is observed. However, 8CB confined in these large pore materials differs from that in the related MCM-41 materials by an additional bulk-like behavior characteristic for the LC. Besides, there are differences between the two loaded SBA adsorbents, most probably related to the particular guest-host interactions.

2. EXPERIMENTAL

The mesoporous molecular sieves SBA-15 and A1SBA-15 were hydrothermally synthesized according to the literature [8], using Pluronic P 123 surfactant. Template organics of as-synthesized materials were removed by calcinations at 773 K in air. There were used some materials with large pores, of 10.2 and 7.5 nm, respectively (Table 1). The 8CB is a nematic LC commercially available (Aldrich). It shows three phase transitions in an accessible temperature interval: crystalline-smectic A (SmA) at 294.1 K, smectic A-nematic (N) at 306.5 K, nematic-isotropic (I) at 313.8 K [9]. The molecular length of the 8CB is ca. 2 nm, whereas the height is 0.67 nm towards the aryl part. A picture of the molecule as obtained by DFT is given in Figure 1. Cyano group is situated along the long axis of the molecule. The LC was loaded inside the pores of SBA materials from a solution in acetone [ 10]. The excess of LC from the external surface of the grains of the molecular sieve was carefully removed by outgassing the sample in vacuum until the LC molecules were located (mainly) inside the pores. The samples were characterized by XRD, nitrogen absorption, electron microscopy, TGDTA, DSC, FTIR and dielectric spectroscopy to investigate the structure of the molecular sieves, the loading with LC, its phase transitions, the vibration modes and the molecular dynamics of the confined molecules. The techniques and the experimental details were described previously [4, 5, 10-12]. Thus, X-ray diffraction patterns were obtained on a STOE powder diffraction system in transmission. Nitrogen absorption was performed with a Micromeritics ASAP 2010 apparatus. Combined TG/DTA curves were recorded on a Setaram TGTDTA92 instrument, in dry air stream at a heating rate of 10 K/min. DSC analysis was completed on a Perkin Elmer DSC-7 apparatus at a rate of 5 K/min under nitrogen atmosphere. FTIR spectra were recorded with a Mattson Galaxy 5020 spectrometer equipped with a microscope or with a Biorad FTS 60A spectrometer connected to a vacuum installation for in situ studies. Broadband dielectric spectroscopy (10 -2 to 109 Hz) measured the complex dielectric permittivity z*: ~*(f) = ~'(f) - i~"(f) where f is the frequency, ~' the real part, ~" the imaginary part, using a Schlumberger frequency response analyzer FRA 1260 and a Hewlett Packard impedance spectrometer HP 4191. During these measurements, the sample temperature was ensured by a nitrogen gas jet and covers a rather large interval (at least 30 K below and over the temperature of phase transitions of the bulk LC). Model function(s) Havriliak and Negami were fitted to the isothermal data. The conductivity contribution to the dielectric loss was described by an additional term ~/fk.

1325 Table 1 Characterization of SBA- 15 adsorbents Sample Si/A1 BET surface, mol ratio m2g1 SBA-15 oo A1SBA-15 9.8 627 *Evaluated on the basis of the pore volume

Pore size, Maximum nm loading with LC, %* 10.2 61.5 7.5 52.7 of the empty molecular sieve.

Figure 1. Sketch of 8CB molecule.

3. RESULTS AND DISCUSSION The main characteristics of the loaded samples are given in the Table 1. It is worthy to note that the adsorption isotherms of N2 at liquid nitrogen temperature, leading to the surface area and pore size exhibited distinct steps due to capillary condensation, which suggests the uniformity of pore size in each adsorbent.

3.1 Thermal analysis It was already demonstrated that TGA provides information on the content of template and water in as-synthesized nanomaterials [13]. We have used combined TG/DTA technique to have an additional look at the surface hydroxyl groups of the SBA samples and especially to pursue their loading with the liquid crystal. Figure 2 presents the results obtained for loaded samples. Physisorbed and hydrogen bonded water is removed up to 473 K (endothermic). On the detemplated unloaded SBA materials, a dehydroxylation process of silanol and A1-OH groups starts (exotermic) above 473 K and proceeds up to 1173 K; on loaded samples, one observe instead the strongly exotermic oxidation of the organic liquid crystal. Besides, 8CB/SBA-15 presents a clear DTA peak at ca. 500 K. Estimated loadings are reported to the corresponding dry samples and are presented in Table 2. DSC measurements were used to study the phase transition temperatures for the liquid crystal embedded in the pores of the SBA materials. The temperature of the N-I transition was clearly shifted downward (Table 2). The freezing temperature under these confined conditions is also depressed when comparing with the bulk, but changes of the other phase transitions are less evident due to peak rounding. The effects of the surface anchoring and finite sizes on the temperatures of phase transitions of the bulk LC confined to restricted geometry is already known: The N-I transition temperature increases if the surface aligns the LC molecules [14] to form a boundary layer more ordered than the bulk. At the same time, the finite size effects decrease the N-I transition temperature and round the heat capacity peak [15]. Besides these effects, extensively studied in regular geometries, in the case of SBA mesoporous materials the systems present an inherent randomness of the pore geometry as it

1326

100

(t; (/)

o -~ .E Ey) (1)

- ......

a)

b)

200

...,,

8O ~- 100 o

60 40

- 8CB/SBA-15 ~ .............. 8CB/AISBA-15

460

a) "1-

.

660 860 Temperature, K

10'00

0

~f..-'/ ..............8CB/AISBA-15 . . . . 80B/SBA-15 460

660 860 Temperature, K

1600

Figure 2. TG (a) and DTA (b) curves for SBA materials loaded with 8CB.

was discussed for aerogels, Controlled Porous Glass and Vycor glass [16]. This randomness is also the source of rounding the peaks. It is quite probably that the mechanism responsible for the changes observed for our loaded samples is related to the finite size and randomness effects.

3.2 FTIR spectroscopy Typical spectra of loaded samples are given in Figure 3. Assignment of the observed bands follows the literature of bulk 8CB [18-20] and related compounds [21]. The interactions between the LC molecules and the SBA host results in the changes in spectral parameters of some fundamental bands of both interacting components. The FTIR spectra in range of the stretching of the OH groups show the formation of H-bonds involving these groups of the molecular sieves while the absorptions in the range of the stretching of CN groups (of the LC molecules) show typical shifts (Table 2) characteristic to bonds with the surface of the molecular sieves, mostly OH groups (shifts of a few cm "1) and also Lewis sites. Such changes in the spectra were discussed also for other composite materials containing the same LC and other molecular sieves [12, 17]. Therefore, it was reasonable to assume that the LC molecules interact with the SBA matrix forming hydrogen bonds of the type Si-OH...NC- in the interface layer. Additionally, in the case of A1SBA-15 materials, a strong shift (30 cm -1) toward higher frequency shows the formation of coordination bonds of the A1...NC- type. The variation of the integrated intensity of the band due to CN stretching vibration as function of the sample temperature is represented in Figure 4. A general tendency to decrease with the increasing temperature is observed. A continuous decrease of the components of the Table 2 Properties of loaded samples Sample Loading found by TGA, % 61.1 8CB/SBA-15 37.6 8CB/A1SBA-15

AT of N-I transition from DSC, K -1 -2.5

AVCN, cm -1 6 4, N30

1327 CN band was also found for the 8CB/A1MCM-41 sample [12]. However, up to 475 K, the decrease of the intensity integrated upon the whole range (2300-2000 cm "1) is rather small, whereas the changes in the intensity of the two main peaks (due to hydrogen bonded and coordinatvely bonded species) are easy to be observed [22]. This means that a surface species is transformed into the other one. Further studies in this direction are in progress in order to correlate the changes in the species type with the appearance of the low temperature DTA peak (at ca. 500 K). 0

5

40-

"g9

60

........... AISBA-15 8CB/AISBA-15

.,....".i " 'v\:,

t'-

-

80

vCN

1oo

/

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15'00

20'00

'

25'00

'

Wavenumbers,

30'00

'

35'00

'

4000

c m "1

Figure 3. FTIR spectra of A1SBA samples.

-4 500o~ t--

=9

"O (9

L_ (9 r m

''%.....

400.

\,,.

300 200 300

400 500 Temperature, K

\.

\.

600

Figure 4. Variation of the vcN band with the temperature.

3.3 Dielectric measurements

Broad band dielectric spectroscopy is a suitable tool to study the molecular dynamics of the confined liquids and particularly, of confined liquid crystals [23-27]. The guest molecules in nanopores are still mobile showing a different dynamics than the bulk LC; particularly in the case of pores of 2 nm; this dynamics is slower [4]. Two representative pictures for the dielectric behavior of the loaded SBA materials in the frequency range where the bulk LC does not present appreciable losses are shown in Figures

1328 5a and 5b. It is obvious from these that silica SBA loaded material indicates only a small absorption at ca. 100 Hz for a temperature of 275 K, which is shifted toward higher frequency

a)

323 K ~,,

o .

0.450-

or

301 K x --~

343 K

o

1.0

%

0.8

0.375 -OOooo275

o

o 0.6

275 K

- -,~,.XXXxx 9

0.300

243 K ~ 208 K

-~,

6

W~

aaaao

b)

v ~

243~ . . . . .

:~ ~, log (f[Hz])

~ o

oo

log (f[Hz])

Figure 5. Dielectric loss in the low frequency range of a) 8CB/SBA-15 and b) 8CB/A1SBA15 sample.

0.375

% o

--

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275 K 2~163

.

%

0.300 a 0.225

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b)

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9

,

7 log (f[Hz])

.

i

8

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Figure 6. Dielectric loss in the high frequency range of a) 8CB/SBA-15 and b) 8CB/A1SBA15 sample. when the temperature increases. This peak might be due to the surface layer with a slow dynamics, as found for MCM-41 materials. A conductivity contribution to the dielectric loss appears at "high" temperatures. At the same time, the spectra of the loaded A1SBA sample are much more complex than those obtained for loaded SBA sample. A dc conductivity contribution seems to be present for these spectra, probably due to some extra-framework A1 species. Careful consideration of all the possible dielectric process is in progress. Besides, the surface layer contribution seems to be also much more important than in the case of silica SBA material. It is noteworthy that the pores of these SBA adsorbents are of cylindrical-like nature and arranged in a parallel way in a honeycomb-like lattice. The absence of the pore channel intersections guarantees that the pore networking effects are negligibly small [28]. Therefore, the observed dynamics is attributed to the movements of the LC molecules inside of singular pores.

1329 The bulk LC presents dielectric loss only in the high frequency domain [9]. Figures 6a and 6b illustrate the behavior of the corresponding loaded samples in the same range. While loaded SBA sample show clearly absorptions at temperatures higher than 294 K, for which the bulk LC is not in a solid (crystalline) state, the loaded A1SBA sample shows again more complex spectra than the correspondent silica sample. However, the parameters characterizing the bulk-like relaxation process are somehow different from those of the bulk LC, probably due to the substrate influence on the confined liquid crystal [29]. A similar behavior was found also for the bulk-like behavior of the extra pore molecules in 8CB/A1MCM-41 sample [4]. Therefore, the dielectric data indicate the presence of bulk-like LC structures inside the pores. A slowing down of the relaxation dynamics assigned to a surface layer is also observed in the dielectric spectra for the loaded samples. To conclude the confinement of a nematic LC in the nanopores of SBA type materials was investigated for the first time. Strong guest-host interactions were put in evidence for two loaded SBA materials by thermal analysis measurements, FTIR spectroscopy and dielectric measurements. There were differences between silica SBA-15 and A1SBA-15 as concerning these interactions. Confined 8CB may preserve the phase transitions characteristic for the LC behavior provided that the confining pores are sufficiently large. However, other properties highly depend on the present guest-host interactions.

Acknowledgements. The financial support of the Deutsche Forschungsgemeinschaft (Project Ko 1639/2-3) is gratefully acknowledged by some of the authors (L.F., H.K.). S.F. thanks the financial support of Romanian Ministry of Education and Research. REFERENCES 1. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Amer. Chem. Soe., 114 (1992) 10834. 2. Y.H. Yue, A. Gedeon, J.L. Bonardet et al., Stud.Surf.Sci.Catal., 129 (2000) 209. 3. L. Frunza, H. Kosslick, H. Landmesser, E. Hoft and R. Fricke, J. Mol. Catal., 123 (1997) 179. 4. S. Frunza, A. Sch6nhals, L. Frunza, H.-L. Zubowa, H. Kosslick, H.E. Carius and R. Fricke, Chem. Phys. Lett., 307 (1999) 167. 5. L. Frunza, H. Kosslick, S. Frunza, A. Sch6nhals and R. Fricke, J. Non-Cryst. Solids to appear 2002. 6. D. Zhao, Q. Huo, J. Feng, B.F.Chmelka and G.D.Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 7. P. Yang, D. Zhao, D. Margolese and G.D.Stucky, Nature, 396 (1998) 152. 8. H. Kosslick, I. M6nnich, E. Paetzold, G. Oehme and R. Fricke, Micropor. Mesopor. Mater. 44-45 (2001) 537. 9. A. Sch6nhals, H.-L. Zubowa, R. Fricke, S. Frunza, L. Frunza and R. Moldovan, Cryst. Res. Technol., 34 (1999) 1309. 10. S. Frunza, L. Frunza, A. Sch6nhals, H. Sturm and H. Goering, Europhys. Lett., 56 (2001) 801.

1330 11. S. Frunza, L. Frunza, A. SchOnhals, H.-L. Zubowa, H. Kosslick and R. Fricke, Stud. Surf. Sci. Catal., 135 (2001) A21P14. 12. L. Frunza, S. Frunza, A. Sch6nhals, H.-L. Zubowa, H. Kosslick and R. Fricke, J. Molec. Str., 563-564 (2000) 491. 13. M. Kruk, A. Sayari and M. Jaroniec, Stud. Surf. Sci. Catal., 129 (2000) 567. 14. P. Sheng, Phys. Rev. Lett., 37 (1976) 1059. 15. M.D. Dadmun and M. Muthukumar, J. Chem. Phys., 98 (1993) 4850. 16. S. Kralj, A. Zidansek, G. Lahajnar, I. Musevic, S. Zumer, R. Blinc and M.M. Pintar, Phys. Rev. E, 53 (1996) 3629. 17. H.-L. Zubowa, H. Kosslick, E. Carius, S. Frunza, L. Frunza, H. Landmesser, M. Richter, E. Schreier, U. Steinike and R. Fricke, Micropor. Mesopor. Mater., 21 (1998) 467. 18. I. Gener, G. Buntinx and C. Bremard, Micropor. Mesopor. Mater., 41 (2000) 253. 19. K. Merkel, R. Wrzalik and A. Kokot, J. Molec. Str., 563-564 (2001) 477. 20. I. Gnatyuk, G. Puchkovska, O. Yaroshchuk, K. Otto, G. Pelzl and T. Morawska-Kowal, J. Molec. Str., 563-564 (2001) 498. 21. H.-L. Zubowa, U.Bentrup, H. Kosslick, R. Fricke, Stud. Surf. Sci. Catal., 125 (1999) 321. 22. L. Frunza, H. Kosslick, U. Bentrup, in preparation. 23. G.P. Crawford and S. Zumer (eds.), Liquid Crystals in Complex Geometries, Taylor and Francis, London 1996. 24. S.A. Rozanski, R. Stannarius, H. Groothues and F. Kremer, Liq. Cryst., 20 (1996) 59. 25. M. Arndt, R. Stannarius, W. Gorbatschow and F. Kremer, Phys. Rev. E, 54 (1996) 5377. 26. G.P. Sinha and F.M. Aliev, Mol. Cryst. Liq. Cryst. 304 (1997) 309. 27. A. Huwe, F. Kremer, J. Karger, P. Behrens, W. Schwieger, G. Ihlein, O. Weiss and F. Schueth, J. Mol. Liq., 86 (2000) 173. 28. K. Morishige and K. Kawano, J. Phys. IV France 10 (2000) PrT-91. 29. L. Frunza, H. Kosslick, S. Frunza and A. Sch6nhals, in preparation.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1331

Synthesis and characterization o f bimetallic G a , A 1 - M C M - 4 1 and Fe,A1MCM-41

R. Bkjega a, C. Nenu a, R. Ganea a, Gr. Pop a, S. ~;erbanb and T. Blasco c aZECASIN s.a., Spl. Independen,tei 202A, Bucharest 77208, ROMANIA bICECHIM, Spl. Independen,tei 202A, Bucharest 77208, ROMANIA CInstituto de Tecnologia Quimica (UPV-CSIC) Avda. De los Naranjos s/n, 46022, Valencia, Spain The structural and acidic properties of bimetallic Fe,A1-MCM-41 and Ga,A1-MCM-41 were compared with the A1-MCM-41. The competitive presence of A1 and Fe/Ga in the MCM-41 walls and the formation of extra-framework A1 and Fe/Ga species induces changes in the nature of the acid sites and therefore of their catalytic properties.

1. INTRODUCTION Since the researchers from Mobil Oil [1,2] had introduced to the scientific community the new family of ordered mesoporous materials of M41S, a great deal of work focused on the ability of tailoring pore size and controlling the chemical compositions of these materials, in order to be used in catalysis research. From this series, MCM-41 due to its uniform arrangement of straight and unconnected channels with large pore size reflected in its large specific surface areas and narrow pore size distributions had received great attention in material science and catalysis. Isomorphous substitution of silicon with metals is common used to create catalytic active sites. In contrast with zeolites, A1-MCM-41 possesses weak- and middle-strength acid sites similar to amorphous alumina-silica [3,4 ]. However, Fe 3+ and in particular Ga 3+ substitution seems to provide a different distribution of the acid site strength [5]. 2. EXPERIMENTAL SECTION 2.1 Synthesis

The metallosilicates mesoporous samples were synthesized according to procedures reported in literature [3,4]. Three silica sources were used: sodium silicate (27% SiO2, 9% Na20, Merck), tetramethylammonium silicate ( TMA/SiO2=0.5 molar ratio, 10% SiO2 ) and fumed silica (98% SiO2, Sigma). The metal sources were aluminum iso-propoxide (Merck), ferric nitrate (Fe(NO3)a.9H20, Merck) or gallium nitrate

1332

(Ga(NO3)3.8H20, Aldrich). The quaternary ammonium surfactant used was hexadecyltrimethylammonium bromide (C16TMABr, Fluka). The syntheses procedure started with the preparation of the tetramethylammonium silicate solution by mixing appropriate amounts of a tetramethylammonium hydroxide solution (25% TMAOH, Aldrich) and fumed silica. Then, the sodium silicate solution, water and fumed silica were added, under continuous stirring, to the tetramethylammonium silicate solution. By adding a 15% C16TMABr solution to the above silicate mixture, under vigorous stirring, a well-homogenized gel was obtained. Finally, an adequate amount of aluminum isopropoxide or either ferric nitrate or gallium nitrate was added into the surfactant-silicate mixture. The molar chemical compositions of the reaction mixtures were: SiO2: 0.07Na20:(0.08-0.10)TMAOH: 0.017Me203: 0.15C 16TMABr: 60H20, where Me stands for A1; Fe, A1 (Fe/AI=I); Ga, A1 (Ga/AI=I) After stirring for one hour at room temperature the synthesis gels having a pH around 12 were loaded into a 500 ml Teflon-lined autoclave and heated at 100~ for 48 hours, under continuous stirring. After cooling to room temperature, the resulting products were repeatedly washed with distilled water until the pH reached 7.5, separated by filtration and dried in air at ambient temperature. The surfactant was removed from as-synthesized product by calcination in air (static conditions) with a heating rate of I~ from room temperature to 550~ and maintained at 550~ for 6 hours.

2.2 Characterization X-ray diffraction The as-synthesized and calcinated samples were characterized by X-ray powder diffraction (XRD) on a DRON-3 diffractometer using a monochromated CuKa radiation. The diffraction patterns were recorded from 1~ to 10~ (20) with a resolution of 0.02 ~ and a count time of 20s at each point. The diffraction peaks were fitted assuming a Voigt profile function.

Spectroscopic techniques

The IR spectra were recorded between 1600 c m "1 t o 400 cm1 on a SPECORD M80 spectrophotometer using KBR pellets technique. The diffuse reflectance DR spectra was recorded from 50000cm 1 to 11000cm- 1 on a SPECORD M40 spectrophotometer equipped with a reflectance attachment 45/0 ~for powder samples. The spectra were fitted assuming a Gauss profile function. The experimental transitions detected for the Ga, AI.MCM-41 sample were compared with theoretical calculated transitions. The theoretical evaluations of transitions positions and oscillator strength were performed using an ab-initio procedure (ARGUS, RHF, STO6G). The geometry of GaO4" was optimized by molecular mechanics methods.

27A! MAS NMR

The 27A1MAS NMR spectra were recorded at ambient temperature on a VARIAN VXR-S 400 WB spectrometer working at 104.2 MHz with a Doty XC4 probe. The samples were packed in 4 mm silicon nitride rotors and span at c.a. 15 KHz. The

1333

acquisition was carried out using pulses o f 0.5 ~ts corresponding to a flip angle of x/18 tad and delays of 0.5 ~s were used. The chemical shifts are reference towards At(H20)0 +. Acidity

measurements

The acid contents were measured by thermogravimetric studies on samples saturated with cyclohexylamine, prepared following the methodology described by Mokaya & Jones [6]. The cyclohexylamine thermodesorption curves were record_e-d_ using a DuPont 951-thermogravimetric analyzer with a heating rate of 20~ under argon flow. 3. RESULTS AND DISCUSSION The X-ray patterns of the as-synthesized/calcinated samples are typical for a highly ordered mesoporous MCM-41 with four well resolved Bragg reflections (as fig. 1 shows), which can be indexed in a pseudo-hexagonal symmetry with hkl triplets of 100, 110, 200 and 210.

::::i

Fe,~l

0

9

I

2

"

I

4

"

!

6

"

I

8

"

20 C~Ka

Figure 1. XRD patterns of calcinated MCM-41 samples. There are slight modifications of the structural data as a result of the incorporation of the either Ga or Fe in the mesoporous structures accounting for the built in the walls of an important part of Ga or Fe. These are: - an increase of the lattice parameter (ao=2d100/~/3) especially for the Fe, A1-MCM-41 a slight decrease of the intensity of the 100 reflection (110o) in the order A1-; Ga, A1; Fe, A1-MCM-41 which might be described as a loss in the "crystallinity" as a result of an accommodation with larger ions.

1334 an increase of full width at half maximum height of the 100 reflection (FWHM100) in the same order, A1-; Ga,A1-; Fe,A1-MCM-41, as a sign of the distortion of long range order, probable due to the lack in the homogeneity of walls chemical composition, due to Ga or Fe incorporation. an increase of the relative intensities of the 110 diffraction peak to 100 peak ,in the same order A1-; Ga,A1-; Fe,A1-MCM-41, marking an increase of the wall thickness, in agreement with the model structure for MCM-41 proposed by Feuston & Higgins [7]. That means that the increase of the lattice parameter is probable due to the increase of the wall thickness. The structural data are gathered in Table 1. Table 1. Structural data of the .....Sample .............. A1-MCM-41 Ga,A1-MCM-41 Fe,A1-MCM-41

calcinated MCM-41 sample. ao (~) i10o (aml) FWHM ( ~ 47.33 12.8 0.30 47.44 11.0 0.32 51.54 7.2 0.52 .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

ili0/Ii00 0.059 0.056 0.055

However, from the structural results from table 1 it could be concluded that Ga accommodates better in the framework structure than Fe that induces a higher degree of disorder. The IR spectra of the calcinated samples are characteristic to MCM-41 structures and show no major modifications as an effect of Fe or Ga incorporation. The 960 cm 1 band assigned to Si-OH groups (Si-O- groups [8]) appears in all the samples. Some authors [9] associated this band directly with the mesoporous order in the MCM-41 materials, namely with the 100 peak intensities but there is no clear such relationship in our samples, probably due to the quite highly ordered structures.

, A I - M C M - 4 1

M C M - 4 1

A I - M C M - 4 1

.---

8 r0~

e'~

U 200' 4;0' 6;0' 8;o ' 10'00' 12'00' 14'00' 16'00' 18'00' 20'00 wavenumber (cm-1)

Figure 2. IR-spectra of calcinated MCM-41 Fe/Ga,Al-samples.

1o0o6 ' 2dooo ' 3o;oo ' '4o;oo ' 5o;oo ' wavenumber((xn-1)

Figure 3 DRS spectra of the MCM-41 calcinated samples.

1335 The diffuse reflectance spectra for the calcinated Fe,A1-MCM-41 and Ga, A1MCM-41 are presented in figure 3. The spectra of Fe, A1-MCM-41 in both assynthesized and calcinated forms presezat the char_~e transitions assigned-to species FeO4- (at 46800, 38800, 32400, 19600 cm- ) [t0]. Upon calcination very weak transitions bands (at 26100 and 12800 cm-~) assigned to octahedral iron species appeared. We observed that after calcination there is a slight change in colour from white to very pale beige and therefore, one could assume that a weak process of iron extraction from the framework occurred~ The assignment of the transitions bands of the Ga, A1-MCM-41 was performed by comparison with theoretical evaluations as described in the Experimental Section. The intense metal to ligand charge transitions from 36960, 43000, 47770, 275-60 cm1 were attributed t o a tetrahedral environment of gallium. In addition with these intense bands very weak transitions band (at 22324and 31412 cm1) appeared only in the spectrum of the eatcinated Ga, At-MCM-4t . They might be assigned, similar to the Fe, A1-MCM-41 sample, to extra-framework octahedral gallium species. Nevertheless, even the DRS spectra indicate the presence few amounts of extra-framework metal species in the calcinated samples the presence of extra-framework oxide species cannot be excluded [11 ]. 27A1 MAS NMR spectroscopy was used to investigate the coordination of aluminum into the framework after the calcination procedure~ The 27 A1 MAS NMR spectra of the calcinated samples are presented in fig. 4. The predominant peak for all the samples is around 53-57 ppm and is typical for a tetrahedral coordinated A1 [12,13] (fig.4). A peak around 0 ppm due to octahedrally coordinated A1 is observed. Its relative intensity increases in the order A1; Fe, A1; Ga, A1-MCM-41. A second octahedrally coordinated AI appears clearly in the Fe,A1-MCM-41, around -21 ppm, while the peak is asymmetric in the Ga, A1MCM-41 spectra. The peak could tentatively be related to the presence of heteroatoms into the walls. Additionally, we considered a very broad peak underlining the entire spectrum that might be assigned to extra-MCM-41 amorphous aluminum oxide/hydroxide species. The intensity (area under the peak) of this very broad peak is the highest for Fe, A1-MCM-41 samples in agreement with its less structured and lower intensities XRD pattern. The acidity measurements are based on the thermogravimetric desor~on of cyclohexylamine (CHA) from 20~ to 800~ under argon flow. The TG curves are quite similar as figure 5 presents. From the DT curves some temperature steps could be distinguished in spite of the complexities of DT curves and the difficulties to evaluate the strength of acidic sites. The first region up to around 200~ degree is due to water desorption. The amounts of acid site were evaluated in terms of mmol CHA/g of dehydrated material. The results appear in Table 2. The first weight loss could be associated with weak acid sites probably Si-O- groups. The second step, of medium strength (Lewis or Broensted), could be associated with extra-framework A1 or Fe (Lewis) as is claimed that Ga induces strong Lewis sites [5] or Ga/Fe isomorphous

1336

~(~ sv(r-e,~:<3o

I

'

A00

I

'

200

I

'

0

I

-200

'

I

-403

Figure 4.27A1 MAS NMR spectra of the calcinated MCM-41 samples. Asterisks denote spinning sidebands.

100 .---.

o~ Or;

r O

t-r ._

96 A I-M C M -41 96

94 Ga,AI-MC 92

Fe,AI-M

CM-41

90

88

'

i 200

,

i 400

,

680

9

i 800

tern perature (oC)

Figure 5. TGA plots of cyclohexylamine thermodesorption in argon flow

1337 substituted sites (Broensted). The larger amount of such sites obtained for Fe,A1-MCM-41 and especially for Ga, A1- MCM-41 is to be ~ in correlation with the higher amotmt of A1 in an octahedral coordination as the 27A1 MAS NMR data suggested and with flae ability of iron and particular of gallium for isomorphous substitution in the tetrahedral framework positions of the MCM-41 as the XRD and DRS data stand for. The third peak is probably due to strong Broensted acid sites in connection with tetrahedral coordinated framework aluminum. The high temperature weight loss is probably provided by strong Lewis sites generated by extra-framework species A1, Fe and Ga or even which are formed during the desorption process itself. However is difficult to rely on the amount of acid sites evaluated from weight loss measurements because even at high temperature, water is still lost via dehydroxylation reaction (condensation of silanol groups) [ 14,15]. The thermal stability of these samples will be checked. Table 2. Acidic measurements. Sample AI-MCM-41 temperature step (o) amount of acid sites (mmol CHA/g)

I 181-319 5.03

II 319-385 1.81

III 385-472 6.15

IV 472-700 58.4

Fe, A1-MCM-41

temperature step (o) amount of acid sites (mmol CHA/g)

202-302 3.19

302-387 2.55

387-450 3.03

450-760 44.42

Ga, A1-MCM-41

temperature step (o) amount of acid sites .(mm~ C H M ~

191-320 4.82

320-402 5.44

402-440 3.41

440-700 42.83

Benzene alkylation reaction with propanol was selected for catalytic activity test. The results of the catalytic test could be resume in some conclusion [ 16]: the most active catalyst is A1-MCM-41 - propanol consumption in alkylation is maximum for Ga, A1-MCM-41 Fe, A1-MCM-41 has a better stability in time - selectivity towards di- and tri-isopropylbenzene is maximum for Fe,A1-MCM-41 - selectivity calculated as the sum of mono and poly-propylbenzenes is maximum for Fe, A1-MCM-41. -

-

4. CONCLUSION Our studies on the synthesis and characterization of bimetallic Fe, A1-MCM-41 and Ga, A1-MCM-41 allow us to draw some conclusions: - well defined ordered mesoporous Ga, A1; Fe, AI-MCM-41 are formed. the presence of iron or gallium in competition with aluminum induces a slight distortion of long ordering of mesoporous structure, especially for the Fe, A1-MCM-41 sample, marking the incorporation of Fe or Ga into the walls.

-

1338 the presence of iron or gallium in competition with aluminum induces a slight distortion of long ordering of mesoporous structure, especially for the Fe,A1-MCM-41 sample, marking the incorporation of Fe or Ga into the walls. extra-framework (extra-wall) A1 amount is higher in bimetallic Fe,A1-MCM-41 and Ga,A1-MCM-41 in comparison with the A1-MCM-41 sample. Two extra-framework octahedral coordinated A1 species appeared, apparently in connection with Fe/Ga isomorphous substituted sites. these detected structural changes induce a change in the nature of acid sites and therefore of their catalytic properties.

-

-

-

R

E

F

E

R

E

N

C

E

S

1. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, A. Schlenker, J. Am. Chem. Soc.,114 (1992) 10834. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. 3. X.S. Zhao, G. Q. Lu, G. J. Millar, Ind. Eng. Chem. Res., 35 (1996) 2075. 4. F. Schfitch, Ber. Bunseges. Phys. Chem., 99 (1995) 1306. 5. H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Frike, Applied Catalysis A: general 184 (1999) 49. 6. R. Mokaya and W. Jones, J. Mater. Chem., 9 (1999) 551. 7. B.P. Feuston and J. B. Higgins, J. Phys. Chem., 98 (1994) 4459. 8. M.A. Camblor, A. Corma, J. P6rez-Pariente, J. Chem. Soc., Chem. Commun., (1993) 557. 9. R. Melo, E. Urquieta-Gonzfilez in Zeolites and Mesoporous Materials at the Dawn of the 21th Century, Stud. Surf. Science and Catal. ,135, A. Galarneau, F. Di Renzo, F. Fajula, J. Vedrine (eds.), 06p 19 10. D. H. Lin, G. Coudurier and J. Vedrine, Stud. Surf. Sci. Catal., 49 (1989) 1431. 11. W.Carvahalo, P. Varaldo, M. Wallau, U. Schuchardt, Zeolites, 18 (1997) 408. 12. Z. Luan, C.-F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 13. R. Schmidt, D. Akporiaye, M. St6cker, O. H. Ellestad, in Zeolites and Related Microporous Materials: State of the Art 1994, Stud. Surf. Science and Catal., 84, J. Weitkamp, M. G.Karge, H.Pfeifer, W.H61derich (Eds.), Elsevier Science B.V. Amsterdam, (1994) 61. 14. I. Petrovic, A. Navrotsky, C-Y. Chen, M. Davis, in Zeolites and Related Microporous Materials: State of the Art 1994, Stud. Surf. Science and Catal., 84, J.Weitkamp, M.G.Karge, H.Pfeifer, W.H6lderich (Eds.), Elsevier Science B.V. Amsterdam, , (1994) 677. 15. M. Keene, R. Gougeon, R. Denoyel, R. Harris, J. Rouquerol, Ph. Llewellyn, J. Mater. Chem., 9 (1999) 2843. 16. Gr. Pop, R.Ganea, I. R. Tamas, R. Birjega, proceedings of the 7th European Conference on Advanced Materials and Processes, EUROMAT-2001, 10-14 June, 2001, Rimini, Italy, ref.331

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1339

Fischer-Tropsch synthesis on iron catalysts supported on MCM-41 and MCM-41 modified with Cs. A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramihn and S.G. Marchetti

CINDECA, Fac. Cs. Exactas, Fac. Ingenieria, U.N.L.P., CIC, CONICET. Calle 47 N ~ 257 (1900) La Plata, Argentina. MCM-41 and MCM-41 modified with cesium were synthesised and utilised as supports of iron species, to be used as catalysts in the Fischer-Tropsch reaction. X-Ray Diffraction (XRD), Specific Surface Area (BET), Mrssbauer Spectroscopy (MS) in controlled atmosphere, between room temperature (RT) and 15 K, CO chemisorption and Volumetric Oxidation (VO) were used to characterise the solids. The cesium presence produces lower iron reducibility and a unique fraction of very small metallic iron crystals. These structural properties lead to a high turnover frequency and an increase of the selectivity towards light olefins. 1. INTRODUCTION Metallic supported catalysts have been extensively used in the Fischer-Tropsch synthesis fiTS) up to date. Notwithstanding, the design of catalysts with high activity, good selectivity towards to some interesting products like as light olefins and with an important structural and functional stability is a subject to be solved. Amorphous solids, such as A1203, SiO2, C, etc [1, 2], were used as metallic catalyst supports in the Seventies. However, these systems show a broad distribution of metallic crystal sizes, sometimes displaying multimodal distributions. This fact introduces an additional complication since some of the reactions involved in the FTS are "sensitive to the structure" [3] and the presence of a broad range of particle sizes contributes to the selectivity loss. Another difficulties, especially when iron is used, appear when crystalline solids of high surface area, such as the zeolites, are used as support. When these systems are subjected to reduction treatments in order to obtain the desired uniform-dispersed metal particles inside the zeolite structure, Fe migrates out of the pores [4] by mechanisms which are not understood yet[5]. When this fact happens, the beneficial effect of the zeolite channel structure no longer influences the outcome of the reaction. Recently, the appearance of new materials with unimodal and very narrow pore size distribution and pore diameter larger than the zeolite ones, named mesoporous solids, has attracted the attention as metal support. In order to obtain iron catalysts with a very narrow distribution of metallic crystal size we use a mesoporous material named MCM-41, as support synthesised following one of the Beck's recipe [6]. Beating in mind that a high basicity enhances the olefin yield, we prepared other catalyst of iron on MCM-41 modified with Cs.

1340 2. EXPERIMENTAL SECTION Sodium silicate, in acidic medium, as silica source and cetyltrimethylammonium bromide as surfactant were used to obtain the support according to synthesis of Beck et al [6]. The solid was characterised by Specific Surface Area (BET), X-Ray Diffraction (XRD) and Thermal Gravimetric Analysis (TGA) The MCM-41 was split in two fractions. One of them was dry impregnated with a CsCH3COO water solution, at a concentration high enough to yield a solid with 2% Cs20 loading. This solid was dried and subsequently calcined in air stream (150 cm3/min) from 298 to 773 K at a heating rate of 0.2 ~ and kept at 773 K for 1 h. This sample was named CsMCM-41. Both, Cs-MCM-41 and MCM-41 were dry impregnated with Fe(NO3)3.9H20 water solution, at a concentration high enough to yield a solid with c a . 5% w/w of iron. The samples were dried and subsequently calcined in dry N2 stream (60 cm3/min) from 298 to 598 K at a heating rate of 0.2 ~ and kept at 598 K for 1 h. The precursors pFe/MCM-41 and p-Fe/Cs-MCM-41 were characterised by Atomic Absorption Spectroscopy, BET, XRD and Mrssbauer Spectroscopy (MS) at room temperature and 15 K. Both precursors were reduced in 1-/2 stream (120 cm3/min) from 298 to 698 K at 0.28~ and kept at 698 K for 26 h. The catalysts c-Fe/MCM-41 and c-Fe/Cs-MCM-41 were characterised by CO chemisorption, volumetric oxidation and MS in a controlled atmosphere at 298 and 15 K. Measurements of activity and selectivity were carried out in a fixed bed MCM-41 reactor at 543 K, H2:CO ratio of 2:1, atmospheric pressure, 20 cm3/min of total volumetric flow and a space rate of 0.14 s1. The reaction products were analysed by gas chromatography. v

t,--

t,--

m

I...

-

.% 9

.

~_

1

3. RESULTS AND DISCUSSION

Cs-MCM-41

2

.

"

p-Fe/MCM-41

3

4

5

6

7

CI

8

9

20 (o)

Figure 1 X-ray diffraction patterns of MCM-41, Cs-MCM-41 and p-Fe/MCM-41 before and/or alter HC1 treatment.

The X-ray diffraction pattern of the MCM-41 (Figure 1) shows an intense peak at 20 = 2.10 and two additional peaks, less intense, at 20 = 3.8, and 4.30 characteristics of an hexagonal arrangement [6]. The textural properties of this solid are: 29 A pore diameter, and 1084 m2/g of surface area (Sg). Combining the XRD and BET results a 25 A of wall thickness was obtained. During one of the preparation steps of Cs/MCM-41, the Cs introduction produced the hexagonal arrangement destruction, since the peak at 20 = 2.10

1341 disappeared and was not restored after an extraction of cesium compound with a .~ 9 p-Fe/Cs-MCM-41 {:= . . . . . p-Fe/MCM-41 HC1 solution at room temperature. AHowever, an unimodal and narrow pore size distribution was retained, as it was "0 demonstrated by the N2 adsorption measurements. The textural properties of "0 this sample were: 17 /~ pore diameter, and 643 m2/g of surface area. In this sample the Cs would be as Cs20 and/or 0 20 40 60 80 100 120 140 160 180 Cs2CO3, due to the high capacity of Pore Radii (A} Cs20 to react with the CO present in the air. Figure 2: Pore size distribution of p-Fe/MCM-41 The X-ray diffraction pattern of pand p-Fe/Cs-MCM-41 Fe/MCM-41 does not show any peak. This result is attributed to the interruption of the crystallographic planes periodicity due to the presence of Fe oxides inside the channels of the mesoporous structure. This conclusion was obtained by the restoring of the peak at 20 - 2.20 in the XRD pattern after the Fe compounds extraction with hot solution of HC1. The following textural properties were determined in pFe/MCM-41 730 m2/g of Sg, and 25 A_of pore diameter. The BET results in p-Fe/Cs-MCM-41 were 445 m2/g of Sg and 17 A_of pore diameter. The maintenance of the unimodal and narrow pore size distribution was confirmed by the adsorption isotherm (Figure 2). In order to obtain a reliable .. .." value of the average pore diameter, several measurements, with small pressure increments, in the low pressure range were realised. The TGA of both precursors ~M~l showed a weight loss in two steps. The first one between 298 and 373 K assigned to the water surface molecules elimination, and the second one between 546 and 923 K corresponding to the irreversible loss of water associated to the sylanol groups. The M6ssbauer spectra of both precursors (Figure 3) show one doublet ~1~41 p-Fe/Cs-lVlCM-41 at 298 K. When the temperature decreased to 15 K, the spectra of both samples displayed two signals: a - 1 2 - 8 -4 0 4 8 12 - 1 2 - 8 -4 0 4 8 12 paramagnetic doublet and a weak and Ve~oc~y(mn~s) Ve~odty (mn~$) broad magnetic sextet. In p-Fe/MCM41 the fitting was obtained with three Figure 3" M6ssbauer spectra of the precursors at signals: one doublet and two sextets, 298 and 15 K one of them very broad. Instead, in 9

.

"

1342 Table 1: M6ssbauer hyperfine parameters of the precursors. Temp. 298 K

Species t~-Fe203 and/or Fe 3+ ~-Fe203

Parameters 5(mm/s) A(mm/s) 15 K H(T) ~5(mm/s) 2e(mm/s) t~-Fe203 H(T) 15(mm/s) 2e (mm/s) Fe 3+ ~5(mm/s) A(mm/s) *Constant used for the fit.

p-Fe/MCM-41 0.35 + 0.01 0.86 + 0.01 53.2 + 0.2 0.50 + 0.02 -0.16 + 0.05 46.3* 0.47* 0.00" 0.46 + 0.01 0.93 + 0.01

p-Fe/Cs-MCM-41 0.40 + 0.01 0.80 + 0.01 45 + 1 0.5 + 0.1 -0.2 + 0.2

0.56 + 0.02 0.95 + 0.02

p-Fe/Cs-MCM-41 only two signals were necessary: one doublet and a sextet with the background significantly curved. The values of the hyperfine parameters at both temperatures (Table 1) are characteristic of microcrystals of ct-Fe203 and/or paramagnetic Fe 3§ ions coordinated with O--. These O-- ions would become from the dehydroxilation of the sylanol groups located on the MCM-41 surface. The dehydroxilation was verified by TGA experiments, as it was mentioned above. At 15 K, in p-Fe/MCM-41, it was epossible to determine the existence of two .o (n (n magnetic signals assignable to the "core" c-Fe/Cs-MCM.41 c-Fe/MCM-41 (sextet with higher magnetic field) and 298K "shell" (sextet with smaller magnetic field) of c/,-Fe203 "clusters" [7]. Assuming homogeneous semi-spherical particles and using the ratio of the areas of the two sextuplets, it was possible to estimate an average "cluster" diameter of 2.1 nm. Therefore, these "clusters" could be c-Fe/I~M-41 c-Fe/Cs-I~M-41 located inside the channels of the MCM41. This result, analysed in connection with the XRD and BET results, mentioned above, confirm the existence of iron oxide -12-8-4 0 4 8 12 - 1 2 - 8 -4 0 4 8 12 microcrystals situated inside the Wodty (minis) veoety (mn~s) mesoporous solid structure. The remainder doublet was assigned to Fe 3+ Figure 4: M6ssbauer spectra in controlled coordinated with the support, and/or atmosphere of the catalysts at 298 and 15 K superparamagnetic r particles. I .'".:"

,: ...',. 9 "

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1343 Table 2: Hyperfine MOssbauer parameters of c-Fe/MCM-41 and c-Fe/Cs-MCM-41. Species

Parameters

Fe~

H (T) '5 (mm/s) 2e (mm/s) A (mm/s)

Fe 3+

c-Fe/MCM-41 298 K 15 K 33.0 + 0.2 34.1 + 0.2 0.00 + 0.02 0.13 + 0.03 -0.02 + 0.04 -0.01 + 0.05

6 (ram/s) Fe 2+

A (mm/s) ,5 (mm/s) Fe~ ,5 (mm/s) *Constant used for the fit

1.78 + 0.03 1.17 4- 0.01 -0.02 • 0.02

2.16 + 0.02 1.33 • 0.01 -0.02 • 0.01

c-Fe/Cs-MCM-41 298 K 15 K

1.03 + 0.02 0.38 + 0.01 1.93 + 0.03 1.09 + 0.02 0.00 + 0.00"

1.15 + 0.02 0.47 + 0.01 2.20 + 0.02 1.28 + 0.01 0.00"

In p-Fe/Cs-MCM-41, the same iron species were found, but the crystal size of the otFe203 "clusters" must be smaller than in p-Fe/MCM-41 since at 15 K, the magnetic order was not reached yet. The Mrssbauer spectra in controlled atmosphere of both reduced solids, at 298 and 15 K, show three components (Figure 4): in c-Fe/MCM-41, a singlet, a doublet and a magnetic sextet are present. Instead, in c-Fe/Cs-MCM-41, a singlet and two doublets are displayed. The singlet can be assigned to the presence of superparamagnetic Fe ~ (Fe~ [8, 9], the sextet to magnetic Gt- Fe ~ (Fe~ the doublet with the higher isomer shift to Fe 2+ [10] and the remainder doublet to Fe 3+. Tables 2 and 3 show the parameter values and the species percentages (assuming equal recoilless fractions) respectively. Since in c-Fe/Cs-MCM-41 the Fe~ fraction does not appear, we can deduced that in this catalyst, the metallic crystal size is smaller than in c-Fe/MCM-41. Besides, the lower reducibility of the c-Fe/Cs-MCM-41 system, in comparison with c-Fe/MCM-41, may be attributed to the Cs presence inside the pore structure of the MCM-41. This lower reduction capacity is verified by the Fe 3+ presence in c-Fe/Cs-MCM-41. To estimate roughly the average size of the two Fe ~ crystal fractions in c-Fe/MCM-41, we combined the Collective Magnetic Excitation Model [ 11 ] (CMEM) and the CO chemisorption experiments. The Fe~ diameter, estimated using CMEM, assuming that all the magnetic hyperfine field diminution is due to the very small crystallite size [12], is about 21.8 nm. Taking into account the percentage of Fe~ from MS (Table 3) and considering semi-spherical particles of 21.8 nm diameter, it was possible to calculate the theoretical amount of CO that this fraction is able to chemisorb. Assuming a stoichiometry Fe:CO=2:1, a quantity of 2 lamol CO/goat was obtained. We have already mentioned [ 13] that, in amorphous silica, Fe 2+ ions do not chemisorb CO. Since, MCM-41 is made up of amorphous silica walls, the CO consumption by the Fe~ fraction can be obtained subtracting the theoretical CO uptake of Fe~ from the experimental Table 3: Percentages of iron species obtained by M6ssbauer spectroscopy at 15K.

c-Fe/MCM-41 c-Fe/Cs-MCM-41

Fe~ 7•

Species (%) Fe 2+ Fe 3+ 81+3 60+4 22+ 1

Fe~ 12+1 18+3

1344 Table 4: Values of experimental and theoretical 02 uptake and CO chemisorption.

c-Fe/MCM-41 c-Fe/Cs-MCM-41

Exp. 02 uptake (~tmol O2/g) 353 • 18 319 • 16

Theor. 02 uptake (lxmol O2/g) 393 • 293 • 23

Exp. CO uptake (~mol CO/g) 75 • 3 50 • 2

one (Table 4). The average diameter calculated, assuming semi-spherical shape and equal size for all particles of this fraction, is 1.1 nm. This value is coherent with the presence of Fe~ at 15 K [141. Considering the dimensions of the regular arrangement of the support, in c-Fe/MCM41, the Fe~ fraction must be located outside the channels. Beating in mind: (i) the diminution of the pore size and the specific surface area after impregnation and calcination steps, (ii) the XRD results mentioned above and (iii) the particle diameter obtained for Fe~ this fraction would be located inside the channels of the support. In c-Fe/Cs-MCM-41, there is only one size fraction for Fe ~ crystals, since the Fe~ fraction is not present in the MOssbauer spectrum of this catalyst. Therefore, the size estimated for Fe~ from the CO chemisorption results (Table 4), assuming semi-spherical particles, is 1.4 nm. The experimental O2 uptake necessary for the complete reoxidation of the reduced samples is shown in Table 4. There is a good agreement between these values and the theoretical O2 consumption calculated from the percentages of each species obtained from MS at 15 K assuming equal ffactors for all iron species. The crosschecking of volumetric oxidation results with the complex M6ssbauer spectra of these catalysts is the only reliable method [ 15] to verify the M~ssbauer species assignments. The Table 5 shows the activity and selectivity results at different reaction times for cFe/MCM-41 and c-Fe/Cs-MCM-41 in Fischer-Tropsch reaction. Comparing c-Fe/MCM-41 with c-Fe/Cs-MCM-41 at the same reaction time and space rate, it can be seen that the first one presents a CO conversion higher than the second one. The activity per site at the first reaction hour is higher for c-Fe/Cs-MCM-41. However, when the pseudo-steady state is reached, this trend is reversed. Analysing the selectivity results of both catalysts, c-Fe/Cs-MCM-41 produced a higher percentage of CH4 and a higher olefin/paraffin ratio. Finally, there is a low chain growth, as it can be deduced from the low Schultz-Flory coefficient, for both catalysts. The higher initial activity per site of the c-Fe/Cs-MCM-41 can be attributed to the Cs + ions presence in the solid. Theoretical models have demonstrated that the main effect of cations in contact with a metal is electrostatic [ 16, 17], which is essentially of short range. However, a long range effect is possible as a result of a cumulative electrostatic field, generating zones of minimum potential energy at the surface. Consequently, the bond between the Fe ~ and the CO adsorbed becomes stronger, while at the same time, the intra-molecular CO bond is weakened, increasing the catalyst activity. This increased dissociative effect leads towards to a major carbon deposition. Therefore, the catalyst deactivates rapidly, and its activity per site results lower than in c-Fe/MCM-41 at the pseudo-steady state. The higher basicity of c-Fe/Cs-MCM-41 means that less of the weakly bound, active hydrogen is available at the surface and hence hydrogenation activity is lower. This causes a higher content of olefin in the hydrocarbon products [ 18, 19]. Finally, the low cz coefficient values and the high methane production in both catalysts

1345 Table 5: Activity and Selectivity measurements. Reaction Conv. Prod. O/P Cl (%) ot (S-F) Time(h)_............(~ . ...................HC xl_.P4!s!te-s.~................................................................. c-F e/MCM-41 1 2.7 15.5 1.35 45 0.33 12 1.9 11.2 1.55 47 0.30 24 1.8 11.0 1.58 48 0.29 c-Fe/Cs-MCM-41 1 1.9 17.6 2.27 56 0.28 12 0.8 9.2 3.66 66 0.32 24 0.6 7.8 2.01 76 0.13 Conv.: CO conversion; Prod. HC/site.s.: total Hydrocarbon molecules per site of catalyst and per see.; O/P: C2:-C5~/C2-C5 ratio; C1: methane percentage; ot (S-F): Schultz-Flory coefficient Sample

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can be justified taking into account that on very small metallic particles, the chain propagation finishes at low molecular weight hydrocarbon (up to C4) [20]. The small metallic crystal size avoids the presence of enough CHx neighbour groups to produce the propagation chain. This effect is more noticeable in c-Fe/Cs-MCM-41, where the fraction of big metallic iron crystals is not present. 4. CONCLUSIONS Two active catalysts for the Fischer-Tropsch synthesis were obtained when the iron was supported on MCM-41 and MCM-41 modified with Cs. The more important structural differences between them are the lower reducibility and the absence of a big metallic iron crystal fraction, attributed to the Cs presence in c-Fe/Cs-MCM-41. Both characteristics should lead to a low activity per site in CO hydrogenation. Notwithstanding, the initial activity per site is higher in this catalyst due to the promoter effect of the Cs§ ions. The marked increase of the selectivity towards light olefins in c-Fe/Cs-MCM-41 is attributed to the higher basicity of this support. In both catalysts, the chain growth finishes at light hydrocarbons, due to the very small metallic crystal size. ACKNOWLEDGEMENTS The authors acknowledge support of this work by Consejo Nacional de Investigaciones Cientificas y T6cnicas, Agencia Nacional de Promoci6n Cientifica y Tecnol6gica (PICT 14-04315), Comisi6n de Investigaciones Cientificas de la Provincia de Buenos Aires and Universidad Nacional de La Plata, Argentina. They also thank to Qca. Myriam Mihdi for her contribution in the experimental section of this paper. Helpful discussions and encouraging comments by Dr. J.L. Fontes Monteiro that improved the presentation of this paper gratefully appreciated. REFERENCES

1. M.A., Vannice, J. Catal., 37 (1975) 449. 2. J.A. Amelse, J.B. Butt, and L.H. Schwartz, J. Phys. Chem., 82 (1978) 5.

1346 3. M.A. Mc Donald, D.A. Storm, and M. Boudart, J. Catal.,102 (1986) 386. 4. T. Lin, Ph.D. dissertation, Northwestern University, Evanston, Illinois, August (1984). 5. P. A. Jacobs in: Metal Microstructures in "Zeolites. Preparation-Properties-Applications", Studies in Surface Science and Catalysis. Eds. P. A. Jacobs, N. Y. Jaeger, P. Jim and G. Schulz-Ekloff, Elsevier, Amsterdam, 12 (1982) 71. 6. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 7. M. Vasquez-Mansilla, R.D. Zysler, C. Arciprete, M.I. Dimitrijewits, C. Saragovi, J.M. Greneche, J. of Magnetism and Magnetic Materials, 204 (1999) 29. 8. F. Bodker, S. Morup, C.A. Oxborrow, S. Linderoth, M.B. Madsen and J.W. Niemantsverdriet, J. Phys. Cond. Matt. 4 (1992) 6555. 9. J.W. Niemantsverdriet, A.M. van der Kraan, W.N. Delgass and M.A. Vannice, J. Phys. Chem. 89 (1985) 67. 10. B.S. Clausen and H. Topsoe, Appl. Catal. 48 (1989) 327. 11. S. Morup and H. Topsoe, Appl. Phys. 11 (1976) 63. 12. F. Bodker, S. Morup and J.W. Niemantsverdriet, Catal. Lett. 13 (1992) 195. 13. A.M. Alvarez, S.G. Marchetti, M.V. Cagnoli, J.F. Bengoa, R.C. Mercader and A.A. Yeramihn., Applied Surface Science, 165 (2000) 100. 14. F. Bodker, S. Morup, M.S. Pedersen, P. Svedlindh, G.T. Jonsson, J.L. Garcia-Palacios and F.J. Lazaro, J. Magn. Magn. Mater., 925 (1998) 177. 15. S.G. Marchetti, M.V. Cagnoli, A.M. Alvarez, J.F. Bengoa, R.C. Mercader and A.A. Yerami~m, Appl. Surf. Sci., 165 (2000) 91. 16. V. Bonacic-Koutecl~, J. Kouteclc~, P. Fantucci and V. Ponec, J. Catal., 111 (1988) 409. 17. J.W. Niemantsverdriet, "Spectroscopy in Catalysis" VCH, Weinheim (1995) p. 219,. 18. N.G. Gallegos, A.M. Alvarez, M.V. Cagnoli, J.F. Bengoa, S.G. Marchetti, R.C. Mercader and A.A Yeramian, J. Catal., 161 (1996) 132. 19. R. Snel, Catal. Rev. Sci. Eng., 29(4) (1987) 361. 20. L. Guczi, in '~New Trends in CO Activation", Studies in Surface Science and Catalysis, Ed. L. Guczi, Elsevier, 64 (1991) 350.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1347

Coordination and oxidation states o f iron incorporated into M C M - 4 1 K. Lfiz,~", G. P~-Borb61yb,/k. Szegedib, H.K. Beyerb "Institute of Isotope and Surface Chemistry, CRC, H-1525, Budapest, POB. 77, Hungary b Institute of Chemistry, CRC, H-1525, Budapest POB. 17, Hungary

Low iron content Fe-MCM-41 (Si/Fe = 139) samples are characterized under in situ redox conditions by TPR, IR and M6ssbauer spectroscopy. In the mesoporous structure full reversibility of the Fe 3+ <--->Fe z+ transition is found. Different types of low coordinations are suggested for Fe z+. Reductions in hydrogen and carbon monoxide are compared, the possible role of silanolic -OH groups is discussed and propagation of reduction via (H2)+ ions is also suggested. A comparison is made with samples of higher iron contents (Si~e = 77, 20 and 12).

1. INTRODUCTION The interest has been expanding towards the ordered stable mesoporous materials due to the variety of their advantageous properties. The hexagonal structures, MCM-41 (with extended regular pore structure) and HMS (with local order and interconnected secondary mesopores) are also widely studied. Their siliceous original structures can be modified by introducing further constituents into the synthesis mixture. For instance, the incorporation of iron may result in the generation of redox (Fe2+ +-> Fe 3+ transition) or Lewis acidic centers in the matrix - both are important for catalysis. Various aspects of the application of iron containing MCM-41 and HMS have been studied so far; their synthesis [1-4] and their application in catalytic processes either by utilizing the Lewis sites [5,6] or promoting the oxygen transfer by the Fe 2+ ~ Fe 3+ redox function [7-9]. The environment of the iron sites plays an important role in catalytic processes. In order to obtain information on them, these substances have also been characterized by structural methods. Existence of two types of tetrahedral sites was detected by EPR and EXAFS [6,10]. In Fe-HMS, presence of distorted tetrahedral environment is experienced. Upon evacuation the coordination of iron changes to trigonal one, whereas adsorption of water restores the tetrahedral coordination [11]. TPR, IR and in situ EXAFS studies were performed on FeMCM-41 prepared by hydrothermal way and by post-synthesis modification as well. In both cases iron preferred to occupy sites in silanol nests of tetrahedral coordination, presence of single ions was dominating up to high iron contents (3 wt %). In addition, the Fe 3+~ Fe 2+ The financial support provided by the Hungarian National Scientific Research Fund (OTKA T 32249) is gratefully acknowledged.

1348 redox transition was studied in conversion of nitrous oxide, and the behaviour of the mesoporous Fe-MCM-41 was compared to that of the microporous Fe-ZSM-5 under the same conditions [12-14]. In our previous studies Si/Fe = 12, 20 and 77 Fe-MCM-41 (c.a. 7, 4.2 and 1.1 wt % Fe) samples were characterized by TPR, IR, and in situ MOssbauer spectroscopy [15-17]. Lewis acidic Fe 3§ and Fe2§ sites were distinguished by IR detection of adsorbed pyridine, the Fe 3§ Fe 2§ conversion had been reversible for several redox cycles. Various coordinations were identified in the in situ M0ssbauer spectra recorded at 300 and 77 K and possible arrangements for different sites were proposed as well. In additon, the catalytic performance in a simple test reaction, in CO oxidation was also tested. In the detailed Mtissbauer study of sample Si/Fe = 20 (4.2 wt % Fe) existence of a component exhibiting intermediate (Fe2§ 3§ oxidation state was suggested indicating close interactions along Fe-O-Fe chains for a part of iron [ 15]. From TPR profiles presence of clustered iron ions can be suggested for the Si/Fe = 12 and 20 samples [17]. In the present account we report on TPR, IR and MOssbauer spectroscopic studies performed on low iron content (Si~e = 139, c.a. 0.5 wt % Fe) samples. Preparation of samples is attempted in which the single siting is predominant for iron by selecting the low iron content with the high Si/Fe ratio. Changes in the coordination and oxidation states are followed under various in situ treatments accompanied with redox Fe 3+ ~-~ Fe2+ transitions. For a broader interpretation, comparison is made with the higher iron content samples studied previously as well as with features shown by microporous Fe-MFI zeolite samples.

2. EXPERIMENTAL The Fe-MCM-41 sample was prepared by hydrothermal synthesis from a gel of composition 1 SiO2 : 0.0048 Fe203 : 0.204 Na20 : 0.083 Na2SO4 : 0.29 CTMA : 50 H20. CTMA is the surfactant (cetyltrimethylammonium bromide), and 57Fea(SO4)3 was used as iron source to provide good sensitivity for the M6ssbauer measurements. The synthesis was performed at 370 K under autogenous pressure for 170 h. The thermal decomposition of the template was carded out in two steps. First, a heating was applied (ramping with 10 K / min rate) to 753 K in a nitrogen stream, then afterwards, isothermal calcination followed in air at this temperature for 1.5 h. The ordered structure of the synthesized sample was confirmed by X-ray diffraction (XRD) carded out with a Philips 1810 diffractometer by applying monochromatized CuK~ radiation. TPR experiments were carded out in HJAr stream (10:90) using a conventional apparatus equipped with a thermal conductivity detector and a trap to remove the released water. The applied heating ramp was 10 K min-1. FTIR spectra were recorded with a Nicolet Compact 400 spectrometer. Thin wafers of samples were prepared (2 - 3 mg / cm2), treatments were performed in an in situ measuring cell. After treatments the samples were exposed to pyridine, as a probe molecule, at 473 K, and prior to the measurements the samples were cooled down kept still in pyridine atmosphere to, and degassed at 373 K. In situ M6ssbauer spectra were recorded at temperatures ranging from 77 to 620 K, after various treatments (evacuation in 10-1 Pa, reduction in streams either of hydrogen or carbon monoxide, etc.). Spectra were decomposed to Lorentzian-shape lines. Positional data are

1349 related to the center of metallic or-iron spectrum. The estimated accuracy of the positional data is + 0.03 mm/s. Further experimental details are described in Refs. [16,17].

3. RESULTS

3.1. X-ray diffraction (XRD) The ordered mesoporous structure was proven by XRD: from reflections at low angle 4.0 nm d(100) spacing was calculated for the synthesized sample.

3.2. Temperature Programmed Reduction (TPR) TPR profile of the Si~e = 139 sample in the 400 - 900 K region is shown in Fig. la. For comparison, the profiles for samples Si/Fe = 77, 20 and 12 are also shown in the same scale (Fig. 1 b,c,d), as well as a profile for bulk Fe203, hematite. In the SFFe = 139 sample the dominant Fe 3+ --~ Fe 2+ reduction proceeds around 700 K. It might also be mentioned that the reduction is not stoichiometric, only ca. 2/3 of Fe 3+ is reduced to Fe 2§ under the applied dynamic experimental conditions with a fast temperature ramp (10 K min-l). In hematite (Fe203) a part of Fe 3+ is reduced to Fe2§ to form magnetite (Fe304) at 610 K. In samples Si~e = 77, 20 and 12 a shoulder is seen at 620 - 640 K indicating the presence of iron in various coordinations (probably in associated forms, in part). Further reduction of associated Fe 2§ clusters to Fe ~ are shown around 900 K in samples Si~e = 12.

0,05 1614

10

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rt~

tl)

•

a t

400"500

.

|

600

.

i

700

.

i

800

900

Temperature (K)

Figure 1. TPR profiles of calcined FeMCM-41 for the same amount of samples, with Si~e ratios of 139 (a), 77 (b), 20 (c) and 12 (d). For comparison profile for hematite Fe203, is also shown (e).

916'00" 1560" 15'20' 1180" 1~0 Wavenumber (cm-1)

Figure 2. IR spectra of pyridine retained after adsorption at 473 K and subsequent degassing at 373 K on the Si/Fe = 139 sample after calcination and reduction in H2 670 K (bottom) and subsequent evacuation and oxidation at 670 K (top).

1350

3.3. Infrared Spectroscopy (IR) Bands characteristic of ring vibrations of pyridine chemisorbed on Lewis sites were detected by IR spectroscopy after two treatments on the Si/Fe = 139 sample. First, the calcined sample was reduced (90 kPa H2, 670 K for 0.5 h; see Fig. 2, bottom). Then, the sample was evacuated, and treated in oxygen at 670 K; Fig. 2, top). The assignments proposed earlier for samples Si/Fe = 12 and 20 [16] can be adopted: vibrations at 1454 and 1614 cm-~ can probably be assigned to pyridine interacting with Fe 3+ sites, whereas bands appearing at 1449 and 1605 cm ~ originate from pyridine adsorbed on Fe2+ sites. A particular feature is observed on the Si/Fe = 139 sample, that is the permanent appearance of bands at 1447 and 1598 cm -~ irrespective of the oxidation state. In the spectruna of the reduced sample the band at 1447 cm ~ is hidden in the low-frequency side of the band at 1449 cm-~. These vibrations can tentatively be assigned to pyridine bonded to silanolic groups with iron in their close vicinity. Vibrations characteristic for pyridinium ions (attached to Bronsted sites) can hardly be detected at 1548 cm ~.

3.4. M~issbauer spectroscopy Sequential in situ MOssbauer spectra were obtained on the Si/Fe = 139 sample after synthesis and calcination, as well as after and during various treatments (evacuation, reduction in carbon monoxide and hydrogen etc.). Emphasis was laid on the in situ characterization, hence spectra were recorded at "actual reaction conditions" i.e. during treatments and at temperatures ranging from 300 to 670 K. Various oxidation and coordination states of the high-spin Fe 3§ and Fe 2§ components appear in form of different doublets. The position of the centrum of the doublet, namely the isomer shift (IS) value, is related to the electron density (i.e. oxidation state), whereas the distance of the two separate lines of the doublet, i.e. the quadrupole splitting (QS), is correlated to the extent of the electric field gradient (generated by the ligands coordinated) at the iron nucleus. For the assignment of various oxidation and coordination states data compiled from studies on similar substances on ferro/ferrisilicate minerals can be adopted [18]. For the first evaluation (as a very simplyfied approximation) two "thumb-rules" can be adopted: i/ the increase of IS value indicates decreasing oxidation number (increase of electron density: partial reduction), and ii/the increase of symmetry around Fe 3+ decreases QS, whereas the opposite is valid for the quadrupole spiltting of Fe 2§ In certain cases we tentatively assign different coordinations to species identified in the spectra henceforth. However, the mesoporous structure is not as defined as that of those of the ferrisilicates from where the comparing data are derived. Thus, the tentative assignments should be considered with precaution. A further note for interpretation of data is also appropriate, i.e. when comparing spectra recorded at different temperatures, the temperature dependence of IS should also be considered (decrease of the temperature results in the increase in IS with c.a. 0.1 mm/s shift for A T - 100 degree). Some typical spectra are represented in Fig. 3, the extracted data are compiled in Table 1. The 300 K spectrum recorded after calcination (in streams of nitrogen and subsequently air at 750 K - Fig. 3/1) can be described by two Fe 3+ doublets, i.e. even at this low iron content (0.6 wt %) different coordination sites are found. The next step, evacuation at 670 K, results in the appearance of two new components: i / a distorted tetragonal Fea+(d-Tetr) component (IS = 0.28, QS = 1.68 mm/s) at high proportion, and ii/a reduced Fe 2§ one. The former, Fe3+(d-Tetr) has similar parameters as those characterizing the isomorphously substituted Si-O-Fe(OH)-O-Si sites with distorted tetragonal symmetry in zeolites, [19], thus probably existence of a similar coordination state can also be suggested in Fe-MCM-41 as well.

1351

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"

-~,

0K

570K 6

~,

4.

Velocity, m m / s

Figure 3. Sequential in situ Mrssbauer spectra recorded on the Fe-MCM-41 sample (Si/Fe = 139) after serial treatments: calcination 1.) and 5.); evacuation 2.) and 6.); reduction: in H2, 3.), in CO, 7.), and repeated evacuations 4.) and 8.) Formation of Fe z+ component upon evacuation (Fig 3/2) in the mesoporous ferrisilicate might be apparently similar to the autoreduction observed in microporous zeolites. This latter process is observed on extra-framework ions and is connected to a displacement of a tetracoordinated trivalent ion from the framework with simultaneous release of a framework oxygen [20]. In mesoporous structures an alternative interpretation can be put forward. Namely, considering the abundant presence of silanol groups in the structure [16] and adopting the suggestion on the preferred siting of iron in silanol nests [13], dehydratation and simultaneous oxygen release may also result in the observed Fe 3+ ~ Fe z+ reduction. In a following step, the evacuated sample was reduced in hydrogen (spectrum recorded at 620 K is shown in Fig. 3/3). Almost complete Fe 3+ --->Fe 2+ reduction can be detected in the in situ spectrum: tetrahedral ferrous components appear with distorted symmetry in part. The disorted coordination is probably originated from the Fe-OH groups by dehydration in the adjacent -OH groups, whereas the tetragonal coordination might correspond to the reduction of the previous Fe3+(d-Tetr) single sites embedded into the pore wall. It is worth mentioning that these sites are coordinatively unsaturated: with readsorbing a small amount of water present in the system, the overwhelming part of the ferrous form completes its coordination to a state close to octahedral symmetry. (The 77 K spectrm~ is composed from doublets of IS > 1.0, and QS > 2.0 mm/s, respectively - not shown). After a repeated treatment in hydrogen at

1352 Table 1. M6ssbauer parameters derived from selected in situ spectra (IS" isomer shit~ related to metallic a-iron, mm/s; QS: quadrupole splitting, mm/s; RI: relative spectral intensity, %. S" spectrum assignments in Fil~. 1; M: notes the measuring temperature) S Treat m. Comp. IS QS RI IS QS RI Treatm. S 1. As rec. Fe3+(Oct) 0.35 0.68 40 0.33 0.88 62 Calc.(2 ~) 5. 3+

....... Mi 3_09.~_ ..... ge_.(~-o.ct)

2. .......

3.

Evac. (1 st) 670 K M~3o9.~.

..... 037.__1_13

FeS+(d-Tetr) 0.28 Fe3+(Oct) 0.34 2+

..... ge..(oct)

H2 670 K

1.68 1.04

_60.

...... 9.32 .... b48__3.8

44 27

H2+Evac. (2"d)530K M: 530K

1.88 1.15

. . . . . . . 1.o7.__ 2 0 . 3 . . _ _ 3 9 . . . . . . . ! : 1 . 2 . . _ . 2 _ 2 7

Fe3+(Oct) 0.09 FeZ+(d-Tetr) 0.53

0.50 0.84

8 37

....... M i 6.20. K. ..... Fe_2 +.(.T.etr). ...... 0,8_5__._ J..2_1__._ 54 ......

4.

0.29 0.33

FeS+(d-Tetr) Fe2+(d-Tetr) 0.73 Fe2+(Tetr) 0.92

0.81 1.61

32 68

0.02 0.68 o. 8 3

0.13 0.78 0.87

1.77 0.78

.... .M..300.~

60 38

..... 2.._.M.-.3oo.~

18 30

........

Evac (3 rd) 670 K

6.

........

CO 620 K

7.

.... .1..4.5.__. _5..2_.__.M__ 57 o _.K.........

1.78 0.91 1.70

48 18 34

Evac (4 th) 670 K M:570K ,

8. ,,

640 K and collecting the spectrum under evacuation at 550 K a completely reduced state is attained (Fig 3/4). The oxidized state of the sample was restored by an oxidation in air at 670 K (Fig. 3/5). Upon evacuation the adsorbed water was removed, and (in contrast to the first evacuation) only slight autoreduction was detected upon the second evacuation (Fig. 3/6). In the following step the reducing effect of another agent, the carbon monoxide was studied. The spectrum collected at 570 K in CO is shown in Fig. 3/7. The reduction is less expressed than found in hydrogen, a significant amount of iron exist still in ferric state (18 % spectral area). After cooling the sample to 300 K and collecting the next spectrtma under vacuum at 570 K (Fig. 3/8), the relative proportions of the components change, i.e. the ferric state is even more pronounced.

4. DISCUSSION 4.1. Coordination and location of iron ions

The obtained experimental data correspond to the assumption of the dominance of single siting of the iron ions. No evidences were found for the presence of Fe-O-Fe pairs or chains. Namely, in the TPR profile the reduction proceeded in one step at 700 K, wihout consuming hydrogen at lower temperatures. Furthermore, neither intermediate valency states nor octahedrally coordinated components were identified in the M6ssbauer spectra recorded under in situ conditions at higher temperatures. In correspondence, the local environment of iron ions is coordinatively unsaturated: below 400 K the coordination is filled up close to octahedral one by adsorbing water (if present in small amounts). The parameters obtained are close to those characterizing tetrahedral sites in the samples studied under in situ conditions at higher temperatures. This holds for the greater portions of both the ferric and ferrous iron. Further, for the Fe 2§ component different coordinations can be distinguished, exhibiting various extent of distortion.

1353 As for the location of iron, both the emplacements: i/ in the centre of [FeO4/2]- units ("framework" substitution), and ii/in sites abundant in -OH groups in their close vicinity can be proposed. The former assumption is in correspondence with the values of the IS and QS parameters obtained on evacuated samples, they are characteristic for the Fe3+(d-Tetr) coordination similar to those found in microporous zeolites for framework substituted position. However, it should also be mentioned that Bronsted acidity was hardly detected in these samples (the accessibility of these sites is probably restricted for pyridine). The second emplacement, in the -OH abundant environment, is also a possible location, since the -OH streching vibrations are always present at 3745 cm -j even after high temperature treatments [16]. Upon reduction, in particular in hydrogen, further -OH groups may develop by forming Si-O-Fe(OH),, (n = 1 or 2). The actual symmetry depends further on the relative positions of the -OH groups (cis or trans) and, upon further dehydration, additional distortion may develop. 4.2. Reducibility of the iron ions and the Fe 3§ ~-~ Fe z§ redox process

The first important feature in discussing the Fe 3+ --, Fe 2§ process is the difference of the reducing effect of hydrogen and carbon monoxide. In hydrogen, the Fe 3§ -~ Fe ~-§ process is almost complete to Fe 2§ whereas in carbon monoxide the reduction proceeds only for a part of Fe 3+. The difference can probably be attributed to the different net mechanisms: hydrogen may directly interact with single Fe 3+ ions forming Fe2+-OH by being attached to lattice oxigens whereas the main route for reduction with CO is the extraction of oxygen to form CO2. In our previous papers [15,16] various mechanisms and local structural arrangements of intermediate and final redox states were suggested. Here an additional option, the propagation of the reduction processes by (H2) + ions, is considered. The reduction with hydrogen may start with a charge transfer from Fe 3+ to H2 according to -Si-O-Fe 3+= + H2 ~

~Si-O-(H2) + + Fe z+=

and

-Si-O(H2)+---~-=Si-OH + H"

(1)

In further steps silanol groups may play an important role in the promotion of the process as suggested in Ref. [21]. Namely, in systems which contain hydroxyl groups in high density hydrogen atoms may migrate through the structure by jumping from one OH group to another: (H2)+-O 2- + HO- --~ n o + (H2)+-O2-. If the jumping hydrogen atom interacts with a hydroxyl group located in the vicinity of a not yet reduced Fe 3+ framework species the intermediately created (HE) + ion will vanish according to OH O(H2) + OH =Si-O-Fe 3+= + H" --~ =Si-O-Fe3+= --, =Si-OH + Fe2+=

(2)

Furthermore, it can be tentatively suggested that this reaction way is also an essential element in the mechanism of the reduction with carbon monoxide. Then the formation of H2 by reaction of CO with two neighboured silanol groups, similar to the water gas reaction, has to be additionally considered as preliminary step: CO + 2 -SiOH --, =Si-O-Si-- + CO2 + H2. It is self-evident that in this case succesively performed reduction/reoxidation cycles must result in progressive dehydroxylation and, hence, damages of the framework. It is worth to compare the extents of Fe 3+ to Fe 2§ reductions in MCM-41 and e.g. in ZSM5. Under the same experimental conditions the conversion is full in MCM-41, whereas in ZSM5 the process is partial, only the extra-framework ions are reduced at the latter case [19].

1354 4.3. A short comparison of the behaviour of iron in the mesoporous and in microporous structures

For a broader context, it is worth to compare some characteristic features of iron incorporated to MCM-41 and those of iron in zeolites e.g. in ZSM-5. The difference mentioned above (i.e. the complete reduction of Fe 3+ to Fe z+ in hydrogen) can be rephrased so that Fe 2+ can be incorporated in the structure of MCM-41. In contrast, in zeolites this holds only for iron in extra-framework position. The strong retardation of Bronsted acidity is also an important feature for the mesoporous structure, whereas its presence in H form of Fe-ZSM-5 is wellknown. More general, taking into account the lack of the rigid crystalline structure (with strict translation symmetry) in mesoporous structures, it can probably be suggested that the distinction between framework and extra-framework positions is not pertinent for MCM-41; the different environments may easily be converted from one to the other, particularly in redox processes in presence of silanolic groups.

REFERENCES 1. Z.Y. Yuan, S.Q. Liu, T.H. Chen, J.Z. Wang, H.X. Li, J. Chem. Soc., Chem. Commun., (1995) 973. 2. M. Alves, H.O. Pastore, Microporous Mesoporous Mater., 47 (2001) 397. 3. L. Pasqua, F. Testa, R. Aiello, F.Di Renzo, F. Fajula, Microporous Mesoporous Mater. 44-45 (2001) 111. 4. A.B. Bourlinos, M.A. Karakassides, D. Petridis, J. Phys. Chem., B, 104 (2000) 4375. 5. N.Y. He, S.L. Bao, Q.H. Xu, Appl. Catal., A:General, 169 (1998) 29. 6. S.K. Badamali, A. Sakthivel, P. Selvam, Catal. Lea., 65 (2000) 153. 7. W.A. Carvalho, M. Wallau, U. Schuchardt, J. Mol. Catal., A:Chemical, 144 (1999) 91. 8. V. Parvulescu, B.L. Su, Catal. Today, 69 (2001) 315. 9. Z.H. Fu, D.L.Y.in, W. Zhao, Y..D. Chen, D.H. Y'in, J.W. Guo, C. Xiong, L.X. Zhang, Stud. Surf. Sci. Catal., 135 (2001) 29-P-08. 10. F. B61and, B. Enchachaded, L. Bonneviot, Stud. Surf. Sci. Catal., 130 (2000) 2945. 11. A. Tuel, I. Arcon, J.M.M. Millet, J. Chem. Soc., Faraday Trans., 94 (1998) 3501. 12. M. Stockenhuber, M.J. Hudson, R.W. Joyner, J. Phys. Chem., B 104 (2000) 3370. 13. G. Grubert, M.J. Hudson, R.W. Joyner, M. Stockenhuber, J. Catal., 196 (2000) 126. 14. M. Stockenhuber, R.W. Joyner, J.M. Dixon, M.J. Hudson, G. Grubert, Microporous Mesoporous Mater., 44-45 (2001) 367. 15. K. Lfiz~ir, G. P~il-Borb61y, A. Szegedi, H.K. Beyer, Hyperfine Interactions, in press. 16. G. Pfil-Borb61y, A. Szegedi, K. Lfizfir, H.K. Beyer, Stud. Surf. Sci. Catal., 135 (2001) 07-0-03 17. A. Szegedi, G. Pfil-Borb61y, K. Lfizfir, React. Kinet. Catal. Lett., 74 (2001) 277. 18. R. Bums, Hyperfine Interactions, 91 (1994) 739. 19. K. Lfizfir, G. Borb61y, H. Beyer, Zeolites, 11 (1991) 214. 20. P.A. Jacobs, Stud. Surf. Sci. Catal., 29 (1986) 357. 21. O.E. Lebedeva, W.M.H. Sachtler, J. Catal., 191 (2000) 364.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1355

Synthesis and Characterization of In-MCM-41 Mesoporous Molecular Sieves with Different Si/In Ratios W. BOhlmann*, O. Klepel #, D. Michel*, and H. Papp # *Universit~it Leipzig, Fakult~it far Physik und Geowissenschatten, Linn6str. 5, D-04103 Leipzig, Germany #Universit~it Leipzig, Fakult~it far Chemie und Mineralogie, Linn6str. 3, D-04103 Leipzig, Germany

MCM-41 based materials are currently under intense studies with respect to their utility as adsorbents, possible catalysts, supports, and molecular hosts. A series of mesoporous InMCM-41 molecular sieves (Si/In ratio from 5 to 120) in their hydrogen and sodium form have been synthesized using indium nitrate as source of indium. The obtained material were characterized in detail by XRD, l lSIn-MAS-NMR spectroscopy, nitrogen adsorptiondesorption experiments, and temperature programmed desorption. The XRD and nitrogen adsorption measurements demonstrate that indium can be incorporated into the MCM-41 structure down to a Si/In ratio of about 10. The structure of the MCM-41 totally collapses with a further increasing amount of indium in the gel.

1. INTRODUCTION Following the announcement of the novel class of mesoporous M41S molecular sieves by Mobil researchers ~ diverse interest has occurred in the introduction of heteroatoms 2-4, particularly aluminum, into the silica network. To apply the MCM-41 in various processes not only the aluminum incorporation may be useful because further group III elements as boron, gallium and indium should be have beneficial effects which are influenced the properties of the material. While the substitution of boron and gallium into the framework was successfully done 5-9 the synthesis of In-MCM-41 was not described up to now. As in other zeolite systems m'~, indium incorporation is a further pathway to influence the chemical reaction properties of MCM-41 material. In the present work we firstly report that it is possible to obtain In-MCM-41 mesoporous molecular sieves in their sodium and hydrogen form with Si/In ratios >_ 10 which show the typical characteristics as it is well known from systems mentioned above. 2. EXPERIMENTAL

2.1. Synthesis procedure a) Sodium form: Na-In-MCM-41 molecular sieves were synthesized starting from an aqueous solution of cetyltrimethylammoniumbromide (CTAB), Aerosil as silicon source and a suspension of sodium water glass in water as cationic agent. Then the indium source

1356 (In(NO3)3) was added and the gel was stirred for lh. Generally, the molar composition of the resulting gels were 0.004...0.06 In(NO3)3: SiO2:0.31 CTAB" 0.17 Na20:25.7 H20. The obtained starting gel was heated at 90~ for 96 h. The as-synthesized products were filtered and washed first with distilled water and in a second step with methanol to remove residual template molecules. After drying at 90~ for 6 h in air the solid product was calcined for 12 h at 540~ b) Hydrogenform: Pure silica MCM-41 was synthesized from aqueous silica solution in the system SiO2 (tetramethoxysilane as silicon source), ethylenediamine (EN), water, cetyltrimethylammoniumbromide as described by Oberhagemann et al. 7 Again In(NO3)3 as indium source was added to obtain the In-MCM-41. The gel was stirred for 1 h. Generally, the molar composition of the resulting gels were 0.008...0.2 In(NO3)3: SiO2:0.36 CTAB: 2.2 EN: 62 H~O. The obtained starting gel was heated at 95~ for 170 h. After the hydrothermal synthesis the same procedure was used as for the sodium form of the In-MCM-41. 2.2. Sample characterization

X-ray Diffraction. The XRD powder pattem were recorded using a Philips PW 1877 diffractometer with CuKa radiation, 0.05 ~ step size, and 2.5 s step time of each point.

Nitrogen adsorption. Nitrogen adsorption-desorption isotherms were measured at 77.4 K using a ASAP 2010 (Micromeritics) analyzer. The specific surface area and the pore size were determined following the BET procedure. Temperature Programmed Desorption. The TPD experiments were carried out in a home built flow apparatus with helium as carrier gas in a temperature range from 300 to 900 K. For evolved gas detection a mass spectrometer was used. 115In-MAS-NMR measurements. The solid state 115In-MAS-NMR spectra were performed at a resonance frequency of 65.768 MHz at a Bruker MSL 300 spectrometer, zirconia rotors 7 mm in diameter spun at rotation frequency of 4.5 kHz. A recycle delay of 100 ms, short pulses of 2.1 gs, and 100000 scans for each spectrum were applied. External In(NO3)3 (chemical shift of 0 ppm) was used as reference. 3. RESULTS AND DISCUSSIONS The In-MCM-41 molecular sieves were synthesized with Si/In ratios between 5...120. Both the as-synthesized and the calcined materials were investigated using XRD powder diffraction. The obtained XRD pattern of the In-MCM-41 (shown in Figure 1) with a Si/In ratio higher 10 posses the typical low-angle reflections that are characteristic of hexagonal ordered mesophases. At Si/In ratio of 10 the intensity of the reflections are decreased, indicating a transition from the MCM-41 structure to an amorphous material having a undefined structure. The collapse of the MCM-41 structure at lower Si/In ratios which is clearly demonstrated in Figure l a and l b can be attributed to the fact that the amount of indium molecules in the synthesis gel are to large to form the MCM-41 molecular sieves. Both the sodium and the hydrogen form of the molecular sieves show the same tendencies. Obviously, other processes are preferred in the hydrothermal synthesis if the amount of indium molecules increase in the gel leading to an amorphous product. The obtained products have a yellowish gleam which can be attributed to a mixture of indium oxide and indium silicate.

1357

t

Si/In a:

I

5

b: 10 c:20

]l

2

ratio:

I

4

I

I

6 8 2 Theta

I

10

12

Figure 1: Powder XRD pattern of calcined In-MCM-41 with different Si/In ratios prepared using ethylenediamine as cationic agent. The most reliable information about the mesopore structure of solids is obtained from low temperature nitrogen adsorption isotherms, which allow the calculation of specific surface area, pore volume, and mesopore size distribution. 12 The nitrogen adsorption-desorption isotherms are shown in Figure 2. Materials with a Si/In ratio higher than 10 possess the well known type IV isotherms. Considering the isotherm of the material with a Si/In ratio of 10 (compare Figure 2b) it is seen that the type IV character is weakly distinct in comparison with material of higher Si/In ratios. This should be caused by a transition from the MCM-41 structure to a more amorphous material as it can be observed by XRD measurements too. The isotherm of the sample with a Si/In ratio of 5 (Figure 2a) shows that irregular macropores are formed corresponding to a structural collapse of the material. From the adsorption data pore sizes between 38...45 A were calculated which are summarized in Table 1. Furthermore, the BET surface areas are estimated show relatively high values between 735...950 m2/g which are comparable with well known A1-MCM-41 material. 13 The material with Si/In ratio of 10 shows a BET surface area of 410 m2/g which is between the values of MCM-41 (735...950 mVg) and materials without MCM-41 structure (84 m2/g). As the relative pressure increase (p/po>0.3) the isotherms (Figure 2c, 2d) exhibit a sharp inflection characteristic of capillary condensation within the mesopores, and the P/Po position of the inflection point is related to

1358

Si/In ratio: a: 5 b: I0 c: 20 d: Si-MCM41

I

0,0

i

I

i

0,2

I

0,4

i

I

i

0,6

I

m

0,8

I

1,0

Figure 2: Nitrogen adsorption isotherms at 77.4 K of various In-MCM-41 samples (H-Form) the diameter of the mesopores. The sharpness in this step indicates uniform pores. The hysteresis loop at p/po>0.9 reflects that the larger pores are filled at high pressures. Contrary to this findings the isotherms in Figure 2b and 2a are quite different demonstrating that the amount of indium species strongly influence the formation of the structure. The values in Table 1 correspond with the results of the XRD and NMR measurements which indicates that at Si/In >_ 10 the MCM-41 is formed. With increasing amounts of indium in the synthesis gel an amorphous material is obtained indicating that indium is expelled from the structure. Table 1" Specific pore volumes, pore diameters, and BET surface areas estimated from nitrogen adsorption-desorption isotherms S i/In ratio (H-Form) 5* 10 20 volume 0.16 0.41 0.72

specific pore (cm3/g) pore diameter (in A) 78 BET surface area, (in m2/g) 84 *no MCM-41 structure is obtained.

40 414

39 735

52 0.80

Si/In ratio (Na-Form) 30 60 120 0.72 0.76 0.86

38 834

45 617

40 760

38 914

1359 The l15In MAS NMR spectra of the samples ( Figure 3a-c) show two broad signals which are assigned to four- (at about 780 ppm) and six-coordinated (at about -900 ppm) indium species. According of these results it can be established that more than 50% of the employed indium is incorporated into the framework. As it is expected the intensity increases with increasing indium content. Furthermore the spectral intensities of ll5In peaks in samples with Si/In > 10 are unaffected by calcination. It must be mentioned that the highly electric quadrupole interactions (indium: nucleus spin ofI - 9/2) strongly broadens the NMR signals in the solidstate powder spectra. Furthermore it is known 14 that satellite transitions in addition to the central transition take place which are become visible in the recorded spectra. This influences the quality and the signal noise ratio of the 115In MAS NMR spectra. Because of the molecule size of the indium in all spectra a high amount was detected as so called extra-framework species which is slightly dependent on the used Si/In ratio. The spectrum of the material with a Si/In ratio of 5 (Figure 3d) is quite different in comparison with spectra in Figure 3a-c. As it is clearly seen the intense lines are shiited to about 1100 ppm and-1000 ppm, respectively, as a result of a changed environment of the zlSIn nuclei. This is a further hint that no MCM-41 structure was obtained. The NMR results represent a further evidence that it is possible to synthesize In-MCM-41 with a high content of indium into the framework. Further information about the state of incorporation should be possible if the MQMAS (multiple quantum magic-angle spinning) NMR method is applied.

d

Jb F 2000

1500

1000

500

0

-500

-1000 -1500 -2000

ppm Figure 3: 115In-MAS-NMR spectra of ln-MCM-41 mesoporous molecular sieves with a Si/In ratio of 120 (a), 60 (b), 30 (c), and 5 (d). All spectra were recorded at 65.768 MHz with a rotation frequency of 4.5 kHz.

1360 It is well known, that incorporation of aluminum in siliceous materials like zeolites or mesoporous molecular sieves generates Bronsted and Lewis acid centers. Indium, as an element of the 3rd main group like aluminum, should be able to generate acidic centers too. However, one should be expected, that the strength of the In-generated centers is lower in comparison to aluminum. The temperature programmed desorption (TPD) of ammonia has been established as a powerful tool to investigate the acidic properties of solids. ~5 The aim of our investigations was to elucidate in which manner the introduction of indium influences the acidic properties of the materials. The obtained temperature programmed desorption files are shown in Figure 4. Considering the curves (Figure 4a-d) it is seen that all studied samples undergo interactions with the adsorbed ammonia. Except the In-free Si-MCM-41 material (Figure 4e), which did not show any desorption, all curves have a maximum at about 500 K. This maximum should be assigned to weak acid centers. The materials with typical MCM-41 structure (lower Incontent) have an additional shoulder at about 550 K. This could be an indication for the existence of different In-species in these materials.

i

Si a

/ In :

ratio"

5

b: 10 c: 20 d: 43 : "-

a

300

I

400

"

e

500

.

e

a

600

T

I

700

-41

I

I

800

a

I

900

/K

Figure 4: Temperature programmed desorption curves of different In-MCM-41 molecular sieves (a-d) in comparison with pure siliceous material (e).

1361

6,0x10 J

5,0x10 g~

N ~, Z

4,0x10 3,0xl 0 2,0x10

-9

M CM-structurc

-9

non MCM-structure

-9 I

0

I

10

a

I

20

In/% (Si+In=100%)

Figure 5: Comparison of the amount of desorbed ammonia in dependence of the amount of

indium in the MCM-41 material. The amount of desorbed ammonia correlates to the content of indium but not in a linear way. For a more detailed elucidation, we compared the amount of desorbed ammonia in relation to the amount of indium in the mesoporous molecular sieves. As it is shown in Figure 5 two different areas are visible representing the material with MCM-41 structure and non MCM-41 structure. Obviously, the material without MCM-41 structure possess a higher affinity against ammonia which can be described with a higher acidity. This clearly demonstrates that the acidic properties depends on the environment of the indium molecules. Generally, the results of the TPD studies demonstrate that the obtained materials can be applied in several procedures where acidic centers are involved in catalytic processes. 4. CONCLUSIONS A series of indiumsilicate MCM-41 materials prepared from gels with Si/In ratios between 5...120 have been synthesized using indium nitrate as source of indium. The products were characterized by different methods like XRD, NMR, N2 adsorption, and TPD measurements. The quality of the molecular sieves strongly depends on the amount of indium. The results of the XRD and the nitrogen adsorption-desorption isotherms confirm the MCM-41 structure. ~5In MAS NMR indicates that indium is tetrahedrally incorporated into the framework of the molecular sieves with amounts of more than 50% in the calcined samples. However, an increasing amount of indium leads to a collapse of the structure and forms macropores. Generally, it could be demonstrated that it is possible to synthesize In-MCM-41 materials in their sodium- and hydrogen form which have the typical properties of the M41S family and it is of potential interest as catalyst.

1362 A C K N O W L E D G M E N T : This work was supported by the DFG. Contract No: SFB 294/G8

and Contract No: PA194/4-4. REFERENCES

1

10 11 12 13 14 15

T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S.Beck, Nature, 359 (1992) 710. Z. Luan, C.-F. Cheng, W. Zhou and J. Klinowski, J. Phys. Chem., 99 (1995) 1018. C.-G. Wu and T. Bein, J. Chem. Soc., Chem. Commun., (1996) 925. W. B6hlmann and D. Michel, Stud. Surf. Sci. and Cat., 135 (2001) 202. C.-F. Cheng and J. Klinowski, J. Chem. Soc., Faraday Trans., 92 (1996) 289. A. Sayari, C. Danuman and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. U.Oberhagemann, M. Jeschke, and H. Papp, Microporous and Mesoporous Mat., 33 (1999) 165. D. Trong On, P.N. Joshi, and S. Kaliaguine, J. Phys. Chem. 100 (1996) 6743 C.-F. Cheng, H. He, W. Zhou, J. Klinowski, J.A.S. Goncalves, and L.F. Gladden, J. Phys. Chem., 100 (1996) 390 N.H. Heo, S.W.Jung, S.W. Park, M. Park, and W.T. Lim, J. Phys. Chem. B, 104 (2000) 8372 M. Chatterjee, D. Bhattcharya, H. Hayashi, T. Ebina, Y. Onodera, T. Nagase, S. Sivasanker, and T. Iwasaki, Microporous and Mesoporous Mater., 20 (1998) 87 P.T. Tanev and L.T. Vlaev, J. Colloid Interface Sci., 160 (1993) 110 P. Selvam, S.K. Bhatia, and C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237 D. Freude, R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, John Wiley & Sons Ltd, Chichester, (2000), pp 12188-12224 B.M. Lok, B. K. Marcus, C. L. Angnell, Zeolites 6 (1986) 185

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1363

The effect o f niobium source used in the synthesis on the properties o f N b M C M - 4 1 materials Izabela Nowak A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland; e-mail: nowakiza@amu, edu.pl Niobium-containing mesoporous molecular sieves (NbMCM-41) prepared by using different sources of niobia were characterized by means of N2 adsorption, XRD, TEM, SEM combined with microanalysis and IR spectroscopy in the skeletal region. The results showed that the nature of Nb-sources influences the crystallite diameter, volume, and wall thickness of the mesoporous material. Only NbMCM-41 prepared from Nb ammonium oxalate complex exhibits the uniform pore size distribution. 1. INTRODUCTION The relatively large interest of MCM-41-type mesoporous materials that has arisen since 1992 [ 1] lies in the fact that they can be tailored to form pores with narrow size distribution, from 2 to 10 nm. In silica MCM-41 material, silicate matrix surrounds hollow tubes which are arranged in a relatively regular two-dimensional hexagonal array. This opened up a field of fundamental and applied science, where narrow pore size distribution and high surface area could be required. The easy modification of MCM-41 by isomorphously substitution tetrahedral silicon atoms with other novel elements such as AI, Ti, V, B, Ga, etc makes MCM-41 a potentially useful catalyst. By structural ordering, MCM-41 has long-range order which is derived from a regular array of unisized cylindrical mesopores, and manifests itself in X-ray diffraction (XRD) at small angles (2-8 ~ 20), transmission electron microscopy (TEM), and adsorption analysis. On the other hand, scanning electron microscopy (SEM) images reveal the morphologies. The synthesis and properties of niobium and siliceous containing mesoporous sieves of MCM-41 type were first time described by our group [2-3] and simultaneously, Nb-doped mesoporous sieves were synthesized by Zhang and Ying [4]. Recently, it was found a very high activity of NbMCM-41 mesoporous molecular sieves in the oxidation of thioethers to sulfoxides with hydrogen peroxide [5,6] and in the direct conversion of cyclohexene into trans-2-alkoxycyclohexanol in the presence of H202 [7]. The first synthesis of NbMCM-41 mesoporous molecular sieves was carried out using niobium oxalate as a source of Nb [2]. The obtained materials were partially disordered especially when the content of niobium was high (Si/Nb=16). The aim of this study was to synthesize NbMCM-41 materials using the other sources of niobium, especially those available commercially, i.e., niobium chloride and ammonium-oxalate complex of niobium. The another goal was to compare their structure and properties with those found for NbMCM-41 prepared from niobium oxalate and for MCM-41 impregnated with niobium salts.

1364 2.

EXPERIMENTAL

2.1. Syntheses procedure In the previous report [2] it was shown that a mesoporous molecular sieve MCM-41 could be successfully synthesized with niobium oxalate as a source of niobium. The same procedure of the preparation of niobium-containing MCM-41 materials was applied in this study, i.e. niobium oxalate (Nb(HC204)5, CBMM, Brazil), niobium chloride (NbCIs, Aldrich) or ammonium-oxalate complex of niobium (NbO(C204)3(NH4)3, CBMM, Brazil) were added to an aqueous solution of the sodium silicate (27% SiO2 in 14% NaOH, Aldrich) mixed vigorously with surfactant (cetyltrimethylammonium chloride, 25 wt. % solution in water, Aldrich). In the case of niobium chloride, the niobium source was first dissolved in ethanol solution in a dry-box under dry nitrogen flow. The formed gel was stirred for about 0.5 h before 20 g of distilled water was added. The gel was loaded into a stoppered PP bottle and heated without stirring at 373 K for 48 h. The precipitated product was recovered by filtration, extensively washed with distilled water, and dried in air at ambient temperature. The product was finally calcined at 773 K for lh in helium flow and 6 h in air. NbMCM-41 obtained from different niobium sources, i.e. niobium oxalate, ammoniumoxalate complex of niobium or niobium chloride, will be designated in this paper as Nb(O)MCM-4 l-Y, Nb(Oc)MCM-4 l-Y, and Nb(CI)MCM-4 I-Y, where: O stands for oxalate, Oc - oxalate complex, CI - chloride, and Y - Si/Nb atomic ratio in the synthesis mixture, respectively. The chemical analyses showed that the obtained Si/Nb ratios for samples prepared with different sources of niobium were very close to the theoretical of 32 and are as follows: 36 for oxalate, 33 -chloride, and 32 for ammonium complex of niobium.

2.2. Characterization The physico-chemical properties were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption-desorption analysis, scanning electron microscopy (SEM) and infrared spectroscopy (IR). Powder X-ray diffraction data were collected on a TUR-42 diffractometer using Cu Kc~ radiation (~=0.154 nm) with 0.02 ~ step size. For TEM measurements, all calcined materials were crushed in an agate mortar, dispersed in ethanol, and deposited on a microgrid. The transmission electron micrographs were taken on a JEOL-2000 operated at 80 keV. Scanning electron microscopy was performed on a JEOL JSM-5400 using an accelerating voltage of 20 keV with microanalysis. The N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus. The sample was outgassed at 623 K under vacuum prior to the adsorption. The data were analyzed by the BJH (Barret-Joyner-Halenda) method and the surface area and the pore size distribution curve were obtained from the analysis of desorption portion of the isotherm. However, the calculations from adsorption branch of isotherm were performed as well in the case of some pore blocking (not presented in this paper). Infrared (IR) spectra were recorded with a Vector 22 (BRUKER) FTIR spectrometer. The samples were measured by diluting them to 1 wt. % in KBr. 3. RESULTS AND DISCUSSION The Nb-containing mesoporous material prepared with various niobium sources showed the distinct textural and structural characteristics. The calcined NbMCM-41 materials with Si/Nb ratio of 16 (Fig. 1A) gave well-defined hexagonal XRD patterns with a main peak (indexed as [ 100] assuming a hexagonal unit cell) at 20 ~ 2 ~ and up to 3 signals in the region

1365 20=3-8 ~ These reflections are due to the ordered hexagonal array of parallel niobosilica tubes [5]. The materials with less niobium content have got a very sharp signal indexed as [ 100] and not well resolved signals in the higher angle region (Fig. 1B). It is well known that by the means of X-ray diffraction it is not possible to quantify the purity of the material. Corma [8] attributed the apparently "less-resolved" XRD pattern to the formation of smaller although no less ordered MCM-41 crystallites. MCM-41 is considered as crystalline on a macroscopic level because of the regular arrangement of the mesopores in honeycomb fashion; therefore, it is possible to estimate crystallite size perpendicular to the basal plane with the help of the Xray diffraction pattern using the Scherrer equation: d=0,9 9L/(B 9cos0), where B is the peak width at half-maximum, E - wavelength, 0 - Bragg angle [9]. The data in Table 1 suggest that, with higher silica to niobia ratio the smaller A B crystallites were obtained. To elucidate the pore structure of MCM-41 transmission electron 30 microscopy is usually r used. Fig. 2 shows a TEM image of the hexagonal "~: 20 arrangement of uniform, jb -~4 nm sized pores in the samples of Nb(O)MCMlO 41-32 and Nb(C1)MCM41-32. However, the unambiguous analysis of o 4 6 8 2 4 8 lo the pore size and 2| o 2e, o thickness of the pore walls is very difficult and Figure 1. Powder X-ray diffraction patterns of the NbMCM-41 possible without materials prepared with Si/Nb ratio of 16 (A) or 32 (B) by using not additional simulations niobium oxalate (a), ammonium-oxalate complex of niobium because of the focus (b), and niobium chloride (c).

,o

rA.

/,

c

!

!

Table 1. Estimates of d spacing, pore, crystallite, and particle size for various niobium-containing MCM-41 samples.

d spacing, nm

Average pore diameter, nma

Crystallite diameter, lam b

Particle diameter, lamc

Nb(O)MCM-41-16

3.68

3.90

0.035

2.0

Nb(Oc)MCM-41-16

3.74

3.24

0.028

-

Nb(O)MCM-41-32

3.81

4.56

0.025

2.0

Nb(Oc)MCM-41-32

3.56

4.62

0.018

6.0

Nb(CI)MCM-41-32

3.51

3.30

0.019

2.0

Sample

The data was estimated: a _ by using gas adsorption (BJH -des. ), b_ from line broadening of X-ray diffraction, and c_ from optical microscopy.

1366

.::.:." 9 .

~!.. . . . .

..::!::...~:: .

..:..i:: 9

. . . . . . .

~ ~ ' . ' - ' ~ . ~ . - ~ ~ : - ~ " - . r ~ : : 8 ~ . : : '

..<.:.

.

.. ,.. ~ : . : ~

...~ y .: .: :..'~,~:-:! ......".:::~

:.:: 2 ~ ; ~ . ~ 1

Figure 2. Transmission electron micrographs of Nb(O)MCM-41-32 (A) and Nb(C1)MCM41-32 (B). The marker represents 10 nm and arrow indicates less ordered region. problem [10,11]. In the case of all synthesized mesoporous niobium-containing MCM-41 materials, prepared with niobium oxalate or niobium chloride, ordered regions with the hexagonal structure are visible. However, some contrast areas (marked with arrow, where the hexagonal arrangement of the pores is hardly recognized) are also observed. In Fig. 2 it is also possible not only to notice the honeycomb structure, but also the equidistant parallel lines which are related to the hexagonal repeat between tubules [12]. Oberhagemann et al. [13] demonstrated in their paper, that the arrangement of different parts of particle of boroncontaining MCM-41 is misaligned. These parts can be considered as blocks that are tilted against each other in different directions. One can adopt this model to the results in this paper and conclude that well resolved hexagonal structure was obtained, but slightly incoherent. In contrast, the Nb(Oc)MCM-41-16 and -32 show the most disordered structures, which is also confirmed by the less resolved XRD patterns (with less intensity of the [100] signal). The existence of some lamellar phase cannot be excluded. The nitrogen sorption isotherms of all 10oo synthesized samples are of type IV in the FJPAC classification and show a 800 a b distinct feature: a sharp capillary condensation step at a relative pressure o f - 0 . 3 5 (compared to 0.4 for a pure ~ 600 Eo siliceous material prepared under analogous conditions) Fig. 3. c "~" 400 Additionally, no hysteresis loop was detected in this region which prove high -8 200quality mesoporous framework in all Z prepared materials. It is worthy to note that for all samples (especially those o o o prepared with niobium chloride and Relative pressure, P/Po oxalate) one can observe very high Figure 3. Adsorption/desorption isotherms of nitrogen adsorption in mesoporous region at 77 K on NbMCM-41-32 materials prepared by (plateau a t - 5 0 0 cm 3 g-l). Only for using niobium oxalate (a), ammonium-oxalate niobium containing MCM-41 prepared complex of niobium (b), and niobium chloride (c).

f

9

1

9

.

9

!

9

!

.

|

9

1367 with niobium chloride there is no hysteresis between the adsorption and desorption branches at higher partial pressures. However, the biggest one was visible for Nb(Oc)MCM-41-16. It seems that a high content of niobium causes the formation of extraframework niobium species, as it was stated earlier [5,14]. The hysteresis loop is probably caused by capillary condensation in the secondary interparticle mesopores (like as it was stated for vanadium[15] and titanium-containing [16] mesoporous molecular sieves) and the steepest part of this loop is in the p/p0 range close to the saturation pressure. The slope of the linear increase at low pressure is found to be significantly lower compared with the slope at higher pressures for samples prepared with oxalate ions, indicating a much larger surface area of the external surface with the pores [17]. However, the pore volume for these samples is very high, even higher than for Nb(CI)MCM-41-32, as seen from Table 2. This could be due to the additional pore system (maybe mesoporous niobium oxide), but probably not from the external niobium oxide because samples prepared via impregnation of niobium oxalate on siliceous mesoporous sieves ofMCM-41 type show lower surface area and pore volume (Table 2). Table 2. Structural properties of the calcined MCM-41 samples estimated by N2 adsorption/ desorption and infrared measurements. Surface area, m 2 g-1 (I)

Total pore volume,

Pore diameter,

SiMCM-41

1140

1.19

Nb(O)MCM-41-16

1409

Nb(Oc)MCM-41-16

Sample

c m 3 ~-1 (I)

nm(iI)

Wall R = 1960 crn'I/ thickness, nm(III)

1480 cm-1 (IV)

3.46

1.22

0.065

1.37

3.35

1.06

0.318

1185

0.96

3.08

1.39

0.337

Nb(O)MCM-41-32

1355

1.55

3.57

1.00

0.269

Nb(Oc)MCM-41-32

1310

1.52

3.32

0.95

0.330

Nb(CI)MCM-41-32

1334

1.10

3.01

1.06

0.185

Nb(C1)/MCM-41-32 (v)

1047

1.04

-

-

0.051

Nb(O)/MCM-41-16 (x0

433

0.58

-

-

0.063

(I) _ BJH, from desorption branch of isotherm; g0_ calculated using w=Cdloo[pVp/(l+pVp)] 1/2, where c=(8/(31/27t))u2; 19=2.2 gcm3; 011)_ calculated as the difference between ao=2dloo31/2 and w/1.05; (IV)_ from infrared spectroscopy; ~ - samples obtained by impregnation. The maximum in the pore size distribution (Fig. 4) of all niobium-containing MCM-41 at ca. 3.2 nm is typical of MCM-41 and that at ca. 4 nm is ascribed to the intraparticle pores and is similar to those observed for vanadium- and titanium-containing MCM-41 [15,16]. It cannot come from niobium oxide localized inside the pores because the impregnated samples (Fig. 4 C) show different pore distribution patterns. The existence of the intraparticle pores causes the difference between the average pore diameter shown in Table 1 and the pore diameter exhibited in Table 2 and calculated according to the procedure described below. To determine the pore size distribution in cylindrical pores, several methods are known from which based on geometrical considerations is commonly used [18]. In this case, the pore diameter w can be obtained from the pore volume Vp and the lattice spacing d (obtained from

1368

0.5]

A

]

1 ~~MCM-41

$

i"o

4

8

B

~~

12

C

-- Nb(O)MCM-41-32

4

8

12

= SiMCM-41

4

8

12

16

Pore diameter, nm Figure 4. Pore size distribution os mesoporous molecular sieves os MCM-41 type (based on the analysis of desorption portion of the N~ isotherm). X-ray diffraction data) in the following way: w = cdl00[pVp/(l+pVp)] 1/2, where c = (8/(31/2rt))u2; p is the density of pore walls and is equal to 2.2 g cm"3. Additionally, the pore wall thickness, b, is calculated under assumption of the hexagonal pore geometry and is equal to the unit cell parameter, a (a=2(3~/2)d), minus the distance between the midpoints of the sides of the hexagonal cross section (equal to w/1.050) [19]. For the Si/Nb ratio of 32 the greatest wall thickness was observed for the sample prepared with niobium chloride. Other samples (with exception of Nb(Oc)MCM-41-16) show lower values of the wall thickness. 9

....

~

-.

==============================

.........................

~

.::.:-:::::::-:::. ' 9- " " " - : . . -

~,'~--~\ ...

~:--:---

.

.

.

.

- ..............................

: : : : ~ i , . ~ . ~ A ~ a . , - - ~ - - ~ " ' ~ !

":~i:~:i:~:!:!:~!

......

.....

~ .............

..-..

Figure 5. Scanning electron micrographs of the niobium-containing samples synthesized from: A - niobium oxalate, B - niobium chloride and C - niobium complex with Si/Nb ratio of 32 (the marker represents 1 I.tm). The final products prepared with NbCI5 and Nb(HC204)5 show microparticle morphology (Fig. 5 A and B). When one compares transmission micrographs (not shown here) with scanning electron microscopy images (Fig. 5 C) it could be concluded that the addition of ammonium-oxalate complex of niobium into the gel during the synthesis leads to the formation of a lamellar material. A microanalysis performed during the scanning shows that

1369 the distribution of niobium and silicon is regular in ease of samples prepared with niobium complex and chloride. The latter sample do not include sodium chloride that is oRen formed during the synthesis and could be a source of Lewis acidity (Na+). The formation of polarized Si-O~'......Nb ~+ bond due to the incorporation of Nb in the framework of mesoporous molecular sieves is evidenced by means of IR [20,21]. The ratio (R) between the absorbance of a band which could be assigned to the polarized Si-O~. ..... Nb ~+ bond (960 cm"1) and a structure band (480 cm"1) should give information about the position of niobium. As one can see from Table 2 this ratio is bigger for samples prepared with addition of niobium during the synthesis than by postsynthesis treatment- the impregnation.

The effect of Nb sources on the hexagonal arrangement and the characteristic parameters of NbMCM-41 Low-pressure adsorption curves are very similar for all NbMCM-41 materials with Si/Nb=32 (Fig. 3). At higher pressures the plots exhibit upward deviations caused by capillary condensation in primary mesopores. This is not so sharp as in siliceous MCM-41. The main difference in the N2 adsorption/desorption isotherms appeared in the high p/p0 range. The highest loop was found on Nb(Oc)MCM-41-32 and the lowest on Nb(CI)MCM-41-32 (Fig. 3). This is accompanied by the lowest ordering of the Nb(Oc)MCM-41-32 sample estimated on the basis of X R patterns and TEM. However, the samples prepared from ammoniumoxalate complex of niobium exhibit the uniform pore size distribution (Fig. 4) whereas, the other samples showed the existence of two primary mesoporous systems and Nb(O)MCM-41 even secondary mesopores. The surface area of the materials seems to be independent on the Nb source used for the synthesis, when Si/Nb=32 (Table 2). The other parameters like crystallite diameter, pore diameter, wall thickness and total pore volumes are influenced by the nature of Nb sources and they change in the following orders (for Si/Nb=32): Crystallite diameter ~ Nb(O)MCM-41 > Nb(CI)MCM-41 > Nb(Oc)MCM-41 Pore diameter ~ Nb(O)MCM-41 > Nb(Oc)MCM-41 > Nb(CI)MCM-41 Total pore volume ~ Nb(O)MCM-41 > Nb(Oc)MCM-41 > Nb(CI)MCM-41 Wall thickness :::> Nb(CI)MCM-41 > Nb(O)MCM-41 > Nb(Oc)MCM-41 Among the above-presented molecular sieves, Nb(O)MCM-41-32 exhibits the highest crystallite diameter, pore diameter, and pore volume, whereas the highest wall thickness was registered when NbCI5 was used in the synthesis. The influence of Si/Nb ratio on the quality of NbMCM-41 molecular sieves The role of Si/Nb ratio was considered for two niobium sources: oxalate and oxalateammonium complex. The lower Si/Nb ratio (16) the higher participation of the second primary mesoporous system suggesting the presence of the additional mesoporous system (maybe mesoporous niobium oxide). The wall thickness decreases with the decrease of Nb content (from Si/Nb=l 6 to Si/Nb=32) while the total pore volume increases in the same order. The other parameters like surface area and pore diameter in the samples prepared from niobium oxalate and niobium ammonium-complex change in various ways depending on Si/Nb ratio. Location of Nb IR spectroscopic results did not indicate the difference in the ratio R(I960/I480) for the samples prepared from ammonium oxalate complex of niobium exhibiting various Si/Nb ratios (16 and 32) suggesting that in both samples the number of Nb atoms in the lattice is

1370 similar. Taking into account the higher content of Nb in NbMCM-41-16 than in NbMCM-4132 one can conclude that more Nb-extra lattice species exists in the sample with Si/Nb=l 6. The decrease of Nb content is not proportional to the decrease of R value for niobiumcontaining samples prepared with niobium oxalate. This is in agreement with the results of pore size distribution as the second primary mesopores system exists when Si/Nb = 16. 4. CONCLUSIONS The presented results point at the important role of a niobium source used in the synthesis of mesoporous MCM-41 molecular sieves and its influence on the structure of the prepared materal. The catalyst with the uniform pore size distribution was obtained from ammoniumniobium oxalate complex. ACKNOWLEDGEMENT I am grateful to Prof. Maria Ziolek for helpful discussion and significant contribution. Companhia Brasiliia de Metalurgia e Minera~;ho is acknowledged for providing niobium oxalate and ammonium-oxalate complex of niobium. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C Vartuli, J.S. Beck, Nature, 359 (1992) 710. 2. M. Ziolek, I. Nowak, Zeolites, 18 (1997) 356. 3. M. Ziolek, I. Nowak, J.C. Lavalley, Catal. Lett., 45 (1997) 259. 4. L. Zhang, J.Y. Ying, AIChE J., 43 (1997) 2793. 5. M. Ziolek, I. Sobczak, I. Nowak, P. Decyk, A. Lewandowska, J. Kujawa, Microporous and Mesoporous Mater., 35-36 (2000) 195. 6. M. Ziolek, I. Nowak, I. Sobczak, A. Lewandowska, P. Decyk, J. Kujawa, Stud. Surf. Sci. Catal., 129 (2000) 813. 7. J. Xin, J. Suo, X.. Zhang, Z. Zhang, New J. Chem., 24 (2000) 569. 8. A. Corma, Chem. Rev., 97 (1997) 2373. 9. C.G. Sonwane, S.K Bhatia,. Langmuir, 15 (1999) 2809. 10. C.Y. Chen, S.-Q. Xiao, M.E. Davis, Microporous Mater., 4 (1995) 1. 11. S. Schacht, M. Janicke, F. SchOth, Microporous and Mesoporous Mater., 22 (1998) 485. 12. A. Chenite, Y. LePage, A. Sayari, Chem. Mater., 7 (1995) 1015. 13. U. Oberhagemann, I. Kinski, I. Dierdorf, B. Marler, H. Gies, J. Non-Crystalline Solids, 197 (1996) 145. 14. M. Ziolek, I. Sobczak, A. Lewandowska, I. Nowak, P. Decyk, M. Renn, B Jankowska, Catal. Today, 70 (2001) 169. 15. S. Gontier, A. Tuel, Microporous Mater., 5 (1995) 161. 16. M.D. Alba, A.I. Becerro, J. Klinowski, J. Chem. Soc. Faraday Trans., 92 (1996) 849. 17. R. Schmidt, M. StOcker, E. Hansen, D. Akporiaye, O.H. Ellestad, Microporous Mater., 3 (1995) 443. 18. M. Kruk, M. Jaroniec, A. Sayari, J. Phys. Chem., 101 (1997) 583. 19. M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, Ch.H. Ko, J. Phys. Chem. B, 104 (2000) 292. 20. M.A. Camblor, A. Corma, J. P6rez-Pariente, J. Chem. Soc. Chem. Commun., (1993) 557. 21. K.S. Smirnov, B. van de Graaf, Microporous and Mesoporous Mater., 7 (1996) 133.

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 ElsevierScienceB.V. All rights reserved.

1371

I n c l u s i o n of e u r o p i u m ( I I I ) ~ - d i k e t o n a t e s i n m e s o p o r o u s M C M - 4 1 silica. /L Fernandes a, J. Dexpert-Ghys a, C. Brouca-Cabarrecq a, E. Philippot~, A. Gleizes b, A. Galarneau c and D. Brunel c a Centre d'Elaboration de Mat6riaux et d'Etudes Structurales (UPR 8011), rue Jeanne Marvig, BP 4347, 31055 Toulouse Cx 4 France b Centre Interuniversitaire de Recherche et d'Ing~nierie des Mat6riaux (UMR 5085), ENSIACET, 118 route de Narbonne, 31077 Toulouse Cx 4 France c Laboratoire des Mat6riaux Catalytiques et Catalyse en Chimie Organique (UMR 5618), ENSCM, 8, rue de l'Ecole Normale, 34296 Montpellier Cx 5 France

Inclusion of europium tris(2,2,6,6-tetramethyl-3,5-heptanedionate), [Eu(thd)8], and of europium(IH) tris(1,3-diphenyl-l,3-propanedionate) [Eu(dbm)3], in mesoporous MTS (Micelle Templated Silicas) materials such as MCM-41- type silicas has been realized by impregnation as well as by vapour reaction. The immobilisation occurs via the grafting of the Eu ~+ on the free silanol groups of the surface. The luminescence characteristics of the europium ~- diketonates precursors are strongly modified by the reaction, but still exhibit ligand to metal energy transfer. 1. I N T R O D U C T I O N The search for new optioally active materials combining a metalorganic complex and an inorganic matrix [1] has led us to investigate t h e inclusion chemistry of europium-diketonates in MCM-41 silicas. This paper describes the photoluminescence, structural and textural characteristics of materials denoted {Eu(L)x}M obtained by reacting europium ~-diketonates [Eu(L)3] with the hexagonaUy structured mesoporous silica MCM-41 noted as M. One of our goals is to compare materials obtained via the classical "wet" process (the impregnation of the matrix by a solution of the precursor), with those prepared via a "vapour" process (the reaction of the matrix with the sublimated complex). The vapour process requires readily volatile complexes that sublime without decomposition like for the CVD process. Europium tris (2,2,6,6-tetramethyl-3,5-heptanedionate), Eu(CllH19OD3, [Eu(thd)8], a commercially available product, was chosen as it can be used in either process. The "wet" process was also applied to

1372 europium(III) tris(1,3-diphenyl-l,3-propanedionate), Eu(Cl~HllO2)3, [Eu(dbm)3] , which was synthesized in the laboratory. Due to strong absorption in the nearUV region, the ligand dbm is a good sensitizer of visible luminescence of the Eu 3+ cation t h a n k s to the so-called "antenna" effect. This effect is expected to still occur after inclusion in the mesoporous silica matrix.

2 . EXPERIMENTAL 2.1. S a m p l e s preparation, The MCM-41 silica (pore size 3.5 nm) was prepared and characterized following known procedures as described for instance in [2]. After removal of the organic template CTMA (=cetyltrimethylammonium) and activation at 180~ (samples 1-4) or at 195~ (sample 5) under vacuum, the substrate was kept away from ambient air. Eu(dbm) 3 was synthesized according to the method reported in Ref [3]. Complex-included samples were prepared by impregnation of activated MCM-41 (100 mg) by a solution Eu(thd) 3(1,42.10-3M) or Eu(dbm) 3 (1,22.10~M) in cyclohexane, for 24h at 25~ followed by several washings with cyclohexane and drying at room temperature. The ,~ v a p o u r - process consisted in reacting activated MCM-41 (100mg) with Eu(thd) 3 sublimed (100mg). The reactants were introduced in separated open glass ampoules in a pyrex-glass tube. After sealing under vacuum, the tube was heated at 182 ~ for 66 hours, and quenched in air. 2.2 C h a r a c t e r i z a t i o n s Chemical analyses were performed by the Service Central d'Analyse du CNRS. Si and Eu by ICP-AES, C and H after combustion of the organic p a r t ; %(0) calculated from mass differences. . Textural properties have been characterized by N2 sorptiomat 77K on samples outgassed at 423K during 1 2 h under vacuum (10 ~ torr). N 2 sorption experiments have been carried out in a Micromeritics ASAP2000 apparatus. X-ray powder diffraction patterns were recorded on a Seifert diffractometer using a monochromated CuKcz radiation and a set of slots apropriate for m e a s u r e m e n t s at low diffraction angles (20 from 1 to 5~ Energy loss spectra were obtained with a spectrometer mounted on a Philips CM20 (200kV) transmission electron microscope (TEM). IR spectra were recorded on a FT-IR Brucker Vector22. Fluorescence measurements (excitation and emission spectra) were recorded with a Hitachi F4500 spectrophotometer. The X-ray diffraction and EELS investigations were performed on samples resulting from the , , v a p o u r , process immediately after the solid-vapour reaction, and from the ~, w e t , route after impregnation, washings and room t e m p e r a t u r e d r y i n g . The chemicals analyses, the IR spectroscopy, and the visible fluorescence investigation were performed on samples having been submitted to the N 2 sorption process. Self-supported wafers were prepared for the IR investigation and further outgassed under 10 .2torr at 423K for 4 hours

1373 3.

RESULTS

3.1. T e x t u r a l c h a r a c t e r i s t i c s

c~ 600

03

-

pure MCM-41

E o 500 400

~L_

o

300

r

200 [~ >

100

i ~ -.

0 0.0

0.2

0.4 0.6 relative pressure (P/Po)

0.8

Figure 1. N 2 adsorption-desorption isotherms; {Eu(thd)~} n~

1.0 as in Table 1.

Table 1 S t a r t i n g compositions" X ~.,(~d)3(mg) for 100mg of d e h y d r a t e d M C M - 4 1 , chemical analyses results,and t e x t u r a l characteristics" specific area (A), pore volume (V) and pore d i a m e t e r (D) of {Eu(thd)x}M samples ~.

Samples a n~ Process 1 2 3 4 5

wet wet wet wet Vapour

X 25 50 75 100 100

A n a l y s e s (% w e i g h t ) Si Eu C H O 34.5 32.1 30.5 29.4 24.5

4.5 8.2 11.1 13.3 11.0

2.5 0.5 2.8 0.7 4.0 0.8 5.7 1.2 15.7 2.5

58 56.2 53.6 50.1 46.3

N 2adsorption results A (m2/g) V (ml/g) D (rim) 774 708 646 625 338

0.55 0.48 0.44 0.38 0.13

3.6 3.45 3.3 3.2 2.3

The N 2 adsorption/desorption isotherms for {Eu(thd)x}M are displayed in Figure 1 and the t e x t u r a l characteristics are g a t h e r e d in Table 1 The mesoporous s t r u c t u r e is preserved after the inclusion process. However, the specific surface, pore volume and pore size are reduced with respect to the pure MCM-41 ( A~ T = 952 m2/g, pore volume = 0.74ml/g, pore diameter = 3.65nm), and decreased as a function of the complex loadings. The sample prepared according to the vapour process shows the highest pore reduction, in agreement with the largest a m o u n t of organic p a r t found by chemical analyses.

1374

3.2. Quantitative analyses by Energy Loss Spectroscopy (EELS) Results from EELS and chemical analyses are compared in Table 2. Quantitatively, the localized and bulk determinations are coherent. At the spatial resolution of the probe (150 nm), the samples are monophasic and the europium ions are homogeneously dispersed inside the silica matrix. Table 2. Atomic ratios [Eu]/[Si] measured by EELS* versus bulk analyses. ......

s

.pie

.....

........

2

3 .

.

.

.

4 ........

' [Eu]/[Si] EELS 3.3_+0.3% 5.7_+0.5% "7.4_+0.7% 8.1_+0.8% [Eu]/[Si] bulk 2.40_+0.06% 4.72_+0.09% 6.73_+0.10% 8.33_+0.10% 8.30_+0 10% * [Eu]/[Si] ratios calculated as ([Eu]/[O])*([O]/[Si]). [Eu]/[O] determined by comparison with experimental ratio (Eu M-edge / O K-edge) in the standard Eu203. [Si]/[O] by calculated Si and O's K-edge cross sections. .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

3.3. X-ray diffraction. Pure and loaded MCM-41 exhibit, in the low-diffraction angles range, diagrams characteristic of the hexagonal a r r a n g e m e n t of the channels (Figure 2). It has been established t h a t the peak intensities are all the more weakened with respect to pure MCM-41 silica as the electronic density from inserted m a t t e r increases in the tunnels (for instance in Ref [4]). This is the general trend observed here if one considers the Eu(thd) 3 / MCM-41 ratio in the initial mixtures (Table 1). The difference observed between samples n~ (wet) and n~ (vapour) could be linked to the larger amount of ligand measured after reaction for the second one. It must be noticed t h a t none of the diffractograms recorded at larger diffraction angles shows peaks indicative of the free europium complex. '

C"

.

'

'

I

~.a~,~" '

'

/ joure

'

I ......

'

'

'

I

'

'

"

3 4 L _

co rC ,

2

3

4

2 theta Figure 2. XRDiffractograms; {Eu(thd) X}n~

as in Table 1.

,

5

1375

2.3.

IR s p e c t r o s c o p y

pure M C M - 4 1

,m

pure4~ CM- 1

c.>,

.Q

3000

2900

2800

fl) o c .Q 0 .Q

3800

3700

3600 wavenumbercm

Figure 3.IR spectra ; {Eu(thd) x} n~

3500

3400

-1

as in Table 1.

The IR spectra, were normalized to the intensity of MCM-41 lattice vibrations . On Figure 3 are displayed the two more between 2100 and 1750 c m -1 characteristic w a v e n u m b e r s ranges. The sharp absorption at 3741 cm 1 appearing after removal of the adsorbed water in MCM-41 has been conclusively assigned to vo~ in isolated silanol groups on the walls of the tunnels ~ . g ;. ref. [5]). After impregnation of activated MCM41, the intensity of this-vibration diminishes with increasing content of the loaded complex, according to the elemental analyses. In sample n~ the isolated silanol signal completely disappears and it appears a band at 3705 cm -1 t h a t could be due to c o m p l e x - hydrogen bonded silanols. Intensities of vibrations characteristic of the ligands (vcm,c~) in the 3000 - 2800 cm 1 range increase simultaneously from sample n ~ to n~ These observations coherently evidence t h a t the metalorganic species are grafted on the mesoporous walls via the silanol groups.

3.4.

Visible fluorescence The Eu 3§ luminescence displayed on Figure 4 and 5 were recorded at room t e m p e r a t u r e and pressure. The narrow emission lines 5Dj-~7Fj, are characteristic of the europium i m m e d i a t e environment ; this is illustrated for instance by the differences observed between Eu(thd) 3 and Eu(dbm)~ emission spectra in the 550680 n m range (~Dj--~ 7F~3) : The excitation spectra exhibit simultaneously narrow intra-4ff lines and broader features due to the ligand part.

9s p u e q uo!~daosqe pue~.~[ oq~ u! 'uiu OL8 ao O~;g " sA~oaa~ ~;q u ~ o q s sq~uoIOAeA~ SUO!~m!OXO 'mu099-0~g 9uo~s.rm~t "tuu 119 ~uvo~!uotu 'uIu00g-0~ ~ 9uo.t~e~t.oxx "lA~{(uzqp)n~i} pug ~(tuqp)nx jo oouoosout.uzn~I "9 o2n~.t~i

OOZ

.

.

.

I

.

i

1 I

009 I

I

s I !

i

I

(tuu) q],BUOlOABAA OOg O0"lr '

I

'

"

'

'

I

'

'

'

'

00~ I

'

'

~v

9

!

-.

I

l~{(wqp)n3}

I

.........

~

,~.

,

i I I I

C

g .

..

,

I

..,~,

..

.

.

I

.

.

',

'

I,

.,l'

.."

.'

. .

(9~I~- ~'~ ~ uIU 068 zo ( ~

.

r~

I

.

.,,,

.

~

"

s u ~ o q s sq~uoIoAU~ uo.t~u].roxo 'tlIU099-0~g 9uo.tss.tm~t "uzu I I 9 ~u!Jo~!uouI 'uzu00g-0g~" uo.t]u~.tox~t "lAI{X(pq~)n~t} puu ~(pq])n~[ jo oouoosou.tmn~I "~ ozru~.~i

OOZ w

(WU) Li::I6UOIOAI~M

009

I

'

'

.....

I

.

009

'

'

'

'

I

,-, ; . . . . . . . . I

I

"-.

OOg

00"17.

'

'

-'",;

', :'.; I I I I

I I I I

'

'

I

'

'

,, I , , " , :',,

t/

I

'

"''

I

t

I I

'

'

I

'

'

"

',

~1.-"

I

" =

g.

Ii

_~.

g

g( ,Pq::l)n3 -

~i |,

I

.

.,

9

9L~!

1377 The characteristic features of the corresponding Eu(L) 3 precursor are not recovered in the emission spectra of the {Eu(L)~}M hybrids, which unambiguously proves the modification of the Eu surroundings. In the luminescence excitation spectra, the intra-4ff excitations 7Fo.1 --> ~D2 (468 nm) 7Fo.1 --~L 6 (390 nm) appear with different relative intensities. F e a t u r e s relative to the ligand, which give evidence for the , , a n t e n n a effect- also exhibit drastic variations before and after their immobilization on the silica substrate. F u r t h e r investigations are needed to explain accurately these observations. As they stand, the luminescence excitation data corroborate the emission part and prove the strong interaction between the mesoporous substrate and the metal complexes. This interaction affects both the Eu 3§ surroundings and the ligand to metal energy transfers.

4.

DISCUSSION

These investigations of two mixed systems denoted {Eu(L)x}M obtained by reacting europium ~-diketonates [Eu(L) 3] with the hexagonally structured, mesoporous silica MCM-41 (M) must be discussed under several aspects. Two ligands were considered: (2,2,6,6-tetramethyl-3,5-heptanedionate) = (thd), and (1,3-diphenyl-l,3-propanedionate) = (dbm). For {Eu(thd)x}M samples, two reaction routes were compared: the classical "wet" process (the impregnation of the matrix by a solution of the precursor), and the "vapour" process (the reaction of the m a t r i x with the sublimated complex). The two synthesis routes lead to the homogeneous immobilisation of Eu(L) x species within the channels of the mesoporous silica, as concluded from X-ray diffraction and localized analysis (EELS) results. Nitrogen adsorption / desorption experiments prove that the mesoporous structure is preserved after the inclusion process in b o ~ routes. A comparison of the wet and the vapour routes may be done by considering samples n ~ 4 and n~ (Table 1): a higher ligand to europium ratio, stronger reductions of the pore volume, of the hexagonal X-ray diffraction and of the (yon isolated silanols) vibration intensities are observed for the sample elaborated via the vapour process. The m a x i m u m insertion rate, expressed by the [Eu]/[Si] atomic ratio, is 8.5% for both synthesis routes, t h a t is about 2/3 of the n u m b e r of silanols per silicium measured on similar substrates (13.2 % in Ref. [6]). This fact, and the observed diminution of the voH vibration intensity simultaneously with the increase of Eu loading strongly suggest t h a t the immobilisation takes place via Si-O-Eu grafting. The characteristic Eu 3§ emissions of the metal-organic precursors are lost after grafting and the emission spectra are very similar for all the hybrid materials investigated, whatever the synthesis route and even the nature of the ligand (thd or dbm). Ligand to metal energy transfer still occurs after grafting on the silica surface, but t h e observed strong modifications of the Eu 3+ luminescence excitation spectra are not still understood. Similar observations

1378 have been reported in Ref. [7] when the complex Eu(dbm)3phen (phen = 1,10phenanthroline) is intercalated in unmodified- or modified- MCM 41s. 5. C O N C L U S I O N The inclusion of europium ~- diketonates in mesoporous MCM-41 silica has thus been realized by impregnation as well as by vapour reaction. The different characterization methods employed coherently suggest that the immobilisation occurs via the grafting of the Eu 3+ on the free silanol groups of the surface. The luminescence characteristics of the europium 13- diketonates precursors are strongly modified by the reaction, but still exhibit ligand to metal energy transfer (the so-called ,,antenna effect~). It is worthy of note that the conclusions we have reached are valid for samples having been submitted to severe post-synthesis outgassing. Further investigations are now in progress to study the different reaction steps in the synthesis routes.

REFERENCES 1. J. Dexpert-Ghys, C. Picard and A. Taurines. J. of Inclusion Phenomena and Macrocyclic Chemistry, 39:(2001) 261. 2. D. Desplantier-Giscard, O. Collart, A. Galarneau, P. Van der Voort, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 129 (2001) 665 3. K.J. Eisentraut and R.E. Sievers, J. Am. Chem. Soc., 87 (1965) 5254. 4. B. Marler, U. Oberhagemann, S. Vortmann, H.Gies Microporous Mater., 6 (1996) 375. 5. J. Chen, Q. Li, R. Xu and F. Xiao, Angew. Chem. Int.Ed. Engl. 34 (1995) 2694. 6. G. Gerstberger, C. Palm and R. Anwander, Chem. Eur. J. 5, 3 (1999) 997. 7. L. Fu, Q. Xu, H. Zhang, L. Li, Q. Meng and R. Xu, Mater. Sci. and Engineering B28 (2002) 68.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1379

Synthesis, characterization and catalytic properties of mesoporous titanostanno silicate, Ti-Sn-MCM-41 Nawal Kishor Mal,a'* Prashant Kumar,b Masahiro Fujiwaraa and Koji Kuraoka a aAIST

Kansai, 1-8-31 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, JAPAN

bCeramic Section, Industrial Research Institute of Ishikawa, Kanazawa 920-0223, JAPAN

Mesoporous titanostanno silicate, Ti-Sn-MCM-41 was prepared by direct hydrothermal synthesis. Samples were characterized using XRD, SEM, UV-visible, FT-IR, Sorption techniques (N2 and HzO), TG-DTA and 29Si MAS NMR. Sn-containing samples are more hydrophilic in nature and contain more hydroxyl groups in comparison to Si-MCM-41. The selectivity of cyclohexene to cyclohexene oxide in the presence of tert-butyl hydroperoxide over Sn-Ti-MCM-41 is higher than Ti-MCM-41.

1. I N T R O D U C T I O N

Microporous titanostanno silicate (MFI structure) is highly active and selective propylene epoxidation catalyst [ 1]. However, channel of the MFI structure less than 0.6 nm restricts the diffusion of relatively larger molecules such as cyclohexene. Following the discovery of M141S silica mesoporous molecular sieves by Beck et al. [2] in 1992, COrma et al. reported the synthesis of mesoporous organic (-ell3) containing Ti-MCM-41 [3]. Organic (-ell3) containing Ti-MCM-41 is highly active and selective for the epoxidation of cyclohexene [3]. However, regeneration of this catalyst at high temperature may be a serious problem due to presence of methyl group attached to framework silica. In this report, for the first time, we describe the synthesis of mesoporous Ti-Sn-MCM-41, their characterization by using X R , SEM, UV-visible, FT-IR, TG-DTA, sorption techniques (N2 and H20) and 29Si MAS NMR, and their catalytic activity in the epoxidation of cyclohexene in presence of tert-butyl hydroperoxide (TBHP) as oxidant. +NNM is grateful to AIST, Tokyo for STA fellowship.

1380 2. MATERIALS AND METHODS 2.1. Materials and synthesis

The reactant used in this study were silicon ethoxide (99.9%, Aldrich Chem.) (TEOS), cetyltrimethylammonium bromide (96%, Kanto Chem.) (CTMABr), tetramethylammonium hydroxide (25% aqueous, Aldrich Chem.) (TMAOH), titanium ethoxide (Aldrich Chem.) (TEOT), tin(IV) chloride (99%, Wako Chem.) (SnC14), 2-propanol (2-PrOH, Wako Chem.) and H2SO4 (96%, Wako Chem.). Ti-Sn-MCM-41 samples were synthesized in absence of sodium using following molar composition: 1.0 SiO2 : 0.48 CTMABr : 0.67 TMAOH : (0.0 0.033) TiO2 : (0.0- 0.010) SnO2 : (1.0-4.0) 2-PrOH : 0.20 H2504 : 80 H20 with pH ofthe gel was varied between 10.26 to 11.46. In a typical preparation, 24.43 g of TMAOH and 18.22 g of CTMABr were dissolved in 100 g of water by stirring at 308 K to give a clear solution. 20.85 g of TEOS was added and stirred for 30 min. 0.38 g of TEOT (Si/Ti - 60) in 10 g of 2-PrOH was then added followed by 0.26 g of SnC14 (Si/Sn = 100) in 10 g of 2-propanol under stirring for 3 h. Finally, 2.0 g of H2SO4 in 126 g of H20 was added and stirred for 2 h. The resultant gel (pH = 10.31) was aged at room temperature for 1 day then heated statically at 373 K for 4 days under autogenous pressure. The products were filtered, washed, dried at 378 K and calcined at 823 K for 6 h to yield Ti-Sn-MCM-41. Ti-MCM-41 was prepared using the same procedure without addition of SnCI4, where as Si-MCM-41 was prepared without addition of titanium and tin sources. 2.2. Characterization

Elemental analyses of the samples were carried out using ICP (Shimadzu ICPV-1017). Characterization of the samples was carried out using XRD (Shimadzu XRD-6000), N2 sorption at 77 K (Bellsorp 28 instrument), H:O sorption at 298 K and at fixed p/po ratio of 0.5 in electrobalance (Chan, USA), FT-IR (JASCO FT/IR-230, UV-visible (JASCO V-560), and thermogravimetric (TG) and differential thermal analysis (DTA) with heating rate of 10 K min1 (Seiko, SSC/5200). 29Si M_AS NM~ spectra were obtained on a Varion VXP-400. 2.3. Catalytic oxidation reactions

Liquid phase epoxidation reaction of cylcohexene was carried out batch wise in two-necked round bottom flask fitted with a condenser and placed in oil bath at 333 K for 5 h under the reaction conditions; 0.10 g catalyst, 12.2 mmol cyclohexene, 5 ml acetonitrile (solvent), 4.05 mmol TBHP (70% aqueous). The reaction products were analyzed in a capillary GC (HP 5880) using 50 m long silicon gum column and identified by known standards and GC-MS.

1381

2.4. Methods The BET surface area [4] was calculated in the relative pressure range between 0.04 and 0.2. The average pore diameter (APD) was calculated using adsorption branch of isotherms. Total pore volume was determined from the amount adsorbed at relative pressure of 0.99 [4]. The pore size distributions were calculated from adsorption branches of the nitrogen adsorption isotherms using Barrett-Joyner-Halenda (BJH) method [5].

3. RESULTS AND DISCUSSION 3.1. Synthesis, structure and sorption properties Physico-chemical characterization of various titanium tin containing MCM-41 samples are shown in Table 1. All the samples were prepared in absence of sodium because in the presence of sodium formation of SnO2 takes place (confirmed by UV-visible). The Si/Ti molar ratio in product is higher than in synthesis gel for all samples. Where as Si/Sn ratio in product is lower than in synthesis gel except for sample 3 (Table 1). In Fig. 1, XRD profiles of samples 1, 2, 3 and 6 are shown. Four peaks in the XRD patterns of all the samples are observed, which are characteristics of long range ordering of a typical MCM-41 material. Ti-MCM-41 and Ti-Sn-MCM-41 samples have higher interplanar, dl00 spacing than Table 1. Physico-chemical Characterization of Ti-Sn-MCM-41 a Mole ratio in gel Sample

pH

Si/Ti

1

11.46

2

11.15

60

3

10.26

30

.

Si/Sn .

in product

dloo/

ao/

aBET/ Vp/

APD

HeO b

Si/Ti

nm

nm

m2g-1 cm3g"1

/nm

/wt%

3.56

4.11

1 0 8 0 0.93

3.54

16.2

Si/Sn

.

.

oo

68

-

3.68

4.25

1 0 4 5 0.89

3.63

17.3

52

210

3.77

4.35

930

3.67

20.4

200

0.82

4

10.31

60

100

61

89

3.74

4.32

998

0.78

3.61

24.5

5

10.49

60

200

75

164

3.72

4.30

967

0.75

3.56

20.1

6

10.60

60

600

64

405

3.70

4.27

1 0 1 1 0.80

3.58

18.4

7

10.86

60

900

89

533

3.67

4.24

1 0 2 1 0.85

3.59

17.8

8

10.75

100

200

126

174

3.65

4.21

1 0 5 8 0.86

3.58

19.7

9c

-

91

-

2.84

3.28

881

oo

77

-

-

-

adl00: X-ray diffraction (100) interplanar spacing; ao: unit cell parameter = 2d100/3v2; Vp: primary meospore volueme; APD: Average pore diameter = 1.213d100((2.2Vp)/(l+2.2Vp))v2; bSorption capacity of water measured gravimetrically at 298 K and P/P0 = 0.5. CDatataken from ref. 7 for comparison.

1382 Si-MCM-41 (sample 1) due to substitution of Si4+ ions by relatively larger Ti4+ and ~, t~ v

d

Sn 4+

c

spacing is much higher than expected

ions. However, an increase in dl00

compared with Si-MCM-41 because the

e-

b

_.=

content of tin (IV) chloride increases in the synthesis gel the pH of the gel decreases.

a

4 i

2

,

6

1'o

20 (degree)

After lowering the pH of the synthesis gel an increase in dl00 spacing was reported by Wang et al. [6]. For all the samples pH of

Figure 1. XRD profiles of different samples:

the synthesis gel varies between 11.46 and

(a) sample 1, (b) sample 2, (c) sample 3, and

10.26. Difference in dl00 spacing also may

(d) sample 6.

cause due to large uncertainties on low diffraction angle values. Sample 9

(Ti-MCM-41), data taken from reference 7 for comparison of catalytic activity, has much smaller dl00 spacing (2.84) in comparison to our samples because characteristics ofMCM-41 strongly affected by the.synthesis conditions [8]. Another possible reason is that sample 9 shows less defined X R , much broader peak at dl00 plane and do not observe any peak at higher order by Blasco et al [7]. In Fig. 2, N2 adsorption-desorption isotherms and pore size distribution of samples 1, 2 and 6 are shown. All the samples show a typical type IV isotherm with narrow pore size distribution with peak pore diameter at 2.8 nm. However, pore size distributions of samples 2 and 6 are relatively broader compared with sample 1 (Si-MCM-41) probably due to presence of Ti and Sn. BET specific surface area and pore volume of Ti and Sn containing samples are marginally lower than for Si-MCM-41 (Table 1). As the content of tin increases in the samples the H20 sorption capacity of samples gradually increased from 16.2 to 24.5. Thus, tin containing samples are more hydrophilic than Si-MCM-41 (Table 1).

800

--;

600 {3)

>

~'2

A

:~

400

,oJ...I".,._

2.01

! .......

0.0

0.2

0.4

0.6

0:8

Relative pressure (P/Po)

110

B

....... : ..............

08]

200

2.8nm

..... / ~"~'*........................... b

oo! -.,.'!-...,........ -'-.... 2

4

6

Pore diameter (nm)

8

....

10

Figure 2. (A) N2 adsorption-desorption isotherms and (B) pore size distribution curve of (a) sample 1, Sample 2 and (b) sample 6.

1383

* CO 2

r

b

b

t 200

360 " Wavelength

460 (nm)

s6o

4o'oo

" 3o'oo

2o'oo

Wave n u m b e r ( c m "1)

~ooo

Figure 3. UV-visible spectra of (a) sample 2, Figure 4. FT-IR spectra of (a) sample 2 (b) sample 5 and (c) sample 6.

and (b) sample 6.

3.2. UV-visible and FT-IR UV-visible spectra of calcined samples 2, 5 and 6 are shown in Fig. 3. All these samples show single band near 220 nm, is taken as proof of Ti [9,10] and Sn [11,12] incorporation into the silica walls. The absence of band near 250-270 nm or 300-330 nm indicates that no extra framework species (partially polymerized hexacoordinate Ti and Sn species) or completely polymerized TiO2 and SnO2 phase, respectively, existed in these samples [9-12]. However, we observed the extra framework Ti species (band near 270 rim) in the case of sample 3, which contains low Si/Ti ratio (30) in gel. FT-IK spectra of samples 2 and 6 are shown in Fig. 4. Presence of vibration band at 1081 and 1079 cm 1 in the samples 2 and 6, respectively, which are lower than compared to pure Si-MCM-41 (1089 cml), may be considered as a substitution of Ti and Sn in the frame work of Ti and Sn containing samples [13]. Samples 2 and 6 show band at 960 and 962 cm 1, respectively, assigned to the framework vibration of Si-O-M (M = Ti, and/or Sn) bond [13,14]. However, in the case of pure Si-MCM-41, this vibration band is also exit due to presence of excess silanol groups in the calcined material. Therefore, vibration band near 960 cm -1 can not be taken as a proof for substitution of titanium and tin in the structure. 3.3. Thermogravimetric analysis (TGA) Thermogravimetric (TG) and differential thermal analysis (DTA) of the Si-MCM-41 (sample 1) and Ti-Sn-MCM-41 (sample 6) are given in Fig. 5. Thermal patterns of the both samples are qualitatively similar [7]. The total weight losses are 48.1 and 49% for sample 1 and 6, respectively. Total weight losses for other samples at 1073 K remain in the 47-52% range. Four distinct weight losses were observed in thermo diagram [15-17]. Weight loss below 409 K corresponds to desorption of physisorbed water (or ethanol) in the voids formed by crystals agglomeration. Three other weight losses can be distinguished; 409-547 K,

1384

1~ t

20

b

10

60. .

"

i

400

"

,

'

,

600

,

i

800

,

x.~~~0

-10

J

1000

Terrtaerature (K)

Terrl:erature (K)

160o

Figure 5. TG-DTA profiles in air of as-synthesized samples (a) Si-MCM-41 (sample 1) and (b) Ti-Sn-MCM-41 (Sample 6). 547-593 K, and above 593 K are related to breakage, decomposition and combustion of residual organics associated with three exothermic processes. Weight loss above 628 K is attributed to condensation of hydroxy groups.

3.4. 29SiMAS NMR 29Si MAS NMR spectra of calcined form of Ti-Sn-MCM-41 (sample 6) is shown in Fig. 6. Intensity of Si(-OSi)3(-OH) (i.e. Q3 at-99 ppm) is comparable to Si (-OSi)4 (i.e. Q4, at -109 nm) even after calcination at 823 K. It indicates that titanium tin containing samples contains much amount of hydroxy groups. Probably the substitution of titanium and tin in the sample generates many defect sites, which shows considerable contribution :from Q3 species. i

.....

I" 0

"

9

~

~"

9 - - 5

I

~ O

""'"

9

9 --110

"' 0

9

-

-'"

1

- - l S

' tD

9

"

'

9 2CO

Figure 6.29Si MAS NMR ofTi-Sn-MCM-41 (Sample 6).

0

~.

i

1385 Table 2. Catalytic activity in the epoxidation of cyclohexene Sample

Mole ratio in product

Cyclohexene

TBHP efficie-

Cyclohexene oxide

Si/Ti

conversion (%)

ncya (mole%)

selectivity (%)

Si/Sn

2

68

-

19.4

58.2

96

3

52

210

15.5

46.5

98

4

61

89

17.8

53.4

99

5

75

164

16.3

48.9

100

6

64

405

21.0

63.0

100

7

89

533

19.0

57.0

99

8

126

174

16.2

48.6

99

9b

77

-

14.1

-

93

aTBHP efficiency = mole% of TBHP consumed in the formation of cyclohexene oxide, cyclohexadiol, cyclohexene-2-ene-l-ol and cyclohexene-2-ene-l-one, bData taken from ref. 7 for comparison, reaction conditions: 0.3 g catalyst, 56.3 mmol of cyclohexene, 14.0 mmol TBHP, 333 K, 5 h reaction time.

3.5. Catalytic reactions Catalytic activity of titanium tin containing samples in the oxidation of cyclohexene in presence of TBHP as oxidant are given in Table 2. As the contents of titanium and tin increased the cylcohexene conversion increases except sample 3. In the case of sample 3 extra framework Ti species was detected by UV-visible, which is inactive in the reaction. Maximum cyclohexene conversion was obtained with sample 6 (21%). TBHP efficiency defind as the mole% of TBHP consumed in the formation of cyclohexene oxide, cyclohexadiol, cyclohexene-2-ene-l-ol and cyclohexene-2-ene-l-one. TBHP efficiency is 48.9 and 63% over sample 5 and 6, respectively. Cyclohexene oxide is major product over all titanium tin containing samples and other products are cyclohexadiol, cyclohexene-2-ene-1-ol and cyclohexene-2-ene-l-one. However, cyclohexene oxide selectivity is 100% over sample 5 and 6 and no other products were detected. It clearly indicates that in presence of tin cylcohexene oxide selectivity increased. Cycloohexene oxide selectivity over sample 9 is reported to be 93% by Blasco et al. [7].

CONCLUSION In conclusion, mesoporous Ti-Sn-MCM-41 was prepared for the first time in absence of sodium by direct hydrothermal synthesis. The resultant materials possess BET specific surface area of 967 to 1045 m2g~ range, pore volume of 0.75 to 0.89 crn3g~ range and

1386 average pore diameter of 3.56 to 3.67 nm range. UV-visible predicts the presence of tetrahedral coordination of titanium and tin in the samples. Tin containing samples are more hydrophilic in nature compared with pure Si-MCM-41. Cyclohexene conversion and cylcohexene oxide selectivity are 21% and 100%, respectively over Sn-Ti-MCM-41 (Si/Ti = 64, Si/Sn = 405).

REFERENCES

.

2.

L. Nemeth, G. J. Lewis and R. R. Rosin, US patent No. 5,780,654 (1998). C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 3 59 (1992) 710.

.

A. Corma, J. L. Jorda, M. T. Navarro and F. Rey, Chem. Commun., (1998) 1899.

4.

S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309.

5.

E. R Barrett, L. G. Joyer and P. R Halenda, J. Am. Chem. Soc., 73 (1951) 373.

6.

A. Wang and T. Kabe, J. Chem. Soc., Chem. Commun., (1999) 2067.

7.

T. Blasco, A. Corma, M. T. Navarro and J. P. Pariente, J. Catal., 156 (1995) 65.

8.

C.- Y. Chen, H.- X Li and M. E. Davis, Micropor. Mater., 2 (1993) 17.

9.

A. Zechinna, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti and M. Padovan, Stud. Surf. Sci. Catal., 69 (1991) 251.

10. 11. 12.

T. Blasco, M. A. Camblor, A. Corma and J. P~rez-Pariente, J. Am. Chem. Soc., 115 (1993) 11806. N. K. Mal and A. V. Ramaswamy, J. Mol. Catal., 105 (1996) 149. N. K. Mal, V. Ramaswamy, S. Ganapathy and A. V. Ramaswamy, Appl. Catal. A 125 (1995) 233.

13.

N. K. Mal and A. V. Ramaswamy, J. Chem. Soc., Chem Commun., (1994) 1933.

14.

M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal., 48 (1989) 133.

15.

C.- Y. Chen, H.- X Li and M. E. Davis, Micropor. Mater., 2 (1993) 17.

16.

R. Schmidt, D. Akporiaye, M. Stocker and O. Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61.

17.

P. T. Tanev and T. J. Pinnavaia, Chem. Mater., 8 (1996) 2068.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1387

Alternative synthetic routes for NiAI layered double hydroxides with alkyl and alkylbenzene sulfonates Raquel Trujillano, Maria JesOs Holgado and Vicente Rives* Departamento de Quimica Inorg/mica, Universidad de Salamanca, Salamanca, Spain (e-mail [email protected]).

A study is presented on different synthetic routes to prepare layered double hydroxides (LDHs) containing organic sulfonates hosted in the interlayer space. The solids prepared have been characterized by powder X-ray diffraction, FT-IR spectroscopy, mass spectrometry, and thermal methods. Anionic exchange and precipitation in the presence of NaOH have been used, but urea hydrolysis represents the best route to obtain crystalline solids, with low specific surface area, although coprecipitation of amorphous AI(III) oxohydroxides is only avoided when a large excess of urea is used. 1. INTRODUCTION Synthesis of new materials involved in the preparation of composites is deserving much attention in recent years. Usually, swelling layered materials have been used for these purposes, mainly layered clays, both cationic clays, as well as anionic clays, also known as layered double hydroxides (LDHs) or hydrotalcite-like compounds. These LDHs are able to host both organic and inorganic anions (A) swelling their structure; they thereof constitute appropriate materials for the synthesis of nanocomposites. The intercalated anions are also able to be exchanged, depending on the strength of the bonds between the anions and the hydroxyl layers; if such a bond is of the type A-water-layer the anion is simply surrounded by water molecules, and the bond strength to the layers is rather week; otherwise, if the bond is A-layer the bond is strong, the anion becomes grafted to the layers and is not exchangeable

[1].

Several methods have been described in the literature to prepare this sort of materials. The importance of the synthesis procedure on stacking of the layers and location of the interlayer anion has been stressed in different papers. We here report on different methods for the synthesis of LDHs with Ni(II) and AI(III) in the layers, which contain organic surfactants differing in the length of the organic chain (eight or twelve methylene groups, with or without phenyl endings), in the interlayer, in order to ascertain the optimum synthesis conditions to prepare these materials as nanocomposite precursors.

1388 2. E X P E R I M E N T A L

2.1 Synthesis procedures The surfactants used were octanesulfonic acid sodium salt, dodecanesulfonic acid sodium salt, octylbenzenesulfonate acid sodium salt and dodecylbenzenesulfonic acid sodium salt (the corresponding anions will be labelled as OS, DS, OBS, and DBS, respectively). All of them were provided by Fluka (Switzerland) and were used without any further purification, as well as other chemicals. Gases were from L'Air Liquide (Spain). The synthesis methods used to prepare the NiA1 anionic clay with the surfactant in the interlayer spacing were, anionic exchange, and direct synthesis method by coprecipitation with NaOH or urea as precipitation agents.

2.1.1. Anionic exchange The starting material Ni-A1 anionic clay with a Ni/A1 molar ratio of 2 and chloride in the interlayer was prepared by the coprecipitation method described by Miyata [3]. 100 ml of a 0.4 M NiCI2 and 0.2 M A1C13 aqueous solution was dropwise added to 250 ml of decarbonated water; the pH was controlled with a 725 Dosimat coupled to a 691 pH-meter, both from Metrohm, and maintained constant at 8.3 by adding NaOH 1M. The synthesis was carried out under inert atmosphere of N2. The green solid suspension obtained was aged during 24 hours by stirring at room temperature, then washed with decarbonated water and dried under vacuum. The sulfonate salts of the surfactants used were dissolved in decarbonated water (0.4 M). A given amount of the starting material was suspended in 100 ml of decarbonated water and then an aqueous solution of the surfactant solution was added. The amount of the surfactant was twice the anion exchange capacity (AEC) of the clay, calculated from the AI(III) content. The mixture was stirred during three days under inert atmosphere at room temperature, then washed with decarbonated water, centrifuged and dried under vacuum. Samples were labelled as NiA1OS, NiA1DS, NiA1OBS and NiA1DBS. 2.1.2. Direct Synthesis with NaOH as precipitating agent A 200 ml solution of NiC12 (0.16 M) and A1C13 (0.08 M) in decarbonated water with a Ni/A1 molar ratio of 2 was dropwise added to 100 ml of a decarbonated water solution of the surfactant. The amount of the surfactant was twice the AEC of the clay. The pH was controlled with a 725 Dosimat coupled to a 691 pH-meter, both from Metrohm, and maintained at 8.3 with 1 M NaOH. The suspension obtained was magnetically stirred for three days under inert atmosphere at room temperature, and then it was washed with decarbonated water, centrifuged and dried in vacuo. A portion of the suspension was hydrothermally treated in a Teflon lined stainless steel bomb during 5 days at 105 o C, and the solid was then washed and dried in the same conditions above given. Solids obtained by this method were named as NiAIOSNa, NiA1DSNa, NiA1OBSNa and NiA1DBSNa. 2.1.3. Direct Synthesis by precipitation with urea A 200 ml solution of NiC12 (0.8 M) and A1C13 (0.4 M) in decarbonated water with a Ni/A1 molar ratio of 2 was added to a 300 ml solution of the surfactant which amount was twice that corresponding to the AEC of the clay. Urea was added to this mixture with a molar urea:total cations ratio of 10:3 or 20:3. The mixture was stirred during three days under inert atmosphere and reflux conditions at 95 ~ then washed with decarbonated water, centrifuged

1389 and dried under vacuum. Samples thus obtained were called NiA1OSU, NiA1DSU, NiA1OBSU and NiA1DBSU. 2.2. Characterisation Techniques Elemental chemical analysis for Ni, A1, S, and Na were carried out by conventional techniques in Servicio General de An/disis Quimico Aplicado (Universidad de Salamanca, Spain). Powder X-ray diffraction patterns (PXRD) were recorded with a Siemens D-500 instrument, using Cu Kc~ radiation ()~=1.54050 A) and equipped with AT Difract software. FTIR spectra were recorded using a Perkin Elmer FT1730 instrument, using KBr pellets; 100 spectra (recorded with a nominal resolution of 4 cm ~) were averaged to improve the signalto-noise ratio. Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out using TG-7 and DTA-7 instruments from Perkin Elmer, in flowing oxygen (50 ml min ~) at a heating rate of 10 ~ min 1, and using alumina (from Merck) calcined at 1200 ~ as a reference for the DTA studies. Specific surface area assessment and pore size analysis were carried out using a Gemini instrument from Micromeritics. The sample (ca. 80-100 mg) was previously degassed in flowing nitrogen at 150 ~ for 2 h in order to remove physisorbed water in a FlowPrep 060 apparatus, also from Micromeritics, and the data were analysed using published software [4]. Mass spectra were recorded by the FAB (Fast Atom Bombardement) method in a VG-AutoSpec spectrometer; the Cs emission was 1 gA and acceleration voltage 3 5 kV. 3. RESULTS AND DISCUSSION 3.1. Elemental chemical analysis Elemental chemical analysis data are summarized in Table 1, together with the proposed formulae for some of the solids prepared. Table 1 Elemental chemical analysis data and sample Ni* AI* S* NiA1C1 31.11 7 . 3 6 - - NiA1OS 23.94 5 55 5.79 NiAIOSU 21.88 8.35 6.04 NiA1OSNa 25,27 7.32 4.46 NiA1DS 23.74 5.01 5.4 NiA1DSU 18.90 4.16 5.14 NiA1DSNa 18.84 4.41 5.14 NiA1OBS 21.14 4.38 4.69 NiA1OBSU n.m. n.m. n.m. NiA1OBSNa 19.8 4.86 5.82 NiA1DBS 14.92 3,57 3.6 NiAIDBSU 12.19 5.8 4.36 NiA1DBSNa 16.63 3.85 5.37 *weight percentage; tatomic ratio;

....

proposed formulae of the solids prepared Ni/Al~f A1/St Formula 1.94 .... [Nio.66oAlo.34o(OH)2](C1)o.34l.03H20 ....... 1.98 1.14 [Nio.665Alo.335(OH)2](S)o.2941.24H20 1.20 1.64 [Nil.xAlx(OH)2] (S)x nH20 1.59 1.95 [Nil.xAlx(OH)2] (S)x nH20 . 2.17 1.10 [Nio.685Alo.315(OH)2](S)o.2861.14H20 2.08 0.96 [Ni0.676Alo.324(OH)2](S)o.338" 2.08H20 1.96 1.02 [Ni0.662Alo.337(OH)2](S)o.330 1.24H20 2.21 1.11 [Ni0.689Alo.31o(OH)2](5)o.279 nH20 n.m. n.m. [Nil.xAlx(OH)2] (S)x nH20 1.87 0.99 [Nio.652A10.348(OH)2](S)0.351"l.12H20 1.92 1.17 [Ni0.658A10.342(OH)2](S)o.292 2.3H20 0.97 1.58 [Ni~.xAI• (S)x nH20 1.99 0.85 [Ni0.665AI0.335(OH)2](S)o.3941.18H20 n.m. = not measured.

1390 The amount of water has been calculated from the first weight loss in the TG curves. The Ni/A1 molar ratio is in almost all cases close to the expected value of 2. The molar AI/S should be close to 1, as each interlayer monovalent anion balances the positive charge in the layers because the introduction of an AI(III) cation. In some cases the A1/S markedly exceeds the expected value, suggesting precipitation of A1 hydroxides (probably as an amorphous material) outside the LDHs crystallites; so, as the amount of A1 in the LDH layers and of sulfonate in the interlayers cannot be determined, the formula of the corresponding solid has not been calculated. In this same samples, the Ni/A1 is markedly lower than the expected value of 2, which is the minimum value for a stable hydrotalcite. The amount of Na found in some cases is almost negligible, indicating that the surfactant anions are not as sodium salts, but probably inserted in the LDH interlayer. 3.2. Powder X-ray diffraction The PXRD diagram for the starting NiAIC1 material is typical of an anionic clay with interlayer chloride anions [5], the interlayer spacing d(003)(7.8 ~) coinciding with the value reported by Miyata [6]. The diagrams for representative surfactant-exchanged samples are given in Fig. 1. All diagrams correspond to layered materials, with basal spacings ranging from 21 to 30 A. These values are in the range reported by several authors for LDHs with interlayer anions containing hydrocarbon chains [7, 8].

2500

~"

~

0

7~...~NiAIDS

[

i

10

20

I

I

30 40 2 e - Cu Ko~ (o)

./~. x2

I

[

50

60

70

Figure 1. PXRD diagrams of parent NiA1CI and of sulfonate-containing LDHs prepared by anionic exchange. Such a spacing decreases as the length of the hydrocarbon chain does. With respect to the application of different synthesis routes to prepare the same compound, the PXRD diagrams

1391 for samples prepared with octane sulfonate are included in Fig. 2. Spacing decreases and crystallinity slightly increases when the sample has been prepared by hydrolysis with urea; probably this is a result of the smooth increase in pH during hydrolysis of urea, thus providing conditions for a better crystallisation of the solid. It should be stressed that diffraction maxima due to crystalline phases other than the expected LDH were not recorded, so indicating that, if such additional phases exist (as assumed for samples NiAIOSU, NiAIOSNa, and NiA1DBSU, from the elemental chemical analysis data), they should be in an amorphous state, and, probably, its concentration should not be very large.

~"

I 1000

d

g

/~ 0

....

I

I

10

20

I

~SNa.~,,.X 2] I

I

I

I

30 40 50 60 70 2 0 - CU Ka (~ Figure 2. PXRD diagrams of octanesulfonate-containing LDHs. 3.3. Mass spectrometry The PXRD diagrams of samples NiAIDBS and NiA1OBS are rather similar to those of the corresponding sodium sulfonate salts. In order to check the absence of sulfonate species adsorbed on the external surface of the LDH crystallites, the solid samples prepared, as well as the pure sulfonates and the parent NiAICl LDH, were analysed by mass spectrometry. The MS of the pure surfactants showed the signals expected for these molecules. The spectra of the solids prepared and of the original NiA1CI LDH did not show any detectable signal, or they were extremely weak. These results indicate that the LDHs are hardly volatilised in the MS chamber and that the surfactant anions hosted in the interlayer space are strongly held; on the contrary, if the surfactant molecule was adsorbed on the external surface of the crystallites, such a volatilisation would not be hindered. Consequently, these results demonstrate that there is no appreciable amount of "free" surfactant externally adsorbed on the surface of the LDHs prepared. 3.4. FT-IR spectroscopy The FT-IR spectrum of NiA1C1 (not shown) is typical of a hydrotalcite with intercalated chloride [9]. Representative FTIR spectra for some of the solids prepared are included in Fig. 3. The broad absorption in the 3600-3300 cm I is due to stretching mode of OH groups (both from the layers and from intercalated water molecules); it should be stressed that a shoulder

1392 around 3200 cm "~, reported in the literature to be originated by OH stretching mode of hydroxyl groups hydrogen-bonded to intercalated carbonate anions, is not present, thus confirming the absence of carbonate impurities. The strong absorption at 425 cm "1, recorded in the spectra of all samples prepared, is due to M-O vibrations in the layers, and is characteristic of this sort of layered solids [10]. The other bands recorded are mostly due to the interlayer species. Broadly speaking, the bands are in close positions to those of the pure surfactants, although in all cases, a shift towards lower wavenumbers is observed. Sulfonate anions strongly absorb at 1230-1120 cm ~, and also show weak bands in the 1080-1025 cm "~ range, due to the antisymmetric and symmetric modes, respectively, of the SO3 group. If the sulfonate anion contains also aromatic rings, four bands are also recorded close to 1230, 1990, 1130, and 1040 cm 1 (three SO and one S-phenyl vibrations interactions) [ 11 ]. These bands, except that at 1130 cm "a, are recorded also in the spectra of the non-aromatic sulfonates, as well as in the LDHs prepared from them. The bands at 1013-1011 c m l are due to centroantisymmetric vibrations, and are recorded in the spectra of DBS and OBS surfactants, and of their corresponding LDHs. The bands in the 850-600 cm ~ are due to in-plane quadrant bending, and those in 550-400 cm ~ to the benzene ring vibrations, which involve ring bending by quadrant. These bands shift towards lower wavenumbers and their intensities decrease when the surfactant is hosted in the LDHs. Finally, bands in the 2960-2800 and 1025-700 cm ~ ranges are due to different C-H and C-C vibrations of the benzene and alkyl chains.

..

I

v

T

~

DBS

O t-t~

E C" L_

4000

I

3000

I

2000

I

11~00

0

wavenumbers (cm Figure 3. Representative FT-IR spectra of LDHs samples containing the indicated sulfonate.

3.5. Thermal analyses The DTA curve of parent NiA1C1 hydrotalcite shows two endothermic effects close to 130 and 290 ~ due to removal of interlayer water molecules and through condensation of layer OH groups, respectively [12]. The curves for the sulfonate-containing LDHs are completely

1393 different, and the curves are dominated by exothermic effects due to combustion of the organic chains. The synthetic route followed does not have a strong effect on the shape of the curves nor positions of the maxima. The curves corresponding to the samples prepared by ionic exchange are given in Fig. 4. These four curves can be grouped in two subgroups: those corresponding to samples NiA1OS and NiA1DS show two exothermic effects between 214287 and 280-380 ~ with a weak shoulder at 400 ~ for those samples containing aromatic rings, however, two additional effects are recorded, a weak one at ca. 150 ~ and a rather sharp peak at 470 ~ probably, combustion of the aromatic ring should be in some sort of way related to the exothermic effect close to 470 ~

5~

exo (9 o

I

0

I

I

200

400 600 800 temperature (~ Figure 4. DTA diagrams of LDHs samples containing the indicated sulfonate. The TG curves show a first effect due to removal of interlayer water and then a strong weight loss due to combustion of the interlayer surfactant and evolution of water from the layer hydroxyl groups. 3.6. Surface texture

The N2 adsortion-desorption isotherms at-196 ~ correspond in all cases to type II in the IUPAC classification [ 13 ], indicating they are macroporous or non-porous, with unrestricted monolayer-multilayer adsorption. It has been previously reported [ 14] that during adsorption experiments, N2 molecules are unable to enter the interlayer space of LDHs containing simple anions, as chloride, nitrate or carbonate, while if large anions with large formal negative charge (e. g., hexacyanoferrates, polyoxometalates, Keggin-type anions) are hosted in the interlayer space such an access is possible, thus the solids behaving as microporous. In our case, despite the large interlayer space, which would made it accessible to N2 molecules, such a space should be highly populated and packed with monovalent surfactant anions, so leaving not too much room available for the N2 molecules; consequently, microporosity is not observed.

1394 A relationship between the synthesis route and specific surface area development seems to exist. So, samples prepared by anion exchange show a specific surface area similar to that of parent NiA1C1, suggesting anion exchange takes place without destruction of the layers nor the crystallites. On the contrary, samples prepared in NaOH or urea media show different specific surface area development; those prepared by precipitation with NaOH show SBET values in the range 15-80 m g , the specific value probably depending on the drying rate. However, the samples prepared by hydrolysis with urea show SBETvalues close or even lower than 8 m 2 g-1. These results are in agreement with PXRD results above commented, as the most crystalline samples (prepared in urea) show the lower specific surface area values. 4. CONCLUSIONS Among the three synthetic routes tested, urea hydrolysis leads in all cases to the most crystalline materials, although coprecipitation of amorphous compounds of the trivalent cation cannot be avoided; the pure LDH was obtained only when a large excess of urea was used. Changes in specific surface area are in agreement with the change observed in crystallinity, SBETincreasing when the amorphous co-product is formed.

Acknowledgments. Finantial support from MCyT (grant MAT2000-1148-C02-01) is acknowledged. REFERENCES 1. J. Inacio, C. Taviot-Gu6ho and J. P. Besse, Appl Clay Sci., 18 (2001) 255. 2. F. Leroux M. Adachi-Pagano, M. Intissar, S. Chauvi6re, C. Forano and J-P. Besse, J. Mater. Chem., 11 (2001) 105. 3. S. Miyata, Clays Clay Min., 26 (1978) 441. 4. V. Rives, Ads. Sci. Technol., 8 (1991) 95. 5. V. A. Drits and A. S. Bookin, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 37. 6. S. Miyata, Clays Clay Min., 31 (1989) 511. 7. M. Meyn, K. Beneke and G. Lagaly, Inorg. Chem., 29 (1990) 5201. 8. H.-P. Boehm, J. Steinle and C. Vieweger, Angew. Chem. Int. Ed. Engl., 16 (1977) 265. 9. J. T. Kloprogge and R. L. Frost, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 139. 10.M.J. Hern~indez-Moreno, M. A. Ulibarri, J. L. Rend6n and C. J. Serna, Phys. Chem. Solids, 12 (1985) 34. 11. N. B. Colthup, L. H. Daly and S. E. Wiberley (eds.), Introduction to Infrared and Raman Spectroscopy, Academic Press, San Diego, California, 1990. 12. V. Rives, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 115. 13. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 57 (1985)603. 14. V. Rives, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 233.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1395

Spectroscopic Studies on Aminopropyl-containing Micelle Templated Silicas. Comparison of grafted and co-condensation routes D. Brunel a*, A C Blanc a, E Garrone b, B Onida b*, M Rocchia b, J B.Nagy c, D J Macquarrie d*

(a) Laboratoire des Mat6riaux Catalytiques et Catalyse en Chimie Organique, ENSCM, UMR-5618-CNRS, 8, rue de l'Ecole Normale, 34296 Montpellier, c6d6x 5 France (b) Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 1-10129 Turin, Italy (c) Laboratoire de R6sonance Magn6tique Nucl6aire, Facult6s Universitaires Notre-Dame de la Paix, 63 rue de Bruxelles, B-5000 - NAMUR, Belgium (d) Centre for Clean Technology, Department of Chemistry, University of York, Heslington, YORK, YO 10 5DD, England Spectroscopic studies on three aminopropyl-Micelle Templated Silicas indicate significant differences between the interactions of the amine groups and the surface. These differences correlate to the differences in catalytic and stoichiometric activity which have already been reported 1. INTRODUCTION The recent development of routes to novel highly structured and regular silicas such as the M41-S family, and others such as the neutral amine templated and polyether templated Micelle Templated Silicas (MTS) has opened up new areas of inorganic chemistry.[1] Such materials display structural regularity expressed in terms of regular pore size and shape (controllable via template length and synthesis conditions), and high surface areas, typically of the order 1000 m2g1. Such features make these materials eminently suitable candidates for many applications, including catalysis (where regular, controllable pores is of obvious importance in terms of selectivity), adsorption and many other materials applications such as molecular wires and sensors. In terms of catalysis, the MTS group of materials has been well investigated as a catalyst support, with a range of organic species being supported onto its surface, and many catalytic applications investigated.[2] These catalytic materials are accessible via grafting methods first developed for amorphous silicas, relying on the fact that the walls of the MTS materials are broadly similar to amorphous silica. A second methodology for the incorporation of organic groups has been

1396 developed, whereby the organic moiety is incorporated into the material during the synthesis of the MTS.[3] Thus, a silica precursor (tetraethoxysilane, TEOS) and an organosilane (RSi(OMe)3) are co-condensed in the presence of the template (a neutral amine) leading directly to a MTS containing the R group as an intrinsic part of the surface of the material. Removal of template by solvent extraction leaves the material ready for use. Analogous materials can also be prepared using quaternary ammonium[4] and polyether block copolymers.[5] These materials have been used as catalysts in similar reaction types to the grafted MTS and grafted silica analogues, and their physical properties, such as pore size distribution, surface areas and thermal behaviour have been studied. While their overall activity is generally broadly similar, there are significant differences in the detail of their behaviour which hints at important differences in surface chemistry. For example, in the materials where R - CH2 CH2CH2NH2, all three materials catalyse the Knoevenagel condensation of ethyl cyanoacetate with cyclohexanone with similar rates, but the condensation of ethyl cyanoacetate with benzaldehyde is fastest with grafted materials, and is considerably slower with the in-situ materials. Stoichiometric reactions with benzaldehyde to form the imine are likewise different, with very large differences in rate (orders of magnitude) even within the in-situ materials. Despite this striking behaviour, their spectroscopic properties have not yet been investigated in depth. We present here studies on aminopropyl-containing MTS materials prepared from the grafting route and from the direct co-condensation route. Two different co-condensed materials are investigated, as they display intriguing and large differences in the chemistry of the amine.

H20,t- EtOH ~ # ~ _ . , ~ . _ D-C12H25 NH2 @~t{~~l~

~ (RO)aSi

remove template

I RSi(OMe) + (RO)4Si remove._ template

Figure 1. Different routes to preparing the MTS-G and MTS-C catalysts

2. EXPERIMENTAL Grafted MTS materials (MTS-G) and materials prepared by co-condensation MTS-C) were prepared according to previously published methods.[2b,6] MTS-C50 refers to a material prepared by the co-condensation route using a 50vo1% water : 50vo1% ethanol solvent mixture; MTS-C70 to a material prepared by the co-condensation route using a 70vo1%

1397 water: 30vo1% ethanol solvent mixture. These two materials have been shown to have reaction rates with benzaldehyde differing by several orders of magnitude[6]. MTS-C70 reacts completely within 20 minutes, whereas MTS-C50 requires 6 days under the same conditions. MTS-G reacts completely within 2 minutes. Quantitative MAS NMR spectra were obtained using a Bruker MSL400 spectrometer operating at 100.6MHz (13C) and 79.5MHz (29Si) respectively. Infra-red spectra were obtained using self-supporting wafers in a home-made infra-red cell. Wafers were prepared at pressures of <1 tonne to avoid mechanical damage to the pore structure[7]. The wafers were loaded into the cell and evacuated at 10.4 Torr for one hour before being attached to a vacuum line and placed in the instrument beam. The spectra were obtained under a vacuum of 10.4 Torr, using a Bruker Equinox 55 spectrometer operating at 2cm ~ resolution. After evacuation, samples were dosed with increasing quantities of (a) acetone and (b) benzaldehyde in separate experiments. XPS spectra were recorded on a Kratos AXIS HSi instrument with a Mg I ~ X-ray source using a pass energy of 20eV and X-ray power of 225W. 3. RESULTS

3.1 NMR Investigation

The 13C MAS spectrum of MTS-G was recorded under conditions where quantitation of the signals was possible. The spectrum consisted of three broad resonances at 9.8ppm, 26.2ppm and 43.4ppm, corresponding to the three carbons of the aminopropyl group in a ratio of 1 : 1 : 1. These signals are typical of those reported for aminopropyl-containing amorphous silicas, and can be assigned as Si_CH2, SiCH2C-~-I2and CH2NH2 respectively. No resonances are apparent for methoxy groups of the silane, indicating that these are absent in the sample. The corresponding 13C MAS spectrum of the MTS-C50 material indicates a different picture. In addition to the three resonances described above (in this case at 11.2ppm, 27.0ppm and 44.2ppm, there are two other peaks at 15.8ppm and 59.0ppm, which are indicative of residual ethoxy groups. Very approximately, the ratio of ethoxy : aminopropyl is 1 : 1, but the rather noisy baseline precludes more accurate determination. The 29Si MAS M R spectrum of the MTS-G sample in the Q region (-90 to -120ppm) shows a large resonance for Q4 silicons at-110.3ppm (i.e. for Si(OSi)4, with only a very slight shoulder for Q3 silicon at-97.6ppm (Si(OSi)3OR where R is H or alkyl), indicating few silanols. The MTS-C50 sample displays a similar Q4 peak at -109.gppm, but in this case the Q3 peak at-101.3ppm is much more prominent, and is a clear and distinct second peak.

3.2 Infra-red Investigation. The spectra of MTS-G and MTS-C50 were measured as self-supporting wafers after evacuation in vacuo at 150~ to remove surface bound water. After cooling in vacuo, spectra were measured, after which benzaldehyde as dopants was introduced in controlled amounts, and spectra re-measured with progressively greater amounts of carbonyl species present.

1398

A.U. I 0.5

" 1~ MTS-C50

.

.

3500

.

170b

1~

l~bo 1~ .

.

.

.

.

.

.

.

t MTS-C50

.

3000

I

2500

I

2000

'

'1

1500

1800

1700

1600

L.~0)0

1400

W a v e n u m b e r s / cm -~

Figure 2. Selected IR Spectra of the materials, and their interatcion with benzaldehyde On the right are expansions of the interaction of (top) MTS-G and (bottom) MTS-C50 with increasing amounts ofbenzaldehyde.

3.2.1 Spectra of MTS-G At 3740cm 1 there is a very weak isolated SiOH, a broad band at ca. 3650cm 1 due to inaccessible silanols (hidden in the bulk of the silica structure, and therefore persistent and relatively unchanged by external perturbations due to either temperature or dopants). At 3370cm 1 and 3302 cm 1 are the NH2 asymmetric and symmetric stretching vibrations respectively. A weak NH3 § band is visible at ca. 3160cm 1. At lower energies are the CH2 stretches, asymmetric at 2935cm 1 and symmetric at 2863cm 1. Features due to methyl groups (from residual ethoxy groups or methoxy groups of the silane are absent. The broad absorption centred below 3000 c m "1 is due to silanols engaged in H-bonding with NH2 groups. In the low frequency region, the predominant vibration is the NHdefat 1590cm ~. The interaction of the sample with acetone at room temperature (in the infra-red cell, and under vacuum) leads to some changes in the spectrum. In the region 2700 - 3800cm a there is no discernable change. All bands appear the same before and after dosage, with no new features appearing. Thus there is no evidence here of an interaction between the solid and the acetone. In the region 1900 - 1300cm 1, however, there are some clear differences. Specifically, there are two new peaks, at ca. 1700cm 1 and at 1665cm1. The former is due to H-bonded acetone molecules interacting with SiOH units [(CH3)2C=O ....H-O-Si], the latter due to the formation of imines by reaction of NH2 units with acetone. Upon ageing of the sample the weak absorption of the H-bonded acetone has disappeared, either due to desorption, or to reaction with further NH2 groups leading to more imine groups. Slight reduction in the intensity of the NH~r can be seen. Prolonged evacuation of the sample after exposure to acetone does not lead to any further changes in the spectrum, indicating that these changes are irreversible. The adsorption of benzaldehyde leads to a variety of changes in the spectrum. Firstly, the intensities of the N H s t r vibrations does not appear to change significantly, but there is a slight increase in the intensity of the SiOH H-bonded envelope, due to the interaction of

1399 the benzaldehyde C=O with the silanols. Two weak bands at 3043cm 1 and 3079cm -1 appear, which are due to the C-H~tr o f the aromatic ring. At lower energies, there appears an intense signal at 1646cm -~ which again can be attributed to the formation of an imine group on the surface of the material. A very weak absorption at ca. 1690cm -1 can be attributed to H-bonded C=O groups. Additional bands at <1500cm ~ can be assigned to CHdef of the imine groups and the aromatic ring. As for acetone, extensive outgassing of the material after exposure to benzaldehyde did not result in a reversal of these changes, indicating that the formation of imine is irreversible. 3.2.2 MTS-C samples In the infra-red spectrum of MTS-C50, the isolated silanol vibration at 3740cm 1 is prominent, as are H-bonded SiOH species at 3710cm 1 (terminal) and 3560cm 1 (bridged). NH2 and NH3 + stretching vibrations are visible at 3370cm 1, 3303cm 1 and 3160cm -1, as for the MTS-G material. In addition to the two CH2 vibrations, and additional feature is visible at 2983cm 1 which is attributable to a CH3 vibration. Again, the major absorption in the range 1 5 0 0 - 1700cm 1 is the NHdef at 1590cm 1. An absorption due to inaccessible silanols is very weak or absent. The addition of acetone to the sample causes a large increase in intensity in the H-bonding region. The intensity of the isolated silanol decreases considerably, and a broad peak appears and grows between 3650cm 1 and 3200cm 1. The peaks due to NH2 and NH3 + appear to be unchanged and are visible as features on this broad, intense peak. On increasing levels of doping of acetone, the region 1 8 0 0 - 1500cm 1 also changes. Vibrations due to the carbonyl group interacting with silanols are apparent, and at low acetone pressures, there is a peak at 1692cm 1, which shifts on increasing the amount of acetone, to 1698cm -1. A second band at 1706cm ~ is also apparent. The NH2 def band at 1592cm 1 remains unchanged throughout. The addition of quantities of benzaldehyde to the sample leads to substantial changes in the O-Hstr region. Specifically, the isolated silanol peak reduces significantly in intensity, and a very broad, intense peak is seen between 3600 and 3000cm "1. This can be attributed to hydrogen bonding between the O-H groups and the oxygen of the C=O bond. There is no appreciable change in the intensity or position of the NH2 bands. In the region 1750cm 1 to 1500cm ~ there are also changes. A peak becomes evident at 1693cm -1 which is due to the C=O~ of the adsorbed carbonyl. Other weaker peaks appear at 1628cm -1, which is in the same position as the water peak noted under water adsorption, and a very weak shoulder at 1654cm -1, which can be attributed to the formation of an imine between the amine and the carbonyl group. This latter reaction could be the source of water which could be responsible for the small peak at 1628cm 1. MTS-C70 represents an intermediate situation between the two extremes above. In this case both acetone and benzaldehyde show both chemisorptive (via imine formation) and physisorptive (via SiOH H-bonds) behaviour, with an increase in the importance of the chemisorption with time. thus, in this case imine is formed relatively readily, but not as rapidly as with MTS-G. 3.3 XPS Investigation XPS Spectra indicate differences in the three samples. MTS-G shows two peaks in the N(ls) region, assigned to unprotonated NH2 at 399.6eV and to protonated NH3 + (SiO) at

1400 401.7eV. Deconvolution of the spectrum indicates a ratio of 7:3 for these two respectively. On the other hand, the MTS-C samples show the presence of only ca. 5% protonated material. Protonation has been achieved by treatment of the samples with HBF4, whereupon a strong peak at 401.7eV appears in these samples, proving the assignment to be due to protonated species. ._ = _c r x

z

404

402 Binding

400

398

Energy

(eV)

396

404

402 Binding

400

398

Energy

(eV)

396

Figure 3 - XPS spectra of MTS-C50 and 70 (left top and bottom respectively, and of MTS-G, right 4. DISCUSSION

4.1 Comparison of spectra from MTS-G and MTS-C. The M R spectra for the two parent materials indicate some significant differences. In particular, the presence of significant amounts of ethoxy (and possible methoxy) groups in the MTS-C contrasts with their much lower number in the MTS-G sample. This is entirely consistent with other techniques and previously reported results[8] which indicate that there are ca. 1-2 EtO groups per amine chain on the MTS-C materials. The majority of the RO groups in the MTS-C materials derive from the condensation step, where the hydrolysis of the TEOS ethoxy groups is never 100% complete. There is also evidence that the template extraction process (involving hot ethanol) may lead to the formation of further ethoxy groups chemically bound to the surface.[9]. Thus these materials are somewhat rich in ethoxy groups. It is also possible that these ethoxy groups are predominantly attached close to the amine groups, due to base-catalysed condensation reactions. The lack of a calcination step (as is carried out with MTS-G materials prior to grafting, and which removes any such unhydrolysed EtO groups) is also a factor which contributes to this substantial difference in the two materials. The complete absence of alkoxy groups on the MTS-G materials is perhaps a little surprising, but it is known that the grafting process on MTS materials results in almost complete condensation, leading to very low quantities of residual OR groups. Infra red samples of the two systems MTS-G and MTS-C50 in the region 4000 - 2700cm 1 are quite different in many respects. The almost complete absence of isolated silanols is unusual for a calcined material, but is likely to be due to the grafting of trimethoxy aminopropylsilane, which is known to take place mainly on the hydrophobic areas of the surface, J10] thus eliminating most of the isolated silanols. H-bonded silanols from the hydrophilic, silanol-rich areas are visible as a broad peak centred at ca. 3250cm 1. In contrast, the silanol stretching region of the MTS-C50 sample has, in addition to a higher energy H-bonded OH ~, significant quantities of isolated silanols, possibly due to a good dispersion of aminopropyl chains and residual ethoxy groups throughout the surface,

1401 rather than the more concentrated bundles of functional groups, and lack of OR species seen on the MTS-G material. Such a random distribution would lead to significant numbers of silanols which are rendered incapable of H-bonding due to adjacent aminopropyl or OR groups. Similarly, this effect may reduce the ability of the amine groups to interact with the silanols on the surface. It might be expected, but has not yet been demonstrated, that a random distribution of these groups would exist on the surface, as the co-condensation mechanism should not favour segregation of species. Indeed, a species which contained a large number of adjacent, condensed trialkoxy silanes might be expected to be of very limited mechanical stability (due to the reduced functionality of the monomer), and would not lead to the structured materials seen here. The XPS data indicate that the degree of protonation does not appear to affect the reactivity of the three materials. This can be rationalised by considering that the weak acid weak base pair would be in a rapid equilibration with free amine and silanol. Thus the protonation of the amine would not preclude its reaction with an electrophile, so long as a rapid equilibration was possible. However, this does not directly explain the relative reactivities of the samples towards carbonyl compounds. When one considers possible reasons why the in-situ materials are hardly protonated, then possibilities arise. Clearly, the amine groups in the grafted material have ready access to silanols, with which they can interact and exchange protons. It may be, therefore, that the in-situ materials cannot interact with silanols, and thus do not protonate. This is perhaps more likely an explanation than the relative acidities and basicities changing sufficiently to preclude proton transfer, especially given that their catalytic activities in base-catalysed reactions is quite similar. One such possibility is thus that the amine groups in the MTS-C materials are "surrounded" by ethoxy groups, and are thus sterically hindered from reacting with silanols. Such a situation might also render them less active in imine forming reactions with carbonyl compounds. This behaviour is similar to that observed with MTSG which was previously treated with trimethylsilylimidazole in order to passivate the surrounding silanol groups [ 11 ] The higher amounts of alkoxy groups present in the in-situ materials (as evidenced by NMR and by the IR spectra where the CH3 asymmetric stretch of alkoxy is evident in both the MTS-C samples at 2988cm -1, but not in the MTS-G sample) supports this proposition. 5. CONCLUSIONS Dramatic differences in the reactivity of aminopropyl groups in different Micelle Templated Silicas have been seen and can be reproduced using in-situ infra-red measurements. Spectroscopic evidence for such differences may be attributed in part at least to the better accessibility of the amine groups in the calcined and grafted materials compared to those prepared by the one step co-condensation method, where many more ethoxy groups may hinder access of electrophiles.

ACKNOWLEDGEMENTS DJM thanks the Royal Society for a University Research Fellowship, and for travel funds. DJM and DB acknowledge Alliance program (British Council and Minist6re des Affaires etrang6res) for travel funds. The authors are grateful to Dr Karen Wilson (York) for XPS measurements, and to Dr Guy Daelen (Namur) for the NMR measurements.

1402 REFERENCES

1. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. W. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, J. Amer. Chem. Soc., 1992 114 10834; P. T. Tanev, T. J. Pinnavaia, Science, (1995) 267 865; S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science, (1995) 269 1242 2. (a) M. Lasperas, T. Llorett, L. Chaves, I. Rodriguez, A. Cauvel and D. Brunel, Stud. Surf. Sci. Catal., (1997), 108 75; (b) D. Brunel, Microp. Mesop. Mater. (1999) 27 329 3. D. J. Macquarrie, Chem. Commun (1996) 1961; D. J. Macquarrie and D. B. Jackson, Chem. Commun., (1997) 1781; D. J. Macquarrie, Green Chem., (1999) 1 195 4. S.L. Burkitt, S. D. Sims, S. Mann, Chem. Commun., (1996) 1367 5. R. Richer and L. Mercier, Chem. Commun (1998) 1775 6. D.J. Macquarrie D. B. Jackson, S. Tailland, K.A. Utting, J Mater,. Chem, (2001) 11 1843 7. A. Galarneau, D. Desplantier-Giscard, F. Di Renzo and F. Fajula, Catalysis Today, 2001,68, 191 8. D.J. Macquarrie, D. B. Jackson, J. E. G. Mdoe, J. H. Clark, New J Chem., (1999) 23 539 9. D.J. Macquarrie, D. B. Jackson and A. Watson, unpublished results 10. P. Sutra, F. Fajula, D. Brunel, P. Lentz, G. Daelen, J.B. Nagy, Colloids, Surfaces APhysicochem. Eng. 158 21-27 1999 11. A.C. Blanc D.J. Macquarrie, B. Onida, M. Rocchia, E. Garrone, J.B. Nagy, F. Fajula, and D. Brunel, New J. Chemistry, in preparation.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Preparation, characterization, stability and catalytic reactivity transition metals incorporated M C M - 4 1 m o l e c u l a r sieves

1403

of

the

3d

v. P~,rvulescu 1 and B.L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium Mesoporous MCM-41 silica was modified by incorporation one or two of 3d transition metal ions (Ti, V, Cr, Mn, Fe, Co, Ni and Cu, ). The mono- and bimetallic mesoporous molecular sieves and their stability (storage under ambient condition over the course of months and under the reaction conditions) were studied by various techniques, such as: XRD, N2 adsorption-desorption, SEM, TEM, IR spectroscopy and TGA. These modified MCM-41 materials have been shown to be active for the liquid phase oxidation of styrene and benzene with aqueous H202. It has to note that Ti, V, Cr and bimetallic (V-Ti, Cr-Ni) incorporated catalysts are particularly active both for selective oxidation of styrene and benzene, Mn, V-Ti and V-Co only active for benzene and Co, Cu and V-Cu for styrene. The catalytic results obtained with these 3d transition metals incorporated materials revealed a possible correlation between the catalytic activity and d electron configuration in the valence shell of transition metals. 1. INTRODUCTION Transition metal ion-containing microporous molecular sieves have extensively been investigated because of their remarkable activity and selectivity in oxidative transformation of organic molecules [1-6]. The discovery of the M41S family of mesoporous silicates has led to a host of studies aimed at the incorporation of transition metals, like Ti, V, Mo, W, Cr, Fe, Mn or Co, into the structure of these materials and the obtained materials have been used in a series of catalytic reactions and separation processes [7-12]. These metal ions modified materials as its pure silicieous analogues, exhibit a hexagona! array of one-dimensional mesopores, whose diameters can be tuned from 15 to 100 A with a narrow pore-size distribution. The environment of the metal cations in the mesoporous molecular sieves determine their catalytic properties in oxidation reactions and in adsorption processes. Although pure silicieous or metal ions incorporated MCM-41 have such interesting properties and the great perspectives in the preparation of new nanomaterials for electronic, optic and other applications as matrix, the structural stability under ambient atmosphere seems to be braking their large potential applications since it was reported that the humidity can alter the pore structure [13]. The storage ofMCM-41 materials needs therefore a special attention. We report in this paper the results on the preparation and characterization of a series of MCM-41molecular sieves containing redox active metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) and bimetals (V-Ti, V-Co, V-Cu, Cr-Ni). Their catalytic activity in liquid phase oxidation of aromatic hydrocarbons (styrene, benzene) will be evaluated and their structural stability and the change in morphology under ambient and reaction conditions will also be discussed. We try to shed some light on the correlation between the catalytic activity and d electron configuration in the valence shell of metal ions. On leave from Institute of Physical Chemistry" I.G. Murgulescu", Spl. lndependentei 202, Bucharest, Romania *Corresponding author (Fax: 32.81.72.51.14, e-mail: [email protected])

1404 2. EXPERIMENTAL 2.1. Synthesis

Mono- and bimetallic meosoporous molecular sieves were synthesized by hydrothermal treatment (100 ~ for 3 days) on the basis of molar gel composition of 1.00 SiO2: x Men+: 0.48 CTMABr: 0.28 Na20:3.70 TMAOH: 222.00 (where: x= 0.02 for monometallic materials and x=0.04 for bimetallic materials). In the case of bimetallic incorporated samples, M1/M2 molar ratio was fixed at 1.0. The solid products were recovered by filtration, washing, drying in air at 100~ and calcination in a flow of N2 followed by air at 550 ~ The reagents used were sodium silicate (25.5-28.5% silica), cethyltrimethylammonium bromide (CTMABr), tetramethylammonium hydroxide (25 wt% TMAOH in water), Ti(ac.ac.)2, VOSO4-5H20, Cr(NO3)3-9H20, CrO3, Mn(CH3COO)2-4 H20, FeSO4-7H20, FeC13, Co(NO3)2-6H20, Ni(CH3COO)2-4H20, 2-prophanol and H2SO4 2.2. Characterization

The materials obtained were characterized by X-ray diffraction (Philips PW 170 diffractometer), N2 adsorption-desorption (Tristar, Micromeritics), scanning electron microscopy (SEM) with a Philips XL-20 microscope, transmission electron microscopy (TEM) with Philips Tecnai microscope, Fourier transform infrared spectroscopy (FTIR) with Spectrum 2000, Perkin Elmer and TG-DSC analysis (Setaram microbalance). The catalytic oxidation reaction was performed for 24 h in a temperature range of 203343K using an oil bath. A molar ratio of hydrocarbure/acetonitrile/H202 (30%)=1/3.6/3 (for styrene) and 1/-/3 (for benzene), and 70 mg of the catalyst were used. The analysis of the oxidation products was made using a Carlo Erba gas chromatograph with a stainless steel column containing OV-101 connected to a FID detector. The used catalysts were characterized by TEM, SEM and FTIR. Catalyst active component leaching during the reaction was verified. For structural stability and the change in morphology with time, the prepared samples were exposed in ambient atmosphere during the months. Then the samples were characterized. 3. RESULTS AND DISCUSSION 3.1. Characterization

Figures 1 and 2 display the powder XRD pattems of the transition metals-containing MCM-41 molecular sieves. The patterns are typical of mesoporous materials with hexagonal arrangement of channels. Although T-O-T bond angle, an important structural parameter, was Table 1. Characteristics of the mesoporous metallosilicates Metal Ti V Crl (3+) Cr2 (6+) Mn Fel (2+) Fe2 (3+)

S~E~ mZ/g 813 1055 958 862 859 713 736

Q~BET nm 2.5 2.5 2.6 2.7 2.7 2.5 2.8

dloo nm 4.08 4.03 3.88 3.95 3.94 3.85 3.92

ao nm 4.71 4.65 4.48 4.56 4.56 4.45 4.53

Metal Co Ni Cu V-Ti V-Co V-Cu Cr-Ni

SBET m2/g 990 945 985 1096 1013 999 914

Q~BET nm 2.8 2.8 2.8 2.5 2.7 2.7 2.7

dloo nm 3.99 3.97 3.96 3.76 3.85 3.83 3.60

ao nm 4.61 4.58 4.57 4.35 4.60 4.61 4.32

1405 varied due to the variable size of cations (RM(,+)/Ro(2.)=0.35-0.62), only a slight change in the hexagonal structure was observed. The highly ordered and stable hexagonal phase, with a higher lattice parameter ao (Table 1) are obtained when the oxidation state of the cation, introduced during synthesis, is favorable for a tetrahedral coordination and the RM(,+)/~(/_)ratio is situated between 0.3 and 0.5. The metal content here is around 1.8 wt%. In Figure 1 (inserted part) is evidenced the effect of different oxidation state of Cr. The size of Cr 6+ should be smaller than that of Cr 3+ and more suitable for a tetrahedral coordination position. The unit cell parameter is higher compared to that of Cr 3+ ion incorporated material. Similar situation is also observed for Fe(2+) and Fe(3+) incorporated

J

_

2 Cr

I

o~

r/l

" l (D

_~.._./

rae]

\-.

2

4

6 20

..i ,,

8

---_~_J~.

V-Ti-MC'M-41 ,

2

L

I

4

6

,T~-+--/ ..... "

8

20

Fig. 1. X-ray diffraction patterns of the calcined monometallic samples

Fig. 2. X-ray diffraction patterns of the calcined bimetallic samples

800

700

70(t

600

C

Mn C

600 O

e~

500

"~ 500 400

o

400 300

300 > 2o0 / o

100

~J

' , - ~ t _ ~ V-Cu-MCM-41 _/~ "..... ~ - .... Cr-Ni-MCM-41

I

o

,- \ V-Co-MCM-41

/ ~

Fe

200-

/

0,0

I

0,2

I

0,4

1

0,6

I

0,8

Relative pressure, p/p0

1,0

0,0

0,2

0,4

0,6

0,8

1,0

Relative pressure, p/po

Fig.3. The N2 adsorption-desorption isotherms for mono- and bimetallic ions incorporated MCM-41 catalysts

1406 samples. In the case of bimetallic V-Ti, V-Co and V-Cu incorporated samples, the unit cell parameter increases from Ti to Cu in the same sense with increasing d electron in valence shell. The d~00 values, unit cell parameter ao, BET surface area and pore size of all the prepared materials, listed in Table 1, show a mesoporous structure similar to that of Si-MCM41 molecular sieves. It can be seen from Table 1 that, in general, the lattice parameters (a0=2(3 ~/2) d~00) of all the M-MCM-41 are greater than that of Si-MCM-41. This is consistent with the Me-O bonds longer than that of Si-O and gives an evidence of the metal incorporation into the framework. All the materials exhibit a narrow monomodal pore size distribution, determined by BJH method, centered at about 2.65 + 0.15 nm (Table 1). The measured Nz adsorptiondesorption isotherms of the mono- and bimetallic samples appear to be of a similar type (Fig. 3). Most of the isotherms exhibit a sharp inflection at a relative pressure of about 0.36-0.38, characteristic of capillary condensation in uniform mesopores. While the inflection points for

Fig, 4, TEM image of the calcined Co-MCM-41

Fig, 5, TEM image of the calcined Mn-MCM-41 molecular sieves

.~... ~-. l

,

,

.

-d -~

.

I

,-

r,.

r

~.~

-,

-~-.

'

..

o

p,':: Fig, 6, SEM images ofthe Fe-MCM-41 (a) and Ti-MCM-41 molecular sieves (b)

1407 V-, Ti, Fe(2+) and V-Ti-MCM-41 samples are located at lower relative pressure of 0.30-0.34. This indicates their smaller pore size (Table 1). It is very interesting to note that the pore volume of the iron incorporated samples is relatively low and the distribution of pore size is relative large compared to other samples. This was previously observed. However, the Fe containing samples showed higher benzene adsorption capacity [4, 5]. TEM pictures of transition metals (mono- or bimetallic) -MCM-41 molecular sieves confirm the well ordered hexagonal pore system (Fig. 4 and 5). The globular morphology of the all the samples (monoor bimetallic), evidenced by SEM (Fig. 6 and 7), is typical for the mesoporous metallosilicates [4, 5]. SEM micro graphs of the calcined samples show agglomerates of small spheres with 0.2-0.5 lxm in diameter. The IR spectra of the calcined samples show no significant effect of the cation incorporation on the intensity and wavenumber of the silanols. The spectra (no shown here) are similar to those of mesoporous silicates. In the skeletal range, a band at around 960 crn1, usually taken as a proof for the incorporation of the metals in crystalline molecular sieves, can be observed for the metallosilicates (Fig. 7). The intensity of the 960 crnq band increases for the metallosilicates with a higher ordered structure (Cr, Mn, Co, Ni, Cu, V-Co, V-Cu -MCM41 materials). Generally, the band at 960 cm ~ is attributed to a stretching mode of a [SiO4] unit bonded to a M ion in the [SiO4] structure.

Crl .~x i", //~\t ik,, ,'- / '{U/~, /f" \ .; "

,..a,

!

"~ t I

4

B-I

\J ~\'Jl

<\

500

/

k."'

\/

)/ / ....

v'

,' / "

\,,j

,,i

//i

p Y"\

i / ....

t"k.j' /

1000

I _ ...

Cu

/ / / f - " . , . / N~--~T~--~

I

/"'~'i sf"\\ ',,i

'\/<

/

,'

/%

VCu

1500

Wave number -1

Fig. 7. IR spectra of the calcined metallosilicates

C,% 4

2000 Ti V Crl Mn Fe Co Ni Cu Vti VeoVcuCrNi

Fig. 8. Conversion of styrene and benzene at 343K in oxidation reaction on metallosilicates

3.2. Catalytic activity in selective oxidation of styrene and benzene in liquid phase The catalytic activity results obtained for selective oxidation of styrene and benzene with hydrogen peroxide, catalyzed by the mesoporous metallosilicates are presented in Figure 8. All the materials have a significant catalytic activity in oxidation reactions. A high selectivity for benzaldehyde and phenol, respectively, was obtained in all the reaction. It has to emphasize that a very remarkable activity was obtained for Ti, V, Crl, M n MCM-41 catalysts and for V-Ti, V-Co and Cr-Ni bimetallic molecular sieves for selective oxidation of benzene. Ni and V-Cu containing samples show low activity while Co and Cu modified catalysts have practically no activity. For selective oxidation of styrene, Crl and CrNi incorporated catalysts exhibit the highest activity and Ti, V, Co, Cu, V-Ti and V-Cu

1408 incorporated catalysts exhibit a notable activity whereas Mn, Fe, Ni and V-Co modified materials give very low activity. It is interesting to note that for monometallic modified samples, the activity seems to decrease with increasing the number of d electrons in the valence shell of the metal ions. Ti, V and Cr ions are unstable and form ions with a 3d ~ configurations and a high oxidation state. Their electrophilic character increases the possibility to interact with the reactants and therefore favors their activity. For bimetallic samples, the situation is quite complex.

3.3. Stability study The structure stability and the change in morphology during the several months storage under ambient atmosphere and under liquid phase reaction conditions have been studied. A

Fig.9. SEM images of the V-Co-MCM-41 (a) and Cr-Ni-MCM-41 molecular sieves (b) ,

" w l r ~"

9~ .

Fig 10. SEM images of the Cr-MCM-41 materials after 10 months storage under ambient atmosphere (a) and after reaction in H202 (b)

1409

iO n n

Fig. 11, TEM images of the Co-MCM-41 (a) and V-Co-MCM-41 (b) catalysts after oxidation reactions with H202 slight modification in pore structure and morphology, revealed by TEM and SEM (Fig.10a) was observed for the metal cation-containing MCM-41 molecular sieves after several months storage under ambient conditions. These slight modifications were confirmed by the XRD method. A high stability of the ordered hexagonal mesoporous structure under reaction conditions is observed and confirmed by TEM (Fig. 11) and SEM (Fig.10b) images. The structural stability of the materials both under ambient atmosphere storage and liquid phase reaction conditions is favored when the Ru(,+~(2.)ratios have a variation between 0.3 and 0.5. It is observed by SEM that the storage of the materials under ambient condition over the course of months can increase the dimensions while the oxidation reaction in the liquid phase decreases the dimensions of the agglomerates. The morphology of agglomerates of small spheres is still noted, only the size of agglomerates atter first and second cycle reaction decreases. Our very recent results issued from a detailed study on the structural stability of Vmodified MCM-41 materials showed an evident structural transformation to MFI structure over the course of several months under ambient atmosphere if the V-modified samples were synthesized with cationic surfactants in the presence of TMAOH and NaOH using a synthesis method described in ref. [14]. It has be revealed also that the stability of MCM-41, in particular, metal ions modified MCM-41 materials depends strongly on the preparation conditions, such as pH value of gel and the formation of silica anions from silica sources. It is not the aim of this paper to go further to describe this structural transformation under ambient atmosphere. Anyway, these results will be published elsewhere [ 14]. The present paper shows that under the synthesis conditions described in this paper, the prepared samples remains stable in structure and in morphology both under ambient atmosphere storage and the reaction conditions.

1410 4. CONCLUSIONS Highly ordered mono and bimetallic ions-containing mesoporous molecular sieves analogue to MCM-41 can be prepared by incorporation of the 3d transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, V-Cu, V)Ti, Cr-Ni, V-Co) into the framework of siliceous MCM-41. These materials are stable and active in oxidation reactions of aromatic hydrocarbons. Ti, V, Cr and bimetallic (V-Ti, Cr-Ni) incorporated catalysts are particularly active both for selective oxidation of styrene and benzene, Mn, V-Ti and V-Co only active for benzene and Co, Cu and V-Cu for styrene. The activity seems to decrease with increasing the number of d electrons in the valence shell of the metal ions. Ti, V and Cr ions are unstable and form ions With a 3d ~ configurations and a high oxidation state. Their electrophilic character increases the possibility to interact with the reactants and therefore favors their activity. For bimetallic samples, the situation is quite complex. ACKNOWLEDGMENTS

This work was performed within the framework of PAI-IUAP 4/10. VP thanks the SSTC (Federal scientific, technological and cultural office of Premier Minister, Belgium) for a scholarship and a research grant from The University of Namur. REFERENCES

1. 2. 3. 4. 5. 6.

A. Corma, Chem. Rev., 97 (1997) 2373. D.C.M.Dutoit, M. Schneider, P. Fabrizioli and A. Baiker, J. Mater. Chem. 7 (1997) 271. M. Taramasso, G. Perego and B. Notari, U.S. Patent, 4410 501, 1983. V. P~vulescu, C. Dascalescu and B.L. Su, Stud. SurfSci. Catal., 135 (2001) 4772 V. Parvulescu and B.L. Su, Catal. Today, Catal. Today, 69 (2001) 315 J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J.Am.Chem.Soc. 114 (1992) 10834 7. V. Ciesla and F. Schtith, Microporous and Mesoporous Mat., 27 (1999) 131. 8. W.A. Carvahlo, P.B. Varaldo, M. Wallau and U. Schuchardt, Zeolites, 18 (1997) 408. 9. D.Wei, W-T. Chueh and G.L. Hailer, Catal. Today, 51 (1999) 501. 10. A.B.J. Arnold, J.P.M. Niederer, T.E.W. Niel3en and W.F. H61derich, Microporous Mesoporous Mater., 28 (1999) 353. 11. F.Di Rezo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater., 28 (1999) 437. 12. S. Suvanto, J. Hukkam~ki, T.T. Pakkanen and T.A. Pakkanen, Langmuir, 16 (2000) 4109. 13. M. M. L. Ribeiro Carrott, P. J. M. Carrott, A. J. E. Candeias, K. K. Unger and K. S. W. Sing, Fundamentals of Adsorption (F. Meunier, Ed.), Elsevier, 1998, p 69 14. V. Parvulescu and B. L. Su, to be published

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.

Amine-functionalized structures

SiMCM-41

1411

as

carrier

for

heteropolyacid

L. Pizzio, P. Vfizquez, A. Kikot, E. Basaldella Centro de Investigaci6n y Desarrollo en Procesos Cataliticos (CINDECA), Facultad de Ciencias Exactas, UNLP, CONICET, 47 N ~ 257, 1900 - La Plata, ARGENTINA.

Different characterization techniques were used to evaluate the immobilization and acidic properties of Keggin structured Tungstophosphoric (TPA) and molibdophosphoric (MPA) acids impregnated on an amine-functionalized SiMCM41 support. XRD, F T i.r., 31p MAS NMR measurements and evaluation of acidity before and after leaching in ethanol/water mixtures indicated that acidity of these solids makes them attractive for use in replacement of conventional homogeneous catalysts. 1. I N T R O D U C T I O N Widespread interest is now devoted to solid catalysts possessing both active sites and high surface area, because of their potential use in the development of clean processes for the production of fine chemicals. For the synthesis of fine chemicals in acidic media, the study of reactions catalyzed by heteropolyacids (HPA) and related compounds are a very important growing field. Within the HPA, there is a special interest in those that present a Keggin type structure. A disadvantage of HPA as catalysts lies in their relatively low thermal stability, so it has been tried to stabilize them by supporting HPA on several carriers [ 1, 2]. Additionally, the leaching of HPA cannot be excluded when HPA/support catalysts are used in heterogeneous liquid reactions. In order to avoid such phenomena, we studied here the immobilisation and acidic properties of Keggin structured tungstophosphoric (TPA) and molibdophosphoric (MPA) acids, when they are impregnated on an amine-functionalized SiMCM-41. This mesoporous material was selected as the host structure because it has been shown to be an excellent support for preparing bifunctional catalysts due to its high surface area and peculiar porosity.

1412 2. EXPERIMENTAL

2.1.Mesostructure synthesis Pure siliceous, SiMCM-41 support (SBET: 915 me/g) was hydrotermally synthesized at pH= 9-10 in our laboratory according to the methodology described in [3]. A solution of commercial waterglass (SiO2, 26.8%w/w; Na20, 9.2%w/w; H20, 64% w/w) was used as the silica source, employing cetyltrimethylammonium bromide (CTABr, 98%, Aldrich), as the framework structure director. The molar ratio of the starting mixture was 1.89 SiOg:l CTABr:0.738 Na20:0.267 H~SO4:160 H90. The resulting gel was stirred about lh, then it was transferred to a teflon container, and placed in an oven at 100~ for 4 days. To provide a control for the pH, the synthesis was carried through to completion with addition of appropriate 1M H2SO4 solution each 24 h. The solid product was recovered and washed by filtration on a Buchner funnel, and dried in air at room temperature. The surfactant was subsequently removed from the mesostructure by calcination at 650~ for 6 h. 2.2. F u n c t i o n a l i z a t i o n Mesostructure functionalization was performed by addition of 3aminopropyltriethoxysilane to a suspension of organic free SiMCM-41 in refluxing toluene and stirred for 5 h. The solid was filtered, washed in a Soxhlet apparatus with diethylether and dichloromethane and dried at 393 K, by means of Lhsperas technique [4]. 2.3. Catalysts Preparation SiMCM-41 (MS) and the functionalized support (MS-F) were impregnated using the equilibrium adsorption technique (20 ~ 1 g of support was contacted with 4 ml of a solution obtained by dissolving the corresponding HPA in an ethanol-water solvent (contact time: 72 h), under constant stirring. The solution concentration was 110 g W(Mo)/I, using Fluka H3PW12(Mo12)O4o.nH20 as precursors. The solids (HPA-MS and HPA-MS-F) were separated from the solutions by centrifugation and dried at 70 ~ (24 h). In order to evaluate the HPA retention in the mesoporous structure, these samples were leached in ethanol-water, with continuous stirring, for two periods of 24 h (samples HPAMS-L and HPA-MS-F-L, respectively). 2.4. Characterization Textural properties. N2 adsorption and desorption isotherms at 77~ were carried out using a Micromeritics Accusorb 2100 equipment. The pore size distribution curve is obtained from analysis of adsorption branch of the isotherm. F T i.r. s p e c t r o s c o p y . A Bruker IFS 66 equipment, pellets in BrK and a measuring range of 400-4000 cm a were used to obtain the FT-IR spectra of solids. Nuclear m a g n e t i c r e s o n a n c e spectroscopy. A Bruker MSL-300 equipment linked to a "SOLIDCYC.DC" pulse program was utilized to obtain 3ap MAS-NMR spectra of solids. Diffuse reflectance spectroscopy. DRS spectra of the solid

1413 samples were recorded, in the range 200-600 nm, using a l_W-visible Varian Super Scan 3 equipment, fitted with a diffuse reflectance chamber with inner surface of BaSO4. Samples were compacted in a teflon sample holder to obtain a sample thickness of 2 mm. X-ray diffraction. The obtained mesostructure support was characterized by small-angle X-ray scattering (SAXS), using a Phillips PW-1714 diffractometer. C a t a l y s t acidity. A small quantity of 0.1 N nbutylamine in acetonitrile was added to a known mass of solid, and shaken for 3 h. Later, the suspension was potentiometrically titrated with the same base at a flow of 0.05 ml/min. The electrode potential variation was measured with an Instrumentalia S.R.L. digital pHmeter.

3. RESULTS AND DISCUSSION

Supports. XRD diffractograms corresponding to SiMCM-41 before and after functionalization are shown in Fig.1 (MS and MS-F, respectively). It can be seen that MCM-41 was the only phase formed. The pattern shows the strong reflections for the (100) plane of MCM-41 and secondary peaks are also well resolved. The secondary peaks indicated long-range ordering of the MCM-41 structure. Additionally, it can be seen in Fig.1 that organosilane grafting to the mesostructure (MS-F) causes a significant decrease in peak intensities. These results could be attributed to the occurrence of contrast matching between the silica framework and the grafted organic groups [5].

MS MS-F '

0

I

2

'

I

'

I

4 2 thet2

'

I

8

'

1'0

F i g u r e 1. SAXS patterns of SiMCM-41 before (MS) and after functionalization (MS-F).

1414

Figure 2 shows the FT-IR aminopropyltriethoxysflane (ANH).

spectra

MS,

of

3-aminqampyltriethor ysilane ...... v.

\-,

,-r I'

4000

I

3500

I

.-," . . . . " ' - ~ / ' ; ~ '

:

3000

I

I

I

and

_

i! ,4 ! !; !

.

"~i i 'qi,,

t

MS-F

2500 2000 1500 wixenu rrber (cm-~)

i'

I

i:lilt,

I

"l

1000

I

500

F i g u r e 2. FT-IR spectra of MS, MS-F and 3-aminopropyltriethoxysilane (ANH). The main differences between the MS-F spectrum and that of the MS, are due to the presence of small shoulders in the 3000-2750 cm -1 region, assigned to the C-H2 carbon-hydrogen stretching of ANH. The ANH used in the grafting process, present a strong band in the region 1110-1050 cm -1, assigned to the Si-O-C aliphatic groups [6] which overlaps with those belonging to the support. The grafting of MS surface decreases the surface area from 915.5 to 307.3 m2/g. These results suggest that a considerable grafting of silanol groups takes place at the work conditions used. Catalysts. The Keggin primary structure presents the general formula [XM1204o](8-")-, where M are addenda atoms, X is the heteroatom and n is the X valence. The oxygen atoms in this structure fall into four classes of symmetricequivalent oxygen: X-Oa-(M)3, M-Ob-M, connecting two M3013 units by corner sharing; M-Or connecting two M3013 units by edge sharing and terminal OdM. The main characteristic features of bulk TPA (Figure 3a) in FT-IR are observed at 1081 (P-Oa), 982 (W=Od), 888 (W-Oh-W), 793 (W-Oc-W), 595 and 524 (Oa-P-Oa) cm -1. The bulk MPA spectrum shows bands at 1064 (P-O~), 962 (Mo=Od), 869 (Mo-Ob-Mo), 787 (Mo-Or 378 and 342 (bending vibrations) cm 1 (Figure 3b). Support in the HPA/MS and HPA/MSF spectra masks the HPA band placed at the 1100 cm -1 zone.

1415 Anyway, information can still be obtained from the less affected regions which show an intensity increase of the band placed at 962 cm -1 and a small nonoverlapped band at 869 cm -1 that confirm the presence of the undegraded anion for MPA/MS (Figure 3b). TPA/MS catalyst shows the bands at 982 and 793 cm 1 as an increase in transmittance of support bands, while the b a n d at 888 cm -1 is observed without overlapping (Figure 3a).

.

~

?r~\~illl/"~k/~~~ JTPA/

V,

" ~ TPNMS-F/

~ / /

I

,

i

,

i

,

i

,

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,

i

, "i

,

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4000 3500 3000 2500 2000 1500 1000 500

V

4000 3500 3000 2500 2000 1500 1000 500 '

I

'

I

'

I

'

I

'

I

'

I

'

I

TPA/MS-.-F-L

"I'P~S-L

MPNMS-L

c '

wavenumber(cm~)

I

'

I

I o

~,~ .

.

.

.

.

wavenumber(cm-1)

F i g u r e 3. FT-IR spectra of SiMCM-41 and functional~ed SiMCM-41, bulk and supported MPA (TPA) (a, b) and the catalysts after leaching with ethanol/water

(c, d).

1416

TPAIMS

MPA~S

9 i O0

-80

'

i -60

'

I -40

'

I

'

-20 chemical

i 0

'

I

'

20 shifs (ppm)

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'

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'

-20 chemical

I 0

'

I 20

'

i 40

'

i 60

'

I

80

'

1O0

shifs (ppm)

F i g u r e 4. 31P MAS NMR spectra for TPA and MPA supported on MS. Figure 4 illustrates the 31p MAS NMR spectra for MPA and TPA supported on MS. Previously reported bulk acids chemical shifts are between -2.9-and -4.8 ppm for MPA, and -15 ppm for TPA [1, 2]. In our case, spectra obtained exhibits one line at -3.6 ppm for MPA/MS and at -15.1 ppm for TPA/MS. These measurements confirm the presence of the acids, in concordance with the FT-IR results. For TPA/MS-F and MPA/MS-F systems, the main Keggin features appear in the spectra. Small bands and shoulders are also observed in the 800 - 950 cm -1 zone, which could be possibly assigned to the PMo(W)11039 -7 lacunar species. The FT-IR spectra of the catalysts, after leaching with ethanol/water (TPA(MPA)/MSL; TPA(MPA)/MS-F-L) do not present important changes, when they are compared with the spectra before leaching (Figure 3c and 3d). A c i d i t y . The catalyst acidity measurements by means of potentiometric titration with n-butylamine enable the evaluation of the total number of acid sites and their acidic strength. The titration curves obtained for the catalysts, before and after leaching, are shown in Figure 5. As a criterion for interpreting the results obtained, it is suggested that the initial electrode potential (E) indicates the maximum acid strength of the surface sites, being the total n u m b e r of acid sites indicated by the range where the plateau is reached (meq/g solid) [1]. The acidic strength of surface sites can be assigned according to the following ranges: very strong site, E > 100 mV; strong site, 0 < E < 100 mV; weak site, -100 < E < 0 mV and very weak site, E < -100 mV.

1417 o -50

1000 ,-~

-100

% \ %

750

-150

MS

500

0

-200 250

-250 -300

E~" -350 "-"

U.I 1250

0

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I

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,,

i

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--

500

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I

0,5

'

I

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%

250 o ~ -250

MPA/MS-F '

0,0

I 0,5

'

I 1,0 meq/g

'

I 1,5

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' 2,0

~MPA/MS-F-L

~ 0,0

TP/~S-F-L '

I

0,5

'

I

1,0

meq/g

'

1,5

Figure 5. Potentiometric titration of SiMCM-41 and functionalized SiMCM41 (a), supported MPA (TPA) (b, c) and the catalysts after leaching with ethanol/water (d). MS and MS-F were titrated in order to compare their acidities. In Figure 5a, it can be seen t h a t MS presented higher acidity t h a n MS-F. It is evident from this result that the grafting process interchanges some acidic sites of SiMCM-41 by amine groups of 3-aminopropyltriethoxysilane. Additionally, MPA and TPA presented similar acid strength values when they are supported on MS, 1017 mV for MPAfMS and 1062 mV for TPA/MS (Figure 5b and 5c). These results can be attributed to the presence of Keggin structures t h a t remain unaltered onto the MS surface[l]. It was supposed that at least one proton of H3PW(Mo)1204o will react with the OH of silanol leading to a SiOH2 § group, which should act as a counter ion for the polyanion. The acidity decreased considerably for TPA when is supported on MS-F (-80 mV, Figure 5b). For MPA/MS-F, the acid strength is similar to MPA supported on MS, but the plateau of potentiometric curve is very narrow. This behavior could be due to different interaction of MPA and TPA on the functionalized support

1418 with respect to MS as a consequence of the different superficial groups. HPA are linked to the MS-F through proton transfer from the acids to the amine group, resulting in an electrostatic bond between t h e - N H 3 + and the heteropolyanion. In addition, this difference in acidity could be assigned to a change of the HPA proton positions. The protons would be localized on the most highly charged oxygen atoms, they could migrate from bridged to terminal oxygens. It is interesting to point out that acidity do not change for the catalysts after the leaching with ethanol/water when they are supported on MS and MS-F (Figures 5b, 5c and 5.d). The catalytic activities of TPA/MS-F-L and MPA/MS-F-L in the esterification of acetic acid with isoamyl alcohol are being tested [7]. They display high activity and selectivity for the studied reaction. It was determined by atomic absorption spectrometry that no leaching of TPA or MPA occur during the reaction. These results show that HPA supported onto functionalized mesoporous silica seem to be promising solids to be used as heterogeneous catalysts in liquid phase reactions. References 1. P.Vfizquez, M.Blanco and C.Cficeres, Catal. Lett. 60, 205 (1999). 2. L.Pizzio, C.Chceres and M.Blanco, Appl. Catal. A: General 167, 283 (1998). 3. K.J.Edler, J.W.White, Chem.Mat. 1997, 1226 (1997). 4. M.Lasp6ras, T.Lloret, L.Chaves, I.Rodrlguez, A.Cauvel, D.Brunel, Stud.Surf.Sci.Catal. 108, 75 (1997). 5. W.Zhang, M.Froba, J.Wang, P.T.Tanev, J.Wong and T.J.Pinavaia ,

J. Chem.Am.Soc. 118, 9164 (1996). 6. Spectroscopic techniques for organic chemists, James W. Cooper, Ed. John Wiley & Sons (New York) (1980). 7. L. R. Pizzio, P.V. Vhzquez, C.V. Chceres, M.N. Blanco, unpublished results.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1419

Acidity of Mesoporous Aluminophosphates and Silicas MCM-41. A Combined FTIR and UV-Vis-NIR Study Enrica Gianotti a*, Valeria Dellarocca a, Erica C. Oliveirab, Salvatore Coluccia a, Heloise O. Pastoreb and Leonardo Marchese ~ aDipartimento di Chimica IFM, Universit/t di Torino, v. P. Giuria, 7, 10125, Torino - Italy blnstituto de Quimica, Universidade Estadual de Campinas, CP 6154, 13083-970, Campinas, SP, Brasil ~Dipartimento di Scienza e Tecnologie Avanzate, Universit/l del Piemonte Orientale, "A. Avogadro", C.so Borsalino, 54, 15100, Alessandria - Italy. The surface acidity of new mesoporous aluminophosphates, silicoaluminophosphates and [Si]-MCM-41 was studied by means of FTIR and DR UV-Vis-NIR spectroscopies using ammonia as probe molecule. Moreover, a novel Ti-gratted mesoporous ALPO was synthesised and studied by the use of DR-UV-Vis spectroscopy.

1. Introduction

Microporous aluminophosphates materials (A1PO-n) with various crystalline structures, are known as the first example of inorganic molecular sieves composed of a material other than silica [ 1]. In the search for new synthesis methods, that could afford channel systems with pores in the range of mesoporosity, phosphate-based molecular sieves, like cloverite [2] and VPI-5 [3], have been prepared and displayed ring systems larger than the usual 12 T atoms found in large pore zeolites. Despite the large channels and/or cavities, the actual openings in these solids are not larger than 1.2 nm, still limiting their use to small reactants. A further limitation to their application is that their thermal resistance is not significant. It was not until the advent of mesoporous silicates and aluminosilicates that the possibility of preparing aluminophosphates with pore apertures larger than the ones already known turned into a reality [4]. Recently, cetyltrimethylammonium bromide was used as a structure-directing agent to synthesise mesoporous ALPOs and Mg-ALPOs [5]. In this contribution we report the synthesis of mesoporous aluminophosphates (ALPO) and silicoaluminophosphate (SAPO) and their spectroscopic characterisation using FTIR and DR UV-Vis-NIR techniques and NH3 as molecular probe to monitor the surface acidity. MCM-41 was also studied for comparison. In the view of the great interests in Ti-based materials for selective oxidation reactions, mesoporous ALPO was functionalised grafting titanium complexes on the hydroxyl groups (P-OH or A1-OH) present on the ALPO surface. This new material was studied by the use of FTIR and DR UV-Vis spectroscopy for obtaining information on surface hydroxyls and coordination of Ti(IV) centres.

1420

2. E x p e r i m e n t a l

Section

Mesoporous ALPO and SAPO materials were synthesised using cetyltrimethylammonium bromide (CTMABr) as surfactant, aluminium sulphate and orthophosphoric acid; in the case of SAPO, tetraethylorthosilicate (TEOS) was used as source of silicon (Si/AI = 0.38, Si/P = 0.30). Titanium-mesoporous ALPO was synthesised following the procedure used by Maschmeyer et al. [6] to prepare Ti-grafted MCM-41 (Ti = 1.2 wt %). In order to eliminate the surfactant, the samples were outgassed at 500~ and then calcined in 100 torr O2 at the same temperature. MCM-41 was prepared according to literature methods [7]. FTIR spectra on pelletised samples were collected using a Bruker IFS88 spectrometer and UV-Vis-NIR Diffuse Reflectance experiments were performed on a Perkin Elmer (Lambda 19) spectrometer equipped with an integrating sphere attachment. 3. R e s u l t a n d D i s c u s s i o n

In these systems the mesopores are randomly oriented and do not present an hexagonal organisation, as indicated by the appearance of only the (100) peak in the X-ray diffractogram. The study of the thermal decomposition of the surfactant in mesoporous ALPO and SAPO was followed by in situ FTIR spectroscopy. Fig. 1 shows the FTIR spectra of the assynthesised mesoporous ALPO (section A) and SAPO (section B) after outgassing at increasing temperatures from 200~ to 500~ (curves a to e). After outgassing at 200~ water was desorbed and a broad band between 3800-3200 cmq due to H-bonded P-OH and AI-OH groups observed in the ALPO spectrum (Fig. 1A, curve a). At higher outgassing temperatures (curves b-e), dehydroxylation took place and oxygen-sharing-AIOa and-PO4, along with free AI-OH and P-OH groups, were formed (scheme 1). In fact, bands at 3670 cmq, assigned to the stretching of isolated P-OH groups, and bands at 3789 and 3720 cmq, due to the stretching of free AI-OH groups, were present even at 350~ (curve c) and increased in intensity at higher temperatures (curves d and e).

Oj

1

i'

H.

""O /

1

H_

"-O /

I

H..o/H -

-H20

I

u_ l AI 0~'\0 ~/ P~. uk 00~_-O 1 ~ 0 j ~ ' ' O _ -

_

0

_

"~

_

O/H

o

H

o/

0 ~ O \o/P,~n / t \ - . /

-

_

0"7_00 .

_

-

,

o

u

i

~_

Scheme 1 Bands in the range 3050-2800 crnq which decreased with increasing temperature up to 500~ (curve e), and disappeared after calcination at 550~ (curve f), are assigned to the C-H stretching vibrations of the -CH2 (2922, 2851 cmq) and -CH3 (2964 and 2876 cmq) groups of the hydrocarbon chain of the surfactant. The same behaviour is observed for two bands at 1458 and 1580 crnq. The former was assigned to the bending of-CH2 groups, whereas the latter is of unknown nature. The surfactant preserved its cationic form after synthesis as revealed by bands at around 3025 and 1482 crnq assigned to -CH3 stretching

1421 and bending vibrations in -N(CH3)3 + polar heads of the template. In the case of ALPO, the positive charges might be counterbalanced by either bromide ions, introduced during the synthesis, or PO" and A10" groups present at the ALPO/surfactant interface, while for MCM-41 and MCM-48 (spectra not reported) the anionic counterpart is represented by

sio- [81.

The spectrum of SAPO (Fig. 1B) outgassed at 200~ (curve a), showed broad bands at 3750-2500 cm"1 range and a band at 1660 crn"1 respectively due to stretching and bending modes of H-bonded water molecules. It is of note that water molecules are more tightly bonded on SAPO than on ALPO revealing that the surface acid/base properties of the two materials are different. The presence of micropores, where 1-120 molecules can be entrapped, within the inorganic walls of SAPO might also explain this behaviour. A weak band at 3692 cm"~, due to the presence of H-bonded hydroxyls was also found. All of these bands completely disappeared after outgassing at 400~ (curve d). The bands corresponding to the polar heads of the surfactant (3033 and 1475 cm~) decreased in intensity more rapidly than those of the hydrocarbon chains (bands at 2964 for -CH3 and 2925, 2855 and 1465 cml for -CH2) being nearly completely removed upon treatment at 400oc. The decomposition of the surfactant lead to the formation of the P-OH, Si-OH and A1-OH groups (bands at 3670, 3735 and 3789 cml ) and was complete only upon calcination at 550~ (curve f).

...................

l

~

.....

|

H

a

.... 3600 3200 2800 Wavenumber[era"~] 9

3600

3200

2800

1800

1500

..

t

,,

t

// 1800 1500 -

9

I .

Fig-1 - FTIR spectra of mesoporous ALPO (section A) and mesoporous SAPO (section B) recorded after outgassing the sample at: 200~ (a), 300~ (b), 350~ (c), 400~ (d), 500~ (0), after calcination in 100 tort 02 at 550~ (f)

I

1422 Fig.2 shows FTIR spectra in the OH stretching region of mesoporous ALPO, Ti-ALPO and MCM-41 upon calcination at 550~ in 02. In the ALPO spectrum (curve a), bands at 3789 and 3720 crnq are assigned to the stretching mode of free A1-OH groups and the band at 3675 crnq to the stretching mode of isolated P-OH groups. Similar bands were present in the spectrum of Ti-ALPO but with lower intensity (curve b), and this is a clear evidence of the fact that some OH groups were used to gratt titanium ions. MCM-41 (curve d) exhibited only a narrow band at 3745 cm"l due to the stretching vibration of isolated Si-OH groups.

3745 4

It !

C

.]

%

..

. . . . . . . . . . . .

3,2o 3900

!

3 8 0 0 3 7 0 0 3 6 0 0 3 5 0 0 3400 Wavenumber [cm"1]

Fig.2 - FTIR spectra of calcined ALPO (a), Ti-ALPO (b) and MCM-41 (c) NH3 was used as molecular probe to monitor the acidity of hydroxyl groups both in mesoporous ALPO (Fig. 3 section A) and MCM-41 (Fig. 3 section B). The adsorption of 10 mbar NH3 on ALPO (Fig. 3A, curve a) produced bands at 3380, 3280, 1620 and 1460 emq, while simultaneously, the bands due to the stretching modes of both AI-OH (3789, 3720 cm"l) and P-OH groups (3675 cmq) completely disappeared. The bands at 3380, 3280 and 1460 cmq are assigned to the asymmetric and synm~tdc stretching and to the asymmetric bending modes of NH4+ ions formed by proton transfer from the surface hydroxyl groups to NH3 molecules, whereas the band at 1620 cnaq is assigned to the asymmetric bending mode of NH3 adsorbed on Lewis acid sites likely due

1423 to AI ions located on the inner surface of the mesopores [9,10]. After outgassing the sample at room temperature for lh (curve b), the bands due to NH4 + ions decreased in intensity and the band due to P-OH stretching vibration (3675 cm "l) reappeared, although of smaller intensity than that observed before adsorbing NH3 (curve e). Also the band at 1620 cm"~ was still present atter outgassing the sample at room temperature. Only after outgassing at 350~ (curve d), the bands of NH4 § disappeared and the absorptions due to the stretching of A1-OH and P-OH groups were completely restored. NH3 on Lewis acid sites was also desorbed at 350~

//

A

1

B

i:

a r~

C

II

iI

, .. 9

'

,

,

,

,

.

,

..

3600 3300 3000 2700

/;

,

.

, " . " ' - I

1800 1600 1400

"

9

,

d - " - ] 7 . . - - - . ~ - - 7 - - - ' t e

3600 3300 3000 2 7 0 0

,

.

,

,

, l

"'180016001400

Wavenumber [em-~1 Fig.3 - FTIR spectra of NH3 adsorption on mesoporous ALPO (section A) and on MCM-41 (section B). Section A - curve a: 10 mbar NH3, curve b: outgassed sample at room temperature for lh, curve e: outgassed sample at 150~ curve d: outgassed sample at 350~ curve e: sample before NH3 adsorption. Section B - curve a: 10 mbar NH3, curve b: 1 mbar NHa, curve e: outgassed sample at room temperature for lh, curve d: sample before NH3 adsorption.

1424 The adsorption of 10 mbar of NH3 on [Si]-MCM-41 (Fig. 3B, curve a) produced, in the FTIR spectra, bands at 3405, 3330 and 3320 crn"1, overlapped to a broader band centred at 3030 crn"l. The band at 3330 cm "~, due to the asymmetric stretching mode of NH3 molecules in gas phase, disaplpeared by decreasing ammonia pressure (curves b). The broad band centred at 3030 cm" is assigned to the stretching O-H of the silanol groups Hbonded to NH3 molecules (Si-O-H .... NH3). The bands at 3405 and 3320 crn"l are respectively due to the asynm~tric and symmetric stretching vibrations of NH3 H-bonded to silanols. At lower frequency, bands at 1635 and 1625 cm"1 were also formed. The band at 1625 crn"l is due to the asymmetric bending mode of NH3 molecules in gas phase, whereas the band at 1635 crn"1 is assigned to the asymmetric bending mode of the ammonia molecules adsorbed on Si-OH groups. All these bands completely disappeared upon outgassing the sample at room temperature for lh (curve c), and the narrow band at 3745 cm"l, due to stretching mode of free Si-OH groups, was almost completely restored, giving a spectrum which was similar to the one recorded for the bare MCM-41 sample (curve d) [11,12]. These results show that AI-OH and P-OH hydroxyl groups in ALPO are sufficiently acidic to protonate ammonia molecules, and that NH4+ is decomposed at temperatures lower than in acid zeolites. This particular acidity sets this material as intermediate between Si-OH and zeolitic bridged OH acid groups, specific of silicate and/or aluminosilicates structures. These materials might be used with advantages in organic reactions where a mild acidity is necessary. NIR spectra of ALPO (curve a) and MCM-41 (curve b) calcined and outgassed at 550~ are shown in Fig.4. 0.3

-

:

.

.

/ /

.

.

.

.

.

.

.

.

4530

:,,

02

t |

',

7321 t

4610~

/

b,

~17190

~, . , . r

s

o.o

8000-7500

7000

6500"" 50004500-4-000 W avenum ber r "~

Fig.4 - NIR spectra of mesoporous ALPO (curve a) and MCM-41 (curve b)

1425 The ALPO spectrum showed broad bands at 7190 cm~ and at 4530 cm~ with a shoulder at 4610 cm"~, the former being due to the first overtone of the stretching mode and the latter to the combination of stretching and bending modes of isolated OH groups. Such bands are broad and asymmetric in that, different hydroxyl groups (AI-OH and P-OH), are present in ALPO material. The spectrum of MCM-41 showed a band at 7321 crn"~ due to the first overtone of the stretching mode of isolated Si-OH groups (2VOH), and a band at 4530 cml assigned to the combination of the deformation with the stretching OH modes (VoH + ~oH). These bands are narrow because only Si-OH groups are present in MCM-41. Surface OH groups, P-OH and A1-OH in ALPO and Si-OH in MCM-41, are the sites for anchoring metals ions, for example Ti ions. In this way it is possible to obtain a variety of Ti-functionalised mesoporous catalysts with tuned polarity (from MCM-41 to ALPOs) for reactions involving bulky hydrocarbons. In Ti-based materials the active sites for selective oxidations are isolated tetrahedral Ti(IV) centres. DR UV-Vis spectroscopy was used to achieve information about the coordination of Ti(IV) sites in Ti-ALPO mesoporous material (Fig. 5). The spectrum of the as-synthesised sample in air (curve a) showed a broad absorption centred at ca. 250 ran, which after calcination (curve b) became sharper and shifted to 230 nm. These features indicates that the titanium sites existed mainly in tetrahedral coordination, in fact, bands in the 210-230 nm range arise from oxygen to tetrahedral Ti(IV) ligand-to-metal charge transfer (LMCT) [11-14]. The broadness of the band in the spectrum of as-anchored sample was due to the presence of electron-reach pentadienyl ligands bonded to titanium centres.

3

2

0 200

~'\a

250

300

350

400

450

500

Wavelength [nm]

Fig. 5 - DR VU-Vis spectra of mesoporous TiALPO. Curve a: as-synthesised sample in air; curve b: calcined sample in vacuo

1426 In conclusion the synthesis of mesoporous ALPO and SAPO materials and their characterisation by spectroscopic means is reported. ALPO showed acidic properties intermediate between silicas and zeolites which can be useful for catalytic reactions where mild acidity is required. Measurements of the acidity of the meso-SAPO is in progress. These materials are also interesting as support for preparing metal-functionalised catalysts. An example of Ti-gratted material is reported and showed that tetrahedral Ti(IV) sites, the active centres for many selective oxidations, are present. ACKNOWLEDGEMENTS: The Italian MURST (Progetto di Rilevante Interesse Nazionale, Cofin. 2000) and the Brazilian FAPESP (Funda~.o de Amparo h Pesquisa no Estado de S~.oPaulo) are acknowledged.

REFERENCES

1. S.T.Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, J.Am. Chem. Soc., 104 (1982) 1146 2. M. Estermann, L.B. McCuster, C. Baerlocher, A. Merrouche, H. Kessler, Nature 352 (1991)320 3. M.E.Davis, C. Saldarfiaga, C. Montes, J. Garces, C. Crowder, Nature 331 (1998) 698 4. T. Kimura, Y. Sugahara, K. Kuroda, Micropor. Mesopor. Mater. 22 (1998) 115 5. N.C. Masson, H.O. Pastore, Micropor. Mesopor. Mater., 44 (2001) 173 6. T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature, 378 (1995) 159. 7. C.T. Kresge, M.E. Leonowicz, W.J. Vartuli, J.S. Beck, Nature 359 (1992) 710M. 8. L.Pefia V. Dellarocca, F. Rey, A. Corma, S. Cosuccia, L. Marchese, Micron. Mesop. Mater., 44-45 (2001) 345 9. L.H. Little, Infrared Spectra of Adsorbed Species, Academic press, New York, (1966) 10. R.L. Puurunen, A. Root, S. Haukka, E.I. Iiskola, M. Lindblad, A.O.I. Krause, J. Phys. Chem. B, 104 (2000) 6599] 11. L. Marchese, E. Gianotti, T. Maschmeyer, G. Martra, S. Coluccia and J.M. Thomas, I1 Nuovo Cimento, 19D (1997) 1707. 12. E. Gianotti, V. Dellarocca, L. Marchese, G. Martra, S. Coluccia, T. Maschmeyer in preparation 13. P.E. Sinclair, G. Sankar, C.R.A. Catlow, J.M. Thomas and T. Maschmeyer, J. Phys. Chem. B, 101 (1997) 4232. 14. L.Marchese, T. Maschmeyer, E. Gianotti, S. Coluccia and J.M. Thomas, J. Phys.Chem. B, 101 (1997) 8836.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1427

Modification of silica walls of mesoporous silicate and alumino-silicate by reaction with benzoyl chloride. L. Pasqua a, F. Testa b, R. miello b, G. Madeo b and J. B.Nagy~ aDipartimento di Ingegneria dei Materiali e della Produzione, Universit~t Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy bDipartimento di Ingegneria Chimica e dei Materiali, Universit~t degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy. CLaboratoire de R. M. N., Facult6s Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium.

Preparation of mesoporous silicate and alumino-silicate by different synthesis methods and their surface modification are reported. Specific surface area of starting samples ranges between 400 and 1100 m2/g. Surface properties of pore walls of obtained samples were investigated by reaction with benzoyl chloride. FF-IR spectra of modified samples show a peak centred at 1705 cm -1 assigned to the stretching vibration of carbonyl group of benzoic ester. Modified samples show lower specific surface areas and pore volume with respect to the correspondent values for starting materials. The weight ratio between benzoyl and silica was also calculated by thermogravimetric analysis. Mesoporous materials and their reactivity open new perspectives in the possibility of designing specific catalysts. 1. I N T R O D U C T I O N

A new family of mesoporous molecular sieves, named M41S, was recently discovered by Mobil's researchers [ 1]. MCM-41 is the member of the M41S family characterized by a regular hexagonal array of uniformely-sized mesopores. Mesopore diameters can be varied from approximately 20 to ca. 100 A depending on the surfactant employed in the synthesis, the pore size increasing with the chain length of the surfactant. These materials have important applications in catalysis, metal ion extraction, optical applications, etc.. Catalytically active materials have been prepared by introduction of inorganic heteroatoms by grafting metallocene complexes on mesoporous silica creating well-defined active sites [2]. Hybrid inorganic-organic materials are produced when chemically active groups are covalently linked to the inorganic framework of mesoporous materials either by post-synthetic grafting or by simultaneous condensation of siloxane and organosiloxane precursors, the last one containing a non-hydrolysable Si-C bond [3-5]. This method was also applied for bulky chromophores [6]. MCM-41 was also studied as a drug delivery system. In particular, inclusion and delivery mechanism of Ibuprofen, an anti-inflammatory drug, were investigated [7]. Immobilization of enzymes in MCM-41 host was also studied [8].

1428 Molecular design of the active site is one of the elements of greatest importance in heterogeneous catalysis. High reactivity of acyl chlorides of every kind of molecule allows, in fact, fast modifications of pore walls of mesoporous materials representing a valid tool for modelling pores or giving the pores the appropriate chemical functionality, being the synthesis of a chemically-interactive or shape-selective (or both) composite inorganic-organic catalyst the ultimate goal. In this work the preparation of mesoporous silicate and alumino-silicate by different synthesis method is reported. The cationic surfactants cetyltrimethylammonium bromide, cetylpyridinium chloride and the neutral surfactant polyoxyethylene(10)isononylphenylether (Nonfix 10, Condea) were used as structure directing agents while tetraethylorthosilicate, fumed silica, and sodium silicate solution were used as silica sources. Synthesized and modified products were characterized by X-Ray powder diffraction, nitrogen adsorption-desorption, thermogravimetric analysis. Chemical ability of free silanols on surface of pore walls toward organic functional group of guest species were investigated by reaction with benzoyl chloride.

2. EXPERIMENTAL 2.1. Materials and methods The reagents utilized were: cetyltrimethylammonium bromide (CTABr, Aldrich), cetylpyridinium chloride (CPIC1, Aldrich), polyoxyethylene(10)isononylphenylether (Nonfix 10, Condea), benzoyl chloride (Sigma), ammonium fluoride (Carlo Erba), and NaOH (Prolabo). The silicon source was tetraethylorthosilicate (TEOS), fumed silica (SigmaAldrich), sodium silicate solution (Sigma). X-Ray powder diffraction patterns were measured on a Philips PW1710 diffractometer using Cu-Kct radiation (40 Kv, 20 mA) over the range 1~ ~ The N2 adsorptiondesorption volumetric isotherms at 77 K were measured on a Micromeritics Asap 2010 apparatus. Samples were pre-treated under vacuum at 300~ to a residual pressure of 2 l.tmHg. Benzoylated samples were treated at 230~ to the same residual pressure. Surface area of the samples was obtained by BET linearization in the pressure range 0.05 to 0.2 P/Po. Lattice pore volume was obtained from the amount of nitrogen gas adsorbed at the top of the rising section of the isotherms of type I or IV. FT-IR spectra of calcined and modified samples were recorded on a Bruker IFS-28 spectrometer in the absorbance mode with a resolution of 2 cm -1. For each spectrum 256 scans were acquired. 2.2. Synthesis Mesoporous materials submitted to chemical modifications of pore walls were obtained from different synthesis procedures at different temperatures and pH. Sample A and sample B were obtained starting from the gel with the following molar composition: SiO2-0.2NaOH-x AI(OH)a-0.2CTABr-40H20 where x is 0.04 and 0.02, respectively, for samples A and B. 10 grams of fumed silica were added to a solution consisting of AI(OH)3 0.52 g (for sample A) or 0.26 g (for sample B) and 1.35 g of NaOH in 120 grams of distilled water. The gel was aged for 2 hours at room temperature and then transferred to a Teflon-lined container in a thermostated oven at 140~ for 24 h. The synthesis product was then filtered and dried at 80~

1429 Samples C and D were prepared in fluoride medium [9] at 50~ The molar composition of the gel was: TEOS -0.21 CTABr-10 NH4F- x Al(NO3)3-146 H~O where x is 0.01 and 0, respectively, for samples C and D. 13.7 g of TEOS were added to a solution containing 4.98 g of CTABr and 10g of NHnF in 174 g of distilled water. In the synthesis C 0.248 g of Al(NO3)3 9H20 were added before TEOS. The as-synthesized solid was recovered by filtration, washed and dried at 70~ Syntheses of samples E and F were performed in the presence of cetylpyridinium chloride starting from a gel of the following molar composition: SiO2 -0.25 NaOH -0.25 CPC1-x AI(OH)3 -40 H20 where x is 0.02 and 0, respectively, for samples E and F. The resulting gel was stirred for 15 minutes, then transferred to a Nalgene polypropylene bottle and heated under static conditions at 80~ The recovered solid was filtered, washed and dried at 80~ The neutral surfactant polyoxyethylene(10)isononylphenylether (Nonfix 10, Condea) was used in the synthesis of sample G. The molar composition of the gel was: 1SIO2-0.6 NaOH-0.064 Nonfixl0-0.8 HC1-58.1 H20 14.6 g of sodium silicate were added to the surfactant solution (2.9 g of Nonfix 10 in 57.4 g of H20) after complete dissolution of Nonfix 10. Finally, 5.33 g of 37% HC1 were added and the resuking gel was aged for 24 hours at room temperature and heated in oven for 24 hours at 100~ [ 10]. The template was removed by calcination from all the samples at 550~ under flowing air with a heating rate of 1~ followed by a static step of 8 hours. 2.3. Chemical modification

Silica pore walls of calcined mesoporous materials were chemically modified by means of esterification of free silanols with benzoyl chloride. Typically, 0.5 g of calcined mesoporous materials were suspended in 10 ml of anhydrous tetrahydrofuran under nitrogen atmosphere; 1 ml of benzoyl chloride was then added and the suspension was kept under stirring for 20 hours. The modified product was filtered, washed with ethanol, dried in oven at 80~ and submitted to FF-IR analysis. A band at 1705 cm -1 appears in the spectrum after esterification. The same band disappears after the modified sample was hydrolysed at room temperature for 3 hours (buffer solution pH=l, Merck). Nitrogen adsorption-desorption at 77 K on resulting hydrolysed sample shows that the total chemical process of modification and hydrolysis does not affect the pore structure. 3. RESULTS AND DISCUSSIONS

As-synthesized samples have been characterized by X-Ray powder diffraction and thermogravimetric analysis. X-ray powder diffraction pattern of synthesized samples do not exhibit higher diffraction angle peaks but just a broad band in addition to the main reflection peak (hkl 100). The absence of higher angle peaks is typical of poorly ordered porous system on the long range. Calcined samples are characterized by X-ray powder diffraction and nitrogen adsorption-desorption at 77 K. Table l shows the physico-chemical properties of assynthesized materials before and after calcination. Specific surface area of starting samples ranged between 400 and 1100 mZ/g, while pore volume values are comprised between 0.37 and 1.02 cm3/g. Samples C and D, synthesized in fluoride medium at pH around neutrality, show

1430 relatively low pore volume and specific surface area values. At neutral pH hydrolysis rate is low and condensation is fast. In these conditions, the polymerization of silica accounts for increased wall thickness and lower incorporation of surfactant [ 11]. Tablel Unit cell parameters for as-synthesized (ao) and calcined materials (a~,~), specific surface area SBEX,pore volume and surfactant/silica mass ratio for calcined materials. Sample

ao (A)

aca~(A)

BET Surface Area (m2/g)

Pore volume (cm3/g)

CTA/SiOz mass ratio

A B C D E F G

47.3 50.3 49.2 56.0 54.9 49.4 64.2

44.3 49.4 49.2 56.0 52.9 49.6 64.2

777 933 496 400 1034 1150 992

0.56 0.74 0.41 0.37 0.72 1.02 0.80

0.91 0.60 0.26 0.20 0.66 0.96 0.39

Surface properties of pore walls of obtained samples were investigated by reaction with benzoyl chloride in tetrahydrofuran under nitrogen atmosphere at room temperature for 20 hours. Free silanols on external surface of particles and on the surface of pore walls of mesoporous materials are potential electron donor species, so they are able to react with the electrophilic carbon atom of benzoyl chloride. Esterification of free silanols on pore walls was determined by Fr-IR spectroscopy and thermogravimetrical analysis. Porosity of modified samples was evaluated by nitrogen adsorption-desorption at 77 K. Table 2 shows physico-chemical properties of modified materials. Lattice parameters of modified materials are very similar to values of calcined materials presented in Table 1. Table 2 Unit cell parameters (ao), specific surface area SBET, pore volume, FT-IR band of modified materials. Sample

ao (A)

A B C

47.3 46.9 49.2

BET Surface Pore volume Area (m2/g) . . . . cm3/g 759 803 488

0.44 0.63 0.4

FF-IR band Wavenumber (cm -1)

CO-~/SiO2

1705 1705 -

0.040 0.020 0.014

Modification procedure does not affect the texture of the solids. Modified samples show lower specific surface areas and pore volume compared to the correspondent values for starting materials shown in Table 1. The band at 1705 cm -1 in the FF-IR spectra (Figure 2 for sample A) of modified samples is assigned to the stretching vibration of carbonyl group of benzoic ester. This band, not present in the spectra of starting calcined materials (Figure 1 for sample

1431 A), disappears after hydrolysis of ester function. For samples C and D no FT-IR band at 1705 cm -1 was detected. Silanols groups are probably too diluted on the surface of mesoporous materials and the low amount of carbonyl group is not detected by FI'-IR technique. Amount of benzoylation was quantitatively estimated by thermogravimetric analysis for samples A and G. In order to quantify the weight loss that occur at high temperature due to further condensation of silicate walls the calcined sample was thermally treated at 150~ with a

1.3

1.0-

o

0.8

0.5'

3500

'

3000 '

2500

'

20b0

'

15'00'

1000

-1

Wavenumber cm Figure 1. FI'-IR spectrum of calcined sample A.

0.9

17 oo

0.8

0.7 "35'00'30'00'25'00'20'00"15'00' Wavenumber crn-1 Figure 2. FT-IR spectrum of modified sample A.

1000

1432 heating rate of l~ Then a 6 hours static step followed and finally the sample was heated at 850~ with a heating rate of 5~ The same procedure was successively adopted for modified sample. The molar ratios between benzoyl group and silica are shown in Table 2. Figure 3 shows nitrogen adsorption-desorption isotherms of calcined and modified sample A. The main nitrogen uptake for calcined sample A takes place around a relative pressure P/Po=0.3.

600

----~-- Adsorption ----- Desorption

o,'•~500 o

Sample A calcined

400 300

o

Sample A modified

< 200 100 0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative Pressure P/Pn Figure 3. Nitrogen adsorption-desorption isotherms of calcined and modified sample A.

1000

- - m - - Adsorption ~ - - Desorption

8oo

~ 4O0 200 0.0

0.I

l

0.2

0.3

I

0.4

0.5

i

06

0.7

I

0.8

0.9

1.0

Relative pressure P/Po Figure 4. Nitrogen adsorption-desorption isotherms of calcined and modified sample F.

1433

800

~ m ~ Adsorption - - o ~ Desorption

700 600-

Sample G calcined

500. 400Sample G modified

.

~

300-

100

.

0.0 0.1

I.

02

'

0.3

I

0.4

'

0.5

I

0.6

'

0.7

I

0.8

'

0.9

1.0

Relative pressure P/Po Figure 5. Nitrogen adsorption-desorption isotherms of calcined and modified sample G. The isotherm is a Type IV one, reversible without hysteresis loop. It can be noted that the pore filling step of the isotherm moves toward lower relative pressure and the pore volume decreases after chemical modification. Shift of the nitrogen uptake and decrease of pore volume suggest that benzoic ester occupies part of the available space in the starting calcined materials. The same behaviour is shown by samples F (Figure 4) and G (Figure 5) (both calcined and modified) and also by samples B and E (not reported). Pore volume and specific surface areas of samples decrease following esterification. Table 3 shows BET surface area and pore volume ratios between modified and starting calcined materials. Sample C and D show BET surface area and pore volume ratio at the border line with experimental error. Sample A and sample G show the most important deviations from unity for pore volume ratio. Table 3 S B E T and pore volume ratios between modified and starting calcined materials. Sample

A

B

C

D

E

F

G

SBE'r Ratio Mod/Calc PV Ratio Mod/Calc

0.97 0.78

0.86 0.85

0.98 0.98

0.96 0.97

0.92 0.96

0.97 0.86

0.82 0.79

The 13CN/~Rspectra unambiguously show the presence of benzoyl group grafted on the MCM-41 wall. The chemical shifts are quasi identical to those of benzoic methyl ester and different from those of benzoic esters. The highest difference is in the carbonyl chemical shift. The difference stems from the fact that O-SiR3 group is less electron donor than the CH3-O-

1434 group. Chemical properties of silica surface depend on population of free silanols and on their accessibility. Modification rates depend on reactivity of precursors, diffusion limitations and steric factors. The last two factors could be neglected in the case of mesopores and benzoyl chloride. Samples C and D, synthesised in fluoride media, do not reveal chemical modifications. At neutral pH condensation rate is fast, the gelling time of a silica sol is at its minimum [12] and materials with increased wall thickness and low amount of silanols on the pore walls are produced. 4. CONCLUSIONS The above reported results show that new possibilities are open in designing molecular host. Molecules that, on one side, approach silica walls and reacts with free silanols, as it occurs in the case of benzoyl chloride can carry on the opposite side, instead of phenyl group, every kind of organic functional groups. This makes possible to create appropriate active sites in terms of size, polarity, and presence of chelating agents, or metals so that the desired reaction can be catalysed in a tailored cavity as it naturally happens in enzymes. 5. ACKNOWLEDGMENTS

The authors thank the Belgian Programme PAI P5/10 on "Quantum size effects in nanostructured materials". REFERENCES

1. J.S. Beck, J.C. Vartuli, V.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. Mc Cullen, J.B. Higgins and J.L. Schlenker, J.Am. Chem. Soc., 114 (1992) 10834. 2. T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature 378 (1995) 159. 3. S.L. Burkett, S.D. Sims and S. Mann, Chem. Commun (1996) 1367. 4. M.H. Lim and A. Stein, Chem. Mater. 11 (1999) 3285. 5. D. Brunel., Microporous Mesoporous Mater., 27 (1999) 329. 6. C.E. Fowler, B. Lebeau and S. Mann, Chem. Commun, (1999) 201. 7. M. Vallet-Regi, A Ramila, R:P. Del Real and J. Perez Pariente, Chem. Mater., 13 (2001) 308. 8. J.F. Diez and K. J. Balkus Jr., J. Mol. Catal. B: Enzymatic 2 (1996) 115. 9. L. Pasqua, F. Testa, R. Aiello, Proceed."5 ~ Congresso Nazionale Scienza e Tecnologia delle Zeoliti" Ravello (Sa) Italy, l-5 ottobre 2000 p. 41. 10. F. Cavallaro, L. Pasqua, F.Testa and R. AieUo, Abstract 2nd Feza Conference Taormina 1-5 Sept.2002. 11. L. Pasqua, F. Testa, R. Aiello F. Di Renzo and F. Fajula, Microporous Mesoporous Mater., .44-45 (2001) 111. 12. F. Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater.,28 (1999) 437.

ADVANCED MATERIALS AND APPLICATIONS

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1437

Options for the Design of Structured Molecular Sieve Materials J. Sterte, J. Hedlund and L. Tosheva Division of Chemical Technology, Lule~t University of Technology, 971 87 Lulegt, Sweden Options for structuring of molecular sieve materials at the crystal, particle and reactor level are discussed in general and exemplified by work performed in our group. Zeolite crystal manipulation at Lule~ University of Technology (Ltu) includes the preparation of colloidal zeolites as well as zoned materials. Molecular sieve films have been prepared using the seeding approach developed by the group using a variety of zeolite-substrate combinations with a number of different applications intended. Of particular interest has been to develop high flux molecular sieve membranes. A method for the preparation of three-dimensional molecular sieve macrostructures of great interest in catalytic applications has been developed and modified.

1. I N T R O D U C T I O N Structured materials have attracted an increasing interest during the last few years. Although no strict definition has been given, structured materials are usually referred to as materials designed and prepared in a controlled manner to overcome limitations of existing materials prevailing in commercial use. The control could be effected on the crystal level, the particle level or the reactor level and the design involve both the physical and the compositional structure of the material. The borders are not definite but structures at the molecular level (e.g. crystalline structures, isomorphous substitutions etc.) are usually not included. In the book by Cybulsky and Moulijn [1] on "Structured Catalysts and Reactors", the editors choose not to define such catalysts but rather to set the limitations by giving examples. The examples given include (i) Monolith catalysts (honeycombs), (ii) Membrane catalysts, and (iii) Arranged catalysts, the last group including catalysts arranged in arrays and structural catalysts derived from structural packings for distillation and absorption columns. They have thus chosen not to include structures at or below the level of particles with the motivation that randomness in packing always will result in a lack of uniform structure at the reactor level. To limit the scope of structured molecular sieve materials to the three groups given by Cybulski and Moulijn would, however, be to narrow the field too much and would not correspond to the use of the term structured materials in the literature. Molecular sieves are widely used in catalysts as well as in adsorption applications. The reasons are their well known properties in terms of defined pore sizes (selectivity), surface area, ion-exchange capacity, and catalytic activity. Taken alone or in combination, these properties provide possibilities for the preparation of materials with unique properties for a huge number of applications.

1438 To increase the utility of molecular sieves as components in structured materials, further insights in possibilities of manipulating and control properties of molecular sieve crystals, molecular sieve films, self-bonded molecular sieve structures and composite materials comprising molecular sieves are needed. Such insights form the basis for the design of structured molecular sieve materials tailored for optimum performance in applications such as catalysis, separation technology and chemical sensors. This paper intends to give a brief overview of recently developed possibilities for the controlled preparation of molecular sieve materials with a more extensive treatment of such methods developed in our laboratories.

2. MODIFIED CRYSTALS

Tailoring of molecular sieve crystals may involve control of properties such as size (size distribution), shape or compositional gradients. It may also concern modifications aimed at e.g. controlling the properties of the external surface of the crystals. Since single crystals often are primary building blocks for structured molecular sieve materials, possibilities to prepare discrete crystals with the desired size and shape are of great interest. Some preparative procedures recently developed rely on the use of colloidal suspension of molecular sieves as seeds for further growth and in a number of applications the size of the crystals are important since it affects parameters such as diffusion length and ratio between external and internal surface area. Methods for the preparation of stable colloidal suspensions of a number of molecular sieve types have been developed during the last decade. Some relevant examples are given in Table 1.

Table 1 Molecular sieves prepared in the form of stable colloidal suspensions Molecular sieve Appr. Size range (nm) References Hydroxysodalite Zeolite A Silicalite-1

30-50 100-500 >40

Ti-silicalite- 1 (TS 1) ZSM-5 ZSM-2 Faujasite

80-250 100-250 100-500 40-150

Zeolite L Offretite Zeolite Beta

>30 >60 10-200

Schoeman et al. [6] Schoeman et al. [7] Persson et al. [8] Anthonis et al. [9] Li et al. [ 10] Zhang et al. [ 11-12] Persson et al. [ 13] Schoeman et al. [ 14] Schoeman et al. [7] Li et al. [ 15] Verduijn et al. [ 16] Verduijn, [ 17] Camblor et al. [18] Schoeman et al. [ 19]

1439 Most of these examples have been developed within our group using preparative methods involving crystallization from clear synthesis solutions. Although several of the most interesting molecular sieve types are present in this list, a number of interesting zeolite types has not yet been prepared in this form. For some applications it is desirable to grow extremely large single crystals of molecular sieves. During the last years significant progress in this area has been achieved by using nonaqueous growth media for crystallization. Strategies for the preparation of large single crystals have been reviewed by Qiu et al. [2] and some applications of large single crystals are discussed by Scandella et al. [3]. It may also be desirable to limit the surface acidity or to control the adsorption properties of the external surface area of molecular sieve crystals. It can e.g. be of interest to have an outer shell of silicalite-1 on an active core crystal consisting of ZSM-5 or Ti-Silicalite. This can be envisaged to be realized either by a truly zoned crystal, i.e. a single crystal with a compositional gradient from core to shell or by a (large) crystal covered with a polycrystalline film of silicalite-1. These options are further discussed below in connection with molecular sieve films. Another possible way to adapt the performance of molecular sieves in a given application is to modify the surface properties of the crystals [4]. This can be done with the purpose of introducing a diffusion constraint e.g. by chemical vapor deposition. Objectives can also be to deactivate existing catalytic sites at the surface or introducing an additional catalytic function selectively at the surface. Another possible purpose may be to change the hydrophobicity e.g. by application of methods described by Caro et al. [5].

3. Z E O L I T E FILMS Three main routes for the preparation of continuous zeolite films can be distinguished in the vast number of papers devoted to zeolite films and membranes. These routes are the vapor phase method, the method of direct crystallization and seeding methods. Focus will be on the last of these approaches and on work performed at Ltu. In the vapor phase method, first reported by Xu et al. [20], the support is first coated with a parent aluminosilicate gel. The amorphous gel is then hydrothermally treated with vapors of template molecules and converted into zeolite. The technique has been explored by several groups and is well described by Matsukata et al. [21]. This method is versatile but reproducibility appears to be an issue, at least when it is applied for membrane preparation. The method of direct crystallization is the most widely used approach. From a practical point of view, this approach benefits from being a single step method. The preparation is straightforward, the support is treated directly in a synthesis mixture whereupon zeolite crystals nucleate on the support and grow into a continuous film. The main disadvantage is that nucleation must occur in a synthesis mixture. For instance, a synthesis mixture that is capable of creating a sufficient concentration of nuclei on the surface may not exist. The nuclei concentration is dependent of the chemical properties of the support and the nuclei concentration may be low thus ruling out the possibility for preparation of thin and continuous films etc. Despite the limitations, the approach has been successfully demonstrated for the preparation of a large number of zeolites on various types of substrates [22]. In seeding methods, pre-synthesized seeds are somehow concentrated on the support surface and the seeded support is subsequently hydrothermally treated in order to grow the

1440 seeds into a dense film on the support. Advantages with this route are that nucleation no longer must occur on the surface of the support, and the chemistry of the support becomes less important. Instead, nucleation (and limited growth) is carried out in a separate step, during the crystallization of the seed crystals. In the second step, the seed crystals are concentrated and attached to the support. Several techniques are available to accomplish this, such as dip coating, spin coating or electrostatic adsorption, the latter technique facilitating monolayer adsorption. Another advantage with the method is thus that the concentration of nuclei/seed crystals on the surface may be high, provided that the seeding is effective. If the seed crystals are small and adsorbed in a monolayer, it is possible to grow very thin and dense films, which may be difficult/impossible with the alternative methods. In the last step, the seeded support is hydrothermally treated, the seeds grow and a dense film is formed. As opposed to the method of direct crystallization, a synthesis mixture with low nucleation rate can be used. In fact, it is even possible to use a synthesis mixture were no zeolite would form in the absence of seed crystals. For sensitive supports, a synthesis mixture with low alkalinity could be selected. An additional advantage is thus that a wider range of synthesis mixtures can be used for film growth, due to the presence of seed crystals. A disadvantage with the method is that it relies on the existence of small seed crystals, and the fact that it is multi step procedure makes it more labor intensive. The development at Ltu of a seeding technique for preparation of zeolite films was initiated in the mid-nineties, recognizing the facts that extremely thin and continuous films are desired in most applications. This resulted in a method employing colloidal zeolite crystals as seeds and electrostatic adsorption on a modified substrate surface as a means to facilitate a dense momolayer coating [23]. In a first test of the concept, silicalite-1 films were prepared on silicon wafers [24]. Colloidal silicalite-1 seed crystals were prepared according to Persson [8] and the seeds were adsorbed elctrostatically, employing a cationic polymer to reverse the charge of the silicon surface. Films with thickness ranging from about 100 to 800 nm, depending on the synthesis time, were prepared. Since then, the same basic concept has been used, modified in a number of ways to facilitate the growth of various zeolite types on a variety of different supports, see Table 2. SEM top view images of a few examples of zeolitesupport combinations are shown in Figure 1. Table 2 Examples of zeolite films prepared at Lulegt University of Technology References Molecular sieve Support Film thickness (nm) Silicalite- 1 Silicalite- 1 S ilicalite- 1 Silicalite- 1 ZSM-5 ZSM-5 (Si/AI= 10) Zoned MFI A FAU FAU Beta

Silicon wafer Carbon fiber Gold Steel Gold Quartz wafer Quartz wafer Alumina wafer Alumina wafer Steel Tantalum

100-800 -3200 100-800 200-800 200-250 200-4000 6300 200-800 150-2700 2000-6000 - 140

Hedlund et al. [24] Valtchev et al. [25] Engstr6m et al. [26] Wang et al. [27] Mintova et al. [28] Mintova et al. [29] Li et al. [30] Hedlund et al. [31 ] Lassinanti et al. [32] Wang et al. [33] Schoeman et al. [ 19]

1441

Figure 1. A 1000 nm thick silicalite-1 film on silicon wafer (a), a 1500 nm thick ZSM-5 (Si/AI=10) film on porous alumina (b), a 800 nm thick LTA film on alumina wafer (c) and a cracked 6 ~tm thick FAU film on a stainless steel support (d). Besides exploring the possibilities of preparing samples of various zeolite-support combinations, we have studied the preferred orientation of the crystals in the films and explored how to control the preferred orientation [34-36]. It has been found that the preferred orientation of the crystals making up the film is controlled mainly by the orientation of the seed crystals and by competitive growth during the film formation. Recently, compositionally zoned MFI films have been prepared [37]. Silicalite-1 films have been grown on ZSM-5 and vice versa. The aluminum content in the ZSM-5 part has been varied. It appears that truly zoned films, i.e. films with a continuously propagating channel system throughout the entire film, only form if the first layer is silicalite-1 or if the compositional difference between the ZSM-5 part of the film and the silicalite-1 film is small.

4. MACROSTRUCTURES Synthetic zeolites are normally produced as a crystalline powder. Prior to using the zeolites as e.g. catalysts and adsorbents, the powder is usually formed into agglomerates such as spheres, tablets and extrudates. Methods for forming the zeolite powder into agglomerates

1442 include the addition of non-zeolitic binders, generally various types of clays. The binders provide macro particles that conform to the process requirements of activity, pressure drop and attrition resistance. However, the binding additives may also affect the zeolite performance. For instance, since the binder is typically present in amounts of up to about 50 wt.% of zeolite, the binder reduces the adsorption capacity of the zeolite agglomerate. Also, during the preparative procedures, the binder can block the pores of the zeolite thus slowing the rate of mass transfer to and from the zeolite pores. Thus, the preparation of molecular sieve macrostructures (e.g., macroscopic structures with dimensions greater than 0.1 mm) is of great technological interest. Several approaches to prepare zeolite macrostructures have been reported in the scientific literature. Generally, molecular sieve macrostructures can be prepared by transforming shaped monoliths consisting of zeolite precursors into a zeolite using molecular structure-directing agents, e.g. ref. [37], or using other, non-zeolitic monoliths, that are removed after the synthesis. In the latter case, the monoliths are often composed of organic materials, e.g. 3D arrays of polystyrene spheres [38] or polyurethane foams [39], and the macrostructures obtained after the template removal are characterized by a controlled macroporosity. We developed a method for the preparation of self-bonded zeolite macrostructures, the resin templating method [40-42]. Conceptually, the method is based on the use of macroporous anion exchange resins as shape-directing macrotemplates. Anion exchange resins are supplied as spherical beads and are available in various particle sizes. The introduction of zeolite nutrients (e.g., negatively charged silica and alumina species that are present in the zeolite synthesis solutions) into these resins is facilitated by the ability of the macrotemplate to exchange anions. The permanent pore structure of the macroporous resins allows the zeolite crystallization to take place within the beads. In addition, being an organic material, the resin may easily be removed after the synthesis by calcination. Figure 2 represents a schematic illustration of the different stages of production of molecular sieve macrostructures by the resin templating method. Firstly, a mixture of resin beads and zeolite synthesis solution is hydrothermally treated. As a result of the treatment, zeolite is crystallized both in the bulk solution and within the resin. The resin-zeolite composite formed is separated from the zeolite crystallized in the bulk and as a final step the resin is removed by calcination leaving self-bonded zeolite spheres with a shape and size similar to the original resin beads.

9 9

Ion exchange resin

Hydrothermal synthesis

Calcination

~

Ion exchange resin microporous material

~Se!f-bonded body

Figure 2. Schematic illustration of the procedure for preparing self-bonded molecular macrostructures by the resin templating method.

1443 The resin templating method was firstly developed for the preparation of silicalite-1 macrostructures due to the relatively simple reaction system compared to the aluminum containing zeolites [40]. The method was then applied to synthesize zeolite macrostructures such as zeolite beta and ZSM-5 [41,42]. Figure 3a shows typical SEM images of molecular sieve-resin composite particles. These composite beads were similar in shape and size to the original resin beads. Generally, no shrinkage and change in appearance were observed upon removal of the ion exchanger (Fig. 3b). Depending on the synthesis conditions (e.g., synthesis time, synthesis solution, temperature, synthesis solution to resin weight ratio used) hollow spheres or spheres with a reduced size may be obtained as well [6]. The particles building up the sphere interiors always had a size of about 100 nm, which is comparable to the pore size of macroporous resins. Larger zeolite crystals were observed on the sphere surfaces. This is related to the fact that while the surface of the resin beads is open and exposed to the synthesis solution, the resin polymer chain restricts crystal growth within the resin to the size of the resin pores. Besides the self-bonded form achieved by the resin templating method, another advantage is the controlled dual pore structure of the spheres prepared by the method. The calcined macrostructures contain micropores emanating from the molecular sieve (or the amorphous silica that is present in the amorphous or semi-crystalline spheres) and mesopores resulting from the removal of the ion exchanger. The microporosity is dependent on the zeolite type and the ratio between crystalline and amorphous material present. The mesoporosity is predetermined from the resin polymer chains but is also dependent to a certain degree on the material formed within the resin beads. These pore structure features are exemplified in Fig. 4a showing three nitrogen adsorption isotherms, for silicalite-1, ZSM-5 and zeolite beta. The isotherms are all of type IV characteristic of mesoporous materials with a relatively steep increase at low pressures indicating a substantial microporosity. From the corresponding pore size distributions presented in Fig. 4b is seen that the ones for the MFI type materials are very similar (the two samples have similar degree of crystallinity), whereas the one for the zeolite beta is slightly different. Depending on the synthesis conditions and the amount of a high surface area amorphous material present, spheres with specific BET surface areas of up to 1000 mZg-1 were obtained.

O0 l.tm~l

Figure 3. SEM images of silicalite-l-resin composite beads (a) and the silicalite-1 spheres obtained after the removal of the resin (b).

1444 500

0.8

a

b A:

450 0.6

m~ 400 350 @ r,g3

0.4

300

~,!~7 ! iiii .:

250 @ 200 >.

150 -

~

_A_ZX_A_A-ZX-/X-ZXZXaxa-~"

.

0.0 I

0.0

i ,,

0.2

0.2

,

I

0.4

,

I

06.

,

Relative pressure

I

0.8

(p/po)

, , ,

'

,

1.0

10

, ,

,

i ,

, i

,,,l ,,1,

i

i

,

, ,

, , , , , , ,,,,

i i I

100

.

.

,

.

.

,

.

,

.

,,,

,

1000

Pore diameter (D)/A

Figure 4. Nitrogen adsorption isotherms of silicalite-1 (D), ZSM-5 (A) and zeolite beta (o) spheres (a); solid symbols, adsorption; open symbols; desorption; and the corresponding BJH pore size distributions (b). Employing gel type resins in the resin templating method results in the formation of a high surface area amorphous silica macrostructures. This is due to steric hindrance effects: the gel resins are characterized by an "apparent" porosity of no greater than 4 nm, which is not enough to allow zeolite crystal formation within those resins. Materials with very high surface areas, up to 1600 mZg-l,were prepared by the resin templating method using gel resins [43]. In a recent development of the method, a number of modified molecular sieve macrostructures were prepared. Firstly, molecular sieve-resin composite beads are prepared as previously described. In a second step, transition metal anions are introduced into the composite beads by ion exchange employing the residual ion exchange of the resin. Finally, the organic compounds are removed by combustion. The method allows the introduction of basically any transition metal providing that it is available in an anionic form. For instance, vanadium and tungsten containing silicalite-1 macrostructures of various crystallinity and transition metal content have been prepared and extensively characterized [44]. A major advantage of the method is that the transition metals are distributed throughout the molecular sieve spheres, a result of the presence of the three-dimensional resin chains responsible for the ion exchange. By adjusting the conditions for ion exchange (e.g., the pH of the transition metal solution, the solution concentration and solution to composite weight ratio as well as the properties of the molecular sieve material formed within the resin), materials of desirable metal loading and high surface areas may be obtained. In combination with the dual pore structure and controlled macro shape, this makes the modified molecular sieve macrostructures interesting materials for catalytic applications.

1445 5. A P P L I C A T I O N S The main application areas for structured molecular sieve materials are membranes, catalysts and sensors. In cooperation with other research groups, a few of our films have been tested in sensor applications [45,46]. Recently, a greater fraction of our work, briefly described below, has been devoted to zeolite membranes and catalysis applications. ZSM-5 films with high aluminum content have been explored in membrane applications. The zeolite was crystallized in the absence of template molecules and calcination was thus not necessary. Crack formation during calcination could thus be avoided [47]. However, we found that cracks still developed in the membranes at the elevated temperatures needed to dry the membrane [48]. By applying a support masking technique, and grow thin (500 nm) silicalite-1 films, very good membranes with extremely high flux, higher than previously reported, could be prepared with high reproducibility [49,50], see Table 3. Molecular sieves and zeolites are currently used in numerous catalytic applications taking advantage of the unique properties of these materials. Considering this, and the general interest in structured materials, it is not surprising that a significant effort currently is being devoted to the development of structured molecular sieve catalysts. A number of research groups in academia as well as industry around the world are presently active in the development of structured molecular sieve materials primarily for use as catalytic or gas separation membranes and other types of structured catalysts such as catalytic distillation packing materials but also for a number of other technologically sophisticated applications such as films on conventional packing materials and zoned zeolite crystals. There is a number of possible ways to utilize molecular sieves in novel applications such as structured catalysts as illustrated schematically in Figure 5. In cooperation with the Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, South Africa, we are presently studying ZSM-5 films in catalysis applications. Catalysts configured as in a) and c) in Figure 5 are explored. Test reactions are p-xylene isomerisation and triisopropylbenzene cracking. The first results show that defects such as cracks or grain boundaries in the zeolite film may play a major role in these materials. Defects may have a positive effect in a) since they increase the effective diffusivity in the film and thus increases the activity of the catalyst. Defects are not acceptable in c) if they provide additional pathways in the material and thus reduces or eliminates the effect of the outer coating. However, it seems as if the film is thin enough, defects are not formed.

Table 3. Permeation data from mixtures of butanes, hexanes or xylenes for silicalite-1 membranes. The permeance of the favored component is given in 10 -l~ m o l / ( m Z s 9Pa) for each system. System Permeance a Temperature/~ n-butane/iso-butane

9800

9.0

25

n-hexane/2,2-dimethyl-butane

5600

227

390

para-xylene/ortho-xylene

3000

16

390

1446 a

b

c Carrier Zeolite Catalyst

Figure 5. A carrier material (inert or catalyst) can be coated with an inert or catalytically active zeolite film (a), carrier material with catalytic coating (e.g. metal) can be coated with a inert or catalytically active zeolite film (b) or carrier material can be coated with a zoned zeolite film with varying composition i.e. framework aluminum content (c).

6. CONCLUSIONS Structured molecular sieve materials are of great interest in a number of application areas. The potential for eliminating limitations of materials presently used by tailored design of molecular sieve materials for specific applications is considerable. Often, conceptual possibilities are identified but the preparative methods needed for practical realization are lacking. Such methods are however currently being developed concurrently with an increasing understanding of the fundamental mechanisms directing the outcome of preparations of molecular sieve based materials. Thus, methods for the controlled preparation of molecular sieve crystals, films, macrostructures and composite materials with desired properties have been developed during the last decade. At Ltu we have focused our activities at this area of development and made some contributions to the recent progress. Efforts in academia as well as in industry within this area are increasing This is a fact that warrants a further development towards new and exiting materials in years to come.

REFERENCES 1. A. Cybulski and J.A. Moulijn (eds.), Structured catalysts and reactors, Marcel Dekker Inc., New York, 1998. 2. S. Qiu, J. Yu, G. Zhu, O. Teresaki, Y. Nozue, W. Pang and R. Xu, Microporous Mesoporous Mater., 21 (1998) 245. 3. L. Scandella, G. Binder, T. Mezzacasa, J. Gobrecht, R. Berger, H. P. Lang, C. Gerber, J. K. Gimzewski, J. H. Koegler and J. C. Jansen, Microporous Mesoporous Mater., 21 (1998) 403. 4. H. Manstein, K. P. M/511er, W. B6hringer and C. T. O'Connor, Microporous Mesoporous Mater., 51 (2002) 35. 5. J. Caro, M. Noack and P. K61sch, Microporous Mesoporous Mater., 22 (1998) 321. 6. B.J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites, 14 (1994) 208. 7. B.J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites, 14, (! 994) 110. 8. A.E. Persson, B. J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites 14 (1994) 557.

1447 9. M.H. Anthonis, A. J. Bons and J. P. Verduijn, PCT WO 97/25272 (1997). 10. Q. Li, D. Creaser and J. Sterte, Microporous Mesoporous Mater., 31 (1999) 141. 11. G. Zhang, J. Sterte, B. Schoeman, J. Chem. Soc. Chem. Commun, (1995) 2259. 12. G. Zhang, J. Sterte and B. Schoeman, Chem. Mater., 9 (1997) 210. 13. A. E. Persson, B. J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites 15 (1995) 611. 14. B. J. Schoeman, J. Sterte and J.-E. Otterstedt, J. Colloid Interface Sci. 170 (1995) 449. 15. Q. Li, D. Creaser and J. Sterte, accepted for publication in Chem. Mater. 16. J. P. Verduijn, M. M. Mertens and M. H. Anthonis, PCT WO 97/03021 (1997) 17. J. P. Verduijn, PCT WO 97/03019 18. M. A. Camblor, A. Corma, A. Mifsud, J. P6rez-Pariente and S. Valencia, in: Chon et al. (Eds.) Progress in Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, vol 105A, p.341. Elsevier Science, Amsterdam, 1997. 19. B. J. Schoeman, E. Babouchkina, S. Mintova, V. Valtchev and J. Sterte, J. Porous Materials, 8 (2001) 13. 20. W. Xu, J. Dong, J. Li, J. Li, F. Wu, J. Chem. Soc. Chem. Commun. (1990) 755. 21. M. Matsukata and E. Kikuchi, Bull. Chem. Soc. Jpn. 70 (1997) 2341. 22. J. Coronas and J. Santamaria, Catalysis Today 51 (1999) 377. 23. J. Sterte, J. Hedlund and B. J. Schoeman, US Patent No. 6 177 373 (2001). 24. J. Hedlund, B. J. Schoeman, J. Sterte, In: H. Chon, S.-K. Ihm., Y. S. Uh (eds.) Progress in Zeolites and Microporous Materials, Studies in Surface Science and Catalysis, vol 105, p. 2203, Elsevier Science, Amsterdam, 1997. 25. V. Valtchev, B. J. Schoeman, J. Hedlund, S. Mintova and J. Sterte, Zeolites 17 (1996) 408. 26. V. Engstr6m, B. Mihailova, J. Hedlund, A. Holmgren and J. Sterte, Microporous Mesoporous Mater. 38 (2000) 51. 27. Z. Wang, J. Hedlund and J. Sterte, Microporous Mesoporous Mater., in press. 28. S. Mintova, J. Hedlund, B. J. Schoeman, V. Valtchev and J. Sterte, Chem. Commun. 15 (1997). 29. S. Mintova, J. Hedlund, V. Valtchev, B. Schoeman and J. Sterte, J. Mater. Chem. 7 (1997) 2341. 30. Q. Li, J. Hedlund, D. Creaser and J. Sterte, Chem. Commun. 7 (2001) 527. 31. J. Hedlund, B. Schoeman and J. Sterte, Chem. Commun. (1997) 1193. 32. M. Lassinantti, J. Hedlund and J. Sterte, Microporous Mesoporous Mater. 38 (2000) 25. 33. Z. Wang, J. Hedlund and J. Sterte: Synthesis of FAU type films on steel supports using a seeding method, in: A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (Eds.), Studies in Surface Science and Catalysis, Zeolites and mesoporous materials at the dawn of the 21st century, Elsevier Science, Amsterdam, in press. 34. J. Hedlund, S. Mintova and J. Sterte, Microporous Mesoporous Mater. 28 (1999) 185. 35. J. Hedlund, J. Porous Mater. 7 (2000) 455. 36. S. Mintova, J. Hedlund, V. Valtchev, B. Schoeman and J. Sterte, J. Mater. Chem. 10 (1998) 2217. 37. I. Kiricsi, S. Shimizu, Y. Kiyozumi, M. Toba, S. Niwa and F. Mizukami, Microporous Mesoporous Mater., 21 (1998) 453. 38. B. T. Holland, L. Abrams and A. Stein, J. Am. Chem. Soc., 121 (1999) 4308. 39. Y.-J. Lee, J. S. Lee, Y. S. Park and K. B. Yoon, Adv. Mater., 13 (2001) 1259. 40. L. Tosheva, V. Valtchev and J. Sterte, Microporous Mesoporous Mater., 35-36 (2000) 621.

1448 41. L. Tosheva, B. Mihailova, V. Valtchev and J. Sterte, Microporous Mesoporous Mater., 48 (2001) 31. 42. L. Tosheva and J. Sterte, proceedings of the 2n~ FEZA conference, Giardini Naxos, Italy, 1-5 September 2002. 43. L. Tosheva, V. Valtchev and J. Sterte, J. Mater. Chem, 10 (2000) 2330. 44. L. Tosheva and J. Sterte, Chem. Commun. 1112 (2001) 45. S. Mintova, B. Schoeman, V. Valtchev, J. Sterte, S. Mo and T. Bein, Adv. Mater. 9 (1997) 585. 46. R. Bjorklund, J. Hedlund, J. Sterte and H. Arwin, J. Phys. Chem. B 102 (1998) 2245. 47. J. Hedlund, M. Noack, P. K61sch, D. Creaser, J. Sterte and J. Caro, J. Memb. Sci. 159 (1999) 263. 48. M. Lassinantti, F. Jareman, J. Hedlund, D. Creaser and J. Sterte, Catalysis Today, 67 (2001) 109. 49. M. H. Anthonis, A. J. Bons, H. W. Deckman, J. Hedlund, W. F. Lai and J. A. J. Peters, International patent application WO 00/53297 (2000). 50. M. H. Anthonis, A. J. Bons, H. W. Deckman, J. Hedlund, W. F. Lai., J. A. J. Peters, International patent application WO 00/53298 (2000).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1449

C h r o m i u m c o n t a i n i n g zeolite beta m a c r o s t r u c t u r e s Valeri Naydenov, Lubomira Tosheva and Johan Sterte

Division of Chemical Technology, Lulegt University of Technology, S-97187 Lule&, Sweden luto @km. luth.se Chromium containing zeolite beta macrostructures were prepared by a multi-step procedure employing macroporous anion exchange resin beads as shape-directing macrotemplates. In a first step, zeolite beta was crystallized in the pores of the ion exchanger. Secondly, chromium oxoanions were introduced into the resin-zeolite beta composites thus obtained using the residual ion exchange capacity of the resin. Finally, the resin was removed by calcination leaving behind chromium containing zeolite beta spheres. Spheres of various chromium content were synthesized and characterized by AAS, XRD, EDS, UV-vis DRS spectroscopy and nitrogen adsorption measurements. The properties (chromium loading, crystallinity, pore structure) of the Cr-beta spheres prepared were dependent on the starting resin-zeolite composites as well as on the chromium solution to composite weight ratio used for the Cr ion exchange. 1. INTRODUCTION Chromium based catalysts are generally composed of finely divided chromia supported on a carrier such as alumina and silica. The nature of the chromium species in these catalysts is dependent on the support, pretreatment conditions and chromium loading. Chromia supported catalysts are active in ethene and propene polymerization, dehydrogenation reactions such as n-butane to butene, propane to propene, i-butane to i-butene as well as for selective catalytic reduction of NOx [1-13]. The catalysts used in paraffin dehydrogenation and ethylene polymerization are commonly prepared by the incipient impregnation method [1-7]. Molecular sieves are often used as catalyst supports due to their well-defined pore architectures, high surface areas and high thermal stability. Molecular sieve and mesoporous materials, in which chromium was directly incorporated into the support during the synthesis, have been prepared as oxidation reaction catalysts [8-13]. A major disadvantage of using molecular sieves as supports is that they are generally produced as powders, which are difficult to handle and further processing is necessary to produce macroparticles such as pellets and spheres, generally by mixing the zeolites with amorphous binders. The binding additives, however, dilute the adsorption properties of the zeolite and cause diffusion limitations. Recently, a method for producing molecular sieves in the form of spheres using an ion exchange resin as a shape-directing macrotemplate was reported [14,15]. In this method, zeolites are crystallized in the pores of ion exchange resin beads and upon removal of the resin, zeolite spheres with a shape and size similar to the original resin beads are obtained. In this contribution we report on the preparation of chromium containing zeolite beta spheres by

1450 the resin templating method. Chromium anionic species are introduced into the resin-zeolite beta composite spheres using the residual ion exchange capacity of the resin.

2. EXPERIMENTAL SECTION A batch of resin-zeolite beta composite spheres was prepared according to the previously described procedure [14] using a macroporous strongly basic resin (Dowex MSA-1, chloride form, bead size distribution 0.3-1.2 mm). A 0.010M Na2Cr2OTo2H20 (Merck, 99%) solution was employed to introduce anionic chromium species into the composite particles. Two series of experiments were performed utilizing as-synthesized composites (assuming that the resin in these composites is in the OH- form) and composites, in which the resin was converted into a C1- form by passing a 10 wt.% NaC1 (Riedel-de Hahn, >99.8%) solution through an ionexchange column loaded with the composites [16]. The dichromate solution was added to resin-zeolite beta composites dried at room temperature in a beaker at three different solution to composite weight ratios, namely 10, 30 and 50, and the corresponding samples were designated as BetaCrx(OH or C1), where x is equal to 10, 30 or 50. The beakers were placed on a shaker for 48 h. After the ion exchange, the chromium containing composites were separated, rinsed with distilled water, dried at room temperature and calcined at 600~ for 6 h, after heating to this temperature at a rate of 1~ min -~. The chromium content in the zeolite beta spheres was determined by flame atomic absorption spectrometry (AAS, Perkin-Elmer 3100) after fusing the samples with LiBO2 according to the procedure described in ref. [17]. X-ray diffraction (XRD) patterns were collected with a Siemens D5000 powder diffractometer using Cu K~ radiation. Relative crystallinity was evaluated from the area of the most intense zeolite beta reflection peak, which appears at 20 = 22.5 ~ The most crystalline sample (pure zeolite beta spheres obtained from resin-zeolite beta composites in an OH- form) was used as a standard sample for these calculations. UV-vis DRS diffuse reflectance spectra (UV-vis DRS) were obtained with a Perkin-Elmer Lambda 2 UV-vis spectrometer equipped with a Labsphere RSA-PE 20 Reflectance Spectroscopy Accessory and operating in a single beam mode. A white SRS-99 standard reference material was used for a background correction. Calcined spheres were ground into a powder prior to the XRD and UV-vis DRS spectroscopy study. A scanning electron microscope (SEM), Philips XL 30 equipped with a LaB6 emission source, was used to study the morphology of the samples. The distributions of chromium in the zeolite beta spheres was studied by line-scan energy dispersive spectroscopy (EDS) using a Link ISIS Ge energy dispersive X-ray detector. Flat and polished cross sections of the spheres were prepared prior to these measurements by embedding the chromium spheres in an epoxy resin (Epofix, Struers). Nitrogen adsorption desorption measurements were performed with a Micromeritics ASAP 2010 instrument after degassing calcined spheres at 300~ overnight. Specific surface areas were calculated with the BET equation and total pore volumes were obtained by converting the amount adsorbed at relative pressure of 0.995 to the volume of liquid adsorbate. Pore-size distributions were determined by the BJH method (desorption isotherm).

1451 Table 1. Properties of the calcined pure zeolite beta and chromium containing spheres. Sample

Cr content (wt%)

Degree of crystallinity

BET surface area, SBET

Total pore volume, Vp

Beta (OH) Beta (C1) BCrl0(OH) BCrl0(C1) BCr30(OH) BCr30(C1) BCr50(OH) BCr50(C1)

0.7 1.2 2.4 2.9 3.5 4.3

100 70 81 69 65 67 60 60

624 666 624 651 588 607 582 614

0.64 0.73 0.79 0.79 0.75 0.74 0.79 0.76

(%)

(m2~ -1)

(cm3g -l)

3. R E S U L T S A N D D I S C U S S I O N

Visually, the calcined chromium containing zeolite beta spheres were yellowish or greenish colored depending on the chromium content. SEM analysis showed no changes in the appearance of the spheres due to the introduction of chromium and the particles were similar in size and shape to the original resin beads (not shown, see ref. [ 15]). No changes in the morphology of the particles building up the sphere were observed by SEM. The chromium content in the samples increased with an increase in the chromium solution to the resin-zeolite composite ratio (Table 1). Higher chromium loadings were obtained using

C

rae3 r .4,,,a

20

40

60

20 degrees

80

Figure 1. XRD patterns of calcined BCrl0(OH) (a), BCr30(OH) (b) and BCr50(OH) (c) samples and a reference Cr203 XRD pattern (d).

1452

1.0 ,

,

,

,

, , ,

,

,

,

,

, , ,

: : : : :: :i~+: : ::

"

,;,'+i i: i!i

----o---BCrl0(OH) ----+---Beta(C1)

-,,-BC

lO(C1)

r162

~,0.5

e

-

.:~++_r,t,/ii:',::,~

~..

iz~:~,:,iiix

. ~ '

0.0

,

10

,

::::::

~o. ii

%

: :::::: : :iiiii .

.

.

.

.

.

.

\ct~

,

100 Pore Diameter

1000 (D)/A

Figure 2. BJH desorption pore size distributions of calcined zeolite Beta and Cr-Beta spheres. composites in a C1- form. XRD analysis showed that the crystallinity of the chromium samples decreased compared to pure zeolite beta spheres with the decrease being higher for the samples of higher chromium content (Table 1). An interesting observation is the inferior crystallinity of the Beta(C1) spheres compared to Beta(OH). Considering the fact that the conversion of the resin into a C1- form was performed in the presence of zeolite in the pores of the resin, there might be a certain degree of amorphization of the zeolite during the ion exchange. The higher surface area and pore volume for this sample were in accordance with this suggestion (Table 1). Fig. 1 shows XRD patterns of the three BetaCr(OH) samples. By increasing the chromium content the intensity of the reflection peaks of zeolite beta decreased and peaks corresponding to Cr203 appeared. The Cr203 peaks are somewhat difficult to distinguish because of their closeness to the zeolite beta peaks. Nevertheless, the above statement is best exemplified by the Cr203 peak at 20 = 36.2 ~ which can barely be seen in the XRD pattern of BCrl 0(OH) and which is of comparatively high intensity in the XRD pattern of BCr50(OH). The BET surface areas of the chromium containing zeolite beta spheres were lower compared to pure zeolite beta samples (Table 1). On the other hand, total pore volumes were higher. The nitrogen adsorption isotherms recorded for all spheres were of type IV with a substantial microporosity (not shown, see ref. [ 15]). The mesopores present are related to the removal of the ion exchanger whereas the micropores are due to the presence of zeolite beta and high surface area amorphous material. Differences were observed in the BJH pore size distributions of the zeolite spheres prepared. Firstly, by comparing the pure zeolite beta samples it can be noted that the broad mesopore size distribution centered at 400 * for Beta(OH) becomes narrower and shifts to the left (50-60*) for the Beta(C1) sample. The

1453

0

'

'2"0'0

'

'

400 '

'

length, gm

'

'6"0'0

Figure 3. Typical SEM image of a cross-sectioned Cr-Beta sphere (a) and typical EDS line scan analysis of chromium over it (b). changes in the pore structure of Beta(C1) are related to the conversion of the resin in the resinzeolite composites into a C1- form. However, the resin-zeolite composite is a very complex system and the exact reasons for the changes observed are not clear. Pore-size distributions for the chromium containing spheres were similar with a substantial part of the pore volume found in mesopores in the range 80-100 * (Fig. 2). The distribution curves were broader and shifted to the right compared to the one for Beta(C1) and narrower and left-shifted compared to the Beta(OH) one. Again, the reasons for these changes in the pore structure of the chromium containing samples are unclear. The changes are more likely related to changes in the resin polymer chains during the chromium ion exchange. Thus, the type of chromium species exchanged might differ when starting with resin-zeolite composites in an OH- and in a C1- form. It is well known that in Cr(VI) solutions there is an equilibrium between chromate and dichromate ions which is very sensitive to the pH value [1]. The equilibrium is shifted towards chromate ions with an increase of pH. Such an increase occurs when the ion exchange is performed with composites in the OH- form due to the release of OH- ions into the solution. However, a further discussion in this direction would be highly speculative. Although the material appeared homogeneous upon grinding, EDS line scan analysis was used to further investigate the chromium distribution within the zeolite spheres. Figure 3 shows a SEM micrograph of a cross-sectioned sphere (a) and the EDS line scan analysis of chromium over it (b). Chromium was relatively evenly distributed across the sphere. Similar results were obtained for all the chromium spheres.

1454

f

9

d

,

200

I

300

,

I

400

500

600

700

800

Wavelength/nm Figure 4. UV-vis DRS spectra of calcined Cr-beta samples: BCrl0(OH) (a), BCrl0(C1) (b), BCr30(OH) (c), BCr30(C1) (d), BCr50(OH) (e), BCr50(C1) (f).

Further, UV-vis spectroscopy was used to study the chromium species in the chromium containing zeolite beta spheres. Generally, chromate species have absorption bands at 275 and 375 nm, dichromate- at 275, 322 and 445 nm and pseudo-octahedral Cr 3+ - at 625 nm (the Cr 3+ bands at 295 and 465 nm are overlapping with the stronger chromate and dichromate bands) [1,18]. Figure 4 shows UV-vis DRS spectra of the calcined chromium containing zeolite beta spheres. In the spectrum of BCrl0(OH) (Fig 4a) bands at 265, 302, 355, 445 and a very weak band at 600 nm are present. The bands may be associated with the presence of grafted chromates (the bands at 265 and 355 nm) and dichromates (445 nm) and Cr203 (302, 600 nm). By increasing the chromium content the amount of surface dichromates and Cr203 is increased (the bands at 445 and 600 nm, respectively) whereas the surface chromates are decreased (265, 355 nm). The spectra of the chromium samples prepared with the OHcomposites and the Cl-composites were similar and no conclusion about the influence of the counter ion of the resin-zeolite composite can be drawn. 4. CONCLUSIONS Chromium containing zeolite beta spheres were prepared using macroporous anion exchange resin as a shape directing macrotemplate. Firstly, resin-zeolite beta composites were synthesized by crystallizing the zeolite into the resin pores. Chromium was then introduced into the composites as oxoanions using the residual ion exchange capacity of the resin. Finally, the resin was removed by calcination leaving behind chromium containing zeolite beta spheres. The chromium content in the spheres was dependent on the chromium solution to resin-zeolite composite weight ratio as well as on the resin form in the composites OH- or

1455 C1-. The crystallinity of chromium containing samples was inferior compared to pure zeolite beta spheres. The pore structure of the materials prepared consisted of both micropores (the zeolite) and mesopores (from the removal of the resin) with features dependent on the pretreatment conditions. A major advantage of the method reported is that the materials prepared may be directly used as catalysts in e.g. fixed-bed reactors and no further processing to form macroparticles is necessary thus avoiding deterioration effects due to the addition of binders in conventionally prepared zeolite supported catalysts. This makes the chromium containing zeolite beta spheres prepared according to the procedure potentially beneficial oxidation catalysts. ACKNOWLEGMENTS The partial financial support from the Swedish Research Council for Engineering Sciences (VR) is gratefully acknowledged. REFERENCES 1. 2. 3. 4.

B.M. Weckhuysen, I. E. Wachs, R. A. Schoonheydt, Chem. Rev. 96 (1996) 3327. B.M. Weckhuysen, R. A. Schoonheydt, Catal. Today 51 (1999) 223. A. Hakuli, M. E. Harlin, L. B. Backman, A. O. I. Krause, J. Catal. 184 (1999) 349. F. Cavani, M. Koutyrev, F. Trifiro, A. Bartolini, G. Ghisletti, R. Iezzi, A. Santucci, G. Del Piero, J. Catal. 158 (1996) 236. 5. B.M. Weckhuysen, D. Wang, M. P. Rosynek, J. H. Lunsford, J. Catal. 175 (1998) 338. 6. B.M. Weckhuysen, D. Wang, M. P. Rosynek, J. H. Lunsford, J. Catal. 175 (1998) 347. 7. A.V. Salker, W. Weisweiler, Appl. Catal. A: General 203 (2000) 221. 8. Z. Zhu, Z. Chang, L. Kevan, J. Phys. Chem. B 103 (1999) 2680. 9. D. Escalante, L. Giraldo, M. Pinto, C. Pfaff, V. Sazo, M. Matjushin, B. Mendez, C. M. Lopez, F. J. Machado, J. Goldwasser, M. M. Ramirez de Agudelo, J. Catal. 169 (1997) 176. 10. S. Yuvaraj, M. Palanichamy, V. Krishnasamy, Chem. Commun. (1996) 2707. 11. N. van der Puil, Widyawati, J. C. Jansen, H. van Bekkum, in: J. Weitkamp, H. Karge, H. Pfeifer, W. H/51derich (eds.), Zeolites and Related Mesoporous Materials: State of the Art, Amsterdam, Elsevier, 1994, p. 211. 12. T. Chapus, A. Tuel, Y. Ben Taarit, C. Naccache, Zeolites 14 (1994) 349. 13. M. Selvam, M. P. Vinod, Appl. Catal. A: General 134 (1996) L 197. 14. L. Tosheva, V. Valtchev, J. Sterte, Micropor. Mesopor. Mater. 35-36 (2000) 621. 15. L. Tosheva, B. Mihailova, V. Valtchev, J. Sterte, Micropor. Mesopor. Mater. 48 (2001) 31. 16. C. E. Harland, Ion Exchange: Theory and Practice, 2 ed., The Royal Society of Chemistry, Cambridge, 1994, pp. 63-75. 17. C. O. Ingamells, Talanta, 11 (1964) 665. 18. B. M. Weckhuysen, A. A. Verberckmoes, A. R. De Baets, R. A. Schoonheydt, J. Catal. 166 (1997) 160.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1457

Semiconductor nanoparticles in the channels of mesoporous silica and titania thin films Michael Wark l, Hartwig Wellmann 1, Jiri Rathousk~, 2 and Arno~t Zukal 2 1Institute of Applied and Physical Chemistry, University of Bremen, FB 2 - Chemistry, D-28334 Bremen, Germany, e-mail: [email protected]. 2 j. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-18223 Prague, Czech Republic, e-mail: [email protected]. A procedure for the preparation of porous silica thin films was developed, which is based on the use of block copolymers as structure directing agents and the application of the dipcoating technique. The prepared films exhibit an uniform thickness and a large roughness factor indicating the highly developed porosity. The addition of C d 2+ ions to the reaction gel for silica films prior to their formation, followed by the calcination of films at 623 K and treatment with HzS or HzSe is the preferred procedure for the creation of CdS or CdSe nanoparticles. The occurrence of the size-quantization effect confirms that the particles are located within the pores and that their size corresponds to the pore width of the host. The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. Well-organized film is formed only from a completely hydrolyzed precursor. The photocatalytic activity of such organized film is comparable with that of the most active commercial anatase powders. 1. INTRODUCTION The most important feature of ordered mesoporous materials is their ability to form thin films [1]. The motivation for synthesis of mesoporous thin films originates from the appreciation of their technological potential as membranes, sensors, surfaces for heterogeneous catalysis and insulating layers. Pore accessibility, which is of fundamental importance in almost all applications, is highly enhanced in structures which possess threedimensional symmetries, e.g. bicontinuous cubic. Such structures can be obtained using block copolymers as structure-directing agents [2]. The additional aspect of mesoporous films is the possibility to use them as a hosting matrix for nanoparticles, which opens a new field of application, relying on quantum size effects of the confined particles. First syntheses of nanoparticle loaded silica mesoporous films have been reported recently dealing with SiGe [3], Si [4] and Ag [5] nanoparticles. Although most of the published experimental research has focused on silica as the inorganic framework constituent, there is a major stimulus behind the research into the nonsilica mesoporous films, especially titanium dioxide, namely its potential for converting light to electrical energy or chemical energy by its solar-driven band gap excitation. The application of the reaction schemes originally developed for siliceous materials has been found much less successful for the synthesis of mesoporous transition metal oxides due to their facile crystallization and subsequent grain growth, which leads to the loss of the original

1458 mesoporous structure. However, recently a novel approach has been developed, enabling to obtain mesoporous titanium dioxide with highly interesting structural properties based on the usage of amphiphilic poly(alkylene oxide) block copolymers as structure directing agents in non-aqueous solution for organizing the network-forming titanium dioxide species [6]. Only very recently first successful preparations of mesoporous titania films have been reported [7,8]. In the present communication, the loading of silica mesoporous thin films with nanoparticles of CdS and CdSe is reported for the first time. Further the above mentioned synthetic approach for the preparation of titania films will be analyzed with respect to the effects of decisive processing parameters and it will be shown that thus prepared materials can be effectively used in the photocatalytic destruction of an important organic water pollutant, viz. 4-chlorophenol.

2. MATERIALS AND METHODS 2.1.

Materials

2.1.1. Preparation of silica mesoporous thin films loaded with CdS and CdSe A typical synthesis using the surfactant Brij 56 (Aldrich, C16H33(C2H40)10H) was performed as follows. 8.35 g of tetraethylorthosilicate (TEOS) were dissolved in 16 g ethanol, followed by 4 g of 0.1 M HC1 and the sol was heated under reflux for 1 hour. After the solution had cooled to ambient temperature, 3 g of Brij 56 dissolved in 16 g ethanol were added under vigorous stirring and the mixture was allowed to age at ambient temperature for 2 hours. Subsequently, the viscous mixture was used for dip-coating quartz glass or silicon wafers 3 cm • 3 cm in size, which had been previously cleaned with acetone, at a constant velocity of 1 m m s -1. After drying at ambient temperature for 1 hour, the films were calcined in air at 350~ for 2 hours (heating rate: 1~ rain-l). With surfactants PE 9400 (BASF, (C2H40)21(CH(CH3)CH20)47(C2H40)21H) and cetylpyridinium chloride (Aldrich) the compositions of the reaction mixtures were different. With PE 9400, 8.35 g of TEOS dissolved in 16 g ethanol was mixed with 7.2 g of 0.1 M HC1. After heating under reflux for 1 hour and cooling to ambient temperature, 4 g of PE 9400 in 24 g ethanol were added. For the preparation of films with cetylpyridinium chloride, 8.7 g of TEOS, 2.22 g of 0.1 M HC1 and 3 g of cetylpyridinium chloride were dissolved in 36 g ethanol and heated under reflux for 20 h. After cooling to ambient temperature, thus prepared reaction mixtures were used for dip-coating the wafers. In some cases, 0.56 g of cadmium acetate were added to the surfactant solutions in order to incorporate Cd 2+ ions. Alternatively, Cd 2+ ions were introduced by impregnating the silica films with a methanolic solution of cadmium acetate, whose concentration was adjusted according to the desired loading, which was between 1 and 10 wt.% Cd. Cd 2+ ions were transformed into CdS or CdSe nanoparticles by a treatment with H2S or H2Se, respectively. Prior to and after the treatment all the physisorbed species were removed by evacuation. 2.1.2. Preparation of mesoporous titanium dioxide using block copolymers First, 0.9 g of Pluronic P-123 (BASF) were dissolved in 11 mL of ethanol. To this solution, 1 mL of titanium tetrachloride was added under vigorous stirring. The mixture was maintained in an open beaker at 40~ for 5 days, the evaporated ethanol being filled up every 12 h. Thus prepared clear yellowish solution could be stored at room temperature for several

1459 weeks without apparent changes. Films of different thickness were prepared by spreading various amounts of the stock solution on the glass support. The liquid layer was subsequently gelled in air at 40~ for 7 days and calcined at 400~ for 5 h in air. Finally, the titania films were peeled off the support (samples I-A, I-B, I-C, I-D). 2.2.

Measurements The thickness of the films was determined by a Veeco DEKTAK 3030ST profilometer and a NT-MDT Smena B atomic force microscope. Scanning electron micrographs were obtained by a Hitachi S-900 apparatus. Powder X-ray diffraction data were collected with a Siemens D 5005 diffractometer in the Bragg-Brentano geometry using CuKc~ radiation. Adsorption isotherms of nitrogen and krypton were measured at -196~ with an ASAP 2010 instrument (Micromeritics). FT-IR, UV/Vis and X-ray photoelectron spectra were measured by Biorad FTS-60A, Varian Cary and Physical Electronics PHI 5600 spectrometers. Raman spectra were measured using a T64000 spectrometer (Instruments, SA, France) equipped with an Olympus BH2 microscope. Photocatalytic activity of the TiO2 samples was studied using 4-chlorophenol as model pollutant. Photodegradation of this compound was examined employing a tube photoreactor where TiO2 was dispersed in water. After illumination, 4-chlorophenol follows three separate reaction pathways: hydroxylation, substitution and direct charge-transfer oxidation forming 4-chlorocatechol, hydroquinone and non-aromatic compounds as primary intermediates, respectively. The reaction rate was calculated according to the first-order kinetics.

3. RESULTS AND DISCUSSION 3.1.

Mesoporous films with embedded CdS and CdSe Using all the three mentioned templates, i.e. block copolymers Brij 56 and PE 9400 as well as the ionic cetylpyridinium chloride, thin films on quartz glass or Si substrates were obtained by dip-coating. To ensure the homogeneity of the films, the presence of some amount of solvent (ethanol) was important, the thickness of the films being varied between 50 and 800 nm by evaporating different amounts of the solvent in a rotary evaporator. The thickness of the films was determined by profilometrv, which enabled to measure areas of about 1 cm 2. Consequently, the step in the height l~etween film and wafer was determined with a higher accuracy than by atomic force microscopy, allowing to scan only an area of about 10 lam2. The morphology of the films was determined with a high precision by AFM. Fig. 1 shows a typical AFM micrograph of a 300 nm thick film, which was synthesised with the template Brij 56 and was supported on a Si wafer. The roughness over the measured area of 2 ~tm2 was very small, achieving only + 7 nm. Scanning over more extensive parts (about 300 ~tm x 300 ~tm in size) ensured that the roughness obtained was typical of the whole film. Therefore, the formation of islands of particles was ruled out. The observed unevenness, however, also demonstrated that the films were not formed as a single entity but consisted of a large number of individual closely packed structures, whose diameter was between 100 and 200 nm. The characterization of the regularity of the pore structure of the films by X-ray diffraction was difficult due to the very small amount of the scattering material. Nevertheless, reflections with relatively sharp maxima at 20 = 1.4 and 1.6 ~ were found.

1460

nm 14

lO 8

0,01

6

1,0x,

,.-..-,f 1,8

0,13"".

~lWj~3f

0,6

pm

),8 ,

1,2

pm

1,4

1,6

4 2 o

0.4

o,o~;- ~

Figure 1. Atomic force micrograph of a 300 nm thick mesoporous SiO2 film prepared using Brij56 as a structure-directing agent.

Basic data about the porosity of films were obtained by the analysis of Kr adsorption isotherms. From the adsorbed amount of Kr and the geometrical area of the film of about 0.5 cm 2 a roughness factor of about 40 was calculated. Since AFM also for this films showed only a variation of + 10 nm in the surface topology, the high roughness factor demonstrates the presence of a porosity within the film. The N2 adsorption isotherm on the corresponding powdered sample exhibited a strong increase up to p/p0 ~ 0.25, which indicates the presence of a distinct amount of small mesopores with a diameter around 2 nm. The analysis of the isotherm gives a BET surface area of 915 cm2/g and a mesopore volume of 0.432 cm3/g. The measured values in combination with the results of X-ray diffraction clearly demonstrate the formation of silica materials with structured mesopores both in powders and films. Since no Coulomb interactions of cations with the matrix are possible in silica films, postsynthetic loading with Cd-species can only be achieved by impregnating with a suitable cadmium salt. These procedure, however, led to an inhomogeneous distribution of Cd 2§ ions as well as CdS or CdSe nanoparticles within the film. Fig. 2b shows a film obtained by dropping a solution of cadmium acetate in methanol on the film, drying and precipitation of CdSe by H2Se. The dark spots show CdSe particles, which are mainly concentrated at the edge of the film. If the film was impregnated by dip-coating, a more homogeneous distribution of the Cd-salt was achieved (Fig. 2c), but the concentration of the salt could hardly be controlled. The addition of Cd 2§ ions to the reaction gel prior to the formation of films enabled to obtain films, which very homogeneously tumed their color to orange after treatment with H2Se (Fig. 2d). Except for the intense color there are no deviations from the colorless films which were prepared without addition of cadmium acetate to the synthesis gel (Fig. 2a). With templates PE 9400 and cetylpyridinium chloride, the addition of cadmium salt led to the fast formation of a solid gel, from which film preparation by dip-coating was no longer possible.

1461

Figure 2. Photographs of parent and CdSe-loaded mesoporous SiO2 films; unloaded transparent 300 nm thick parent film (a), film with CdSe particles, introduction of Cd 2+ ions by impregnation with a droplet of a cadmium acetate solution (b), film with CdSe particles, introduction of the Cd 2+ ions by dip-coating from a cadmium acetate solution (c) and film with homogeneously distributed CdSe nanoparticles, introduction of the Cd 2§ ions directly to the synthesis gel prior to dip-coating (d). The latter exhibits a homogeneous bright orange color.

The composition of films treated at different temperatures was checked by XP spectroscopy. Calcination at 500~ led to a complete evaporation of highly dispersed Cd species. Thus, a diminution of the calcination temperature was necessary. Temperaturedependent in-situ IR studies in an O2/Ar gas flow showed that surfactant molecules were quantitatively removed from the films quantitatively at 350~ within 2 hours [9]. After calcination at 350~ a Cd content of 2.1 atom-% was detected for a Cd-containing film by XPS, a value which is only 0.3 atom-% less than that of a film dried at 80~ After treatment with H2Se, the Se content of the film calcined at 350~ was higher than that of the non-calcined film, since free pores allowed the penetration of H2Se. However, only about 50% of the Cd 2§ ions reacted to CdSe, probably due to the fact that a considerable portion of Cd 2+ ions was incorporated within the silica walls during the gelation process and was not accessible for the H2Se. In all the samples the binding energy of the Se3ds/2 electrons was between 54.7 and 55.0 eV, which is in good agreement with literature data for CdSe [10]. In the film calcined at 500~ containing almost no Cd, an orange-red color appeared after treatment with H2Se and a considerable amount of Se was found. However, the binding energy of the Se3ds/2 electrons was increased to 55.4 eV, which is typical for elemental selenium. This observation indicates the silica film tends to catalyze the decomposition of H2Se. All the CdS and CdSe loaded transparent mesoporous SiO2 films were homogeneously colored, pale yellow (CdS) and orange (CdSe), respectively, indicating the occurrence of sizequantization effects with respect to the bulk materials, which were yellow-orange (bulk-CdS, absorption edge around ) ~ - 520 nm) and grey-red (bulk-CdSe, absorption edge at )~ = 712 nm). In the prepared films the observed absorptions increased considerably at around )~- 500 nm (CdS) and ~ = 590 nm (CdSe), and the first excitonic shoulders could be deduced from the second derivations of the spectra at )~ = 483 nm (CdS) and 2~ = 557 nm (CdSe).

1462 According to the tight-binding model, introduced by Lippens and Lannoo for the estimation of diameters of semiconductor nanoparticles from the altered opto-electronic features [11 ], mean diameters of about 2.5 nm were calculated for both CdS and CdSe nanoparticles [9]. The flatness of the absorption edges, however, indicates that the size-distribution is rather broad, about 2.5 + 0.8 nm. Nevertheless, this particle size corresponds well to the widths of the host pores (around 2 nm), confirming that the CdS and CdSe particles were formed within the pores, where they were protected against further growth even due to heating at 373 K.

3.2. Mesoporous titanium dioxide prepared using block copolymers The stock solution for the film preparation contains an ethoxide-modified titanium chloride, formed by the reaction: TIC14 + x EtOH ~ TiC14_x(OEt)x + x HC1, where x ~ 2. The formed TiClx(OC2Hs)4_• species, which are rather stable against hydrolysis, associate preferentially with poly(ethylene oxide) moieties to produce a self-assembling complex. The necessary prerequisite for the formation of ordered material is the hydrolysis of titanium-containing species. Due to their stability, this process is strongly dependent on such parameters as the sufficient supply of water vapor and the length of the hydrolysis. Finally, calcination in air removes quantitatively the organic template. Chemical analysis by XPS has confirmed that the product does not contain any detectable amounts of elements other than titanium and oxygen, i.e. the removal of the organic component was complete. Because of the intended application in the continuous effluent decontamination and the aimed study into the effect of the completeness of the hydrolysis on the structure properties of mesoporous titania, the samples were prepared in the form of films of variable thickness. With thin films (samples I-A and I-B, density of 2 mg/cm 2 and 4 mg/cm 2, respectively), the full hydrolysis occurs due to a good accessibility for the air humidity during the aging. This ensures the creation of a highly uniform and regularly arranged porous structure with a narrow pore size distribution as has been proved by SEM and N2 adsorption (Fig. 3).

I

8

6

,

I-C

4

2

0 0,0

J 0,4

J 0,8

/ 1,2

I 16,

i 2,0

P/Po Figure 3. SEM image of the thinnest film I-A (left) and adsorption isotherms of N2 at-196~ (right). The start point of each isotherm is shifted by P/P0 = 0.4. The solid symbols denote desorption.

1463 Table 1. Structure parameters of films prepared using block copolymers Dpc 9 Sample da SBET~ (mg/cm 2) (m2/g) (nm) I-A 2 105 4.4 I-B 4 94 4.6 I-C 6 127 4.0, 5.2 I-D 8 104 6.2,16.0 a density of the film, b BET surface area, c mean pore size (two values correspond to a bimodal porous structure).

With medium (sample I-C, density of 6 mg/cm 2) and thick films (sample I-D, density of 8 mg/cm 2) the hydrolysis is far from being complete. Consequently, larger pores are formed in addition to smaller ones during the calcination of the non-hydrolyzed fraction. This leads to the formation of a bimodal porous structure (Fig. 3, right). The structure parameters of all the films studied are given in Table 1. X-ray diffractograms and the Raman spectra evidence that all the samples contain a pure anatase phase. The presence of an amorphous titania component is probable because X-ray diffractograms exhibit decreased intensity of reflections due to anatase in comparison with a pure reference material (Bayer). This component does not seem, however, to exhibit any characteristic Raman signal, which would distinguished it from anatase.

3.3. Photocatalysis It was recently demonstrated that mesoporous titania prepared using ligand assisted templating methods has low photocatalytic activity compared to the crystalline phase despite its high surface area [12]. This low activity is due to the incomplete extraction of the surfactant and the amorphous titania channel walls. The authors conclude that partially crystallized titania is essential for obtaining high photocatalytic activity. It this study we have found that by optimizing the synthesis condition a highly active photocatalysist can be synthesized using block copolymers, whose activity compares well even with the best commercial materials (such as PKP 09040, Bayer). There are, however, severe requirements, which should be met. The preparation of a highly active photocatalyst requires the complete hydrolysis of the precursor, as that is the case with samples I-A and I-B. Consequently such a photocatalyst is characterized by a regularly arranged porous structure with a narrow pore size distribution. Rate constants of the decomposition of 4-chlorophenol calculated according to the first-order kinetics are given in table 2. Table 2. Decomposition of 4-chlorophenol Sample Rate constant of the decomposition of 4-chlorophenol (10 4 S"l) I-A I-B Non-optimum films Bayer

3.49 3.42 1.4-2.4 3.37

1464 4. CONCLUSIONS Procedures for the formation of thin, nanostructured, transparent and crack-free films of silica up to a thickness of about 800 nm have been developed by use of block copolymers as structure-directing agents. The crack formation is avoided by lowering the calcination temperature to 350~ The films contain uniform mesopores, whose diameters are about 2 nm. The mesopores can be used to host highly dispersed CdS and CdSe nanoparticles, showing size-quantization effects. The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. Well-organized film is formed only from a completely hydrolyzed precursor. The photocatalytic activity of such organized film is comparable with that of the most active commercial anatase powder. ACKNOWLEDGMENTS

This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (contract No. A4040804). REFERENCES 1. S. Pevzner, O. Regev and R. Yerushalmi-Rozen, Curr. Opin. Colloid. Interface Sci., 4 (2000) 420. 2. D. Zhao, P. Yang, N. Melosh, J. Feng, B.F. Chmelka and G.D. Stucky, Adv. Mater. 10 (1998) 1380. 3. Y.S. Tang, S.J. Cai, G.L. Jin, K.L. Wang, H.M. Soyer and B.S. Dunn, Thin Solid Films, 321 (1998) 76. 4. O. Dag, G.A. Ozin, H. Yang, C. Reber and G. Bussiere, Adv. Mater. 11 (1999) 474. 5. Y. Pyuto, J.-M. Berquier, C. Jacquiod and C. Ricolleau, Chom. Commun. (1999) 1653. 6. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 7. L. Kavan, J. Rathousky, M. Gr~itzel, V. Shklover and A. Zukal, J. Phys. Chem. B 104 (2000) 12012. 8. D. Grosso, G.J. de A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.-A. Albouy, A.Brunet-Bruneau and A.R. Balkenende, Adv. Mater. 13 (2001) 1085. 9. H. Wellmann, J. Rathousky and M. Wark, Thin solid films, 2001, submitted. 10. G.E. Muilenberg (Ed.), Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corporation, 1979. 11. P.E. Lippens and M. Lannoo, Phys. Rev. B 39 (1989) 10935. 12. V.F. Stone and R.J. Davis, Chem. Mater. 10 (1998) 1468.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1465

Spin-coating induced self-assembly of pure silica and Fe-containing mesoporous films N. Petkov, S. Mintova* and T. Bein* Department of Chemistry, University of Munich, Butenandtstr. 11-13 (E), 81 377 Munich, Germany Pure silica and Fe-containing mesoporous thin films have been prepared from preformed silica/triblock copolymer/ethanol precursor solutions by spin- coating. The Xray diffraction results show transformation of the mesophase structure from onedimensional hexagonal to three-dimensional cubic structure by increasing solvent evaporation rate or by thermal transformation during the calcination process. Thus cubic pure silica and Fe-containing mesoporous films have been obtained possessing accessible mesopore channel system suitable for variety of nanotechnolgical and catalytic applications. The films show very smooth and homogeneous morphology and can be prepared with controllable thickness by varying the conditions of the spin-coating procedure. The UV-Vis spectroscopy and thermal analysis results suggest that Fe species are preferentially introduced inside the mesoporous channel system. I. INTRODUCTION The ability to utilize the structural and functional characteristics of the mesoporous films provides an excellent avenue for numerous potential applications ranging from catalysis to the development of electronic and optical devices on a nanometer scale [ 1-4]. Although excellent progress has been made in the preparation of mesoporous films with desired mesophase structure and surface morphology there are still several drawbacks from the standpoint of nanotechnology. Many envisioned nanotechnological applications could benefit by the preparation of continuous mesoporous films that possess threedimensional accessible pore system and provides easy contact to the support which offers a host channel matrix for the growth of nanomaterials. Recently evaporation induced self-assembly route for the preparation of continuous mesoporous films showing smooth and homogeneous surface has been proposed [ 1, 2, 57]. This approach employs preformed silica/surfactant/solvent coating solutions with surfactant concentrations below the critical micellar concentration that are dip- or spincoated on different substrates [5-7]. The rapid solvent evaporation induces self-assembly of the silica/surfactant precursor species and leads to the formation of homogeneous and very smooth mesoporous film. Through the variation of the initial surfactant concentration [5, 7, 8] and the type of the surfactant [9], mesoporous films with different "Correspondingauthor: [email protected]

1466 mesophase structure including cubic and three-dimensional hexagonal structures have been prepared. Furthermore the evaporation induced self-assembly method gives an opportunity for incorporation of different functional groups in the micellar interiors by mixing the precursor silica/surfactant/solvent assemblies with the desired molecular functionality and subsequent coating [ 1, 2]. In this study we report on the preparation of mesostructured pure silica and Fe-loaded films with different mesophase pore systems on silicon wafers. Evaporation induced selfassembly approach performed via spin- coating is used to deposit thin mesoporous films from pure silica and Fe-containing silica/nonionic triblock copolymer/ethanol precursor solutions. 2. EXPERIMENTAL Clear precursor silica/surfactant assemblies were prepared by mixing acid hydrolyzed tetraetoxyisilane (TEOS 98%, Aldrich) at 60~ with ethanol solution of PEO-PPO-PEO triblock copolymer (Pluronic 123 - BASF). Iron was introduced during the mixing of the two solutions as Fe(NO3)3.9H20 and resulted in yellow transparent sol. The final molar ratio of the coating solution was 1TEOS: 0.017PEO-PPO-PEO: 0.06HCI: 60EtOH: 10H20:(0.1 or 0.2)Fe(NO3)3.9H20. The pure silica sample is designated as MSil, whereas the samples prepared with 0.1Fe(NO3)3.9H20 and 0.2Fe(NO3)3.9H20 are designated as MSilFel and MSilFe2, respectively. The coating solutions were aged for 2 h prior to preparation of the thin films. The mesoporous films were deposited on acetone cleaned silicon wafers (25 x 25 mm) by spin-coating (SCS P6700) with rotation speed of 4000 rpm at room temperature (RT). For comparison thick films were prepared by slow evaporation of the same coating solutions deposited drop-wise on precleaned Si-wafers at room temperature for 24 h. These samples are labeled as MSi2, MSi2Fel and MSi2Fe2 for the pure-silica and Fe-containing samples, respectively. Finally the mesoporous films were thermally treated in flowing N2 at 300~ for 12 h, followed by calcination in flowing air at 500~ for 6 h. The mesophase structure of the as-synthesized and calcined films was determined by X-ray diffraction in the 00-20~ scan mode (Scintag XDS 2000 cooled Ge detector, Cu K~ radiation). The surface morphology was evaluated from atomic force microscopy images (AFM) (Nanoscope N S E - Digital Instruments). The thickness of the mesoporous films was determined from the scratched parts of the films by scanning electron microscopy (SEM) (Philips XL 40). The degree of loading of Fe species into the mesoporous materials was determined by thermal analysis (TA) (DuPont Instruments - 951 TA) and UV-Vis spectroscopy (Hitachi U-3501 Spectrophotometer). 3. RESULTS AND DISCUSION 3.1. Structure of the mesoporous films

The structure of the mesoporous films deposited on Si wafers has been determined by X-ray diffraction. Figure 1 represents the X-ray diffraction patterns of the as-deposited pure-silica and Fe-containing mesoporous films. The pure-silica film shows several diffraction peaks that can be indexed as the (111), (200), (210), (220), and (440) reflections of the Pn3m cubic space group with high unit cell size a - 184 A (Figure 1a). The measured and calculated 20~ values for the primitive cubic Pn3m space group are presented as an inserted table in Figure 1. Similar results have been reported before for

1467 the primitive cubic mesophase structure identified as a bicontinuous Pn3m prepared by spin- coating of silica/cationic surfactant solutions [8]. However cubic mesophase structure showing Im3m symmetry has been observed for the dip-coated mesoporous films prepared with triblock copolymers with higher molar ratio of ethylene oxide to propylene oxide [9]. Our data for the films prepared by spin- coating of silica/triblock copolymer mixture suggest the existence of the bicontinuous cubic mesophase structure with Pn3m symmetry. After calcination the observed space group symmetry is preserved, but the reflections appear at higher 20 ~ values due to the contraction of the silica framework (Figure 2a). The unit cell size of the calcined pure-silica mesoporous film is a= 116 A and corresponds to 37 % contraction of the silica framework. The XRD patterns of the as-deposited MSilFel film shows three peaks with decreased intensity that can be indexed as (100), (200) and (300) reflections of the highly ordered one-dimensional hexagonal mesophase structure with channels oriented parallel to the substrate surface (Figure l b). The unit cell size of the corresponding as-deposited onedimensional hexagonal structure is a = 108/~. Upon calcination several reflections are observed, and they can be indexed in primitive cubic symmetry of the same bicontinuous Pn3m mesophase structure obtained already for the pure-silica sample (Figure 2b). The unit cell size of the calcined MSilFel film is a = 115 A, and it has similar value to that of the pure-silica sample. The X-ray results reveal that mesophase transformation has taken place during the calcination process leading to the formation of three-dimensional mesophase structure due to shrinkage of the silica framework. (100)

hkl 20 ~ 100 1.06 200 2.02

hkl 20 ~ 20cal 200 1.70 1.70 211 2.08 2.08

(C)

~. (200) . ~ 0 0 3.05

(19o) A

II i[

!1

9~= , ] ]

,, I,c)

. . . . . . hkl 20 ~ 100 0.94

%~ ;200] "3""" 200 1.87 vv) 300 2.71

(b) ,,,,,

xlt)t)

~" (111~? 00) ~i ~ \ .~~ o ( ~

)

hkl 111 200 211

20 ~ 1.33 1.53 2.03

20cal 1.33 1.53 1.89

hkl 111 200 211

20 ~ 20cal 1.31 1.32 1.52 1.52 1.98 1.86

(b)

(200) ,1,,,11 1, 1 l)gll v

i

0.7

~(21~ I(21~)~ k..----

(440)

111 0.83 0.83,'aX 200 0.93 0.96I, )

~440

(a)

2.62 2.63 X40

i

i

2.7

4.7

2 theta (degrees)

(

Figure 1. XRD patterns of the as-deposited (a) MSil, (b) MSilFel, and (c) MSilFe2 mesoporous films.

1

2 2 theta (~egrees)

4

Figure 2. XRD patterns of the calcined (a) MSil, (b) MSilFel, and (c) MSilFe2 mesoporous films.

1468 Similar mesophase transformation process has been observed before for the cubic mesoporous films prepared by dip- coating of silica/cationic surfactant solution [5]. The as-deposited MSilFe2 film shows one very intensive and two less intensive reflections that are consistent with one-dimensional hexagonal mesophase structure with unit cell size of a = 96 A (Figure lc). In comparison to sample MSilFel the mesophase order and the unit cell size is decreased probably due to the introduction of higher amount of Fe salt in the coating mixure. The calcined MSilFe2 film shows only two very broad and low intensive reflections similar to (200) and (211) reflections of the bicontinuous cubic Pn3m mesophase structure (Figure 2c). Apparently the introduction of the Fesource in the synthetic mixture provokes formation of one-dimensional mesophase structures (as-deposited samples) but enables the formation of three-dimensional cubic mesophase through thermal transformation during the calcination process. In order to elucidate the role of the solvent evaporation during the formation of mesoporous films, thicker films are prepared by slow evaporation of the same coating solutions deposited drop-wise on Si-wafers at room temperature. The XRD patterns of the as-deposited films are presented in Figure 3. The results show that preferentially onedimensional hexagonal mesophase structures with channels running parallel to the substrate surface are formed from the three coating solutions. The corresponding unit cell size of the one-dimensional hexagonal mesophase structures are: aMsi2-- 115 /~ aMSi2Fel= 106 ~ and aMSi2Fe2 = 103/~. (100) A i II ~ (200) l l L A

hkl 20 ~ 100 1.00 200 1.96 3002.95

(c)

(c)

, ,

(100) A ~i II .~-~ I!

(200) i

(300)

hkl 100 200 300

20 ~ 0.94 1"86 (b) 2.78

.~~

111 1.89 1.89 200 2.20 1.18

O

(100) (100)

hkl 20 ~ 100 0.88

(200)

,

~

(300)

JLJ i

0.7

hkl 20 ~ . . . . . 100 1.86 tzuu) _ 200 3.88

]~176 0 ~175~ (a)

i

2.7 2 theta (degrees)

i

4.7

Figure 3. XRD patterns of the asdeposited (a) MSi2, (b) MSi2Fel, and (c) MSi2Fe2 mesoporous films.

1

2

3

2 theta (degrees)

4

5

Figure 4. XRD patterns of the calcined (a) MSi2, (b) MSi2Fel, and (c) MSi2Fe2 mesoporous films.

1469 The observed decrease in the unit cell size from the pure-silica to the Fe-containing films is attributed to the introduction of Fe salt in the synthetic mixture. After calcination of film MSi2, the X-ray pattern exhibits two reflections that can be straightforwardly assigned to (100) and (200) reflections of one-dimensional highly ordered mesophase structure (Figure 4a). The unit cell size of the calcined sample is a = 48 A and corresponds to 58 % shrinkage of the silica framework. For the calcined MSi2Fel film two reflections that are indexed as (111) and (200) values are detected and assigned to the primitive cubic Pn3m symmetry (Figure 4b). The same mesophase transformation process has been observed for the thinner films prepared by spin- coating of solutions MSilFel and MSilFe2. The unit cell size of the thicker film (MSi2Fel) is a - 81 A, and it is much smaller than that of the MSilFel cubic film (a - 115 A) suggesting higher stability of the silica framework towards thermal contraction. One broad reflection shifted to higher 20 ~ values of about 2.4 20 ~ was recorded for the sample MSi2Fe2 alter calcination (Figure 4c). This pattern suggests decreased mesostructural order of the sample, and it is attributed to more disordered mesophase structure. Obviously the spin-coating process induces formation of three-dimensional mesophase structures due to the rapid evaporation of the solvent, which promotes the formation of spherical micellar interiors on the substrate surface and leading to self-assembling in a cubic mesostracture. In contrast, the slow evaporation of the solvent promotes slower silica condensation and enables formation of tubular micelles on the substrate surface, which are self-assembled into hexagonal arrangement of the mesoporous channels parallel to the surface. 3.2. Morphology and thickness of the mesoporous films The surface features and the thickness of the mesoporous films are evaluated by contact mode AFM images and by scanning electron micrographs of the scratched parts of the films. Figure 5 shows the AFM image of the calcined MSil mesoporous film, where a sausages-like type of morphology is seen on the film surface. At higher magnification, irregular objects with dimensions of less than 0.5 ~tm could also be seen. The film roughness calculated from the AFM image is estimated to be around 9 nm. The SEM image of the calcined MSil sample shows that very thin (- 90 nm) and continuous film along the whole Si-wafer was prepared with no cracks and corrugation (Figure 6). The Fe-loaded thin mesoporous film deposited using the same spin-coating conditions and calcination procedure is shown in Figure 7. The observed sausages-like type of surface morphology is slightly distorted in the MSilFel compared to the MSil film. The roughness of the Fe-loaded film is similar to that of the pure silica film deposited under the same spin-coating conditions. The thickness of the calcined MSilFel film is about 120 nm, which is slightly higher than the thickness of the pure-silica film (Figure 8). The Fe-loaded film is continuous and no surface inhomogenates or cracks were observed alter calcination. The as-deposited thicker films (data not shown) exhibit continuous but uneven surface morphology alter slow evaporation of the coating solutions at room temperature. Upon calcination the MSi2, MSi2Fel and MSi2Fe2 films show cracks and they partially delaminate from the substrate.

1470

Figure 5. AFM image of the calcined cubic MSil film.

Figure 6. SEM image of the calcined cubic MSi 1 film.

The thickness of these films determined from the SEM images is estimated to be in the micrometer range (~ 21am). The above results demonstrate that the surface morphology and the film thickness of pure silica and Fe-containing films deposited by spin- coating are analogous. In the both cases, very smooth and homogeneous films with nanometer thickness suitable for nanotechnological applications can be prepared. In addition, the surface morphology and the thickness of the films can be easily controlled by changing the solvent evaporation rate. The slower solvent evaporation results in thicker micrometer films showing uneven and bumpy surface. 3.3. Fe-loading of the mesoporous materials The UV-Vis spectra of the calcined pure-silica and Fe-containing samples obtained aider slow evaporation of the coating mixtures at room temperature are depicted in Figure 9.

Figure 7. AFM image of the calcined cubic MSilFel film.

Figure 8. SEM image of the calcined cubic MSilFel film.

1471

1.1

110

.•(c)

100

~ 90

~ so

O

70 -

O

~0.8

(c)

~ 6o ~-_

__

- .

__

:

b)

_

50 0.5

t

250

450 Wavelenght (nm)

I

650

Figure 9. UV-vis spectra of (a) MSi2 (b) MSi2Fel, and (c) MSi2Fe2 samples after delaminating from the substrate.

(a)

40 30

130

230

330

430T(C~

Figure 10. TG-analysis of (a) MSi2 (b) MSi2Fel, and (c) MSi2Fe2 samples alter delaminating from the substrate.

As expected, the pure-silica sample shows no absorbance in the range of 250-700 nm (Figure 9a). On the other hand, the Fe-containing samples show absorbance in the region 250-400 nm with maximum at 290 nm indicative for the ligand to metal Fe 3§ charge transfer (Figure 9 b, c). On the other hand the absorbance bands in the range of 450-600 nm related to the absorbance of bulky Fe203 or FeO(OH) particles are absent in our samples [10]. The XRD measurements taken at high 20 ~ angles and the microscopy confirm the absence of large bulky particles of Fe oxides. Our results suggest the encapsulation of Fe oxide nanoparticles and clusters inside the mesoporous channels upon calcination and are consistent with the previously published result [ 10]. The degree of Fe-loading and the stability of the mesoporous films were determined by TG-analysis. The TG curves of the pure-silica and Fe-containing mesoporous samples are shown in Figure 10. The MSi2 sample shows that the surfactant is removed in a single decomposition step in the temperature interval of 160-230 ~ and the weight loss is estimated to be around 52 wt. %. In the case of the Fe-loaded mesoporous sample the weight loss behavior is different compared to the pure-silica sample. The combustion of surfactant takes place at higher temperature interval (250-330 ~ and leads to lower weight loss of 46 wt. % and 37 wt. % for MSi2Fel and MSi2Fe2 samples, respectively (Figure 10 b, c). The difference of 8 wt. % and 15 wt. % for the MSi2Fel and MSi2Fe2 samples compared to the pure-silica sample is probably due to the incorporated Fe species into the mesoporous host. 4. CONCLUSION Evaporation induced self-assembly via spin- coating has been employed for the formation of mesoporous films from preformed pure-silica and Fe-containing silica/triblock copolymer/ethanol solutions. The X-ray diffraction results reveal the

1472 transformation from one-dimensional hexagonal to the three-dimensional cubic mesophase structure by increasing the solvent evaporation rate or by thermal transformation during the calcination process. The rapid solvent evaporation of the coating mixture induces formation of three-dimensional micellar interiors, which selfassembled in cubic mesophase structure on the silicon substrate. Varying the solvent evaporation rate and silica condensation process in the coating solutions can control the different morphology and thickness of the films. The results from UV-Vis spectroscopy and TG analysis suggest that the iron species are preferentially introduced inside the mesoporous channel system. The mesoporous films containing Fe-nanoparticles are suitable supports for variety of catalytic applications. The growth of carbon nanotubes by catalytic decomposition of hydrocarbons is currently being investigated. REFERENCES

1. H. Fan, Y. Lu, A. Stump, S. T. Scott, T. Baer, R. Schunk, V. Perez-Luna, G. P. Lopez and C. J. Brinker, Nature, 405 (2000) 56. 2. D. Doshi, N. Huesing, M. Lu, H. Fan, Y. Lu, K. Simmons-Potter, B. G. Pottr Jr., A. Hurd and C. J. Brinker, Nature, 290 (2000) 107. 3. G. Wirnsberger and G. D. Stucky, Chem. Mater., 12 (2000) 2525. 4. R. Hayward, P. Alberius-Henning, B. Chmelka and G. D. Stucky, Micropor. Mesopor. Mater., 44-45 (2001) 619. 5. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y. Guo and J. I. Zink, Nature, 389 (1997) 364. 6. C.J. Brinker, Y. Lu, A. Sellinger and H. Fan, Adv. Mater., 11 (1999) 579. 7. M. Ogawa and N. Masukawa, Micropor. Mesopor. Mater., 38 (2000) 35. 8. I. Honma, H. Zhou, H. Kundu and A. Endo, Adv. Mater., 12 (2000) 1529. 9. D. Zhao, P. Yang, N. Melosh, J. Feng, B. Chmelka and G. D. Stucky, Adv. Mater., 10 (1998) 1380. 10. S. E. Dapurkar, S. K. Badamali and P. Selvam, Catal. Today, 68 (2001) 63. 11. M. Iwamoto, T. Abe and Y. Tachibana, J. Mol. Catal., 155 (2000) 143.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Guanidine Catalysts Supported Catalysts for Organic Chemistry

on Micelle T e m p l a t e d

1473

Silicas. N e w

Basic

D. J. Macquarrie *a, K. A. Utting a, D. Brunel b, G. Renard b and A. Blanc b (a) Centre for Clean Technology, Department of Chemistry, University of York, Heslington, YORK, YO 10 5DD, England (b) Laboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS, 8, Rue de l'Ecole Normale- 34296 Montpellier, France. The preparation of guanidines covalently linked to the framework of Micelle Templated Silicas is discussed. These materials can behave as efficient catalysts in base catalysed epoxidation. Control over surface chemistry is very beneficial in enhancing selectivity and efficiency with respect to utilisation of the oxidant hydrogen peroxide. Such systems contribute to the search for cleaner manufacturing technologies. The same catalysts are active in the Linstead variation of the Knoevenagel reaction. Initial work aimed at preparing chiral guanidines attached to Micelle Templated Silica is also presented. 1. INTRODUCTION The development of novel catalysts supported on inorganic solids such as silica has received a great deal of attention recently. With the drive towards cleaner chemical processes intensifying, the search for efficient catalytic routes to products and intermediates is increasingly important, and progress is being made in acid, base and oxidation catalysts, many of which are based on the new highly structured solids such as the MCMs and related materials[1 ]. Many important synthetic protocols exist which rely on soluble base catalysts, and we, and others, have in the past addressed some of these with simple amines attached to silica.[2-7] While these catalysts are very effective in a range of important reactions involving mild base catalysis, they fail in other reaction types where more powerful bases are required. The types of organic bases which are of interest are the guanidines, considerably more powerful than the simple amines commonly used so far. One such reaction type which is of importance in synthetic chemistry, and which is amenable to guanidine catalysis is the base catalysed epoxidation of electron-deficient alkenes (Scheme 1).[8,9] This reaction is particularly appealing, since it delivers functionality efficiently, is of importance in synthesis, and requires only the clean, cheap oxidant hydrogen peroxide, as terminal oxidant. There is also the as-yet untapped potential to develop chiral guanidines supported on structured inorganic matrices which would allow the enantioselective preparation of important building blocks for natural product synthesis, such as 1 in Scheme 1, a structural motif in a range of antiobiotics of the manumycin family.

1474 0

0

0

Scheme 1. Epoxidation of electron deficient alkenes. Previous work in the area of has been carried out by the groups of Jacobs,[ 10] Brunel[11 ] and Jaenicke. [ 12] Jacobs' approach was to attach the bicyclic guanidine triazabicyclo[4,4,0]undec-5-ene (TBD) to a silica support modified with the glycidyl group. Ring opening of this epoxide led to the attachment of the guanidine unit. This product was active in the epoxidation reaction mentioned earlier (with excellent selectivity towards the organic component, but poor utilisation of the hydrogen peroxide (ca. 20% selectivity), as well as being a good catalyst for both the Michael and Knoevenagel reactions. The catalyst described by Brunel was prepared by displacement of the chlorine of a chloropropyl-micelle templated silica with the same bicyclic guanidine, followed by liberation of the free base by treatment with a stronger guanidine base. The resultant catalyst was found to be efficient for the base catalysed ring-opening of epoxides to form monoglycerides. A similar approach was used by Jaenicke to provide a catalyst active for the same ring-opening reaction.[12] We now present our results on the development of supported guanidines and their application in model reaction types. The achiral guanidines are based on tetramethylguanidine, rather than TBD. Chiral guanidines are also prepared, based on the work of Isobe, who has developed routes to soluble chiral guanidines.[ 13-16] 2. PREPARATION OF CATALYSTS Our approach to achiral guanidines is based on the reaction of chloropropyl silanes, either attached to a silica surface or prior to grafting, with 1,1,3,3-tetramethylguanidine (TMG). Removal of HC1 liberates the free guanidine, and is accomplished using 1-methyl-TBD, a stronger base than TMG. Various routes were attempted, with these being summarised in Scheme 2. Catalysts were prepared by either grafting onto a pre-formed Micelle Templated Silica (MTS-G) of by direct sol- gel preparation of an organically modified MTS (MTS-D). Preliminary attempts to attach TMG via epoxide ring-opening were unsuccessful, probably due to steric hindrance from the methyl groups impeding attak of the imine N on the epoxide. Thus, emphasis was placed on the nucleophilic displacement of the chloride and subsequent liberation of the free guanidine. The guanidine silane 2 is known[17], and was prepared in a straightforward manner, the product being very moisture sensitive, as might be expected. This was grafted directly to a Micelle Templated Silica to give 5, but templated sol-gel preparations resulted in only amorphous material.

1475

(MeO)3Si~/~XCI

(a)

=" (MeO)3Si ~ (/

1

3

7

4

1

N--------~NMe2~ ) ~ NMe2 8

/~

NMe2 N~"~ 2 NMe2 " ~ (b)

NMe2 ~ ~ N _ _ . ~ N~---~NMe2

5

NMe2 NMe2

6

N----"~NMe2 NMe2

Scheme 2. Summary of synthetic routes to catalysts. (a) Displacement of chloride by TMG, followed by liberation of free base; (b) templated sol-gel synthesis of modified silica; (c) grafting onto preformed MTS This is likely to be due to the guanidine unit becoming protonated in the aqueous environment of the system, something which is known to hinder tempated synthesis.[18] Both templated sol-gel and grafting routes to the chloropropyl systems 3 and 4 proceeded in a straightforward manner, and displacement of the chloride was easily achieved. Silylation with N,O-bistrimethylsilylacetamide was carried out before attachment of guanidine. This improves the ease with which physisorbed guanidine can be removed, by substantially reducing the interaction of the guanidine with the surface of the silica, as well as reducing the interaction of the basic chemisorbed guanidines with the surface. Positive effects on hydrogen peroxide utilisation have also been noted in Ti-MTS systems in epoxidation reactions after surface passivation,[ 19,20] and it was hoped that these would also be found here. A summary of the physical parameters of the succesfully prepared materials is given in Table 1. Table 1 Physica! parameters of chosen catalysts Material Parent MTS 5 7 8

SSA (m 2g-1) 972 221 1281 807

Pore diameter (nm)

loading (mmol g-l)

3.0 1.9 2.4 2.5

1.7 1.4 0.4

1476 3. CATALYTIC RESULTS The above catalysts were investigated in the epoxidation of 2-cyclohexenone (Scheme 1), and in the Linstead-Knoevenagel condensation (Scheme 3).[21,22]

HO2C~"'+~CO2H

~ C H O

THF

-H20

H~ ,.,4,CHO CeH1/C~C'h"CsHll 10

06H13~~/COOH 9

\

COOH I -cO2 C5H1'~ 0H2002H 9

Scheme 3. Linstead-Knoevenagel condensation The epoxidation of 2-cyclohexenone proceeded smoothly at room temperature in methanol (Table 2). Best results were obtained when the hydrogen peroxide was added incrementally over the period of the reaction, rather than all at the start. Using this method, we were able to obtain good selectivities towards the enone, and towards hydrogen peroxide. The major byproduct consisted of small amounts (2-3%) of a product derived from the addition of methanol to the enone were always observed (even in blank reactions with no catalyst); this product could be suppressed by changing solvent to a bulkier alcohol such as i-propanol, although this reduced the rate and conversion significantly. Selectivities towards enone were excellent (up to 89%) and match those in the literature. Importantly, selectivity towards hydrogen peroxide were substantially improved, due to the reduction in the amount nonproductively decomposed. This is due to surface passivation, and has been noted in other TiMTS systems[19,20] In this way we were able to improve the hydrogen peroxide utilisation from the 21% reported[ 10] to over 50%. This corresponds to a substantial decrease in the final volume of the reaction mixture, and to a substantial decrease in the quantity of water present. The condensation of malonic acid with heptanal is a key step in the synthesis of coconut oil lactone, and is carried out by the Linstead variation of the Knoevenagel condensation, usually using tris(hydroxyethyl)amine as catalysts and solvent. Here, the solvent chosen was THF, largely because solubility problems with malonic acid in most simple organic solvents preclude effective heterogeneous catalysis. Catalytic quantities of the guanidine catalysts can be used in THF to bring about the Linstead condensation, as summarised in Table 3 and Scheme 3. Condensation of the two components to give product 9 is relatively simply achieved in the cases attempted, but two main difficulties were encountered:- firstly, the formation of condensation products (10) from two molecules of aldehyde dimerising were high (14%) in one catalyst (8), but in the slightly less active 5, these products were minimised. The major loss of selectivity came as a result of the incomplete decarboxylation of the

1477 intermediate diacid, which can be isolated (and converted to the product in a separate reaction step). This stems from the low temperature of reflux in THF, with higher boiling solvents not dissolving sufficient malonic acid to allow reaction to proceed. Table 3. Results of supporte J guanidine catal, rsis of the Linstead condensation. _atalyst Conversion Selectivity tumover 8 95% 58% 158 5 78% 40% 31

Time 48h 48h

4. CHIRAL SUPPORTED GUANIDINES The approach investigated to obtain chiral guanidines is represented in Scheme 4. The first variation, involving the condensation of the chiral cyclohexane-diamine (instead of 11) with urea in a high temperature step[ 13-16] was only partly successful, with very low yields being obtained. Subsequent steps have not yet been studied.

H2N Ph

NH2

/ '., Ph

H2N

11

~ ,, II 0

/NH 2

O ,,,,LL,. -- HN NH.,, ~

Ph

'

2xMel

[

12

Ph

\

MeN'~NMe+ [.,,

Ph 14

[MTS]---NH2

13

Hx

.HCI

oxalyl chloride

",, Ph

,,Ph

S / ' ~ L _ N___~~I---1"

~ MT

NMe [

Ph

CI

Ph

=Mej

O ,~

--L /N~ph

removal of HCI

"

15

final catalyst 16

Scheme 4 - Scheme for the preparation of chiral guanidine supported on MTS Instead, we have concentrated our initial efforts on the second route involving 1,2-diphenyl, 1,2-diaminoethane 11, Scheme 4. Here, we have successfully prepared the supported chiral guanidine on a Micelle Templated Silica, in the form of its HC1 salt. Efforts are ongoing to liberate the free base, and to evaluate the product. The first three steps up to 14 proceeded smoothly, and according to the literature. Coupling of 14 with a trimethylsilyl-passivated aminopropyl grafted micelle templated silica was carried out after drying of the aminopropylsilica under vacuum (lmbar) at 150~ overnight. The crude reaction mixture containing 14 was cooled to room temperature, and excess oxalyl chloride was removed under vacuum, leaving a suspension of 14 in chlorobenzene. To this

1478 was added the activated aminopropyl silica, and the mixture stirred for 48 hours. Triethylamine was added and stirring continued for 3 hours. Filtration and washing with methanol and DMF yielded the product 15. Elemental analysis indicated a 0.66mmol/g loading of guanidine (and 0.66mmol/g C1).

5. EXPERIMENTAL All solvents used were analytical grade, and were not purified further unless specified. Reagents were purchased from Aldrich, and were used as received. The MTS used was prepared following a published route[6], and was calcined to remove template shortly before use. Grafting was carried out following published methods.[6] The templated sol-gel method has been described previously.[23,24] Silane 2 was prepared following a literature method.[17] Porosity and surface area measurements were carried out using dinitrogen on a Micromeritics ASAP2100 instrument. NMR spectra were recorded on a Bruker Model AM 300 spectrometer operating at 75.470 MHz with Fourier transform. The instrument setting were the following : 90 ~ pulse of 4.80 ms; proton decoupling power : 30 G; contact time : 5.10 .3 s; delay time : 5 s; rotor spinning speed : 5 KHz

5.1 Passivation of chloropropyl supports Chloropropyl-MTS (2.00g) was refluxed in dry toluene (75ml). To the suspension was added N,O-bis trimethylsilyl acetamide (5.6ml, 27.5mmol) and the reaction mixture refluxed for 2.5h. After cooling, the solution was filtered and washed with toluene (25ml). The solid was subjected to Soxhlet extraction with methanol for 6 hours, and subsequently dried at 100~ 5.2 Grafting of guanidine onto chloropropyl-containing supports The material from the previous experiment (1.3g) was activated at 150~ for 2h before reaction. The activated solid was then suspended in dry toluene (30ml), and 1,1,3,3tetramethylguanidine (TMG, 1.00g, 8.7mmol) and 1-methyl-l,5,9-triazabicyclodecane (TBD, 0.3ml, 2.0mmol) were then added. The mixture was heated to reflux and held at this temperature for 7h. After cooling to room temperature, the mixture was filtered, and the solid washed thoroughly with toluene (3x 20ml), methanol (20ml) 1:1 v/v methanol / water (20ml) methanol (2x 20ml) and then extracted in a Soxhlet apparatus containing dichloromethane / diethyl ether (1:1) for 18h. This yielded 1.3g of product, the physical parameters of which are recorded in Table 1. 5.3 Grafting of guanidine silane 2 to MTS To activated (150~ vacuum, 18h) MTS (2.00g) was added dry toluene (50ml) and silane 2 (5mmol, 1.4g) The mixture was heated to reflux and held there for 15 hours. After cooling, the solid was filtered and washed thoroughly with methanol (3x50ml) and then extracted (Soxhlet, ether, dichloromethane 1:1). NMR (~3C, CPMAS) 10.5, SiCH2; 24.7, SiCH2CH2; 40.4 4x NCH3; 47.8 (CH30); 51.9 (CH2N=C); 162.2 (N=C(NMe2)2.

5.4 Epoxidation To a solution of 2-cyclohexenone (0.96g, 10mmol) in methanol (10ml) at 20~ the catalyst was added (0.10g) followed by hydrogen peroxide (30% v/v in water) dropwise. The reaction

1479 was followed by GC, using 2-phenyl-2-butanol in methanol as external standard, and products identified by GC-MS and by comparison with an authentic standard. Hydrogen peroxide was added until no more conversion was seen, and the total quantity added used to determine the selectivity towards oxidant. 5.5 Linstead-Knoevenagel condensation Heptanal (1.14 g, 10 mmol) and malonic acid (1.04 g, 10 mmol) are added to THF (30 ml). To this mixture was added the catalyst (150rag, activated at 100~ overnight under vacuum) and the mixture was heated to reflux under nitrogen atmosphere. Reaction is followed by GC, using dodecane in THF as external standard. 3-Nonenoic acid and 2-pentyl-2-nonenal were identified by GC/MS and by comparison with authentic compounds. Dicarboxylic acids are not detected by GC. After two days, the reaction is stopped and the catalyst filtered and washed with THF. The solvent is removed and the crude mixture recovered. Isolation of products was achieved by flash chromatography (SDS silica 60 AC.C/70-200 mesh, cyclohexane/diethyl ether/formic acid 90:10:1). 5.6 Attachment of chiral chloramidinium salt to AMPS (a) preparation of passivated aminopropyl-MTS. To 6.0g aminopropyl-MTS[6] (preheated at 150~ under vacuum) was added trimethylsilylimidazole (14g). The suspension was stirred under nitrogen for 8 hours at 60~ The solid was filtered and washed with toluene, DMF, methanol, acetone (each 2x35ml) before being extracted (Soxhlet, 50:50 ether : dichloromethane). The product was then dried at 60~ for 1 day. (b) Coupling reaction. The urea 14 (0.85g, 3mmol) was heated at 70~ in 20ml of chlorobenzene and oxalyl chloride (0.5g, 4mmol) added. Heating was continued for 14h. The flask was then cooled to 25~ and a vacuum applied to remove excess oxalyl chloride. After the removal of oxalyl chloride was complete, the passivated aminopropyl-MTS was added ((0.7g) and the reaction stirred for 48h. Triethylamine (0.2g) was then added and stirred for a further 3 hours. The solid was then filtered and washed with methanol and DMF. Soxhlet extraction with ether : dichloromethane (50:50) was then carried out overnight before drying at 50~ Loading from elemental analysis was 0.66mmol/g guanidine. 6. CONCLUSIONS The preparation of a range of guanidines supported on Micelle Templated Silicas is presented. It is possible to prepare these materials without significantly damaging the structure of the solid matrix, and control over surface properties is also achieved. Catalytic activity is good in two reaction types. Initial attempts to prepare chiral supported guanidines are underway, and supported salts have been prepared. Liberation of the free base is currently being undertaken 7. ACKNOWLEDGEMENTS DJM thanks the Royal Society for a University Research Fellowship, and for travel funds to carry out this work. KAU thanks the EPSRC for a Project Studentship. DJM and DB are grateful to the British Council / EGIDE Alliance scheme for travel funds. Authors are grateful to Dr Francois Fajula for consistent support, and Anne Derrien for useful preliminary work. They wish also to thank Dr Annie Finiels for NMR analysis and Dr Patrick Graffin for GCMS analysis.

1480 REFERENCES

1. "Fine Chemicals Through Heterogeneous Catalysis", ed. R A Sheldon, H Van Bekkum, Wiley-VCH (Weinheim) 2001 2. D J Macquarrie and D B Jackson, Chem. Commun., (1997) 1781 3. J E G Mdoe, J H Clark and D J Macquarrie, Synlett, (1998) 625 4. D J Macquarrie, Green Chem., (1999) 1 195 5. M Lasperas, T Llorett, L Chaves, I Rodriguez, A Cauvel and D Brunel, Stud. Sure Sci. Catal., (1997), 108 75 6. D Brunel, Microp. Mesop. Mater. (1999) 27 329 7. B M Choudary, M L Kantam, P Sreekanth, T Bandopadhyay, F Figueras, A Tuel, J. Mol. Cat., A, (1999) 142 361 8. T Genski, G Macdonald, X D Wei, N Lewis, R J K Taylor, Synlett, 795 (1999) 9. C.L. Dwyer, C. D. Gill, O. Ichihara and R. J. K. Taylor, Synlett, 2000, 704 10. Y V Subba Rao, D E de Vos, P A Jacobs, Angew. Chem. Int. Ed. Engl., (1997) 36 2661 11. A Derrien, G Renard, D Brunel, Stud. Sure Sci. Catal., (1998) 117 445 12. X Lin, G K Chuah, S Jaenicke, J. Mol. Cat., A, (1999) 150 287 13. T Isobe, K Fukuda, T Ishikawa, Tetrahedron Asymmetry, (1998) 9 1729 14. T Isobe, K Fukuda, T Ishikawa, J. Org. Chem., (2000) 65 7770 15. T Isobe, K Fukuda, T Ishikawa, J. Org. Chem., (2000) 65 7774 16. T Ishikawa, Y Araki, T Kumamoto, H Seki, K Fukuda, T Isobe, Chem. Commun (2000) 245 17. T Takago (to Shin-Etsu Chemical Industry Co. Ltd.), Ger. Often. 2 827 293 (1979) [US Patent, 4 248 992 (1981) 18. R J P Corriu, A Mehdi, C Rey6, C R Acad. Sci, Paris, t.2, S6rie IIc 35 (1999) 19. T Tatsumi, K A Koyano, N Igarishi, Chem. Commun., (1998) 325 20. M B D'Amore, S Schwarz, Chem. Commun., (1999) 121 21. S E Boxer and R P Linstead, J. Chem. Soc., (1931) 740 22. R P Linstead and E G Noble, J. Chem. Soc., (1933) 557 23. D J Macquarrie, Chem. Commun, (1996) 1961 24. D Macquarrie, D B Jackson, J E G Mdoe and J H Clark, New J. Chem, 1999 23 539

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1481

A t t e m p t s on generating basic sites on m e s o p o r o u s materials* X.W. Yan, X.W. Han, W.Y. Huang, J.H. Zhu** and K. Min Department of Chemistry, Nanjing University, Nanjing 210093, China

The stability of A1-MCM-48 could be obviously enhanced by post-synthetic treatment. Basic mesoporous materials were thus prepared by introducing basic guest through microwave irradiation and impregnation along with ion exchange, exhibiting a considerable basicity in dehydrogenation of 2-propanol. 1. INTRODUCTION Basic selective catalysis is a developing field in the application of zeolite. Moreover, limited to their small pore size, zeolites are not efficient for large size molecule reactions such as pharmaceutical chemical reactions; therefore new basic material with uniform mesoporous size distribution is desirable. A family of mesoporous molecular sieves designated as M41S was developed on 1990's [1 ], and since then the research of novel mesoporous materials and their properties has attracted more and more attention [2-3]. Hexagonal mesoporous molecular sieve MCM-41, the representative of M41S, has been used to prepare basic catalyst either as ion-exchanged substrate for alkali-containing derivative [4], or as support for composite with basic organic group grafted [5]. Another important member of M41S series is the MCM-48 with cubic (Ia3d) mesophase that is expected to be a better candidate for preparation of basic catalyst because of its interwoven three-dimensional channel system [6]. However, the question on the preparation of satisfactory mesoporous supports and the generation of basic sites in an environmentally benign way still remains to be answered. For prepare basic zeolite, it is known that introducing basic guests is easier to create strong basic sites on zeolite than adjusting the chemical composition of host framework [7], and existence of aluminum in support is proven to be beneficial for preparing solid base [8]. Consulting these facts, we study the preparation of aluminum-containing MCM-48 (denoted as A1-MCM-48) and the modification with magnesia by the use of microwave radiation. *The key laboratory of chemical engineering and technology of Jiangsu province and Analysis Center of Nanjing University financially support this subject. ** Corresponding author, E-mail: [email protected], FAX: 0086-25-3317761.

1482 2. E X P E R I M E N T A L

A1-MCM-48 was prepared in basic condition, with the CTAB (Cationic surfactant cetyltrimethylammonium bromide) used as a template and TEOS (tetraethyl orthosilicate) as the silica source. The alumina source was fresh aluminum hydroxide gel, which was obtained from the recovery of the precipitation of aluminum sulfate solution and equivalent sodium hydroxide. In a typical procedure, a clear solution was obtained by dissolving 35 g CTAB in 115 g distilled water and 53 g of 1.5 mol/L sodium hydroxide solution was added dropwise into the emulsion formed by 33 g TEOS and 7.8 g distilled water. After hydrolyzing for about 3 rain, the TEOS emulsion was poured into the CTAB solution under vigorous stirring for 0.5 h. Then the aluminum hydroxide gel was added with vigorous stirring. The obtained mixture was sealed in a Teflon-lined stainless autoclave and kept at 373 K for 3 d, and the synthesis molar composition was TEOS:0.06AI(OH)3:0.6CTAB:0.4NaOH:60H20. The product was recovered by filtration, washed with distilled water and dried at ambient temperature. In a typical process of hydrothermal post-synthetic treatment, as-made product was mixed thoroughly with distilled water at 1 g/100 ml ratio [9], sealed again in the autoclave and kept at 373 K for 14 d, followed by recovery, washing and drying. The obtained material was calcined in N2 at 823 K for 1 h followed by in air for 5 h to remove the organic surfactant. Basic samples were prepared in the different ways as described below: (1). 2g MCM-48 was stirred in 200 ml solution of cesium nitrate and refluxed at 373 K for ion exchange, then filtrated, washed with adequate distilled water and dried at ambient temperature to obtain Cs+-A1-MCM-48, denoted as Cs+/ie sample. (2). 10 g NaX zeolite, supplied by Nanjing Inorganic Chemical Factory, was added to 100 ml NaC1 solution of 1 mol/L then vigorously stirred for 10 h under 353 K, followed by drying at 373K over night. This procedure was repeated for another 3 times. In succession, a 0.23 mol/L CsNO3 solution was used as the substitution of NaC1 solution, and ion exchanging was conducted for another 4 times as described above. The residue was washed, dried, and calcined in air at 823 K for 4 h. (3). 0.2 g A1-MCM-48 was added into 25 ml 0.02 mol/L magnesium acetate solution. After volatilization of the liquid, the residue was calcined in air at 773 K for 4 h to yield magnesium acetate impregnated A1-MCM-48, and denoted as MgO/imp sample. (4). MgO or CaO, dried at 473 K prior to use, was ground with A1-MCM-48 at a given ratio and named as MgO/mix or CaO/mix sample; some of them was then irradiated in a domestic microwave oven for 3-40 rain [10]. The finally obtained material was denoted as MgO/mw or CaO/mw sample. To characterize the resulting sample, XRD patterns were recorded on a Rigaku D/max-rA diffractometer employing CuKo~ radiation, from 0.5 ~ to 10 o for mesophase and 20 o to 60 o for alkaline-earth metal oxide detection. The X-ray tube was operated at 30 kV and 50 mA, 40 kV and 70 mA respectively. Nitrogen adsorption-desorption isotherms at 77 K were measured using a Micromeritics ASAP 2000 instrument, and the sample was activated at 573 K in vacuum line. The data were analyzed by Barrett-Joyner-Halenda (BJH) method using Hasley equation for multi-layer thickness. Pore size distribution curve of the sample came from the

1483 analysis of adsorption branch of isotherm, and the pore volume was taken at the P/Po=0.9869 signal point. The element molar ratios of components in the sample were calculated with the data investigated on a Shimadz VF-320 X-ray fluorescent spectrometer. All samples were melted in the flux agent before measurement. The X-ray tube was operated at 40 kV and 60 mA. To evaluate the basic catalysis function of sample, decomposition of 2-propanol was performed in a conventional flow-type reactor with a WHSV of 2.4/h, in the manner as described previously [11 ]. Although the reaction had been carried out for several hours, only the data of 0.5h was cited to compare the catalytic properties of the sample. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of as-made A1-MCM-48, calcined and post-treated samples, and the patterns of as-made A1-MCM-48 can be indexed as a cubic mesophase belonging to Ia3d space group [6]. Since MCM-48 is less reliable to be impregnated in electrolyte aqua solution overnight, it was post-synthesis treated to improve its stability. As evident in the XRD patterns, sharp and more peaks emerged in the range of 20 between 3~ and 6~ along with the substantially greater intensity of peaks, indicating that the stability and long range mesoscopic ordering of the pores of MCM-48 has been promoted during the posttreatment. The enhanced stability has a significant influence, as demonstrated later, on the existence of mesoporous structure in the basic derivatives prepared under rigorous conditions. Figure 2A shows the N2 adsorb-desorption isotherms of post-treated A1-MCM-48 and Cs+/ie sample. They are type IV adsorption isotherms with two hysteresis loops. The sharp loop in the P/Po range of 0.2 to 0.4 corresponds to capillary condensation within uniform mesopores, and its steep slope reflects the uniform pore size of the sample. Another larger and longer loop

~-., ..',! r !li

9~_

i/ iio '."

9

'~04

bz"

i,,,

I'C Jl~i o40'"'o403 /

IV

/

2

li

I I :

v . . O ";, r

",-,,'I-'4.: ""

4

''r.... 03 "6

8

10

2e Figure 1. Powder X-ray diffraction patterns of as-made A1-MCM-48 (curve a), its calcined sample (curve b, dotted line), and its post-treated sample with subsequently calcination (curve c, dashed line), which has been indexed.

1484

i--

700

8

A

m 600 131

7

!4

E 500

,s

o

-o 400 r ..Q

'..0 300

3

t~ "13

< 200

o 2

13_

(1.)

E 100 o 0

>

5

012'014'016 018" Relative Pressure (P/Po)

0.0

1.0

1 o lO

100 1000 Pore Diameter (A)

Figure 2. (A) N2 adsorb-desorption isotherms of (a) the post-synthesis treated A1-MCM-48 and (b) Cs+/ie sample; (B) Pore size distribution curve of the post-synthesis treated A1-MCM-48. between P/Po of 0.5 and 1.0 can be probably attributed to a wide distribution range of the inter space among the sample particles. Influence of Cs + ion exchange on the structure of MCM-48 emerged on the isotherm of Cs+/ie sample whose slope in the first hysteresis loop of appeared from 250 to 350 cm3/g STP, lowered about 100 units than that of A1-MCM-48 (from 300 to 500 cm3/g STP). The total adsorption capacity and mesoporous volume of Cs+/ie sample seems to be ascended after ion-exchange procedure, but the mesoporous structure is well maintained proven by the clear first hysteresis loop. In the BJH plot of pore volume versus pore diameter (Fig.2B), a very narrow pore size distribution of A1-MCM-48 with a pore diameter of 3.4 nm was observed, and its fwhm of about 0.6 nm indicates a well-defined and uniform pore-size distribution. Besides, this sample has a BET surface area of 983 m2/g and a pore volume of 1.07 cm3/g (see Table 1). Figure 3 show the XRD patterns of three derivatives from A1-MCM-48, Cs+/ie, MgO/imp and MgO/mw samples along with the MgO/mix sample. Among them the Cs+/ie and MgO/mix

A

:..

,,

a

d

2

4

6 2--e

8

10

20

'

3'0

'

4'0

'

2e

Figure 3. XRD patterns of the sample derived from post-treated A1-MCM-48. a) MgO/mw; b) Cs+/ie, c) 10%MgO/imp and d) 10%MgO/mix sample.

5'0

'

60

1485 samples had a XRD patterns same as that of the parent material, indicating no obvious collapse formed even though the former was treated in hot water. However, impregnation in electrolyte solution for a long time left no distinct fine structure in the 20 range of 30-6~ on the XRD spectrum of MgO/imp sample, despite the cubic mesophase still preserved enough for further tests. This disadvantage results from the effect of ionic strength to accelerate hydrolysis, since if the sample was impregnated in a concentrated solution of 0.5 mol/L Mg(Oac)2 under the same condition, only a broad peak remained in the XRD patterns (not shown). For the sample of MgO/mw, only the intensity of XRD peaks was slightly lowered as seen in Fig 3A, though the dispersion of MgO by microwave radiation was conducted in an open system and the host was exposed to moisture during the process. The use of microwave radiation seems not only energy and time efficient to disperse basic guest on mesoporous materials, but also in favor of avoiding structural damage of the support. Figure 3B reveals the dispersion of MgO on A1-MCM-48. There was no MgO phase on the XRD patterns of impregnated sample, but a small one survived on that of MgO/mw and a large one in the MgO/mix sample. Through the comparison of MgO/mw sample and the mixture, it is clear that the majority of MgO has been dispersed on A1-MCM-48 by microwave radiation, similar to that reported on zeolite KL and NaY [ 10]. What is amazing about the survived XRD peak of MgO is whose intensity kept similar in MgO/mw sample, no matter how the radiation time was prolonged to 40 min or the mass ratio of MgO to the support was halved to 0.05/1. Abundance of silica in mesoporous support seems not beneficial for dispersion of MgO by microwave radiation, since the same amount of MgO cannot be dispersed on the porous silica either. However, on the zeolite ZSM-5 with a similar Si/A1 ratio as the MCM-48, a perfect dispersion of MgO has been observed [ 10], which indicates the more important role-played by geometric structure of the support than the chemical composition for dispersion of the basic guests. Intrinsic property of A1-MCM-48 host structure is another factor for the preservation of MgO particles. As mentioned below, some tiny fragments may be caused in the channel of A1MCM-48 by microwave radiation and wrapped the MgO particles, forming an obstacle for continuous dispersion of MgO. Table 1 lists the porosity characteristics of the basic derivatives, and among them the surface area and pore volume as well as the d value is smaller than that of A1-MCM-48. The pore size

Table 1. Structural properties of A1-MCM-48 and its basic derivatives Sample d" Surface area Pore size b (nm) (m2/g) (nm) A1-MCM-48 3.9 983 3.4 Cs+/ie 3.7 850 3.3 MgO/imp 3.6 814 3.4 5%MgO/mw 3.7 612 3.9 ! 0%MgO/mw 3.8 527 4.2 a) d (211) spacing, b) BJH adsorption average pore diameter.

Pore volume (cm3/g) 1.07 0.84 0.78 0.69 0.66

1486 Table 2. Decomposition of 2-propanol over the solid catalysts Reaction condition: in N2 (20 mmol/h), WHSV=2.4/h, 0.5h. Sample Temp. 1 Conv. 2 Selectivity Sample (K) (%) (%)3 A1-MCM-48 Cs+/ie

773 100 0 5%MgO/mix 673 100 2.60 723 100 9.66 773 100 15.13 5%MgO/mw MgO/imp 723 100 0 773 100 0.38 NaX 673 100 0.70 10%MgO/mw 723 99.56 0.11 773 100 2.58 10%CaO/mix CsX 673 69.37 20.93 723 87.50 1 3 . 4 1 10%CaO/mw 773 97.56 21.58 1Temp means the reaction temperature; 2 It represents the 3 The selectivity of acetone in the products of reaction.

Temp. 1 Conv. 2 (K) (%)

Selectivity (%)3

673 99.9 0.91 723 100 0.46 773 100 1.31 673 100 0.64 723 100 4.20 773 99.97 7.31 723 100 0.84 773 100 1.08 723 85.97 11.07 773 97.60 22.22 723 100 8.75 773 100 29.69 conversion of 2-propanol.

of Cs+/ie or MgO/imp sample was unchanged, but a rather enlargement was observed on the pore size of MgO/mw sample which seems to be accelerated by loading MgO: the more MgO loaded, the more obvious phenomena appeared. The different variation in the structure of A1MCM-48 result from different preparation methods. In aqueous solution, the host decomposed exteriorly with water immersing in it. For radiation, however, microwave penetrates through the wall of A1-MCM-48 and affects both outer and inner of the host at the same time, causing more remarkable crumbling in the inner pore especially in the presence of MgO. The role of MgO played in this procedure is not clear yet but seems very important. Mesoporous materials and zeolites can be microwave-assisted synthesized [12], so it is not unusual for them to be stable in microwave radiation though their surface became very hot [10,13]. The MgO guest adsorbed microwave energy and thus interacted with the host during the dispersion process to form tiny particles in which the MgO is wrapped, but a further study is desirable. Table 2 illustrates the performance of A1-MCM-48 and the basic derivatives in the reaction of 2-propanol decomposition. A1-MCM-48 was inactive for dehydrogenation of 2-propanol but active for dehydration due to the existence of acid sites. Acetone formed on Cs+/ie and MgO/mw sample at 673 K and its selectivity increased unequally with temperature, even higher than that on zeolite NaX, a typical basic zeolite (Table 2). For comparison the catalytic properties of MgO/mix were also listed in Table 2, but the acetone selectivity above 723 K was lower than MgO/mw. Although the catalysis function of MgO/mw sample for 2-propanol dehydrogenation rose totally from the introduced basic component, loading MgO of 10 wt.-% or more on the host did not enhance the selectivity of acetone in products. On the MgO/imp sample, however, no acetone was found up to 723K and only negligible acetone selectivity appeared at 773 K. Similar phenomena had been reported on the NaY sample impregnated

1487 Table 3. The component molar ratio of the basic mesoporous materials Sample Si/Cs Cs+/ie 36.65 MgO/imp 10%MgO/mw -

Si/A1 11.60 11.23 9.59

with Mg(NO3) 2 followed by calcination of 873K, on which the formed MgO was high dispersed on zeolite and lost its intrinsic catalytic properties [7]. In comparison with those zeolites loaded MgO [ 10], the resulting basicity on MCM-48 is relative weak. This difference, in our opinion, results from the geometric structure of the host. For instance, ion exchange with Cs + is a common way to enhance the basicity of molecular sieve, and the Cs +exchanged in MCM-48 presents a better catalytic performance than those derived from MgO loaded indeed. The reason is simple; every Cs § ion can act as the separate active site but the MgO disperses in the form of particle so the actual reactive area of MgO is much small according to its atomic amount. However, the basicity of Cs+/ie sample, represented by the selectivity of acetone in this probe reaction, was obviously weaker than that of zeolite CsX as demonstrated in Table 2. Microscopic structure of the surface assumedly takes charge of this difference. Basic zeolite CsX owns quasi-crystal framework, whereas mesoporous material is built with amorphous silica wall that is lack of both Br6nsted and Lewis site [ 14]. Additionally, the Si/A1 ratio of zeolite X is much lower than mesoporous MCM-48, which means the former can provide more acid sites than the latter for ion exchange. Since the concentrations of acid sites in hydrogenated mesoporous alumino-silicate are lower than the concentration of fourcoordinate aluminum [14], A1-MCM-48 host is unable to provide the anticipated number of active site for alkali matter though even its A1/Si ratio is low (ca circa 0.10). Moreover, as the component ratios in Table 3 reveals, the Si/A1 ratio of Cs+/ie and MgO/imp sample is actually higher than that of MgO/mw sample. This variation results from the especial preparation process; Unlike dispersion of MgO under microwave irradiation that occurs on the surface of solid in dry circumstance, ion exchange or impregnation is usually performed in aqueous solutions and therefore dealumination may take place in some extent. As the result, the ratio of Si/Cs is obviously higher than Si/A1 (Table 3), indicating a relatively low concentration of Cs § is exchanged on the parent MCM-48. This is not extraordinary since Cs § can only substitute the ion on the original Br6nsted site in host, so the number of Br6nsted site provided by the host limits the amount of exchanged Cs +ion. A further proof on this comes from the report on the sample of MCM-41 and zeolite NaY loaded KNO 3 [15] in which the latter exhibited a stronger basicity than the former. Clearly the lack of acid sites in mesoporous materials makes it inefficient to introduce basicity through ion-exchange method. For these intrinsic reasons, many methods, which can be used on zeolite to create strongly basic sites, do not work on the mesoporous molecular sieves. Consulting the performance of MgO/mw sample in 2-propanol dehydrogenation, microwave radiation is proven to be one optimistic way to get more resultant basic mesoporous catalyst.

1488 Without dependence on the amount of acid sites in support, microwave-assisted dispersion can introduce more basic guest on mesoporous host than alkali ion exchange; rather, the operation is much simple without any pollution or waste. Moreover, the obtained basicity in the sample can be adjusted by choosing suitable type and amount of guest material and modifying the surface of the host [10,13]. As the proof to pursue this point further, CaO was dispersed on A1MCM-48 by microwave radiation to replace MgO, and formed basic sites on the mesoporous material. As expected, these new basic sites exhibited a high catalytic activity for dehydrogenation of 2-propanol therefore more acetone formed at 773 K on CaO/mw than that on CsX zeolite, which would be discussed elsewhere in detail. From the results discussed above some conclusions can be made. Hydrothermal postsynthetic treatment significantly enhanced the stability of A1-MCM-48, so the host can keep its mesoporous structure in electrolyte aqua solution or microwave radiation. A considerable basicity can be introduced on the host of A1-MCM-48 by use of ion exchange, impregnation or microwave-assisted dispersion of basic guest such as MgO or CaO, possessing catalytic activity for dehydrogenation of 2-propanol. Among these methods, microwave radiation is proven to be a cost-effective and environmental benign process for preparing the basic mesoporous materials. ACKNOWLEDGEMENT We appreciate Professor Q.H. Xu and Y.Q. Liang (Nanjing University) for their helpful advice.

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359(1992) 710. 2. RT. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368(1994) 321. 3. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, and G.D. Stucky, J. Am. Chem. Soc., 120(1998) 6024. 4. K.R. Kloeststra and H. Van Bekkum, J. Chem. Soc., Chem. Commun., (1995) 1005. 5. I. Rodriquez, S. Iborra, A. Corma, F. Rey and J.L. Jorda, Chem. Commun., (1999) 593. 6. V. Alfredsson and M.W. Anderson, Chem. Mater., 8(1996) 1141. 7. J.H. Zhu, Y. Chun, Y. Wang and Q.H. Xu, Chin. Sci. Bull., 44(1999) 1926. 8. J.H. Zhu, Y. Chun, Y. Qin and Q.H. Xu, Micropor. Mesopor. Mater., 24(1998) 19. 9. Q. Huo, D.I. Margolese and G.D. Stucky, Chem. Meter., 8(1996) 1147. 10. Y. Wang, J.H. Zhu, Y. Chun and Q.H. Xu, Micropor. Mesopor. Mater. 26(1998) 175. 11. J.L. Dong, J.H. Zhu and Q.H. Xu, Appl. Catal. A, 112(1994) 105. 12. D.S. Kim, J.S. Kim, J.S. Chang and S.E. Park, Stud. Surf. Sci. Catal. 135(2001) 333. 13. B.I. Whittington and N.B. Milestone, Zeolites, 12(1992) 815. 14. J. Weglarski, J. Datka, H.Y. He and J. Klinowski, J. C. S. Faraday Trans., 92(1996): 5161. 15. Y. Chun, J.H. Zhu, Y. Wang, D.K. Sun and Q. H. Xu, Proc.12th Intern. Zeol. Conf., (Eds: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins), MRS 1999, Vol.2, 989-996.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1489

A p p l i c a t i o n o f zeolite in the health science" novel additive for cigarette to r e m o v e N - n i t r o s a m i n e s in smoke* Y. Xu, Y. Wang, J.H. Zhu**, L.L. Ma, L. Liu and J. Xue Chemistry Department, Nanjing University, Nanjing 210093, China

This paper reports the especial function of zeolite to remove N-nitrosamines along with the latest progress in new cigarette containing zeolite. N-nitrosamines in cigarette smoke can be strongly adsorbed and catalytic degraded by zeolite additive without any change in the taste of cigarette.

1. INTRODUCTION In the realm of health science, one of the new areas of study is the application of zeolites, because zeolite has the unique function of selective adsorption and catalysis. Among many efforts involved in slow release drugs [1], enzyme mimetic drugs [2], anti-tumor drugs [3] etc, a noteworthy example is the zeolite additive in cigarette to remove carcinogenic agents like Nnitrosamines [4-6]. Unlike the improved cigarette filter wrapped zeolites for adsorption of toxic chemical, zeolite additive in cigarette rod could perform catalysis when it approaches the hot zone in the burning cigarette, especially to decompose N-nitrosamines, the most active carcinogenic compounds in smoke [4-5]. Of the N-nitrosamines about half could be eliminated from the mainstream (MS, inhaled by smokers) or the sidestream (SS, the smoke formed in between puffs), and the taste of the cigarette was kept according to the results obtained in the laboratory [4]. However, many suspicions exist on the zeolite additive concerning how the Nnitrosamines are adsorbed in zeolite and if the addition of zeolite changes the properties of cigarette when the experiment is preformed in an industry scale. In this paper we try to explore the adsorption manner of N-nitrosamines in zeolite through analyzing the degradation products of N-nitrosodimethylamine (NDMA), N-nitrosopyrrolidine (NPYR) and Nnitrosohexamethyleneimine (NHMI) on different zeolites by use of GC-MS technique and * National Advanced Materials Committee of China and Analysis Center of Nanjing University financially support this subject. ** Corresponding author, E-mail:[email protected], FAX: 0086-25-3317761.

1490 TPSR (temperature programmed surface reaction) method. Moreover, the latest progress in new cigarette containing zeolite is also reported. 2. EXPERIMENTAL NaY, NaZSM-5, NaA and KA zeolites are commercially available in powder form. HZSM-5 zeolite was obtained by an ion exchange method from NaZSM-5 [7], and the surface basicity of NaY was enhanced by dispersion of magnesia with microwave irradiation [8]. NDMA, NPYR and NHMI had been purchased from Sigma and dissolved in methylene chloride [7] TPSR of N-nitrosamines adsorbed on zeolite was carried out in a flow reactor, an "on line" HP-5890 II GC with HP-5972 MSD was employed to explore how the carcinogenic compound was degraded on zeolite [6]. The test cigarette samples containing 3% of zeolite were prepared in two methods as reported by Meier et al. [4,9]. One was hand-rolled by mixing zeolite with finely cut tobacco until the mix looked perfectly uniform [4]; for another zeolite was added in solution and sprayed onto tobacco before cigarette manufacturing [9]. 4 or 30 cigarettes were smoked in the glass-made chamber designed by Caldwell [10] or Miyake [11 ], and the smoldering smoke or mainstream was collected to measure the observed content of N-nitrosamines. All of the smoke was pulled through 60-100 mL citrate-phosphate buffer at a rate of 1 L/min so that the N-nitrosamines could be absorbed. The buffer solution was extracted with methylene chloride then the combined organic fractions were dried over a bed of anhydrous sodium sulfate and concentrated to a final volume of 25mL. N-nitrosamines were chemically denitrosated and analyzed by use of a SP-830 spectrophotometer (ColeParmer)[7]. For the analysis of tobacco specific N-nitrosamines (TSNA) in cigarette smoke, Heinr Borgwaldt 20 port and 1-port smoking machines were used under the standard condition [12]. Thermal Energy Analyzer (Thermo Electron 502B) and HP 5890B GC equipped SPB-5 fused silica capillary column were employed and N-nitrosoamine-N-Propylamine (NDPA) were utilized as ISTD. 3. RESULTS AND DISCUSSION Figure 1 reveals the strong adsorption of N-nitrosamines on zeolites. When the zeolite NaY adsorbed NPYR was evacuated at 553 K, there was no characteristic Mass signal of NPYR with m/e of 100 appeared on the MS spectrum (Fig. 1A). This fact indicates the absence of NPYR desorption from the adsorbent though the temperature is close to the boiling point of the carcinogenic compound. Some fragments with m/e of 30, 41, 44 and 53 emerged on the spectrum along with the signal of water (m/e of 18) and N2 (m/e of 28), and they could be tentatively assigned to the degraded products of NPYR such as N20 or (CH3)2N§ (m/e = 44), 2methyl-l-H-pyrrole (m/e=53), and 1-nitroso-pyrrolidine (m/e of 30 and 41 ) [6]. A further confirmation came from Figure 1B in which zeolite NaY adsorbed the N-nitrosamines from tobacco and was evacuated at 514 K. No any characteristic Mass signal of NDMA (m/e =74) and NPYR (m/e = 100) [13] or TSNA such as NNN (N-nitrosonornicotine, with m/e of 178)

1491

18

160

28

,,,-->.

160

A

~" 120

=. 120

8 80 '8 ~ <

80

44

8

.~ 40

40, 0

<

2c

40

2O

0

60

i

8O

100

i

,,

20

120

160

45

,.

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6O

i

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i

100

i

120

m/e

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t

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co- 80.

o 80 r--

-2 = 40 < 00

44

28 .

_~ 40 <

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.

.

20

18

40

,

,

60 ' 80 ' 1 0 0 ' 1 2 0 m/e

00

2O

4558 40 ' 60 ' 80

i

100

i

120

m/e

Figure 1. Mass spectrum of zeolite NaY (A and B), NaZSM-5 (C) and NaA (D) adsorbed NPYR (A) or N-nitrosamines from tobacco (B-D) and evacuated at 533 K (A), 514 K (B), 573 K (C) and 529 K (D).

and NAT (N-nitrosoanatabine, with m/e of 189) was observed. Similar phenomena were also observed on zeolite NaZSM-5 and NaA as shown in Fig. 1C and 1D. Figure 2 shows the influence of zeolite pore structure on the adsorption and catalytic decomposition of N-nitrosamines. As the pore size of zeolite decreased from 0.7 nm (NaY zeolite) to 0.3 nm (KA zeolite), adsorption and degradation ofNDMA, NPYR and NHMI were dramatically suppressed. On the zeolite NaY, most of the adsorbate was decomposed between 550-670 K, and the maximum concentration of NOx in the product could exceed 0.05 mmol/g (Fig.2A). However, N-nitrosamines began to degrade near 600 K on zeolite KA, forming a small amount of NOx with the maximum concentration less than 0.0008 mmol/g as shown in Fig 2B. To explore the adsorption characteristic of N-nitrosamines on zeolite, NaA and KA were employed for their structural difference is only the pore size. On the former with pore size of 0.4 nm, NPYR adsorbed and decomposed at 713 K [6], giving off the maximum NOx concentration of 0.03 mmol/g during the TPSR process. Contrarily only 0.0003 mmol/g of NOx was detected on the latter under the same conditions (Fig.2C). This difference was also observed in TG-DTA test, in which only trace amount of NPYR was detected from zeolite KA,

1492

0.08

A

O3

Np

.0.0008-

lu liNDMA

B

NDMA _

~ 0.0006

o

E 0.06. E x o 0.04

/~

Z

~ oooo, "6 ~ o ooo~

[]

o

-~ 0.02

< 0.00 450 500 550 600 650 700 750 Temperature (K)

o

E E Ox z v

o

o

E <

NHMI zx-~-~, u ID'/~

p.O.o. / ~-O-o-\o/~

30~ ' 4d0 ' 560 ' 6d0 ' 7d0 ' 860 Temperature (K)

0.035 /k 0.030 C 0.025 NPYR on NaA/ 0.020 0.015 0.010 0.005 0.000300 400 500 600 700 Temperature (K)

Z

/on_

800

Figure 2. NOx desorption in the TPSR process of N-nitrosamines on zeolite NaY (A), KA (B), along with zeolite NaA and KA (C) via the temperature.

00100 i o E E 0.0075

; ~ Z SM-5

0.10

A

o

E 0.08~

~ Z

o oo o

o

1~ 0.0025 o

<E 0.0000

400

NaYozx /~/'5%MgO/NaY

(~ 0"06J

x

Z o

aZSM-5 VW --

.

. . . . . 500 600 700 Temperature (K)

o

800

0.04~ 0.02

E 0.00 400

'

|

'

|

'

i

--~

500 600 700 Temperature (K)

,

t~

800

Figure 3. NOx liberated during the TPSR process of NPYR on (A) zeolite ZSM-5 and (B) the zeolite NaY before and after modification of magnesia.

1493

0il ~.

r

/ I

In the main stream

=5

r

"C::

~0

1

2

3

4

5

6

7

Number of sample

"5~~8o .~ 40

s

c~.e

o

1

2

3

4

5

6

7

8

9

Number of sample

Figure 4. Variation of N-nitrosamines content in the smoke of sample cigarette before (B) and after (A) added of zeolite. Sample 1-7: Virginia type cigarette; Sample 8-9: blended type cigarette.

indicating the negligible adsorption capacity of zeolite KA for NPYR in comparison with that of NaA. N-nitrosamines molecule was assumed to enter the channel of zeolite A through the insertion of the characteristic group of-N=N-O [7], therefore its adsorption would be hindered if the pore size of adsorbent became too small, and the amount of NOx formed during TPSR process was thus dramatically decreased. Based on the facts mentioned above, the possibility of N-nitrosamines adsorbing and reacting on the external surface of zeolite seems to be excluded. Otherwise, both KA and NaA zeolite should exhibit the similar catalytic property in TPSR experiments since their external surface area is almost the same. Figure 3 exhibits how the surface acid-basic properties of zeolite affect the TPSR of NPYR. Since N-nitrosamines possess a weak basicity, the acid sites of zeolite can thus play an important role in adsorption and degradation of N-nitrosamines [6-7]. Consequently, much more NPYR were degraded on zeolite HZSM-5 than on NaZSM-5 between 400-600 K, forming the maximum concentration of NOx twice higher than that on NaZSM-5 (Fig.3A). Surface basicity of zeolite also affects the TPSR of NPYR, however, the difference shown in Fig

1494 Table 1 ......prope~!.es..0fthe Virginia type c.iga!'ette before and after adding ze01ites ...Sampl e . . . . . . . . . . . . . . . . . . . . . . . . . A 1 ..... A2 . B 1 .......... B2 Added zeolite(wt.-%) 0 3.0 0 3.0 Average weight (g/cig) 0.97 0.95 0.96 0.93 Draw resistance (mmH20) 105.35 99.55 98.55 95.10 The amount of TSNA in MS (nmol/cig) 0.68 0.44 0.76 0.30 in SS (nmol/cig) 3.01 2.25 4.38 2.96 Decrease (%) in MS 35.3 60.5 in SS 25.2 32.4

3B were fainter than that in Fig 3A. Although the basicity of 5%MgO/NaY was significantly enhanced [8], the NOx produced on the sample was similar to that on NaY zeolite. In addition, the temperature at which the maximum concentration of NOx was reached, was changed from 593 K (NAY) to 613 K (5%MgO/NaY) revealing the declined activity of 5%MgO/NaY for catalytic degradation of NPYR. Judged on these results it is very likely that surface acid sites on zeolite are beneficial for the degradation of N-nitrosamines, but enhancing the surface basicity of zeolite seems not necessary. Figure 4 demonstrates the elimination of N-nitrosamines in smoke by use of zeolite additive in cigarette. In the case of cigarette containing zeolite of 3 wt.-%, the detected total amount of N-nitrosamines was significantly reduced in both side-stream and main stream, coincided with what reported by Meier [4,9]. As seen in Fig.4, the decrease of N-nitrosamines varied from 41.9% (sample 7) to 66.7% (sample 6) in side-stream while from 53.9% (sample 7) to 81.5% (sample 6) in the mainstream depended upon the type and the composition of cigarette, which will be discussed elsewhere in detail. The results of TEA test listed in Table 1 confirm the function of zeolite additive to remove Nnitrosamines from the smoke of Virginia type cigarette, since the concentration of TSNA is obviously lowered in both MS and SS. Although the molecular volume of TSNA is larger than that of N D M A or NPYR, they are also degraded on zeolite when the cigarette is burning. On the other hand, more TSNA seem to be removed from the smoke of burley tobacco than that of Virginia tobacco, because the eliminated TSNA could reached 19.2 nmol/cig in SS according to the report of Meier [4], but it was only 0.8-1.4 nmol/cig in Table 1. One of the reasons is that the average total TSNA in burley is much more than that in flue-cured tobacco [14], but the catalytic behavior of zeolite in the different type of smoke is unclear and need to study. Considering the fact that the pore size of zeolite additive is only 0.4-0.7 nm, it is reasonable to argue that these TSNA molecules may enter the channel of zeolite by insertion with the group o f - N - N = O , if the adsorption of TSNA is the necessary step for their catalytic decomposition. Table 1 also lists some important parameters of the new cigarette containing zeolites. With the zeolite of 3 wt.-% attached to the tobacco fibers, the average weight of cigarette kept constant

1495 8O

o~ r~

bysprayingjO ~ O

60

hu mi miYi tZX~i ~ ~'~'- by xing

"g: 40

o~02o ~ 0

/ 0

,:k

1 2 3 4 5 Amountof NaY(wt.-%) (~)

,

I

'

I

'

I

'

I

Figure 5. Influence of addition manner of zeolite on the removal of N-nitrosamines in smoke.

Table 2 Taste report on the Virginia type sample cigarette* Sample Reference Added 3 wt.-% of zeolite Brilliance 3.5 3.5 Aroma 26.0 26.1 Harmony 3.3 3.1 Offensive-taste 10.9 11.0 Irritancy 11.5 11.5 After-taste 11.6 11.7 Total 66.5 66.9 * 11 adjusters in Jiangsu province (China) gave this report.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

within the error of measurement but the draw resistance deceased slightly though the true reason is unclear up to date. Figure 5 reveals the importance of addition method of zeolite in tobacco. Spraying 1 wt.-% of zeolite NaY on the tobacco could remove about 40% of N-nitrosamines from the smoke, similar to that of mixing 2 wt.-% of the zeolite. The reason, as expected, is that zeolite additive can be highly dispersed on tobacco. As the result, the N-nitrosamines in smoke, no matter whether it is the original ones in the tobacco or that formed thermally by NOx with some amines during the burning process of tobacco, could be adsorbed and/or degraded by the zeolites before they escaped to atmosphere [5-6]. Table 2 lists the taste report on the sample cigarette before and after addition of zeolite; The mark is usually proportional to the satisfactory content of taste, and the six indexes relect the style of cigarette. Judged on the data in Table 2 it can be safe to infer that addition of zeolite in the cigarette only caused a minor variation in harmony, but the total mark was still higher than the reference. That is to say, the taste and the style of cigarette are kept unchanged, which ensures tobacco industry to sell the product. The cost of zeolite additive is another absolutly

1496 important factor for the production of the environmental benign cigarette. However, the zeolites with a pore size less than 0.7 nm, as the main component of the novel additive, can be synthesized from very cheap raw materials such as clay, so the added manufacture cost for the new cigarette is inappreciable. Furthermore, an especial process to disperse the additive on tobacco has been invented and successfully applied in those experiments carried out in factory [ 15], finally the added cost for the product cigarette was lowered to about 0.01 US$ per carton. With repect to protecting the health of both smoker and non-smoker, zeolites are expected to be applied in tobacco industry for reducing unwanted components in smoke, and the reported results provid evidence that dispersing zeolites on tobacco would be an effective strategy to reduce the content of N-nitrosmines in smoke. For practical reasons and considering the complexities of the system, more investigations are desirable because only a limited number of experiments has been performed. The function of zeolites for adsorbing and degrading other carcinogenic compounds such as polycyclic aromatic hydrocarbons and radicles need to be explored, and in vitro tests can be utilized to assess the efficiency of zeolite additive in cigarette. ACKNOWLEDGE The authors are grateful to Dr. M. S. Rhee (Korea GTRI) for the help in TEA test, to Dr. M. W. Meier (ETH, Switzerland) and Dr. R.R. Baker (BAT, U.K.) for their beneficial discussion. REFERENCES

1. A. Dyer, S. Morgan, R Wells and C.J. Williams, J. Helmintol., 74(2000)137. 2. P.C.H. Mitchell, Chem. Ind., (1991) 308. 3. H.C. Weiner, Immunol. Today, 18(1997) 335. 4. M.W. Meier and K. Siegmann, Micropr. Mesopr. Mater., 33(1999) 307. 5. J.H. Zhu, B. Shen, Y. Wang and D. Yan, Chin. Sci. Bull., 46(2001) 705. 6. J.H. Zhu, B. Shen, Y. Xu, J. Xue, L.L. Ma and Q.H. Xu, Stud. Surf. Sci. Catal., 135(2001) 320. 7. J.H. Zhu, D. Yan, J.R. Xia, L.L. Ma and B. Shen, Chemosphere, 44(2001) 949. 8. Y. Wang, J.H. Zhu, J.M. Cao, Y. Chun and Q.H. Xu. Micropr. Mesopr. Mater., 26(1998) 175. 9. W.M. Meier, J. Wild and F. Scanlan, EP 0740907A1 (1996). 10. W.S. Caldwell and J.M. Conner, J. Assoc. Off. Anal. Chem., 73(1990) 783. 11. T. Miyake and T. Schibamoto, J. Chromatogr. A, 693(1995)376. 12. D. Haoffmann and I. Hoffmann, J. Toxicol. Environ. Health, 50(1997) 307. 13. B. Crathorne, M.W. Edwards, N.R. Jones, C.L. Waiters and G. Woolford. J. Chromat., 115(1975)213. 14. S. D'Andres, R. Boudoux, J-M. Renaud and J. Zuber, Presentation given at the 2001 joint meeting of the CPRESTA smoke & technology study group in Xi'an. 15. Y. Wang, J.H. Zhu, B. Shen, u Chun and Q.H. Xu, CN 99114106.7 (1999).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1497

Direct synthesis o f Z S M - 5 crystals on gold modified b y zirconium-phosphonate multilayers S. Dumml, J. Warzywoda and A. Sacco, Jr. Center for Advanced Microgravity Materials Processing, Chemical Engineering Department, 147 Snell Engineering Center, Northeastern University, Boston, MA 02115, USA Thin zeolite films are of interest in selective membranes and in dilute gas sensors. The effect of composition, pH, aging, and duration of synthesis on integrity of thin ZSM-5 films on Au sanfaces was evaluated. Substrates were prepared by sputtering Au onto mica supports, followed by modification of these Au surfaces by zirconium-phosphonate multilayers to help induce nucleation. These substrates were then immersed into synthesis solutions. When high pH (N13) solutions were used, crystallization on the substrate san'face could not be achieved due to peeling off the Au layer from the support. Low pH (--7.5) solutions resulted in large ZSM-5 crystals on the substrate stn'face, but without formation of a continuous film. Crystal growth was monitored by visually evaluating substrates after different periods of time during synthesis. Small crystals were observed on the surface at the earlier stages of crystallization. It was also observed that fully-grown crystals were formed on the substrate surface in half of the time required for the completion of crystallization (all gel consumed) in the bulk solution, and that there were more crystals on the surface earlier in the synthesis compared to when synthesis was complete. In all cases studied, crystals grew in isolated "islands"; never as a continuous film. Aging gels at room temperature had no effect on the films formed. 1. INTRODUCTION The synthesis of thin films ofzeo-type materials has attracted attention for applications in membrane separations and chemical sensors because of their uniformity of pore size and resistance to high temperatures. Various concepts/techniques used in preparation of selfsupporting and supported zeolite films have been summarized recently [1]. Most of supported films of molecular sieves on metal, ceramic, or glass surfaces have been prepared either by direct in situ conventional hydrothermal [2-6], microwave heating [7], and gas phase [8] crystallization or by hydrothermal treatment of supports with deposited seeds [912] or precursor material [13-15] in molecular sieve precursor solutions. Zeolite films have been also fabricated by embedding previously synthesized crystals in a glassy silica matrix [16]. In many instances the resulting films have shown lack of oriented crystal growth and contained crystals differing in size. Utilization of self=assembled organic layers on substrate surfaces [17] is a promising method for molecular sieve thin film synthesis with control of the orientation and size of crystals and of film thickness by anchoring the previously

1498 synthesized crystals to supports via self-assembled layers [18-20]. Using this concept, direct in situ synthesis of metallophosphate molecular sieves has been studied on the Au substrates with self-assembled zirconium phosphonate (Zr-P) multilayers [21, 22]. Zr-P multilayers have alkanethiol head groups that attach themselves on Au maqaces. At the other end there is a phosphonic acid group, which forms the surface of the film when the multilayers are assembled. Unlike many self-assembled monolayers of alkanethiols on Au surfaces, Zr-P multilayers were found to be thermally stable at high temperatures, i.e., at the synthesis temperature of ahmiinophosphate molecular sieves of 180 ~ [21]. Thermal stability of the organic interface layer on the substrate muqace is critical when a high silica zeolite is synthesized on substrates, since typical syntheses of high silica molecular sieves are usually carried out at temperatures _>150 ~ Zr-P multilayers are ideal in that respect. Despite the higher thermal stability of Zr-P multilayers compared to other self-assembled monolayers, synthesis conditions can still be harsh considering the high pH (~13) required for zeolite crystallization. Since high silica ZSM-5 has been synthesized from solutions with a wide range of composition and pH [23-25], ZSM-5 is an ideal zeolite to investigate the suitable conditions for direct synthesis of aluminosilicate molecular sieves on self-assembled organic layers. The strong interaction of the phosphonic acid group with silica has been utilized in forming Zr-P multilayers on silica surfaces, similar to how thiol interacts with Au to form multilayers on Au [26]. Since Zr-P multilayers self-assembled on the Au surface form a surface rich in phosphonic acid groups, this interaction between phosphonic acid and silica was considered to be useful for nucleation of high silica zeolites on phosphonic acid modified surfaces. Direct in situ synthesis of the aknninosilicate zeolite ZSM-5 on the Au surfaces modified by self-assembled Zr-P multilayers was investigated. Initially, the synthesis conditions for the growth of ZSM-5 crystals were explored, considering the stability of the substrate in the synthesis solution during the course of synthesis. Then, the growth of crystals on the substrate surface during synthesis was evaluated. 2. EXPERIMENTAL Substrates were prepared by sputtering 72 nm Au layers on mica supports, using a Denton Vacuum Hi-Res 100 Chromium Sputtering System, followed by self-assembly of the Zr-P multilayers on the Au surfaces. After evacuating the chamber to 2.66x10 5 Pa, the sputtering was carried out with a deposition rate of 5 A/s at 0.66 Pa of ultra high purity Ar (99.999%, Matbeson). Mica supports (0.3x40x10 ram, SPI Supplies) coated with Au (99.999%, Denton Vacuum) were directly immersed into a solution of 1.0 mM 11-mercapto1-undecanol (0.005 g) (MUD, 97%, Aldrich) in ethanol (20 mL) (99.8%, Riedel-de Haen) for 48 hours. After being rinsed by ethanol and deionized water (resistivity>IS M~'cm) to remove any molecules that were not adsorbed onto the Au surface, these Au-coated mica supports were immersed into a solution of 0.2 M phosphorus oxychloride (0.614 g) (POC13, 99%, Aldrich) and 0.2 M 2,4,6-collodine (0.484 g) in acetonitrile (20 mL) under a nitrogen atmosphere for 1 hour. Then, they were rinsed with acetonitrile (99.93%, Aldrich) and deionized water, and immersed into another solution of 5.0 mM zirconyl chloride octahydrate (0.032 g) (99.5%, Riedel-de Haen) in water (20 mL) for 2 hours. The substrates were rinsed with ethanol and deionized water, followed by immersing into a solution of 1.25 mM

1499 dodecane biphosphonic acid (0.097 g) in ethanol (20 mL) for 1 day. Finally they were rinsed with deionized water. The presence of Zr-P multilayers on the substrate surfaces was verified by immersing facedown (to avoid settling) the Au=coated mica supports with and without Zr= P multilayers into a dispersion of~1-3 ~m ZSM-5 crystals in deionized water and comparing if and where crystals attached. If they attached, it was assumed that the attachment was duo to reaction with the Zr-P multilayers. The syntheses were studied by immersing the substrates facedown into the synthesis solutions with composition A (60 SiO2:1.5 A1203:7.5 Na20:7.5 (TPA)20:1800 H20) and composition B (60 SiO2:0.1 A1203:2.5 Na20:8 (TPA)20:2400 H20), and hydrothermally treating at 150 and at 175 0(2, respectively. In a typical synthesis utilizing composition A, ahaninate solutions were prepared by dissolving 0.049 g of sodium aluminate (NaA102"0.14H20, EM Science) and 0.104 g of NaOH (97%, Aldrich) in 1.030 g of deionized water. Another solution was prvpared by dissolving 0.767 g of tvtrapropylammonium bromide (>99%, Fluka) in 4.119 g of dvioni~d water. After the solutions prepared were mixed, 1.733 g of colloidal silica (Ludox HS-40, Aldrich) was adde~ to the mixture. To prepare solutions with composition B, almninate solutions were madr by dissolving 0.003 g of sodium aluminate, and 0.030 g of NaOH in 2.502 g ofdcionized water. The other solution was prepared by dissolving 0.647 g of tetrapropylammonium bromide in 3.250 g of deionized water. Aiter the solutions prepared were mixed, 1.370 g of Ludox HS40 was added to the mixture. After mixing the final solutions, they were placed in 10-ml Teflon-lined stainless steel autoclaves. The products were filtered, washed with deioniz~d water, and dried at 80 ~ The solutions pH were typically ~13 for both composition A and composition B. For the low pH syntheses, the pH of the synthesis solutions was adjusted by adding phosphoric acid until the pH decreased to ~7.5. Verification of preferred (111) orientation of Au on mica, and the identification of synthesized ZSM-5 crystals was accomplished by X-ray powder diffraction (XRD, Bruker D5005). The appearance of the substrates and the size and morphology of the ZSM-5 crystals were imaged using scanning electron microscopy (SEM, Hitachi S-4700). Atomic force microscopy (AFM, Digital Insmanents Dimension 3000 scanning probe microscope with a NanoScopr IIIa controller) was utilized to measure both film thickness and roughness of the Au coatings. 3. RESULTS AND DISCUSSION Initially, a series of ZSM-5 syntheses was performed using Teflon-lined autoclaves without placing any substrate into the synthesis solutions, to characterize the crystals formed at various conditions of composition, pH, and temperature. Then, syntheses at the same crystallization conditions were carried out with the substrates immersed into the synthesis solutions. 3.1. ZSM-5 synthesis in the bulk solution

Bulk syntheses were performed to characterize the resulting crystals, synthesized at high and low pH, from synthesis solutions with composition A at 150 ~ and composition B at 175 ~ ZSM-5 crystals were grown in 2 days from composition A at a pH of ~13. These crystals were in the form of small aggregated particles -~1-3 9m in size (Figure la). Composition B, at a typical pH of~13, resulted in interpenetrated-twin crystals with size ~50

1500 ~tm (Figure lb) which were grown in 1 day. When the pH of the solutions of composition A was decreased to -7.5, no crystallization occurred even after 60 days. Therefore, composition A was not used in this investigation at low pH. On the other hand, lath-shaped crystals --130 jam in size (Figure lc) were grown from composition B at pH of-7.5 in 30 days.

3.2. Analysis of the substrate surfaces Figure 2 illustrates the morphology of Au sputtered on mica supports imaged by SEM and AFM. As shown in Figure 2, deposited Au formed grains-30-60 nm in size. The roughness of 72 nm thick Au layers measured by AFM was -0.8 nm. XRD patterns of the Au-coated mica showed only Au (111) reflections, thus suggesting preferred (111) orientation. Before the ZSM-5 syntheses on substrates were attempted, the presence or absence of the self-assembled Zr-P multilayers had to be determined. When previously synthesized ZSM-5 crystals (Figure la) were contacted with the substrate (mica/Au/Zr-P), the crystals attached randomly on the surface, as shown in Figure 3. When analogous experiments were performed using pure Au surfaces, crystals did not attach to the Au surfaces. This was taken to mean that the crystals must have attached to the Zr-P multilayers. Also, the randomness of attachment was the first indication that the organic layers most likely formed islands on the Au surfaces, instead of a continuous film.

i~

.

,,

,

5.~!i

:: :; 2...z

Figure 1. ZSM-5 obtained using (a) composition A at pH -13 (150 ~ pH --13 (175 ~ and (c) composition B at pH -7.5 (175 ~

(b) composition B at

1501 0.5

tam

0

larn

0.5

Figure 2. SEM (a) and AFM (b) image of the Au surface sputtered onto a mica support.

Figure 3. SEM images of ZSM-5 crystals attached randomly to the substrate surface. 3.3. ZSM-5 synthesis on the substrate surfaces

Initially, synthesis parameters were explored to obtain suitable conditions for the ZSM-5 film growth, and to determine which parameters can be changed and which are restrictive; and how they affect the crystallization. When syntheses were performed at high pH (-13), the Au layers peeled off from the mica support. This eliminated high pH synthesis solutions from use, as well as the use of composition A, since it was only possible to obtain crystals from composition A when the pH was high. Low pH (-7.5) syntheses using composition B resulted in a small number of ZSM-5 crystals with size -200 ~tm (Figure 4) randomly attached to the substrates, i.e., neither continuous film growth nor preferred orientation of crystals was obtained. Absence of a continuous film had previously been reported during direct synthesis of metallophosphates on support modified with Zr-P multilayers [21, 22]. This could be attributed to the hypothesized "island-like" formation of the Zr-P organic

1502 layers on Au surfaces. Another parameter investigated was aging of the synthesis solution at room temperature (-20 ~ prior to immersion of the substrate. A portion of the composition B, low pH synthesis solution, including the substrate, was placed into the oven at 175 ~ Another portion was aged for 2 days at room temperature before the substrate was immersed and synthesis started. There was no observable difference in the appearance of the substrates and attached crystals when these two samples were compared. A second set of experiments was carried out using composition B at low pH without aging of the synthesis solution. In these experiments, the growth of crystals was monitored at different stages of crystallization by removing substrates from solutions after predetermined times throughout the synthesis. When the substrates were taken out at the early stages of crystallization (e.g., after 8 days) there was gel attached to the substrate surfaces, but it did not form a continuous film (Figure 5a). At later stages, after 11 days, poorly developed and randomly arranged small-5-10 btm ZSM-5 crystals (Figure 5b) were found on the substrates in addition to large, fully-grown, well-developed lath-shaped crystals (similar to those in Figure 4). When the substrate was removed from the solution after 15 days, the crystals on the substrate were fully-grown, although the crystallization in bulk solution was not complete, i.e., there was the unreacted material still present in the bulk solution. These experiments, as well as those performed to complete synthesis) suggest that the number of crystals that are attached to the substrate surface at the earlier stages of synthesis is higher than the number of crystals that are attached to the surface when the crystallization in the bulk solution is complete. Therefore, it can be concluded that only some of the crystals that nucleate on the substrate surface stay attached on the surface throughout the synthesis. This would suggest that it may be very difficult to obtain a continuous film as has been observed in this study. Control experiments using Au-coated mica without self-assembled Zr-P multilayers did not result in any crystals attached to the Au surfaces. This study has shown that the presence of Zr-P multilayers resulted in directly grown ZSM-5 crystals attached to the Au surface, but continuous zeolite films were not obtained. The small number of attached crystals could be due to their large size, which may cause the crystals to fall off, and/or the island-like appearance of Zr-P multilayers self-assembled on the Au surfaces.

.

Figure 4. ZSM-5 grown on the substrate from composition B at low pH.

1503

Figure 5. Images of substrates at the early stages of the synthesis showing (a) amorphous gel after 8 days, (b) groups of small crystals after 11 days.

4. CONCLUSIONS Direct synthesis of zeolite ZSM-5 crystals on the Au surfaces was studied by changing the pH and composition of synthesis solution, duration of synthesis, and room temperature aging of solutions before immersing the substrates. The substrates were prepared by coating a thin layer of Au on the mica surfaces, followed by the self-assembly of Zr-P multilayers on these Au surfaces. At a typical pH for ZSM-5 synthesis solutions (--13), the Au layers were observed to peel off of the mica supports. Lowered solution pH (-7.5) restricted the composition range that could be used to synthesize ZSM-5 in the bulk solution, thus only one composition investigated was utilized in growing ZSM-5 on the substrates. Syntheses using lower pH resulted in large crystals of ZSM-5 on the substrate surface without formation of a continuous zeolite film. Aging of the synthesis solutions with lower pH at room temperature before placing the substrates in these solutions did not help forming a continuous ZSM-5 layer on the substrate surfaces. This was attributed to the island-like formation of the Zr-P multilayers on the Au surface, as well as to a possible gravity (size) effect. Syntheses utilizing lower pH solutions extended the crystallization time in the bulk solution from 1 to 30 days, but fully-grown crystals were formed on the substrate surface in half of that time.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support of NASA. REFERENCES

1. J. Caro, M. Noack, P. KOlsch and R. Schiller, Microporous Mesoporous Mater., 38 (2000) 3. 2. V. Valtchev, S. Mintova and I. Vasilev, J. Chem. Soc., Chem. Commun., (1994) 979. 3. C.-N. Wu, K.-J. Chao, T.-G. Tsai, Y.-H. Chiou and H.-C. Shih, Adv. Mater., 8 (1996) 1008.

1504 4. J.C. Jansen, J.H. Koegler, H. van Bekkum, H.P.A. Calis, C.M. van den Bleek, F. Kapteijn, J.A. Moulijn, E.1L Geus and N. van der Pull, Mieroporous Mesoporous Mater., 21 (1998) 213. 5. E.I. Basaldella, A. Kikot, J.O. Zerbino and J.C. Tara, in: A. Galameau et al. (Eds.), Proc. 13th International Zeolite Conf., Elsevier, Amsterdam, 2001, paper 20-P-14. 6. F.S. Scheffler and W. Schwieger, in: A. Cralarneau et al. (Eds.), Proc. 13th International Zeolite Conf., Elsevier, Amsterdam, 2001, paper 20-P-17. 7. J.H. Koegler, A. Ararat, H. van Bekkum and J.C. Jansen, in: H. Chon et al. (Eds.), Progress in Zeolite and Mieropomus Materials, Stud. Surf. Sei. Catal., vol. 105, p. 2163. 8. R. Althot~ B. Sellegreen, B. Zibrowius, IC Unger and F. Sehfith, in: M.Oeeelli, H. Kessler (Eds.), Synthesis of Porous Materials: Zeolites, Clays and Nanostructtres, Marcel Dekker, New York, 1997, p. 139. 9. L.C. Boudreau and M. Tsapatsis, Chem. Mater., 9 (1997) 1705. 10. J. Hedkmd, B.J. Sehoeman and J. Sterte, in: H. Chon et al. (Eds.), Progress in Zeolite and Mieroporous Materials, Stud. Surf. Sei. Catal., vol. 105, p. 2203. 11. V. EngstrSm, B. Mihailova, J. Hedkmd, A. Holmgren and J. Sterte, Microporous Mesoporous Mater., 38 (2000) 51. 12. I. Kumakiri, Y. Sasaki, W. S h i m i ~ T. Yamagushi and S. Nakao, in: A. Galarneau et al. (Eds.), Proc. 13th International Zeolite Conf., Elsevier, Amsterdam, 2001, paper 20-P-16. 13. M.E. Gimon-Kinsel, T. Munoz, Jr., A. Ayala, L. Washmon and ICJ. Balkus, Jr., in: M.M.J. Treacy et al. (Eds.), Proe.12 th International Zeolite Conf., Materials Research Society, Warrendale, PA, 1999, p. 1779. 14. A.M.J. van der Eerden, D.C. Koningsberger and J.W. Geus, in: M.M.J. Treacy et al. (Eds.), Proc. 12e~International Zeolite Conf., Materials Research Society, Warrendale, PA, 1999, p. 637. 15. N.B. Milestone, F. Mizukami, Y. Kiyozumi, K. Maeda and S. Niwa, in: M.M.J. Treacy et al. (Eds.), Proe. 12th International Zeolite Conf., Materials Research Society, Warrendale, PA, 1999, p. 1833. 16. T. Bein, K. Brown and C.J. Brinker, in: P.A. Jacobs, R.A. van Santen (Eds.), Zeolites: Facts, Figures, Future, Elsevier, Amsterdam, 1989, p. 887. 17. G.M. Whitesides, Scientific American, Sept. 1995, p. 146. 18. Y. Yan and T. Bein, J. Phys. Chem., 96 (1992) 9387. 19. S. Mintova, B. Schoenmn, V. Valtehev, J. Sterte, S. Mo and T. Bein, Adv. Mater., 9 (1997) 585. 20. G. Cho, I.-S. Moon, Y.-G. Shul, K.-T. Jung, J.-S. Lee and B.M. Fung, Chem. Lett. 1998, p. 355. 21. S. Feng and T. Bein, Nature, 368 (1994) 834. 22. S. Feng and T. Bein, Science, 265 (1994) 1839. 23. M.J. Eapen, S.V. Aware, P.N. Joshi, A.N. Kotasthane and V.P. Shiralkar, in: M.L. Oceelli, H. Robson (Eds.), Molecular Sieves, vol. 1, Synthesis of Microporous Materials, Van Nostrand Reinhold, New York, 1992, p. 139. 24. G.H. Kuehl, US Patent No. 4 797 267 (1989). 25. R. Kumar, P. Mukherjee, R.K. Pandey, P. Rajmohanan and A. Bhaumik, Mieroporous Mesoporous Mater., 22 (1998) 23. 26. C.D. Bain and G.M. Whitesides, Adv. Mater., 4 (1989) 110.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1505

Square root relationship in growth kinetics of silicalite-1 membranes P. Nov6.k1, L. Brabec 1, O. ~olcov~i2, O. Bortnovsky1, A. Zik~inov~tI and M. K o 6 , ~ 1. 1j. Heyrovsl~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej]kova 3, 188 22 Praha 8 Czech Republic, [email protected] 2Institute of Chemical Processes Ftmdamentals, Academy of Sciences of the Czech Republic, Rozvojov~i 135, 165 02 Praha 6, Czech Republic Conceivable limiting models of growth kinetics of polycrystalline layers involving diffusion of low molecular silicon containing species, Brownian motion of nanoparticles and their sedimentation are analyzed from the point of view of their application to the preparation of zeolite-based membranes. The activation energies derived using these models were evaluated and the effect of support orientation was quantified. A criterion of a relative importance of colloidal particle sedimentation with respect to the Brownian motion was formulated. 1. INTRODUCTION With some exceptions, diffusion limited kinetics of zeolite crystals growth is assumed to be ruled out based on relatively high activation energy of growth kinetics measured. Barrer [ 1] gives for activation energy of linear growth of zeolite crystals values between 49.4 and 65.3 kJ/mol (NaX) and for aluminum free MFI the value 46.0 kJ/mol. For growth rates of colloidal TPA-silicalite-1 particles Schoeman et al. [2] estimated values of activation energy to be z 42 kJ/mol. The above findings support the idea that the rate of species incorporation into the crystals is the rate determining step of the overall growth kinetics. At present there is a strong interest in the literature in preparation of zeolite films and layers on various supports. The key questions regard the effect of substrate on the film morphology, the kinetics of film formation and the role of colloidal particles and seeding in the layers growth. Examples of the papers on this topic are refs. [3-5]. The growth kinetics of consolidated polycrystalline zeolite layers from non-agitated clear solutions on various supports might be governed by law other than that governing a linear growth of isolated crystals. The aim of the present study has been to contribute to answering questions on (i) the time development of membrane yield, intercrystalline void space in membranes and their dependence on the temperature of synthesis batch, (ii) the effect of support quality and its orientation on the growth kinetics. The synthesis method by Kyiozumi et al. [6] allowed a good membrane manipulation to obtain quantitative characteristics of pure zeolitic layers. 2. THEORETICAL In principle there are three mechanisms of the transport of silicon containing species to the support: (i) diffusion of low molecular species, (ii) Brownian motion of colloidal particles and

1506 (iii) their motion under the action of an extemal force (in our case it is the effect of gravitation). All the above mechanisms may act simultaneously. The process of species transport to the surface would be followed in the case of low molecular species by the kinetic step of species incorporation into existing zeolitic material. This process is always accompanied by crystal dissolution. When, however, colloidal particles are being deposited on the support a rate of their attachment and detachment may also play a role. The processes behind a consolidation of contingent colloidal particles on the supports surfaces have not yet been described satisfactorily. 2.1. Relative importance FsBof the sedimentation with respect to the Brownian motion Based on the solution of the first passage time problem [7] we deduced the following formula: FsB = fi/fB

=

(1)

(1/r~)/(1/rB)

Impacts of particles moving to the support either exclusively by sedimentation or exclusively by Brownian motion take place with frequencies fi and fs, respectively (subscript s and B stands for sedimentation and Brownian motion, respectively). Time constants r~ and r8 are the respective average values of the first passage time of a moving particle through an absorbing barrier located at the support top, provided the motion of the particle starts at time t = 0 at a distance x from the support. The first passage time problem was solved under simplified conditions that there are reflecting barriers at the top of the liquid column above the support i.e. at x = L and at cylindrical autoclave wall. The averaging of the first passage times was carried out over all the initial positions x e < 0, L>. Thus, L

r~ = 1/L. f (x/vA dx) = L/(2v~) (2) o where vs stands for sedimentation velocity of spherical particles of time independent radius r

given by the Stokes law cf. e.g.[8]. The formula for rB can be obtained in a straightforward way by the modification of the procedure of the solution of the Planck-Fokker equation shown in ref. [9], i.e." (3)

rB = L 2/(3DB)

where DB denotes the diffusion coefficient of the translational Brownian motion which can be estimated for the particles of spherical shape using the theory of the motion of non-interacting colloidal particles based on Stokes-Einstein relation as [ 10] 9 (4)

D8 = RT/(NAJ)

where NA is the Avogadro number and f = 6zr/r. As r/we used the viscosity value for liquid water at 170 ~ (r/= 1.63 x 10.4 Pa.s). Thus, the resulting form of F,B criterion reads as: Fss = KsB. (Pparticle

" ,t3~,ater) .

L / T . r s . cos a

(5)

Here KsB = (8z/9)(NA.g/R), pi are the respective densities, T is the temperature of the synthesis 2 batch, L =(LB)/Ls is a characteristic distance both for sedimentation and Brownian motion.

1507 Table 1 Values of Do, to, r~ and Fso for the liqu!d column height L = 1 cm at 170 ~ particle diameter [nm]

"

,

.

.

.

.

.

10 ~6 30 40 50 .

.

.

6O

100 .....

'

2oo

1000

avs = ks. P with ks

Brownian diff. coeff. Do [cm2/s] 4 x 106 2 x 10"6 i.3 x 10 ~ 1 x 10-6 8 x 10"7 6.7 X 10-7 4 x 10-7 2x10. ~ 4 X 10s 1;46 x 107 m'~.s''

ro = L2/(3Do) [days] 1.0 2 3 4 5 6 10 19 96 ' ' . . . .

r~ = L/(2vs) [days] a 159 40 18 10 6.4 4.4 1.6 0.40 01016 . . . . . .

Fso = ro/ rs

,

.

.

0.006 0.045 0.2 0.4 0.8 1.3 6.2 48 . . . . . 6000 . . . . .

.

.

.

.

.

We took for our experimental arrangement Lo = Ls = L and a represents the angle between the normal of the support and the direction of the gravitational field. The most interesting result is the dependence of the Fso on the third power of colloidal particle size. It follows from the above analysis that there is a region of particles size with prevailing importance of Brownian motion on one side or sedimentation on the other, cf. Table 1.

2.2. Selected limiting models of layer growth kinetics It is conceivable to treat the growth kinetics of zeolitic layers using simplified kinetic models of both kinds of particle movement. (i) Assumption: The growth kinetics of crystal layer on the horizontally oriented support (yield Yh ) is controlled simultaneously by the Brownian motion to the support from a liquid column of the depth L and the rate of particle attachment to the growing layer. We consider the range of synthesis times t for which L >> Lxaz (Lae/z is the width of the mass transfer zone at time 0. The solution is subject to a radiation boundary condition at layer surface accounting for the respective rates of diffusing particles attachment and detachment [11]:

Yh = -hL exp(hZ Dt)erfc(h4r-Dt )- I + -~-77Th

(6)

where h = ko/D, ka is the rate constant of species attachment to the layer surface and D is the corresponding diffusion coefficient of a low molecular silicon containing species or coefficient Do characterizing the Brownian motion. The first term can be neglected for sufficiently large v/t. Thus, after an induction period, the function is reduced to a straight-line and the yield is Yh = V/(4D/~L 2) V/t - D/(kdL). Denoting the slope of the straight line Yh vs. V/t as k, one can estimate D as:

D - re. (kL/2) 2.

(7)

An important feature of this model is that slope of Yh VS. V/t is proportional to L s or when plotted Yh.L vs. V/t for various depths of liquid column above the support, the experimental points should fall into a narrow stripe along the straight-line.

1508 (ii) Assumption: The growth kinetics of crystal layer on the horizontally oriented support (yield Yh) is controlled exclusively by the sedimentation of colloidal particles of radius r which is invariant of synthesis time t. Thus, Yh (t) reads as:

(8)

Yh = (Vs. t)/L for synthesis time t _~ tm= = L/vs; Yh = 1 for t > tm=

For the sedimentation velocity of silicalite-1 colloidal particles in water at 170 ~ holds: vs = 2 7 1 1 ks. r with ks = 1.46 x 10 m .s-. A close value would correspond to compact colloidal silica particles. Table 1 gives an insight into the effect of sedimentation and Brownian motion. When plotting (Yh.L) vs. t, the experimental points should fall- similarly as in the case (i) into a narrow stripe along the corresponding dependence, however, the time dependence for both models is different. 3. EXPERIMENTAL The synthesis of silicalite-1 layers was carried out in 100 ml Teflon lined autoclaves. The starting molar composition was 10 SiO2" 1 TPABr 91 NaOH" 800 H20. Tosil (30 wt.% SiO2, pH = 9.0, supplied by Silchem Ltd., Czech Republic) served as the SiO2 source. SiO2 particles exhibit the size distribution plotted in Fig. 1 and measured by dynamic light scattering on a HORIBA LB-500 instrument. The bottom of the reaction vessel was covered either with mercury or with a Teflon disk. The reaction temperature was 170 or 180 ~ synthesis lasted 3 - 300 h. Membranes were characterized by SEM, helium density and XRD. Mass yields on horizontal and vertical areas were evaluated as Yh = m h / m t h e o r and Yv = mv/mtheor, respectively, where mtheor i s t h e mass of the total amount of SiO2 contained in the synthesis batch and mh, mv are masses of respective layers.

30 maximum: at 24 nm 2.5

standard deviation: 13 nm

2.0 0~ ._z,1,5

I

a 1.0 0.5 0.0

I

ex .,m~ I 0 20

40

t 60

80

100

120

140

nm

Fig. 1" Distribution of SiO2 particle size in Tosil (clear colloidal solution)

4. RESULTS AND DISCUSSION A typical membrane morphology is shown in Fig. 2. The quantity (Yh.L) was plotted vs. t m (Fig. 3). The linear form of Yh vs. t m is consistent with the growth kinetics controlled by diffusion of low molecular species or by a Brownian motion of colloidal particles. The respective diffusion model described above can be used for the growth kinetics in the region of low and medium yields (< 0.5). The full triangles in Fig. 3 belong to the synthesis on Teflon support. The importance of a diffusion or Brownian transport is also evident from Fig. 4. After synthesis at 180 ~ weighing of bottom as well as wall layers was performed and the yields Yh and Yv were plotted vs. t I/2 . The sedimentation can be excluded when considering the growth of the vertical layer, however, it cannot be neglected in the mass transport to the bottom.

1509

"

~'~.-"-.";Z',,~

-'9

" :~

-x.,~',X,'~,~- , ' -

~-+~<~.]:E}7. ........

"-'

9

~,

~,~.-"

,~

< ~<~

~{

:~

~ ~

~,~

S;;,;,,L,~ ~:"<.7.i.~s

Fig. 2: A silicalite-1 membrane synthesized at 170 ~ t o p - the whole membrane and a view to the cut; bottom- smooth side (contact with mercury) and rough side (contact with solution)

/ Q,'

o

o ,i

,$

,'o

o

E

,,'"

/

S >-

s~

E .__..

| ,i i

x A,~"

.= 0 . 3 0

0,," /

"m

/

:-f 0.40

,,'"

,,"

.ai

8

Iti ill

/

/

| 0.20

E "

| / 8o ,i~," 6

'"F"o

/0

o

x

f

,

/

,," ,,,'0 ,/

//

4

t ~r2 [h~2]

Fig. 3" Yh.L for L= 9 mm (X), 16 mm (OHg, A-Teflon) and 31 mm (O) (at 170 ~

0

,/

O" 0 ,0 o

,,'"(3 I

'

I

'

2

t lr2

'

[h wx]

'

14

16

Fig. 4: Synthesis at 180 ~ yields from the bottom ( 0 ) and the wall ( 0 )

1510

4O00

o)

3000

smooth side

2000 1000

(o40)

1

10

'

I

'

20

:::it .c_

(060)

(0100)

I

30

I 10

20

10

20

'

I

40

'

1

50

rouo.e 30

40

50

2O00

1000

0

I

'

I

30

'

I

40

'

I

50

2e

Fig. 5: XRD of the membrane synthesized at 170 ~ h and that of a silicalite-1 powder The XRD pattern of the upper (rough) and bottom (smooth) side of the zeolite layer grown on mercury and that of a silicalite-1 powder sample are shown in Fig. 5 and indicate that the crystal layer consists of a

pure silicalite-1 phase. XRD pattern of the mercury side of the membrane clearly demonstrates the preferred orientation of crystals where the most of b-axes of the crystals are perpendicular to the mercury plane, i.e. the ac-faces of the crystals are parallel with the Hg surface. 0k0 diffraction lines (020, 040, 060, 080 and 0100) of the mercury side of the layer are more intensive in comparison with those of the solution side which are almost identical to these found in the XRD pattern of silicalite-1 powder. This indicates that the crystals of the solution side of the layer are not oriented. A diffusion coefficient was estimated fi'om the experimental slope of Yh vs. t I/2 according to the formula (7) as D - 1.5 x 10-6 cm2.s1. This value is practically the same for 170 and 180 ~ and it is at least one order of magnitude lower than that calculated for monomeric Si species at synthesis temperature. This estimation was made using diffusion coefficients of monomeric Si species measured by Jander and Jahr [12] at room temperature and extrapolating the data to 170 ~ using the activation energy Eo = 10 kJ/mol. Thus, rather a Brownian motion of nanoparticles is more probable to represent the determining step of crystal growth rate than a diffusion of low molecular species. Our value of D corresponds to the particle size of ca 25 nm (of. Table 1).

It appears that aider decreasing the reaction temperature to 160-140 ~ the activation energy of growth (calculated with the assumption that a diffusion process is still rate controlling) increases and reaches the value range (> 40 kJ/mol) given in the literature for a kind of a surface reaction. Such a behaviour suggests that in this temperature region a consecutive combination of two rate steps affect the layer growth. 5. CONCLUSION The silicalite-1 layer growth rate at 170-180 ~ on vertically oriented support is most likely controlled by Brownian motion of nanoparticles. The mass yield on horizontal area increases more rapidly with time due to an additional contribution of colloidal particles sedimentation. These horizontal yields are inversely proportional to the height of solution column. Mass yields on mercury and Teflon do not differ.

1511 ACKNOWLEDGEMENT

This study was supported by Grant Agency of the Czech Republic via grants No. 203/99/0522 and No. 104/01/0945. The authors thank to Mr. A. Gaba (Labimex, Prague) and Mr. H. Wachernig (Retsch, Haan) for arranging the measurement by dynamic fight scattering. REFERENCES

1. R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, pp. 152-153. 2. B.J. Schoeman, J. Sterte, J.-E. Ottersted, Zeolites 14 (1994) 568. 3. V. Valtchev, S. Mintova, L. Konstantinov, Zeolites 15 (1995) 679. 4. J. Hedlund, S. Mintova, J. Sterte, Microporous Mesoporous Mater. 28 (1999) 185. 5. V. Valtchev, S. Mintova, Microporous Mesoporous Mater. 43 (2001) 41. 6. Y. Kyiozumi, F. Mizukami, K. Maeda, S. Niwa, Advanced Materials 8 (1996) 517. 7. G.H. Weiss in Advances in Chemical Physics vol XIII, I. Prigodine (ed.), J. Wiley, London, 1967. 8. R.J. Stokes, D. F. Evans, Fundamentals of Interracial Engineering, Wiley-VCH, New York, 1997, p. 98. 9. N.N. Tunicky, V.A. Kaminsky, S. F. Timashev, Metody fizikokhimicheskoi kinetiky, izd. Khimia, Moskva, 1972, pp. 14-23. 10. T.G.M. van de Ven, Colloidal Hydrodynamics, Academic Press, London, 1989, p. 70. 11. J. Crank, The Mathematics of Dii~sion, Clarendon Press, Oxford, 1956, pp. 34 - 37. 12. G. Jander, K.F. Jahr, Kolloid-Beihefie, W. Ostwald (ed.), vol. 41, T. Steinkopf, Dresden 1935.

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1513

Transport characteristics o f zeolite m e m b r a n e from d y n a m i c e x p e r i m e n t s Arlette Zikfinovfi b, Bohumil Bernauer a*, Vlastimil Fila a, Pavel Hrabfinek a, Jifi Hradil r Vladislav Krystl a and Milan Ko6ifik b

a

Institute of Chemical Technology, Technickfi 5, 166 28 Praha 6, Czech Republic

b j. Heyrovsk2~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, 182 23 Prague 8, Czech Republic c Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsk2~ Sq. 2, 162 06 Prague 6, Czech Republic The analysis of the Wicke-Kallenbach dynamic experiments in semi-open arrangement showed that for single component and binary mixture permeation the effective diffusivities can be estimated from responses to step changes in partial pressure of components at the cell open side. The various techniques were used to study single component permeation of nitrogen and binary nitrogen - methane mixture and to evaluate M a x w e l l - Stefan diffusion coefficients of the species. Comparison of the results from single component permeation and counter-current binary permeation is presented.

1. INTRODUCTION Prediction of the separation performance of molecular sieve materials requires knowledge on multicomponent diffusion and adsorption parameters. Measurements of transport related characteristics of microporous zeolitic matrices using uptake sorption techniques such as gravimetric or piezometric ones fail frequently due to very low overall time constants of transient processes and also to rate mechanisms superimposed on intracrystalline diffusion. Consequently there exist orders of magnitude differences between values of transport parameters measured with different techniques. These techniques can be divided in macroscopic (or transport) methods and microscopic (or spectroscopic) ones. The macroscopic methods provide the value of flux as a function of concentration gradient. Examples of such methods are uptake measurements in a static or flow system, frequency response techniques or various modifications of chromatographic methods. Examples of microscopic techniques to measure self-diffusion of components in the absence of a concentration gradient are the NMR methods, quasi-elastic neutron scattering or light scattering1.

Corresponding author. E-mail: [email protected]

1514 A large effort in a development of zeolite-based membranes offers new routes to investigation of zeolitic transport cf. e.g. 1-3. Permeation through a membrane composed of randomly oriented and intergrown non uniform zeolitic crystals is different from permeation in a single crystal and the diffusion in intercrystalline voids can interfere with true intracrystalline transport. For zeolites such as those of MFI type which contain after synthesis in their structure molecules of organic species there is a simple test of zeolite membrane integrity based on impermeability of as-synthesized zeolite layers. Thus, e.g. a long-time invariability of an air pressure difference with respect to atmospheric pressure in a vessel separated from the ambient atmosphere by an as-synthesized membrane would indicate that intercrystalline voids have no significant influence on the permeation experiment. Apart from diffusion in a perfect zeolite channel system there are grain boundaries within apparent single crystals of silicalite-1. The occurrence of grain boundaries is enhanced in polycrystalline randomly oriented zeolite layers. In isolated silicalite-1 crystals there was investigated the effect of grain boundaries in detail by light microscopy4-6. In references 4'5 a counter diffusion of iodine against large organic molecules was investigated and it was evidenced that at high crystal loading the boundaries represent a leakage for mass transport. By contrast, the grain boundaries represent in the same type of silicalite-1 crystals (90~ a barrier for mass transport of isobutane 6. The membrane technique seems to be an appropriate tool to study the mass transport in zeolitic materials. A benefit would be in particular when results are directed to check the extent of crystal coalescence in zeolite polycrystalline layers and that of the interfacial area of grain boundaries in them during the synthesis. The aim of the present work is to analyze the transient and steady state diffusion and permeation measuring techniques to estimate the transport related parameters of polycrystalline zeolite layers. In the present work we focus our attention on coarse grained layers of Silicalite-1 grown on c~-alumina disks. As model sorbing species to study the zeolitic transport we selected deliberately relatively weakly adsorbing components N2 and CH4. Phenomenological model to analyze the mass transport in zeolitic matrix will be that based on Maxwell-Stefan formulation 7. 2. EXPERIMENTAL 2.1. Materials

The membrane supports used in this study consist of an asymmetric a-alumina filtration elements in the form of disks Q 20 mm, thickness of 3 mm prepared in our laboratories in cooperation with Carborundum Electric company. The details of synthesis and characterization can be found elsewhere 8. The body of the disk consisted of two layers:(i) ~ 2.8 mm in depth with average pore diameter of 12 ~tm and (ii) ~ 0.2 mm in depth of the a-alumina grain size 3 ~tm and a maximum defect of the radius 0.8 gm (estimated by the bubble point method). The magnitude A of the area of the composite membrane exposed to the gas phase was A = 3.0x 10-4 m 2. Silicalite-1 layers were synthesized on the surface of the supports following the procedure by Giroir-Fendler et al. 9. The synthesis mixture was composed of silica (12g of Aerosil 380, Deggusa), 100 ml 1.0 M solution of TPAOH (Fluka). The synthesis took place in Teflon lined stainless steel autoclaves. Disks were fixed in Teflon holders to keep them in vertical position and favored the silicalite-1 growth on the side containing the fine pores. Synthesis and crystallization took place at 180~ for 72 hours.

1515 The calcination in air flow 300 ml/min was performed to remove the template using temperature program: sample was heated 0.5~ to 550~ kept at 550~ for 24 hours, cooled 2~ to 30~ 2.2. Measurement of permeation flows The experimental arrangement used in this study was as follows: 1) A standard Wicke-Kallenbach cell (W-K cell) operated in steady state is shown in Fig.

9

•(2) )

1 The measured quantities of the system for the open W-K cell are the input (P/o1),~ ;o

t P/~I)~-~

~

e/~2)I

I Figure 1. Schematic representation of Wicke- Kallenbach cell.

and output (~o),fi(2)) partial pressures of component i in the compartments (1) and (2) of the cell and the corresponding input (Fo('), Fo(2)) and output ( f ( 1 ) , f (2)) total molar flow rates. The global flux densities of the species (Ni) can be calculated from mass balances of the cell. 2) A semi-open W-K cell was constructed to observe transient processes exclusively. Its operation can be described using the schematic representation of the standard W-K cell depicted in

V(2)

Figure 1. The compartment (2) was closed i.e. Fo(2) = 0 . Volume of the compartment (2) was stirred with a magnetically driven propeller and was equipped with a septum to take samples for GC analysis 9The membrane was tightened with a Viton "O" ring. Sufficiently high molar flow Fo(1) ( Z Ni << Fo~ was kept up in compartment (1) to maintain the gas composition at the membrane inlet identical to feed concentration 9 The validity of this assumption was checked by the examination of the transient response variability in compartment (2) with respect to the changes of inlet mixture volumetric flow rate Fo(1) . The measured responses were invariable for Fo(1) > 60 ml/min. The semiopen W-K cell was equipped with a difference pressure manometer to indicate changes in total pressure difference between the compartment (1) and (2) during the transient process. A precise barometer measured the total ambient pressure Pa. A rough estimate of the thickness L of the composite layer was made using SEM. The effective membrane thickness L was also an adjustable parameter in the model referred to below (cf. Eqs. (2) to (5)). The simulation procedure provides an adequate value of L even for transient processes very close to quasi-stationary ones i.e. for low values of the parameter H defined by Eq. (1). 3) The non-stationary permeation of gases through the membranes was also measured using a semi-open permeation apparatus with a reservoir of the testing gas of the volume V = 2.0x 10 -3 m 3 and with the gas pressurized at experiment beginning to the pressure difference A Po ~ (0.5, 5 ) bar over Pa. AP(t) was monitored by a precise gauge. 3. RESULTS AND DISCUSSION Results of non-stationary diffusion measurements of binary mixtures N2 - CH4 performed at 298 K in the semi-open W-K cell under counter-diffusion conditions are

1516

1 -

A g 2 ) c n 4 (t) / A g l ) c n 4 (t) iil

i 00

.0.51

t 4000

t Is] 6000

-1

-1.5 -2

-

-2.5 -

9

-3-3.5

9

_

-4 -4.5

_

-5

80 00

exemplified in Figure 3 as the plots In {1 A P(2)CH4 (t) / A P(1)CH4 (t)}. The data are based on transient responses of the CH4 content in the volume U 2) to a step change in the composition in chamber (1) from the CH4 molar fraction XCH4 O - ) "- 0 ( f l O W of pure nitrogen) to XCH4(O+) -- 0.2 (flow of the corresponding mixture nitrogen + CH4). Overall change A Pz-(t) was found to

Figure 2 In {1 - A P(2)CH4 (t) / A pa) CH4 (t)} -- t plot for diffusion of C H 4 against N2 through silicalite-1 membrane. Results from two identical measurements are presented

(m,~).

be less than 0.05 x Ap(1)CH4(0+). Thus, we neglected at this stage the pressure change. It can be seen that the plots exhibit in a broad interval of the 1 - A P(e)ci-i4 (t) / A pr (t) values i.e. between 1x 10-2 and 5x 10 .2 a linear shape y = - a t. This is the consequence of a low value of the parameter H (cf. Eq. (1)) which represents the ratio of the amount of sorbing species accumulated at equilibrium in the membrane to that accumulated in the volume V(2) = 68 cm 3.

HCH4 = LA(K~

(2)/RT) = 1.13x 10.2 << 2

(1)

As an estimate of L we took L = l x l 0 -4 m and (K~ CH4)t-- q satbCH4 is the initial slope of the sorption isotherm (of Langmuir type) for CH4 on silicalite-1. Based on data by de Graaf et al. 2 we used the value q satbcH4 = 1.03x 10 -2 mol/(m3.pa). The estimation of Maxwell-Stefan surface diffusion parameters was performed using the Vignes interpolation formula for D1213 and IAS (Ideal Adsorbed Solution) model 14. Calculation of the theoretical response curves for nonlinear regression requires the simultaneous solution of the following set of partial differential equations for each component. The component balance in the zeolite layer

OO-'---'~i= - - 1 L 0~"

[zm(~)i ]

(2)

Z m OX

where 0/, ~ and z represents the surface occupancy by i-th species, dimensionless flux of i-th species and dimensionless distance measured along the normal to the membrane, respectively.

1517 The relationship for the flux is derived from the generalized Maxwell-Stefan equations applied to the diffusion in microporous structure7.

-- Z

dO/ = ~ OJ(~')i'-~--'Oil~ -- J ~-lffs dz j=l a-" tj

j=l

i= 1,2,....N

(3)

j~i

Oln(pi)

F 0. = 0; ~

(4)

c30j

Boundary conditions were formulated as the balance equations for ideally mixed compartments (1, 2) and in the most general form can be written as

dYli dr 2i

dr

o

= a l (Yli -- Yi )

E

+ Pl (~)i(Z -" O) I

Yi

NCOMP

Z (I:)j (Z

-" 0)

j=l

NCOMP

=a2(Y2i-Y2;)+fl2 ~ , ( z = l ) + y , ~ / ( z = l )

(5a)

1

(5b)

j=l

where aj,fl/ are dimensionless parameters and Yli,Y2i,Y,,Yzi are gas phase molar fractions of i-th component in outlet and inlet mixture, respectively. The numerical solution of the set equations (2) - (5) was calculated by finite element orthogonal collocation method using 5th order Legendre polynomial basis and collocation constants were computed by FORTRAN routine JACOB115. o

o

0.20

Y2,CH4 0.15

dllk

A W

e

0.10 0.05 0.00 ,~ 0

I

I

I

I

30

60

90

120

150

t[min Figure 4 Molar fraction of CH4 in permeate (compartment (2)) as a function of time. Experimental data (,) and model prediction based on the optimized diffusion parameters presented in Table 1( ~ ) .

1518 The coefficient of thermodynamic factor matrix F (equation (4)) were calculated by analytical expressions presented elsewhere 14. The resulting system of nonlinear algebraic/differential equations was solved by DDASSL 16 or DDASPK 17 packages. The model was used in nonlinear parameter estimation which was done by GREG package ~8. Examples of estimated parameters are presented in Table 1. Figure 4 gives a comparison of the experimental and calculated molar fraction of CH4 on permeate side of membrane. Table 1 Estimated diffusion parameters species methane (counter-diffusion) nitrogen (counter-diffusion) 0

~lP(t)/APo -0.5

-1.5

~;//~: (s -1) 30.1 + 1.7 15.6 + 4.8

t [s] t )00

The value of the parameter L obtained by simulation is L = 4.1 x 10-5 m. The evaluation of data from semi-open permeation apparatus is reduced to the evaluation of the slope - a = - l/(Pls)i of the asymptote straight line to the dependence In AP(t)/A Po for large times. The quantity (~t~s)i represents a contribution to the overall

-2.5

Figure 3. lnAP(t)/A Po vs. t for permeation of N2 through silicalite-1 membrane at AP0 = 1 bar, T = 25~ time constant of the transient process due to the quasi stationary state in the system. Two necessary conditions have to be fulfilled to obtain the linear plot of In AP(t)/A Po vs. t in a form of a straight line y = - a t for all t _> 0 : (i) the sorption capacity of the membrane for the tested species is negligible with respect to the reservoir accumulation capacity of the volume V and at the same time (ii) the permeability of the membrane does not change during the transient process as a result of species immobilization. Figure 2 shows the results of permeation measure-ment of N2. For the total time constant (pJ)i of the transient permeation process in the region of a linear sorption isotherm we deduced the formula (1) consistent with formulas presented for macroporous bodies 1~ 9 (l-t0i = L2/2Div + L2/ (DivHi )

(6)

Here L denotes an effective thickness of the zeolite layer, D i v is the diffusion coefficient of the species i through the membrane layer in the Fickian approach and Hi is defined as:

Hi = LA (Ki)t /(V/RT)

(7)

1519 It represents the ratio of the amount of sorbing species accumulated at eqilibrium in the membrane (of the volume LA) to that accumulated in the volume V. (Ki)t is the slope of sorption isotherm at the partial pressure Pi - Pa of the permeating species. For sorption descibed by Langmuir type isotherm equation (7) makes possible to estimate the upper bound to Hi which corresponds to the condition Pa = 0 (evacuation of the line at the membrane outlet). Thus, the estimate of (KN2)t for N2 is (K~ = qsat,N2 bN2 >- (KN2)t and qsat,N2bN2 - - 3.8x 10- 3 mol/(Pa.m 3) 12 and the estimate of HN2 i s "

(8)

HN2 < 5 . 3 3 x 1 0 -5

This result implies that the first term in equation (6) which characterizes the nonstationary diffusion in the membrane is negligible as compared to the second one. The evaluation of the data from the experiment exemplified in Figure 3 is as follows: From the slope of the plot one obtains a = 1.614x 10 .4 s and then using equation (9) the quantity kN2/L. Here ku2 stands for the permeability of membrane for pure N2: kN2/L = ~. V/(ART) = 4.34x 10 -7 mol/(s.m2pa)

(9)

The corresponding value of the parameter kN2/L 2 c a n be estimated from (9) using the thickness L = 4.1 x 10 -5 m obtained from the above diffusion experiment: kN2/Z 2

=

(10)

1.059x 10 -2 mol/(s.m 3 Pa)

and the corresponding value of the quantity DN2v/L 2 c a n DN2v/L 2 = kN2/(L 2 Ks

) =

be estimated as"

3.03 s-1

(11)

Here Ks (Ks = 3.49• .3 mol/m3/pa) is the initial slope of the secant of the sorption isotherm between the pressures at the inlet membrane side PN2 = 2x 105 Pa and that at the outlet pressure side PN2 = 1• 105 Pa at the beginning of permeation process. The Maxwell-Stefan diffusion coefficient Div is related to the Fickian diffusion coefficient by the relation 7 : DiV = D i z /I~i = Div / ( 0 lnPi / 0 lnqi)

(12)

Using the Langmuir isotherm one obtains for 1--'N2the formula: 1-'N~ = ~

1

(13)

1--0N2

where

|

= -

qN 2 -

q sat,N2

Thus, at the membrane inlet (PN2 = 2x 10 5 Pa) 1"N2(0) "- 1.192 for t = 0 and at the experiment membrane outlet (for any t _>0 PN2 = lxlO 5 Pa) 1-'N2 = 1.087. Taking the mean value ofI-'N2 = 1.14 one obtains DN2v/L 2 = 2.66 s -1 .

1520 4. CONCLUSIONS The Maxwell-Stefan diffusion coefficient of N2 obtained from transient permeation experiments was found to be reproducibly lower as compared with that obtained from transient diffusion experiment with the mixture C H 4 - N2. This result was obtained at the conditions that a contingent change of the total pressure in the chamber (2) of the W-K cell were neglected. Thus, for the diffusion coefficient of N2 in silicalite-1 gives permeation technique of single component the value 4.47x 10.9 mZ/s whereas the diffusion measurements in binary mixture the value 2.62x10 8 mZ/s. For diffusion coefficient of CH4 one obtains from diffusion measurements in binary mixture the value 5.05x 10-8 mZ/s. ACKNOWLEDGEMENT

This research was supported by the Grant Agency of the Academy of Sciences of the Czech Republic as Grant No 4040901 and Grant Agency of Czech Republic, grant no. 104/01/0945. Professor W.E.Stewart (University of Wisconsin) and his coworkers are acknowledged for permission to use GREG package in our parameter estimation problem. REFERENCES

1. J. K~irger and D.M Ruthven., Diffusion in zeolites and others microporous solids, Wiley, New York, 1992. 2. J.M. van de Graaf, F. Kapteijn and J. A. Moulijn, AICHE J. 45 (1999) 497. 3. F.Kapteijn, J. A. Moulijn and R. Krishna, Chem. Eng. Sci. 55 (2000) 2923. 4. M. Ko~ifik, J. Komatowski, V. Masafik, P. Novfik, A. Zikfinovfi, J. Maixner, Microporous and Mesoporous Materials 23 (1998) 295. 5. V. Masafik, P. Novfik, A. Zikfinovfi, J. Komatowski, J. Maixner, M. Ko~ifik, Collect. Czech. Chem. Commun. 63 (1998) 321. 6. O. Geier, S. Vasenkov, E. Lehmann, J. K/irger, R.A. Rakoczy, J. Weitkamp, Stud. Surf. Sci. Catal 135 (2001) 154. 7. R. Krishna, Chem. Eng. Sci. 45 (1990) 1779. 8. J. Pavlfi, J. Kudovfi, A. Zikfinovfi, M. Ko6ifik, P. Uchytil, O. Solcovfi, J. Ro6ek, V. Fila, B. Bemauer, V. Krystl, P. Hrabfinek, Chemick6 listy (in Czech), submitted. 9. A. Giroir-Fendler, A. Julbe, J. Ramsay, J.-A. Dalmon, Patent WO 95/297/751. 10. A. Burghardt, J.M. Smith, Chem. Eng. Sci. 34 (1979) 267. 11. D. Amost, P. Schneider, Chem. Eng. Sci. 49 (1994) 393. 12. T.C. Golden and S.J. Sircar, J. Colloid. Interface. Sci. 162 (1994) 182. 13. A. Vignes, Ind. Engng. Chem. Fundam. 5 (1966) 189. 14. B. Bemauer, M. Kocirik, V. Fila, V. Krystl and J. Fulem, Computers&Chemical Engineering, submitted. 15. J. Villadsen, M. Michelsen, Solution of Differential Equations Models by Polynomial Approximation, Prentice Hall, New York 1978. 16. K. E. Brenan, S. L. Campbell, and L. R. Petzold, Numerical Solution of Initial-Value Problems in Differential-Algebraic Equations, Elsevier, New York, 1989. 17. P. N. Brown, A. C. Hindmarsh, and L. R. Petzold, SIAM J. Sci. Comp., 15, 1467(1994). 18. W.E. Stewart, M. Caracotsios, J. P. Sorensen, AICHE J. 38 (1992)641.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1521

Incorporation of zeolites in polyimide matrices P. Sysela, M. Fry(,ovfia, R. Hobzovfi~, V. Krystlb, P. Hrabfinekb, B. Bernauer b, L. Brabec c and M. K o ~ i ~ c aDepartment of Polymers, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic bDepartment of Inorganic Technology, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic r Heyrovslc~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague 8, 182 23 Czech Republic 3-Aminopropyltriethoxysilane terminated polyimide precursors (polyamic acids) and Silicalite-1 were used to prepare zeolite-filled polyimide films. The accessibility of the pores of the zeolite built in both non-treated and treated polyimide matrices was studied using sorption of iodine from the vapour phase. Transport properties of the filled films were investigated using small probe molecules of hydrogen and methane. 1. INTRODUCTION Polymeric membranes have been successfully applied in numerous processes of gas separation. Principal requirements of the membrane technologies are on increasing permeability at sufficiently high selectivity. In this respect pure polymeric membranes reached their limit [ 1]. A promissing route to membranes of improved permeabilities consists in incorporation of microporous materials into polymers. Thus, the incorporation of zeolites into a rubbery polymeric membrane may result in an improvement of its properties both in separation of gases and pervaporation [2,3]. Lower selectivies have been achieved by incorporation of zeolites into glassy polymers. One reason of this result was generation of voids at the polymer zeolite interface [4-6]. This drawback may, however, be overcompensated in many respects with outstanding mechanical and chemical stability of some high-performance glassy polymers at elevated temperatures. Aromatic polyimides (PI) exhibit very good chemical, mechanical and dielectric stability at temperatures up to 200-250~ They are mostly used in (micro)electronics, aviation industry, aerospace investigation and as polymeric separation membranes [7]. Non-porous polyimide membranes show high separation factors for separation of

1522 mixtures of permanent gases but low permeability for both permanent gases and organic vapours [8]. Asymmetric membranes [9], hollow-fiber membranes [10] and non-porous membranes [ 11] based on polyimides, poly(ether-imide)s and poly(amideimide)s crosslinked with poly(ethylene adipate), respectively, were successfully employed in separation of organic vapours from permanent gases. The aim of the present work is to examine feasibility of Silicalite-1 - PI composites which would exhibit (i) a sufficient interracial adhesion of phases, (ii) accessibility of zeolitic phase for sorbing molecules, (iii) an enhanced flow of species at reasonable selectivities. 2. EXPERIMENTAL 2.1. Chemicals

Pyromellitic dianhydride (PMDA) and 4,4"-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were heated to 180~ overnight in a vacuum before use. 4,4"Oxydianiline (ODA) (all Aldrich, Czech Republic) and p-aminopropyltriethoxysilane (APTES) (ABCR, Germany) were used as received. N-Methyl-2-pyrrolidone (NMP) (Merck, Czech Republic) and N,N-dimethylformamide (DMF) (Aldrich) were distilled under vacuum over phosphorus pentoxide and stored in an inert atmosphere. 2.2. Procedures

Polyimide precursors, polyamic acids (PAA), were prepared in a 250 ml two-necked flask equipped with a magnetic stirrer and a nitrogen inlet/outlet. PAA based on a dianhydride and ODA with uncontrolled number average molecular weight (M.) were prepared by the reaction of equimolar amounts of the dianhydride and the diamine in NMP or DMF (solid content 15 wt.%) at room temperature for 24 h [7]. A typical example of the Pl-aminopropyltriethoxysilane terminated PAA (6FDAODA) with Mn = 10000 g mol is as follows: 6FDA was dissolved in NMP or DMF and the terminating agent (APTES) was added to the reaction mixture and allowed to react with 6FDA for 2 h. 6FDA was then added and the reaction (solid content 15 wt.%) was allowed to proceed at room temperature for 24 h. Zeolite crystals were prepared according to the protocol by Kornatowski [12]. They were S ilicalite-l-90~ with S i/A1 ratio cca 350 and the lenght Lc cca 190 ~tm. The most crystals are of prismatic form with a minimum crosses and rosettes. The template (tetrapropylammonium bromide) was removed in a single stage calcination process in the flow of air (60 ml min l ) using the heating programme applied to a shallow bed of crystals. ZSM-5 was supplied by PQ Zeolites (Conteka) and NaY by VURUP Bratislava. Membranes were prepared by dispersion of the zeolite in PAA solution in NMP or DMF (by stirring for 2 h) and subsequent casting of the mixture on a Teflon substrate.

1523 After solvent removal the films were heated in subsequent steps up to 230~ for 2 h. The content of zeolites in the films was 10 wt.%.

2.3.

Instrumental techniques

Sorption experiments using Iodine Indicator Technique (involving light microscopy) [13,14] were performed with crystals embedded in polyimide matrix. Fine iodine particles placed at the beginning of the experiment into the cell represented the source of iodine vapour. The sorption kinetics was monitored by taking coloured photographs of the filled films at different contact times. The photographs provided information on uniformity of colouring process and the time interval necessary for reaching a limiting intensity of crystal colouring. Optical observations of sorption by transmission light microscopy were done with a microscope Peraval Interphako (Carl Zeiss Jena) coupled with a digital camera Nikon Coolpix 950. A sorbent loaded with volatile species was examined in a closed glass cell with a total thickness of about 2.5 mm (including the glass) and an internal space of a cylindrical shape (15 mm in diameter and 1.2 mm in depth). All the observations were performed at room temperature under air. The permeability and selectivity of the composite membranes were investigated using Wicke-Kallenbach cell and small probe molecules as hydrogen and methane [151. 3. RESULTS AND DISCUSSION

3.1. System selection The characterization of the zeolites tested in this study is given in Table 1. Distribution of both ZSM-5 and NaY inclusions in polyimide films was strongly nonuniform (based on observations by light microscopy - crystal aglomerates form dark patterns) probably due to their hydrophilic character. The most uniform distribution of zeolitic inclusions was obtained with Silicalite-1 crystals (Figure 1). We were not successful in the preparation of self-standing PI-zeolite films with PI Table 1 Characterization of zeolites Crystal length

(~m)

Silicalite- 1 ZSM-5 NaY

190 1-3 1

Pore size

Si / A 1

0.5 0.5 0.8

350 25 2

(nm)

Hydrophilicity

very low middle high

1524 based on PMDA and ODA (both with uncontrolled or controlled Mn). The most probable reason of miscarriage was PI (PMDA-ODA) chain rigidity. On the contrary, PI-zeolite films based on 6FDA and ODA (both with uncontrolled or controlled M,) were self-standing up to 50 wt.% zeolite content. All composites based on PAA (6FDA-ODA) with uncontrolled Mn (see Figure 2 for its preparation) and zeolites summarized in Table 1 exhibited very high permeabilities and very low selectivities. One reason of this result was undoubtedly a generation of interfacial voids at the polymer-zeolite interface. This phenomenon is typical for glassy polymer- zeolite combination [4].

~,,

~

.

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Figure 1. Light micrographs of PI (6FDA-ODA)-zeolite films based on p-aminopropyltriethoxysilane terminated PAA (6FDA-ODA) with theoretical M, = 10000 o g mol 1 and 10 wt,~ of a) Silicalite-1, b) ZSM-5 and c) NaY

1525

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Figure 2. Preparation of PI (6FDA-ODA) with uncontrolled Mn To improve the adhesion between the matrix and the zeolite we prepared PAA (6FDA-ODA) with controlled Mn terminated with APTES (see Figure 3 for its preparation). The theoretical Mn was 10000 g mo1-1. The imidization was accompanied with the formation of Si-O-Si bonds between the polymer matrix and the zeolite surface bearing hydroxyl groups (Figure 4). The silylation of the zeolite with APTES was studied as a tool to improve zeolite incorporation in the polyimide matrix [6].

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o c--- ~

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o

o

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/

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Figure 3. Preparation of PI (6FDA-ODA) with controlled M. terminated with APTES

1526

/

O--C2H 5

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\

O--C2H 5

OH

HO--

'-~ N--(CH 2)3--Si--OH

HO--

/

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H20 ....

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- H20

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,---~N--(CH2)3--Si--OH

\

OH

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_

,-.-.--,N--(CH 2 ) 3 - - S i - - O - -

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Figure 4. Formation of chemical bonds between polymer matrix and zeolite surface The permeabilities of hydrogen and methane of the filled PI prepared in the presence of APTES were very low. The untreated PI (6FDA-ODA)-Silicalite-1 film was tested so far. The permeability coefficient of methane was 1.2x10 q5 mol mqslPa "l and selectivity for the mixture hydrogen/methane amounted to 23. It has been also found that the zeolite channels in such filled films were practically inaccessible for iodine.

3.2. Zeolite channel accessibility The procedures to improve the zeolite channel accessibility were evaluated via iodine sorption. The PI films based on PAA (6FDA-ODA) with Mn = 10000 g mol 1 terminated with APTES and filled with 10 wt.% Silicalite-1 were used in the present study. The iodine sorption was enhanced by the following operations: a) N-methyl-2-pyrrolidone (b.p. 202~ was substituted by a lower-boiling N,Ndimethylformamide (b.p. 153~ during the PAA preparation (Figure 5a) b) Silicalite-1 was soaked with low boiling solvent before incorporation to PAA solution (Figure 5b) c) filled film was evacuated at elevated temperatures up to 200~ for 48 h (Figure

5e)

d) PI layers which cover zeolite inclusions at either membrane sides were removed by a mechanical treatment (grinding) (Figure 5 d) They are compared with the pure Silicalite-1 (Figure e) and the sample prepared from untreated sample using NMP (Figure 5 f).

1527

~~iI.I~. ,~

:

Figure 5. Light micrographs of PI (6FDA-ODA) (10000 g mol "1, APTES terminated) S ilicalite-1 films prepared under various conditions or pure Silicalite-1 (see the above text for the specification, contact time of iodine with samples was of 3 h) This work was supported by the grant CEZ:MSM 223100002 and by Grant Agency of the Academy of Sciences of the Czech Republic via Grant No. 4040901 and A 1040101.

1528 REFERENCES

1. C.M. Zimmerman, A. Singh and W.J. Koros, J. Membr. Sci. 137 (1997) 145. 2. M.D. Jia, K.V. Peinemann and R.D. Behling, J. Membr. Sci. 57 (1991) 289. 3. H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, J. Membr. Sci. 35 (1987) 39. 4. J. M. Duval, A.J.B. Kemperman, B. Folkers, M.H. Mulder, G. Desgrandchamps and C.A. Smolders: J. Appl. Polym. Sci. 54 (1994) 409. 5. I.F.J. Vankelecom, E. Mercks, M. Luts, and J.B.Uytterhoeven: J. Phys. Chem. 99 (1995) 13187. 6. I.F.J. Vankelecom, S. Van den broeck, E. Merckx, H. Geerts, P. Grobet and J.B. Uytterhoeven, J. Phys. Chem. 100 (1996) 3753. 7. C.E. Sroog: Prog. Polym. Sci. 16 (1991) 561. 8. P. Sysel, V. Sindel~, K. Friess, V. Hynek and M. Sipek, Proc. 5th European Technical Symposium on Polyimides&High Performance Functional Polymers, Montpellier, 1999, ISIM, Montpellier 1999, PVI.2. 9. X. Feng, S. Sourirajan, H. Tezel and T. Matsuura, J. Appl. Polym. Sci. 43 (1991) 1071. 10. S. Deng, A. Tremblay and T. Matsuura, J. Appl. Polym. Sci. 69 (1998) 371. 11. V. SindelfiL P. Sysel, V. Hynek, K. Friess, M. Sipek and N. Castaneda, Collect. Czech. Chem. Commun. 66 (2001) 533. 12. J. Komatowski, Zeolites 8 (1988) 77. 13. V. Masafik, P. Novfik, A. Zik~-aov~i,J. Kornatowski, J. Maixner and M. Ko~i~ik, Collect. Czech. Chem. Commun. 63 (1998) 321. 14. M. Ko~i~ik, J. Komatowski, V. Masafik, P. Novhk, A. Zik~inovfi, and J. Maixner, Microporous and Mesoporous Materials 23 (1998) 295. 15. P. Sysel, V. Sindel~, M. Mare~ek, B. Bernauer, Polyimide, Proc. 5th European Technical Symposium on Polyimides&High Performance Functional Polymers, Montpellier 1999, ISIM,Montpellier, 1999, p. 262.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1529

The formation m e c h a n i s m of ZSM-5 zeolite membranes + Yongsheng Li l, Jianlin Shi l*, Jinqu Wang2 and Dongsheng yanl 1The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai China, 200050. 2Institute of Adsorption and Inorganic Membrane, Dalian University of Technology, Dalian China, 116012 Continuous ZSM-5 zeolite membranes have been prepared by multi in-situ crystallization (MISC) and temperature-varying hydrothermal synthesis (TVHS), respectively. The membrane formation process was discussed, and the formation mechanism was proposed. During MISC under constant temperature, the crystals formed in the last synthesis grew discontinuously; they would dissolve first and thus induce more nuclei to form. At last, all nuclei grow together into integral zeolite membranes. For TVHS, crystals on the support grew into large ones by a re-nucleation and re-growth process on the crystal surface, in this way the final crystals lost their typical morphology and the intercrystalline gaps disappeared, and the dense membrane was formed. 1. INTRODUCTION During the last few years, zeolite membranes have drawn great attention due to their applications, ranging from catalysis and selective adsorption to separation and sensing of molecules. Many efforts have been reported on finding optimal conditions for high-quality membranes synthesis, especially for continuous and oriented membranes grown on various supports. Zeolite membranes are usually prepared by multi in-situ hydrothermal synthesis [1-12] or secondary growth [13]. Recently, Li et al [14] have reported that high-permeance ZSM-5 zeolite membranes could be prepared by temperature-varying synthesis. There are many parameters, which could be used to govem zeolite growth on supports during synthesis. Therefore, the nucleation and crystallization process, and consequently the membrane formation process have not been well understood and fully controlled. Some interesting results in silicalite-1 film formation have recently been reported and the formation mechanisms proposed. Nakazawa et al [15] applied field emission scanning electron microscope (FE-SEM) to +This work is supported by SINOPEC (497016) and the National Natural Science Foundation of China (50172057) "Corresponding Author: Tel: 0086-21-62512990. Fax: 0086-21-62513903 Email address: [email protected]

1530 observe the early stages of MFI (Si-ZSM-5) film formation on a quartz substrate. The results indicated that a gel layer, most probably consisting of silica/tetrapropylammonium composites, was formed at first. Then there were two crystallization routes. In one route, the spherical composites were formed, enlarged and aggregated, followed by crystallization. In the other route, the crystallization started within but then grew out of the gel layer, den Exter et al[ 16] found that the stability (crack formation) of the membrane was determined by the orientation of crystals by prepared silicatite-1 as b- and (a,b)- oriented monolayers on silicon wafers. Koegler et al [17] proposed a model for the growth of silicalite-1 crystals from zeolite synthesis mixtures. Nucleation and crystal growth occurred at the interface between gel particles and liquid, where both the silicon source and template were present in abundance. In the process of growth on a support surface, a precursor gel layer plays an important role as an anchoring site for the template molecules. Iwasaki et al [18] directly observed the growth process of zeolite crystals on various substrates using an optical microscope. It was found that the growth behavior was dependent on the nature of the substrate surface, i.e. the anchoring of the crystals. For the secondary growth, Tsapatsis et al [19] confirmed that the attachment of newly formed nuclei or growth of the crystals on the precursor film could eliminate gaps in-between the particles in the precursor seed layer(s) on the substrate. Lin [20] and co-workers studied the microstructure evolution of the supported zeolite films during calcination for template removal by high-temperature X-ray diffraction and in-situ gas permeance. They found that the MFI zeolite crystal framework shrinked during template removal at 350-500~ In contrast, after template removal, the MFI zeolite framework expanded while the substrate shrinked upon cooling. A compression stress developed in the zeolite film during the cooling process when the zeolite crystallites were chemically bonded to the support after template removal. This stress induced cracks in the supported MFI zeolite films without an annealing step prior to template removal. The above studies reported the zeolite crystal crystallization or the cracks formation during zeolite film preparation. However, the exact mechanism responsible for continuous zeolite membrane formation on porous ceramic supports was not addressed. In this work, we investigated the evolution of ZSM-5 zeolite membrane on porous ceramic supports by different synthetic routes and the formation mechanisms were proposed. 2. EXPERIMENTAL

2.1 Synthesis solution The clear synthesis solution was prepared by mixing tetraethyl orthosilicate (TEOS), n-butylamine (NBA), Aluminum Sulfate, Sodium Hydroxide and deionized water. The composition of the solution was 1A1203"100SiO2:16Na20:10000H20:25NBA. 2.2 Hydrothermal treatment ZSM-5 zeolite membranes were prepared on porous ~-A1203 tubes by MISC and VTHS, respectively. The supports used here are of 3-5pm-pore diameter, 35-40% porosity, 10mm i.d., 12mm o.d., and 250mm length. Before synthesis, the tubes were washed with 1M HC1,

1531 deionized water and dried at 100~ for a day. Then, the tube was placed in a Teflon-lined stainless steel autoclave and held vertically in the solution by the Teflon cap after each end of it was wrapped with Teflon tape. For MISC, the hydrothermal synthesis was repeated two or three times under the same conditions until the membrane was impermeable to N2 after being dried at 120 ~ for 2 days. For VTHS, the synthesis was first conducted at low temperature (130 ~ and then the autoclave was moved to high-temperature (170 ~ oven for another crystallization time. It needs only one time to prepare continuous membrane. The detailed of the preparing procedures were described in the previous study [ 14]. 2.3 Membrane Characterization

After calcination, the morphology of the membranes was examined by scanning electron microscopy (SEM) (JEOL-1200X, 40kV), X-ray diffraction (XRD) (Rikagu RINT 2000, CuK~z) and single gas permeation. 3. RESULTS AND DISCUSSION The XRD patterns of the ZSM-5 membranes prepared under different procedure are shown in Fig.l, which indicates that the membranes formed on the supports are ZSM-5 zeolite membranes. The gas permeation results indicate that the membranes are continuous. The membranes have high gas permeances, especially for those prepared by temperature-varying synthesis, the H2 permeance is up to 2.4x 106mol.m-2.sl.Pa 1, at room temperature.

O cz-A1203 * ZSM-5

t

|

,

*

,

tl,

~

0

0

!

~D

9

5

10

15

.

20

25

30

35

40

20/0 Fig. 1 XRD patterns of ZSM-5 zeolite membranes. A: MISC; B" VTHS

1532 3.1 F o r m a t i o n of Z S M - 5 zeolite m e m b r a n e s by M I S C

The SEM photographs of the membrane, which was prepared by MISC, are shown in Fig.2. From Fig.2 (A), it can be seen that the crystals in the membrane are uniform, which indicates that the crystals grew into large ones through the same process. From Fig.2 (B), we cannot find interface between the zeolite layers, although the membrane was achieved by repeated synthesis. Moreover, the thickness did not increase with the synthesis times. These show that the uniform and complete zeolite layer was obtained by a homogenous growth of all nuclei in the last synthesis.

A

B

Fig. 2 Micrographs of ZSM-5 membranes prepared by MISC. A: Top view; B" Cross section

70 60 50 40

30 ~

~o

20 10 I

1

I

I

~

2

i

3

I

i

4

Synthesis time Fig. 3 Zeolite membrane weight increment as a function of synthesis times. In order to determine the influence of synthesis time on the net weight increment of zeolite membrane, the composite membrane was weighed after each synthesis. The weight increment

1533 as a function of synthesis time is shown in Fig.3. It can be seen that the membrane weight increased by 15 percent after the first synthesis. In the second synthesis, it increased quickly, up to 60 percent. While in the third and fourth synthesis, only 20, 5 percent increment was found, respectively. This indicates that the crystals on the surface of the support grow discontinuously. In the succedent synthesis after the first one, they may dissolve first, and create proper conditions for the latter nucleation and crystallization. Thus, large amounts of crystals could form after the first synthesis. That the membrane weight-increasing rate decreased abruptly in the third synthesis shows that large coverage of crystals on the support surface has dual effects. One is that it can prevent formation of more crystals; the other is that some crystals probably grow again after their dissolving, which results in the small weight increment, and the crystals are in the same size and shape. This is consistent with what we have observed from SEM photographs (Fig. 2). The process of ZSM-5 zeolite membranes prepared by MISC, which we propose, is depicted schematically in Fig. 4. Besides the composition of synthesized sols and the synthesis conditions, the support plays an active role on the formation of membrane [21]. Thus, it is more difficult and complicated than the preparation of zeolite powder. So far, the mechanism of zeolite powder formation is still under discussion. However, as well known that the gel would form in the synthesis solution is the first stage, which is also true for the zeolite membrane, as reported by Nakazawa et al [ 15] and Koeglar et al [ 17]. The gel layer is formed first in the solution on the surface of the support (quartz or wafer substrate) by the agglomeration of primary gel particles present in solution, which are transported to the support by Brownian motion. This gel layer formation is relatively independent of the type and positioning of the support in the autoclave. The gel layer has a low template content, as have been verified with FT-IR measurement [17]. While for porous supports, some gel particles would aggregate in the pores (as shown in Fig.4 (a)), and presence of lots of crystals in the pores and pore openings demonstrated that the template content is not low. More recent 1H-13C CP/MAS studies [22] on a closely related synthesis have also suggested a close contact between TPA and the solid silicate phase. So, it is believed that the crystal growth on porous support is different from that on non-porous substrates [15,17]. High porosity of the support provides more diffusion paths for the particles in the solution moving into pores, which results in the existence of template both in the pores and the gel layer. During synthesis, the density of gel layer increased continuously and the dissolving of the gel layer proceeded simultaneously. At a stage where the gel layer formation is slowed down, nucleation will develop in the gel, both in the relative large pores and the outer surface of the support (as shown in Fig, 4 (b)). For relative small pores, few nuclei formed as only small amount of sol can diffuse into them, and the gel density is low. Additionally, aluminate ions would leach from the substrate in strongly alkaline solution, so that the content of A13+ in the pores is higher than that around the outer surface. As reported [21] that A13+ can prevent gel zeolitization, so during crystallization, the nuclei on the pore openings or on the outer surface will grow larger under the presence of large amount of gel without the hindrance of A13+. While for the nuclei in the pores, they will grow out of the gel layer. Therefore, different

1534 orientation of crystals occurred (as shown in Fig. 4(c)), which determine the pinholes and piled holes existed in the membrane. In order to eliminate the pinholes and piled holes, especially for porous supports, it needs repeated synthesis. In the second synthesis, the crystals formed in the first synthesis did not grow into dense membranes as Tsapatsis [19] has described. More favorite conditions for the nucleation on the area uncovered by crystals was obtained due to the dissolving of the crystals formed in the first synthesis, consequently induced more nuclei to form quickly (as shown in Fig.4 (d), (e)). Finally, all nuclei grow gradually into large ones. With the synthesis, the intercrystalline gaps disappeared, and the continuous membrane was formed (as shown in Fig.4 (f)). Strictly speaking, the intercrystalline gaps can't be absolutely removed by repeated synthesis, because they are determined by the nature of the support surface. Also, this is different from that of flat supports because few crystals could deposit on the outer surface of the vertically positioned support.

I

I

I

I I

~

I

I

I

~

i

I I

Crystallization

I

iGel formation

"1

support

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(b)

Crystallization Repeated J t Nuclei growth J

Nuclei formation introduction

Crystals dissolution

9

<

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(0

Repeated synthesis

(e)

(d)

The final membrane Fig. 4 Formation process of ZSM-5 zeolite membranes by MISC

3.2 Formation of ZSM-5 zeolite membranes by TVHS. During TVHS, the first stage is also the gel formation. But the gel formed more slowly at the first stage than that at high temperature in MISC. Before the temperature is raised, the formation process of the gel is the same as that at 170~ in MISC. At temperature as low as 130 ~ there are plenty of nuclei formed, as shown by Li et al [23]. However, these nuclei grow slowly. At elevated temperature, no more new nuclei could form because the material

1535 species in the solution has been consumed by the formation of large amounts of nuclei in low temperature. Therefore, these nuclei would grow into larger crystals quickly, and the continuous zeolite layer was formed. Although no isolated, new nuclei formed, as temperature was increased, repeated nucleation could probably occur on the existing crystals surface. It can be seen from the photographs of top view of the membrane (Fig. 5(A)) that the crystals grow into large ones by repeated nucleation and re-growth, so that they lost their typical morphology. Obviously, this is different from those formed at constant temperature. Especially when the synthesis sol was renewed during temperature varying process, re-nucleation on the crystal surface is more apparent. The intercrystalline gaps could be removed absolutely (see Fig.5 (C)). Therefore, it needs only once to prepare integral and dense membranes. The intrinsic difference between MISC and VTHS is that the latter do not go through crystal dissolving and crystal continuous growing process, but the re-nucleation and re-growth occurred on the crystals surface.

A

B

Fig. 5 Micrographs of ZSM-5 zeolite membranes prepared by VTHS. A: Top view; B: Cross section; C: Top view (x 5000)

1536 4. CONCLUSION The formation process of ZSM-5 zeolite membrane prepared by different synthesis routes was studied. The nature of the support determines the pinholes and piled holes in the membrane. For multi in-situ crystallization, dissolving of the crystals formed in the last synthesis could induce formation of more nuclei. All nuclei grow into large ones unifromly to form continuous membrane. But the intercrystalline gaps can't be absolutely removed by repeated synthesis. For temperature-varying synthesis, large amounts of crystals could form at low temperature. After the temperature was raised, re-nucleation and re-growth occur on the crystal surface. It needs only one synthesis to achieve dense membrane by this method. REFERENCES 1. E.R. Geus, M.J. den Exter, H.van Bekktun, J. Chem. Soc., Faraday Trans., 88(1992)3101. 2. H.H. Funke, A.M. Argo, C.D. Baertsh et al, J. Chem. Soc., Faraday Trans.,92(1992)2499. 3. M.D. Jia, B.S. Chen, R.D. Noble et al, J. Membr. Sci., 90(1994)1. 4. Y.S. Yan, M.E. Davis, G.R. Gavalas, Ind. Eng. Chem. Res., 34(1995)1652. 5. Z.A.E.P. Vroon, K. Keizer, M.J. Gilde et al, J. Membr. Sci., 113(1996)293. 6. K. Kusakabe, S. Yoneshige, A. Murata et al, J. Membr. Sci., 116(1996)3020. 7. J. Coronas, J.L. Falconer, R.D. Noble, AIChE J. 43(1997)1797. 8. R. Lai, G.R. Gavalas, Ind. Eng. Chem. Res., 37(1998)4275. 9. K. Aoki, K. Kusakabe, S. Morooka, J. Membr. Sci., 141 (1998)197. 10. K. Kusakabe, T. Kuroda, A. Murata et al, Ind. Eng. Chem. Res., 36(1997)649. 11. J.H. Dong, Y.S. Lin, Ind. Eng. Chem. Res., 37(1998)2404. 12. J.C. Poshusta, V.A. Tuan, J.L. Falconer et al, Ind. Eng. Chem. Res., 37(1998)3924. 13. M.C. Lovallo, M. Tsapatsis, AIChE J. 42(1996)3020. 14. Y.S. Li, X.F. Zhang, J.Q. Wang, Sep. Purif. Tech., 25(2001)459. 15. T. Nakazawa, M. Sadakata and T. Okubo, Microp. Mesop. Mater., 21 (1998)325. 16. M.J.den Exter, H.Van Bekkum, C.J.M. Rijn et al, Zeolites, 19(1997) 13. 17. J.H. Koegler, H.van Bekkum and J.C.Jansen, Zeolites, 19(1997)262. 18. A. Iwasaki, T. Sano and Y. Kiyozumi, Microp. Mesop. Mater., 38(2000)75. 19. M.C. Lovallo, A. Gouzinis and M. Tsapatsis, AICHE J., 44(1998)1903. 20. J.H. Dong, Y.S. Lin, M.Z.C. Hu et al, Microp. Mesop. Mater., 34(2000)241. 21. R. Lai, Y.S. Yan and G.R. Gavalas, Microp. Mesop. Mater., 37(2000)9. 22. S.L. Burkett, M.E. Davis, Chem. Mater., 7(1995)920. 23. Q. Li, D. Creaser and J. Sterte, Microp. Mesop. Mater., 31 (1999) 141

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1537

Mesoporous molecular sieves for albumin A.Y.Eltekov and N.A.Eltekova Institute Physical Chemistry Russian Academy of Sciences, Leninsky prospect 31, 119991 Moscow, Russia In presented work we studied the adsorption of bovine serum albumin (BSA) from aqueous solutions by silica gels, silochrom and porous glasses at 293 and 313 K in order to estimate the sizes of self- organized structures of protein macromolecules in bulk solution and also the temperature effect on the adsorption behavior of protein macromolecules. 1. INTRODUCTION Adsorption study of biopolymers and proteins by porous silicas is of considerable interest for possibility to estimate the characteristics of porous structure of silicas and to determine the self- organized structure ofbiopolymers and proteins in bulk solution [1, 2]. The characteristics of the adsorption of biopolymers from aqueous solutions and the adsorption of synthetic polymers such as polystyrenes, polyolefins, polyacrylates are crucial dependent on the type of the monomer unit of the macromolecular chain and the temperature. Unlike the adsorption of synthetic nonelectrolyte polymers, the adsorption of biopolymers and proteins strongly depends on the pH value and the ionic strength of the aqueous solution [2 - 7]. Many biopolymers, including proteins, form ternary and quaternary self- organized aggregates in aqueous solutions [2, 4, 6, 8]. 2. THEORY

The equation for the description of equilibrium in liquid phase physical adsorption systems was used in following form [9] nff = nmJ3(k-1)a(1-a)/[ l+(j3k- 1)a]

(1)

where na is excess adsorption value (by Gibbs) in mg/m2, nm - is limited value of total content for adsorbed protein, 13 is displacement coefficient, k is the constant of adsorption equilibrium and a is the protein activity in water solutions. The equation (1) differs from Semenchenko equation [9] by 13coefficient. The Henry constant equation

1538 K H = nO/a

(2)

for extremely dilute solution (when a ~ 0) has following form K H = nml3(k- 1)

(3)

Practically, the excess (by Gibbs) adsorption na was calculated by the formula n a = (C o - C) V / m S

(4)

where C O and C are the initial and equilibrium concentrations of the aqueous protein solution, respectively, V is the volume of the aqueous protein solution and m and S are the mass and specific surface area of silica sorbent. 3. EXPERIMENTAL In this work we studied the adsorption of bovine serum albumin (BSA) from aqueous solutions by silica gels, silochrom and porous glasses. The experiments were performed at 293 and 313 K in order to estimate the sizes of self-organized structures of protein macromolecules in bulk solutions and also the temperature effect on the adsorption behavior of protein macromolecules.

3.1. Samples We used globular protein - bovine serum albumin (BSA) (Serva, Germany) without additional treatment. Twice distilled water was used as the solvent. Table 1 summarizes the characteristics of protein. Table 1 Characteristics of BSA macromolecules: molecular mass M (Da), globule size d (nm), isoeletric point IP (pH) Protein

M

d

IP

BSA

67000

11.6x2.7x2.7

4.7-5.0

The samples of silica gel KSM, KSK2 and SO95 (GOB VNIINP, Nizhnii Novgorod, Russia), silochrom $80 (Luminofor, Stavropol, Russia) and porous glasses PS (GOB VNIINP, Nizhnii Novgorod, Russia) were used as adsorbents. The samples of PS 20 and PS30 were prepared at the laboratory headed by S.P.Zhdanov in the Institute of Silicate Chemistry RAN in St.Petersburg. The characteristics of the silica samples are given in Table 2.

1539 Table 2 Characteristics of silicas: specific surface area S (m2/g), pore diameter dp (nm), pore volume Vp (cm3/g) Silica

S

dp

Vp

KSM

520

3

0.6

KSK2

340

14

1.2

PS20

74

20

0.7

PS30

50

30

1.2

PS40

100

41

1.6

$80

100

55

1.3

PS70

45

70

1.5

SO95

24

80

0.7

PS120

30

120

1.5

PS160

23

160

1.5

3.2. Method

The adsorption experimental procedure was described in detail elsewhere [8, 9]. For adsorption study 0.1 - 0.5 g of silica sample (preliminary dried in a vacuum camera at 373 K for 2 h) and 5 ml of aqueous protein solution of a desired concentration were mixed in 10 ml ampoules. The ampoules were sealed and kept in a thermostat at a constant temperature until the adsorption equilibrium was established. The concentration of aqueous protein solution before and after adsorption was measured on an laboratory liquid interferometer. 4. RESULTS AND DISCUSSION

There are four levels of self- organization of protein macromolecules. The primary protein structure is determined by the chemical bonding (via covalent peptide bonds) of the amino acids in the protein molecule. The secondary (helical) structure is controlled by the self- organization of the polypeptide network under the action of hydrogen bonding between the peptide groups. The ternary and quaternary structures are governed by the configurations of the linear and helical sections of the protein chains (this type of selforganization is governed by the intermolecular interaction between the linear and helical sections of the protein chain). 4.1. Concentration effect

Table 3 shows the dependences (isotherms) of BSA adsorption values (n6) for three samples of porous glass on equilibrium concentration of BSA in aqueous solutions (C). The

1540 adsorption values refer to unit surface area that permits the estimation of the effect of the specific surface area of the sorbents. Table 3 Experimental values of BSA adsorption (n~ mg/m2) on silica sorbents from aqueous solutions at 293 K Silica

BSA concentration (C), m~/ml 1

2

3

4

5

6

PS160

0.77

0.92

1.04

1.06

1.11

1.07

PS30

0.46

0.58

0.65

0.67

0.68

0.68

PS20

0.13

0.15

0.16

0.18

0.21

0.20

Note that the adsorption values are largely determined by the size of pores in the porous glass and by self- organized structure ofBSA macromolecules in aqueous solution. The excess adsorption of BSA from aqueous solutions on the negatively charged surface of three porous glasses is positive and increases with the increasing of the pore size in the adsorbents. Sieve effects were observed for the interaction of high molecular polystyrene molecules on mesoporous silicas [2, 8, 9]. The adsorption values of BSA on all three porous glasses in the range of equilibrium concentration from 3 to 6 mg/ml are practically independed of the equilibrium concentration. This suggests that the accessible part of the surface of the porous glass is saturated with protein. The adsorption of proteins from dilute aqueous solutions on the surface of solids has been examined in detail in many works [ 10 - 18]. It was found that electrostatic forces essentially influence the adsorption interaction between charged protein macromolecules and a charged adsorbent surface. Protein macromolecules, as a whole, are hydrophilic in aqueous media; however, they contain hydrophobic sites. Electric charged at the surface are neutralized by counterions from the ambient aqueous solution, resulting in the formation of an electric double layer. When a protein macromolecule approaches the surface, the electric double layers can overlap with each other. As a result, the electric charges are reversibly redistributed, and ions from the bulk solutions penetrate into the adsorption protein layer. The transport of ions is accompanied by variations in the thermodynamic functions. It was experimentally found [5] that a moderate number of ions from the solution were sufficient to neutralize the charge at the points of contact of a protein molecule and the adsorbent surface. During adsorption, the degrees of hydration of the protein macromolecule and the adsorbent surface may change [10 - 13]. As a result, the hydrophilic parts of the macromolecule and the sorbent surface retain their initial states, whereas the hydrophobic sites are dehydrated, thereby promoting the hydrophobic interaction between the protein and the adsorbent surface. The process of dehydration results in a significant decrease in the Gibbs free energy of the adsorption system.

1541

4.2. Adsorption constant Table 4 shows the comparison of BSA adsorption parameters from equation (1) and (2) for three porous glasses. Table 4 BSA adsorption parameters: limited adsorption value n m (mg/m2), constant of adsorption equilibrium 13k and Henry constant K H (ml/m2) Adsorbent

nm

13k

KH

PS160

1.15

1300

1.47

PS30

0.72

1100

0.85

PS20

0.18

1100

0.21

The results in table 4 show good correlation the adsorption parameter n m with the experimental adsorption values for all three porous glasses (see table 3). Note that the values of adsorption equilibrium constant 13k are close for all three porous glasses. Hence the constant of adsorption equilibrium for BSA-water-porous glass systems does not strongly depend on a pore size of adsorbents. On the contrary the values of K H are more sensitive to the pore structure of adsorbents and change in the same manner. The migration of protein macromolecules from the bulk solution to the liquid-solid interface can be accompanied by a rearrangement of its structure [14, 15]. In a bulk solution, or far from the region of action of adsorption forces, hydrophobic parts of the protein are hidden in the interior of the macromolecule. Intramolecular hydrophobic interactions keep the secondary structure of the protein in the form of helices and layers. Near the adsorbent surface, the hydrophobic sections of the macromolecule tend to approach hydrophobic sites at the adsorbent surface. When the intramolecular hydrophobic interactions in a protein macromolecule are replaced by intermolecular hydrophobic interactions, the secondary protein structure may change with an increase in the conformation entropy of the protein molecule. Using Fourier-transform IR spectroscopy [ 10] and evanescent fluorescence spectroscopy [ 11 ] has been found that the adsorption of a protein was accompanied by changes in its secondary structure.

4.3. Temperature effect Table 5 shows the dependences of the maximum values of the excess adsorption for BSA from aqueous solutions by porous silica samples at 293 and 313 K (isopycns). Table 5 Maximum amounts of BSA adsorption (mg/m2) on silica sorbents from aqueous solutions at 293 and 313 K Silica

KSM

K S K 2 PS20 PS30 PS40 $80

PS70 SO95 PS120 PS160

dp, nm

3

14

20

30

41

55

70

80

120

160

293 K

-0.01

0.02

0.18

0.72

1.05

1.07

1.16

1.17

1.13

1.15

313 K

0.01

0.06

0.41

0.78

1.14

1.18

1.44

1.48

1.47

1.43

1542 Note that the maximum adsorption values are largely determined by the size of pores in the porous silicas. The maximum values of the excess adsorption of BSA from aqueous solutions increase with an increase of pore size of silica adsorbents. The fact that the excess adsorption of BSA from aqueous solutions on the porous silica KSM at 293 K is negative can be explained by the electrostatic repulsion of protein molecules and by the sieve effect (negatively charged solvated BSA macromolecules, whose average at 293 K size exceeds 12 nm, cannot penetrate into pores of KSM silica sample). Because of this, the pores accumulate water molecules, which leads to an increase in the concentration of BSA macromolecules in the bulk solution. Sieve effects were observed for the interaction of high molecular polystyrene molecules on mesoporous silicas [8, 9]. The fact that the isopycns level off to form a plateau is indicative of the formation of dense adsorption layers at the hydroxylated surface of silica. These layers may be as thick as 1.0 nm. At 293 K and pH 7.0, the maximum adsorption values for BSA on silicas with pores larger than 40 nm in diameter are similar, 1.1 - 1.2 mg/m2. At 313 K and pH 7.0, the maximum adsorption values for BSA on silicas with pores larger than 40 nm in diameter are similar, 1.15 - 1.18 mg/m 2, and the second plateau of maximum adsorption values on silicas with pores larger than 70 nm in diameter are similar, 1.4 - 1.5 mg/m2. Obviously the adsorption of BSA from aqueous solutions by silicas is accompanied by changes in protein self-organized structure in bulk solution under a high temperature. A protein molecule is attached to the adsorption surface by different forces. Since proteins are polyelectrolytes, they carry both positive and negative charges. The positive charges are thought to be placed at the NH+3 groups in the peptide chain, while the negative charges are located at the COO- groups. Because of this the electrostatic interaction primarily manifests itself during the adsorption of the protein at hydrophilic areas. When a protein molecule interacts with a hydrophilic surface, carrying a charge of the same sing, electrostatic repulsion arises. The hydrophobic (dispersion) interaction of proteins is observed during their adsorption on the hydrophobic surface of carbonaceous sorbents or hydrophobized silicas. However, loose protein molecules can be adsorbed on a hydrophilic surface carrying a charge of the same sing as the protein molecule despite the action of electrostatic repulsion [ 16, 17]. 4.4. Pore size effect

When protein macromolecules are sorbed on a porous silica, they are accumulated near pore openings. The rate of diffusion of protein macromolecules in the bulk solution is small, but the rate of their diffusion into pores, whose size is comparable with the diameter of solvated macromolecules, is still lower because of steric hindrances. The time required to attain the state of equilibrium for the adsorption ofBSA macromolecules on silochrom $80 is 4 h, the analogous characteristic for PS20 is above 80 h. The accumulation of protein macromolecules near pore openings may be accompanied by their association. This means that, namely, associates of protein macromolecules penetrate into pores. This phenomenon is not observed for nonporous and macroporous adsorbents. Another factor influencing the dependence of the adsorption value on the pore size is the localization of strongly adsorbed protein macromolecules at openings leading inside

1543 the pore system at the early stage of the interaction between the protein and adsorbent. For example, the localization of protein macromolecules near pore openings results in their narrowing by the thickness of the adsorption layer. A similar effect was observed [2] for separation of BSA and lysozime on silica gel with pores 28 nm in diameter. As a result BSA is eluted from the column before lysozime, whose molecules are smaller and can freely penetrate into the adsorbent pores. The diffusion ofBSA macromolecules near silica adsorbent grains slows down. The electrostatic charge of the hydrophilic silica surface (zero-charge point at pH 2 - 3 [4]) is responsible for the repulsion of BSA macromolecules (isoelectric point is pH 4.7 [12]). However, the rearrangement of the structure of protein macromolecule occurs under the action of hydrophobic forces. Due to the availability of hydrophilic sites, BSA macromolecules are strongly adsorbed near pore openings, resulting in a significant decrease in the pore size ( by the thickness of the adsorption layer comprised of firmly bound BSA macromolecules). Thus, negatively charges BSA macromolecules are sorbed spontaneously (at pH 7.0) on negatively charged areas of the hydrophilic silica surface. The adsorption occurs under the action of two forces: hydrophobic attraction (dispersion forces) and the electrostatic repulsion between likely charged molecules and the adsorbent surface [ 17, 18]. The results of Table 5 suggest that in dilute aqueous solutions (C = 2 - 8 mg/ml) at 293 K BSA macromolecules have self-organized structures which are 25 nm in size and at 313 K BSA macromolecules have self- organized structures which are 25 nm and 65 nm. This suggests that BSA macromolecules exist in aqueous solutions in the form of associates and solvated self- organized structures because X-ray analysis shows that the sizes of these molecules are 7 nm. Thus the results obtained it possible estimate the sizes of BSA macromolecules in dilute aqueous solutions and the thickness the adsorption layers composed of unfolded BSA molecules at the silica surface. 5. CONCLUSION Studying the adsorption of BSA on the surface of uniformly porous silica from aqueous solutions made it possible to determine the influence of the porous structure on the self-organization of adsorption macromolecules. A comparison of the apparent sizes of the protein macromolecules calculated from the adsorption isotherms and the XRD sizes suggests that adsorption layers strongly (irreversibly) bound to the surface near pore openings are formed. These layers hinder or completely block the migration of macromolecules into cavities smaller than 20 nm for BSA. The self-organization of the adsorption layers may be due to hydrate interactions of the hydrophobic groups of the macromolecules and electrostatic interactions leading to the formation of an electric double layer. The effective sizes of the macromolecules calculated without due regard for the thickness of the adsorption layers and electrostatic repulsion may be overestimated by a factor of 2 - 3. The contribution from the association of the protein molecules accumulated near pore openings in the adsorbent was considered.

1544 REFERENCES

1. S.P.Zhdanov, A.V.Kiselev and Yu.A.Eltekov, Kolloidn.Zh., 39 (1977) 354. 2. Yu.A.Eltekov, A.V.Kiselev and T.D.Khokhlova, Chromatographia, 6 (1973) 187. 3. A.Basvin and D.N.Lyman, J.Biomed.Mater.Res., 14 (1980) 393. 4. K.Aoki, T.Takagi and H.Terada, Serum Albumin, Kodansha, Tokyo, 1984. 5. R.Iler, Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties and Biochemistry, Wiley, N.Y., 1979. 6. S.Kondo, E.Amano and M.Kurimoto, Pure Appl.Chem., 61 (1989) 1897. 7. T.D.Khokhlova and Yu.S.Nikitin, Zh.Fiz.Khim., 67 (1993) 2090. 8. N.A.Eltekova and Yu.A.Eltekov, Ross.Khim.Zh., 39 (1995) 33. 9. N.A.Eltekova and Yu.A.Eltekov, Izv. Ross.Akad.Nauk, Ser.Khim., 9 (1996) 2204. 10. J.R.Durig (ed.), Chemical, Biologycal and Industrial Application of Infrared Spectroscopy, Wills, London, 1985. 11. V.Hludy, D.R.Reinecke and J.D.Andrade, J.Colloid.Interface Sci., 111 (1986) 555. 12. W.Norde and J.P.Favier, Colloids Surf., 64 (1992) 87. 13. T.Arai and W.Norde, Colloids Surf., 51 (1990) 1. 14. J.Benesch, A.Askendal and P.Tengvall, Colloids Surf., B 18 (2000) 71. 15. D.Leckband and S.Sivasankar, Colloids Surf., B 14 (1999) 83. 16. J.Talbot, G.Terjus, P.R.Van Tassel and P.Viot, Colloids Surf., A 165 (2000) 287. 17. S.M.O'Connor, S.H.Gehrke and G.S.Retzinger, Langmuir, 15 (1999) 2580. 18. C.F.Wertz and M.M.Santore, Langmuir, 15 (1999) 884.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Characterizing the novel porous superbase K§

1545

by probe adsorption:

A Raman study W.Y. Huang, Y. Wang*, Q. Wang and Q.Yu Chemistry Department, Nanjing University, Nanjing 210093, China Superbasicity of potassium modified ZrO2 can be characterized by Ex-situ IR of chemisorbed carbon dioxide and In-situ Raman spectra of chemisorbed pyrrole. All of the basic sites detected by these methods are lattice oxygen species that are strong electron -pair donor (EPD) Lewis basic site. Several surface carbonates are identified as bicarbonates, monodentates and bidentate species. As an H-bonding donator, moreover, pyrrole interacts with surface basic site to form a strong colinear NH-O through the H-bond between the basic site and the chemical adsorbed pyrrole, resulting in a sharp band at 1136 cml. INTRODUCTION Solid superbases are extremely desirable for environmental benign catalysis, and many efforts have been done for their preparation involving dropping potassium species on alumina [ 1-2]. Possessing both weak acidic-basicity and redox properties, zirconia is a novel candidate for preparation of superbases as proven recently by using modification of potassium salt [3]. Although the catalytic behavior of K*/ZrO2 in butane isomerization has been thoroughly described in our previous studies related to its basic properties, it is necessary to investigate the guest-host interaction occurred in this composite. Incorporation of potassium ions into ZrO 2 induces this interaction that Will influence the formation of superbasic sites. However, it is unclear how to characterize the basic sites on superbases, especiallyto use Raman spectra of adsorbed probe molecule up to date. This prompts us to explore the possibility of using pyrrole and its derivatives such as pyrrolidine and N-methylpyrrolidine to characterize the novel superbase K§ . As an H-bonding donor, pyrrole molecule was reported to be a useful acidic probe for evaluating the 02. basicity of various alkali ion-exchanged zeolites or metal oxides [4-5]. When pyrrole is non-dissociatively adsorbed through a N H O bridge with moderately basic surface O2-centers, the shift of NH-stretching frequency can reflect the basicity of 02. center. In the present work, we examine the possibility of characterizing the superbasic sites by in-situ Raman spectra of pyrrole chemical adsorption. Besides, TG-DTA *Corresponding author, E-mail: aiko@public 1.ptt.js.cn, FAX: 0086-25-3317761

1546 and ex-situ IR methods are employed to investigate decomposition of potassium salt on ZrO2 and CO 2 adsorption of basic sites respectively. 2. E X P E R I M E N T A L The zirconia (Toray Ltd, Torayceram, SA =120 m 2 g-l) used as support was a porous material with a pore volume of 0.23 ml.g ~ and an average pore size of 6.66 nm: K+/ZrO2 samples were prepared by grinding support with potassium salt at a given weight ratio and then thoroughly mixed with right amount of water. The paste was dried at 110~ overnight and heated in air at desired temperatures for 5 h. Pyrrole (99%, Aldrich) was distilled before use and pyrrolidine (99%, Aldrich) and N-methylpyrrole (99%, Aldrich) was used as supplied. A typical adsorption experiment of probe molecular involved the evacuation of sample (ca.50mg) at 500 ~ for 3h in an in situ quartz cell ~onnected to a vacuum system. After the sample was cooled down to room temperature, it was exposed to the vapor of probe molecule for 30 min, then evacuated for 5-15 min. In the Raman experiments, spectra were recorded on a Brulcer RFS-100 Furrier spectrometer with a 1064 nm Nd-YAG laser at a power level of 350 mw, and the region 200-3500 cm 1 was scanned with the resolution of 4 cm -1. In thermogravimetric-differential thermal analysis (TG-DTA), about 20 mg of sample was heated in a platinum crucible with a rate of 20 ~ under nitrogen flow in a meter thermalanalyzer SDT 2960. Ex situ IR spectrum was recorded at room temperature using a JASCO FT/IR spectrometer with a resolution of 4 cm 1 in the range from 4000 to 1000cm -1. 3. RESULTS A N D D I S C U S S I O N 3.1 Interaction of K § with ZrO~ support

Raman spectra of ZrO 2 heated at various temperatures are shown in Fig. 1. A mean crystal phase exists in the ZrO: as shown in Fig.la, resulting in the Raman bands of metastable tetragonal form at 268, 473 and 645cm 1. After calcination at 300~ (Fig.lb) for 5h in air, the relative intensity of the metastable tetragonal bands: keep unchanged, while calcination at 500~ (Fig. lc) weakens the intensity of the characteristic band at 268 cm 1. Besides, the band of tetragonal phase at 645 cm ~ was overlapped by the one at 638 cm -1 assigned to the characteristic vibration of monoclinic phase, which indicated the transformation of part of the metastable tetragonal form into the monoclinic form. For the spectrum of ZrO2 calcined at 700~ (Fig.ld), the band at 268 cm -~ disappeared completely meanwhile the intensity of band at 638 cm ~ increased obviously. Furthermore, the bands of monoclinic form at 502, 476 and 307 cm 1 became stronger. Clearly the calcination at 700~ led to the complete transformation of ZrO2 from metastable form to the monoclinic form. Figure2 shows the spectrum of 20%KNO3/ZrO 2 after calcination at the same temperature as that for the free ZrO2. Different from those spectra of free ZrO2, there was no obvious phase transformation to be observed. When the sample was calcined at 300~ and 500~ (Fig.2b and 2c) in air, the relative intensity of metastable tetragonal form keeps unchanged in certain extent, and the bands of metastable

1547

5

"'2".

>,

04 0

if) t--

I'~ 0

g I13

....,

g

r-

b

E

800

700

600

500

400

Wavenumber/cm-1 Figure 1. Raman spectra of ZrO 2 (a) dried and calcined at (b) 300~ (c) 500~ (d) 700~

300

200

800

700

600

500

400

300

200

Wavenumber/cm -~

Figure 2. Raman spectra of 20%KNO3/ZrO 2 (a) dried and calcined at (b) 300~ (c) 500~ (d) 700~

tetragonal form at 645 and 268 cm -~ were also presented. In addition, the band intensity of potassium nitrate at 716 cm -1 became weaker. It is noticeable that the metastable tetragonal crystalline is still maintained in large extent even when the sample was heated at 700~ (Fig.2d), while calcination at 700~ diminished the band at 716cm 1 completely, indicating the decomposition of KNO 3 on the surface of ZrO 2 as shown in Fig.2d. After calcination at 700~ the free ZrO 2 transferred completely to monoclinic form, but on the sample loaded K salt the stability of metastable tetragonal form seemed to be significantly enhanced because it still remains in a large extent. These Raman results are in good agreement with those XRD tests on the same sample as reported previously [3]. Obviously, such stabilization is related to the introduction of potassium salt, probably due to the interaction of K ions with ZrO 2. Duwez et al. [6] reported the stabilization of high-temperature tetragonal crystal after incorporation of a Ca 2+ into ZrO2, and those Ca 2+ ions were assumed to occupy the position of Zr 4+ and formed an O 2- vacancy. Therefore these K § ions were inferred to occupy the position of octa-coordinated Zr 4+ which is more stable than the hepta-coordinated Zr 4+, due to interaction of K salt with zirconia [3].

3.2 Formation of superbasic sites In order to investigate the basic site of 20%KNO3/ZrO2, this sample was heated at 600~ for 5h in air and consequently kept in air for 24h at room temperature. Formation of carbonate-like substance on the sample was observed by IR spectroscopy. As shown in Fig.3, new bands of carbonates-like species appeared at 1344,1260 and 1030 cm -1 (Fig.3b), meanwhile the 1380 cm I band of N-O asymmetric stretching vibration of NO3- disappeared. Moreover, the band at 1615 cm 4 became significantly broader to 1580-1540cm ~, the former was assigned tentatively to bicarbonate species due to the binding of CO2 to surface OHgroups, while the latter could be attributed to an asymmetric stretching vibration of surface

1548

b S"

,

18oo

i

16oo

,,

,

i

1380

1615 ,

1

120o 1400 Wavenumber/crn 1

i

,

1000

Figure 3. IR spectra of 2 0 % K N O 3 / Z r O 2 (a)dried (b) calcined at 600~

1800

,

I

1600

,

I

i

1400

1200

1000

Wavenumber/cm 1

~-Figure 4. IR spectra of ZrO2 (a) dried (b) calcined at 600~

monodentate and bidentate species [7]. As a comparison, no such new characterizing bands were observed on the spectra of free ZrO2 after calcination at 600~ in air as shown in Fig.4b. Fukuda and Tanabe examined the adsorption of CO2 on CaO and observed the phenomena that the monodentate carbonate complex changed into bidentate carbonate complex when the evacuation temperature was increased from room temperature to 300~ [8]. The bidentate carbonate species was proposed to be formed by CO2 binding not only to surface oxygen but also to surface metal ions, while monodentate carbonate species can be formed by the CO2 adsorbed on surface basic sites 0 2. even if at room temperature. On the surface of 20%KNO3/ZrO2 sample, the surface carbonate species presented not only in the form of monodentate but also in the form of bidentate, as demonstrated in Fig.3, providing some information on the basicity of the sample. Carbon dioxide is known to tentatively absorb on the electron-pair donor sites on the surface since it is a strong-pair acceptor. Besides, the surface lattice oxygen contributed to the producing monodentate carbonate species while bidentate carbonate species were formed by linkage to Mn§ site)[9-11 ]. In addition, different basic samples will give different adsorpti6r/::states of CO2. As Fukuda and Tanabe reported [8], monodentate carbonate species were the predominate of CO 2 adsorbed on the surface of MgO, while on the surface of CaO only monodentate species were formed at room temperature. Consulting these results it is reasonable to infer that calcination for the sample of 20KNOg%/ZrO2in air made KNO3 decomposed over the surface of support. As a result, basic sites formed on the sample and adsorbed CO2 from atmosphere to form the monodentate carbonate and bidentate carbonate species. Moreover, the monodentate carbonate can be assumed resulting from the adsorption of CO~ on the stronger EPD oxygen sites of surface and bidentate carbonate species is located in the oxygen which is adjacent to K § ions. For the sample of 20 O~KNO3/ZrO2, a high activity was observed in the isomerization of cis-2-butene at 0~ when this catalyst was evacuated at 500~ 12]. Formation of basic sites on the K§ sample should account for the high catalytic activity, no matter how they were generated

1549 a[1.5

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Figure 5. TG-DTA profile of (a) KNO 3 (b) ZrO2 (c) 7.5%KNO3/ZrO2 (d) 14% KNO3/ZrO2 (e) 20% KNO3/ZrO 2 (f) 27% KNO3/ZrO 2 samples. below 500~ in vacuum or 600~ in air through the decomposition of KNO3. Pure KNO3 cannot decompose until the temperature rise above 600~ as shown in Fig.5a. However, the decomposition temperature of KNO 3 loaded on ZrO2 was become much lower than that of pure KNO3. As evident in the TG-DTA curves of K + modified samples shown in Fig.5, there is only one endothermic peak centered about 82~ resulting from the desorption of the physical adsorbed water in free zr02 (Fig.5b). On the TG-DTA curve of 7.5%KNO3/ZrO2 sample, two peaks centered about 82~ and 546~ and the second one corresponded to decomposition of the guest material (Fig.5c). When the loading amount of KNO3 increased to

1550 14 wt.-% and 27 wt.-%, three endothermic peaks emerged at 87~ 551~ and 830~ for the former (Fig.5d) and another three appeared near 82~ 556~ and 884~ for the latter (Fig.5e). The temperature of the final weight loss in the KVZrO2 samples that usually occurred above 800~ increased with the increase of loading amount of potassium salt, from 830~ (14%KNOJZrO2) to 884~ The reason, as what revealed on K+/A1203 [ 1], is the different interaction states of the loaded KNO 3 with the support. Among the supported of K + species, some of them was highly dispersed and interacted very strongly with the surface of ZrO 2 but another may be still the micro-crystal KNO 3 with the similar properties as the pure KNO 3 owing to lack of interaction with support. As expected, the KNO 3 interacted with support can decompose at the temperature much lower than that needed for pure KNO3, meanwhile those without interaction still need high temperature for decomposition. Therefore the contribution to the formation of superbasic sites only comes from those highly dispersed K + species, decomposed near 550~ and represented'~y the second peak in TG-DTA spectra, whereas those decomposed above 800~ is not associated with the generation of strong basic sites. 3.3 Adsorption of probe molecular on K+/ZrO2 Pyrrole can be used to detect strong basic sites because of its very weak acidity. Acting as an H-bonding donator, pyrrole interacts with surface basic site to form a strong colinear NH-O band complex that results in a strong characteristic bond in the ring-stretching region. As shown in Fig.6, the band at 3135 cm -t for pure liquid pyrrole (Fig.6a) can be attributed to CH stretching vibration [13], and the two weaker bands at 1468 and 1380 cm t may be assigned to C=C and C-C stretching vibration. The strong band at 1145 cm -1 is the NH bending vibration [14]. The spectra of free ZrO2 (Fig.6b) evacuated at 773 K gave no new band, since both the basic sites and acidic sites on the surface of ZrO 2 were too weak, so the adsorbed pyrrole was thus completely desorbed in the evacuation process even at room temperature. For the sample of 14%KNO3/ZrO 2, however, the spectra of pyrrole adsorption is very different (Fig.6c). A sharp band at 1136 cm 4 due to the NH band deformation was observed, while the NH bending q.~

.-~ u) oN

-

u

d

co

V

[ 3500

i 3000

.,

, 2500

i

.

2000

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Figure 6. Raman spectrum of (a) liquid Pyrrole, (b)ZrO2, (c) 14%KNO3/ZrO2, (d) 20%KNO3/ZrO2

1551 e3 Lo o

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W a v e n u m b e f f c m -1 Figure 7. Raman spectrum of (a) liquid Pyrrolidine (b) ZrO2,(c ) 14%KNOJZrO2, (d)20%KNO3/ZrO 2, (f) 27%KNO3/ZrO2 (g) 34%KNO3/ZrO 2 vibration of pure pyrrole appeared near 1145 cm 4. This bathochromic shift may result from the formation of H-bond between surface basic site and NH band of chemical adsorbed pyrrole molecule, because the formation of H-bond lowered the strength of NH band so that the vibration of the latter shifted down to lower frequency. As the amount of potassium salt on ZrO2 increased to 20 wt.-%, the intensity of the peak at 1136 cm -1 was obviously enlarged (Fig.6d), mirroring the increase of basic sites. Moreover, a series of new bands appeared at 3095,1362 and 1321 cm "1, the former could be assigned to the v[NH-O] species [15] and the two latter probably represented the C=C and C-C:~mg stretching vibration [16]. Binet and coworkers [5] studied the dissorciate and undissociate adsorption of pyrrole on oxides and propose that the degree of ionicity/covolency of the 02. surface species may parallel their basicity. When the 02. species was very basic, the easily polarizable hydrogen in pyrrole will be bridge-linked to the 02. and result in [-NH-O] species that bond at 3100-2700 cm -~ range. Based on these facts, the weak band at 3095 cm ~ on 20%KNO3/ZrO ~ can be tentatively assigned to [-NH-O] species. Although pyrrolidine possesses a stronger basicity than pyrrole, it is still an acid probe molecule when it interacts with a superbase. After pyrrolidine adsorbed on the K+/ZrO2 samples, a shift of N-H bending vibration from 1454 to 1321cm ~ emerged on the spectrum as shown in Fig.7, accompanied with a small one for the C-N ring stretching from 1280 to 1257 cm -~. In contrary no obvious shift was observed in the C-H stretching band at 2963 cm -1. Moreover, the C-H deformation vibration at 899 cm 4 was split into two weak bands at 912 and 871cm -~ respectively, which can be tentatively attributed to the polarity

1552 change of C-H band and will be discussed elsewhere in detail. The use of N-methylpyrrole on the sample of K+/ZrO2 seems not successful, because no obvious adsorption was observed on the obtained spectrum of 20%KNO3/ZrO2 (Figure was not shown). Probably the replacement of the H atom connected to N atom by the group of-CH3 hinders the probe molecule to interact with the basic sites by the formation H-bond. Another possible reason is strength of adsorption. The adsorbed species may be physically eliminated during the evacuation at room temperature, and a further investigation is thus desirable. CONCLUSION (1). Strong interaction exists between KNO3 on and ZrO2 and results in the stabilization of metastabte tetragonal form of ZrO2 and lowers the decomposition temperature of K N O 3 o n Zr0s than that of crystalline KNO3. (2). Two states of KNO3 locate on the surface of ZrO2. One is the high dispersed K + species that decompose around 550~ and play a important role in the formation of superbase, another is the bulk KNO3 that decompose above 700~ but not participate to create superbasic sites. (3). The strong basic sites on the potassium modified ZrO2 sample can be characterized by the adsorption of pyrrole, and a strong coliner NH-O species is formed by H-bond with surface basic sites. N-methylpyrrole is not a convenient probe since it hardly chemically adsorbs on the surface of solid superbase. REFERENCES

1.Y. Wang, J.H. Zhu and W.Y. Huang, Phys. Chem. Chem. Phys., 3 (2001) 2537. 2.T. Baba, H. Handa and Y. Ono, J. Chem. Sot., Faraday Trans., 90 (1994) 187. 3.Y. Wang, W.Y. Huang, Y. Chun, J.R. Xia and J.H. Zhu, Chem. Mater. 13 (2001)670. 4. M. Huang and S. Kaliaguina. J. Chem. Soc. Faraday Trans., 88 (1992) 751. 5. C.Binet, A.Jady, J. Lamotte and J.C. Lavalley, J. Chem. Soc., Faraday Tran. 92(1996) 123. 6. P. Duwez and F. H. Brown, J. Am. Ceram. Soc., 35(!952) 109. 7. J. A. Lercher, C. Colombier and H. Noller, J. Chem_=Soc., Faraday Trans., 80(1984) 949. 8. Y. Fukuda and K. Tanabe, Bull. Chem. Soc. Jpn., 46(1973) 1616. 9. B. W. Krupay and Y. Amenomita, J. Catal., 67(1981)362. 10. Y. Morikawa and Y. Amenomiya, J. Catal. 48(1977) 120. 11. J. V. Evan and T. L.Whateley, J. Chem. Soc., Faraday, 63(1967)2769. 12.Y. Wang, W.Y. Huang, Z.Wu, Y.Chun and J.H.Zhu, Marterial letter, 40(2000)198. 13. T.D. Klots,R.D. Chirico and W.V. Steele, Spectrochimaca Acta.50A 4(1994)765 14. R.C.Lord, Jr and F. A. Miller. J. Chem.Phys. 10(1942)328 15. J. C. Lavalley, Catalysis Today, 27(1996)377 16. B.Tian and G.Zerbi. J. Chem. Phys., 92(1990)3886.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1553

The S y n t h e s i s and C h a r a c t e r i z a t i o n o f Zeolite Z S M - 5 and Z S M - 3 5 F i l m s b y Self-transformation o f Glass Jinxiang Dong, Wugang Fan, Guanghuan Liu, Jinping Li Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, P.R. China

Zeolite ZSM-5 and ZSM,35 films have been synthesized successfully by selftransformation of glass discs. The synthesis was performed by the vapor phase method. Ethylenediamine (EDA) was used as the organic template agent_ The synthesis conditions were studied. The phase of the zeolite film was dependent on the ratio of EDAJH20. The properties of the Zeolite ZSM-5 and ZSM-35 films were investigated by calcination, aqueous vapor treatments at 493K and were characterized by XRD, SEM. The films didn't peel off atter this series of treatments~ This showed that the films connected with the substrate firmly.

1. INTRODUCTION Zeolites have been widely used in many fields including catalysts, separation, ion exchange, sensing etc. In most cases, zeolites are preferred as powder composition whereas they should have suitable configurations for practical uses (powder zeolites are used as washing builders). Zeolites in membrane or film configuration have been an attractive subject for several years because their potential in separation and catalysis. A membrane is an intervening phase separating two phases and/or acting as an active or passive barrier to the transportation of matter between p ~ s e s adjacent to it under driving force. It can be gas, liquid or solid. Generally, it has greater lateral dimension than its thickness[1 ]. Zeolite films or membranes can be classified as asymmetric films (supported) or symmetric films (self-supported). The sencond type is constituted by a pure zeolite phase; the first type is a zeolite thin layer formed on different supports. Many materials can be used as supports, such as glass[2], quartz[3], stainless steel disc[4], silicon[5], alloy[6], polymeric[7], ceramic[8]~ alumina[9], mercury[10], filter paper[ll] etc. The combination of catalysis and distillation is one the new trend in chemical engineering to reduce the enormous space taken up by distillation towers. If the zeolitic films could be employed as elements, it would require the films to be of high thermal stability and of a certain mechanical strength. Symmetric films have good

1554 engineering properties (strength, hardness) and are more suitable for this technique, it can be prepared by direct synthesis, pressuring sol,gel method and sQ on, but this kind of film can't be connected to the support material very well and can be removed easily. Comparing with traditional commonly used hydrothermal synthesis systems, the vapor phase method[12], which avoids the direct contact of the solid phase and the liquid phase, can decrease the waste of liquids and enhances the productivity of zeolite. It has been employed for preparing zeolitic films, for example, Dong et al. formed ZSM-5 and ZSM-35 self-supported films by this way[ 13], Matsukata and Nishiyama using vapor phase synthesized a layer of mordenite and analcime on porous alumina [14]. Recently, Dong et al. prepared a ZSM-5 film on boron glass disc in the vapor phase of ethylamine (EA) and H20 by the same method[ 15]. In this work, some conditions of synthesis of self-transformation zeolite ZSM-5 and ZSM-35 film on glass discs were investigated. The influence of different factors in synthesis process was studie& The stability of the films and the joint situation of the films with the substrate after calcination and aqueous vapor treatments were investigated by XRD and SEM. The results showed that the film firmly connected with the substrate.

2. EXPERIMENTAL AND CHARACTERIZATION 2.1 Materials

Glass disc (30mm• for the convenience of XRD characterization) was used as the starting substrate. The prior treatment of the glass disc was a complete immersion into an erosion solution for 2-~3h at room temperature and then a cleaning with distilled water. EDA (A.R.>99%), triethylamine (Et3N)(A.R.>99%) and n-propylamine (PrNH2) (>98.5%), n-butylamine (ButNH2)(>98~5%) in reagent grade were used for the synthesis. The synthesis method (vapor phase method) was described in a previous paper [12]. The glass disc was mounted in a special autocalve. The liquid phase at the bottom of the autoclave is composed of template and H20 at different ratio. The calcination of the assynthesized glass film was conducted in a crucible oven_ The temperature was raised at a rate of 1K/min from room temperature to 823K; then kept constant for 6h in air, and then allowed to cool to room temperature. The aqueous vapor treatment of the zeolite films was carried out in the same autoclave of the synthesis. However, the liquid phase was distilled water. The heating treatment was carried out at 493K for 72h, at the same time, XRD and SEM inspected phase and morphology change of the zeolite film respectively. The glass disc should be carefully brushed and cleaned before XRD examination in order to get rid of the peeled off zeolite after the treatments.

1555 2.2 Characterizations The XRD identification was carried out with a Rigaku D/max 2500 X-ray powder diffractometer. Cu Ko~ radiation was used. The relative crystallization of zeolite ZSM-5 film was calculated by making the summation of intensities of the four peaks 20=7.92,23.10,23.27,23.98, and divided by highest crystallization. For ZSM-35, the eight peaks 20 = 9.23, 22.21, 22.30, 22A3~ 22_92, 23.47, 23 .76,24.07 were used as reference peaks. The morphology of a small piece of zeolite film detached from the integrated glass disc was observed with SEM (JEOL JSM-35). 3. RESULTS AND DISCUSSION 3.1 Synthesis conditions The synthesis of zeolite films was different from the traditional hydrothermal synthesis. The basic factors which influence film formation included liquid phase composition (template agent), the type of substrate, crystallization time and reaction temperature. Firstly, different kinds of glasses were used to discover the possibility of preparing this type of self-transformation film(Table 1). The glass discs without boron (common glass) and the boron glass discs (B203=12.8wt%) were used as starting substrate to synthesis. It was found that the common glass transformed with EDA as template agent in the steam of water and EDA. Whenthe EDA/H20=0.30(mol ratio), at 493K for 288 h, ZSM-5 films can form on the glass disc. However, the films had lower crystallization than that forming on boron glass surface and it peeled off after calcination treatment(823K,6h). So in the following experiments, boron glass discs were used as starting substrate. Secondly, different organic amines were tried as template to investigate the possibility of synthesis. In most experiments, EDA was used as an effective template, which will be discussed in details. When the Et3N was added to the aqueous solution of EDA, the ZSM-5 film could also be formed on the glass disc. Yet, it was also found the PrNH2 and ButNH2 were invalid_ as template for the glass disc transformation at 493K for 288h(Table2), although they can be used as template for synthesizing the powder of zeolite ZSM-5. The reason for the selectivity of template seemed to be the different dissociation constants in water. Table 1 The influence of glass category The category of glass Boron glass Size (mm) 30•215 Phase ZSM-5

Common 8 1 a s s 30•215 ZSM-5

Common glass 30• ZSM-5

1556

Table2 The comparison of different of template a~ents Run 1 2 Template EDA EDA/Et3N Mol ratio (template/H20) 0.30 1.37/0.83 Table3 Synthesis conditions of zeolite film Run t EDA/H20 (mot ratio) 6.5 Phase FER

3 PrNH2 0.36

on Boron-glass disc 2 3 2.4 1. t FER FER+MFI

4 ButNH2 0.30

4 0.60 FER+MFI

5 0.40 MFI

6 0.30 MFI

Thirdly, the influence of ratio of EDA and H20 on the zeolite film formation on glass was investigated at 493K for 288h. The results showed that when the molar ratio in 2.4-6.5, zeolite ZSM-35 film was obtained; when the ratio was changed from 0.30-0.40, zeolite ZSM-5 fdm was synthesize_& Between these ranges there was a film of mixed crystal grew on the glass disc (Table3). From the synthesis results, it can be seen that an appropriate ratio EDA/H20 was of great importance for the transformation of the glass disc.. Fourthly, the influences of crystallization temperature on the self-transformations of the glass disc were investigated at 453-493K for 240h. The results showed that in this temperature range the two films could be formed. Finally, the influence of crystalline time on the synthesis of zeolite film was investigated by choosing 144h, 19211, 240h, 288h, 384h and 400h respectively at 493K. For zeolite ZSM-5 film, it was found that the crystallization increased along with the crystallization time until the time was up to 384h(Fig.1). The growth of ZSM,35 film seemed to be slower than that of the ZSM-5 film. = 1.20 N

"~

0.8

--

----0-- ZSM-5 film

r~

~,o.6

-

L)

o0.4

~

0.2

~

o o

t

-~

I

I

I

I

I

I

I

I

I

50

100

150

200

250

300

350

400

450

Time (h)

Fig. 1 The growth of zeolite films with the heating time

ZSM-35film

1557 3.2 The influence of the a f t e r - t r e a t m e n t on the rdms

The organic amine molecules should be removed before zeolite can be used as catalyst and adsorbent by calcination~ According to the process showed in 2.2, the two kinds of zeolite films were calcined and then treated with aqueous vapor to investigate the properties of the films. The film grew on the common glass disc would peel off after calcinations. Fig.2 and Fig.3 were the XRD patterns of the zeolite ZSM-35 and ZSM-5 film that were formed by self-transformation of boron glass disc respectively.

tt'i

ttlt

(a)

~-""---"~ ~l

(b)

r

j .aJ

c)

5

10

15

20 20

25

30

Fig.2 XRD patterns of ZSM-3 5 film (a) After aqueous vapor treatment (b) After calcination (c) As-synthesized film (d) Starting substrate

5

10

15

20

2e

(a)

(b)

,

(c)

25

30

Fig.3 XRD patterns of ZSM-5 film (a) Film after calcination (b) After calcination (c) As-synthesized film (d) Starting substrate

From the X R patterns, it can be seen that (I) The glass disc was amorphous. (II) The zeolite film was synthesized. (III) After calcination at 823K for 6h, aqueous vapor treatment at 493K for 72h, the phase of the zeolite ZSM,5 and ZSM-35 didn't change. The crystallization even was improved after aqueous vapor treatment.

1558

(a) The as-synthesized ZSM-5film

(a) The as-synthesized ZSM-3 5film

(b) The ZSM.5film after calcination

(b) The ZSM-35film after calcination

(c)After aqueous vapor treatment Fig.4 SEM images of ZSM-5

(c) After aqueous vapor treatment Fig.5 SEM images of ZSM-3 5

1559

Fig.6 The surface of starting substrate From the series of SEM images (FigA and Fig.5), it can be seen that the two kinds of zeolite films not only maintain a good phases stability(XRD results) but also have good morphology after calcination and aqueous vapor treatment. It is proven that this kind of self-transformation film is effectively connected to the substrate and it is resistant to peeling off comparing with other films. Fig.6 showed that the surface of the starting substrate after chemical treatment. The surface of the glass disc was rough. It seemed suitable for the growth of zeolite film on it. This glass disc can be partly transformed into zeolite films in the vapor of EDA and H20. XRD and SEM showed that zeolite films grown on the glass disc was resistant to calcination and aqueous vapor treatments. 4.CONCLUSIONS We studied the synthesis conditions of zeolite ZSM-5 and ZSM-35 film formation on glass by the self-transformation with the vapor phase method. Zeolite films were obtained using EDA and H20 as liquid phase. When the EDA/H20 varied from 6.5-2.4, the FER zeolite phase was obtained; when the ratio was from 0.40-0.30, MFI was obtained. This kind of zeolite film connect with the substrate welt and can endure calcination and aqueous vapor treatments, so it has good quality that maybe could find future application.

ACKNOWLEDGEMENT

The authors gratefully thank the f'mancial assistance provided by Natural Science Foundation of Shanxi Province (20011012).

1560 REFERENCES

1.A. Tavolaro, Adv. Mater., l l , No~12(1999)975. 2.J.G.Tsikoyiannis, W.O. Haag. Zeolite, 12(1992) 126. 3.G.J.Myatt, P.M.Budd, C.Price, S.W.Carr, J.Mater.Chem.2(1992) 1103. 4.T.Sano, H.Yanagishita, Y.Kiyozumi, F.Mizukami, K.Haraya, J.Membr.Sci., 95(1994)221. 5.J.C.Jansen, W.Nugroho, H.van Bekkum. Proc.9th Int~ Zeolite Conf.,1(1992)247. 6.Y.Kiyozumi, K.Maeda, F.Mizukami, Stud.Surf.Sci.Catal.98(1994)278. 7.M.W.Anderson, K.S.Pachis. J.ShLS. W.Carr, L Mater. Chem.,(1992)255. 8.M.D.Jia, K.V.Peinemann, R.D.Behling. J.Membr. Sci., 82(1993) 15. 9.NANishiyama, K~Ueyama, M.Matsukata, Stu& Surf. SckCatal.,105(1997)2195. 10.Y.Kiyozumi, F.Mizukami, K.Maeda, M. Toba, S.I.Niwa, Adv.Mater.,8(1996)517. 11.T.Sano, Y.Kiyozumi, K.maeda, M.Toba, S~Nivca,F.Mizukami, Proc,_9th Int. Zeolite conf., 1(1992)239. 12.W.Y.Xu, J.X.Dong, J. Chem. Sock, Chem. Commun.,(1990)755. 13.J.X. Dong, T.Dou, X.Zhao, L.Gao, J. Chem. Sot., Chem. Commu.,(1992)1056. 14.M.Matusakata, N.Nishiyama, K~Ueyama,Microporous Mater.,7(1996)299. 15.W.Y. Dong, C. L. Long Chem.Commun.,(2000)1067.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1561

Preparation of mesoporous materials as a support for the immobilisation of iipase. A. Macario, V. Calabr6, S. Curcio, M. De Paola, G. Giordano, G. Iorio, A. Katovic Dipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria via P. Bucci, 187030 Rende (CS), Italy In this paper the immobilisation of the lipase enzyme on a M41S type mesoporous material support is presented. Different M41S type materials, with a pore diameter ranging from 37 A to 48 A, were synthesised starting from different silica sources, in order to immobilise, by physical adsorption, lipase with a spherical molecular diameter close to 41 A. The efficiency of the immobilisation carried out at an optimal pH=7, different temperatures and time, were tested by UV Spectrophotometry. The maximun amount of bound lipase resulted to be around 472 mg at 0~ compared to the 362 mg at 25~ and 119 mg at 40~ Indeed, the maximun immobilisation efficiency is around 47%. The activity of enzyme immobilised was tested by hydrolysis of triglycerides in olive oil into fatty acids and the lipase immobilised has 78% of the activity of the free enzyme. 1. INTRODUCTION The most important industrial applications of lipase regard the synthesis and the hydrolysis of glycerides. The principal glycerides inquiries are in the food, pharmaceutic and cosmetic industries [1]. In particulary, the mono and diglycerides are often used as emulsifier and some triglycerides are used as economic substitute of cocoa butter [2,3]. The glycerides are, today, produced by inorganic catalyst at high temperature (130~176 consequently, with high energy cost [4]. Recently, owing to the problems related to high reaction and separation costs, new methods for glycerides synthesis were proposed as possible alternatives. Particularly, an enzymatic process, in which a biocatalyst is used, allows to carry out the reactions under milder conditions. The advantages of using immobilised enzymes, as compared to their native state, pertain the utilization of processes that are easier to control and to operate, allowing the reutilization of catalyst that may be readily separated from the substrate. Enzyme molecules are, generally, immobilised by entrapment within a porous matrix, by physical adsorption or by chemical based methods and, usually, maintain most of their catalytic activity. In recent years, different enzyme immobilisation techniques were reported [5-9]. As far as lipase is concerned, no study regarding its immobilisation into the internal surface of a mesoporous material has been presented. M41S are hydrophobic materials that allow immobilising enzyme through a solvent containing both lipase and the support, under stirring. Physical adsorption is, therefore, attained.

1562 Aim of the present study is the preparation and the characterization of a novel biocatalyst with lipase immobilised in MCM-41 support. Many mesoporous materials, having pores diameter comparable to spherical molecular diameter of used lipase, were synthesized. The effect of the synthesis time, the different silica sources and the different organic cations on the pore size of the support were investigated. The amount of lipase immobilised in the synthesised supports, with a pore size comparable to the dimensions of lipase, has been quantitatively evaluated. Finally, the activity and stability of immobilised enzyme, as regards the hydrolysis of triglycerides, were tested.

2. EXPERIMENTAL 2.1 Materials The enzyme used is produced by NOVO and its trade name is Palatase: a purified 1,3specific lipase from Rhizomucor miehei produced by submerged fermentation of a genetically modified Aspergillus oryzae micro-organism. Optimun working conditions are reported as 40~ and pH = 7.5. Calculated MW is 29536 g/mol. The enzyme spherical molecular diameter has an influence on the support synthesis. The lipase spherical molecular diameter was estimated as 41 A, on the basis of experimental evidences [ 10]. The synthesized support was MCM-41. The following materials were used for purely siliceous MCM-41 synthesis: silica gel (BDH) with a specific surface of 550 m2/g; Z6osil silica with a specific surface of 175 m2/g; cetyltrimethylammonium bromide (CTABr 99%, Aldrich); tetraethylammonium hydroxide solution (TEAOH 40%, Fluka), tetramethylammonium hydroxide solution (TMAOH 25%, Fluka). 2.2 Synthesis and characterization of MCM-41 support Pure siliceous MCM-41 with different pores diameter were synthesized from the following molar composition, with the optimal CTA/Si molar ratio [ 11 ]:

0,07 X20- 1 SiO2- 0,12 CTABr- 30 H20 in which X represents the cations (TMA + or TEA +) added as hydroxides. The mixture was heated at 140~ in a Teflon-steel autoclave. The time of crystallization was varied from 1 to 12 days. The solid phases were recovered by filtration and washed with distilled water and, then, dried at 100~ for 24 h. Samples were calcined in air at 550~ for 10h in order to remove the template. The long-range structures of the MCM-41 materials were characterized by XRD patterns collected on a Phillips PW 1710 diffractometer with CuKcz radiation. The samples were scanned from 1 to 10~ (20) in steps of 0,02 ~ with a count time of 1 second at each point. The specific surface area, the mesopore volume and the pore diameter of MCM41, after calcination, were determined by N2 adsorption isotherms obtained at 77K using an ASAP 2010 Micromeritrics instrument.

2.3 Lipase immobilization 0,4 g of calcined MCM-41 were mixed with 50 ml of 0,5 M phosphate buffer pH 7 containing 1 g of lipase and was stirred for different times at three different temperatures (0~ 25~ and 40~ The mixture was, then, filtered and the MCM-41 containing lipase

1563 Table 1 Activities of lipase in its native state Amount of free Lipase Temperature of (mg) Immobilisation (~ 119,0 40 362,2 25 471,7 0

Activities (U/L) 8820 11760 15400

was washed twice with deionised water and dried at 25~ ovemight. The filtrate was collected in a small graduated cylinder to measure its exact volume. The amount of immobilized enzyme was calculated from the difference of the absorbance at 280 nm before and after addition of the support. The enzyme concentration of the initial solution and of the filtrate was calculated by calibration curve determined by Perkin-Elmer Spectrophotometer UV. A linear regression equation is applicable to obtained data: C o n c e n t r a t i o n = 1 3 , 8 8 3 * A b s o r b a n c e , with a correlation coefficient of 0,9971. The absorbance of the initial lipase solution was 1,636 corresponding to a concentration Co of 22,71 mg ml 1. The amount of enzyme adsorbed on the MCM-41 (Win, [mg]) was determined from the following lipase mass balance: WIL = CoV0 - CfVf, in which Co is the initial enzyme concentration (mg mll); V0 is the initial volume of lipase solution (ml); Cf is the enzyme concentration of the filtrate (mg mll); Vf is the volume of the filtrate (ml). 2.4 Activity of immobilised Lipase. The activity of immobilised lipase was tested and compared to the activity of the lipase in its native state by the Tietz and Fiereck method [12] that involves hydrolysis of triglycerides contained in olive oil to give fatty acids, diglycerides and, a lesser extent, monoglycerides, and glycerol. The activity assay procedure is the same as reported in the Tietz and Fiereck method, both for soluble and immobilised lipase [12]. Lipase activity in its native state, in the same amount that has been immobilised at the three tested temperatures, is reported in table 1.

3. RESULTS AND DISCUSSION 3.1 Structural characteristics of the MCM-41 support The dl00 spacing and the hexagonal lattice parameter (ao) of the samples synthesized with different silica sources and different organic cations at different reaction times are reported in the tables 2, 3 and 4. The organic cations strongly affect the pore diameter of as synthesised M41S materials. As matter of fact, in the presence of TMA + ions in the initial reaction mixture, the pore diameters increase when the reaction time is prolonged (see samples 1-7 table 2). On the contrary, independently of the silica source, the presence of TEA + ions in the synthesis hydrogel induce a reduction of the pore diameter value, for a longer reaction time (see samples 8-14, table 3, and 15-17 table 4). When the silica Z6osil was used, the decrease of pore diameter is detected after 6 days of reaction time, whereas, when silica gel from BDH was used, the larger pore diameter is showed after only 2 days of reaction at 140~ Probably, the different behaviour observed in the samples synthesised with different organic

1564 Table 2 XRD dloo spacing, hexagonal lattice parameter of uncalcined samples synthesized with Z6osil as silica source and TMA + organic cation, reacted at 140~ [ 14]. Sample Synthesis time [h] dloo [ A ] ao [ A ] Final phase 1 24 37,3 43,1 MCM-41 2 68 41,2 47,6 MCM-41 3 120 43,1 49,8 MCM-41 4 144 43,5 50,2 MCM-41 5 168 43,9 50,7 MCM-41 6 192 44,1 50,9 MCM-41 7 288 45,7 52,8 MCM-41 Table 3 XRD dloo spacing, hexagonal lattice parameter of uncalcined samples synthesized with Z6osil as silica source and TEA + as organic cation, reacted at 140~ [ 14]. Sample Synthesis time [h] dloo [ A ] ao [ A ] Final phase 8 24 41,2 47,6 MCM-41 9 68 42,1 48,6 MCM-41 10 120 42,6 49,2 MCM-41 11 144 47,2 54,5 MCM-41 12 168 42,7 49,3 MCM-41 13 192 42,6 49,2 MCM-41 14 288 40,9 47,2 MCM-41 Table 4 XRD dl00 spacing, hexagonal lattice parameter of uncalcined samples synthesized with BDH as silica source and TEA + as organic cation, reacted at 140~ [ 14]. Sample Synthesis time [h] dloo [ A ] ao [ A ] Final phase .... 15 48 47,8 55,2 MCM-41 16 120 44,1 50,9 MCM-41 17 144 42,7 49,3 MCM-41 cations and silica source are due to the different interaction of organic cation and silica [ 13] and to the different length of organic chain. In table 5 are reported the wall-thickness of calcined and uncalcined samples. The wallthickness of uncalcined samples was calculated by the geometric methods [14], where the volume of the N2 adsorbed was substituted with the surfactant volume calculated by thermal analysis, while the wall-thickness of calcined samples was calculated considering the difference between the unit cell and the pore diameter calculated by N2 adsorption. The contraction of the unit cell after calcination is observed only for the samples prepared with BDH as silica source (N~ and 17), on the contrary, the samples 7 and 11 show an unit cell opening and, then, a wall-thickness reduction. Since the contraction of unit cell after calcination is caused by condensation of the Si-OH unit in the channel wall, the reason of this difference, probably, is the high specific surface area of the BDH silica and its greater acidity. In any case, the pore walls thickness is comparable to the largest wall-thickness reported in

1565 Table 5 XRD dl00 spacing, hexagonal lattice parameter and wall-thickness of uncalcined and calcined samples. Sample before calcination after calcination dl00 [A] ao [ ,~ ] wall-thickness dloo [ ,h ] ao [ ,h ] wall-thickness

[X]

7 11 15 16 17

45,7 47,2 47,8 44,1 42,7

52,8 54,5 55,2 50,9 49,3

[A]

14,9 15,3 13,5 11,6 9,3

48,1 49,0 44,1 40,8 41,9

55,5 56,6 50,9 47,1 48,4

14,0 14,7 15,2 -

Table 6 Properties of the MCM-41 calcined samples, obtained from the BJH method. Sample Specific surface area Mesopore Volume a0 Pore diameter [m2 g-l] [cm3 g-l] [A] [A] 7 11 15

950 1028 1087

0,95 1,08 0,77

55,5 56,6 50,9

41,5 42,0 35,7

literature, from other authors, for the MCM-41 materials [15]. In table 6 are reported the results of N2 adsorption/desorption analysis. It can be observed that pore size of the samples increases when the ao parameter enlarge. When the Z6osil was used as silica source, large pore volume and lower specific surface area were measured. However, for all samples the large pore volume indicates that no structural collapse occurs during calcination. The sample 11 shows the highest pore diameter, while the sample synthesized with BDH as silica source (15) has the highest specific surface area but smaller pore diameter. Therefore, in order to immobilize the highest amount of lipase, the sample 11 was chosen. 3.2 I m m o b i l i s e d lipase on M C M - 4 1

In order to better understanding the role played by temperature, three different immobilisation experiments have been performed at the temperatures of 0~ 25~ and 40~ For these experiments two characteristic quantities, i.e. the ratio between the filtrate volume and the volume of initial lipase solution (Rv) and the ratio between the weight of dried MCM-41 after and before the immobilization (Rw), were considered and evaluated after 24 hours. Results are summarised in the following table 7 and figure 1. These results show that the adsorption is favoured at low temperatures and the higher amount of lipase immobilised is at the temperature of 0~ Figure 1 shows the dynamic evolution of adsorbed lipase on M41S materials at different temperature. The initial trend of the curve shows a very fast adsorption. When the time is prolonged, the amount of adsorbed enzyme increase slowly. This behaviour is observed for all temperatures tested; besides, the time necessary to achieve the plateau region is independent of the temperature and is around 1080 minutes (18 hours). In agreement with physical adsorption theory, the amount of lipase adsorbed decreases with

1566 Table 7 Rv and Rw of the immobilisations carried out at three Experiments T (~ 1 0 2 25 3 40

different temperatures for 24h. Rv Rw 0,90 3,050 0,92 1,325 0,96 1,275

5001

A

400

....................

!

300

.... ~

~ .............................

200

...............................................................

[]

1

i ............................

~

Wil (mg)

1

lOO o

0

500

1000

1500

Time (rain.)

Figure 1 Dynamic evolution of adsorbed lipase on MCM-41 at three different temperatures. increasing temperature. Since the rate of a chemisorption increases with temperature, we suppose that only a physical adsorption on M41S occurs. The maximum amount of immobilised lipase is equal of 472 mg. The maximum immobilisation efficiency, defined as the ratio between the amount of immobilised enzyme and its initial value, evaluated at 0~ is equal to 47,2%. Compared to lipase immobilisation on other supports, such as membrane [16-17] or zeolites [7], characterised by immobilisation efficiency of about 33%, the proposed process is seems suitable and promising. One of the most important characteristics of immobilised enzymes is the preservation of their catalytic activity throughout the immobilisation procedure. Immobilised and native lipase performances were tested, in exactly the same conditions, for the hydrolysis of triglycerides, contained in olive oil, to fatty acids. The activities of immobilised lipase on M41S type materials are shown in table 8. It can be observed that the highest activity of bounded lipase

1567 Table 8 Activity of lipase immobilised on MCM-41 Temperature of Activity (U/L) Immobilisation (~ 0 12040 25 5600 40 2800

With RESPECT to the activity of the free enzyme 78% 48% 32%

Table 9 Stability of immobilised lipase after repeated use Activity (U/L) Activity (U/L) Cycles Number Enzyme Immob. at 0~ Enzyme Immob. at 25~ 1 12040 5600 2 8400 5040 3 6720 4480 4 5320 4060

Activity (U/L) Enzyme Immob. at 40~ 2800 2100 1540 1484

Table 10 Activity related at first hydrolyses cycle of the lipase immobilised at 0~ Number of Relative Activity (%) cycles 0~ 25~ 40~ 1 100 46,5 23,3 2 69,8 41,9 17,4 3 55,8 37,2 12,8 4 44,2 33,7 12,3 is that corresponding to an immobilisation performed at 0~ its value is 78% of that exhibited by native enzyme. Although lipase lost about 22% of its original activity throughout the immobilisation, this result is satisfactory if compared to the activity loss observed during immobilisation on other types of supports [7,8,16,17], that might even reach 65%, with respect to the initial enzyme activity. In table 9 and 10 the stability and the relative activity of immobilised lipase on M41S materials, after four repeated hydrolysis cycles, are reported. The highest activity was measured at 0~ although this corresponds to a more significant decay after different process cycles. In any case, we observe a high activity for the samples prepared at 0~ Further studies are to be devoted to the comprehension of the deactivation phenomena that affect lipase under such particular immobilisation conditions.

4. CONCLUSIONS First of all, the silica source and the nature of organic cations strongly affect the properties of as synthesised materials. In fact, the samples synthesised from BDH silica gel and TEA + ions show a larger pore diameter after shorter reaction time. The use of Zdosil silica source and TEA + ions in the initial reaction mixture favours the formation of M41S

1568 materials with larger pore diameter, wall-thickness, mesoporous volume and good specific surface area. With different synthesis procedure, it is possible to modulate the characteristics of mesoporous materials in order to optimiser lipase immobilisation. Lower temperatures drastically influence the immobilisation of lipase and, then, the activity and stability of enzymatic catalyst with respect to triglycerides hydrolysis. Finally, the obtained results show that the immobilisation of lipase on mesoporous materials is a very promising alternative as compared to the enzyme immobilisation on other support. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Kim S.M., Rhee J.S., Production of medium-Chain Glycerides by immobilized Lipase in a Solvent-Free System, JACOS, Vol. 68, N ~ 7 (1991). Macrae A.R., Biocatalysts in organic Synthesis, Proceedings of an International Symposium, 14-17 april 1985. Katchalski-Katzir E., Immobilised enzymes - learning from past successes and failures, Tibtech Vol. 11 (1993). Mariani E., Chimica Industriale Applicata, UTET, (1983). Lie E. and Molin G.,J. Chem. Tech. Biotechnol. 50 (1991) 549-553. Goncalves A., Lopez J., Lemos F., Ribeiro F., Prazeras D. M. F., Cabral J., and Aireshbarros M. R., J. Mol. Catal. B-Enzymatic 1 (1996) 53-60. Knezevic Z., Mojovic L., and Adnadjevic B.; Enzyme Microb.Technol., 1998, vol. 22, March. Felipe Diaz J., Kenneth J. Balkus Jr.; J. Molec. Catal. B." Enzymatic 2 (1996) 115-126. Jing He, Xiaofen Li, D.G. Evans, Xue Duan, Chengyue Li; J. Molec. Catal. B: Enzymatic 11 (2000) 45-53. Perry' s Chemical Engineers' Handbook, sez. 17-20, table 17-10 Katovic A., Giordano G., Bonelli B., Onida B., Garrone E., Lentz P., Nagy J.B., Microporous and Mesoporous Materials; 44-45 (2001) 275-281. Tietz NW, Fiereck EA, A specific method for serum Lipase determination, Clin. Chim. Acta 13:352, 1966. R.M. Barrer, Zeolites, 1 (1981) 130; Galarneau A., Desplantier D., Dutartre R., Di Renzo F., Microporous and Mesoporous Materials; 24 (1999) 297-308. Coustel N., Di Renzo F., Fajula F., J. Chem. Soc., Chem. Commun., 1994,967. Bosley J.A., Clayton J.C., Peilow A.D., Immobilisation of Lipase for use in non-aqueous media, Uniliver research, 1995. Taylor F., Lipase Membrane Reactor for Continuos Hydrolisis of Tallow, in Engineering of~with Lipase, Ed. by F.X. Malcata, NATO ASI Series, Vol. 317; pp. 455-472.

ADSORPTION, DIFFUSION, SEPARATION AND PERMEATION

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1571

Adsorption and diffusion of linear and dibranched C6 paraffins in a ZSM-5 zeolite E. Lemaire a, A. Decrette a, J.P. Bellat a*, J.M. Simon a, A. M6thivier b and E. JolimaRre b a LRRS, UMR 5613 CNRS Universit6 de Bourgogne, 9 av. A. Savary, BP 47870, 21078 Dijon Cedex, France b Institut Frangais du P6trole, 1-4 av. du Bois Pr6au, 92852 Rueil-Malmaison, France

The adsorption of n-hexane and 2,2-dimethylbutane on a commercial ZSM-5 zeolite is studied under isothermal and isobaric conditions. ZSM-5 exhibits two different behaviours with linear and dibranched alkanes. A substep at 4 molec.uc 1 is observed on the adsorption isotherm of n-hexane at 348 K. A singular adsorption-desorption process is evidenced on the adsorption isobar at 5.5 kPa with 2,2-dimethylbutane at a temperature close to 343 K when the sample is activated at 298 K under vacuum. The diffusivities have been determined by fitting directly the uptake curves with a numerical resolution of second Fick's law based on finite difference method. Microporous diffusion seems to be rate limiting but surface barrier could also be significant. The diffusivities are almost independent on the filling and are 100 times greater with the linear alkane than with the dibranched one.

1. I N T R O D U C T I O N Over the last years, refining and petrochemicals have become aware of environmental and human health protection. New legislative standards on pollutant emissions impose to reduce some dangerous compounds as aromatic hydrocarbons from the gasoline. However, this reduction would be detrimental to the octane number and a good way to maintain it is to increase the content of highly branched alkanes and to reduce that of linear and monobranched ones. In order to satisfy this octane requirement, C6-C~0 isomerization units will become more common. So it is necessary to be able to separate the different isomers of alkanes after isomerization. MFI-type zeolites may be used to perform such a separation. The adsorption of linear and branched paraffins on silicalite or ZSM-5 is subject to an extended research, often oriented to chemical engineering applications as for example the separation of gas by permeation through a zeolitic membrane 1' 2, 3. Indeed, in the last decade, a lot of experimental data on the diffusion of single paraffins and their mixtures in MFI type zeolites have been published 4' 5, 6. However, the data on adsorption equilibria are less numerous. Regarding the C6 paraffins the adsorption isotherm of n-hexane on silicalite has been largely studied 7, 8, 9, 10 but few results about the adsorption of its branched isomers like 2methylpentane or 2,2-dimethylbutane are quoted in the literature ~' 12. A relatively good agreement is observed among the data on adsorption equilibria given by the different authors. * To whom correspondence should be addressed. Email: [email protected]

1572 On the other hand, the diffusion data are much more scattered. They mainly depend on the technique used (macroscopic or microscopic approach). The adsorption experiments are usually performed on well crystallized samples, synthesized at the laboratory scale and few studies have been done on commercial adsorbents. However, the size and the morphology of zeolite particles, which depend on the synthesis procedure as well as the thermal treatment of the adsorbent before adsorption, may influence the adsorption properties of the material. Moreover, the adsorption of C6 paraffins on MFI zeolite is always studied at constant temperature. No adsorption data under isobaric conditions, which is a convenient way to study the effect of the temperature on the adsorption equilibria, are given in the literature, at our knowledge. This work is then devoted to the adsorption and diffusion of single C6 paraffin isomers in the gas phase as n-hexane and 2,2-dimethylbutane on a commercial ZSM-5 zeolite in order to use it in adsorption/separation process of linear and branched alkanes.

2. EXPERIMENTAL The adsorbent was supplied by the Institut Frangais du P6trole. It was a commercial template-free ZSM-5 without binder. The Si/A1 ratio was of 500. As shown in the scanning electron micrograph in Figure l, the powdered sample is composed of spherical aggregates of different sizes, with a diameter between 1 and 5 ~tm. Each aggregate is composed of interpenetrating crystals with the size in range between 0.2 and 1 gm. The microporous volume estimated by nitrogen adsorption at 77 K is of 0.184 cm3.g-1. This value is in a good agreement with the crystallographic porous volume of ZSM-5 zeolite (0.18 - 0.19 cm3.g -1)13. However, the porous volume determined on the adsorption isotherm at the relative pressure p/po = 0.95 is 0.204 cm3.g-1. This indicates the presence of a great external surface and secondary mesopores. The adsorptives n-hexane (HEX) and 2,2-dimethylbutane (22DMB) were provided by Prolabo with specified purities over 99% and were dried over a 4A zeolite before application. The adsorption isotherms of single HEX and 22DMB on ZSM-5 were measured at 348 and 443 K for pressure ranging from 10 -4 to 10 kPa by using a home made manometric device. The sample (m = 400 mg) was first activated in situ at 673 K under secondary vacuum (10 .5 kPa) for 12 hours and then, cooled to adsorption temperature. The adsorption isotherms were drawn step by step. The adsorbent was submitted to a first amount of pure alkane vapor introduced into the adsorption cell. Once a plateau of pressure was reached, a following equilibrium step was performed by varying the pressure. Kinetics of adsorption (uptake curves) was followed by recording the pressure as a function of time and by this way, the diffusion coefficients were calculated. The adsorption of alkanes under isobaric conditions was performed by means of a Mc Bain

Figure 1: Scanning electron micrograph of the commercial ZSM-5 zeolite. The particles are far from having the coffin shape usually observed on ZSM-5 zeolite.

1573

thermobalance. Prior each adsorption experiment, the zeolite (m = 15 mg) was activated in situ under secondary vacuum either at 673 K for 12 hours or at 298 K for 2 hours. The sample was then submitted to a constant hydrocarbon vapor pressure of 5.5 kPa at 298 K. This step corresponded to the first adsorption up to the quasi-saturation of the micropores. The desorption branch was then drawn by increasing step by step the temperature of the sample under constant pressure from 298 K to 673 K. Additionnal adsorption-desorption cycles were performed by increasing or decreasing the temperature.

2. RESULTS AND DISCUSSION 2.1. Adsorption isotherms

The adsorption isotherms are shown in Figure 2 and 3. They are reversible and exhibit a general type I shape of the IUPAC classification. Nevertheless the adsorption isotherm of n-hexane shows in the low pressure region a slight substep, which occurs at a filling close to 4 molec.uc 1. This substep is more noticeable on Figure 4 with a logarithmic scale for the pressure axis. This phenomenon has been observed by other authors 7' 15 before and with other adsorptives like p-xylene 14 and tetrachloroethene15. The substep is interpreted as the signature of an adsorption mechanism on two different energetic sites. According to Richards et al. 7 the first part of the isotherm would correspond to the adsorption of 4 molecules of n-hexane in the sinusoidal channels and the second one to the packing of the molecules in the straight channels, sinusoidal channels and intersections. However this adsorption model is questionable and the origin of the substep on the adsorption isotherm of molecule with a size close to that of the pore opening is still subject to discussion. Concerning the adsorption capacities, our data are self-consistent and are comparable with the data published for these systems (Table 1). The slight differences observed in the adsorption capacities are attributed to few defects present in our sample. It is worth pointing out on the adsorption isotherms of n-hexane at 348 K that the plateau is not exactly horizontal in the pressure range 2 - 10 kPa. This cannot be attributed to an adsorption on the external surface because the relative pressure is too much low (p/po < 0.08). It seems that the last two molecules of n-hexane encounter difficulties to be adsorbed in the channels. At high filling the steric hindrance becomes important in the micropores and the packing of the last molecules progressively occurs by force of the pressure. As expected, the adsorption capacity 8 -~

~ 6

HEX

4

9

3 O

"z2

--o

.....

o '

9

2

~

1 I

0

2

4 6 p/kPa

8

10

Figure 2" Adsorption isotherms of C6 alkanes on ZSM-5 at 348 K [open circles: experience; solid line: DSL model for HEX and SSL model for 22DMB.

0

2

I

I

4 6 p/kPa

I

8

10

Figure 3: Adsorption-desorption of C6 alkanes on ZSM-5 at 443 K [open circles: experience; solid line: SSL model].

1574

6 O

4

i

Figure 4: Adsorption isotherm of n-hexane on ZSM-5 at 348 K. A diffuse substep marc be noticed at the filling of 4 m o l e c . u c . [open circle symbols: experimental; solid line: DSL model].

~z2 1

0.001

0.01

I

] .....

1

0.1

10

1

p/kPa is lower for the dibranched isomer than for the linear isomer. In the low pressure region the slopes of isotherms decrease according to the sequence HEX>22DMB, indicating that the adsorption affinity of the adsorbent decreases in the presence of methyl groups branched along the carbon chain. The adsorption isotherms are fitted with the Dual Site Langmuir (DSL) model or with the Single Site Langmuir (SSL) model defined by the well-known relations: a

KIP

(DSL)

Na = Nsl 1 + KiP

(SSL)

N a =

+Na2

K2P 1 + K2P

Kp

N~ 1 + Kp

where Nas is the amount adsorbed at saturation (relative pressure p/po = 1). These models are commonly used for the adsorption of alkanes in MFI zeolites and give generally a good fit with the experimental data. The values of the Langmuir parameters are given in Table 2 even though they may lack physical significance as outlined by Ruthven 16. As shown on Figures 2,

Table 1" Comparison of amount of C6 alkanes adsorbed on ZSM- 5 under a pressure of 2 kPa and corrected diffusivities with few data found in the literature. Reference This work (Do at zero filling) Richards et al. 7 Cavalcante et al. 11

T/K 348 443 343 373 423 448 473

Wu et a117 Sun et al. 18 Millot et al. 19 Post et al. 2~

373 473 343 353 348 473 423

,,

N a /molec.uc "l HEX 22DMB 6.3 2.5 2.0 1.1 7.0 4.4 1.15 7.5 7 1.2 -

-

Do / mZ.s -1 HEX 22DMB 1.6 10 -18 2.6 10 -16 3.3 10 "16 3.3 10 15 8.9 10 -13 9 10 -16 -

1.5

-

1.0 0.7 -

4.4 10 -12 1 10 -17

1575 Table 2: Parameters of the DSL and SSL models. The maximal adsorbed amount molec.uc -~. The constants K are given in kPa -~. HEX DSL

Nasl

or

Nas2 K2

K1

SSL

348 4 70 4 0.9

Nas is expressed in

22DMB 443 5 0.36 -

348 2.89 5.48 -

443 2.8 0.34 -

3 and 4 the fit of the adsorption isotherms is reasonable. Nevertheless, Langmuir's models do not offer sufficient accuracy for estimation of the thermodynamic correction factor for the measured diffusivity values (see below). Moreover two adsorption isotherms are not sufficient to derive adsorption enthalpies and entropies derived from the Langmuir parameters. 2.2. Adsorption isobars The adsorption isobars of n-hexane and 2,2-dimethylbutane are displayed on Figures 5 and 6. They show that the adsorption process is strongly dependent on the activation temperature of the zeolite. After activation at 673 K under vacuum, the adsorption-desorption isobars of the two C6 alkanes have the same shape with reversible adsorption-desorption cycles, while after activation at 298 K a singular behavior is observed with 2,2-dimethylbutane. Indeed, during the first desorption, an unusual increase of the adsorbed amount is observed between 343 and 403 K (Figure 6). Such a phenomenon is thermodynamically inconsistent and has never been observed on ZSM-5. Above 403 K, the adsorbed amount decreases as expected and the adsorption-desorption cycles become reversible and similar to those observed after activation at 673 K. Several hypotheses can be put forward to explain this singularity as for example, an appearance of new accessible adsorption sites, an activated adsorption process or a structural change of the adsorbent. It is worth noticing that the well-known monoclinicorthorhombic phase transition of ZSM-5 occurs at a temperature closed to 343 K under vacuum 21. Even so in the state of this work this particular behavior cannot be clearly elucidated, we think that the more convenient explanation is to consider an adsorption process on two different energetic sites with a maximal adsorbed amount estimated at 3 molec.uc 1. 3 6 O

-

d 2

4-

O

~Z 2 0 200

Z 9 300

400 500 T/K

600

700

Figure 5: Adsorption-desorption isobar of n-hexane on ZSM-5 under 5 kPa [open circle: activation at 673 K; full circle: activation at 298 K].

10 200

300

400

500 T/K

600

700

Figure 6: Adsorption-desorption isobar of 2,2-dimethylbutane on ZSM-5 under 5.5 kPa after activation at 298 K [full circle: first desorption; open circles: second adsorption-desorption cycle].

1576 Both sites would be accessible in the orthorhombic phase while for steric reasons only the weak energetic sites would be occupied in the monoclinic phase. Thus, at low temperature three molecules would be adsorbed on the weak energetic sites and progressively desorbed as the temperature increases. At 343 K the phase transition occurs and the strong energetic sites become accessible. Then the molecules are preferentially adsorbed on the strongest energetic sites, which exhibit a better adsorption affinity than the others at this temperature. This would explain why the adsorbed amount increases between 343 K and 403 K. Obviously, above this last temperature the molecules adsorbed on these strong sites are desorbed in turn. Whatever the activation temperature, after a first adsorption-desorption cycle, the isobars exhibit at low temperature a plateau characteristic of the adsorption in a finite volume. The amount of alkanes adsorbed at 300 K are given in Table 3. Let assume the adsorbate as a liquid, the volume of n-hexane adsorded at this temperature is of the same order of magnitude as the microporous volume of the zeolite, n-hexane occupies almost the channel volume as opposed to 2,2-dimethylbutane, which occupies a significant lower volume. 2.3. Diffusion

Kinetics of adsorption of C6 isomers has been studied only for samples activated at 673 K under vacuum with a transient macroscopic method by means of the manometric apparatus. As adsorption experiments are performed on a thin bed of pure crystalline powder, the intracrystalline diffusion is considered as the rate limiting step of the adsorption process. The diffusion coefficients of the adsorbed species are determined from uptake kinetic curves by solving numerically second Fick's law under specific conditions. According to the SEM picture (Figure 1), the model of the zeolite particle is taken as spherical with a radius rc of 1 gm. The diffusion coefficient is assumed to be constant and isotropic over each analysis. Under these conditions Fick's law can be reduced to its spherical expression: 0Na0t = DI02Na0r 2 + r0r20Na 1 where D is the Fick diffusion coefficient, N a the adsorbed amount, r the spherical radius from the center of the zeolite particle and t the time. Fick's law is integrated over time and space using a Finite-Difference method, called Forward Time Center Space 22. The particle is divided into 1000 spherical layers of constant thickness 1 = 1 nm. The computational procedure is as follows. At the initial step, the amount of adsorbed molecules is distributed homogeneously between all the layers in equilibrium with a fixed volume of gas, Vg, under a pressure given by the adsorption isotherm at a fixed temperature. Then the zeolite particle is submitted to a higher pressure of gas according to the experimental conditions. The outer surface layer (layer 1) is assumed to be in instantaneous equilibrium with the gas phase given by the isotherm curve. Second Fick's law is then applied to the inner layers allowing the filling of the zeolite. The mass balance on the whole system gives the evolution of the Table 3: Adsorption capacities derived from the adsorption-desorption isobars at the temperature of 300 K. HEX 22DMB N a / molec.uc 1 7.3 2.81 V a / cm3.g-1 0.166 0.065 P/Po 0.22 O. 11

1577 quantity of gas in the volume Vg and the adsorbed amount. This numerical resolution of the diffusion equations requires the specification of only one boundary condition: during adsorption, the gas phase concentration on the outer surface of the zeolite particle reaches instantaneous equilibrium as soon as the adsorbent is brought into contact with the gas. This condition is fixed by the adsorption isotherm. The corrected diffusivities are calculated by means of the Darken equation: 0LnNa m D D O= D ~ 0Lnp F The thermodynamic factor F is determined by numerical derivation of the experimental adsorption isotherms. Figure 7 shows that the uptake curves are rather well described with the second Fick law. However less accurate fits have been obtained for certain loadings. These can be attributed to experimental errors. Surface barriers at the outer layer of the zeolite particle are not excluded. As shown on Table 1 the values of the corrected diffusivities are in good agreement with those given in the literature for 2,2-dimethylbutane but they are much lower for n-hexane. This could be attributed to nature of our commercial adsorbent. The corrected diffusivities are quasi-independent of the loading compare to the experimental errors (Figure 8). As expected, the corrected diffusivities increase with the temperature and are higher with the linear alkane than with the dibranched one.

3. CONCLUSION The ZSM-5 zeolite exhibits two different behaviors with n-hexane and 2,2-dimethylbutane according to the experimental conditions and the activation temperature. Under isothermal conditions and after activation at 673 K a substep is observed on the adsorption isotherm at 348 K with n-hexane. This linear paraffin probes almost the whole porosity of the adsorbent while the dibranched paraffin probes less than 40 % of the porosity. Under isobaric conditions and after activation at 298 K under vacuum, the zeolite shows a singular behavior with 2,2-dimethylbutane, which is not observed after activation at 673 K. These particular phenomena could be interpreted as the result of an adsorption mechanism on different energetic sites. On the other hand, the monoclinic-orthorhombic structural change that

011 -~

1014

0

_

HEX

"~ 0.4

~

~

i 500

1000

1500

2000

t/s Figure 7: Uptake curves for HEX and 22DMB on ZSM-5 at 348 K for the first loading,

t

22DMB 10 "20

0

~U

0

1

2

3

I

I

I

I

4

5

6

7

8

N a/molec.uc -1 Figure 8: Corrected diffusivities of HEX and 22DMB in ZSM-5 at 348 K versus loading.

1578 undergoes the ZSM-5 zeolite at 343 K is suspected to play an important role in this special adsorption process. From the kinetic point of view, the numerical resolution of the second Fick law with a Finite-Difference method allows to determine the diffusivities by fitting the uptake curves. Microporous diffusion seems to be rate limiting but surface barrier could also be significant. The diffusivities are almost constant with the loading and are more than 10 times greater for the linear alkane than for the dibranched one. A similar study with a monobranched alkane (2-methylpentane) and with linear and branched alkanes mixtures is in progress but it seems obvious that the adsorption selectivity would be governed by kinetics. The adsorption mechanism of alkanes in MFI zeolites is very complex as with aromatic or chlorinated compounds. In order to elucidate the special behaviors observed in this work, detailed studies of the adsorption of linear and branched C6 paraffins by differential calorimetry, in situ X-Ray diffraction and molecular dynamics are in progress.

REFERENCES 1. B. Millot, A. M6thivier, H. Jobic, H. Mouedded and J.A. Dalmon, Microporous and Mesoporous Materials, 38 (2000) 85. 2. R. Krishna and D. Pashek, Separation Purification Technology, 21 (2000) 111. 3. R. Krishna, L.J.P. van den Broeke, The Chemical Engineering Journal, 57 (1995) 155. 4. J; Xiao and J. Wei, Chemical Engineering Sciences 47 5 (1992) 1143. 5. C.L. Cavalcante and D.M. Ruthven, Ind. Eng. Chem. Res., 34 (1995) 185. 6. W. Zhu, F. Kapteijn and J.A. Moulijn, Microporous and Mesoporous Materials, 47 (2001) 157. 7. R.E. Richards and L.V.C. Rees, Langmuir, 3 3 (1987) 337. 8. Y. Yang and L.V.C. Rees, Microporous Materials 12 (1997) 117. 9. L. Song and L.V.C. Rees, J. Chem. Soc. Faraday Trans., 93(4) (1997) 649. 10. B. Millot, A. M6thivier and H. Jobic, J. Phys. Chem. B 102 17 (1998) 3210. 11. C.L. Cavalcante and D.M. Ruhven, Ind. Eng. Chem. Res., 34 (1995) 34. 12. B. Millot, A. M6thivier, H. Jobic, I. Clemen~on and B. Rebours, Langmuir 15 (1999) 2534. 13. D.H. Olson, W.O. Haag, R.M. Lago, J. Catalysis 61 (1980) 390. 14. D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J. Phys. Chem. 85(15) (1981) 2238. 15. F. Bouvier and G. Weber, J. Thermal Analysis 54 (1998) 881. 16. D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley and Sons, New York, 1984. 17. P. Wu, A. Debede and Y.H. Ma, Zeolites 3 (1983) 118. 18. M.S. Sun, O. Talu and D.B. Sha, J. Phys. Chem. 100 (1996) 17276. 19. B. Millot, PhD Thesis, Universit6 Claude Bernard Lyon I, France, 1998. 20. F.M. Post, J. van Amstel, H.W. Kouwenhoven, Sixth International Zeolites Conference, Proceedings, Reno, NV, July 1983, Olson D.H. and Bisio A. Eds, Butterworth; Guilford UK, 1984, 517. 21. W.C. Conner, R. Vincent, P. Man and J. Fraissard, Catalysis Letters 4 (1990) 75. 22. W.H. Press, S.A. Teukolsky, W.T. Vetterling and B.P. Flannery, Numerical Recipes in Fortran 77,Cambridge University Press, 1992.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

ADSORPTION OF INDOLE AND BENZOTHIOPHENE ZEOLITES WITH FAUJASITE STRUCTURE

1579

OVER

J.L. Sotelo, M.A. U guina, V.I./kgueda.

Chemical Engineering Department. Faculty of Chemistry. Complutense University of Madrid. 28040, Madrid, Spain. ingenieriaq@quim,ucm.es This work addresses the adsorption of indole and benzothiophene as model heteroring compounds of nitrogen and sulfur impurities, present in liquid fuels like gasoline and diesel, over zeolites with faujasite structure. It has been studied the influence of the Si/A1 molar ratio, the exchanged cation and the presence of toluene considered as a model aromatic compound present in liquid fuels. Cristallinity (by XRD), exchange level (XRF), and basic properties (CO2 TPD) of different adsorbents have also been determined. The Si/A1 molar ratio and the alkali cation exchanged have a big influence on the selective adsorption of indole and benzothiophene. Competitive adsorptives present in fuels, such as toluene, affect to the selectivity to benzothiophene adsorption rather than the corresponding to indole. Thermogravimetric analysis of the spent adsorbents shows a strong indole adsorption. Finally, a comparative study between nitrogen and sulfur adsorption confirms that nitrogen compounds could be selectively removed from fuels. 1. INTRODUCTION European legislators have introduced new specifications for low sulfur content in fuels in order to reduce exhaust emissions. For instance, a maximum sulfur level of 150 ppm and 350 ppm for gasoline and gas-oil respectively has been established. More restrictive specifications (< 50 ppm of S for both fuels) will be reached by 2005 [1]. Hydrodesulfurization (HDS) process is extensively used to remove sulfur in fuels. Deep desulfurization conditions will be required to achieve these novel specifications. For hydroprocessing, nitrogen-containing compounds are the most common poisons due to their strong adsorption on catalyst sites. The HDN of N-containing heterorings requires more severe conditions than those for other heteroring compounds [2]. Part of the nitrogen is strongly bounded to the catalyst and can not be removed during regeneration [3]. HDN produces NH3, resulting in strong deactivation of the catalyst [4]; several studies propose novel catalysts in order to decrease nitrogen compounds effects [5]. In this context, adsorption appears as a soft technology for nitrogen removal before hydrotreatment to improve HDS at milder operation conditions [6-7]. Besides, sulfur refractory compounds to HDS could be removed either before or after hydrotreatment [8-11 ]. Low silica zeolites (A and X) can be used for the selective adsorption of polar or polarizable molecules such as water, carbon dioxide or sulfur and nitrogen containing molecules [12].

1580 The aim of this work is to study the adsorption of a N-containing heteroring compound (indole) using several zeolites (FAU framework) as adsorbents. The influence of an aromatic compound (toluene) on the adsorption capacity has been studied. Finally, the competitive adsorption of indole and a S-containing compound (benzothiophene) has also been analyzed for these materials. 2. EXPERIMENTAL

2.1. Adsorbent preparation Several adsorbents based on zeolites with FAU structure with different Si/A1 molar ratio have been used. Commercial NaY and NaX zeolites were obtained from Grace Davison and Aldrich, respectively. Low silica X zeolite (LSX) was synthesised on the laboratory according to Ktihl [13], using sodium aluminate (Carlo Erba), sodium hydroxide (Merck), potassium hydroxide (Merck) and sodium silicate (Merck) as raw materials. The starting gel was previously aged and then crystallised at 343K. The solid obtained in this way was filtered and washed with 0.01M NaOH solution to avoid protonation. Alkali-exchanged LSX zeolites were prepared by conventional ion-exchange procedure: 10 ml of 0.5M aqueous solutions of sodium, potassium and cesium chlorides were added per gram of zeolite at 333K under stirring for 30 minutes. Then the zeolites were filtered and a fresh solution of alkali metal chloride was added. After the last exchange, the zeolite was washed with 0.01M solution of alkali hydroxide until no chloride ions were detectable in the filtrate. The as-synthesised NaKX zeolite (LSX) was exchanged three times with sodium chloride solutions to obtain the NaLSX sample. In the same way, KLSX and CsLSX samples were prepared from the NaLSX zeolite using potassium and caesium chloride solutions, respectively. 2.2 Characterization of the adsorbent BET surface area was calculated from nitrogen adsorption-desorption measurements at 77K using a Micromeritics Asap-2010 instrument. X-ray diffraction (XRD) patterns were performed on a Philips diffractometer (X'pert MPD) with CuK~ radiation and Ni filter. Infrared spectra were recorded on a Nicolet 510P FTIR instrument with a resolution of 2 cm -1 using the KBr wafer technique. Chemical composition was determined by X-ray fluorescence (XRF) in a Philips PW-1480 instrument. 27A1 and 29Si MAS NMR spectra were obtained on a Varian VXR-300 spectrometer equipped with a Jacobsen probe. Basic properties of the alkali ion-exchanged zeolites were determined by temperatureprogrammed desorption (TPD) of CO2. Zeolites were outgassed under a helium flow at 773K, then cooled to 373K and saturated with CO2 for lh. Afterwards, the zeolites were purged with helium at the same temperature to remove the physisorbed CO2. Finally, TPD was run at a heating rate of 10 K-min -1 to 773K. TGA of spent adsorbents were performed on a Exstar 6000 of Seiko Instruments Inc. with the module TG/DTA 6200 under a He flow of 30 cm3"min -1, with a heating rate of 10 K.min -1 up to 873K. 2.3. Adsorption experiments Adsorption experiments were carried out in closed Pyrex tubes with Vyton seals to avoid vapour losses, placed in a thermostatic bath. Initial mixture was prepared with indole

1581 (Aldrich) in anhydrous cyclohexane (Merck) with a nitrogen content of 500 ppm. 5 g of this mixture were stirred with different weights of adsorbent at 298 K during 48 h. All adsorbents were previously activated by calcination for 2 h at 723 K. After adsorption, the adsorbents were recovered by centrifugation and dried at 298 K before TGA. To determine the influence of a competitive adsorptive on the capacity and selectivity to the nitrogen compound, 5% wt of toluene (Merck) was added to the initial mixture. Nitrogen and sulfur adsorptions were compared adding benzothiophene to the mixture as a compound with the same indole structure (500 ppm of sulfur). Nitrogen concentration was measured by GC. Liquid phase sulfur analyses were performed by XRF, in a Bruker $4 Explorer wavelength-dispersive X-ray spectrometer based on multi-layer analyser, using S-Ka spectral line in 40 mm PE-HD cells, with 2.5 bun Mylar film windows in atmospheric helium mode. Acquisition time was 100 s.

3.RESULTS AND DISCUSSION 3.1. Adsorbent Properties Chemical composition, CO2 TPD, and surface area of the adsorbents studied are summarized in Table 1. Figures 1 and 2 present CO2 TPD of the adsorbents. Table 1 Properties adsobren_t of the, Si/A1

Adsorbent NaY NaX NaLSX KLSX CsLSX _

_

~

__,_~s ....................................................

,

~

,

Na K Cs (Na+K+Cs) (Na+K+Cs) (Na+K+Cs)

CO2 TPD

Tm~x BET area

(molar)

(molar)

(molar)

(molar)

(mmol-g -1)

(K)

(m2-g-1)

2.8 1.3 1.1 1.1 1.1

1 1 0.95 0.25 0.45

~ -0.05 0.75 ~

~ -m ~ 0.55

1.12"lff~ 1.49-10 -3 2.23.10 -3 1.57-10 -3 9.57.10 -4

533 593 608 583 503

844 847 765 715 507

~

Table 1 shows that the higher A1 content logically corresponds to the low silica X (LSX) zeolite samples prepared, with a Si/A1 ratio of 1.1. The CO2 TPD measurements show a foreseeable increase with the A1 content of the adsorbent, as the number of adsorption sites increases too. Lower values of CO2 TPD are obtained for cation exchanged materials due to the higher atomic weight and volume of these cations, hence the number of basic sites per gram of adsorbent decreases and the possible hindrance to CO2 adsorption should be taken into account. The temperatures corresponding to the maximum CO2 desorption (Figure 2) increase in the order: CsLSXKLSX>>CsLSX.

1582 0,15 ":'. 0,14


,."

j"

0,13-

6

",,

NaY - . . . . . . NaX -.......... NaLSX

"',

0,15

I ........... NaLSX

0,14

.."

,.

0,12

0,12-

' " ...... '"

/ .............KLSX "'.. [ ~ CsLSX

"J"........,, ' , : - \ ,I" ",, ",,

o,13

...:: ......,, ..,.

.

~9

.. .... .." "" "

.,,.,.

.,% .........:=;::;""

~

_,.Sf::'"

.......... "....... "'~-~--.

"..........'::'::::-'"

0,11

0,11 460

560 660 Temperature (K)

760

400

Fig. 1. Influence of Si/A1 on CO2 TPD

500 600 Temperature (K)

700

Fig. 2. Influence of alkali cation on CO2 TPD

3.2 Adsorption equilibrium Indole adsorption capacities have been determined varying the Si/A1 molar ratio, the exchanged cation and considering the presence of a competitive adsorptive (toluene). Benzothiophene (BT) adsorption capacities and the competitive adsorption of indole and benzothiophene have also been studied. 3.2.1 I n d o l e a d s o r p t i o n

Table 2 shows the maximum adsorption capacities of indole (mg of nitrogen adsorbed/g of adsorbent) in the absence of toluene and when a 5% wt of toluene is present as a competitive adsorptive for the different adsorbents. The indole adsorption capacity is lower as both Si/A1 molar ratio and the electronegativity of the exchanged cation decrease.

.

Table 2. Maximum adsorption capacities of indole. Adsorbent NaY qD (mg N/g of adsorbent) 0% tol 33 qD (mg N/g of adsorbent) 5% tol 23.5 .

.

.

.

.

.

.

-

~ , ~ . ~ , _ ~ .

NaX 23 21.5

NaLSX 10 9.8 ~

-

~-.

.

KLSX 5 5 .

.

.

,. . . . . . . . . . . . CsLSX 4 4 - _

.

.~

.

-.~,

For a better analysis of toluene effect on the indole adsorption, selectivity for each adsorbent are obtained by dividing the maximum adsorption capacities with and without toluene, avoiding the effect of the variables related to the sorbent, like surface area. 100

0 NaY

NaX

NaLSX KLSX

Fig. 3. Selectivities to indole adsorption.

CsLSX

1583 As shown in figure 3, selectivity would be enhanced by both decreasing the Si/A1 molar ratio, and exchanging faujasites with low electronegative cations, despite the reduction in the adsorption capacities. The decrease in selectivity is due to the lower aluminum content, i.e. the lower number of adsorption sites. Exchanged adsorbents present higher selectivities due to their basic properties that favour the adsorption of polar molecules. TGA plots of the recovered adsorbents (Figures 4 and 5) show a first weight loss at temperatures lower than 523 K that corresponds to solvent thermodesorption and to indole that is adsorbed weakly. A second peak appears at temperatures higher than 523 K due to indole strongly adsorbed. Figure 5 also shows that the higher the Si/A1 molar ratio, the higher the indole desorption temperatures. The temperature of desorption also increases when faujasites are exchanged with less electronegative cations. These effects could be explained considering two factors: the interaction of rt electron cloud of the indole molecule with the cations, and the interaction of hydrogen atoms present in the molecule with the basic oxygen atoms of the zeolite framework [14]. The lower the Si/A1 molar ratio, the less the charge density on the cation, resulting in a lower interaction with the rt electron cloud, so the temperature of desorption should be lower. As the cation electronegativity is lower, its charge density and the interaction with the rt electron cloud decrease, although the basic character is enhanced, being stronger the interaction with hydrogen atoms. For aromatics such as toluene, interaction with adsorption sites is produced through cations rather than through the oxygen basic sites because of its rt electron cloud. Therefore, toluene should have a strong effect on adsorbents with high Si/A1 molar ratio or exchanged with high electronegative cations. ,"! ," ~.!

NaY - . . . . . . NaX .......... NaLSX

].i [.~

300

:',, .'i i:"~! t'.~ f i~

560 T (K)

660

760

.............. KLSX ............. CsLSX

~/-";!k..,:.'~',

Indole desorption

460

........... NaLSX

860

Fig. 4. Si/A1 influence on TGA of indole.

300

400

Indole desorption

500

600

T 0~)

700

800

Fig. 5. Cation influence on TGA of indole.

3.2.2 Benzothiophene adsorption Figures 6 and 7 show the benzothiophene adsorption isotherms. The amount of BT adsorbed decreases in all cases when a 5% wt of toluene is present as a competitive adsorptive.

1584

E -~

6o1"1 4 50 ~

o

...........

,:.-.'......... :

"o 3 0

...-~,~..-.~x ~

~ta~

I ......zx...... N a L S X

[ ..... ....

,,," 20

/"

o

~o 0

100

200

300

C q (ppm S)

400

Fig. 6. Adsorption isotherms of BT without toluene.

.~ "" "

$..o--

;

~ r

~0

.v

,v' . ~ ,

20

E lO

NaY

---<>--- N a X

__. . . . . ~7 - - - - v - - - - K L S X

.~.'"

4

30

--u-- NaY ---o--- NaX

..-o

0

O"

......... ~.-

.o .......

......v- ..... K L S X ..... o ..... C s L S X

. . . 0 . . 0 ....... O" .... 0

~ ...... o .......

160

-.....zx...... N a L S X -o

...0"

260 360 Ceq(ppm S)

460

"

Fig. 7. Adsorption isotherms of BT with a 5% of toluene.

Faujasites appear as good adsorbents for sulfur-heteroring compounds such as benzothiophene, because of the higher capacities of adsorption, even when a competitive adsorptive is present. Benzothiophenes and dibenzothiophenes are the most refractory compounds to HDS. Hence, faujasite could be used to eliminate selectively sulfur-heteroring compounds from fuels. Selectivities to benzothiophene adsorption (figure 8) increase as the Si/A1 molar ratio decreases, due to the higher amount of adsorption sites. Ion exchanged faujasites show a different trend when LSX is exchanged with Na or K and when LSX is exchanged with Cs. Selectivity increases as the electronegativity of the cation exchanged is lower. For Cs exchanged LSX, selectivity decreases attributed to the reduction in adsorptive sites per gram of adsorbent, and to steric effects that should be the highest for BT. Also, the presence of two types of cation adsorption sites (Na and Cs) due to the low exchanged level reached for CsLSX adsorbent (table 1) should be considered. TGA plots of recovered solids show a desorption peak at temperatures lower than 573 K due to solvent and benzothiophene desorption. Figures 9 and 10 show slight differences in the desorption temperatures. Thermodesorption temperatures of indole (figures 4 and 5) are higher than those observed for benzothiophene (figures 9 and 10) because indole interaction with the adsorbent is stronger due to its higher polarity. These results explain the higher selectivity to indole (figure 3) in comparison with the benzothiophene ones (figure 8) for all the adsorbents. 80

~60

~20

lXhY lXhX ~ ~ Fig. 8. Selectivities to benzothiophene adsorption.

1585

............ N a L S X

/'x

: ...........

/I.'": ........

r~

\ t~i. y

,,i

...,..........!%... ..... 400

500

x

BT desorption------ ~

des~176

"..",,

300

:

A/".,../:';:":'~,\~

x

600

T(K)

700

300

400

500

-_,,

-,,

600

700

T (K) Fig. 10. Alkali cation influence on BT TGA.

Fig. 9. Si/A1 influence on BT TGA.

NaY, and CsLSX adsorb BT stronger than the other adsorbents as it was explained before. CsLSX TGA shows two peaks at temperatures higher than 573 K due to the two cation adsorption sites. Therefore, these materials could interact strongly with toluene. Their lower selectivities could be also explained taking into account the strongly interaction with toluene in spite of their higher temperatures of desorption. 3.2.3 Indole vs. benzothiophene adsorption Figure 11 shows the resulting indole and BT concentrations in equilibrium with different weights of adsorbent when the Si/A1 molar ratio is changed. In binary mixtures indole is selectively removed, whereas BT is only significantly adsorbed when indole has been practically retained from solutions. Lower amounts of adsorption sites are needed to begin sulfur adsorption when Si/A1 increases, despite the lower amount of adsorption sites per gram of adsorbent. Benzothiophene adsorption is favoured on NaLSX, according to the higher slope of the curve in figure 11. Faujasites exchanged with low electronegative cations (figure 12) did not show different trends than those explained before. 600 -

B...T.T

.... [] .... N a Y

.... o .... N a X

.... zx.... N a L S X

Indole

---B--- NaY

--o--- NaX

---A---NaLSX

600 -

500- 1~":0 400-

'~, ,

300,

r/3

500-

\,,",, 9

,.?o...o ....,,........... "

""i,"., ,

9

-

~.~ 2 0 0 .

""-'A " ..

[]'.

g ~

100,

,,

~

:.. ". ~

c~--.....

/--II-'-------~-

0'

0,0

011

Indole

" . ~9

",&

r..)~

&...

".....9"-*". . . . . . : ~ : ; : ~

0 1 2 " 0 1 3 '

Fig. 11. Si/A1 influence on competitive adsorption of indole and BT.

.... O---- C s L S X -- ~--- CsLSX

"...v-~._r

~,

"v,

.... 4~

..... " ..... "..

~,,

1000"

0,4

0,0

"......... v -

.......

v-2

200 -

-.

Weight of adsorbent ( g )

.... v---- K L S X ---v--- KLSX

"A

300-

,

"-'-

,,

BT

.... zx----NaLSX ---A--- NaLSX

;~-,-f1: . . - : ~ ........ ",~-: : ~ . -. . ~. .... ~~x-..~ ........ ^v ....... ....... :: . . . . .O .................

400 -

".. "..

". ,,.

~.

B...~T

Indole

"& /X'..

o11

o12

Weight of adsorbent ( g )

o13

"V 0,4

Fig. 12. Cation influence on competitive adsorption of indole and BT.

TGA plots of spent adsorbents (figures 13 and 14) show a first thermodesorption peak at temperatures lower than 473 K due to solvent, weakly adsorbed benzothiophene and indole, as well as a second peak at temperatures higher than 573 K due to strongly adsorbed indole.

1586 ,,',

~"

NaY

i /il:.~,., / ;~;.

" ''-

BTdesorption

I i I ["" \ ", /

]

11

~ -

.......... NaLSX

- ...... NaX

/I

1/,

............. K L S X

..........

llI

~" |1 !i/ii

!I ~~'~~ ~i/~11."::'. . .-.'":'~."').~",..,.BT~""f desorption ............

Indole desorption

-......... - ~ Indoledesorption

.....

300

400

500

600

700

800

T (K) Fig. 13. Si/A1 influence on TGA in BT and indole competitive adsorption,

900

300

400

500

600

700

800

900

T (K) Fig. 14. Cation exchanged influence on TGA in BT and Indole competitive adsorption.

4. C O N C L U S I O N S From the results obtained in this work, adsorption can be considered as a soft technology for fuel denitrogenation and desulfurization. Selective adsorbents for HDS poisoning compounds such as indole have been prepared from low silica FAU zeolites. Low electronegative metal exchange improves indole adsorption selectivity of these zeolitic materials in presence of other adsorptives, although the adsorption capacities are reduced. Thus, poisoning nitrogen compounds for HDS catalysts could be selectively removed from fuels and therefore a better performance in the higher desulfurization levels could be reached. 5. A C K N O W L E D G M E N T S Authors thank the financial support from FEDER European Project 2FD 1997-1873. 6. R E F E R E N C E S

1. 2. 3. 4. 5. 6.

European Directive 1998/70/CE. E. Furimsky, F.E. Massoth, Catal. Today, 52 (1999) 381-495. E. Furimsky, Ind. Eng. Chem. Res. 12 (1996) 4406-4411. Seunghan Shin et al, Appl. Catal. A: General 205 (2001) 101-108. Senzi Li et al, Appl. Catal. A: General 184 (1999) 1-9. Zeuthen P. et al., A Combined process for improved hydrotreating of diesel fuels, EP1057879 (2000). 7. G. Gadja et al., Process for the removal of nitrogen compounds from an aromatic hydrocarbon stream. US Patent N ~. 5744686 (1998). 8. J.L. Sotelo et al, Stud. Surf. Sci. Catal., 135 (2001) 227. 9. A.S.H. Salem and H.S. Hamid, Chem. Eng. Technol., 20 (1997) 342. 10. H.A. Zinnen et al., Removal of Organic Sulfur Compounds from FCC Gasoline using Regenerable Adsorbents, US Patent No. 5 935 422 (1999). 11. R. Bartek, Removal of Sulfur from a Hydrocarbon Stream by Low Severity Adsorption, US Patent No. 5 919 354 (1999). 12. M.E. Davis, Ind. Eng. Chem. Res., 30 (1991) 1675-1683. 13. G.H. Kilhl, Zeolites, 7 (1987) 451-457. 14. D. Barthomeuf, Catal. Rev., 38, No. 4 (1996) 521.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1587

Determination of Microporous Structure of Zeolites by t-Plot Method - State-of-the-Art P. Hudec a, A. Smie~kovda, Z. Zidek a, P. Schneider b, O. Solcovd b aDepartment of Petroleum Technology and Petrochemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinsk6ho 9, 812 37 Bratislava, Slovak Republic bInstitute of Chemical Process Fundamentals, Academy of Science of Czech Republic, 165 02 Prague 6, Czech Republic e-mail: phudec@chelin, chtf. stuba, sk,

The possibility of the use of t-plot for the determination of changes in the microporous structure of zeolites is demonstrated for various methods of t-plot construction. By modified BET-isotherm the value of C-constant of non-microporous part of zeolites was determined. Characteristics of different zeolites by Harkins-Jura master isotherm, modified BET-isotherm and Lecloux n-method were very close each to other. The range of t-plot linearization is discussed and the possibility of the differentiation between ultra- and super-micropores is shown.

1. INTRODUCTION Pure zeolites or zeotypes with perfectly crystalline structure possess only micropores determined by their pore structure predicted on the base of crystallographic data. Modification of zeolites and related zeotypes by acid or steam dealumination very often leads to changes in their microporous structure and to the creation of a secondary porous structure, obviously mesoporous [1-4]. Undesired dealumination by microatmosphere of water vapor can occur even during the removing of organic templates after synthesis or during the calcination of ammonium or H-forms before various use, if zeolites are heated in not perfect shallow-bed conditions or without sufficient flow of dry carrier gas. It is very important to characterize these structural changes from the point of view of selectivity in catalytic and sorption processes. The most enhanced method for the characterization of changes in the surface and porous properties of solids is physical adsorption of nitrogen at the temperature of liquid nitrogen-196 ~ Results of sorption data obtained with different man-made or commercial equipments are the most frequently treated with BETisotherm, which is a standard method for nitrogen adsorption data treatment. Even though the BET-method is not valid for microporous solids, because the volume filling of micropores instead of the multilayer adsorption of nitrogen there proceeds, values of the specific surface SBET are generally used for the characterization of zeolite samples. To obtain more information

1588 by separation of adsorption in micropores and mesopores, several methods has been used: tplot method [5], ors-plot method [6] or Dubinin-Radushkevitch isotherm [7]. One of the best ways is the analysis of nitrogen adsorption data by the t-plot method which provides the volume of micropores, specific surface of mesopores and the external specific surface. The t-plot method compares adsorption isotherm of a porous sample with an (master) isotherm of nonporous solid of similar surface chemical nature. The dependency of adsorbed volume Va versus t - the statistical thickness of adsorbed layer of nitrogen, linearized in the range of the second adsorbed nitrogen layer formation, gives slope proportional to the specific surface of mesopores and external surface, and intercept on Va axis gives the specific volume of micropores. The txs-plot is similar comparative method, replacing t by cx~ - ratio (Va/Vx), taking the value cz~-I at P/P0=0.4. Dubinin-Radushkevitch isotherm in general overestimate the micropore volume so it is not very frequently used for zeolites evaluation. Although the t-plot and czs methods are frequently used for the evaluation of the microporous structure of zeolites, details (master isotherm used, shape of the t-plot or CZs-plot, range of linearization) are usually not mentioned [8-11 ]. Only rarely detailed information about the master isotherm are given as well as comments on the problems of interpretation of different shapes oft-plots and suitable ranges for t-plot linearization [12-14]. In this work, we summarize the results of t-plot application on a wide range of various basic as-synthesized zeolite samples and comment the suitability of different t-plot application, linearization and interpretation of results.

2. EXPERIMENTAL 2.1. Zeolite samples Zeolite samples Ca-LTA (A-type, Si/AI=I), Na-MOR (mordenite, Si/A1-5.2, Na-FAU (Y-type, Si/Al=2.5), Na-MFI (ZSM-5 type, Si/Al=14.2) and NaK-ER/ (T-type, Si/Al=3.2) came from Research Institute for Petroleum and Hydrocarbon Gases (VURUP-Slovnafi), Bratislava. The purity of prepared zeolite samples was verified by XRD and SEM measurements. Ammonium forms were prepared by repeated treatment with 1M anamonium nitrate solution. Dealuminated forms of zeolites (USY, DB-ERI, DB-MFI) have been prepared by deep-bed treatment of ammonium forms at 780~ for 3 hrs followed by subsequent acid leaching with 0.1M HC1. ~/-Al:O3 was prepared by extrudation of peptized boehmite with following calcination at 500~ 2.2. Experimental Techniques Nitrogen adsorption was performed with Sorptomatic 1900 (Erba Science, Italy) and ASAP 2100 (Micromeritics, USA). Before measurements, samples were evacuated at 400~ overnight. 2.3. Calculation methods Adsorption data were basically treated by standard two-parameters BET isotherm in range of P/P0=0.03-0.3.

1589 Three types of micropore evaluation were used for the adsorption data treatment: i) For the characterization of the changes in the microporous structure of zeolites, the de Boer's t-plot method is one of the most used tool [5]. But it is strongly recommended to use the appropriate reference isotherm, corresponding to a nonporous material of similar chemical nature of surface. From this reason for the evaluation of zeolites the Harkins-Jura model [15], measured for nonporous A1203 is mainly used [8,12,14]:

t (nm) = O.1

13.99

(1)

o o34- lo ( o) The linearization of t-plot in range from t=0.354nm (the statistical thickness of one adsorbed layer of nitrogen), corresponding to P~0 about 0.08, up to t=0.6-0.7 nm (formation of second adsorbed layer of nitrogen), corresponding to P/P0=0.44-0.56, gives line, whose slope is proportional to the external + mesoporous surface St and intercept on Va axis gives the micropore volume Vmi,o. ii) The recommendation of Schneider to treat the adsorption data of physical adsorption of nitrogen by modified BET-equation, including Vr~cro [ 16]:

Va

(cm3/g)

=

Vmicro

P am.C.-~, Po

-k-

1=,

(2)

(1- "---) [1 + (C- 1). A-] eo" eo gives the possibility by non-linear regression regression of adsorption data in BETrange (P/P0=0.03-0.3) directly to extract the Vr~cro value from adsorption data and consequently to determine the value of CBETof non-microporous part of solid materials. From a m - volume of monolayer the specific surface of non-microporous part of solids is determined. iii) Lecloux's n-method of t-plot more precisely reflects the chemical nature of surface using in calculation the value of CBET [17]. For zeolites, the standard treating of adsorption data by BET-isotherm gives generally the negative values of C. The CBETconstant determined by non-linear regression of Eq.2 corresponds to the adsorption of nitrogen only on external surface and on mesopore surface (if it is present) of zeolite crystals. Thus, this value allows to construct on its base the t-plot of Lecloux: P k-t=t~

Po

1-(k

P)

eo

p p 1- (m + 1)(k - ) m +m(k ~_)m+l 1 C-1 --+--(k

c

c

Po

P)

t'o

Po

-(k

P)m+l

/'o

wherq t0=0.354 nm, k=0.95 and m=4.18. The received t-plot is treated by the same way as in i).

(3)

1590 3. RESULTS AND DISCUSSION

We verified the suitability of Harkins-Jura model of t-plot by measuring of porous solids with similar chemical composition as zeolites - microporous alumosilicates. Figure 1 shows the adsorption isotherms, BET-isotherms and t-plots (Harkins-Jura) of samples of NaY, 3,-A1203 and SiO2. The adsorption isotherm of Na-mordenite without hystheresis loop is typical for all pure zeolites - pore volume filling of primary micropores at very low pressure and adsorption on external surface. Isotherms of silica and alumina are typical for different mesoporous materials. BET-isotherms in range of P/P0-0.03-0.3 for alumina (SBET=222 m2/g, C=132) and silica (SBEx--382 m2/g, C=109) are perfectly linear with positive values of intercept. The BETisotherm for NaY has u-shape, where the linearization in the same range of P/P0 gives a negative value of intercept and consequently negative C-value-Table 1. The corresponding tplots on Figure 1c shows that the Harkins-Jura master isotherm, derived from adsorption data on nonporous A1203 could be suitable as master isotherm for the evaluation of crystalline alumosilicates. For measured mesoporous silica and alumina, t-plots linearized in range of t=0.35-0.6 give practically zero intercept and from their slope (max.+0.002 cm3/g), surface calculated are St= 221 and 382 m2/g for alumina and silica respectively. For NaY the t-plot is linear even to greater values of t (over 1.0 nm), corresponding to P/P0 up to 0.8. This fact is related to the adsorption in second and even in the third layer on microporous materials without mesopores, perfectly described by used master isotherm. For the t-plot method it is strongly recommended to choose the master isotherm according the value of C-constant of BET-isotherm. For zeolites, as it was shoved above, the C-constant calculated by standard application of BET has negative value. For this reason, many authors shill the linearized range of BET-isotherm towards the very low relative pressure to obtain positive intercept and consequently the positive C-constant, obviously very high. Such value, derived from BET, which is not valid for micropores, cannot be taken as real. This problem can be solved by use BET-isotherm modified by adding V~c,o to the linearized form of standard BET (Eq.2), and the determined C-constant use also in Lecloux t-plot. The results of such treatments of adsorption data of different zeolites are compared in Table 1. From the results it is seen that akeady by nonlinear regression of modified BETisotherm it is possible to obtain relatively good valued, close to those derived by Harkins-Jura t-plot. The determined CBETCOnstant of extemal surface (and also mesoporous surface in the case of USY) for zeolitic materials was in range 24-65. The values of St and Vn~cro, determined by Lecloux n-method on the base of above-mentioned C-constant, were in very good agreement with those obtained by Harkins-Jura t-plot. The differences in Vmicroare about 0.002 cm3/g, and in St 2 - 4 m:/g. Both t-plots were linearized in range of t=0.35-0.6 ran, i.e. in the range of the second layer of nitrogen formation. Harkins-Jura and Lecloux t-plots are compared in Figure 2 for Ca_A-zeolite. Both t-plots are practically identical in the abovementioned range, but the linearity of Harkins-Jura t-plot continues up over 1 n m - similarly as in Figure 1 for NaY sample. The results confirm the suitability of the use of the Harkins-Jura tplot for microporous alumosilicates and allow to linearize t-plot from 0.35 nm up to 1 nm. The accuracy of CBETvalue in range of 20-60 has no a serious influence on the results of Lecloux tplot, as it is demonstrated in Table 3. Recently we have demonstrated [18] that after dealumination of narrow pore zeolite erionite (DB-ERI) the creation of a secondary micropores -supermicropores can be determined

1591 400

a)

350 a.. v.oo ,,,

E u

>

Si02

300 250

NaY

200 150 100

b~203

50 I

I

I

0.2

0.4

0.6

1

0.8

PIPo 0.008

b)

-

0.007

Al=O3.o-'" .I~

o.oo5 o.oo4

~..

0.003 -

O

n

~),~176

~

~ o

0.006 -

="

~

~ o o

~

0.002 -

.

0.001 -

. A~~ ~ -

~:6::---'",

0.000 0

.-~176

~176

"~

~

~

..0"

SiO2 ~176176176 .~

k

,,o ~176176176176

,~~ ~

.O"~

~

.O" ~

~

~o

-A""

.~

O ..o

.~

.6 -''

..o

.oo

9

NaY

"

I

01

I

02

0.4

03

PIPo

300 O. p. r

,,,

E v

A

C)

250

zx

NaY

200

............ 00

A

." ~ .

0

1

0

02

-o . . . .

&'"

jL.dK

.A"

.A.-A" SiO2

..--'~o<>,~~~ ... ":.~'~

50-

~ ~176

o- o oe.ooe-e-o- o--e- ..~-,6 . . . . . . .

150 100-

~

A...--""

[

04

. ,..o'*"

1

06 t(nm)

~"

1

08

- "G'""

.. . . . . . . "" o

AI203

4

1

12

Figure 1. Adsorption isotherms (a), BET-isotherms (b) and t-plots (c) o f NaY, A1203and SiO2

1592 Table 1 Comparison of surface characteristics of various microporous materials obtained by different methods Standard t-plot Schneider's t-plot BET Harkins-Jura [ 15] modified.BET [ 16] Lecloux [ 17] Sample SBET C St Wmicro S "BET Vmicro C St Vmicro (m2g-') (m2g1) (cm3g1) (m2g1) (cm3g1) (m2g1) (cm3g1) CaA 349 -41 14 0.186 20 0.186 24 12 0.188 NaY 573 -35 25 0.290 36 0.285 65 27 0.290 USY 521 -43 115 0.215 96 0.226 34 119 0.218 ERI 274 -51 11 0.130 15 0.128 35 11 0.130 MOR 304 -48 11 0.148 20 0.145 53 11 0.149 MFI 268 -45 49 0.112 41 0.117 32 49 0.116 Table 2 Influence of C-constant value on results of Lecloux t-plot for various zeolites ERI MFI NaY USY C St Vmiero St Wmicro St Vmicro St Vmicro (m2/g) (cm3/g) (m2/g) (cm3/g) (m2/g) (cm3/g) (m2/g) (cm3/g) 20 10.1 0.131 42.2 0.119 22.7 0.293 110.7 0.224 30 10.6 0.131 45.2 0.117 24.1 0.292 1 1 7 . 3 0.219 40 10.9 0.130 47.7 0.115 25.1 0.291 121.5 0.216 50 11.2 0.130 49.8 0.114 25.8 0.290 124.4 0.214 100 11.7 0.130 56.6 0.110 27.4 0.289 1 3 1 . 3 0.209 150 Ix.

130

A

110

I--

01

"E

90

:~

7o

x ......

X

x"Xo

~~mi~='=~

o

o

0

x

....

O.-~

o - Harkins-Jura x -

50

~..~x

Lecloux

I

I

I

I

0.2

0.4

0.6

0.8

1

t(nm)

Figure 2. Comparison between t-plots Harkins-Jura and Lecloux for CaA zeolite

1593

,

..................

o

o

A

150

o

"

..-

~,

O

.....

-

0 m

,.

..x-

100

:.~---"-~-

"0"

E]- . . . .

o

0

0.2

o--

,

USY

',

:.+

! i

a.-9-n"nm -

,,

~)" " O "

o

n.

9 .O..O-

9

--

l ~ - - [ T ' " "D" " ~

......

DB-ERI

. . . . . . . . . . . .

': 0.45

9 .........

t)

Silicalite

! !

A

tx "

.n. -. .-.

:

....

,. . . . . .

: ""

o 50

,

. ~". . -

...-

:

! ,

...--n [

- .---" ...-. _ - - - -

A" ' & ' A ' "

.*

.~.

',~ . =, . : . : .

a , "*

o- -*

............

.-~

:

..-". . . . . . . . . . . . . . . .a. . . .

>

;

.N . . .a. Y : ,~ . , . . , . . . , . . , . ~ , . . , . . .

200 -

I

0.35[

, '

0.7 0.4

0.6

0.8

1

t(nm)

Figure 3. Examples of various types of t-plot shapes and range of their linearization from the shape of t-plot in the range of t=0.35-0.45, connected with the shape of adsorption isotherm. This area corresponds to the cooperative adsorption process in wider micropores at P/P0 0.02-0.2 after primary process in prinaary micropores (ultramicropores) at very low P/P0 (generally under 0.01) - this process involves enhanced adsorbent-adsorbate interactions [ 19]. The intercept of the linearization of the upper part of t-plot (above t=0.45) gives the total volume of all micropores, the volume of the secondary micropores could be estimated from the lower part of t-plot - Figure 3. The next type of shape of t-plot is also reflecting the shape of adsorption isotherm- as is was akeady published, for ZSM-5 zeolites with Si/A1 over about 50 up to silicalite a doublestep isotherm was observed, reflecting in double-step t-plot - Figure 3. The lower and upper parts of t-plot are linear generally at t=0.2-0.4 nm and t=0.5-0.8 nm, and the values of intercepts of these two linearization are in the same ratio as the liquid and solid nitrogen [20]. As we have shown, the adsorption isotherms as well as t-plots of ZSM-5 zeolites from good, microporous type for zeolites with Si/AI=14-20 via curved type changes after hydrothermal dealumination characteristic for supermicropore creation to two-steps curves for high dealuminated samples (Si/AI>45) [21 ].

4. CONCLUSIONS The t-plot method is very powerful method for the evaluation of microporous mesoporous structure of zeolites. The results showed that the Harkins Jura master isotherm is

1594 applicable for zeolites, in which the vales of 25-65 of the CBEx-COnStant of non-microporous part was determined by modified BET-isotherm. Results of Lecloux t-method on the basis of determined C-constant were very close to those of Harkins-Jura. We can to suggest to incorporation the non-linear regression of modified BET-isotherm (Eq.2) into soitwares of commercial sorption equipments to calculate C-constant for Lecloux t-method. The general range for t-plot linearization is in t=0.25-0.6, i.e. in the formation of the 2/3 of the second layer of nitrogen. For the pure zeolitic materials this linear range can be extended even to t-1 nm. Very sensitive seems to be the t-plot method to the all anomalies and irregularities in ultramicropore system (under 0.7 nm) that is typical for the most of zeolites. The curving of tplots in range oft=0.35-0.45, connected with curving of adsorption isotherms in range P/P0 up to 0.2, corresponds to the adsorption in supermicropores. In this case the linearization of the upper part of t-plot gives the total micropore volume, and from the lower part it is possible to estimate the volume of supermicropores. REFERENCES

1. J.Lynch, F.Raatz and P.Dufresne, Zeolites, 7 (1987) 333. 2. S.Cartlidge, H.-U. Nissen and R.Wessicken, Zeolites, 9 (1989) 346. 3. A.Zukal, V.Patzelovfi and U.Lohse, Zeolites, 6 (1986) 133. 4. B.Chauvin, P.Massiani, R.Dutartre, F.Figueras, F.Fajula and T.Des Courieres, Zeolites, 10 (1990) 174. 5. J.H.de Boer, Lippens B.C., Linsen B.G., Broekhoff J.C.P., van den Heuvel A. and Osinga Th.V.J.Colloid Interface Sci. 21 (1966) 405. 6. K.S.W.Sing, Chem.Ind., London 1968, 1520. 7. M.M.Dubinin, Chem.Rev., 60 (1960) 235. 8. L.E.Aneke, W.A. de Jong and P.J.van den Berg, J.Royal Neth. Chem.Soc., 99 (1980) 263. 9. G.Leofanti, P.Genoni, M.Padovan, G.Petrini, G.Trezza and A.Zecchina, Characterization of porous solids II (Eds.F.Rodriguez-Reinoso et al.), Elsevier, Amsterdam 1991,553. 10. E.F. Sousa-Aguiar, A.Liebsch, B.C.Chaves and A.F.Costa, Microporous and Mesoporous Materials, 25 (1998) 185. 11. G.Leofanti, M.Padovan, G.Tozzola and B.Venturelli, Catalysis Today, 41 (1998) 207. 12. P.Hudec, J.Novansk~, S.Silh~, T.Trung, M.Zfibek and J.Madar, Ads. Sci. Technol., 3 (1986) 159. 13. R.L.Mieville, J.Colloid and Interface Science, 41 (1972) 371. 14. M.F.L.Johnson, J.Catal. 52 (1978) 425. 15. W.D.Harkins and G.Jura, J.Am.Chem.Soc. 66 (1944) 1366. 16. Schneider P.: Applied Catalysis A: General 129 (1995) 157. 17. Lecloux A.J., Catalysis- Sci. and Technology, Vol.2, (Akad.Verlag, Berlin 1983) 172 18. P.Hudec, A.Smie~kov~, Z.Zidek, E.Rojasovfi, Stud. Surf. Sci. Catal., 130 (2000) 2903. 19. P.J.M.Carrot and K.S.W.Sing, Characterization of porous solids (Eds. K.K.Unger et al.) Elsevier, Amsterdam 1988, p.77 20. P.Hudec, A.Smie~kov~i, Z.~;idek, M.Zfibek, P.Schneider, M.Korifik and J.Koz~xlkov~i, CoECzech. Chem. Commun., 63 (1998) 141. 21. P.Hudec, A.Smie~kov~i, Z.~dek, E. Sabo and B.Lipt~kovfi, Stud. Surf. Sci. Catal., 135 (2001) 29-P-26.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1595

B i n a r y m i x t u r e a d s o r p t i o n o f w a t e r and ethanol on silicalite Y. Oumi ", A. Miyajima a, J. Miyamoto b and T. Sano a a School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan; E-mail:[email protected], Fax: +81-761-51-1625 b BEL Japan Inc., Midori, Sumida-ku, Tokyo 130-0021, Japan Single component and binary mixture adsorption behaviors of water (H20) and ethanol (EtOH) were investigated on silicalites with different crystallinity (structure-defects) by a combination of volumetric and gravimetric methods. No influence of the crystallinity of silicalite on the pure EtOH adsorption was observed. On the other hand, the amount of H20 adsorbed on the silicalite with less structure-defects was significantly smaller. The total adsorption amount in the binary mixture adsorption was strongly dependent on the EtOH/H20 ratio in the initial mixture. From the analysis of the binary adsorption data, it became clear that EtOH molecules adsorb preferentially on silicalite with less structure-defects and that more selective adsorption of EtOH takes place under the lower equilibrium pressure.

1. INTRODUCTION In recent years, zeolite membranes have attracted considerable interest due to their potential use in several industrial applications such as gas or liquid separation membranes, sensors and electrodes [1-4]. When the zeolite membranes are used as separation membranes and sensors, a detailed understanding of adsorption process of molecules is needed. Although there are a large number of literature concerning adsorption experiments with various polar and non-polar molecules, most of researches are related to the single component adsorption behavior. Some researchers have only investigated the binary adsorption behavior using the FT-IR, NMR and computer simulation techniques [5-11 ]. However, there are few studies on the simultaneous measurements of adsorption isotherms by combining volumetric and gravimetric methods. We have already reported a high alcohol permselectivity of silicalite membrane [ 12]. From single component adsorption studies of water and alcohol on powdered silicalite and ZSM-5 zeolite crystals prepared under various hydrothermal synthesis conditions, it was found that the high alcohol permselectivity is attributable to the high hydrophobic properties of silicalite crystals and that the amount of water adsorbed is strongly influenced by the number of silanol groups of zeolite crystals [13]. This suggests that the separation performance of zeolite membrane is affected by the crystallinity of crystals. From such a view point, we study an influence of the crystallinity of silicalite crystals on the adsorption behavior of binary mixture of H20 and EtOH by a combination of volumetric and gravimetric methods.

1596 2. EXPERIMENTAL

2. 1. Synthesis of silicalite crystals Synthesis of silicalite crystals was performed using colloidal silica as a silica source, tetrapropylammonium bromide (TPABr) or tetrapropylammonium hydroxide (TPAOH) as a structure-directing agent. The chemical compositions of starting synthesis mixtures prepared are as follows; (1) 0.1TPABr-0.05Na20-SiO2-40H20, (2) 0.1TPAOH-SiO2-40H20, (3) 0.36TPAOH-O.39HF-SiO2-34H20. The hydrothermal synthesis conditions were listed in Table 1. The silicalite crystals obtained were filtered, washed thoroughly with deionized water, dried at 120~ and calcined at 500~ for 20 h to remove the structure-directing agent. 2.2. Characterization The identification of zeolites obtained was achieved by X-ray diffraction (Rigaku RINT 2000). The crystal size was examined by scanning electron microscopy (Hitachi S-4100). Textural properties were determined by nitrogen adsorption (Bel Japan Inc. Belsorp 28SA). Before adsorption measurements at -196~ the powdered zeolites (ca. 0.1 g) were evacuated at 400~ for 10 h. 29Si MAS NMR spectra were recorded on a Varian UNITY INOVA400 spectrometer at 79.5 MHz with 3 kHz spinning speed and 7.6 gs pulses for 2,000 scans. DSS was used as a chemical shift reference. The IR spectra for the framework vibration were recorded on a FT-IR spectrometer (JEOL JIR-7000) with a resolution 4 cm-1 at room temperature. The sample was pressed into a self-supporting thin wafer (ca. 6.4 mg/cm 2) and was placed in a quartz IR cell with CaF2 windows. Prior to the measurements, each sample was dehydrated under vacuum at 400~ for 2 h. 2.3. Single component and binary mixture adsorption isotherms The single component and binary mixture adsorption experiments of pure H20 and EtOH vapors on the silicalites were carried out at 27~ by a Belsorp 18 and a FMS-BG adsorption apparatus (Bel Japan Inc.), respectively. Prior to adsorption measurements, the powdered silicalites (ca. 0.3 - 1.0 g) were evacuated at 400~ for 6 - 10 h. The binary mixture adsorption experiments of H20 and EtOH vapors were performed by combining volumetric and gravimetric methods. The EtOH/H20 ratio in the initial mixed vapor was varied from 100/0 to 0/100. 3. RESULTS AND DISCUSSION

3.1. Synthesis and characterization of silicalites In general, it is recognized that zeolite crystals synthesized in the presence of F- are larger as compared with zeolite crystals synthesized under a conventional hydrothermal method, and contain less structure-defects [ 14]. To clarify an influence of crystallinity of silicalite on the adsorption behavior, therefore, the silicalite crystals were synthesized with and without HF. The hydrothermal synthesis conditions are listed in Table 1. For all the obtained products, the X-ray diffraction diagrams showed no peaks other than those corresponding to MFI type zeolite structure. Fig. 1 shows the SEM images of silicalite crystals synthesized. The shape of silicalite crystals was a coffin type. The large differences in the crystal size and the aspect ratio were observed among the silicalite crystals synthesized with and without HF. By addition of HF, a large silicalite crystal with the high aspect ratio was obtained. Table 1 also

1597 Table 1

Hydrothermal synthesis conditions and characteristics of silicalites obtained.

Synthesis conditions BET Pore volume a) Chemical composition Temp. Time surface / ,cm3rl:_..:_~x ~ n q u m ) g _1 of starting synthesis gel /~ / hr area/m2g -1 0.1TPABr-0.05Na20160 24 407 0.18 SIO2-40H20 0.1TPAOH-SiO2-40H20 160 48 419 0.19 0.36TPAOH-0.39HF200 88 391 0.17 SIO2-34H20 a) Determined by the Dubinin-Radushkevich method. b) Weight loss calculated from TG curve (300~ -1100~ Sample No.

Weight lossb) /wt% 0.95 0.93 0.43

Fig. 1 SEM images of the silicalite crystals synthesized with (a) TPABr, (b) TPAOH and (c) TPAOH/HF. summarizes the characteristics of the silicalite crystals obtained at various synthesis conditions. No difference in the BET surface area and pore volume was observed. However, the large difference in the weight loss from 300 to 1100 ~ in the TG curve was observed between the silicalite synthesized with TPABr or TPAOH (sample No. 1 or 2) and the silicalite synthesized with TPAOH/HF (sample No. 3). This indicates that the number of silanol groups (structure-defects) of silicalite synthesized with TPAOH/HF is smaller than that of silicalite synthesized without HF, if the weight loss is assumed to be attributed to the dehydroxylation process (2SiOH ~ SiOSi + H20). To get further information concerning the silanol groups (structure-defects) of silicalites, the IR and 29Si MAS NMR spectra of the silicalites were measured. Fig. 2 shows the IR spectra of the silicalites in the 4000-3000 cm -1 region. The IR spectrum of the silicalite synthesized with TPABr or TPAOH exhibited two peaks at ca. 3500 and 3740 cm 1. The peaks at ca. 3500 and 3740 cm -1 are assigned to the

(c)

I

I

I

4000 3800 3600 3400 3200 3000 Wave numbers (cm- 1) Fig. 2 IR spectra of various silicalites synthesized with (a) TPABr, (b) TPAOH and (c) TPAOH/HF.

1598 hydrogen bonding adjacent silanol groups and the isolated silanol groups, respectively. In the spectrum of the silicalite synthesized with TPAOH/HF, although the peaks at ca. 3500 and 3740 cm -1 were also observed, their intensities were considerably weaker than those of the silicalite without HF. Fig. 3 shows 29Si MAS NMR spectra of the silicalites. The resonance lines between-109 and-115 ppm could be assigned to Q4 [Si(0A1)] unit [15]. As no line of Q3 [Si(1A1)] unit appears in the spectra owing to the very low A1 content of silicalite, the small line observed at ca. -103 ppm could be assigned to Q3 [Si(0A1)] ~unit belonging to SiOH defect centers in the zeolite framework. No line at c a . - 1 0 3 ppm was observed in the spectrum of the silicalite synthesized with TPAOH/HF, indicating the number of structure-defects is significantly smaller. From the above results, it may be concluded that the crystallinity of the silicalite synthesized with TPAOH/HF is higher than that of silicalite without HF.

\

3. 2. Single component adsorption of H20 and EtOH vapors on silicalites At first, the single component adsorption isotherms of pure H20 and EtOH vapors on the silicalites with different crystallinity were measured using a conventional volumetric method. Fig. 4 60

I

-90

-100

I

(a)

I

i

-110 -120 -130 -140 ppm

Fig. 3 29Si MAS NMR spectra of various silicalites synthesized with (a) TPABr, (b) TPAOH and (c) TPAOH/HF. 60

(a)

(b)

"7

"7

~o 50

5o 40

40 e~

o

30

cD

3o

,.Q

CD

"~ 2o r.r

20

9 10 c.q 7:

e=.,=a lO b o.5

P/Ps

1

0~

0

!

0'.5 P/Ps

Fig. 4 Adsorption isotherms of pure (a) H20 and (b) EtOH vapors at 27~ on various silicalites synthesized with ( 0 ) TPABr, (O) TPAOH and (A) TPAOH/HF.

1

1599 shows the adsorption isotherms of pure H20 and EtOH vapors. The amount of water absorbed was strongly dependent on the synthesis condition, whereas no difference in amount of EtOH adsorbed was observed. Namely, water adsorption on the silicalite synthesized with TPAOH/HF hardly occurred at the low P/Ps. It is well known that adsorption of water on zeolite involves the specific interaction between water molecules and the hydrophilic centers, which are silanol groups or protons associated with the framework aluminums [ 13]. Therefore, these results indicate that there is a large difference in the adsorption behavior between H20 and EtOH molecules. The EtOH adsorption on silicalite is hardly influenced by the structure-defects in the zeolite framework. Probably, EtOH molecules adsorbs on silicalite in a similar manner as non-polar molecules. 3.3. Binary mixture adsorption of H20 and EtOH vapors on silicalites To calculate the individual isotherm of each component from the adsorption data of mixed vapors, the adsorbed amount and the equilibrium partial pressure of each vapor have to be determined with high accuracy. As the dead volume of the FMG-BG instrument used for measurement of the binary adsorption isotherm with a combination of volumetric and gravimetric methods was ca. 300 ml and larger than that (ca. 180 ml) of the Belsorp 18 (for single component adsorption), the accuracy of the FMG-BG instrument was evaluated by pure H20 adsorption. It was found that there is no difference in the amount of H20 adsorbed on silicalite measured using two instruments. Therefore, next, the simultaneous measurement of the adsorption isotherms of H20 and EtOH on various silicalites was conducted by combining volumetric and gravimetric methods. Fig. 5 shows the adsorption isotherms of binary mixture of H20 and EtOH under the dosing ratio of each vapor of 50/50. EtOH molecules were found to adsorb preferentially on both silicalites synthesized with TPABr and TPAOH/HF. The total amount adsorbed on the 0.14

0.14 (a)

0.12 ~- 0.10 "~ 0.08 ra~

"7 -

9

*-, 0.06 -

-o

0.04 0.02

9

9

9

9

9

000

9

9

O

O

O

O

"~ 0.10

O

tD

"~ 0.08

_ .oO

_

9

o

0

E 0.04

I

0

O

-~ 0.06

AAAAAAAA

0

O

(b)

0.12

I

1

I

2

P/kPa

0.02

A A A I

3

0

_ . ot,? 0

^,^ ~AI_A ~

1

AA

2 P/kPa

Fig. 5 Adsorption isotherms of binary mixture of H20 and EtOH at 27~ on the silicalites synthesized with (a) TPABr and (b) TPAOH/HF. 9 Total adsorption amount, 9 Amount of adsorbed EtOH, /k Amount of adsorbed H20

3

1600 0.14

(a)

~ o 0.12

~3 [] A [] A A 4:) [] A [] A DA DA DA

o

~0 0.12

a~

e~o

~9 0.10 o

0.14

0.08 0.06 0.04 0.02

Co)

9 o" ~

0.10 ~0.08

- t& :

[]

9 o

~ 0.06 t o g /

~

o

[]

g'~ ./~

.A

0.04 0.02

0 0 1 2 3 2 3 4 P/kPa P/kPa Fig. 6 Adsorption isotherms of binary mixture of H20 and EtOH at 27~ on the silicalites synthesized with (a) TPABr and (b) TPAOH/HF. EtOH/H20 ratio in the initial mixed vapor : & 100/0, 9 80/20, F1 60/40, /k 50/50, ~ 20/80, 9 0/100

0

1

/

4

silicalite synthesized with TPAOH/HF was smaller than that on the silicalite synthesized with TPABr. However, the mole fraction of EtOH on the silicalite synthesized with TPAOH/HF was larger than that on the silicalite synthesized with TPABr. Taking into account the fact that no difference in the amount of adsorbed EtOH was observed between these silicalites as shown in Fig. 4-(b), it is suggested that the adsorption behavior of binary mixture on silicalite is strongly influenced by the crystallinity of silicalite. Additionally, to investigate an influence of the EtOH/HzO ratio in the initial mixed vapor on the binary adsorption behavior, the dosing ratio of each vapor was varied from 100/0 to 0/100. Fig. 6 shows the total adsorption isotherms of binary mixtures on the silicalites synthesized with TPABr and TPAOH/HF. For both silicalites, the total adsorption amount was strongly dependent on the EtOH/HaO ratio. Namely, the total adsorption amount decreased with an increase in the mole fraction of H20. Fig. 7 shows the relationship between the mole fraction of EtOH in the initial mixture and the mole fraction of EtOH adsorbed on silicalite. From a comparison of the mole fraction of EtOH in the adsorbed phase with the initial composition of binary mixture, it became clear that the mole fraction of EtOH adsorbed on the silicalite synthesized with TPAOH/HF was larger than that on the silicalite synthesized with TPABr, indicating more selective adsorption of EtOH on silicalite crystals with less structure-defects. It was also found that the lower the equilibrium pressure, the higher the EtOH selectivity.

4. C O N C L U S I O N S Silicalite crystals with different crystallinity were synthesized with and without HF and their single component and binary mixture adsorption behaviors of EtOH and H20 were

1601 investigated. It was found from the single component adsorption experiments that the H20 adsorption is strongly influenced by the crystallinity, namely, the amount of H20 adsorbed increased with an increase in the number of structure-defects. On the other hand, no difference in amount of EtOH adsorbed was observed. It was also found from the binary mixture adsorption experiments that EtOH molecules adsorb preferentially on silicalite with less structure-defects and that more selective adsorption of EtOH takes place under the lower equilibrium pressure. These findings suggest that in order to improve the alcohol permselectivity of silicalite membrane, an increase in the crystallinity of silicalite crystals is needed. 1.0

(a)

1.0

/-=

0.8

0.8

.==o ~ 0 . 6 ~'Y~ g0.4

.g So.2 0[

///

. ,...~

0.6

///

0.4

N 0.2

0.4

0.6

0.8

The mole fraction of EtOH in initial mixture

1.0

0.2 O[ 0

I

0.2 0.4 0.6 0.8 The mole fraction of EtOH in initial mixture

1.0

Fig. 7 Relationship between the mole fraction of EtOH in the initial mixture of EtOH and H20 and the mole fraction of EtOH adsorbed on the silicalites synthesized with (a) TPABr and (b) TPAOH/HF. Equilibrium pressure: Vq 0.1 kPa, O 1 kPa, A 3 kPa.

REFERENCES

1. J. Neel, "Pervaporation Membrane Separation Processes", R. Y. M. Huang (ed.), Elsevier, Amsterdam, (1994) 1. 2. T.Q. Nguyen and K. Nobe, J. Membrane Sci., 23 (1987) 11. 3. M.H.V. Mulder, A. C. M. Frank and C. A. Smolders, J. Membrane Sci., 23 (1995) 41. 4. J. Coronas and J. Santamar/a, Sep. Purif. Meth., 28 (1999) 127. 5. R.L. Portsmouth, L. F. Gladden and M. J. Duer, J. Chem. Soc. Faraday Trans., 91 (1995) 963. 6. S. Ashtekar, J. J. Hastings and L. F. Gladden, J. Chem. Soc. Faraday Trans., 94 (1998) 1157. 7. S. Ashteker, A. S. McLeod, M. D. Mantle, P. J. Barrie, L. F. Gladden and J. J. Hastings, J.

1602 Phys. Chem. B, 104 (2000) 5281. S.U. Rege and R. T. Yang, Chem. Engng. Sci., 56 (2001) 3781. F. Karavias and A. L. Meyers, Mol. Simu., 8 (1991) 51. L. F. Gladden, M. Hargreaves and P. Alexander, Chem. Eng. J., 74 (1999) 57. V. Lachet, A. Boutin, B. Tavitian and A. H. Fuchs, Langmuir, 15 (1999) 8678. T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami and K. Haraya, J. Membrane Sci., 95 (1994) 221. 13. T. Sano, T. Kasuno, T. Takeda, S. Arazaki and Y. Kawakami, Stud. Surf. Sci. Catal., 105 (1994) 1771. 14. E. Nigro, R. Mostowicz, F. Cera, F. Testa, R. Aiello and J. B. Nagy, Stud. Surf. Sci. Catal., 105 (1997) 309. 15. C. A. Fyfe, G. T. Kokotailo, G. J. Kennedy and C. de Schutter, J. Chem. Soc., Chem. Commun., (1985) 306.

8. 9. 10. 11. 12.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1603

Influence o f water adsorption on zeolite Beta C. Flego, G. Pazzueorfi, C. Perego EniTecnologie S.p.A. - Via Maritmao, 26 - 20097 S. Donato Mil. (MI) - Italy The influence of water on acidity mad catalytic performances of zeolites has been widely studied and controversial results have been proposed ha literature. The re-hyda'ation of zeolite Beta is here evaluated as a tool to recover the Bronsted acidity, that can be partially lost during thernaal treatments (e.g. regeneration). Introduction of controlled amoums of water influences both the A1 siting and the acidity distribution in zeolite Beta and consequently the densit3~ of the active sites.

1. INTRODUCTION Water strongly influences the catalytic behaviour of zeolites, modifying their catalytic performances mad lifetime [1]. Investigation of the adsorption of water is therefore of great impol~tance with respect to practical applications. Treatments with water vapour may influence the A1 distribution even at mild temperatures, with or without changes of its overall content in the zeolite [2-6]. Attention to the re-hydration of zeolite Beta has been paid in order to both verify the possible recovering of the Bronsted acidity lost during thermal treatmlents (e.g. regeneration by coke combustion causes dehydroxylation and abstraction of AI from the framework) or avoid the detrimental effects of the Lewis acid sites during reaction (e.g. selective poisoning) [5-7]. Both phenomena are responsible of modification or reduction in the number of the active sites. A series of Beta zeolite samples with similar SiO2/A1203 molar ratio were chosen in order to determine the changes in acid sites distribution related to the different locations of A1 atoms mad flaeir coordination. The conditions of water adsorption on zeolite Beta were studied (i.e. temperature and time) mad their influence on the fomaation of the active sites (i.e. in framework acid A1-OH groups [8]) was evaluated.

2. EXPERIMENTAL 2.1 Catalysts Beta zeolite samples were synthesised with SIO2/A1203 molar ratio of 20-24, according to [4]. [31 sample was mildly dealuminated to SIOJA1203=33, by post-synthesis treatmem with 0.1 M HCI (2h at 100~ giving rise to ~3 smnple. 131 sample was also thermally treated in subsequent cycles (550~ 5h, air flow), giving rise to 136, and subsequently washed wifll HC1, giving rise to [37 smnple. As a reference was synthesised one zeolite Beta (t]8) wifla B instead of AI in the framework (Si/B= 1.5). All samples were analysed in H%form, after exchange with CH~COONH4, but 134 and ~5 obtained by washing with diluted HC1 solution. Na was present in traces (<210 ppm),

1604 but in ]35, poorly washed in order to maintain this element in larger amount. In Table 1 the list of the samples and their main physico-chemical characteristics is reported. 1.2 Physico-chemical characterization Water adsorption (amount, strength) was followed by TG (ThermoGravimetric) equipment. The powdered sample was evacuated in N2 flow at 200~ till constant weight (no further desorption of moisture was registered), followed by water adsorption through interaction with steam (24 ml/min N2 flow saturated with 0.0336 mbar H20) at different temperatures (25, 100 and 150~ up to 6 hours. Acid sites and hydroxyl distribution were evaluated by FT-IR spectroscopy. The acid sites distribution was determined by pyridine adsorption at 200~ and stepwise desorption (in the 200-500~ range, 1 h, dynamic vacuum 2* 10.3 mbar) on pure pellets, after conventional evacuation (500~ lh, dynamic vacuum 2* 10.3 mbar) and after water interaction under controlled conditions (T, t), following the procedure described above. The density of the acid sites was obtained from the intensity of the IR signals at 1545 and 1455 cm j, applying the extinction coefficient of [9]. A1 coordination and hydroxyl distribution were evaluated by UV-Vis-IR (UltraVioletVisible-InfraRed) spectroscopy with diffuse reflectance on powdered samples. The in situ treatments are similar to those described above. 3. RESULTS AND DISCUSSION 3.1 AI siting in zeolite Beta Zeolite Beta was proven to be characterised by high flexibility of the coordination sphere of its A1 atoms and by structural disorder [5, 10]. These characteristics did not allowed to simply relate the density of the active sites (i.e. in framework acid A1-OH groups) with the whole A1 content (i.e. SiOJA1203 molar ratio, as determined by chemical analysis) and other tools have therefore to be applied for its determination (Table 1). From acidity determination it is possible to discriminate between unsaturated A1+3 cations (Lewis sites) and protonic species (Bronsted sites), as the IR analysis of the OH region was proven to give information of the siting of A1-OH groups in the zeolite [4]. Table 1 List of Beta zeolite samples and some of their physico-chemical characteristics Sample SiOJA1203 Na total Lewis total Bronsted AlOHa A 1 O Hb chemical (ppm) acid sites acid sites 3782 cm ~ 3665 cm -~ analysis (pmol/g) (pmol/g) (n.a.) (n.a.) 131 20 200 392 329 0.012 0.799 [32 23 105 272 222 0.024 0.765 133 33 140 109 201 0.016 1.161 134 24 30 144 225 0.001 0.858 135 23 570 207 166 0.010 1.153 136 23 210 212 108 0.013 0.926 137 29 170 80 154 0.034 0.496

A1OHc 3605 cm -~ (n.a.) 0.351 0.343 0.280 0.411 0.335 0.328 0.141

n.d, '= not determined; n.a. = intensity of the IRsignal normaiised by the peiiet thickness

1605 The total amount of the Lewis acid sites in this series follows a similar ranking of the overall A1 content, while that of the Bronsted sites, as determined by pyridine adsorption, varies because of the high diversity of the A1-OH groups. The location of A1-OH groups is evaluated from the IR spectra in the OH region (Figure 1), its distribution is reported in Table 1. Framework tetrahedral (Td) species (corresponding to IR signal at 3605 cm~; A1-OHc, Table 1) predominate in samples [31, 132 and [34. AI(Td)-OH groups only partially bonded to the structure (IR signal at 3782 cm~; AlOHa, Table 1) are present in 133 and [36 due to a "incoming" A1 abstraction. In 133 and 137 (samples dealuminated with different procedures) and in [36 (obtained by subsequent cycles of calcination) the high intensity of the IR signal at 3665 cm l (A1-OHb, Table 1) confirms the presence of a high density of extraframework octahedral (Oh) A1-OH groups. These sites do not contribute significantly to the Bronsted acidity because of their low acid strength [5]. The presence of Na in larger extent decreases the density of the Bronsted acid sites of 135 and increases the distribution of the weakly acid extraframework AI-OH species. Qualitative evaluation of A1 distribution, even of neutral A10, species, was obtained by UV spectroscopy in the 200-380 nm range (Figure 2). Well defined signals can be related to the ordered distribution of A1 atoms both in Td (215-230 nm) and Oh coordination (ca. 270 nm), while one broad absorption with maximum at ca. 270 nm is proposed to be caused by distorted Oh A1 atoms, as in [33 and 137 (partially dealuminated samples). This series of Beta zeolite samples, with a similar A1 content, shows therefore a large diversity of A1 sites, as above identified: Lewis acid sites (Td or Oh A1~-~cations, in framework or extraframework), Bronsted A1OH sites (partially or completely bonded to the framework Td sites, extraframework Oh sites) and neutral A10, agglomerates. %

---'-"~

A

~2

~7

3800

3~500 cm-1 3400

Figure 1: IR spectra of the OH region of Beta zeolite samples

3"2002(J0

'

i

260

i

320

nm

380

Figure 2: UV spectra of some Beta zeolite samples

3.2 Water influence on A! siting The different A1 distribution in zeolite Beta greatly influenced the further behaviour of the materials after water vapour treatments.

1606 The amount of H20 molecules adsorbed by some Beta zeolite samples and [38 (B substituted zeolite Beta without A1) is evaluated in the range 21-150~ at the saturation (sat.) and by extrapolation of the first plateau region to zero pressure (pres.) (Table 2). Water is physisorbed at 21~ at similar extent in all samples (9.9-10.8 mmol H20/g), due to weak bonds and to not-specific interaction with the surface of the zeolite [11 ]. The saturation amount decreases with the increase of the temperature for all samples and at 150~ only the water molecules chemically bonded to the materials are adsorbed (see isotherms at 150~ in Figure 3). The low pressure adsorption is proposed to be due to the interaction of water molecules with hydrophilic A1 (or B) sites of the zeolite [ 11, 12]. Two different behaviours are observed in these samples for the amount determined at zero pressure: (i) the B containing zeolite has a maximum of adsorption at 100~ (ii) in A1 containing samples the density of the highly reactive species towards water increases with the temperature, supporting the idea that water bonding is an activated process. The isosteric heat (%) of water adsorption, calculated at a given loading, is similar for the zeolite Beta with A1 and very far for that with B in the framework, that is in the range of a physisorption phenomenon [13]. This confirms the hypothesis that A1 atoms are the sites responsible of water bonding in the presence of activated interaction (Table 2). 0,8 ~

-~ 0,6 E E 0,4

~.~

-

.

. . . .

" ~ ~ " ~ - " - - : - - ~ i

+

131

9 t"q

z 0,2

"i .

0

.................. .

.

+133

.

+[38

!

!

!

3

6

9 H2Oin (mmol/g)

12

Figure 3: Water adsorption at 150~ Table 2 Results of water adsorption isotherms Sample" Sat.100oC zer0-pres.100oc (mmol H20/g ) (mmoi H20/g ) [31 2.275 0.026 [32 1.967 0.121 [33 2.083 0.082 [38 2.155 0.148 .

.

.

.

.

.

.

.

.

sat.150oc (mmol H20/g ) 0.403 0.709 0.497 0.265 .

.

.

.

.

.

.

.

.

,,,,

Zero-pres.150oc (mmol H20/g ) 0.262 0.331 0.209 0.082 ,

qst (kcal/mol) 15.10 15.53 13.80 4.42 ,

,,,..,

However the nature of these active sites is controversial in literature: both hydration of the proton associated with A1 sites [11, 12, 14, 15] and adsorption on Lewis sites [ 16, 17] are proposed. In these experiments the strongly adsorbed water (determination at zero pressure and at 150~ follows a similar ranking of the AI(Td)-OH groups

1607 (especially those partially bonded to framework, but also the A1-OH in framework): [32>131>[33. In order to more deeply evaluate the interaction of water with zeolite Beta, the water interaction was also followed by spectroscopic analyses. The evolution of the OH groups during water adsorption at 150~ are shown in Figure 4 for [~2. 50 ~

act H20

%T 40

10 4000

3800

" l3600 cm-I 3400 ' [~

3;00

' 3000

Figure 4" IR spectra (OH region) of [32 after interaction with increasing H20 amount at 150~

2 3800

cm-1

3200

3800

cm-1

3200

3800

cm-1

3200

Figure 5: OH region of IR spectra after activation and after adsorption of small H20 amounts In the very early stages of water vapour interaction, the density of AI(Td)-OH groups partially bonded to the framework decreases (lower intensity of the IR signal at 3782 cm -~) and that of the extraframework AI(Oh)-OH groups increases (higher intensity of the IR signal at 3665 cm-~). At the same time a change in the coordination is observed by UV analysis: a large absorption grows up at ca. 270 nm due to Al(Oh) species. The reversible Td-Oh inter-conversion under mild treatment conditions has been already observed [6, 10], even it is not clear which type of the AI(Td) sites (partially or completely in framework) was considered. The acid AI(Td)-OH active site presumably bonds one H20 molecule through H-bonds [18], without significantly modifying its coordination and the length of its bonds. The corresponding IR signal (3605 cm -~) remains almost constant in intensity at the first stages of reaction and does not disappear even at high H20 coverage, in agreement with the findings of [14]. When the process continues, another water molecule is adsorbed by AI(Td)-OH and forms neutral dimeric species [ 19], which give rise to and grow the IR signal at 3661 cm -~ (Figure 4 and [~1 in

1608 Figure 5). This signal overlaps that of extraframework A1-OH species (IR signal at 3665 cm-~), which seem to be poorly modified by water vapour adsorption (133 in Figure 5) [2, 14, 15, 20]. Only at high H20 coverage, all A1-OH groups and part of SiOH's (i.e. from the decrease in intensity of the IR signal at 3747 cm -~) contribute to water adsorption. The extent of adsorption as determined by TG analysis and by spectroscopic evolution follows the same ranking: the largest modifications are observed in 132, the lowest in p3. The B containing sample hardly interacts with water (138 in Figure 5). In 131 the IR signal at 3782 cm -~, characterised by a low intensity in the de-hydrated form, increases its intensity after interaction with small amount of water (Figure 5), because of the high instability of this kind of A1 sites and the easiness to break their bonds with the framework (A1-O-Si) in favour of the formation of new A1-OH bonds. Any direct information of the behaviour of the unsaturated A1§ cations is available and only by pyridine adsorption/desorption the determination of their density and strength is possible. The acidity determination after water sorption evidences a large density of protonic groups, especially of weak acid strength, as are supposed to be extraframework and partially in framework A1-OH's [4]. In Figure 6 the changes in total density of both Lewis and Bronsted sites are depicted after re-hydration with respect to that registered after de-hydration. Beside A1-OH species, also the unsaturated A1+3 cations interact with water, in agreement with [16]. 2OO

A (lamol/g)

'00 0

-

100-

-200-

~T

i

'

L

,,,. ,i, [] Lewis 9 Broensted

131 132 p2+ p2* [33 I]4 135 [36 137 Figure 6: Changes in acid sites distribution after adsorption of small amounts of water on [3 zeolite samples; p2+ and 132" after interaction with medium and high water level, respectively,

'"~ !''""", b

....-.--.v...

",,,l,l !fIvf! v i ~

I

'~

\\ ....

1700 1600

1500

~

1400 cm-1

Figure 7: IR spectra of pyridine on [32 after adsortion/desorption at 200~ (a), 500~ (b) and after rehydration at 150~ (c).

The presence of one water molecule per Lewis site still allows the interaction of pyridine with the unsaturated A1+3 [17], while a larger number of H20 molecules prevents further probe molecules from bonding [2]. The conversion of Lewis acid sites into Bronsted ones by partial hydrolysis has been also proposed [21] and is characterised by a slow reaction rate. By contrast at low temperatures of adsorption highly hydrated species Al(H20)6 +3 are formed [16]. The total acidity determination depicted in Figure 6 supports this theory. By contrast, the large amount of Na in p5

1609 keeps from such a transformation, because the hydration of the cation is favoured with respect to that of A1§ [ 13, 14]. The extent of water interaction also influences the acid sites distribution. On 132 the total density of the Bronsted sites, after the large increase at the first stages of adsorption, decreases with the increase of H20 coverage, while the density of the Lewis sites is always lower than the starting value. Not all the Bronsted sites originated with the presence of water are equally stable at the following water treatments, due to the high diversity of the A1-OH species. The conversion of unsaturated species into protonated ones after H20 adsorption at 150~ is shown in Figure 7. On [32, previously contacted with pyridine and evacuated at 500~ the density of the Lewis sites decreases from 214 to 12 pmol/g, while that of the Bronsted sites increases from 12 to 93 lamol/g (Figure 7). In analogy with these data, the disappearance of Lewis sites has been proven after adsorption of ammonia at 100~ [5]. From the experimental evidences above described, the following behaviours of A1 can be drawn in presence of water. (i) When A1 is mainly in framework (i.e. high intensity of the IR signal at 3605 cm j in [31, [32, 134) water bonds preferentially this kind of site: the larger its amount with respect to the overall AI content, the larger is the water adsorption. Acid AI(Td)-OH sites in framework need high energy to break their bonds with Si tetrahedra. Therefore they initially H-bond water and only during re-hydration at high temperature (or at long time water interaction) they can be partially abstracted from zeolitic framework, weakening their acid strength. (ii) Beta zeolite samples containing a large amount of A1-OH partially in framework (i.e. high intensity of the IR signal at 3782 cm ~ in [32 and [37 samples) can easily interact with water distorting the original coordination of A1 atoms from Td to Oh (i.e. extraframework AI(Oh)-OH species), with decrease of the intensity of the IR signal at 3782 cm ~, in favour to an increase of the intensity of that at 3665 cm ~, and of the density of the weak acid sites. (iii) Beta zeolite samples ([31, [~3, 135, 136) characterised by a low intensity IR signal at 3782 cm -~ (i.e. with a low density of this kind of sites or with only two AI(Td)-OH groups per site) interact also easily with water, breaking a part of the Si-O-A1 bonds and giving rise to a larger density of weakly acid AI(Td)-OH (i.e. a larger intensity of the IR signal at 3782 cm-t). Also in this case, H20 vapour eventually causes the abstraction of A1 from the framework and gives rise to new extraframework Al(Oh)-OH's (weak Bronsted acid sites). (iv) When extraframework Al(Oh)-OH's (i.e. high intensity of the IR signal at 3665 c m 1) and neutral (Oh) A1 agglomerates (i.e. high intensity of the UV absorption at 270 nm) prevail, as in [33, [36, ~7 samples, water is mainly sorbed through H-bonds, in agreement with [14]. The so modified Al(Oh)-OH's become too weak to bond pyridine and the density of the Bronsted sites decreases. (v) The higher the content of unsaturated A1§ cations, the larger is their decrease in the presence of water vapour ([31 vs. the other Beta zeolite samples). In particular the weak Lewis sites disappear, while the strong ones seem not to be modified, but in [~5. The different siting (in and out of framework) of these A1 cations presumably influences

1610 their reactivity. Unfortunately the determination of their position as the amount of neutral A1Ox agglomerates is at the moment not possible, baring the way to a whole description of their evolution during water adsorption. 4. C O N C L U S I O N Water adsorbed on zeolite Beta gives rise to different effects as a function of the location of the A1 atoms in the zeolite and the amount of water introduced. At low H20 coverage, water decreases the density of the Lewis acid sites and favours the formation of new weak Bronsted sites. At high H20 coverage, the total density of acid AI(Td)-OH sites is suppressed due to the formation of highly hydrated Oh species. One controlled water vapour adsorption is proposed as a tools to recover part of the Bronsted acidity, usually lost during post-synthesis thermal treatments (i.e. regeneration). These new weak acid sites are AI(Td)-OH species, that could play an active role in catalytic reactions. REFERENCES

1. A.D. Schmitz, C. Song, Catal. Lett. 40 (1996) 59 2. F. Deng, Y. Du, C. Ye, J. Wang, T. Ding, H. Li, J. Phys Chem. 99 (1995) 15208 3. D. Freude, T. Fr6hlich, H. Pfeifer, G. Scheler, Zeolites 3 (1983) 171 4. I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker Jr., R. Millini, C. Perego, G. Bellussi, J. Phys. Chem., 98 (1994) 4627 5. P.J. Kunkeler, B.J. Zuurdeeg, J.C. van der Waals, J.A. van Bokhoven, D.C. Koningsberger, H. van Bekkum, J. Catal. 180 (1998) 234 6. L.C. de Menorval, W. Buckermann, F. Figueras, F. Fajula, J. Phys. Chem. 100 (1996) 465 7. S.-B. Liu, J.-F. Wu, L.-J. Ma, T.-C. Tsai, I Wang, J. Catal. 132 (1991) 432 8. C. Flego, I. Kiricsi, C. Perego, G. Bellussi, Stud. Surf. Sci. Catal. 94 (1995) 405 9. J. Take, T. Yamaguchi, K. Miyamoto, H. Ohyama, M. Misono, in New developments in zeolite science and technology, Y. Murakami, A. Iijima, J.W. Ward Eds., Stud. Surf. Sci. Catal., 28 (1986) 495 10. P. Ratnasamy, R.N. Bhat, S.K. Pokhriyal, S.G. Hegde, R. Kumar, J. Catal. 119 (1989) 65 11. D.H. Olson, W.O. Haag, W.S. Borghard, Microp. Mesop. Mater. 35 (2000) 435 12. N.Y. Chen, J. Phys. Chem. 80 (1976) 60 13. B. Hunger, M. Heuchel, S. Matysik, K. Beck, W.D. Einicke, Thermochim. Acta 269/270 (1995) 599 14. A. Jentys, G. Warecka, M. Derewinski, J.A. Lercher, J. Phys. Chem. 93 (1989) 4837 15. T. Sano, N. Yamashita, Y. Iwami, K. Takeda, Y. Kawakami, Zeolites 16 (1996) 258 16. F. Deng, Y. Du, C. Ye, J. Wang, T. Ding, H. li, J. Phys. Chem. 99 (1995) 15208 17. K.H. Bourne, F.R. Cannings, R.C. Pitkethly, J. Phys. Chem. 74 (1970) 2187 18. F. Wakabayashi, J.N. Kondo, K. Domen, C. Hirose, J. Phys. Chem. 100 (1996) 1442 19. J.N. Kondo, M. lizuka, K. Domen, F. Wakabayashi, Langmuir 13 (1997) 747; R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto, A. Zecchina, J. Phys. Chem. 99 (1995) 11937 20. F. Wakabayashi, J.N. Kondo, K. Domen, C. Hirose, Catal. Lett. 38 (1996) 15 21. F. Di Renzo, B. Chiche, F. Fajula, S. Viale, E. Garrone, Stud. Surf. Sci. Catal. 101 (1996) 851

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1611

Diffusion and adsorption o f h y d r o c a r b o n s from a u t o m o t i v e e n g i n e e x h a u s t in zeolitic a d s o r b e n t s D. Caputo a*, M. Ei6 a** and C. Colella b aDepartment of Chemical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, N.B., Canada E3B 5A3 bDipartimento di Ingegneria dei Materiali e della Produzione, UniversitY. di Napoli Federico II, P.le Tecchio 80, 80125 Napoli, Italy Diffusion characteristics of hydrocarbons typical of automotive engine exhaust were determined by the Zero Length Column (ZLC) chromatographic method in silicalite and ferrierite pellets. The data clearly show that intracrystalline diffusion is too fast to exert any significant influence on the sorption kinetics in adsorbent pellets, and the dominance of meso/macropore diffusional resistance was confirmed by replicate experiments with different particle size fractions. Surface diffusion makes a significant contribution to the meso/macropore diffusivity for toluene in silicalite pellets leading to large differences in effective diffusivities relative to other sorbates, in particular ethylene. Ethylene, iso-butane and iso-butene adsorption and diffusion in ferrierite pellets showed a peculiar behavior due to immobilization effects involving both zeolite crystals and possibly mesoporous walls of the composite particles. At low concentration levels sorbate species diffused as single species in their respective binary mixtures. 1. INTRODUCTION Future legislative restrictions on passenger car emissions require the development of new after-treatment systems. In particular, exhaust hydrocarbon concentration levels need to be significantly reduced during cold start, because the largest fraction (about 80%) of the total organic pollutants flows out during the short time following the engine start-up (200 seconds), when the catalyst temperature is lower than its light-off value [1]. In recent years, several alternative technical solutions have been proposed for reducing cold start hydrocarbon emissions of engine exhaust, but the most practical one for car manufacturers seems to be the combination of catalytic and adsorbent functions on the same cordierite support (ACS, Adsorber-Coated Substrate) [2,3]. The main performance feature required for a successful adsorbent material is its high trapping capacity for hydrocarbon species at relatively low temperature range (25-150 ~ and low sorbate concentration levels.

*Present address: Dipartimentodi Ingegneriadei Materiali e della Produzione, Universith di Napoli Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy. **Corresponding author.

1612 Microporous materials such as zeolites, due to their favourable adsorption properties at these conditions are considered appropriate candidates [3]. Any future efforts related to a proper design of the "trapping" device to control cold start emissions will also have to address diffusive properties of a proposed adsorption system. Since the cold start is a relatively short process (usually less than 200 seconds) the kinetics will play a very important role in assuring that maximum (equilibrium) capacity of the adsorption system is achieved. Furthermore it is also important to investigate diffusion behavior of representative sorbates in their respective mixtures to assess any adverse effects on adsorption kinetics due to interactions between different species. Thus the main objective of this study was to determine diffusive properties of selected hydrocarbons (single component systems) as well as their respective binary mixtures on MFI- and FER-type zeolites. In addition equilibrium data obtained in our earlier studies [4-6] were also used in the analysis of kinetic data. 2. EXPERIMENTAL 2.1. Materials

Silicalite (MFl-type zeolite structure) and ferrierite (FER-type zeolite structure) samples were kindly provided by Prof. R. Aiello (Universita' degli Studi della Calabria, Rende, Cosenza, Italy) as powders. All experimental runs were carried out on pellets with average radius of 360 and 650 lam, which were prepared by sieving the fragments of crushed pellets. The pellets were formed by applying high pressure on the original powdered materials. Ethylene, iso-butene, iso-butane and toluene were chosen as representative hydrocarbons typical of engine start-up exhaust gases [4-6]. 2.2. Diffusion runs

Diffusion measurement for both pure components and their respective binary mixtures were performed using the ZLC (Zero Length Column) chromatographic technique [7]. A very small amount of pelleted adsorbent (1.5-2.0 mg) was placed in the ZLC column and equilibrated with sorbate diluted in a helium flow to obtain a low concentration required by the ZLC theory. Furthermore sorbate concentrations used in experimental runs were similar to the typical concentration levels in the exhaust gases. The effluent sorbate concentrations from the ZLC column were measured using an on-line mass spectrometer (Dycor Dymaxion, Ametek). Details of the experimental method and apparatus have been described elsewhere [7,9]. 2.3. Adsorption runs

Adsorption properties were investigated by a gravimetric technique using a McBain-type adsorption balance. A detailed description of the apparatus and methods employed in the adsorption runs can be found in a previous paper [4]. 3.

THEORY

The analysis of the ZLC desorption curve involves solving Fickian diffusion equation with appropriate initial and boundary condition, i.e., zero concentration of sorbate species on the surface of the adsorbent particle [7]. For a linear equilibrium system with uniform spherical particles the normalized effluent gas concentration is given by:

1613

c _- ~ .

2L

Co ~: [p~ + L ( L - 1)]

e x p ( - f l ~ Dt / R 2 )

(1)

where [3ns are eigenvalues given by the root of the equation: /3, cot fin +L-1 = 0 and

L=

1 gv R2 1 purge flow rate R 2 -= 3 (1 - ~)z KrtD 3 crystal volume KHD

(2)

(3)

In Eqn. (3) ~ is the voidage of the ZLC bed, v the interstitial gas velocity, z the ZLC bed depth and Ktl the dimensionless Henry's Law constant. Diffusivity coefficient D or time constant D/R 2 can be extracted from the model by fitting Eqn's 1-3 with an experimental ZLC curve for the entire time range. In equations (1-3) D/R 2 represents Dflrc2 if the diffusion process is micropore controlled, where Dc and rc are intracrystalline diffusivity and radius of a crystal particle, respectively. On the other hand, if the diffusion process is controlled by meso/macropores (secondary pore structure in a composite adsorbent pellet) the D/R 2 is replaced by (TpDp/Rp2)/[Tp +(l-yp)Ktt], the effective diffusional time constant [8], in which Dp denotes the pore diffusivity, Rp the equivalent pellet radius and ~,p the particle porosity. Adsorption equilibrium data were analysed using the virial equation [8]: Kn.p/q =exp (2A t.q+3/2A 2.q2+ ......... )

(4)

where p is the partial pressure, q is the relative amount adsorbed and A, are the virial constants. According to equation (4), a plot of In(p/q) vs. q should be linear at concentrations above the limit of validity of Henry's Law. Therefore extrapolation of such a plot to p---~0 provide a simple method for determining the Henry's constant KH [5,8], which is an useful parameter in estimating the adsorption affinity at low pressures. The typical hydrocarbons present in an automotive engine exhaust are usually at low pressure range (<1 torr). 4. RESULTS AND DISCUSSION

The ZLC measurements were performed for ethylene, iso-butane, iso-butene and toluene in the temperature range o f - 3 0 to 120 ~ depending on the sorbate and zeolite sample. The concentration of sorbate in helium was set at 0.12 vol. %. This is low enough to ensure substantial linearity of the equilibrium isotherms (region of validity of Henry's Law) even at the lowest temperatures, as well as keeping it at the same time within the concentration level of the exhaust gas. The zeolite pellets are characterized by micropores (zeolite crystals), meso/macropores (secondary pore structure) and a different pore size distribution, depending on the agglomeration process. In order to evaluate the relative significance of both micro and meso/macro pore diffusion, two different particle sizes of the pelleted sample were used (360 and 650 ~tm).

1614 Representative desorption curves for toluene on different size of silicalite pellets are depicted in Figure 1. The diffusivity data obtained for ethylene, iso-butane and toluene and the activation energy (E) values are summarized in Table 1. It is interesting to note that diffusion results obtained for iso-butene (olefine representative) in silicalite pellets (650 lam) were almost identical to the results of iso-butane, and are not reported in Table 1. From the ZLC curves shown in Figure 1 it is evident that desorption rate is strongly dependent on particle size. An analysis of the time constants obtained with different particle sizes (R~= 360 and R2=650 ~tm, Table 1) shows that the ratio is very close to the expected theoretical ratio of square of the particle radii (R22/R~2= 3.3) for all the sorbates studied, thus providing clear proof of meso/macropore diffusion control. The same results involving diffusion of iso-butane in silicalite pellets were earlier reported by Jiang et al. [9]. Inspection of the data in Table 1 shows that diffusivity of ethylene in silicalite pellets is much higher than iso-butane (iso-butene) and toluene, while the corresponding activation energy is lower. Furthermore due to the very high diffusion rates of ethylene reliable ZLC curves were only possible to obtain at relatively low temperatures (below 0~ The term D/R 2 in Table 1 represents effective diffusional time constant as explained in the theoretical section. The more dominant term for this parameter is Henry's law constant, which is the main reason for relatively high activation energies, approaching heat of adsorption limits, shown in Table 1. Adsorption equilibrium measurements in silicalite [4-6] showed that ethylene had much lower adsorption affinity, because Kit value was about one order of magnitude lower in comparison with other sorbates (see Table 3). Therefore it is expected that its effective diffusivity is about one order of magnitude higher. By extrapolating D/R 2 values of ethylene to a higher temperature range (0-120~ a rough analysis involving the comparison of D/R 2 values with other sorbates could be easily made. The expected difference of about one order of magnitude is basically encountered for iso-butane and iso-butene. However the difference between ethylene and toluene is almost three orders of magnitude at the extrapolated higher temperatures, i.e., 60-120~ which is too large to be explained based on the equilibrium differences alone. In an earlier study related to diffusion in zeolite 5A pellets by Ruthven and Xu [ 10] it was demonstrated that the mass transfer through meso- and macro-pores of relatively strongly adsorbed species could be attributed to a dual diffusion mechanism, involving molecular diffusion in larger pores or macropores and surface diffusion in smaller mesopores of the same secondary pore structure. The proposed mechanism can be quantitatively defined by a simple additive rule:

ypDp = yp' Dp' + KsDs

(5)

where yp and Dp represent particle porosity and apparent pore diffusivity (all mechanisms). The first term on the right-hand side of Eqn. (5) represents contribution of the molecular flux due to transport process in macropores, where Dp' and yp' denote molecular diffusion and macropore porosity (a fraction of the total meso/macropore porosity) respectively. In the second term parameters Ks and Ds denote respective surface adsorption equilibrium constant and surface diffusivity. For strongly adsorbed molecules, like toluene, the surface diffusion could become an important contributing factor to the total mass transfer.

1615

1L 0.1 0.1

II

b

o

0.1

0 ~

0.1 0

50

100

150

200

250

300

time, sec

Figure 1. Experimental (symbol) and theoretical (solid line) ZLC curves for toluene in silicalite pellets with an average radius of 360~tm (a) and 650~tm (b).

0

10

20

30

40 50 time, see

60

70

80

Figure 2. Experimental (symbol) and theoretical (solid line) ZLC curves for ethylene in ferrierite pellets with an average radius of 360~tm (a) and 650~tm (b).

Table 1. Diffusivity data for ethylene, iso-butane and toluene in the silicalite pellets D/R12x10 3 D/R22x103 Ratio of Activation energy E*** (~ (sec "1) (secl) D/R 2 (KJ-mol "1) ..... ethylene* 27.8 -30 41.1 -20 72.3 23.6 3.1 -10 121.8 37.0 3.3 0 191.9 61.0 3.1 37.3 iso-butane** 30 9.9 45 18.7 6.1 3.1 60 41.4 13.0 3.2 80 76.7 25.5 3.0 39.0 toluene** 60 2.7 80 6.4 2.1 3.0 100 13.1 4.2 3.1 120 25.4 7.9 3.2 R1 = 360 gm'; R2=650 gm; R2Z/R'12='3.3; ' *purge flow rate = 40 ml/min; **purge flow rate = 60 ml/min; ***Average value between ER1 and E~. T

1616 Table 2. Diffusivity data and activation energy (E) for ethylene in the ferrierite pellets T (~

D/Rl2x 103 (sec -i)

D/R22x103 (sec-')

Ratio of D/R 2

10 8.1 30 18.1 5.8 60 65.3 20.0 80 156.2 47.2 Ri = 360 ~tm; R2-650 lam; R22/R!e= 3.3; between ERi and ER2.

Activation energy E* (KJ.mo1-1)

36.0 3.1 3.3 3.3 purge flow rate - 60 ml/min; *Average value

For most agglomerated particles porosity yp' of the large macropores is very small in comparison with the total porosity of a particle [10], thus leading to a small value of molecular diffusion flux due to that mechanism (the first term in Eqn. (5)). On the other hand toluene is relatively strongly adsorbed in silicalite, and it is plausible to expect, that the surface diffusion can become more dominant mechanism than Knudsen diffusion for the overall diffusion in mesopores. Based on the above arguments the mesopore contribution to total flux in the secondary pore structure can be attributed mainly to the surface diffusion, thus becoming comparable to the limited contribution of the molecular diffusion flux in the macropores. This generally leads to much lower effective diffusivities for toluene, and consequently very large differences in comparison with ethylene effective diffusivities as shown in Table 1. Representative desorption curves of ethylene in ferrierite pellets with an average radius of 360 lam and 650 ~tm are shown in Figure 2. Corresponding numerical values of these diffusivity data are summarized in Table 2. Analysis of the D/R 2 ratios for different particle sizes showed a very close agreement with the theoretically expected ratio (3.26), thus clearly indicating the meso/macropore diffusion controlled process, as in the case of silicalite pellets. Attempts to obtain ZLC curves for other sorbates, i.e., iso-butane, iso-butene and toluene in the ferrierite sample have failed to produce meaningful results indicating either absence of adsorption or very strong immobilization (ZLC curves for these sorbates were practically identical with the blank ZLC curves, i.e., without sorbates). Equilibrium results for iso-butane and iso-butene reported in our earlier studies [4-6], as well as K values reported in Table 3 showed limited but not negligible adsorption capacities of these species in the ferrierite, which is in some contradiction with the ZLC results. This indicates that the butanes are most likely very strongly immobilized or trapped in the ferrierite crystal structure. It is plausible that immobilization proceeds via formation of strong sorbate complexes with active sites in the crystal channels. Further studies that should involve TPD and FTIR measurements will be necessary to obtain a closer insight into this problem. On the other hand the size of ferrierite micropores (0.5 nm) is too small to allow larger toluene molecules (about 0.6 nm) to penetrate in the crystal structure. Similar conclusion was obtained from the equilibrium results [4-6]. If ethylene data in Table 1 are extended to higher temperature range for comparisons with the data in Table 2 it is evident that the effective diffusivity of ethylene diffusion in ferrierite is about two orders of magnitude lower than in silicalite. This large difference can only be partly attributed to the surface diffusion through mesopores in a similar way as discussed in the preceding section concerning the diffusion in silicalite pellets.

1617 Table 3. Henry constants K (torr-~) at 25 ~ computed from equilibrium data by virial equation (5). . . . . . . . . . . Zeolite sample\Sorbate Silicalite Femerite

ethylene ....... isobutene 0.95 8.47 1.97 0.17

isobutane 0.10

toluene 10.26 -

It is clear from the equilibrium data for ethylene (Table 3) that its affinity for the ferrierite sample is not much greater than for its silicalite counterpart; thus surface diffusion is not likely to play an important role. Another plausible explanation could be related to the phenomena of immobilization or "trapping" of ethylene molecules on the surface of femerite crystals that comprise walls of the mesopores, as a part of the secondary pore structure. This "trapping" could also act as a promotor for ethylene oligomerization inside the mesopores. In a number of previous studies, i.e., Ei6 et al. [11], immobilization inside zeolite crystals has been reported, but not in the mesopores of composite particles. It remains to be seen in the more detailed future studies if immobilization phenomena could be extended to mesopores. To assess interaction effects of different species a series of diffusion measurements involving binary mixtures, i.e., toluene/iso-butane and toluene/ethylene in silicalite pellets were carried out in a co-current mode of operation. As expected results showed that the species in mixtures behaved as pure species indicating negligible interactions between them at low or relatively low concentration levels (Figure 3). Equilibrium data for the sorbates used in this study in silicalite and ferrierite were reported in our earlier study [4, 6]. Representative isotherms for ferrierite sample are shown in Figure 4. Analysis of data for the estimation of Henry's law constants at 25~ as summarized in Table 3 was carried out in the present work. From the ispection of Henry constants in Table 3, it is obvious that silicalite exhibits strong adsorption affinities for toluene and iso-butene, but weak affinity for ethylene. 14 l,

12 ~t~1 0 o

8 6

0.1

~o

9

9

9

4 2

20

-40

60

80

100

120

time, see

Figure 3. Comparison between experimental ZLC curves for isobutane in silicalite pellets (360~tm), as single component (open symbol) and in mixture with toluene (solid symbol).

O-

0

'

'

'

i

2

,

,

,

i

,

,

,

i

.

.

4 6 Pressure, torr

.

.

i

8

,

,

,

10

Figure 4. Adsorption isotherms at 25 ~ of ethylene (triangle), isobutene (square) and isobutane (circle) on ferrierite.

1618 In contrast, ferrierite exhibits the strongest affinity towards ethylene, and weak affinities for iso-butane and iso-butene. As expected no toluene was adsorbed in small-port ferrierite. 5. CONCLUSIONS The experimental data provide a reasonably consistent and coherent picture of the kinetic behavior. Intracrystalline diffusivities of hydrocarbons in silicalite and ferrierite samples are too fast to have any significant influence on the sorption rate in pellets formed from small (-2 pm) crystals. The sorption-desorption rate in pelleted adsorbents is controlled by a meso/macropore diffusion. The dominant mechanism for lighter hydrocarbons is molecular diffusion through larger macropores and Knudsen diffusion in mesopores, while diffusion of more strongly adsorbed toluene through silicalite pellets is controlled by molecular and surface diffusion in macro- and meso-pores respectively. In binary mixtures the sorbates behave as single component systems. From the kinetic and equilibrium data analysis it seems that combination of silicalite and ferrierite adsorbents could provide a viable option for "trapping" of the cold start exhaust gases. However, a further study is necessary to more properly address effects of immobilization involving the light hydrocarbons, in particular ethylene in ferrierite, and its potentially adverse effects on the "trapping" device's performance. 6. ACKNOWLEDGMENTS Italian National Research Council ( C N R - Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II) is gratefully acknowledged for financial support. One of us (D.C.) is grateful to University of Naples Federico II (Short Mobility Program for Researchers) and Natural Sciences and Engineering Research Council of Canada (NSERC) for grants which enabled him to take part in this investigation. REFERENCES

1. S. Kubo, M. Yamamoto, Y. Kizaki, S. Yamazaki, T. Tanaka and K. Nakanishi, SAE Techical Paper No. 932706 (1993). 2. K. Kollmann, J. Abthoff and W. Zahn, SAE Techical Paper No. 940469 (1994). 3. C.T. Goralski, Jr., T. Chanko, J. Lupescu and G. Canti, SAE Techical Paper No. 2000-010654 (2000). 4. P. Corbo, F. Migliardini, D. Caputo, F. Iucolano and C. Colella, Transaction of SAE (Society of Automotive Engineering), No. 2000-01-3094 (2000). 5. D. Caputo, P. Corbo, F. Iucolano, F. Migliardini and C. Colella, Proceedings EUROMAT 2001 (CD-ROM), AIM (Ass. Ital. Metallurgia), Milano, 2001. 6. P. Corbo, F. Migliardini, R.Aiello, F. Crea, D. Caputo, C. Colella and F. Iucolano, SAENA Technical Paper Series No. 2001-01-066 (2001). 7. M.Eid and D.M. Ruthven, Zeolites, 8 (1988) 40. 8. D.M. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley and Sons, New York, 1984. 9. M. Jiang, M. Eid and D.M. Ruthven, in Proceedings of the 7th Fund. of Adsorption Conf., Nagasaki, Japan, May 20-25,2001, in press. 10. D. M. Ruthven and Z. Xu, Chem. Eng. Sci., 48 (1993) 3307. 11. M. Ei6, A. Micke, M.Kof:irik, M. Jama and A. Zikanova, Adsorption, (2002) in press.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1619

Kinetic processes during sorption and diffusion of aromatic molecules on medium pore zeolites studied by time resolved IR spectroscopy H. Tanaka, S. Zheng, A. Jentys and J. A. Lercher Institute for Chemical Technology, Technische Universit~t Mianchen, Lichtenbergstrage 4, D-85747 Garching, Germany The transport of (alkyl-)aromatic molecules from the gas phase to the catalytically active sites inside the channels of ZSM-5 was studied. The sorption of the molecules on the external surface and inside the pores was followed independently by time resolved IR spectroscopy and two mechanisms for the transport, i.e., the adsorption on the external surface followed by a consecutive diffusion to sites inside the pores and the sorption of the molecules on both sites directly from the gas phase, were discussed. On the basis of the results presented the second pathway appears to be more likely, as the very low sticking probability indicates that the number of collisions of the molecules at sites in the pore entrance region is sufficiently high for a direct sorption. Nevertheless, for both pathways discussed the reorientation of the molecules from their gas phase geometry to the sorbed state is proposed to control the rate of the sorption process.

1.

INTRODUCTION

Medium pore zeolites have attracted substantial academic and industrial interest as shape-selective catalysts, because of their remarkable para-selectivity in reactions leading to di-alkyl-benzene molecules [1, 2]. In order to understand the individual reaction steps and, thus, to elucidate the reasons for the shape selectivity of these materials, knowledge about the sorption and transport of reactant molecules from the gas phase to the active sites inside the pores on a molecular level is essential. One of the key questions with respect to the sequence of transport steps is how reactant molecules enter into the micropores before their subsequent transport to the active sites inside the pores of the molecular sieve. In principle, two conceptually different transport pathways might exist. (i) The reactant molecules pre-adsorb on the external surface of the zeolite crystals and, after entropically and enthalpically controlled re-orientations [3], they enter into the micropores and diffuse to the acid sites inside the pores. (ii) The reactant molecules enter directly from the gas phase into the pores and adsorb on the sites located in the channels of the molecular sieve. The first transport mechanism proceeds through a sequence of steps including a pre-adsorbed state of the molecules on the outer surface of the crystals. This additional step in the transport process was found to be potentially the rate-limiting step and the effects observed are summarized under the term "surface barrier" [4, 5]. Several models have been applied to describe the underlying processes including structural models [6], linear and non-linear isotherms [7, 8], non-equilibrium effects [9] and entropic effects

1620 during the re-orientation of molecules at the zeolite outer-surface [3]. In contrast, the direct transport of molecules from the gas phase to sites inside the pores is primarily determined by their collision frequency with the surface and the sticking probability on a particular surface site under the conditions investigated. A pre-adsorbed state on the surface is not required and, therefore, the sorption of the reactant molecules on the external surface and inside the pores is decoupled. The probability of successful collisions, described by the sticking coefficient, however, will critically depend on the geometry and possible orientations of the reactant molecules in the gas phase as well as on the structural properties of the pore openings of the molecules sieve. In contrast to the adsorption process, it is more likely that the transport of the molecules from the sites inside the micropores to the gas phase occurs without an additional surface state at the outer surface. Note that for reactions involving shape selective transformations of substituted benzenes both transport steps are important for controlling the selectivity. Here, we use time resolved IR spectroscopy coupled with repeated adiabatic small variations of the volume (leaving the total number of gas molecules in the closed system constant) to study the details of reversible sorbate uptake.

2.

EXPERIMENTAL

2.1. Sample H-ZSM-5 with a Si/A1 ratio of 45 and a particle size of 0.11am was used. The concentration of SiOH and SiOHA1 groups (determined by pyridine and NH3 sorption) was 0.83 mmol.g -1 and 0.261 mmol.g -1, respectively. Adsorption of di-tert-butylpyridine indicated that 28 % of the SiOHA1 groups (BrCnsted acid sites) were located in the pore mouth region of the zeolite [10] and, therefore, were directly accessible for molecules from the gas phase.

2.2. Experimental The sample was prepared as a self supporting wafer (approx. 20mg) and activated in vacuum (<10 .7 mbar) at 823 K (heating rate 10 K.min -1) for 1 h. The sorbate molecules (benzene, toluene, p-xylene and o-xylene) were adsorbed at 403 K with an equilibrium partial pressure of 6x10 -2 mbar. The changes in the coverage of the hydroxyl groups during stepwise changes in the partial pressure of + 3x10 -3 mbar were followed by time resolved IR spectroscopy. The experiments were carried out in a newly developed system consisting of a vacuum IR cell attached to a high vacuum system, which allows varying the partial pressure of sorbate molecules periodically via a magnetically driven bellows sealed separator plate used to modulate the total volume of the vacuum chamber. The volume was typically varied + 5% around the equilibrium pressure in order to minimize adiabatic effects during the compression and expansion of the gas. The frequency of the volume modulation was synchronized with the IR spectrometer to allow the collection of IR spectra with a sufficient signal-to-noise ratio at a high time resolution using the rapid scan technique (typically 4000 interferograms per spectra, time resolution 500 ms). In principle, this technique utilizes the periodicity of the experiment to collect only a certain fraction of the interferograms during each cycle. The number of

1621 interferograms collected in each cycle is determined by the scanning rate (i.e., number of interferograms per second) and the time resolution required in the final spectra. For this technique it is essential to ensure that the process studied is completely reproducible in each cycle, as the final spectra are obtained from the co-addition of the interferograms collected in the individual adsorption-desorption cycles. For the spectra discussed 4000 interferograms (at a resolution of 8cm -1) were necessary to obtain a sufficient data quality as small pressure changes (+ 3x10 -3 mbar) in the presence of an equilibrium pressure of 6x10 -2 mbar were studied. The time resolution between the spectra was 500 ms, the modulation frequency of the volume was 0.0167 Hz. The variation of the surface coverage during the adsorption-desorption cycles was calculated from the intensities of the IR bands at 3745 cm -~ (SiOH groups) and at 3610 cm -~ (SiOHA1 groups) assuming that one molecule adsorbs per hydroxyl group by: I~SiOH ACsioH = ~ X CSiOH JsioH I~ABAS A CBAS "- ~ X CBAS ABAS

C and AC describe the concentration of hydroxyl groups and the variation in the number of sites occupied during the ad- and desorption cycles. A and AA are the integral intensity and change in the intensity of the corresponding IR bands. The indexes SiOH and BAS indicate SiOH and SiOHA1 groups, respectively.

3.

R E S U L T S AND DISCUSSION

The changes in the IR spectra during a stepwise change in the toluene partial pressure of +_3x10 -3 mbar at an equilibrium pressure of 6x10 -2 mbar at 403 K are shown in Fig.1. To clearly visualize the changes in the IR spectra during the experiment the first spectra was subtracted from the subsequent spectra in the series. Therefore, bands increasing in intensity after the adsorption are pointing upwards, while bands decreasing in intensity are pointing downwards. In the first half of the cycle shown in Fig.1 the adsorption of toluene on the zeolite led to an increase of the intensities the toluene ring vibrations (1492 cm -1) and C-H stretching vibrations (2850-3000 cm -1) and in parallel to a decrease of the hydroxyl stretching vibrations of the SiOH (3745 cm -1) and SiOHA1 groups (3610 cm-1). In addition, a broad band around 3500 cm -1, assigned to hydroxyl groups of Br0nsted acid sites interacting with toluene via hydrogen bonding interactions (perturbed hydroxyl groups), was observed. Due to the selection of the first spectra (at t = 0 s) as reference for the subtraction the intensities of all bands decrease to zero during the desorption process (second half of the cycle).

1622

.0004~-~.0003'-~.0002.,~ .0001 -

O-.0001-, -.0002

35;0

3000

25100

2000

15;0

~i.0~-~10

Time

[sec]

Wave number [cm -1] Figure 1: Changes in the IR spectra during stepwise changes in the toluene partial pressure of 6x10 -2 + 3x10 -3 mbar at 403 K (Time resolution between the spectra 0.5 s, modulation frequency of the volume 0.0167 Hz)

The coverage of SiOH and SiOHA1 groups after adsorption of the different molecules with a partial pressure of 6x10 -2mbar are summarized in Tab. 1. Table 1: Coverage of SiOH and SiOHA1 groups after adsorption of the alkyl-benzene molecules at a partial pressure of 6x10 -2 mbar Sorbate Benzene Toluene p-Xylene o-Xylene

SiOH group 0.04 0.06 0.11 0.12

SiOHA1 group 0.38 0.64 0.67 0.67

The changes in the coverage of the SiOH (3745 cm -1) and SiOHA1 (3610 cm -1) groups, determined from the intensity of the corresponding OH stretching frequencies, during the ad- and desorption of benzene, toluene, p-xylene and o-xylene at 403 K are shown in Figs. 2 to 5.

1623 0.8 0.6 "i-

03 9

o E -1 ~-" 0

0

%.o oO oo ~'

/

[

1.0 0.8 0.6

0.4

0.4 0.2 C

0.0 -0.2

0.2 0.0

0

10

20

30

40

50

60

-0.2

0

10

20

T i m e [s]

30

40

50

60

T i m e [s]

Figure 2: Changes in the coverage of SiOH (o) and SiOHA1 groups (m) during sorption of benzene at 403 K. 0.8

0.6

0.6 "i" 03

o E

O

0.4

/

c

0.4 0.2

0.2

0.0 -0.2

0 0

10

20

30

40

0.0

9 50

60

-0.2

0

10

20

T i m e [s]

30

40

50

60

T i m e [s]

Figure 3" Changes in the coverage of SiOH (o) and SiOHA1 groups (m) during sorption of toluene 403 K.

'T, 03 --

o E

"-1

(O

0.8

0.25

0.6

0.20 0.15

0.4

0.10 0.2

0.05

O

0.0

0.00

-0.2 0

10

20

30 T i m e [s]

40

50

60

-0.05

mm

0

10

20

30

40

nnnnl

50

T i m e [s]

Figure 4: Changes in the coverage of SiOH (o) and SiOHA1 groups (m) during sorption of p-xylene at 403 K.

60

1624 0.10

0.8 0.6 0 E 0 <1

9

0.06

9

9

0.04 ii '? 9

0.4 3

0.2

~.o~ oo.~

0.0 -0.2

0.08

o

0

10

20

30

40

50

60

0.02

./n

0.00

9

-0.02 f -0.04

Time [s]

mm~ i

l i

9

0

10

20

30

40

5O

Time [s]

Figure 5: Changes in the coverage of SiOH (o) and SiOHA1 groups (n) during sorption of o-xylene at 403 K. The initial rates of the sorption processes of the aromatic molecules on the SiOH and SiOHA1 groups, calculated from the initial slope of the concentration changes, are summarized in Table 2. Table 2: Initial sorption and desorption rates of the (alkyl)benzene molecules at a variation of the partial pressure of +3• -3 mbar (equilibrium partial pressure 6x 10 -2 mbar)

benzene toluene p-xylene o-xylene

dCsi/dt [(nmol/g)s -~]

dCBAs/dt [(nmol/g)s -1]

adsorption desorption 52 59 56 64 105 97 198 177

adsorption desorption 145 172 88 75 43 35 8 9

Independently of the kinetic diameter of the sorbate molecules, the adsorption and desorption processes were found to occur at approximately the same rate. For benzene and toluene, the sorption on the SiOH (surface) sites was slower compared to sorption on the SiOHA1 sites inside the pores, while for the xylene isomers the ad- and desorption from the gas phase on the SiOH groups located on the external surface was faster. With increasing kinetic diameter of the molecules the changes in the coverage during the dynamic pressure change strongly decreased for the SiOHA1 groups, while the changes in the coverage of the surface SiOH groups were not affected. Under the conditions studied the pressure increase of 6x10 -3 mbar (between both equilibrium positions) led to an increase of the collision frequency of benzene with the surface of 4.4x1021 s-ira -2. During the pressure step the additional benzene uptake was 6x1014 molecules.s-lm -2, which results in a sticking probability of 1.4x10 -7 for the sorption on the SiOH groups. Note, that this is in perfect agreement with the sticking probability observed for a single pressure step of toluene on ZSM-5 which is in the order of 3.0x10 -7 (T=403 K, p: 0--) 1 mbar). For all other sorption processes a similar sticking coefficient was found.

60

1625 The extremely low sticking coefficients indicate that the reorientation of the molecules from the gas phase geometry to the sorbed state controls the rate of the sorption process. The enthalpy of the sorption process, which increases from benzene to the xylene isomers due to the larger number of atoms capable to polarize the surface groups, determines the coverage under equilibrium conditions. For the sorption on sites inside the pores the reorientation becomes more demanding for toluene and the xylene isomers compared to benzene due to additional contributions resulting from the reorientation of the alkyl groups. Therefore, the number of molecules adsorbed during a given pressure step decreases form benzene to xylene, although the total coverage (controlled by the enthalpy) increases. For o-xylene (kinetic diameter 7.1 * ) steric constraints limit the access into the pores of ZSM-5 (5.1x5.5 and 5.3x5.6 /k) and, therefore, only sites in the pore mouth region were accessed within the time scale of the dynamic experiment. The decreasing concentration of molecules adsorbed inside the pores and the slower transport process during the dynamic experiment led to a faster equilibration of the sorption process of the xylene isomers on the SiOH groups located on the outer surface of the crystals. At present, the experiments do not allow to unequivocally differentiate between a consecutive transport process via a pre-adsorbed state on the surface and an independent sorption process on the surface and inside the pores. The extremely low sticking probability indicates that the number of collision form molecules from the gas phase leading to an adsorption is extremely low. Under these conditions it is not plausible that a pre-adsorption on the outer surface is required, as the number of collisions of the molecules at sites in the pore entrance region is sufficiently high for a direct sorption on sites inside the pores. In addition, the similar rates observed for the adsorption and desorption processes further indicate that a pre-adsorbed state is not required, as a complete reversibility of the transport including the pre-adsorbed species on the crystal surface appears unlikely. In contrast, the connection between the uptake rates on the SiOH and SiOHA1 groups could point out that both processes are coupled, however, the lower amount of molecules sorbed on sites inside the pores for xylene compared to benzene leads to a higher apparent partial pressure over the sample during the dynamic pressure changes, which will also increase the rate of sorption of the SiOH groups. Independently which model is used to describe the transport into the pores of the zeolite, the experiments clearly indicate that the reorientation processes, either after the sorption on the external surface in a pre-adsorbed state or before the molecules can enter into the pores, control the transport of aromatic molecules to sites inside the pores of a molecular sieve.

4.

CONCLUSIONS

Two models for the transport of (alkyl-)aromatic molecules from the gas phase to the catalytically active sites inside the channels of ZSM-5 were discussed. Based on the results presented it can not be univocally concluded, if the molecules enter into the pores via a precursor surface state on the outer surface of the crystal or directly form the gas phase. However, based on the low sticking probabilities for the sorption processes we favor the direct transport pathway over the consecutive model. Whatever mechanism prevails, the transport of molecules to sites inside the zeolite channels and the sorption

1626 on the crystal surface are linked, either via the pre-adsorbed surface state or via the apparent partial pressure of the molecules in the gas phase. The (entropically controlled) reorientation of the molecules from their gas phase geometry to the sorbed state was found to be the rate controlling step in the dynamic sorption process, while the enthalpy primarily determined the sorption capacity under equilibrium conditions. The rates of the sorption and transport steps from the gas phase to the active sites inside the pores were found to be identical for the adsorption and desorption processes. This indicates that for reactions with methyl-substituted benzene molecules the transport pathways for reactant molecules from the gas phase to the active sites inside the pores and of the product molecules from the active sites to the gas phase are the same. Therefore, a modification of the catalyst surface and the region around the pore openings, e.g., by deposition of tetra-ethoxysilane (TEOS), might affect the energy and entropy of the molecules during the required re-orientations and, therefore, can influence the transport kinetics of the reactant and product molecules without altering the diameter of the pore entrances.

ACKNOWLEDGMENTS The financial support of the Bayerische Forschungsverbund Katalyse (FORKAT II) is gratefully acknowledged.

REFERENCES 1. W.W. Kaeding, G. C. Baffle and M. M. Wu, Catal. Rev.-Sci. Eng. 26 (1984) 597. 2. N.Y. Chen, W. E. Garwood and F. G. Dwyer, in "Shape Selective Catalysis in Industrial Application", Marcel Dekker, New York, 1996 3. J . A . Z . Pieterse, S. Veefkind-Reyes; K.Seshan and J.A Lercher, J. Phys. Chem. B 104 (2000) 5715. 4. J. K~irger and J. Caro, J. Chem. Soc., Faraday Trans. I, 73 (1977) 1363. 5. M. Billow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt, W. Schirmer and J. K~irger, J. Chem. Soc. Faraday Trans. I, 76 (1980) 597. 6. M. Balow, Z. Chem., 25 (1985) 81. 7. M. Kocirik, P. Struve and M. Btilow, Z. Phys. Chem., 267 (1986) 446. 8. M. Kocirik, P. Struve, K. Fiedler and M. Btilow, J. Chem. Soc. Faraday Trans. I, 84 (1988) 3001. 9. A. Micke, M. Btilow and M. Kocirik, J. Phys. Chem., 98 (1994) 924. 10. S. Zheng, H. R. Heydenrych, A. Jentys and J. A. Lercher, J. Phys. Chem. B. submitted (2001).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1627

C a l o r i m e t r i c study o f C2H4 a d s o r p t i o n on synthetic Zeolites with N a + and Ca 2+ cations. I.V.Karetina ~ G.Ju.Zemljanova # and S.S.Khvoshchev ~ ~

Scientific Center in St.Petersburg, Russia

WESTPOST PO BOX 109, Lappeenranta, Fin-53101, Finland #Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St.Petersburg, Russia The heats of C2H4 adsorption on sodium and calcium forms of synthetic faujasites, chabazites, erionites and zeolites A are calorimetricaUy measured. The Ca 2+ ions are preferential adsorption centers and the substitution of Na + by Ca2+ leads to an increase of adsorption heats at low coverages. Most of calcium cations in erionites and in zeolites A are screened and do not form adsorption complexes with C2H4 1. INTRODUCTION Ion-exchange cations in zeolites are usually active adsorption centers for molecules, which are capable of interacting specifically with the localized charges of the surface. The energies of interaction between such molecules and isolated Ca 2+ ions should always be higher than ones for Na + ions. The same correlation must also remain when the coordination conditions of Na + and Ca 2+ions in zeolite framework are identical or similar. The substitution of Na + by Ca2+ in low-silica FAU leads to an increase of ethylene adsorption heats at low coverages [ 1]. The regions of high heats correspond to the formation of Ca2+... C2H4 adsorption complexes at site II in the supercages. Thirty ethylene molecules are sorbed, all in the supercages, one coordinating laterally to each site II Ca 2+ ion [2]. The purpose of the present work is to determine the trends of changes in the heats of C2H4 adsorption on synthetic faujasites with different Si/AI ratios, on synthetic chabazites, erionite and zeolites LTA with the substitution of Na + by Ca2+. No information is available on the calorimetric heats of C2H4 adsorption by these zeolites with Ca 2+ ions. Isosteric heats have been determined for CaLTA [3]. Calorimetric measurements have only been made for LTA with some transition metal ions and with Li+ [4, 5]. 2. EXPERIMENTAL SECTION Prior to the determinatiom of adsorption heats all samples were dehydrated in three steps: 1. evacuation at ambient temperature (2 - 3 h), 2. evacuation at slow increase of temperature until 673 K (2-3 h), 3. evacuation at 673 K (20 h). Measurements were made on DAK-I-I microcalorimeter (USSR) at 303 K. Adsorption was measured by the volumetric adsorption system connected to the calorimeter. Sodium LTA, FAU, CHA and potassium-sodium ERI were synthesized in Grebenshchikov Institute of Silicate Chemistry.

1628 Tablel Chemical composition of dehydrated crystals Sample Chemical composition of dehydrated crystal s 0.98Na20A12Oy2.06SiO2 Na-LTA 0.91CaO0.13Na20A12Oy2.04SiO2 Ca-LTA Na-FAU(X) 0.92Na20A12Oy2.28Si02 Ca-FAU(X) 0.98 CaO0.01Na20A12032.34S iO2 Na-FAU(Y) 1.05Na20A12Oy5.0SiO2 Ca-FAU(Y) 1.06CaO0.06Na20A1203 5.0SIO2 Na-CHA 1.01Na20A12Oy4.5 Si02 Ca-CHA 0.94CAO'0.01Na20"A1203' 4.5SIO2 Na,K-ERI 0.75Na200.31K20A12037.6SiO2 Ca, K-ERI 0"67 CAO0.34 K20 0. 09Na20A!203 7.93 S!.O2 ..... 3. R E S U L T S A N D D I S C U S S I O N

Calorimetric heats of ethylene adsorption on zeolites FAU and LTA are shown on Figure 1 and 2. At low coverages the heats of C2I-I4 adsorption on zeolite Ca-Y are significantly higher than those on Na-Y. The first adsorbed molecules interact with Ca2+ cations in sites II. The number of C2H4 molecules (- 10 per unit cell) adsorbed with high heats practically coincides with the number of Ca2+ cations in these sites (11.4 ions per unit cell of the dehydrated calcium faujasite of similar composition [6]). The remaining Ca2+ cations should not be involved in interactions with adsorbed molecules because they are located in sodalite cages and hexagonal prisms (sites I' and I). About 25 Ca2+ cations per unit cell of dehydrated Ca-X zeolite occupy sites II and the remaining cations (-~18 per unit cell), as in the case of Ca-Y, are located in sites I' and I [6]. Accordingly the region of coverages corresponding to the interactions of adsorbed molecules with cations in sites II for Ca-X is considerably wider than that for Ca-Y (up to - 25 molecules per unit cell). Position of the step on the heat curve is in line with this value

70- kq,_kJ/mol

70fq' kJ/mol

60.

6 0 ~

50. 4

TA

50 0

.

30]

~

40 30

a, molec./u.c.

a, molec./u.c. ,

'

fo'

2'0

'

3'0

Figure 1. Heats of ethylene adsorption on synthetic faujasites

'

'

1

i

2

,

!

3

,

,

4

Figure 2. Heats of ethylene adsorption on zeolites LTA

1629 In the unit cell of the dehydrated CaLTA five Ca 2+ cations are located at 6-membered oxygen rings and one is at 8-ring [7]. The latter must be very weakly bonded with framework since its distances to the nearest oxygen atoms exceed considerably the sum of Ca 2+ and 0 2ionic radii. It may thus be expected that these cations would be the most active adsorption centers. High values of adsorption heats at small coverages on CaLTA in comparison with NaLTA (Figure 2) may apparently be evidence of strong interactions between the first adsorbed C2H4 molecules and Ca ~+ at 8-rings. The next molecules are adsorbed on Ca 2+ at 6-rings and their interaction energies are near to those for C2H4 adsorption on NaLTA. Calorimetric heats of ethylene adsorption on synthetic chabazites and erionites are shown on Figure 3 and 4. Sodium cations in chabazites occupy seven different crystallographic sites [8]. Because of the difference in coordination of these cations with framework oxygen atoms, their successive involvement in interaction with adsorbed molecules results in a smooth decrease in C2H4 adsorption heats. In dehydrated Ca- chabazites almost all the Ca 2+ ions (- 2 per unit cell) are located near the 6-rings of hexagonal prisms [9]. Half of them are preferential adsorption centers for C2H4 because the high values of the adsorption heats are observed only at small coverages (< 1 molec./u.c.). In hydrated erionite the cancrinite cages are occupied by K + ions, and the Ca 2+ ions are located in the large cavities [10, 11 ]. On dehydration these cages are populated by Ca 2+ ions and the K + ions move to the large cavities. Hence, in dehydrated erionite only a small number of Ca 2+ions can be located in large cavities. On hydration some of the Ca2+ ions return to the large cavities and their sites are again occupied by K + ions. This internal ion-exchange process should not proceed in the case of C2H4 adsorption because of the weakness of adsorption interactions. Only the Ca 2+ cations, which are located in the large cavities of the dehydrated zeolite, are involved in the interaction with C2H4 molecules. This results in very narrow range of high adsorption heats for Ca, Kerionites.

kJ/mol

q, kJ/mol 80

801q'

60

60- o ~ ~K-ERI

40

40-

a, molecJu.c. '

o'.5

"

l'.o

"

Figure 3. Heats of ethylene adsorption on synthetic chabazites

Figure 4. Heats of ethylene adsorption on synthetic erionites

1630 4. CONCLUSIONS Ca2+ ions located in 6-membered oxygen rings of faujasite supercages are preferential adsorption centers for ethylene. The heats of adsorption in the ranges of coverages corresponding to the interactions of adsorbed molecules with such ions are higher than those for appropriate sodium zeolites. Only half of similar cations in chabazites are involved in direct interaction with adsorbed ethylene molecules. Most of calcium cations in erionites and in zeolites A are screened and do not form adsorption complexes with CzH4.

REFERENCES

1. T.M.Amelitcheva, A.G.Bezus, L.L.Bogomolova, A.V.Kiselev, N.K.Shoniya, M.A.Shubayeva, S.P.Zhdanov, J.Chem.Soc.Faraday Trans., 74, (1978) 306. 2. S.B.Jang, M.S.Jeong, Y.Kim, K.Seff, J.Phys.Chem. B, 101, (1996), 3091. 3. K.Pilchowski, F.Wolf, U.Kroll, V.Reissing, Z.phys.Chem., 265, (1984) 127. 4. D.Amari, J.L.Ginoux, L.Bonnetain, J.chim.phys.phys.chim.biol., 87 (1990) 1083. 5. D.Amari, J.L.Ginoux, L.Bonnetain, J.Yherm.Anal., 37, (1991) 2507. 6. W.J.Mortier, Compilation of Extra Framework Sites in Zeolites, Butterworth, Guildford, UK, 1982. 7. R.L.Firor, K.Seff, J.Am.Chem.Soc., 100, (1978), 3091. 8. W.J.Mortier, J.J.Pluth, J.V.Smith, Mater.Res.Bull., 12 (1977), 241. 9. W.J.Mortier, J.J.Pluth, J.V.Smith, Mater.Res.Bull., 12 (1977), 97. 10. W.J.Mortier, J.J.Pluth, J.V.Smith, Z.Kristallogr., 143, (1976), 319. 11. J.L.Schlenker, J.J.Pluth, J.V.Smith, Acta Crysallogr., B, 33 (1977), 3265

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1631

A1-MCM-48: synthesis and adsorption properties for water, benzene, and nitrogen M. Rozwadowski, *aM. Lezanska, a R. Golembiewski,a K. Erdmann, a and J. Kornatowskib aFaculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torun, Poland bLehrstuhl II for Technische Chemie, Technische Universit~t Mtinchen, Lichtenbergstr. 4, 85747 Garching bei Mtinchen, Germany

Mesoporous aluminosilicate molecular sieves with the MCM-48 structure have been synthesised using the method of grafting aluminium onto the purely siliceous material. The products were characterised with XRD and adsorption techniques. The obtained materials exhibited high surface areas and high total pore volumes. Adsorption of nitrogen, benzene, and water on the materials with various contents of A1 proceeded similarly despite different physicochemical properties of the adsorptives as well as different adsorbent-adsorbate interactions.

1. INTRODUCTION The M41S family of molecular sieves [ 1,2], comprising materials like MCM-41, MCM-48, and MCM-50, has extensively been studied [3-7] with respect to their syntheses and properties since the discovery in the early 1990's. The M41S materials can be considered as mesoporous molecular sieves due to the presence of uniform pores with sizes in the mesopore range (2-10 nm). These properties give rise to various potential applications in the field of catalysis and adsorption. The type, shape, and dimensions of supramolecular templates used for a synthesis determine the resulting pore structure. Thus, the mesoporous materials are distinguished as hexagonal (MCM-41), cubic (MCM-48), or unstable lamellar (MCM-50) phases. Considering the chemical composition, these molecular sieves occur in pure silica form (i.e., as silicates) or as metal-containing derivatives (i.e., aluminosilicates and metalaluminosilicates), which is the situation similar to that in the zeolite family. Most of the studies on characterisation and application of these materials have been devoted to the hexagonal mesoporous material (MCM-41) because of its easy synthesis. Information on the preparation and properties of MCM-48 is less available [8,9]. In this work, we report on the synthesis of A1-MCM-48 and on influence of the Si/A1 ratio of the reagents and, consequently, of the Si/A1 ratio of the products, on adsorption properties of the latter. As probe molecules for the adsorption investigations, we have chosen nitrogen and benzene as nonpolar compounds and water as a polar one to learn more about both the structure and adsorption properties of the examined mesoporous solids.

1632 Table 1 Conditions for grafting A1 onto the MCM-48 material Sample

Aluminium source

Solvent

no.

1 2 3 4 5 6 7 8

parent material aluminium chloride aluminium isopropoxide aluminium tri-sec-butoxide aluminium isopropoxide aluminium isopropoxide aluminium isopropoxide aluminium isopropoxide

n/a chloroform isopropanol n-butanol n-hexane n-hexane n-hexane n-hexane

Si/A1 of reagents

product a

n/a 30 30 30 32 15 5 2

n/a 26.0 27.4 43.6 34.5 12.7 3.8 3.5

aDetermined with the atomic absorption spectroscopy, n/a = not applicable.

2. E X P E R I M E N T A L

Purely siliceous MCM-48 was synthesised from a mixture containing suspension of SiO2 (Ultrasil, Degussa) in water and both tetramethylammonium hydroxide and cetyltrimethylammonium chloride as templates [10]. The synthesis was carried out in an autoclave under autogeneous pressure at 423 K for 8 h. The product was filtered, washed with water, dried, and then calcined at 803 K under air for 5 h. The samples of A1-MCM-48 were prepared by grafting aluminium onto the parent siliceous material. Prior to the grafting, the calcined MCM-48 samples were evacuated at 10-3 Pa and 423 K. The grafting processes were performed according to the procedures reported in the literature [11,12]. Aluminium chloride, isopropoxide, and tri-sec-butoxide were chosen as the sources of aluminium (Table 1). The obtained A1-MCM-48 samples were filtered, washed with a solvent as shown in Table 1, dried, and then calcined as given above. X-ray diffraction (XRD) powder patterns were recorded on a Siemens D 5000 diffractometer using the CuK~ radiation. Adsorption of nitrogen was measured at 77 K with a Micromeritics ASAP 2010 instrument. Adsorptions of benzene and water were determined at 298.2 K with use of vacuum devices equipped with McBain balances and MKS Baratron gauges.

3. RESULTS AND DISCUSSION Various aluminium sources and solvents used in the syntheses of A1-MCM-48 result in different relative consumption of substrates in the applied grafting processes as indicated by the Si/A1 ratios (Table 1). Nevertheless, the products show a typical system of uniform cubic pores, reflected in a set of peaks in the low-angle XRD powder patterns (Fig. 1). The interplanar spacings (d211) and unit cell parameters (a0) as derived from the XRD data are given in Table 2 for some of the obtained materials. Table 2 contains also some structural parameters derived for the studied samples from the nitrogen adsorption data. As seen, the samples exhibit relatively high surface areas (above 1000 m 2 g- 1) and high total pore volumes (above 0.7 cm 3 g-l). The BET specific surface areas

1633

211

"-'2, :3

"E

I

2

"'

' .....

! .......

4

'

I

6

'

20 [degree]

!

8

10

Figure 1. XRD powder pattems of the parent and some grafted MCM-48 materials (for sample designations, see Table 1).

(SBET) were calculated using the standard Brunauer-Emmett-Teller method. The total surface areas (St), the external surface areas (Sext), and the primary mesopore volumes (Vp) were obtained with use of the high-resolution CZs-plot method [15] and the LiChrospher Si-1000 macroporous silica gel (S~zT = 25 m z g-l) [16] as a reference adsorbent. The values of St were calculated from the slope of the initial part of the eta-plots. Those of Se,,t and Vp were derived from the slope of the linear part of the as-plots within the range between the pressure corresponding to the termination of nitrogen condensation in the primary mesopores and that associated with the onset of capillary condensation in both the secondary mesopores and macropores [13,16]. The total surface area of the studied material was assumed to be the combined surface area of all mesopores and macropores. The external surface area was considered to be that of the macropores and secondary mesopores together. The surface area of the primary mesopores (So) was equal simply to the difference SrSext. The total pore volume (Vt) was derived from a single-point adsorption, taken as the last experimental point of an isotherm (P/Ps =- 0.99), by converting the volume of the adsorbed gaseous nitrogen to that of the liquid one. The calculations made with the Gts-plot method did not indicate the formation of micropores in any sample. Isotherms of the nitrogen ad/desorption are of type IV (IUPAC classification), each exhibiting a step with the inflection point at p/ps = 0.3-0.35 (Figs. 2 and 3). They were used to characterise porosity of the studied materials. The intracrystalline cubic pores are here referred to as the primary mesopores whereas the remaining mesopores as the secondary ones.

1634 Table 2 Characteristics of the MCM-48 materials examined: interplanar spacings (d211), unit cell parameters (a0), BET specific surface areas (SBE~), total surface areas (St), extemal surface areas (Se• surface areas (Sp) and volumes (Vp) of primary mesopores, and total pore volumes (Vt) Sample no.

[nm]

d211

[nm]

a0

[m 2 g-l]

[m 2g-l]

[m 2g-l]

1 2 3 4 5 6 7 8

3.71 3.37 n/d n/d 3.40 n/d 3.32 n/d

9.10 8.26 n/d n/d 8.33 n/d 8.14 n/d

1315 1208 1252 1207 1245 1188 1051 1030

1294 1174 1195 1160 1213 1164 1010 995

284 206 193 157 259 136 154 209

SBET

St

Sext

Sp

Vp

Vt

1010 968 1002 1003 954 1028 856 786

0.718 0.672 0.712 0.703 0.662 0.673 0.538 0.513

0.958 0.864 0.907 0.855 0.884 0.794 0.713 0.711

[m 2 g-l] [cm 3 g-l] [cm 3 g-l]

n/d = not determined. The inflection point corresponds to condensation of nitrogen inside the primary mesopores. Before the condensation, a multilayer adsorption occurs. The narrow range of p/ps in which the isotherm steps occur demonstrates that the aluminium atoms do not significantly affect the lyophilic character of the surface with respect to nitrogen. Each of the isotherms shows also a narrow ad/desorption hysteresis loop at p/Ps > 0.45. It indicates the presence of the secondary mesopores of a wide range of sizes, from c a . 4 nm up to those of macropores [ 13,14]. These

,,-,--, "7

600

,._, 600 4 n

13... 1-03 03

m

~m~Jm~ma

400-

oE 400

.13 O "o

~

03 "o (D t'~ 0

"~ 200-

200

E _=

E

~

i

O

O > 0 "l~l~'-r~T'-~'-r~---l'~'--'r------T--'---r------T~

0.0

0.2

0.4

pips

0.6

0.8

0 1.0

Figure 2. Isotherms of nitrogen adsorption (open) and desorption (full symbols) at 77 K for the parent and grafted MCM-48 materials. The effect of different aluminium sources used for the grafting; <~ - 1, [] - 2, A - 3, 9 - 4 (sample numbers as in Table 1).

0.0

0.2

0.4

pips

0.6

0.8

1.0

Figure 3. Isotherms of nitrogen adsorption (open) and desorption (full symbols) at 77 K for the parent and grafted MCM-48 materials. The effect of different aluminium contents of the materials; <~ - 1, [] - 5, A - 6 , o - 7, O - 8 (sample numbers as in Table 1).

1635 pores origin from, e.g., void space between adjoining particles of the MCM-48 material. The wide range of sizes of the secondary mesopores suggests that the particles are relatively small. In the case of the samples 2-6, the sorption capacities for nitrogen are very similar to one another and seem to be independent of the A1 content although differences in the metal contents are considerable. The samples 7 and 8 containing over 25% A1 reveal a significant decrease in the sorption capacity. This suggests occurrence of stronger interactions between the adsorbate and these two Al-rich adsorbents. The Barrett-Joyner-Halenda (BJH) method was applied to calculate the average pore size and pore size distribution from the nitrogen desorption data. We used a relation in which a correction for the thickness of a statistical film covering the pore walls was added to the Kelvin equation [13,16]. Each differential pore size distribution obtained with the BJH method for the parent MCM-48 and selected A1-MCM-48 samples demonstrated a relatively narrow intense peak with the maximum in the range of 2.2-2.4 nm and a very weak peak at ca. 3.8 nm (Fig. 4). This confirms that the synthesised materials show a regular pore structure. The above results show that both the specific surface areas (SBET and St, see Table 2) and average pore diameter (Fig. 4) decrease with the increasing aluminium content of the materials. The adsorption isotherms of benzene, like those of nitrogen, are of type IV (Figs. 5 and 6). Similarly, uptake of benzene proceeds v i a a multilayer adsorption at lower relative pressures, which is followed by a capillary condensation. The position of the (slight) step on the isotherms is practically independent of the aluminium content of the material and its inflection point occurs at P/Ps = 0.1-0.2. Such a narrow range of the relative pressures indicates that the aluminium centres do not substantially influence the lyophilic character of the surface in relation to benzene, in the same way as for nitrogen. 0.20

-

t-

015-

E .o.

-6

', <~ 0.10-

O

-~ "E

PdoD,~

0.05-

o_

0.00

115

210

215

310

3'5

pore diameter [nm]

410

45

Figure 4. BJH pore size distributions obtained from nitrogen desorption for the parent and grafted MCM-48 materials. The effect of different aluminium contents of the materials; <1 - 1, [] - 5,/x - 6 , 9 - 7 (sample numbers as in Table 1).

1636 Interestingly, the sorption capacity for benzene depends on the A1 content in a similar way as for nitrogen. It may imply two possible explanations: either the interactions with the benzene molecules change only at very high contents of A1 or, more probably, the effect is connected with an actually occurring decrease in the pore volume (see Table 2). The latter might be explained by the deposition of alumina species in the pores. 10

"T, 0

E

10

8

8 "T

@~sooo oo O oO

6

tO

o

o O

O

0

0

0

0

O

a. 4

,.i.-,,

o "o c~

o t~

2

2

P '

0.0

I

'

0.2

I

0.4

',

I

pips

0.6

,

'1

'i'

0.8

'

0.0

1.0

Figure 5. Isotherms of benzene adsorption at 298.2 K for the parent and grafted MCM-48 materials. The effect of different aluminium sources used for the grafting; <1 - 1, [] - 2, A - 3, O - 4 (sample numbers as in Table 1).

,

I

0.4

50

40

40

"7

I

,

piPs

0

o

o

~20 o

~z. 20 o "o

q

10

'

I

0.2

,

I

0.4

,

pips

I

0.6

,

I

0.8

0

,

1.0

Figure 7. Isotherms of water adsorption at 298.2 K for the parent and grafted MCM-48 materials. The effect of different aluminium sources used for the grafting; <1 - 1, [] - 2, A - 3, o - 4 (sample numbers as in Table 1).

1.0

o

000

o

[]

o A

q

o

oOAr]

10

0.0

|

0.8

zxqzx ~ ~x

i'-

o

. N

I

0.6

30

o 30 E

0

,

Figure 6. Isotherms of benzene adsorption at 298.2 K for the parent and grafted MCM-48 materials. The effect of different aluminium contents of the materials; <1 - 1, [] - 5, A 6, O - 7, O - 8 (sample nos. as in Table 1).

50

"7

I

0.2

0

1

0.0

'

I

0.2

,

I

0.4

,

pips

I

0.6

,

I

0.8

,

1.o

Figure 8. Isotherms of water adsorption at 298.2 K for the parent and grafted MCM-48 materials. The effect of different aluminium contents of the materials; <1 - 1, [] - 5, A 6, O - 7, + - 8 (sample nos. as in Table 1).

1637 The adsorption isotherms of water are also of type IV, showing a distinct step with the inflection point at p/ps = 0.5-0.6 (Figs. 7 and 8). When the aluminium content in the adsorbent increases, the height of the step decreases and its position shifts to lower relative pressures (Fig. 8). Thus, the grow of the number of the adsorption centres results in a higher adsorption during the initial stage (at the lower relative pressures) and, consequently, in an earlier start of the capillary condensation of water. These effects are relatively small for samples 1-6 and much larger for 7 and 8, i.e., similarly to the observations for nitrogen and benzene. Such changes for the relatively small and polar water molecules strongly support the suggestion that A1 species really cause a decrease in the pore volume of the Al-rich materials. This agrees with the discussed changes in both the pore volume (Table 2) and pore size distribution (Fig. 4). At the relative pressure equal ca. 0.6, the samples become nearly saturated and then the adsorption rises with P/Ps relatively little while the isotherms are parallel to one another. Like in the case of A1-MCM-41 [14], an increase in the A1 content, i.e., an increase in the number of hydrophilic centres, unexpectedly causes a decrease in the sorption capacity (such a decrease was also observed for nitrogen and benzene, as mentioned above). Since the sorption capacities for water are clearly lower than those for benzene and nitrogen (see Figs. 2, 3, and 5-8), it is likely that formation of clusters of liquid water around the A1 centres and clogging of the pores occur in addition to the deposition of alumina species in the pores. In general, adsorption of all three examined adsorptives proceeds similarly despite their different physicochemical properties as well as different adsorbent-adsorbate interactions. 4. CONCLUSIONS Grafting aluminium onto the MCM-48 material enabled preparation of highly ordered mesoporous samples as indicated by XRD and the BJH pore size distribution. The samples exhibit high surface areas and high total pore volumes. The BET surface area rises with the decreasing A1 content of the material. This effect involves an increase in the surface area of the primary mesopores and a decrease in the external surface area. It might result from a superior ordering of the materials with the decrease in the A1 content. Adsorption of all three examined adsorptives proceeds similarly despite their different physicochemical properties as well as different adsorbent-adsorbate interactions. Nitrogen and benzene fill the pores to a higher extent as compared with water. ACKNOWLEDGEMENT This work was supported in part by the State Committee for Scientific Research (KBN).

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.W. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834.

1638 3. J.C. Vartuli, W.J. Roth, J.S. Beck, S.B. McCullen, and C.T. Kresge, in "Molecular Sieves Science and Technology", H.G. Karge and J. Weitkamp (eds.), Springer Verlag, Berlin, 1998, Vol. 1, pp. 97-120. 4. S. Biz and M.L. Occelli, Cat. Rev. Sci. Eng., 40 (1998) 329. 5. A. Corma, Chem. Rev., 97 (1997) 2373. 6. M.E. Raimondi, and J.M. Seddon, J. Lig. Cryst., 26 (1999) 305. 7. W.J. Roth and J.C. Vartuli, Stud. Surf. Sci. Catal., 135 (2001). 8. R. Ryoo, S.H. Joo, and J.M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 9. S.R. da Rocha and L.D. Fernandes, Stud. Surf. Sci. Catal., 135 (2001). 10. C.T. Kresge, M.E. Leonowicz, W.J. Roth, and J.C. Vartuli, US Patent No. 5 098 684, (1992). 11. L.Y. Chen, Z. Ping, G.K. Chuah, S. Jaenicke, and G. Simon, Micropor. Mesopor. Mater., 27 (1999) 232. 12. R. Mokaya and W. Jones, J. Chem. Soc., Chem. Commun., (1997) 2185. 13. M. Rozwadowski, M. Lezanska, J. Wloch, K. Erdmann, R. Golembiewski, and J. Kornatowski, Phys. Chem. Chem. Phys., 2 (2000) 5510. 14. M. Rozwadowski, M. Lezanska, J. Wloch, K. Erdmann, R. Golembiewski, and J. Kornatowski, Langmuir, 17 (2001) 2112. 15. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. 16. M. Kruk, M. Jaroniec, and A. Sayari, Langmuir, 13 (1997) 6267.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1639

A f r e q u e n c y - r e s p o n s e study of the kinetics of a m m o n i a sorption in zeolite particles Gy. Onyesty/tk, a J. Valyon a and L. V. C. Rees b alnstitute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary bDepartment of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, UK

The dynamic sorption properties of synthetic zeolite NaX and mordenite were studied using the batch-type frequency response (FR) method. FR rate spectra of NH3 sorption were recorded at 200 ~ and pressures in the range of 1.0 - 4.0 Torr. The rate of diffusion controlled the rate of mass transport in particles larger than 0.2 mm. Commercial NaX beads, manufactured with binder additive showed higher diffusion resistance than binderless particles of the same zeolite. The rate of the sorption step governed the rate of NH3 transport in powder samples (<0.064-mm sieve fraction). These spectra reflected the energetic heterogeneity of the sorption sites. The FR rate spectra of NH3 sorption were compared for synthetic and natural mordenites.

1. INTRODUCTION Commercial adsorbents are commonly characterized by their equilibrium sorption properties, such as, sorption capacity and selectivity. However, when the economics of separation or purification technologies are considered, the dynamic properties of the adsorbents, such as, the rate of mass and heat transports are, also, of decisive importance. The dynamic parameters of sorption mass transport can be calculated from the "rate spectra", determined by the relatively simple frequency-response (FR) technique [1-3]. In principle, the rate spectra gives information about the nature of the rate controlling transport step, and can distinguish parallel transport processes by their different time constants. The best-fit theoretical FR function can give the kinetic parameter. Theoretical FR functions, describing mass transport in porous solid, were derived and published by Yasuda [1], and Jordi et al. [2]. Up to now, however, very few FR studies have been devoted to sorption systems of commercial importance. Many important adsorption technologies, for instance, the purification of air or hydrocarbons from NOx, SOx, water, or ammonia use pelleted zeolite as the adsorbent. It is well known that the sorption kinetics depends on the size and porosity

1640 of the sorbent pellets. Ammonia and water molecules have very similar sizes and both establish strong field-dipole interactions with the base-exchanged cation sorption sites of the zeolites. In the present work ammonia was used as a probe molecule to study and compare the dynamic properties of different zeolite sorbents. It was shown that in most commercial adsorbents the diffusion resistance of the macropores of the pellets determines the rate of ammonia sorption.

2. EXPERIMENTAL The studied molecular sieve 13X powder (Pw) and beads (Bd), designated as NaXPw and NaX-Bd, respectively, were obtained from Lancaster Synthesis, U. K. The synthetic zeolite Na-mordenite was a powder product (NaM-Pw) of the Stidchemie GmbH, Germany. For comparison, sorbent particles were prepared from sedimentary tuffs, containing zeolite mordenite as the main mineral component. The samples, designated as M1 and M2, were obtained from quarries near to the villages of Bodrogkeresztfir and Mhd, respectively. Both villages are located in the Tokaj Mountains, Hungary. The ammonium form of the zeolites, designated as NH4X, NH4M, NH4M1 and NH4M2, were obtained by exhaustive ion exchange with NH4C1 solution. The zeolite powders were compacted to pellets without binder additive using a die and 1600 Kg/cm 2 pressure. The pellets of the synthetic zeolites, the rock grains, as well as, the 13X beads were ground and sieved. The particle size fractions, smaller than 0.064 mm are referred to as powder. NaX-BdPw indicates a powder sample, obtained from the beads by grinding. The particles prepared from the powder by pressing and sieving are designated by the suffix PwPr and followed by the size range of the used sieve fraction. For the FR measurements a batch-type FR system was applied and 50 mg of sample was used. In order to avoid bed effects sample particles were placed in the FR chamber homogeneously distributed in a glass wool plug. Before the FR spectrum was recorded, the sample was outgassed at 200 ~ in high vacuum for 1 h, then contacted and equilibrated with ammonia under the conditions of the FR experiment. Spectra were recorded in the 1.0 - 4.0 Torr (1 Torr= 133.33 Pa) pressure range at 200 ~ A square-wave modulation was applied to the volume of the system. The modulation frequency was varied between 0.01 and 10 Hz. The pressure response, arising from the volume modulation, was recorded with and without sorbent in the FR chamber. From the amplitude ratio and the phase difference of the pressure waves a response wave function was derived for each modulation frequency. The in-phase and the outof-phase components of the response functions were plotted against the modulation frequency to derive the FR rate spectrum.

1641 3. RESULTS AND DISCUSSION If the time constants of the heat and mass transports are comparable the FR spectrum contains non-isothermal response characteristic components. A process can be considered isothermal if the heat transport is rapid relative to the mass transport. The near-equilibrium adsorption-desorption processes of NH3, followed by the FR method, were expected to be slow because of the strong ammonia-zeolite interaction. If the heat transport was much slower than the ammonia mass transport the system had to be treated as adiabatic. On the basis of the pressure response function adiabatic and isothermal FR systems cannot be distinguished. Nevertheless, models assuming isothermal process over isotropic and spherical particles gave a good fit of the FR responses. In absence of particle size heterogeneity and bed effects the FR spectra could be usually fitted using a model involving a single kind of mass transport resistance, i.e., the rate-controlling mechanism of the transport could be distinguished. A single resistance gives one in-phase step and the corresponding outof-phase peak at the modulation frequency that is in resonance with the perturbed

3 NaX-PwPr

0.5-0.36 mm

JI

3 NaX-Bd

0.5-0.25 mm

2 ~o c

0

c~o u~

n,

.................

2

'NaX-Pw ...................

C

- ..........

D

1

0

0.01

0.1

1

10

0.01

0.1

Frequency, Hz

1

10

, 0 100

Figure 1. FR rate spectra of NH3 sorption in molecular sieve 13X granules and powders. The NaX powder (NaX-Pw) was granuled by pressing (NaX-PwPr). The manufactured NaX beads (NaX-Bd) were powdered by grinding (NaX-BdPw). 50 mg of sample was used. The mean NH3 pressure was 1.0 Torr. Symbols (D) and (o) represent experimental in-phase and out-of-phase response functions, respectively. The full lines give the best-fit characteristic curves derived assuming rate-controlling isothermal diffusion in uniform spherical particles (A and B), or rate controlling isothermal sorption (C and D).

1642 transport process. It follows from the theory [1,2] that the intensity of the response is proportional with the change in the amount sorbed/desorbed associated with the process, while the resonance frequency gives the time constant of the process. In the case of a rate-controlling diffusion resistance the high frequency end of the out-ofphase FR peak approaches the high-frequency tail of the in-phase curve asymptotically. If the slowest step is the NH3 sorption on energetically homogeneous sorption sites the corresponding in-phase curve and the out-of-phase peak intersect at half the step height and at the maximum of the out-of-phase peak. The experimental FR spectra suggest that diffusion in the macropores controls the rate of ammonia transport in particles having diameter larger than 0.2 mm (Figures 1 to 4). In contrast, the rate of the sorption step was found to control the rate of sorption mass transport in small zeolite particles (<0.064 mm), referred to as powder, finely distributed in the FR chamber. However, the FR spectra of the different preparations showed noticeable differences. The NH3 FR rate spectra given in Figure 1 were recorded with various 13X molecular sieve sorbents. A comparison of the resonance frequencies suggest that the manufactured 13X beads present higher resistance to diffusion transport than the

3

3 NaM-Pw

2

~ ~ g

NaM-PwPr 0.5-0.36 mm

(D1

1

(/) E O

~0 (n t~

rY2

2

C

........ '

.........

........ ' ........ ] ........ ' NH4M-Pw tD NH.M-PwPr 0.5-0.36 mm

1 0 0.01

0 2 1

0.1

1

10

0.01

0.1

Frequency, Hz

1

10

0 100

Figure 2. FR rate spectra of NH3 adsorption in zeolite mordenite (M) granules and powders. The powder of the Na- and the NH4-forms (NaM-Pw and NH4-M-Pw) were pelleted by pressing to get samples NaM-PwPr and NH4M-PwPr. For further details see the legend of Figure 1.

1643

similar size 13X particles, which do not contain binding additive (cf. Figures 1A and 1B). Moreover, the intensity of the responses show that the beads and the powdered beads have lower sorption capacities than would be expected from the known amount of the non-sorbing additive. For the powders of these same samples the signals appear at resonance frequencies which suggest that the additive may affect also the strength of interaction with the sorption sites (Figure 1C and 1D). The FR rate spectra shown in Figure 2 for the ammonia sorption over the Na- and NH4-forms of synthetic mordenite demonstrate that both the sorption time constant, Kj, and the intensity of the response, Kj/•_j, depend on the nature of the base-exchange cations. The strong resonance signal obtained with the NH4M-Pw sample near to 7 Hz was assigned previously to ammonia sorption over NH4 + ion sorption sites [4, 5]. The two resonances appearing in the spectrum of the NaM-Pw are at lower frequencies indicating that NH3 is more strongly bound to the Na + ion sorption sites than to the NH4+ ions (Figure 2A and 2C). The strength of interaction with the sorption sites seems to affect also the diffusional resistance of the macropores: The diffusional transport is faster in the particles of the NH4-mordenite than in similar particles of the Na-mordenite (Figure 2B and 2D). .5-

9

0.5

0 E El. (/)

0.0

I ,-,.._

NH4M1-Pw

NH4M2 0.125-0.10 mm

0.5

1.0

NH4M2-Pw

NH4M2 1.4-1.0 mm

0.5

0.01

1.0

0.0 1.0

0.5

0.1

1

10

0.01

0.1

Frequency, Hz

1

10

0.0 100

Figure 3. FR rate spectra of NH3 sorption in sorbents prepared from mordenitecontaining rocks. The mordenite content of the rocks, obtained from quarries in the region of the Tokaj Mountains, Hungary, were 66 % (M1) and 59 % (M2). Prior to the FR examination the indicated sieve fractions of the ground rock samples were exhaustively ion exchanged with NH4CI solution. The conditions of the FR experiments were the same as described at Figure 1.

1644

In Figure 3 NH3 FR spectra for natural mordenite samples are shown. The NH4forms of the mordenite-containing tuffs were thermally decomposed and the ammonia evolved was determined by a titrimetric method. Based on the ion-exchange capacities the mordenite content of the M1 and the M2 samples were estimated to be 66 and 59 %, respectively. The relative intensity of the highest frequency FR resonances corresponds approximately to the mordenite contents. However, signals were obtained at lower frequencies for both samples. These signals, which are virtually absent in the FR spectra of the synthetic NH4M sample (Figure 2C), may arise from the strong interaction of the sorptive ammonia with the non-zeolitic rock components. Besides this energetic heterogeneity, the presence of non-zeolite components introduces also textural heterogeneity in the rock particles. The experimental FR spectra of the M2 particles, shown in Figure 3B, could not be fitted by a simple characteristic curve of a process, rate-controlled by diffusion resistance in a macropore network. Very strongly hindered diffusion was observed in grains, larger than 1.0-mm size. Adsorption and desorption rate constants, ka and kd, and transport diffusivities, DM, were determined from the experimental sorption and diffusion time constants. The linear pressure dependence of the sorption time constant, K_j, on the mean ammonia 250 -,

t /

A /

I-I NaX-Pw

II NaX-BdPw ~' NaM-ew

9

// __/

I J

I-I NaX-PwPr

II NaX-Bd ~ NaM-ewer

B

W~

/

~"

12"0 I

,15o

~1.o ~

lOO

5

0.5

0

1

2

3

Pe' Torr

4

0

10

20 30 R 2 xl0 s, m2

0.0 40

Figure 4. Plot of the (A) sorption time constant, ., vs. Pc, the mean NH3 pressure, and (B) the characteristic diffusion time, R2DM1, vs. R :!, the squared particle sizes.

1645 Table 1 Rate constants of ammonia sorption (ka and kd) in zeolites, and ammonia diffusivities in sorbent particles (DM) prepared from the zeolites Sample ID ka, s-1 Torr -1 kd, s-~ Sample ID DM, m 2 s q NaX-Pw

48.6

5.0

NaX-PwPr

2.66

NaX-BdPw

11.5

3.5

NaX-Bd

1.76 x 10-8

NaM-Pw

11.8

3.5

NaM-PwPr

1.82

NH4M-Pw

24.6

11.0

NH4M-PwPr

3.73 • 10.7

•

•

10 -7

10 -7

Note: Pw=Powder, Bd=Beads, BdPw= ground beads, PwPr=pressed powder particles. pressure (Po), shown in Figure 4A, suggests that the process can be represented by the Langmuir sorption rate equation: •_j = ka~ PC + k~

(1)

Thus, ka and kd were obtained as the slope and the intercept, respectively, of the K_j v s . Po plots. Similarly, for particles larger than 0.2 mm size, linear correlations were found between the characteristic diffusion time and the squared particle size (Figure 4B), showing that the diffusion resistance of the macropores controls the rate of transport in the sorbent particles. The ammonia transport diffusivities in the sorbent grains (DM) were calculated from the slopes of the lines in Figure 4B. The dynamic parameters of the ammonia sorption are given in Table 1. The results of the present study clearly show that the efficiency of a dynamic sorption technology can be improved considerably by decreasing the mass transport resistances within the sorbent particles. The crucial step of adsorbent manufacturing is the pelletization of the zeolite powder. Sorbents are often obtained from geological zeolitic formations simply by grinding and sieving the mined rock. The textural properties of the rock often do not favor the use of the grains as sorbent in dynamic sorption separation processes. Application of methods, such as, leaching the grains by strong mineral acids or bases, or granulation of the powdered rock, may result in better adsorbents [6]. The development of sorbents having improved dynamic properties is in progress.

1646 CONCLUSIONS Besides structural and chemical properties, the sorbent texture is of significant influence on the sorption dynamics and, thereby, on the economics of the adsorption technologies. The FR method can be a useful tool in the development of better commercial adsorbents. ACKNOWLEDGEMENTS

We gratefully acknowledge the Royal Society, London for funding under the Joint Projects with Central/Eastern Europe Scheme which allowed this work to be undertaken. The excellent assistance of Mrs. Agnes Wellisch is gratefully acknowledged. REFERENCES 1. 2. 3. 4.

Y. Yasuda, Heterogeneous Chemistry Reviews, 1 (1994) 103. R.G. Jordi and D.D. Do, Chem. Eng. Sci., 48 (1993) 1103. L.V.C. Rees and D. Shen, Gas Sep. & Pur., 7 (1993) 83. Gy. Onyestygtk, J. Valyon and L.V.C. Rees, in "12th International Congress on Catalysis" (A. Corma, F.V. Melo, M. Mendioroz and J.L.G. Fierro, eds.) Studies in Surface Science and Catalysis, Esevier, Amsterdam, 2000, Vol. 130, p. 2921. 5. J. Valyon, Gy. Onyestyhk and L.V.C. Rees, Langmuir 16 (2000) 1331. 6. L. Kotsis, J. Argyel~in, P. Szolcsfinyi and K. Kutics, React. Kinet. Catal. Lett., 27 (1985) 143.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

An attempt to correlate the non-isothermal heterocyclic compounds on a NaY zeolite

1647

desorption

behavior of

B. Hunger a *, I. A. Beta c, C. Engler a, E. Geidel d, O. Klepel b and H. B6hlig a a Wilhelm-Ostwald-Institut fiir Physikalische und Theoretische Chemie, b Institut far Technische Chemic, Universit~it Leipzig, D-04103 Leipzig, Germany

c Department of Physics, UMIST, Manchester M60 1QD, UK d Institut far Physikalische Chemie, Universitat Hamburg, D-20146 Hamburg, Germany

The strength of interaction of various heterocycles with a NaY zeolite was investigated by using temperature-programmed desorption (TPD). The global and local softness of the molecules was computed and an attempt was made to find correlation between the softness of the molecules and the strength of their interaction with the NaY. 1. INTRODUCTION Detailed knowledge of the interaction of adsorbed molecules with zeolites is of great interest regarding the broad spectrum of zeolite applications as catalysts and adsorbents [1, 2, 3]. However, little attention has been paid in the past to comparing the strength of interaction for molecules with different structures and properties and to find relationships between the molecular structure and the nature of interaction with the zeolites. In this contribution the strength of interaction of various 5-ring and 6-ring heterocyclic organic compounds with a zeolite NaY was investigated by using the temperatureprogrammed desorption technique. Taking into account the electrophilic nature of the interaction of the cations with the adsorbed molecules, the global and local softness of the molecules were computed and an attempt was made to find correlation between the softness of the molecule and the strength of its interaction with the zeolite.

2. EXPERIMENTAL The NaY (Si/A1 = 2.6) zeolite was a commercial material supplied by Chemie AG Bitterfeld-Wolfen, Germany. In addition, a siliceous faujasite (Si/A1 = 100) was also investigated. The TPD experiments were carried out in a flow apparatus with helium as carrier gas (50 cm3/min). For evolved gas detection both a thermal conductivity detector (TCD) and a *Author to whom all correspondence should be addressed: E-mail: [email protected]

1648 quadrupole mass spectrometer (Leybold, Transpector CIS System) with a capillary-coupling system were used. The samples were equilibrated with water vapor over a saturated Ca(NO3)2-solution in a desiccator. For each experiment 50 mg of the hydrated zeolite were used in a mixture with 1 g lquartz of the same grain size (0.2- 0.4 mm). At first the samples were heated at 10 K min in the helium flow up to 673 K. The zeolites were afterwards cooled down to the adsorption temperature ( 3 0 0 - 400 K) and loaded with the organic probe to saturation or by injection of small amounts. The saturated samples were flushed with helium until no further desorption was observed. The linear temperature program (10 K min 1) was then started. The desorbed amounts of probes were determined by calibration of the TCD signal and the intensity of the corresponding amu response.

3. COMPUTATIONAL DETAILS The quantum chemical calculations were carried out in the density functional formalism with the B3LYP hybrid functional [4] using the 6-31++G** basis set for all atoms as implemented in the Gaussian 98 program package [5]. Full geometry optimizations were performed for the neutral molecules M, while for the molecular ions M + single point calculations at the optimized geometry of the neutral molecules were carried out in the spin resolved formalism. The global softness cr was calculated according to the HSAB concept [6] as the inverse of the global hardness 11 which itself is defined as the difference between ionization energy I and electron affinity A: 11 -- (I-A)/2. The latter quantities were approximated with I = -EHoMO and A = "ELuMO respectively according to Koopmans' theorem. Local (atomic) softness for electrophilic attacks crx was calculated as [7]: c~x = cr [qx(N)-qx(N-1)]

(1)

The electronic populations qx of the atoms X in a molecule/ion with N and (N-l) electrons respectively Were calculated with the NPA population analysis [8, 9].

4. RESULTS AND DISCUSSION Figures 1 and 2 show the desorption profiles of all compounds under study. The desorbed amounts are given in Table 1. Only Cyclopentane was not adsorbed under the chosen experimental conditions. Decomposition of the probe during non-isothermal desorption was observed in some cases at higher temperatures (pyrrolidine, 1methylpyrrolidine). All profiles show a pronounced peak in the temperature region of about 350 - 600 K. This peak was also observed during TPD after adsorption of a greater amount of the compound (Figure 3). The shoulder at lower temperatures of the profiles can be assigned mostly to weakly bonded molecules interacting non-specifically with the zeolite. This assignment is supported by the observed desorption profile of tetrahydrofuran on a siliceous faujasite (Si/A1 = 100) which shows only a desorption peak at about 400 K (Figure 4). The main peak of all TPD profiles can be attributed to a stronger, direct interaction with the sodium ions in the supercage. For desorption profiles with just one maximum, the peak temperature can be used as a relative measure of the strength of the interaction of the adsorbed molecules with the

1649 zeolites [e.g. 10]. On the other hand, the global softness of the molecule (Table 1) can be used to characterise their ability to interact with the sodium cations of the 9 zeolite. The plot of the peak 8 temperatures against the global softness (see Figure 5) shows the overall trend of increasing strength of interaction with 67 increasing global softness. However, 5 there is significant scattering of the points and therefore it is not possible to 4 o fit functions of certain types. In the following we will try to explain the o deviations from the trend. The contribution of the interaction of the CH and NH groups with the framework oxygen of the zeolite which has been found to take place by spectroscopic studies in case of furan [11], pyrrole [12], and methylpyrrole [13] is not sufficient to explain the scattering of 300 400 500 600 700 800 points in Figure 5. The most important T/K factor which affects the interaction energies is the structure of the adsorption complexes. To support this argument we Figure 1. Normalized desorption profiles: mention that for example, while 1: cyclopentene, 2: furan, 3: 2-methylfuran, cyclopentene, furan, 2,5-dihydrofuran 4: 2,5-dihydrofuran, 5: tetrahydrofuran, and tetrahydrofuran have almost the 6: pyrrole, 7: 1-methylpyrrole, same global softness, peak temperatures 8: 2-methylpyrrole, 9: 2,3-dihydrofuran of their desorption profiles vary in an interval of almost 100 K. In case of cyclopentene and furan the interaction with the cations of the zeolite occurs mostly through their n electrons. For tetrahydrofuran, which does not possess n electrons, the main interaction would be between the negative charge localized in its oxygen atom and the cation. The data in Table 2 show that the greatest local softness was obtained on the oxygen atom, which is in full agreement with our model of adsorption complex for tetrahydrofuran. In case of 2,5-dihydrofuran and 2,3-dihydrofuran besides the high local softness of the oxygen atom there are significant values for the carbon atoms of the double bond which leads to additional interactions with the cations. The overall strength of interaction will be therefore higher than what would be estimated from the global softness of these molecules. In case of pyridine, pyrazine and pyrimidine the nitrogen atoms have the greatest local softness which means that they are the preferred sites for the interaction with the sodium cations. This leads to the formation of relatively strongly bonded adsorption complexes with higher interaction energies than what one would predict from their global softness. Spectroscopic studies and theoretical computations provide evidence for this kind of interaction [ 14]. The interaction between nitrogen atoms and the sodium cations represents also the dominant interaction for pyrrolidine and 1-methylpyrrolidine. ~D

,,..~

r~

,

I

,

I

,

I

,

I

|

1650 Up to now we have discussed molecules in which the heteroatom possesses the greatest local softness and therefore represents the interaction centre with the cations. However, this is not the case for the benzene molecule and another explanation is needed for its high desorption temperature. Quantum chemical computations also show a relatively high interaction energy of the benzene molecules with the A1+ cations [15]. The computed interaction energy increases in the order furan < benzene < pyrrole < pyridine which is in agreement with the variation of the peak temperature of their desorption profiles. Experimental evidence also shows a stronger interaction of benzene with the zeolite NaY compared to furan. The heat

4 o o

I

400

l

I

500

,

I

600

~

I

700

,

800

T/K

Figure 2. Normalized desorption profiles: 1: benzene, 2: pyridine, 3: pyrazine, 4: pyrimidine, 5" pyrrolidine, 6: 1-methylpyrrolidine of adsorption of 78.7 kJ/mol determined by Barthomeuf et al. [16] for benzene on a zeolite NaY is significantly higher than the desorption energy of furan at 67 kJ/mol calculated by us from the desorption profile [ 17]. We also calculated a desorption energy for tetrahydrofuran of 104 kJ/mol which is much higher than for benzene. This result i.e. that the interaction of benzene with the zeolite is stronger than that of furan but weaker than that of tetrahydrofuran could be explained with the fact that in case of the benzene molecule there is a perfect aromatic system compared with furan which exhibits only a fair degree of aromaticity. If we consider only the molecules in which the carbon atoms of the ring possess the greatest local softness and plot the peak temperatures of their desorption profiles against the global softness (triangles in

o

r/)

2

i

300

I

400

,

I

500

,

I

600

J

I

700

T/K Figure 3. Desorption profiles: 1: 2-methylpyrrole- solid line: 4.7, dashed line: 2.4 molecules per supercage; 2" benzene- solid line: 3, dashed line: 2 molecules per supercage

1651 Figure 5) a good linear relationship results (regression line in Figure 5, r2 = 0.929). This implies that for such molecules the global softness could be used as measure for the strength of their interaction with the cations. In case of molecules with .2 heteroatoms which interact directly o r~ with the cations, the local softness ~D should be used instead of the global softness. 300 400 500 600 700 The corresponding correlation T/K is shown in Figure 7. For pyrimidine and pyrazine the sum of the local Figure 4. Desorption profiles of tetrahydrofuran softness of the nitrogen atoms (localized on two docking centers) and in case of 2,3-dihydrofuran and 2,5-dihydrofuran the sum of the local softness of the oxygen atom and the highest value of the carbon atoms was used. Apart from pyridine and 1-methylpyrrolidine a reasonable correlation is observed. Regarding that the local softness of pyrrolidine and 1-methylpyrrolidine is almost the same, the somewhat weaker interaction in case of 1-methylpyrrolidine could be explained with sterical hindrances caused by the methyl group. |

i

,

I

i

i

,

Table 1 Desorbed amounts and global softness of the compounds compound chemical desorbed amount composition [molecules per supercage] cyclopentane CsH10 no adsorption cyclopentene

global softness CY

0.2411

2.5

0.3163

3.5

0.3153

4.4

0.3158

2.0

0.3537

5.4

0.3148

3.0

0.3387

5.6

0.3616

1-methylpyrrole

C5H8 C4H80 C4H60 C4H60 C4H40 C5H60 CaHsN CsHTN

4.7

0.3648

2-methylpyrrole

C5HTN

2.4

0.3866

pyrrolidine

C4H9N

2.0

0.4043

tetrahydrofuran 2,5-dihydrofuran 2,3-dihydrofuran furan 2-methylfuran pyrrole

1-methylpyrrolidine

CsH11N

2.0

0.4281

benzene

C6H6

2.0

0.3036

pyridine

CsHsN

2.0

0.3269

pyrazine

CaH4N2 CaH4N2

2.7

0.3764

2.0

0.3526

pyrimidine

1652 650 600

~ 12

550 [.-,

500

14~74

~75~3 ~

13

0

6

11~

450 400 350 I

,

I

,

I

,

I

,

I,

,

I

,

|

,

0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 Global Softness 5. Relationship between the peak temperature and the global softness of the compounds (description of the compounds see Figure 6)

Figure

1

5

1

2 (1)

1

5

2 4

1

N

-~4~'/ )32 (11)

5

H

2 4 (12)

3

I

3

2

5

2

(4)

(5)

H

H I

5 N76 4~~/ CH3

(8)

(9)

1

1

6

2

6

5

3

5

4 (13)

1

I

5 N1 4~ /~3

1

5

(3)

6 CH3

I

6

5 4

(2)

0 6 5 N1 2 5 ~ ? CH3 4~ /x~3 4 3 (6) (7)

6=3 I1

1

(10)

N3 4 (14)

54~,/N~3 1 2

60

5

4 (15)

Figure 6. Numbering of atoms of the compounds: 1: cyclopentene, 2: tetrahydrofuran, 3: 2,5-dihydrofuran, 4: 2,3-dihydrofuran, 5: furan, 6: 2-methylfuran, 7: pyrrole, 8: 1-methylpyrrole, 9: 2-methylpyrrole, 10: pyrrolidine, 11: 1-methylpyrrolidine, 12: pyridine, 13: pyrazine, 14: pyrimidine, 15: benzene

2

3

1653

Table 2 Local softness of the compounds *

cyclopentene

local softness O'xcarbon atoms CI:-0.0024, C2, C5:-0.0148, C3, C4:0.0873

tetrahydrofuran

C2, C5:-0.0155, C3, C4:-0.0032

O1:0.1569

2,5-dihydrofuran

C2, C5: 0.0172, C3, C4:0.0473

O 1: 0.0798

2,3-dihydrofuran

C2:-0.0082, C3:-0.0154, C4: 0.1112, C5:0.0386

O1: 0.0879

furan

C2; C5: 0.0844, C3, C4:0.0368

O1: 0.0032

2-methylfuran

C2: 0.0802, C3: 0.0453, C4:0.0307 C5: 0.0853, C6:-0.0136

O 1: 0.0024

pyrrole

C2; C5: 0.1031, C3, C4:0.0372

N 1: -0.0090

1-methylpyrrole

C2; C5:-0.0048, C3, C4: 0.0832, C6:-0.0138

N1: 0.0850

2-methylpyrrole

C2: 0.0991, C3: 0.0490, C4:0.0300 C5: 0.1047, C6:-0.0170

NI: -0.0102

pyrrolidine

C2, C5: -0.0308, C3, C4:-0.0044

N 1: 0.2254

compound

local softness (5"X" heteroatoms

1-methylpyrr01idine C2, C5:-0.0316, C3, C4:-0.0042, C6:-0.0311

N1: 0.2230

pyridine

C2, C6: 0.0031, C3, C5: 0.0175, C4:0.0290

NI: 0.1561

pyrazine

C2, C3, C5, C6:0.0110

N1, N4:0.1046

pyrimidine

C2: 0.0059, C4, C6: 0.0137, C5:0.0213

N1, N3:0.1017

* Calculation of local softness of benzene is not possible on the quantum chemical level here used (single configuration formalism). Since the HOMO is degenerated and therefore a multiconfiguration formalism for the benzene cation is needed.

600 3

6

550

500

450

I 0'.12

,

I , I , 0.16 0.20 Highest local softness

I 0.24

Figure 7. Relationship between the peak temperature and the highest local softness of compounds (line only for eyes): 1: 2,5-dihydrofuran, 2: tetrahydrofuran, 3: pyridine, 4: 2,3-dihydrofuran, 5: pyrimidine, 6: pyrazine, 7: pyrrolidine, 8: 1-methylpyrrolidine

1654 From the numerous examples presented in this paper we can conclude that the global and local softness of the molecules is a useful concept to qualitatively characterize and compare the strength of their interaction with the cation-exchanged zeolites. Depending on the structure of the adsorption complexes either global or local softness can be of primary relevance. For cases in which the interaction with the cation is delocalized (for example molecules with double bonds or n electron systems) the global softness is the relevant quantity. For molecules with heteroatoms in which the interaction with the cation occurs mainly through the electronegative atom (mostly N or O) the interaction is localized and its strength can be characterized through the local softness.

ACKNOWLEDGEMENTS

This work has been partially supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Graduiertenkolleg ,,Physikalische Chemie der Grenzfl~ichen" at the University of Leipzig and the Fonds der Chemischen Industrie.

REFERENCES ~

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

K. Tanabe, W.F. H61derich, Appl. Catal. A: General, 181 (1999) 399. P.B. Venuto, Stud. Surf. Sci. Catal., 105 (1997) 811. J. Weitkamp, U. Weig, S. Ernst, Stud. Surf. Sci. Catal., 94 (1995) 363. A.D. Becke, J. Chem. Phys., 98 (1993) 5648. M.J. Frisch et al. Gaussian 98, Revision A.3 (1998). R.G. Pearson, Coordination Chemistry Reviews, 100 (1990), 403. A. Chatterjee, T. Iwasaki, T. Ebina, J. Phys. Chem. A, 103 (1999) 2489. A.E. Reed, R. B. Weinstock, F. Weinhold, J. Chem. Phys., 83 (1985) 735. A.E. Reed, F. Weinhold, J. Chem. Phys., 83 (1985), 1736. B. Qian, H. Jiang, Y. Sun, Y. Long, Langmuir, 17 (2001) 1119. I.A. Beta, H. Jobic, E. Geidel, H. B6hlig, B. Hunger, Spectrochim. Acta A, 57 (2001) 1393. H. F6rster, H. Fuess, E. Geidel, B. Hunger, H. Jobic, C. Kirschhock, O. Klepel, K. Krause, Phys. Chem. Chem. Phys., 1 (1999) 593. J. D6bler, H. F6rster, H. Fuess, E. Geidel, B. Hunger, C. Kirschhock, O. Klepel, E. Poschnar, Phys. Chem. Chem. Phys., 1 (1999) 3183. J. D6bler, E. Geidel, B. Hunger, K.H.L. Nulens and R.A. Schoonheydt, Stud. Surf. Sci. Catal., 135 (2001) 1845. D. St6ckigt, J. Phys. Chem. A, 101 (1997) 3800. D. Barthomeuf, B.H. Ha, J. Chem. Soc., Faraday Trans. I, 69 (1973) 2147. I.A. Beta, H. B6hlig, J. D6bler, H. Jobic, E. Geidel, B. Hunger, Stud. Surf. Sci. Catal., 135 (2001) 1889.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1655

Determination of Diffusion Coefficient for Cu(ll) Retention on Chemically Activated Clinoptilolite R. Pode a, T. Todinca a, A.Iovi a, R. Radovet a, G. Burtic~a Department of Environmental Engineering, University ,,Politehnica" of Timisoara, Victoriei No. 2, Et. 2, 1900 Timisoara, Romania a

P-ta.

The present paper studies the influence of the initial pH of solutions on Cu (II) retention on chemically activated clinoptilolite. The calculation of the diffusion coefficient, which characterised the process from a kinetic point of view, was carried out by two methods: BoydAdamson equation and numerical simulation. The Cu(II) diffusion within the solid matrix of zeolite was considered as the rate-limiting step. For the numerical simulation, the estimation of the diffusion coefficient was obtained by comparing experimental values to those obtained by simulation. Both methods led to nearly equal values of the diffusion coefficient.

1. INTRODUCTION The ion exchange on zeolites is a multistep mass transfer process in a homogeneous system. At present, the accepted theory stipulates that the diffusion process determines the rate of ion exchange. A general opinion is that the ion exchange is controlled by the internal diffusion because the diffusion across the liquid film that surrounds the particle is not determined by the properties of zeolite. Moreover, under enhanced hydrodynamic conditions the effect of diffusion across the liquid film can be removed. The diffusion coefficient is one of the most important parameters that characterize the kinetic of ion exchange [ 1,2]. The present paper aims at determining the diffusion coefficient for Cu(II) retention on chemically activated zeolite. Two methods were taken into consideration: Boyd-Adamson equation and numerical simulation.

2. EXPERIMENTAL

2.1. Kinetic studies The material used for experiments was clinoptilolitic zeolite originating from the Marsid (Romania) deposit. The chemical composition was determined according to [3] and the weight percentage was as follows: SiO2 - 70.06%; A1203 - 11.77%; Fe203 - 0.67%; CaO 3.36%; M g O - 0.55%; K 2 0 - 2.20%; N a 2 0 - 0.40%; TiO2- 0.18%; I.L.-10.61%. The average diameter of the zeolite particle was 30 ktm. The kinetic of Cu(II) ion exchange on chemically activated clinoptilolite as Na form [4] was studied in Cu (II) solutions of

1656 unchanged concentration and various pH values. Samples of 50 mL solution were contacted with 1 g clinoptilolite and maintained in suspension in a thermostat Shaker Bath at 25~ At well-determined durations, the solid phase was separated by centrifugation and the solution was analysed. The initial pH of the solutions was measured by a Menom E5000 pH meter. The Cu(II) concentration was determined by atomic absorption spectroscopy. The device was a Varian SpectrAA 110 spectrophotometer.

2.2. Determination of the diffusion coefficient by Boyd-Adamson method. The calculation of the diffusion coefficient according to Boyd-Adamson equation [2] is based on the presumption that the kinetics of the ion exchange on zeolites is controlled by the diffusion of ions within the zeolite particle. Based on kinetic experimental data obtained at various pH, the exchange degree U(t) was calculated as a function of time. The product B t was calculated from Reichenberg's tables [5]. B is the characteristic frequency [see 1] and t the exchange time [sec]. The plot B " t versus time allowed the determination of the characteristic frequency, B. The diffusion coefficient was calculated from the equation D = B R2/7c2; R was the average radius of the zeolite particle. 2.3. Determination of the diffusion coefficient by numerical simulation. The numerical simulation was based on the following characteristics of clinoptilolite: density- 1.66 g/cm3; volume of 1 g zeolite - 0.60241 cm3/g; radius of the zeolite particle- 15 10-4 cm; average number of particles in 1 g of zeolite - 4 . 2 6 107 particles/g. The volume of liquid corresponding to one particle (VL) was 1 . 1 7 3 4 10-6 cm3. The experiments for determining the diffusion coefficient were carried out with 50 mL of solution for 1 g clinoptilolite. The use of the proposed mathematical model required data on ion exchange equilibrium. Solutions of various initial Cu(II) concentrations and pH were contacted with 1 g clinoptilolite and maintained in suspension in a thermostat Shaker Bath at 25~ until equilibrium set up. After the separation of phases, Cu(II) concentrations were determined by atomic absorption spectroscopy. 3. RESULTS AND DISCUSSION

3.1. Kinetic studies Figure 1 shows the dependence of Cu(II) retention capacity on chemically activated clinoptilolite versus time at various pH values. The resulting kinetic curves showed the positive influence of the initial pH increase on the retention capacity. The exchange capacity at equilibrium increased as initial pH of the solutions increased too. The duration of equilibrium set up was not influenced by initial pH. 3.2. Determination of the diffusion coefficient by Boyd-Adamson method The data regarding the dependence of the exchange degree and the product B t versus time for the studied processes are shown in Table 1. The plot B " t as a function of time for three initial pH values allowed the calculation of the characteristic frequency, B.

1657 ~o

"~

0.40

I

n "~-I-

l

i

....

"

r

I

!

0.32+

I

_,_

,-t,~ 0.24t-y~'~,..~'

O.l~|~: .... i .....

N r=

t~

0.08 0.o4 0.00

0

40

' ;

i

,

,

~. . . . .

i- -

.,'-.....

i--1

.....

i--1

j

7 ~ . ~ _ a . - . , , - ~ - - ~ - .....- - - ~ _ ~ ,. . . . .

I

i

,.

t .....

.,'-....

f

.

.....

80

"t . . . .

1

j .....

,,.:~.o--~

120 160 Time, min

_

I

200

240

Figure 1. Cu(II) retention capacity on chemically activated clinoptilolite as a function of time (Cu(II) initial concentration of the solution, Co = 405 mg/L). Table 2 shows the values of the characteristic frequency and diffusion coefficient for the studied process (2 Na+r Cu2+). Table 1 Dependence of the exchange degree and product B "t versus time. pH Time, s U(t) Bt 3 300 0.24 0.057 3 600 0.39 0.167 3 720 0.42 0.199 3 900 0.47 0.259 3 1200 O55 0.382 4 300 030 0.093 4 600 0.44 0.222 4 900 0.53 0.348 4 1200 0.62 0.522 5 300 0.39 0.167 5 600 0.54 0.365 5 900 0.69 0.500 5 1200 0.67 0.647

1658 Tabel 2 Values of the characteristic frequency and diffusion coefficient for the studied processes. pH B 104, S"1 D "101~ c m 2" S"1 3 3.6 0.82 4 4.7 1.07 5 5.3 1.21 The obtained data indicated the increase of the diffusion coefficient as initial pH of the solutions increased. The values of the diffusion coefficient are in agreement with those reported by literature [6,7]. 3.3. Determination of the diffusion coefficient by numerical simulation

Estimation of parameters of adsorption isotherms The relation between Cu (II) concentrations within liquid and zeolite particle, respectively, at equilibrium was expressed by Langmuir isotherms: %,,, .K,,, .c A c~ = 1+ K,,, 9cA

(1)

where: c~- ion concentrations within zeolite at equilibrium, mequivalent/g; CA- ion concentrations in the bulk of solution, mequivalent/cm3; CAzm- saturation capacity, mequivalent/g; K m - equilibrium constant, cm3/mequivalent. The parameters in Langmuir's isotherms were determined by minimizing the sum of square deviations for the experimental values as against the predicted data by model. The way the Langmuir's isotherms approximated the experimental values is shown in Figure 2. Table 3 put together CA~ and Km values from Langmuir's isotherms at various pH values. One can notice the positive influence of the pH increase on the retaining capacity again. The changes determined by pH on ion exchange capacity were probably due to the modification of the ratio between the ion exchange process and the adsorption one at the same time with the pH change. Table 3 The parameters ofLangmuir's isotherm at various pH values. Ion exchange process

pH 3

2Na + r

C u 2+

4

5

CAzrn, meq/g 0.3282 0.3310 0.3410

K~

cm3/meq 2813 3891 6215 ,

,

1659 0.4 t."

pH = 5,0

t~ ...,.

r

11,)

0.3

I

E

4,0

d

= 3,0

O

N 0.2 ._= co t~ L_

o

e-,

0.1

O O

0 0.002

0.004

0.006

0.008

0.01

Cu 2§ concentration in solution, [m equivalent/cm 3]

Figure 2.The Langmuir's isotherms for the ion exchange 2 N a + <=> Cu 2 + at various pH values.

Estimation of effective diffusivity coefficient

References [1, 8] indicated that the rate-limiting step of the ion exchange process was ion diffusion within the solid matrix of zeolite. Equation (2) described the modification of an "A" ion concentration within the zeolite particle along the radius and in time.

OCA~ = D, ~ ~92c'~

e----t-

20CA~ ]

"~.Or ~ +-'r &

(2)

where: D e - effective diffusivity coefficient within the solid matrix of the zeolite, cm2/s ; r - radius of the zeolite particle, cm ; c ~ - Cu (II) ion concentration within zeolite, mequivalent/g; The boundary conditions were as follows: 9 On the surface of the zeolite particle (r = rp), the ion concentration within the first layer of zeolite was in equilibrium with the surrounding solution (according to Langmuir's isotherms equation (1)); 9 To the core of the zeolite particle (r = 0), the concentration gradient was null:

(

a~. ~

& )r=0

= 0

(3)

1660 The evolution of cation concentrations within the surrounding liquid was given by equation (4):

do A,

VL" dt = - D , . p . 4 . ~ r . r p .

(4)

2 ( COOA.]

Or )r=rp

where: VL - volume of liquid corresponding to a zeolite particle; CAL cation concentration within the corresponding liquid; -

rp- external radius of the zeolite particle. The material balance under dynamic regime for the "A" ionic species in a spherical volume element of "dr" thickness, when diffusion is the rate limiting step, is described by the following equation:

4 --.~.(r3. -r3._l).p dcm. 9 " =De .4.Tr.r. 3

dt

2

(Cdz, 9 n+]--CAz,n) -De .4.~.r2._, (Cdz'n--Cdz'n-1) 9 dr

dr

(5)

To increase the accuracy of the numerical solution, the discretization of the partial derivatives equation (2) was processed using a variable increment of radius, but the ratio of two consecutive sections remained constant. Taking into account the equation (5), the change of Cu(II) concentration within the "n" liquid ring in time was as follows:

4

--- 7C" (r3 - P43_1)-p .

3

dcA~, (cA,.+, -cA~,) (cA,, -cAa._,) = D e - 4- ~r- r, 2- D e 94-zc- r',-i dt 0.5(dr.+, +dr,) 0.5(dr. +dr._,)

(6)

To calculate the effective diffusivity coefficient, the experimental data were compared to those resulted by numerical simulation for several values of the coefficient. The value, for which the sum of square deviations of the experimental data as against predicted data by model was minim, was chosen as the most probable value of the coefficient. The numerical simulation program used 20 cross-sections along the radius of the zeolite particle and was written under MATLAB software. Figure 3 present comparatively the experimental kinetic data and those obtained by numerical simulation for 3 values of the effective diffusivity coefficient at an initial pH of 3 for Cu(II). Table 4 shows the values of the diffusion coefficient obtained by the two methods. The two methods led to nearly equal values of the diffusion coefficients. The numerical simulation offered the possibility of monitoring the concordance between the experimental results and those obtained by numerical simulation. In addition, the numerical simulation allowed the estimation of the kinetic parameters even when the rate-limiting step is not the internal diffusion.

1661 14

t

r

E

I I

I I

I

t

i I I

I

t

I

O

1

t

l

r

t

I

t

._> 1 t3v

1

I

I

t

T

T

i

t

l

I

•

-

-

-;

-

I

I q

-

--

E =._o

--= lO -kkX- ~,~

1

0 ffl

tO

~

f

~

I

-1~ c m 2 / s

/

. . . . . . . . . .

0 t-

t

De=0.5*10

De~0.7*10-1~ 7.........

-

O 0

o

6

0

t

l

1

I

I

I

3000

6000

I

9000

12000

15000

Time, [s]

Figure 3. Comparison between experimental and simulated data at various diffusion coefficients for the ion exchange process 2 N a + r

Cu 2+ (pH = 3).

Table 4 Values of the diffusion coefficient obtained by the two methods pH Values according to Predicted values, B oyd-Adamson, 10 ]0cm 2s ] 1010cm2s-1

Accepted as real by comparison to predicted values,

3 4 5

0.7 1.0 1.2

101Ocm2s -1

0.82 1.07 1.21

0.5 0.9 0.8

0.7 1.0 1.2

0.9 1.2 1.4

4. CONCLUSION The investigation of the influence of the initial pH of solutions on Cu(II) exchange process on chemically activated clinoptilolite showed the positive influence of pH increase on exchange capacity during time and at equilibrium. The kinetic study resulted in determination of the diffusion coefficient by two methods: B o y d - Adamson and numerical simulation. When the numerical simulation was used, the estimation of the diffusion coefficient resulted by comparing experimental data to values obtained by simulation.

1662 The numerical simulation method allowed recording of the concordance between experimental results and predicted ones by simulation all the way of the kinetic determination. The two methods led nearly equal values of the diffusion coefficient. REFERENCES

1. R.H. Perry and D.W.Green (eds.), Perry's Chemical Engineers'Handbook, Seventh Mc Graw-Hill Edition, 1997. 2. M.Cruceanu, E. Popovici, N. Balba, N.Naum, L. Vladescu, R. Russu and A. Vasile, Zeolitic Molecular Sieves (Romanian), Ed. Stiintifica ~i Enciclopedica, Bucharest, 1986. 3. H.Minato, Natural Zeolites - Sofia'95 (G. Kirov, L. Filizova and O. Petrov eds.), Pensoff Publishers, Sofia - Moscow, Bulgaria (1997) 282. 4. G. Catana, L. Frunza, D. Crisan, R. Pode and G. Burtica, Annals of"A1. I. Cuza" Univ.Iasi, Vol. I, Series of Chemistry (1992) 200. 5. D.Reichenberg, J.Am.Chem.Soc., No.75(1953) 597 6. R. Pode, A. Iovi, G. Burtica and. M. Sava, Annals of "A1. I. Cuza" Univ.Iasi, Vol. III, Series of Chemistry (1995) 85. 7. E. Popovici, G. Burtica, R. Pode, I. Bedeleanu and I. Calb, Natural Microporous in Environmental Technology (P. Misaelides et al., eds.), Kluwer Academic Publisher, Holland (1999) 345. 8. P.M. Armenante and D. J. Kirwan, Chemical Engineering Science, No. 44 (12) (1989) 2781.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1663

Dynamics of sorption columns in dewatering of bioethanol using zeolites Martin Boldig a'b, Karel Melzoch a, Jan Pokorny c, Milan Ko~i~ik b a

The Institute of Chemical Technology in Prague,Technick~. 5, 166 28, Praha 6

b j. Heyrovsl~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, 182 23 Praha 8, E-mail: [email protected] c The Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8 1. INTRODUCTION There is an increasing interest in using bioethanol as an additive to fuels. Classical technologies of residual water removal from ethanol via an azeotropic distillation using ternary systems suffer from numerous drawbacks. Thus, alternative methods of ethanol dewatering as sorption 1 or pervaporation techniques using membranes are searched. The aim of the present work is to test potentiality of selected zeolitic sorbents to remove residual water from ethanol-water mixtures with composition close to that of azeotrope at normal boiling point (7"6 - 78 ~ where the mass fraction WH20 in both the phases WH20 = 0.05. The emphasis is also laid on the analysis of heat effects the bed of adsorbent and a conceivable coupling of energy balance of the sorption process with that of azeotropic distillation. Another subject of interest is behaviour of the sorbent in adsorption desorption cycles and a long term stability of the zeolite in hydrothermal conditions during the bed regeneration. 2. EXPERIMENTAL SECTION

2.1. Materials Ethanol used was fine spirit from distillery B I O F E R M - LIHOVAR KOLiN a.s. with water content corresponding to the azeotropic composition i.e. 96.0 % vol.; the content of impurities was as follows: volatile acids (as acetic acid) <10 mg/1; alcohols with C,, _> C3 <1 mg/1, aldehydes < 5mg/1; esters < 30 mg/1; total solid residue < 30 mg/1. Sorbents: (i) Synthetic zeolites 3A, extrudates in the cylindrical form of the diameter of 1/16". Made in USA, by Linde Air Products Company a Division of Union Carbide and Carbon Corporation; apparent density J:)Hg "- 1.1 g/cm 3, volume of transport pores in the extrudates Vp - 0.26 cmag-1, porosity 13of transport pores 13- 0.286. (ii) Natural clinoptilolite from the locality Ni~.n37 Hrabovec, Slovakia, apparent density P r i g - - 1.1 g/cm3; BET-surface = 25 mE/g (measured by the sorption of N2 at 77 K).

1664 Table 1 Composition Of clinoptilolite tuff from the deposit Ni~n~ Hrabovec [2] composition content % by mass composition

content % by mass

SiO2

67.1

CaO

2.90

TiO2

0.24

Na20

0.68

A1203

10.6

K20

2.96

Fe203

1.72

H20

12.8

MnO

0.03

P205

0.30

MgO

0.73

XRD phase analysis has shown in addition to clinoptilolite also crystobalite and as minorities muscovite and/or iUite.

2.2 Experimental arrangement and operations 2.2.1. Sorption bed preparation For preparation of beds of zeolite 3A original extrudates were used. For preparation of adsorption beds of clinoptilolite we crushed the original clinoptilolite particles. After crushing the sorbent was sieved to obtain the grains of the size 1.0 m m - 2.0 mm and 2.0 m m - 2.5 ram. Adsorption columns were glass tubes of internal diameter of 2.8 cm. Some of the columns were provided with thermocouples to measure temperature of the gas at different positions on the column axis and also in the vicinity of the column wall. The length of the packing varied in the range between 4 cm and 22 cm. 2.2.2. Activation of the samples Prior to a series of adsorption desorption cycles the sorbents were brought to an initial standard state by heating to 310~ for 24h in a stream of nitrogen. The heating rate of the bed was 3~ Volumetric flow rate of nitrogen was 150 ml/min, in the upward direction. To study adsorption desorption cycles we varied the length of the desorption period tsteady measured from the time point of reaching the temperature plateau Tsteaay. within the range 0 to 24 h. Tsteady used ranged between 150 ~ and 310 ~ The overall regeneration time tr = ttrans + tsteacly where ttrans is the time of attaining the temperature Tsteady (i.e. tr = 95 min for Tsteady 310 ~ 2.2.3. Experimental arrangement for sorption with a thermostated column All the adsorption dynamics was measured at 363K with P/P0 in the column inlet = 0.17. Inlet gas composition of the azeotropic composition was prepared by pumping the liquid azeotropic mixture into the evaporator using a peristaltic pump. The total evaporation of the mixture takes place in the evaporator. The evaporator consists of a 2m long copper tube placed in the water bath, heated to 90~ No carrier gas was admixed to the vapour mixture EtOH + water.

1665 The column was thermostated to 90~ by placing it in the water bath with a circulating system. Vapour leaving the column was condensated and the collected fractions were analysed by gas chromatography or K. Fischer titration (using automatic coulometer WTK 901). A schematic representation of the experimental arrangement is shown in figure. 1

5

2

6

Figure 1. Adsorption apparatus with thermostated column wall 1 - reservoir of original liquid mixture, 2 - peristaltic pump, 3 - thermostated bath, 4 - termostated adsorption column, 5 - condenser, 6 - reservoir of condensate

2.2.4. Experimental arrangement for sorption with an heat insulated column Prior to the beginning of adsorption run the activated column coated with a heat insulation (2 cm of glass wool) was placed to a furnace that preheated it to 90~ Other details of the experiment were the same as described in figure 1 and section 2.2.3. 3. R E S U L T S AND D I S C U S S I O N

3.1. Shape of breakthrough curves for various column regimes of heat exchange The examples of breakthrough curves measured on a bed of zeolite 3A in a thermostated column and in heat insulated column at otherwise identical conditions are shown in figure 2. A similar representation for a bed of clinoptilolite can be seen in figure 3. The column length H was in all the above cases H ~ 15.5 cm. There is a marked effect of heat exchange regime on breakthrough time tB and therefore also on the corresponding usable sorption capacity qs. This will be discussed in section 3.2. Also the L~rz (length of mass transfer zone MTZ) is considerably higher in the case of heat insulated column. The effect of heat insulation is more pronounced for the bed of clinoptilolite.

1666

W/Wo

w/w o m

0,5

i

0,5

.=,,

~

[

0

J

50

100

t [min]

150

Figure 2. Comparison of breakthrough curves on zeolite 3A: 1 - thermostated column; 2 - heat insulated column; 3 -breakhrough concentration w/w0 = 0.06

0

100

200

t [min]

Figure3. Comparison of breakthrough curves on clinoptilolite: 1 - thermostated column; 2 - heat insulated column; 3 - breakthrough concentration w/w0 = 0.06

3.2. Comparison of sorption data from static and dynamic sorption measurements An upper bound on usable sorption capacity of selected zeolites was estimated as that based on gravimetric sorption measurements at T = 303 K with a sorbent mass = 10 g performed after standard activation procedure (Tstea+ = 310 ~ t~teaay = 24 hours, nitrogen stream 150 ml/min). The results are summarized in Table 2. Table 2 Static sorption data of water at T = 303 K Zeolite 3A UCC 0.511 0.201 Clinoptilolite p/p0 0.117 0.330 0.511 qm0 [g/gsorbent] 0.072 0.082 0.089 The interpolation gives for P/P0 = ().17 qm0 = 0.184 [g/g] and [g/g] for clinoptilolite. p/p0 0.117 qH20 [~Jgsorbent] 0.181

0.330 0.194

0.942 0.221

0.964 0.263

0.942 0.964 0.125 0.199 for zeolite 3A and qm0 = 0.074

Table 2 exemplifies the adsorbed amounts qB evaluated from breakthrough curves up to breakthrough time tB of the relative water concentration w/wo = 0.06 and qtot which represents the total sorption capacity per unit mass of sorbent evaluated from the breakthrough curves. Most experiments with clinoptilolite were carried out with grain size 1.0 m m - 2.0 mm. The samples denoted by exhibited the grain size 2.0 m m - 2.5 mm. The level of relative pressure p/po = 0.17 implies that at equilibrium there is no complete filling of zeolite micropores and therefore one expects a non-negligible effect of bed temperature on equilibrium adsorbed amount.

1667 Table 3 Sorbed amount from sorption dynamic experiments at 90~ p/p0 = 0.17 Adsorption dynamics with thermostated column Experiment Sorbent ms qB qtot NO

[~]

1 1 2 3 1 2 1

[g/~sorbent]

[~J~sorbent]

3A 3A

86.6 58.7 58.7 58.7 Clinoptilolite* 98.2

0.115 0.146 0.104 0.140 0.097 0.143 0.090 0.143 0.050 0.053 0.046 0.057 3A 76.1 0.113 0.145 Adsorption dynamics with isolated column Sorbent ms qB qtot

Experiment NO

1 2 3

3A

1

Clinoptilolite

2 3 4 1 1

3A Clinoptilolite ** Instantaneous breakthrough

[g]

[g/gsorbent ]

[g/~sorbent]

19.5 39.1 58.5 19.8 39.5 59.3 79.0 76.1 75.2

0.098 0.103 0.126 0.0 ** 0.029 0.039 0.036 0.083 0.035

0.154 0.145 0.154 0.059 0.065 0.069 0.063 0.142 0.055

qB/qtot .

.

.

[]

.

.

.

.

AA

0,5 -

~--~ .................. -F ......................... ,

0

20

H [cm]

40

Figure 4. Plot of q~qtotV vs.length H of the bed m -thermostated column vith zeolite 3A A -heat insulated column (zeolite 3A) 9 -heat insulated column (clinoptilolite)

This can be seen by a comparison of data from static measurements at 30~ (cf. Table 2) with the corresponding qtot values in Table 3. The average value of qtot for water on 3A on thermostated column is 0.143 [g/gsorbent] and on heat insulated column 0.148 [g/gsorbent ] the respective values for clinoptilolite are 0.055 [g/gsorbent] and 0.062 [g/gsorbent]- Thus, the adsorbed amounts for 3A at 90~ are on average about 2 1 % lower as compared to those measured at 30~ in static conditions. For clinoptilolite the corresponding difference amounts to 19 %. Figure 4 shows the plot of the ratio qB/qtot of the bed qB denotes usable bed capacity (breakthrough w = WB = 0.05) and qtot the total capacity up to the time t ~ oo. It can be seen that with heat insulated columns there is needed a considerably higher H to reach the same level of sorption capacity usage

1668 as compared to thermostated ove. The effect of bed length variation on the shape of brakthrough curves development for heat insulated column, of 3A and clinoptilolite can be seen in figure 5 and 6 respectively. A characteristic feature of the clinoptilolite bed is a sharp breakthrough on one side and a pronounced tailing on the other of the breakthrough curve. wlwo

I

0,5

2 t

0 |.

_ _

_

,

0

.~"

3 Breakthroug_h_c__o_n_c_ep_t_r_ati_o_h,

50

100

150

t [mini Figure 5 Breakthrough curves on zeolite 3A on heat insulated columns adsorption for variable length of bed: 1 - l = 4 cm, 2 - l = 8 cm, 3 - l = 12 cm. wlwo 1

.

0,5-

1

0-0

.

.=,,

~=.4~-~

..,

2

.

.

Breakthrough concentration - .... 7--

. 50

100

150

t [min]

Figure 6 Breakthrough curves on clinoptilolite on heat insulated column for variable length of bed: 1 - l = 4 cm, 2 - l - 8 cm, 3 - l - 12 cm., 4 - l =16 cm 3.3. T e m p e r a t u r e curves The time development of the temperature in the fluid phase measured at different positions of heat insulated beds are shown in figure 7 (zeolite 3A) and figure 8 (clinoptilolite). The maximum temperature increase ATmax was for 3A zeolite ATmax = 67~ and for clinoptilolite ATmax = 94~ A comparison of the temperature and breakthrough curves on the heat insulated columns measured at the end of the bed (H =12 cm) is shown in figure 9 (zeolite 3A) and figure 10 (clinoptilolite).

1669 T [~

T [~ 170

190

160 150 140

170 150

130

130

120 110

110

100

90

90 = 50

0

t [mini

100

0

150

Figure 7 Temperature curves during sorption on the bed of 3A (heat insulated column). Position in the bed: 1 - l = 4 cm, 2 - l = 8 cm, 3 - I = 12cm

I

~ ~

, !22 I

150

-

---~-

-130

120

0,5

I t

I

~- 100 L.____ 0

90 50

100

150

j 190 ~- 180

~-I 140 0,5

100

T [~

w[%]

;160

1 1

t [min]

Figure 8 Temperature curves during sorption on the bed of clinoptilolite (heat insulated column). Position in the bed: 1 - l = 4 cm, 2 - l = 8 cm, 3 - l = 12 cm

T [~ 170

w [%1

50

150

t [min]

Figure 9 Mutual position of a) temperature and b) breakthrough curve for bed from zeolite 3A (H = 12 c m ) a t t h e bed outlet, The curves overlap partially each other and increased temperature. Thus, a considerable tails of the breakthrough curves) is due to the system seems to tend to a constant pattern

Ol

!

0

50

17o

160

150 140 130 120 110 100 90

100

t [min]

Figure 10. Mutual position of a) temperature and b) breakthrough curve for clinoptilolite bed (H = 12 cm) at the bed outlet. the adsorption proceeds at beginning at portion of adsorbed amount increase (in the cooling of the bed with fluid phase. The behaviour cf. breakthrough curves 3, 4 in

1670 figure 6 and temperature curves 3, 4 in figure 8.The system behaviour in sorption dynamics offers a possibility to utilize a portion of the heat retained in the column at the instant of breakthrough for the bed regeneration. Another way of sorption heat utilization would be using a lower starting temperature of the bed in the adsorption stage of the process. The latter way would represent milder hydrothermal conditions and it might thus prolong the operation life of the bed. 3.4. Effect of regeneration time on the usable sorption capacity The breakthrough curves measured on beds activated during regeneration times tr are exemplified in figure 11. Here tr = ttrans + tsteady where ttrans is the time necessary to reach the temperature T m ~ of sorption using the linear heating program 3~ (i.e. tr = 95 rain) and tsteady is the period of the duration of the experiment at the temperature of sorption, tsteady Table 4 Effect of regeneration time on the usable sorption capacity 9fzeolite 3A Regeneration qB qtot Total sorption capacity from time tsteady weighing of the column

[min] 0.0 120 1440

[g/g sorbent] 0.095 0.119 0.135

[g/g sorbent] 0.109 0.133 0.146

[~/tg sorbent] 0.117 0.143 0.156

The sorption capacity obtained by weighing is to about z 7 % higher as compared to that obtained from breakthrough curves. The most interesting result is that the main portion of adsorbed water removal takes place during the heating period ttrans. The continuation of the experiment with tsteady 2 hours increases the total usable sorption capacity about 22 % and a further prolongation of the desorption experiment to 24 hours brings about an increase of sorption capacity only to 9.8 % as compared to the case with tsteady = 2 hours. After 20 adsorption-desorption cycles the W/Wo X -ray patterns of the sorbents did not show any change suggesting damage of the lattice. =

1 -[

~

5. ACKNOWLEDGEMENTS

0,5 t

1

This research was supported by the Grant Agency of the Czech Republic as Grants No 104/01/0945, 203/99/0522 and 104/00/1007.

O! 0

6. REFERENCES

100

200

Figure 11.Comparison of breakthrough curves for 3 different regeneration times: 1) tsteady = 0 h; 2) tsteady = 2 h; 3) t~te~ay - 2 4 h.

1. C. Colella, M. Pansini, F. Alfani, M. Cantarella, A. Gallifuoco, Microporous Materials 3 (1994) 219.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1671

A d s o r p t i o n properties of M C M - 4 1 materials for the V O C s a b a t e m e n t G. Calleja*, D.P. Serrano, J.A. Botas, F.J. Guti6rrez Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, c/Tulip~in s/n, 28933, M6stoles, Madrid, Spain, e-mail : [email protected] The hydrophobic/hydrophilic properties of MCM-41 materials prepared by different methods have been investigated by toluene and water TPD measurements. The best combination of adsorption capacity and selectivity for toluene versus water has been obtained with a pure silica MCM-41 sample prepared by the sol-gel route, which has been assigned to the different nature of its internal surface compared to MCM-41 prepared by hydrothermal treatment. These results indicate that MCM-41 materials present interesting properties for application in processes of VOCs adsorption in air.

1. INTRODUCTION MCM-41 materials are mesoporous solids characterised by a hexagonal array of cylindrical mesopores with a narrow pore size distribution, large BET surface area and high pore volume. Accordingly, MCM-41 solids present remarkable features for their application in adsorption [1]. Moreover, they can be synthesised as pure silica materials or as aluminosilicates over a wide range of Si/A1 ratios, which makes possible to modify and adjust their hydrophobic/hydrophilic properties. Volatile organic compounds (VOCs) are major pollutants that must be controlled under the increasingly stringent environmental regulations. Their emission into air and water generates a variety of pollution problems, since many volatile organic compounds are toxic or carcinogenic. Likewise, VOCs such as hydrocarbons are important contributors to photochemical smog, being precursors of photochemical ozone. One of the major sources of the hydrocarbons released to the environment is their use as fuels. Therefore, the presence in the air of VOCs, coming from different sources, is currently an environmental issue. Among the commercial technologies available for VOCs abatement, adsorption over activated carbons is one of the most widely applied alternatives. Recently, zeolites and mesoporous MCM-41 materials have appeared as potentially interesting adsorbents due to their hydrothermal and chemical stability, as well as to the possibility of fine tuning their hydrophobic surface properties [2,3]. Moreover, the high stability of MCM41 materials makes more feasible their thermal regeneration in regards to activated carbons, leading to longer adsorption-desorption operation cycles. *Corresponding author This research has been funded by the regional government of Madrid through the Strategic Group Programme (Direcci6n General de Investigaci6n, Comunidad de Madrid).

1672 In the present work, the adsorption-desorption of toluene and water has been studied over MCM-41 materials, prepared through different methods, with and without aluminium, in order to establish their organophilic and hydrophobic properties. Thereby, temperature programmed desorption (TPD) experiments have been carried out, which provide information not only on the adsorption properties, but also on the feasibility of adsorbent regeneration by temperature increase.

2. EXPERIMENTAL 2.1. Adsorbent preparation A pure silica MCM-41 sample was prepared following a method, based on a sol-gel approach, recently published [4]. This sample was named as MCM-41 (sg). In a first step, 73.7 g of cetyltrimethylammonium chloride (CTAC1) were dissolved with 22.5 g of HC1 (35% in water) under slow stirring. Then, 20 g of tetraethylorthosilicate (TEOS) were added, the mixture being stirred for 75 minutes at room temperature. In a second step, 2 wt% ammonia solution was added dropwise until the pH of this mixture was adjusted to 4. Then, the solid obtained was filtered and washed with distilled water and dried at room temperature. The final product was obtained by air calcination at 550~ for 10 h. A1-MCM-41 sample was synthesised following a procedure similar to the previous one, using aluminium isopropoxide as alumina source [4]. Two solutions were prepared under stirring: solution A, composed by 20 g of TEOS and 0.7 g of aluminium isopropoxide and solution B, formed by 36.5 g of CTAC1 and 6.6 g of HC1. When both solutions were perfectly homogenised, solution A was added to solution B, the mixture being stirred for 3 h. Thereafter, 54 g of 2 wt% ammonia solution were added dropwise and stirred for 1.5 h. The obtained sample was filtered and washed with distilled water and room temperature dried. The final product was obtained by air calcination at 550~ for 10 h. A second pure silica MCM-41 sample was synthesised by a hydrothermal treatment according to a method earlier reported [ 1], being named as MCM-41 (ht). Thereby, 20.4 g of CTAC1 were mixed with 212.7 g of distilled water and 23.9 ml of triethylamine at room temperature. When the solution was perfectly homogenised, 20 g of TEOS were added dropwise under stirring, and the resultant mixture was stirred for 4 hours. Next, the synthesis mixture was placed in a PTFE-lined stainless steel autoclave and heated at 110~ for 48 h. The solid product was recovered by filtration, washed with distilled water and air dried. Finally, the surfactant was removed by calcination at 550~ in air for 6 h. A HZSM-5 sample has been also synthesised, to be used as reference, from an ethanolcontaining gel according to a procedure published in a previous work [5].

2.2. Experimental techniques The aluminium content of A1-MCM-41 and ZSM-5 samples was quantified by Inductively Coupled P l a s m a - Atomic Emission Spectroscopy (ICP-AES) analysis in a Varian Vista AX spectrometer. XRD spectra were recorded on a Philips X'PERT MPD diffractometer with Cu K~ radiation. FTIR (Fourier Transform IR) spectra were obtained at atmospheric conditions in a Mattson Infinity Series FTIR spectrophotometer using the KBr wafer technique. A Micromeritics TRISTAR 3000 instrument was used for obtaining the nitrogen adsorption-desorption isotherms at 77 K. Previously, the samples were outgassed under

1673 vacuum at 210~ for 6 h. Specific surface areas of the studied materials were calculated using the standard BET method. Pore size distributions were estimated using the BJH method [6-8]. Toluene and water temperature programmed desorption (TPD) experiments have been carried out in a Micromeritics AutoChem 2910 system with a Thermal Conductivity Detector (TCD) using He as carrier gas. Previously, the adsorbent sample was introduced in a quartz tube and outgassed by treatment at 400~ for 15 min in 30 cm 3 min -1 He flow with a heating rate of 50~ min 1 from room temperature. Thereafter, the sample was cooled down at 40~ and saturated at this temperature by repeated pulse injection of separated either toluene or water (amounts between 0.1 and 2 gl) in helium flow. Once the sample has been saturated by the adsorbate, the He flow was maintained for 2 h in order to remove the reversibly adsorbed toluene or water, which is denoted by the recovery of the base line. Then, the TPD step was started with a heating ramp of 15~ min -1 between 40 and 400~ the experiment being continued keeping constant this last temperature for a 15 min period. The toluene or water concentration in the effluent is continuously monitorized by the TCD detector during all the experiment. Two parameters are derived from the TPD curves: the adsorbate amount retained and desorbed by the solid (WToL and WH20) and the temperature corresponding to the maximum of the desorption peak (TToL and TH20).

3. RESULTS AND DISCUSSION

3.1. Properties of the adsorbents Table 1 summarises the chemical composition and the textural properties of the four adsorbent samples investigated. The high BET surface area and pore volume of the MCM-41 materials are in contrast with the lower values corresponding to the HZSM-5 sample. Moreover, the Si/A1 ratios indicate that the zeolite is more acidic and should be more hydrophilic than the Al-containing MCM-41 sample. Figure 1 illustrates the XRD spectra of the three as-synthesised MCM-41 samples, showing a strong and wide reflection at low angles, which is typical of mesostructured materials. It is remarkable that, in the samples prepared by the sol-gel route, the peak is shifted towards higher angles compared to the MCM-41 obtained by hydrothermal treatment. Accordingly, high differences are observed in the corresponding unit cell dimensions (a0), calculated assuming hexagonal symmetry: 3.85, 3.71, and 5.36 nm for MCM-41 (sg), A1-MCM-41 and MCM-41 (ht), respectively.

Table 1 Properties of the MCM-41 and ZSM-5 samples ..... Sample SBET(m 2 g-l) Dp (nm) MCM-41 (sg) 1065 2.3 MCM-41 (ht) 914 2.4 A1-MCM-41 1310 1.8 ZSM-5 370 0.55 a Mesopore volume, measured at P/P0=0.5

Vp (cm 3 g-l) 0.73 a 0.69 a 0.66 a 0.19

Si/A1 O0 oo 47.6 30.5

1674

MCM-41 (ht) MCM-41 (sg)

5

AI-MCM-41

20 (Degrees)

Fig.1. XRD spectra of the as-synthesized MCM-41 materials

3.2. Temperature programmed desorption experiments (TPD) Figure 2 illustrates the desorption curves for toluene and water obtained in the TPD experiments on the MCM-41 samples and HZSM-5 zeolite. Table 2 summarises the amounts of toluene and water desorbed (in mg of adsorbate by g of adsorbent). It must be noted that the adsorbed-desorbed amounts of toluene and water, showed in Table 2, correspond to results obtained in dynamic experiments carried out in continuous helium flow and with a previous physidesorption step to remove all the reversibly retained adsorbate. Accordingly, the adsorption amounts obtained account only for the adsorbate which is strongly linked to the adsorbent by direct interaction with the internal pore walls, being significantly smaller than the adsorption capacity corresponding to the total pore filling of the adsorbent. Thus, for MCM-41 (sg) samples the amount of toluene detected in the TPD experiment is around 3 wt%, whereas the overall adsorption capacity of this type of materials for hydrocarbons is typically in the range 40-50 wt%. However, as the former amount corresponds with the toluene molecules interacting directly with the MCM-41 surface it can be considered a suitable measurement of the adsorbate-adsorbent affinity. The TPD curves for toluene over the MCM-41 samples and the HZSM-5 zeolite show only one desorption peak, located between 100 and 150~ However, the picture is somewhat more complex for the case of the water desorption since, in addition to a major peak, several minor peaks are detected at higher temperatures over the MCM-41, whereas a shoulder is observed in the water TPD curve over the HZSM-5 zeolite. These results suggests the presence of a variety of adsorption sites for water with different strength and/or the possible existence of dehydroxilation phenomena at high temperature during the TPD experiments.

1675 2.0

2.0

MCM-41

c._~ o~ D (o I-

(sg)

MCM-41

1.5

1.5 -

1.0

1.0-

0.5

0.5-

0.0

, 50

.

, 100

.

, 150

.

,

.

200

, 250

.

, 300

.

, 350

.

,

0.0

400

2.0

a

, 50

.

,

~ 150

.

, 200

.

, 250

.

,

.

300

~

AI-MCM-41

, 350

.

., 400

ZSM-5

1.5

,-:~

!-

.

100

2.0

1.5

(ht)

a

a

1.0

1.0

0.5

0.5

0.0 50

100

150

200

250

300

350

-~r 400

0.0,

, 50

'100

Temperature (~

'150

'200

'250

Temperature

'300

'3~0"

480

(~

Fig.2. TPD results of toluene (a) and water (b), over the different adsorbents

In order to quantify the hydrophilic/hydrophobic properties of the adsorbents, a hydrophobic index (HI) has been defined according to the method proposed in the literature [9], being calculated as the ratio between the adsorbed-desorbed mass of toluene to the adsorbed-desorbed mass of water between 40 and 400~ in the TPD experiments (HI = WToIJWH20). The results obtained have been also included in Table 2.

Table 2 Toluene and water TPD results over the different adsorbents Sample MCM-41 (sg) MCM-41 (ht) WTOL(mgTOlJg) 28.19 5.38 TTOL(~ 116.2 102.9 ATToL (~ 5.9 -7.4 WH20 (mg~lzo/g) 1.67 0.60 TH20 (~ 87.4 74.8 ATH2o (~ - 12.6 -25.2 HI (w/w) 16.9 8.97

A1-MCM-41 33.62 118 7.7 10.16 115.4 15.4 3.31

ZSM-5 65.54 147.3 37 24.74 118.4 18.4 2.65

1676 According to Figure 2 and Table 2, the order of the maximum temperature peak for toluene in the studied samples is as follows: MCM-41 (ht) << MCM-41 (sg) - A1-MCM-41 < HZSM5. A similar trend is also observed for the total amounts of toluene adsorbed-desorbed from the samples. These results show the strong interaction of toluene with the HZSM-5 zeolite, although its adsorption by the MCM-41 materials prepared by the sol-gel route is also significant. It is interesting to note that both pure silica MCM-41 (sg) and A1-MCM-41 behave quite similar regarding toluene adsorption, which indicates that the presence of aluminium in this materials does not modify significantly their interaction with the adsorbed toluene molecules. In contrast, the MCM-41 (ht) sample presents a low affinity for toluene. In regards to the water adsorption-desorption, the following order is derived for the adsorption strength according to the WH20 and TH2Odata: MCM-41 (ht) < MCM-41 (sg) < A1MCM-41 < HZSM-5. As expected, both HZSM-5 and A1-MCM-41 exhibit a hydrophilic character, which is in agreement with the presence of aluminium in these two materials, leading to the generation of both Br6nsted and Lewis acid sites. It is interesting to note that in the case of water the two MCM-41 samples prepared by the sol-gel route present quite different results, with the pure silica MCM-41 adsorbing much less water than the A1-MCM41 sample, which is also related to the aluminium content and acidic character of the latter. Similar conclusions are obtained if the samples are compared in terms of the values of the ATToL and ATH2o parameters, calculated as the difference Td-Tb. Td is the temperature corresponding to the peak maximum for toluene or water desorption and Tb is the boiling temperature of the adsorbates (at atmospheric pressure, Tb toluene = 110.3~ and Tb water = 100~ If the value of the difference Td-Tb is positive, the surface is philic to the adsorbate, while the surface is phobic to that adsorbate if this value is negative [10]. According to the results shown in Table 2, the pure silica MCM-41 synthesised by hydrothermal treatment is phobic to both toluene and water, whereas the A1-MCM-41 and HZSM-5 samples are philic for both adsorbates. Interestingly, the pure silica MCM-41 (sg), synthesised by the sol-gel route, is the only material that is philic to toluene and phobic to water. The values of the hydrophobic index (HI) are in agreement with the above commented results. HZSM-5 and A1-MCM-41 present the lowest HI values, confirming they are hydrophilic materials. On the contrary, the pure silica MCM-41 samples are hydrophobic solids. Especially interesting in regards to its possible application for the removal of VOCs is the case of the MCM-41 (sg) sample, with HI = 16.9. A great difference is observed between the adsorption properties of the two pure silica MCM-41 samples. Toluene is retained in rather larger amounts on the MCM-41 prepared by the sol-gel route. The ratio of toluene/water adsorption in this sample is higher by a factor of 1.9 compared to the MCM-41 obtained by a hydrothermal treatment. In order to find out the origin of this discrepancy between both MCM-41 materials, their properties have been studied and investigated in more detail. Thus, Figures 3.a and 3.b show the N2 adsorption-desorption isotherms at 77 K and pore size distributions, respectively, corresponding to the MCM-41 samples. The three isotherms in Figure 3.a are of type IV, which are typical of mesoporous materials. The inflection points observed at relative pressures in the range 0.15-0.20 indicate that capillary condensation within the pores occurs after the monolayer adsorption on the MCM-41 walls. For relative pressures above 0.5, the MCM-41 isotherms present an almost constant adsorption zone, whereas at high relative pressures a significant increase in the adsorption is observed due to multilayer formation on the external surface of the particles.

1677 600 -----o~ M C M - 4 1

6-

n I(/) E~ E o

(sg)

---o--- M C M -41 (ht) ~AI-MCM-41

400 4-

I

~~ozf

200

>

+

MCM-41 (sg) MCM-41

2-

(ht)

4

---~--- A I - M C M - 4 1 0

o.0

.

.

.

o12

o14

.

.

o16

o18

0

1.0

i 1

,

i 2

P/Po

~

3

,1 i 4 D

,

I 5

,

i 6

,

i 'rTv~I 7 8

(nm)

Fig.3. N2 adsorption on the MCM-41 materials: a) isotherms, b) pore size distributions

Significant differences can be appreciated in the pore size distributions of Figure 3.b. The A1-MCM-41 sample presents smaller pores compared to the other two materials, with an average pore diameter just below the border between micro- and mesopores. The average pore size of both pure silica MCM-41 is very similar, although the sample prepared by hydrothermal treatment presents a quite narrower pore size distribution, which is in agreement with the steeper jump observed in the isotherm. The lower uniformity of pore size in the MCM-41 materials synthesized by the sol-gel route suggests the presence of a larger disorder in these solids, which may be due to both an irregular array of the pores and to the existence of a higher concentration of defects in the pore walls. Figure 4 illustrates the FTIR spectra of the three MCM-41 samples. The broad absorption band in the region around 3500 cm -1 can be ascribed mainly to the hydrogen bridges of the water adsorbed in the samples. As the water is expected to be adsorbed mainly over the

u =o

4o'oo

'

3o'oo

'

~o'oo

~o'oo

o

W avenumber (cm 1)

Fig.4. FTIR spectra: a) MCM-41 (sg); b) MCM-41 (ht), c) A1-MCM-41

1678 silanol groups of the MCM-41 materials, this adsorption could be related to the their concentration in the surface. This band is more pronounced for the MCM-41 materials prepared by the sol-gel route compared to that obtained by hydrothermal treatment, suggesting the presence of a higher concentration of silanols in the former, which may arise from a lower condensation degree of the pore walls. This fact is in agreement with the larger disorder observed from the pore size distribution and is probably the reason to explain the different behaviour exhibited by the two pure silica MCM-41 in the TPD measurements. Thus, the different types of defective silanol groups present in the MCM-41 (sg) materials are probably the origin of its higher affinity for both adsorbates, especially for toluene, which in turn determines its remarkable adsorption properties. Likewise, the smaller FTIR absorption band for MCM-41 (ht), in comparison with the other two MCM-41 materials, is in good agreement with its smaller water adsorption observed in the TPD measurements (see Figure 2 and Table 2).

4. CONCLUSIONS Toluene and water TPD experiments have shown that the pure silica MCM-41(sg) material, prepared at room temperature by a sol-gel approach, is a very interesting adsorbent for VOCs removal as it combines the hydrophobic properties of pure silica materials with the toluene affinity of Al-containing samples (A1-MCM-41 and HZSM-5). This behaviour is probably due to a lower condensation degree of the pore walls and a higher defective silanol concentration in this MCM-41 sample. In contrast, the pure silica MCM-41 sample prepared by a hydrothermal method presents a more uniform mesopore array and lower silanol content. However, although this material exhibit a hydrophobic character its interaction with toluene is also very weak, which hinders its application for the adsorption of aromatic hydrocarbons. It is remarkable that the pure silica MCM-41 synthesised by the sol-gel route is, among the four samples investigated, the only material that is philic to toluene and phobic to water. REFERENCES

1.

W. Lin, Q. Cai, W. Pang, Y. Yue, and B. Zou, Microporous Mesoporous Mater., 33 (1999) 187. X.S. Zhao, G.Q. Lu and X. Hu, Microporous Mesoporous Mater., 41 (2000) 37. 3. T. Martin, A. Galarneau, D. Brunel, V. Izard, V. Huela, A.C. Blanc, S. Abramson, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 135 (2001) 178. J. Aguado, D.P. Serrano, and J.M. Escola, Microporous Mesoporous Mater., 34 (2000) 43. M.A. Uguina, A. de Lucas, F. Ruiz and D.P. Serrano, Ind. Eng. Chem. Res., 34 (1995) 451. S. Brunauer, P.H. Emmett, and E. Teller, J. Am. Chem. Soc., 60 (1938) 309. 7. E.P. Barret, L.G. Joyner, and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 8. M. Kruk, M. Jaroniec, and A. Sayari, Langmuir, 13 (1997) 1737. 9. H.K. Seo, J.W. Oh and S.J. Choung, Stud. Surf. Sci. Catal., 135 (2001) 328. 10. X.S. Zhao, G.Q. Lu and X. Hu, Colloids and Surfaces A, 179 (2001) 261. ,

.

.

.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1679

Adsorption o f linear and branched paraffins in silicalite : T h e r m o d y n a m i c and kinetic study. I. Gener a, j. Rigoreau a G. Joly a, A. Renaud a and S. Mignard a a LACCO, Chimie 7A, UMR 6503, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex The adsorption and diffusion of linear, monobranched and dibranched C6-isomers in silicalite at 298 K have been investigated using microgravimetric, volumetric and microcalorimetric techniques. The sorption capacities and diffusion time constants are discussed in terms of the shape-selective characteristics of the sorbent and of the size of the sorbates. The separation of monobranched and dibranched C6-isomers should be essentially induced by kinetic effect even though thermodynamic consideration should also be taken into account. The adsorption phenomenon should be correlated to the steric hindrance and not to the existence of preferential sorption sites. 1. INTRODUCTION The separation of the dibranched paraffins from a mixture of linear and monobranched paraffins is of a great industrial interest because of its potential application for the octane number enhancement (see Table 1). This separation is difficult and expensive by using classical separation processes (distillation, crystallization, solvent extraction) because of the close physical properties of these hydrocarbons. The molecular shape selectivity of zeolites should allow a discrimination based on differences in diffusivity, adsorptivity or reactivity. The potential applications of zeolites as shape selective adsorbents for the separation of alkane isomers have been widely studied [ 1] essentially with 5A but the use of silicalite, MFI, mordenite, beta and MCM22 type zeolites has received an increasing interest during the past six years [2-18]. In this work, we have measured the thermodynamic properties (adsorption isotherm, adsorption heat) and kinetic properties (di~sion time constant) of the linear and branched paraffins with six carbon atoms on a silicalite at ambient temperature. 2. EXPERIMENTAL SECTION 2.1. Materials

The proprietary silicalite powder was kindly provided by the French Institute of Petroleum (IFP). The crystals were essentially of a spherical shape of 1-3 ~tm. The crystallinity, determined by XRD, was about 98%. Hexane (n-C6), 2-methylpentane

1680 (2MP), 3-methylpentane (3MP), 2,2-dimethylbutane (22DMB) and 2,3-dimethylbutane (23DMB) (purity > 99,9%) were provided by Fluka Chemic AG. The physical characteristics of the sorbates are listed in Table 1. Table 1 Physical characteristics of the paraffins used Sorbates

Criticaldiameter (rim)

Length(nm)

Vapor pressure 298 K (bar)

Octane number RON

n-C6 2MP

0.49 0.54

0.10 0.94

0.201 0.282

31 75

3MP 23DMB

0.54 0.58

0.94 0.81

0.253 0.425

76 96

22DMB

0.63

0.81

0.313

104

2.2. Experimental Both adsorption and diffusion measurements were performed with a SARTORIUS 443 microbalance at constant volume and constant pressure at 298 K. The zeolite samples (about 70 mg) were outgassed under primary vacuum at 623 K for 12 hours prior to the sorption measurements. After cooling to 298 K, the sorbate pressure was increased step by step in order to obtain the entire equilibrium adsorption isotherm. For each step, the amount of sorbate introduced to the system was kept small enough in order to consider the diffusion process to be isothermal and to assume constant dit~sivity. For each increment (uptake), the mass increase is recorded v e r s u s time. Calorimetric measurements were performed with a SETARAM DSC 111 instrument at 298 K linked to a volumetric system allowing the introduction of very small sorbate quantity. The zeolite (about 100 mg) was activated under secondary vacuum at 623 K for 12 hours. After cooling to 298 K, the sorbate was introduced into the system as vapour. Sorbate pressure was increased step by step similarly to the gravimetric measurements. The volumetric data were used to determine the amount adsorbed at 298 K. The heat flow is recorded v e r s u s time in order to determine differential heat of adsorption for each step. 3. DIFFUSION MODEL The adsorption rate curves were measured at 298 K. The diffusion in the crystals is assumed to follow Fick's equations. The mathematical solution for the transient diffusion equation involving a spherical particle in terms of uptake of sorbate by the solid assumes the well-known form given by Crank [ 19] : 6 o~ 1 n~2D Qt .=l-~-~,[~-~.exp( .... 1.2-)3 Q~o

(1)

1681 where Qt and Qoo are the adsorbed amounts at time t and at sorption equilibrium, respectively. A simplified and convenient solution for short times is [20] : Qt_6 Q~o_-~ 1]r2,

(2)

For long times, the solution is of the form [20] :

(3) The diffusion can be adversely affected by heat effects or when the sample contains a wide distribution of crystal size. The short time response is less susceptible to thermal effects or the wide distribution of adsorbent particle size, so it was mainly used in this study for the calculation of diffusivity coefficient.

4. RESULTS AND DISCUSSION 4.1. Adsorption equilibria The isotherms of the monobranched and dibranched C6-isomers on silicalite at 298 K are shown in Figure 1. They are found to be of Type I form in the Brunauer's classification. Isotherms were fitted with the classical Langmuir equation (see Figure 2) : P_IAP Q-Q~K 'Q-s-~

(4)

The parameters K and Qs, have been then been calculated and reported in Table 2. Table 2 Langmuir con stants (K). and m0n~ capacities (Q~) on silicalite at 298 K Sorbates K (bar-1) Q~ (mmol.g-1) Q~ (molecule.UC-~) n-C6 214.1 1.32 7.6 2MP 70.9 0.97 5.6 3MP 52.4 1.19 6.8 23DMB 50.9 1.31 7.5 22DMB 32.7 0.74 4.2 Silicalite is an hydrophobic or organophilic zeolite. Therefore, adsorption of organic molecules occurs through the volume filling of micropore by physisorption at low relative pressures. For P/P0=0.1, more than 60% of the total capacity is filled for all the C6-isomers. The total monolayer adsorption capacities of C6-isomers as well as Langmuir constants exhibit a decreasing trend with increasing critical diameter of the sorbate molecules indicating that a shape selectivity occurs. The 10-ring pore opening of

1682 silicalite leads to a shape selectivity for C6-isomers which have molecular dimensions close to those of the silicalite micropores (++0.6 nm). 1,6

0,3

1,4

0,25

"~

1

E "~

0,8

o

&

&

_&a&=

9149

-nn _ 9 9 9 9 9 1 4dD.+o 9e- 9 r i B , l i e" 9 1 4 ee 9 1 7 69 =n,,oQ

l 9

0,6

i,.

0,4

X

-

X

~

~

-

a,, ,,,,,

X

xxXX

9

_,=.tllV

21C N

X

~ 0,15

0,1

,

|

+

i

,

t

i

i

,

9mu

I

~ "

9

A

-On, ~.~,** ,,,,o,

Z

0,05 1

..', ="

00

9

0!

gO U

X

v

9

0,2 I=

~F"

0,2 0

X

o

,

O,O 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 PIPO

Figure 1. Adsorption isotherms for n-C6 (.), 2MP (.), 3MP (=), 23DMB (A), 22DMB ( . ) on silicalite at 298 K.

0

0,05

0,1 P/bar

0,15

0,2

Figure 2. Langrnuir plot for n-C6 (.), 2MP (-), 3MP (.), 23DMB (A), 22DMB ( , ) on silicalite at 298 K.

The steric hindrance is more pronounced for 22DMB which is preferentially adsorbed at the channel intersection. For this compound the monolayer capacity (Qs = 4.2 molecules.UC -]) is related to the total occupation of the channel intersections. Nevertheless, 2MP, 3MP and 23DMB total capacities are very closed. Thus, it seems difficult to separate a mixture of these compounds by equilibrium-based adsorption. The differential heats of adsorption as a function of the coverage for C6-isomers in silicalite at 298 K are plotted on Figure 3. As shown in Figure 3, all the differential calorimetric isotherms of C6-isomers have the same shape. Whatever the sorbate molecule, no initial decrease in the differential adsorption heat has been noticed as it can be observed for other hydrocarbons [21]. So, there are no real preferential adsorption sites for the C6-isomers owing the non-specificity of C6-isomer surface bond. The plateau corresponds to the micropore filling. It is not perfectly horizontal since there is a tiny positive slope. This small rise of differential adsorption heat can be ascribed to sorbate-sorbate interactions which increase when the coverage increases. The plateau length depends on the nature of sorbate molecule and it is directly correlated with the monolayer capacity (Qs). The final abrupt decrease of the differential adsorption heat essentially corresponds to the adsorption on the external surface. The plateau position is quite the same for all the C6-isomers except for the 22DMB. The experimental values of the differential adsorption heat at zero coverage are in agreement with the values found in literature (see Table 3) except for the 23DMB. Surprisingly, this compound is highly adsorbed at very low P/P0 values (see Figure 4).

1683

70 ..:,

0,9-

80

]

0,8 i

~'l:~. 0,7 .q

=

0

E 0,6

'~ o,4 ,o

:

0,1

10

0,000

0

0

0,2

0,4 0,6 0,8 1 cove rag e l m m oi.g-'

1,2

Figure 3. Differential heats of adsorption of n-C6 (.), 2MP (.), 3MP (.), 23DMB (A), 22DMB (,) on silicalite as function of the coverage at 298 K. Table 3 Adsorption heats at zero coverage Sorbates Technique n-C6 microcalorimetry microcalorimetry isosteric method microcalorimetry microgravimetry 2MP microcalorimetry microgravimetry molecular simulation TPD 3MP microcalorimetry microgravimetry TPD chromatography 23DMB microcalorimetry microgravimetry 22DMB microcalorimetry microgravimetry chromatography

. . . . . . .

~

m 0,3 0 0,2

0,001

0,002

0,003

0,004

0,005

PIP0

Figure 4. Adsorption isotherms for n-C6 (.), 2MP (.), 3MP (-), 23DMB (A), 22DMB (.) on silicalite at 298 K for low P/Po domain.

on sili.calite,' comparison with others studies Reference Differentialiadsorption .heat OO/m,ol) this work 62 [22] 69.8 [23] 76 [ 14] 72 [24] 87 this work 59 [3] 62.7 [ 12] 63 [6] 60.9 this work 56 [3 ] 67, 7 [6] 68.4 [8] 66.4 this work 66 [3 ] 54.3 this work 54 [3] 54.3 [8] . . . . . . 55

_

As expected, the lowest adsorption heat (54 kJ.molt ) is observed for the 22DMB. From the adsorption isotherms and differential adsorption heats of the C6-isomers, we may conclude that monobranched (2MP, 3MP) and dibranched (23DMB) compounds

1684 exhibit adsorption properties rather close with respect to silicalite. This can be explained by their dimensions (critical diameter) which are very close. 22DMB is adsorbed with difficulty because of its high diameter whereas n-C6 which is smaller is highly adsorbed.

4.2. Kinetics

Sorption kinetics plots for the sorption of C6-isomers in silicalite at 298 K are reported in Figure 5.

1

.4'

0,9 O,8

o~ t

.4'

0,r

05

0,S

4DI

~ 0,6 O,4

9

II

9

,o

II

i i II

IN

9

IN

9

I

0,3

$

O,2 0,1

Ii:

o_ 0

~

n~ 4E 2

4

It 6

8

tlr~(S1~)

10

12

14

Figure 5. Qt/Q= v e r s u s the square root of time plots for the sorption of n-C6 ( . ) , 2MP (e), 3MP (-), 23DMB (A), 22DMB ( . ) in silicalite at 298 K. The values of the diffusion time constants for the sorbates, obtained from the slopes of the initial linear section of the plots according to equation (2), are given in Table 4. Table 4 Diffusion time constants at 298 K Sorbate D/r 2 (s "i) n-C6 .... 9.~) 10-3 2MP 5.8 10 -3 3MP 4.1 10 -3 23DMB 3.1 10-5 22DMB 2.6 10-5 The diffusion time constants calculated are held constant for all the uptake steps. From the diffusion curves and the diffusion time constants, it clearly appears that the diffusivities of the C6-isomers decreases with increasing chain branching as it has already been observed in H-ZSM5 zeolite [2].

1685 In order to evaluate the separation potential of the zeolite for linear and multibranched C6-isomers, a very simple quantitative approach has been adopted. Using the short term solution (equation. 2) and assuming a linear isotherm, a simple model for effective separation was obtained (see Table 5) : ot/r_KA ~-~-B-~~DA ~b-~-~~~/2

(s)

Table 5 Effective separatio n factors between C6-isomers on silicalite at 298K Linear Monobranched Separation factor n-C6 2MP 3.9 n-C6 3MP 6.4 Linear Dibranched Separation factor n-C6 23DMB 75.8 n-C6 22DMB 128.0 Monobranched Dibranched Separatio n factor 2MP 23DMB 19.2 2MP 22DMB 32.5 3MP 23DMB 11.9 3MP 22DMB 20.2 The contribution of the Langrnuir constant ratio on the effective separation factor values of linear and monobranched isomers represents more than 60%. This lets appear an equilibrium driven separation. Since the effective separation factors of monobranched and dibranched isomers are much larger than that of linear and monobranched isomers, the separation of monobranched and dibranched C6-isomers is considerably better than that of linear and monobranched isomers on silicalite. In this case, the effective separation factor increase is due to the increase of the diffusion coefficient ratio. This observation lets appear that a separation between mono and dibranched C6-isomers may be essentially induced by kinetic effect. Furthermore, the effective separation factor of the monobranched isomers (2MP or 3MP) and 22DMB factor is always larger than that of the monobranched isomers (2MP or 3MP) and 23DMB. This difference is due to the difference between the Langmuir constant ratio which is larger for monobranched C6isomers (2MP or 3MP) and 22DMB system and shows an equilibrium driven separation. 5. CONCLUSION In conclusion, the thermodynamic characteristics (sorption capacities and differential adsorption heat) of the C6-isomer series adsorbed on silicalite at 298 K are not very different except for the 22DMB. The adsorption phenomenon is strictly correlated to the steric hindrance and not to preferential sorption sites. The kinetic study has revealed an important difference between the diffusion time constants of monobranched and dibranched C6-isomers. The silicalite separation potential for mono and dibranched C6-

1686 isomers is larger than for linear and multibranched C6-isomers. However, the separation is not only induced by kinetic effect. Indeed, a very simple quantitative approach has revealed an equilibrium driven separation for linear and monobranched C6-isomers and a kinetic induced separation for monobranched and dibranched C6-isomers. REFERENCES

1. R. M. Dessau, ACS Symp.Ser., 135 (1980) 123. 2. V. R. Choudhary, V. S. Nayak, A. S.Mamman, Ind. Eng. Chem. Res. 31 (1992) 624. 3. C. L. Cavalcante and D. M. Ruthven, Ind. Eng. Chem. Res. 34 (1995) 177. 4. C. U Cavalcante and D. M. Ruthven, Ind. Eng. Chem. Res. 34 (1995) 185. 5. T. J. H. Vlugt, W. Zhu, F. Kapteijn, J. A. Moulijn, B. Smit, R. Krishna, J. Am. Chem. Soc. 120 (1998) 5599. 6. B. MiUot, A. Methivier, H. Jobic, I. Clemencon, B. Rebours, Langmuir 15 (1999) 2534. 7. H. Du, M. Kalyanaraman, M. A. Camblor, D. H. Olson, Mic. Mes. Mater. 40 (2000) 305. 8. E. Jolimaitre, M. Tayakout-Fayolle, C. Jallut, K. Ragil, Ind. Eng. Chem. Res. 40 (2001) 914. 9. J. F. Denayer, W. Souverijns, P. A. Jacobs, J. A. Martens, G. V. Baron, J. Phys. Chem. B 102 (1998) 4588. 10. H. A. Begum, N. Katada, M. Niwa, Mic. Mes. Mater. 46 (2001) 13. 11. H. H. Fur,&e, M. G. Kovalchick, J. U Falconer, R. D. Noble, Ind. Eng. Chem. Res. 35 (1996) 1575. 12. R. L. June, A. T. Bell. D. N. Theodorou, J. Phys. Chem. 94 (1994) 1508. 13. K. Huddersman, M. Klimczyk, J. Chem. Soc. Faraday Trans. 92 (1996) 143. 14. F. Eder, J. A. Lercher, Zeolites 18 (1997) 75. 15.J. Xiao, J. Weil, Chem. Eng. Sci. 47 (1992) 1143. 16. J. R. Hufion, D. M. Ruthven, R. P. Danner, Mic. Mater. 5 (1995),39. 17. M. A. Jama, M. P. F. Delmas, D. M. Ruthven, Zeolites 18 (1997) 200. 18.D. Schuring, A. O. Koriabkina, A. M. de Jong, B. Smit, R. A. van Santen, J. Phys. Chem. B 105 (2001) 7690. 19. J. Crank, The Mathematics of diffusion, Oxford Press, London, 1975. 20. J. Karger, D. M. Ruthven, Diffusion in zeolites and other microporous solids, John Wiley and Sons (eds), New York, 1992. 21. J. M. Guil, R. Guil-Lopez, J. A. Perdigon-Melon, A. Corma, Mic. Mes. Mater. 22 (1998) 269. 22. H. Stach, U. Lohse, H. Thanun, W. Schirmer, Zeolites 6 (1986) 74. 23. Y. Yang, L. V. C. Rees, Mic. Mater. 12 (1997) 117. 24. P. Wu, A. Debebe, Y. H. Ma, Zeolites 3 (1983) 118.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1687

L o c a t i o n and transport properties o f a m m o n i a m o l e c u l e s in a series o f faujasite zeolite structures as studied b y F T - I R and 2 H - N M R spectroscopies Fr6d6ric Gilles a'w Jean-Luc Blin a, Helge T o u f a r b and Bao-Lian Su a'* a: Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61 Rue de Bruxelles, B-5000 Namur, Belgium. b : Tricat Zeolites GmbH, Chemiepark, Tricat-StrafSe, D-06749 Bitterfeld, Germany. The interaction, the location and the transport properties of ammonia molecules in a series of cationic faujasite zeolites have been investigated by means of infrared and 2H-NMR spectroscopies. The results have been correlated with the nature of counter-ions, the Si/A1 ratio, the negative charge of oxygen atoms of the zeolite framework and the acido-basicity of the studied zeolites. The present work evidenced experimentally for the first time two kinds of interaction between ammonia and cationic zeolites. The ammonia can interact not only with alkali cations via the lone pair on nitrogen atoms but also with the negatively charged oxygen atoms of the framework via its hydrogen atoms. However, the Lewis acidity of counter-ions is the dominating factor for the ammonia interaction. The correlation times and the activation energies of ammonia in Na+-exchanged faujasite zeolites have been determined from 2HNMR spin-lattice relaxation experiments. At low temperature, a rotation around 3-fold axis of the ammonia molecule is detected. At high temperature, an isotropic motion of ammonia is evidenced. Moreover, an enhancement of the ammonia mobility from Na-X to Na-Yd has been observed and explained. 1. INTRODUCTION Due to their def'med uniform pore size, structure and tunable acid-base properties [1-4], zeolites have been widely used in catalytic and separation processes. It is well known that two elementary steps, adsorption of guest molecules on active sites and their diffusion within the cavities of zeolites, occur in the majority of the industrial processes, which involved zeolites. The knowledge of the molecular motion and the understanding of the host-guest interactions, which control the diffusion, the adsorption of reactants and products and subsequently the activity, the selectivity of reaction and efficiency of separation, may therefore lead us to develop new materials with advanced performances. In a recent past, different research groups have used trichloromethane, acetylene, benzene, methylamine and others as probe molecules [5-9] to investigate the acid-base properties of cationic zeolites. These studies have greatly enlarged the knowledge on these materials. However, because of the large size of these molecules, only a part of the channels and cavities of the zeolites are accessible to them. Therefore, the overall basicity of these materials is not accurately measured. Moreover, in these studies, the cooperative role of the counter-ions in adsorption and in catalysis is not studied in detail. In the present work, the ammonia, a smaller molecule which posses a dipolar moment, has been used as probe molecule in order to characterize the overall properties of the zeolites. * Corresponding author ([email protected] Tel :+32 (0)81.72.45.31, Fax: +32 (0)81.72.54.14) wFRIA fellow

1688 Our recent study on the adsorption of ammonia in a series of cationic-exchanged faujasite zeolites, realized by means of TPD-MS technique, revealed the decomposition of ammonia molecules to hydrogen and nitrogen at low pressure even at room temperature [ 10]. This very important observation suggests that such materials could be good catalyst candidates for the amination of methanol to produce the methylamines. This implies also, but more significantly, that ammonia will be competitive from different points of view, despite its unpleasant odor, to methane or methanol and may be used as the hydrogen source for green automotive fuel cell. 2. EXPERIMENTAL SECTION

2.1. Materials Starting materials Na-Y (Si/A1 = 2.5) and Na-X (Si/A1 = 1.2) were provide by Union Carbide. The Na-LSX was provided by Tricat Zeolites GmbH. The Li+, K +, Rb + and Cs+ exchanged Y and X zeolites were obtained by ion exchange of Na-Y and Na-X respectively. The dealuminated Na-Y was obtained by treatment of NHn-Y in a 0.4M solution of (NHn)aSiF6 at 363 K for 3h. The obtained material with Si/A1 ratio equal to 3.4 was further exchanged with a NaC1 solution to give the Na+-exchanged dealuminated Y (Na-Ya).

2.2. Samples characterization The XRD patterns were obtained with a Philips PW 170 diffractometer. The crystal morphology was studied using a Philips XL-20 scanning electron microscope (SEM) with conventional sample preparation and imaging techniques. The specific surface areas were measured by nitrogen adsorption using a Micromeritics ASAP 2010. 2.3. Infrared study This was performed on a self-supported zeolite wafer pressed with a pressure of 5 tons/cm2. The zeolite wafer was first calcined in a Pirex IR cell with two NaC1 windows in oxygen overnight and then in vacuum for 6-8h at 723 K. After pretreatment of the zeolite wafer, the IR cell was slowly cooled to room temperature and the spectrum of zeolite phase alone was recorded as reference using a Perkin-Elmer Fourier Transform Spectrum 2000 spectrometer. The adsorption of known and increasing amounts of ammonia was then performed on wafer. After 0.5h equilibration at room temperature, the spectra of adsorbed ammonia were then recorded. The desorption experiments were carried out at different temperatures for 0.5h to evaluate the adsorption strength of ammonia in studied zeolites. 2.4. 21-I-NMR spectroscopy Powder samples were packed into NMR tubes (5.0 mm with restriction) fitting exactly the double-bearing Brucker zirconia rotor. The sample powders were treated under the same condition as IR studies. The adsorption of known amount (32 molecules / u.c.) of deuterated ammonia was carried out. After introduction of deuterated ammonia, the NMR tubes were maintained in liquid nitrogen for 5 minutes to ensure quantitative adsorption and then sealled. Deuterium NMR spectra were recorded on a Brucker MSL-400 spectrometer at a frequency of 61.4MHz. Spin-lattice relaxation rates were measured using the inversion recovery method followed by quadrupole echo sequence for observation of the signal, i.e. 180~176 90~ Spectra were recorded as a function of temperature, allowing at least 30 minutes for the samples to reach equilibrium. The activation energy was calculated from the slope of the Arrhenius plot of the spin-lattice relaxation time [11 ].

1689 3. RESULTS AND DISCUSSION 3.1. Zeolites characterization

The crystallinity of our materials was checked using Xray diffraction. This technique reveals that the introduction of a larger cation such as Cs + in the zeolites framework can cause a decrease of the peaks in intensity but not a significant modification in the crystallinity of the materials. The scanning electron microscopy shows the octahedral morphology of the starting material Na-X (Fig. 1A) and that the ion-exchanges do not affect this morphology since the other samples exhibit the same morphological features (Fig. 1B for K+-exchanged sample). The obtained N2 adsorption-desorption isotherms are typical of microporous materials with specific surface areas located between 405 and 717 mZ/g. The characteristics of all the studied faujasite zeolites are summarized in Table 1.

....; ~,~,.,,

'

' ;'~'

~,~

-. '@,'9.

Fig. 1. SEM micro graphs ot A : Na-X and B : K-X

3.2. FT-IR study of ammonia-cationic faujasites interaction

The gaseous ammonia is a tetragonal molecule with C3v symmetry. The group theory reveals that this molecule presents six fundamental modes of vibration. Two of them are doubly degenerated, this is why only four peaks are observed in mid-infrared spectroscopy at 3336, 3414, 931 and 1627 cm -1 (Fig. 2A). These four peaks are assigned to symmetric (v l) and asymmetric (v3) stretching vibrations of N-H bond and to symmetric (v2) and asymmetric (v4) bending vibrations of H-N-H angles, respectively [12]. Adsorption of ammonia on a series of faujasite zeolites gives very complicated spectra in the range of 4000-1300 cm-1 (Fig. 2B). Due to the interaction between ammonia and zeolite, the group of symmetry of the ammonia molecule changes and a split of the degenerated modes v3 and v4 occurs. This is why a higher number of peaks are present for adsorbed ammonia than for gaseous ammonia. Figure 3 reports the IR absorbance spectra of Na-Y in the range 4000-2800 cm"1 (A) and 1800-1400 cm-1 (B) after adsorption of different amounts of ammonia (1 to 20 mol./u.c.). The adsorption of ammonia gives five main peaks at 3398, 3384, 3369, 3317 and 3267 cm l and two shoulders at 3464 and 3197 cm l in the region of Table 1, Comp0sifi0nfom e l e u s i s Zeolite

Si/A1 ratio

Na-L SX Li-X Na-X K-X Cs-X Li-Y Na-Y K-Y Rb-Y Cs-Y

1.0 1.2 1.2 1.2 1.2 2.5 2.5 2.5 2.5 2.5

and surface area of all the studied zeolites Composition/u.c.

Na96A196S i960384 Li80Na6A186Si1060384 Na86A186Si1060384 K66Na20A186Si1060384 Cs20Na66A186Si1060384 Lin6Na10A156Si1360384 Na56A156SiI360384 K42NalnA1568i1360384 Rbx6Na30A156Si1360384 Csl 5Na41A156Si1360384

Specific surface area (m2/g)........... 629 659 527 518 405 717 620 631 550 446

1690

[~ 0.2

I

3317 ~ ~0.2 ~ I 3263AI~0 .21652 \A,,/"1643 B ~ 3464 //J~/ 31971 // 710

~ 4000

3500

3000

2500 2011

Wavenumber (cml)

Fig. 2. IR spectra of ammonia. A" gaseous and B : adsorbed on Rb-Y

,

[a 4000

~a 3500 ~ _ _3000 1700/ ~ Wavenumber (cm -1)

. 1500 ~

Fig. 3. IR spectra of adsorbed NH3 on Na-Y in the range (A) 4000-2800 and (B) 18001400 cm-~. a: 1 ; b : 5 ; c: 10; d: 15and e : 20 mol./u.c.

4000-2800 cm -~ (Fig. 3A) and two peaks at 1652 and 1643 crn~ and one shoulder at 1510 crn in the range 1800-1400 cm -~ (Fig. 3B). These peaks are present for any ammonia loading from one to twenty molecules. Taking into account the isotopic displacement when deuterium atom replaces the hydrogen one in ammonia molecule, these peaks can be assigned as follow : the peak at 3317 cm ~ can be attributed as the v~ symmetric stretching mode of the adsorbed ammonia molecule. The splitting of the v3 asymmetric stretching mode and the 2v4 combination bands are responsible for the appearance of the peaks located at 3384, 3398 and 3263 cm ~, respectively. Two shoulders characteristic of the amine group, at 3464 and 1510 cm-1 can be detected after adsorption of one molecule of ammonia (1 mol./u.c.). Their observation suggests that a decomposition of ammonia on the zeolite occurs. This confirms our results obtained by TPDMS. Indeed, we have shown that such a phenomena takes place. The low intensity of these shoulders is related to the weak quantity of decomposed ammonia molecules. When the IR spectra of gaseous ammonia and adsorbed ammonia are compared, a bathochrome shift of the stretching modes is observed whereas an hypsochrome shift is present for the v4 bending modes (Table 2). The same behavior is observed for the other alkali-exchanged faujasite zeolites. Only the Av values are different and reflect the interaction strength between the ammonia molecule and the zeolite. The higher the interaction, the higher the bathochrome shift. This shift to the lower wavenumber, when varying the counter-ion from Na + to Cs +, reflects the weakening of the N-H bond of the ammonia due to its interaction with the zeolite. Nevertheless, we can observe that this shift is more important when the ionic size of the counter-ion is higher. This observation is not in agreement with the infrared desorption experiments. Indeed, we have shown that the desorption temperature of the ammonia increases with the increasing of the Lewis acidity of the counter-ion. As we can see from Figure 4, Av value of the v3 stretching mode to the low wavenumber increases with increasing negative charge on the oxygen atoms of the framework. This leads us to propose that an interaction other than the cation-nitrogen interaction, takes place. We

1691 Table 2.

Wavenumber

(cm -1) of the different vibration bands of gaseous and adsorbed

(NH3)g

(NH3)ad onNa-LSX Li-X Na-X K-X Cs-X Li-Y Na-Y K-Y Rb-Y Cs-Y

NI-I 3

-~O a

V1 (A1)

Av

v2 (A1)

v3 ( g )

Av

v4 ( g )

0.428 0.406 0.412 0.445 0.429 0.346 0.350 0.383 0.392 0.408

3334 3308 3312 3306 3305 3316 3317 3310 3310 3306

26 22 28 29 18 17 24 24 28

966/931 b c c c c c c c c c c

3414 3358 3383 3368 3343 3369 3383 3384 3378 3378 3375

56 31 46 71 45 31 30 36 36 39

1626.5 1654 / 1633 1643 / c 1652 / 1639 1656 / 1637 1640/c 1641 / 1635 1643 / c 1644 / 1632 1646 / 1630 1639 / 1632

negative charge on the oxygen atoms of the framework calculated from the Sanderson electronegativity equalization principle [13], b the two observed values are attributed to the inversion doubling effect [12], r not observed peak a

suggest that interaction between the oxygen atoms of the framework of the zeolites and the hydrogen atoms of the ammonia molecules exists. The scheme of this double interaction is represented in figure 5. In 1997, Br~indel et al [14], in a paper dealing with a computational study of zeolite-adsorbate interactions for NH3 on H-faujasite, have already suggested such interaction. However, from our best knowledge, our present observations provides for the first time experimental prove for this double interaction. Even if the nitrogen-cation interaction is the dominating force for the adsorption, the weakness of the N-H bond is strongly affected by the O-H interaction. From desorption experiments and TPD-MS measurements, the nitrogen-cation interaction is stronger than the O-H interaction. However, the direct influence of the O-H interaction is the cause of the weakening of the N-H bond observed by infrared spectroscopy. So, the higher the negative charge of the oxygen atoms, the weaker the N-H bond, the higher hypsochrome shift of N-H stretching band.

70-

.at_

, ,,'|IH

Na .... /N .....

60 ' ~ 50

o

<1 40

30 84

-o 45

/

O

si

\

-

/

A1 \

O O

-0.'40

-o55

~o

Fig. 4. Wavenumber displacerment of the v3 band of ammonia as function of the negative charge of the oxygen atoms of the framework

Fig. 5. Scheme of the postulated double interaction ammonia-zeolite for a Na +zeolite

1692

3.3. 2H-NMR study of motion and transport properties of ND3 in a series of Na +exchanged faujasite zeolites It is well known that different types of molecular T A reorientation could be !25 kHz lkHz distinguished by 2H-NMR ..1_. lineshape which depends on the way in which the quadrupolar interaction is averaged[ 15]. Figure 6 depicts the 2H-NMR spectra of deuterated ammonia in Na-Yd at different temperature. At low / temperature, a Pake-type powder pattern which shows a splitting between the two -l{~}b'' 0''100(} -11)00' 6 '10'00 "-300' 0 ' 3 0 0 ' horns of the spectrum, Chemical shift ( p p m ) related to the fl angle (angle between the N-H bond and Fig. 6.2H-NMR spectra of the adsorbed ammonia on the molecular axis of Na-Yd at A" 118 ; B 9168 and C 9288 K rotation), is observed (Fig. 6A). Since the splitting for a static N-H bond is -156 kHz [ 16], approximately three times the observed value (Table 3), it may be concluded that a rotation around the 3-fold axis of the molecule is occurring. With increasing temperature, the Pake-type powder pattern, characteristic of the rotation of ammonia molecules around the 3-fold axis, disappears (Fig. 6B). Only one peak is detected, being indicative of the isotopic motion of ammonia. If the temperature is further raised (depending on the sample), the thermal agitation allows the ammonia molecule to jump between adsorption sites at a rate that is sufficiently fast to average completely the quadrupolar tensor. The same behavior is observed for the Na-Y and Na-X zeolites. Only the transition temperature from the isotopic to the rotational motion of the ammonia molecule changes. This temperature varies from 178 K for the Na-Ya to 189 K for the Na-Y and 218 K for the Na-X zeolite. This transition temperature is indicative of the interaction strength between ammonia molecule and zeolite. Indeed, the more important the interaction, the higher the transition temperature as it can be correlated with the infrared results. Table 3, Activation energy of diffusion, rotation angle and DQCC values for the Na-X, Na-Y and Na-Yd zeolites Na-X Na-Y Na-Yd Av (kHz) 58.4 55.3 59.8 DQCC (kHz) 77.9 73.7 79.7 fl(o) 35.3 36.5 34.8 E, (kJ/mol) 10 + 0.2 6.8 + 0.3 4.5 + 0.1 xc (s) 27 x 10.8 3 x 10.8 2.7 x 10.8 Transition Temp. (K) 218 189 178 -8o 0.412 0.350 0.322

1693

A -~.01

o o

~o

o

c

3.6

-

o

o

o

3.3-

o o

o

~'3.0-

o

o 0

~.01

0

o 0

0.tJ04' 0.006 '0.0030

o

o~

Oo

'

Ax

"

o o

o

0

~

2.7-

o

o

0

AYE

0.0645

1/Temperature (K-~)

o.;o4

2.4

0.01 T ' o.;~

o.oo3s

0.0040 0.1)645 1/Temp (K-1)

0.t)050

Fig. 8. The Arrhenius plot for NH3 in Na-Y

Fig. 7. T1 versus 1/temperature for NH3 in A" Na-X ; B "Na-Y and C 9Na-Yd Diffusion of ammonia molecules in zeolite framework at high temperature has been studied by the measurement of the spin-lattice relaxation time (Tj). The 2H-NMR spin-lattice relaxation times for the isotopic motion of deuterated ammonia adsorbed in Na-X, Na-Y and Na-Yo as a function of temperature are shown in Figure 7. Because of technical limitation, only a part of the parabola curve is observed for each sample. The decreasing part is present for the Na-X zeolite whereas the increasing part for the Na-Y and Na-Yo samples. The diffusion activation energy of the ammonia molecules can only be determined from the Arrhenius plot (Fig. 8 for Na-Y) of the Tl relaxation time. The activation energy, the observed quadrupolar constant and the fl angle values for three Na+-exchanged faujasite zeolites are presented in Table 3. We can observe that the activation energy for jumping from one adsorption site to other increases with the decrease of the Si/A1 ratio, in other words with increasing the negative charge of the oxygen atoms of the zeolite framework as shown in Figure 9. These observations are in quite good agreement with the infrared results presented above that the stronger interaction between ammonia and zeolite is observed for zeolite having the higher negative charge on oxygen atoms when the counter-ion is same and only the S i/A1 ration varies. From the 2 H - N M R study, it is obvious that the interaction of ammonia with Na-X material is stronger than that with the Na-Y zeolite. "~ 10 However, in 1996, a study on the behavior of benzene in Na-X and Na-Y zeolites revealed that ~ 8 the activation energy of diffusion in Na-Y (23.5 + 0.9 kJ / mol) was higher than that in Na-X (14 + 6 ~o 0.6 kJ / mol) [15]. Therefore, the mobility of .. benzene in Na-X is higher than that in Na-Y. our present results are contrary to what observed by 1.5 3.0 Bull et al in the case of benzene. This difference Si/AI ratio can be explained by the two different modes of host-guest interaction proposed. Indeed, it should Fig. 9. Evolution of the activation be reminded that ammonia can interact not only energy as a function of the Si/A1 ratio with cation but also with oxygen framework of

1694 the zeolite. This double interaction that depends both on the Lewis acidity of the counter-ion and as the basicity of the framework oxygen atoms whereas benzene molecule interacts with cation via its electronic cloud. The supplementary interaction observed between hydrogen atoms of ammonia and oxygen atoms of zeolite framework will certainly reduce the mobility of ammonia and increase the activation energy. 4. CONCLUSIONS The adsorption of ammonia in zeolites is the result of two interactions. The first one is the electrostatic interaction between the lone electron pair on the nitrogen of ammonia and the counter-ions of the zeolite framework. This interaction indirectly affects the N-H bond and increases with the Lewis acidity of cations. The other one is a hydrogen-bonding type between the oxygen atoms of zeolite and the hydrogen atoms of the ammonia molecules. This last interaction is stronger with increasing the negative charge of the oxygen atoms of the framework. However, the overall interaction strength is dominated by the cations-lone electron pair interaction. The ammonia decomposition observed previously by TPD-MS has been also detected by infrared. We have performed solid state 2H-NMR spin-lattice relaxation measurements on deuterated ammonia in the zeolites Na-X, Na-Y and Na-Yd over the temperature range 118308 K. The correlation times and activation energies decrease with decreasing Si/A1 ratio and increasing the negative charge on oxygen atoms. This represents a significant enhancement of ammonia mobility from Na-X to Na-Yd that is in good agreement with the infrared results. ACKNOWLEDGMENT F. Gilles thanks FNRS (Fonds National de la Recherche Scientifique) for a F.R.I.A. scholarship. This work was performed within the framework of PAI-IUAP 4/10. REFERENCES 1. R. Le Van Mao, N. T. Vu, S. Xiao, A. Ramsaran, J. Mater. Chem. 4(7) (1994) 1143 2. P.B. Venuto, Microporous Mater. 2 (1994) 297. 3. Jaimol T., Moreau P., Finiels A., Ramaswamy A. V., Singh A. P.,Appl. Catal.,A 214(2001) 1. 4. J. Horniakova, D. Mravec, M. Kralik, J. Lesko, P. Graffin, P. Moreau, Appl. Catal., A 215 (2001) 235. 5. H. KnSzinger in "Handbook of Heterogeneous Catalysis" Ertl G., Kn6zinger H. and Wertkamp J. (Editors), Weinheins, Germany, 1997. 6. D. Jaumain and B.-L. Su, CataL Today, 73 (2002) 187 7. B.-L. Su, V. Norberg and J. A. Martens, Langmuir, 17(4) (2001) 1267. 8. B.-L. Suand V. Norberg, Langmuir, 16(14) (2000) 6020. 9. F. Docquir, H. Toufar and B.-L. Su, Langmuir, 17(20) (2001) 6282. 10. F. Gilles, F. Docquir and B.-L. Su, Adsorption Science and Technology, D. D. Do (Ed.), Sidney (2000) 578 11. L. M. Bull, N. J. Henson, A. K. Cheetham, J. M. Newsam and S. J. Heyes, Jr. Phys. Chem. 97 (1993) 11776. 12. G. Herzberg in "Molecular spectra and molecular structure" Van Nostrand Reinhold Company, New York, (1945). 13. W.J. Mortier, J. Catal., 55 (1978) 138. 14. M. Br/indel and J. Sauer, Journal of Molecular Catalysis A : Chemical 119 (1997) 19. 15. S.M. Auerbach, L. M. Bull, N. J. Henson, H. I. Metiu and A. K. Cheetham, J. Phys. Chem. 11)0 (1996) 5923. 16. S.W. Rabideau and P. Walstein, J. Chem. Phys. 45(12) (1966) 4600.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1695

Characterization of Mesoporous Solids: Pore Condensation and Sorption Hysteresis Phenomena in Mesoporous Molecular Sieves Matthias Thommes l, Ralf K6hn 2, and Michael Fr6ba 3 1 Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, F1 33426, USA, E-mail: [email protected] 2 Institute of Inorganic and Applied Chemistry, University of Hamburg Martin-Luther-King-Platz 6, 20146 Hamburg, Germany 3 Institute of Inorganic and Analytical Chemistry, Justus-Liebig University of Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany We report results of an experimental study of pore condensation and hysteresis phenomena of nitrogen and argon in pristine MCM-48 and MCM-41 silica materials (pore diameter range: 3.5 - 5 nm). An analysis of nitrogen and argon adsorption/desorption isotherms obtained on a MCM-48 silica sample (pore diameter 4.6 nm) at 77 K and 87 K, respectively, leads to the conclusion, that - although MCM-48 consists of a unique, three-dimensional pore network- the desorption branch of the argon sorption hysteresis loop (type H1) corresponds to the pore diameter. However, the sorption hysteresis behavior for argon sorption at 87 K and 77 K appears to be peculiar in MCM-41 and MCM-48 materials of smaller pore diameters (i.e. in the range between 3.5 and. 4.2 nm). A clear correlation between the desorption branch of the observed argon hysteresis loops and the pore diameter is not possible anymore in this pore diameter range. Further, a detailed comparison of the pore condensation and hysteresis behavior in MCM-48 and MCM-41 silica materials of nearly equal pore diameters reveals that sorption hysteresis loops are wider in the pseudo-one-dimensional pores of MCM-41 compared to the three-dimensional pore system of MCM-48 silica. 1. INTRODUCTION Recent advances in the synthesis of periodic, mesoporous silica materials as for instance MCM-41, MCM-48, SBA-15 etc., have attracted much interest during recent years [ 1]. These materials are ideal candidates for the testing of theoretical models of adsorption, pore condensation and hysteresis, because of their uniform pore structure and morphology. Pore condensation represents a transition from a gas-like state to a liquid-like state in presence of a bulk fluid reservoir, which occurs at a pressure p less than the saturation pressure P0 at gasliquid coexistence of the bulk fluid. In this sense, pore condensation can be considered as a shifted gas-liquid phase transition due to confinement [2,3]. A characteristic feature associated with pore condensation is the occurrence of sorption hysteresis, i.e., capillary evaporation occurs at a relative pressure, which is lower than the capillary condensation pressure. Details of sorption hysteresis depend on the thermodynamic state of the pore fluid and on the texture of the adsorbent, i.e., the presence of a pore network. However, the origin of sorption hysteresis is still a matter of discussion [2]. There are essentially three models

1696 which contribute to the understanding of sorption hysteresis: (i) Independent (single) pore model [2,5] : Sorption hysteresis is considered as an intrinsic property of a phase transition in a single, idealized pore, reflecting the existence of metastable gas states. The hysteresis loop expected for this case is of type H1, according to the IUPAC classification [see ref. 6 ]. (ii) Network model [2,4]: Sorption hysteresis is explained as a consequence of the interconnectivity of a real porous network with a wide distribution of pore sizes. If networkand pore blocking effects are present, a hysteresis loop of type H2 is expected. (iii) Disordered porous material model: This model represents a more realistic picture, which takes into account, that the thermodynamics of the pore fluid is determined by phenomena spanning the complete pore system [7]; the thermodynamic metastability of low and high density phases of the pore fluid plays here also an important role. Pore condensation hysteresis observed in MCM-41 is a prominent example, where hysteresis can be explained by the independent pore model, i.e. the occurrence of metastable states of the pore fluid associated with pore condensation. However, hysteresis is much more complex in disordered porous materials and in addition to the thermodynamic metastabilities of the pore fluid, aspects described in the models (ii) and (iii) have to be considered as well. As indicated above, it is still not sufficiently known how factors like the pore geometry, the dimension of confining space and the texture of the porous material affect the mechanism of pore condensation, the nature of hysteresis and thus the shape of the sorption isotherm. A better understanding of these problems is crucial with regard to a comprehensive pore size analysis of mesoporous materials. In this paper we focus on some important aspects of the pore condensation and hysteresis behavior of nitrogen and argon in MCM-41 and MCM-48 silica materials of pore diameters in the range between 3.5 and 5 nm. We also compare the pore condensation and hysteresis behavior of argon in the pseudo-one-dimensional pore system of MCM-41 silica materials with appropriate results obtained in the three-dimensional pore network of MCM-48 silica. 2. EXPERIMENTAL The MCM-48 and MCM-41 silica materials were synthesized by standard procedures described elsewhere [8]. Variations in pore diameter were obtained by utilization of tetraalkylammonium bromides with different chain length; details can also be found in ref. [8]. The sorption isotherms of nitrogen and argon at 77 K (i.e. 77.35 K) and 87 K (i.e. 87.29 K) were determined in a pressure range from p/p0=0.025 to 1 using a static volumetric technique (Autosorb I C, AS1 software version 1.24, Quantachrome Instruments, Boynton Beach, USA). Experimental details are described elsewhere [9,12]. 3. RESULTS AND DISCUSSION Different methods were used to analyze the nitrogen (at 77 K) and argon sorption isotherms (at 87 K) shown in figures 1 - 7 to obtain surface and pore size characteristics for the MCM-41 and MCM-48 silica materials. The results are summarized in table 1. BET surface areas were determined from nitrogen isotherms at 77 K in a range of relative pressures (P/P0) prior to the occurrence of pore condensation (MCM-41A: 0.05 - 0.17; MCM-41B: 0.05 0.2; MCM-41 C, MCM-48D, and MCM-48E: 0.05 - 0.3) by assuming a cross-sectional area

-

1697 Ads. SBE'T,N2 [m2/g]

MCM-41B

MCM-41C

MCM-48D

MCM-48E

1070

897

982

1092

SBE-r,Ar [mZ/g]

903

775

781

918

dp,Nz(BJH) [nm]

2.54

3.03

2.91

3.33

dp,Nz(NLDFT) [nm]

3.66

4.25

4.09

4.57

0.76 (P/po. = 0.40)

0.77 (P/Po = 0.42)

0.81 (P/po = 0.39)

0.97 (P/Po = 0.43)

Vp,N2 [ 10-6m3/g]

Table 1. Characterization of MCM-41 and MCM-48 silica with respect to specific surface area (SBET), pore volume (Vp) according to Gurvich, mode pore diameter according to the BJH method (dp, N2(BJH) and DFT (dp, N2-NLDFT). of 0.162 nm 2 for N2. In addition, BET surface areas were also calculated from the argon sorption isotherms (at 87 K) in the above given relative pressure ranges by assuming the cross-sectional area of Ar as 0.138 nm 2 [6]. As also reported by other groups [9] the BET specific surface areas derived from nitrogen isotherms are significantly higher (ca. 15- 20 %) than the BET surface areas obtained form argon sorption isotherms at 87 K. The reasons for this observation are discussed elsewhere [9,10], but the argon surface areas are here considered to be more realistic [9,10]. The pore volumes (Gurvich) given in table 1 were calculated from the plateau of the sorption isotherms after pore condensation has occurred in the primary mesopores of the MCM-41 and MCM-48 silica samples. The desorption points were used for pore size analysis by applying the methods of Barrett, Joyner and Halenda (BJH) [6] and Nonlocal Density Functional Theory (NLDFT) [11]. As to be expected, the pore diameters obtained from the application of the N2-NLDFT method are at least 1 nm higher as compared to the N2-BJH results [ 11 ]. Figure la shows the sorption isotherms for nitrogen and argon sorption in the MCM-41B sample at 77 K and 87 K, respectively. Pore condensation occurs at a smaller relative pressure in case of the nitrogen adsorption/desorption isotherm, indicating that here the effective adsorption potential for nitrogen sorption on silica is more attractive as compared to the argon/silica system. The pore condensation is much sharper in the argon isotherm and the amount adsorbed after the pores are filled, is higher because of the smaller size of the argon molecule and the higher density of liquid argon (Pl = 1.396 g c m -3, at 87.29 K) compared to the liquid nitrogen density (pl = 0.870 g cm3). Both isotherms are reversible, i.e., no sorption hysteresis can be observed and the pore size obtained does of course not depend on whether adsorption or desorption data are used for the calculation. This is illustrated in Figure l b, where it is shown that the NLDFT pore size distributions obtained from the adsorption branch of the reversible argon isotherm and the desorption branch of the reversible nitrogen isotherm agree very well. The two NLDFT-models used for this pore size analysis were developed from the authors of ref. [ 11 ]. These models are dedicated to describe nitrogen sorption (at 77 K) and argon sorption (at 87 K) in siliceous pores of cylindrical pore geometry. Whereas the pore condensation step observed in reversible sorption isotherms is considered to represent the equilibrium vapor-liquid transition [5], the situation is more complicated if a hysteresis loop is present, as in case of argon sorption at 87 K in MCM-48E. Fig. 2a. shows this sorption isotherm in comparison with the nitrogen isotherm obtained at 77 K, which does not exhibit sorption hysteresis. Hence, it is interesting to compare pore size

1698 60O 500

~

:E 400

~

0.20-

"--am N2: des

0.15

~Ar:

/,~

ad

0.10

~'300 0.05 ~200 100

~

0.0

-----am N2 (77 K) ~ Ar (87 K)

0.00 ~ ~ ~ ~ "

012 ' 0:4 ' 0:6 ' 018 ' 110

2.0

'

i

2.5

RELATIVE PRESSURE P/Po

Fig. la. N2 (77 K) and Ar sorption (87 K) in MCM-41B. 900

,

310

,

3.'5

,

!

4.0

,

415

,

PORE DIAMETER [nm]

Fig. lb. NLDFT pore size distributions from Ar adsorption and N2 desorption. 0.30

N2: ads

A |

/~/ IIi

----a---N2:

des

I1~1

~ 6~176 E OOl

_~ o.~o

4001

~3ooI ~ ~ -

200-] ~ z x 100 t,, 0.0 0:2

--.--ads---o--des / N2 (77 K)[

+ 0:4

~ 0:6

Ar (87 K) / 0:8 1:0

RELATIVE PRESSURE P/P0

Fig. 2a. Nitrogen and argon sorption isotherms of MCM-48E obtained at 77 K and 87 K.

~, o.,o

~' 0.05 0.00

3.o

315'4'.o'4'.5'51o'515' PORE D I A M E T E R lnml

Fig. 2b. NLDFT pore size distributions obtained from the isotherms shown in Fig.2a.

and pore size distribution curves obtained from nitrogen adsorption/desorption and argon adsorption/desorption isotherms using again the appropriate equilibrium NLDFT-models for argon sorption (87 K) and nitrogen sorption (77 K) in cylindrical silica pores. As clearly shown in figure 2b, the pore size distribution curve obtained from the argon desorption branch is in perfect agreement with the results obtained from the adsorption/desorption branches of the reversible nitrogen isotherm, whereas however significant deviations are observed in case the argon adsorption branch is chosen for the NLDFT pore size analysis. Hence, - although MCM-48 silica consists of a unique, three-dimensional pore network- the desorption branch of the hysteresis loop corresponds to the pore diameter, which is in accordance to similar results obtained for the isolated, cylindrical pores ofMCM-41 silica [5,11].

1699 800 700 600

---o-9 ---b--

Ar Ar Ar Ar

(87 K): (87 K): (77 K): (77 K):

ADS DES ADS D

'~ 500 '400

~

300 200 100 00

0.2

0.4 0.6 RELATIVE PRESSURE

P/Po

08

Fig. 3. Argon sorption isotherms obtained in MCM-41C at 87 and 77 K. (The P0 value at 77 K corresponds to undercooled liquid Ar, i.e., 230 Torr). Accordingly, the origin of sorption hysteresis seems to be here primarily associated with the development of metastable fluid states during the pore condensation process, and capillary evaporation (i.e. the desorption branch of the hysteresis loop) represents the equilibrium gas-liquid phase transition (independent pore model of hysteresis). However, the pore condensation and hysteresis phenomena appear to be more complicated for materials consisting of smaller pores as in the case of MCM-48E, i.e., pore diameters smaller than 4.6 nm. For instance, the argon (87 K) sorption isotherm for MCM-41C (see Fig. 3) reveals a triangular hysteresis loop of type H2, similar as in case of the presence of pore blocking effects.At 77 K the hysteresis loop is more pronounced as to be expected because of the lower temperature [10,12], but in contrast to 87 K the sorption isotherms exhibits now parallel adsorption and desorption branches, i.e. type H1 hysteresis. Thus, the shape of the hysteresis loop for MCM-41C changes from H2 to H1 just by a variation of temperature, indicating that phenomena like pore blocking are not dominant for the H2 hysteresis observed at 87 K. The H2-type hysteresis observed at 87 K may result from the proximity to a lower limit of pressure (P/P0 ca. 0.38, i.e., tensile strength effect) below which hysteresis cannot occur anymore [see also ref. 9]. In addition, a detailed comparison of pore condensation and sorption hysteresis in the three-dimensional pore network of MCM-48, and the one-dimensional pores of MCM-41 silica (independent cylindrical pores) has been performed. Figures 4 and 5 show nitrogen sorption isotherms in MCM-41C and in two MCM-48 silica materials together with the appropriate NLDFT pore size distribution curves. The surface and pore size characteristics of MCM-48D and MCM-41C are given in table 1 (for a characterization of MCM-48C: see ref.[12]). MCM-48D and MCM-41C have similar mode pore diameters (slightly smaller for MCM-48) and narrow pore size distribution curves, whereas MCM-48C is less ordered, which leads to a pore size distribution curve, which is much wider as compared to the one obtained for MCM-41C. The nitrogen sorption isotherms reveal pore condensation steps, but no hysteresis, however hysteresis could be observed for argon sorption at 87 K and 77 K. The influence of temperature- and pore size on the occurrence of hysteresis in MCM-48 and MCM-41 is discussed elsewhere [9,12].

1700

800

~E

'~ 0.2-

600

0.1-

400 d -~ ads

200-

des

~o--

MCM-48D

~z~

MCM-41C

0.0.

0.0

012'014'016'018'1.

.

3.0

'0

70o

!

.

4'.0

4'.5

,

5.0

PORE DIAMETER [nm]

RELATIVE PRESSURE P/P0

Fig. 4a. N2 sorption: MCM-41C, MCM-48D.

.

3:5

Fig.4b. NLDFT pore size distributions. 0.2-

_., 600 500 400 =1 300

y

0.1-

ads

des

0.0-

0.0'012'014'016'018'110 RELATIVE PRESSURE P/P0

Fig. 5a. N2 sorption: MCM-41 C, MCM-48C.

3.0

'

15

3.

'

I

4.0

,

,

4.5

,

PORE DIAMETER [nm]

Fig. 5b. NLDFT pore size distributions.

Figure 6 shows a comparison of argon sorption isotherms at 77 K obtained in MCM-41 C and MCM-48 D (P0 corresponds here to solid argon and was continuously measured during the experiment). The width of the sorption hysteresis loop observed in MCM-48 D is significantly smaller compared to MCM-41C and is located within the broad hysteresis loop of MCM-41C. It is also interesting to note, that capillary evaporation of argon (i.e., the desorption branch) occurs now for MCM-41 at a smaller relative pressure compared to MCM-48 D. A qualitatively similar behavior is observed, if one compares the argon sorption behavior at 77 K in MCM-48C and MCM-41C. As indicated in figure 7, capillary evaporation occurs for MCM-48C and MCM 41C at essentially the same relative pressure. In contrast, the pore condensation/evaporation steps of the reversible nitrogen isotherms occur for MCM-41C at a higher relative pressure as compared to both MCM-48 silica. Accordingly, one has to assume, that in contrast to the situation for larger mesopores (the MCM-48 E sample of 4.6 nm pore diameter), the desorption branches of the argon sorption isotherms obtained in smaller

1701 700 -

700

600 -

600 ,~ 500

4~ 500-

400

400-

~ ~

~ ~

300

~200

adL 0.2

0.4

=~

ads des / __~_._ ~MCM_48C/ ~ MCM-41C//

.___dd et~__sMC M_48 D

200- ~

0.6

100 ,, 0.0 ' 012 ' 014 ' 0;6 ' 0'.8 ' 110

100 0.0

300-

0.8

1.0

RELATIVE PRESSURE P/Po Fig. 6. Ar sorption (77 K): MCM-48D/-41C.

RELATIVE PRESSURE P/Po Fig. 7. Ar sorption (77 K): MCM-48C/-41C.

mesopores are not clearly correlated with the pore diameter. A similar behavior was found for argon and krypton sorption at 87 K [15]. The observation of wider argon hysteresis loops in MCM-41, as compared to MCM-48, and the peculiarities associated with the location of the desorption branch, may be caused by different reasons: (i) Differences in texture and pore structure: The pore diameter of MCM-41 is slightly higher and an increase of the width of the hysteresis loop with increasing pore diameter is expected for both, MCM-48 [12] and MCM-41 [13]. Small differences in the width of the pore size distribution could in principle play a role, but as indicated in figure 5b, MCM-48C exhibits a much wider pore size distribution curve as compared to MCM-41, whereas MCM-48D exhibits a slightly sharper pore size distribution, as compared to MCM41 C, i.e. these effects are apparently not dominant with regard to the observed differences in the hysteresis behavior. It should also be noted, that in contrast to MCM-41 C, the sorption isotherms obtained on both MCM-48 materials indicate a small secondary hysteresis loop, which is caused by interparticle condensation. In the case of nitrogen sorption the regimes of primary mesopore condensation and interparticle pore filling regions are separated. However, these two regions merge to one hysteresis loop in case of argon sorption at 77 K. The question here is, whether the evaporation of argon from the interparticle voids could affect the argon capillary evaporation process from the primary mesopores of MCM-48, i.e. the location of the desorption branch of the hysteresis loop. A very recent Monte Carlo simulation study for Xe sorption in cylindrical silica pores (pore diameter range: of 3 - 4 nm) indicated, that special aspects of the pore geometry (e.g., closed-ended-, open-ended- and single-ended pores) and the pore length could also have an effect on width of the hysteresis loops, and in particular on the location of the desorption branch [14]. However, the relevance of these results for our experimental observation has to be investigated in more detail. (ii) Differences in the dimensionality of the pore space: Phase transitions of fluids confined to such narrow mesopores are expected to be affected by the dimensionality of the confined pore space. For instance, phase transitions in narrow pseudo-one-dimensional geometry (e.g. cylindrical pores) are expected to be rounded due to finite size effects and the shape of the coexistence curve of a confined fluid is expected to be different for a pseudo-one-dimensional geometry compared to a three-dimensional system [2]. It can be expected, that such effects have also an

1702 influence on the width of the hysteresis loop, associated with the gas-liquid phase transition. Further experimental and theoretical work is necessary to achieve a more comprehensive understanding of the observed differences in sorption hysteresis behavior between MCM-48 and MCM-41 [ 15]. 4. CONCLUSIONS The pore condensation and hysteresis behavior of nitrogen and argon was studied in pristine MCM-48 and MCM-41 silica samples in the pore diameter range from 3.5 and ca. 5 nm. An analysis of nitrogen and argon adsorption/desorption isotherms at 77 K and 87 K obtained in a MCM-48 silica (pore diameter 4.6 nm) leads to the conclusion, that in this welldefined, interconnected pore network the desorption branch of the observed H1 hysteresis loop is correlated with the pore diameter. This is in agreement with the assumption, that hysteresis is due to the occurrence of metastable states associated with pore condensation (i.e. adsorption branch of the hysteresis loop) as it was also found for the pseudo-one-dimensional pore system of MCM-41 [e.g,.5]. However, the sorption hysteresis behavior for argon sorption at 87 K in MCM-48 and MCM-41 silica materials appears to be somewhat peculiar for smaller pores, i.e. the pore diameter range between 3.5 nm and ca. 4.2 nm. Our results indicate, that in this region a clear correlation between the desorption branch of the observed argon hysteresis loops and the pore diameter is not possible anymore. A detailed comparison of pore condensation and hysteresis in the three-dimensional pore network of MCM-48, and the one-dimensional pores of MCM-41 silica materials of nearly equal pore diameter, reveals that the widths of hysteresis loops appear to be significantly wider for MCM-41 compared to MCM-48. These observations could be related to the differences in the texture of MCM-48 and MCM-41, but more experimental and theoretical work is needed in order to achieve a comprehensive understanding of the experimental observations. 5. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

U. Ciesla and F. Sch~ith, Microporous Mesoporous Mater. 27, (1999), 133. L.D. Gelb, K. Gubbins, etal.,Rep. Prog. Phys. 62, (1999), 1573. M. Thommes and G.H. Findenegg, Langmuir 10, (1994), 4270. S. Cordero et al, Studies in Surface Science and Catalysis 128, (2000), 121. A.V. Neimark, P.I. Ravikovitch, A.Vishnyakov, Phys. Rev. E 62, (2000), R1493. F.Rouquerol, J. Rouquerol, & K. Sing, Adsorption by Powders & Porous Solids, Academic Press: 1999. L. Sarkisov and P.A. Monson, Studies in Surface Science and Catalysis 128, (2000), 21. RalfK6hn, Dissertation, Universitaet Hamburg, 2001. M. Kruk and M. Jaroniec, Chem. Mater. 12, (2000), 222. M. Thommes, R. K6hn, M. Fr6ba, Applied Surface Science, in press (2002). A. V. Neimark, P. I. Ravikovitch, Microporous and Mesoporous Materials 44-45, (2001), 697; A.V. Neimark, P.I. Ravikovitch, M.Grfin, F. Sch~ith, and K.K. Unger, J. Coll. Interface Sci. 207, (1998), 159. M. Thommes, R. K6hn, and M. Fr6ba, J. Phys. Chem. B. 104, (2000), 7933. C. G.Sonwane, S. K. Bathia, N. Calos, Ind. Eng. Chem. Res. 37, (1998), 2271. L. D. Gelb, Molecular Physics, accepted (2002). M. Thommes, R. K6hn, and M. Fr6ba, to be published, (2002).

NATURAL ZEOLITES

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1705

Ion exchange selectivity of phillipsite Alessandro F. Gualtieri, E lio Passaglia, Ermanno Galli Dipartimento di Scienze della Terra, Universit~ di Modena e Reggio Emilia, Via S.Eufemia 19, 41100 Modena, Italy Phillipsite is one of the main components of the sedimentary zeolite-rich rocks called

zeolitites used in a variety of applications (i.e., in the field of pollution abatement and

agriculture). The interest for the understanding of the cation exchange properties for a proper and efficient use in industrial applications prompted in 1995 a long term project on the systematics of cation exchanged phillipsites, their crystal chemistry and properties. Natural phillipsite samples with different Si/(Si+AI) ratio were selected for the research project and exchanged with a number of either monovalent and divalent cations. In this paper we describe a general picture of the changes which occur in the structure of phillipsite in response to the cation exchange either in terms of framework relaxation/contraction and cation migration, ion exchange selectivity and properties, and their prediction. I. INTRODUCTION Phillipsite [PHI] is a natural monoclinic zeolite with a general formula K2(Cao.5,Na)4 (AI6SiI0032).12H20 [1]. The framework is built up by layers of tetrahedra with eight- and four-rings roughly perpendicular to the a axis. The layers are vertically linked by 4-rings forming double crankshatts with the 4-rings of the layer. Its crystal structure was firstly solved in the space group B2mb [2-3] which corresponds to the topological symmetry, and then refined in the space group P2dm [4] with a~,-9.8 A, b~14.0 A, c~8.6 A, 13~124~ and two extraframework sites (I and II). Site I on the mirror plane (010) along one of the larger channels of the type I octagonal prism, is usually populated by large size cations such as K + or Ba2+. Site II is in a general position in the cage formed by type II octagonal prism and adjacent type I octagonal prism. This site is usually populated by Ca 2+ and Na +. Another site was recently discovered and labelled II' [5-6]. The position of the extraframework sites is shown in Figure 1. Besides clinoptilolite and chabazite, phillipsite is one of the main components of the sedimentary zeolitized tufts (zeolitites) used in a variety of applications and especially in the field of pollution abatement and agriculture. The interest for the understanding of the cation exchange properties for a proper and efficient use in industrial applications prompted in 1995 a long term project on the systematics of cation exchanged phiUipsites, their crystal chemistry and properties. Natural phillipsite samples with different Si/(Si+AI) ratio were selected for the research project and exchanged with a number of monovalent and divalent cations [5-11 ]. This contribution is the last step of the project. Using the now available structural data of all the cation exchanged phillipsites, it is possible to describe a general picture of the changes which occur in the structure of phiUipsite in response to the cation exchange either in terms of

1706 fi'amework relaxation/contraction and cation migration, ion exchange selectivity and properties, and their prediction. 2. EXPERIMENTAL S E C T I O N The samples selected for the project are (1) Vallerano (Rome) with R=Si/(Si+AI)=0.63; (2) Perrier (Puy du D6me,France) with R=0.72; (3) Monte Lungo (Berici, Vicenza) with R=0.76. The structural characterization of the cation exchanged forms exchanged with N a+, Ca ++, K +, Ba++, Sr++, Cs+, NH4+, Mg ++ and Li+ is described in the cited references [5-11]. Cation exchanged forms were obtained using mono-cationic chloride solutions [6] and the sample characterization of the natural and exchanged forms was accomplished using microprobe analyses, SEM, thermal analyses, and FTIR. The structures were refined using the Rietveld method. The powder data of the exchanged sample were collected using a conventional lab source for the Na +, Ca++, K +, Ba~, Sr++, Cs+, Mg ++and Li+ forms and synchrotron radiation for the NH4+ form. Data were all refined using the GSAS sottware package [12].

L

t/ _

i

i

]_

ii

ii

i|1 ii

b ii

i

Fig. 1. Position of the extmframeworkcations in phillipsite: black spot=site I; grey spot=site II; white spot=site II'; W1 ...W5 = sites of the water molecules.

1707 3. RESULTS AND DISCUSSION The systematics on the crystal chemistry of cation exchange phillipsites allowed to define the ion selectivity of phillipsite. The nature of the cation in the extmframework sites is mainly ruled by its size as shown in Figure 2. Site I may host K +, Ba ++, Sr++, Cs +, NI-h +. The average coordination number and average (I)-O distance are rather variable (9-12 and 2.81-3.21 A) mainly depending on the nature of the cation and number of water molecules. Although there are always at least five connections with the framework oxygen atoms of the cradle (O1, 03, 05, 08, and O9), the cation may not be found at centre of gravity of the cradle. Only K + and Cs + are located more or less at the centre of gravity as they have connections with all the surrounding framework oxygen atoms (see above labels). NH4 + instead is shifted towards 0 9 and do not form bonds with 08. Sr++ is even more shifted upward as it is not connected to 03, 0 8 and 09. The anomalous distribution of Sr ++ is the cause for the cell modification in sample (1) with 2.63 Sr ++ atoms per unit cell. The cell is doubled with a'=16.503(9) and pseudo-B centred with Sr ++ atoms distributed in an ordered way in the two adjacent cavities. Site II is not selective as it may host Na +, Ca ++, K +, Ba ++, Sr ++, Cs +, Nq-h+, Mg ++ and Li+. The average coordination number is 6-8 with an exceptional 9-fold coordination for K + in the Kexchanged sample (3). The average (II)-O distance is consequently highly variable (2.38-3.39 A) depending on the nature of the cation. Site II' may host Na +, Ca ++, Mg ++ and Li+. The configuration with both site II and II' is energetically favoured with respect to a configuration with the site II only (that is, with a population > 50%) because the cation positive charge is more evenly distributed over a larger number of framework oxygens.

o

A

.............

[~

site I site II site I!'

v

w

ionic r a d i u s (A)

Fig. 2. Occupancy of the phillipsite extrafmmework sites depending on the ionic radius. Lower line= not populated. Upper line=populated.

1708 ........ ,~,,,,, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

,~

...........

t

....

: ::

:::

: :

:

. . . . . . . . . . . .

~,,o1

__

I I

I

io.l . !o., 1 |

i i

i

~

.....

s

o...I

e~

:

-

I I

!

,

Fig. 3. When the population of site II+II'ca.0.5, site II' is populated (right column). See the text for the point indicated by the arrow. It is possible to predict a priori whether the site II' was populated by the so called atomic sterical occupancy coefficient C (=crd with c and r, the atomic charge and ionic radius of the atom in site II, and d, the average cation-anion distance) [7]. It was found that when C>5.73, site II' is empty. Besides it is clear from Figure 3 that whenever the population of site II+II' exceeds ca. 0.5, the cation preferably migrates from II to II'. The point indicated by the arrow in Figure 3 corresponds to the Li-exchanged sample (1) where Li § fully occupies site II' and (Na+Ca) migrates at the window intersecting channels running along _aand b.

1,6

!

1,4

-

1,2

-

.................................

1,0-

O

o,.

t

"

. . . . . . . . . . . . . . . . . . . . . . . .

O

0,6 0,4 0,2

I .....

,

9

, ....

,

,

........

,

. . . . . . . . .

---

:,

.............

Fig. 4. Plot of the bond strength (BS) of site II vs. occupancy of site II'. Left column=not populated; right eohann=populated.

1709 The plot in Figure 4 shows that whenever the calculated bond strength of site II exceeds 0.7, site II' is not populated. On the other hand, when BS(II)<0.7, the positive charge compensation offered by site II only is not sufficient and a configuration with site II+Ii' is favoured for an optimal compensation of the negative framework charge required at the bottom of the cavity. The framework oxygens heavily involved in the net charge compensation in cavity of type 1I are O1, 03, 06, 07. As a cons~luence of the structure control of phillipsite over ion selectivity, we have found that Na, Ca, Mg, and Li exchange is invariably incomplete because they cannot populate site I; K::>Na exchange is sometimes incomplete because Na may be retained in site II'. At R=0.63, Cs, NH4, Ba, Sr exchange is complete because they populate either site I and II. At R=0.75, Na is invariably retained in site II'. If the number of extraframework cations decreases more room is available for water usually distributed in a disordered way. R is possible to predict the number of water molecules W in a structure by the total cation steric volume TCSV=4/3x[~rtw~with r, ionic radium of atom i and w, the weight fraction of that atom in the unit cell, using the linear relationship W=O.531(TCSIO+I5.1with R2=0.848 (Figure 5). Using this full structural data set of phillipsite, we are able to predict the efficiency and ion selectivity of phillipsite for any element. The flow chart in Table 1 reports the possible predictable information for any ion exchange of phillipsite. With the microprobe analysis of the phillipsite phase in the zeolitite, we are able to predict the distribution of the cations in sites I, II, and eventually II' (diagrams in Figure 2, 3, and 4). If we take an exchanging ion (lat's say a heavy metal or others), we are able to predict the degree of exchange (ion selectivity) for each site in the structure.

~81 16

:

:

W= -0.531(TCSV)+15,1 r~_.0,848

I

14

12

10

6

i

0

.

,

,,i

2

,

,

i'

-i

4

6

,

w

|

'i'

8

10

12

14

Total cations steric volume (TCSV)

,,

,

,

,

,,,

,,

,,,

,,

16

,

,

,

_L

Fig. 5. Plot of the TCSV (see text for details) vs. the predicted number of water molecules in the structure ofphillipsite.

1710 Thus, we have an indirect theoretical CEC. Besides, knowing the number of exchanged ions, we are also able to predict the number of water molecules in the exchanged phillipsite (diagram in Figure 5). This model has been applied for the prediction of Pb 2+ ion exchange (see Table 2) and the results predict very well the behavior reported in the literature [13-14]. In fact, Table 3 clearly shows the Pb 2+ selectivity of phillipsite is very high. Accordindly, Pansini at al. [13] reported that column runs data support the possibility of performing a massive lead removal from large volumes of water by using moderate quantities ofphillipsitecontaining materials. In agreement, Shanableh and Kharabsheh [14] found that the phillipsite additive enhanced the sorption capacity of the soil and achieved satisfactory leaching reduction results for Pb. Recently, Colella et al. [15] have also experimentally proved that phillipsite is highly performant as far as the Pb-exchange is concerned. On the other hand, Shanableh and Kharabsheh [14] reported poor leaching reduction results for cadmium and nickel. Again this result is easily predicted using our model since the ionic radius of both cations is smaller than 1 A so that the cannot exchange for K (or other large cations) in site I. As a consequence, the cation exchange is incomplete. In addition, although cadmium and nickel may exchange cations in site II, residual Na in expected in site II'. 4. CONCLUSIONS This paper is the last step of a long term project on the systematics of cation exchanged phillipsites, their crystal chemistry and properties using natural phillipsite samples with different Si/(Si+AI) ratio exchanged with a number of either monovalent and divalent cations. A conclusive general picture on the changes which occur in the structure of phiUipsite in response to the cation exchange either in terms of framework relaxation/contraction and cation migration, and ion exchange selectivity was presented. Table 1. Flow chart for the prediction of ion exchange in phillipsite. Type of phillipsite rich ze01itite

(limitation~nly PHI for nowt) .

.

.

.

.

.

.

l

Type of application = tI i .....................................

. . . .

type ofexchanging ~on [

Microprobe analysis of the zeolite if not available

..................

Cation number and predictable distribution in the structure

........................................................

~

i

P

rediction of the distribution of lhe exchanging ion in the structure

l

................................................................................................................................................................................... Prediction ofthe number of water molecules, ion selectivity and CEC for that ion ! LS

1711 Table 2. Flow chart for the prediction ofPb 2+exchange in phillipsite. ! Typeofz~ id~iwtth i [ vauermop~ I

.N.,~ ...~.c~.~,~.... . . . . . . . . . . . . . . . . . . . . .

. Available microprobe analysis:

K in s~e I i i K. Ca in site II i i Na in sfte 11' (slte pop..s~l.) ! ......... j

i Type of applkatloa = PI~" ionic rsdim (CN 6 - 8 ~ I 0 . . ~ s L.. .

~.~

. . . . . . . . . . . . . .

t ' .~

Pb=~K, C a ~ l l ~ ~ )

11 I Predicted W=10.7 (r

check TG). complete Pbexcbanp in tke ~ ~

~

!

Using the now available structural data base, we are able to predict the efficiency and ion selectivity of phillipsite for any element. The example of Pb 2+ was presented. Our approach with the structural data modelling is complementary to the experimental technique of ion adsorption (exchange). The coupling of the two methods yields a full clear picture of the ion exchange in phillipsite because on one hand, adsorption curves interpret the kinetics of the process of ion adsorption and allows to draw the selectivity patterns, and on the other hand, structure refinements interpret the process of ion exchange a at a molecular level. Since Italian tufts (e.g. Neapolitan yellow tuff) commonly contain a mixture of phillipsite and chabazite [16, 17] it is important in view of a possible application to study also the crystal chemistry of natural cation exchanged ehabazite to predict the ion exchange in the real cases of zeolite rocks. REFERENCES

1. G. Gottardi, E. Galli, in: Natural Zeolites, Springer, Berlin, 1985, p. 409. 2. R. Sadanaga, F. Marumo and Y. Tak6uchi, Acta Cryst. 4 (1961) 1153. 3. H. Steinfink, Acta Cryst., 15 (1962) 644. 4. R. Rinaldi, J.J: Pluth, and J.V. Smith, J.V., Acta Cryst., B30-10 (1974) 2426. 5. A.F. Gualtieri, E. Passaglia, E. Galli, in: Proc. of Zeolite 97 International Conference, Ischia (Naples, Italy), Natural Zeolites for the Third Millenium. C. Colella and F.A. Mupton eds., (2000) 93.

1712 6. E. Passaglia, A.F. Gualtieri, E. Galli, in: Proc. of Zeolite 97 International Conference, Ischia (Naples, Italy), Natural Zeolitesfor the Third Millenium. C. Colella and F.A. Mupton eds., (2000) 259. 7. A.F. Gualtieri, E. Passaglia, E. Galli, A. Viani, Micro. And Mesoporous Mat. 31 (1999) 33. 8. A.F. Gualtieri, D. Caputo, C. Colella, Micro. And Mesoporous Mat. 32 (1999) 319. 9. A.F. Gualtieri, Mat. Science Forum (2000) in press. 10. A.F. Gualtieri, Acta Cryst. B56 (2000) 584. 11. A.F. Gualtieri, in: Proc. of the 13th International Zeolite Conference, Montpellier, France, 8-13 July 2001, Zeolites and mesoporous materials at the dawn of the 21 st century, Studies in Surface Science and Catalysis, A.Galameau, F. Di Renzo and F. Fajula eds., 135 (2001) 147. 12. A.C. Larson, 1LB. Von Dreele, in: GSAS, LANL, Los Alamos, NM, (2000), p. 86. 13. M. Pansini, C. Colella, D. Caputo, M. de' Gennaro, A. Langella, Microporous Mat., 5 (1996) 357. 14. A. Shanableh and A. Kharabsheh, J. of Hazardous Mat., 45 (1996) 207. 15. C. Collr E. Torracca, A. Colella, B. de' Gennaro, D. Caputo and M. de' Gennaro, in: Proc. of the 13th International Zeolite Conference, Montpellier, France, 8-13 July 2001, Zeolites and mesoporous materials at the dawn of the 21 st century, Studies in Surface Science and Catalysis, A.Galameau, F. Di Renzo and F. Fajula eds., 135 (2001) 148. 16. M. de' Gennaro, C. Colella, E. Franco and R. AieUo, Ind. Miner., 186 (1983) 47. 17. A.F. Gualtieri, E. Passaglia and E. Marchi, in Porous Materials in: Environmentally friendly processes. Studies in Surface Scince and Catalysis, I. Kiricsi, G. P/fl-Borb61y, J.B. Nagy, H.G. Karge eds. 125 (2000) 707.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1713

Sorption of Ammonia from Gas Streams on Clinoptilolite Impregnated with Inorganic Acids K. Ciahotn~,a, L. Melenovha, H. Jirglovha, M. Boldigb and M. Ko~,ifikb a Institute of Chemical Technology, Technickh 5, 166 28 Praha 6, Czech Republic, e-mail: karel, ciahotny@vscht, cz b j. Heyrovsle) Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, 182 23 Praha 8, Czech Republic

The contribution deals with ammonia-removal from waste air by adsorption using lowcost sorbents. The natural zeolite clinoptilolite from the deposit Ninny, Hrabovec, Slovakia was tested for ammonia removal from the waste air. Measurements of breakthrough curves for ammonia were performed using fixed beds of clinoptilolite (i) in its natural form and (ii) on clinoptilolite samples pre-treated with acids. The parameters of the experiments were selected to simulate the conditions ruling in animal breeding farms. The pre-treatment of clinoptilolite with acids increased sorption capacity of clinoptilolite for ammonia. 1. INTRODUCTION Ammonia represents a frequent pollutant of air atmosphere and its occurrence leads to the eutrophication of water streams. Significant sources of ammonia pollution are due to agricultural production. Ammonia is also a cause of a bothersome odour in the neighbourhood of animal breeding farms. On the other side, ammonia is an important source of nitrogen for crops production. For this reason there is a permanent interest in searching for ways to utilise the ammonia from animal breeding in crops farming. Conceivable processes to recover ammonia from air are first of all those based on adsorption. The world-wide emissions of ammonia are estimated at 25 to 35 rail t/a. Cca 85 % of this amount is produced in the agriculture. To reduce the ammonia-emissions and the emissions of other gaseous pollutants, the ,,Proposal for a Directive of the European Parliament and of the Council on national emission ceilings" [1] for certain atmospheric pollutants has been prepared. In accordance with this directive, the ammonia pollution should be significantly reduced by the year 2010 (see Tab 1.). The total NH3 emission level was evaluated in 1990 5472.15 kt NH3/a. The Table 1 shows the proposal of ammonia emission abatement in selected countries in 2010. Thus, the world-wide abatement of ammonia emission in 2010 is estimated to 1 mil ton NH3/a. Suitable low-cost adsorbents for this purpose can be found among natural zeolites. Among them, the clinoptilolite appears to be the most promising one from the point of view of its potential use.

1714 Table 1 Examples of proposals for the abatement of ammonia pollution Emission level NH3 Emission limit NH3 Country (1990) (2010) (kt/a) (kt/a) Austria 81 66 Belgium 107 74 Bulgaria 144 108 Czech republic 156 101 Denmark 122 69 Germany 764 550 Hungary 124 90 Italy 466 419 Netherlands 226 128 Slovakia 62 39 U.K. 333 297

Emission decrease (2010/1990)

(%)

- 19 - 31 - 25 -35 -43 -28 -27 -10 -43 - 37 -11

A large deposit (of about 7 mil ton) of clinoptilolite has been discovered in Slovakia close to the settlement N i ~ , Hrabovec. The thickness of this deposit is of about 100 m, it is located close to the earth-surface and the content of the clinoptilolite in the rock is between 65 % and 85 % by mass [2]. The clinoptilolite can be used to remove small molecules as water, ammonia and H2S from various media and even for separation of species as separation of air. The aim of the present work is to examine a feasibility of sorption technology applicable in agricultural production which would: (ii) use clinoptilolite to decrease ammonia emission and (i) utilise the sorbent saturated with ammonia as fertiliser in crop production. The application of the above-mentioned fertiliser would bring the following benefits: (a) release of ammonia from the clinoptilolite into soil would be slow and therefore a rapid elution of the nitrogen into water streams due to an increased precipitation would be prevented. This would be an important regulation step to prevent the eutrophication of water streams. (b) the clinoptilolite is by its nature a material compatible with the soils. This is first due to its cation-exchange properties and second to its sorption capacity for water, which would buffer extreme changes in soil humidity throughout the year.

2. EXPERIMENTAL 2. 1. Materials

Natural clinoptilolite

The principal features of the clinoptilolite tuff from the deposit Ninny, Hrabovec are summarized in Table 2. Phase analysis by X R showed in untreated samples crystals of cristobalite ~ 10 % and minority phase muscovite or illite (could not be recognized).

1715 Table 2 Composition of clinoptilolite tuff from the deposit Ni~n~ Hrabovec [2] composition composition content % by mass

content % by mass

SiO2

67.1

CaO

2.90

TiO2

0.24

Na20

0.68

A1203

10.6

K20

2.96

Fe203

1.72

H20

12.8

MnO

0.03

P205

0.30

MgO

0.73

Modified clinoptilolite samples To increase the adsorption capacity of clinoptilolite for ammonia, the treatment (impregnation) of the adsorbent by selected inorganic acids has been carried out. Adsorbents used were: (i) the natural clinoptilolite (not treated) (ii) clinoptilolite pre-treated with H2SO4, (iii) clinoptilolite pre-treated with H3PO4and (iv) clinoptilolite pre-treated with HNO3.

Ammonia Ammonia used was 99.98 % supplied by Linde Technoplyn in steel bottles.

2. 2. Characterization of texture properties of clinoptilolite samples The porous structure of clinoptilolite samples was analysed by measuring of N2 sorption at 77 K on Coulter SA 3100+. The BET-equation was used to evaluate the BET-surface of natural and modified samples. The accessible pore volume of the samples brought (prior to the dynamic measurements) to a standard saturation was evaluated from the sorption amount of N2 at Prel = 0.98 using the density of liquid N: at 77 K.

2. 3. Experimental arrangement to measure the sorption dynamics The laboratory apparatus shown in Figure 1 was used to study ammonia sorption from a stream of air at temperatures of 20 ~ and 50 ~ The gaseous mixtures containing ammonia used in the dynamic experiments were prepared by mixing of air and pure ammonia. The source of air stream was a membrane pump using the air from the ambient atmosphere (supply 2.3 m3/h). In all the experiments the concentration of ammonia in the air was selected from the interval 50 to 400 mg/m3 and the relative humidity amounted to about of ~ 45 %. Thus, the concentration range of ammonia was adjusted to simulate the conditions in animal breeding farms. The concentration of ammonia in the gas stream was measured using IR analyser Horiba VIA 510. A set of experiments was performed using air dried with a bed of molecular sieve 5A. In other series of experiments the humidity of the air was adjusted using a gas saturator. The humidity of the gas stream was measured using Dewpoint meter MBW DP-3D (Testo).

1716 Flow-meter

Mixing container

|

. . . . . . . . . . .

.

.

.

~ ~'l~-]f(;:~--- ofsorption i

...." ' A J r p u m p

Balance

.

~ ~ ~ [ Thermostat r ~ r ~ [ ,. to keep the ]I~_:-~[--[~ [-]---' temperatare

NH3 ~.

Adsorbers filled with clinot~tilolite

~

~irflow-meter

~? i ~

~_

I

[~l == U. . . .

~-~

.beds __

Exhaust gas

"i 11

u

NH3 analyser

Gasometer

Figure 1. Schematic layout of the laboratory apparatus 3. RESULTS AND DISCUSSION The effect of tuff treatment with the acids The effect of acids on the grains of tuff is a complex process and it is difficult to decide what portion of extracted species can be attributed to a dissolution of non-zeolitic phases on one side and to the modification of the HEU framework on the other. An insight into the processes involved can be obtained from a chemical analysis of the extract. Table 3 gives the fractions of species extracted from the tuff using selected acids. It can be seen from Table 3 that the dealumination of the tuff increases in the series H3PO4, HNO3 and H2SO4. The dealumination of the tuff is accompanied by the removal of K and Ca but not in the proportion, which would correspond to a removal of tetrahedrally coordinated A1. A complicating factor of this analysis is a low solubility of Ca species in H2SO4. High part of potassium was extracted using sulphuric acid or nitric acid while phosphorous acid extract this cation from the structure of zeolite sample only in the low degree. Calcium is partly extracted using phosphorous acid or nitric acid. The pre-treated samples of clinoptilolite were dried at 120 ~ using a vacuum drier. After drying a part of acid remained in porous structure of clinoptilolite. This residual acid causes the chemisorption of ammonia and decreases pore - volume of the adsorbent. The basic properties of used adsorbents are given in Table 4. Some of the pre-treated samples were leached using distilled water to constant pH (pH = 6.5) of leach. After drying, the leached samples were also used in the dynamic experiments. The breakthrough curves were measured up to the complete saturation of the sorbent by ammonia. Figure 2 exemplifies the breakthrough curves of ammonia on clinoptilolite.

1717 Table 3 Portion of extracted species from the clinoptilolite using inorg, acids in % of total content Acid Acidportion of extracted species concentration Si A1 K Ca Fe H2SO4 20 % < 0.1% 18.8 % 47.8 % 1.1% 5.2 % 30 % <0.1% 22.1% 56.2 % 0.4 % 11.1% 40 % < 0.1% 16.4 % 60.6 % 0.3 % 8.3 % H3PO4 20 % < 0.1% 6.1% 5.4 % 26.2 % 1.8 % 30 % < 0.1% 6.8 % 11.3 % 30.6 % 3.8 % 40 % < 0.1% 4.8 % 20.0 % 22.1% 2.6 % HNO3 20 % < 0.1% 13.4 % 49.4 % 32.5 % 0.8 % 30 % <0.1% 16.5 % 60.1% 37.1% 1.4 % 40 % < 0.1% 11.7 % 64.3 % 23.6 % 0.9 %

Table 4 Selected properties of adsorbents prepared clinoptilolite-sample Acid'concentration in the natural form pre-treated 20 % with H2SO4 30 % 40 % 30 % - after leaching pre-treated 20 % with H3PO4 30 % 40 % 40 % - after leaching pre-treated with HNO3 20 % 30 % 40 % 30 % - after leaching

BET-surface 26 m2/g 9.7 m2/g 3.7 m2/g 3.0 m2/g 21.0 m2/g 10.1 m2/g 6.8 m2/g 5.7 m2/g 12.3 m2/g 20.3 m2/g 12.5 m2/g 12.1 m2/g 27.2 m2/8

pore volume 0.093 cm3/g 0.047 cmS/g 0.024 cmS/g 0.013 cm3/g 0.024 cm3/g 0.06 cm3/g 0.041 cm3/g 0.039 cm3/g 0.058 cm3/g 0.081 cm3/g 0.071 cm3/g 0.060 cm3/g 0.086 cm3/~

The form of the breakthrough curves is exemplified in Fig.2 for two bed lengths of clinoptilolite impregnated with H3PO4. The inlet concentration of NH3 in these experiments was 360 mg/m3, temperature 25 ~ C, flow rate 200 1/h and relative gas humidity 45 %. The sorption kinetics was found to be limited by a radial diffusion into quasihomogeneous particles with diffusion coefficient D ~ 1.4 x 10-s cm2/s and a slower rate step acting in series to the radial diffusion. The slow sorption rate does not represent any serious limitation for the sorption phase of the process. On the other hand slow kinetics would be of advantage when using spent sorbent as soil conditioner/fertilizer because of retarded release of nutrients into the soil. Table 5 shows the sorption capacities of the clinoptilolite samples for ammonia. The sorption capacities were evaluated using the mass balance of ammonia in the column. The

1718 integration of the breakthrough curves was carried out up to the time of the complete saturation of the sorbents. 40o 350

~

.....

~

~-" 250

go=.2oO15o =I O'J

lOO

O[

.

0

5

.

. 10

. 15

.

. 20

25

30

time (hr)

Fig. 2

Breakthrough curves of ammonia on clinoptilolite impregnated with H3PO4 1: inlet concentration of ammonia, 2: L = 12.5 crn, 3: L = 15.0 cm

Table 5 Sorption capacity of clinoptilolite for ammonia (mg NH3/g sorbent) temperature of the sorption clinoptilolite-sample 20 ~ 35 ~ in its natural form 10.8 mg/g 12.1 mg/g pre-treated with 30 % H2SO4 21.6 mg/g 27.2 mg/g pre-treated with 30 % H3PO4 20.0 mg/g 17.9 mg/g pre-treated with 30 % HNO3 19.1 mg/g 18.2 mg/g

50 ~ 11.9 mg/g 29.7 mg/g 15.0 mg/g 17.5 m~g .....

inlet concentration ofNH3:360 mg/m3, flow rate: 200 dm3/h, humidity of gas mixture: 45 % In the next step we investigated the influence of the gas-humidity on the sorption capacity of clinoptilolite for ammonia. The results of the tests shows Table 6. Table 7 shows the results of the overall sorption capacity of pre-treated samples for ammonia. The results are compared with those for samples after leaching. Conceivable contributions to the overall capacity enhancement are those due to adsorption, absorption, chemical reaction and ion exchange. The results in Table 7 give an estimate of the sorption capacity of impregnated samples due to ammonia hold up in the transport pores of the tuff and to that in the zeolitic phase.

1719 Table 6 The influence of gas-humidity on the adsorption capacity of clinoptilolite for ammonia (mg NH3/g adsorbent) clinoptilolite-sample gas-humidity (by 20 ~ gas-humidity (by 20 ~ 95 % rel. < 0.1% rel. temperature of adsorption 20 ~ 50 ~ 20 ~ 50 ~ 9.4 mg/g 9.6 mg/g in its natural form 34.3 mg/g 31.4 mg/g 8.9 mg/g 8.0 mg/g aktivated by 350 ~ 32.3 mg/g 31.6 mg/g 46.6 mg/g 25.6 mg/g pre-treated with 30 % H2SO4 24.3 mg/g 24.7 mg/g inlet concentration ofNH3:360 mg/m3, flow rate: 200 dm3/h Table 7 Comparison of useful sorption capacity for ammonia of pre-treated samples and samples after leaching (mg NH3/g sorbent) clinoptilolite sample temperature of adsorption temperature of adsorption 20 ~ 50 ~ pre-treated samplesafter pre-treated samplesafter samples leaching samples leaching 21.6 mg/g 14.5 mg/g 29.7 mg/g 13.5 mg/g pre-treated with 30 % H2504 25.3 mg/g 10.4 mg/g 24.6 mg/g 10.7 mg/g pre-treated with 40 % H3PO4 19.1 mg/g 17.1 mg/g 17.5 mg/g 16.8 mg/g pre-treated with 30 % HNO3 29.7 mg/g pre-treated with 30 % H2SO4 34.9 mg/g + 40 % H3PO4 inlet concentration ofNH3:360 mg/m3, flow rate: 200 dm3/h, humidity of gas mixture: 45 % The highest sorption capacity for ammonia was that of the sample pre-treated first with 30 % H2SO4 and subsequently with 40 % H3PO4. This result is comparable with the sorption capacity of the activated carbon SS4-P (product of Chemviron-Carbon) developed for ammonia removal. The sorption capacity of this activated carbon for ammonia estimated under conditions similar those used in the present work amounts to 49.3 mg NH3/g. The price of this special sorbent is, however, by a factor 20 higher [3]. 4. CONCLUSIONS The sorption capacity of clinoptilolite for removal of ammonia from the gas mixture (ammonia + air) were determined from dynamic experiments on fixed beds of sorbents in a laboratory scale. The results show that natural (not dehydrated) clinoptilolite exhibits also a non-negligible sorption capacity for ammonia-removal from waste air. The sorption capacity of clinoptilolite for ammonia depends on the operating conditions. In the case of water vapour co-adsorption

1720 from the air the sorption capacity for ammonia decreases from cca 34 mg/g adsorbent (for dry air) to eta 9.4 mg/g adsorbent (for air containing ~18 g/m3 H20). The pre-treatment of clinoptilolite with selected inorganic acids increases its sorption capacity for ammonia. This enhancement is assumed to be due to a combined effect of several mechanisms. The effect of chemisorption would increase with increasing acid-valence due to the stoichiometry. This effect was confirmed. The dependence of sorption capacity of clinoptilolite pre-treated with acids on gas humidity is not significant for low and moderate humidity levels. However, when the gas humidity approaches ~ 95 % the effect of ammonia absorption obviously contributes to the overall sorption capacity. This is caused due to an additional increase of water content by the corresponding neutralization reaction. Thus, the humidity exceeds the dew point and a moving boundary between the liquid phase and gas phase is formed in the bed. The influence of temperature on the sorption capacity of clinoptilolite in the range between 20~ and 50 ~ is weak. The best sorption capacity was achieved by a two stage pretreatment using H2504 an subsequently H3PO4 cf. ref. [4]. ACKNOWLEDGMENTS The financial support of the work by the Grant Agency of the Czech Republic via Grant No 104/00/1007 is gratefully acknowledged. REFERENCES

1. Directive of the European Parliament and of the Council on national emission ceilings for certain atmospheric pollutants 599 PC 0125, 1999. 2. E. Kubinyiov/t, Zeolity z lokality Ni2n~ Hrabovec - ekologickh surovina; Sbornik p~ednhgek Zeolity- ekologickh surovina, MZP CR, 1992. 3. K. Ciahotn3~, M. Korifik, L. Melenovh, O. Pachtov~i, Sorption of ammonia from air on a natural zeolite; 13. Deutsche Zeolith-Tagung, Erlangen, March 2001. 4. K.Ciahotn~,, M. Korifik, Special sorption material for ammonia and amines removal from gaseous mixtures, CZ Patent No. 11 735 (2001).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1721

Microtopographic features and dissolution behavior of natural zeolite surfaces studied by Atomic Force Microscopy (AFM) M. Voltolini a, G. Artioli a'b, M. Moret c aDipartimento di Scienze della Terra, Universitb, di Milano, via Botticelli 23, 1-20133 Milano, Italy bCentro CNR Geodinamica Alpina e Quaternaria, Via Mangiagalli 23, 1-20133 Milano, Italy ~Dipartimento di Scienza dei Materiali, Universit~ di Milano-Bicocca, via R. Cozzi 53, 120125 Milano, Italy Atomic Force Microscopy (AFM) has been used to observe surface features at the micrometer scale in several zeolite samples. In zeolites exhibiting tabular habit, such as heulandite, yugawaralite, and stilbite, the AFM investigation of the well developed {010} form allowed direct mapping of the elementary steps visible on freshly cleaved surfaces. The steps have a thickness invariably corresponding to half-unit cell and they are commonly involved in growth spirals. Dissolution experiments using diluited acidic solutions have also been performed, both under in-situ and ex-situ conditions, in order to observe the structure and/or defect-controlled formation of dissolution pits. Regularly shaped etch pits have been observed on laumontite (both on the { 110} prism and on the {-201} pinacoid) and in chabazite, whereas heulandite shows very irregular dissolution figures. In situ dissolution experiments on yugawaralite and chabazite showed anomalous surface swelling and deformation in proximity of structural defects prior to intensive fracturing of the surface layers. 1. INTRODUCTION The study of mineral surfaces is of importance whenever a mineral is undergoing chemical reactions due to its interaction with the environment. The observation and interpretation of surface features (microtopography, surface structure, crystal chemistry, etc.) are essential to understand a number of key processes such as crystal nucleation and growth, dissolution, chemisorption, and surface transport and diffusion. AFM is an ideal surface probe, and its unequalled sensitivity perpendicular to the surface provides the survey of microtopographical features with high sensitivity. Furthermore AFM experiments may be performed with the probe tip submerged in solution, so to follow surface reactions in situ. To date rather few AFM investigations on zeolites have been performed. This is mainly because of their high chemical activity that produces a strong interaction between the zeolite surface and the probe tip. Furthermore the surface structural features are most commonly obscured by adsorbed species. Interesting studies on surface microtopography have been performed on natural heulandite "[1, 3] and mordenite [4, 5]. As far as zeolite crystallization

1722 is concerned, to our knowledge only the zeolite LTA and faujasite have been investigated by AFM [6, 9] and only one zeolite dissolution experiment is present in the literature [10], whereas a number of experiments have been performed on sulphates [ 11, 13] carbonates and other minerals. In favorable conditions molecular resolution of zeolitic surfaces has been achieved [ 14, 16]. 2. M I C R O T O P O G R A P H Y The microtopographic studies have been performed on zeolites having tabular morphology, using the {010} pinacoid as the measurement surface, either natural or obtained by cleavage. Three natural zeolites contanining 5-membered rings have been investigated: heulandite, stilbite, and yugawaralite.

2.1 Experimental The AFM experiments were performed in air using a Nanoscope III instrument (Digital Instruments, Santa Barbara, Ca.) equipped with a 12.5 l.tm piezo-scanner (D) and commercial Si3N4 tips mounted on triangular cantilevers having elastic constant k = 0.06 nN/nm. The tipsurface force was calculated to be about 10 nN. Images have been generally reproduced using different scan angles and scan rates to exclude instrumental artifacts and have been collected recording both the deflection and the height signals in constant force mode. An etched synthetic mica sample has been measured to control the absolute spatial calibration of the scanner. All freshly cleaved surfaces were glued onto a cylindrical flat steel sample holder. The zeolites samples are from the following localities: yugawaralite- Poona, India; stilbiteFunningstjordur, Faer Oer Islands; heulandite from Osilo, Sardegna, Italy.

2.2 Elementary steps measurements Heulandite has a b cell parameter in the range 17.82- 18.03 A, depending on composition. The height of the elementary steps measured on the Osilo heulandite by AFM is about 0.90 + 0.05 nm, corresponding to half of the unit cell. Figure 1 shows a sequence of steps observed on the (010) surface with the related height profile. The steps found on the (010) face of the Indian yugawaralite crystals (b axis in the range 13.95-14.01 A) have a thickness of about 0.72 + 0.05 nm. Those shown in Figure 2 form a double growth spiral. Similar results have been obtained on the Faer Oer stilbite sample: the steps are about 0.92 + 0.05 nm high, with a b axis of 18.20 - 18.31 A, as shown in Figure 3. In Figure 4 we can also see an image of the surface of an elementary step at molecular resolution. This image was obtained in deflection mode with a scan rate of 20 Hz after an instrument sabilization of about one hour. Further investigation of the same surface indicates that the rows of bright spots appearing in the figure are vertices of the framework tetrahedra. The edges of the single steps in the natural surfaces are o~en not linear, but rather curved or irregular (Figure 1 and 2), whereas the edges of multiple steps formed by stackings of elementary layers tend to be consistent with the lattice directions.

1723

-i

0~93om I I

-^ \

0.00

X [l~'tt]

4.06

Figure 1. Elementary steps on the (010) face of heulandite. The height profile of the surface is shown.

0.00

Figure 2. Double growth spiral on a (010) face of yugawaralite.

0 9 2 nm t-,.1

0.00

1.28

Figure 3. Elementary steps on the stilbite surface. The angles between straight step edges are compatible with the crystal morphology.

Figure 4. Deflection image showing the surface of an elementary step of stilbite at molecular resolution.

1724 2.3 Growth Spirals

An important feature that may be commonly observed on these samples are growth spirals. A number of them were observed on the (010) surfaces of yugawaralite and stilbite. No growth spirals have been observed on heulandite, although they have been reported in the literature (Yamamoto et al., 1997). The presence of growth spirals may provide some insights on the growth mechanisms of natural zeolites, for example following the BCF theory (Bennema and Gilmer, 1973), although none of such structures has yet been observed on zeolites by in situ crystal growth experiments. Some examples of various kinds of growth spirals are shown below.

i

, 0.500 10.000

Figure 5. Single counter-clock-wise (CCW) growth spiral on yugawaralite.

-,

uM/div nM/div

,,,

~,

1.5

~

PN

Figure 6. A double clock-wise (CW) growth spiral on yugawaralite.

,,--~ 3 0 . 0

X 2

0,500 15.000

p./diu nN/div

",

i '\,

..

.,,--""

~ ,.J1.0

.t.~,

i........o , s

Figure 7. Another CCW double growth spiral on yugawaralite.

2,50

5. O0

Figure 8. Multiple growth spiral on stilbite. Height mode image.

1725 3. DISSOLUTION Dissolution experiments have been performed on a number of zeolite samples in acidic solutions using a commercially available glass fluid cell mounted in the optical head of the AFM. The best results have been obtained with zeolites characterized by a relatively high A1/Si ratio: laumontite, heulandite, yugawaralite and chabazite. 3.1 Experimental

In the in situ experiments HC1 or H2804 at different concentrations acted as the flowing solvents in a conventional fluid cell. Only in particular cases a stagnant solution was used in order to limit the chances of fluid spills and to work without the o-ring The stainless steel sample holder has been covered with thermal glue to avoid corrosion and to hold firmly the sample at the desired orientation. The solution flux was slow (about 150 IA/min) to avoid any possible interaction with the cantilever. On heulandite only e x situ experiments have been successful because of the extremely low dissolution rate. 3.2 Laumontite

This zeolite proved to be highly suitable for dissolution experiments, mainly because of its high A1 content and in spite of its low stability, due to the laumontite to leonhardite transition by partial dehydration. We can see that both in HC1 and in H2SO4 solutions (at different concentrations, from 0.05 to 0.2 M) regularly shaped etch pits occur on the { 110} prism and on the {-201} pinacoid.

Figure 9. Etch pit forming on the { 110} prism of a laumontite crystal in 0.05 M H2804.

Figure 10. Etch pits on the (-201} pinacoid of laumontite in 0.1 M sulphuric acid.

1726 3.3 Heulandite

The dissolution rate of heulandite was found to be too slow to be appreciable with in situ investigations, so that the etched samples were studied ex situ after they had been exposed to acidic solutions for an increasing amount of time, in the range 5-48 h. The results obtained are similar to the ones already present in literature (Yamamoto et al., 1996), although our dissolution rate appears to be lower, possibly due to the different composition of the specimen. We observed on the surface many irregularly shaped etch pits with variable depth: this chould indicate a diffusion controlled dissolution mechanism. The sample shown in Fig. 11 has been kept in H2SO4 0.1 M for 24 hours. 3.4 Chabazite

Dissolution experiments on this zeolite have been carried out both in HC1 and H2S04 solutions on the (101) natural face. Swelling and fracturing of the surface was observed, and the degradation of the crystal surface is also evident at a macroscopic scale. Experiments in HC1 solutions on one chabazite sample have shown the reactivation of the dissolution process in pre-existent etch pits having a shape similar to the (101) face (Figure 12). There was no evidence of the production of new dissolution figures during the experiment. The depth of these etch pits is in the range 50-100 nm.

5.z o X

0.500 U N / , i , 500. 000 nN/di v

"

",>:

i

~/ / ,/,//,-/2,0

.//I,5 ~N

x

/'/0.5

Figure 11. Etched (010) surface in heulandite: note the very irregular shape of the dissolution figures.

X 2

2.000 vm/div 1.000 pm/div

6

~'---~

./

~'~VN

Figure 12. Naturally etched surface of the chabazite sample.

(101)

1727

3.5 Yugawaralite This zeolite has shown a dissolution behavior similar to chabazite: sulphuric acid solutions produce clear swelling and fracturing of the surface. In this case the concentration of the acidic solution had to be rather high (0.9 M) because of the remarkable resistance of yugawaralite to acid dissolution. A sequence of images showing the formation of swelling areas on the yugawaralite surface is presented in Figure 13. 4. DISCUSSION AND CONCLUSIONS The microtopographic observations provide some useful indications about the behavior of zeolite surfaces during the growth process. All three zeolites characterized by AFM microtopography show a bidimensional channel system parallel to the (010) structural plane, which is also the cleavage plane and the most developed morphological face in all these zeolite species. The suggested crystal growth mechanism is by step advance (a terrace- ledge - kink process) on steps having b/2 thickness. The observed growth spirals are essential to the process insofar they continuously provide new kink sites necessary to the crystal growth. Several kinds of growth spirals have been observed: single, double and multiple spirals with both CW and CCW sense of rotation. All step layers involved in growth spirals are b/2 thick. As far as the dissolution experiments are concerned, we observed a pronounced difference among various zeolite species. In laumontite we have observed a perfect structure-controlled dissolution, as evidenced by the perfect correspondence between etch pit geometry and face shape. Other regular dissolution figures have been observed only in naturally etched chabazite surfaces. This behaviour is strongly in contrast with the diffusion-controlled dissolution found in heulandite. A rather different behavior has been observed in yugawaralite and chabazite. The reasons causing the swelling of these surfaces are at present not clear. The fact that we observed this phenomenon only in presence of sulphuric acid and in zeolites with a significant amount of Ca, may suggest that localized supersaturation conditions just under the surface induces the nucleation of Ca sulphates with consequent surface swelling. However until now we have no evidence of nucleation of new species, and furthermore swelling is not observed in -

~

~~

Figure 13. Evolution of the surface of yugawaralite in H2SO4 (0.9 M). The arrow indicates the swelling (about 20 nm high) and the forming fracture. Images are unfiltered and were taken about 20 min apart using the deflection signal.

1728 laumontite, which is also Ca rich, albeit with a higher AI/Si ratio. Another possible cause of the deformation and fracturing of the crystal surfaces could be the stress caused by the increased volume of the leached surface layer (see Casey and Bunker, 1990). In such a case the water molecules and the AI content of the surface layer ought to be involved in the process. ACKNOWLEDGMENTS

The work has been carried out in the frame of the project "Mineral surface chemical reactions: intercalation and sorption processes" (Coordinator Prof. G. Artioli), and financed by MURST COFIN 2000. REFERENCES

1 Sugiyama Ono S., Matsuoka 0., Yamamoto S.; Microporous and Mesoporous Materials 48 (2001) 103-110. 2 Binder G., Scandella L., Schumacher A., Kruse N., Prins R.; Zeolites 16 (1996) 2-6. 3 Yamamoto S., Sugiyama S., Matsuoka O., Honda T., Banno Y., Nozoye H.; Microporous and Mesoporous Materials 21 (1998) 1-6. 4 Sugiyama S., Yamamoto S., Matsuoka O., Honda T., Nozoye H., Qiu S., Yu J., Terasaki O.; Surface Science 337-339 (1997) 140-144. 5 Yamamoto S., Sugiyama S., Matsuoka O., Honda T., Banno Y., Nozoye H.; Chemical Phisical Letters 260 (1996) 208-214. 6 Agger J. R., Pervaiz N., Cheetham A. K., Anderson M. W.; Angew. Chem. Int Ed. Engl. 35 (1996) 1210-1213. 7 Agger J. R., N. Pervaiz, A.K. Cheetham, M.W. Anderson, J. Am. Chem. Soc. 120 (1998) 10754-10859. 8 Sugiyama S., Yamamoto S., Matsuoka O., Nozoye H., Yu J., Zhu G., Qiu S., Terasaki O., Microp. Mesop. Mater. 28 (1999) 1-7. 9 Anderson M. W., Agger J. R., Thornton J. T., Forsyth N.; Angew. Chem. Ed, Ingl.35 (1996) 1210-1213. 10 Yamamoto S., Sugiyama S., Matsuoka O., Komura K., Honda T., Banno Y., Nozoye H.; J. Phys. Chem. 100 (1996) 18474-18482. 11 Bosbach D., Rammennsee W.; Geochimica et Cosmochimica Acta 58 (1994) 843-849. 12 Bosbach D., Hall C., Putnis A.; Chemical Geology 131 (1998) 143-160. 13 Putnis A., Junta-Rosso J. L., Hochella jr M. F.; Geochimica et Cosmochimica Acta 59 (1995) 4623-4632. 14 MacDugall J. E., Cox S. D., Stucky G. D., Weisenhom A. L., Hansma P. K., Wise W. S.; Zeolites 11 (1991)429-433. 15 Yamamoto S., Matsuoka O., Sugiyama S., Honda T., Banno Y., Nozoye H.; Chemical Phisical Letters 260 (1996) 208-214. 16 Komiyama M., Tsuijimichi K., Oumi Y., Kubo M., Miyamoto A.; Applied Surface Science 121/122 (1997) 543-547.

Studies in SurfaceScienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa(Editors) 9 2002 ElsevierScienceB.V. All rights reserved.

1729

O c c u r r e n c e a n d c r y s t a l s t r u c t u r e of m a g n e s i a n c h a b a z i t e E. Passaglia and O. Ferro Dipartimento di Scienze della T e r r a - Universit~ di Modena e Reggio Emilia Piazza Sant'Eufemia 19, 41100 Modena, Italy The chemical composition of three chabazite samples associated with offretite showed Mg contents as high as those found in a few other samples described in literature. The crystal structure refinement of one of these samples revealed that Mg is hosted in just one extra framework site and is totally surrounded by water molecules. 1. I N T R O D U C T I O N Chabazite is one of the most widespread natural zeolites. It may be found as micro- and macrocrystals in amygdales of massive volcanic rocks (commonly termed "hydrothermal" samples) and as submicroscopic authigenic crystals in diagenetic pyroclastic rocks (commonly termed "sedimentary" samples). According to the current zeolites nomenclature [1], chabazite is actually a zeolite series because, on the basis of the most abundant extra-framework cation, chabazite-Ca, chabazite-Na and chabazite-K species have been identified. An accurate review of the reliable chemical compositions of "hydrothermal" samples (80) from literature revealed that besides the dominant extra-framework cations (Ca, Na, K), Sr is constantly present reaching high values as 0.72 a.p.f.u., Ba is nearly absent or in very low amounts (< 0.15 a.p.f.u.). Mg, usually present in amounts lower than 0.30 a.p.f.u., reaches remarkable values in four samples out of which two exhibit a complex twinning ("herschelitic habit") and are epitaxially overgrown by fibrous crystals of offretite (a Mg-rich zeolite). In this study, three new occurrences of Mg-rich chabazite samples intimately associated with offretite are described and the structure refinement of a magnesian sample is reported. 2. E X P E R I M E N T A L

The occurrence and paragenesis of the studied chabazite samples are: a) Mont Semiol, Montbrison, France: clear-white rhombohedral crystals associated with offretite, mazzite and phillipsite; b) Gedern, Germany: clear-white lamellae overgrown by milky-white offretite fibres; c) Herbstein, Vogelsberg, Germany: clear-white flattened prisms overgrown by milky-white offretite fibres. Sample from Mont Semiol exhibits the typical and most frequent morphology of

1730 chabazite: simple pseudo-cubic rhombohedra corresponding to the shape of the unit cell. On the contrary, the morphology of the other two samples is very unusual and is, very likely, due to complex twinning resulting in a particular "herschelite" habit with large dominance of the {0001} pinacoid where the hexagonal offretite crystals can epitaxially grow. Under a binocular microscope, small crystal fragments (average 3 - 4 for each occurrence) were selected and, in the case of the samples from Germany, carefully hand-separated from the overgrown offretite fibres. The fragments were tested for mineralogy and purity by an X-ray Gandolfi camera and then enclosed in epoxy resin and polished for electron probe microanalysis. EPMA were performed on an ARL-SEMQ instrument using wavelength-dispersive mode, 20 ~m diameter electron beam size, 15 kV accelerating voltage, and 10 nA probe sample current. Reference standards were microcline (K), anorthite (Ca), albite (Na, Si, A1), olivine (Fc), diopside (Mg), Sr-anorthite (Sr) and celsiana (Ba). The paucity of pure material especially for the samples overgrown by offretite hindered the determination of their water contents. The single crystal structure refinement of the sample from Mont Semiol was performed using a Siemens P4R diffractometer with graphite monochromatized MoKa radiation ( ~ - 0.71073 A) and equipped with rotating anode generator. The unit cell parameters were derived by a least square fit using 30 medium 0 reflections. After verifying the -3m Laue symmetry, 8252 reflections were collected in 0- 20 scan mode. The intensities were corrected for absorption by ~scan system and reduced to IFI 2. 3. R E S U L T S A N D D I S C U S S I O N 3.1 C h e m i s t r y The electron microprobe point analyses for each sample (average 1 0 - 12) were highly consistent showing variations of the major elements within 3% of the estimated instrument errors indicating a high degree of chemical homogeneity within each sample. In particular, no point analysis of the chabazite samples overgrown by offretite showed Mg values as high as those known for offretite crystals confirming the X-ray analysis results about the absence of offretite zones in the analysed fragments. Fe and Ba contents were below the detection limits. The anhydrous unit cell content of the chabazites calculated by the averaged microprobe point-analyses along with the respective balance errors [2] are given in Table 1. The reliability of the chemical formulae is supported by both the framework contents (Si+A1) very close to half of the oxygen atoms and charge balance errors (E) lower than 7%. The chemical formulae allow the following remarks: concerning the framework content [R = Si/(Si+AI)], samples from Mont Semiol and Gedem are remarkably richer in Si (R = 0.72 and 0.75, respectively) than the sample from Herbstein (R = 0.65); among the extraframework cations, Ca is the dominant one in all samples, Mg shows anomalously high values up to 0.95

1731 a.p.f.u, in the sample from Herbstein, K content although variable is remarkable, Na content is negligible. Table 1 Uni t cell contents (24 Oxygens) of chabazites. E % = balance error. Sample

Mont Semiol

Gedern

Herbstein

8.71 3.32 0.46 0.80 0.02 0.07 0.56 + 4.1

8.98 3.05 0.50 0.74 0.04 0.34 + 6.6

7.85 4.19 0.95 0.96 0.04 0.17 + 4.0

Si A1 Mg Ca Sr Na K E (%)

According to the quoted Rule 5 of the zeolite nomenclature [1], all the samples can be classified as m a g n e s i a n chabazite-Ca. The high Mg contents of the studied chabazites is emphasized if compared with those from literature (Figure 1). Ca + Na

40/-

Mg

80

--

eo

-kSO

40

20 K + Sr

Figure 1. Compositional diagram showing the extra framework cation content of chabazites. D a r k area = 73 reliable analyses from literature. Crystals with "herschelitic habit" overgrown by offretite from O Adamello, Italy [3], @ Fitt~, Italy [4], @ Gedern and O Herbstein, G e r m a n y (this study). Crystals with "rhombohedral habit" associated with offretite from O Mont Semiol, France (this study) and without offretite from | P e n t l a n d Hills and | Narre Warren, Australia [5].

1732 The distribution of the points in the diagram show that the highest Mg contents are found for offretite overgrown chabazites (Nos. 1, 3 and 4). Unfortunately the complex twinning of the crystals of these samples inhibited their use for single crystal structure refinement. Therefore, this study was carried out on the sample from Mont Semiol (No. 5). 3.2 S t r u c t u r e r e f i n e m e n t The chabazite structure displays a tetrahedral framework built up by double six-membered rings in ABC sequence with fully or partially disordered (Si, A1) distribution and a large number of partially occupied extra framework sites with very irregular coordinations in which cations and water molecules are spread over [6, 7, 8]. The fully disordered (Si, A1) distribution of the Mont Semiol sample was determined by the average distances T-O for the six independent tetrahedral positions resulted in a preliminary refinement in the P - 1 space group. The structure was then refined in the space group R - 3 m using SHELX-97 [9]. The utilised atomic scattering curves were: Si for T, Ca for C2 and C4, Mg for C3, O for W. Starting coordinates for the framework atoms were taken from [7]. The extra framework cations and water molecules were localised by difference Fourier synthesis. The final reliability indices for the anisotropic model and other experimental and refinement details are reported in Table 2. Fractional atomic coordinates and interatomic distances are reported in Table 3 and Table 4, respectively. Anisotropic temperature factors were refined for framework and extra framework atoms with the exception of the extra framework sites with very low occupancy.

Table 2 Experimental and refinement details. Structural Formula Space group; Z Unit cell parameters (.~, ~ Crystal size (ram) 28 range (o) Data Range Collected reflections Unique reflections Internal R(F 2) Number of refined parameters Rhkt in the anisotropic approximation wR(F 2) for observed reflections

Apmax(e/A 3) Apmin(e/A 3)

[(Ca, K)1.51Mg0.GT][(Si, A1)120241.13.20H2O R-3m; 1 a = 9.382(2), a = 94.57(1) 0.08 • 0.09 • 0.24 4-70 -15 < h <1, -15 < k < 15, -15 < l < 15; 8252 1333 0.0652 69 0.044 for 1106 Fo > 4(~(Fo); 0.054 for 1333 0.115 0.57 -0.54

1733 Table 3 Fractional coordinates and isotropic, Uiso, or equivalent isotropic, Ueq, thermal parameters (/~2). Ueq = 1/3[Ull + U22 + U33 -b 2 (U12 + U13 +U23) cos a]. Site x y z Ueq or Uiso* Occ. (%) T .10548(4) .33341(4) .87760(5) .0121(1) O1 .2602(2) -.2602(2) 0 .0362(6) 02 .1520(2) -.1520(2) 89 .0265(4) 03 .2545(2) .2545(2) .8948(3) .0314(5) 04 .0279(2) .0279(2) .3254(3) .0294(4) C2 .2329(3) .2329(3) .2329(3) .0394(7) 39(1) C3 .4028(5) .4028(5) .4028(5) .046(3) 33(1) C4 .5850(6) .5850(6) .2366(8) .043(1)* 12(1) W1 89 89 0 .112(3) 100 W3 .221(1) .321(1) .515(1) .162(4) 54(1) W4 89 -.376(1) .376(1) .14(1)* 37(2) W5 .543(4) .543(4) .295(6) .11(2)* 14(1) W6 .268 (2) .268(2) . . . . .268(2) .13(2) 34(2) Table 4 Cation, oxygen and water molecule distances less than 3.30 C2---O3 [x3] 3.217(4) C3---C2 2.531(8) C4--O2 [• ----04 [• 2.845(4) --C3 2.90(1) ----03 --C3 2.532(9) ---C4 [• 2.943(9) ---C3 --W3 [• 2.72(1) - - W 3 [• 2.193(9) --C4 [• - - W 4 [• 2.24(2) --Wl --W5 [• 2.16(6) --W3 [• --W6 2.01(2) --W3 [• --W5 [•

2.807(5) 2.508(8) 2.941(8) 2.922(9) 2.353(8) 2.51(1) 2.88(1) 2.72(3)

The results of the structure refinement revealed the presence of three partially occupied cation sites (C2, C3 and C4) and five sites (Wl, W3, W4, W5 and W6) occupied by water molecules; sites are coded as reported by [7]. According to the chemical composition of the Mt. Semiol sample (Table 1), C2 site (along the t e r n a r y a x i s [111] near a 6-ring of the D6R cage) and C4 site (in the large cage about 2 A apart from the plane of the 8-ring window) are partially occupied by Ca and K (Figure 2 a). The shortest C4-O distance 2.508 A, compared with the shortest C2-O 2.845 A, suggests that K occupies uniquely C2 position as found in K-exchanged chabazites [7], whereas Ca is spread over both sites. C3 site (along the ternary axis [111] near the centre of the large cage) is partially occupied by Mg uniquely coordinated by water molecules (Figure 2 a). Such statement is supported by both the completely "hydrated" feature of C3 site and reliable C3--W distances.

1734

z

Z

y

c

Figure 2. Part of the chabazite cage showing cation sites (a); octahedral (b) and tetrahedral (c) coordination polyhedra of Mg in C3 site. The occupancy factor of Mg in C3 site corresponds to 0.67 atoms per. unit cell and is not in full agreement with the reported content from the chemical analysis (0.46). This excess is also observed for the Ca and K and is probably due to an underestimation of exchangeable cation by microprobe analysis as indicated by the positive balance error (+ 4.1%). Two coordinations for Mg (Figure 2 b, c) were hypothesized according with the C3--W coordination distances and W--W bond distance compatibilities. The former has a six-fold distorted arrangement (Figure 2 b), with Mg--W distances in good agreement with those observed in the other natural zeolites containing Mg as extra framework cation: offretite [10] mazzite [11], ferrierite [12]. In such six-fold coordination two m related polyhedra are possible depending on the alternated way in which Mg can result bonded to three of the six W3 and three of the six W4 (Table 4). The latter is a four-fold coordination (Figure 2 c) which is unknown for zeolites, but present in the Mg-spinel [13] and in a few silicates [14, 15, 16]. W1 is a site fully occupied by water at the centre of the 8-ring window, as reported by several authors [7, 17, 18]. The same site is found only partially occupied by other authors [19]. W3, W4, W5 and W6 site, accepted as water sites, are inside the large cage with a low occupancy [7]. The atomic coordinates of water sites slightly deviate from those reported in literature [7] as a consequence of the involvement of these water molecules in the coordination shell of a small cation as Mg. 4. C O N C L U S I O N S The usual very low and subordinate Mg content of "hydrothermal" chabazites noticeably increases up to about 1 a.p.f.u in samples associated with offretite

1735 especially in the case of crystals epitaxially overgrown by. Accordingly, the crystallization of such anomalous Mg-rich chabazite is very likely explained by the high concentration in Mg of the percolating solution needed for the growth of offretite, a Mg-rich zeolite species. The structure refinement of a crystal with about 0.5 a.p.f.u, of Mg showed this cation exclusively allocated in one extra framework site (C3) and completely coordinated by water molecules. Consequently, C3 site may show higher Mg occupancy in samples with higher Mg contents.

Acknowledgements: The work was made possible through the financial support of MIUR (Ministero Istruzione, Universith e Ricerca). The CNR (Consiglio Nazionale delle Ricerche) is acknowledged for financing the electron microprobe laboratory at the Dipartimento di Scienze delia Terra of Modena e Reggio Emilia University.

REFERENCES 1. D.S. Coombs et al. Can. Mineral., 35 (1997) 1571. 2. E. Passaglia, Am. Mineral., 55 (1970) 1278. 3. E. Passaglia and A. Tagliavini, Eur. J. Mineral., 6 (1994) 397. 4. E. Passaglia, A. Tagliavini and R. Gutoni, N. Jb. Miner. Mh., 1996 (1996) 418. 5. W.D. Birch, Mineral. Soc. of Victoria, Spec. Publ. 2 (1989) 91. 6. J.V. Smith, F. Rinaldi and L.S. Dent Glasser, Acta Cryst., 16 (1963) 45. 7. A. Alberti, E. Galli, G. Vezzalini, E. Passaglia and P.F. Zanazzi, Zeolites, 2 (1982) 303. 8. F. Mazzi and E. Galli, N. Jb. Miner. Mh., 1983 (1983) 461. 9. G.M. Sheldrick, SHELX-97: Program for the solution and refinement of crystal structures. Siemens Energy and Automation, Madison, WI, 1997. 10. A. Alberti, G. Cruciali, E. Galli and G. Vezzalini, Zeolites, 17 (1996) 457. 11. E. Galli, Soc. It. Mineralogia e Petrologia- Rendiconti, 31(2) (1975) 599. 12. A. Alberti and C. Sabelli, Z. Kristallogr., 178 (1987) 249. 13. T. Yamanaka and Y Takeuchi, Z. Kristallogr., 165 (1983) 65. 14. J.V. Smith., Am. Mineral., 38 (1953) 643. 15. N. Nguyen, J. Choisnet and B. Raveau, J. Solid State Chem., 34 (1980) 1. 16. T. Armbruster and R. Oberhaensli, Am. Mineral., 73 (1988) 585. 17. M. Calligaris and G. Nardin, Zeolites, 2 (1982) 200. 18. M. Calligaris, G. Nardin and L. Randaccio, Acta Cryst., B38 (1982) 602. 19. I.K. Butikova, Yu.F.Shepelev and Yu.I. Smolin, Crystallogr. Rep., 38(4) (1993) 461.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1737

Treatment of Urban Dump Leachates with natural Zeolite packed bed column T. Rodriguez F. a, E.Acevedo del Monte ~, G. Mori b, B.Rafuzzi b a GME. Gruppo Marbox Engineering S.r.l., Piazza della Vittoria 12/5, Genoa

b Consorzio Cuoio Depur S.p.A., San Miniato, San Romano (Pisa) It is presented the result of the treatment of Urban Dump Leachates (UDL) with a Cuban natural Zeolite (64% as Clinoptilolite-Mordenite) packed bed exchange column. The main target of the test works was to reduce conveniently the (high) content of ammonium and other contaminant elements from UDL and at the same time to establish convenient ammoniumorganic charge ratios in the treated UDL for its conventional depuration process. The mean ammonium content in the studied leachates was about 3900 mg/1 with a COD mean value of 15000 mg/l. During the pilot plant test works, ammonium, potassium and COD were removed 97, 98 and 13-22 % respectively. The Zeolite bed did not trap the heavy metals. The characteristic of the exhausted Zeolite gives the possibility to reuse it for soil amendment in agricultural applications. Relatively low volumes of urban dump leachates could be treated industrially by cationic exchange system with natural Zeolite. The combination of this process with the conventional one for UDL could optimize the last. 1. INTRODUCTION The Urban Dump Leachates (UDL) are characterized by a wide variability in its composition as well as their high content of contaminants substances. Among main contaminants we find high content of ammonium and a great content of organic substances expressed by high values of Chemical Oxygen Demand (COD), which frequently are in inconvenient ratios, making difficult to apply the conventional depuration process. For ammonium removal from UDL it has been developed technologies that consider the ammonium stripping process with the possible recovery of Ammonium salts through a chemical reaction into Sulfuric acid solution, but these technologies are not always recommendable. The high selectivity of natural Zeolites for ammonium and potassium cations makes possible to consider them as a useful solution for many environmental problems related to the presence of the already mentioned contaminants in wastewaters of diverse origin. The ammonium removal with natural Zeolites has been widely described by many authors. This particular use has been reported frequently for drinking water, aqua culture and

1738 wastewater treatment [1] where the ammonium content generally does not exceed the hundred's of mg/1. Exceptionally in some papers has been reported the use of natural Zeolites for ammonium removal through cationic exchange process in swine sewage with 1000 mg/l NH4 + [2], nevertheless it is not considered a viable method for the treatment of this kind of sewage. The aim of the present work was to study the possible application of a cationic exchange column process with a natural Zeolite packed bed for the UDL treatment. Adsorption of nutrient especies was monitored to verify the possible use of exhausted Zeolite in soil amendment for agricultural use. At the same time certain regulations were established for the NH4+/COD ratio in the UDL treated.

2. MATERIALS AND METHODS Cuban natural Zeolite (64% as Clinoptilolite-Mordenite) from Tasajeras deposit, with grain size into the range 1-3 mm, was used for the development of the experimental work in a pilot plant column. Typical chemical composition of the Tasajeras deposit's Zeolite is shown in Tablel. Table 1 Chemical composition of Tasajeras' Zeolite Comp. SiO2 A1203 CaO Fe203 Na20 MgO K20 FeO TiO2 P205 H20 % 64.3 13.7 5.0 2.7 2.2 1.2 1.2 0.8 0.4 0.1 3.4 The Zeolite content, as Clinoptilolite-Mordenite, was determined by semi quantitative XRay diffraction spectrometry method; and the total cationic exchange capacity (1.35 meq/g) was determined by cationic exchange with ammonium chloride and potassium chloride solutions alternatively for exchange and regenerative process, evaluating the effluents by atomic adsorption spectrometry. The UDL composition from the historical recorded data (from 1991 to 2001) showed that NH4§ content was into the range 2000-4000 mg/1, with mean pondered value of 3931 mg/l, while the mean pondered value of COD was very close to 15000 mg/1. The exchanger column was designed with three outlet valves to study the progress of exchange process into the column. The outlet valves were situated at different distances (A, B and C) from the bottom. The UDL was pumped through the zeolite bed ;with a size range of particles 1-3 mm from inlet at the bottom of the column, at a flow rate of 0.28 VB/h. The experimental conditions were the following: Outlets valves position Large from the bottom, cm Zeolite weight, k8

A 60 9,5

B 120 20,5

C 180 31,5

Samples of the outlet effluents A, B, C, were analyzed by Molecular Adsorption Spectrophotometry (Nessler method) for ammonium content, Atomic Adsorption Spectrophotometry was employed for metallic cations, and COD was determined by Dichromate method. The principal characteristics of UDL are shown in Table 2.

1739 Table 2. Characteristics of UDL pH Elect.Conductivity, 20 ~ l.tS/cm C.O.D., mg/l NH4+-N, mg/1 Organic Nitrogen as (N), mg/1 CI', mg/1 Na +, mg/1 K+, mg/1 Cd 2+, mg/l Cu 2+, mg/l Pb 2+, rag/1 Zn2+, mg/1

9,39 27620 14941 3435 758 4237 3228 2172 0,03 0,27 0,32 0,69

9,12 29300 16115 3305 1340 3884 2972 2136 0,04 0,23 0,28 0,45

3. RESULTS The maximum exchange capacity of the Zeolite used was 1.29 meq/g, determined at room temperature in a stirring system using an ammonium chloride solution method[4]. The breakthrough curve for NH4+ is shown in Fig 1. The main experimental results, considering 100 mg/l of ammonium as a breakthrough concentration (BTC) are summarized in Table 3 Table 3. BTC point operation parameters Outlets Volume UDL treated, 1 Effect.CEC mg/g ..A . 21.08 9.00 B 59.01 12.07 C 101.16 13.47

for NH4+ meq/g 0.50 0.67 0.75

The maximum ammonium exchange capacity obtained was 0.78 meq/g. The COD removal, during the tests, was measured and its behavior is shown in Fig. 2. The mean values obtained for the studied parameters for the UDL treated sampled at outlets B and C, at the breakthrough point, are showed in Table 4. Representative samples from a total weight of exhausted zeolite, was taken to evaluate the content of heavy metals adsorbed into the column. These results are showed in Table 5. 4. DISCUSSION

For the established experimental conditions the effective exchange capacity of natural zeolite was increased with the bed length for NH4+ and K +, up to 0.94 meq/g, with high removal efficiency of 73% for both cations. The organic material evaluated as nitrogen was significantly retained into the bed, probably due to a filtration process reaching values of 61.9-76.4 %.

1740

----g-- B --O--C --e--A

40OO ~3000 "-F

z~2000 1000 0 0

2

4

6

8 10 12 14 16 18 q['ln~ hr

Fig 1. Ammonium concentration curves of the effluents from outlets A-B-C.

+A!

70 ~60

~ B --O--C

9.- 50

O40 u

~30 o

~20 10 0c 0

2

4

6 8 Tlmeh

10

12

14

Fig. 2. Behavior of the COD for the effluents from outlets A-B-C

1741 Table 4. Mean Values of parameters for UDL at BTC point. Parameter Outlet B Removal% Outlet C pH 7,45 7,36 Elect.Conduct. at 20 ~ 12850 56,1 15300 COD, rag/1 12278 23,8 14171 Ammonium as (N), mg/1 101 96,9 268 Organic N, mg/l 316 76,4 511 Chloride (C1-), mg/1 3037 21,8 3460 Sodium (Na), mg/1 3252 0.0 3172 Potassium (K), mg/1 70 96,7 72 Cadmium (Cd), mg/1 0,08 0,0 0,04 Copper (Cu), mg~ 0,36 0,0 0,34 Lead (Pb), mg/1 0,54 0,0 0,46 Zinc (Zn), mg/1 0,45 0,0 0,36

Removal% 47,8 12,1 91,9 61,9 10,9 0.0 96,6 0,0 0,0 0,0 20,0

Table 5 Characteristics of exhausted Zeolite Parameter Values pH 8.9 Copper (Cu), mg/kg 21.4 Lead (Pb), mg/l 6.7 Nickel Ni, mg/kg 0.78 Cadmium (Cd), mg/kg 0.06 Chromo total mg/kg 3.5 Zinc (Zn), mg/kg 48.8 Mercury Hg, mg/kg 1.9 Ammonium NH4 +, % 1.1 Phosphorous total, % P 0.03 Potassium % 0.70 The COD removal by the exchange column was more effective during the first four hours of test work. During that lapse, a relatively influence of bed length on COD removal was observed up to reach a maximum value of 65 %. Nevertheless the mean effective removal value was reduced to 22 and 13 % at outlets B and C respectively. This retention can be attributed to mechanical forces inherent to the nature of the filtration process Natural zeolite used showed a preferential adsorption for NH4+ and K § cations with removal of 97-98 %, while heavy metals were not practically adsorbed by the zeolite bed as it is showed in Table 4 and Table 5. During the pilot plant test it was demonstrated that the exchange natural zeolite packed column was capable to adsorb the main nutrient species contained in the UDL, converting it in an enriched nitrogen and potassium subtract.

1742 An appropriated conventional depuration process, of course, is needed to conclude the UDL decontamination, but it can be considered the possibility to combine it with natural zeolite treatment can give several advantages. An adequate technological process is currently in progress in terms of engineering design to apply the results showed in this paper. 5. CONCLUSIONS On the basis of the high selectivity of natural zeolites for NH4+ and K § cations, an appropriated volume of UDL can be treated by cationic exchange process obtaining a reusable product for agricultural applications as its has already studied. The effective cationic exchange capacity for NH4+ and K + was 0.93 meq/g, with a exchange efficiency of 73 %. At the same time a COD removal up to 22 % was obtained. REFERENCES

1. D.B.Shah, O.Talu, D.T. Hayhurst, and X.-C Lu "Ammonium removal from industrial wastewater by ion exchange with packed-bed Clinoptilolite". Zeolite'93, Boise, Idaho, June20-28, 1993. 2. E. Passaglia, S. Azzolini. "Italian Zeolite in wastewater purification: Influence of zeolite exchangeable cations on NH4+ removal from swine sewage. Materials Engineering 1994,Vol.5 No2, pp 343-355. 3. Howard Sherry. The cation exchange properties of Zeolite. A review. Zeolite'93, Idaho, June 20-28, 1993. 4. T.Rodriguez, G.Mori. Report about the pilot plant test of urban dump leachates depurations with Cuban natural zeolite. Italy, 2001.(not publicized)

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1743

Phosphorus removal from wastewater by bioaugmented activated sludge with different amounts of natural zeolite addition J. Hrenovic and D. Tibljas Faculty of Science, University of Zagreb, Zagreb, Croatia The influence and performance of different amounts of NZ additive in enhanced biological phosphorus removal (EBPR) system with different initial concentration of phosphorus (1-100 mg T-P L -1) were investigated. The efficiency of P-removal was in correlation with the amount of NZ additive used. The improvement of P removal in regards to the EBPR was lower with 0.5 % NZ addition than with 1.5 % NZ addition for each range of starting P load in wastewater. The amount of P removed increased with increasing starting P load. Significant stronger decrease of COD has been achieved in reactors with NZ, and stronger by larger NZ additive used. The application of NZ addition in EBPR process enables increase in sludge concentration (increase of MLSS) in the system and decreased volume of excess sludge (lower SVI). The probable mechanism of P-removal in EBPR system with NZ addition is a combination of adsorption of P onto NZ particles, increased metabolic activity of activated sludge, P uptake by P-accumulating bacteria biosorbed on the NZ particles and suspended in solution.

1. INTRODUCTION Enhanced biological phosphorus removal (EBPR) in conventional activated sludge wastewater treatment is based on the enrichment of activated sludge with phosphateaccumulating organisms. These microorganisms (also called P-removing or poly-P bacteria) are able to store intracellular phosphorus as polyphosphate. The requirement to achieve phosphorus removing bacterial population in an activated sludge system is exposure of activated sludge to anaerobic and aerobic conditions [1 ]. Bacteria from Acinetobacter genus, such as A. calcoaceticus, have become the model organism for biological phosphorus removal since it was isolated from a phosphorus-removing activated sludge plant [2]. Under anaerobic conditions P-removing bacteria transport volatile fatty acids (e.g. acetic, propionic acid) present in or produced from the intermediate metabolism of heterotrophic populations in wastewater, into the cell and subsequently convert and store these as polyhydroxy-alkanoates (PHA, e.g. poly-beta-hydroxy-butyrate). The energy for this transport and storage is supplied by hydrolysis of intracellularly stored polyphosphate to orthophosphate, which is released from the cell to the liquid. Under aerobic conditions, anaerobically formed PHA are catabolized using oxygen as electron acceptor to generate energy for cell growth, poly-P synthesis, glycogen formation and maintenance, resulting in the uptake of phosphate [3,4]. Natural zeolites (NZ) are the main absorptive, low-cost material used in agricultural and industrial situations [5]. Jardin and Popel [6] mention the impact of zeolites as an aluminium source for phosphate fixation by adsorption. The presence of high levels of Ca, A1 and Fe

1744 oxides in adsorptive materials such as zeolite suggests that these materials can absorb phosphorus and are potentially valuable for the use in the removal of phosphorus from wastewater [5]. Significant degree of phosphate elimination from domestic water has been achieved with addition of NZ [7]. Tertiary treatment of the effluent from wastewater stabilization pond in column with NZ tuff showed high efficiency of phosphate removal [8]. Successful reduction of phosphorus from dairy [9] and pig slurry [10, 11] with addition of NZ has been observed. Zeolite may be used alone or in combination with soils to improve phosphorus removal in constructed wetlands [5]. Improved phosphorus removal in EBPR system has been achieved using NZ as a support material [12]. The efficiency of a zeolite-containing material in most of the applications, such as ion exchange, selective sorption or catalysis, is in correlation with the amount, structure and composition of the zeolitic component itself. Similar correlation could not be observed when clinoptilolite-containing tufts were used as additive in water treatment or for water filtration [13, 14]. One of the promising approaches to improve the efficiency and increase the capacity of biological wastewater treatment plants (hydraulically or biologically overloaded plant), without increasing size, is based upon application of NZ in the aeration basin. Although the additions of zeolites in wastewater treatment evidently reduce the final concentration of phosphorus, the mechanism of this reduction is unclear. The objective of this study was to investigate the influence and performance of different amounts of NZ addition in EBPR system with different initial concentration of phosphorus.

2. EXPERIMENTAL SECTION

2.1. Natural zeolite The 0.25-0.80 mm fraction of NZ tuff (Aegean Region, Turkey) was used in this study. NZ consists of more than 70 % clinoptilolite - heulandite + mordenite, minor quartz and opal-CT, estimated by X-ray powder diffraction method. The chemical composition of the NZ (wt %) is: SiO2 68.5; TiO2 0.07; A1203 11.18; Fe203 1.21; MnO 0.02; MgO 0.71; CaO 1.98; Na20 0.33; K20 3.88; P205 0.01; H20 5.51; H20 + 5.78. The NZ tuff was washed three times with demineralised water and then dried at 105~ for 16 h before using for experiments. 2.2. Experimental operation Laboratory-scale batch experiments in alternated 24 h anaerobic/24 h aerobic stages were carried out for the synthetic medium, used to simulate the sewage, with the following composition (in mg Ll): Na-acetate, 500; Na-propionate, 40; glucose, 40; peptone, 100; MgSO4, 10; CaC12, 6; KC1, 30; yeast extract, 20; KH2PO4, variable. The concentration of KHzPO4, the only sole source of P in tests, varied from 4 up to 440 mg L 1 to obtain a concentration of P in the wastewater ranging from 1 to 100 mg L-1. The fresh activated sludge was obtained from the aeration tank of a municipal wastewater treatment plant and acclimatized for two weeks in the mineral solution with mixing and aerating at room temperature. The synthetic wastewater was inoculated with activated sludge bioaugmented with polyphosphate-accumulating bacteria Acinetobacter calcoaceticus DSM1532. The pH of the experimental reactors was regulated at 7.0+0.1 pH units with 1M NaOH or 1M HC1 only at the start of each run. Temperature was kept at 20~ After 24 h anaerobic stage the each

1745 reactor volume was divided in two reactors. In one reactor 5 g L -1 or 15 g L 1 of NZ was added and the other reactor was left without NZ addition. The aeration (about 4 L min l ) was provided by aquarium pumps.

2.3. Analytical methods The control parameters in reactors were: pH, total phosphorus concentration (T-P), chemical oxygen demand (COD), mixed liquor suspended solids (MLSS), sludge volume index (SVI) and plate counts of A. calcoaceticus. Samples were taken from the reactors three times per each anaerobic and aerobic stage. The samples were filtered before measurements through the nitrocellulose filters Sartorius pore diameter 0.2 gm. All measurements were done according to the Standard Methods for the Examination of Water and Wastewater [ 15]. pH-values were measured with Crison micro pH 2000 pH-meter. T-P concentration in water was measured after persulfate oxidation by stannous chloride method in a Cary UV-visible spectrophotometer at 690 nm. T-P concentration in activated sludge was determined after perchloric acid digestion. COD was determined by open reflux method. MLSS were determined after drying at 105~ h. SVI was calculated after 30 min sludge settlement. Bacterial number of A. calcoaceticus was determined as colony forming units (CFU) on the nutrient agar. Serial dilutions (10 -1 t o 10-8) of the one mL sample were prepared. Dilutions (0.1 mL) were plated (spread plate method) onto nutrient agar to obtain a viable cell count. Plates were incubated at 30~ for 72h. After period of incubation, colonies were counted and CFU L -1 was calculated. Dissolved oxygen and temperature were controlled with Jenway 9071 dissolved oxygen meter. 2.4. Calculations On the basis of measured parameters following values were calculated: Percentage of released phosphorus: P-released (%) = (B - A) / B x 100 Percentage of phosphorus removal: P-removal (%) = (A - C) / A x 100 Phosphorus release ratio per MLSS: P-release ratio (mg g-i) __ (B - A) / a Phosphorus uptake ratio per MLSS: P-uptake ratio (mg g-l) = (A - C) / b Phosphorus release ratio per cell A. calcoaceticus: P-release ratio (mg cell -1) = (B - A) / c Phosphorus uptake ratio per cell A. calcoaceticus: P-uptake ratio (mg cell -1) = (A - C) / d Where: A - phosphorus load at time zero (T-P m g L "l) B = phosphorus load at the end of anaerobic stage (T-P m g L "l) C = phosphorus load at the end of aerobic stage (T-P mg L -1) a = MLSS at the end of anaerobic stage (g L l ) b = MLSS at the end of aerobic stage (g L -1) c = CFU L -1 after incubation at the end of anaerobic stage d = CFU L 1 after incubation at the end of aerobic stage

3. RESULTS AND DISCUSSION Activated sludge enriched with polyphosphate-accumulating bacteria A. calcoaceticus shows EBPR characteristics [ 16]. Significant improvements of P removal in EBPR system have been achieved by addition of NZ (Table 1, 2). Whereas P eliminations in EBPR system were

1746 similar in both control experiments, P elimination increased increasing NZ additive. The efficiency of P-removal was in correlation with the amount of NZ additive used. The improvement of P removal in regards to the EBPR was lower with 0.5 % NZ addition (0.23; 1.12; 3.38; 6.28 mg L -~) than with 1.5 % NZ addition (0.59; 2.20; 11.16; 60.61 mg L -~) for each range of starting P load in wastewater. The amount of P removed increased with increasing starting P load. Just for the highest P load (load 4) in experiment with addition of 0.5 % NZ the amount of P removed was lower than for load 3, both in reactor with and without NZ additive (Table 1). It was due to the decay of phosphate-accumulating bacteria in reactors, indicated by low numbers of A. calcoaceticus and low pH. Higher anaerobic P release resulted in higher aerobic P uptake. Table 1 The performance of bioaugmented activated sludge system using 5 g L 1 NZ* additive and without NZ additive by different initial phosphorus loads

Period Influent T-P (mg L I) COD (g 02 L 1) MESS (g E -1) 109 CFU L "1 pH- value Anaerobic stage(end) T-P released (mg L -1) COD (g 02 L -1) MESS (g L 1) 101~CFU L -1 pH- value P-re lease (%) Aerobic stage(end) T-P removed (mg L -1) COD (g 02 L "l) MESS (g E l ) 1011CFU L "1 pH- value P-removal (%) Aerobic stage(end)* T-P removed (mg L "l) COD (g 02 L l) MESS (g L 1) l0 ll CFU L -1 p H - value P-removal (%)

Load 1

Load 2

Load 3

Load 4

1.93 2.05 1.23 2.3 7.00

13.24 2.10 1.41 12.3 7.02

49.35 2.05 1.11 3.3 7.01

98.48 2.10 0.62 12.3 7.01

1.59 1.37 2.91 4.8 6.46 82.38

6.62 0.65 1.83 6.5 6.51 50.00

28.64 0.90 2.47 5.0 6.64 58.03

15.33 0.86 1.69 8.0 6.82 15.57

1.40 0.77 2.92 6.0 8.68 72.54

7.12 0.51 2.55 5.4 8.52 53.78

18.28 0.49 2.66 7.2 8.60 37.04

12.77 0.46 2.34 2.0 8.40 12.97

1.63 0.42 3.03 1.8 8.59 84.46

8.24 0.36 2.70 2.0 8.66 62.24

21.66 0.31 2.70 3.0 8.56 43.89

19.05 0.28 2.84 1.2 8.05 19.34

1747 Table 2 The performance of bioaugmented activated sludge system using 15 g L l NZ* additive and without NZ additive by different initial phosphorus loads Period Influent T-P (mg L -1) COD (g 02 L "l) MESS (g E -1) SVI (ml g-l) 109 CFU L "1 pH- value Anaerobic stage(end) T-P released (mg L "l) COD (g 02 L "l) MLSS (g L-1) SVI (ml g-l) 10 l~ CFU L "1 pH- value P-re 1ease (%) Aerobic stage(end) T-P removed (mg L -1) COD (g 02 E -1) MESS (g L l) SVI (ml g-l) 1011CFU L "l pH- value P-removal (%) T-P increase in activated sludge (%) Aerobic stage(end)* T-P removed (rag L1) COD (g 02 L "l) MESS (g g "1) SVI (ml g-l)

l011CFU L-1

pH- value P-removal (%) T-P increase in activated sludge (%)

Load 1

Load 2

Load 3

Load 4

2.34 2.25 1.75 23.19 3.2 7.03

9.89 2.25 1.49 26.86 2.2 7.02

64.49 2.36 1.29 24.81 5.5 7.02

157.69 2.36 1.04 30.77 4.0 7.03

1.95 1.40 1.89 20.11 5.4 6.40 83.33

5.00 0.73 1.59 25.16 4.3 6.41 50.56

13.99 0.82 1.66 24.09 4.8 6.41 21.69

13.51 1.27 1.32 30.30 3.6 6.50 8.57

1.62 0.41 1.95 20.06 6.0 8.66 69.23 19.23

4.76 0.53 1.71 24.07 5.9 8.62 48.13 20.56

26.32 0.60 1.80 23.77 5.9 8.57 40.81 28.31

26.18 0.70 1.42 28.50 7.8 8.61 16.60 20.02

2.12 0.19 3.27 17.58 3.9 8.63 90.60 3.67

6.96 0.39 3.08 16.26 3.3 8.72 70.37 7.57

37.48 0.30 2.02 20.08 3.2 8.63 58.12 27.41

86.79 0.29 2.31 24.53 3.4 8.49 55.04 36.02

In all cases, numbers of A. calcoaceticus increased for one order of magnitude at the end of anaerobic stage, and than for one order of magnitude at the end of aerobic stage. Numbers of A. calcoaceticus in reactors without NZ additive were similar and significantly (p<0.05) higher than in reactors with NZ addition (Table 1, 2). The difference in numbers was higher

1748 by larger NZ addition. In additional experiment, we confirmed that used NZ tuff did not have antibacterial properties. Microbiological examination of NZ tuff at the end of aerobic stage showed NZ particles surrounded with biosorbed bacteria. The contribution of bacteria from A cinetobacter genus in adsorbed bacterial population was around 80 %. Higher NZ additive provided larger surface area of substrate particles available to be biosorbed with Paccumulating bacteria. Zeolite particles are good carriers of bacteria, which absorb on the zeolite surface resulting in increased activated sludge activity. The biological activity of the zeolitic tuff is determined by the processes on the outer surface of the embedded zeolite crystallites accessible for microorganisms through cavities, macro- and mesopores. Paccumulating bacteria, either attached to the NZ or suspended in solution, participated in the process of the P removal. The dimensions of bacteria are comparable to the sizes of zeolite crystallites and also to the corresponding inter-crystalline pores; it seems therefore conceivable that P-accumulating bacteria become activated by biosorption on the outer surface of zeolite crystallites accessible [14]. Surfactant modified zeolites can absorb anions [ 17]. So it seems also possible that improvement of P-removal by NZ addition can be due by bacteria biosurfactant production. Significant (p<0.05) stronger decrease of COD has been achieved in reactors with NZ, and stronger by larger NZ additive used (Table 1, 2). It can be ascribed to the increased metabolic activity of activated sludge and biosorbed phosphorus accumulating bacteria onto zeolite particles. Successful reduction of COD from pig slurry using zeolite has been observed [ 10]. The results in regards to MLSS show better metabolic and growth conditions of activated sludge in the presence of NZ, especially by larger NZ additive used (p<0.05). SVI (Table 2) was significantly (p<0.05) lower in reactors with NZ additive, showing better settling properties of the activated sludge than system without NZ additive. The addition of NZ did not have significant influence on the final pH of the effluent (Table 1, 2). In each system, decrease of the pH values during anaerobic release and increase during the aerobically P uptake can be seen. Although the amounts of P removal were higher in reactors with NZ addition than in EBPR system, increases of P content in activated sludge at the end of experiment were lower (Table 2). These indicate the role of NZ in the process of P removal in the EBPR system with NZ addition through the adsorption of P onto NZ particles. Just by the highest starting P load the increase of P content in activated sludge was higher in reactor with NZ addition, thus suggesting important role of the biological component in the process of P removal. Calculated P elimination in mg P/g NZ was enormously high (0.14 - 5.79). Predominantly negative surface charge and the lack of sorption sites in zeolite suggest the absence of P sorption [9]. Phosphorus can be incorporated into zeolites as surface phosphite or phosphate groups [ 18]. P-release and P-uptake ratios per MLSS (Table 3) and per cell of A. calcoaceticus were low (Table 4). For the pure culture of A. calcoaceticus in the wastewater of t9he same composition much higher P-release (4.3 x 108 to 8.4 x 10"7) and P-uptake (8.0 x 10 to 2.8 x 10 ) ratios has been observed [19]. The probable mechanism of P-removal in EBPR system with NZ addition is a combination of adsorption of P onto NZ particles, increased metabolic activity of activated sludge, P uptake by P-accumulating bacteria biosorbed on the NZ particles and suspended in solution. The NZ addition performed well in success of EBPR wastewater treatment system. The use of the NZ additive resulted in lower sludge production and high treatment efficiencies. The application of NZ addition in EBPR process enables increase in sludge concentration (increase of MLSS) in the system and decreased volume of excess sludge (lower SVI). The P-

1749 Table 3 Phosphorus release and uptake ratios per MLSS in bioaugmented activated sludge system using 5 g L 1 NZ* (a) and 15 g L 1 NZ* (b) additive and without NZ additive by different initial phosphorus loads

Experiment (a) (b)

Experiment (a) (b)

Load 1 0.546 1.032 Load 1 0.479 0.538* 0.831 0.648*

P release ratio (mg P g-1 MLSS) Load 2 Load 3 3.617 11.595 3.145 8.428 P uptake ratio (mg P g-1 MLSS) Load 2 Load 3 2.792 6.872 3.052* 7.922* 2.784 14.622 2.260* 18.554"

Load 4 9.071 10.235 Load 4 5.457 6.708* 18.437 37.571"

Table 4 Phosphorus release and uptake ratios per cell Acinetobacter calcoaceticus in bioaugmented activated sludge system using 5 g L -1 NZ* (a) and 15 g L 1 NZ* (b) additive and without NZ additive by different initial phosphorus loads

Experiment (a) (b)

Experiment (a) (b)

Load 1 3.31 e -11 3.61 e -11

Load 2 Load 3 1.02 e "1~ 5.73 e -1~ 1.16 e "1~ 2.91 e"1~ P uptake ratio (nag P 8 1 MLSS) Load 1 Load 2 Load 3 2.33 e "12 1.32 e 11 2.54 e 11 9.06 e "12. 4.12 e "11. 7.22 e "11. 2.70 e "12 8.07 e"12 4.46 (11 5.44 e-12. 2.11 e-11. 1.17 e-1~

Load 4 1.92 e -1~ 3.75 e -1~ Load 4 6.39 e 11 1.59 e -1~ 3.36 e "11 2.55 e "10.

removal efficiency using 1.5 % NZ additive was over 50 % even at P-shock loading. The results support that the proposed biological reactors with NZ addition are very effective in treating high strength phosphorus bearing wastewater. The results of this study indicate that NZ is worthy of further consideration as an EBPR amendment, especially where phosphorus removal is priority. The application of NZ in EBPR wastewater treatment can be used as a low-cost, efficient and energy-saving technique. The sludge and zeolite produced through such biological treatment for phosphorus removal from wastewater can be used as a good slow release fertilizer [17]; thus indicating this process as an excellent means of solving environmental problems while, at the same time, recycling huge amounts of energetic substances from waters.

REFERENCES 1. Jones M. and Stephenson T. (1996): The effects of temperature on enhanced biological phosphate removal. Env. Tech. 17, 965-976.

1750 2. Fuhs G.W. and Chen M. (1975): Microbiological basis of phosphate removal in the activated sludge process for the treatment of wastewater. Microb. Ecol. 2, 119-138. 3. Mino T., van Loosdreeht M.C.M., Heijnen J.J. (1998): Microbiology and biochemistry of the enhanced biological phosphate removal process. Wat. Res. 32, 3193-3207. 4. Kortstee G.J.J., Appeldorn K.J., Bonting C.F.C., van Niel E.W.J., van Veen H.W. (2000): Recent developments in the biochemistry and ecology of enhanced biological phosphorus removal. Biochemistry (Moscow) 65, 332-340. 5. Sakadevan K. and Bavor H.J. (1998): Phosphate adsorption characteristics of soils, slags and zeolite to be used as substrates in constructed wetland systems. Wat.Res. 32, 393-399. 6. Jardin N. and Popel H.J. (1994): Phosphate fixation in sludges from enhanced biological P-removal during stabilization, In: Chemical water and wastewater treatment III, Klute R. and Hahn H.H. (eds.), Springer, Berlin. 7. Lopez-Ruiz J.L., Lopez-Aleala J.M., Torres-Fernandez J.C., Rodriguez-Fuentes G. (1997): Elimination of phosphates by natural zeolites. 5th International Conference on the Occurrence, Properties, and Utilization of Natural Zeolites-Ischia, Program and Abstracts, De Frede, Napoli, 209-211. 8. Gharaibeh S.H. and Dwairi I.M. (1996): Removal of nutrients from sewage effluent in stabilization ponds using natural zeolite. Chemisehe Technik 48, 215-218. 9. Lefcourt A.M. and Meisinger J.J. (2001): Effect of adding alum or zeolite to dairy slurry on ammonia volatilization and chemical composition. J. Dairy Sci. 84, 1814-1821. 10. Venglovsky J., Paeajova Z., Sasakova N., Vueemilo M., Tofant A. (1999): Adsorption properties of natural zeolite and bentonite in pig slurry from the microbiological point of view. Vet. Med. 44, 339-344. 11. Sasakova N., Vargova M., Venglovsky J (1999): Anaerobic stabilization of the solid fraction of pig slurry amended with zeolite (Clinoptilolite). Slovak Veterinary Journal 24, 206-210. 12. Hrenovic J., Orhan Y., Buyukgungor H., Tibljas D. (2001): Phosphorus removal from wastewater in upgraded activated sludge system with natural zeolite addition. Proc. 13th International Zeolite Conference, Montpellier, France, 372. 13. Papp J. (1992): Einsatzmoglichkeiten von Zeolith in der Abwassertechnik. AWT Abwassertechnik, Heft 2. 14. Papp J. and Olah J. (1997): The texture- and morphology-related biological activity of clinoptilolite in sewage treatment. 5th International Conference on the Occurrence, Properties, and Utilization of Natural Zeolites-Ischia, Program and Abstracts, De Frede, Napoli, 244-245. 15. APHA (1992): Standard Methods for Examination of Water and Wastewater. American Public Health Association, 18th edn, New York. 16. Hrenovic J., Orhan Y., Buyukongor H. (2000): Phosphorus removal from wastewater by activated sludge and supplied activated sludge. Biotechnology 2000 - The World Congress on Biotechnology - 11th International Biotechnology Symposium and Exhibition, Berlin, Book of Abstracts 3, 473-475. 17. Armbruster T. (2001): Clinoptilolite-heulandite: applications and basic research. Proc. 13th Intemational Zeolite Conference, Montpellier, France, 13-27. 18. Hannus I., Fejes P., Fonesca A., Nagy J.B., Parker W.O., Szendi Z. (1996). Interaction of phosphorus trichloride with zeolites. Zeolites 16, 142-148. 19. Hrenovic J. (2001): Effect of the various carbon sources and growth conditions on phosphate release and uptake by Acinetobacter calcoaceticus. Acta Bot. Croat. 60, 85-96.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1751

Zeolitized tuffs as p e d o g e n i c substrate for soil re-building. Early evolution o f zeolite/organic m a t t e r proto-horizons A. Buondonno a, E. Coppola a, M. Bucci a, G. Battaglia a, A. Colella b, A. Langella c and C. Colella d aDipartimento di Scienze Ambientali, Seconda Universit/~ degli Studi di Napoli, Via Vivaldi 43, 81100 Caserta, Italy bDipartimento di Scienze della Terra, Universit/l di Napoli Federico II, Via Mezzocannone 8, 80134 Napoli, Italy CDipartimento di Scienze Geologiche ed Ambientali, Facolt/t di Scienze, Universit/~ del Sannio, Via Port'Arsa, 11, 82100 Benevento, Italy aDipartimento di Ingegneria dei Materiali e della Produzione, Universit/l di Napoli Federico II, P.le Tecchio 80, 80125 Napoli, Italy An investigation was undertaken aiming to evaluate the suitability of zeolitized turfs as an

anthropogeomorphic material for soil re-building in degraded and desertified areas. Four artificial soil proto-horizons were prepared utilizing fine limestone gravel or Neapolitan

yellow tuff as inorganic components, and sewage sludge or pellet manure as organic parent materials. The proto-horizons evolution was followed by analyzing leachates periodically collected over 80 days. After 80 days, the body of proto-horizons was also analyzed. Our results showed that the release of C- and N- compounds in the leachates was strictly dependent on the nature of the parent organic material. The presence of Neapolitan yellow tuff in the proto-horizon bodies clearly favored an advantageous evolution and stabilization of parent organic matrices, in optimizing the carbon to nitrogen ratios by increasing the C/N values, and by protecting the organic matter against a disproportionate degradation. Our findings led to conclude that the zeolitized tuff is a promising material in pedotechnique strategies for soil reconstruction and fight against desertification; on the contrary, the organic matter appeared to be excessively demolished in the presence of limestone gravel. 1. I N T R O D U C T I O N Soil degradation and desertification are today some of the most relevant environmental issues. Over the last decades, the global rate of soil degradation has dramatically increased [1 ]: about 80% of the world's agricultural lands are now affected by erosion, and eve~ year 75 billion metric tons of topsoil are washed or blown away, which causes 120,000 km" to be lost to agriculture, with a connected world-wide annual cost for soil erosion at approximately 400 billion dollars. In particular, desertification from natural climatic change, human

1752 activities, or both, now threatens 45 million km 2, a full one-third of the Earth's land surface. Worldwide population growth will reduce the average area per person for food and fiber production from 0.27 ha today to < 0.14 ha in 2040 [2]. When the appropriate conditions subsist, a possible way to re-build lost soils and to restore the environmental balance is to use inorganic and organic materials, even waste or byproducts [3,4], as "anthropogeomorphic" materials [5] for pedogenic substrates of artificial soils. In our concept, such a primarily heterogeneous substrate, when correctly used, might evolve as a "proto-horizon" in developing an artificial anthropogenic soil compatible with the surrounding pedoclimatic environment. On these bases, our research aimed to evaluate the suitability of zeolitized tufts as pedogenic mineral component of proto-horizons for soil re-building. In fact, zeolitized materials are often found as pedogenic substrate in volcanic areas [6-9]. Furthermore, their peculiar cation exchange and adsorption properties [ 10], which result in high surface activity, are likely to favor the basic physical-chemical reactions involved in the pedogenic processes. Finally, notwithstanding zeolitized materials are used for various purposes [11], their application in pedotechnique for soil re-building has not yet been considered. This paper deals with the preliminary investigation on the use of Neapolitan yellow tuff (NYT) for soil re-building in strategies of environmental restoration of devastated limestone quarry areas of Campania region (southern Italy). In particular, observations on the early evolution of organic matter/NYT substrates, compared with organic matter/limestone gravel substrates, will be discussed. To follow the evolution without disturbing the proto-horizons, analyses were carried out on leachates periodically collected over 80 days. After 80 days, the body of proto-horizons was also analyzed. Attention was primarily paid to variations concerning carbon and nitrogen, which are the foremost elements involved in the evolution of soil organic matter, with special reference to humification processes [12-14]. 2. MATERIALS AND METHODS

2.1. Starting Materials Starting mineral materials were (a) coarse or fine limestone gravel (CLG or FLG, respectively) from limestone quarries near Caserta (Campania region, Italy), and (b) Neapolitan yellow tuff, NYT, from Marano, (Napoli, Italy); this material represents the more recent (12,000 a b.p.) tuffaceous formations of the Phlegraean Fields (Napoli, Italy), covering an area of about 13 km 2 [15]. The NYT sample used for the present investigation, with the following chemical composition (wt%): SIO2=52.91, A1203 = 14.73, Fe203=4.03, MgO = 1.08, CaO = 2.07, K20= 7.57, Na20= 2.76, P205 = 0.11, H20= 14.10 [16], had a cation exchange capacity (CEC), measured by the cross-exchange method [ 17], equal to 1.90 meq g-l, and a zeolite content, estimated by the vapor desorption procedure [ 18] equal to 54%, with phillipsite = 37% and chabazite = 17%. 2.2. Organics Matrices utilized as organic matter source were (a) sewage sludge, SS, from the "Regi Lagni" plant (GI.RE.LA. Company, Villa Literno, Caserta), and (b) pellet mixed manure, PM, produced by SCAM Company (Modena). Table 1 shows the main properties of the starting mineral and organic materials.

1753 Table 1 Selected properties of starting mineral and organic materials FLG NYT SS PM OC a g kff 1 n.d n.d. 186.6 275.0 N b g kg" n.d. n.d. 20.6 46.5 OC/N 8.1 5.9 EC c dS m "1 0.06 0.16 4.05 17.13 pH r 9.1 8.3 7.7 7.2 n.d. = not detectable; aorganic carbon; ~total nitrogen; CElectrical Conductivity and pH in material/water suspension 1:2.5 w/v. 2.3. Proto-horizon model preparation Suitable amounts of FLG (Q 2.00-4.75 mm) or NYT ( ~ < 0.30 mm) were mixed with SS or PM to arrange the four proto-horizons FLG-SS, FLG-PM, NYT-SS and NYT-PM, each with a final organic carbon (OC) content of 62.5 g kg ~, and with N content and C/N ratio of 7.7 g kg l and 8.1 for FLG-SS and NYT-SS, and of 10.6 g kg l and 5.9 for FLG-PM and NYT-PM, respectively. Table 2 reports the main properties of the proto-horizons at starting experiment. Table 2 Selected properties OC g kg" N g kg- " OC/N EC dS m 1 pH

of proto-horizons at starting experiment FLG-SS 62.50 7.70 8.12 2.10 7.6

FLG-PM 62.50 10.60 5.90 6.86 7.4

NYT-SS 62.50 7.70 8.12 2.46 7.4

NYT-PM 62.50 10.60 5.90 6.39 7.1

FLG or NYT alone were used as reference models. Small-scale cylindrical lysimeters ( 0 10 cm, height 40 cm) were filled to 20 cm with CLG limestone gravel ( 0 4.75-9.50 mm), on which the proto-horizons or the reference materials (10 cm depth) were superimposed. Three lysimeters were replicated for each model. The artificial soil systems were water-saturated at starting time and after 1, 7, 14, 21, 35, 50 and 80 days, and then leachates were collected and analyzed. After oven-drying in air stream at 40 ~ the proto-horizon bodies were carefully removed from lysimeters, softly crumbled and ground avoiding disruption of carbonate grains, sieved to O < 2.0 mm and then analyzed.

2.4. Leachate and proto-horizon body analysis Leachates were analyzed for the following parameters: (a) pH, Electrical Conductivity (EC) and nitric nitrogen (N-NO3) according to the Italian standard methods for water analysis [19], and (b) dissolved organic carbon (DOC), according to the method by 0hlinger and Gerzabek [20].

1754

Proto-horizon bodies were analyzed for pH, EC, organic carbon (OC) and total nitrogen (N) according to the Italian standard methods for soil analysis [21]; pH and EC were determined in material/water suspension 1:2.5 w/v. Data from leachates or from proto-horizon bodies were respectively expressed as ml or mg per kg ofproto-horizon bodies on the dry weight (105 ~ basis. To make the comparison easier, data for carbon and nitrogen evaluated after 80 days in proto-horizons have also been expressed as percentage variation (A%) with respect to the initial value of the corresponding sample, as A% = 100"[(X0a-Xs0a)/X0a], where X is the value of OC or N parameters at starting experiment (0d) or after 80 days (80d). 3. RESULTS AND DISCUSSION 3.1. Leachates The different prow-horizons showed an initial water holding capacity increasing as FLG-PM < FLG-SS < NYT-PM < NYT-SS, with values corresponding, respectively, to 168.3, 305.1,385.8 and 513.9 mg water per kg proto-horizon. This is evidently due to the concurrent effect of the zeolitized material and the sewage sludge, both able to retain huge amounts of water. The pH values widely varied as a function of the nature of the starting materials (Table 3). Leachates from simple FLG or NYT models were definitely alkaline, with average pH values of 8.4 for both samples. Differently, pH from proto-horizons usually showed an initial increase from near neutral to sub-alkaline or alkaline values during the first 15 days of observation, followed by a gentle decrease with final pH values ranging from 7.1 (FLG-SS) to 7.5 (FLG-PM). The highest pH values were observed in leachates from proto-horizons containing pellet manure, especially in the presence of carbonate matrix (pH FLG-PM = 7.7 and pH NYT-PM = 7.5, on average). In any case, the largest variations were observed within 40d from the start of the experiment. The leachates from PM-containing proto-horizons also showed particularly high EC values (Table 3), with starting values of 14.27 and 12.69 dS m -1, and average values of 3.58 and 3.04 dS m 1, for FLG-PM and NYT-PM, respectively. Leachates from NYT-SS and FLG-SS models showed initial EC values of 4.25 and 3.76 dS m -1, with average values of 1.63 and 1.44 dS m l , respectively. As a comparison, the average EC values for simple FLG or NYT leachates were 0.10 or 0.14 dS m-, respectively. For all proto-horizon leachates, the EC evidently decreased to less than 1.10 dS m -1 during 40 days, with a final moderate increase ranging from 0.80 for FLG-SS to 2.00 for NYT-SS. The labile organic matter in leachates showed a large initial increase within 40d, followed by a plateau trend (Table 3). Much more dissolved organic carbon (DOC) was leached from PM-containing than from SS-containing proto-horizons. Up to 3718 or 3812 mg of DOC per kg proto-horizon were cumulatively collected from FLG-PM or NYT-PM, respectively (Table 3), whereas 846 or 955 mg kg -1 DOC were cumulatively collected from FLG-SS or NYT-SS models. After 80 days, 29, 30, 35 or 41 mg of leached nitric-nitrogen (N-NO3) were cumulatively collected per kg FLG-SS, NYT-SS, FLG-PM or NYT-PM, respectively (Table 3). However, the N-NO3 leaching trends showed a particular behavior: an initial substantial release followed by a plateau trend in leachates from NYT-PM and FLG-PM, and a more gradual,

1755 continuous release from FLG-SS and NYT-SS. Consequently, the DOC/N-NO3 ratio followed distinct tendencies as a function of the organic matrix of proto-horizon: in the presence of PM, the ratio rapidly declines within 8 days, and hence assumes practically constant values until 80 days; differently, in the presence of SS, the ratio still declines from 8 to 80 days. Table 3 Characteristics of leachates from proto-horizons over 80 days experiment 1d a 8d a 15d a 22d a 36d a 50d a

pH FLG-SS FLG-PM NYT-SS NYT-PM

80d a

7.1 7.2 7.0 7.0

7.3 7.7 7.4 7.6

7.3 8.0 7.5 7.7

7.2 7.9 7.5 7.6

7.1 8.0 7.4 7.7

7.1 7.6 7.4 7.7

7.1 7.5 7.1 7.4

3.76 14.27 4.25 12.69

1.73 4.33 1.57 2.60

1.07 1.82 1.08 1.53

1.06 1.27 1.04 1.15

0.86 1.09 0.65 0.90

0.79 1.07 0.80 1.01

0.80 1.25 2.00 1.38

Ec b

FLG-S S FLG-PM NYT-S S NYT-PM

DOC r FLG-SS FLG-PM NYT-SS NYT-PM

281.6 2327.2 384.5 2591.8

471.2 3036.4 575.1 3076.2

552.1 3185.1 654.7 3233.1

699.3 3440.2 810.8 3528.6

760.9 3600.2 867.7 3637.1

818.6 3688.9 916.5 3753.6

846.1 3718.3 955.4 3811.5

N-N03 c FLG-SS FLG-PM NYT-SS NYT-PM

0.9 3.6 0.9 4.5

5.1 27.0 7.1 32.7

7.4 31.0 11.0 37.2

9.0 32.0 15.6 38.5

13.6 33.5 20.7 39.5

21.0 34.0 24.5 40.0

29.1 34.7 30.1 40.9

DOC/N-N03 FLG-SS 296.4 91.8 74.1 77.4 55.8 39.0 29.0 FLG-PM 646.5 112.6 102.7 107.4 107.4 108.4 107.2 NYT-SS 427.2 80.9 59.5 55.7 42.0 37.4 31.7 NYT-PM 575.9 93.9 87.0 91.6 92.1 93.8 93.2 adays from experiment start; bdS ml; Ccumulative amounts, mg per kg proto-horizon. These data clearly suggest that the release of C- and N-compounds in leachates, arising from the evolution of organic matter, strictly depends on the nature of the parent organic material; in particular, the pellet manure seems to promptly evolve toward a steady stadium, whereas the sewage sludge appears to be somewhat recalcitrant, and less susceptible to reach a stable equilibrium within the observation period.

1756 3.2. Proto-horizon bodies and carbon and nitrogen budget Table 4 reports analytical data for the different proto-horizons bodies determined after 80 days. Table 4 Characteristics ofproto-horizons after 80 days FLG-SS FLG-PM OC g kg -1 46.42 33.12 N g kg ~ 6.26 9.01 OC/N 7.41 3.68 EC dS m l 1.73 3.10 pH 7.5 7.5

NYT-SS 59.50 6.70 9.22 1.28 7.6

NYT-PM 45.47 7.30 6.44 2.31 7.5

According to the characteristics of the organic matrices, the starting conditions ofproto-

horizons revealed a strongly unbalanced organic carbon to nitrogen ratio (Table 2), with values corresponding to 8.1 for proto-horizons with SS, and even 5.90 in the presence of PM. Such values of OC/N ratio less than 10.0 are typical clues to inadequate humification processes in soil, particularly when the organic matter is prevailingly decomposed and bioutilized rather than transformed in more stable humic substances. As expected, the final content of both carbon and nitrogen in the proto-horizons after 80 days (Table 4) was substantially lower than the respective initial value (Table 2). However, the decrease rates of C and N notably differed from sample to sample. As a rule, according to the above observations, more carbon was proportionally lost from proto-horizons with PM than from those containing SS (Table 5). Table 5 Relative variation (A%) of Organic Carbon and Nitrogen content, and of Organic Carbon to Nitrogen ratio in proto-horizon bodies at 80d from starting experiment FLG-SS FLG-PM NYT-SS NYT-PM OC -25.71 -47.01 -4.65 -27.25 N -19.14 -14.62 -12.99 -31.13 OC/N -8.19 -37.98 +13.59 +9.00 Besides, the average A% of OC was -16% in the presence of NYT, versus -36% in the presence of FLG, whereas the average A% of N was -22% in the NYT-proto-horizons, against -17% in the presence of FLG -proto-horizons. As a consequence, the OC/N ratio raised up to 9.2 in NYT-SS and 6.4 in NYT-PM, while dramatically fell down to 7.4 in FLG-SS and 3.7 in FLG-PM (Table 4). It must be also mentioned that all proto-horizons bodies exhibited final pH values near 7.5, not far from the respective initial values. Differently, the electrical conductivity strongly decreased, with values about 2.3-3.1 dS m l in proto-horizons containing pellet manure, and < 2 dS m l in those with sewage sludge.

1757 4. CONCLUSION Our results suggest that: 9 the organic matter in the pellet manure can evolve more quickly than that present in the sewage sludge; 9 the final pH and EC conditions are compatible with pedogenic processes; 9 the proto-horizons formed by Neapolitan yellow tuff as inorganic anthropogeomorphic material favored an advantageous evolution of parent organic matrices, in optimizing the carbon to nitrogen ratios by increasing the C/N values, and by protecting the organic matter against a disproportionate degradation; on the contrary, the organic matter appeared to be excessively demolished in the presence of limestone gravel. These last effects are particularly remarkable, and deserve further investigations and explanations. In the first instance, it can be postulated that the high surface activity of zeolitized materials could positively affect the fundamental bio-chemical-physical reactions of humification processes. It is also relevant to evaluate the experimental results from the environmental point of view. Taking also into account the cumulative amounts of carbon transferred in the leachates, the net loss of C after 80 days from proto-horizon bodies was 16.1 or 29.4 g per kg of FLGSS or FLG-PM, and 2.9 or 17.0 g per kg NYT-SS or NYT-PM, respectively. Assuming that this carbon was transformed in CO2, the average loss in terms of greenhouse-effect gas emission corresponds to 224.4 t CO2 per ha of an artificial soil with limestone gravel, against 83.6 t CO2 per ha for an artificial soil with zeolitized materials. Considering as reference that about 1 t C ha l , equivalent to 3.7 t CO2 ha ~, is annually assimilated by forest in neo-tropical sites [22], the amount of C saved and stored by proto-horizons with Neapolitan yellow tuff undoubtedly appears important. In conclusion, the zeolitized tuff is a promising material in pedotechnique strategies for soil reconstruction and fight against desertification, including the protection of soil organic matter. Future research steps will regard the pedogenic evolution of the proto-horizons under the mineralogical, structural and chemical aspects, evaluating the weathering and the transformation of the parent materials, also as influenced by the presence of active rizosphere.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1759

N e a p o l i t a n y e l l o w t u f f for the recovery o f soils polluted by potential toxic elements in illegal d u m p s o f C a m p a n i a R e g i o n E. Coppola a, G. Battaglia a, M. Bucci a, D. Ceglie b, A. Colella c, A. Langella d, A. Buondonno a and C. Colella e aDipartimento di Scienze Ambientali, Seconda Universit~t degli Studi di Napoli, Via A. Vivaldi 43, 81100 Caserta, Italy bprocura della Repubblica, Tribunale di S. M. Capua Vetere, Piazza della Resistenza, 81055 S. M. Capua Vetere, Caserta, Italy r di Scienze della Terra, Universit& di Napoli Federico II, Via Mezzocannone 8, 80134 Napoli, Italy dDipartimento di Scienze Geologiche ed Ambientali, Facolt~t di Scienze, Universitg del Sannio, Via Port'Arsa 11, 82100 Benevento, Italy eDipartimento di Ingegneria dei Materiali e della Produzione, Universit~t di Napoli Federico II, P.le V. Tecchio 80, 80125 Napoli, Italy The Neapolitan yellow tuff (NYT) was utilized as a component of an organo-mineral sorbent/exchanger soil conditioner with pellet manure (NYT/PM) to limit the mobility of Cd and Pb in heavily polluted soils from illegal dumps in Low Volturno river basin (Campania Region, southern Italy). The NYT/PM mixture (1:1, w/w) was added to soil at the rates of 0%, 25%, 50% or 75% (w/w). The overall significant effect of the amendment with NYT/PM on soil properties was the substantial reduction of the availability of the toxic elements, with special reference to Pb, connected with an increase of cation exchange capacity and electrical conductivity and a moderate decrease of soil pH. The observed effects were primarily ascribed to the Pb-selectivity of phillipsite and chabazite present in the tuff matrix. On the whole, our findings suggest the suitability of NYT as natural exchanger material to be utilized in strategies for the remediation of polluted sites. 1. INTRODUCTION The so-called "heavy metals", better designated as "potential toxic elements", PTE [ 1], are the foremost cause of the chemical degradation and pollution of soil. The PTE reach soil and accumulate in it through various natural and anthropic factors and processes [2], including criminal activities. These activities represent a threat for those areas, such as the Campania Region of southern Italy, where the waste emergency favors the illegal disposal of toxic

1760 waste, with subsequent pollution of soils and water, and very great hazard for all biota, including human beings. In particular, in the case of pollution by PTE, these easily move from soil to man directly, or along the food chain. Since PTE can be neither swept nor washed away, a possible way to reduce their dangerousness is to restrict their mobility in the soil/environment system by means of sorption/exchange reactions in soil and by uptake and accumulation in plant [1,2]. From this standpoint, natural zeolites have been proved to effectively take up some toxic elements by cation exchange [3,4]. A recent study also suggested the use of organo-clinoptilolite fertilizers for the remediation of mine-waste sites [5]. On these bases, a study was started aiming to evaluate the suitability of zeolitized turfs as components of organo-mineral sorbent/exchanger soil conditioners to limit the mobility of PTE. This paper deals with the preliminary investigations on the effects of Neapolitan yellow tuff (NYT)/organic matter conditioner on the mobility of Cd and Pb in soils of illegal dumps in Low Voltumo river basin (Campania Region, southern Italy).

2. M A T E R I A L S AND M E T H O D S 2.1. Study area and toxic waste The Low Volturno river basin belongs to the Campanian Plain, and it is bounded by Tyrrhenian sea on the West, Mount Massico and Roccamonfina Volcano in the North, Mount Maggiore and Tifatini chain on the East, and by the Phlegraean Fields to the South. The area, covering about 60,000 ha, represents one of the most valuable agricultural lands of southern Italy, with buffalo farming and fruit, tomato, tobacco, vegetables and cereals as prevailing crop production. Illegal dumps have been discovered and seized in various sites between the towns of Capua and Castelvolturno, not far from the left bank of the Volturno river. The toxic material deposited was pellet foundry slags, containing considerable amounts of Cd and Pb (Table 1).

Table 1 Main properties of pellet foundry slags Feature mean Total Cd, mg kg ~ 724 Total Pb, mg kg "1 32954 Extractable Cd a, mg kg "l 83 Extractable Pb a, mg kg -1 1211 b, dS m l pH b EC

17.2 10.2

min 318 9937 36 48 1.9 9.9

max

1305 59900 164 3540 38.6 10.7

Particle size O >16 mm 4.7 4 6 16 mm > O >2 mm 46.3 45 48 O <2 mm 52.3 46 60 aExtractable by DTPA; bElectrical Conductivity (EC) and pH in material/water suspension 1:2.5 (w/v).

1761 Slags were clearly alkaline, with high electrical conductivity, and with coarse to fine particle size. It is noteworthy that the density of the material was 1.30-1.35 kg dm 3 for particles with ~ <2 mm, and 2.57-2.62 kg dm 3 for particles ranging from 2 to 16 mm. Both particle size and density of slags are similar to those of soil materials: this implies that toxic slags are easily incorporated by soil, thus increasing the pollution hazard. The surface of contaminated sites widely varied from about 100 to 25,000 m 2. However, larger amounts of toxic slags were amassed per unit area in smaller sites.

2.2. Soils Soils have been classified according to USDA Soil Taxonomy [6]. The dominant pedotype in the investigated dump areas is mixed, mesic Vertic Xerofluvent (calcareous). Twelve polluted soils were collected from illegal dumps, and non-polluted reference soils were also sampled from uncontaminated neighborhoods. Soils were analyzed according to the Italian standard methods for soil analysis, and Cd and Pb were extracted by DTPA (diethylenetriamine-pentaacetic acid) [7]. A Pollution Index (PI) for Cd or Pb was calculated by: P1-Cd = [(DTPA-Cd ps- DTPA-Cd rs) / DTPA-Cd rs] PI-Pb = [(DTPA-Pb ps- DTPA-Pb rs) / DTPA-Pb rs]

(1) (2)

where DTPA-Cd ps or DTPA-Pb ps represent the amounts of Cd or Pb extracted from polluted soils, and DTPA-Cd rs or DTPA-Pb rs represent the respective amounts extracted from the reference non-polluted soils.

2.3. Organo-mineral sorbent/exchanger conditioner Starting materials for the preparation of organo-mineral sorbent/exchanger conditioner were: 9 Neapolitan yellow tuff, NYT, from Marano, (Napoli) (zeolite content: phillipsite = 37%; chabazite = 17%; cation exchange capacity (CEC) = 1.90 meq/g; pH = 8.3; EC = 0.165 dS rel; O <0.30 mm) [8-12]. Details on NYT characteristics are reported by Buondonno et al., this volume [ 13]. 9 Pellet manure (PM) (C - 275.0 g kgl; N = 46.5 g kgl; pH - 7.2; EC = 17.130 dS m'l), produced by SCAM, Modena. The organo-mineral NYT/PM mixture was prepared by thoroughly mixing NYT and PM at 1 to 1 ratio by weight. 2.4. Preparation of amended soil systems and design of laboratory experiments The soil samples utilized for the experiments were collected from surface horizon (Ap, 40 cm depth) of a polluted soil representative of a large cultivated area (about 25,000 m 2) where waste materials were disseminated and mixed with soil particles. Four amended soil systems (ASS) were prepared by adding the NYT/PM conditioner to soil samples at the ratio of 0, 25, 50 and 75% conditioner/soil; such amended soil systems will be indicated as ASS-0, ASS-25 ASS-50 and ASS-75, respectively. Five-hundred grams of each ASS replicate were placed into l-liter stoppered, large-neck vessels together with water (soil:water = 1:2 w/v). The mixtures were then equilibrated by end-over-end shaking for 24 h at 120 oscillations-per-minute (opm), quantitatively transferred

1762 into 2-liter large polyethylene cups (O 20 cm), and finally dried for 2 weeks in a stream oven at 40 ~ After the first week, samples were daily inspected and stirred to ensure uniformity in the drying process. After drying, ASS samples were collected, suitably crushed and sieved at O <2 mm, and then analyzed for DTPA-extractable Cd and Pb. Extracted Cd and Pb were determined by Atomic Absorption Spectrophotometry (AAS) using a Perkin Elmer Analyst 100 apparatus. Three replicates of each ASS were prepared. Data were expressed on dry weight (105 ~ basis.

3. RESULTS AND DISCUSSION 3.1. Soil pollution The disposal of toxic slags induced a substantial alteration of physical and chemical soil properties (Table 2). The incorporation of waste material in soil caused an artificial enrichment of the coarser sandy fraction, shifting the soil texture from silty loam to sandy loam. Consequently, a dilution of the whole soil body occurred, thus decreasing the organic carbon and the clay content, with subsequent decline of the cation exchange capacity. The contamination was also associated to a moderate increase of soil pH and electrical conductivity.

Table 2 Selected properties of polluted and reference soils Feature Polluted soils mean min max Sand, g k~ 622 324 777 Silt, g k g 316 200 492 Clay, g kg "l 62 9 184 ......

-1

mean 287 577 136

Reference soils min max 91 541 375 740 77 260

OC a, g kg-1 CEC b, meq g-1

5.9 0.047

3.9 0.036

9.5 0.074

16.9 0.187

6.2 0.104

28.7 0.351

EC c, dS m "l pH c

0.453 8.8

0.228 8.4

0.762 9.2

0.342 8.0

0.201 7.6

0.701 8.4

DTPA-Cd, mg kg l 9.58 1.01 17.03 0.16 0.09 0.30 DTPA-Pb, mg kg l 45.84 20.8 97.49 6.02 1.22 12.92 aOrganic carbon; bCation Exchange Capacity; CElectrical Conductivity and pH in soil/water suspension 1:2.5 (w/v). The amounts of DTPA-Cd and -Pb extracted from the polluted soils were dramatically higher than the respective amounts extracted from the reference soils. The rate of contamination was particularly severe for Cd, which showed an average PI-Cd = 58.9, but still important for Pb, with an average PI-Pb = 6.6. Extreme Pb and Cd contamination prevailingly occurred in small areas, where large amounts of toxic material were massively accumulated. However, critical levels of DTPA-Pb and DTPA-Cd were also detected in large

1763 areas, where waste materials were spread on cultivated soil surface and frequently incorporated into the Ap surface horizons. 3.2. Effects of amendment with NYT/PM conditioner on polluted soil

The polluted soil utilized in the experiment had DTPA-Cd = 4.8 mg kg -1, and DTPAPb = 55.7 mg kg l , with P I - C d = 31.0 and P I - P b = 9.5. As expected, the amendment with NYT/PM conditioner induced an extensive, albeit non-linear increase of organic carbon (Figure 1, left scale) and cation exchange capacity (Figure 1, right scale), thus providing the amended soil systems with substantial exchange/sorption sites for Cd and Pb. We also found that both the electrical conductivity and pH of the amended soil systems were notably affected by the NYT/PM additions (Figure 2, left scale and right scale, respectively). The pH definitely fell from 9.1 in ASS-0 sample to 8.6 and 8.3 in ASS-25 and ASS-50 samples, respectively, whereas remained practically unchanged at 8.3 in the ASS-75 sample. On the other hand, the EC almost linearly increased from ASS-0 to ASS-75, with values corresponding to 0.76 or 8.37 dS m -1, respectively (Figure 2). Therefore, the ionexchange equilibrium in the experimental soil systems was altered as a consequence of the various amendments, and then the mobility of both Pb and Cd was significantly reduced (Figure 3, left scale and right scale, respectively). The effect was more substantial for Pb, whose P o l l u t i o n I n d e x was lessen to 3.8 already after a 25% NYT/PM addition rate, and reached an average value of 1.7 in the ASS-75 sample. Conversely, the amount of DTPA-extractable Cd moderately declined in ASS-25 and ASS-50 samples, with DTPA-Cd = 4 or 3.7 mg kg 1, respectively, whereas it was reduced to 2.8 mg kg 1 in the ASS-75, with a final P I - C d = 17.7.

150 125

- 0.150 ---0-- OC --X-- CEC

- 0.125

100

0.100 ,..,,

75

0.075 g

50

0.050 ~

!

25 0

................ ~. . . . . . ASS-0 ASS-25

0.025

,................ , ............... [ 0.000 ASS-50 ASS-75

Figure 1. Effects of NYT/PM amendment on organic carbon (OC) content and cation exchange capacity (CEC) of polluted soil.

1764

-

9.2

-

9.0

-

8.8

~6 |

=Z ~ cj4

_

-

_

8.6

8.4 t

. . . . . . . . . . .

ASS-0

I:

l

ASS-25

T

i

ASS-50

8.2

ASS-75

Figure 2. Effects of amendment on electrical conductivity (EC) and pH of polluted soil.

60

6 -90 - -

50 "7

Pb

----X--Cd

40

4

"Te~

E .Q 30

3

-~

e~o

<

!

< 20

[-.,

10 . . . . . . . . .

ASS-0

t

.

.

.

.

ASS-25

i

.

.

.

.

.

.

.

.

.

ASS-50

"'i

.........................

"'""

ASS-75

Figure 3. Effects of amendment on cadmium and lead mobility in polluted soil.

0

1765 The distinct behavior of Cd and Pb is likely attributed to their different interactions with the various components of the investigated systems, e.g., soil, zeolitized tuff and organic matrix. In fact, several factors and processes may govern the Cd and Pb mobility [1,2,14], such as: 9 the nature and amount of the exchanger/sorbent; 9 the specific affinity of each cation for selective exchange/sorption sites; 9 the pH, composition and ionic strength of the soil solution; 9 the relative stability of metal complexes with organic ligands; 9 the competition with other metal ions and the interaction with counter-anions. It is therefore difficult to give an estimation of the peculiar influence of each of the above factors and processes on the final Cd and Pb budget. Nevertheless it is undeniable that phillipsite and chabazite, present in the Neapolitan yellow tuff, may exert a prevailing role on the differential Cd and Pb mobility observed in the experimental systems, considering that both zeolites are much more selective for Pb than for Cd (equilibrium constants, Ka, turned out to be 22.8 and 4.1 for Na/Pb pair in phillipsite and chabazite, respectively, and 0.025 and 0.57 for Na/Cd pair in phillipsite and chabazite, respectively [ 15]). 4. CONCLUSIONS Our findings showed that, in agreement with literature [5], a conditioner prepared as a mixture of zeolitized tuff and pellet manure was suitable to amend soil polluted by anthropic activities. The amendment was particularly effective in reducing the Pollution Index of Pb more than that of Cd, due to the selectivity of zeolite minerals in the tuff matrix. In the light of what here discussed, research will continue aiming: 9 to evaluate the effects of the amendment with zeolitic tuff/pellet manure conditioner on the plant response; 9 to elucidate the competitive or synergistic effects of soil clay minerals/zeolitic minerals/organic matter interactions in controlling the global ion equilibrium in amended soil systems, also considering the effects on the dynamics of nutrient beneficial elements. REFERENCES

1. 2. 3. 4. 5. 6.

B.J. Alloway (ed.), Heavy metals in soils, 2nd ed., Blackie Academic and Professional, Glasgow, UK, 1990. A. Kabata-Pendias and H. Pendias, Trace elements in soils and plant, 2nd ed., CRC Press, Boca Raton, Florida, USA, 1992. C. Colella, Mineral. Deposita, 31 (1996) 554. M. Pansini, Mineral. Deposita, 31 (1996) 563. P.J. Leggo and B. Led6sert, Miner. Mag., 65 (2001) 563. USDA (United States Department of Agriculture- Soil Survey Staff), Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys, 2nd ed., Agriculture Handbook No. 436, USDA, Natural Resources Conservation Service, New York, 1999.

1766

10. 11. 12. 13. 14. 15.

MIPAF (Ministero per le Politiche Agricole e Forestali- Osservatorio Nazionale Pedologico e per la Qualit/t del Suolo), Metodi di Analisi Chimica del Suolo, No. 1124.2, FrancoAngeli Editore., Milano, Italy, 2000. M. de' Gennaro and A. Langella, Mineral. Deposita, 31 (1996) 452. A. Buondonno, C. Colella, E. Coppola, M. de' Gennaro and A. Langella, in Natural Zeolites for the Third Millennium, C. Colella and F.A. Mumpton (eds.), A. De Frede Editore, Napoli, Italy, 2000, p. 449. C. Colella, M. de' Gennaro, E. Franco and R. Aiello, Rend. Soc. Ital. Min. Petr., 38 (1982-83) 1423. M. Pansini, C. Colella, D. Caputo, M. de' Gennaro and A. Langella, Microporous Materials, 5 (1996) 357. M. de' Gennaro and C. Colella, Thermochimica Acta, 154 (1989) 345. A. Buondonno, E. Coppola, M. Bucci, G. Battaglia, A. Colella, A. Langella and C. Colella, Zeolitized tufts as pedogenic substrate for soil re-building. Early evolution of zeolite/organic matter proto-horizons: This volume. B.H. Wiers, R.J. Grosse and W.A. Cilley, Environmental Science & Technology, 16 (1982)617. C. Colella, in Natural Microporous Materials in the Environmental Technology, P. Misaelides, F. Macasek, T.J. Pinnavaia and C. Colella (eds.), NATO Sciences Series No. E 362 (Applied Sciences), Kluwer A.P., Dordrecht, The Netherlands, 1999, p. 207.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1767

Application of Jordanian faujasite-phillipsite tuff in ammonium removal Khalil M. Ibrahim Department of Earth & Environmental Sciences, Institute of Land, Water and Environment Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan. [email protected]

Faujasite-phillipsite tuff (FAU) from Jabal Hannoun area was evaluated in ammonium removal from wastewater compared with chabazite-phillipsite tuff (CHA) from Tell Rimah area. Both samples are characterized by good physical and chemical properties, which enable them to be used as ion exchangers under column operation condition. In the presence of considerable concentration of competing ions such as Na +, Ca § K § and Mg § under conditions of different ammonium concentrations, the FAU exhibits successful performance in removing NH4 § from wastewater compared with the efficiency of CHA for 50 and 25 mg NH4+/I concentration. The study indicates that the efficiency of the FAU is about 2.2 to 2.0 times greater than CHA. Unlike the selectivity series of pure natural phillipsite and synthetic faujasite reported in the literature, FAU selectivity series is: Mg+2> K+> Ca +2>NH 4+. This study remarks that regeneration of the FAU samples with Na using column operation is far more efficient than Na-loading by soaking in concentrated NaC1 solution. In this regard, a considerable enhancement in the performance of regenerated FAU samples exceeds 100% is noted compared with their performance in the other experiments. 1. INTRODUCTION It was demonstrated that the presence of ammonia in water has far more serious implications than merely serving as an index of recent pollution [ 1]. It is therefore important to remove NH4 + from industrial wastes and from drinking water. The main physical-chemical processes considered for ammonium removal are biological nitrification, de-nitrification, chlorination [2,3]. Removal of NH4 + from solutions by ion exchange, due to the relative simplicity of the application can be considered a valid alternative. Organic ion exchangers which are very selective to NH4 +, are usually used for the purpose but their cost is prohibitively high. Zeolites are the most important inorganic ion exchangers. Natural zeolites, including clinoptilolite, phillipsite, chabazite, mordenite and erionite, frequently display good selectivity for NH4 +, which makes them valuable tools for the purification of industrial wastewater [4]. This conclusion is based on the results of thorough investigations including both laboratory and pilot plant studies [5]. In Jordan, as a consequence of increasing emissions from anthropogenic sources, the general abundance of NH4 + in the important Amman-Zarka groundwater basin has reached intolerable and alarming levels. In some of the groundwater observation wells, NH4 + is between 26 mg/1 and 115 mg/1. The level permitted by National Standard 202 is 5 mg NH4+/I,

1768 but more than eight industries violate standard 202 and discharge effluents with NI-I4+ concentration from 8.3 to 414 mg/1 [6]. Even though industrialists realize the problem, they always care about the costs. They need solutions that offer efficiency at a very low cost. In order to evaluate the performance of the faujasite-phillipsite tuff (FAU) in ammonium removal from wastewater compared with a chabazite-phillipsite tuff (CHA), four sets of experiments were carried out using simulated solution under dynamic conditions in an ion exchange column. The degree of ion exchange was investigated at different ammonium concentrations. 2. EXPERIMENTAL SETUP Two zeolitic tuff samples prepared by mineral processing technique [7] were used in this work. The FAU sample is collected from Jabal Hannoun area, where an economic faujasite tuff deposit was discovered by [8]. The zeolitic tuff of this locality was evaluated for industrial and environmental application by [9,10]. The CHA sample is collected from Tell Rimah area. The Tell Rimah zeolitic tuff was also evaluated for industrial and agricultural applications [ 11,12]. The two samples exhibit suitable grain size, have suitable zeolite content and cation exchange capacity. They are characterized by good attrition resistance and high packed-bed density. Such properties enable these fractions to be used as ion exchangers under column operation condition. The physical properties of the FAU and CHA samples and their chemical analysis are summarized in Table 1 and 2. As shown in Table 2, the FAU samples are characterized by higher Na20 and LOI content and lower K20 and MgO content compared to the CHA. Zeolitic samples were Na-loaded as described in [9]. The sets of experiments will be referred to as S1 - $4. The experiments were operated under fixed conditions. Table 3 summarizes the feed composition and Table 4 shows the operation conditions under which the experiments have been carried out. In S 1 and $2, the NH4§ was prepared in the presence of other interfering and competing cations (Na§ K § Mg 2§ and Ca2§ in order to simulate conditions of wastewater. $3 and $4 were performed in the absence of competing cations (Table 4). Table 1 Physical properties of the enriched zeolitic tuff samples used in the experiment. Grain size (mm) Packed-bed density (g/cm3) CEC (meq./g) Attrition resistance (wt loss %) Zeolite grade (%)(1)

FAU -0.25+0.125 1.12 3.55 5.3 faujasite =57 phillipsite =35

~l~Zeolitic grade was evaluated by X-ray diffraction following [7] method.

CHA -0.25+0.125 1.04 3.24 5.4 chabazite =53 phillipsite =43

1769 Table 2 Chemical analysis of the zeolitic tuff Wt% SiO2 A1203 Fe203 MgO CaO Na20

FAU 40.01 15.13 1.06 0.79 10.18 4.91

CHA 44.96 17.54 2.27 3.10 8.50 0.30

Wt% K20 TiO2 P205 MnO LOI Total

FAU 0.92 0.21 0.33 0.09 26.58 100.21

CHA 3.24 0.43 0.24 0.07 19.03 99.68

Table 3 Operation conditions of column experiments 5.8 5.0 4.3 12.0 0.5

Bed Length, (cm) Bed Weight, (g) Bed Volume, BV (ml) Flow Rate (ml/min) Breakthrough Point (mg/l)

Table 4 Feed composition and dynamic data Feed Composition (mg/1) Ca

++

Na + K+ Mg ++ NH4+ Dynamic Parameters Breakthrough Volume (BV) Total volume at exhaustion (BV) % Efficiency ($2/S 1• 100) % Efficiency ($3/$2• 100) % Efficiency ($4/$3• 100)

S1

$2

$3

$4

20 51 11 10 50

20 57 12 10 25

0 0 0 0 25

0 0 0 0 25

CHA

FAU

CHA ,,

FAU

CHA

FAU

FAU

73 593

161 291

110 1280 151

221 404 137

345 1512

691 1349

1400 1828

314

313 203

1770 Evaluation of the efficiency of a product as an ion exchanger can not be considered complete without testing the efficiency of the exchanger to regeneration after exhaustion. In the literature, different regenerants were used, during this investigation NaC1 regenerant was used, a widespread and low cost material. Several exhausted samples from previous experiments were regenerated for the experiments $4, which were carried out under the same conditions of $3. A solution with a high concentration of the regenerant (0.1M NaC1) was prepared and fed to the bed at a very slow flow rate (3 - 4 ) ml/min in order to optimize results of regeneration. Based on a report to the Federal Water Pollution Control Administration, USA, [1] reported that the maximum allowable concentration of the total ammonia in water is 1.5 mg/1. According to the instruction of the European Community Council of 15 July 1980, the maximum permissible level of ammonium in drinking water is 0.5 rag/1. The performance of the FAU compared with CHA, was evaluated at 0.5 mg/1 total ammonia in water.

3. RESULTS AND DISCUSSION In the S1, the initial NH4 + concentration in the simulated influent was 50 mg/1. Under these conditions indicated in Table 4, the FAU and CHAsamples display a considerable efficiency to remove NH4 + from the wastewater of 161 BV and 73 BV, respectively. A considerable improvement of efficiency was noted in the experiment $2 where the NH4 + concentration was reduced to 25 mg/1, keeping the other factors unchanged. In detail, the FAU efficiently treated about 221 BV of ammonium-polluted wastewater and CHA treated about 110 BV. This indicates that the efficiency of FAU in the $2 experiment increased of about 137% compared to S 1, whereas CHA increased of about 151%. In the $3 and $4 experiments, the NH4 + concentration was also 25 mg/1. The efficiency of the FAU in the $3 further increased of about 213% compared to $2 and of about 103% in $4 experiment compared to $3~ Fig.1 reports the NH4 + breakthrough curves obtained for the FAU, which show a progressive increase in the performance of the sample from S 1 to $4. Fig. 2 compares the selectivity for NH4 + of the two samples in experiments $2 and $3. According to [13], breakthrough curve characterized by high symmetry and limited width, indicate higher selectivity. The self-sharpening boundary of the breakthrough curves in FAU confirms remarkable higher selectivity for ammonium even in the presence of interfering cations. The CHA gave rise to a self-diffusing boundary of the breakthrough curves indicating decreasing selectivity. Fig. 3 reports the breakthrough curves of Ca, K, Mg and NH4 + from the FAU obtained in experiments $2. Interpretation of these breakthrough curves allows to define the selectivity sequence. In general, the least selected cation is that which reaches the breakthrough point first. The reported selectivity series as emphasized by the results of the experiments, is: Mg+2> K+> Ca+2>NH4+. This series is different from the selectivity series of pure natural phillipsite and synthetic faujasite reported by [14,15]. This may be related to the combined effect of the two zeolites occurring together in each sample (Table 1), the complex effect of interfering cations in solution and the influence of their ionic strength.

1771 50

OOO 9

()

O

40

~- FAU-S2

30

il

20

+

Z

i

--O-- FAU-S 1 [ ---A--- FAU-S3 ~FAU-S4

lO

q

0

500

1000

1500

2000

'

-

-

I

2500

3000

Effluent Volume (BV Fig 1 the NH4 + breakthrough curves obtained for the FAU

20

-

+

D

p

ix

9

10

----I--- CHA-S2 CHA-S3

12]

FAU-S2

l-1 0

250

500

~FAU-S3 750

1000

1250

1500

1750

Effluent Volume (BV)

Fig. 2 Comparison between the efficiency of FAU and CHA

A surprising result is that regenerated FAU sample, in experiment $4 exhibits better performance compared with its previous results. It is remarkable the enhancement of its performance between $3 and $4 which exceeds 100% (Fig. 1). This unexpected result implies that regeneration of the samples, using the method described in this section, is far more efficient than regeneration of the samples using the procedure described by [9], i. e. by soaking in concentrated solution of regenerant (1M NaC1)for two weeks with continuous shaking (batch operation). As a matter of fact, successful Na-loading or regeneration of FAU samples requires continuous contact with concentrated NaC1 solution under dynamic condition in order to drive substitution reactions away from equilibrium conditions.

1772 E 25o..

v

--O-- K+

= 20-

o -.~

Mg++

~ 15-

~0 lO~

o E

--a--- Ca++ NH4+

5-'

~ 0~0

..:.:

100

200

300

400

Effluent Volume (BV) Fig. 3 Selectivity of FAU during S2 experiment

4. CONCLUSIONS A faujasite-phillipsite tuff sample collected from Jabal Hannoun area and a chabazitephillipsite tuff sample from Tell Rimah area, south-east Jordan, were evaluated in wastewater treatment. A comparison between efficiency of the samples in NH4+removal from wastewater effluent was carried out with 50 and 25 mg NH4+/I solutions in the presence of Na +, Ca +2, K § and Mg +2 as interfering cations. The study indicates that the efficiency of the FAU is about 2.2 to 2.0 times greater than CHA. Unlike the selectivity series of pure natural phillipsite and synthetic faujasite reported in the literature, FAU exhibits the following selectivity series: Mg+2> K+>Ca+2>NH4+. This study remarkss that treatment ofNH4 + loaded wastewater by using regenerated FAU samples will lead to considerable enhancement in the performance of the FAU exceeding 100%. This unexpected result implies that regeneration of the samples using column operation is far more efficient than loading the samples with Na by using soaking in concentrated NaC1 solution. REFERENCES

[1] B. Mercer, L. Ames, C. Touhill, W. Van Slyke and R. Dean, J. Wat. Poll. Cont. Fed., 24 (1970) R95-107 [2] A.F Cassel., T.A. Pressley, W.W. Schuk and D.F. Bishop, AIChE, Symp. Ser., 68 (1972) 124:56. [3] H.M. Abd E1-Hady, A. Grunwald, K. Vlckova and J. Zeithammerova, in: Studies in surface science and catalysis. A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (eds.), Elsevier, 13 5 (2001) 3 l-P-09. [4] C. Colella, Mineral Deposita, 31 (1996) 554. [5] M. Pansini, Mineral Deposita, 31 (1996) 563-575. [6] R. Gedeon, in: Water pollution in Jordan, Friedrich Ebert Stit~ng, (1993) 51. [7] K.M. Ibrahim and S.D.J. Inglethorpe, Mineral Deposita, 31 (1996) 589.

1773 [8] K.M. Ibrahim and A. Hall, Eur. J. Mineral., 7 (1995) 1129. [9] K.M. Ibrahim, Environmental Geology, 40 (2001) 440. [10] C. Colella, D. Caputo and B. de' Gennaro, in: Studies in surface science and catalysis. A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (eds.), Elsevier, 135 (2001) 3 l-P-13. [11] K.M. Ibrahim, A.M. Ghrir and H. Khoury, in: Studies in surface science and catalysis. A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (eds.), Elsevier, 135 (2001) 31-O-01. [12] K.M. Ibrahim, Nasser T. Ed-Deen and H. Khoury, Environmental Geology, 41 (2002) 547. [13] A. Nastro and C. Colella, Ing. Chem. Ital., 19 (1984) 40. [14] D. Vaughan, in: Natural zeolites; occurrences, properties, use. L. Sand and F. Mumpton (eds.), Pergamon Press, Oxford, (1978) 353. [15] G. Gottardi and E. Galli, Natural zeolite, Springer-Verllag, (1985) 409.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1775

E v i d e n c e of the relationship occurring b e t w e e n zeolitization and lithification in the y e l l o w facies o f C a m p a n i a n Ignimbrite (southern Italy) Langella A. a, De Simone p.b, Calcaterra D. b, Cappelletti p.c, de' Gennaro M. c a Facolt/: di Scienze, Universit~t del Sannio, via Port'Arsa, 11 - Benevento, Italy b Dipartimento di Ingegneria Geotecnica, Universit/t Federico II, Piazzale V. Tecchio 80 Naples, Italy c Dipartimento di Scienze della Terra, Universit/l Federico II, Via Mezzocannone, 8 -Naples, Italy

An attempt to correlate different facies of an important volcaniclastic formation of southern Italy, the Campanian Ignimbrite, with their zeolite content and physico-mechanical properties was carried out. The following parameters were measured: total zeolite content, solid unit weight (7s), porosity, and uniaxial compressive strength. Samples were formerly grouped on the basis of the displayed colour in four categories each of them characterized by a different zeolite grade. An increase of zeolite content determines a decrease of porosity and a good positive correlation with UCS values. The relationship existing between zeolite grade and UCS becomes less pronounced or even lost, when zeolite content reaches about 50 % wt. Here, UCS values scatter in a quite wide range (3 - 6 MPa). It is hypothesized that the lithification of zeolite-bearing rocks is the result of two combined effects playing a different role during the zeolitization process: the transformation of the amorphous fraction in zeolite and the rock texture. In the first stage of the zeolitization process the role played by the rock texture (clasts/matrix ratio, pumice concentration, fractures, etc.) is totally masked by the overwhelming behaviour of zeolite itself up to that threshold value, above which the influence of these two leading parameters is inverted.

1. INTRODUCTION In Campania Region as well as in other areas of Southern Italy, volcanic tufts have been widely used as building stones since Greek-Roman ages. This material reached high architectural relevance in some monuments of the most important italian "cities of art" as well as in minor villages. The large availability and workability along with some good physicomechanical features of the stone favoured its large use as building stone. These petro-physical features have always been related to the lithification of the rock as a consequence of welding processes or authigenic mineralization of feldspars and/or zeolites. However, the current knowledge has not yet scientifically proved if the secondary minerogenetic processes can somehow affect the physical parameters of the rock. The present study represents a first approach to the evaluation of any relationship between the zeolite grade (or feldspar grade) of the rock and its petro-physical features, with particular

1776 reference to compressive strength. Besides, for zeolite-bearing rocks, an accurate method for the determination of some physical parameters such as solid and dry unit weight seems necessary to assess; accordingly, a new method for the determination of these physical parameters was validated.

2. MATERIALS Samples here considered belong to a widespread volcaniclastic formation of centralsouthern Italy, the Campanian Ignimbrite (37.000 y.b.p.). It is a trachytic pyroclastite mainly constituted by scoriae and pumice in a cineritic matrix. Two different lithofacies can be recognized [1 ]: a yellow facies, characterized by the presence of zeolites (mainly chabazite) and a grey one, with epigenetic feldspars. This latter facies can be further subdivided in three different subunits: a basal layer, with an eutaxitic structure, an intermediate layer, displaying an iso-orientation of scoriae and an upper layer characterized by chaotic texture and generally roundish scoriae [ 1]. Samples were collected from an archaeological site resting over Campania Ignimbrite outcrops and located in the middle course of Calore river in Hirpinia (Felette - Torte Le Nocelle, Avellino province). They belong to different lithofacies of the formation, from the weakly coherent grey facies to the lithified yellow one.

3. M E T H O D S Rock mineralogy was investigated by XRD analyses using a Philips PW1730/3710 automated diffractometer with a graphite monochromator and CuI~ radiation, operating at 40 kV and 30 mA. Measurement conditions using Philips APD 3.6 software included a 20 range from 3 to 100 ~ a step size of 0.02 ~ 20, and a count time of 4s per step. Quantitative mineralogical analyses were performed using the Reference Intensity Ratio (RIR) technique [2]. Morphological characterization was performed by Scanning Electron Microscopy (Jeol JSM 5310, CISAG). Solid unit weight (Ts) was determined on samples of rocks crushed and ground to a grain size lower than 150 ~tm. The mass of the sample is measured after drying in an oven at 110~ cooling in a desiccator and then placing it in the pycnometer (capacity: 70 ml), in accordance with the Suggested Methods for porosity/density determination of ISRM [3]. The same procedure is indicated in the ASTM Standard Test Method for Specific Gravity of Soils (ASTM D 854). The porosity (n) of a sample is usually determined by means of the relationship n=l-Td 7s, with 7d dry unit weight. As above cited, a specific procedure suitable to zeolitized material was introduced and hereafter commented. Uniaxial compressive strength was determined on cylindrical specimens (diameter of 54 mm and height to diameter ratio >2), using a Tritech 50 Compression Machine by Wykeham Farrance International, following the Suggested Methods for determining the Uniaxial Compressive Strength of lSRM [3].

1777 Table 1" Mineralogical quantitative evaluation (weight %) of the Felette site samples Smectite Biotite Feldspars Phillipsite Chabazite Analcime

Total

Tot. zeolite

Yellow

Mean Max Min Dev. St.

6.0 8.0 3.6 0.9

0.7 1.4 0.4 0.2

18.2 21.1 14.9 1.6

3.7 7.4 3.0 0.9

50.4 60.6 46.2 3.9

0.6 1.4 0.0 0.3

79.7 90.0 73.5 3.8

54.7 64.2 50.3 4.0

Yellow -Grey

Mean Max Min Dev. St.

2.6 6.4 0.0 2.1

0.8 2.0 0.4 0.5

18.3 24.9 12.4 3.4

2.1 3.2 0.7 0.8

28.5 39.1 19.5 6.6

0.9 1.4 0.0 0.4

53.2 65.3 34.4 10.6

31.6 43.5 21.5 7.5

GreyYellow

Mean Max Min Dev. St.

1.4 5.3 0.0 2.1

0.8 1.5 0.2 0.4

22.5 31.1 16.3 4.6

3.5 6.0 1.3 1.8

4.4 7.1 2.2 1.8

1.1 1.4 0.6 0.2

33.6 47.1 24.0 7.2

9.1 10.1 7.5 0.7

Grey

Mean Max Min Dev. St.

3.4 7.8 0.0 2.8

0.6 0.7 0.5 0.1

22.3 26.1 18.9 2.6

1.0 1.6 0.0 0.6

2.3 4.7 0.9 1.5

0.6 1.5 0.0 0.6

32.6 36.9 27.4 3.9

3.9 6.2 2.6 1.4

4. M I N E R A L O G I C A L P R O P E R T I E S All the studied samples were collected by a distal outcrop of the Campanian Ignimbrite formation. The vent area of this huge volcaniclastic deposit is supposed to be located within the Phlegraean Fields (Naples). In the investigated sector of the formation, the deposit is characterized by chromatic variations from grey to yellow passing through a series of intermediate hues. The relationship between these different typologies are not always well defined as a consequence of the limited exposure of the formation. The available data, however, allow to infer that the yellow areas represent the nucleus of the flow unit that, towards the most peripherical sectors, keeps the original greyish colour. The mineralogical quantitative analyses have been carried out on samples representative of the various lithotypes, distinguished from a chromatic point of view, in order to verify possible compositional differences and to evaluate any relationship existing between zeolite content and some physico-mechanical parameters. As expected, chabazite and phillipsite are the most abundant authigenic phases (Table 1) and the zeolite content constantly grows by passing from the weakly coherent grey facies to the yellow lithified one. Glass content is not reported and should be evaluated by the difference to 100 of the total weight %. SEM observations were also carried out on the four identified lithotypes, in order to point out the microtextural relationships of the constituents. It is quite remarkable the evolution of the zeolitization process by passing from the grey facies, through the grey-yellow and yellowgrey facies, to the yellow one (Figure 1).

1778

~i,o~,'~"..:~ ,

~"i~'~'~

. . . , , ---

..,:

,~ ~'~ . . . . .

~,.!i~

Figure 1 Micrographs of the typologies of tuffs identified in Felette site; l a: grey facies; 1b: grey-yellow facies; 1c: yellow-grey facies; 1d: yellow facies Micrograph of Figure l a reports a glass shard still well preserving its smooth surface typical of the unzeolitized glassy precursor and just slightly affected by very tiny acicular crystals. XRD analyses likely suggest this phase as being phillipsite (Table 1). Figure 2b (grey-yellow facies) also shows a glass surface that starts to be partially affected by larger crystals of authigenic phases (phillipsite). The evolution of the process can be observed in Figure 1c (yellow-grey facies) where the whole surface of the former glass shard is coated by smectite and large chabazite crystals can also be observed. Quantitative mineralogical analyses confirm for this lithotype a quick increase of chabazite content whereas phillipsite remains constant. Finally, Figure 1d evidences that glass has been totally replaced by zeolites (rombohedral chabazite crystals) which still preserve the overall original shape of the glass shard. Phillipsite is no longer involved in this process as its content remains constant. A concomitant slight increase of smectite is also recorded. The complex of information achieved by the mineralogical characterization enables to group all the analysed samples in 4 categories defined on the basis of the zeolite grade and falling within the following ranges: 0 - 5%; 6 - 10%; 11 - 4 0 % ; > 40%.

1779 5. GEOMECHANICAL PROPERTIES Measurements of solid unit weight for zeolitized tufts carried out with the standard procedures generally provide not reliable results and, above all, hardly reproducible data. This behaviour is due to the presence of zeolites whose capability of water absorption-desorption, even at temperatures lower than 110~ is well known. This behaviour is also displayed by the different lithotypes of Campanian Ignimbrite studied in the present paper as evidenced by Figure 2. Specimens were weighted soon after drying at 110~ and then at different times, until 24 hours. The adsorbed water content Wzwas calculated with reference to the initial weight. The adsorption is very fast in the early stages of the process, especially in the case of the yellow facies where more then 20% of the desorbed water is adsorbed in the first half hour. Adsorption then continues at a rate which rapidly decreases and the equilibrium is reached at 24h. Thus, the usual procedure that encompasses the reference weight useful for the measurement of dry and solid unit weight can not be applied to zeolite-bearing materials. In this case, a re-equilibration of at least 24h at controlled R.H. conditions is required. This measurement allows to correctly define another important physical property such as porosity. This parameter can provide useful information on the mechanical behaviour of the rocks and in particular on the Uniaxial Compressive Strength. The porosity values determined in this way once again evidence an overall grouping of the samples, as previously mentioned, with decreasing values of porosity as the zeolite content increases (Figure 3). As far as the comparison between mineralogical composition and geotechnical properties is concerned it can be best performed in terms of zeolite content and uniaxial compressive strength (Figure 4).

10 I-1 Yellow Yellow-Grey A Grey-Yellow C Grey

8! o

6 - -

d[ II.

>0 or a ~

~

2 ~

,(

ab

) 0

r, '

0

4

i

i

i

8

12

16

20 Time (h)

Figure 2. Typical adsorption curves for Felette tufts.

24

1780 0,65

[21Yellow

0,61

u

O Yellow-Grey 0 () C)

= 057

A Grey-Yellow

AA C

O Grey

~D A

o

o 0,53

O~O

<2 -, _.

0,49 0,45 1

10

100

Zeolite Content (%) Figure 3. Porosity distribution as a function of zeolite content It is well evident the role of zeolitization in lithification, expressed in mechanical terms by the increase of the uniaxial compressive strength. What should be also noticed is the quite regular separation of the representative points of the various facies. This result is not surprising if one considers that sampling was first carried out keeping in mind a distribution based on the aforementioned four hues. It is likely expected a more regular and uniform relationship between these two parameters by a careful and thick sampling. 100 I-'] Yellow 0 Yellow-Grey A Grey-Yellow 0 Grey

75

c12

E

0]3 ff]

50

L"

0

O o

~ 0

~ b~

25

0 ~]0 0,1

1

Uniaxial Compressive Strength (MPa) Figure 4. Zeolite content versus UCS

10

1781 6. DISCUSSION AND CONCLUSIONS The various samples of the Felette tuff analysed in this study have been initially grouped into four categories according to a chromatic criterion (i.e. grey, grey-yellow, yellow-grey, yellow facies). Both mineralogical and geotechnical measurements have put forward that each property groups exactly in the same way. This implies that chromatism corresponds to the different zeolite content, whose crystallization always leads to the formation of accessory phases, such as hydrated iron oxides, which give to the bulk mass a typical yellow or sometimes reddish colour. An important evidence, even though partially expected, is the modification of some physical properties of the rock as a consequence of the zeolitization process. The decrease of porosity as the zeolite content increases (Fig. 3) is to be related to the reduction of macropores among the glass shards when phillipsite crystals first, and chabazite in the following stages of the process, take over the shard surfaces. It is necessary to remark that the used analytical methodologies do not account for the microporosity, a peculiar feature of the zeolitic framework. However, according to the experimental results, it should be inferred that microporosity does not influence the other properties here considered. The existing relationship between zeolite content and UCS values (Fig. 4) can be considered as a further proof of the relationship between zeolitization and lithification as already reported for other Italian volcaniclastic products. A typical example is provided by the Neapolitan Yellow Tuff formation, characterized by a thick sequence of volcaniclastic materials showing an incoherent facies at the bottom and the top of the deposit and a lithoid, deeply zeolitized central portion [4]. Notwithstanding this relationship, it should be remarked that the good correlation existing between these two parameters becomes less pronounced or even lost, when zeolite content reaches about 50 % wt. Here, UCS values scatter in a quite wide range (3 - 6 MPa). This sharp increase of UCS values cannot be justified by the slight increase of zeolite content above the threshold mean value of about 50%. Consequently, other factors should be invoked to account for this behaviour. The present research evidenced that the zeolitization process mainly takes over the preexisting glassy matrix as a consequence of the progressive variation of the chemical-physical conditions of the system. However, textural relationships between the main constituents, namely glass shards, are substantially preserved (Fig. 1). In the first part of the process the zeolite net, formed at expenses of this amorphous fraction, gradually contributes to increase the steadiness of the rock framework as demonstrates the good correlation between zeolite content and UCS values (Fig. 4). When the zeolitization process seems to be completed (yellow facies) zeolite content starts to fluctuate within a narrow range (50-60%). UCS values linked to this group of samples, on the contrary, scatter in quite wide range (3-6 MPa) thus suggesting that, beyond that zeolite content threshold, parameters ruling the behaviour of the tuff should be searched in the rock texture, in its widest meaning (clast, lithics, and pumice concentration, their grain size, microcrackings, etc.). As a matter of fact, in the first stage of the zeolitization process the role played by the rock texture is totally masked by the overwhelming behaviour of zeolite itself up to that threshold value, above which the influence of these two leading parameters is inverted. Further investigation based on a careful observation of the relationship between the zeolitized matrix and clasts is however needed to confirm this hypothesis.

1782 AKNOWLEDGMENTS The Authors would like to express their appreciation to the students, especially eng .Giovanni Cerbone, whose meticulous laboratory work has contributed greatly to the measurements reported in this paper REFERENCES

1. Di Girolamo P., Petrografia dei tuff campani: il processo di pipernizzazione (tufo>tufo pipemoide>pipemo), Rendiconti Accademia Scienze Fisiche e Matematiche, s. IV, 35, Napoli, (1968) 5-70. 2. Chipera, S.J. and Bish, D.L. (1995) Multireflection RIR and intensity normalizations for quantitative analyses: applications to feldspar and zeolites. Powder Diffraction, 10 (1), 4755 3. Brown E.T. 1981. Rock Characterization Testing and Monitoring. Pergamon Press. 4. de' Gennaro M., Cappelletti P., Langella A., Perrotta A. and Scarpati C. (2000) - Genesis of zeolites in the Neapolitan Yellow Tuff: geological, volcanological and mineralogical evidences. "Contribution to Mineralogy and Petrology", 139 (1), 17-35.

ION EXCHANGE AND M O D I F I C A T I O N

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1785

C h a r a c t e r i s a t i o n of iron c o n t a i n i n g m o l e c u l a r sieves - the effect of T - e l e m e n t on Fe species P. Decyk, M. Trejda, M. Ziolek,* A. Lewandowska A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland ziolek@ amu.edu.pl Surface properties of Fe-modified micro- (ZSM-5) and mesoporous (MCM-41) molecular sieves exhibiting various T elements (Si, A1, Nb) in the framework have been studied by means of XRD, adsorption of N2, ESR, and FTIR spectroscopy combined with the adsorption of NO. Fe oxide species, tetrahedral coordinated Fe 3§ and isolated Fe 2§ cations have been identified depending on the kind of the matrix. The presence of niobium in the mesoporous framework enhances the oxidative properties of iron modified samples. 1. INTRODUCTION The subject of this paper refers to a rich field of inclusion chemistry in periodic mesoporous hosts [ 1] which has been explored in the context of the discovery of the ordered hexagonal MCM-41 and cubic MCM-48 mesoporous sieves [2]. Large pore MCM supports are of a great interest for numerous catalytic applications, some of which require the introduction of accessible ions into the pores. Niobium-containing MCM-41 has been synthesised the first time in 1997 [3,4] and has been recently applied as a matrix for transition metal cations [5,6]. Fe-containing catalysts have been intensively studied since their activity in HC-SCR of NO (selective catalytic reduction of NO with hydrocarbons) has been found [7]. Fe, Cu and other transition metals have been applied as active species supported on molecular sieves, mainly on ZSM-5 and mordenite. However, HC-SCR catalysts tested so far are not sufficiently active and selective even in the initial stage, are easy degradated by sulfur oxides, and exhibit too narrow temperature window. As several reaction steps occur in HC-SCR process it is difficult to find a catalyst that accelerates all these reactions with satisfactory activity and durability. Therefore, the bifunctional catalysts have been prepared and studied. In this contribution we wish to compare the surface properties of Fe containing meso- and microporous sieves which exhibit various T elements in the framework (A1, Si, Nb). A special focus will be on the bifunctional catalyst that contains niobium in the framework and iron in the extra framework position. Mesoporous matrices for iron have been chosen due to the possible application of the prepared catalysts in the oxidation of bulky molecules. This paper is devoted to the characterisation of iron species formed on the molecular sieves surfaces. 2. EXPERIMENTAL

The parent materials used for the modification with iron were: ZSM-5 zeolite (Degussa, Si/A1 = 31), MCM-41, and AI- and NbMCM-41 (Sift = 32) synthesised in our laboratory according to the procedures described in [3,8]. The F e - ion exchange was carded out in three manners depending

1786 on the parent material: i) traditional ion-exchange procedure (IE) used for all molecular sieves, ii) template ion exchange ( T I E ) - applied in the case of mesoporous molecular sieves containing template (i.e. before calcination), and iii) solid state ion exchange (SS) applied for ZSM-5 zeolite. The prepared materials were characterised using the following methods: XRD, N2 adsorption/desorption, ESR, FTIR + NO adsorption. XRD patterns were obtained on TUR 42 diffractometer with CuI~ radiation. FTIR spectroscopy was used for characterisation of Fe species in the catalysts and complexes formed after NO adsorption. Infrared spectra were recorded with a VECTOR 2 2 (BRUKER) FTIR spectrometer using an in situ cell. The samples were pressed, under low pressure, into w a f e r - 10 mg cm"2 and placed in the cell. The catalysts were activated under vacuum at 723 K for 2 hours. NO was adsorbed at room temperature. All spectra were recorded at room temperature. The subtracted spectra are presented. The ESR measurements were conducted after evacuation of the catalyst at various temperatures and after adsorption of NO. ESR spectra were recorded at 77 K on a RADIOPAN SE/X 2547 spectrometer. The patterns were obtained at VESR= 8.9 GHz. 3. RESULTS AND DISCUSSION

3.1. Description of the catalysts Four various matrices (shown in the experimental part) for iron have been used in this study. Table 1 Catalyst Ion-exchange Si/T a Fe/T a F e procedure (wt.%) exhibits the composition of the materials used. Fe-ZSM-5 (IE) IE 31 0.30 1.0 The template ion exchange Fe-ZSM-5 (SS) SS 31 0.75 2.4 (TIE) procedure leads to the higher level of Fe loading in Fe-MCM-41 (IE) IE 1.1 Fe-MCM-41 (TIE) TIE 2.9 mesoporous materials (with Fe-AlMCM-41 (IE) IE 32 0.33 1.0 the exception of A1MCMFe-AlMCM-41 (TIE) TIE 32 0.33 1.0 41) than the traditional ion Fe-NbMCM-41 (IE) IE 32 0.23 0.6 exchange (IE) technique. Fe-NbMCM-41 (TIE) TIE 32 0.70 2.1 The solid state ion exchange applied in microporous a - T = A I o r N b in the f r a m e w o r k ZSM-5 gives rise to the higher Fe content than that obtained after ion exchange (IE). The hexagonal arrangement of mesopores is preserved after modification with iron as shown from N2 adsorption/desorption isotherms and XRD patterns. It has been found that the metal species formed after the modification strongly depends on the molecular sieve structure, a nature of T element in the framework, and a technique of the cation including. FTIR and ESR spectroscopy have been applied to determine the Fe species on the catalyst surface. Table 1. The composition of the catalysts used in this study.

3.2. Infrared spectroscopy study NO adsorption combined with FTIR spectroscopy is widely used technique for the characterisation of iron species on the solid surfaces [for example 9-13]. Nitrogen oxide was found to be adsorbed more strongly over Fe 2+ sites than Fe 3+ ones. Segawa et al. [14] stated that the position of IR bands due to NO adsorbed on iron cations is rather insensitive to the

1787 valence state of iron; rather it depends on location of Fe cations. Therefore, the IR band positions might be determined by the structure of matrix used for the Fe modification. The FTIR spectra of Fe-ZSM-5 zeolite after adsorption of the increasing amounts of NO (Fig. 1A) are comparable with those presented by Sachtler et al. [11]. The IR band at 1878 cm -1 is assigned to mononitroslYl complex [Fe2+(NO)]. It seems to cover another one at a lower wavelength (at -1860 cm- ) suggesting the presence of mononitrosyl complexes located on iron at different exchange sites. The increase of the NO amount (>1.5 mbar of NO) results in the formation of a distinctly resolved band at 1814 cm -1 assigned to dinitrosyl complex [Fe2+(NO)2] although the second mode of this complex at -1920 cm -1 is not well visible (Fig. 1Ac). This complex is not stable and a short evacuation at room temperature (RT) causes its disappearance and the formation of the corresponding iron mononitrosyl complex formed at the same iron sites which gives rise to the 1768 cm -1 band (Fig. lAd). Therefore, one can state that Fe 2+ species accessible for the formation of dinitrosyl complex is different than that characterised by -1880 cm -1 mononitrosyl [Fe2+NO]. The increase of the contact time of NO with the zeolite (16 h at RT) leads to the growth of the 1632 cm -1 band intensity (Fig. 1Ae). The origin of this band will be discussed later. Nitrosyl complexes formed upon NO adsorption on Fe-A1MCM-41 (TIE) are presented in Fig.lB. The IR spectra are different from that registered on Fe-ZSM-5 although both matrices are aluminosilicates. The double bands at 1821 and 1847 cm -1 origin from two various complexes because their intensity do not change parallel depending on the amount of NO exposed. At a low NO coverage (0.5 mbar) both IR bands exhibit the highest intensity. Upon increasing the NO pressure (1 and >1.5 mbar of NO) they are gradually eroded (but not parallel) and one component (1821 cm -1) almost disappears after a short evacuation at RT. Less intensive features appear at 1926 and 1756 cm -I with the increasing NO doses. The described behaviour is similar to that related in the literature [12] for Fe-silicalite samples. Thus, taking into account the discussion in the literature and our results, one can postulate the presence of two various Fe 2+ species on Fe-A1MCM-41" less and more coordinatively unsaturated species. The first one gives rise to the formation of mononitrosyl complex [FeZ+(NO)] absorbing in the IR at 1847 cm -1. The second, more coordinatively unsaturated, is connected with two NO molecules forming dinitrosyl complex [FeZ+(NO)2] (a doublet at 1926 and 1756 cml). The less stable mononitrosyl characterised by the band at 1821 cm -1 is most probably formed on six fold coordinated Fe2+ ions at a surface of small iron-oxide particles. IFe-ZSM-5 (IE)

a

2200

2000

A

1800

1600 -1

,

i

1400

Fe-AIMCM-41 (TIE)

I

2200

,

I

2000

,

1821

I

1800

,

B

I

16010

i

I

1400

Wavenumbers, cm Wavenumbers, cm Fig 1. FTIR spectra of aluminosilicate molecular sieves after NO adsorption at RT of: a) 0,5 mbar; b) i mbar; c) >1,5 mbar; and d) after evacuation 15 min at RT; e) after 16 h at RT.

1788 Fe-NbMCM-41 (TIE)

,,.~18191752

'A

Fe-MCM-41 (TIE)

B 1818

a

34 d e

22100'

20100'

18100'

16100 '

Wavenumbers, cm 1

1410 0

2200

2000 1800 16.100 Wavenumbers, cm

1400

Fig 2. FTIR spectra of mesoporous molecular sieves after NO adsorption at RT of: a) 0,5 mbar; b) 1 mbar; c) >1,5 mbar; and d) atter evacuation 15 min at RT; e) atter 16 h at RT The FTIR spectra of NO adsorbed at RT on Fe-NbMCM-41 (TIE) and Fe-MCM-41 (siliceous) look similar in the range of 1800 - 1900 cm"] (Fig. 2 A,B). The difference is observed in the 1620-1650 cm"] region where NO2 species absorbs. The bands in this region are more intensive when NO is adsorbed on Fe-NbMCM-41. The behaviour of the band at 1819 cm"~ is like that at 1814 cm"~ on Fe-ZSM-5 zeolite, i.e., with the increasing of NO doses its intensity increases and a short evacuation at room temperature causes the decrease of its intensity. Simultaneously, the intensity of the 1752 cm"~band increases. Therefore, one can assigned the first band (1819 cm'~), which is accompanied by a small band at 1916 cm "1, to dinitrosyl [Fe2+(NO)2], whereas that at 1752 cm"] to mononitrosyl species. When Fe-NbMCM-41 sample was contacted with NO at RT for 20 hours (>1.5 mbar of NO) (Fig. 3) the IR bands became more intensive. Moreover, a shoulder at --1876 cm"1 is visible. It is due to mononitrosyl species located on a different iron sites than that responsible for 1752 cm1 band. The heating of the sample with the 1817 increasing temperature causes the decrease of 1817 and 1752 cm"~ bands intensities and the band at 1876 cm"] from mononitrosyl [Fe2+(NO)] becomes more pronounced. a 1986 1752 1 ,4 Therefore, one can suggests that this complex b is more stable at higher temperatures. It is worthy to notice the formation of nitro, c nitrito, nitrato species characterised by the bands at 1624 cm~ and also in-~1400 cm1 region (Fig. 3). These bands were discussed by Sachtler et al. [11, 15] and we assigned them I , I = I = I , I 2200 2000 1800 1600 1400 to the complexes formed on the species V~Ive'UTI:e~, cm"1 included oxo or peroxo ions. As the bands at --1400 cm] were observed only when NO was Fig. 3. The effect of heating of Feadsorbed on Fe-NbMCM-41 material, one can NbMCM-41 (TIE) in the presence of suggest that they are due to the presence of NO (>1.5 mbar) after: a) 20 h at RT; b) niobium in the sample. In fact, our previous 0,5 h at 473 K; c) 0,5 h at 673 K; d) 0,5 h study [16] showed such bands after NO at 723 K adsorption on the parent NbMCM-41 material.

1789 As concerns the 1 6 0 0 - 1650 crn"1 region, the highest intensity (taking into account the same experimental conditions) of the bands in this range is noted on aluminosilica micro- and mesoporous molecular sieves containing iron (Fig. 1 A,B). This band most probably covers more than one component. It might be due to the nitro groups bonded to iron ions existing at a different location, as concluded by Sachtler et al. [11] for NO adsorbed on Fe-ZSM-5. A shoulder at a lower wavelength, better visible on Fe-AIMCM-41, might be assigned to NO2 weakly adsorbed on the ferric oxide nano clusters. The same band is located at N1620 cm 4 when NO is adsorbed on Fe-NbMCM-41. It seems interesting that the nitro groups are formed slowly in the presence of NO and that its intensity increases upon evacuation or/and a longer contact time (16-20 h). Some authors [11] speculate that a superoxide ion (02") or peroxo species may be involved in the formation of nitro groups. This species can be formed in the interaction of oxygen, generated from the decomposition of NO or traces of oxygen in the system, with the vacancy in the binuclear site [HO-Fe2+-N-Fe2+-OH]2+.

3.3. Electron spin resonance spectroscopy study Fe-ZSM-5 (IE)

The ESR experiments were conducted for the identification of the "7 paramagnetic species on the surface of ::3 the catalysts used. The ESR spectra of i,.,,.i g _ . ~ g=4 ~ ,., ._.g, Fe-ZSM-5 (IE) evacuated at 773 and ill 973 K and scanned at 77 K are shown E: t-in Figure 4 a,b. The activation at 773 K ~~--3.93 _ c leads to the appearance of two ESR signals (g = 4.26 and g = 2.003). The first signal (g = 4.26) is typical of Fe 3+ , I i I paramagnetic cations in a strong 200 400 Field [mT] rhombic distortion of the tetrahedral Fig. 4. ESR spectra (registered at 77 K) alter: a) sites [11,17]. A narrow ESR signal (g = evacuation at 773 K, 2h, b) evacuation at 973 K, 2.003) can origin from coke formed 2h, c)adsorption 1 mbar of NO and evacuation at from the residual template but also it RT for 10m in. can origin from Fe 3+ in octahedral coordination as isolated ions at cationic Fe-AIMCM-41 (TIE) positions [ 18]. Moreover, Sachtler et al. Ig=6.1 I [11] attributed a sharp line at g=2.0 to superoxide ions (02") which are associated with iron ions. The increase of the evacuation temperature to 973 K causes the appearance of a small signal at g = 6.23 assigned in the literature [11,19] to Fe3+ in the coordination of the less distorted tetrahedron. Moreover, a broad signal described by g ~ 2.0 appears. It 200 400 600 800 can be attributed to the Fe-O-Fe species Field [mT] with ferri-, ferro-, and/or Fig. 5. ESR spectra (registered at 77 K) after: a) antiferromagnetic behaviour [20]. This evacuation at 873 K, 2h; b) evacuation at 973 K, 2h. signal is better visible when the spectrum

I

1790 is scanned at RT (not shown here). The ESR spectra of Fe-AIMCM-41 (Fig. 5 a,b) are similar to that of Fe-ZSM-5. The same signals are registered in the low field region (g>3) which are attributed to isolated Fe 3+ ions in different coordination environments, as well as lines at g<3 originating from Fe-oxo species. However, the increase of the evacuation temperature to 973 K causes the decrease of the intensity of all the ESR signals showing that Fe 3+ species in aluminosilica mesoporous matrix are less stable than when they are located in microporous ZSM-5 zeolite.

Fe-MCM-41 (TIE)

A

Fe-MCM-41filE)

~ / ~ g=2.2

g=4.27,..~__.B Ba

GainX10

S

e -

.......

I -~E

, . . . . .

200

..

.

.

.

.

400

Field [mT]

I

0 = 4 . 2 7 g -- 3 .93 c

100

120

140

160

Field [ mT ]

Fig. 6 A, B. ESR spectra (registered at 77 K)" after a) evacuation at 573 K, 2h, b) evacuation at 773 K, 2h, c) sorption 1 mbar of NO and 12h at KT. Figure 6 A,B shows the ESR spectra of Fe-MCM-41 (TIE) material. They differ from those described above showing the domination of the signal at g = 2.2 and a very weak one from Fe 3+ (g = 4.27 - Fig. 6B). These signals are visible after evacuation at 573 K but completely disappear when a higher evacuation temperature is applied (773 K). The above results allow us to propose that iron is Fe-NbMCM-41-32 (TIE) not grafted to the silicate framework as a Bourlinos et al. [21] observed in Feplanted MCM-41 samples. A broad -. = g=3.96 signal (g =-2.2) can be assigned to some ._~ b kind of iron-oxide species. Hagen et al. [18] observed a similar signal in H-[Fe,g=3.96 AI]- MFI samples and stated that they are due to aggregated hydroxide/oxide c species, whereas Sachtler et al. [11] observed a similar ESR line over Fe203/HMFI and attributed it to iron in 200 400 Field [roT] small Fe203 particles. It is worthy to Fig. 7. ESR spectra of Fe-NbMCM-41 (TIE) notice that all paramagnetic Fe 3+ species (registered at 77 K); after: a) evacuation at 773 K, disappear upon evacuation of the sample 2h, b) evacuation at 973 K, 2h, b) sorption 1 mbar at 773 K suggesting either the reduction of Fe 3+ to Fe 2+ or the formation of other of NO, c) without any treatment forl2h at RT. iron ESR silent species. Iron located in the mesoporous matrix containing niobium (Fe-NbMCM-41 (TIE)) gives rise also to a broad ESR signal at g = 2.10, due to a kind of Fe-hydroxide/oxide species (Fig. 7 t _ l ~

- - _

,,,-

t-"

I

=

I

1791 a,b). The participation of that Fe-oxo species in the whole paramagnetic Fe species is higher on Nb-containing matrix than on aluminosilica one. However, two other species (g = 4.27 and g = 2.002), the same as registered on Fe-ZSM-5, are more pronounced. The evacuation at 973 K does not lead to the appearance of Fe coordinated in a less distorted tetrahedron (g = --6) like on Fe-ZSM-5 and Fe-AIMCM-41. Nitrogen oxide is a good probe molecule for the identification of Fe 2+ isolated species if its adsorption is combined with the ESR measurements. Although it is a stable radical, NO in its ground state (2I'I1/2) does not exhibit an ESR signal unless the degeneracy of its 7t orbitals is removed [22]. The signal should be observable if the orbital moment of the electron is quenched, i.e., if the degeneracy among the n orbitals is removed by its environment [23]. The electric field associated with the Fe 2+ isolated cations might bring about such quenching of the orbital moment. The adsorption of NO on all the samples studied gives rise to the appearance of ESR signals in the range between 3 0 0 - 350 mT and another one in the low field (g=3.93) - Figures 4, 6B, 7. The latter origins from (NO)2 biradicals. The lines in the 300 - 350 mT region are not typical of mono or biradical of nitrogen oxide, i.e. a triplet from the hyperfine structure of N from NO or (NO)2. Most probably the superposition of ESR signals from mono, biradicals of nitrogen oxide and Fe-oxo species occurs. However, the fact that nitrogen oxide radicals appear after the adsorption on all the materials indicate the presence of Fe 2+ isolated cations, but on Fe-MCM-41 their amount is negligible. 4. CONCLUSIONS 9 All catalysts studied exhibit the presence of Fe 2+ isolated cations, but their amount is negligible on Fe-MCM-41, as shown from ESR (mono- and biradicals of NO adsorbed on Fe 2+) and FTIR studies (mononitrosyls [Fe2+NO]). 9 Fe-MCM-41 presents the highest concentration of Fe oxide species and the lowest amount of tetrahedral coordinated Fe 3+. 9 The presence of niobium in mesoporous matrix of MCM-41 sample increases the oxidative properties of the iron modified material (Fe-NbMCM-41) resulted in the formation of nitrito/nitrato species after NO adsorption (FTIR spectra). ACKNOWLEDGEMENT This work was partially supported by Polish State Committee for Scientific Researches, grant No 3 T 09A 102 19 REFERENCES

1. K. Moiler and T. Bein, Chem. Mater., 10 (1998) 2950. 2. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S. Beck, Nature, 359 (1992) 710. 3. M. Ziolek and I Nowak, Zeolites, 18 (1997) 356. 4. L. Zhang and J. Y. Ying, AIChEJ, 43 (1997) 2793. 5. M. Ziolek, I. Sobczak, I. Nowak, M. Daturi, and J.C. LavaUey, Topics in Catalysis, 11 (2000) 343. 6. M. Ziolek, I. Nowak, I. Sobczak, and H. Poltorak, Stud. Surf. Sci. Catal.130 (2000) 3047. 7. M. Iwamoto, H. Yahiro, Y. Yu-U, S. Shundo, and N. Mizuno, Appl. Catal., 70, L1,1991.

1792 8. S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W.Chu,D.H. Olsort, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. 9. L.J. Lobree, In-Chul Hwang, J.A. Reimer, and A.T. Bell, J. Catal, 186 (1999) 242. 10. K. Hadjiivanov, H. Knozinger, B. Tsynstsarski, and L. Dimitrov, Catal. Lett., 62 (1999) 35. 11. H.-Y Chela, EI-M. El-Malki, X Wang, ILK van Saxlten,and W.M.R Sachtler, J. Mol. Catal. A, 162 (2000) 159. 12. G. Spoto, A. Zecchina, G. Berlier, S. Bordiga, M.G. Clerici, and L. Basini, J. Mol. Catal. 158 (2000) 107. 13. M. Stockenhuber, M.J. Hudson, and R.W. Joyner, J. Phys. Chem. B, 104 (2000) 3370. 14. K.I. Segawa, Y. Chert, J.E. Kubsh, W.N. Delgass, J.A. Dumesic, and W.K. Hall, J. Catal., 76 (1982) 112. 15. H.-Y. Chen, T. Voskoboinikov, W.M.H. Sachtler, J. Catal., 180 (1998) 171. 16. M. Ziolek, I. Sobczak, A. Lewandowska, I. Nowak, P. Decyk, M. Renn, and B. Jankowska, Catal. Today, 70 (2001) 169. 17. A.V. Kucherov and A.A. Slinkin, Zeolites, 8 (1988) 110. 18. A. Hagen, F. Roessner, I. Weingart, and B. Spliethoff, Zeolites, 15 (1995) 270. 19. S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal., 158 (1996)486. 20. A. Bruckner, R. Luck, W. Wicker, B. Fahlke, and H. Mehner, Zeolites, 12 (1992) 380. 21. A.B. Bourlinos, M.A. Karakassides, and D. Petridis, J. Phys. Chem., 104 (2000) 4375. 22. P.H. Kasai and R.M. Gaura, J. Phys. Chem., 86 (1982) 4257. 23. P.H. Kasai and R.J. Bishop, Jr., J. Am. Chem. Soc., (1972) 5560.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1793

T h e r m a l d e c o m p o s i t i o n o f s o d i u m azide in various m i c r o p o r o u s materials Gy. Onyesty~ Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary Thermogravimetry and infrared spectroscopy were used to study the reactions within solid NaN3/zeolite mixtures. H-form zeolites were converted to Na, H-form already at room temperature, while hydrazoic acid (HN3) was evolved. Hydrazoic acid and NaOH was formed in the reaction with H20, bound to any of the cationic zeolite forms. In absence of mass transport limitations the rate of mentioned reactions was shown to depend on the protonic acidity of the zeolite, i.e., on the zeolite structure and composition. The NAN3, not consumed in mentioned reactions decomposed to metallic sodium and nitrogen at about 400-460 ~ 1. INTRODUCTION The zeolitic environment offers unique opportunities for the growth of small metallic clusters within the cages of the lattice. The most common procedure for metal introduction into zeolites is the ion exchange that is followed by the reduction of the metal ion with a suitable reducing agent. Metal clusters can be obtained in the zeolite cages, for instance, by reducing the metal ions with hydrogen gas. However, some cations, such as, Co- or Fe ions, are difficult to reduce by hydrogen when they are balancing the negative charge of the zeolite framework. In order to achieve reduction, vapour of Hg, Cd, or Na was used as reducing agent [ 1-3]. It was shown that sodium vapour can reduce the Fe 2+ ions in zeolites, resulting in highly dispersed iron clusters [4, 5]. Many studies have reported that metallic sodium can be formed in situ in the zeolites by thermally decomposing zeolite-embedded NaN3 salt [6-15]. When formed in Fe-zeolites the sodium was found to readily reduce the Fe 2+ ions to iron nanoclusters of narrow cluster size distribution [16-18]. However, it wa~ found that less than about 30 % of the iron could be reduced to the metallic state. Moreover, experiments were of poor reproducibility. Different aspects of the reaction between the sodium metal and the zeolitic cations were addressed in numerous papers, but the reason of the unexpectedly low extent of reduction and the poor reproducibility remained unknown. Na-zeolites, loaded with sodium azide can be prepared by impregnating the zeolite with a methanolic solution of sodium azide [11-15]. Due to the parallel conversion of the methanol and the NAN3, this method was not applied to prepare azide-loaded catalytically active transition metal zeolites. In the present study the reactions, occuring within the mechanical mixtures of solid zeolite and sodium azide were investigated. The results showed that, besides thermal decomposition, NaN3 can react also with the OH groups of the zeolite and with the zeolite-bound water. The conversion in latter reactions depends on the structure and composition of the zeolite and on the experimental conditions.

1794 2. E X P E R I M E N T A L

The zeolites used were Na-Y/Na55.9[A155.6Si136.40384]/(obtained from Grace, USA); Na-A

/Nall.9[Alll.6Si12.4048]/ and Na-X /Na79.E[A183.5Si108.50384]/ (Wolfen, Germany), and Namordenite /Na7.1[A17.2Si40.7096]/ (Stidchemie, Germany). The zeolite H-ZSM-5 sample /Na0.03H5.77[A15.8Si90.20192]/ was a gift from the Zentralinstitut ffir Physikalische Chemie, Berlin, Germany. The zeolite clinoptilolite was of natural origin (Horseshoe Dam, Arizona, USA). The clinoptilolite content of the examined rock was more than 96 wt %. The studied zeolite H-Beta /Sa0.2H4.1[A14.2Si59.90128]/ was our preparation. It was synthesized as prescribed in ref. [19]. The various cationic forms of the zeolites were prepared by conventional ion-exchange methods. The rest of the used materials, such as, the MgO and NAN3, were Merck products with a purity of >99%. Mixtures were prepared by grinding zeolite and sodium azide together in an agate mortar. No any pretreatment was given to the mixed components. The amount of NaN3 relative to the dry mass of the zeolite was the range of 43 to 299 mg/gzeolitr Some samples were produced by impregnating zeolite with aqueous solutions of NaN3, or NaN3 and NaOH. The thermal analysis of the samples was carried out in the 20-1000 ~ temperature range using Derivatograph 1500 Q type thermal balance (MOM, Hungary). During the measurement, sample was flushed with nitrogen or, in some experiments, with oxygen. The heating rate was 10~ For the infrared (IR) spectroscopic examinations samples were compressed to selfsupporting wafers. The spectra of the wafers were recorded using a stainless steel UHV cell and a Nicolet Impact 300 type spectrometer. Spectra were recorded at room temperature after in-situ heat treatments, each for 30 min, at higher and higher temperatures in lower than 10.5 mbar vacuum.

3. RESULTS AND DISCUSSION

Thermogravimetric (TG) and differential thermal analysis (DTA) curves of the solid mixtures and the pure components are shown in Fig. 1. On the DTA curves (Fig. 1, dotted lines) peaks of the endothermal dehydration of the zeolite and the exothermal decomposition of sodium azide can be distinguished. For the pure NaN3 an exotherm DTA peak appeared around 390 ~ (Fig. 1A). When MgO/NaN3 was examined similar curves were obtained as for the pure azide. At about the decomposition temperature of the pure azide DTA peak was obtained also with the NaNa/zeolite mixture, suggesting that some of the NaN3 is in weak interaction with the zeolite. This NaN3 fraction remained, most probably, out of the zeolite micropores. The peaks in the 400-470 ~ temperature range indicate hindered exothermic NaN3 decomposition processes (Tables 1 and 2). We assign this processes to the decomposition of the azide in the zeolite cavities [7]. The decomposition temperature depends on the azide content and also on the structure and composition of the zeolite. With increasing azide content the highest rate of decomposition, i.e., the peak temperature decreases. The peak at 469 ~ (Fig 1D), appeared as most intense in the presence of oxygen flushing gas. This peak is attributed to the decomposition of sodium nitrite that is known to be formed from the azide in reaction with 02 at elevated temperature.

1795

f

'0

'

o

TG

|

|

|

A

.C:) ~ ~

|

'

|

.O

C

o

O~1

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DTA

t-.

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.d L_

t'-

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NaN

~176

3

i

i

I

"13 t"

B

t~

N a - Y / N a N a in N 2

_~ ,

9

I

,

,

I

,

P~

oo

<3

Na-Y 0

I

100

9

.:

oo

2:

9

N a - Y / N a N a in 0 2 ,

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200

,

i

300

,

I

400

~]

,

20

P 9 ~

i

..o D

E

~

,

,

i

500

,

,

0

I

100

I

,

200

,

i

300

,

,

400

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500

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600

Temperature, ~

Fig 9 1 9TG (solid line) and DTA (dotted line) curves of NaN3 (A), Na-Y (B) and Na-Y/21 9 wt% NaN3 mixture (C) in nitrogen flow; and (C) in oxygen flow (D). The weight loss due to the azide decomposition, in the zeolite/azide mixtures was found generally lower than that calculated from the stoichiometry (2NaN3---~2Na~ + 3N2'~). For the different preparations and experimental conditions the deficits of the weight loss are given in Tables 1 and 2. The deficit was found to increase with the time elapsed between the preparation of the zeolite/azide mixture and the TG measurement (Table 1). If the dehydrated Na-Y zeolite was ground with NaN3 under hexane the deficit of the decomposition weight loss was smaller, but it could not be fully eliminated. Results suggested that water in the zeolite was responsible for the reaction consuming sodium azide. Zeolite ,~ H20 + NaN3 ~ " NaOH + HN3'~ (1) Only a fraction of the product H N 3 w a s retained in the zeolite 9It is not surprising that reaction (1) proceeds in acids, stronger than the hydrazoic acid (pKa = 4.77 at 25 ~ The conversion in reaction (1) depends strongly on the type the cations (Tables 1 and 2). It is known that the heterolytic dissociation of water in the electrostatic field of the trivalent cations of the La-Y generate strong Bronsted acid sites. The highest azide deficit was obtained with this zeolite indicating that the protonic acidity is related with the activity of the zeolite in the azide conversion (Table 1). The same can be concluded if the activities of the Na-X and the Na-Y zeolites are compared (Table 2). The addition of NaOH to the Na-Y support also inhibited reaction (1) either by shifting its equilibrium to the left or by decreasing the acidity of the zeolite (Table 1).

1796 Table 1 Characterization of the surface and decomposition reactions in the NaN3/zeolite Y mixtures. Sample a

NaN3 c o n t e n t , mg/gzeoliteb (sample condition)

Deficit of weight loss, c %

DTA peak temperature, ~

467 43 58 43 (3 months) 100 85 (3 months) 47 457 425 128 (3 months) 43 391 422 171 (3 months) 32 393 214 (3 months) 39 390 405 413 214 26 382 214 e 15 420 Na-Y/NaN3 f 214 (400 Kg/cm 2, 0.6-1 mm) 29 381 421 214(2000 Kg/cm 2, 0.3-0.6 mm) 31 380 413 214(2000 Kg/cm 2, 0.6-1 mm) 16 393 214(2000 Kg/cm 2, 4-5 mm) 0 381 Na-Y/NaN3 g 214 31 400 214 (1 wt % NaOH) 0 395 214 (3.7 wt % NaOH) 0 405 Fe-Y/NaN3" 214 28 411 Co-Y/NaN3 h 214 17 402 Ni-Y/NaN3 h 214 41 401 La-Y/NaN3 h 214 56 413 a Powdered mixtures were examined. Where it is indicated the selected particle size fraction was used. bThe TG measurement was carried out right after the preparation of the mixture, or after the storage time indicated in parentheses. The NaN3 content was given relative to the weight of the water-free zeolite. c In percents of the weight loss of stoichiometric NaN3 decomposition. a The effect of storage time. e Co-grinding under hexane. f The effect of granulation pressure and particle size. The powder was pressed to pellets, the pellets were crushed and sieved to obtain the particles. g The effect of impregnation with NaOH. h The effect of the exchange cation.

N'a-Y/NaN3 ~

Interestingly the azide loss in reaction (1) could be decreased or eliminated, if the mixed powder of Na-zeolite and NaN3 were compressed into pellets immediately after mixing. The azide loss was found smaller for the larger and for the or more compact particles, suggesting that the strong intercrystalline barrier to mass transport of HN3 within the particle suppresses the HN3 release and, possibly, the hydrolysis of the NaN3 salt. Zeolite Na-A and Na-X have about the same basicity, however the channel size of zeolite A is smaller than that of zeolite X. It seems probable, therefore, that the extent of hydrolysis of the azide salt and the azide loss was smaller for the Na-A due to the high resistance of its channels against the transport of

1797 Table 2 Characterization of the surface and decomposition reactions in different NaN3/support mixtures. Sample a

N a N 3 content,

mg/gzeoliteb MgO/NaN3 Na-A/NaN3 Na-X/NaN3 Na-Y/NaN3 Na-mordenite/NaN3 NH4-clinoptilolite/NaN3 H-Beta/NaN3

215 215 215 215 215 215 66 d 133 215 215 215 215 215

Deficit of weight loss, r % 0 9 19 26 12 ?r 100 11 ? 0 0 5 ?r

DTA peak temperature, oC 390

382 389

428 417 413 409 457 9

397

463 404 403

H-ZSM-5/NaN3 Na-ZSM-5/NaN3 382 429 NH4-ZSM-5/NaN3 375 451 466 Cu-ZSM-5/NaN3 422 a Powdered mixture. b Given relative to the weight of the water-free zeolite 9 r The TG curve could not be determined. The rapid nitrogen evolution blew out the sample from the sample holder. d This amount is equivalent with the amount of zeolitic OH-groups in the support.

1.5

381

the reactant NaN3 into the pores and the product H N 3 o u t of the pores (Table 2). Relative to that found with NaX and NaY the azid loss was also low with Namordenite, and even less with zeolite ZSM-5 and clinoptilolite. The activity difference of these zeolites can be can be interpreted in terms like channel size and network configuration. Mordenite contains unidimensional straight channels, while molecular sieve X and Y have three dimensional channel network. Although the pore sizes are comparable I (12-member ring channels), the diffusion resistance of the mordenite pores is undoubtedly higher than that of the pores WaveruT~r / c m -1 of molecular sieve X and Y. The azide Fig. 2. FTIR spectra of (A) H-Beta and its anion has 0.51-nm length and 0.35-nm mixture with (B), 13.3 wt % (C) 21.5 wt% NAN3. diameter. The diffusion of the HN3 and Spectra were recorded after 30-min evacuation of NaN3 molecules is probably very much the samples at 300 ~ hindered in the medium size (10-member

1.0

~0.5

%oo

3000

2000

1798 ring) channels of zeolite ZSM-5 and clinoptilolite. The sodium azide was rapidly converted ~ 4 o over zeolite H-Beta. If NaN3 was added to ethe H-Beta sample in an amount equivalent 2066 with the amount of the acid sites in the .Q 3 zeolite all of the NaN3 was converted in the o reaction with the water and/or the OH..Q2 #1 groups, present in the zeolite, at < 9". i: temperatures lower than the decomposition (..9 :! 9 temperature of the NAN3. Product HN3 was 9 c " . 5 :: :: 1 released. After thorough investigation of the Na-Y/ NaN3 system Fejes et al. [7-10] concluded that the sodium vapour, 0 4000 3000 2000 developing from the sodium azide, is W a v o n u m b e r / c m -1 oxidized to Na + ions by the acidic protons of Fig. 3. Comparison of the FTIR spectra of (A the zeolites. Our findings suggest that the and B) NaX/12.8 wt% NaN3 (dotted lines), and reaction with the NaN3 can neutralize the (C and D) NIX/12.8 wt % NaN3 mixtures Bronsted acid sites without decomposing (solid lines) after 30-min evacuations at (A and first to sodium metal and nitrogen. The IR C) room temperature and (B and D) 300 ~ bands at 3607 cm ] and at 3738 cm -] indicates that the H-Beta contains strong acid zeolitic OH groups and also weak acid silanol groups (Fig. 2, spectrum A). Both kind of hydroxyls were eliminated in the reaction with the sodium azide salt at temperatures, much lower than the temperature of NaN3 decomposition (Fig. 2, spectrmn B). The bands at 3393 and 2081 cm -1 stem from azides, HN3 and NAN3, remained in the sample (Fig. 2, spectrum C). As it was shown the cationic forms of the zeolites also induce reaction between H20 2070 and NaN3 (eq. 1). The reaction was followed by IR spectroscopy. The results for Na-X and the Ni-X is compared in Fig. 3. After O treatment at 300 ~ the OH-bands almost o~ disappeared, showing that most of the acidic 8E protons were replaced by Na-ion. The Ni-X ..Q was more active than the Na-X sample. The O characteristic asymmetric stretching band of the retained azide appeared at about 20602090 cm -]. (The spectrum of KBr/NaN3 pellet showed the azide stretching band at "'" ~ 2120 cm-1.) 0 The IR spectrum of the Na-Y/21.4 wt % 4OO0 3000 2000 NaN3 sample was recorded immediately after -1 Wavenumber / cm preparing the sample and then again 23 days Fig. 4. FTIR spectra of the NAY/21.4 wt % later (Fig. 4). The bands around 3400, at NaN3 mixture recorded (A) right after 3703 and 1640 cm -1, which can be assigned to the vibrations of adsorbed water, remained preparing the sample, and (B) 23 days later. o

1 2 0 9 1 i

t

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...

9.

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....

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.

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.

9

.

1799 the same. However the azide band at 2070 cm 1, present at the beginning, completely 1,0 disappeared by the end of the experiment. The new band at 1449 cm -1 indicates that NaOH was formed. The water polarized by the electrostatic field within the pores of the Na-faujasites, such as, zeolite X and Y gives a vOH IR band at about 3690-3700 cm 1. The 0,5 frequency of this band is lower than that of the silanol groups suggesting that the polarized water has acidic character. Obviously, the zeolite-bound water is an acid, strong enough to hydrolyze the NaN3 salt. The low temperature azide loss was , oo" 2000 ' ' 180 relatively low for the mordenite, the Wavenunt::er / a ' n -~ clinoptilolite and for the ZSM-5 Fig. 5. FTIR spectra of Na-mordenite/21.5 wt preparations (Table 2). The reactions in the % NaN3 sample calcined in high vacuum at Na-mordenite/NaN3 system was followed as (A) 100, (B) 400, (C) 500, and (D) 600 ~ a function of evacuation temperature by IR spectroscopy (Fig. 5). Interestingly, the intensity of the peak around 2070 cm -1 reached a maximum after treatment at 400 ~ It was shown earlier, that the absorption coefficient of the azide group strongly depends on its ionic environment [20 ].The increased intensity of the 2070-cm -1 can suggest that the treatment at elevated temperature increased the azide concentration in the micropores, where the azide ions are affected by a strong electrostatic field. Above 400 ~ decomposition of the azide takes place. Similar results were obtained, when the ZSM-5/NaN3 and clinoptilolite/NaN3 systems were examined. In the pores of these zeolites the transport of the hydrazoic acid and its sodium salt was slow, but it could be accelerated by increasing the temperature. All the water was removed from the narrow pores to 400 ~ while azide could hardly penetrate the micropores resulting in very low azide loss.

8_

4. CONCLUSIONS In the NaN3/zeolite systems, where zeolites are in the H-form and/or bind water, sodium ions and hydrazoic acid can be formed already at room temperature. At high-temperature (>400 ~ sodium metal and nitrogen can be generated only from that fraction of the sodium azide, which was not converted in reaction with the hydroxyl groups or water. The loss of sodium azide can be decreased or prevented if fully dehydrated zeolites zeolites are used. Impregnation of the zeolite with sodium hydroxide suppresses the NaN3 hydrolysis. Forming large pellets quickly after mixing hinders the HN3 transport and, consequently, suppresses the hydrazoic acid release. The entrance of the NaN3 into the pores of the narrow pore zeolites is strongly hindered. These zeolites can be fully dehydrated at temperature that is lower than the decomposition temperature of sodium azide.

1800 ACKNOWLEDGEMENTS

The excellent assistance of Mrs. Agnes Wellisch is gratefully acknowledged. REFERENCES

1. R.M. Barrer and J.L. Whiteman, J. Chem. Soc. A (1967) 19. 2. D. Fraenkel, and B.C. Gates, J. Am. Chem. Soc. 102 (1980) 2478. 3. J. A. Rabo, C. L. Agnell, P. H. Kasai and V. Schomaker, Disc. Farady Soc. (1966) 329. 4. F. Schmidt, W. Gunsser, and J. Adolph, ACS Symp. Ser. 40 (1977) 291. 5. J.B. Lee, J. Catal. 68 (1981) 27. 6. R.M. Barrer, E.A. Daniels, and G.A. Madigan, J. Chem. Soc. Dalton Trans., (1976) 1805. 7. P. Fejes, I. Hannus, I. Kiricsi, and K. Varga, Acta Phys.Chem. Szeged, 24 (1978) 119. 8. I. Hannus, I. Kiricsi, K. Varga, and P. Fejes, React. Kinet. Catal. Lett., 12 (1979) 309. 9. I. Kiricsi, I. Hannus, A. Kiss, and P. Fejes, Zeolites, 2 (1982) 247. 10. I. Hannus, Gy. Tasi, I. Kiricsi, J.B. Nagy, H. Fr6ster and P. Fejes, Thermochimica Acta, 249 (1995) 285. 11. L.R.M. Martens, P.J. Grobet, and P.A. Jacobs, Nature, 315 (1985) 568. 12. L.R.M. Martens, P.J. Grobet, W.J.M. Vermeiren, and P.A. Jacobs, Stud. Surf. Sci. Catal., 28 (1986) 935. 13. L.R.M. Martens, W.J.M. Vermeiren, P.J. Grobet and P.A. Jacobs, Stud. Surf. Sci. Catal., 31 (1987) 531. 14. M. Brock, C. Edwards, H. F6rster and M. SchrSder, Stud. Surf. Sci. Catal. 84 (1994) 1515. 15. E. J. Doskocil and R. J. Davis, J. Catal. 188 (1999) 353. 16. H.K. Beyer, Gy. Onyestyfik, B.J. JSnsson, K. Matusek, and K. Lfizfir, in Procc. of 12th IZC, (eds. M.M.J. Treacy, B.K. Marcus, M.E. Bisher and J.B. Higgins) Materials Research Society, Warrendale, Pennsylvania, USA, 1999. p2875. 17. K. Lfizfir, L.F. Kiss, S. Pronier, Gy. Onyestyfik and H.K. Beyer, M6ssbauer Spectroscopy in Material Science, (eds. M.Miglierini and D. Petridis) Kluwer, Amsterdam, Netherlands, 1999. p291. 18. K. LAzAr, H.K. Beyer, Gy. Onyestyfik, B.J. J6nsson, L.K. Varga and S. Pronier, NanoStructured Materials, 12 (1999) 155. 19. Z. Gabelica, N. Dewaele, L. Maistrau, J. B. Nagy and E. G. Derouane, Zeolite synthesis, (eds. M.L. Occelli and H.E. Robson) ACS Symp. Ser. 398 (1989) 539. 20. R. Sreekumar, R. Radmakumar and P. Rugmini, Chem. Comm. 12 (1997) 1133.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1801

M o d i f y i n g t h e a c i d i c p r o p e r t i e s of P t Z S M - 5 a n d P t Y zeolites b y a p p r o p r i a t e l y varying reduction methods A. Tamfisia, K. Niesz a, I. Pfilink6 b'*, L. Guczi c and I. Kiricsi a aDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich B. t6r 1, Szeged, H-6720 Hungary bDepartment of Organic Chemistry, University of Szeged, D6m t6r 8, Szeged, H-6720 Hungary cChemical Research Center of the Hungarian Academy of Sciences, Institute of Isotopes and Surface Chemistry, P.O. Box 77, Budapest, H-1525 Hungary

PtZSM-5 and PtY catalysts were prepared by the wet ion-exchange method from their Naforms. The modified zeolites were reduced by either NaBH4 or gaseous H 2. Reduction with NaBH4 did not alter the original Lewis acidity of the samples (Lewis acidity due to the sodium ions), while reduction with

H2

generated new Bronsted and Lewis acid sites. Reduction with

NaBH4 did not modify the ZSM-5 crystal lattice, however, treatment with Hz led to some structural decomposition. Thus, Lewis sites due to extraframework alumina and Bronsted sites due to hydroxyl groups of the heeled defect sites were formed. For the Y zeolite, after reduction the platinum atoms moved out from the ion-exchange positions and started to migrate toward the supercage. On their way they aggregated and the large crystallites pushed apart portions of the zeolite crystal. The detrimental effect was of a higher extent when reduction occurred with gaseous Hz. The resulting SiO2-AI203 supported catalyst contained many "true" Lewis sites due to extraframework alumina species as well as acidic OH groups connected to defect sites, however, it has lost shape-selective properties connected to the zeolitic structure. 1. INTRODUCTION Zeolites containing transition metal in their structure offer the combination of the acidic sites of the zeolites in a constrained environment as well as the properties of the metal catalysts [1]. These trifunctional catalysts (acid-base properties, shape-selective behavior and hydrogenationdehydrogenation ability) may be used effectively in environmentally benign processes, since Research leading to this contribution was financed through a grant from the National Science Foundation of Hungary (OTKA T034184). The support is gratefully acknowledged.

1802 there are a lot of possibilities in fine-tuning their activities and selectivities, thus, facilitating catalysts for virtually waste-free chemical transformations. Carefully chosen reduction methods allow the fine modification of acidity upon creating the other vital functionality, hydrogenationdehydrogenation that is [2]. In many previous studies the particle size of the transition metal after reduction was of major concern [3] and influencing the acidic properties by varying the methods of reduction was largely neglected. Particle sizes of Pd [4, 5] or Pt [6-11] were influenced by the method of preparation as well as with the methods of reduction or oxidation-reduction. Reduction mainly occurred with H2 and its effect on varying acidity, especially types of acidity, was generally not emphasized. Comparative studies with other methods of reduction are scarce [12]. Thus, this study, in which the effects of various reducing treatments on the types of acidity of Pt-exchanged zeolites of two distinctly different structures are investigated, should fill considerable gap. 2. EXPERIMENTAL 2.1 Materials and treatments

The platinum containing zeolites were prepared by the wet ion-exchange method starting from NaZSM-5 (Si/AI=40) or NaY (Si/AI= 13.8) [13]. After the ion exchange the zeolite samples were dried and calcined at 573 K for 5 hours in air. Then, the zeolites were reduced by one of the three methods: (i) NaBH 4 (aqueous solution, room temperature, 24-hour stirring) or (ii) H2 (573 K, 4 hours, gas flow, N2/H2 mixture with gradual increase in H2 content) or (iii) the Pt-containing zeolite reduced by NaBH4 was treated by H2 under the conditions of (ii). The model compound for the catalytic reaction was 1-butene from Aldrich Chemical Co. and was used as received. 2.2 Methods of characterization The resulting samples were characterized by derivatography, powder X-ray diffractometry and BET surface area measurements. Thermal behavior of the substances was investigated by a Derivatograph Q instrument. The powdered samples were placed on a platinum sample holder and studied under the following conditions: mass sample 100 mg, heating rate 10 degree/min, temperature range 300 to 1000 K in air. X-ray diffractograms were registered on well-powdered samples with a DRON 3 diffractometer in the range of 3 o _ 43 ~ For X-ray source the K 1 line of the copper anticathode was selected by the monochromator. BET measurements were performed in a conventional volumetric adsorption apparatus

1803 cooled to the temperature of liquid nitrogen (77.4 K). Prior to measurements the host samples were pretreated in vacuum at 723 K for 1 h under continuous evacuation. The acid-base properties were studied by pyridine adsorption followed by IR spectroscopy. Self-supported wafers (10 mg/cm 2) were prepared from the powdered zeolites and placed into the sample holder of the in situ IR cell. The temperature of the wafer was slowly increased to 723 K under continuous evacuation. After 2 h the sample was cooled to room temperature and the background spectrum of the zeolite was recorded. Resolution was 1 cm 1, and 64 scans were accumulated on a Mattson Genesis I FT-IR spectrometer. For the acidity measurements 1.33 kPa of pyridine was introduced into the cell at ambient temperature, which was then heated to 473 K. After 1-h equilibration the cell was evacuated for 1 h at the same temperature. The sample was cooled to ambient and the spectrum was recorded. For calculating Bronsted and Lewis acidities bands at 1540 cm -~ and 1450 cm ~ were used. The absorbances were integrated, specific areas were calculated and compared. The acidity of the catalysts was also characterized by the double bond isomerization of 1butene in a recirculatory batch reactor. 100 mg of the catalyst was placed into the reactor and pretreated at 723 K for 1 h under continuous evacuation. The reactor was cooled clown to reaction temperature (323 K) followed by introducing 66.6 kPa of 1-butene into the reactor. The gas mixture was analyzed by gas chromatography (Hewlett Packard 5710 GC, flame ionization detector, 4.5-m-long all-glass column packed with 30% dimethylsulfolane on Chromosorb W. 3. RESULTS 3.1. N a Z S M - 5 and its modified varieties

It was found that the various heat and reducing treatments of the ion-exchanged ZSM-5 zeolite did not result in significant modifications in its original structure. The BET surface areas did not change much (Table 1, column 2). The thermal behavior also remained similar. Two types of weight losses could be observed. Both were assigned to water losing processes. The first step at around 373 K was attributed to the loss of crystal water, while the high-temperature step indicated dehydroxylation. The total weights lost for the variously treated samples are very similar as well (Table 1, column 3). The XRD spectra of the platinum-containing samples resembled that of the parent sample to a great extent, except that weak Pt reflection could be observed (Figure 1). IR spectrum taken in the range of framework vibration (400 cm-1-1400 cm 1) verifies the results of the X-ray measurements.

1804 Table 1 Characteristic data on the variously treated catalysts Catalyst

BET area (m2/g)

total weight loss

Bronsted sites/Lewis

by 1200 K (%)

sites d

NaZSM-5

322

4.90

0/0.34

PtZSM-5(B a)

310

5.17

0.06/0.42

PtZSM-5(H b)

302

5.70

0.28/0.13

PtZS M-5 (BH c)

309

5.20

0.05/0.53

NaY

595

6.51

0/0.84

PtY(B a)

410

5.85

0/0.60

PtY(H b)

265

5.97

0.25/0.54

a : reduced with NaBH4 b: reduced with gaseous H 2 c: reduced with NaBH4, then, postreduction with gaseous H2 d: ratio of integrated areas

The structure remained largely intact irrespective to ion exchange and the various heat and reducing treatments. No new band appeared at 930 cm -~, which is generally accepted as the indication of framework defects [ 14].

I

As far as acidity is concerned the parent sample displayed only negligible Bronsted acidity. The intense band at 1445 cm 1 is assigned to pyridine bonded coordinatively to the sodium ions (Figure 2, spectrum a). Upon reduction

with

NaBH4 the

overall

picture remained the same. Negligible amount of Bronsted acid sites were found and the band at 1445 cm ~ could

a

40

30 2 Theta 20

10

also be detected (Figure 2, spectrum e). If this sample was further treated with H2

gas ("postreduction"), changes did

Figure 1. XRD spectra of (a) NaZSM-5, (b) PtZSM-5 not occur (Figure 2, spectrum d). reduced with NaBH4, (e) PtZSM-5 reduced with However, the acidity of the H2-reduced gaseous H2 sample changed significantly (Figure 2,

1805 spectrum b). While the other reduction methods did not generate new type of acid sites and only slightly modified the concentration of Lewis sites, reduction with gaseous hydrogen resulted in the appearance of Bronsted acidity (Table 1, column 3). At the same time the band assigned to pyridine bonded to sodium ions diminished and a new band could be detected at 1455 cm 1 indicating the appearance of "true" Lewis acid sites, generated by the release of framework aluminum. In 1-butene isomerization the samples all showed near to one as the ratio of cis-2-

Absorbance (a.u.)

butene to trans-2-butene, indicating that the compounds

were

acidic,

indeed

[5].

However, there were significant differences in the rate of double bond isomerization between the Pt-containing zeolites reduced by the two different methods.

Initially,

the

PtZSM-5 reduced by H 2 proved to be significantly more active than the one reduced by NaBH4. The ratio of the initial rates was 29.3:1. 1550

3.2. NaY and its modified varieties

1500

1450

Wavenumber (cm")

Reduction of the platinum-exchanged NaY,

Figure 2. Acidities of the ZSM-5 samples by especially by gaseous hydrogen, caused signi- pyridine adsorption followed by IR spectroscoficant changes in the zeolite structure. The py, (a) NaZSM-5, (b) PtZSM-5 reduced with BET surface areas decreased dramatically gaseous H2, (e) PtZSM-5 reduced with NaBH4, (Table 1, column 2). The diminishing lines in (d) PtZSM-5 reduced with NaBH4, then by H 2 the X-ray diffractograms indicate significant loss of crystallinity (Figure 3). At the same time large platinum clusters were formed as shown by the intense and broad reflections around 40 degree. The method of reduction influenced the acidity of the ion-exchanged Y zeolite much the same way as was experienced with the ZSM-5 zeolite. However, the effects were larger, probably due to the lower Si/A1 modulus of the faujasite structure. The parent material only contained Lewis acid centers due to the presence of sodium ions: a band at 1445 cm 1, pyridine coordinatively attached to the sodium ions, could be detected in the case of this sample as well (Figure 4, spectrum a). Reduction with NaBH4 did not alter the acidity of the parent NaY zeolite: beside the combination band, the one at 1445 cm ~ was only found (Figure 4, spectrum c). "True" Lewis centers appeared when reduction was performed by H2. Moreover, reduction with gaseous hydrogen resulted in the appearance of Bronsted acidity (Table 1, column 3 and

1806

Figure 4, spectrum b). In

1-butene

isomerization

these

samples, similarly to those of the ZSM-5, the cis-2-butene to trans-2-butene ratios were close to one, indicating again that the compounds were acidic, indeed [15]. As far as the initial rates are concerned, PtY reduced by H2 was 15.1 times more active than the one reduced by NaBH4. 4. DISCUSSION ......

40

30

.

.

.

.

.

.

.

.

.

20

.

10

When the effects of the two reducing Figure 3. XRD spectra of (a) NaY, (b) PtY reduagents are compared, one can see significed with NaBH4, (c) PtY reduced with gaseous H 2 cant difference. While NaBH4 does not influence the acidic properties of the parent sample, 2 Theta

only Lewis centers due to the presence of sodium ions could be observed, H2 treatment generated

Asorbrcau

AI

new Lewis sites as well as appreciably amounts of Bronsted sites. Beside the reducing agent, the structure of the zeolite is also a major contributing factor in determining the final properties of the platinum-containing catalysts. The original zeolite structure is

b

preserved for ZSM-5. Irrespective to the method of reduction the majority of the platinum ions remained in or near the ion-exchanged positions even after reduction. Reduction with H2 had some detrimental effect, however, minor extent of decomposition of the ZSM-5 crystal lattice did occur and extraframework alumina took part in

1570

1530

1490

1450

Wavcnumbcr (cm-')

the formation of"true" Lewis sites. For the fauj asite zeolite, after reduction the platinum atoms Figure 4. Acidities of the Y zeolite samples by pyridine adsorption followed by IR spectmoved out from the ion-exchange positions and roscopy, (a) NaY, (b) PtY reduced with started to migrate toward the supercage. On their gaseous H2, (e) PtY reduced with NaBH 4 way they aggregated and the large crystallites pushed apart large portions of the zeolite crystal. The detrimental effect was of a higher extent

1807 when reduction occurred with gaseous H2. These events resulted in the formation of SiO2-A1203supported catalyst containing many "true" Lewis sites due to extraframework alumina species as well as acidic OH groups connected to defect sites. Obviously, the reduced fauj asite samples lost their shape-selective properties. 5. CONCLUSIONS The method of reduction significantly altered the properties of transition metal containing ZSM-5 and Y zeolites. Catalysts with Bronsted and Lewis acidities and metal functionality also (PtZSM-5 and PtY) could only be obtained when H2 was the reducing agent. The extent of reduction was higher for PtY, in turn, however, this catalyst largely lost its third functionality. Shape selectivity disappeared with the extensive disruption of the original faujasite structure. Results presented in this work open a novel way to prepare catalysts with tailor-made acidity, preserving hydrogenation-dehydrogenation activity at the same time. Combination of the reduction methods allows acidity to be fine-tuned, e.g., reduction with NaBH 4 first transforms part of the platinum ions to platinum metal clusters without the generation of acid sites. Upon further reduction in H2 the rest of the platinum ions are reduced with concomitant formation of acidity. REFERENCES 1. J. Wang, Q. Li and J. Yao, Appl. Catal. A 184 (1999) 81. 2. P. Gallezot, A. Alarcon-Diaz, J.-A. Dalmon, A.J. Renouprez and B. Imelik, J. Catal. 39 (1975) 334. 3. J. Chupin, N.S. Gnep, S. Lacombe and M. Guisnet, Appl. Catal. A 206 (2001) 43. 4. G. Koyano, S. Yokoyama and M. Misono, Appl. Catal. 188 (1999) 301. 5. B. Pommier and P. G61in, PCCP 1 (1999) 1665. 6. E.S. Shpiro, R.W. Joyner, K.M. Minachev, P.D.A. Pudney, J. Catal. 127 (1991) 366. 7. P. Gallezot, A. Alarcon-Diaz, J.-A. Dalmon, A.J. Renouprez, B. Imelik, J. Catal. 39 (1975) 334. 8. M. Guerin, C. Kappenstein F. Alvarez, G. Gianetto and M. Guisnet, Appl. Catal. 45 (1988) 325. 9. J. Wang, Q. Li and J. Yao, Appl. Catal. A 184 (1999) 181. 10. S. Ciccariello, A. Benedetti, F. Pinna, G. Strukul, W. Juszczyk and H. Brumberger, PCCP 1 (1999) 367. 11. E. Baburek and J. Nov~ikov/~, Appl. Catal. A 190 (2000) 241. 12. I. Manninger, Z. Pa~,l, B. Tesche, U. Klenger, J. Halfisz and I. Kiricsi, J. Mol. Catal. 64 (1998) 361.

1808

13. M. Guerin, C. Kappenstein, F. Alvarez, G. Giannetto and M. Guisnet, Appl. Catal. 45 (1988) 325. 14. P. Fejes, I. Hannus and I. Kiricsi, Zeolites 4 (1984) 73. 15. P. Fejes and D. Kall6, Acta Chim. Hung. 39 (1963) 213.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1809

Vibrational Studies o f Iron Phthalocyanines in Zeolites Peter-Paul H.J.M. Knops-Gerrits 1., Fr6d6ric Thibault-Starzyk2, Rudy Parton 3 1 D6partement de Chimie, Universit6 Catholique de Louvain (UCL), Batiment LAVOISIER, Place L. Pasteur n~ B-1348 Louvain-la-Neuve, Belgium Tel : (32)10 -47 29 39 Fax: 47 23 30, [email protected] 2Laboratoire Catalyse & Spectrochimie, ISMRA-CNRS, 14050 Caen (cedex) France Tel +33-231 452 810; Fax +33-231 452 822; [email protected] 3 DSM Research, P.O Box 18, 6 1 6 0 MD Geleen

Tel +31 46 4761293 ; Fax+31 46 4761173; [email protected] * To whom correspondence should be addressed. The range ofzeolites which are suitable as host for phthalocyanines is limited, because of the size of these ligands. Generally, faujasite structures are preferred. Synthesis of phthalocyanines in the internal space of zeolites is exceptional in that mostly, the ligand is itself synthesized in situ before complexation of the metal ion. Moreover, depending on the metal source (salts, carbonyl complexes or metallocenes) distinct synthesis procedures can be recognized. FT-IR and Raman data are presented for the structure of phthalocyanines encapsulated in faujasite type zeolites. A new conformational model for the encapsulation of metal-phthalocyanines in the supercage of zeolite Y resolves the apparent contradiction between molecular modelling and spectroscopy of these catalysts. In the present model, two aromatic rings of the phthalocyanine are pushing against the supercage walls. 1. INTRODUCTION The range ofzeolites which are suitable as host for phthalocyanines is limited, because of the size of these ligands. Generally, faujasite structures are preferred [1-8], but the use of VPI-5 [3] has also been claimed. Synthesis of phthalocyanines in the internal space ofzeolites is exceptional in that mostly, the ligand is itself synthesized in situ before complexation of the metal ion. Moreover, depending on the metal source (salts, carbonyl complexes or metallocenes) distinct synthesis procedures can be recognized. When salts are used, the phthalocyanines are synthesized by tetramerization of dicyanobenzene on cation exchanged zeolites at temperatures between 523 and 573 K [1-5]. Although the complexation of the TMI by the formed phthalocyanine is thermodynamically favorable, it generally proceeds only slowly. The degree of incorporation of TMI in zeolite hosted phthalocyanines decreases in the sequence Co > Ni > Cu > Fe. Therefore, after synthesis, large quantities of transition metal are still ligated by the lattice, or are present as

1810 oxides [1, 4, 6, 8]. Lattice coordinated cations can be removed by exchange with NaC1, but the exchange of the TMI with Na + is incomplete [6-9]. Moreover, considerable amounts of TMI may migrate to the surface during dehydration and Pc synthesis, as shown by XPS-analysis of [Rh(Pc)]-Y and [Rh(Pc)]-X [5]. To overcome the unfavorable kinetics of the chelation of TMI by the Pc chelate, procedures using other metal sources have been proposed. 2. MATERIALS AND METHODS 2.1. Synthesis FePcY, FeTNPcY and CoPcY were prepared as previously described [3, 5] MePcY is synthesized by mixing 5 g MeY (obtained by ion exchange from NaY zeolite Ventron, anhydrous unit cell composition: Na54 (A102)54 (SIO2)138) dried at 473 K with 3.15 g DCB (8 DCB per supercage). The mixture is heated at 453 K for 24 h. Raw MePcY is treated in a soxhlet extractor with acetone, until the extraction solvent is colorless. 2.2. FT-IR and Raman spectroscopy Raman spectra were recorded on a Renishaw Raman Microscope. The zeolite samples, in powder form, were placed on a glass microscope slide. The power on the sample was about 2mW/mm 2. The collection time varied from sample to sample and was between 15 and 20 minutes. The spectra were background corrected and a Fourier deconvolution procedure, described elsewhere 3, was applied to resolve the overlapping bands in the OH stretching region. FT-IR and Raman spectra were recorded on a Bruker IFS66 instrument equipped with a Ge detector. The zeolite samples, in powder form, were pressed in metal sample-holders. The complexes were used in KBr pellets (~-1 mg of MPc or MPcY with ~-300 mg ofKBr). FT-Raman experiments were performed with 1064 nm, 300 mW Nd-Yag laser excitation. 2.3. The molecular mechanics calculations Calculations were carried out using the software Hyperchem (Auto-desk, Inc.), using a completed version of the MM+ force field, i.e. a completed version of Allinger's MM2. Partial calculations were first performed with a reduced version of the zeolite cluster, composed of the supercage only. These results allowed us to improve the time needed for calculations with a 30 A cluster.

OH

0

FePcY

O

,Buoo. J-coo. FePcY

r

~COOH

Scheme 1 : catalytic cyclohexane oxidation with FePcY and tBuOOH.

1811 2.4. The catalytic oxidations Oxidations were carried out under stirring at 25~ using 0.5 g of catalyst, 25 mmol of each substrate, 0.5 g of chlorobenzene as internal standard for the GC, and 50 ml of acetone as the solvent. The oxidant was t-butyl hydroperoxyde (TBHP : 70 % solution in water). Analysis of the products was done by gas chromatography on a CP Sil 5 CB column from Chrompack. 3. RESULTS AND DISCUSSION One of the alternative procedures involves adsorption of TMI-carbonyl complexes on the zeolite. These complexes may or may not be decomposed prior to the in situ synthesis of the Pc ligand [7,10-12]. Decomposition can be performed thermally or photochemically and results in formation of metal clusters in the zeolite. Photochemical decomposition is preferred, in order to suppress the migration of the metal to the outer surface. As the transition metal is present as metal clusters in the faujasite supercage, and as only one phthalocyanine can be synthesized per cage, the presence of unchelated TMI is unavoidable. If, on the other hand, the decomposition step is omitted, the CO ligands are directly replaced by 1,2-dicyanobenzene. However, this procedure also leaves some unchelated TMI in the zeolite, as the rate of decomposition of the carbonyls is higher than the rate of formation of [TMI(Pc)]. Therefore, it is advisable to use more stable complexes, such as metallocenes, as precursors. Zakharov et al. first applied ferrocene and cymantrene in [TMI(Pc)]-Y synthesis [13]. The amount of unchelated TMI can be minimized by selecting a proper synthesis temperature [14]. An analogous synthesis procedure was used by Parton et al. [3 ], who claimed, based on chemical analysis, that there was almost no residual iron in their [Fe(Pc)]-Y. However, because of the high stability of ferrocene, large amounts of free base phthalocyanines are synthesized. Fortunately, at low loading, these do not interfere with the catalytic activity. Using a metallocene as TMI-source moreover allows to apply molecular sieves without cation exchange capacity as host for [TMI(Pc)]. Indeed, when ferrocene is mixed with dry VPI-5 and dicyanobenzene, [Fe(Pc)] complexes are formed inside the channels of this neutral, aluminophosphate molecular sieve [3 ]. [TMI(Cp)2]-VFI + 4 DCB + H20 --~ [TMI0~c)] -VFI + 2 Cp + 2 H + + 1/2 0 2

(1)

Cyclization ofphthalocyanines starting from dicyanobenzene is a two-electron reduction process. When TMI-carbonyl complexes are used, these two electrons are supplied by the metal, which is oxidized to the divalent state: [TMI(CO)m] + 4 DCB

--~

[TMI(Pc)] + m CO

(2)

1812 Table 1. Raman bands corresponding to the symmetric deformation of the macrocyclic ring in encapsulated and free phthalocyanines (in cm-1) 9 *(632nm excitation)

FT-Raman Resonance Raman* Symmetry group

H2Pc 720

D2h

H2Pc Y 720 718 D2h

CoPc 751 746 D4h

CoPc Y 750 748 D4h

FePc 748 744 D4h

FePc Y 748 751 D4h

In case salts or metallocenes are used as the metal source, some water must be added as an electron source [3]. The subsequent complexation results in the liberation of two protons. These protons can be trapped efficiently by cyclopentadienyl anions, but when salts are used as the metal source, the protons cause surface acidity: 4 DCB + H20

~

Hffc + [TMI(Cp)2] Hffc + TMI-Y

--~

H2Pc + 1/2 02

(3)

[TMI(Pc)] + 2 cyclopentadiene

(4)

[TMI(Pc)] + I-I2-Y

(5)

The acidic sites can interfere with the catalytic activity of the phthalocyanines and may even cause dealumination or loss of crystallinity of the zeolite [5]. There is as well some spectroscopic evidence for protonation of the coordinating nitrogen atoms. This results in a symmetry reduction from D4h to D2h and a splitting of the Q-bands in the Vis-NIR region [15]. In IR spectra of [Co(Pc)]-X, a band at 1020 cm 1 was attributed to protonation of the inner nitrogen atoms [16]. XPS measurements on [Ni(Pc)]-Y showed non-equivalency of the chelating nitrogen atoms [6]. Instead of synthesizing the phthalocyanines in sial, they can be used as a template during zeolite synthesis. Phthalocyanines are suitable for such a synthesis as they are thermally and chemically extremely stable. Consequently, no residual TMI is formed during synthesis. A major problem is, however, to keep the phthalocyanines monomolecularly dispersed in the aqueous zeolite synthesis medium. By careful control of the synthesis gel chemistry, Balkus et al. obtained a zeolite X material in which 50% of the unit cells were occupied by Pc, even after severe extractions [4] . Various [TMI(Pc)]'s were also successfully used as templates for A1PO-5 and AIPO-11 synthesis [5]. However, in view of the relative size ofthe Pc ligand and the pore system of these AIPO's, it is likely that the complexes are mainly located at structural defects. As discussed previously, [TMI(Pc)] zeolites ot~en contain free base H~Pc. Distincnon between free base and metallatedphthalocyanines is facilitated by the symmetry change upon chelation. The symmetry of free base Hffc is D2h, whereas [TMI(Pc)] belongs to Dnh. In IR spectroscopy, this symmetry difference results in the splitting of the conjugated isoindole band (at 1332 cm -a) and the C-H in-plane deformation band (at 1287 cm 1) into doublets for the free base H~Pc's (at 1336 and 1322 cm 1, and at 1304 and 1278 cm -1 respectively ; see Figure 1). Moreover, the TMIN vibrations around 900 cm 1 are also typical for metallated Pc [3 ].

~'E~ .~,~

t~

~=

=r.

z

E,.-, ,~~

C

295

~, .oo

_~92

~593 f~09 ' - - I__485

~"-':=>e.4

~----__.~682se5 ,

;>833 ~'~ 762

~21186 1 ~ 1158 ~ .... ..1108 11008

~1160-1185 1,,zQ ~---1~ . . . . ";1007 S ~-_87(,~1"

~'~---'~1341 r 1309

k _ f 1340 "1308

!>1..

Raman Absorbance (a.u.)

457"C--.__

580~ t

736 ~----~

793~ - ' ~

436--~

574"--'~

7346 ~ ~ ~ 4 ~

781 - : ~

~ 1163~ 1 1 2 0 ~ 1 1 2 0 ~ 1091 1,,10 t/ 1076-~'~--~-'"'-~ 1003

. . . . . . . . . . . . . . . . .

1399~. 1426 1334~1334--- --------~

1,,1<

Infra Red Transmission (a.u.)

1814 the subject of XPS investigations by Romanovskii and Gabrielov [6,18]. Fe, Ni and Co occur in the divalent state, while Os and Ru are trivalent. In the latter case, the positive charge of the complex must be compensated by the anionic zeolite lattice. Rh on the contrary is easily reduced to Rh0I) or even to Rh(I) during Pc synthesis. This implies that negatively charged [Rh(I)(Pc)] complexes may be entrapped in the anionic zeolite medium [5]. The close resemblance between zeolite entrapped and dissolved Pc's has been proved by a multitude of techniques, such as M6ssbauer spectroscopy [19], cyclic voltammetry [20] and EPR spectroscopy [21,22]. While the pore system of the VPI-5 molecular sieve provides sufficient space for the Pc ring, the faujasite supercage may be somewhat tiny. The size of a Pc ring is about 1.5 nm, whereas the free

+

't>

Figure 2. Structure of (A) NaY zeolite + (B) FePc and (C) FePc encaged in Y zeolites. diameter of a supercage is only 1.3 nm. Therefore, based on molecular graphics analysis, a

saddle-type deformation of the aromatic system was proposed. In this model, the four benzene

rings of the chelate occupy the four twelve-membered ring openings of the supercage [2,8]. Some indirect (and weak) evidence for this saddle-type deformation comes from the broadening of the Nls lines in XPS, and from a coordination number of 3.6 (instead of4)and Fe-N distances of 1.84 nm (instead of 1.83 nm), determined by EXAFS [12]. Very small shifts of the IR C=N and C=C stretching frequencies have also been associated with this saddle-type deformation [ 12]. Several other effects of occlusion on Pc properties have been documented. In the DRS spectra of occluded Pc, B- and Q-bands are red-shifted ; the latter become also weaker [5,11,12]. This behavior is analogous to the red shift of the Q-band of crystalline [TMI(Pc)] upon pressure increase. Thus, [TMI(Pc)] must be in a spatially constrained environment, which results in effects similar to those of high pressures [15]. Moreover, the optimal synthesis temperature of[Co(Pc)] in the Y supercage is 542 K, which is 15 K higher than in the absence of the zeolite [24]. The confinement of the complexes to the inner zeolite volume is even more difficult for the substituted phthalocyanines. It has e.g. been demonstrated that nitro-substituted FePc's can be exclusively located at the outer surface of zeolite Y [25]. For tetra-t-butyl substituted FePc's, some evidence in favor of encapsulation in NaY has been presented [26]. The relative intensities of the B-band (280 nm) and the Q-bands (550, 580 nm) are different for encaged and adsorbed substituted Pc's. The Q-band, which arises from 7t --~ re* transitions in the inner Pc aromatic system, is strongly suppressed for the encaged complex, whereas the intensity of the B-band,

1815 which is due to transitions in the peripheral benzene rings, is markedly enhanced. Furthermore the Q- and B-bands, as well as some IR bands are shifted to lower frequencies. Encaging of tbutyl substituted Pc reduces the EXAFS coordination number from 3.9 to 3.6 and increases the Fe-N interatomic distance from 0.185 nm to 0.190 nm. All these spectral differences are attributed to the structural deformation of the Pc plane and a slight departure of the Fe atom from the Pc plane. Catalytic results on the other hand disfavor the encapsulation hypothesis, since large substrates, such as stilbene, are epoxidized on the t-butyl substituted phthalocyanines. It is unlikely that this occurs in supercages, which are occupied by t-butyl substituted Pc's [26]. Cytoehrome

P-450:

Fe=O

H-R

A <1

I

B I

I I

I I

I

OH n

~

I

I

I

I

I

I

R*nm

GIF

~

I I !'--~

R

M

FePcY

J,

12 <1

H ~

FePeY

4,

M - - O ~ H - R ~

9

6

3 2

1 0

I

I

I

I I

kHlk D

I

0

0.4 I

I

0.8

I

I 2~

I

1.2 I

I

>

13 ~

Figure 3. Catalyticdata ofFePc encaged in Y zeolitesas a cytochrome P-450 model in the cyclohexane and adamantane oxidation. For cyclohexane oxidation with FePcY and tBuOOH, a kinetic isotope effect of 9.0 + 0.2 is observed in Figure 3. This figure is in agreement with values reported for hydrogen abstraction by an electrophilic Fe=O species via a symmetric linear transition state. The mechanistic similarity between the enzyme and its model is confirmed by the relative reactivities of different C-H bonds. With FePcY and tBuOOH, C-H bonds on tertiary carbon atoms are about eleven times as reactive as C-H bonds on secondary carbon atoms in adamantane oxidation. This value again falls well within the range expected for iron oxo chemistry. The presence of one or two ferryl groups in the active site gives interesting differences with f.e. MMO active sites [27]. ACKNOWLEDGMENTS PPKG thanks the UCL & ESA Prodex for research grants, FTS thanks CNRS for support.

1816 REFERENCES

.

5. 6. .

.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

G. Meyer, D. W6hrle, M. Mohl, G. Schulz-Ekloff, Zeolites, 4, (1984) 30. N. Herron, G. Stucky, C. Tolman, J. Chem. Soc. Chem. Commun., (1986) 1521. R. Parton, L. Uytterhoeven, P. Jacobs, Stud. Surf. Sci. Catal., 59 (1991) 395; D. Huybrechts, R. Parton, P. Jacobs, Stud. Surf. Sci. Catal., 60 (1991) 225 ; R. Parton, D. Huybrechts, P. Buskens, P. Jacobs, Stud. Surf. Sci. Catal., 65 (1991)47; R. Patton, D. De Vos, P. Jacobs, Zeolite Microporous Solids: Synthesis, Structure and Reactivity, E. Derouane et al. (Eds.), Kluwer Academic Publishers, (1992) 552. Z. Weide, Y. Xingkai, W. Yue, J. Mol. Catal. (China), 5 (1991) 168. K. Balkus, A. Welch, B. C_made,J. Inclus. Phenom. Molec. Recogn.Chem., 10 (1991) 141. E. Shpiro, G. Antoshin, O. Tkachenko, S. Gudkov, B. Romanovskii, K. Minachev, Stud. Surf. Sci. Catal., 18 (1984)31. A. Zakharov, B. Romanovskii, Vest. Mosk. Univ. Ser. Khim., 20, (1979) 94. B. Romanovskii, Acta Phys. Chem., 31 (1985) 215. N. Herron, J. Coord. Chem., 9 (1988) 25. E. Ignatzek, P. Plath, U. HOndorf, Z. Phys. Chem. Leipzig, 268 (1987) 859. /k Zakharov, B. Romanovskii, J. Incl. Phen., 3 (1985) 389. T. Kimura, A. Fukuoka, M. Ichikawa, Shokubai, 30 (1988)444. T. Kimura, A. Fukuoka, M. Ichikawa, Catal. Lett., 4 (1990) 279. A. Zakharov, B. Romanovskii, D. Luka, V. Sokolov, Metalloorg. Khim., 1 (1988) 119. A. Zakharov, Mendeleev Commun., 80 (1991). H. Diegruber, P. Plath, Z. Phys. Chem. Leipzig, 266 (1985) 641. G. Schulz-Ekloff, D. W6hrle, V. Iliev, E. Ignatzek, A. Andreev, Stud. Surf. Sci. Catal., 46 (1989)315. H. Diegruber, P. Plath, G. Sehulz-Ekloff, M. Mohl, J. Mol. Catal., 24 (1984) 115. B. Romanovskii, A. Gabrielov, J. Mol. Catal., 74 (1992) 293. M. Tanaka, Y. Sakai, T. Tominaga, A. Fukuoka, T. Kimura, M. Iehikawa, J. Radioanal. Nucl. Chem. Letters, 137 (1989) 287. F. Bedioui, E. De Boysson, J. Devynck, K. Balkus, J. Electroanal.Chem., 315 (1991) 313. V. Iliev, L. Prahov, A. Andreev, E. Ignatzek, G. Schulz-Ekloff, Proc. 6th Int. Symp. Heterog. Catal., Sofia, Part 2 (1987) 79. T. Borisova, L. Izmailova, E. Kotov, B. Romanovskii, Zh. Fiz. Khim., 60, 1195 (1986). C. Tolman, N. Herron, Catal. Today, 3 (1988) 235. K. Balkus, J. Ferraris, J. Phys. Chem., 94 (1990) 8019. R. Parton, C. Bezoukhanova, J. Grobet, P. Grobet, P. Jacobs, Proc. Int. Symp. on Zeolites and Microporous Crystals, Nagoya (Japan), August 22-25 (1993). M. Ichikawa, T. Kimura, A. Fukuoka, Stud. Surf. Sci. Catal., 60 (1991) 335. P.P. Knops-Gerrits, P.A. Jacobs, A. Fukuoka, M. Ichikawa, F. Faglioni, W.A. Goddard 111,J. Mol.Cat., A., 166, (2001) 3-15.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1817

The effect of dealumination on the AI distribution in pentasil ring zeolites

J. D6de6ek, V. G~bov6. and B. Wichterlovh J. Heyrovsle) Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, CZ-182 23 Prague 8, Czech Republic* The effect of zeolite dealumination on the distribution of aluminium atoms in the framework of mordenite, ferrierite and ZSM-5 was investigated. In general, "A1 pairs" [A1-O-(Si-O)I,2A1] located in one ring and forming cationic sites for divalent cations, and isolated "single" A1 atoms far distant from each other and unable to balance divalent complexes are present in the framework of silicon rich zeolites (Si/AI _> 8). Number of "A1 pairs" [A1-O-(Si-O)I,2-A1] located in one ring and their distribution in the individual cationic sites in the zeolite channel system were estimated using Vis spectroscopy of "bare" Co(II) ions in dehydrated zeolites. The sensitivity of framework A1 atoms to dealuminination is not uniform and depends on the nature of Si-A1 sequences (A1 pairs of different types, single A1 atoms), zeolite type and on the conditions of dealumination procedure. 1. INTRODUCTION There are two highly important properties of silicon rich zeolites relevant to catalysis. High acid strength of the protonic sites and exceptional redox activity of the exchanged transition metal ions. Both these properties are controlled, besides the zeolite topology, by the concentration and distribution of aluminium in the framework. Due to low fi-amework aluminium content in the silicon rich zeolites (Si/A1 _> 8), as ZSM-5, ferrierite, mordenite and beta zeolite, "AI pairs" [A1-O-(Si-O)I,2-A1] and "single A1 atoms" far distant from each other can be present in these zeolites (1,2). Distribution of such Si-O-A1 sequences raises also varieties in the aluminium distribution at the cationic sites bearing metal ions and controlling distances between protonic sites. Presence of A1 pairs is necessary in zeolite tings coordinating cations or their complexes exhibiting two positive charges (2), while single A1 atoms are sufficient for monovalent counter ions or complexes. The geometry and charge of zeolite tings accommodating A1 pairs or single A1 atoms affects coordination of the cations in zeolites (2), their redox properties (3,4) and also acidity of protonic sites (5).

* Financial support of the Grant Agency of the Czech Republic under projects # 101/00/0640 and 104/99/0432 and of the Ministry of Education under the EC COST program, project # D 15/0014/00 - OC D 15.20 is highly appreciated.

1818 D:ealumination of zeolites by acid or hydrothermal treatment represents method enabling to modify zeolite properties. This procedure is often used for optimization of properties of zeolite based catalyst and this has a significant industrial impact. It is necessary to point out that the changes in the aluminium distribution in the zeolite framework caused by dealumination should significantly affect properties of dealuminated zeolites. However, the effect of aluminium distribution in the framework (i.e. single A1 atoms, A1 pairs and position of A1 pairs in the channel system) is not taken in account when the effect of dealumination on the catalytic properties of zeolites is discussed. This is due a lack of information on the distribution of Al atoms in the framework of dealuminated silicon rich zeolites and on the lack of knowledge of relationship between A1 distribution in the parent zeolite, conditions of dealumination procedure and resulting Al distribution in dealuminated zeolite. All this is because a method for determination of Al distribution in silicon rich zeolites has not been available. 29Si MAS NMR experiments enable to distinguish only Al-OSi-O-Al pairs from single Al atoms and not Al-O-(Si-O)2-Al pairs. However, particularly these Ai-O-(Si-O)2-A1 pairs often represent substantial fraction of Al pairs in silicon rich zeolites (as reported for ZSM-5 in Ref. 6). Recently, we reported an indirect method for the estimation of the distribution of Al atoms in silicon rich zeolites (7,8). The suggested approach for estimation of the number of Al pairs and single Al atoms present in the zeolite and of the distribution of Al pairs among local framework structures (cationic sites) is based on the monitoring of distribution of "bare" divalent Co(II) ions at the cationic sites of Co-zeolites with maximum degree of Co ion exchange. The "bare" divalent Co(II) ions in dehydrated zeolites, coordinated exclusively to framework oxygens, require for balancing of their charge sufficiently close two negative AlO2 charges, i.e. a ring containing two Al atoms. Therefore, the Co-zeolites under study could not contain Co(II) ions with extraframework ligand, which would enable them to be charge balanced by a single AIO2- entity. Known absorption coefficients of Co(II) ions located in individual cationic sites and in various zeolite structural types provides to estimate concentration of Co(II) ions, and thus also concentration of AI pairs at these sites (for details see Refs 9-11). In this paper, an attempt is made to study the effect of dealumination procedure on the aluminium distribution in silicon rich zeolites of mordenite, ferrierite and ZSM-5 structure using Co(II) ions as probes. Aluminium atoms at different sequences and local structures (single Al atoms, Al pairs in different cationic sites) exhibit different tendency to dealumination. Moreover, it is shown that conditions of the dealumination significantly affect A1 distribution in dealuminated zeolites. 2. EXPERIMENTAL Sample preparation. Na-mordenite (Si/A1 8.5; Institute of Oil and Hydrocarbon Gases, Bratislava, Slovak Republic) and NaK-ferrierite (Si/A1 8.4, Na/A1 0.3, K/A1 0.7, TOSOH Co., Japan) were dealuminated with oxalic acid. Na-ZSM-5 (Si/A1 14.1; Institute of Oil and Hydrocarbon Gases, Bratislava, Slovak Republic) was dealuminated both with oxalic and nitric acid. Dealumination in oxalic acid occurred in 0.5 M acid (100 ml per 1 g of zeolite) at 60 ~ for 5 hrs (ferrierite, ZSM-5) and 10 hrs (mordenite). Dealumination in nitric acid occurred in three steps equilibration of zeolite with 0.5 M acid (3 x 100 ml per 1 g of

1819 zeolite) at RT for 3 x 24 hours. Subsequently, samples were washed by distilled water, dried, converted to the sodium form by ion exchange with 0.5 M sodium chloride solution (3 x 100 ml per lg of zeolite) at RT and carefully washed by distilled water. The chemical composition of dealuminated Na-zeolites (denoted as Na-ferrierite-O, Na-mordenite-O, Na-ZSM-5-O (dealumination with oxalic acid) and Na-ZSM-5-N (dealumination in nitric acid)) obtained alter their dissolution by Atomic Absorption Spectroscopy, is given in the Table. XRD analysis reflected good crystallinity of all these materials. CoNa-zeolites with maximum loading of Co(II) ions were prepared by ion exchange of parent zeolites and dealuminated Nazeolites with 0.05 M cobalt nitrate solution at RT repeated three times. After the ion exchange, the samples were washed three times with distilled water and dried on the air. The chemical composition of dealuminated CoNa-zeolites, obtained after their dissolution by Atomic Absorption Spectroscopy, is given also in Table 1. Optical spectroscopy. Prior to spectra monitoring, samples were dehydrated at 770 K under a vacuum of 7x10 a Pa with a ramp of 5 K/min in two steps: 370 K for 30 min and 770 K for 3 h. After dehydration, the samples were cooled to RT and sealed. UV-Vis-NIR spectra were measured using a Lambda 19 Perkin-Elmer UV-Vis-NIR spectrometer equipped with a diffuse reflectance attachment with an integrating sphere coated by BaSO4, and BaSO4 as a standard. Details were described elsewhere (9-11). The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(I~) = (1-R~)2/2R~, where R~ is the diffuse reflectance from a semi-infinite layer and F(I~) is proportional to the absorption coefficient. Table 1 Chemical composition of zeolites zeolite Na-ZSM-5 Na-ZSM-5-O Na-ZSM-5-N NaK-ferrierite Na-ferrierite-O Na-mordenite Na-mordenite-O *(Na+K)/A1

Si/AI 14.1 18.1 17.3 8.4 10.8 8.5 10.2

Na/A1 0.94 1.04 0.98 1.05" 0.99 1.01 1.04

zeolite CoNa-ZSM-5 CoNa-ZSM-5-O CoNa-ZSM-5-N CoNaK-ferrierite CoNa-ferrierite-O CoNa-mordenite CoNa-mordenite-O

Si/A1 14.1 18.1 17.3 8.4 10.8 8.5 10.2

Co/A1 0.41 0.20 0.39 0.31 0.19 0.35 0.41

NaJA1 0.03 0.45 0.08 0.28" 0.42 0.12 0.23

3. R E S U L T S and D I S C U S S I O N 3.1 The effect of the dealumination on the AI distribution - Ai pairs and single A! atoms

Ion exchange capacity of dealuminated zeolites for Na ions close to 100 % of the theoretical level evidences that AI atoms in these zeolites are Td coordinated in the framework A102- groups and presence of extraffamework A1 species can be excluded. Vis spectra (not shown in the Figures) evidence exclusive presence of Co(II) hexaaquacomplexes in the as prepared Co-zeolites (details see in Refs 7,8). UV-Vis spectra of dealuminated Co-zeolites with maximum Co(II) loading are shown in Fig. 1, for spectra of parent Co-zeolites see Refs 7-11. Only spectrum of dealuminated Co-mordenite exhibits charge (CT) band at 30 000 cm1, characteristic of Co-Ox-Co bridging species (7,8). However, the absorption intensity of this

1820

09

!

Figure 1 UV-Vis DR spectra of dehydrated dealuminated Co-zeolites with maximum Co(II) loading. Coferrierite-O (m), Co-mordenite-O ( - - - ) , I Co-ZSM-5-O ( ..... ) and Co-ZSM-5-N

r 0.6 0.3

i

:

:

~.,..

;

i 0.0

10000

/ I

"\

:

//

/

(..... )

d

.:/"

20000 30000 w a v e n u m b e r (cm -1)

40000

species is significantly higher compared to that of Co(II) ions (ca 100 times). Thus, we should conclude that these bridging Co species, which reflect close single A1 atoms (when only Co(II) hexaaquacomplexes are present in the as prepared zeolites) represent negligible fraction of A1 atoms in the framework of dealuminated Co-mordenite (less than 5 % of all A1 atoms). In all the other zeolites, such close single A1 atoms are not present. The effect of the dealumination on normalized Vis spectra of dehydrated Co-zeolites is shown in Fig. 2. Absorption bands in Vis region correspond to the d-d transitions of Co(II) ions. Only absorption bands reported for "bare" Co(II) ions in a, 13 and ? sites of mordenite, ferrierite and ZSM-5 (see later and Refs 2,9-11) were found atter decomposition of the spectra to Gaussian bands (not shown in the Figures). The exclusive presence of "bare" Co(II) in dehydrated Co-zeolites ions was confirmed by spectroscopy in NIR region (9-11). Presence of water molecules and Co-OH groups was excluded.

z

z 15600

20600

25000

ls6oo

20600 wavenumber (cm -1)

25000

Figure 2 Effect of dealumination on the normalized Vis DR spectra of dehydrated Co-zeolites with maximum Co(II) loading. Parent Co-zeolite (- - -), zeolite dealuminated in oxalic (--) and nitric (..... ) acid.

i z

15600

z0600 wavenumber (cm -1)

25000

1821

,~ 6o 0

Oxalic acid

=•,.80

8o

MOR

40

"<2O

20

14.1

0

17.3 Oxalic acid

8O

-~ 6O g

8.5

10.2 Oxalic acid

6o

FER

40

ZSM-5

E~ 40

'~ 20

,~ 20

0

14.1

Si/AI

18.8

8.4

Si/AI

10.8

Figure 3 Effect of dealumination with oxalic and nitric acid on the relative concentration of single A1 atoms (///) and A1 atoms in pairs (~") in ZSM-5, ferrierite and mordenite. Using absorption coefficients of Co(II) ions in or, 13 and 7 sites of mordenite, ferrierite and ZSM-5 reported elsewhere (9-11), the concentration of Co(II) ions in individual cationic sites was estimated. When only "bare" Co(II) ions are present in maximum Co(II) loaded zeolites, the concentration of Co(II) ions reflects the concentration of A1 pairs at the individual sites and the sum of concentrations of A1 pairs in individual sites corresponds to the concentration of A1 pairs in the zeolite. The effect of the dealumination on the number of A1 pairs in mordenite, ferrierite and ZSM-5 is depicted in Fig. 3. As follows from this Figure, the effect of dealumination using oxalic acid on Al distribution depends on the zeolite structural type. Dealumination dramatically affects Al distribution in ZSM-5 and ferrierite. A1 atoms in A1 pairs are removed preferentially and thus, number of single AI atoms significantly increases. The effect of dealumination using oxalic acid on mordenite is significantly weaker. Moreover, on contrary to ferrierite and ZSM-5, single Al atoms are less resistant to the dealumination than AI atoms in pairs. Also conditions of the dealumination affects Al distribution in dealuminated zeolites, as it is reflected in the Al distribution in ZSM-5 dealuminated using oxalic and nitric acid. The preference of Al atoms in pairs to be removed during dealumination by nitric acid is significantly lower compared to dealumination using oxalic acid. Observed differences in the sensitivity of Al atoms to dealumination correspond to 27Al MAS NMR experiments on beta zeolite, when different sensitivity of Al atoms located in different framework T sites to dealumination was evidenced (12). The Al sites exhibiting

1822 preference for dealumination should correspond to A1 sites enabling formation of A1 pairs, as it was suggested in Ref. 13. 3.2 The effect of the dealumination on the distribution of A! pairs in cationic sites The d-d transitions of "bare" Co(II) ions in Vis region reflect the local geometry of framework oxygens at cationic sites coordinating the Co ion and thus, local arrangements accommodating A1 pairs. From the characteristic spectral components, three different cationic sites (~, 13 and y) were suggested for mordenite, ferrierite and ZSM-5. The ~ sites represent deformed six-member tings composed of two five-member rings. This site is located in the main channel of mordenite and ferrierite and straight channel of ZSM-5. The 13 sites correspond to twisted eight-member ring in mordenite pocket and to approximately planar deformed six-member tings located at the intersection of the straight and sinusoidal channel of ZSM-5 and in side channel of ferrierite. The "boat shaped" 7 sites are accessible from the mordenite pocket and ferrierite side channel and from the sinusoidal channel ofZSM-5 (for details see Refs 2,9-11). The effect of the dealumination on the relative concentration of AI pairs in the (~, 13 and 3/cationic sites, related to the concentration of the r 13- and ~'-type Co ions obtained from the quantitative analysis of the Co(II) Vis spectra of Co-zeolites with maximum cobalt loading is given in Fig. 4. There is no correlation between local site arrangement or accessibility of site accommodating A1 pair and preference/resistance of A1 pair to dealumination. A1 pairs in most accessible cc site of mordenite and ferrierite are resistant to dealumination, while A1 pairs in r site of ZSM-5 are dealuminated preferentially. The AI pairs in ~/site in ferrierite are sensitive to dealumination while in ZSM-5 and mordenite A1 pairs located in this sites are stable. A1 pairs located in most hidden site of mordenite (13) are dealurninated preferentially.

.8~t

80-

FER

t

;));))))

~/../(../..~

~o

~//////A

~ ao-

((((L/./~

"//.4/.4//..

,,,j

~J 1.1

~4o o

~

4o-

~HHH~

z//////,

~6o-!

-

"HHH~

~2o /

...80[

ZSM-5 6o-

L

7

MOR

(Z

Figure 4 Effect of dealumination o5 mordenite, ferrierite and ZSM-5 on the relative concentration of A1 pairs in cationic sites of or, 13 and y type. Parent zeolite (~:~'), zeolite dealuminated using oxalic (///) and nitric (1\\) acid.

1823 Thus, the effect of the accessibility of the site to the dealumination using oxalic acid can be excluded. Also conditions of dealumination significantly affect distribution of A1 pairs, as it is shown for ZSM-5 treated with acetic and nitric acid. A1 pairs in the 13 site are removed preferentially in nitric acid, in the ot site by oxalic acid. 3.3 General remarks

As follows from Figs 3 and 4, dealumination significantly affects A1 distribution in silicon rich zeolites. Both concentration and relative concentration of single A1 atoms and A1 atoms in pairs and the distribution of Al in pairs in individual sites, i.e. in specific local arrangements and positions in the channel system, is changed. These changes should be reflected in various physiochemical properties of dealuminated zeolites, such as ion exchange capacity of zeolite for polyvalent cations or their complexes, distance of reaction centers (A1 pairs and thus close centers should be preserved during dealumination procedure) or position/accessibility of the reaction centers in the zeolite channel system. It is necessary to point out that the effect of the dealuminafion of zeolite on Al distribution seems to be hardly predictable and experimental monitoring of AI distribution is important. As it was mentioned above, A1 distribution should affect activity of zeolite based catalysts. Dealumination represents, beside zeolite synthesis, possible way how to control A1 distribution in zeolite and thus, tune its properties. On the other hand, the differences in the sensitivity of individual A1 species present in zeolite structure to dealumination should play role in often reported different behaviour of several zeolites of the same structural type and with close chemical composition during dealumination (14,15). A1 distribution in several zeolites of the same structural type and chemical distribution should be different (7). Thus, the differences in the concentration of A1 species sensitive or resistant to dealumination should result in different tendency of the zeolite framework to the dealumination.

4. CONCLUSIONS Different Si-A1 sequences present in silicon rich zeolites (single A1 atoms and A1-O-(Si-O)I,2-A1 pairs located at different cationic sites exhibit different tendency for removing from the framework during acid treatment. The sensitivity/resistance of individual A1 species varies with zeolite structural type and conditions of dealumination procedure. Dealumination posses a possible tool for the control of A1 distribution in zeolites, which affects physicochemical properties and catalytic activity of zeolites.

1824 REFERENCES

1. N.O. Gonzales, A.K. Chakkraborty, A.T. Bell, Catal. Lett. 50 (1998) 135. 2. B. Wichtedov~t, J. D6de6ek, Z. Sobalik, Catalysis by Unique Metal Ion Structures in Solid Matrices. From Science to Application, eds. G. Centi, B. Wichterlov/t and A. Bell, Kluwer Academic Publishers, Dordrecht, 2001, p. 31 - 53. 3. B. Wichterlovb, J. D6de6ek, Z. Sobalik, A. Vondrov~. and K. Klier, J. Catal. 169 (1997) 194. 4. R. Bulb.nek, B. Wichterlov~t, Z. Sobalik and J. Tithe,, Appl. Catal. B: Environmental, in press. 5. J. Sauer, P. Ugliengo, E. Garrone and V.R. Saunders, Chem. Rev. 94 (1994) 2095. 6. G.C. Gobbi, G.J. Kennedy and C.A. Fyfe, Chem. Lett. (1983) 1551. 7. J. D~de6ek, D. Kauclo) and B. Wichtedov~i, Chem. Commun., 11 (2001) 970. 8. J. D6de~,ek, D. Kauclo), O. Gonsiorov~i and B. Wichtedovh, Phys. Chem. Chem. Phys., submitted. 9. J. D6de6ek and B. Wichterlov~i, J. Phys. Chem. B, 103, 1462 (1999). 10. D. Kauclo), J. D6de6ek and B. Wichterlov/t, Micropor. Mesopor. Mater. 31 (1999) 75. 11. J. D6de6ek, D. Kauclo) and B. Wichtedovh, Micropor. Mesopor. Mater. 35-36 (2000) 483. 12. J.A. van Bokhoven, D.C. Koningsberger, P. Kunkeler, H. an Bekkum and A.P.M. Kentgens, J. Am. Chem. Soc., 122 (2000) 12842. 13. O. Bortnovsky, Z. Sobalik, B. Wichtedov/t, Micropor. Mesopor. Mater., 46 (2001) 265. 14. M. MOiler, G. Harvey and R. Prins, Micropor. Mesopor. Mater, 34 (2000) 135. 15. A. Omegna, M. Haoua, A. Kogelbauer and R. Prins, Micropor. Mesopor. Mater 46 (2001) 177.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1825

Pb(ll) Ion Exchange on Zeolite- Supported Magnetite. Characterization of Process by Effective Diffusivity Coefficient V. Pode a, T. Todincaa, R. Pode a, V. Dalea a and E. Popovici b Department of Environmental Engineering, University ,,Politehnica" of Timisoara, Victoriei No. 2, Et. 2, 1900 Timisoara, Romania a

P-ta.

b Department of Chemistry, University "A1. I. Cuza" of Iasi, Bvd. Copou No. 11, 6600 Iasi, Romania This paper studied the retention capacity of Pb(II) on zeolite - supported magnetite (ZESMAG). The kinetic determinations showed the positive influence of initial concentration on Pb(II) retention capacity (0.64 mequivalent Pb(II)/g ZESMAG at initial Pb(II) concentration of 710 mg/L as compared to 0.44 mequivalent Pb(II)/g ZESMAG at initial Pb (II) concentration of 465 mg/L). The process equilibrium was characterized by a Langmuir's isotherm and the estimation of the effective diffusivity coefficient was obtained by comparing experimental kinetic data to those resulted by numerical simulation. The effective diffusivity coefficient increased as initial concentration increased. The simulation offered the possibility of monitoring the compliance between experimental results and those obtained by numerical simulation all the way during kinetic determinations. 1. INTRODUCTION Gangue and other wastes landfills near the abandoned mining exploitations and industrial sites contain heavy metals and radionuclides, which are risk factors upon the environment. The action of the environmental factors on those waste products determines a complex process of dissolving cationic pollutants, which enter the soil and aquifers and affect their quality negatively. The use of alkaline washing technologies for metals contained in the abandoned landfills or settling ponds allows the reclamation of some metals. However, these technologies are extremely complex and are economically justified only when the reclaimed metals are valuable products [ 1, 2]. Romania has rich deposits of natural zeolites, which are partially exploited and used at a low cost. The use of ion exchange on zeolites technology for industrial wastes and soils polluted by heavy metal ions is not so easy to accomplish due to difficulties linked to the separation of phases (zeolite - polluted material) subsequent to the pollution abatement process.

1826 The problem can be solved by a changing process of zeolite surface, i.e., the deposition of magnetite film on the surface of zeolite particles. The deposited film gives magnetic properties to the zeolite particle and helps the separation of phases [3-7]. The previous research of our team pursued the determination of optimal conditions for preparing zeolite-supported magnetite (ZESMAG) and the characterization of the new adsorbent material [8]. This paper dealt with kinetic studies for Pb(II) retention on ZESMAG. The results were used to determine the effective diffusivity coefficient by numerical simulation. 2. EXPERIMENTAL 2.1. Kinetic studies on Pb(UD retention process on Z E S M A G

The material used for research was micronized volcanic tuff originating from the Mirsid area (Romania). The main zeolite was clinoptilolite and other minerals were albite and aquartz [9]. The chemical composition of the volcanic tuff was determined according to [ 10] and was expressed as % wt: SiO2 62.2, A1203 11.65, Fe203 1.30, CaO 3.74, MgO 0.67, K20 3.30, Na20 0.72, TiO2 0.28, I.L (ignition loss) 9.14. The granular sort taken into experiments was previously chemically activated to increase the exchange capacity. The chemical activation was carried out by treating the tuff with a 2 M NaCI solution at a solid: liquid ratio of 1:5, stirrer time of 2 hours. After activation, the volcanic tuff was washed with distilled water and dried at 105~ for 8 hours. ZESMAG was prepared by a chemical process of coveting volcanic tuff particles with synthesized magnetite according to the following reaction: FeSO4 + 2FeC13 + 8NH4OH ~ Fe304 + (NH4)2SO4 -t- 6NH4C1 + 41-120 The working conditions were as follows: Fe304/zeolite mass ratio: 1/5; concentration of ferrous-ferric solution: 5%; temperature: 90-95~ ammonia excess 25% - 100%; ammonia addition time: 45 minutes. Previous research indicated that these conditions were the best for preparing an adsorbent material with good magnetic properties [8]. Solutions of Pb(NO3)2 characterized by Pb(II) concentrations of 465 mg/L and 710 mg/L, respectively, at a initial pH of 3, were used for the kinetic study. This pH value was considered that did not effect the structure of magnetite film deposited on zeolite particles and avoided Pb(II) hydrolysis during experiments. Identical samples of 0.5 g ZESMAG were contacted with 50 ml solutions in a shaker bath stirrer and maintained at 25~ At welldetermined durations, the solid phase was separated by centrifugation. The Pb(II) concentration within the liquid phase was determined by means of a Varian Spectr AA 110 atomic absorption spectrophotometer. 2.2. Determination of the effective diffusivity coefficient.

The use of mathematical model for determining the effective diffusivity coefficient required some studies at equilibrium. For that purpose, samples of 0.5 g ZESMAG were contacted with 50 mL Pb(NO3)2 solutions, at various Pb(II) initial concentrations and maintained in a shaker bath at 25~

1827

until the equilibrium set up. After phases separation by centrifugation, the concentration of Pb(II) within the solutions was determined by atomic absorption spectrophotometer. The main characteristics of ZESMAG were as follows: density- 2.92 g/cm3; average radius of particles - 1 8 . 7 5 10-4 cm; volume of one particle - 2 . 7 108 cm3; average number of particles in one gram of ZESMAG - 1.27 " 107. These data were taken into consideration for determining the effective diffusivity coefficient. Because 100 mL solution was used for 1 g ZESMAG, the volume of liquid corresponding to one particle (VL) was 7 . 8 8 10-6 cm3.

3. RESULTS AND DISCUSSION 3.1. Kinetic studies on Pb(l]) retention process on ZESMAG.

The kinetic plots obtained for two initial concentrations of solutions showed the increase of Pb(II) retention capacity as the initial concentration increased (Figure 1). In addition, the new adsorbent material (ZESMAG) showed a good Pb(II) retention capacity (0.64 mequivalent/g at initial Pb(II) concentration of 710 mg/L). 3.2. Determination of the effective diffusivity coefficient.

Estimation of parameters of adsorption isotherm The parameters in Langmuir's isotherms were determined by minimizing the sum of square deviations for experimental values as against predicted data by model. The way the Langmuir's isotherms approximated the experimental values is shown in Figure 2. 0.7 .&

0.6

&

_

A

A

~k

9

I)

9

A

[2]

0.5

9

~

0.4

~

0.3

= o

0.2

~

0.1

9

9

9

[1] kO

. ,,.,..~

0

k

0

,

100

200

300

400

500

Time [min] Figure 1. Pb(II) retention capacity versus time at various initial concentrations of solutions; 1-initial Pb(II) concentration of 465 mg/L, 2- initial Pb(II) concentration of 710 mg/L

1828 The relation between Pb (II) concentrations within liquid and ZESMAG particle at equilibrium, was expressed by Langmuir's isotherm: c~, =

9cA,- 9K m "cA I+K., .c A

(1)

where: c~- ion concentrations within zeolite at equilibrium, mequivalent/g; CA- ion concentrations in the bulk of solution, mequivalent/cm3; CA=,- saturation capacity, mequivalent/g; Km- equilibrium constant, era3/mequivalent.

0.8 " o al .2 r

0.7

I

I

I

I

0.6

0.9

0.6

E - 0.5 ID 0 ID

N 0.4 C C

o

,4--* t~ I,.= r

t" 0 0

0.3

0.2 0.1 0

0.3

1.2

P b 2+ c o n c e n t r a t i o n in solution , [ m e q u i v a l e n t / c m 3]

1.5 x 1 0 -3

Figure 2. Langmuir's isotherm for Pb(II) retention on ZESMAG The values of parameters in the Langmuir's isotherm were: Cram= 0.757 mequivalent/g and Km= 13 151 cm3/mequivalent.

Estimation of the effective diffusivity coefficient

References [ 11, 12] indicated that the limiting-rate step of the ion exchange process was ion diffusion within the solid matrix of the zeolite. Equation (2) described the modification of an "A" ion concentration within the zeolite particle along the radius and in time.

1829

CqCA~-- D, ( o2cA~ 2 OCaz)

-~---

"lv Or 2 + r

Or

(2)

where: D e - effective diffusivity coefficient within the solid matrix of the zeolite (ZESMAG), cm2/s; r - radius of the zeolite particle (ZESMAG), cm ; CAz- Pb(II) ion concentration within zeolite (ZESMAG), mequivalent/g; The boundary conditions were as follows: o On the surface of the zeolite particle (r = rp), the ion concentration within the first layer of zeolite was in equilibrium with the surrounding solution (according to Langrnuir's isotherms equation (1)); o To the core of the zeolite particle (r = 0), the concentration gradient was null:

(a ~ )

Or Jr=O

=0

(3)

The evolution of cation concentrations within the surrounding liquid was given by equation

(4):

VL" dc& dt = - D , . p . 4 . z . r p

2.(OcA~ ~, Or Jr=rp

(4)

where: VL - volume of liquid corresponding to a zeolite particle(ZESMAG); CaL cation concentration within the corresponding liquid; "

rp- extemal radius of the zeolite particle (ZESMAG). The material balance under dynamic regime for the "A" ionic species in a spherical volume element of "dr" thickness, when diffusion is the rate limiting step, is described by the following equation:

-4. z . ( r 3 -r~,).p. 3

dCAz, n

dt

=De

n+l--C'Az,n) - D e .4.z.rZ,q 9(C'Az'n--Caz'n-1) .4.x.r, 2 (CAz, 9 dr dr

(5)

To increase the accuracy of the numerical solution, the discretization of the partial derivatives equation (2) was processed using a variable increment of radius, but the ratio of two consecutive sections remained constant. Taking into account the equation (5), the change of Pb(II) concentration within the "n" liquid ring in time was as follows:

4 --.

-r:_,). p .

d,

2 ,,~,+l-CA~,,_D.4.z.rZ

= D e .4. z. r. 90.5(dr.+, +dr.)

1

0.5(dr.

+drn_l)

(6)

The numerical simulation program used 20 cross-sections along the radius of the zeolite particle and was written under MATLAB sottware.

1830 v

i

i

i

i

I I I I

I I I I

I I I I

I I I I

~

~

-~

-I

9000

12000

I

4

I

I

I

"d 0

,._._,

De=0.5,10-10 em ~ 2 /s 2

---

-r

e=0.75 * 1

cm2/s I

+O

=

10-1~ cm2/s I I |

0

0

I

I

3000

6000

.,,

~:~-~ _

,

I

,

,

,,

,,

,~

15000

Time, [s]

Figure 3. Comparison between experimental and simulated data at various effective diffusivity coefficients for Pb(II) retention on ZESMAG (Pb(II) initial concentration: 465 mg/L.) To calculate the effective diffusivity coefficient, the experimental data were compared to those resulted by numerical simulation for several values of the coefficient. The value, for which the sum of square deviations of the experimental data as against the predicted ones by model was minim, was chosen as the most probable value of the coefficient. Figure 3 shows a comparison between the experimental kinetic data and those obtained by numerical simulation for three values of the effective diffusivity coefficient at an initial concentration of 465 mg Pb(II)/L. Table 1 shows values of the effective diffusivity coefficient obtained by numerical simulation at two concentrations as against the predicted values. Table 1 Values of effective diffusivitv coefficients Initial concentration, Predicted values, 1010cm2s1 mg Pb (II)/L 465 710

0.5 1.0

0.75 2.0

1.0 3.0

Accepted as comparison to values, lOl~ 0.75 2.0

real by predicted

The values of the effective diffusivity coefficient increased as initial concentration increased.

1831 4. CONCLUSION The experiments in these paper showed the adsorption properties of a new synthesized material (zeolite - supported magnetite - ZESMAG), for Pb(II) ion from solutions. The Pb(II) retention capacity was 0.64 mequivalent/g ZESMAG at an initial Pb(II) concentration of 710 mg/L. The retention capacity for Pb(II) ion increased as initial concentration increased. The kinetic study resulted in the determination of the effective diffusivity coefficient. The equilibrium of Pb(II) retention process on ZESMAG was characterized by a Langmuir's isotherm. The comparison of kinetic data obtained by simulation at various values of the effective diffusivity coefficient to those from experiments allowed the determination of the really accepted ones. The numerical simulation offered the possibility of monitoring the compliance between the experimental data and those obtained by simulation all the way during kinetic determinations. The numerical simulation allowed the estimation of the kinetic parameters even when the limiting-rate step was not diffusion.

REFERENCES 1. J. E. Dutrizac and R.J.C. Mc Donald, Min. Sci. Engineering, 6 (1979) 59. 2. S. Tataru and A. Ardeleanu, Utilization of Ores by Dissolution and Biotechnology (Romanian), Foundation "Vasile Goldis", Arad, 1998. 3. I.B. Serova, V.A. Nikashina and B.A. Rudenko and S.S. Meshalkin, Natural ZeolitesSofia'95 (G.Kirov, L.Filizova and O.Petrov eds.), Pensot~ Publishers Sofia-Moscow (1997) 115. 4. S.E.Berkovich and A.Nikashima, Neorgan.Materiali, 26(5) (1990) 1035. 5. V.G.Aleynikov, V.I. Kolychev, T.N. Burushkin, A.A. Chuyko and N.Pikalov, The method of production of ferromagnetic sorbent, Soviet Union Patent No. 1577129 (1988) 6. K.Hoving and J.W.M.Walterbos, Zeolite-coated magnetic particles, Eur. Patent Appl. No.0149343 (1985) 7. I.B.Serova, V.A.Nikashima and B.B. Rudenko, Composition for the preparation of ferromagnetic ion-exchanger, Russian Patent No.208146 (1994) 8. V. Pode, V. Georgescu, V. Dalea, R. Pode and E. Popovici, Studies in Surface Science and Catalysis, Zeolites and Mesoporous Materials at the Dawn of the 21 st Century (A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine eds.), Elsevier, 135 (2001) 360. 9. G. Catan~., L. Frunza, D. Crisan, R. Pode and G. Burtica, Annals of "AI.I. Cuza" Univ. Iasi, Series of Chemistry, Vol. I, (1992) 200. 10. H. Minato, Natural Zeolites - Sofia '95 (G. Kirov, L. Filizova and O. Petrov eds.), Pensott Publishers, Sofia - Moscow (1997) 282. 11. R.H. Perry and D.W. Green, Perry's Chemical Engineers' Handbook, Seventh Mc Graw Hill Edition, 1997. 12. P.M. Armenante and D.J. Kirwan, Chemical Engineering Science, 44 (12) (1989)2781.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1833

Galliation o f beta zeolite b y the p H control m e t h o d Y. Oumi a, S. Kikuchi a, S. Nawata b, T. Fukushima b and T. Sano a a School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan ; E-mail: [email protected] b Tosoh Corporation, Shin-nanyo, Yamaguchi 746-8501, Japan The post-synthetic galliation of BEA by treatment of dealuminated zeolite with acidic gallium solutions was investigated by means of XRD, 71Ga MAS NMR, FT-IR and nitrogen adsorption. It was found that the gallium species in the solution are easily incorporated into the framework of dealuminated BEA zeolite by controlling the pH value of the solution below 7. The cumene cracking activity of the galliated BEA zeolite at pH 4 was comparable to that of the galliated BEA zeolite prepared by the direct hydrothermal synthesis method. 1. INTRODUCTION In general, the number of tetrahedrally coordinated aluminums in the zeolite framework greatly influences the physicochemical properties of zeolites such as thermal stability and sorptive, ion-exchange and catalytic abilities. To control the physicochemical properties of zeolites, therefore, isomorphous substitution of framework aluminums by various metals as well as dealumination has been widely investigated. The isomorphous substitution is achieved by the direct hydrothermal synthesis or the post-synthesis treatment. The post-synthetic replacement of tetrahedrally coordinated framework aluminums by hetroatoms is a suitable method if direct synthesis of the materials fails or synthesis is difficult to achieve. The post-synthesis replacement is usually attained by zeolite treatment in alkali media, pointing out dissolution of a part of zeolite framework [ 1-5]. Zeolite beta (BEA) is a 12-membered ring zeolite with a large pore structure and has a potential as catalyst in the petrochemical industry. Owing to its potential commercial importance, the isomorphous substitution of aluminums in the framework by other metals such as B, Fe and Ga has been studied by the post-synthesis under alkali conditions as well as the direct hydrothermal synthesis [6]. Recently, we found that aluminum species in the solution, which are eliminated from the framework of BEA zeolite by HC1 treatment, are easily reinserted into the framework by controlling the pH value of the solution (below pH 7), indicating the reversibility of dealumination-realumination process of BEA zeolite [7,8]. Dissolution of zeolite framework during the post-synthesis treatment hardly occurred due to acidic conditions. From such a viewpoint, we have now studied the incorporation of various metals into the BEA framework by the pH control method. In this paper, we describe the post-synthetic galliation of BEA zeolite by treatment of dealuminated zeolite with acidic gallium solutions and the characteristics of galliated zeolites.

1834 2. EXPERIMENTAL 2.1. Dealumination and galliation of BEA zeolite The protonated BEA zeolite (Si/A1 = 21) from Tosoh Co., Japan was used as the parent zeolite. For dealumination of the BEA zeolite, 5 g of the BEA zeolite was treated with an 8 mol dm 3 HC1 aqueous solution (20 g) at 80 ~ under stirring for 2 h. The dealuminated BEA zeolite was filtered off, washed thoroughly with deionized water and then suspended into an aqueous solution of Ga(NO3)3. A certain amount of a 0.2M NaOH aqueous solution was added to control the pH of the solution, which was stirred at 80~ for 30 min. The product was filtered off, washed thoroughly with deionized water (60~ 1L) and dried at 120~ for 12 h. Then the sample was exchanged two times with a 2M NH4NO3 solution at 80~ for 2 h and washed thoroughly with deionized water, and calcined again at 550~ for 8 h to obtain the protonated BEA zeolite. 2.2. Characterization The identification of zeolites obtained was achieved by X-ray diffraction (Rigaku RINT 2000). The bulk chemical composition was measured by X-ray fluorescence (XRF, Philips PW2400). Textural properties were determined by nitrogen adsorption (Bel Japan Belsorp 28SA). Before adsorption measurements at -196~ the powdered zeolites (ca. 0.1 g) were evacuated at 400~ for 4 h. 71Ga MAS NMR spectra were recorded on a Varian UNITYINOVA 400 spectrometer at 121.95 MHz with 10 kHz spinning speed and 1.6 [ts pulses for 30,000 scans. Ga(NO3)3" 8H20 was used as a chemical shift reference. The IR spectra for the framework vibration were recorded on a FT-IR spectrometer (JEOL JIR-7000) with a resolution 4 cm -1 at room temperature. The sample was pressed into a self-supporting thin wafer (ca. 6.4 mg/cm 2) and was placed in a quartz IR cell with CaFE windows. Prior to the measurements, each sample was dehydrated under vacuum at 400~ for 2 h. The IR spectra of chemisorbed pyridine on the protonated BEA zeolite were also measured at room temperature. The adsorption of pyridine was carried out at 150~ for 1 h, and then evacuated at 150~ for 30 min to remove the excess and weakly adsorbed pyridine. 2.3. Catalytic testing The cumene cracking was performed in an atmospheric pressure flow system. The sample catalyst placed in a quartz tube reactor of a 10 mm inner diameter was dehydrated at 400~ for 1.5 h in a nitrogen stream. The temperature was then brought into a reaction temperature (250~ The reactant was fed into the catalyst bed with microfeeder. Nitrogen was used as a carrier gas (40 ml/min). The contact time (W/F) was 24.1 g h mo1-1, and the partial pressure of cumene was 7.9 kPa. On-line product analysis was done on a Shimadzu GC-17A gas chromatograph (FID) with GL-Science TC-1 capillary column (30 m).

3. RESULTS AND DISCUSSION 3.1. Galliation and characterization of BEA zeolite Post-synthetic galliation was performed in the liquid phase. Namely, the BEA zeolite dealuminated with 8M HC1 aqueous solution was suspended into an aqueous solution of Ga(NO3)3 and the pH value of the solution was varied by adding a 0.2M NaOH aqueous solution. Fig. 1 shows the powder X-ray diffraction patterns and nitrogen adsorption

1835

.300

(A) (9

.2

I

tO

(dl

~-- 250 i.~ 200 E

.

t~

-~ o

i 5

(B)

"7,

~~

oo

100

~ 50 z~ 0

,

..

I

,,

15

25 35 45 0 0.5 1 2 0 (degree) Relative pressure (P/P0) Fig. 1 Powder X-ray diffraction patterns (A) and N2 adsorption isotherms (B) of the parent BEA, the dealuminated BEA and the products obtained at various pH values. (a) II : the parent BEA, (b) x : the dealuminated BEA, (c) /~ : pH 2, (d) O : p H 4 , (e) r--] : p H 6 , (f) ~ : p H 9 isotherms of the parent BEA zeolite, the dealuminated BEA zeolite and resulting products obtained at various pH values. For all the products, the X-ray diffraction diagrams of the products showed no peaks other than those corresponding to BEA zeolite and the intensities of the peaks were almost the same as those of the parent BEA zeolite. However, the reduction in the peak intensities was observed for the product obtained at pH 9, indicating slight structural degradation. Except for the product obtained at pH 9, all isotherms exhibited the type I isotherm and the amounts of nitrogen adsorbed on the BEA zeolites obtained at various pH values were comparable to that of the parent BEA zeolite. Table 1 lists the characteristics of BEA zeolites galliated at various pH values. The Si/Ga ratio of the galliated BEA zeolite decreased with an increase in the pH of the solution. Except for the BEA zeolite galliated at pH 9, a large reduction in the BET surface area and the pore volume was not observed, indicating no structural degradation. Table 1 Characteristics of the parent BEA, the dealuminated BEA and the galliated BEA obtained at various pH values. Si/Metal ratio BET surface Pore volume No. Sample Si/A1 Si/Ga area/m2g -1 /cm3(liquid)g -la) 1 Parent BEA 21 -625 0.27 2 Dealuminated BEA 894 -584 0.25 3 BEA galliated at pH 2 893 86 565 0.26 4 BEA galliated at pH 4 929 38 548 0.25 5 BEA galliated at pH 6 826 36 527 0.25 6 BEA galliated at pH 9 904 19 477 0.24 7 BEA(Ga) b) -26 594 0.27 a) Determined by Dubinin-Radushkevich method. b) Prepared by the direct hydrothermal synthesis method (135~ 14 days) [9].

1836

_.=

400

Fig. 2 SEM images of (a) the parent BEA and (b) the BEA galliated at pH 4.

I

I

200

0

I

--200

ppm Fig. 3 71Ga MAS NMR spectra of Ga-containing BEA zeolites prepared by (a) the pH control method and (b) the impregnation method. * denotes a spinning side band.

Fig. 2 shows the SEM images of the parent BEA zeolite and the BEA zeolite galliated at pH 4. Although the broken pieces of zeolite crystals were observed in the SEM image, no crystal dissolution seems to occure during the post-synthtic galliation treatment. To clarify the chemical state of galliam species in BEA zeolite, the galliated BEA zeolites were characterized by 71Ga MAS NMR. Fig. 3 shows the 71Ga MAS NMR spectrum of the BEA zeolite galliated at pH 4. As a refernce, the 71Ga MAS NMR spectrum of the Ga-loaded zeolite prepared by an incipient impregnation method was also shown in Fig. 3. In the galliated sample, the 71Ga MAS NMR spectnma shows the single sharp peak assigned to the tetrahedra!ly coordinated galliums at ca. 153 ppm [9,10]. A weak additional peak assigned to non-framework galliums was also observed around 0 ppm. On the other hand, the intensity of the peak at ca. 153 ppm was considerably weak for the Ga-loaded sample. This indicaes that most of the gallium species in the BEA zeolite perapred by the post-sythetic galliation are located in the zeolite framework. In the previous paper concerning realumination of dealuminated BEA zeolite [7,8], we found that the structural degradation of BEA zeolite occurred when the rapid incorporation of a large amount of aluminums into zeolite framework took place. Therefore, the pH of the solution for the post-synthtic galliation of BEA zeolite was changed stepwise. Namely, the solution was maintained at each pH (2, 3, 4) for 1 h. The Si/Ga ratio of the BEA zeolite galliated stepwise was 23 and smaller than that of the BEA zeolite galliated without stepwise treatment (sample no. 4 in Table 1). No reduction in the peak intensities of XRD pattren was observed, indicating no structural degradation. Fig. 4 shows the 71Ga MAS NMR spectra of the galliated BEA zeolite and the BEA(Ga) zeolite prepared by the direct hydrothermal synthesis method. The peak intensity was normalized based on 1 g of zeolite. It may be noted

1837

,!

4000 I

400

200

....

ppm

|

0

,,

t

--200

Fig.4 71Ga MAS NMR spectra of Ga-containing BEA zeolites prepared by (a) the pH control method (stepwise) and (b) the direct hydrothermal synthesis method.

I

I

3500

,,,

3000

Wavenumber (cm 1) Fig. 5 IR spectra of various protonated BEA zeolites. (a) the parent BEA, (b) dealuminated BEA, (c) the BEA galliated at pH 4 (stepwise) and (d) the BEA(Ga) prepared by the direct hydrothermal synthesis method.

that the the peak intensity at ca. 153 ppm of the galliated BEA zeolite is comparable to that of the BEA(Ga) zeolite, suggesting the high potential of the stepwise pH control method. Next, to get further information concerning the post-synthetic galliation, the IR spectra of the protonated BEA zeolites were measured. The BEA zeolites were protonated in a 2 M NHaNO3 aqueous solution at 80~ for 2 h followed by calcination at 550~ Fig. 5 shows the IR spectra of the protonated BEA zeolites in the 4000-3000 cm -I region. The IR spectrum of the parent BEA zeolite exhibited four peaks at ca. 3610, 3670, 3740 and 3782 cm -I. The peaks at ca. 3610 and 3740 cm -I are assigned to the acidic bridged OH of Si(OH)A1 and isolated silanol groups, respectively. It is considered that the peak at ca. 3670 cm 1 is assigned to the hydroxyl groups of aluminum species where the A1 atom is connected to the zeolite framework only by one or two remaining chemical bonds, whereas the peak at ca. 3782 cm 1 to the terminal hydroxyl groups bonded to non-framework species of A1OOH [ 11]. The three peaks at ca. 3610, 3670 and 3782 cm 1 disappeared after dealumination with HC1, confirming elimination of aluminum atoms. Simultaneously, the clear increase in the peak intensities at ca. 3740 and 3500 cm -1 assigned to the isolated and hydrogen bonding adjacent silanol groups, respectively, was observed. In the spectrum of the galliated BEA zeolite, the peak at ca. 3620 cm -1 appeared, while the broad peak at ca. 3500 cm -1 disappeared. The peak at ca. 3620 cm -1 is assigned to the acidic bridged OH of Si(OH)Ga [ 12] and its peak intensity was comparable to that of the BEA(Ga) zeolite prepared by the direct hydrothermal synthesis method. This strongly suggests that the post-synthetic galliation by the pH control method proceeds through

1838 the incorporation of gallium species in the solution species into hydroxyl nest generated by dealumination [ 13]. From the above results, it may be concluded that the post-synthetic galliation by the pH control method is very effective for preparation of Ga-containing zeolites.

3.2. Acidity and catalytic activity The acidic property of galliated BEA zeolites was examined with IR spectra of adsorbed pyridine. Fig. 6 shows the IR spectra of pyridine adsorbed on the parent BEA zeolite, the dealuminated BEA zeolite and the Ga-containing BEA zeolite in the region of 1650-1400 cm -1. The parent BEA zeolite exhibited several peaks due to Lewis bound pyridine (L: 1456 and 1618 cm -1) and pyridinium ion on Bronsted acid sites (B: 1546 and 1641 cm -1) as well as the shoulder peaks due to hydrogen bonded pyridine (1446 and 1596 cm-1). For the dealuminated BEA zeolite, very weak peaks due to hydrogen bonded pyridine were observed at 1446 and 1596 cm -1. On the other hand, the IR spectrum of adsorbed pyridine on the BEA zeolite galliated at pH 4 revealed the characteristic peaks due to pyridine species adsorbed on Bronsted and Lewis acid sites again. No difference in the peak intensities at 1546 and 1641 cm -1 was observed between the galliated BEA zeolite and the BEA(Ga) zeolite prepared by the direct hydrothermal synthesis method. These results strongly suggest that the acidic sites are regenerated by gaUiation and that most of the gallium species present in the galliated BEA zeolites are located in the tetrahedrally coordinated framework positions. The catalytic activityies of the galliated BEA zeolites were evaluated by the test reaction of cumene cracking, which requires medium to strong Br6nsted acid sites [ 14]. The conversion profiles of cumene on various BEA zeolites are shown in Fig. 7. The galliated BEA zeolite 60

~ 8

50

._~ 40

o

(b)

w

30 2o

,

0

1600 1500 Wavenumber (cm 1)

1400

Fig. 6 IR spectra of pyridine adsorbed on various protonated BEA. (a) the parent BEA, (19) the dealuminated BEA, (c) the BEA galliated at pH 4 (stepwise), (d) the BEA(Ga) prepared by the direct hydrothermal synthesis method.

0

,

,

9

5"

0.5

1

1.5

2

2.5

3

Time on stream (h), Fig. 7 Conversion profiles of cumene on various protonated BEA. Temperature - 250~ W/F - 24.1 g h mo1-1, II : the parent BEA, • :the dealuminated BEA, O : the BEA galliated at pH 4 (stepwise), /k : the BEA(Ga) prepared by the direct hydrothermal synthesis method.

1839 showed the catalytic activity although the activity was lower than that of the parent BEA zeolite. The catalytic activity of the galliated BEA zeolite was nearly the same as that of the BEA(Ga) zeolite. As no differenec in the Si/Metal ratio is observed between the parent BEA zeolite and the galliated BEA zeolite, the difference in the cumene conversion seems to be attributable to the difference in the acidc properety. This was confirmed from a comaprison of the peak intensities of pyridine adsorbed on the parent BEA zeolite and the galliated BEA zeolite as shown in Fig. 8. Pyridine vapor was adsorbed onto the sample at 150~ for 1 h and then IR spectra were recorded at various stages of pyridine desorption, which was continued by evaporation at progressively highly temperatures (250-600~ The intensities of peaks assigned to pyridine associated with both BrSnsted and Lewis acid sites decreased gardually with evaporation temperaure. In the case of the galliated BEA zeolite, the peaks due to pyridinium ions on BrSnsted acid sites disappeared after 500~ On the other hand, in the case of the parent BEA zeolite, the peaks were still observed even after evacuation at 600~ This strongly suggests that the BrSnsted acid sites generated on the galliated BEA zeolite are less acidic than those of the parent zeolite containing aluminums.

(A)

(D 0 r k., 0

<

L

i

1600

I

~B+

,~Ln L

I

i

1500 1400 Wavenumber (cm~)

1600 1500 1400 Wavenumber (cm~)

Fig. 8 IR spectra of pyridine adsorbed on (A) the parent BEA and (B) the galliated BEA at various evacuation temperatures.

(a) 250~ (b) 300~ (c) 400~ (d) 500~ (e) 600~ 4. CONCLUSIONS

It was found from all above results that the gallium species in the solution are easily incorporated into the framework by controlling the pH value of the solution below 7 (acidic condition). The efficiency of galliation increased with an increase in the pH value. The physicochemical properties of the galliated BEA zeolites were the same as those of the

1840 BEA(Ga) zeolite prePared by the direct hydrothermal synthesis method. Taking into account the fact that synthesis of highly crystalline Ga-containing zeolites generally needs longer crystallization time, the post-synthetic galliation by the pH control method is very useful for preparation of Ga-containing zeolites. ACKNOWLEGEMENTS The authors would like to thank Mr. J. Ashida at Varian Technologies Japan Ltd. for MAS NMR measurement.

71Ga

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

X. Lin, J. Klinowski and J. M. Thomas, J. Chem. Soc., Chem. Commun., (1986) 582. L. Aouali, J. Jeanjean, A. Dereifn, P. Tougne and D. Delafosse, Zeolite, 8 (1988) 517. Z. Zhang, X. Liu, Y. Xu and R. Xu, Zeolites, 11 (1991) 232. J. Datka, B. Sulikowsk and B. Gil, J. Phys. Chem., 100 (1996) 11242. X. Zaiku, C. Qingling, Z. Chengfang, B. Jiaqing and C. Yuhua, J. Phys. Chem. B, 104 (2000) 2853. R. Fricke, H. Kosslick, G. Lischke and M. Richter, Chem. Rev., 100 (2000) 2303. Y. Oumi, R. Mizuno, K. Azuma, S. Nawata, T. Fttkushima, T. Uozumi and T. Sano, Microporous Mesoporous Mater., 49 (2001) 103. Y. Oumi, R. Mizuno, K. Azuma, S. Nawata, T. Fukushima, T. Uozumi and T. Sano, Stud. Surf. Sci. Catal., 135 (2001) 209. K.J. Chao, S. P. Sheu, L.-H. Lin, M. J. Genet and M. H. Feng, Zeolites, 18 (1997) 18. J. E. Hazm, P. Caullet, J. L. Paillaud, M. Soulard and L. Delmotte, Microporous Mesoporous Mater., 43 (2001) 11. I. Kiricsi, C. Flego, G. Pazzuconi, W. O. Parker, R. Millini Jr., C. Perego and G. Bellussi, J. Phys. Chem., 98 (1994) 4627. M. L. Occelli, H. Eckert, A. Wolker and A. Auroux, Microporous Mesoporous Mater., 30 (1999)219. S. Dzwigaj, M. J. Petre, P. Massiani, A. Davidson, M. Che, T. Sen and S. Sivasaker, J. Chem. Soc., Chem. Comm., (1998) 87. R. Mokaya and W. Jones, J. Catal., 153 (1995) 761.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1841

Ion e x c h a n g e b e h a v i o u r o f two synthetic phillipsite-like phases C. Colella a, B. de Gennaro a, B. Liguori a and E. Torracca b aDipartimento di Ingegneria dei Materiali e della Produzione, Universith Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy bDipartimento di Ingegneria Meccanica e Industriale, Universit~ di Roma 3, Via della Vasca Navale 79, 00146 Roma, Italy Two phillipsite-like phases have been synthesized at 80~ treating a rhyolitic pumice in a mixed Na+-K+ alkaline environment, having different Na+/K+ molar ratios. The synthesized samples, displaying distinct X-ray diffraction patterns, have been characterized as regards their ion exchange properties. Ion exchange isotherms for the cation pairs NaJNH4, Na/Ba, N a ~ and Na/Ca have been obtained and the relevant thermodynamic equilibrium constants calculated with the help of a computer program. Both phillipsite-like phases revealed an excellent to good selectivity for Ba, NH4 and K and a substantial unselectivity for Ca, in substantial accordance with previous results obtained with a sedimentary phillipsite. Nevertheless, the different shapes of some isotherms and therefore the different occupancy of peculiar ion exchange sites indicate that the two synthetic phillipsites are structurally distinct, even if this supposition should be confirmed by a structural analysis. 1. INTRODUCTION Phillipsite is one of the most common natural zeolites, if we consider in general minerals of both hydrothermal and sedimentary origin. The sedimentary phillipsite-rich formations of economic interest are, however, concentrated in a few locations in the world, especially in Italy, Germany and, to a lesser extent, in Spain (Canary Islands) [ 1]. Phillipsite-bearing rocks (either tufts or ignimbrites), which often present as accompanying zeolitic phases chabazite and/or analcime, have proved to be employable in dozen environmental, industrial or agricultural applications [2], mostly in consideration of the good ion exchange selectivity for several toxic or noxious cations [3]. Phillipsite is readily obtained by synthesis. Amorphous alumino-silicate systems (for instance natural glasses) with medium to high silica content are prone to give phillipsite by hydrothermal treatment at low temperatures (<100~ in alkaline media, provided both Na + and K + are present in the reaction environment [4,5]. Some years ago two phases, both resembling phillipsite, but displaying apparently different X-ray diffraction patterns, were obtained by reacting in the same conditions similar reaction mixtures having, however, different K+/Na+ molar ratios [6]. Having in mind that isotypical phases with different chemistry may exhibit distinct ion exchange properties [7], this paper aims at (1) characterizing from a chemical point of view

1842 the two synthesized phillipsite-like phases, (2) investigating some ion exchange equilibria involving them, (3) correlating their ion exchange behaviour with chemical and/or structural features, also in the light of the known ion exchange properties of an analogous mineral phase [3]. 2. EXPERIMENTAL SECTION

2.1. Materials Synthetic phillipsite was obtained by reacting for 7 days, at 80~ rhyolitic pumice from Lipari island (Messina, Italy) in 1 molal mixed NaOH-KOH solutions with a pumice-to-water ratio (w/w) equal to 1/10 [6]. The selected samples, designated as Phl and Ph2 (M and Ph in the original paper [6]), were obtained from systems characterized by K+/Na+ molar ratios equal to 0.09 and 0.67, respectively. Their X-ray diffractograms (Philips PW 1730 apparatus) revealed the presence of a phillipsite-like phase as the only crystalline component of the synthesized products. No attempts were made to ascertain the purity of the samples, which might therefore contain some unreacted amorphous material. The two phillipsite-like samples were pre-exchanged in their Na+-form (see below) and stored at room temperature over saturated Ca(NO3)2 solution (R.H. near 50%). Selected chemical analyses were performed using standard methods (see below). Water content was estimated by thermogravimetry (Netzsch STA 409 thermoanalyzer). 2.2. Cation exchange capacity Cation exchange capacity (CEC) of the two phillipsite-like samples was determined using the cross-exchange method [8]. Accordingly, two 1-g zeolite samples, placed on gooch filters, after extensive washing with distilled water, were percolated at about 60~ up to exhaustion by 0.5 M NaC1 or KC1 solutions, prepared by using reagent-grade Carlo Erba RPE chemicals (purity 99.5%). The obtained monocationic forms (Na + or K +) were then re-exchanged under the same conditions with potassium or sodium, respectively. Na + and K + concentrations in the eluates of the second exchange cycle, evaluated by atomic absorption spectrophotometry (AAS, Perkin Elmer AA 2100 apparatus), were used to calculate the CEC values, taking also into consideration the A1 content of the samples. It is to be observed that the CEC value can be ascribed to phillipsite only, because the contribution of the non-crystalline exchanging phases (essentially unreacted pumice) may be considered negligible [8]. 2.3 Ion exchange runs The two synthesized samples were used for the equilibrium studies in the presence of the Na/K, Na/Ca, NaJNH4 and Na/Ba cation pairs. Sodium forms of the two phillipsite samples, obtained through an analogous exhaustive procedure, like that used in the estimation of the CEC, were allowed to react at 25 • 0.1 ~ in sealed teflon test tubes with solutions, containing varying amounts of Na § and one of the cations K § Ca 2§ NH4§ or Ba 2§ at 0.1 total normality, prepared starting from the relevant reagent-grade Carlo Erba RPE chlorides. Reversibility ion exchange tests were performed following the recommendations of Fletcher and Townsend [9]. The reaction time was fixed at 3 days, which was beforehand proved to be sufficient to attain equilibrium. At equilibrium, the liquid phase was analyzed for the cation concentrations, whereas the concentration of the exchangeable cations in solids was calculated by mass balance.

1843

2.4. Analytical procedures

Alcali metals were determined by AAS using the standard addition method; ammonium concentrations were measured colorimetrically using the Nessler's reagent with an AQUAMATE UV-Vis spectrophotometer (Spectronic Unicam); A1 and Fe (coming from an acid attack of the zeolite samples), Ca and Ba were estimated titrimetrically with EDTA [ 10].

2.5. Computation of the thermodynamic parameters The measured equilibrium data were plotted under the form of ion exchange isotherms, reporting the equivalent fraction of the ingoing cations in the solid phase as a function of the equivalent fraction of the same cations in solution. From the same data were the thermodynamic exchange parameters calculated with the help of a computer program following a standard procedure [11 ]. A critical step in this procedure is the computation of the cation activity coefficients in solution. In this study these coefficients have been estimated following two different methods, proposed by Ciavatta [12] and Pitzer [13], and compared with each other.

3. RESULTS

3.1. Characterization of the synthetic phillipsite-like samples Table 1 reports the main reflections of the X-ray diffraction patterns of the two synthesized phillipsite-like phases (Phl and Ph2), compared with those of a typical diagenetic phillipsite (NP), coming from the huge formation of Neapolitan yellow tuff (Campi Flegrei, NE Naples, Italy) [ 15]. Indexing is purely indicative; no attempts were made to refine the structures of the two synthetic phases. The diversity of the three patterns reported in Table 1 is evident, although a closer similarity between the Ph2 and NP patterns is undeniable. Differences in Phl and Ph2 patterns may certainly arise from a possibly different framework and extraframework composition. It is to be observed, however, that Na +- or K+-exchanged samples, obtained by extensive exchange of the two original samples with Na + or K +, respectively, gave rise to similar but not identical X-ray diffraction patterns, which accounts for the fact that the two samples might be structurally distinct. This is not surprising, considering that minor differences in structure are possible in isotypical phases, made homoionic through ion exchange procedures [ 16]. Selected chemical analyses were carried out on Na+-exchanged Phl and Ph2 samples (Table 2). From these data it can be seen that exchanged Na + is equal or slightly lower than total Na (in Phl this might account for the presence of minor amounts of impurities in the sample). The fact that Na + released is higher than A1 but lower than A1 + Fe content might be connected to the possible partial incorporation of Fe in the aluminosilicate framework. In this case the reduced amount of K + uptaken following to exchange with Na +, could be explained supposing that some Na + is replaced by hydronium ions. It is in any case noteworthy that in both zeolites A1 content is nearly coincident with K + uptaken. This suggests that A1 content may be considered as the maximum available CEC in the selected experimental conditions. On these bases, the calculated CEC values, which have been used throughout this study, are 3.48 mequiv./g and 3.15 mequiv./g for Phl and Ph2, respectively. Accordingly, neglecting the possible presence of Fe in the framework, the unit cell formulas of the Na-exchanged samples turned out to be Na4.5[Ala.sSill.5032]'12.8H20 and Na4.0[A14.0Si12.0032]'12.5H20,

1844 respectively. The Si/A1 ratio in the two samples, i.e., 2.6 and 3.0, respectively, accounts for their higher acidicy compared with the most common natural phillipsites formed by diagenesis [ 15].

Table 1 X-ray diffraction patterns of synthetic and natural phases connected to phillipsite Phl hkl

d

Ph2 I

101 8.17 8 020,200 7.09 67 210 121 5.37 6 220 5.00 43 002 301 4.29 4 131,311 4.09 58 202,022 212 321 410 141 103 3.240 24 420,240,331 3.167 100 113 123 2.949 12 042,402 2.889 10 501,341 2.744 18 422,242,151,511 133,313 2.676 62 * NP = Natural phillipsite from sedimentary

NP*

d

I

d

I

8.15 7.13

19 60

5.37 5.03 4.97 4.29 4.11 4.08

26 26 38 14 17 24

8.17 7.07 6.33 5.35 5.02

6 58 5 13 19

4.28 4.10

3 28

3.675

5

3.258 3.231 3.188 3.145 2.941

50 52 100 31 52

3.93 3.665 3.440 3.265 3.245 3.174

4 6 2 20 22 100

2.746 2.687 2.678 deposits

2.966 2.920 36 2.737 33 2.684 24 2.675 in Marano (Napoli, Italy) [ 14].

Table 2 Analytical data for the Na forms of the two synthetic phillipsite-like phases Measured quantity

Phl

Ph2

Total Na § (mequiv./g) Na § released (mequiv./g)* K § uptaken (mequiv./g)* A1 content (mequiv./g) A1 + Fe content (mequiv./g) Weight loss (%) * Following to a Na § -~ K § exchange.

3.95 3.67 3.48 3.50 3.82 17.9

3.24 3.26 3.15 3.15 3.45 17.6

14 3 19 19 19

1845 3.2. Ion exchange equilibria Figures 1 and 2 report the profiles of the ion exchange isotherms obtained for the two zeolites Phi and Ph2, respectively. A close inspection and comparison of the various curves enable to make a number of observations that are reported in the following. 9 Na§ NH4 § and N a § exchanges. Phi and Ph2 evidence distinct behaviours, in that the relevant isotherms exhibit a regular shape (convex curve) for the former zeolite and a very distinct plateau with an inversion of selectivity at about 90% of the equivalent fraction of the ingoing cation for the latter zeolite. The latter behaviour is practically identical to that found for a sedimentary phillipsite [3], demonstrating the close similarity between these two phases. 9 2 N a + . ~ - B a 2+ exchange. The isotherms are convex for both synthetic phillipsite-like phases, demonstrating a good selectivity for Ba 2+, in agreement with the results obtained for the natural phillipsite [3].

U~

1

1

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.2

0.4 E

0.6

/

0 0

0.8

~

0.2

9

0.4 E

NH4(s)

0.6

0.8

0.6

0.8

1

Ba(~

0.8

0.(

0.6 ,3

0.4

0.4

0.2

0.2

0

0

0.2

0.4

0.6 E

K(s)

0.8

1

0 0

/ 0.2

0.4 E

Ca(s)

Figure 1. Isotherms at 25~ for the exchange of various cations X z§ (top left: NH4§ top right: Ba2+; bottom left: K +; bottom right: Ca 2+ ) into Na-Phl at 0.1 total normality. Ex(s): X equivalent fraction in solution; Ex(z)" X equivalent fraction in the zeolite. Open circles: forward points; filled circles: reverse points.

1846

J

f

0.8

0.6

0.4

0.2

1

0 0

0.2

0.4

0.6

E

0.8

0

0.2

0.4

0.6 E

NH4(s)

0.8

Ba(s)

( 0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

%''

'0'.2

'014

' 016 E

018

1

03

0.2

0.4

0.6 E

K(s)

0.8

1

Ca(s)

Figure 2. Isotherms at 25~ for the exchange of various cations X z+ (top left: NH4+; top right: Ba2+; bottom left: K+; bottom right: Ca 2+) into Na-Ph2 at 0.1 total normality. Ex(s): X equivalent fraction in solution; Ex(z)" X equivalent fraction in the zeolite. Open circles: forward points; filled circles: reverse points.

Table 3 Ka values of exchange reactions at 25~ in two synthetic phillipsites Cation pairs Phi Ph2 Ciavatta Pitzer Ciavatta Na + ~ K + 8.94 9.00 10.50 Na + ~ NH4 + 7.15 7.26 8.98 2Na + ~

Pitzer 10.57 8.85

Ca 2+

0.11

0.11

0.14

0.14

2 N a + --~ Ba 2+

30.03

29.02

29.46

28.47

2+ e x c h a n g e . Also in this case the isotherms present a similar behaviour: the curves are S-shaped for the presence of a selectivity reversal in the range of 58-60% of the ingoing cation substitution in the framework. This behaviour is dissimilar to that of the

9 2Na + ~-Ca

1847 mineral [3], which presents a very distinct limit (close to 50%) to the replacement o f N a + for Ca 2+. Table 3 reports the equilibrium constants, Ka, calculated with the help of a computer program following a standard procedure [11]. It is to be observed the excellent agreement between the values obtained following the two procedures indicated by Ciavatta [12] and Pitzer [ 13], which can therefore been used indifferently. The data confirm, from a qualitative point of view the trend already found for the sedimentary phillipsite, i.e., elevated selectivity for Ba 2+, K + and NH4 +, in this order, and a substantial unselectivity for Ca 2+ [3]. The selectivity series for both zeolites will be therefore: Ba > K > NH4 > Na > Ca. 4. DISCUSSION AND CONCLUSION An attempt can be made to interpret the collected ion exchange data in the light of the phillipsite structure [17,18], which, according to a recent investigation [19], is reported to contain three cation extraframework sites: site I, located on the mirror plane (001); site II, located in a general position near the intersection of the two sets of channels; site II' located in a position near to site II. Site I is preferentially populated by large-size cations; site II, which has a low occupancy (below 50%), is usually populated by small-size cations, but may be occupied also by large-size cations; site II' is usually empty, but, if necessary, it may be partially populated, preferentially by small-size cations, only if the nearest site II is unoccupied. Coherent interpretations may be worked out from the behaviours of the two synthetic phillipsites with the Na/Ba and Na/Ca cation pairs. Ba 2+, in fact, confirming its affinity for the phillipsite framework, already observed in the natural sample [3], presents an easy accessibility either to site I or to site II. As regards Ca 2+, it is apparent that this cation can readily substitute for Na + only in roughly half the sites of the synthetic phillipsites, presumably in type II sites, whereas occupancy of site I is manifestly difficult. It is interesting to point out that a distinct, but still coherent, behaviour is shown by the natural phillipsite [3], which presents a clear limit to the replacement of Na + for Ca z+ at about 50%. In this case the apparent higher selectivity of site II for Ca 2+ results in the change from an inversion point (in samples Phl and Ph2) to a plateau in the isotherm curve of the mineral [3]. As regards K + and NH4 + the observed differences in the behaviour of the samples Phl and Ph2 are more hardly explainable without the help of the structural analysis. Ph2, which behaves very closely as the natural counterpart [3], presents a plateau in the isotherm at about 90% conversion. This might be interpreted in terms of difficulties in populating the less selective site II (or site II' if site II is saturated, i.e., occupied at about 50%). The anomalous behaviour of Phl, compared to that of Ph2 and the natural phillipsite, which may account also of its distinct X-ray diffraction pattern, can not be interpreted easily. It may be supposed a poomess of cristallinity or a partial structure distortion, presumably due to the not completely favourable synthesis conditions in which it has been produced (scarce presence of K +, compared to Na +) [4,6,19]. It may be concluded that the two synthetic phillipsites, which have been demonstrated by the present ion exchange studies to be similar but not identical, confirm the selectivity sequence, presented by the sedimentary mineral [3], pointing out the favourable perspective of use of this zeolite for the removal of Ba and NH4 from wastewaters, as laboratory experiments have already proved [20,21 ].

1848

REFERENCES

1.

.

,

.

5. .

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

C. Colella, M. de' Gennaro and R. Aiello, in Natural Zeolites: Mineralogy, Occurrence, Properties, Applications, D. L. Bish and D. W. Ming, Eds., Reviews in Mineralogy & Geochemistry, Mineralogical Society of America, Vol. 45, Washington, D.C., 2001, p. 551. C. Colella, in Handbook of Porous Solids, F. Schtith, K. Sing and J. Weitkamp, Eds., Wiley-VCH, Weiheim, Germany, 2002, in press. C. Colella, E. Torracca, A. Colella, B. de Gennaro, D. Caputo and M. de' Gennaro, in Zeolites and Mesoporous Materials at the Dawn of the 21 st Century, A. Galameau, F. Di Renzo, F. Fajula and J. Vedrine, eds., Studies in Surface Science and Catalysis, No. 135, Elsevier, Amsterdam 2001, p. 148 ( paper 01-O-05 in the CD-Rom). C. Colella and R. Aiello, Rend. Soc. Ital. Min. Petr., Milan, 31 (1975) 641. M. de' Gennaro, C. Colella, E. Franco and D. Stanzione, N. Jb. Miner. Mh. No. 4 (1988) 149. C. Colella, R. Aiello and V. Di Ludovico, Rend. Soc. Ital. Min. Petr., Milan, 33, No.2, (1977) 511. M. Adabbo, D. Caputo, B. de Gennaro, M. Pansini and C. Colella, Microporous and Mesoporous Materials, 28 (1999).315 C. Colella, M. de' Gennaro, E. Franco and R. Aiello, Rend. Soc. Ital. Min. Petr. (Milan), 38 (1982-83) 1423. P. Fletcher and R.P. Townsend, J. Chem. Soc. Faraday Trans. I, 77 (1981) 497. G. Schwarzenbach and H. Flaschka, Complexometric Titration, Methuen, London, 1969, 490 pp. D. Caputo, R. Dattilo and M. Pansini, Proc. III Conv. Naz. Scienza e Tecnologia delle Zeoliti, R. Aiello (Ed.), AIZ (Associazione Italiana Zeoliti), Napoli, Italy, 1995, p. 143. L. Ciavatta, Annali di Chimica, 70 (1980) 551. K.S. Pitzer, in Activity Coefficients in Electrolyte Solutions, K.S. Pitzer, ed., CRC Press, Boca Raton, Florida, 1991, p. 75. A. Colella, Laurea Thesis in Geological Sciences, Earth Science Department, Federico II University, Naples, Italy, 1997, 126 pp. M. de' Gennaro, M. Adabbo and A. Langella, in Natural Zeolites '93, D.W. Ming and F.A. Mumpton, eds., I.C.N.Z. (Int. Comm. Natural Zeolites), Brockport, New York, 1995, p. 51. A. Martucci, A. Alberti, M. Sacerdoti, G. Vezzalini, P. Ciambelli and M. Rapacciuolo, in Natural Zeolites for the Third Millennium, C. Colella and F.A. Mumpton, eds., De Frede, Napoli, 2000, p. 45. H. Steinfink, Acta Cryst., 15 (1962) 644. R. Rinaldi, J.J. Pluth and J.V. Smith, Acta Cryst., B30 (1974) 2426. A.F. Gualtieri, E. Passaglia and E. Galli, in Natural Zeolites for the Third Millennium, C. Colella and F.A. Mumpton, eds., De Frede, Napoli, 2000, p. 93. C. Colella and R. Aiello, Occurrence, Properties and Utilization of Natural Zeolites, D. Kallb and H.S. Sherry, eds., Akad6miai Kiadb, Budapest, 1988, p. 491. B. de Gennaro and A. Colella, Proc. EUROMAT 2001 (CD Rom), AIM (Ass. Ital. Metallurgia), Milano 2001, (Abstract in "Conference Abstracts", p. 353).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1849

C o m p e t i t i v e E x c h a n g e o f Lead(II) and C a d m i u m ( I I ) f r o m A q u e o u s Solution on Clinoptilolite

S. Berber-Mendoza, R. Leyva-Ramos, J. Mendoza-Barron and R. M. Guerrero-Coronado

Centro de Investigacion y Estudios de Posgrado, Facultad de Ciencias Quimicas Universidad Autonoma de San Luis Potosi Av. Dr. M. Nava. No 6, San Luis Potosi, SLP, Mexico, CP 78210

The experimental data for the single exchange isotherms for Pb(II) and Cd(II) were adjusted quite well by the Langmuir isotherm and the exchange capacity for the Pb(II) ion is about 2.3 times that for the Cd(II) ion. The results of the competitive exchange showed that the ion exchange isotherm for Pb(II) was not significantly dependent upon the Cd(II) concentration, this means that Pb(II) ion was exchanged more selectively than the Cd(II) ion. However, the exchange isotherm for Cd(II) was considerably affected by the presence of Pb(II) ions. The exchange capacity for Cd(II) diminished drastically increasing the concentration of Pb(II) ion. Therefore, both ions compete for the same cationic sites of the zeolite but the zeolite is much more selective for the Pb(II) ion than for the Cd(II) ion.

1. INTRODUCTION Natural zeolites are crystalline, hydrated aluminosilicates with a framework consisting of a network of SiO4 "4 and A104 -5 tetrahedra linked to each other by sharing all of the oxygen atoms. The isomorphous substitution of Si +4 by A1§ in the tetrahedral zeolite structure gives rise to a net negative charge on the framework. This negative charge is usually balanced by exchangeable cations which for natural zeolites are mainly sodium, potassium, calcium and magnesium. The fact that these cations, which are relatively innocuous, can be exchanged for heavy metal cations present in aqueous solution, makes natural zeolites very useful for removing toxic cations present in water. Natural zeolites have been employed as cation exchangers in environmental pollution control for the removal of ammonia from municipal wastewaters, caesium and strontium from radioactive waste and heavy metals from industrial wastewaters (1). The heavy metal exchange on natural zeolites have been investigated in various studies (25). The cation exchange capacity of zeolites is dependent upon the pH and temperature of the solution, the physicochemical and morphological characteristics of the zeolite as well as the physicochemical properties of the cation to be exchanged. The cation selectivity is the preference that a particular zeolite exhibits for one cation over others. Kirov et. al. (2) reported that the selectivity of clinoptilolite is as follows: Pb +2 > Zn +2 > Mn +2 > Cd +2. This selectivity was determined by comparison of the individual ion exchange isotherms for each of the cations tested.

1850 Besides the factors mentioned above, the exchange of a metal cation on natural zeolites is also affected by the presence of other metal cations which can also be exchanged and thus compete for the same cationic sites. The effect of competitive cations on exchange in natural zeolites has not been studied and in general, research on this subject has been mainly focused to evaluating the exchange of a single metal cation and determining its exchange isotherm. Ion exchange of a metal cation in a natural, unmodified zeolite is a multicomponent process. The metal cation is exchanged from the solution to the zeolite and the exchangeable ions (K +, Na +, Ca +2 and Mg +2) are transferred from the zeolite to the solution. In addition, the hydronium ion H30 + has also been reported as exchanging from solution to zeolite, although the magnitude of this exchange is insignificant and can be considered negligible (6). The aim of this work is to determine the simultaneous exchange isotherms for Cd(II) and Pb(II) from an aqueous solution on clinoptilolite, and to examine the effect of Cd(II) on the Pb(II) exchange isotherm and that of Pb(II) on the Cd(II) isotherm.

2. E X P E R I M E N T A L M E T H O D S

The zeolitic rock used in this study was from a deposit located in San Luis Potosi, Mexico. The zeolite sample was washed several times with deionized water and dried for 24 hours in an oven set at 110~ The zeolitic rock has a BET surface area of 22.2 m2/g and a density of 2.32 g/cm 3 (6). Identification of the crystalline species present in the zeolite-rich rock was done by X-ray diffraction analysis. X-ray diffraction patterns were obtained with a Rigaku, DMAX 200 diffractometer. Pb(II) and Cd(II) concentrations in aqueous solution were determined using atomic absorption spectrophotometric methods. Absorbance of a sample was measured with a double beam Varian SpectrAA-20 atomic spectrophotometer and metal ion concentration was determined by comparing absorbance with a previously prepared calibration curve. Experimental exchange isotherm data was obtained in a batch exchanger consisting of a 500 mL Erlenmeyer flask to which 480 mL of a solution of known initial concentrations of Pb(II) and Cd(II) and a predetermined mass of zeolite were added. The adsorber was submerged in a constant temperature water bath. The solution was kept in continuous agitation by means of a Teflon-coated stirring bar driven by a magnetic stirrer. The metal cation solution and zeolite remained in contact until both cations reached equilibrium. Samples were taken at various times to follow the progress of exchange, and metal ion concentration for each sample was determined as described above. The equilibrium was attained when two consecutive samples showed no change in metal ion concentration. Preliminary experiments revealed that 21 days was sufficient to reach equilibrium. The molar uptake of the cation exchanged was calculated by performing a mass balance for each cation.

4. RESULTS AND DISCUSSION The zeolitic rock used in this study was characterized in a previous study (6). X-ray diffraction analysis revealed that rock is composed mostly of clinoptilolite and also contains calcite, quartz and some feldspars such as microcline and albite. The experimental cation exchange capacity, CEC, evaluated by sorption of NH4 § ions is 1.55 meq/g (6). The values for all the zeolite properties are within the interval of values previously reported (2,7,8).

1851 Experimental data for the single exchange isotherms of Pb(II) and Cd(II) were fitted to the Langmuir isotherm expressed mathematically as follows" qm,iKiCi

qi = - -

I+KiC i

9

(1)

The constants for this isotherm were obtained by a method of least squares employing an optimization algorithm. Langmuir isotherm constants and average percent deviation are shown in Table 1. Average percent deviation was calculated according to the following equation: %Dev =

1F

1

~ qexp - qcalc 100% . N i=l L q exp

(2)

As shown by the average percent values reported in Table 1, a reasonable fit to the experimental data was obtained with the Langmuir isotherm. Experimental ion exchange data for Pb(II) and Cd(II) are shown in Figure 1 along with the Langmuir isotherms. The qm,i value of the Langmuir isotherm for a particular ion can be considered the maximum exchange capacity of zeolite for that ion. Comparing the qm,i value for Pb(II) to that for Cd(II) shows that the exchange capacity for the Pb(II) ion is 2.3 times that for the Cd(II) ion. This indicates that the zeolite has a larger exchange capacity for the Pb(II) than for Cd(II) ion. Similar results were reported by other authors (2,4). Selectivity is the preference that a zeolite has for one cation over others and is evaluated by comparing the exchange isotherms on zeolite for each one of the ions separately. Such a comparison fails to take into consideration that when two cations are exchanging simultaneously onto the zeolite, both ions may compete for the same cationic sites of the zeolite. The exchange isotherm for a given ion onto zeolite establishes the relationship between the cation concentration in solution and the mass of the cation exchanged onto the zeolite once equilibrium is reached. The exchange isotherm for a single ion determines the exchange capacity of the zeolite for that ion when it is the only ion present in solution. The effect of the presence of a competitive cation on the exchange isotherm of another cation can be studied by comparing the exchange isotherms of either of the ions for different initial concentrations of the competitive ion. This method is limited since the initial concentration of the competitive ion is not its equilibrium concentration and the isotherm depends on the concentration at equilibrium rather than on the initial concentration. Figures 2 and 3 illustrate the method for representing simultaneous ion exchange isotherms. Table 1 Langmuir isotherm constants for single Cd(II) and Pb(II) exchange at pH 4 and 25~ Ion

% Deviation

qm,i

Ki

(m-tool/g)

(L/m-mol)

Pb +2

0.581

23.05

20.76

C d +2

0.259

2.504

23.62

1852 0.6 "

~) rael r

o i

-

"

'

"

"

"

'

"

"

"

'

O

O '

. . . .

O

..,

"

'

"

"

"

'

"

"

"

'

"

-'

0.5 ~

0.4

I

d 0.3 ~ ' 0.2

0

I

43

1)

_~--

_ m~"'"~'~~~"

o 0.1

N

.

0.0 0.0

.

02

.

"0,,Pb(II) ] 1 " ~ Cd (II) 1

.

04

06

08

~0

12

~.4

Concentration at equilibrium, m-moles/L Figure 1. Single exchange isotherms for Cd(II) and Pb(II) onto zeolite at pH 7 and 25~ Figures 2 and 3 illustrate that the zeolite exchange capacity for Cd(II) decreased drastically when the initial Pb(II) concentration was increased and that the zeolite capacity for Pb(II) decreased very slightly when the initial concentration of Cd(II) was increased. Cd(II) and Pb(II) competed for the same cationic sites but Pb(II) was exchanged at additional sites that were not available to Cd(II), and thus the zeolite exchange capacity for Pb(II) was much greater than for Cd(II). 0.3

Initial concentration of Pb(II) (m-mol/L) Pb=0.00 "'L, Pb=0.24 "-e, Pb=0.82 ~*-, Pb=l.11 "o,. Pb=l.4 "c],,, P b = l . 9 8

O

E E |

,_v, 0.2

O r 0.1

,_.... o

o.~

~176176176176176

9

0.0 0.0

........ :i?-ii17;-i

0.2

0.4

0.6

0.8

...........

. . . . . . . . . . . . . . . . .

1.0

1.2

1.4

1.6

1.8

2.0

Cd(II) concentration at equilibrium, m - m o l / L

Figure 2. Effect of initial concentration of Pb(II) on exchange isotherm of Cd(II). It is well documented that the exchange isotherm of a cation onto zeolite can be affected by the presence of other cations since all can compete for the same cationic sites. Two or more cations exchanging simultaneously onto zeolite present a much more complex exchange isotherm than that of a single cation. The exchange isotherm for two ions can be expressed by an isotherm in three dimensions by graphing the mass exchanged for either of the two cations against the concentration of both cations to obtain an exchange surface. This method for representing the isotherm of two components has been used to describe the adsorption of metal cations onto activated carbon (9) and onto bioadsorbents or biomass (10).

1853 0.7 Ca0

--~ o

0.6

9

0. i

............

0.4

~-a~u

w" .....

9

.......

9

~ ~ . t-.r....

. .........................

......... ::::::::::::::::::::::::::::

....

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0.3

~~0.2 ~ ,~ 0.1 ~

~ of Cd(II)(m-mol/L) ]'"J~Cd,~.0 "*,., Cd =0.089 "'m Cd=0.667 Cd=1.245 xo. Cd=1.824

0.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pb(II) concentration at equilibrium, m-mol/L Figure 3. Effect of initial concentration of Cd(II) on exchange isotherm of Pb(II). The exchange isotherm for two cations onto zeolite can be represented mathematically by the Langmuir bicomponent isotherm expressed below: qi =

qm,iKiCi 1 + KiC i + KjCj

(3)

"

The isotherm constant values for this model are generally considered to be the same as the Langmuir exchange isotherm constants for each cation alone (Table 1). Experimental data for competitive or simultaneous ion exchange of Pb(II) and Cd(II) were interpreted according to this isotherm. The experimental molar uptake values of Cd(II) and Pb(II) were compared to the molar uptake values of Cd(II) and Pb(II) predicted with the bicomponent Langmuir isotherm, as plotted in Figure 4. As shown in this figure, the bicomponent Langmuir isotherm underestimated the molar uptake of Pb(II) while overestimated the molar uptake of Cd(II). Percent deviations obtained using the bicomponent Langmuir isotherm varied from-117.4% to 92.3% for Pb(II) and from-159.6% to 67.9% for Cd(II), and the average absolute percent deviations estimated with equation (2) are 23.2% for Pb(II) and 44.8% for Cd(II). Thus, the bicomponent Langmuir isotherm did not interpret the experimental data for competitive exchange of Cd(II). The bicomponent Langmuir isotherm provides a reasonable fit to bicomponent exchange data when qm,i is very close to qmj, however, in this case qm,Pb is 2.3 times greater than qm,cd. This could explain why this isotherm does not adequately represent the experimental data. Garke et. al. (11) proposed that when qm,i/qm,j is greater than 1, the bicomponent Langmuir isotherm can be modified as follows: qCd =

qPb =

KcdCcd [(1 + KpbCpb)qm,cd - KpbCpbqm,Pb8 ] 1 + KcdCcd + KpbCpb + (1- 8)KcdCcdKpbCpb KmbCeb[0 + KcdCcd)qm,vb -- KcdCcdqm,cd ] 1 + KcdCcd + KpbCpb + ( 1 - 8 ) KcdCcdKpbCpb

,

(4)

,

(5)

1854 where: 8=qm,cd/qm,Pb.The parameter 8 corrects the Langmuir isotherm since qm,cd/qm,Pbis less than 1. Data for the experimental molar uptake of Cd(II) and Pb(II) were compared to molar uptake of Cd(II) and Pb(II) predicted with the modified bicomponent Langmuir isotherm. This isotherm overestimated both molar uptake of Pb(II) and molar uptake of Cd(II). The percent deviations obtained with the modified bicomponent Langmuir isotherm varied f r o m 117.4% to 75.7% for Pb(II) and from-94.4% to 67.9% for Cd(II), and the average absolute percent deviations estimated with equation (2) were 21.8% for Pb(II) and 40.3% for Cd(II). Thus, the modified bicomponent Langmuir isotherm did not adequately fit the experimental data for competitive exchange of Cd(II). The bicomponent Langmuir isotherm can be modified applying a factor to account for interaction between ions i and j and represented as rli,j. This model has been employed successfully by Ho and Mckay (12) and Leyva-Ramos et. al. (9) to represent bicomponent adsorption isotherms of Co(II) and Ni(II) onto peat and Cd(II) and Zn(II) onto activated carbon. This modified bicomponent Langmuir isotherm is expressed mathematically below. q m,CdKCd (C Cd //7Cd,Cd )

m

qcd -- 1 + Kcd(eCd/r]Cd,Cd)

+ Kpb(Cpb/r]pb,Cd)

(6)

'

q m,pbK pb (C Cd/r]pb,Pb )

(7)

q~b -- 1 + Kcd (Cc~/'TCd,~b) + K~b(C~b/'7~b,~U)

is the interaction factor of cation i on the exchange of cation j. As in the other bicomponent Langmuir models, the values of the constants are the same as the Langmuir isotherm constants for each cation alone (see Table 1). The best values for the interaction factors were obtained by fitting the experimental data according to equations (6) and (7) using a method of least squares based on an optimization algorithm. The best values of these factors are: rlCd,Cd= 4.38, qPb,Cd= 3.98, rlpb,Pb= 1.12, qCd,Pb= 3.10. w h e r e rli,j

e~

.

o 0.5

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

@

W

"

E

"•0.5

,, 0.4

0.4

~., 0.3

"~~ 0.3 =

O

"6 0.2

o~

E

.~

~176

.t

.

0.1

0.2

0.3

.

.

.

.

.

.

.

.

.

.

.

.

.

-6 0.2 E

0.1 o 0.0 0.0

.

I-~.

Cd I

0.4

0.5

Experimental molar uptake, m-mol/g Figure 4. Comparison between the experimental molar uptake and the molar uptake predicted with the bicomponent Langmuir isotherm.

o 0.1 ._ ,-o 0 ~- 0.0 r 0.0

o ~

o~ ooi~ 0.1

0

~o ........ 0.2

0.3

I'w,,.

Pbl

Cd I Ii..o." .'<.'.Cdj

0.4

0.5

Experimental molar uptake, m-mol/g Figure 5. Comparison between the experimental molar uptake and the molar uptake predicted with the bicomponent Langmuir isotherm modified with the interaction factor.

1855 Figure 5 shows experimental molar uptake of Cd(II) and Pb(II) compared to those predicted by the bicomponent Langmuir isotherm modified with the interaction factor. This isotherm interpreted the experimental data reasonably well, both for molar uptake of Pb(II) and Cd(II). The percent deviations obtained varied from-94.8% to 85.2% for Pb(II) and from-64.5% to 92.6% for Cd(II) with average absolute deviations of 21.5% for Pb(II) and 17.8% for Cd(II). Accordingly, the bicomponent Langmuir isotherm modified with the interaction factor best interpreted the experimental data for competitive exchange for both ions. Experimental data of competitive exchange isotherms for Cd(II) and Pb(II) is shown in figures 6 and 7, respectively. The bicomponent Langmuir isotherms modified with the interaction factors are also graphed in these figures. 4. CONCLUSIONS Experimental data for the individual exchange isotherms of Cd(II) and Pb(II) fit reasonably well to the Langmuir isotherm, and the exchange capacity of zeolite for the Pb(II) ion is approximately 2.3 times greater than for the Cd(II) ion. The Pb(II) ion is more selectively exchanged than the Cd(II) ion. The exchange capacity for Cd(II) decreases considerably as the concentration of Pb(II) ion is increased. Therefore; both ions compete for the cationic sites of the zeolite; but zeolite is much more selective for the Pb(II) ion than for the Cd(II) ion. The Langmuir isotherm modified with the interaction factor is the isotherm model that best fits the experimental data for competitive exchange of Pb(II) and Cd(II) ions.

13,6

~r 0.:2

Figure 6. Exchange isotherm for Pb(II) onto zeolite in the presence of Cd(II). 25~ and pH 4.

1856 0.1~

0,1~ 0.10

~,

0,o6 ~.~e~

.

Figure 7. Exchange isotherm for Cd(II) onto zeolite in the presence of Pb(II). 25~ and pH 4.

ACKNOWLEDGEMENTS

This study was funded by CONACYT through grant No. 31338-U. REFERENCES 1. S. Kesraoui-Ouki, C. R. Cheeseman and R. Perry, J. Chem. Tech. Biotechnol., 59 (1994) 121. 2. G. Kirov, L. Filizova and O. Petrov (eds.), Natural Zeolites- Sofia '95, Pensoft Publishers, Sofia, Bulgaria, 1997. 3. M.J. Semmens and W. P. Martin, Water Res., 22(5) (1988) 537. 4. M. J. Zamzow, B. R. Eichbaum, K. R. Sandgren and D. E. Shanks, Sep. Sci. Technol., 25(13-15) (1990) 1555. 5. L. Curkovic, S. Cerjan-Stefanovic and T. Filipan, Water Res., 31(6) (1997) 1379. 6. M.V. Hemandez Sanchez, Master Thesis, Universidad Aut6noma de San Luis Potosi, Mexico (1999). 7. S. Kesraoui-Ouki, C. R. Cheeseman and R. Perry, Environ. Sci. Tech., 27 (1993) 1108. 8. M.W. Ackley and R.T. Yang, Ind. Eng. Chem. Res., 30 (1991) 2523. 9. R. Leyva Ramos, L.A. Bernal Jacome, L. Fuentes Rubio and R.M. Guerrero Coronado, Sep. Sci. Technol., 36(16) (2001) 3673. 10. K.H. Chong and B. Volesky, Biotechnol. Bioeng., 47 (1995) 451. 11. G. Garke, R. Hartmann, N. Papamichael, W.D. Deckwer and F.B. Anspach, Sep. Sci. Technol., 34(13) (1999) 2521. 12. Y.S. Ho and G. McKay, Adsorption, 5 (1999) 409.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1857

C o m p e t i t i v e Ion E x c h a n g e o f Transition M e t a l s in L o w Silica Zeolites C. Weidenthaler, Y. Mao, and W. Schmidt Max-Planck-Institut mr Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mtilheim, Germany

Ternary ion exchange experiments were performed on the zeolites Na-A, N a ~ - X , and NaY using Mn 2+/Ni 2+, Mn 2+/Co 2+, and Ni 2+/Co 2+ mixtures as exchange solutions. The different zeolites show distinct preferences for specific transition metal cations, e.g. zeolite A and X prefer Mn 2+ o v e r C o 2+ and/or Ni 2+. The reason for that are the different hydration energies of the cations which have to be applied for the stripping of water molecules from the hydration shell of the cations and changes of the coordination of the cations by the different zeolite frameworks. After repeated ion exchanges, a significant over exchange is observed for all systems, which is especially pronounced at elevated temperatures. That effect is more distinctive for the low silica zeolites A and X but also observed for zeolite Y. The cation exchange conditions have a significant influence on the thermal stability of the zeolites A and X. However, due to the slightly higher pH values of the Mn 2+ containing ternary ion exchange solutions, the stability of these zeolites are significantly higher compared to those exchanged in ion exchange system containing only Co 2+ and/or Ni ~+. 1. INTRODUCTION Ion exchanged zeolites are used as catalysts in various reactions as for hydrocarbon cracking [1,2,3] or the oxidation of alkenes with transition metal exchanged zeolites [4]. Exchanged zeolites can also be used as precursors for the synthesis of nano-sized mixed transition metal oxides [5]. In that case, the high dispersion of the transition metals on an atomic scale in the aluminosilicate matrix is advantageous for the crystallization of the dense oxide due to a homogenous distribution of the crystallization seeds and a reduction of the crystallization temperatures. To adjust a distinct chemical composition for the synthesis of specific new materials, it is necessary to have a detailed knowledge of the ion exchange behavior of the precursor materials. As there are many references in literature concerning the exchange of one type of cation into a zeolite, there are only a few studies on zeolites exchanged simultaneously with more than one type of cation [6,7,8]. In this work, we present results on the simultaneous incorporation of two of the transition metals Mn 2+, Co 2+, andNi 2+ into the Na + forms of the zeolites A, X, and Y. 2. E X P E R I M E N T A L

The starting zeolites Na-A (Na12Al12Si12048), N a , K - X (Na78K18A196Si960384), and Na-Y (Na59A159Si1330384) w e r e provided by A1Si-Penta GmbH. The chemical compositions of the zeolites were checked by X-ray fluorescence analyses. For the ion exchange, equal amounts

1858 of 0.05 M CoNO3 96H)O, NiSO4 96H20, and Mn(NO3)2 94H20 salt solutions were mixed resulting in a 0.05 M Me 2+ solution, in each case containing two of the three listed salts. The pH values of the salt solutions were 4.9 for MnNi or MnCo, and 5.0 for CoNi. The zeolites were stirred in the salt solutions for a given time, filtered, washed, and dried. In some cases the temperature of the exchange solution was additionally increased to 70~ to enhance the amount of exchanged cations. The exchange procedures were repeated several times, whereby the number of exchange cycles and the exchange times were varied systematically. The cation contents of all samples were determined by EDX analyses on an Oxford EDX unit attached to a Hitachi S-3500N scanning electron microscope. The crystallinity of the samples and the structural changes were studied by X-ray powder diffraction experiments on a Stoe STADI P 0-0 diffractometer to which a HDK 60/S 1 unit from Johanna Otto GmbH was attached for high temperature experiments.

3. RESULTS 3.1. Mn-Ni exchange The different zeolites have a specific preference for different cations, as expected from the binary exchanges. Some of the examined systems also show a significant influence of the temperature of the exchange solution on the mobility of the exchanged cations and therefore on the degree of exchange. At room temperature almost no Ni 2+ cations are exchanged into zeolite A, whereas about 20 % of the Na + can be exchanged by Ni 2+ at 70~ (Fig. l b). The low affinity of zeolite A for Ni 2+ at room temperature was also reported by Gal et al. [9]. 0.8 ....................................................................................................... Texch" = 2 5 ~ 0.7 Mn/AI

a)

0.6 <

0.5

Mn/AI

0.4

I Mn/AI Ni/AI

0.3 0.2

Ni/AI

0.1 0

-

jlllllrl

Mn,Ni-A

I[1111111 Mn,Ni-X

0.8

b)

Texch" = 7 0 ~

0.7 0.6 i

<

Mn,Ni-Y

Mn/AI Mn/AI

0.5

Ni/AI

0.4 0.3

Mn/AI I

Ni/AI

0.2 o.1 o

Na/AI Mn,Ni-A

Na/AI Mn,Ni-X

Mn,Ni-Y

Figure 1. Comparison of the Mn2+/A13+, Ni2+/A13+, and Na+/A13+ ratios of the zeolites A, X, and Y after five ion exchanges at a) 25~ and b) 70 ~

1859 The exchange Ni 2+ into Na-A is significantly enhanced at elevated temperatures due to water stripping from the hydration shell of the metal cation. Higher temperatures result also in higher amounts of transition metal cations in zeolite X, but neither Mn 2+ nor Ni 2+ seem to be strongly favored (Fig. l b). The increase of cation uptake with increasing temperature for cations like Co 2+, Ni 2+, and Zn 2+ was already described by Maes and Cremers [10]. At elevated temperatures the cations can diffuse through the six-membered rings and occupy sites in the [3-cages. For zeolite Y, Ho~evar and Dr~aj [ 11 ] found that Ni 2+ looses its hydration shell at 453 K almost completely, which enables the cations to migrate into the smaller cages. The results shown in Figs. l a and l b, obtained by the simultaneous exchange of Mn and Ni into zeolite Y, reveal nearly equal amounts of Mn and Ni exchanged into zeolite Y. The increase of temperature has almost no influence on the degree of exchange (Fig.lb). In contrast to zeolite A and X, an almost stoichiometric exchange is observed in zeolite Y (no strong over exchange). However, Na + cannot be removed completely.

3.2. Mn-Co exchange Zeolite A prefers the incorporation of Mn 2§ instead of Co 2§ The amount of Co 2§ can be slightly enhanced by increasing the temperature (Figs. 2a and 2b). Na § is completely removed at elevated temperatures. In total, the amount of Co 2+ exchanged into zeolite A is higher in comparison to Ni 2+. 0.8 ........................................................................... a~/

Texch = 2 5 " C

o.r 0.6 0.5

Mn/AI

"~ 0.4 0.3

{ l

Mn/AI I

N I

MnlAI Co/AI"

tllllllll Na/AI

0 Mn,Co-A

b)

0.8

Mn,Co-X

......................................................................................

0.7

Mn,Co-Y

Texeh"- 70~

0.6 0.5

Mn/AI

Mn/AI

ColAI

~' 0.4 G} 0.3

IIIIIrlIIE

0.2 0.1 0

Mn,Co-A

Mn,Co-X

Mn,Co-Y

Figure 2. Comparison of the Mn2+/A13+, Co2+/A13+, and Na+/A13+ ratios of the zeolites A, X, and Y after five ion exchanges at a) 25~ and b) 70 ~ For zeolite X an increase of the temperature favors the exchange of Co 2+ (Figs. 2a and 2b) as already observed for Ni 2+. The Na + cations cannot be removed completely since the cation sites in the smaller cages (e.g. site I) are not fully accessible. The sum of positive charge is

1860 more than needed for balancing the framework indicating an over exchange at both exchange temperatures. Bae and Serf [12] localized O H anions coordinating Co 2+ as well as H30 + cations in the structure of Co 2+ exchanged zeolite X. Thus, the extra charges may be compensated by O H anions or oxidic species in the zeolite cages. Zeolite Y shows more or less equal selectivities for Mn 2+ and Co 2+. An increase of the temperature increases the amount of exchanged cations (Fig. 2b). For the zeolites X and Y the Na + cations cannot be removed completely.

3.3. Ni-Co exchange Fig. 3 shows the results of the chemical analyses after the ternary ion exchange performed at 70~ From the binary exchange systems it is known that Ni 2+ and Co 2+ exchange only very little against Na + at room temperature. Therefore, experiments at 25~ were omitted for that system. 0.8

0.7

..............................................................................

Co/AI

Texch. = 70~

0.6 0.5

I

Co/AI ~

0.4 0.3

Co/AINi/AI

I

0.2 0.1 0 Ni,Co-A

Ni,Co-X

Ni,Co-Y

Figure 3. Comparison of the Co2+/A1a+, Ni2+/A13+, and Na+/A13+ ratios of the zeolites A, X, and Y after five ion exchanges at 70 ~ In the ternary exchange system Ni-Co-Na the zeolite A prefers the exchange of CO 2+ o v e r Ni 2+ and Na + (Fig. 3), whereas zeolite Y exchanges nearly equal amounts of Co 2+ and Ni 2+ (Fig. 3). The exchange always results in an strong over exchange for zeolite A and an almost stoichiometric exchange in zeolite Y. For all zeolites, Na + cannot be exchanged completely.

3.4. Thermal stability of the exchanged zeolites Low silica zeolites, like zeolites A and X, exchanged by Ni 2+ cations are known to be very unstable [9,13,14]. In comparison to the single systems, the thermal stability of the zeolites is increased significantly when Mn 2+ and Ni 2+ are incorporated simultaneously into these zeolites as shown by the Figs. 4 and 5. The in situ powder patterns of Ni 2+ exchanged zeolite X in Fig. 4 show that the crystal structure of Ni-X collapses after dehydration of the zeolite. When Mn 2+ is exchanged additionally to Ni 2+, the Mn,Ni-X zeolite is stable up to temperatures of about 900~ (Fig. 5). Interestingly, the Mn,Ni-X exchanged at 70~ (Fig. 5) is more stable than the Mn,Ni-X exchanged at 25~ (Fig. 6), which collapses at temperatures between 800 and 900~ For zeolite A exchanged simultaneously with Mn 2+ and Ni 2+, the same thermal stability is found. Mn,Ni-A exchanged at 70~ is stable to temperatures of at least 900~ while the one exchanged at 25~ collapses at temperatures between 800 and 900~ The thermal stability of Ni-A is much lower, its structure decomposes readily at a quite low temperature of about 600~ [14]. For the stability of Mn,Ni-Y, the ion exchange temperature is of less importance. As shown in Figs. 7 and 8, the structures of both zeolite Y samples are stable to at least 900~

1861

~ ~~ ;

1;

1~

2'1

2;

2Theta [o]

2;

3;

T [~

loo

=oo

/- 500

3~ 6oo

Figure 4. XRD patterns of Ni-X exchanged 5 times for 30 min at 70~ temperatures between 25 and 600~

1

recorded at

iocl00 900

9

13

17

21

25

29

33

37 1000

2Theta [o]

Figure 5. XRD patterns of Mn,Ni-X exchanged 5 times for 30 min at 70~ temperatures between 500 and 1000~

recorded at

T[oc ] 500 0

900 9

13

17

21

25

29

33

37 1000

2Theta [o]

Figure 6. XRD patterns of Mn,Ni-X exchanged 5 times for 30 min at 25~ temperatures between 500 and 1000~

recorded at

A very similar thermal stability as for the Mn,Ni-exchanged zeolites is observed for the Mn,Co-exchanged zeolites. The structures of Mn,Co-A and Mn,Co-X exchanged at 25~ collapse somewhat above 800~ while the structures of the respective zeolites exchanged at 70~ are stable to at least 900~

1862

j~.,~.4 T [~ 500 ,.~/" 600

j

700 1~3

1~7

2'1 25 2Theta [o]

2'9

3~3

3'7 1000

Figure 7. XRD patterns of Mn,Ni-Y exchanged 5 times for 30 min at 70~ temperatures between 500 and 1000~

/ ~lJIkJ~kJ 1~ ~ L J ~ ~ / - 7 o o / LULJk__JL_ ~ L L ~ L ~ ~ L ~ ~ / 8 o o 9

13

17

21

25

29

33

recorded at

T[~ 500 o

9OO 37 1000

2Theta [~ Figure 8. XRD patterns of Mn,Ni-Y exchanged 5 times for 30 min at 25~ temperatures between 500 and 1000~

recorded at

The number of structural defects might be enhanced by prolonged calcination of these zeolites at 900~ which is also probable for the Mn,Ni-exchanged zeolites A and X. However, the structures of these zeolites are preserved at 900~ at least for the time needed for a XRD run (about 30 min). Similar as in the case of the Mn,Ni-exchanged zeolite Y, the structure stability of Mn,Co-Y is less affected by the exchange temperature; Mn,Co-Y exchanged at 25 and at 70~ is stable to at least 900~ At higher temperatures and/or at prolonged heating at 900~ (Mn,Ni)A1204 or (Mn,Co)A1204 spinels crystallize from the amorphous solid independent of the starting zeolite. 4. DISCUSSION In the ternary systems Mn-Ni-Na and Mn-Co-Na, the exchange of Mn 2+ instead of Ni 2+ and Co 2+ into zeolite A is preferred. As expected from results of binary exchange experiments, a clear preference of the cations in the following order can be observed Mn 2+ > Co 2+ > Ni2+>Na + for zeolite A. The amounts of Mn 2+ in zeolite A is significantly decreased at 70~ compared to that at 25~ while the amounts of Ni 2+ or Co 2+ are increased. Mn 2+ and Co 2+ are exchanged more easily into zeolite A than Ni 2+ because of the lower energies needed for the dehydration of the cations, which is mandatory for the incorporation of the cations into the

1863 narrow pores of zeolite A. For zeolite X, the amounts of Mn 2+ increase slightly when the exchange is performed at 70~ compared to that at 25~ The preference of zeolite X for the second cation is different to zeolite A, it prefers the incorporation of Ni 2+ over Co 2+. Zeolite Y seems to exchange Ni 2+ or Co 2+ to the same extent as Mn 2+ at 25~ as well as at 70~ In the ternary exchange system Co-Ni-Na at 70~ Co 2+ is significantly favored by zeolite A and slightly preferred by zeolite X. Zeolite Y exchanges both Co 2+ and Ni 2+ to the same amount against Na +. In general, Mn 2+ is exchanged easily into all three zeolites, whereby zeolite A prefers the co-exchange of Co 2+ and zeolite X that of Ni2+; for zeolite Y no such clear preference is observed. The different preferences of the zeolites may result from preferred co-ordinations of the cations in the zeolite cages and by size exclusion at lower temperatures due to large hydration shells, which are stripped at slightly elevated temperatures. The hydration shells of the transition metal cations prevent the incorporation of the cations into the zeolite channels, especially of narrow pore zeolites like zeolite A. Removal of the water molecules from that shell enables the incorporation into the zeolites. The hydration shell of Mn 2+ is less strongly bound to the cation compared to those of Co 2+ and Ni 2+ and Mn 2+ is thus easier exchanged into the zeolites at lower temperatures. Apparently, a significant over exchange is observed for all three zeolites, which is less pronounced for zeolite Y. Since one negative framework charge only balances one positive charge from a cation, either additional anionic species must be present in the zeolites and/or the additional cations are located in amorphous salt crusts on the surface of the zeolite crystals. The fact that O H species may be found in heavy metal exchanged zeolites has already been reported [12,15,16,17]. Nevertheless, salt incrustations on the surface of the zeolites cannot be excluded. Surface analyses on the exchanged zeolites by XPS will be performed soon, which probably will help to elucidate this question. At 70~ Na + is usually completely exchanged by Mn 2+ and Ni 2+ or Co 2+ in the zeolites A and X while for zeolite Y a rather large amount of residual Na + is found at 25 and 70~ An easy explanation would be that Na + cations on sites S 1 and S 1' are not exchanged because the transition metal cations cannot migrate through the six T-atom rings of the ]3-cages to access these sites. Assuming a statistical site occupation of A1 atoms in the framework of the zeolite Y structure, 9.8 Na + cations should occupy sites S 1 and S 1', thus, resulting in a Na+/A13+ ratio of 0.17 from non-exchangeable Na + on those sites. Within the experimental error, the Na+/A13+ ratio found in all exchanged zeolite Y samples more or less matches that value. However, Maes and Cremers found transition metal cations in zeolite Y at moderate temperatures even at low transition metal loading [18]. A reasonable explanation would be that the exchange times of 30 min are too short to exchange the cations on sites S 1, and S 1' in zeolite Y. The large amount of residual Na + in zeolite Y is thus caused by a very slow exchange on these sites. Zeolite X with a higher aluminum content has slightly larger ring openings and the cations on sites S1 and S I' are easier accessible, thus resulting in a faster exchange. An alternative explanation would be that Na + is retained in a salt incrustation on the surface of the zeolite crystals. However, that crust would have to be rather thick to contain such a large amount of Na + cations. Since no such crust was observed on the crystallites by electron microscopy, we assume that the large amount of residual Na + in zeolite Y is due to the slow exchange kinetics on the sites within the [3-cages (SI') and in the double six-rings (S 1). For the same reason, we suggest that most transition metal cations in the zeolites are coordinated additionally by hydroxide ions as mentioned above, which would explain the significant over exchange by transition metal cations.

1864 Similar results as for the Mn-Ni-Na and Mn-Co-Na systems were obtained for the Co-NiNa system. Strong over exchange of the transition metal cations and large amounts of residual Na + cations. Here, the same arguments as discussed above can explain those results. The crystal structure and the stability of the exchanged zeolites are strongly influenced by the experimental conditions during the exchange and the type of exchanged cations. Low pHvalues of the exchange solutions may lead to a partial destruction of the zeolite structures resulting in a reduced thermal stability of the exchanged zeolites. The low stability of the NiX zeolite is probably due to the low pH of the Ni 2+ salt solution. The Mn 2+ and the Mn2+/Ni2+ solutions have a significant higher pH and, therefore, cause less damages to the zeolite framework. However, the higher thermal stability of the Mn,Ni and Mn,Co containing zeolites A and X obtained at 70~ compared to those obtained at 25~ cannot be explained by the different pH values. The autoprotolysis due to the transition metal cations should be enhanced at elevated temperature, resulting in a decrease of the pH. The fact that the pH of the exchange solution is not the only factor affecting the thermal stability of zeolites exchanged by two transition metal cations is demonstrated by Sarbak [19]. He showed that a successive ion exchange of zeolite X by Co 2+ and Mn 2+ leads to a material with a significant higher thermal stability than that of the pure Co-X. A damage due to the low pH of the Co 2+ solution should also decrease the thermal stability of the Mn,Co-X. The thermal stability of mixed transition metal cation containing zeolites will be discussed in detail in a forthcoming paper. REFERENCES 1. J.G. Vassilakis and D.F. Best, Novel Zeolite Compositions Derived from Zeolite Y, U.S. Patent 5 013 699 (1991). 2. L.L. Upson, P.J. Van de Gender, W. Van Dijk, Metal-tolerant FCC Catalyst and Process, U.S. Patent 5 173 174 (1992). 3. M.R. Klotz, FCC Catalyst and Process, U.S. Patent 5 294 332 (1994). 4. F. Farzaneh, S. Sadeghi, L. Turkian, and M. Ghandi, J. Mol. Catal. A, 132 (1998) 255261. 5. W. Schmidt and C. Weidenthaler, Chem. Mat., 13 (2001) 607-612. 6. S. Ahmed, S. Chughtai, and M.A. Keane, Sep. Purific. Technol., 13 (1988) 57-64. 7. M.A. Keane, Microporous Mater., 5 (1995) 359-368. 8. J.S. Kim and M.A. Keane, J. Colloid. Interf. Sci., 232 (2000) 126-132. 9. I.J.Gal, O. Jankovic, S. Malcic, P. Radovanov, and M.Tudorovic, Trans. Faraday Soc., 67 (1971) 999-1008. 10. A. Maes and A. Cremers, J. Chem. Soc. Faraday Trans., 71 (1975) 265-277. 11. S. Ho6evar and B. Dr~aj, Proc. 5th Int. Conf. on Zeolites, L.V.C. Rees (ed.), Heyden, London (1980) 301-310. 12. D. Bae and K. Seff, Micropor. Mesopor. Mater., 33 (1999) 265-280. 13. I.J. Gal, P. Radovanov, J. Chem. Soc. Faraday Trans. I, 71 (1975) 1671-1677. 14. C. Weidenthaler, W. Schmidt, Chem. Mater., 12 (2000) 3811-3820. 15. W.C. Patalinghug, K. Seff, J. Phys. Chem., 94 (1990) 7662-7665. 16. D. Bae and K. Seff, Micropor. Mesopor. Mater., 40 (2000) 219-232. 17. C. Nardin, L. Randaccio, E. Zangrando, Zeolites, 15 (1995) 684-688. 18. A. Maes and A. Cremers, Adv. Chem. Ser., 121 (1973) 230-239. 19. Z. Sarbak, Cryst. Res. Technol., 28 (1993) 979-987.

STRUCTURE ANALYSIS AND M O D E L L I N G

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Computational

Methods

for

1867

the

Design

of

Zeolitic

Materials

M. Elanany, K. Sasata, T. Yokosuka, S. Takami, M. Kubo, A. Miyamoto* Department of Materials Chemistry, Graduate School of Engineering, Tohoku University Aobayama 07, Sendai 980-8579, Japan Recently we developed an accelerated quantum chemical molecular dynamics program "colors", which is approximately 5000 times faster than the first principle quantum chemical molecular dynamics calculation. In the present study we have applied our Colors program to the investigation of the structure and dynamics of big periodic models for crystobalite (96 atoms) and H-mordenite (145 atoms). Our results show clearly the proton migration in-between the two oxygen atoms named O16 and O18 in H-mordenite model which are more energetically preferable than other two oxygen atoms named O41 and O19 surrounding the A1 atom. The change of the proton charge during the simulation has been observed. 1. INTRODUCTION Zeolites are microporous crystalline aluminosilicates [1 ], which have been subjected to extensive studies in the past decades. Zeolite materials are of prime industrial import, in several applications heterogeneous catalysis [2,3], ion exchange [4] and membranes. While the structures of these materials in general are well known, many works still have been done in order to understand their reactivity at the atomic level in sorption, cracking and polymerization reactions. Considerable attention has therefore been paid to developing and applying the dynamics simulation as a complement tool to the experimental measurements. Modeling and simulation have obvious benefits in this area as the value of simulation arises generally from its ability to deliver, in a reliable manner, information about the structure, properties or performance of a system in a more rapid, detailed and cost-effective way than experimental work [5]. Classical molecular dynamics (CMD) has been used for solving various physical problems [6]. CMD simulation is most useful for studying materials properties at non-zero temperatures, in cases where quantum mechanical effects in the ionic degrees of freedom are small. The central problem in CMD is usually to find a classical inter-atomic potential, which produces many observable physical quantities, by exploring the quantum mechanics of the electrons. There are several accurate quantum mechanically based methods for solving total energies of low symmetry systems such as Density Functional Theory DFT, which gives reliable results for many systems with the only input being the positions and atomic numbers of the atoms. However, this computational method doesn't allow for inclusion of real dynamics. There is a remarkable exception, the method developed by Car and Parrinello (CP) [7]. CP use DFT combined with MD and update electronic and ionic degree of freedom in unison. However the CP method is computationally demanding and feasible for relatively small systems, up to few tens of atoms, involve the simplest zeolites such as Chabazite (36-46 atoms/cell) [8], Sodalite (36 atoms/cell) [9]. Extension to medium size unite cells include Offretite (54 atoms/cell) [10] and Gmelinite [11,12] have been studied recently. An altemative is to use fight-binding (TB) quantum chemical molecular dynamics method, which was proposed by Laasonen and

1868 Niemminen [ 13]. The TB quantum chemical molecular dynamics method is much faster than the CP method because it is based on the extended Hfickel method for quantum chemical calculations. The extended Hiackel method was first proposed by Wolfsberg and Helmholz [ 14] and widely applied to various systems by Hoffman [ 15]. Various improvements have been made in the extended Hfickel method such as the self-consistent-charge and configuration [16], the self-consistent-charge [17], a distance-dependent Wolfsberg-Helmholz constant or the atom superposition and electron delocalization [18], adding a two-body repulsive energy term [19], and the ab initio parameterization [20-22]. To integrate these improvements in unison with MD we developed our accelerated quantum chemical molecular dynamics program "Colors", which is successfully applied to various systems, plasma oxidation of a silicon surface [23], the dynamics of hydrogen in silicon crystal [24], mechanical polishing process of Si surface [25], and chemical reaction dynamics of dioxins decomposition [26]. 2. COMPUTATIONAL DETAILS 2.1. Theoretical bases of Colors program In colors program the total energy and force expressed by Eqs. 1 and 2, respectively. n

2

E : Z m , v,

occ

ZiZjeZ/l~s+~_,~_~ Erepul(Rij)

/2+~-~6k+~-~-~

i=1

k=l

i>j

(1)

.........

i>j

Where the first, second, third, and fourth terms are the kinetic energy of atoms, the orbital energy of valence electrons, the Coulomb interactions between core charges, and the short-range repulsion between atoms, respectively.

F~=s ~ C: (OH/ Ol~j) f k+s ~ C~ (oS / Ol~s) f k- Z z, zs e2 / t~.2+Z OEr,v,,,(l~s) / O~j j~i k=l

j~i

k=l

j~i

jr

The short-range repulsion term grepul('R~ was represented by Eq. (3). Eret~t(I~j)=(bo)exp[(ao -r~j)/bo] ........ (3) The parameters for colors have been determined on the basis of first principle density functional calculation to realize high accuracy, such as the valence state ionization potential VSIP of each atomic orbital and the Slater exponent of atomic orbital. These parameters were calculated at different charges of the atoms. On the basis of the improvements in Wolfsberg-Helmholz formula by Anderson [18] and Calzaferri et al. [19], we used the following expression for Hrs: 1 H,~ =-~K,~(H,~ +H~,)Sr,

(4)

Krs = l + (tCrs + A2 - A4tCrs)eXpE-(~rs(t~.j - d o ) ~

(5)

zX=(H,~-H~)/(H, +H~)

(6)

Where do is the sum of the orbital radii. As shown in Eqs. (4)-(6) we determined Kr~ namely tr and6 ~, for each pair of the atomic orbitals for further improvement of the accuracy in the chemical bonding calculations. 2.2. Structure

Purely siliceous crystobalite structure model at low temperature was considered in our study. The unite cell volume is 1377.4 /~3, which contains 96 atoms (64 O and 32 Si). Hmordenite model has the experimental unite cell volume 2789.9 A s that contains 145 atoms ( 96

1869 O, 47 Si with one A1 and one H, which compensates the charge deficiency introduced by the Si/A1 substitution at the position 144). The most stable position for O-H acid group is pointing outward from the framework to the 12 memberd ring channel. In mordenite structure the largest aperture is the 12 membered-ring of the dimensions 6.5x7.0/~ extending along c axis. 2.3. Static and Dynamic calculations

A-Static: First principles density functional calculations were done with the Amsterdam Density Functional ADF program [28] and the DMol 3 program [29]. In ADF calculations, the triple zeta plus polarization functions (TZPP) basis sets were used, while the energies were calculated using the generalized gradient approximation GGA with Becke's exchange functional and the Perdew correlation functional. In DMol 3 calculations, double numerical atomic basis sets with polarization functions DNP were used, while the energies were calculated using the GGA with Becke's exchange functional and the Perdew correlation functional. B-Dynamic: All our dynamic simulation steps were performed at temperatures around 300 K. In crystobalite study the total number of dynamic simulation steps is 1000 with output interval for MD (10 steps) and time intervals (0.1 fs). In the case of H-mordenite 800 steps with output interval for MD (10 steps) and time intervals (0.2 fs). 3. RESULTS AND DISCUSSION Figures 1 and 2 show a comparison of the radial electron density distributions of oxygen 2s and 2p orbitals, respectively. 0.025

r ~=

0.025

o STOI

0.02

ADF

=

, 0.02

0.015

o =~

~ A D F

"

~ 0.015

0.01

i

0.005

0.01

[] O.OO5

0~0

o.-

1

2

3

0

1

2

r

3

4

0

1

2

r

r

Fig. 1. The electron density of oxygen 2s orbital at different charges 0 (a), 2 (c).

-1 (b), a n d -

Figure 3 shows the single Zeta Slater-type orbital for oxygen 2s (a) and double Zeta Slater-type orbital for oxygen 2p used in Colors calculations. Although, ADF uses triple Zeta function to describe the atomic orbital, Colors can accurately reproduce the same atomic orbital with single or double Zeta Slater-type orbital by employing charge dependent Slater exponent.

~, "o

0.05

--ABE I o STO

0.04

i

0.03

~"

0.05

0.02

~=

=

0.02

--

0.01

g;

0.01

0

1

2 r

3

4

o

0.05

colors

~,

0.04 ~~= 0.03

!

0-

~ADF

0

--ADF

o STOI =

0.04

e

o.o3

,T,

0.01

o.o2

0 .

0

1

2 r

3

4

0

.

.

. 1

.

. 2 r

3

4

1870 Fig. 2. The electron density of oxygen 2p orbital at different charges 0 (a), -1 (b), and -2 (c). 4

4

3

3

3.5

2.5

====================================

2.5

2

1.5

1.5 1

1

0.5

0

zetal

0.5 ''

I

I

-2

-i

I

0

charg

I

1

e

F i g . 3. O r b i t a l e x p o n e n t s a n d O~ysgoen 2 P ( b ) w i t h

I

2

0

3

zeta2

+

I

-3

I

-2

-i

I

0

charge

I

1

i

2

of oxygen 2S (a) with single zeta Slater-type orbital double zeta Slater-type o r b i t a l u s e d in " C o l o r s "

-60 -70 -80

t -2

-i

0 c h al: q e

i

2

Fig. 4. VSIP o f oxygen 2s and 2p at different charges

Figure 4 shows the valence state ionization potential VSIP for Oxygen 2S and 2P calculated by ADF at different charges.

I-4--~F I

20 .#.

1

~10 ,-----

..-,oI

~o

0

,-

j l- o

-20

3

g "~

o

co

0

-lO -20

-20 !

~stance IA1

.

c

nt,n~e IAI

Distance[AI

Fig. 5. Di-atomic potential energy curves calculated by ADF and Colors A1-O (a), Si-O (b), and O-H (c).

Figure 5 shows the diatomic potential energy curves for all diatoms exist in our models. ADF calculations were completely reproduced by colors to obtain good fitting parameters. First we applied our parameters to small models of Si(OH)4, Si207H6 and Si309H6 molecules shown in Figure 6 in order to confirm the effectiveness of our parameters to describe bigger models of crystobalite and H-mordenite. The results obtained are summarized in Table 1. We can see that the binding energy values calculated by colors are very close to those calculated by ADF. Moreover, the atomic charges calculated by colors are in good agreement with ADF results especially Hirshfeld atomic charges [30, 31 ]. The difference in atomic charges between Mulliken [31-34] and Hirshfeld for the ADF calculations may be due to the large basis sets in the ADF

1871 calculations. It is generally considered that a larger basis set leads to a larger error in the Mulliken atomic charge when the Hirshfeld charge is more realistic. The agreement between the Mulliken charge in Colors and Hirshfeld charge in ADF can be explained by the fact that Colors uses only a minimum basis set for each orbital and the Slater exponent is fitted with a real electron density distribution as shown in Figures 1 and 2. As the conventional extended Hiickel method usually leads to overestimation of charges in polarized molecules, the agreement here of charge calculated by Colors and ADF indicates directly to our improvement of the colors program methodology.

...n

B Q

Q ....

~H

Fig. 6. Si(OH)4, Si207H6 and Si309H6 models calculated by ADF and Colors at0K

Table 1. Results of Colors and ADF for Si(On)4, Si207H6and Si309H6 molecules

Molecule

Binding energy

Mulliken charge

(ev)

Si

O

-49.72

1.01

-0.55

-76.45

1.7

-112.18

1.0

H

Hirshfeld charge Si

O

H

0.29

0.56

-0.28

0.15

-0.86

0.47

0.61

-0.32

0.16

ADF

-0.56

0.27

0.51

-0.27

0.16

ADF

ADF

Si(OH)4

Si207H6

Si309H6

1872

Fig. 7. MD of crystobalite model at 0 step (a) and 1000 step (b) at 300 K. Figure 7 shows the dynamic simulation at 0 step (a) and 1000 step (b). It is clear that our parameters maintain the crystobalite structure over the whole dynamics simulation steps reflecting the validity of these parameters. 0.8 0.6 ,

0.4

g

0.2

0~ -O.2 ~

__

200

400

600

800

1(~0 i

-0.4

Fig. 8. Charge analysis if Si and 0 in the crystobalite model The charge of one Si atom and one O atom during the simulation is shown in Fig. 8. There is a little change in the charge of Si and O all over the simulation steps with average charges for Si and O are (0.57) and (-0.27), respectively. These charges are very close to the Hirshfeld charges of Si (0.554) and O (-0.277) obtained by Dmol 3 single point calculation.

e-,,I,,

o

o

o

o

~,~o

i

. ~'~ii~.~i~,'.i~irl ~,~II~*~I~I~-~IIII~II~I~I~'IIII~~'I'I~'-.....

i

~,,=

............................... ..... ~ ~~-ii-~~ ...............ii~i....

~...................................................................

i I" ii~i~i~i~.~i~iiii~ii~i~t~.~ ~ii~i-_ i.,. i~~i~i~i~ii~i~

~ii~ i-~i~

1874 In Figure 9 (a) for H-mordenite at 0 step we put numbers to distinguish different oxygen atoms around the A1 atom. Figures 9 (b) and (c) are snapshots taken from the dynamics simulation at 310 and 360 steps, respectively. From the snapshots we can see clearly the scenario of migration of the proton in-between O18 and O16 which are energetically more preferable than the other two oxygen atoms named O19 and O41 [12]. The interaction of the proton with the oxygen atoms draws oxygen atoms toward the center of the channel [ 11 ] producing some distortion of the structure. 0.4 0.3 :~

.

.

.

.

11

u.t~

. 11

0.2

O18--H

0.5

144

--

O16~H

144

~.

............. O 1 9 ~ H

144

~-o.1 0

Jr

0.4

............. O 4 1 - - H 144

~

A, ,.

I

~ H 144 I ......---~--~ S i 105

L

0.3

0.2 0.1

' ~ -0.1

0

-0.2

0

Step

200

400

600

800

Stop

Fig.10. Bond population analysis (a), and charge analysis (b) at different simulation steps.

It is interesting to introduce Figures 10 (a), which shows the bond population analysis of the proton with the different four oxygen atoms around A1 atom. We can see how the bond population ofH-O 18 is decreasing while it is increasing for H-O 16 in the region 300 - 400 steps. On the other hand the bond population H-O41 is very week and for H-O19 is close to zero indicating the week interaction of the proton with these atoms. It is interesting also to introduce Figure 10 (b), which describes the change of the proton charge during the simulation steps. This may be due to the change of the proton position relative to the oxygen atoms. We can draw some observations for this opinion. At the first part the proton charge decreases gradually corresponding to the decrease in the H-O18 bond population at the first part of Figure 10 (b) followed by charge increase for the same reason. At around 300 step there is an interaction between the two bond population curves for H-O 18 and H-O 16, as the proton position is far from the two oxygen atoms O18 and O 16 the charge of the proton has a lower value and so on. Table 2. The average charge of O, Si and A1 at 0 and 360 steps O Si 0 step -0.258 0.509 360 step -0.228 0.446

A1 0.534 0.518

Table 2. shows the charge analysis of the whole atoms in the H-mordenite model at two different steps 0 and 360. Generally, we can say that as result of the dynamics of all atoms in the model the interatomic distances are slightly longer decreasing the effective charge. At 360 step the charge of the all oxygen atoms have lower negative values at the same time all silicon atoms have lower positive values.

1875 4. CONCLUSION We applied successfully our new accelerated quantum chemical molecular dynamics program to big cell size models of crystobalite and H-mordenite. We confirm that Colors program is fast, reliable agrees with the first principle calculations, and has a promising applications for studying more complicated systems.

REFERENCES

1. W. M. Meier, D. H. Olson, Ch. Baerlocher, Atlas of zeolite types, fourth edition, Elsiever, London, (1996). 2. Jens Weitkamp, Solid State Ionics., 131 (2000) 175. 3. J. Dweyr, in: P. J. Grobet, W. J. Mortier, E. F. Vansant, G. Schulz-Ekloff (Eds.), Innovation in Zeolite Material Science, Elsevier, Amesterdam, 1988. 4. D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Willy and Sons, London, 1973, reprentid R.E. krieger, Malabar, FL, 1984. 5. J. Andzlem, A. Alvarado-Swaisgood, F. Axe, M. Doyle, G. Fitzgerald, C. Freeman, A. Gorman, J. Hill, C. Kolmel, S. Levine, P. Saxe, K. Stark, L. Subramanian, M. vanDaelen, E. Wimmer, J. Newsam, Catal. Tod. 50 (1999)451. 6. G. Ciccotti, D. Frankel, I. McDonald, Simulation of Liquid and Solids (Amesterdam: North Holland), (1987). 7. R. Car, M. Parrinello, Phys. Rev. Lett. 55 (1985) 2471. 8. Y. Jeanvoine, J. G. Angyan, G. Kresse, J. Hafner, J. Phys. Chem. 102 (1998) 5573. 9. K. T. Thomson and R. M. Wentzcovich, J.Chem.Phys.108 (1998)8584; k. T. Thomson, R. M. Wentzcovich, A. McCormick and H. T. Davis, Chem. Phys. Lett. 283 (1998) 39. 10. L. Campana, A. Selloni, J. Weber, and A. Goursot, J. Phys. Chem. B 101 (1997) 9932. 11. L. Benco, Th. Demuth, J. Hafner, J. Chem. Phys. 111 (1999) 7537. 12. L. Benco, Th. Demuth, J. Hafner, F. Hutschka, J. Chem. Phys. 111 (2000)3 73. 13. K. Laasonen, R. M. Nieminen, J. Phys. 2 (1990) 1509. 14. M. Wolfsberg, L. Helmholz, J. Chem. Phys. 20 (1952) 837. 15. R. Hoffman, J. Chem. Phys. 39 (1963) 1397. 16. C. J. Ballhausen, H. B. Gray ; Molecular orbital theory, W.A. Benjamin, Inc. A. Anderson, (1965). 17. M. Zemer, M. Gouterman, Theoret. Chem. Acta. 4 (1966) 44. 18. A. B. Anderson, J. Chem. Phys. 62 (1975) 1187. 19. G. Calzaferri, L. Forss, I. Kamber, J. Phys. Chem. 93 (1989) 5366. 20. P. Blaudeck, T. Frauenheim, D. Porezag, G. Seifert, E. Fromm, J. Phys. Condens. Matter. C4 (1992) 6389. 21. M. Menon, K. R. Subbaswamy, Phys. Rev. B 47 (1993) 12754. 22. N. Bemstein, E. Kaxiras, Phys. Rev. B. Conden. Matter. 56 (1997) 10488. 23. A. Yamada, A. Endou, H. Takaba, K. Teraishi, S. S. C. Ammal, M. Kubo, K. G. Nakamura, M. Kitajima, A. Miyamoto, Jpn. J. Appl. Phys. 38 (1999) 2434. 24. H. Takaba, A. Endou, A. Yamada, M. Kubo, K. Teraishi, K. Nakamura, K. Ishioka, M. Kitajima, A. Miyamoto, Jpn. J. Appl. Phys. 39 (2000) 2744.

1876 25. T. Yokosuka, H. Kurokowa. S. Takami, M. Kubo, A. Imamura, A. Miyamoto, Jpn. J. Appl. Phys. in press 26. A. Suzuki, T. Kusagaya, S. Takami, M. Kubo, A. Imamura, A. Miyamoto, Chemisophere, under contribution. 27. E. J. Bearends, D. E. Elis, P. Ros, Chem. Phys. 2 (1973) 41. 28 B. Delly, J. Chem. Phys. 92 1 (1990) 508. 29. F. L. Hirshfeld, Theor. Chim. Acta, 44 (1977) 129. 30. F. L. Hirshfeld, H. Hope, Acta Crystallogr. Sect. B 36 (1980) 406. 31 R. S. Mulliken, J. Chem. Phys. 23 (1955) 1833. 32 R. S. Mulliken, J. Chem. Phys. 23 (1955) 1841. 33 R. S. Mulliken, J. Chem. Phys. 23 (1955) 2338. 34. R. S. Mulliken, J. Chem. Phys. 23 (1955) 2343.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1877

A theoretical investigation on pressure-induced changes in the vibrational spectrum of zeolite bikitaite E. Fois a, A. Gamba a, G. Tabacchi a, O. Ferro b, S. Quartieri c, G. Vezzalini b a Department of Chemistry, Physics and Mathematics, University of Insubria Via Lucini 3, 1-22100 COMO (Italy) b Department of Earth Sciences, University of Modena and Reggio Emilia Largo S. Eufemia 19, 1-41100 MODENA (Italy) c Department of Earth Sciences, University of Messina Salita Sperone 31, 1-98166 MESSINA - S.Agata (Italy) In this paper, the vibrational spectra of the natural zeolite bikitaite obtained from ab initio MD simulations are discussed. The calculated spectra, in line with experimental IR and Raman spectra of other zeolitic systems, predict that applied pressure significantly affects the O-H and T-O vibrational frequencies. The observed broadening of the OH stretching band is attributed to pressure induced changes in the host-guest and guest-guest hydrogen bond interactions. 1. I N T R O D U C T I O N

Confinement of materials in ordered matrices is currently of primary interest for applied research. It is known that a low-dimensional system shows chemical and electrooptical properties which are, in general, significantly different from the ones of the corresponding bulk material and can be of relevant technological interest for the tailoring of new kinds of materials [1]. Zeolitic frameworks hosting low-dimensional systems may be taken as useful models to study the factors governing at atomic level the stability and the properties of confined materials [2]. One of these interesting model structures has been found in the natural zeolite bikitaite (Li2 [A12Si4012].2H20)[3,4]. This is a rare lithium zeolite characterized by a high framework density and by a monodimensional system of channels in which water molecules and Li cations are hosted. It is composed by sheets of 6-membered TO4 rings laying in the ab plane and connected to each other by pyroxene-chains of tetrahedra. Non-crossing channels, whose section in the ac plane is an eight membered ring of tetrahedra, run parallel to the b direction (Figure la). Each extraframework Li + cation is tetrahedrally coordinated to three framework oxygens and one water oxygen. Experimental and theoretical investigations on bikitaite at ambient conditions proved that water molecules form one

1878 dimensional chains held together by hydrogen bonds (HB), whereas no water-framework HB exist [5,6]. Such a peculiar one-dimensional water chain, that runs parallel to the channel direction, has been found to date only in another high density lithium zeolite characterized by the same one-dimensional 8-ring channels, the synthetic Li-ABW [7,8].

( b ) ~..~..

~~

~

Figure 1: Ball-and-stick representation of the bikitaite structure projected on the ac plane, at ambient pressure (a) and at 9.0 GPa (b). Black spheres represent H atoms, light grey spheres Si atoms, grey spheres O atoms and dark grey spheres A1 and Li atoms. Theoretical studies on bikitaite [6] allowed to explain the stability of the guest "floating" water chain on the basis of long-range electrostatic host-guest interactions. At difference from bikitaite, water molecules in Li-ABW are hydrogen bonded to each other as well as to framework oxygens, thus indicating as additional stabilization factor the presence of short-range interactions [9]. Both experimental and simulated vibrational spectra account for the different dynamical properties of water in these two related zeolites and have allowed to assess the leading role of weak interactions (such as hydrogen bonds) in influencing its vibrational behaviour [9,10]. In this respect, it would be of interest to study these water chains under different conditions with respect to the ambient ones. In particular, we are interested to investigate how the stretching and bending frequencies of water molecules in bikitaite are affected by high pressure (HP). Little is known about the behaviour of zeolites under hydrostatic pressure. This is partly due to the difficulty in finding non-penetrating pressure transmitting media that behave hydrostatically over a wide range of pressure. Moreover, crystal structure refinements are often prevented by a number of factors that cope to decrease resolution, thus allowing only the experimental determination of cell parameters. This problem is particularly severe for X-ray powder diffraction experiments [11,12]. The usefulness of techniques that can integrate the experimental data with atomicscale information is therefore clearly evident. In this context, the use of a b - i n i t i o methods

1879 may lead to significant progress in this largely unexplored area, providing additional information not accessible through experiment. A combined theoretical-experimental approach has already been successfully applied to the study of zeolites scolecite [11] and bikitaite [12] under HP. As far as bikitaite is concerned, both experiment and calculations have proved that this zeolite is remarkably stable under HP, as no amorphization or pressure induced phase transition has been observed up to 10 GPa [12]. 2. D E T A I L S O F T H E C A L C U L A T I O N S Simulations on bikitaite have been performed using the Car Parrinello ab initio molecular dynamics method (CPMD) [13], which has proved to provide a satisfactory description of many condensed-phase systems, including zeolites. The method allows one to obtain reliable information at microscopic level on both static and dynamical properties. In particular, simulated IR spectra satisfactorily reproduce shifts in the O-H stretching and bending frequencies resulting from changes in the chemical environment [6]. Moreover, they allow to single out the contributions of distinct modes or groups of atoms to the total spectra, leading to an increased resolution and providing additional information not accessible through experiment. We report here only the technical details adopted in the simulation runs. For a more detailed description of this methodology, the reader is referred to Ref. [14]. Two constant volume CPMD [15] simulations of bikitaite were performed using the experimentally determined cell parameters at the pressures of 5.7 GPa and 9.0 GPa, reported in Ref. [12]. A periodically repeated triclinic supercell containing two crystallographic unit cells along the b direction was adopted. We used the same plane-waves cutoff, density functional approximations, pseudopotentials and MD simulation parameters (i.e. integration time step, fictitious mass) adopted in previous theoretical studies on bikitaite at ambient conditions ([5,6]). After equilibration, the time evolution of the system was followed for 5.0 ps for both simulations. Simulated vibrational spectra were calculated by Fourier transforming the velocity autocorrelation function obtained from the MD trajectory, and compared with the ambient pressure ones reported in Ref. [5]. 3. R E S U L T S A N D D I S C U S S I O N In zeolites, strong covalent T-O bonds with a partial ionic character are present, therefore long range electrostatic forces play a dominant role. On the other hand, the presence of short-range forces arising from HB can also be of significant relevance [16]. Hydrogen is incorporated in zeolites mainly in the form of guest water molecules, which are in most cases hydrogen bonded to framework oxygens and extra-framework cations. Information on the strength of these bonds are generally drawn from vibrational frequencies of O-H covalent bonds [17]. As far as HP conditions are concerned, studies on a series of minerals have evidenced a decrease in the OH stretching frequency, which is attributed to an

1880

, , , ,

i~

~

i , , ,

~ i w , , ~ l J , ~ ,

ii

- - -

~ ,

i

i

i~

~ i

~,

~ ~

1 atm

............ 5 . 7 G P a i

9.0 GPa

ii^i ~,:,

l

~.z

i

/

I I , , I , , , I I 0

500

1000

i

1500

2000

co (cm -I)

2500

3000

3500

4000

Figure 2: Simulated vibrational spectra for bikitaite at 5.7 and 9.0 GPa. The vibrational spectra at ambient pressure is also reported. increase of hydrogen bonding character [16,17]. In fact, the presence of an HB results in a broadening of the potential energy curve of the covalent OH bond and a decrease of spacing between vibrational levels. As an effect, the stretching frequency decreases with the strengthening of the O...H interaction. However, this simplified picture is inadequate for three centered hydrogen bonds [16], which are often found in minerals at HP. Volume shrinking due to applied pressure can in fact bring two oxygens close enough to a guest water proton to allow formation of two weak HB's. Presence of bifurcated hydrogen bonds may significantly affect O-H stretching frequencies. Before discussing our simulated bikitaite spectra, let us briefly summarize the results of IR and Raman studies on zeolites under HP. Velde and Besson [18] found that the OH stretching band in analcime was splitted in two components with increasing pressure and attributed such change to a pressure-induced deformation of the framework, resulting in two different HB lengths. Huang [19] reported a red shift of the three O-H stretching peaks in zeolite Y for pressures lower than 1.9 GPa, while for higher pressure the three bands collapsed to a broad, unresolved profile. Also the water bending modes are influenced by pressure, and such changes are attributed to the increased strength of hydrogen bonds. Moreover, the three bands associated with T-O stretching modes are shifted to higher energy with increasing pressure, suggesting a shortening of the T-O bonds. Another study reports a slight pressure-induced blue shift of the T-O stretching modes for the zeolite LTA [20]. Belitsky et al. [21] found for the pressure-induced phase transition

1881

natrolite I - natrolite II significant shifts in the T-O R a m a n bands, that point to a strong deformation of the primary structural units TO4. Also the HP R a m a n spectra of Gillet et al. [22] indicate strong modifications in the O-T-O, T-O-T bands as well as in the O-H stretching bands of zeolites scolecite and mesolite. On the whole, these experimental data on vibrational behaviour of zeolites under HP [19-22] seem to support the idea that tetrahedral units are indeed not rigid. Rather, the T-O bonds slightly contract as a response to external pressure, thus resulting in a decrease of the tetrahedral volume. The results of our calculations on bikitaite [12] are in agreement with such description, predicting a negative change in the SiO4 and A104 volumes of the order of 2% and 3% respectively at a pressure of 9.0 GPa. The simulated bikitaite spectra at 9.0 and 5.7 GPa are compared with the ambient pressure one in Figure 2. It is to point out that the simulated bikitaite spectra at 1 atm were found to be in good qualitative agreement with the micro-IR one reported in Ref. [5]. Underestimation of the absolute values of the stretching frequencies is a well-known artifact due to the use of fictitious masses in CP simulations, and prevents quantitative agreement with the experiment. On the other hand, relative frequencies calculated with respect to a reference state (for instance, an isolated water molecule in the gas phase) have proved to well compare with the corresponding experimental frequency shifts. We first examine the region typical of framework modes, i.e. between 200 and 1200 cm -1, focusing our attention on the three bands at highest frequency (i.e. 980, 880 and 660 cm -1 at ambient pressure) commonly attributed to the T-O stretching modes [19,201. Remarkably, they undergo a significant blue shift at 5.6 GPa, passing to 1000, 920 and 740 cm -~ respectively, while the frequency shift from 5.7 to 9.0 GPa is lower (1020, 920 and 740 cm-~). On the whole, these data indicate that pressure-induced strenghtening of the T-O bonds occur in bikitaite, in line with the findings of recent HP IR and Raman studies on other zeolites [19,201 and with the calculated shortening of T-O distances at HP in bikitaite [12]. In general, the HP-induced volume contraction in silicates is attributed to three mechanisms. The first one involves the rotation of rigid TO4, the second one the distortion of intra-tetrahedral O-T-O angles, while the third and less significant one, implies a decrease of the T-O bond lenght [23]. Since the less energetically costly mechanism is the rotation of rigid structural units [24], current models describing the HP behaviour of silicates normally treat the effects of the third mechanism as negligible. Our calculations, together with the above quoted experimental data [19-22], indicate that shortening of TO bonds in zeolites under HP seems to be more effective than what predicted by such models. At ambient pressure, the OH stretching band is broadened in the region betweeen 2900 and 3400 cm -t. The band profile is structured, in agreement with the experimental one, owing to the presence of four crystallographically different protons experiencing different interactions with their environment. At HP, the band still shows distinct peaks but is significantly broader than at 1 atm. The full width at half maximum, 300 cm -1 at 1 atm, increases to 410 cm -1 at 5.7 GPa and to 560 cm -1 at 9.0 GPa. Moreover, the band is broadened in the low frequency region, indicating that the compression leads to stronger perturbing effects of HB to the H20 vibrational modes. This may be due to different

1882

HaW1

H aW2 v

1 atm

............ HbWl HbW2

3

I

i

2200

I

i

I

a

2400

I

i

I

,

2600

2800

~

3000

/

"--,,

8200

HaW1 HaW2 ............ HbWl

A

,. I.~

3600

i

i

3800

5.7 G P a

I~

HbW2

v

3400

A /

/ 2200

2400

2600

2800

2200

2400

2600

2800 co

,,,',,

/

,,/

[/ '.,

',, I

J

3000

3200

3400

3600

3000 (cm-1)

3200

3400

3600

~

I

3800

3800

Figure 3" Contributions of the four crystallographically different OH bonds to the vibrational spectra at 1 atm, 5.7 GPa and 9.0 GPa. mechanisms, as at HP both a strenghtening and an increase in the number of HB are observed. In bikitaite, the a and c cell parameters decrease under compression and two framework oxygens come close enough to a water hydrogen to form bifurcated hydrogen bonds. In addition, the decrease of the b parameter is accompanied by a shortening of the O-O separation between water molecules and by a consequent strengthening of the inter-water hydrogen bonds already present at ambient pressure. In order to have a better understanding of the role played by these mechanisms, we calculated the distinct contributions of the four hydrogens to the total spectra (Figure

1883

3). W1 and W2 represent the two water molecules in the unit cell. In each molecule, a proton (Ha) is hydrogen bonded to the adjacent molecule in the chain, while the other one (Hb) points towards framework oxygens and at ambient pressure is not involved in HB. At ambient pressure, the four stretching bands are centered at distinct frequencies. OH bonds involving an hydrogen-bonded proton have lower stretching frequencies, in line with the previous discussion. The O-Ha bands are also broader, as both O and Ha are involved in strong HB with two adjacent water molecules. We also notice that the stretching frequencies of the two O-H bonds in the same molecule are rather close to each other in W1, and much more separated in W2. This strikingly different behaviour of the two water molecules could be rationalized by considering the difference between the two O-H bond distances in each molecule, that in W2 is three times larger than in W1 (i.e. 0.018 A. vs. 0.006 A)[12]. At 5.7 GPa, the Hb atoms come at distances from Ofrarn e short enough to form hostguest HB. As an effect, both the O-Hb peaks are red-shifted. The frequency shift is slightly more pronounced for W1, which at 5.7 GPa forms a significantly stronger waterframework HB (the calculated Hb..Ofram e distances are 1.885 and 2.081 A for W1 and W2 respectively) [12]. Moreover, hydrogen bonds between water molecules, already present at 1 atm, are significantly shorter at 5.7 GPa (from 1.886 to 1.714 A) [12]. This leads to a red-shift of the O-Ha stretching frequencies as well. The global result is a broadening of the O-H stretching band with respect to 1 atm (Figure 2). Remarkably, a further increase of the pressure, from 5.7 to 9.0 GPa, brings about a rather unexpected blue-shift of both the O-Hb stretching frequencies. We justify this result by considering that, even though at 9.0 GPa the Hb atoms form a higher number of HB they are indeed weaker than those at 5.7 GPa. In fact, the MD simulation showed that at 9.0 GPa two and sometimes three framework oxygens are in competition to form HB's with Hb. As a consequence, water-framework HB's involving different framework oxygens are continuously broken and formed, resulting on average, in a decrease of the hydrogen bonding strenght [12]. In particular, W1, that at 5.7 GPa was involved in a single and rather strong water-framework HB, at 9.0 GPa forms a three centered bond, characterized by significantly larger Oirame-H b distances (2.163 and 2.079 A), therefore suggesting weaker HB interactions. The inter-water average HB distance is 1.719 A at 9.0 GPa, therefore the average Ha-O stretching frequency undergoes no significant shift with respect to 5.7 GPa. On the whole, the above mentioned effects result in a further broadening of the total stretching band at 9.0 GPa (Figure 2). Applied pressure also affects OH bending frequencies. Figure 2 shows that at 5.7 GPa the bending band is shifted towards higher frequencies with respect to 1 atm, and it is also significantly broader. The opposite trend is observed in passing from 5.7 to 9.0 GPa: the bending peak is red-shifted and the band becomes narrower. Again, these effects find a microscopic explanation in the pressure-induced formation of new HB. As discussed above, the perturbing effect of water-framework HB on the vibrational modes of water molecules is larger at 5.7 GPa, when a lower number of stronger hydrogen bonds are formed. Work is in progress in order to perform experimental micro-IR spectra of bikitaite

1884

under HP. A c k n o w l e d g e m e n t s . This work was supported by Italian MIUR (COFIN2001 "Le zeoliti, materiali di interesse per l'industria e l'ambiente: sintesi, struttura, stabilitg e applicazioni.") and Italian CNR. REFERENCES

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

A.P. Alivisatos, Science, 271 (1996) 933. G.A.Ozin, A.Kuperman and A.Stein, Angew.Chem.Int.Ed.Engl., 28 (1989) 359. K. Sts A Kvick and S. Ghose, Zeolites, 9 (1989) 303. G. Bissert and F. Liebau, N. Jb. Miner. Mh., H6 (1986) 241. S.Quartieri, A.Sani, G.Vezzalini, E.Galli, E. Fois, A.Gamba and G.Tabacchi, Microp. Mesop. Mater. 30 (1999) 77. E.Fois, G.Tabacchi, S.Quartieri,G. Vezzalini, J. Chem. Phys., 111 (1999) 355. E. K. Andersen and G.P. Sorensen, Zeit. Kristallogr. 176 (1986) 67. P.Norby, A.N.Christensen and I.G.K.Andersen, Acta Chem.Scand.A40 (1986) 500. E.Fois, A.Gamba, G.Tabacchi, S.Quartieri, and G.Vezzalini, J.Phys.Chem.B, 105 (2001)3012. E.Fois, A.Gamba, G.Tabacchi, S.Quartieri, G.Vezzalini, Phys. Chem. Chem. Phys. , 3 (2001)4158. P.Ballone, S.Quartieri, A.Sani and G.Vezzalini, Am. Mineral, in press O.Ferro, S.Quartieri, G.Vezzalini, E.Fois, A.Gamba, G.Tabacchi, submitted to Am. Mineral. R. Car and M. Parrinello, Phys. Rev. Lett., 55 (1985) 2471. M. Parrinello, Sol. St. Comm., 102 (1997) 107. J.Hutter, P. Ballone, M. Bernasconi, P. Focher, E. Fois, S. Goedecker, M.Parrinello and M.Tuckerman, CPMD Version 3.0; MPI fiir FestkorperfSrschung and IBM Research, 1990-1996. C.T.Prewitt and J.B.Parise, in: R.M.Hazen and R.T.Downs(eds),High-Temperature and High-Pressure Crystal Chemistry, Reviews in Mineralogy and Geochemistry, 41 (2000), p.309. H.D. Lutz and C. Jung, J. Molec. Struc., 404 (1997) 63. B. Velde and J.M. Besson, Phys. Chem. Minerals, 7 (1981) 96. Y. Huang, J. Mater. Chem., 8 (1998) 1067. Y. Huang, E.A. Havenga, Chem. Phys. Lett., 345 (2001) 65. I.A.Belitski, B.A.Fursenko, S.P. Gabuda, O.V.Kholdeev and Y.V.Seryotkin Phys. Chem. Minerals, 18 (1992) 497. P.Gillet, J.M.Malezieux and J.P.Itie', Am. Mineral, 81 (1996), 651. N.L.Ross, in: R.M.Hazen and R.T.Downs(eds), High-Temperature and High-Pressure Crystal Chemistry, Reviews in Mineralogy and Geochemistry, 41 (2000), p.257. K.D.Hammonds, V.Heine and M.T.Dove, J.Phys.Chem.B, 102 (1998) 1759.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1885

Flexible A l u m i n i u m C o o r d i n a t i o n o f Zeolites as f u n c t i o n o f T e m p e r a t u r e and W a t e r content, an in-situ m e t h o d to d e t e r m i n e A l u m i n i u m C o o r d i n a t i o n s J.A. van Bokhoven*, A.M.J. van der Eerden and D.C. Koningsberger Debye Institute, Department of Inorganic Chemistry and Catalysis, Utrecht University, Sorbonnelaan 16, 3584 CH Utrecht, The Netherlands *Corresponding author: [email protected]

The aluminium coordinations in zeolites H-Beta and H-Y have been quantitatively investigated as a function of temperature in the presence and absence of water. In-situ A1 K edge X-ray Absorption Spectroscopy shows that a framework tetrahedrally coordinated aluminium is stable in inert to at least 725 K. However, in the presence of water, already at room temperature, part of the framework tetrahedral aluminium is converted to an octahedral coordination. This octahedral aluminium is not stable in inert at 375 K, where it quantitatively reverts to the tetrahedral framework coordination.

1. INTRODUCTION The importance of the aluminium coordination in zeolites for the development of active and selective catalysts for many reactions has been well documented. A generally used method for zeolite activation is acid or base leaching and / or steaming. Processes occurring during these treatments greatly affect activity and selectivity. Nonetheless, limited information on the aluminium coordinations during these treatments are reported, due to lack of clear methods to do this. Here we report the application of A1 K edge XAFS spectroscopy on zeolites that allow direct structural information about the aluminium atoms during a zeolite treatment. The aluminium coordination is determined as a function of temperature in the presence and absence of water. A1 K edge X-ray Spectroscopy allows the quantitative determination of aluminium coordinations by recognition of coordination-specific features in the near edge spectra. Spectra can in principle be obtained at high temperature and pressure. It takes about 5-10 minutes to take a near edge scan, allowing the detection of changes in structure at such time scale. The applicability of the quantitative determination under in-situ conditions is a great advantage over 27A1 MAS NMR, which is the dominant method to determine the aluminium coordination. Although recently serious progress has been made in the quantitative determination of aluminium coordinations in zeolitic samples and use of multiple quantum magic-angle spinning (MQ MAS) increased significantly the resolution 1'2, still a set-up allowing the determination of in-situ data is not readily available. Normally, before a (27A1

1886 MAS) NMR measurement, samples are pretreated (hydrated) in a controlled water environment in order to decrease the so-called quadrupolar couplings constant (QCC). Certainly, under these conditions, the resolution in 27A1 MQ MAS NMR spectra is very high. However, increasing the temperature in a dry environment causes a very large increase in QCC, resulting in a loss of resolution or even of the signal itself3. As A1 K edge spectroscopy allows the quantitative determination of aluminium coordinations at any temperature, we have developed a set-up enabling measurements at high temperature under controlled gasenvironment 4. Here, the combination of the two techniques provides unique information on the changing aluminium coordination in zeolites as function of treatment. 7A1 MAS NMR is used under standard (and hydrated) conditions, providing high resolution information on samples, whereas A1 K edge X-ray Spectroscopy is used to determine the changing aluminium coordinations under non-ambient conditions. We will show how the aluminium coordinations are changing as a function of treatment conditions. The influence of temperatures up to 725 K and the presence of water are shown to affect the aluminium coordinations.

2. EXPERIMENTAL

2.1. Samples Zeolite NH4-Beta was obtained from Delft University. Template removal has been done very carefully in order to prevent decomposition of the zeolite during this treatment, using a method developed by Kunkeler et al. 5. The heat-treatment of NH4-Beta with heating rate of 10 K per minute (max T = 725 K) is followed in-situ with A1 K edge XAS. The obtained H-Beta was exposed to water in Helium and subsequently to gaseous NH3 and measured with 27A1 MAS NMR and A1 K edge XAS. NH4-Y was obtained via BP-Amoco. 27A1 MAS NMR showed no octahedral A1 was present. Both samples were checked for crystallinity using XRD, nitrogen physisorption and transmission electron microscopy. The formation of octahedral aluminium in a calcined sample was followed by XAS at room temperature by treatment in Helium saturated with water. The stability of the octahedral aluminium was investigated by treating the sample in inert at 400 K.

2.2. AI K edge X-ray Absorption Spectroscopy The recently developed in-situ set-up, ILEXAFS (In-situ Low Energy X-ray Absorption Fine Structure Spectroscopy), has been used 4. It consists of a large vacuum vessel in which a small stainless steel in-situ chamber, containing the sample, is placed. A sample is pressed in a wafer and placed in a boronnitride cup that can be heated up to 800 K under a maximum gas-pressure of 1 bar. The vacuum in the large vessel is protected from the gas-pressure in the in-situ chamber by thin supported (7-15 pm) beryllium or Kapton windows. The design allows simultaneous fluorescence and (total) electron yield detection. Here, only fluorescence detection, via a gas proportional counter (GPC), integrated with the in-situ chamber to increase the solid angle of detected fluorescence radiation, has been used. The initial intensity of the X-ray beam has been measured via the drain current of either a thin 6 ~m Au or 4 ~m Cu mesh (with apertures of 60 and 88% respectively) positioned in the large vacuum vessel. A double crystal monochromator with YB66 crystals has been used. Measurements are performed at station 3.4 at the SRS, Daresbury (UK).

1887 2.3. AI coordinations from AI K edge spectroscopy H4-Beta Near edge spectra at the A1 K edge show features that are characteristic for different aluminium coordinations. Using a fingerprint of these features 6, different aluminium coordinations can be recognized in a sample. By altering the conditions of a measurement, such as temperature and / or gas-environment, (small) changes in aluminium ~~ H-Beta wet coordination are readily detected by comparing the differences between spectra. The characteristic features used to distinguish different coordinations are summarized below6: The K edge of aluminium metal is located at 1560 eV, while the spectra of tetrahedrally coordinated Beta (NH3) aluminium oxides show an edge at 1566 eV and octahedrally coordinated at 1568 eV. Moreover, an octahedrally coordinated aluminium shows a doublet (4 eV split) as whiteline (first intense peak in the spectrum) and displays a small pre edge feature at 1566 eV, except when a perfect octahedron 6. In addition, tetrahedral aluminium I 80 dO 4-0 2() () -20 I ppm shows a very characteristic broad peak a t - 2 0 eV Figure 1.27A1 MAS NMR spectra of zeolite above the absorption edge. The position of this Beta, showing a reversible and quantitative peak is sensitive to the A1-O bondlength v. In transition of tetrahedral A1 (--55 ppm) into spectra of octahedrally coordinated aluminium, a octahedral (0 ppm). comparable peak is visible at --50 eV above the absorption edge. 2.4. 27A1MAS NMR 27A1 MAS NMR was performed on a Chemagnetics Infinity 600 with a magnetic field strength of 14.1 T. Magic angle spinning was performed at a rotation speed of 27 kHz using a Chemagnetics 2.5 mm HX MAS probe. To circumvent saturation of the signal and ensure a linear response, n/18 pulses using an RF field strength of 36 kHz were used. Chemical shifts were taken relative to aqueous Al(NO3)3. The relaxation delays were 0.5 s, which was determined to be sufficient for quantitative analysis. 1000 Scans were recorded.

_j

3. R E S U L T S / D I S C U S S I O N As-synthesized zeolites contain tetrahedrally coordinated aluminium in the framework. In Figure 1, the 27A1 MAS NMR spectrum of zeolite NH4-Beta is shown. This spectrum shows two partly overlapping peaks in the range 50-60 ppm, that have been interpreted as tetrahedral coordinated aluminium at different crystallographic framework T-positions using multiple quantum (MQ) MAS NMR 8. (For an explanation of the spectra of H-Beta and H-Beta (NH3) vide infra). Figure 2 shows the corresponding A1 K near edge spectrum of the same sample. It shows the characteristic features of a tetrahedrally coordinated aluminium: A sharp whiteline

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Figure 3. AI K edge spectra of NH4-Beta, while heated in inert. Every spectrum corresponds to an increase of about 60 K. Some respective temperatures are provided.

is visible at 1567 eV, a broad peak a t - 2 0 eV above the edge and in between some finestructure. Figure 3 shows the near edge spectra of NH4-Beta while heating to 725 K in vacuum. Each spectrum represents a temperature interval of about 60 K. It is obvious that no clear changes in the spectra occur as function of temperature and upon removal of NH3. It is therefore inferred that all tetrahedral aluminium in the framework of zeolite Beta is stable in inert to a temperature of 725 K. No octahedral, penta- or tri-coordinated aluminium is formed under these conditions. After cooling down the sample to room temperature and exposing the sample to He, saturated with water by bubbling it through water at room temperature, changes in the spectra occur. In Figure 2 (spectrum H-Beta wet), it is obvious that an increase in intensity in the range 3-10 eV is observed. Previously we have decomposed similar spectra and concluded that this variation in intensity is due to the creation of octahedral coordinated aluminium. This conclusion is also found by the ZVA1MAS NMR spectrum (Figure 1) of a sample that has been treated in the laboratory under identical conditions. This spectrum (of a hydrated sample) shows a loss of one of the two tetrahedral aluminium framework peaks and the appearance of octahedral coordinated aluminium. The total intensity under the spectra of NH4-Beta and H-Beta are identical, showing no 'NMR-invisible' aluminium or other coordinations are present in the samples. Both NMR and the A1 K near edge show an identical change in variation from tetrahedral into octahedral, without the creation of any other coordination. From the A1 K near edge data it is concluded that the transition from tetrahedral to octahedral takes place at room temperature, induced by the presence of water in the gasphase. The reversible transition of tetrahedrally coordinated aluminium to octahedrally coordinated has been well-documented for zeolite Beta 9, but is also reported for other zeolites l~ Moreover, different crystallographic T-sites have reportedly a different tendency to dealuminate 8. In order to investigate the behavior of aluminium coordination in other zeolites, a sample NH4-Y zeolite was investigated. Figure 4 shows the near edge spectra of this sample. A calcined sample (at 725 K), called H-Y dry, shows all characteristics of a tetrahedrally coordinated aluminium. When this sample is exposed at room temperature to He saturated

1889 3.0

with water, a clear increase in intensity in the range 1 5 6 7 - 1675 "~,. ................ H-Y wet eV is visible, while the whiteline 2.4 ;"",. .............. H-Y w e t 375 K and the broad peak at 1583 eV v decrease in intensity. This is indicative of the creation of ol.8 octahedral coordinated aluminium 2~ < at the cost of tetrahedrally ~ N 1.2 coordinated aluminium. The same ,,.,.,o .,,e/ [ phenomenon is observed, as with zeolite Beta. In an acidic zeolite 0.6 H-Y, the addition of water at room temperature results in the 0.0 formation of octahedral 1555 1560 1565 1570 1575 1580 1585 1 5 9 0 1595 Energy (eV) coordinated aluminium. Figure 4. A1 K edge spectra of zeolite Y: H-Y It has been reported that this (Solid line), H-Y wetted (dotted) and H-Y wetted octahedral coordinated at 375 K (dash-dotted). aluminium, that is established to be connected to the framework, can be reverted into a tetrahedral coordinated aluminium by ion exchange with an alkali-ion or by the addition of NH3 in the gas phase at elevated temperature.9 The 27A1 MAS NMR and XAS spectra of zeolite H-Beta treated with NH3 (at 375 K), are given in Figures 1 and 2 respectively, and are both identical to the spectrum of the parent NH4-Beta. This confirms a complete reversal of aluminium coordination into the original tetrahedral framework coordination. We have previously shown that for zeolite H-Beta, a treatment with NH3 in the gas-phase, completely recovers the tetrahedrally coordinated aluminium, yielding identical isotropic chemical shifts 8. This strongly suggests that an identical aluminium coordination is recovered and the framework is completely intact, as shown here by both NMR and A1 K edge XAS. In order to investigate the stability of the octahedral aluminium in zeolite Y, we evacuated the gas-environment of the H-Ywet sample. Only after heating the sample to -375 K (in vacuum), the octahedral coordinated aluminium completely disappeared and a spectrum of a tetrahedral coordinated aluminium is recovered (Figure 4). This experiment shows that the octahedral coordinated aluminium is not stable to a slight heat treatment in vacuum. It is reverted into a tetrahedral aluminium by removing H20 from the gas-phase at -375 K. This process can be repeated during several cycles. H-Y dry

i

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i

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i

|

i

,

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4. CONCLUSIONS The combination of in-situ A1 K edge with 27A1 MAS NMR provides unique structural information on the changing aluminium coordinations in zeolites. In-situ A1 K edge X-ray Absorption Spectroscopy allows the quantitative determination of aluminium coordinations under non-ambient conditions. 27A1 MAS NMR under hydrated conditions yields high resolution spectra, especially when MQMAS is applied. A study on zeolites shows that part of the tetrahedral framework aluminium shows a high flexibility and is able to transform-reversibly- into an octahedral aluminium. In an acidic

1890 zeolite (H-Beta and H-Y), octahedrally coordinated aluminium is formed at room temperature, only when water is present in the gas-phase. This octahedral aluminium is unstable in vacuum at slightly elevated temperatures and completely reverts to a framework tetrahedral aluminium. This process can be cycled several times.

Acknowledgements The NMR facility at The University of Nijmegen is thanked for providing magnet time for the NMR measurements. Gerda Nachtegaal is thanked for her help during the NMR measurements. Dr. A. Smith is acknowledged for his help during the EXAFS-run at station 3.4 of the SRS Daresbury. The SRS, Daresbury (UK) is kindly thanked for providing beamtime under beamtime award number 37410. References 1 Frydman, L., Harwood, J. S., Jr. Am Chem. Soc. 117, 5367 (1995) 2 Medek, A., Harwood, J. S., Frydman, L,J. Am. Chem. Soc. 117, 12779 (1995) 3 Seiler,M., Wang, W., and Hunger, M., Jr. Phys. Chem. B 105,8143 (2001) 4 van der Eerden, A. M. J., van Bokhoven, J. A., Smith, A., Koningsberger, D. C., Rev. Sci. Instrum. 71-9 3260 (2000) 5 Kunkeler. P. J., Zuurgeed, B. J., van der Waal, J. C., van Bekkum, H., van Bokhoven, J. A., Koningsberger, D. C., J. Cata1180, 234 (1998) 6 van Bokhoven, J. A., Sambe, H., Ramaker, D. E., Koningsberger, D. C., J. Phys. Chem. 103 7557 (1999) 7 van Bokhoven, J. A., Ramaker, D. E., Koningsberger, D. C., dr. Phys.: Condens. Matter 13 10393 (2001) 8 van Bokhoven, J. A., Koningsberger, D. C., Kunkeler, P., van Bekkum, H., Kentgens, A. P. M., J. Am. Chem. Soc. 122(51) 12842 (2000) 9 Bourgeat-Lami, E., Massiani, P., Di Renzo, F., Espiau, P., Fajula, F., and Des Couri6res, T., Appl. Catal. 72 139 (1991) 10 Wouters, B. H., Chen, T.-H., and Grobet, P. J.,J. Am. Chem. Soc. 120, 11419 (1998)

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1891

Structure analysis of Boron-silicalite and of a "defect-free" MFI-silicalite b y s y n c h r o t r o n radiation single crystal X-ray diffraction M. Milanesio 1, D. Viterbo 1, L. Palin 2, G. L. Marra 3, C. Lamberti4, R. Aiello 5, F. Testa 5 1 DiSTA, Universittt del Piemonte Orientale, Corso T. Borsalino 54, 1-15100 Alessandria. 2 ESRF, Diffraction Group, BP 220, F-38043 Grenoble Cedex, France. 3 Polimeri Europa, S.p.A., Istituto Guido Donegani, Via G. Fauser 4, 1-28100, Novara (I) 4 Dipartimento di Chimica IFM, Universith di Torino, Via P. Giuria 7, I-10125 Torino. 5 Dipartimento di Ingegneria Chimica e dei Materiali, Universit~t della Calabria, 1-87030 Rende (Cs), Italy. We report the structure determinations, obtained by synchrotron radiation single crystal X-ray diffraction experiments, of as synthesized orthorhombic MFI boron-silicalite and of pure silicalite. The synthesis in fluoride medium yielded comparatively large twinned single crystals. XRD powder data on silicalite samples were also recorded. The absence of any measurable signal due to Si(OH) species in the 29Si NMR spectra of silicalite indicates that the samples are almost free of Si vacancies (which are usually rather common in MFI materials) and that their defectivity is very low. This study is thus complementary to a previous neutron diffraction study [Artioli et al., Acta Cryst. B56 (2000) 2], where a defective silicalite had been characterized. All carbon and nitrogen atoms of the disordered tetrapropylammonium (TPA +) template molecules have been positioned. These molecules, located at the intersection of the straight and sinusoidal channels, are similar to those found by Van Koningsveld et aL [Acta Cryst. B34 (1987) 127] in the parent ZSM-5 system. The presence in silicalite of a residual electron density peak near sites T9 and T10, similar to that found in Fe-silicalite [Milanesio et aL, J. Phys. Chem. B 104 (2000) 9951], is consistent with the recent NMR results [Fyfe et al. , J. Am. Chem. Soc., (123) 2001 6882] indicating the presence of a SiOatzF group at T9. This residual peak is much smaller in B-silicalite. XRD powder data on silicalite confirm the single crystal results, indicating that they are not biased by twinning. 1. INTRODUCTION Silicalite, first synthesized at the Union Carbide laboratories by the Flaningen group [1], is an aluminum free zeolite, with the MFI structural topology [2]. Silicalite samples with very different morphology and crystal quality and dimension can be obtained using different synthesis conditions. Silicalites synthesized following the original recipe [1] have well defined crystallites with a size of 2-3.5 ~tm and a Pnma orthorhombic symmetry. After calcination, they exhibit a monoclinic P21/n symmetry. On the chemical ground, they have A1 and Na impurities and show a very low density of internal defects i.e. Si vacancies. Silicalites synthesized following an alternative method [3] have a significantly smaller crystal size (0.20.3 l.tm), yielding a high surface area powder with a much lower level of Na and A1 impurities but a much higher density of internal defects [4,5,6]. The lack of one or more adjacent silicon atoms in these defects, is balanced by the presence of hydroxylated nano-cavities in the framework, also referred to as hydroxyl nests. The absence of impurities and the presence of

1892 internal hydroxyl nests have significant consequences on the long-range structural order. Indeed, samples prepared as described in Ref. 3, after calcination, maintain an orthorhombic Prima symmetry. A powder neutron diffraction characterization of defective silicalite has been recently reported by Artioli et al. [5], showing that vacancies do not occur randomly among the 12 symmetry independent tetrahedral (T) sites of the orthorhombic structure, but are preferentially located at sites Si6, Si7, Sil0, and Sil 1. We describe here a synchrotron radiation XRD study of a defect-free silicalite and of a B-silicalite silicalite single crystals exhibiting comparatively large dimensions (-25x40x110 l.t m 3) with respect to those obtained by the Flanigen method [ 1]. These crystals, synthesized as described in Ref. [7], are twins, but the twin law could be easily found. 29Si solid state NMR studies [8] showed only the chemical shift due to Si(OSi)4, while no SiOH(OSi)3 signal was detected. This indicates that Si vacancies are virtually absent in this material, which then represents an example of an almost perfect MFI structure. This structural study can then be considered as complementary to the previous work by Artioli et al. [5] on defective silicalite. Beside the interest of silicalite itself, its discovery demonstrated that the A1 containing ZSM-5 zeolite [9] was not the only possible microporous material with MFI structure, and opened the route to the synthesis of a great number of new microporous materials. In fact, the isomorphous substitution of Si by other tetrahedrally coordinated heteroatoms such as B nI [10], A l Ill [7], Ti TM (Ti-silicalite or TS-1) [11], Ga nI [12] and FenI [13] in small amounts (up to 2-3 % wt.), provides new materials showing specific catalytic properties in oxidation and hydroxylation reactions, related to the coordination state of the heteroatom [ 14]. Moreover, MFI-type materials with trivalent metals present in tetrahedral sites have shown a tremendous impact as new shape-selective industrial catalysts, having tunable acidic strength. In fact, the acidic strength of the protons in the bridged Si(OH)M m (M = B, A1, Fe, Ga) groups depends on the nature of the trivalent heteroatom. Indeed, the choice of M(III) critically affects this acidity property according to the sequence A1 > Fe --Ga >> B [15]. Moreover, the recent discovery of an A1 containing natural zeolite (mutinaite) with the MFI topology [ 16], makes this structure of relevance also in the mineralogical field. For all these reasons, in the last decade, a large number of studies aiming at a systematic characterization of metal-substituted MFI materials have been reported. Here we describe the crystal structure of B-silicalite and compare it with that of silicalite obtained with the same synthetic procedure in fluoride medium [7]. From the crystallographic point of view, even though the structure of the hosting MFI matrix has been satisfactorily clarified, the distribution of the heteroatoms (M) over the 12 symmetry independent T sites of the orthorhombic MFI framework is still open to debate. This is an important problem, since the localization of the M atoms may play an important role in understanding the catalytic properties of the material. The low fraction of M atoms that can be inserted into the MFI framework makes the experimental localization of these atoms by diffraction measurements very difficult. For these reason the most interesting speculations concerning the Ti distribution in TS-1 are so far based on computational chemistry results [ 17,18], and only few crystallographic studies have been reported. Among those, we shall recall synchrotron radiation powder XRD analyses on TS-1 [19,20], a synchrotron radiation single crystal XRD study on Fe-silicalite [21] and powder neutron diffraction studies on TS-1 [18,22,23]. Both theoretical [17,18] and experimental [18-23] investigations give rather contradictory results on the location of the heteroatoms. It is evident that an accurate crystallographic study of the reference material, i.e. pure and defect-free silicalite, could be of great help in validating the faint experimental evidences used by the different groups to support the attributions made in their papers. Since it is well known that template burning, even under mild conditions, causes

1893 a partial migration of the heteroatoms from the framework into extra-framework positions, the single crystal diffraction studies have been mainly carried out on the as synthesized crystals, containing the tetrapropylammonium (TPA) template. The accurate location of the highly disordered TPA in silicalite yields thus an important point of reference for the other single crystal studies. This is particularly true for our single crystal study of Fe-silicalite [21], where the location of Fem at sites T9 and T10 was inferred on the basis of: (i) an increase of the average T-O distance; (ii) the presence of two peaks (2.28 and 0.67 e/]k3) in the electron density map near T9 and T10; (iii) the higher anisotropy of the thermal displacement parameters observed for T9 and T10, which was attributed to the disorder generated by iron insertion. At a recent congress Aubert et al. [24] presented a poster illustrating a single crystal study of silicalite and their results are very close to those described in this paper. We wish to thank the authors for the stimulating discussion, the helpful exchange of ideas and the comparison of our results. 2.

EXPERIMENTAL

2.1. Sample preparation and methods Silicalite and B-silicalite samples exhibiting crystals of comparatively large dimensions (-20x30x100 ~tm3) were synthesized in fluoride medium [7]. The crystals are twins, but the twin law could be easily found from the exceptions to the systematic absences expected for the P n m a space group. The twinning is due to a 4 twin axis along the [0 0 1] direction [25,26]. Our single crystal diffraction measurements were carried out at room temperature, on silicalite and B-silicalite [27], at the Materials Science beamline I D l l of the European Synchrotron Radiation Facility (ESRF), using a Bruker CCD detector. Of the latter derivative data were collected on two crystals: an as synthesized one and a crystal which had been heated at 483 K for 3 h in an ozone/oxygen flux [28], in an attempt at removing the template without damaging the crystal. For comparison also powder data were collected on the as made silicalite sample at the Powder Diffraction beamline BM16 of ESRF [29], employing a 9 channel detector. The cell dimensions and the reduction of the single crystal data were carried out using the programs SMART [30] and SAINT [31]. In order to account for a slight spot splitting at high angles, due to the twinning, the data reduction was performed using a comparatively large integration box. The most relevant crystal data are reported in Table 1. 2.2. Structure refinements Using as starting model the structure of ZSM-5 determined by Van Koningsveld et al. [32], the refinement by full matrix least squares and difference Fourier recycling was carried out using the SXELX97 program [33], which is capable of handling data from twinned crystals. All framework atoms were refined with anisotropic displacement parameters. As expected, no direct indication of the location of the boron substituent in B-silicalite was obtained. The location of the disordered TPA § template, in all the crystals, was performed by several difference Fourier cycles and the refinement of the template atoms was carried out using geometrical restraints on bond distances and angles and isotropic displacement parameters. The occupancies of the disordered atoms were found by refining the population parameters, while keeping the thermal displacement parameters fixed. The crystal of B-silicalite heated in ozone gave diffraction data as good as those of the as synthesized crystal, but the ref'mement

1894 indicated the presence of TPA +, although with a slightly lower population. This shows that template burning under such mild conditions does not damage the crystals, but is far from being complete. For this reason we will only describe the results of the refinement of the as synthesized crystal. The Rietveld refinement of the microcrystalline silicalite sample was carried out by means of the GSAS software [34], employing as starting model the structural resuk of the silicalite single crystal refinement. Table

1 -

Summary of crystal data for silicalite and B-silicalite.

Compound a b c V Density (calc)

Silicalite 20.042(3)/~ 19.990(3) 13.414(2) 5374(1) A 3

Observed reflections Data / restraints / parameters Goodness-of-fit on IFol2 R indices [1>2 t~ (I)] R indices (all data) Largest diff. peak and hole, Weight (calculated)

3389 [I>2c(I)] 4462 [I>2o'(I)] 6348 / 36 / 410 6419 / 36 / 410 0.888 0.926 R1 = 0.0589, wR2 = 0.1452 R1 =0.0426, wR2 =0.1118 R1 = 0.1093, wR2 = 0.1673 R1 = 0.0625, wR2 = 0.1178 0.644, -0.698 e ]k-3 0.585, -0.655 e A 3 w = 1/[ cy2(lFo]2)+(P) 2] w h e r e P = (lEo]2 + 21Fc]2)13

2.036 g c m -3

B-Silicalite 19.968(3)/~ 19.955(3) 13.372(2) 5328(1) A 3 2.054 g c m -3

3. RESULTS AND DISCUSSION The location of the disordered template is not straightforward, but must be done with great accuracy in order to be sure that the remaining significant electron density peaks can be correctly interpreted. The high quality of the single-crystal data obtained at beamline ID11 of ESRF [27] allowed a good elucidation of the TPA structure, which is consistent with that found in ZSM-5 and in Fe-silicafite. The template molecules are located at the intersections of the straight and the sinusoidal channels. The N atom lies on a mirror plane and two distinct disordered images of the tetrahedral arrangement of the propyl arms are obtained (Figure 1). The presence in pure silicalite of a residual electron density peak, located near sites T9 and T10 and only slightly smaller than that found in Fe-silicalite [21], disagrees with our previous interpretation of this peak as due the to the alkaline counterion. Indeed, a recent extended NMR analysis [35] has unambiguously proven that at site T9 a live-coordinate SiO4/2F group is present in silicalite crystals prepared in fluoride media. The residual electron density peak should therefore be interpreted as due to the fluoride anion linked to the T9 site located in a four ring, where F- may stabilize the strained Si-O-Si bonds. The formation of a fivecoordinate group also induces the observed lengthening of the T9-O distances. In any case, this new interpretation is still consistent with our original indication that T9 and T10 (also in the four ring) might be the preferred substitution sites for the heteroatom.

1895

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i ....... i ....... i ....... i ....... i ....... i ....... ~__

i i

3

4

5

6

7

L

8

9

_

10 11 12

T site Figure 2 -Comparison of the 12 different tetrahedral distances in four different MFI structures In B-silicalite the residual electron density peak is much smaller and no lengthening of the T9O distances is observed. Indeed the lengthening due to the formation of a five-coordinate group might be well compensated by the shortening due to B substitution.

1896 As far as the framework is concerned it is interesting to analyze the trend of the T-O distances in the 12 independent tetrahedral sites. In figure 2 the average T-O distance around each site is reported for silicalite and B-silicalite and is compared with that of ZSM-5 [32] and Fesilicalite [21]. It may be seen that site T9 has the largest T-O average distance in silicalite and Fe-silicalite, which is significantly larger than in ZSM-5 and B-silicalite. This observation is in keeping with the formation of a five-coordinate SiO4tzF- group at T9 in silicalite, as indicated by the above mentioned residual electron density peak due to F-. Indeed in the trigonal bipyramid around T9 a significant lengthening of the axial T-O distance opposite to the Fanion is the main responsible of the overall lengthening of the average T9-O distance. In Fesilicalite a similar pattern is found, and, even though there is no direct indication that T9 is the preferred substitution site, we can still suppose that the heteroatoms will tend to go into this most strained four-ring framework position and contribute to the T-O lengthening. This seems to be confirmed by the resuks of B-silicalite, where the much smaller residual electron density peak indicates a smaller percent of trigonal-bipyramid sites and a small T-O lengthening, which is compensated by B substitution. 4. CONCLUSIONS The synchrotron radiation single crystal refinement of the structures of silicalite and Bsilicalite allowed us to revise our previous interpretation of a residual electron density peak found in Fe-silicalite as due to the counterion. In fact, the presence of a similar peak in silicalite contradicted this hypothesis and prompted us to search for a new interpretation, which was offered by the recent NMR study by Fyfe et al. [35] indicating the presence of a penta-coordinated SiO4t2F- group at site T9. Indeed, earlier on Koller et al. [36] proposed the presence of five-coordinate silicon in high silica zeolites prepared in fluoride media, but only the most recent work gave conclusive evidence of the location and features of the SiO4/2Fgroups. The residual electron density peak found in Fe-silicalite and in silicalite at less than 2.0/~ from T9 is than due to the fluorine, as confirmed also by the fact that when it is taken into account site T9 assumes a trigonal-bipyramid coordination, with a lengthening of the axial T-O distance opposite to the F. Despite this change of interpretation the following factors still point to T9 as a preferred substitution site in substituted silicalites: a) From a thermodynamic point of view one might expect that substitution occurs at the less stable position in the framework, such as the five-coordinated site; b) The presence of a much smaller electron density peak in B-silicalite indicates a smaller population of five-coordinated sites, and the smaller average T-O lengthening is compensated by the shortening due to B substitution; c) In Fe-silicalite both the five-coordinate sites and the Fe substitution contribute to the average T-O lengthening. ACKNOWLEDGMENTS This project was supported by MURST (Cof'm 2000, Area 03). XRD measurements were performed at beamlines IDll and BM16 of the ESRF storage ring within the public user program. We are indebted to Drs. Kvik and Fitch (ESRF), for the constant technical support during data collection, to F. Crea, (University of Calabria), for his help in the synthesis of the samples, to A. Zecchina and S. Bordiga (University of Torino) for critical discussion. L. P. acknowledges an INFM grant. M.M acknowledges the "G. Donegani" foundation for a grant in 2001.

1897 REFERENCES

1. 2. 3. 4

5. 6.

7. 8. 9. 10. 11.

12.

13.

14. 15.

16.

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17. (a) A. Jentys, C. R. A. Catlow, Catal. Lett. 22 (1993) 251; (b) R. Millini, G. Perego, K. Seiti, Stud. Surf. Sci. Catal. 84 (1994) 2123; (c) Y. Oumi, K. Matsuba, M. Kubo, M. Inui, A. Miyamoto, Microporous Mater. 4 (1995) 53; (d) K. S. Smirnov, B. van de Graaf, 7 (1996) 133; (e) S. L. Njo, H. van Koningsveld, B. van de Graaf, J. Phys. Chem. B 101 (1997) 10065; (f) G. Ricchiardi, A. de Man, J. Sauer, Phys. Chem. Chem. Phys. 2 (2000) 2195. 18. C. A. Hijar, R. M. Jacubinas, J. Eckert, N. J. Henson, P. J. Hay, K. C. Ott, J. Phys. Chem. B 104 (2000) 12157. 19. C. Lamberti, S. Bordiga, A. Zecchina, A. Carati, A.N. Fitch, G. Artioli, G. Petrini, M. Salvalaggio, G. L. Marra, J. Catal. 183 (1999) 222. 20. G. L. Marra, G. Artioli, A. N. Fitch, M. Milanesio, C. Lamberti, Microporous Mesoporous Mater. 40 (2000) 85. 21. M. Milanesio, C. Lamberti, R. Aiello, F. Testa, M. Piana, D. Viterbo, J. Phys. Chem. B 104 (2000) 9951. 22. C. Lamberti, S. Bordiga, A. Zecchina, G. Artioli, G. L. Marra, G. SpanS, J. Am. Chem. Soc. 123 (2001) 2204. 23. P. F. Henry, M. T. Weller, C. C. Wilson, J. Phys. Chem. B. 105 (2001) 7452. 24. (a) E. Aubert, F. Porcher, M. Souhassou, Y. Dusausoy, C. Lecomte, 20 t~ European Crystallographic Meeting, Krakow, 25-31 August 2001, Abstract No. O.M3.P38; (b) E. Aubert, F. Porcher, M. Souhassou, V. Petricek, C. Lecomte, J. Phys. Chem. B 106 (2002) 1110. 25. G. D. Price, J. J. Pluth, J. V. Smith, J. M. Bennett, R. L. Patton, J. Am. Chem. Soc. 104 (1982) 5971-5977. 26. C. Weidenthaler, R. X. Fischer, R. D. Shannon, O. Medenbach, J. Phys. Chem. 98 (1994) 12687-12694. 27. D. Viterbo, M. Milanesio, L. Palin, C. Lamberti, Beamline 1Dll at ESRF, Proposal CH1027, June, 6-11, 2001. 28. D. Mehn A. Kukovecz, I. Kiricsi, F. Testa, E. Nigro, R. Aiello, G. Daelen, P. Lentz, A. Fonseca, J. N. Nagy, in "Zeolites and Mesoporous at the Down of the 21 ~t Century Materials" (Eds: A. Galarneau, F. Di Renzo, F. Fajula, J. Vedrine) Proceeding of the 13th International Zeolite Conference, Montpellier, 8-13 July 2001. 29. D. Viterbo, M. Milanesio, L. Palin, C. Lamberti, Beamline BM16 at ESRF, Proposal CH917, February, 7-13, 2001. 30. SMART - 9 1998 Bruker AXS, Inc., Madison Wisconsin 53719 USA. 31. SAINT - 9 1994-1996 Bruker AXS, Inc., Madison, Wisconsin 53719, USA. 32. H. Van Koningsveld, H. Van Bekkum, J. C. Jansen, Acta Cryst. B34 (1987) 127. 33. G. M. Sheldrick, SHELAZ-97, University of G~3ttingen, Germany, (1997) (Web site:http://shelx.uni-ac.gwdg.de/SHELX/). 34. A.C. Larson and R.B. Von Dreele, "General Structure Analysis System (GSAS)", Los Alamos National Laboratory Report LAUR (1994) 86-748; (Web site: http://www.ccp 14.ac.uk/solution/gsas/index.html). 35. C. A. Fyfe, D. H. Brouwer, A. R. Lewis, J. M. Chezeau, J. Am. Chem. Soc. 123 (2001) 6882-6891 36. H. Koller, A. Wtilker, L. A. Villaescusa, M. J. Diaz-Cabafias, S. Valencia, M.A. Camblor, J. Am. Chem. Soc. 121 (1999) 3368-3376.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1899

D e n s i t y F u n c t i o n a l T h e o r y m o d e l l i n g E P R spectra o f Cu(II) in Y zeolite D.Berthomieu, J.M.Ducrrr, A.Goursot Laboratoire de Matrriaux Catalytiques et Catalyse en Chimie Organique, UMR CNRS- 5618, ENSCM, 8, rue de l'Ecole Normale, 34296 MONTPELLIER Crdex 5, France bertho@univ-montp2, fr Electron paramagnetic resonance (EPR) experiments have shown that Cu(II) occupy different positions in the Y zeolite. We have studied the EPR properties of different cluster models in which copper is located at different cristallographic sites and different positions. The calculated results showed that the unpaired electron of copper is delocalized on the zeolite and that the hyperfine coupling constants depend linearly on the spin density of the models whatever are the structures and the calculation methods. These calculated constant values are close to experimental values when Cu(II) occupy ideal positions, ie. when it is four-coordinated at the center of a four or six-membered ring.

1. I N T R O D U C T I O N Zeolites are materials containing channels and pores used for adsorption and catalysis. In particular Cu(II) cations are incorporated into zeolites to produce reactive catalytic sites for the selective catalytic reduction of NOx emissions by NH3. The efficiency of this process led it to be used in stationary sources for removing NOx polluants from gazeous effluents. A catalytic cycle has been proposed to explain the reduction process which leads to the formation of H20 and N2 [1,2]. Various treatments and activation procedures are used to generate reactive metal ion states, which are often paramagnetic. The location and structure of the reactive metal ion sites are of great importance for understanding their reactivity towards different adsorbates. Experimental information on cation location such as EPR spectra is difficult to interpret and allows essentially to distinguish the number of different paramagnetic cations. Modelling is thus the best complementary technique to characterize the possible sites by comparison between predicted and experimental EPR spectra. With the recent development of DFT approaches to calculate the EPR g- and the hyperfine coupling Atensors, it is possible to obtain a sufficiently good understanding of the local structure of the catalytic sites. From EPR spectra it has been proposed that the Cu(II) transition metal ion (TMI) occupy different positions in the Y zeolite. It is generally assumed that these positions are sites I/r and II/II' [3-8]. However the real positions of Cu(II) are still subjected to debate. We have studied different models in which copper is located at different accessible sites i.e. site II

1900 and III of the supercage, as proposed by recent experiments [1,2]. We will show that the calculated A-tensors are close to experimental values when Cu(II) occupy ideal positions, ie. when it is four-coordinated at the center of a four or six-membered ring.

2. COMPUTATIONAL DETAILS Density functional calculations have been performed using the deMon-KS program[9] and the Gaussian98 code [10]. Calculations labelled m-BP correspond to a mixed scheme where the density has been obtained at the local level (with a Vosko-Wilk-Nusair correlation functional for the potential) [11] and the energy calculated at the gradient-corrected (GGA)[12] level using Becke's corrections for exchange [13] and Perdew's corrections for correlation [14]. The full GGA calculations with non-local corrections for potential and energy, namely the Becke exchange [13] and the Perdew correlation functional [14] are labelled BP86. We have also used the B3LYP hybrid methodology (B3LYP) [15,16]. All model clusters contain at least 2 A1 in order to obtain a neutral Metal-Zeolite system. The geometries were optimized using the m-BP (CuYB, iCuYB), BP86 (iCuYB.) or B3LYP (CuYF1, CUYF1., CUYF2) calculations. In these cases, the relaxation of the clusters have been performed keeping fixed the terminal H atoms used to saturate the dangling bonds. The CuYF1MM and iCuYB,MM structures were obtained by Molecular Mechanics (MM) optimization, using periodic boundary conditions and a random distribution of A1 for a Si/A1 ratio of 2.5 (Cerius2, [17] cvff-aug-ionic force field). The MM optimization has been performed using Co 2+ parameters for copper, since adequate Cu 2+ Lennard Jones parameters are missing. These approximate positions for Cu 2+ were optimized quantum chemically (BP86 calculation), the framework atoms being kept fixed at the MM solid geometry. This procedure has been adopted in order to take into account the long range interactions between the cluster and its surroundings, in particular neighboring cations. We used the Wachters basis set (without f orbitals) for Cu [ 18] for all calculations. In case of calculations using deMon code, all electrons basis sets of DZVP quality were used for all other heavy atoms and a DZ basis for hydrogen atoms. In Huzinaga's notation their contraction patterns are (6321/521/1"), (621/41/1") and (41) for Si and A1, O and H respectively. The associated auxiliary basis sets used to fit the density and the exchangecorrelation potential (deMon code) are, with respect to the same atoms, (5,4;5,4), (5,2;5,2) and (5,1;5,1), where the usual deMon notation is used. Using Gaussian98 program, the 631G(d) basis set for the others atoms was used. 3. RESULTS AND DISCUSSION

3.1 Hyperfine coupling constants The hyperfine coupling of Cu(II) due to the interaction between its electronic and nucleus spins is a good indication for the knowledge of its close environment. The hyperfine coupling tensor can be separated into isotropic and anisotropic terms. Their expressions, at the frst order approximation, are the following [ 19] :

1901

(Sz/-, ~p.vp.%_,(%(rNi)16(rN~)[~(r Ni))

Aiso (N) : ~4n g e [~e gN [SN

1 1 ,~_p Tk,(N) : 2 g , P ~ g N P N ( S z ) - E P , , (*,lrNiS(r~i 6 k , - 3rNikrNi,)l*~ ) I.tv

where 13e, 13N, ge and gN are the Bohr magneton, nuclear magneton, free electron and nuclear g-values, respectively. P,v a-p is the spin density matrix element related to orbitals ~), and d~v,whereas rNi represents the position of the electron with respect to the nucleus N. Following Munzarova et al [21 ], we have characterized the anisotropic term using the Aaip parameter, obtained from the components (-Adip, -Adip, 2Adip) of the Tkl traceless tensor (axial symmetry). Indeed, this parameter is very useful for the comparison of computed values and experiment results, being also equal to (A//- A• where A//and A• are the experimental components. The isotropic term, or Fermi contact term, still remains a challenge for theoreticians due to the difficulty to calculate it accuratly. We report in Table 1 the calculated Aiso and mdip values for an isolated Cu 2+ ion. Table 1 Hyperfine coupling constants for an isolated Cu 2+ (units 10-4 x cm 1) using different calculation methods ROHF UHF B3LYP BP86' mBP86 BP91 i

Aiso

0.0

-127.2

-90.0

-107.3

-59.8

-130.8

Adio

112.2

104.4

107.7

106.5

106.6

107.5

Spin contamination <$2> = 0.7502 (DFT), 0.7503 (B3LYP), 0.7506 (UHF) These calculations show large variations of the Fermi term depending on the method used. In contrast, the Adip values are very comparable. This stability with respect to the method allows us to compare mdip terms with experimental results. 3.2 Dipolar coupling constants Adipof Cu(ll) in sites II and III We have studied Cu(II) at the most accessible positions in the zeolite supercage, i.e site II and III. To model these sites, we have used a cluster model approach, with a six-membered ring cluster for site II and a four-membered ring cluster for site III. The site II model involves the TMI in ideal position only whereas in the case of site III Cu can occupy an ideal position or can be located at the border of the four-membered ring (Figure 1). Copper is fourfold coordinated in ideal positions and twofold coordinated in the second site III. As shown in

1902 Figure 1, the geometry optimization has led to several conformers. In fact, due to the number of degrees of freedom, we can still expect other conformers with close energies. The two structures CuYF1 and CuYF1, have been also recently reported [20], with only sligth differences in the geometries. We did not obtain any geometry threefold coordinated copper in contrast with what is usually proposed for site II based on X-Ray experiments [21]. The calculated structures contain either four short bond lengths or three short and one longer bond lengths. In contrast to site II, Cu(II) is sligthly out of the ring plane when located in ideal site III positions.

2.01 ..,..""

~ i (0.511

2.04

/ (0.50) e

1.96

CUYFI

2.14

~Q(0.42)

CUYFI '

I

/w e:.e"

.......

//

%".....

/.,"

a\(o.55)

2.13

?,o

~,-~

CUYFIMM

CUYF2 ~

1903

:

5

2.031~

CuYB

1.94

2.00 ~

iCuYB,

~]p 2.64

iCuYB

J~

(0.~48) I]D1"95

1-.99

._~_.

"~

,i'll[

1..96

2.03. i~. . (0042)~~1.96,, . .

d -

2.01 iCuYB 'MM

Figure 1. Cluster models for sites II and III. We reported the Cu-Oring(in A) and the copper spin density values (in paranthesis) calculated using BP86. Cu black balls, H small white balls, O small dark balls, Si gray balls, A1 clear gray balls. We have analyzed the calculated spin density on copper since it is related to the EPR hyperfine anisotropic coupling, as shown in the mathematical expression. The variation of the copper coordination induces large differences in the spin density values: they are very small for a bi-coordinated copper whereas they are much larger in the case of a four-coordinated copper. For all the models, the spin density on copper was calculated to be much smaller than one, which indicates that the unpaired electron is delocalized on the zeolite, as illustrated Figure 2. The spin density is delocalized over the TMI and the O atoms, especially those of the ring. It depends essentially on the Cu-O bond lengths, increasing when the Cu-O bond lengths are decreasing. This property is very dependent on small variations of the geometry. The calculation of the exact density would require to know the real geometry of the TMI site. Our results also show that GGA-DFT and B3LYP calculations lead to different Cu-O bond lengths, with a generally higher spin density on Cu from B3LYP (0.65 for CUYF1, 0.62 for CuYn., 0.68 for CuYF2, 0.47 for iCuYB and 0.57 for iCuYw) than from BP86 calculations (Figure 1).

1904

+ Figure 2 : Spin density contours of iCuYB model. The calculated Adi p values for the models of sites II and III and using different methods are plotted on Figure 3, as a function of the Cu spin density. This function shows a linear relationship. The Adip value of an isolated Cu(II) (d 9) has also been reported on the curve, as a reference for the maximum spin density value.

Adip 10 "4 x cm 4 x

10 I 80J '~

60 40

"* " * x *

./"

20

/ r

0

0,2

s!telII (BP86) s!te II (B3LYP) s!te!!I (BP86) site!II (B3LYP) isolated Cu(II)

T

0,4 0,6 0,8 Cu(ll) spin density

1

Figure 3: Calculated Adip values as a function of the calculated spin density

Experimental EPR results for Y zeolites are summarized in Table 2. Two or three signals are generally reported, depending on the solids. Two ranges of g values can be

1905 considered: g//between 2.36-2.39 corresponding to A//between 112.0-145 and g//between 2.32-2.35 corresponding to A//between 145.8-179 (in 10-4x cm -1) Table 2: Experimental EPR values in 10.4 x cm -~units A// At. Pierloot [20] Yu [8]

Schoonheydt [4] Carl [22] Conesa [5]

Liu [3] Levi Matar [23]

Nacaehe [24]

139 179 137 179 116 147 128 157.5 140 170 168 112.0 145.8 145 149.7

12.5 17.4

19.0 22.0

g//

g•

2.38 2.33 2.39 2.33 2.387 2.332 2.385 2.354 2.380 2.328 2.327 2.365 2.332 2.36 2.32

2.07 2.07

2.07 2.073 2.067

2.07 2.06

The experimental mdip range of values related to the first group is 33.2-44.2 and the second one is 42.5-53.6 (in 10.4 x cm-1). The comparison between these experimental Adip values with our results using the linear correlation allows to estimate the ~ experimental >) Cu spin density to be between 0.35 and 0.45. Moreover, it shows that a bi-coordinated Cu(II) at site III has a too small Adip value to fit with any observed EPR signal. This study also shows that the Adip values vary strongly with the model structures and underlines the necessity to know the exact structure of the Cu(II) site. Despite this difficulty, useful information can be obtained from the comparison of the experimental and theoretical results. When Cu(II) is in ideal positions, both sites II and III can be assigned to experimental signals, depending on the cluster geometries. Among the structures, those cut from a simulated solid lead to the best agreement with experiment. The model iCuYB is also a possible candidate. The results of this study indicate that both site II and site III are possible sites for Cu(II) location, with larger hyperfine coupling for site II models. The estimate of the ~ experimental >>spin density confirms that the unpaired electron is strongly delocalized on the zeolite. This result corroborates the charge tranfer effect calculated for Cu(II)-Y model [25]. 4. CONCLUSION The calculated anisotropic hyperfine coupling constants of cluster models representing sites II and III of Cu(II)-Y have shown the existence of a linear relationship with the copper spin density, for both sites II and III, independently on the calculation method. The

1906 comparison of the calculated and experiment values lead us to consider four-coordinated Cu(II) at these sites as paramagnetic species leading to the observed EPR signals. REFERENCES

1. Kieger, S.; Delahay, G.; Coq, B.; Neveu, B. J. of Catalysis 1999, 183,267. 2. Kieger, S.; Delahay, G.; Coq, B. Appl. Catal. B 2000, 25, 1. 3. Liu, S.-B.; Lin, T.-S.; T-C., Y.; Chen, T.-H.; Hong, E.-C.; Ryoo, R. J. Phys. Chem. 1995, 99, 8277. 4. Schoonheydt, R. A. CataL Rev.-Sci. Eng. 1993, 35, 129. 5. Conesa, J. C.; Sofia, J. J. Chem. Soc. Faraday Trans. 1979, 75,406. 6. Turkevitch, J.; Ono, Y.; Sofia, J. Jr. Catal. 1972, 25, 44. 7. Ichikawa, T.; Kevan, L. J. Phys. Chem. 1983, 87, 4433. 8. Yu, J.-S.; Kevan, L. J. Phys. Chem. 1990, 94, 7612. 9. Casida, M. E.; Daul, C.; Goursot, A.; Koester, A.; Pettersson, L.; Proynov, E.; St-Amant, A.; Salahub, D. R.; Duarte, H.; Godbout, N.; Guan, J.; Jamorski, C.; Leboeuf, M.; Malkin, V.; Malkina, O.; Sim, F.; Vela, A. deMon,KS3, 1996 10. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; J. A. Montgomery, J.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; G. Liu; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; A1-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; C. Gonzalez; Head-Gordon, M.; E. S. Replogle, a. J. A. P. Gaussian98,Revision A.9, Gaussian, lnc., Pittsburgh PA: 1998 11. Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. 12. Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. 13. Becke, A. D. Phys. Rev. A 1988, 38, 3098. 14. Perdew, J. P. Phys. Rev. B 1986, 33, 8822. 15. Lee, C.; Yan, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. 16. Becke, A. D. Jr. Chem. Phys. 1993, 98, 5648. 17. Cerius2. Molecular modeling software for materials research, Accelerys, Biosym Technologies, San Diego USA: 1993 18. Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. 19. Munzarova, M.; Kaupp, M. J. Phys. Chem. A 1999, 103, 9966. 20. Pierloot, K.; Delabie, A.; Groothaert, M. H.; Schoonheydt, R. A. PCCP 2001, 3, 2174. 21. Maxwell, I. E.; de Boer, J. J. J. Phys. Chem. 1975, 79, 1874. 22. Carl, P. J.; Larsen, S. C. J. Phys. Chem. B 2000, 104, 6568. 23. Levi, Z.; Matar, K.; Raitsimring, A. M.; Goldfarb, D. Pure and AppL Chem. 1992, 64, 799. 24. Nacache, C.; Ben Taarit, Y. Chem. Phys. Letters 1971, 11, 11. 25. Berthomieu, D.; Krishnamurty, S.; Coq, B.; Delahay, G.; Goursot, A. J. Phys. Chem. B 2001, 105, 1149.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

M o l e c u l a r M o d e l i n g : a c o m p l e m e n t to e x p e r i m e n t materials u s e d in separation t e c h n o l o g i e s b y a d s o r p t i o n

1907

for

designing

porous

S. Girard, b C. Mellot-Draznieks, a G. F6rey, a and P. Pullumbi b Institut Lavoisier (IREM), UMR CNRS 8637, Universit6 de Versailles, 45 avenue des EtatsUnis, 78035 Versailles cedex, France

a

b Air Liquide, Centre de Recherche Claude-Delorme, B. P. 126, Les-Loges-en-Josas, 78354, Jouy-en-Josas Cedex, France*

In studies of complex molecular systems like zeolites or other inorganic solids, experimental studies have preceded by far the theoretical ones. During the last decade, computational chemistry has had a favorable impact almost in all branches of crystalline microporous research ranging from phase determination to structural characterization and property prediction. An important effort has been focused on developing simulation tools to describe thermodynamic and transport properties of confined fluids in the micropores together with a realistic representation of the catalytic phenomena. In this work we show how molecular modeling coupled to experiment can be used to select/design novel adsorbent materials with better separation properties for given gas mixtures. We illustrate how various computational approaches may be combined to cover major aspects of inorganic materials: i) computational design of new inorganic structures, using the AASBU method (Automated Assembly of Secondary Building Units) with the generation of virtual libraries of crystal structures; ii) lattice energies minimizations in order to anticipate stabilities of hypothetical crystal structures built of pre-defined SBUs; iii) anticipation of calcined crystal structures upon template extraction from existing as-synthesized structures; iv) finally, Grand Canonical Monte Carlo (GCMC) calculations used to correlate microscopic features of the adsorbent/sorbate system to macroscopic properties of interest, such as adsorption isotherms and isosteric heats used in engineering process optimizations.

1. INTRODUCTION Air separation for oxygen production is an important operation in chemical processing industry as well as in energy conversion applications. Classical cryogenic air separation is gradually giving way to new technologies involving either Pressure Swing Adsorption (PSA), Vacuum Swing Adsorption (VSA) or membrane permeation. PSA and VSA technologies have become increasingly competitive and are already adopted for accomplishing small-tomedium scale oxygen production for a variety of applications in steel industry, enhanced combustion, soil and wastewater cleanup. In addition to the operating parameters of the

1908 process itself, one of the important factors influencing the performance of the production unit is the adsorption properties of the adsorbent material. The structure of the adsorbent strongly determines the shape of adsorption isotherms and has a direct impact on the capital and power consumption costs, which are the main components of the overall costs of the produced gases. For this reason the understanding of the dependence of adsorption isotherms upon the microscopic features of the adsorbent is of prime importance. The research for new adsorbents with improved selectivity and stability having more favorable geometries has been the main factor of cost reduction in separation technologies by adsorption during the last decades. The introduction of a new adsorbent material needs the re-design of the process in order to determine optimal process cycles. From this point of view the approach in which molecular modeling is used prior to experiment for generating and pre-selecting novel "virtual" adsorbent structures based on estimates of their respective adsorption properties, followed by process optimization simulations, rather than relying solely on costly experimental programs involving synthesis, structural characterization and performance assessment, can effectively reduce the cost of the research for novel adsorption-based separation processes. Three-dimensional (3D) frameworks linked by T-O-T bonds, where T is an atom in tetrahedral coordination (Si, A1, B, Ga, P, etc.) represent interesting alternatives to experimentally know ones. Molecular simulation of silicates and substituted silicate frameworks ("tectosilicates") has become amply adequate to examine possible structures of potential interest [ 1]. In this work, the generation of virtual libraries of inorganic structures using pre-defined secondary building units (SBUs) has been carried out by using the AASBU method (Automated Assembly of Secondary Building Units) [2]. Its applicability to solidstate inorganic chemistry structure prediction has been demonstrated using simple building units from known families of inorganic structures [2-4]. Refining of virtual crystal structures by minimization methods is now a routine matter, limited only by the availability of appropriate interatomic potentials. Calculations were carried out using the GULP package (General Utility Lattice Program) [5]. Unfortunately, non-empirical calculations are currently computationally too demanding to be used routinely for lattice minimizations, especially in the case of low-symmetry solids with large unit cells. Relative stabilities of minimized structures showing a clear stability to density trade-off relationship are given. One of the most powerful concepts in the synthesis of complex solids is the "template" or Structure-Directing Agent (SDA) approach. An organic molecule is used to imprint certain structural features to the solid under construction. The choice of the SDA is crucial for the synthesis of targeted open-framework architectures. It is incorporated in the final structure, compensating charges and tailoring the size and shape of micropores. The non-covalent interactions heavily control the resulting structure as in the case of the mesoporous materials which are formed in a cooperative assembly process from organic surfactants and inorganic building blocks that crosslink during synthesis [6]. The removal of the template is often a critical step and the production of the related stable open-framework structure cannot be easily anticipated. In this work we will outline the computational methodology that has very recently [7-9] been adopted in order to investigate the thermal stability upon calcination treatment anticipating the crystal structure of the related open-framework template-free material. The next step is the prediction of adsorption isotherms, heats of adsorption and Henry constants for the predicted structure in

1909 order to evaluate its performance prior to synthesis and template removal. Monte Carlo simulations are appropriate for correlating microscopic features of the adsorbent/sorbate system to macroscopic properties of interest to process engineers. Adsorption isotherms and isosteric heats of adsorption are predicted via calculations carried out in the Grand Canonical (GC) statistical ensemble in which the chemical potential, the volume and the temperature are kept fixed. For Henry's constants the simulations are performed in the Canonical (C) ensemble where the number of adsorbed molecules, the volume and the temperature are held fixed. It is to notice that the outlined methodology illustrated with different examples is not exhaustive in its search of new structures and has not been fully validated. Its interest is, however, related to its potential to rationally focus an experimental program by minimizing the range of exploring trails. 2 T H E O R Y AND C O M P U T A T I O N A L DETAILS 2.1. Predicting new inorganic structures : the A A S B U method The first step of the AASBU method is the choice of the Secondary Building Unit. In this study, the sodalite or 13-cage (truncated octahedron 4668) was extracted from the experimental sodalite structure [ 10] as well as the double-4-ring (D4R) building unit, which was extracted from the experimental cloverite [10]. Both units are shown in Figure 1. The ligand or coordinating atoms (L) are considered as linkage points in all the following steps. The interatomic potentials define the rules that control the possible assembly of the SBUs, treated as rigid bodies during the simulation, with the inter-SBU interactions parameterized on an atom-atom basis by a Lennard-Jones expression for the energy of interaction between pairs of atoms i and j:

go ~_ ~.ij [ ( f i~ /,.i;1 )12 _ 2 ( ri] F/;1 )61

(1)

The parameterized force field has a highly attractive potential well, with a minimum at a very short Li...Lj separation distance in order to glue together the SBUs at the linkage points during subsequent simulation steps. A repulsive potential between Mi...Mj pairs prevents SBUs from overlapping each other together with a small attractive potential between Li...Mj pairs used to discourage undesirable local minima corresponding to proximate but unconnected SBUs. The repulsive potential between Li...Bj pairs prevents a SBU to link to an

Figure 1. The sodalite and D4R cages. Metal atoms (M) are shown in gray, bridging atoms (B) are shown in white cylinders and ligand atoms (L) are shown in black balls. Only ligand atoms are allowed to link to other ligand atoms of a different cage.

1910 atom, which is already completely connected inside its own SBU. The force field parameters have been reported recently in the literature [2,3] together with the definition of cost function or "energy" for a given configuration of SBUs in a unit-cell. It is calculated as the sum over all SBUs in the Unit cell of the Lennard-Jones terms involving Li...Lj, Mi...Mj, Li...Mj and Li...Bj pairs, with atoms i and j belonging to two different cages: (2)

E total -" Z (E L..L + EM ...M + E L..M + E L..B ) SBUs

The magnitude of the cost function provides an estimate of the degree of connectivity of a given arrangement of SBUs. Starting with the selected SBU and applying the force field and going through the steps defined in [2,3] virtual periodic structures are generated.

2.2. Lattice energy minimization and structure prediction Energy minimizations are carried out in order to anticipate both the stability of the generated virtual structures and the crystal structure of the calcined materials starting from the knowledge of the as-synthesized structure, using the simple ionic shell model, developed by Gale and Henson for A1POs [11] and the lattice energy minimization code GULP [4]. The robustness of their formal charge shell model forcefield has been demonstrated through the accurate reproduction of experimentally determined structures of A1POs [12] together with estimations of their relative framework stabilities that are consistent with thermodynamic data. We have recently extended this type of approach to the realm of gallophosphates [ 13,14]. The total lattice energy of a structure is given by: Es,r --" ~

( qiqj k ( rc . . . . . hell) 2 + ~ { A ij e x p ( - r ij ) - C ijrij - 6 -+i i ~ j [. IOij r'ij

-Jr- ~ Kijk(Oijk -- 00) 2 i* j:xk

(3)

where qi and qj refer to the charges of the ions, and A O, Pij and C 0 are short-range potential parameters defined in [5]. 0o is the equilibrium tetrahedral bond angle, rij denotes the interatomic distance. The first sum in (3) counts the self-energy of polarizable ions arising from their deformation using the shell model defined in [ 15] in which an ion is represented by a core and a shell coupled by a harmonic spring; the second sum includes pair interactions (Coulombic term together with a short range 6-exp Buckingham potential). The third sum consists of three-body angle-bending terms. Coulombic energy is calculated using the Ewald summation; Buckingham potentials were summed over all interatomic pairs; three-body potentials were calculated only between strong bonds.

2.3. Monte Carlo Simulations to predict cations configuration and adsorption properties The simulation of adsorption properties of microporous adsorbents using GCMC calculations needs as input the structural model of the adsorbent and the force field describing sorbate/sorbent and sorbate/sorbate interactions. In the case of electrically non neutral frameworks, insertion of the required complementary cations may be carried out via a Monte Carlo packing procedure followed by structure optimization [ 16], or even via (N, V,T) Monte Carlo simulations [17]. In the Monte Carlo packing procedure only short-range non-bonded terms are used to gauge the viability of each new cation position introduced.

1911 Regarding the simulation of adsorption properties in zeolites, the interaction potential is the most important ingredient. We have adopted a simplified interaction potential [ 18] including only a dispersive-repulsive short-range potential, represented by a Lennard Jones 6-12 potential combined with electrostatic interactions between partial charges on the adsorbent and guest atoms, or more generally, multipole-multipole interactions according to:

i [Ai

9

j

rij 12

"i qiq t

rij 6

(4)

rij

where Aij is the repulsion constant and Bij the dispersion constant and qi the point partial charges located at the atomic positions of the adsorbent and sorbate molecules. The LJ potential parameters for the adsorbate-zeolite and adsorbate-adsorbate interactions as well as the magnitude of the partial charges localized at the atomic positions of the adsorbent/sorbate atoms has been detailed in [ 19]. Simulations were performed using Cerius 2 suite of softwares. [2O] 3. RESULTS AND DISCUSSION

The AASBU method has been systematically used for screening over the first 70 (resp. over 20) space groups using one sodalite (resp. one D4R) cage per asymmetric-unit for each calculation. The results obtained have recently been reported in the literature [3] and apparently new structures have been discerned. Several known structures have been obtained: for example, sodalite (SOD) was obtained using the sodalite cage as SBU, ACO and AFY were obtained using D4R cage, while LTA structure was obtained either with sodalite or D4R SBUs. The final symmetry was found to be Im-3m for SOD and ACO, P-3 lm for AFY and Pm-3m for LTA structure, in agreement with the symmetry given in [10]. Simulations in progress are being carried out with the sodalite cage to generate FAU and EMT topologies. Table 1. Predicted parameters for hypothetical models generated using the sodalite (M1-M5) or D4R (T1-T5) cages as a SBU.

Model M1 M2 M3 M4 M5 T1 T2 T3 T4 T5

Space group in

Unit cell parameters

Final space

calculation

a/~,

b/~

c/A

13/o

group

C2/c

Ama2

21.55 12.54 12.80 12.79 12.80

12.28 12.54 12.80 12.79 12.80

20.25 60.92 20.07 10.05 40.17

90.96 90.00 90.00 90.00 90.00

R-3m P63/mmc P-3ml P63/mmc

Cm P -1 P na21 P -1 P na21

6.69 9.30 17.71 19.69 12.67

13.99 14.20 10.05 9.93 19.56

28.21 7.17 8.19 11.25 8.15

90.00 90.00 90.00 114.74 90.00

C2 Pm P2/c

C2/m

Fmmm Cmmm Pnma C2/m

Pna21

1912 It is to notice that not only the space groups but also other predicted parameters (unit cell parameters, atomic coordinates) coincide well with experimentally observed ones. Space groups and cell parameters of some of the unknown generated structures using the sodalite (resp. D4R) cage as a SBU are reported as models M1-M5 (resp. T1-T5) in Table 1. It is to notice that the connection of the SBU cages produces known [21 ] or new cages in each of the structures. In order to evaluate the relative stabilities of the all-silica frameworks their lattice energies per tetrahedral unit are compared to the energy of the SiO2 quartz. Accordingly with what has been reported for known silicas [22], the calculated lattice energies of the M1-M5 structures correlate well with their densities. An as-synthesized structure usually contains templating agents, water molecules, bridging hydroxy groups as well as fluorine atoms which are included in the framework. The latter species lead to an increase of the coordination number of A1 atoms from 4 to 5 or 6. In the following we briefly describe how one can computationally proceed to the prediction of the calcined form. The method consists in removing from the experimentally-defined assynthesized structure all the species known to be removed upon calcination, thus generating a highly distorted initial structure. Upon constant pressure lattice energy minimization in the space group of the original structure, the distorted framework converges into a zeolitic topology. Its energy is then compared to that of existing structures of the same class to evaluate its relative stability. To illustrate this approach, we report here the prediction of the calcined form of the as-synthesized aluminophosphate MIL-34, [A14(POn)4OH'CaH10N].Once the crystal structure of the as-synthesized MIL-34 was determined, the calcined template-free one, namely A1PO4, could be anticipated using appropriate interatomic potentials [8], before it was obtained experimentally. The predicted open-framework compound showed an unknown zeolitic topology. The reliability of the simulation allowed the Rietveld refinement of the powder pattern of the calcined phase, validating this approach (Fig. 3). This simulation method has also been successfully applied to a series of gallophosphates [ 14].

Figure 3. a) Predicted structure of calcined MIL-34 by lattice energy minimization along [ 110]. b) Comparison between experimental (up) and simulated (down) XRD powder pattern

1913 The removal of the SDA without breaking down the framework is not always an easy task because the stability of the as-synthesized structure often depends on the stabilizing templateframework interactions as well as on the experimental procedure employed during calcinations. For these reasons it is sometimes very helpful to estimate the adsorption properties of the final material prior to its calcination. Here we report GCMC simulations to predict N2 and 02 adsorption in Lithium and Sodium Low Silica X-zeolite (LSX) but the approach is quite general and could be applied to any kind of crystalline porous adsorbent. Typical runs of 2* 10 6 Monte Carlo steps from which the first 200000 are used for equilibration and not included in the averaging are sufficient to sample configuration space. The predicted adsorption isotherms of single component simulations in NaLSX and LiLSX are reported in Figure 4 and compared to their experimental counterparts. The general trends are well reproduced with a systematic tendency to overestimate the adsorption of nitrogen for low loadings and underestimate that of oxygen. On the contrary the N2 loading is underestimated at higher pressures. This indicates that the balance between van der Waals and Coulombic interactions could still be improved by slightly modifying the adopted point charges at atomic positions. It is possible in principle, to improve the description of sorbent-sorbate interactions by using sophisticated expressions of the potential functions with higher order dispersion and induction terms recently discussed in ref. [23] but it still requires the calculation of the electrostatic field inside the zeolite, which in its turn depends on the choice of the point charges assigned to the atoms. It seems [24] that parameterization of polarization contribution to the potential energy by choosing partial charges cannot be carried out without resorting to a fitting procedure of some type.

40.00 l D "c" 30.00 -~ 0

z m 20.00

o,,, /,

C

O10"007/

g"

I~ ="

D' ' ~

" "~

--

LiLSX_O2_C NaLSX_N2_C

o

NaLSX_O2_C

0.00 ~ 1.00

LiLSX_N2_E .. LiLSX_O2_E

------- NaLSX_O2_E

o 0.50

a O

NaLSX_N2_E

/% A

0.00

LiLSX_N2_C

----O

DI I

[]

1.50

Pressure (bar)

Figure 4. Predicted (C) and experimental (E) single component isotherms for nitrogen and oxygen in NaLSX and LiLSX zeolites.

1914

4. C O N C L U S I O N The rate of scientific discovery of novel superior adsorbents could be significantly accelerated through a judicious combination of experiment with computation strategies. The successful d e v e l o p m e n t of novel materials lies in their rational design and can be achieved through an understanding of fundamental interactions at the molecular level. W e have outlined a multifaceted modelling approach that although far from being complete can help in focussing the experimental effort in the search of new adsorbents. In the particular field of the separation of N2 and O2 in zeolites, the molecular simulation approach enhances the fundamental understanding of the basic microscopic p h e n o m e n a and is appropriate for establishing correlations between the microscopic features of the sorbent/sorbate systems and their macroscopic properties such as isotherms and adsorption heats. It can be successfully used to c o m p l e m e n t experimental studies.

REFERENCES 1.

M.B. Boisen Jr, G. V. Gibbs, M. O'Keefe and K. L. Bartelmehs, Microporous Mesoporous Materials, 29

2.

C. Mellot Draznieks, J. M. Newsam, A. M. Gorman, C. M. Freeman and G. F6rey, Angew. Chem. Int. Ed.,

(1999) 219-266. 39 (2000) 2270-2275. 3.

S. Girard, P. Pullumbi, C. Mellot-Draznieks, G. F6rey, Stud. Surf. Sci. Catal., 135 (2001) 254.

4.

C. Mellot-Draznieks, S. Girard, G. F6rey, J. C. Sch6n, Z. Cancarevic, M. Jansen, Chem., Eur. J. (in press).

5.

J.D. Gale, J. Chem. Soc., Faraday Trans., 93 (1997) 629.

6.

X.S. Zhao, G. Q. Lu and G. J. Millar, Industrial Engineering Chemical Research, 35 (1996) 2075-2090.

7.

S. Girard, C. Mellot-Draznieks, J. D. Gale and G. F6rey, Chem. Commun., (2000) 1161-62.

8.

T. Loiseau, C. Mellot-Draznieks, C. Sassoye, S. Girard, N. Guillou, C. Huguenard, F. Taulelle and G. F6rey, J. Am. Chem. Soc., 123 (2001) 9642-9651.

9.

S. Girard, A. Tuel, C. Mellot-Draznieks and G. F6rey, Angew. Chem. (in press).

10. W.M. Meier, D.H. Olson, Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier, London, (1996). 11. J.D. Gale and N. J. Henson, J. Chem. Soc. Faraday Trans., 90 (1994) 3175-3179. 12. N.J. Henson, A. K. Cheetham and J. D. Gale, Chem. Mater., 8 (1996) 664-670. 13. S. Girard, J. D. Gale, C. Mellot-Draznieks, G. F6rey, Chem. Mater. 13 (2001), 1732. 14. S. Girard, J. D. Gale, C. Mellot-Draznieks and G. F6rey, J. Am. Chem. Soc., 124 (2002) 1040-1051. 15. B.G. Dick and A. W. Overhauser, Physical Review, 112 (1958) 90-103. 16. A.M. Gorman, C. M. Freeman, C. M. K61mel and J. M. Newsam, Faraday Discuss., 106 (1997) 489-494. 17. C. Mellot-Draznieks, S. Buttefey, A. Boutin, A. H. Fuchs, Chem. Commun. (2001) 2200-2201. S. Buttefey, A. Boutin, C. Mellot-Draznieks, A. H. Fuchs, J. Phys. Chem. B, 105 (2001) 9569-9575. 18. K. Watanabe, N. Austin and M. R. Stapleton, Molecular Simulation, 15 (1995) 197-221. 19. J. Ligni~res and P. Pullumbi, Fundamentals of adsorption, 6 (1998) 719-725. 20. Cerius2, Accelrys Inc. San Diego. 21. J.V. Smith, Chem. Rev., 88 (1988) 149-182. 22. N. Henson, A. K. Cheetham and J. D. Gale, Chem. Mater., 6 (1994) 1647-1650. 23. A.H. Fuchs and A. K. Cheetham, J. Phys. Chem. B, 105 (2001) 7375-7383. 24. C.J. Jameson, H.M. Lim, A.K. Jameson, Solid State NMR, 9 (1997), 277-301.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

NMR-

1915

crystallographic studies o f almninophosphate AIPO4-40

C. M. Morais a'b, C. Fernandez a, V. Montouillout a, F. Taulelle c and J. Rocha b a Laboratoire Catalyse et Spectrochimie, CNRS UMR 6506, ISMRA, Universit6 de Caen Basse-Normandie, 14050 Caen, France b Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal CRMN et Chimie du Solide, CNRS FRE 2423, Tectonique Mol6culaire du Solide, University Louis Pasteur, 67070 Strasbourg, France

1. INTRODUCTION In solid-state NMR of inorganic materials, double-resonance (or ~H triple-resonance) experiments often involve a half-integer quadrupolar nucleus that has the disadvantage of exhibiting second-order anisotropic broadening. However, the introduction of twodimensional multiple-quantum magic-angle spinning (MQMAS) has been the source of a tremendous improvement in solid-state NMR of half-integer quadrupolar nuclei. By correlating triple- (or higher) quantum with observable single-quantum coherences Frydman et al. demonstrated that it is possible to remove the second-order quadrupolar interaction leading to high-resolution spectra [ 1]. Taking advantage of the highly resolved dimension of the experiment, double resonance schemes such as CP [2], REDOR [3] or HETCOR [4,5] have been proposed to obtain a better insight into the connectivity of solids. Thus, MQMAS has become an important part of new pulse sequences in solid-state NMR of quadrupolar nuclei. It is also becoming widely used to solve structural problems in applied materials. More recently, Delevoye et al. [6] have demonstrated that, in aluminophosphates, the resolution in the spin 89 dimension may be improved by decoupling the quadrupolar nucleus, while acquiring the spin 89free induction decay. Microporous aluminophosphate AIPO4-40 was initially thought to possess the structure of silico-aluminophosphate SAPO-40 [7]. However, because the latter contains four distinct 31p sites and the 31p MAS NMR spectrum of AlPO4--40 exhibits more than four resonances [8] the two materials cannot have exactly the same structure. Indeed, a subsequent Rietveld refinement of powder X-ray diffraction data found that although the framework topology of both materials is the same (AFR) the AIPO4-40 symmetry is lower (space group Pc2~n) [9]. The reason for the lower symmetry is the presence of an ordered arrangement of hydroxyl groups bridging between framework A1 atoms across a fourmembered ring. As a result, and according to the proposed structure, there are eight 31p and eight 27A1 (two five-coordinated and six four-coordinated) atoms in the asymmetric unit. In the present paper we show that this recent structure refinement does not fully agree with the NMR evidence.

1916 2. EXPERIMENTAL SECTION All NMR experiments were performed on a Bruker Avance 400 (9.4T wide bore magnet) spectrometer operating at 104.3 MHz and 162.1 MHz for 27A1 and 31p nuclei, respectively. 4 mm double-resonance and triple-resonance probes were used, employing MAS rates of 10 to 12.5 kHz. 1 kW high-power linear amplifier was used for 27A1 multiple-quantum excitation and decoupling, whereas a 500 W linear amplifier enabled 31p excitation. The 4 mm double-bearing triple-channel Bruker probe allowed for high-power decoupling radio-frequency (RF) fields beyond 60 kHz on the 27A1 channel. Bruker true band-pass filters with more than 90 dB of rejection on 27A1 and 31p frequencies and with less than 0.5 dB of insertion loss were used on the respective channels. Pure absorption mode 3Q- and 5Q-MAS NMR spectra were obtained by using the Z-filter sequence [10] and hypercomplex acquisition. The 27Al_31p 3Q-HETCOR spectrum was recorded as described in [5]. Except for the 5Q-MAS NMR spectrum, the acquisition of the twodimensional data sets was performed in a rotor-synchronized fashion by advancing the evolution time tl in increments equal to the rotor period. The relevant experimental parameters are given in the figure captions. All 31p and 27A1 chemical shifts are plotted in ppm relatively to aqueous 85% H3PO4 and Al(n20)63+, respectively. Drs J. Lourengo and M. F. Ribeiro, Instituto Superior Tecnico, Lisbon, Portugal kindly provided the A1PO4-40 sample that is the same sample as studied in ref. [8]. 3. RESULTS AND DISCUSSION

3.1.31p NMR spectra In a previous report, Rocha et al. have shown that the deconvolution of the non-

decoupled 31p spectrum of AIPO4-40 requires the presence of at least nine lines with intensity ratios not very different from one [8]. However, this analysis was no trivial due to the broadening of the lines that often affects the phosphorous spectra of aluminophosphates. We have recently shown that 27A1 decoupling may be used to remove residual J-coupling interactions between 31p and 27A1 in microporous aluminophosphates [6]. Figure la and lb show the A1PO4-40 31p MAS NMR spectra recorded, respectively, without and with 27A1 decoupling during the acquisition. The latter is a highly resolved spectrum whose deconvolution requires at least sixteen peaks (Figure l c). The chemical shifts and relative intensities obtained from this deconvolution are collected in table 1. 3.2. 27A! NMR spectra As the conventional 27A1MAS spectrum of the AIPO4-40 is not resolved, it is necessary to use MQMAS. The 27A1 3Q and 5QMAS NMR spectra reveal the presence of at least fifteen resonances (Fig 2a and b). The MQMAS method not only produces highly resolved 27A1 NMP, spectra, making easy the determination of the aluminum coordination, but also allows the extraction of additional parameters such as the quadrupolar coupling constants, the isotropic chemical shifts, and the access to the distribution of these parameters.

1917 Table 1. 31p Chemical shifts and relative intensities resulting from the deconvolution of the AIPO4-40 31p MAS NMR spectrum (see Figure 1c). Peak

8 31p (ppm) -13.7 -14.4 -15.3 -16.2 -17.1 -18.0 -18.8 -20.2 -20.8 -21.2 -22.5 -24.8 -26.5 -28.4 -29.6 -30.0

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

13]p (%) 3.7 5.2 8.5 2.6 2.3 7.9 2.8 2.8 3.2 5.8 5.6 12.5 11.9 19.4 3.6 2.3

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b)

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.

.

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.

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.

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Figure 1. alp MAS NMR spectra of AIPO4-40 recorded with (a) no decoupling, low-power decoupfing; (c) simulated 2 7 Al-decoupled spectrum. A 2.5 l~S (30 ~ pulse was used. Decoupling RF field was experimentally optimized to 12 kHz. were acquired with 16 transients. The recvclint~ delay was 80 s and the M_AS rate

(b) with 27A1 31p excitation Both spectra was 10 kHz.

1918

ppm

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30 40

60 50

40

,

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.

.

.

.

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.

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,

.

.

.

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.

.

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.

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............................

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g~ ....

50I

'1

I

I

I'

50

40

30

20

10

plxn

F2 Figure 2 .

27A13Q

(a) and 5Q (b) MAS N M R spectra of aluminophosphate A1PO4-40.

1919 MQMAS sequence strongly depends on the quadrupolar coupling. Thus, MQMAS is not directly quantitative. However, the quantification of aluminum atoms can take full benefit of the high-resolution of the MQMAS spectra when the two-dimensional spectra are analyzed simultaneously with conventional MAS NMR spectra. We have developed a simulation program (MASAI) that allows iterative fitting of both the MAS and twodimensional MQMAS spectra, taking into account the distribution of interaction parameters [ 11,12]. The results of the best fitting of both MAS and MQMAS spectra are reported in Table 2. Relative intensities, quadrupolar coupling constants and isotropic chemical shift are given for all crystallographic A1 sites. 8 lines from 37.9 to 46.3 ppm are attributed to the tetra-coordinated aluminum. The 7 lines from 19.4 and 22.6 ppm are attributed to the penta-coordinated aluminum. As only fifteen sites are detected for aluminum instead of the sixteen sites found for phosphorous it is likely that the intensity of one of the lines is too low to be detected. 3.3. MQHETCOR NMR correlation spectra 2D MQHETCOR may be used to help the assigmnent of the 27A1 and 31p AIPO4-40 MAS NMR resonances. In this sequence, the MQMAS experiment is followed by a CP transfer in order to correlate different nuclei through dipolar couplings. Work reported recently, using the MQHETCOR sequence, display heteronuclear 27AI/31p spectra correlating neighboring nuclei [5].

Table 2.

27A1 isotropic chemical shifts, relative intensities and quadrupolar coupling constants

resulting from the MASAI refinement of AIPO4-40MAS and MQMAS spectra. Peak 1 2 3 4 5 6 7

5~o 27A1(ppm) 19.4 19.6 20.2 20.9 22.1 22.2 22.6

127A1 (%) 4.4 2.2 4.4 5.7 6.5 6.5 6.5

CQ (MHz) 4.31 1.55 2.37 4.85 2.38 3.09 2.01

8 9 10 11 12 13 14 15

37.9 39.2 39.6 40.6 43.9 44.0 45.9 46.3

6.5 6.8 10.6 5.0 8.7 10.9 10.9 4.4

2.24 2.28 3.67 2.33 1.65 2.57 3.24 4.02

1920 Obviously, optimal resolution in both dimensions is essential, in order to reach an unequivocal structure determination. While multiple-quantum excitation provides a highresolution spectrum in the 27A1 dimension, high-frequency MAS is usually sufficient to remove the dominant heteronuclear dipolar interaction in the 31p acquisition domain. Nevertheless, the 31p MQHETCOR resolution is greatly improved by using 1H/27A1 double-resonance decoupling. Figure 3 shows the 2D MQHETCOR spectrum of AIPO4-40 using the pulse sequence described by Fernandez et al. [5]. However, the correlation spectra show that the study of the AIPO4-40 A1-P connectMty is not straightforward due to the large number of lines present. A structural interpretation of these results can however be established by using the NMR crystallography technique and this work is in progress.

ppm_

!

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20

i

25 30

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45 1 50 55 60

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-5

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31p Figure 3. 27A1-31pcorrelation spectrum of AIPO4-40 obtained using the cross-polarization 3Q-HETCOR method. The 27A1-31pcorrelation (3Q-HETCOR) spectrum was recorded using 27A1decoupling during the acquisition.

1921 4. CONCLUSION In this communication, we show that the NMR evidence is not yet consistent with the recently proposed structure refinement for the AIPO4-40 aluminophosphate. Indeed, we detect at least sixteen 31p sites and ill'teen 27A1 sites. However, their attribution is not trivial and needs additional work. 31p31p correlation spectrum using double-quantum NMR spectroscopy [ 13] with 27A1 decoupling may help. A structural interpretation of these results by using the NMR crystallography is in progress. REFERENCES

1. L. Frydman and J. S. Harwood, J. Am. Chem. Soc. 117 (1995) 5367 2. M. Pruski, D. Lang, C. Fernandez and J-P. Amoureux, Solid State Nucl. Magn. Reson. 7 (1997) 327 3. C. Fernandez, D. Lang, J-P. Amoureux and M. Pruski, J. Am. Chem. Soc. 120 (1998) 2672 4. S.E. Ashbrook and S. Wimperis, J. Magn. Reson, 147 (2000) 238 5. C.Fernandez, C.Morais, J.Rocha and M. Pruski, Solid State Nucl. Magn. Reson., in press 6. L. Delevoye, C. Fernandez, C. Morais, V. Montouillout and J. Rocha, submitted 7. N. Dumont, Z. Gabelica, E. G. Derouane and L. B. McCusker, Microporous Mater. 1 (1993) 149. 8. J. Rocha, J. P. Louren~}o, M. F. Ribeiro, C. Fernandez and J.-P. Amoureux, Zeolites, 19 (1997) 156. 9. V. Ramaswamy, L. B. McCusker and Ch. Baerlocher, Microporous Mater. 31 (1999) 1. 10. J.-P. Amoureux, C. Fernandez, S. Steuemagel, J. Magn. Reson. 123 (1996) 116. 11. A.A. Quoineaud, V. MontouiUout, C. Fernandez, S. Gautier, S. Lacombe, in preparation 12. C. Fernandez, A.A. Quoineaud, V. MontouiUout, S. Gautier, S. Lacombe, Proceedings of the 13th International Zeolite Conference, Montpellier, July 2001, ELSEVIER, eds. A. Galarneau et al. 13. J.Gottwald, D. E. Demco, R. Graf, H.W. Spiess, Chem. Phys. Lett. 243 (1995) 314.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1923

Structural characterization of borosilicates synthesized in the presence of ethylenediamine Stefano Zanardi a, Alberto Alberti a, Roberto Millini b, Giuseppe Bellussi b, Giovanni Peregob a

Dip. di Scienze della Terra, Univ. di Ferrara, C.so Ercole I d'Este 32, 1-44100 Ferrara (Italy)

b EniTecnologie S.p.A., Via. F. Maritano 26, 1-20097 San Donato Milanese (MI - Italy) Hydrothermal treatment of an aqueous solution containing tetramethoxysilane, boric acid and ethylenediamine (EN) led to the crystallization of B-MFI (BOR-C) accompanied by small amounts of B-ferrierite. Single crystals X-ray diffraction studies were performed on a small prismatic crystal of BOR-C, unit cell dimensions a = 19.869(2), b - 19.661(3), c = 13.207(2) A, space group Pnma, and on a platy FER-type crystal with unit cell dimensions a = 18.512(2), b = 13.865(1), c - 7.317(1) A, space group Immm. Different routes (unit cell volume, T-O distances, refinement of B and Si scattering curves) reveal the incorporation of boron in the framework corresponding to 14+1 and 6.5+0.5 B atoms/unit cell in MFI and FER structures, respectively. Three different EN sites in BOR-C and two different EN sites in BFER (all with partial occupancy) were found; the total number of EN, almost one half than boron incorporated in the framework, confirms that EN should be in dicationic form to compensate the negative charge of the framework. Molecular modeling simulations confirmed that those found in the refinements of the X-ray diffraction data are really the most preferred location sites for EN molecules in the two porous systems. 1. INTRODUCTION Crystallization of high-silica zeolites strongly relies with the use of organic additives, usually quaternary ammonium cations, which act as structure directing agents (SDA' s) or, in a few cases, as real templates [1 ]. The central role of the organic additives in determining the nature and the porous structure of the crystalline products has been assessed by several experimental [2,3] and theoretical [4-7] studies, leading to the conclusion that the crystallization of novel microporous structures could be achieved only through the use of increasingly large and complex cations. Indeed, several new zeolites have recently been synthesized by using this approach [3]. The use of charged organic molecules, however, is not mandatory for the synthesis of zeolites, since they can be crystallized even in the presence of neutral molecules such as amines, which favor the formation of some interesting structures such as ERB-1 [8], MCM-22 [9] (MWW), MCM-35 (MTF) [10], as well as of several chlatrasils [2]. In spite of that, they received less attention in the literature with respect to the organic cations and their role in zeolite nucleation and growth was not considered at least until the recent studies of Rollmann et al., who related the structure of small amines to the zeolite products obtained [ 11,12]. These

1924 authors found that small amines really play a pore-stabilizing role and, in general, the zeolite predicted to be better stabilized by the non-bonded amine-framework interactions is effectively obtained. Another interesting feature concerns the Na/A1 molar ratio in the products, often lower than 1. That means that, part of the amine molecules should be protonated in order to compensate the negative framework charge and that in spite of the strong basicity of the synthesis gel (pH >12). Among the small amines considered, ethylenediamine (EN) is in our opinion one of the most interesting because it is characterized by the lowest C/N ratio. According to Rollmann et al., EN favors the crystallization of ZSM22 (TON), ZSM-23 (MTT) or ZSM-48 with SIO2/A1203 = 200 [11], while upon decreasing the SIO2/A1203 ratio to 2 0 - 40, ZSM-35 (FER) and ZSM-5 (MFI) are obtained [12]. Controversial results were reported for the borosilicate system. According to Taramasso et al., B-containing MFI (BOR-C) with an estimated B content of 9.8 atom/unit cell (the highest level of B incorporation in MFI actually reported) can be crystallized from alkali-free reaction mixtures containing EN [13]. On the contrary, Gies et al. reported the crystallization of A1free FER from a synthesis mixture containing Si(OCH3)4, H3BO3, EN and H20, and provided evidences supporting the conclusion that an EN-boric acid complex acts as a template for the crystallization of FER; the same authors concluded that B is a non-framework constituent, though some framework incorporation of B was not excluded [ 14,15]. With the aim to clarify the role of EN in the crystallization of these phases and to evaluate their real framework composition, we have undertaken the structural characterization of assynthesized B-FER and BOR-C crystals crystallized according to the recipe reported in [14]. Moreover, location of EN molecules within the FER and MFI porous structures was examined by using molecular modeling tools and compared with the results of the structural characterization. 2. EXPERIMENTAL 2.1. Synthesis Synthesis was performed according to the recipe reported in [14]. A mixture with composition 0.2B203.SiO2.4EN.110 H20 was heated at 453 K for 56 days. A solid product was recovered, mainly constituted by BOR-C together with significant amount of a glassy phase and traces of B-FER. The overall composition of the as-synthesized material, determined by elemental analysis, was Sio.949B0.05102.0.098EN. 2.2. Characterization

Single crystal X-ray diffraction data were collected at room temperature on a BOR-C prismatic crystal (dimensions 25x25x50~tm) and on a platy FER-type crystal (dimensions 100x60x20gm), using a Nonius KappaCCD diffractometer equipped with a CCD detector; the MoKc~ radiation ()~ = 0.71069 A) was used. In the case of BOR-C crystal, three sets of frames were measured: the first set of 10 images, with a q~rotation width of 1~ and an exposure time of 360 sec. per frame was used for initial cell determination; a second set of 66 images with a q0 rotation width of 2 ~ and a third set of 16 images with a co rotation width of 2 ~ both with an exposure time 2800 sec. per frame, were used for data collection. A total of 10485 reflections were collected up to 53 ~ 20, 5455 unique with t~q = 0.304. For the B-FER crystal, four sets of frames were measured: i) the first set (10 frames), with

1925 a q~ rotation width of 1o and an exposure time of 180 sec. per frame, was used for initial cell determination; ii) one q0 rotation set (total 90 frames), and iii) two co rotation sets (total 65 frames), with a rotation width of 2 ~ and an exposure time of 1410 sec. per frame, were used for data collection. A total of 7024 reflection were collected up to 55 ~ 20, 1244 unique with R e q -- 0.062. The package DENZO-SMN [ 16] was used for the refinement of unit cell parameters and the data reduction, while the SHELX-93 [ 17] program was used for structure analysis. The Si and B content of the crystals was determined by Wavelength Dispersive Spectroscopy (WDS) using an Oxford WDS-600 spectroscope interfaced with a Jeol JSM840A scanning electron microscope. BN and SiO2 were used as reference materials.

2.3. Computational details An approach based on a combination of molecular dynamics (MD), Monte Carlo packing and energy minimization techniques was used for determining the locations and the nonbonded interactions of EN within the MFI and FER porous structures [ 18]. The simulation started with a high temperature (1000 K) molecular dynamics run (0.2 ns long, with a time step of 1 fs) on the isolated EN molecule, in order to explore its conformational space and to generate 1000 conformations (extracted from the trajectory one every 200 fs). After energy minimization, these conformations were stored in an archive file for use in the subsequent Monte Carlo packing step. 12 and 4 EN conformations were randomly extracted from the archive file were packed in the MFI and FER supercells (corresponding to one unit cell, periodic boundary conditions applied), respectively. Energy minimization was then applied in order to find the lowest energy sorption sites for the EN molecules. In both cases, 100 packed structures were generated and energy minimized. The lowest energy MFI/12EN and FER/4EN conformations were successively further refined by impulse dynamics cycles. The purely siliceous frameworks, built from crystallographic data, were kept fixed during the calculations. On the basis of the results reported by Lewis et al. [7], the electrostatic interactions between the SDA molecules and the framework atoms were neglected. All the calculations were performed with the MSI Catalysis 4.0.0 and Cerius 2 software packages [19]; molecular mechanics and dynamics calculations run by Discover [20] employed the cvffforcefield. 3. RESULTS AND DISCUSSION

3.1. Structural characterization Though the synthesis was carried out according the recipe proposed by Gies and Gunawardane [ 14], the results obtained were completely different. In fact, the solid recovered was mainly constituted by a MFI-type molecular sieve accompanied by small but significant amounts of a glassy phase. Only trace amounts of the expected FER-type molecular sieve were observed in the sample, as indicated by the presence of the weak (200) reflection located at 9.54 ~ 20 in the XRD pattern. This is quite surprising, since the only difference between the two preparations was the use of a 200 ml stainless-steel autoclave instead of a 2 ml sealed silica tube. Though no clear explanations were obtained for the unsuccessful reproduction of the synthesis of FER, it is possible that a slight variation of the composition of the reactant mixture, due to the partial dissolution of the silica tube by the alkaline solution, may play

1926 some role. Optical microscope observations revealed that the solid is mainly constituted by large prismatic crystals (BOR-C), accompanied by rare large and thin platelets (B-FER). Structure refinements of B-FER and BOR-C X-ray single crystals diffraction intensities were carried out using SHELX-93, starting from the crystallographic data reported for the orthorhombic MFI [21] and FER [ 14] structures (space group Pnma and Imrnrn, respectively). Unit cell parameters were a = 19.869(2) A, b = 19.661(3) A, c - 13.207(2) A for BOR-C, and a = 18.512(2) A, b = 13.865(1) A, c = 7.317(1) A for B-FER. The refinement converged to a final R(F) factor of 0.128 [ I>4~(I)] and 0.075 [ I>5~(I)] for BOR-C and B-FER, respectively. The complete description of the structures, including fractional atomic coordinates and occupation factors, is reported elsewhere [22]. Here we focus the attention on the main features of these structures, trying to derive information about the effects of the EN molecules on the properties of the crystalline products. To do that, we have to answer to the following questions: a) is boron really incorporated in the MFI and FER frameworks and to what extent? b) is there a correlation between the number of EN molecules packed in the pores and the number of B atoms in the framework? c) does EN play any structure directing role with respect to MFI and FER? Since the sample under investigation is constituted by three different phases (MFI, FER and an amorphous one) elemental analysis does not provide any useful information about the real framework composition of the two crystalline borosilicates. Therefore, their composition was determined with four different methods: i) by WDS analysis of the crystals; ii) using the well known equation relating the unit cell volume (Vx) of a sample containing a molar fraction x of a heteroatom M to that of the pure silica parent structure (Vsi): Vx -- Vsi - Vsi[ 1- (dM.o/dsi.o)3]x (1) where dM-o and dsi-o a r e the tetrahedral M-O and Si-O bond distances, respectively [13]. For BOR-C: dB-o = 1.46 A, dsi-o = 1.595 A [21], Vsi = 5332 A 3 [21]; for B-FER: dB-o = 1.46 A, dsi-o = 1.599 ,s [23], Vsi = 1954.3/~k3 [23]. iii) from the Si and B occupancies in the T-sites, obtained from the refinement of the scattering curves; iv) from the values of the T-O bond distances, assuming a linear variation of the average T-O bond distance derived from the refinements, dT-o (1.573 and 1.571 A, for BOR-C and B-FER, respectively) with the B content; dB-o and dsi-o as in point ii. The framework compositions derived from the applications of the methods i - iv are reported in Table 2. In both cases there is a clear evidence of a remarkable substitution of B for Si, valuable 14+1 B atoms/unite cell in the case of BOR-C, and 6.5+0.5 B atoms/unit cell in B-FER. In the case of BOR-C, the B content resulting from WDS analysis is significantly lower with respect to that derived crystallographically. The WDS analysis, however, is probably affected by the interference of the glassy material, which taking into account the overall chemical composition of the sample (see above) should have a much lower amount of B with respect to the BOR-C crystals. This discrepancy was not observed for B-FER, where a nice agreement exists between the framework composition derived from WDS and crystallographic analyses (Table 2). These results contrast with the hypothesis formulated by Gies et al. about the presence of B as non-framework constituent in Al-free FER [14,15]: the trivalent metal atom is effectively and extensively incorporated in the framework while we did not observe any extraframework B species.

1927 Table 2 Framework composition of as-synthesized BOR-C and B-FER determined with the different methods. BOR-C B-FER WD S [Si845B11.50192] [ S i29.5B 6.5072] Equation (1) [Si81.4B14.60192] [Si30.1B5.9072] Refinement of the Si/B occupancy [8i81.3B16.10192] [Si29.4B6.6072] T-O bond distances [Si80.5B15.50192] [8i29.2B6.8072] The refined structures of BOR-C and B-FER are shown in Figure la and b. Three and two crystallographically independent EN molecules were found in BOR-C and B-FER structures, respectively. In BOR-C, one (EN1) is located in the sinusoidal 10-ring channel parallel to [100] direction, lying on the (010) plane, the other two (EN2 and EN3) in the straight 10-ring channel (Fig. l a). The refined occupancies of these molecules correspond to 2.80(16), 2.96(12) and 1.20(8) EN molecules/unit cell, respectively, giving an overall number of 6.96(20) EN molecules in the unit cell. Another partially occupied extraframework site, assigned to water oxygen, was found in the interception of the straight and sinusoidal channel (Fig. l a). Only EN molecules were found in the B-FER porous system: one (EN1) is located in the FER cage, lying on the (001) plane, the other one (EN2) in the 10-ring channel, lying on the (010) plane (Fig. lb), corresponding to 2.0(12) and 1.62(8) EN molecules/unit cell, respectively (total number of EN molecules per unit cell, 3.62(15)). In both sites, the EN molecules occupy statistically two crystallographic positions related to each other by an inversion center. Though unambiguously recognized as EN molecules, their geometry does not correspond to the ideal one. However, because of the small dimensions of EN molecule with respect to the MFI and FER pores, one has to admit the existence of different possible positions of the molecule close to the preferred location. Therefore, what derived from the crystallographic refinement should be considered as an average of the different situations existing in the crystals. The total number of EN molecules per unit cell found for the two structures is lower than that required for a complete pore filling (12 and 4, for MFI and FER, respectively). That can be explained with the existence of some defects in the stacking of the molecules; the corresponding voids space could be filled with other extraframework species (e.g. H20). It is worth noting that, in both structures, the number of EN molecules correlates well with the number of B atoms incorporated in the framework, since the ratio N/B is close to 1. Since the synthesis was performed in the absence of alkali metal ions, the negative framework charge should be compensated by the charged organic molecules. That implies that most of EN molecules should be in dicationic (H3N-CHzCHzNH3) 2+ form or, because of the weakness of the Bronsted acidic sites in boralites [1], they simply interact with the protons forming N'"H-O hydrogen bonds. These results are perfectly in line with what reported by Taramasso et al.: the maximum amount of B, which can be incorporated in the framework, depends on the number of charged cations hosted in the pores [ 13]. Another interesting consequence of this observation is that an EN-boric acid complex, the formation of which is highly probable because of the large excess of diamine with respect to H3BO3 in the synthesis mixture, really act as a template in the crystallization of BOR-C and B-FER, as postulated for Al-free FER [14,15].

1928

(a)

(b)

t~ .L

(c)

(d)

Figure 1. Location of EN molecules determined experimentally for BOR-C (a) and B-FER (b) and lowest energy location of the same molecules in the MFI (c) and FER porous systems (d).

3.2. Modeling studies The packing method proposed by Freeman et al. proved to be an efficient tool for determining the location and energetics of organic molecules docked or packed in microporous materials [ 18]. We used this approach both to confirm the results obtained in the structural characterization of BOR-C and B-FER, to determine the maximum loading of EN molecules in the two pore systems and to evaluate the existence of any structure directing role of EN towards the formation of these structures. Table 3 Average stabilization energies of EN molecule packed in MFI and FER pores (data in kJ'molm). The number of EN molecules/unit cell are given in parentheses. MFI FER

EN 1

EN 2

EN 3

-68.0 (2.80(16))

-65.2 (2.96(12))

-51.9 (1.20(8))

-79.4 (2.0(12))

-73.9 (1.62(8))

---

1929 Application of these method correctly predicted the location of the EN molecules in MFI and FER porous structures, as clearly shown in Figure 1. In MFI, the lowest energy conformation was found with the twelve EN molecules distributed around the three well defined sites, corresponding to those derived from the crystallographic analysis (compare Figures 1a and c). Depending on the location, the EN molecules are predicted to stabilize the system in a different way (Table 3). The lowest energy sites are located in the sinusoidal 10membered ring (10MR) channels, in the same region found for the EN 1 molecule (Figure 1a). The second site, slightly less stable than the previous one, corresponding to EN molecules located in the straight 10MR channels (EN2, Figure l a). The third EN molecule is located across the straight 10MR channels and points towards the sinusoidal 10MR channel (EN3, Figure l a). With respect to EN1 and EN2, which display similar average van der Waals energies, EN3 is significantly less stable (Table 3) and that accounts well with the lower occupancy of this site derived from the crystallographic analysis (see above). Two different preferred sites for the diamine molecule were predicted in FER (Figure 1d). The low energy sites are located within the FER cages and correspond to the positions found for EN1 molecule (Figure l b). In this case, the dimensions and shape of the cage allow several different but isoenergetic conformations for EN1 and this supports the explanation given for the distorted geometry of EN1 found in the crystallographic analysis (see above). The other sites, corresponding to EN2 molecules (Figure lb), are located in the linear 10MR channels. The position predicted for EN2 along the [001 ] direction is similar with respect to that found experimentally, but it lies on the (100) plane instead of on the (010) (Figure 1d). The difference between the average van der Waals energies of the two molecules is 5.5 kJ.mol 1 (Table 3), again in line with the relative occupancy of the two sites. Upon comparing the stabilization energies of the EN/framework systems reported in Table 3, one has to admit that FER seems to be better stabilized with respect to MFI, in agreement with the results reported by Gies et al. [ 14,15], but not with those here reported. In reality the difference observed can hardly support any preference for one or the other microporous borosilicate. It is probable that subtle differences in the synthesis procedures may influence the nature of the products. As a matter of fact, Bellussi and Perego who always obtained crystalline products constituted by mixtures of B-FER and BOR-C, with B-FER content ranging between 20 and 80 wt% [24]. 4. CONCLUSIONS Single crystal X-ray structure analysis of borosilicates with MFI and FER topology, synthesized from an aqueous solution containing TMOS, boric acid and ethylenediammine, unambiguously showed the presence of boron in the tetrahedral sites. Different routes evidenced the incorporation of 14+1 B and 6.5+0.5 atoms in MFI and FER frameworks, respectively, with a substantial disorder (Si,B) in the tetrahedra. The number of EN molecules/unit cell, determined from the refinement of the X-ray diffraction data, is lower than that expected on the basis of a full loading of the channels (12 and 4 EN/unit cell, for MFI and FER, respectively) but is related to the number of B atoms/unit cell. On the basis of this evidence, it is possible to hypothesize that the number of EN molecules found in the crystals is determined by the extent of B incorporation. That implies a role of the boric acidEN complex postulated by Gies and Gunawardane [14] in the crystallization mechanism of the two borosilicates.

1930 The location of EN derived by using molecular modeling tools, was favorably compared with that obtained by the X-ray structure refinement. The systems are stabilized by favorable host/EN nonbonded van der Waals interation energies; however, the degree of stabilization changes from a site to another It is to note that site characterized by the highest stabilization energy corresponds to the site with the highest occupancy derived by X-ray refinement. REFERENCES

1. R. Szostak, Molecular Sieves. Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989. 2. H. Gies, B. Marler and U. Werthmann in: Molecular Sieves Science and Technology. Vol I: Synthesis (H. G. Karge and J. Weitkamp, Eds.), Springer, Berlin, 1998, p. 35. 3. S. Ernst in: Molecular Sieves Science and Technology. Vol I: Synthesis (H. G. Karge and J. Weitkamp, Eds.), Springer, Berlin, 1998, p. 64. 4. T.V. Harris and S. I. Zones, Stud. Surf. Sci. Catal., 84 (1994) 29. 5. R. G. Bell, D. W. Lewis, P. Voigt, C. M. Freeman, J. M. Thomas and C. R. A. Catlow, Stud. Surf. Sci. Catal., 84 (1994) 2075. 6. P.A. Cox, A. P. Stevens, L. Banting and A. M. Gorman, Stud. Surf. Sci. Catal., 84 (1994) 2115. 7. D.W. Lewis, C. M. Freeman and C. R. A. Catlow, J. Phys. Chem., 99 (1995) 11194. 8. R. Millini, G. Perego, W. O. Parker, Jr., G. Bellussi and L. Carluccio, Microporous Materials 4 (1995) 221. 9. M.E. Leonowicz, J. A. Lawton, S. L. Lawton and M. K. Rubin, Science 264 (1994) 1910. 10. P. A. Barrett, M. J. Diaz-Cabafiaz and M. A. Camblor, Chem. Mater. 11 (1999) 2919. 11. L. D. Rollmann, J. L. Schlenker, S. L. Lawton, C. L. Kennedy, G. J. Kennedy and D. J. Doren, J. Phys. Chem. B 103 (1999) 7175. 12. L. D. Rollmann, J. L. Schlenker, C. L. Kennedy, G. J. Kennedy and D. J. Doren, J. Phys. Chem. B 104 (2000) 721. 13. M. Taramasso, G. Perego and B. Notari, in: Proc. 5th Int. Zeolite Conf., (L.V.C. Rees Ed.), Heyden, London, 1980, p. 40. 14. H. Gies and R. P. Gunawardane, Zeolites 7 (1987) 442. 15. R. P. Gunawardane, H. Gies and B. Marler, Zeolites 8 (1988) 127. 16. Z. Otwinowski and W. Minor, Meth. Enzymol. 276 (1997) 307. 17. G. M. Sheldrick, SHELXL-93, University of G6ttingen, G6ttingen, Germany, 1993. 18. C. M. Freeman, C. R. A. Catlow, J. M. Thomas and S. Brode, Chem. Phys. Lett., 186 (1991) 137. 19. Catalysis User Guide, Release 4.0.0, Molecular Simulations, Inc., San Diego, CA, 1996. 20. Discover User Guide, Release 2.9.8, Molecular Simulations, Inc., San Diego, CA, 1996. 21. H. van Koningsveld, H. van Bekkum and J.C. Jansen, Acta Cryst. B43 (1987) 127. 22. G. Perego, G. Bellussi, R. Millini, A. Alberti and S. Zanardi, Microporous Mesoporous Mater., submitted. 23. R. E. Morris, S. J. Weigel, N J. Henson, L. M. Bull, M. T. Janicke, B. F. Chmelka and A. K. Cheetham, J. Am. Chem. Soc. 116 (1994) 11849. 24. G. Bellussi and G. Perego, It. Patent 1,203,913 (1987) assigned to Eniricerche S.p.A.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1931

Molecular dynamics simulation o f water confined in zeolites P. Demontis, G. Stara and G.B. Suffritti Dipartimento di Chimica, Universit~t di Sassari, Via Vienna, 2, 07100 Sassari (Italy) FAX" +39.79.212069 Extended nanosecond scale MD simulations of water in bildtaite, silicalite and natrolite are illustrated. These systems are representative of different kinds of absorption of water in zeolites: hydrogen-bonded linear chains of water moleculars (bikitaite), water in hydrophobic materials (silicalite) and water molecules held in fixed ordered positions (natrolite). Results are in line with the experimental results and with Car-Parrinello simulations, when available. The different microscopic behaviour of water in each considered zeolite is discussed. 1. INTRODUCTION The behaviour of water in porous media has received recently a renewed interest, both from experimental and theoretical viewpoint. For instance, the experimental evidence of possible phase transitions of water at low temperatures has been studied in ordered mesoporous silica materials [1], where the confinement lowers the freezing point, and in hydrated phyUosilicates [2]. On the theoretical side, new statistical models have been developed for studying the phases of water-like liquids in porous media [3]. Among the different approaches the Molecular Dynamics (MD) simulation technique seems to be very promising in order to gain a better microscopic description for the behaviour of water in porous materials [4,5]. A recent example is the simulation of the behaviour of water in carbon nanotubes [6] which evidenced some unexpected features like the formation of onedimensionally ordered chains of water molecules, transmission bursts of water through the nanotube and sharp two-state transitions between empty and filled states on a nanosecond time scale. In the present work, extended nanosecond scale MD simulations of water in bikitaite, silicalite and natrolite are illustrated. These systems are representative of different kinds of absorption of water in zeolites: hydrogen-bonded linear chains of water molecules (bikkitaite), water in hydrophobic materials (silicalite) and water held in fixed ordered positions (natrolite).

1932 2. M O D E L AND C A L C U L A T I O N S

Standard MD simulations with periodic boundary conditions were performed in the microcanonical ensemble. A sophisticated empirical model for simulating flexible water molecules and flexible zeolite framework developed in this laboratory [5] has been used. Moreover, new empirical potential functions have been developed for representing lithium and sodium - water interactions, as the ones previously proposed for simulating aqueous solutions containing ions did not reproduce satisfactorily the behaviour of water in zeolites. In particular, a problem in representing lithium - water interactions arise because water molecules are located in planes which are roughly perpendicular to the Li - O distance, whereas for all previously available empirical Li - water potential models the preferred orientation of water molecule was parallel to this distance, with high barrier to flap motion. The special features of Li + - H20 interactions were recently discussed by Lyubartsev et al. [7] in an ab initio MD simulation study of lithium salt aqueous solutions. Therefore, a new model fitting both ab initio [8] results for the Li + - H 2 0 system and experimental data for bikitaite was performed. The final form of the Li + - water potential function reads:

VL~o,~ (r) =

1

4rcc o

qLiqi~,o + ALio,I~ exp(--BLio,H r) --]- C~i O,H r

(1)

S(~,r)

r

where S(cy , r ) i s a switching function given by:

S(cy,r)=

1 exp(_cyr2)

/f /f

nm nm

r<0.5 r>0.5

which is necessary because the r

(2) -2

lattice sums do not converge, as in Eqn. (1)

C ~o ~ - 2 C ~ . .

The representation of sodium - water interactions differs from that adopted in Ref. [5], because a term proportional to r -6 was added to the Na + - O pair potential function in order to fit better both the energy and the Na + - O distance. The final form is:

VNaO'g(r) = 4rt~;0

qNaqH, O + ANaO,H exp(_BNao,Hr ) _ CN6a0 + DNaO, 4 H r

r

r

(3)

The parameters included in Eqns. (1-3) are collected in Table 1. Water was assumed to interact with Si and A1 atoms via a Coulomb potential only. The potential functions for interactions between an oxygen atom of the zeolite framework (Of) and an oxygen (O) or an hydrogen (H) atom of the water molecule were represented by:

Vof

O

=

1 qQqo AQO BQO C@o 4rcc ~

r

t

r12

- - + ~ r 6

r 4

(4)

1933

1 qQqH AQH V o f H = 4~r~~ +- r r 7

B@H

(5)

r 4

and the values of the parameters are reported in Table 1. They are slightly different from those reported in Ref. [5], because they were optimised in order to fit better the experimental sorption energy in all-silica zeolites. The evaluation of the Coulomb energy was performed using the efficient method recently proposed by Wolf et al. [9] and extended in our laboratory to complex systems [10]. Runs lasting at least 1 ns (using an elementary time integration step of 0.5 fs) were carried out at different temperatures and loadings in order to compare the computed results with the available experimental data. Besides structural properties (average co-ordinates and temperature factors), the absorption energies, the vibrational spectra, the diffusion coefficients ant the time correlation functions of water molecule rotations were evaluated using standard methods. The analysis of the clustering of water molecules were also carried out. The simulation box for bikitaite was made of 3x4x3 unit cells, resulting in a pseudomonoclinic cell with a = 2.5713, b = 1.9758, c = 2.28363 nm and fl = 114.52 ~ and containing 936 atoms, including 72 water molecules. The accurate neutron diffraction structure by Stahl et al. [11] was assumed. This system is of special interest because it shows parallel straight channels where hydrogen-bonded linear chains of water molecules run along the axis of the channels, parallel to regular rows of lithium ions sticking to the channel surface. Table 1. Values of the parameters included in Eqns. (1-5). Energies are obtained in kJ/mol if the distanced are in units of 10 - l ~ m (formerly A) Charges are in units of e. Interaction

A

Li +- 0 Li +- H Na +- 0 Na +- H Of- 0 Of- H Charges

B

C

2.68 105

5.15

397.2

0.5

4.14

7.07

410.8

0.5

1.0 106

4.526

3139.5

4.897 105

7.07

10 7

8.36 105

31.0 102

2.09 103

1.8 102

qL i +

qN a +

1.0

1.0

D

cr

62.8 460.5

2.09 102

qo -0.65966

qH 0.32983

qoI

- 1.0

1934 For silicalite, two unit cells superimposed along c yielded an orthorhombic simulation box of 2.0022xl.9899x2.6766 nm 3 containing 624 atoms. Silicalite is considered a "hydrophobic" zeolite, because water - water interactions are stronger than the water framework ones. The box includes 16 water molecules, corresponding to the loading of the samples used for experimental measurement of diffusion coefficient. Its framework structure [12] comprises two different channel systems, each defined by ten-membered rings. Straight channels with an elliptical cross section are parallel to b and sinusoidal channels with nearly circular cross section run along a. The resulting intersections are elongated cavities. Natrolite is a natural fibrous zeolite, containing 16 water molecules per unit cell, which are held in fixed ordered positions by an equal number of sodium ions lined around the axis of channels running along c. It was chosen because its structure [13], vibrational spectrum [ 14], and water mobility [15] are known with good accuracy. The simulation box consisted of 3 unit cells superimposed along c, yielding a pseudo-orthorhombic simulation box of 1.8272x 1.8613x 1.9779 nm 3 including 552 atoms.

3. RESULTS AND DISCUSSION The results for bikitaite, silicalite and natrolite will be discussed separately, as the behaviour of water in the different zeolites is different. In general, the simulations were able to reproduce the available experimental data, and when the agreement was not quantitative, is at least qualitatively correct. However, in some cases the goodness of some results could be changed (either into better or into worse) on the basis of the correspondence between the simulated temperature (which is the one defined by classical statistical mechanics) and the experimental one, belonging to systems which are of quantum nature. Some suggestion were put forward by Jobic et al. [16], but the problem deserves a more throughout discussion, which is beyond the scope of the present paper. 3.1 Bikitaite

The experimental knowledge of the behaviour of water in bikitaite is given by neutron [13] and X-ray [ 17] diffraction structural data, completed by IR spectra [17] and NMR data [18], Differential Thermogravimetry (DTG) and high-temperature X-ray diffraction [19]. From neutron diffraction studies it appears that the libration motion of water molecules is relatively large even at low temperature (13 K) and, indeed, NMR experiments [18] evidenced that a flip motion of the water molecules already occurs at a relatively low temperature (224 K) with a correlation time of 10-7 s. The NMR experiments were performed in the temperature range 224 - 418 K, detecting flip motion but no diffusion. The corresponding relaxation time follows closely an Arrhenius trend with values from 10-7 s (224 K) to 10-11 s (418 K). The fitted value of activation energy for the flip motion is 30_+2 kJ/mol. Recent X-ray diffraction experiments at high temperature [19] showed that at about 375 K the dehydration process begins and the complete dehydration is achieved at about 725 K. The apparent discrepancy between X-ray and NMR results can explained by considering the different observation time, which is much smaller for NMR, so that slow diffusive motion could have escaped in fitting NMR data.

1935 Overall, the water molecule chains embedded in bikitaite show a high stability. CarParrinello MD (CPMD) studies [20] demonstrated that these chains are stabilized by a permanent electric dipole parallel to the channel direction, which is coupled to an opposite dipole of the framework, stemming from a special arrangement of the aluminosilicate tetrahedra. Previous CPMD simulations reproduced well the experimental structure and vibrational spectrum of bikitaite, but, as their duration was limited to a few ps due their computational cost, failed in detecting the flip motion of water molecules at room temperature which shows relaxation times several orders of magnitude longer. Before undertaking long MD simulations using empirical functions for the interatomic interactions, intended as an extension of the CPMD ones, we verified that our model could reproduce the experimental structure (at different temperatures) and vibrational spectra, as well as CPMD results. The structural properties computed using our model agree well with the experiment both at low temperature (13 K for experiments, 50 K for simulations, in order to mimic approximately the average zero point vibrational energy) and at room temperature (within a few tens pm). Also the computed vibrational spectrum is satisfactory. The hydrogen bonding between the water molecules was well represented both from a geometrical and energetic viewpoint. Indeed, average intermolecular energy was about 15 kJ/mol per molecule, in line with the results of simulations in carbon nanotubes [6] where similar water molecule chains are present. Once the good quality of the potential model is ascertained, one can examine the long time scale results. By following a well-established theory [21], a quantitative comparison with NMR relaxation times is achieved by considering the second order rotational correlation function:

c: (t) =

[.(0)..(t)l>

(6)

where u(t) is the versor of the HOH plane and P2 is the second-order Lagrange Polynomial. Using Eqn. (6), the relaxation times for flip motion of water molecules were evaluated. They are compared with the experimental values in Table 2. They follow an Arrhenius trend, and computed activation energy is 26 kJ/mol. Therefore, the simulations underestimate the relaxation times by about one order of magnitude, as the energy barrier is too low. However, as recalled above, a discussion has arisen about the adequacy of the temperature computed from classical mechanics simulations for the comparison with experimental data. Therefore the computed relaxation times could compare more favourably with experiment. Finally, no diffusion was detected in the simulated system (fully hydrated infinite crystal), even at the highest temperature. However, it is possible that, in real (finite) crystals water can escape from the free ends of the channels, inducing a defect-driven stepwise diffusion, which eventually leads to dehydration, as experimentally observed [19]. This problem deserves further investigations, which are in progress.

1936 Table 2. Experimental (NMR) [21] and calculated relaxation times (s) for the flip motion of water molecules in bikitaite T (K) Exp. Cal. a

316 7+5 10 1~ 1.7 10 -11

424 4+2 10 -11 2.27 10 -12

611 3+ 1 10 "12 a 2.55 10 -13

800 7+2 10 -13 a 6. 10 -14

Extrapolated from the Arrhenius fit of the experimental data

3.2 Silicalite Experimental data about the diffusion of water in silicalite at room temperature and with a loading of water in the range 4 - 12 molecules per unit cell have been obtained from Pulsed Gradient Field NMR spectra by K ~ g e r and co-workers at 300 K [22]. They obtained a value of 4• 10-9m2s-1 nearly independent of loading, to be compared with 8.6 10-9m2s1 resulting from simulations. Although experimental data are still lacking at lower temperatures, the simulations may suggest a possible behaviour of the sorbed water. Thus the mobility of the water molecules was checked at different temperatures: 100 K, 130 K, 160 K, 200 K, 250 K, 300 K and 350 K. The runs lasted 3 ns (using a time step of 0.5 fs). In Fig. 1 the mean square displacements (MSD) of water molecules vs. time are reported. At room temperature the MSD shows a linear dependence on time. At 250 K the diagonal components of the diffusion tensor, which usually follow the ordering Dz< Dx < Dy, begin to change this ordering and Dx becomes close to Dy, until, at 200 K, Dx > Dy. For lower temperatures (100 - 200 K) the MSD's do not show a linear dependence on time, at least in the considered time scale. Moreover, they are nearly independent of temperature. Nevertheless, the sorbed fluid does not appear completely frozen, as the mean linear displacement is of the order of 0.3 nm after 800 ps and it is still growing (the thermal vibration amplitude is - 0.1 nm at these temperatures). From the distribution of the water molecules in the channels (not shown), it appears that by lowering the temperature from 350 K to 200 K the distribution of the water molecules is spread over all or almost all the available space of the micropores. For lower temperatures (100 - 200 K) the water molecules seem to group around some stable positions, mostly in the intersections between straight and sinusoidal channels. At 100 K a diffuse distribution along the straight channels emerges again. The analysis of the lifetime of the water molecule clusters shows that by lowering the temperature from 350 K to 200, in correspondence with "normal" diffusion the molecules spend most of the time as monomers or dimers, with lifetimes of the order of one ps, at most. In the temperature range 130 - 200 K, long-lived clusters made of 5 - 6 water molecules and lasting some tens of ps are formed. On the basis of the average distributions in the channels one may assume that these clusters are located in the intersections, hindering each other the diffusion, thus giving rise to the observed irregular, single-file like diffusion behaviour. Finally, at 100 K the longlived clusters are formed by 7-8 water molecules, and become so large than they cannot be accommodated in the intersections, but line up in the straight channels, showing a low mobility because of the low kinetic energy available.

1937 //]-4000

]

| 9

,

..

:,

.::~

": ' :

'

:'

400

600

time (ps)

!~: 300 K

O0 K

160K 200

I

~::"i'~

I/ ~

' 200

~

~ 400

~

"

/

225 K

20o K

600

time (ps)

Figure 1. Mean square displacement of the centre of mass of the water molecules v s . temperature. Left: low temperatures, showing an irregular trend. Right: high temperatures. In this case the trend is linear, and the slopes, corresponding to the diffusion coefficient, follow an Arrhenius behaviour.

The formation of long-lived clusters could be interpreted as a phase transition (freezing) of the adsorbed water in the temperature range 200 - 225 K. These findings suggest extending the investigation to different loadings in order to understand better the behaviour of water in silicalite. 3.3 Natrolite We shall shortly mention the results of preliminary simulations of natrolite. The optimised potential functions improved the reproduction of both experimental structure and vibrational spectrum. In particular, the more accurate representation of the w a t e r framework interactions was able to detect the flip motion of the water molecules at room temperature, in agreement with the experimental evidence [23]. The measured activation energy is 36 + 3 kJ/mol, so that very long simulations will be necessary for a reliable comparison between the model and the experiment, as well as for the study of the diffusion of the adsorbed water leading to dehydration at high temperature. 4. CONCLUSIONS The example which are briefly illustrated in this paper show that MD simulations can be an effective tool for studying the behaviour of water confined in the pores of zeolites. Different distributions and mobilities have been demonstrated, depending on the structure, on the kind of extraframework ions and on temperature. Further investigations are in progress in order to refine the models and to extend the simulations to different systems. The final goal is to achieve some general rules for the behaviour of water in confined geometries.

1938 AKNOWLEDGMENTS

This research is supported by Italian Ministero dell'Universit~t e della Ricerca Scientifica e Tecnologica (MURST) and by Universit~t degli studi di Sassari. REFERENCES

1. S. Sklari, H. Rahiala, V. Stathopoulos, J. Rosenholm, P. Pomonis, Micropor. Mesopor. Mater., 49 (2001) 1 2. R. Bergman and J. Swenson, Nature (London), 403 (2000) 283 3. T.H. Truskett, P.G. Debenedetti and S. Torquato, J. Chem. Phys. 114 (2001) 2401 4. P. Demontis, G.B. Suffritti, Chem. Rev., 97 (1997) 2845 5. P. Cicu, P. Demontis, S. Spanu, G.B. Suffritti and A. Tilocca, J. Chem. Phys. 112 (2000) 8267 6. G. Hummer, J.C. Rasaiah and J.P. Noworyta, Nature (London), 414 (2001) 188 7. A.P. Lyubartsev, K. Laasonen and A. Laasonen, J. Chem. Phys. 114 (2001) 3120 8.X. P6riole, D. Allouche, J. P. Daudey, and Y.H. Sanejouand, J. Phys. Chem. 101 (1997) 5018 9. D. Wolf, P. Keblinki, S.R. Phillpot, and J. Eggebrecht, J. Chem. Phys., 110 (1999) 8254 10. P. Demontis, S. Spanu, G.B. Suffritti J. Chem. Phys. 112 (2001) 8267 11. K. Stahl, A. Kvick and S. Ghose, Zeolites, 9 (1989) 303 12. H van Koningsveld, J.C. Jansen and H. van Bekkum, Zeolites, 10 (1990) 235, 13. G. Artioli, J.V. Smith and A. Kvick, Acta Crystallogr. C 40, 1658 (1984) 14. G. Gottardi and E. Galli, Natural Zeolites, Springer-Verlag, Berlin, 1985 15. N.K. Moroz, E.V. Khopolov, I.A. Belitsky, B.A. Fursenko, Micropor. Mesopor. Mater., 42 (2001) 113 16. H. Jobic, K. Smimov, D. Bougeard, Chem. Phys. Lett., 344 (2001) 147 17. S. Quartieri, G. Vezzalini, A. Sani, E. Galli, E.S. Fois, A. Gamba and G. Tabacchi, Micropor. Mesopor. Mater., 30 (1999) 77. 18. K.Larsson, J. Tegenfeldt and A. Kvick, J. Phys. Chem. Solids, 50 (1989) 107 19.G. Vezzalini, O. Ferro, S. Quartieri, A.F. Gualtieri, G. Cruciani, E. Fois, C. Ceriani, and A. Gamba, in: Recent Research Reports, 13th International Zeolite Conference, French Zolite Group, Montpellier, 2001, 09-R-04 20. E.S. Fois, G. Tabacchi, S. Quartieri and G. Vezzalini, J. Chem. Phys., 111 (1999) 355 21. R.G. Gordon in: Advances in Magnetic Resonance, J.S. Waugh (ed.), Academic Press, New York, 1968, Vol. 3, pag. 1 22. J.K~rger and D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, John Wiley & Sons, New York, 1992 23. R.T. Thomson, R.R. Knispel, and H.E. Petch, Can. J. Phys., 52 (1974) 2164

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1939

E X A F S and optical s p e c t r o s c o p y characterisation o f silver w i t h i n zeolite matrices S. G. Fiddy a, N. E. Bogdanchikova b, V. P. Petranovskii b, J.S. Ogden a and M. AvalosBorja b aDepartment of Chemistry, University of Southampton, Southampton SO 17 1BJ, United Kingdom bCentro de Ciencias de la Materia Condensada UNAM, Apdo. Postal 2681, 22800 Ensenada, B.C., Mexico

The study of silver-zeolite samples by a combination of EXAFS and Diffuse Reflectance UV-Visible spectroscopy has been utilised to discover the effect of the structural type and Si/A1 ratio of zeolite matrix as well as influence of reduction temperature on the size, structure and oxidation of silver species stabilised inside zeolite voids. The preparation of silver-zeolite samples at high temperature using mordenite with medium Bronsted acid strength or at low temperature using mordenite with high acid strength leads to fast oxidation of reduced silver species. After reduction at medium temperatures silver clusters interpreted as Ag8~ and Ag8~+ are formed in mordenites with intermediate Bronsted acid strength. The reduction of silver samples at high temperature utilising mordenite with a very small concentration of weak Bronsted acid sites leads to the formation of amorphous Ag particles. Deformation of the eight-atom clusters is more prominent in the erionite cavities, where space is more severely restricted than in mordenite channels. It is proposed that the charged Ag8~+ clusters are more deformed than neutral Ag8~ clusters.

1. INTRODUCTION The study of the physical and chemical properties of small metal particles and clusters is very important for many scientific and technological fields such as solid state physics, surface science and heterogeneous catalysis. Zeolites are perfect matrices for the stabilisation of metal clusters due to the wellordered zeolite intracrystalline environments. However, zeolite pore structure is not the only characteristic that helps to stabilise the metal clusters. Such parameters as the SIO2/A1203 ratio and the location of the acid sites on the internal and external zeolite surface in line with the reduction conditions can all have a major influence on the stabilisation of metal species. Ag-zeolite materials are active in various catalytic reactions such as NOx decomposition [1, 2] NO2 reduction in exhaust lean-burn combustion [3], NOx reduction with hydrocarbons [4 and references therein]. Therefore, the characterisation of active silver states in catalytic reactions and the development of methods for the regulation of the contributions of silver in different states is a very important area of chemistry.

1940 In our previous work, it was shown that variation of the 8iO2/A1203 ratio and the reduction temperature lead to changes of the silver environments in mordenite [5]. The contribution of silver cations Ag +, charged and neutral clusters Agnm+, silver subcolloidal particles with size ca. 1 nm and larger metal silver particles change under the variation of these parameters [5, 6]. For the interpretation of silver species with prominent bands at 322 and 293 nm in the UV/VIS spectra of Ag-erionite, EXAFS characterisation was used. The results support an interpretation given previously that the absorption peaks at 322 and 293 nm in the optical spectra belong to cluster species Ag8~ and Ags 5+ in the channels of the crystalline framework [7]. Subsequent ab initio calculations also confirmed these conclusions [8]. EXAFS is a very valuable technique for the characterisation of the structure of metal clusters with a number of publications concerning the EXAFS study of silver species in different matrices for example in SiO2 [9], solid argon [ 10], zeolite 4A [11 ], zeolite ZSM5 [12, 13]. Other physico-chemical methods such as ESR, single crystal X-Ray diffraction, UV/VIS spectroscopy, NMR, etc. are also successfully used for the characterisation of the structure of silver clusters [ 14]. This work represents a continuation of the search for factors, which are responsible for variation of silver states in zeolites. Within this presentation, a number of different silverzeolite samples are characterised by EXAFS and Diffuse Reflectance UV-Visible spectroscopy. By a combination of these two techniques, important information can be concluded about the effect of the structural type and the 8iO2/A1203 ratio of the zeolite matrix on the size, structure and oxidation of silver clusters stabilised inside zeolites. 2. EXPERIMENTAL In this work, the mixed K+-,Na+-erionite sample with SIO2/A1203 molar ratio 8 and protonated forms of mordenites with MR from 10 to 128 were used as the zeolite matrix. The silver exchanged forms were obtained by ion exchange in 0.1 N aqueous solution of AgNO3. The excess solution was removed, samples were dried in vacuum a t - 3 4 5 K and were heated in a H2 flow at a fixed temperature ranging from 20 to 500~ for 4.5 h. The samples are abbreviated as AgM and AgE for Ag-mordenite and Ag-erionite, respectively, followed by the value of MR and reduction temperature in ~ (e.g., AgM15100). The silver content, measured using an X-ray Fluorescence Spectrometer SEA 2010, was 0.6 - 2 wt. % for AgM samples and 14 wt. % for AgE. Diffuse Reflectance UV-Visible spectra were recorded under ambient conditions on a Perkin Elmer 330 spectrometer with a standard diffuse reflectance unit, using undoped erionite and mordenite as a references. Silver K-edge EXAFS data were recorded at room temperature using the synchrotron source at Daresbury, UK, operating at 2 GeV, and samples were studied in both transmission and fluorescence mode, with data sets extending typically to k = 14 ~-1. The Ag edge position was calibrated using silver foil, and background subtractions were carried out using standard polynomials within the program PAXAS [15]. Subsequent curve-fitting utilised the single scattering curved wave theory incorporated in EXCURVE, with phaseshifts and backscattering factors calculated by normal ab initio methods [ 16]. Fourier transformation was performed on k3-weighted EXAFS oscillations in the range of 2 - 12A. Transmission electron microscopy (TEM) characterisation was carried out using a JEOL 2010 instrument with a point-to-point resolution better than 0.19 nm.

1941 3. R E S U L T S AND D I S C U S I O N

For this study, six Ag-zeolite samples were chosen. The variation of SIO2/A1203 molar ratio leads to the change in the strength of Bronsted acid sites that determine the stability of Ag8~ and Ag85+ clusters in H+-mordenites [6]. The strength of acid sites decreases in the following sequence: M15 > M10 > M30 > M72 > M128 [6]. The mixed K +-, Na +erionite sample does not contain proton sites. Results of EXAFS measurements are summarised in Fig. 1 and Table 1. In Table 1, CN represents coordination number; R - interatomic distance, ~2 -Debye-Waller factor, Rfactor- accuracy parameter of EXAFS calculations, AEo - the difference between the calculated Fermi energy level and the known values for that element. AEo is typically between -10 to 10eV, for exact models reaching to 0. Errors derived from EXCURV98 are given in parentheses. The true accuracy of bonded and non-bonded interatomic distances is considered to be 1.4% and 1.6% respectively. Precision on 1 st shell coordination numbers is estimated to be ca. 5-10% and between 10-20% for non-bonded shells. For all samples R-factor (accuracy parameter of EXAFS calculations) is < 50 %. Spectra of diffuse reflectance spectroscopy of all studied samples are presented in Fig. 2. Table 1 Ag K-edge EXAFS (AFAC = 0.8 and Ak = 2.9 - 10.3 A ~) derived structural parameters for Ag-zeolite samples Sample AgM30-500 AgM15-100 AgM10-100 AgM72-200 AGE8-100 AgM 128-300

Shell

CN

O

2.6(+0.3)

Ag

R (/k)

(fit2)

2or 2

2.26(•

0.025(•

-

-

-

O

2.6(+0.2)

2.28(+0.01)

0.028(+0.002)

Ag

0.4(+0.2)

2.73(+0.04)

0.034(+0.009)

O

2.0(+0.3)

2.34(+0.02)

0.030(+0.006)

Ag

3.6(+0.3)

2.83(+0.01)

0.034(+0.003)

O

2.2(+0.3)

2.34(+0.02)

0.024(+0.005)

Ag

3.0(+0.2)

2.82(+0.09)

0.027(+0.002)

O

2.4(i0.2)

2.35(+0.01)

0.028(+0.002)

Ag

2.0(+0.2)

2.78(+0.09)

0.028(+0.002)

O

2.0(i0.3)

2.32(+0.02)

0.021(i0.005)

Ag

2.4(+0.3)

2.83(+0.01)

0.022(+0.002)

R-factor

(%)

AE0

36.9

3.3(+0.9)

31.0

4.2(+0.7)

50.0

-4.5(+0.9)

45.9

-4.4(+0.8)

35.6

-2.4(-t-0.7)

49.3

-3.1 (-1-0.9)

1942

':,

'

/ "

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9 A v tO .

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,

.

i

"i

5

,/; ";

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m

-.

.

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,

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2

3

4

5

6

7

8

9

10 k(A_11

o

l

2

3

,

3

4

s

R(A)

Figure 1. Ag K-edge EXAFS and Fourier Transform of samples: 1 - AgM30-500, 2 AgM15-100, 3 - AgM10-100, 4 - AgM72-200, 5 - AGE8-100, 6 - AgM128-300. 6

5 C ..0 s,.,. 0 ..0

200

300

400

500

Wavelength,

600

700

800

nm

Figure 2. Diffuse reflectance UV-Vis. spectra of samples: 1 - AgM30-500, 2 - A g M 15100, 3 - A g M 10-100, 4 - AgM72-200, 5 - AGE8-100, 6 - A g M 128-300.

6

1943 The results reveal that the studied samples could be divided into three distinct groups. 3.1. Samples with dominance of silver oxidised forms For samples of the first group including AgM15-100 and AgM30-500, the EXAFS and Fourier transform spectra indicate a high proportion of Ag-O distances. In both cases, approximately 2.6 oxygen atoms at a distance of 2.26 - 2.28 A could be fitted successfully. These results are consistent with previous data on Ag§ oxygen) bonds in zeolite (ca. 2.25 A [14]). The contact between Ag + cation and water oxygen is characterised by longer distances (2.4 - 2.5 A) [ 17]. Ag+-framework oxygen interactions were further confirmed by the fitting of an Ag-Ag distance. For the AgM30-500 sample, Ag-Ag fitting only lead to worse EXAFS fitting parameters suggesting that only Ag-O was present, whereas for the sample AgM15-100 it produced an improved fit but with a low Ag-Ag coordination number (0.4) and low distance (2.73 A). It has been proposed previously that the metal-metal bond distance gradually contracts with the decrease of metal cluster size [ 18]. In bulk metallic silver the first (Ag-Ag) shell lies at 2.89 A~. Ag-Ag distances measured for eight-atom silver clusters lies in the range 2.78 - 2.83 A [7 and see the results below]. Obtained coordination numbers and distances suggested that Ag + cations were the main species present in both samples, however in sample AgM15-100 there also appears to be a small contribution from silver dimers. Optical measurements are consistent with the EXAFS interpretation. For both samples the peaks agreeing with Ag + cations were observed (~ < 230 nm). For AgM15-100 a large peak at ~ = 310 nm, associated with Ag2+ dimers and weak peaks at 240, 260 and 390 nm assigned to Agnm+, Ag42+ clusters and subcolloidal particles, respectively [6], were also observed (Fig. 2). The dominance of silver oxidised forms is due to strong metal-support interaction occurring at high temperatures (500~ for the AgM30-500 sample and the very high strength of Bronsted acid sites for the AgM15-100 sample [6], which leads to fast oxidation of reduced silver species. 3.2. Samples with dominance of Ag8 ~ and Ag8 ~ clusters The samples of the second group including AgM10-100, AgM72-200 and AGE8-100 were prepared at medium temperatures in zeolites with Bronsted sites possessing intermediate or low strength [6]. Fourier transforms of all these samples contain two prominent features related to the presence of Ag-O and Ag-Ag distances. The Ag-O distance for each sample can be typically found within the range 2.34 - 2.35 A, that is ca. 0.08 A higher than that observed for the cationic silver zeolite species, suggesting that these distances are due to physical contact between clusters occluded within the zeolite cavities/channels and framework oxygen from the zeolite matrix. Ag-Ag distances are measured in the range 2.78 - 2.83 A. In bulk metallic silver the first (Ag-Ag) shell lies at 2.89 A. For these samples formation of Ag8 clusters is suggested from the results of theoretical ab initio calculations [8]. On the basis of the analysis of the variation within coordination numbers, Ag-Ag and Ag-O distances, and optical measurements (peaks at ca. 290 and 320nm, Fig. 2), the difference in Ag8 clusters structure could be suggested. To fully understand these differences it is important to compare the pore structure of mordenite and erionite. Mordenite represents a two-dimensional system of crossing channels, which are characterised by different elliptic cross-section with axes: (a) 0.29 0.57 nm; (b) 0.67 - 0.70 nm [19]. Erionite has elongated cavities with diameter 0.63 nm and length ca. 1.5 nm connected by small windows with a diameter 0.25 nm. The cavities

1944 of neighbouring channels are interconnected with twisted 8-member rings (with sizes 0.36 x 0.52 nm) [19]. Hence, mordenite and erionite have pores with very similar crosssection (0.67 - 0.70 nm in mordenite and 0.63 - 0.63 nm in erionite), but these pores are represented by channels in mordenite and by elongated cavities in erionite. The similarity of the pore size leads to the formation of silver cluster with alike electron structure (peaks at 322 and 293 nm for AGE8-100 and peaks at 320 and 285 nm for AgM72-200 in optical spectra). The two peaks observed in the visible range for the AGE8-100 sample have been assigned to electron transfer between the clusters within the zeolite structure [7]. Results show that Ag8 clusters stabilised in relatively spacious mordenite channels possessing medium acid strength (AgM10-100) are more deformed than Ag8 clusters stabilised by weaker protons (AgM72-200). The increase in cluster deformation for AgM72-200 is characterised by a decrease in coordination number for Ag-Ag interactions (from 3.6 to 3.0) while Ag-Ag distances do not change (ca. 2.83 _~). Deformation of the Ag8 clusters increases significantly (coordination number for Ag-Ag interactions diminish to 2.0 and Ag-Ag distances decrease to 2.78 A) when they are stabilised in the cavities of erionite, where space is greatly restricted. It is interesting to note that silver clusters stabilised in 2-propanol solutions were characterised by peaks at 325 and 295 nm [20]. The positions of these peaks related to silver clusters are closer in position to the peaks for clusters in erionite (322 and 293 nm) than for clusters in mordenite (320 and 285 nm). In the solution, silver clusters are found in an isotropic environment. That suggests that in the tiny erionite cavities, silver clusters are in more isotropic surrounding that in the wider long mordenite channels. In the optical spectrum of AgM72-200 sample, the two peaks at 322 and 285 nm are assigned to the cluster species Ag8~ and Ag8~+, respectively [6]. In the spectrum of AgM10-100, the peak at 290 nm (attributed to Ag8~+) practically disappeared and only the peak at 323 nm (assigned to Ag8~ is observed. For these samples Ag-Ag distances are practically identical, but coordination number for Ag-Ag bond for sample possessing selectively Ag8~ clusters (3.0) is less that that for the sample with mixture of charged and neutral eight-atom cluster (3.6). This implies that Ag8~+ cluster is more deformed than Ag8~ According to Ref. [6] neutral Ag8~ clusters are stabilised by weak acid sites. The charged Ag8~+ clusters are stabilised by strong acid sites, which induce electron density transfer from silver cluster to acid site. This strong interaction between strong acid site and cluster could induce more significant deformation of cluster than weak acid site.

3.3. Sample with dominance of Ag particles The AgM128-300 sample represents the third group. The AgM128-300 sample is prepared at high temperature with mordenite possessing a very small concentration of weak Bronsted acid sites. According to optical data, it contains small Ag particles on the external mordenite surface. In Fig. 2, a very broad peak with maximum position ca. 400 nm was observed. This kind of broad peak is typical for plasma surface resonance band for small particles. Particles with size c a . 1 - 30 nm are observed in micrograph of AgM 128-300 sample (Fig. 3). EXAFS results showed that Ag-Ag distance for these particles (2.83 A) is high and comparable with that observed for Ag8 clusters in mordenite but the coordination number for Ag-Ag interactions is low (2.4). These results suggest that these Ag particles are amorphous. These EXAFS parameters could also be possible if the contribution of silver clusters in the sample is high but could not be registered by optical spectroscopy due to the high fraction coverage of the zeolite surface by Ag particles "masking" smaller silver clusters

1945 located inside pores. However, TEM charcterisation (Fig. 3) shows that the fraction coverage of the zeolite surface by Ag particles is not high for AgM128-300 and therefore, the second hypothesis is not proposed to be highly unlikely.

Figure 3. TEM micrograph of sample AGM128-300 4. CONCLUSIONS The characterisation of a range of silver species formed within zeolite matrices by a combination of EXAFS and Diffuse Reflectance UV-Visible spectroscopy has allowed conclusions to be drawn concerning the effects that the structural type, SIO2/A1203 ratio of zeolite matrix and reduction temperature exerts on the size, structure, deformation and oxidation of silver clusters stabilised inside zeolites. 1. The preparation of silver-zeolite sample using mordenite with medium Bronsted acid strength at high temperature (500 ~ or using mordenite with the very high strength at low temperature (100 ~ leads to fast oxidation of reduced silver species mainly into Ag§ cations. 2. After reduction at medium temperatures silver clusters interpreted as Ag8~ and Ag8~§ are formed in mordenites with intermediate Bronsted acid strength. 3. The reduction of silver samples at high temperature utilising mordenite with very small concentration of weak Bronsted acid sites leads to formation of amorphous Ag particles on external zeolite surface.

1946 4. Deformation of the eight-atom clusters is higher in the erionite cavities, where space is greatly restricted, than in mordenite channels. The charged Ag88+ clusters are more deformed than Ag8~ ones. ACKNOWLEGEMENTS

This work has been supported by CONACYT through grant No 31366-U and UNAM through grant PAPIIT-UNAM No IN 115800. The authors thank J.M. Corker for the performance of EXAFS experiments, and E. Flores, I. Gradilla, F. Ruiz, y G. Vilchis for technical assistance in optical and TEM studies. REFERENCES

1. M. Anpo, M. Matsuoka, H. Mishima, H. Yamashita, Res. Chem. Intermed. 23 (1997) 197. 2. M. Anpo, M. Matsuoka, Y. Shiyona, H. Yamashita, E. Giamello, C. Morterra, M. Che, H.H. Patterson, S. Webber, S. Ouellette, M.A. Fox, J. Phys. Chem. 98 (1994) 5744. 3. J. A. Martens, A. Cauvel, A. Francis, C. Hermans, F. Jayat, M. Remy, M. Keung, J. Lievens, P.A. Jacobs, Angew. Chem. Int. Ed. 37 (1998) 1901. 4. Y. Traa, B. Burger, J. Weitkamp, Micropor. and Mesopor. Mater. 30 (1999) 3. 5. N.E. Bogdanchikova, E.A. Paukshtis, M. Dulin, V.P. Petranovskii, Y. Sugi, T. Hanaoka, T. Matsuzaki, X. Tu, S. Shin, Inorg. Mater. 31 (1995) 487. 6. N.E. Bogdanchikova, V.P. Petranovskii, R. Machorro, Y. Sugi, V.M. Soto and S. Fuentes, Appl. Surf. Sci. 150 (1999) 58. 7. J.S. Ogden, N. Bogdanchikova, J.M. Corker, V.P. Petranovskii. Europ. J. of Phys. D. 9 (1999) 605. 8. V.S. Gurin, N.E. Bogdanchikova, V.P. Petranovskii, J. Phys. Chem. B 104 (2000) 12105. 9. T. Yokoyama, T. Ohta, Jap. J. Appl. Phys. 29 (1990) 2052. 10. P.A. Montano, J. Zhao, M. Ramanathan, G.K. Shenoy, W. Schulze, Z. Phys. D atoms, Molecules and Clusters 12 (1989) 103. 11. T. Miyanaga, H. Hoshino, H. Endo, H. Sakane, J. Synchrotron Rad. 6 (1999) 442. 12. S. Bordiga, C. Lamberti, G. Turnes Palomino, F. Geobaldo, D. Arduino, A. Zecchina Micropor. Mesopor. Mater., 30 (1999) 129. 13. S.M. Kanan, M.A. Omary, H.H. Patterson, M.Matsuoka, M. Anpo, J. Phys. Chem. B, 104 (2000) 3507. 14. T. Sun, K. Serf, Chem. Rev. 94 (1994) 857. 15. N. Binsted, "PAXAS": Microcomputer program for pre- and post-edge background subtractions. University of Southampton. UK, 1988. 16. S.J. Gurman, N. Binsted, I. Ross, J. Phys. C 17 (1984) 143; C 19 (1986) 1845. 17. G.J. Herdman, G.W. Neilson, J. Mol. Liquids 46 (1990) 165. 18. A. Balerna, E. Bernieri, P. Picozzi, A. Reale, S. Santucci, W. Burattini, S. Mobilio, Phys. Rev. B 31 (1985) 5058. 19. D.W. Breck, Zeolite Molecular Sieves. Structure, Chemistry, and Use (A WileyInterscience Publication, John Wiley & Sons, New York-London-Sydney-Toronto, 1974). 20. B.G. Ershov, E. Janata, A. Henglein, J. Phys. Chem. 97 (1993) 339.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1947

M o l e c u l a r d y n a m i c s s i m u l a t i o n s of s t a t i c a n d d y n a m i c p r o p e r t i e s of w a t e r a d s o r b e d in c h a b a z i t e S. Jost, S. Fritzsche, R. Haberlandt University of Leipzig, Institute for Theoretical Physics, Augustusplatz 10-11, D-04109 Leipzig, Germany As pores of zeolites under normal conditions are always filled with water, it is of great interest to get information about the structure and the dynamics of the adsorbed water molecules. Molecular dynamics (MD) simulations are used to get structural informations which are experimentally not accessible and to examine the diffusional process of water inside a zeolite. It is found, that the water molecules build up a partial hydration shell at the calcium atoms, which are attached to the oxygen atoms of the zeolite walls on their other sides. In the case of full water loading there are more water molecules than free places in the first hydration shells. Therefore, there is a part of water molecules which is able to move, leading to a diffusion coefficient, which increases with increasing number of guest molecules for a certain range of loadings. The special structure (trigonal space group R3m [1,2]) causes diffusion anisotropy, which is examined in further detail. The results for the diffusional part of the study are compared with experimental data.

~'::':!:i~:..':.i:~:!:ii:~ ~~'..~k.~.~ :.

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iiilii , i ~ i ~

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.;iiiii~i:~i,!ili!:i:ii.:~:

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::::i:!:!:?:'....... ::" ~:..'.i::~:~i.'.':i:':~'"":~ ~:~ ~

:"::::::::::::"

Z • Figure 1. Left: A typical example of natural chabazite crystals, found in t~ep~ice (Czech republic). The size of the crystallites is approximately lcm. Right: Scematic view of the chabazite framework.

1948 Table 1 Partial charges of the atoms in units of e. OL1 denotes an oxygen atom between two silicon atoms, OL2 an oxygen atom between one silicon and one aluminum atom q(H) q(OH~.O)q(OL1)q(OL2) q(Si) q(A1) q(Ca) +0.33 -0.66 -1.025 -.12 +2.05 +1.75 +2.0

1. T H E M O D E L / C O M P U T A T I O N A L

DETAILS

The interatomic potentials are modelled as a combination of empirical and quantum chemical potentials from the literature. Besides the intramolecular water potential, all potentials are pair potentials. These potentials consist of one short ranged part of LennardJones or Buckingham type and a long ranged Coulombic part. As the calculations need very long computational times, we had to make a compromise in using sophisticated models and decided to ommit lattice flexiblility except for the cations, which are free to move. For the same reason we truncated all forces with a shifted forces correction at the cutoff r o u t = l lJ~. To improve the computational efficiency we used the RESPA algorithm [3], a multiple time step algorithm, with three different time levels. The intramolecular forces were calculated every 0.25 fs, the short ranged part of the interatomic forces every 0.5 fs and the long ranged part every 2 fs. With these technics the runs took approximately two months on our HP J5600 workstation, because for the diffusional part of this study calculations of up to 10.000.000 long ranged time steps (20 ns) were necessary. The physical situations under study vary from partially loaded zeolite (1 quarter of the possible loading) to the fully loaded zeolite, i.e. 13 water molecules per unit cell, and from room temperature (here: T = 300 K) up to very high temperatures (T = 600 K) to make diffusion faster. For the water-water interaction the BJH potential [4] is used, a well tested empirical model of valence force type. This model was initially develloped to fit the internal viTable 2 The pair potentials, without the Coulombic part, energies in kJ/mol, distances in A. The Ca-O potential is used for the oxygen of the lattice as well as for the oxygen of the water molecules. = l12010/(r s's6) - 1.0465. e x p [ ( - 4 . 0 ) . ( ( r - 3.4)2)] Vo-o 1.0465 9exp((- 1.5) 9 ((r - 4.5)2)) ( H20-H20, only) -26.1/r 9a - 41.86/(1 + exp[40.0 9 (r - 1.05)]) VO-H -16.744/(1.0 + exp[5.493 9 (r - 2.2)]) = 418767/(1 + exp[29.9 9 (r - 1.968)]) VH-H = - 1 5 7 2 / r 2 + 259700 9exp(-3.49 9r) VCa-O -

VCa-H Vca-ca gAl,Si-O Vo-o (nur H20-Gitter)

=

626/

= =

5560/r 2 + 1217632. exp(-6.79 9r) - 5 6 . 5 / r 6 + 1028/r 12 - 3 1 0 0 / r 6 + 3500000/r 12 + 15/(r 4)

+ 120200 9

xp(-6.79 9

1949 0.7

3.25 Mol./cage, T=30()K ,' 6.5 Mol./cage, T = 5 0 0 K .......... 13 Mol./cage, T = 6 0 0 K ..... ,~....

0.6 0.5 .o (3L

o

13..

X

,"\

//~".,.

0.4

//

0.3

, ," ",.~".,

,,'""

,, ,.

,

";

0.2

0.1

.--~(

,-

it...

. ._-.--';.: ............. .-"

,

,

, ~

0

3

4

5

1

2

........ 6

Figure 2. Probability distribution to find a calcium ion with a defined number of water molecules in its first hydration shell for different physical situations

brations, but was even succesfully used for hydration problems. It is combined with a potential for the interaction with calcium [5], which was fitted to quantum chemical calculations. The potentials for the framework guest interactions are taken from [6], except for the electrostatic charges, where we used the ones suggested in [7], because this choice ensures electrostatic neutrality for zeolites of general chemical compositions. 2. R E S U L T S A N D D I S C U S S I O N S

2.1. The hydration shell of the ions One major point in the structural analysis of the system is the hydration shell of the extra framework cations (calcium). The coordination number of calcium with respect to water molecules is influenced rather by the total amount of water in the system than by the temperature (see Fig. 2). For the fully loaded zeolite, the mean coordination number is 5 with a none vanishing part of ions with 3, 4 and 6 water molecules around them. For the half loading, not all possible places around the ions can be used, so the mean coordination number is is only 3 and the values, which occur vary from 1 to 5. In all cases the distance between the calcium ions and the oxygen atoms of the nearest water molecules is Ar ~ 2.4/~ (Fig. 4). Very interesting is the situation for the lowest situation under study, with a mean loading of 3.25 molecules per unit cell. In this case there is a big amount (~ 33%) of calcium ions without any water molecules in its direct neighbourhood. This amount is too big to be explained just by a random occupancy of equivalent cations. Indeed, further structural analysis shows, that these water free cations occupy the centers of the hexagonal prisms. These prisms connect the cages in the z-direction and are too small for a water-ion-complex. If we take into account, that the cations with 1 water molecule in the first hydration shell are likely to be placed closer to the border of the hexagonal

1950

0.9

i

i

i

0.5

1

1.5

0.8 0.7 0.6 ~

0.5

o 12.

0.4 0.3 0.2 0.1 0

0

2

Figure 3. Probability distribution to find a water molecule with a defined number of calcium ions in its immediate neighbourhood for different physical situations

prisms, but inside as well, we find, that almost all hexagonal prisms are occupied (There are two calcium ions per unit cell, but only one hexagonal prism.) Altogether there are two preferred types of sites, which are preferably occupied by the calcium ions: In the case of partially dehydratrion, the ions occupy the sites in the hexagonal prisms and then in the 8 rings of the framework. These site are not in the center, because the calcium ions are too small to bridge the 8 rings, but at the outer part. There the ions are usually attached to an oxygen atom of the framework, which has one aluminum atom as a direct neighbour. This behaviour is found quite similar in X-ray diffraction studies [8]. 2.2. T h e s t r u c t u r e of t h e a d s o r b e d w a t e r The other major point of intererest is the structure of the water molecules. The most important interaction with the framework system takes place with the calcium cations.

'

'

goo' goh .......... ghh ...........

6 _

'

5

i

i i

'

'

4

6

+o0.~'

ghda .......... ndca ...........

I

"I /

4 3 2

i.il 0

2

1 I

!

I

4

6

8

10

0

0

V//

2

8

10

Figure 4. Left" Radial distribution functions of water. Right: Radial distribution functions and running integration numbers of water with calcium. Both at T - 600 K and for 6.5 water molecules per unit cell

1951

u z ......)4....... rj~ L.M v-,

E

"T O

I

T"v

a 0.1

300

350

400

450

500

550

600

T/K

Figure 5. The main elements of the diffusion tensor for different temperatures and the half maximum loading.

Although the aluminum and the silicon have partial charges in the same order of magnitude as the calcium, their charges are screened by the neighbouring oxygens. For the lower loadings under study, almost all water molecules have exactly one neighbouring cation, only a few are stuck between two ions and very few have no cation in their immediate neighbourship. At the full loading, the situation has changed. Still most water molecules are attached to exactly one cation, but approximately 38 % have no cation as a direct neighbour. As these water molecules are relatively loose bound, they form a free phase of water inside the zeolite, which can easily diffuse. Another question was, weather the water molecules are able to build up a liquid like network. It turned out, that the structure is disturbed by the zeolite but most significant features are still there. The water can still build up hydrogen bonds, but the number is remarkably reduced to an average number of no more than one hydrogen bond per molecule. This implies, that the other hydrogen atoms are bound somewhere else, at the oxygen atoms of the lattice. 10

t'Xl

E

"T, O v

(:3 0.1

I

1.6 1.8

I

2

I

I

I

I

2.2 2.4 2.6 2.8

I

3

I

,.ft.

3.2 3.4

I000 K / T

Figure 6. Arrhenius plot of the mean diffusion coefficient for 6.5 molecules per unit cell

1952 30

|

,

4

5

,

,

,

7

8

,

,

,

,

25

:

E

20 15

--..- 10 a "1'-"-'

5

3

6

9

10 11 12 13

Water molecules per unit cell Figure 7. The mean diffusion coefficient versus loading at T = 600 K

2.3. Diffusion Due to the symmetrie of the crystal, diffusion in chaba~ite is anisotrop. It can be deduced from the the structure using a jump model, that diffusion in the x-y-plane should be a little bit faster than in the z-direction. If there are no further diffusion barriers Dz/D=y = 0.8 was predicted by B~r et al.[9]. But in the same study, they measured the diffusion coefficients with two different P FG NMR methods, a powder diffraction probe and a measurement with an oriented single crystal. The two methods agreed quite well and yielded Dz/D=y ,~ 0.4. This difference between theory and experiment was explained by the assumption of an additional diffusion barrier in z-direction in the middle of the cages. Diffusion of water in chabazite is a very slow process on the time scale of molecular dynamics simulations, so we had to heat up the system to get reliable diffusion coefficients up to T--- 600 K. At this temperature we found the following behaviour(fig. 7)" For a very low loading there was very little diffusional motion, leading to a very small diffusion coefficient. With increasing loading, the diffusion coefficient increases, up to approximately 75 percent of the maximium loading. For higher loadings the diffusion coefficients decrease with increasing loading. The explanation of this anormal diffusional behaviour is straight forward. A small number of water molecules is bound very strong to the cations and therefore not able to move. When almost all places in the first hydration shells are occupied, the remaining molecules can move faster. With further increasing number of water molecules, the number of mobile molecules increases and this leads to an increasing average movement. On the other hand, more molecules means more possible collision partners, decreasing the average displacement of one molecule. At approximately 10 molecules per unit cell this effect gains control over the complete process, leading to a decrease in the diffusion coefficients with further increase in the number of molecules. The anisotropie coefficient (fig. 5) varies in a range 0.7 < Dz/D=y < 1, in all situations,

1953 Table 3 The mean residence time of water in the free and the adsorbed phase for different loadings (in water molecules per cavity) and different temperatures. free phase bound phase Loading 3.25 6.5 13 3.25 6.5 13 >10ns >10ns >10ns 300K >10ns >10ns >10ns 400K 5ns 0.8ns 500K 0.4ns 3.5ns 600K > 10ns 0.14ns 0.57ns > 10ns 1.03ns 0.94ns

in which the diffusion coefficient can be calculated with sufficient reliability. This is very close, clearly within the errors of this study, to the theoretical prediction of B~ir et al.[9] Dz/Dxy = 0.8. But there is a significant deviation to the experimental value reported in the same study to be Dz/Dxy ,~ 0.4. The agreement is a little bit better with the tracer diffusion experiments[10], where no significant deviation from isotropy was reported. Unfortunately, we cannot confirm the assumption of an additional diffusive barrier with this study. But as it is impossible to make these calculations for room temperature, there might be such a low barrier, so it has no importance at these temperatures anymore.

2.4. Adsorption / desorption at the ions The other important dynamic process which is examined, is the exchange between the two phases of water molecules in the chabzite. As shown above, the water molecules in the first hydration shells of the calcium ions and the free water molecules are clearly distinguished. The question, whether there is an exchange betwewn these phases is important for the mechanism of diffusion, which can be observed. If there is no exchange, the ions with the hydration shells would block a lot of free space especially directly in the windows between the cages. If there is a frequent exchange, the ions can serve as a temporary station in the diffusion path. For room temperature adsorption and desorption to the first hydration shell is a very rare event in MD time scales, because there occur less than 0.05 events per particle and nanosecond, leading to less than 50 events in total, so these numbers cannot be evaluated. For T - 600 K we find residence times in the order of 1 ns. For our system, this means that exchange between the two phases is an improtant element in the diffusioal process, because the water molecules interchange between the two phases quite often. In those situations, when no exchange takes place, the diffusion coefficient is very low. 3. C O N C L U S I O N S The used model describes very reasonable the static properties of the system. The places for the cation agree well with experimental studies for a partially dehydrated zeolite as well as for a completely hydrated zeolite. In our model system, the water molecules build up a hydrogen bondend network even under these confined circumstances. They form a partial hydration shell around the calcium ions, which are attached to oxygen atoms of the walls on the other sides. We found a diffsion anisotropy in the order of magnitude, which is caused by the symmetrie of the crystal, but we found no evidence for an additional

1954 diffusional barrier. The exchange between the first hydration shell of the calcium ions and the free water is an essential process for the diffusion, as there is no measurable diffusion without it. REFERENCES

1. W.M. Meier and D. H. Olson. Atlas of Zeolite Structure types. Butterworths, London, second edition, 1987 2. J.V. Smith and F. Rinaldi. Acta Cryst., 16(1963)45 3. M. Tuckerman, B.J. Berne, G.J. Martyna, J. Chem. Phys. 97(1992)1990 4. P. Bopp, G. Jancso, K. Heinzinger, Chem. Phys. Lett. 98(1983)129 5. M. M. Probst, T. Radnai, K. Heinzinger, P. Bopp, B.M. Rode, J. Phys Chem 89(1985)753 6. P. Cicu, P. Demontis, S. Spanu, G.B. Suffritti, A. Tilocca, J. Chem. Phys. 112(2000)8267 7. Eugenio Jaramillo, Scott M. Auerbach, J. Phys. Chem B 103(1999)9589 8. W.J. Mortier, J.J. Pluth, J.V. Smith, Mat. Res. Bull. 12(1977)241 9. N.K. B~ir, J. K~rger, H. Pfeiffer, H. Sch~ifer, W. Schmitz, Mic. Mes. Mat. 22(1998)289 10. S. V. Garyainov, I.A. Belitsky, Phys. Chem. Min. 22(1995)443

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1955

Correlations in Anisotropic Diffusion of Guest Molecules in Silicalite-1 S. Fritzsche a, J. Kgrger b aUniversity Leipzig, Institute for Theoretical Physics, Augustusplatz 9-11, D-04109 Leipzig, Germany bUniversity Leipzig, Institute for Experimental Physics I, Linnstr. 5, D-04103 Leipzig, Germany Analytical expressions for the correlation between the components of the diffusion tensor in the case of anisotropic diffusion of methane in silicalite-1 have been derived. The asymmetry in the different jump probabilities is taken into account. For comparison the same sytem is investigated by Molecular Dynamics simulations. Comparison of the derived dependencies with the results of the MD simulations yields satisfactory agreement. 1. I N T R O D U C T I O N

,< r

II

X Figure 1. Topology of the channels in silicalite-1

The diffusion of guest molecules is of crucial importance for many technical applications of zeolites as catalysts or molecular sieves [1]. Most popular applications are the use as ion exchangers in phosphate-free washing agents and the use as catalysts in the gasoline production. The migration of particles inside the zeolite is usually the process that controls the time scale and therefore the efficiency of such applications. Therefore, diffusion of guest molecules in zeolites is the subject of many experimental and theoretical investigations. Overviews can be found in [2-8]. The influence of the anisotropy of the channel systems in some zeolites (e.g. silicalite-1) on diffusion is examined in the present paper. The geometrical structure of the channel system in this zeolite is visualized in Fig. 1 (the

1956 more realistic shape can be seen in fig. 2) showing three unit cells including the axes of the channel system, straight channels in y- and zig-zag channels in x-direction. A consequence of this structure is that e.g. long-range movements of a particle in z-direction are only possible as sequences of moves both in in the straight and zig-zag channels. Therefore, the components of the diffusion tensor are correlated [9]. Similarly, an interrelation between the diffusivities in different directions has been predicted to occur in zeolites of type ZSM-11/silicalite-2 [10] and chabazite [11]. In earlier papers the memory effects in the sequence of random jumps have been neglected [12], or have been taken into account only partially by introduction of a two-step model of diffusion [13-15]. If such effects are neglected, the principle values of the diffusion tensor, i.e. the diffusivities in x-, y- and z-direction are determined by the topology and they are related to each other by simple reciprocal addition [9-12] Dz

=

Dx

+

Dv

with l~, ly and lz denoting the unit cell extensions in x-, y- and z-direction. Eq. (1) served as a guide for correlating the data of diffusion anisotropy in numerous experimental [16,17] and simulation studies [13-15,18-21] Though in many simulation studies in particular for small guest molecules the correlation between the diffusivities as provided by eq. (1) proved to provide a reasonable estimate, there were also notable deviations. In ref. [18] these deviations were quantified by introducing a memory parameter fl =

(2)

l~/Dz

It is obviously equal to 1 if eq. (1) is valid, i.e. in the case of totally negligible memory. 2. A N A L Y T I C A L T R E A T M E N T The parameters Pv,v, Pv,-v, Pv,x = Pv,-~ (and Px,~, P~,-x, P~,v - P~,-u), denote the probabilities that a displacement from intersection to intersection along a straight channel (zig-zag channel) is followed by a displacement in the same direction, in the opposite direction and along the other type of channels, respectively. Since the sum of all probabilities to proceed from a given intersection must be equal to one, the relations P~,~ + P~,-~ + 2Px,v -- Pv,v + Pv,-v + 2Pv,x - 1

(3)

are valid. The probability Pv (P~) that an arbitrarily selected segment of the diffusion path is along the straight (zig-zag) channels has to obey the stationarity condition =

+

(5)

yielding Pv

=

p~

=

2 - (Px,x "+"Px,-x "+"Py,y -k py,-y) 2 Pv,x 2 - (Px,x + P~,-~ + Pv,v + Pv,-v)

(6) (7)

1957 or equivalent expressions resulting by application of eq. (3). For particles without memory (i.e. the case considered in refs. [9-12]) we have p~,~ =

;~,_~ = p~,~ = p~/2

(8)

;~,~

; ~ _ ~ = ;~,~ = p~/2,

(9)

=

The mean square displacement of a particle in a given direction, e.g. the x-direction follows from the single shifts in that direction as

with n~ denoting the total number of displacements between adjacent intersections along the zig-zag channels and the respective displacements lxi being equal to + a (of. fig. 1). The component D~ of the diffusion tensor can be calculated from this quantity by the Einstein relation for large times t Dx = < x2(t) > (11) 2t ' with < x2(t) > denoting the molecular mean square displacement during t. Similar relations hold for the y- and z-directions where the displacements are lyi - i b and lzi = =l=c, respectively. The number of displacements in z-direction is the same as that in x-direction, since both displacements occur only simultaneously (see fig. 1). The fraction of the number of displacements in y-direction and that in x-direction must converge in the long-time limit to the ratio of the probabilities py and px

lim ---n~ PY--. t-,~r nx Px The mean square displacements are calculated [22,23]:

(12 )

l~

= nl ~ + 2 ~ < Z,l~+~ > + 2 ~ < l,l,+~ > + . . . (13) i--1 i=l with n standing for nx or ny or nz, I for a, b, or c and li for l~i, lyi or lzi, respectively. The mean values of the products of subsequent displacements < l~l~+l > have to be treated

separately for displacements within one channel type (i.e. in x- and y-direction), and for z-direction. As soon as a molecule changes from one channel type into the other, previous displacements along this channel are not correlated anymore with future ones along the same channel. Subsequent displacements in x- and y-direction are thus easily found to obey the relations < lxilx(i+j) >

-

(Px,x - Px,-x) j a 2,

< lyily(i+j) >

=

(py,y - py,-y)J b~.

(14) (15)

Inserting these relations into eq. (13) yields, for sufficiently large values of nx and ny --

lxi

=

n~

1

n--y

aS 1 + ( P x , z - P~,-~) 1 - (;~,~ - ;~,_~)'

lyi

__

b2 1 + ( p y , y - py,_y)

1 - (py,y - py,_y)"

(16)

(17)

1958 While the probabilities px,~ and p~,_~ (py,y and p~,_y) that subsequent displacements along the crystallographic x- (y-) coordinates are parallel or antiparallel directed, are given by the very model applied, the equivalent probabilities p+ and p_ for displacements in zdirection have to be provided by additional consideration. It may be deduced from fig. 1 that subsequent displacements in z-direction are antiparallel (parallel) oriented if they are separated by an even (odd) number of displacements along the straight channels. Summing over all probabilities belonging to either of the two cases yields OO

(18) i=0 O0

p+

=

4p~,ypy,~

~[(py,y + py,_y)2]i.

(~9)

i-0

The first two terms on the right hand side of eq. (18) stand for the cases of continuation of propagation along the zig-zag channels, while the sum including the prefactor considers all cases of even-numbered subsequent displacements along the y-channels. In all these cases the displacements in z-direction are opposed to each other. Eq. (19) runs over the odd numbers of intermediate displacements along y-direction and thus stands for the probability that subsequent displacements in z-direction are parallel. By summing up and making use of eq. (3) eqs. (18) and (19) may be transferred into p-

=

2(1 - p~,~ - p~,~) 1 + py,y + py,_y

P+

=

1 § py,y + py,_y"

(20)

2p~,~

(21)

With the corresponding replacements (px,~ or py,y by p+, p~,_x or py,_y by p_ and a or b by c) one finally obtains from eqs. (16) and (17) in the limit nz --+ oc

!

l~

= c~ 1 + (p+ - p _ ) = c~p_+

nz

1 - (p+ - p _ ) ---

2 C

Px,y

1 - Py,x - P~,y'

p_

(22)

where in the second equation we have made use of p+ § p_ - 1. Finally by use of the Einstein relation (eq(ll)) and the expressions for the mean square displacements as given by eqs. (16), (17) and (22) the memory parameter (eq. (2)) results to be

/~ =

p~c py A + Px B

(23)

with the notations A =

1 - (p~,~ - p~,_~) 1 + (p~,~ - p~,_~)

(24)

B

=

1 - (pu,y - py,_y) 1 + (py,y- p~,_y)

(25)

C

=

1-py,x-p~,y. Px,y

(26)

1959

yz plane

xy plane

Figure 2. Definition of the intersection regions. The circles mark those regions that are treated as intersection regions. The lowest energy values connected with small minima outside of the intersections are about -18 kJ/mol while the maximum in the center of the intersection is at about -9 kJ/mol. The energy difference between adjacent lines in the picture amounts to 1 kJ/mol. The outmost isopotential lines correspond to-5 kJ/mol.

With the eqs. (8) and (9), eq. (23) may be easily shown to fullfill the condition 3 = 1 for molecular propagation without memory. 3. M D S I M U L A T I O N S The relevance of the obtained analytical expressions has been investigated in extensive MD simulations, which are described in detail in ref. [24]. The simulations have been

1960

Table 1 Results of MD simulations for different radii r of intersection regions. The D values are in 10 -8 m2/s. The notations no-mem, mem and MD refer to data analysis without memory effects, with memory effects and the MD data. r/A 2.0 2.5 3.0 5.0 0.19345 0.16695 0.13477 0.04369 P~,~ 0.32031 0.43876 0.56070 0.87158 0.24078 0.19379 0.14724 0.04161 Px,y 0.37762 0.33034 0.26652 0.08525 Py,y 0.34844 0.4351210.54964 0.86070 Py,-y 0.13891 0.11877 0.09365 0.02751 Py,x 0.63348 0.61635 0.60295 0.59967 Py 0.36547 0.37775 0.38350 0.39653 P~ Ttx 3634 4 768 6 661 30 353 6443 ny 8 007 11023 47611 0.921 nx (no mem) 1.21 1.68 7.64 0.714 D~ (mem) 0.691 0.677 0.720 D~ (MD) 0.718 0.718 0.718 0.718 Dy (no mem) 1.61 1.99 2.74 11.81 Dy (mem) 1.70 1.62 1.53 1.49 Dy (MD) 1.68 1.68 1.68 1.68 (-o mere) 0.263 0.338 0.469 2.09 (mere) 0.160 0.152 0.146 0.153 Dz (MD) 0.17 0.17 0.17 0.17 1.40 1.42 1.42 1.41 fl (mem, eq. (23)) 1.33 1.33 1.33 (MD, eq. (2)) 1.33

carried out for a rigid lattice of pure silicalite-1 with methane as the - spherically shaped guest molecule at a loading of one molecule per channel intersection (corresponding to four molecules per unit cell) and a temperature of 300 K. The interaction parameters for the methane/lattice interaction have been taken from the spherical model potential derived in [25]. In the simulation procedure we have essentially followed our previous studies of the same host-guest system [19,26,25]. In the present study we have considered runs with an unperturbed evaluation part (after thermalization) of 5.106 simulation steps. The temperature was adjusted in the thermalization part of the run using a procedure described in [27,25] which enables runs in the microcanonical ensemble with a predefined value of the temperature. As the time step was 5 fs the length of the examined trajectory corresponded to a total time of 25 ns. A declaration of the limits of the channel intersection regions is not free from arbitrariness. One possible choice is illustrated by fig. 2. It shows the isopotential lines for the centre of a single methane molecule in three planes: a cut through the straigth channel in the yz-plane at x - 0, a cut through the zig-zag channel in the xz-plane at y = 0 and a cut through the straigth channel in the xy-plane at z = 0. The circles correspond -

1961 to cuts through spherical regions of radius 3 _~ which can be interpreted as intersection regions. The simulations have been carried out for different radii of such spheres. Table 1 provides a summary of the obtained simulation results. It particularly includes numerical values for all probabilities introduced in this study. For illustrating the data scattering, the values of both Px,y and Px,-y are presented which are found to differ by several per cent though they should coincide. The D values obtained from MD have been calculated from 4 moments of the displacement as described in [28] for the isotropic and in [25] for the anisotropic case. In addition to these diffusivities from the MD simulations, tab. 1 also contains the diffusivity data which would result by use of the Einstein relation together with eqs. (16), (17) and (22) with the indicated probabilities (case 'with memory' marked by (mem) in the table) and with their simplifications by eqs. (8) and (9) (case 'without memory' marked by (nomem) in the table). 4. C O N C L U S I O N S Summing up infinite series containing the conditional probabilities of passages through the straight and the zig-zag channels it is possible to derive formulas for the components of the diffusion tensor for methane in the silicalite-1. The results do not only mirror the topology of the channel system but, they also take into account memory effects. The derived analytical expressions presented in this paper yield values that are much closer to MD results than those from all earlier analytical treatments of the problem. Nevertheless, the agreement can probably be improved by further examinations. The interpretation of the results also remains a task for more detailed studies. 5. A C K N O W L E D G E M E N T The authors thank the Deutsche Forschungsgemeinschaft (SFB 294) for financial support.

REFERENCES 1. R. Haberlandt, S. Fritzsche, and H. L. Vhrtler, Simulation of microporous systems: Confined fluids in equilibrium and diffusion in zeolites, in Handbook of Surfaces and Interfaces of Materials, edited by H. S. Nalwa, volume 5, pages 358-444, Academic Press, San Diego, London, Boston, New York, Sidney, Tokyo, Toronto, 2001. 2. J. K~rger and D. M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992. 3. N. Chen, J. T.F. Degnan, and C. Smith, Molecular Transport und Reaction in Zeolites, VCH, New York, 1994. 4. D. N. Theodorou, R. Snurr, and A. T. Bell, Molecular dynamics and diffusion in microporous materials, in Comprehensive Supramolecular Chemistry, Edt. G. Alberti and T. Bein, volume 7, pages 507-548, Pergamon, Oxford, 1996. 5. P. Demontis and G. B. Suffritti, Chemical Reviews 97, 2845 (1997). 6. S. Bates and R. van Santen, Adv. Catal. 42, 1 (1998). 7. F. Keil, R. Krishna, and M.-O. Coppens, Chem. Engin. Journal 16, 71 (2000). 8. R. Haberlandt, S. Fritzsche, and H. L. Vhrtler, Simulation of microporous systems:

1962 Confined fluids in equilibrium and diffusion in zeolites, in Handbook of Surfaces and Interfaces of Materials, edited by H. S. Nalwa, volume 5, pages 358-444, Academic Press, San Diego, London, Boston, New York, Sidney, Tokyo, Toronto, 2001. J. K~rger, J. Phys. Chem 95, 5558 (1991). 10. J. K/irger and H. Pfeifer, Zeolites 12, 872 (1993). 11. N.-K. B/ir, J. K/irger, H. Pfeifer, H. Sch/ifer, and W. Schmitz, Microporous Mesoporous Mater. 22, 289 (1998). 12. D. Fenzke and J. K/irger, Z. Phys. D 25, 345 (1993). 13. J. K/irger, P. Demontis, G. B. Suffritti, and A. Tilocca, J. Chem. Phys. 110, 1163 (1999). 14. P. Demontis, J. K/irger, G. B. Suffritti, and A. Tilocca, Phys. Chem. Chem. Phys. 2, 1455 (2000). 15. P. Demontis, G. B. Suffritti, and A. Tilocca, J. Chem. Phys. 113, 7588 (2000). 16. J. Caro et al., J. of Phys. Chem. 97, 13685 (1993). 17. J. Caro, M. Noack, K. KSlsch, and R. Sch/ifer, Microporous and Mesoporous Materials 38, 3 (2000). 18. E. J. Maginn, A. T. Bell, and D. N. Theodorou, J. Phys. Chem 100, 7155 (1996). 19. S. Jost, N.-K. B/ir, S. Fritzsche, R. Haberlandt, and J. K/irger, J. Phys. Chem. B .

lO2, 375 (199s).

20. 21. 22. 23. 24. 25. 26. 27. 28.

T. J. H. Vlugt, C. Dellago, and B. Smit, J. Chem. Phys. 113, 879 (2000). F. Jousse, S. M. Auerbach, and D. P. Vercanteren, J. Chem. Phys. 112, 1531 (2000). J. R. Manning, Phys. Rev. 116, 819 (1959). A. R. Allnatt and A. B. Lidiard, Atomic Transport in Solids, Cambridge University Press, Cambridge, 1993. S. Fritzsche and J. K/irger, J. Phys. Chem., in preparation. S. Fritzsche, M. Wolfsberg, and R. Haberlandt, The Importance of Several Degrees of Freedom for the Diffusion of Methane in Silicalite-1, submitted to Chem. Phys. S. Fritzsche, R. Haberlandt, S. Jost, and A. Schiiring, Molec. Sim. 25, 27 (2000). S. Fritzsche, Untersuchung ausgew~hlter Nichtgleichgewichtsvorg~nge in Vielteilchensystemen mittels statistischer Physik und Computersimulationen, Habilitation Thesis, University of Leipzig, 1998. S. Fritzsche, R. Haberlandt, J. K/irger, H. Pfeifer, and K. Heinzinger, Chem. Phys. Lett. 198, 283 (1992).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

1963

A combined anomalous XRPD, EXAFS, IR, UV-Vis and photoluminescence study on isolated and clustered silver species in Y zeolite C. Prestipino 1'2, C. Lamberti 1'2'3, A. Zecchina 1'3, S. Cresi 1, S. Bordiga 1'3, L. Palin4, A. N. Fitch4, P. Perlo ~ and G. L. Marra6 1 Department of Inorganic, Physical and Material Chemistry, Via P. Giuria 7, 10125 Turin (I) 2 INFM Unit~ di Torino Universit~ (I) 3 FNSTM Unith di Torino 4 ESRF, Diffraction Group, BP 220, F-38043 Grenoble Cedex, France 5 Centro Ricerche FIAT, Torino (I) 6 Polimeri Europa, S.p.A., Istituto Guido Donegani, Via G. Fauser 4, 1-28100, Novara (I) In this contribution we report on structural (XRD and EXAFS), optical (UV-Vis DRS and photoluminescence) IR (adsorption of CO) characterization of a virtually homoionic Ag-Y zeolite (Si/A1 = 2.63). Our study shows that the zeolite is a virtually 100% exchanged silver faujasite, showing almost isolated Ag+ counterions (EXAFS estimates that clustered species represent less than 2% of the whole silver). Synchrotron radiation XRPD measurements (ESRF, BM16), performed at the Ag-K edge [2` = 0.486103(2) A], just before [E -0.486093(2) A] and far away [2, = 0.491153(2) A], allowed us to locate the nearly totality of the expected Ag+ counterions: 52.0(4) out of 52.9 per unit cell, located in five different sites. Two out of the five are located in the supercage, and thus accessible to small ligand molecules, as detected by IR spectroscopy that singled out the presence of two distinct Ag+-'-CO adducts. A subsequent EXAFS (ESRF, BM29), UV-Vis DRS and photoluminescence spectroscopic study on the aggregation of Ag clusters upon thermal reduction of Ag-Y zeolite will be also briefly reported. 1. INTRODUCTION Noble metal-exchanged zeolites are active catalysts in economically important processes. In particular, Ag(I)-exchanged zeolites show high activity in several catalytic and photocatalytic processes [1-4] which have been performed by exploiting the presence of both isolated Ag+ ions and aggregated Agn clusters. Among them we can mention the photochemical dissociation of H20 into H2 and 02 [5,6], the disproportionation of ethylbenzene [7], the selective reduction of NO by ethylene [8] and the photocatalytic decomposition of NO [9]. In this contribution we will report on structural (XRD and EXAFS), optical (UV-Vis DRS and photoluminescence) IR (of CO) characterization of a virtually homoionic Ag-Y zeolite (Si/A1 -- 2.63). Our study shows that we are dealing with a virtually 100% exchanged silver faujasite showing only isolated Ag+ counterions (EXAFS estimate that clustered species represent less than 2% of the whole fraction of silver). The effect of sample activation at increasing temperature on the aggregation of silver clusters will also be briefly commented.

1964 2. EXPERIMENTAL 2.1. Materials and activation procedures Ag+-Y was prepared starting from a NH4-Y sample (Si/AI = 2.63), synthesized in Istituto Guido Donegani through a conventional exchange with a solution of AgNO3. A nearly total exchange has been obtained (corresponding to one Ag + ion for every AI framework atom), as monitored by the total erosion of the absorption bands of internal bridged Bmnsted -Si-(OH)-AI- groups centered at 3643 and 3547 cm q The first step in the characterization of a catalyst, and thus of a zeolite, concerns a thermal treatment able to remove all the molecules pre-adsorbed on the catalytic active centers coming from the ambient atmosphere (activation process). This process is needed in order to guarantee the study of well defined systems [10]. Once this step has been achieved, measurements can be performed, in situ, either on the as activated sample (i.e. zeolite under vacuum conditions) or~ after having dosed a well defined amount of high purity gas on the sample. Now, the achievement of the activation process is very critical for silver exchanged zeolites because an increase of the activation temperature (suitable to remove the most strongly bonded molecules) has the disadvantage of facilitating the aggregation of Ag + ions into Agn clusters [11,12]. Such clusters, needed for some catalytic application (vide supra), are undesired if the aim of the study is the location of isolated Ag + ions. Based on our previous study on the Ag-ZSM-5 zeolite [13,14], an activation temperature of 120 ~ has been adopted. Conversely, part of the as exchanged powder has been subjected to thermal treatments at increasing temperature (200-400 ~ with the aim to study the progressive aggregation into silver clusters. 2.2. Methods We have performed Ag-K edge x-ray absorption measurements, using the synchrotron radiation emitted by the bending magnet of the BM29 beamline at the ESRF (Grenoble, F), equipped with a Si(311) monochromator detuned to avoid harmonics. Each EXAFS spectrum, measured in transmission mode, was recorded three times under the same experimental conditions, and extracted z(k) have been averaged before the EXAFS data analysis. Standard deviation calculated from the averaged spectra was used to estimate the weight of statistical noise in the evaluation of the error associated with each structural parameters. The EXAFS data analysis has been performed following standard procedures [15] and using Michalowicz's programs [16]. The angle/energy calibration has been obtained by measuring the edge position of the corresponding metal foil (measured simultaneously with the sample by means of a third ionization chamber). Powder diffraction patterns were collected at room temperature on the powder diffraction beam line BM16 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The beam line was used with a collimating mirror before the Si(111) water-cooled, double-crystal monochromator, set to deliver the desired wavelength [;~ = 0.486103(2), or 0.486093(2) or 0.491153(2) A], in all cases calibrated using the NIST Si standard 640b. The optional focusing mirror after the monochromator was not used. The sample capillary was spun on the axis of the diffractometer, while the detector bank was scanned from 20 = 0.5 to 20 = 32 ~ at a rate of 0.5~ at low angle and at a rate of 0.1~ at high angle. Data were collected in a continuous scanning mode, with the electronic sealers and the 20 encoder reading around 6 times per second. High-angle regions were scanned more than once to improve the statistical quality of the pattern. The total data acquisition lasted ca. 8 h for each sample. For powder XRD measurements,

1965 zeolite powder, activated under dynamic vacuum at 120 ~ for lh, was transferred (in vacuo) into a boronsilicate capillary with a l-ram diameter. The capillary was sealed and then mounted on the sample spinner on the axis of the diffractometer, which maximizes the number of crystallite orientations presented to the incoming radiation and minimizes the effect of any preferred orientation in the sample. More details on the XRPD acquisition and Rietveld refinement strategies have been reported elsewhere [17-20] where the same instrument has been used to characterize other zeolitic systems. For IR measurements, we have used an IR cell, designed to allow in situ high-temperature treatments, gas dosage and low-temperature measurements. The IR spectra were recorded, at 2 cm "~ resolution, on a BRUKER FTIR-66 spectrometer equipped with a MCT detector. UV-Vis diffuse reflectance experiments have been performed with a Varian CARY5 spectrophotometer. Photoluminescence spectra have been recorded on a SPEX Fluorolog-2 spectrofluorometer equipped with a xenon UV-Vis-NIR excitation lamp, whose light is filtered before reaching the sample by an excitation monochromator; the photoluminescence emission is than selected by a second monochromator (emission monochromator) before reaching the photomoltiplier. This photoluminescence apparatus allows thus to perform both emission and excitation scans, the former by fixing the excitation and scanning the emission monochromator, the latter by fixing the emission and scanning the excitation monochromator [21]. Both monochromators allow a sampling step up to 1 nm in the 2001000 nm (50000-10000 cm "~) spectral range. All the reported spectra have been collected with an integration time of 1.0 s per point and a sampling step of A~ = 1.0 nm. Both diffuse reflectance spectroscopy (DRS) and photoluminescence experimental set-up allow to measure the samples under controlled atmospheres. 3. RESULTS AND DISCUSSION 3.1. Isolated Ag + cations: anomalous XRPD, EXAFS and IR study The tridimensional structure of zeolite-Y is generated by connecting sodalite units with hexagonal prisms to give a framework characterized by big empty cavities (supercages) with a diameter of about 13 A. It is recognized that the cations are mainly located in a few well-defined sites [22], see Figure 1. Site I is located in the center of the hexagonal prism, closely surrounded by six oxygens of the two bases of the prism. Cations located in this site are almost totally inaccessible to guest molecules. Cations in site I' are located on the external basis of the hexagonal prisms, just inside the sodalite cage; these cations are surrounded by three oxygens of the basis of the prisms and are accessible only to molecules able to penetrate through a six-membered ring from the supercages into the sodalite cavities. This penetration is not possible even for small molecules like CO [23]. Owing to Coulombic repulsion, the simultaneous occupation of adjacent I and I ~ sites is forbidden. Sites I r and II are located in the middle of the six-membered ring forming the frontier between the supercage and the sodalite cage, just inside the sodalite cage and the supercage, respectively. For the same reason mentioned before, adjacent sites cannot be simultaneously occupied. Sites I, r , i r and II are coordinated to the framework oxygen of the zeolitic walls and are peculiar of dehydrated Y. In presence of water molecules cations are solvated and occupy positions located in the middle of the cages. Figure 1 also reports the position of the five different extraframework Ag + sites obtained from the Rietveld refinement, labelled as sites I (8.2), r(17.4), Ha (6.6), Hb (15.2) and I'm(4.6) (in parenthesis the occupancy per unit cell is given). It is worth noticing that two different sites II have been found and arbitrarily labelled as IIa and IIb. Four of them occupy positions typical of dehydrated cations. On the contrary I'~ is in the middle of the sodalite cage: this implies that such ions must be coordinated to undesorbed water molecules. In fact, in

1966

Fig. 1. Representation of the Y zeolite framework and cation location. In the fraction of the framework located in the upper fight part of the figure O atoms are represented in dark grey an the T atoms (Si or A1) in light grey. For clarity in the remaining part of the framework only light grey sticks have been adopted. Ag + cations are represented as spheres and are labelled with I, I', lla, lib and l'm in different greys scales. The supercage cavity, where guest molecules can be hosted, can be noted in the centre of the figure. anhydrous conditions, the positively charged cations are in contact with the zeolitic walls, where they interact with the negatively charged oxygen atoms of the framework. The fact that only less than 5 out of 52 cations are solvated by coordinated water molecules (being 48 in form of anhydrous cations coordinated to the zeolite framework) proofs that the activation procedure was carefully selected. Figure 2 shows the high resolution XRPD pattern collected at ~, = 0.491153(2) A and at ~, = 0.486103(2) A. Of interest is the change in relative intensity between some of the observed peaks by moving across the Ag K-edge. The Rietveld refinement, performed simultaneously on the three data sets, allows to obtain the zeolite framework and to locate the nearly totality of the expected (on the basis of the Si/A1 ratio) Ag + counterions: 52.0(4) out of 52.9 per unit cell. This is a rather remarkable result if compared with what obtained for the cases of Cu+-Y [20] and of Rb§ [18] (also BM16 data) systems, where we were able to locate "only" 41.0(5) and 48(1) cations respectively. This noticeable improvement is ascribed to the following arguments: (i) the higher scattering power of Ag; (ii) the simultaneous use of three independent frames; (iii) the benefit of the anomalous effect.

1967 i

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10

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,,,

I 30

(Degrees)

Fig.2. Observed, calculated and difference profiles and reflection positions of the AgLY zeolite. Top: Z, = 0.491153(2) A; bottom: ~, = 0.486103(2) A. We have so used the output of the Rietveld refinement to simulate the EXAFS data reported in Figure 3. In theory we have to simulate 5 EXAFS signals representing the contribution to the overall signal coming from the Ag absorbers located in the 5 different sites found by XRPD. The contribution coming from the hydrated I'm site has been ignored due to the facts that: (i) less than 10% of the absorbers occupy this site and (ii) a rather high Ag-OH2 Debye-Waller factor is expected. Moreover, since sites I' and lib show a similar local environment, (three oxygen atoms at very close distance: 2.46 against 2.47 A), the corresponding contributions were merged in a single one. As a result, 3 different contributions have been used to simulate the experimental EXAFS signal. For each contribution, the Ag-O distance was fixed to the crystallographic value (2.31 A for lla,

1968 2.61 A for I and 2.465 A for I' and Hb), while the coordination number was obtained from the "true" number of first shell O neighbours multiplied by a weighting factor obtained form the relative occupancies given in the Rietveld refinement output. Ref. [20] describes the procedure for the Cu+-Y case in great details. The two superimposed spectra at the bottom of Figure 3, represent the experimental EXAFS signal, filtered on the Ag-O peak (vide infra vertical dotted arrows in Figure 5a), and the best fit obtained by adding the three contributions described above, where the optimized parameters were one common AE and three different Debye-Waller factors. The quality of the EXAFS fit, obtained with only 4 independent parameters, on a so complex sample, represents the definitive prove of the quality of the Rietveld refinement of the XRPD data.

IIa

0.5 a.u.

I o.o5A I'+ IIb

ir

J

...... exp.

IIa +I', + IIb+ !

|

4

.

.

.

.

.

8 k (A') 12

w

'

16

Fig. 3. Best fit and simulated EXAFS spectra of the contribution to the overall signal of silver cations located in, from top to bottom: site IIa (I' and IIb merged) and I and sum of the three simulated contributions (full line) superimposed with the experimental first shell filtered kg(k) function.

2225

2200 2175 2150 2125 Wavenumber (cm-~)

Fig. 4. Room temperature IR spectra collected at increasing CO equilibrium pressures dosed on Ag+-Y zeolite. Two Ag+CO adducts are clearly observed, corresponding to CO adsorbed on IIa and IIb sites: low and high frequency bands respectively.

1969 Figure 4 shows the IR spectra of increasing doses of CO on Ag+-Y zeolite. Two distinct CO stretching bands are clearly observed at 2195 and 2186 em"t, which reflect the two Ag+CO adduets formed on the two cationic sites available in the supereage cavity, i.e. the only two accessible to CO. The high frequency band is ascribed to the adduet formed on lib sites owing to the higher polarization power of silver cations which are less shielded by framework oxygen atoms owing to the higher Ag+-O distance (2.465 A vs. 2.31 A of the IIa site). We can so conclude this section by underlying how the high resolution XRPD study has been able to explain both EXAFS and IR evidences.

3.2. Clustered silver species: preliminary EXAFS, UV-Vis and luminescence evidences

Clear evidences of the progressive clustering undergone by Ag-Y upon increasing the activation temperature is reported in Figure 5 with EXAFS, photolumineseenee and DRS UV-Vis spectroscopy.

(a)

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120 ~ x 20 250 ~ x 4 400~

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W a v e n u m b e r cm -1 Fig. 5. Effect of increasing the activation temperature of Ag-Y zeolite on the kS-weighted, phase uncorrected, FT of the EXAFS signal, on the excitation (250-400 nm range) and emission (400-650 nm range) photoluminescence spectra and on the DRS UV-Vis spectra, parts (a), (b) and (e) respectively.

1970 The Ag-O and Ag-Ag contribution are well resolved in the FT of the EXAFS data since occurring in the 1.35-2.15 A and 2.15-2.90 A ranges respectively. Due to the much higher scattering power of Ag with respect to O neighbors the FT of Ag-Y activated at 120 and 250 ~ have been magnified by a factor of 20 and 4 respectively. For these two samples only a first shell Ag-Ag contribution is observed, reflecting the presence of dimers, trimers and tetramers clustered species. Conversely the EXAFS signal of the sample activated at 400 ~ is dominated by the typical feature of the metal in FCC crystals reflecting the much higher size of silver clusters. As dearly shown in parts (b) and (c) of Figure 5, the progressive clustering of silver atoms has strong influences of the optical properties of the material [11,24,25,]. Photolumineseence spectra exhibit excitation components at 270, 310 and 335 nm and emission bands at 435 and 534 nm which relative intensities change upon changing the sample activation temperature. The same holds in the UV-Vis DRS spectra, where the sharp and well defined absorption at 35115 cm "1 (285 nm), typical of isolated silver species in site I is progressively reduced upon increasing of the absorption edge at 29000 cm l (345 nm) typical of silver metal. REFERENCES 1. T. Sun and K. Serf, Chem. Rev., 94 (1994) 857, and refs. therein. 2. D. Lai, J. Li, P. Huang and D. Wang, J. Mater. Gas. Chem. 3 (1994) 211. 3. Y. Inoue, K. Nakashiro and Y. Ono, Mieroporous Mater. 4 (1995) 379. 4. K. I. Hadjiivanov, Mieroporous and Mesoporous Mater., 24 (1998) 41. 5. P.A. Jacobs, J.B. Uytterhoeven and H.K. Beyer, Chem. Commun., (1977) 128. 6. G. Calzaferri, S. Hug, T. Hugentobler and B. Sulzberger, J. Photochem, 26 (1984) 109. 7. T. Baba and Y. Ono, Zeolites, 7 (1987) 292. 8. S. Sato, Y. Yu-u, H. Yahiro, N. Mizuno and M. Iwamoto, Appl. Catal., 70 (1991) L 1. 9. M. Anpo, M. Matsuoka and H. Yamashita, Catal. Today, 35 (1997) 177. 10. J.M. Thomas, Chem. Eur. J. 3 (1997) 1557. 11. L.G. Gellens, W.J. Mortier, R.A. Schoonheydt and J.B. Uytterhoeven, J. Phys. Chem., 85 (1981) 2783. 12. L.G. Gellens, W.J. Mortier, J.B. Uytterhoeven, Zeolites 1 (1981) 11. 13. S. Bordiga, C. Lamberti, et al. Microporous Mesoporous Mater., 30 (1999) 129. 14. S. Bordiga, G. Turnes Palomino, D. Arduino, C. Lamberti, A. Zecchina and C. Otero Are~n, J. Mol. Catal. A 146 (1999) 97. 15. F. W. Lytle, D. E. Sayers and E. A. Stem, Physica B 158 (1989) 701. 16. A. Michalowicz, J. Phys. IV France 7 (1997) C2-235. 17. G.L. Marra, A.N. Fitch, A. Zecehina, G. Ricchiardi, M. Salvalaggio, S. Bordiga and C. Lamberti, J. Phys. Chem. B, 101 (1997) 10653. 18. C. Lamberti, S. Bordiga, A. Zecehina, A. Carati, A.N. Fitch, G. Artioli, G. Petrini, M. Salvalaggio and G.L. Matin, J. Catal., 183 (1999) 222. 19. G.L. Marra, G. Artioli, A. N. Fitch, M. Milanesio and C. Lamberti, Microporous Mesoporous Mater., 40 (2000) 85. 20. G. Turnes Palomino, S. Bordiga, A. Zecchina, G. L. Marra and C. Lamberti, J. Phys. Chem. B, 104 (2000) 8641. 21. C. Lamberti, S. Bordiga, M. Salvalaggio, et al. J. Phys. Chem. B, 101 (1997) 344. 22. J.V. Smith, Adv. Chem. Ser. 101 (1971) 171. 23. S. Bordiga, D. Scarano, G. Spoto, A. Zecchina, et al. Vib. Spectrosc., 5 (1993) 69. 24. R. Kellerman and J. Texter, J. Chem. Phys. 70 (1979) 1562. 25. G.A. Ozin and F. Hugues, J. Phys. Chem. 87 (1983) 94.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1971

D F T a n d I R studies on c o p p e r sites in C u Z S M - 5 : s t r u c t u r e - r e d o x c o n d i t i o n s - d e n o x activity r e l a t i o n s h i p E. Broclawik a, j. Datka b, B. Gil b and P. Kozyra b aInstitute of Catalysis, PAN, ul. Niezapominajek 8, 30-239 Krakow, Poland bFaculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland In this work we have studied by quantum chemical (DFT) modelling electronic and geometrical properties of copper centres in 13 position in ZSM-5. The results were compared with our earlier data concerning oc sites. It was evidenced that the properties of both Cu 2+ and Cu + in c~ sites differ from those in [3 sites. This is the most pronounced for Cu + the positive charge of which is to the largest extent neutralised by framework oxygens when located in c~ sites. Consequently Cu + in ct site has the highest energy of HOMO orbital, it is therefore the best electron donor to ~* antibonding orbital of NO and activates the most the adsorbed molecule. 1. I N T R O D U C T I O N CuZSM-5 zeolite was intensively studied due to its exceptional properties and activity in deNOx (for review see ref. 1). Nevertheless, detailed information on probable positions and coordination of exchanged copper cations in the context of their activity towards NO is still missing. It is known e.g. from photoluminescence that among many possibilities two positions: c~ and 13 are prevailing while one of them (c~) is suggested to be catalytically active [2,3]. Thus we have undertaken DFT calculations for representative models of both copper sites in CuZSM-5. The aim of this study was to provide information on structural and coordination properties of copper cations in various sites and oxidation states, and on their interaction with NO, in hope to find factors responsible for activity differences. The framework of the ZSM-5 zeolite provides three kinds of exchangeable cationic sites denoted as ec, [3 and 3' [3,4], amongst them the two first are more abundant and preferentially occupied by copper ions. This gives rise to distinct speciation of copper positions reflected in its adsorption properties and reactivity. In addition, Cu + sites are produced from Cu 2+ exchanged cations by the process of self-reduction during dehydration and a number of hypotheses have been proposed as to the structure and coordination of both copper forms [5,6]. The specific geometrical and electronic structure of self-reduced Cu species is primarily related with the reduced coordination along with the supramolecular effects due to the zeolite framework acting as a generalised ligand. This leads to their exceptional ability to switch from electron-acceptor to electron-donor properties and, in consequence, to activate adsorbed NO. This study concerns the interaction of Cu 2+ and Cu + in ZSM-5 zeolite with NO molecules. The focus has been devoted to elucidation of speciation and molecular structure of the intrazeolite adducts formed upon adsorption and self-reduction. These processes have been investigated by DFT method and the results were compared with IR data.

1972

1.1. Quantum chemical modelling DFT calculations were carried out for cluster models by Dmol software of MSI 9 [7]. Dmol code is the implementation of numerical scheme for solving Kohn-Sham equations. We have chosen standard calculational parameters e.g. local VWN exchange-correlation potential and numerical DNP basis set. Inner core orbitals were frozen during calculations. This choice was promoted by the compromise between computational efficiency and expected accuracy. It is believed that for pure DFT methods (not based on the Hartree-Fock scheme) the results are only moderately dependent on the basis set, and local approximation to exchange-correlation potential, although approximate, is clearly derived from basic physical principles. On the other hand cluster modelling of the solid is always burdened with some degree of arbitrariness. In principle, there exist several embedding schemes to simulate remaining part of the solid, which range from boundary atom freezing to very sophisticated methodologies mixing quantum chemical calculations with semi-classical simulations. We did not use more elaborated embedding schemes for the sake of easy access to molecular properties, including full vibrational analysis. The properties to be discussed here are geometrical parameters, charge distribution, one-electron energy levels and stretching frequencies of NO obtained from diagonalisation of the Hessian matrix. Thus the approach is in principle qualitative and the discussion is focused on differences between the sites hosting copper cations in oxidised or reduced forms. Models utilised in our calculations were cut off the MFI structure taken from MSI databases included in the software. Two models were selected: i) basket (M7) model composed of two fused 5T rings forming a 6T ring which simulated framework environment for copper cations in ~ position on main channel wall and ii) deformed 6T ring (Z6) postulated as framework environment for 13 copper position in sinusoidal channel. The first model (M7) has already been elaborated by us and studied in more detail previously [8-11]. Here we attempt to describe the other site in a parallel manner and compare properties of the both with emphasis put on subtle differences which could shed some light on their difference in activity towards NO. In M7 and Z6 parent structures two Si cations were substituted with A1 atoms with two protons compensating negative charge of substituted system. Geometry of all clusters has been optimised with constrained protons in terminal OH groups to keep MFI structure. In the next step Cu 2+ cation was substituted for two protons or Cu + for one proton to mimic the site after cation exchange and self-reduction. Initial position of the copper cation was selected in the centre of 6T ring in both models, final geometry of the system was the optimised one. Similar approach has also been pursued by many authors, within both semiclassical [12] and more rigorous quantum chemical regime [13-16]. We have taken the advantage to merge our experience with that of other groups, which added credibility to selected models. On the other hand, our approach enabled extending the study with explicit examination of the interaction between the site and the sorbed NO molecule and discussion of its effects on the molecule on electronic level. Thus, the most important objective of our study was the interaction between both copper forms located in the two specified models of ZSM-5 framework and NO. In each case the molecule was put in approximate bonding distance from the proper copper site and the final geometry of the complex was determined from calculations along with the details of electronic structure and properties. In the last step full vibrational analysis was performed to calculate NO stretching frequency which is a measure of the activation of NO and could be compared with IR spectra.

1973 2. R E S U L T S AND DISCUSSION Figure 1 shows schematically the structure of the MFI framework taken from MSI databases. Parts of the framework selected as M7 and Z6 models of ct and [3 sites, respectively, are marked with bold lines. After exchanging two silicons, which were separated by two other Si centres but were closest in the distance, with A1 centres these models were subjected to saturation and optimisation procedures according to the rules described in methodology. Results concerning properties of copper centres in tx position have already been published [10,11,17], here we concentrate on details of Z6 model of the 13 site for copper centre and, what is the most important, on comparative analysis of the two. Figure 2 shows already optimised geometrical structure of Cu 2§ (a) and Cu + (b) centres in Z6 model. Only the atoms forming the basal six-ring are labelled apart from the extra proton compensating the charge which has been added to the terminal AI(OH) group in the case of Cu + centre. It can be clearly verified that while divalent copper forms four bonds with 9

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i

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Coordination of C u 2 + (left) and Cu + (right) in Z6 model of 13 site in ZSM-5 (all distances given in ,~)

1974 Table 1. Bond distances between copper cations and bridging oxygens in M7 (c~, data taken from refs. 10,11,17) or Z6 (p) cluster models (below 2.6 A, index sT denotes oxygen in 5T ring of M7, otherwise in 6T ring), mean framework distortion (RMS) and charges on exchanged copper. Centre / Site

Cu 2+/Z6(13)

Site

Site with NO Rcu-o [A]

Qcu

RMS

Rcu-o [a]

Qcu

1.94, 1.96, 2.20, 2.25

+0.48

0.376

1.95, 1.97, 2.14

+0.45

CuZ+/]VI7(ot) 2.03, 2.03, 2.05, 2.05

+0.50

0.242

2.02, 2.03, 2.105T

+0.43

Cu +/Z6([3)

1.90, 1.94, 2.60

+0.32

0.268

2.01, 2.16, 2.23

+0.37

Cu +/M7(cz)

1.93, 2.01, 2.035T

+0.28

0.178

2.02, 2.06, 2.065T

+0.36

bridging oxygens, the reduced form makes two strong and one weak Cu-O bonds. The cluster undergoes substantial distortion to keep Cu + bonded in its centre in variance with more symmetrical or more rigid six-ring fragments of the framework where copper migrated after reduction towards more packed structural fragments [ 10,11,17]. The distance between two A1 atoms diminishes from 5.91A for p site in Cu2+ZSM-5 to 5.81 A for Cu+ZSM-5. Next the interaction of both copper centres (p site in Cu2+ZSM-5 and Cu+ZSM-5) with nitrogen oxide (II) has been examined within the same paradigm. Table 1 shows the comparison of structural and selected electronic properties of Cu 2+ and Cu + centres in our models of cz and ]3 sites in ZSM-5, before and after NO sorption. In both models (cz and 13) Cu 2+ has roughly fourfold coordination and it becomes lowered to at most triple coordination by copper reduction from Cu 2+ to Cu + and/or NO sorption. In the cz site, however, copper coordination is much more symmetric with respect to Cu-O bond lengths but reduction of coordination is always accompanied by the cation migration towards more packed five-ring fragment of the model. In the case of Z6 model of the p site, asymmetry of copper coordination is much higher and clusters distortion after copper exchange is larger than those calculated for the M7 model. The penalty paid for keeping Cu + bonded by the six-ring is its strong deformation reflected by high value of distortion parameter defined as mean square root deviation between cartesian coordinates of the atoms forming the ring (RMS). In principle structural and electronic properties of Cu 2+ and Cu + in cz and p sites are very similar, there are, however, subtle but systematic differences between both sites. Along with already discussed changes in coordination, we observed also the changes of copper charges. We have already proposed that framework oxygens acting as generalised ligand making partly covalent bonds with copper cations reduce the charge on Cu 2+ and Cu +. General conclusion may be drawn that the more packed is oxygen environment, the less positive is the charge on copper cation. Cu 2+, which in ZSM-5 acquires charge around 0.5, shows still prevailing tendency to withdraw electrons from environment. On the contrary, Cu +, which has actual charge around 0.3, may act apparently as an electron donor. Tables 2 and 3 show properties of the systems composed of a or 13 copper sites and NO molecule. NO becomes strongly bonded by all copper centres in a tilted position, the effect of its interaction with the copper cation dramatically depends on copper oxidation state while subtle but systematic differences dependent on the site structure may be also observed.

1975 Table 2 Calculated properties of adsorption complexes between NO molecule and c~ sites (data taken from refs. 10,11,17) or 13 sites in CuZSM-5: adsorption energy, equilibrium Cu - NO bond distance and angle, intramolecular NO bond length. Centre / Site Cu2+-NO

Cu+-NO

Eads kcal/mol

RCu-NO A

Cu-N-O angle (deg)

RNO ft,

Z6(13)

41

1.85

124

1.14

M7(oQ

40

1.80

128

1.15

Z6([3)

17

1.84

132

1.18

M7((x)

24

1.84

132

1.19

C u 2+ in a or 13 sites binds NO molecule more strongly than Cu +, in the letter case also dependence of adsorption energy on the site structure may be noticed. From the data contained in Table 2 one can develop the impression that the et site has enhanced ability to activate NO when hosting Cu + (larger increase of RNO) while the effect of Cu 2+ in 13 position shows in bigger strengthening of the NO bonding. This property correlates with the tendency to donate or withdraw electrons illustrated in Table 1. The information listed in Table 3 summarises our findings concerning properties of copper centres in both oxidation states in two distinct sites in ZSM-5 and the effect of their interaction with NO. It may significantly aid our understanding of geometrical and electronic factors, which are responsible for activation ability of copper centres. The data listed in Table 3 indicate clearly that donor-acceptor properties of the copper centre which coincide with its activation ability towards NO have well documented source in its bonding scheme within ZSM-5 framework. Unique tendency of copper to self-reduction is obviously the most important factor of its activity. The second factor, however, seems to be partly covalent interaction of Cu + with oxygen ligands in ZSM-5: the more packed is oxygen environment, the higher is Fermi level of the site and the stronger is tendency of copper to donate electrons to NO molecule which causes its activation. This tendency increases uniformly down the table. It is seen when considering calculated properties: the position of HOMO orbital, change of charge on both Cu cation and NO molecule and change of NO bond

Table 3. Effect of NO interaction with et or 13 sites in CuZSM-5" energy of the HOMO orbital of the cluster, change of charge on copper and on NO caused by adsorption, changes in NO bond lengths (ARNo), calculated and measured NO stretching frequency (vNO). Centre / Site

EHOMO

AQcu

AQNo

[eV] Cu2+-NO

ARNo A

VNO (DFT)

VNO (IR)

cm -1

cm -I

Z6(13)

-6.482

-0.03

+0.30

-0.02

1912

1913 a)

M7(oQc)

-6.443

-0.04

+0.25

-0.01

1904

1895 a)

Z6(13)

-5.317

+0.05

-0.03

+0.02

1778

M7(oQc) -5.170 +0.08 -0.03 a) from ref. 3 b) from ref. 9 c) from refs. 10,11,17

+0.03

1754

Cu+-NO

1809 b)

1976 distance, as well as properties which were parallel measured and calculated such as NO stretching frequency. IR spectrum of NO adsorbed at relatively low coverage on Cu-ZSM-5 shows two distinct bands of Cu2+-NO at 1905 cm 1 and of Cu + NO at 1809 cm -1. The former one is split and Wichterlova reported [3] two submaxima at 1895 and 1913 cm 1. According to the data presented in ref. 3 it may be supposed that 1895 cm -~ band corresponds to NO bonded to Cu 2+ in ct sites and 1913 cm 1 band corresponds to Cu 2+ in [3 sites. These experimental values agree well with our values calculated by DFT (1904 and 1912 cm 1 for sites c~ and [3, respectively). Unfortunately, the IR band of NO bonded to Cu + at 1809 cm ~ is not split. Maybe such a splitting could be seen when recording the spectra of adsorbed NO at liquid helium temperature (what distinctly narrows IR bands). Such experiments will be done in our laboratory in the future. As mentioned above, our DFT results (Table 3) showed that Cu + in cz sites activates the most NO molecule what is evidenced by the most distinct N-O distance elongation and the lowest NO stretching frequency. This result agrees very well with the experimental data presented by Wichterlova [3]. 3. C O N C L U S I O N S 1. CH 2+ in ct sites has square planar coordination, whereas Cu 2+ in [3 sites form two shorter and two longer bonds to framework oxygens. The coordination of Cu 2+ in [3 positions deforms the framework stronger than in tx sites. 2. Cu + in c~ sites form three strong bonds to framework oxygen, whereas in [3 sites it forms two strong and one much weaker bond. Similarly as in the case of Cu 2+, the distortion caused by Cu + in [3 sites is much larger than in c~ sites. 3. The location of both Cu 2+ and Cu + in zeolite framework diminishes strongly the positive charge on cations to: 0.48-0.50 for Cu 2+ and 0.28-0.32 for Cu +. This is due to strong influence of framework oxygens. Cu + in c~ sites has the lowest positive charge (0.28) due to higher number of oxygens in close vicinity than in 13 sites. 4. Cu 2+ in both ct and [3 sites binds NO with similar strength. The bonding of NO to Cu + sites is weaker than to Cu 2+ - it is the weakest for Cu + in 13 sites. 5. NO molecule interacting with Cu 2+ donates electrons to the cation, therefore the cation looses some positive charge. This strengthens the NO bond as evidenced by NO distance shortening (by 0.01 - 002 A). This effect is more pronounced in [3 sites than in ct sites (shorter NO bond and higher NO stretching frequency). The values of NO stretching frequencies calculated by DFT agree very well with experimental values. 6. NO molecule interacting with Cu + acquires electrons from the cation, therefore the cation gains positive charge. This weakens the NO bond as evidenced by NO distance elongation (by 0.02 - 003 A). This effect is much stronger in-ct sites than in [3 sites (longer NO bond and lower NO stretching frequency). 7. Our DFT calculations showed that Cu + in ot sites activate the most NO molecule, what agrees well with experimental suggestions of Wichterlova [3].

Acknowledgement This study was sponsored by the grant of KBN 3 T09A 010 17.

1977 REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

M Iwamoto, in Studies in Surface Science and Catalysis, vol. 130, Proceedings of the 12th ICC, Granada, A. Corma, F. V. Melo, S. Mendioroz and J. L. G. Fierro (Eds.), 2000, Elsevier, p. 23. B. Wichterlova, J. D6d6cek, Z. Sobalik, A. Vondrova and K. Klier, J. Catal. 169 (1997) 194. B. Wichterlova, J. D6d6cek, Z. Sobalik, Proc. 12th Int. Zeolite Conference in Baltimore, M. M. J. Treacy, B. K. Marcus, M. E. Bisher, and J. B. Higgins, Editors, MRS, 1999, p. 941. J. D6d6cek, D. Kaucky, B. Wichterlova, Micropor. Mesopor. Mater., 35-36 (2000) 483. W.K. Hall, J. Phys. Chem. B, 101 (1997) 1979. G . T . Polomino, P. Fisicaro, S. Bordiga, A. Zecchina, E. Giamello and C. Lamberti, J. Phys. Chem. B, 104 (2000) 4064. DMol, Insight II release 96.0, User Guide, San Diego: Molecular Simulations, 1996. E. Broclawik, J. Datka, B. Gil and P. Kozyra, Phys. Chem. Chem. Phys., 2 (2000) 401. E. Broclawik, J. Datka, B. Gil, W. Piskorz and P. Kozyra , Topics in Catalysis, 11/12 (2000) 335. E. Broclawik, J. Datka, B. Gil and P. Kozyra, Topics in Catalysis, in print. E. Broclawik, J. Datka, B. Gil and P. Kozyra, J. Phys. Chem., submitted. D. C. Sayle, C. Richard, A. Catlow, J. D. Gale, M. A. Perrin and P. Nortier, J. Phys. Chem. A, 101 (1997)3331. D. Nachtigallova, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 1 (1999) 2019. M. J. Rice, A. K. Chakraborty and A. T. Bell, J. Catal., 194 (2000) 278. P. Nachtigall, D. Nachtigallova and J. Sauer, J. Phys. Chem. B, 104 (2000) 1738. D. Nachtigallova, P. Nachtigall and J. Sauer, Phys. Chem. Chem. Phys., 3 (2000) 1552. E. Broclawik, J. Datka, B. Gil and P. Kozyra, Int. J. Mol. Sci., 2002, in print.

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Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1979

Diffusion o f W a t e r in Silicalite b y M o l e c u l a r D y n a m i c s Simulations: A b Initio based interactions C. Bussai a'b, S. Hannongbua a, S. Fritzsche b, and R. Haberlandt b aDepartment of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.* bDepartment of Molecular Dynamics/Computer Simulations, Institute for Theoretical Physics (ITP), Faculty of Physics and Geoscience, University of Leipzig, Augustusplatz 10 - 11, 04109, Leipzig, Germany. The silicalite-1/water potential function has been developed using quantum chemical calculations at the Hartree-Fock level using the 6-31G* basis sets. The silicalite-1 crystal structure is represented by three fragments, in which the chemical compositions are OloSiloH20, O308i22H44 and O355i29H58. Ab initio calculations have been performed for 1,032 fragment-water configurations where water coordinates are generated inside the fragments. The intermolecular silicalite-1/water potentials developed from those data points have been used in the molecular dynamics simulations. The obtained diffusion coefficients at 298 K of 3.3x10 9 m2.s -1 and at 393 K of 6.7x10 9 m2.s "1 are in agreement with those of the PFG-NMR measurements. 1. INTRODUCTION Ze01ites are microporous crystalline solids with well-defined structures. Generally they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within their pores. Due to their unique porous properties, major uses are in petrochemical cracking, ion exchange (water softening and purification), and in the separation and removal of gases and solvents [1]. To study these, several experiment tools have been achievable on one hand, on the other hand, theoretical tools, e. g. Molecular Dynamics simulations, become more feasible [2]. Such theoretical methods require knowledge of the interaction potentials. A crucial method to obtain such potentials is the use of ab initio calculations. Several function modification attempts and great success have been made by Sauer et al., with e. g., the QMPot method [3 ]. In this study, an alternative choice in deriving potential function parameters is proposed. Numerous silicalite-1/water energy points have been generated using quantum chemical calculations at the Hartree-Fock (HF) level using the 6-31 G* basis sets. Molecular dynamics simulations have been performed using the newly developed ab initio potential and diffusion coefficients for water molecules in the silicalite-1 have been investigated.

1980 2. CALCULATION DETAIL

2.1. Development of the intermolecular pair potential To develop intermolecular potential :)-7 ,!)" functions representing the interaction between two molecules in all configurations, numerous coordinations of the second molecule around the first one have to be generated. The interaction ... energies of all configurations have to be calculated and the obtained data points must then (c) be fitted to an analytical form. Due to the size of the silicalite-1 lattice, in Figure 1. Schematic representations of which a Crystallographic cell [4] consists of 96 Si the (a) linked domain, (b) straight and and 192 O atoms, it is rather difficult to take into sinusoidal channels, (c) intersection. account the whole lattice in the quantum chemical calculations. Therefore, the silicalite-1 crystal structure was represented by three fragments (Figures la-lc), for simplicity, named single, intersection and double rings with their chemical compositions O10Si10H20, O30Si22H44 and O35Si29H58, respectively. More details of the classification have been given elsewhere [5]. Ab initio calculations at the HF level with the extended 6-31G* basis sets have been performed for all water configurations generated inside those silicalite-1 fragments. Experimental geometries of water [6] and silicalite-1 [4] have been used and kept constant throughout. All calculations are performed using the G98 program [7]. More than 1,000 ab initio data points were fitted to an analytical function of the form

[8]:

AE(w, s)

-

cab q q}

i

j [ ro6

+ 7~7- + - - ~ + r~ij

rij

rij

'

,

(1)

where 3 and 288 denote the numbers of atoms in a water molecule (w) and the silicalite-1 (s) unit cell, :respectively. The constants A ij , Bij and C/j are fitting constants and rij is the distance between atom i of water and atomj of silicalite-1. Also, qi and qj are the atomic net charges of atoms i and j in atomic units, as obtained from the population analysis of the isolated molecules in the quantum chemical calculations. Superscripts a and b on the fitting parameters have been used to classify atoms of equal atomic number but different environmental conditions, for example, oxygen and silicon atoms of silicalite-1 in the different channels. The third polynomial term (Cjr3ig) was added in order to obtain better numerical fitting. The silicalite-1/water fitting parameters were summarized in Table 1. Concerning an assignment of a negative or positive value to the fitting parameters, physical meaning,of the atomic-based pair potentials is not achieved. Instead, it is a one-to-one correspondence between the predicted (by the potential function) and the observed (by the ab initio calculation) interaction energies. An advantage of this approach is that it is a one-to-one correspondence between the predicted and the calculated interaction energies. Analogously, as well as for better numerical fitting, the third polynomial term (CJr3,g) was added and not considered separately. Some examples are those in references [9-11 ].

1981 Table 1 Optimization parameters for atom i of water interacting with atom j in each channel of the silicalite-1 lattice. Subscripts sd and st denote sinusoidal (zig-zag) and straight channels, respectively.

O O O O i H H H H

2.2.

Sisa Sist Osa Ost j Sisd Sist Osa O~t

Molecular

qi

qj

-0.87 -0.87 -0.87 -0.87

1.57 1.67 -0.78 -0.84

qi

qj

0.43 0.43 0.43 0.43

1.57 1.67 -0.78 -0.87

Dynamics

A 6 1 (A kcal.mol ) - 9043.97 - 4159.83 1371.19 - 110.79

B (A12kcal.mo1-1) 1161167.97 989963.68 -21045.58 51208.44

C 3 (A kcal.mol 1) 1418.92 617.02 -351.61 -110.82

A (A kcal.mo1-1) 3724.97 2077.13 -406.18 34.87

B (A12kcal.mo1-1) -4314.90 -8925.29 689.37 32.84

C (A3kcal.mol 1) -792.37 -415.82 222.32 102.59

6

Simulations

The crystallographic cell [4] of Silicalite-1 contains 288 atoms (Si960192), with lattice parameters a = 20.07 A_, b = 19.92 A_ and c = 13.42 A_. Simulations have been carried out at 298 K and 393 K with the time step of 0.5 fs for the system consisting of 2 silicalite-1 unit cells. The box contains 2 water molecules per intersection of the silicalite-1. Periodic boundary conditions have been applied. The potential proposed by Bopp, Jancso and Heinzinger [12] was employed to describe 9 = . . ~...~,' , . water-water interactions while the newly . 2 developed potential shown in eq. (1), with the m 0 optimal parameters summarized in Table 1, =0 u was used to represent the silicalite-1/water interactions. According to [ 13.14] the use of -~9 -2 Ewald summations can be avoided in systems with tot:al charge zero if shifted force m AEsc~~".-.. /~' "n,D|1 |,== 9 m~ll/ potentials are applied instead. The equations D AEFIT "~ .... ~ of motion are solved by means of the , I , I , I , Velocity-Verlet algorithm. The evaluation -2 -1 0 1L/~ 2 part of each run corresponds to trajectory Figure 2. Silicalite-water interaction energies length of 10 ns after 0.5 ps thermalization during which the total energy is adjusted to a (AE) obtained from the ab initio calculations (AEscF) with the extended 6-31G* basis sets value that leads to the wished average kinetic and from the potential function (AEFIT) All energy. Therefore, the evaluation part can be done in the microcanonical ensemble without ab initio and fitted data points were also compared in the insert. perturbing the trajectories.

1982 3. RESULT AND DISCUSSION 3.1. Quality of the Silicalite-1/Water Potential With the analytical potential shown in eq. (1), the lattice-water interactions in the straight channel have been calculated and plotted in Figure 2. Here, the oxygen atom of the water molecule moves from one perpendicular surface to the opposite side along the vector r (see Figure 2), its dipole moment points parallel to vector r and its molecular plane is parallel to the window of the lattice. The ab initio interaction energies at the same lattice-water configurations have been calculated and given also for comparison. Good agreement between the two curves clearly illustrates the reliability and quality of the fit. This conclusion was, again, confirmed by the plot shown in the insert of this Figure, where all 1,050 ab initio and fitted energies have been compared. Some comments could be made concerning the quality of the ab initio interaction energies given in this study. Discrepancies and reliabilities of the data points due to the size of the fragments, the calculated methods and the basis sets used as well as an error due to the unbalance of the basis set, basis set superposition error, have been intensively examined and discussed in some previous papers [5,8]. 3.2. Characteristics of the Silicalite-1/Water Potential To visualize characteristics of the silicalite-1/water potential function, the interaction energies ~for different orientations of the water molecule in the straight channel have been computed according to eq. (1). The changes of the energies as a function of the distances along r were plotted in Figure 3. Curves 1 and 2 in Figure 3 show the minima at L < 0, and the interaction energies for L > 0 increase more slowly than those for L < 0. This occurrence can be clearly understood as water molecules in these configurations (at the right of this Figure) approach the surface at L > 0 by pointing hydrogen atoms toward the oxygen atoms of the lattice, i.e., attractive Coulomb interactions between the hydrogen atoms of water and the oxygen atoms of the surface compensate the water-surface repulsion. This leads to a slow increase of the interactio, n energy and hence an asymmetry of the lattice-water potential. The difference between the shapes of the two curves indicates how sensitive the obtained function is. That means it is able to classify the two orientations of the water molecule which differ only by rotating the molecule by 90 ~ around its dipole vector. The situation is very similar for curves 2 and 5, in which the 0 minima take place at L > 0 and the interaction energies for L < 0 increase 2-- ) faster than those for L > 0. For curves 3 and 6, the shapes are much more ~'~ -4 % symmetric than the other curves. The 5 - 4 - ~':~ reason is that the water molecule in these configurations approaches the 6-'" ~ 0 1 -2 -1 2 lattice, both for L > 0 and L < 0, by pointing its dipole vector parallel to the Figure 3. Silicalite-water interaction energies surface. Curve 6 is broader than curve (AE) obtained from the potential function 3 because in curve 3 water moves according to eq. (1) for different orientations of a toward the surface in configurations for water molecule which distances from the surface to the

i!!

1983 two hydrogen atoms are identical. For curve 6, at the same position of water as in curve 3, one hydrogen atom is closer to the surface than the other (see legend of Figure 3). This fact confirms the ab initio interaction energies reported [5]. The sensitivity of the silicalite-1/water potential to different environments has been clearly monitored, in addition to that due to water orientation as shown in Figure 3. The difference between the interactions in the straight and the zig-zag channels is consistant with the energy data analyzed intensively in our previous work [5,8]. 3.3. Diffusion coefficients

The self-diffusion coefficients are calculated from the particle displacements. In [1517] the process of self-diffusion was quite generally related to the moments of the propagator. The propagator p (r, r 0 ,t ) represents the probability density to find a particle at position r at time t when it was at relation [15] In

--

ro at time

t = 0. The n th moment of the propagator is defined by the

(2)

_

P ( r , r 0 ,t ) is the solution of the diffusion equation for the initial concentration C (r,t = 0)= 5(r - r 0). In the case of isotropic diffusion and of a homogeneous system the propagator results to be

P(r'r~ )- (4~Dt )-~ exp{ - (r4Dt -roY)

.

(3)

Although zeolites are not homogeneous the propagator can be represented in this way if the displacements exceed the size of the inhomogeneities [16]. Then p ( r , r 0 ,t ) depends only on the difference ~, -r01. For shorter times this is not true. As the transition time to the Gaussian behavior and the final D values were the quantities of main interest in the present paper an averaging over r 0 has been carried out. The resulting propagator depends only upon ~, -r0l for all times. But, it attains the shape shown in eq. (3) (or its equivalents for the different components of the diffusion tensor in the anisotropic case, see below) only for sufficiently long times. The first four moments can be calculated from eqs. (2) and (3) in the case of isotropic diffusion [ 15] and of the anisotropic system, the corresponding equations for each direction, corresponding to the x-, y- and z-axes [18]. In this case, the diffusivity D is one third of the trace of the diffusion tensor:

D = I~3(Dx + Dy + D~)

(4)

The good agreement (within the range of fluctuations) of the final D values calculated for 298 K using different moments indicates that the diffusion time used in the evaluation procedure exceeds the correlation time. The self-diffusion coefficients calculated in this way at 298 and 393 K are summarized in Table 2.

1984 Table 2 The self-diffusion coefficients calculated in this way at 298 and 393 K are summarized. Temp (K)

MD Simulation Dx (m 2s'')

Dy (m2s "1)

Dz (m2s q )

298

2.6•

-9

6.5x10 9

7.9•

393

5.7•

.9

1.3•

1.4x10 -9

.8

-1~

D(m2s -1) 3.3•

"9

6.7•

9

It can be seen from these results that the largest component of the diffusion tensor is Dy values are about two times larger than Dx at both temperatures and about seven times larger than Dz at 298 K and even larger at 393 K. This is consistent with the physical structure of the silicalite-1 crystal, which consists of zigzag channels lying in the xz-plane and the straight channels lying parallel to the y-axis. This causes the significant difference of the elementary diffusion rates in different directions.

Dy. The

Considering the diffusion through silicalite-1 as a random walk of independent steps between the channel intersections, the main elements of the diffusion tensor may be shown to be correlated by the relation [ 19] c2 Dz

=

a 2. b 2 ~

Dx

(5)

Dy'

where a, b, and c are the unit cell lengths. Eq. (5) implies that the correlation time of propagation is shorter than the mean time it takes a molecule to travel from intersection to intersection. Possible deviations from this case, i.e. correlated motion between the channel intersections, may be accounted for by introducing a parameter (6)

The case/3 = 1, obviously represents the above considered case of completely random steps. /3 > 1 indicates preferential continuation of the diffusion path along one and the same channel, while/3 < 1 stands for molecular propagation with interchanges between the two channel types more probable than at random. The values o f f l calculated in this study are equal to 1.04 at 298 K and to 1.25 at 393 K. In agreement with the behavior found for alkanes, e.g. [20], where/3 = 1.2 and 1.3, a tendency is observed that the xenon molecules and the methane molecules, respectively, in silicalite-1 prefer to remain in the same type rather than to change into a segment of the other channel type at a channel intersection.

1985

3.4. Radial distribution function

4.0

g(r)

/

n(r) 4.0

The oxygen-oxygen radial distribution functions g for the water molecules at the two 298 K 3.0 3.0 temperatures have been calculated and displayed in 2.0 2.0 Fig. 4. In inhomogeneous systems, g(rl,r2) depends upon rl also and is not simply g(r) with r=lrl-r2l . 1.0 1.0 But, as a first approximation, we have done the evaluation of g(r) like in a homogeneous isotropic 0 2 4 6 8 10 system. This is equivalent to an averaging over the sites rl taking as a weight function the relative Figure 4 Oxygen-oxygenR/raAdial number of events when the rl are found during the distribution functions and MD run. Note, that due to the asymmetry of the corresponding running integration silicalite-1 lattice the function g(r), defined in this numbers for water molecules obtained way, does not converge to 1.0 for distances of the from the simulations at 298 K and 393 order of 10 A. g(r)=l would correspond to a K. homogeneous distribution in space that can be observed in systems with a structure on molecular level only at a length scale that is larger than the size of the inhomogenities i.e. the channel structure in the present case. The radial density distributions show a first maximum at 3.5 A followed by a pronounced shoulder centered at 4.4 A. In order to see the number of neighbors the integral n(r) of g(r) is also displayed in Fig. 4. It can be seen that e.g. within a distance of 7 A around a given water molecule there are in average only two other water molecules. Although, the first minimum is not well-defined, it can be concluded that the simulations did not show any clustering of water molecules in the silicalite-1 channels for the examined temperatures and concentrations of guest molecules. 4. CONCLUSION To justify the quality of the model in representing a real system, MD simulations have been performed at 298 K and 393 K for a loading of 2 water molecules per intersection of the silicalite-1. The diffusion coefficients have been calculated according to the method described in [21] from different moments of the particle displacements. The results obtained at 298 K and 393 K are 3.3x10 9 and 6.7x10 -9 m2.s-1, respectively. These values are in satisfactory agreement with those from PFG-NMR measurements [22] at the same loading and temperature. ACKNOWLEDGEMENTS Computing facilities provided by the Austrian-Thai Center for Chemical Education and Research at Chulalongkorn University, the National Electronic and Computer Technology Center, Bangkok, Thailand and the Computing Center at Leipzig University are gratefully acknowledged. This work was financially supported by the Thailand Research Fund (TRF) and the Deutscher Akademischer Austauschdienst (DAAD). C. B. acknowledges a DAADRoyal Golden Jubilee Scholarship, Grant No. A/99/16872, and a Royal Golden Jubilee Scholarship, Grant. No. PHD/0090/2541. R.H. and S.F. also thank the DFG (Sonderforschungsbereich 294) for financial support.

1986 REFERENCES

D. W. Breck, Zeolite Molecular Sieve, Wiley, New York, 1974; J. K~irger and D. M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley-Interscience, New York~ 1992. M.P. Allen and D. J. Tildesley, Computer simulation of liquids, Clarendon Press, Oxford, 1990. U. Eichler, M. Brandle and J. Sauer, J. Phys. Chem., 101 (1997) 10035. 4. D. H. Olson, G. T. Kokotailo, S. L. Lawton, W. M. Meier, J. Phys. Chem. 85 (1981) 2238. 5. C. Bussai, S. Hannongbua, R. Haberlandt, J. Phys. Chem. B 105 (2001) 3409 6. W. S. Benedict, N. Gailar, E. K. Plyler, J. Chem. Phys. 24 (1956) 1139. 7. M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart, J. A. Pople, Gaussian 98, Revision A, Gaussian, Inc., Pittsburgh, P A, 1998. C. Bussai, S. Hannongbua, S. Fritzsche, and R. Haberlandt, Chem. Phys. Lett. in press. W. L. Jorgensen, M. E. Cournoyer, J. Am. Chem. Soc. 101 (1978) 4942. 10 G. Karlstr6m, P. Linse, A. Wallqvist, B. J6nsson, J. Am. Chem. Soc. 105 (1983) 3777. 11 S. Udonsub, S. Hannongbua, J. Chem. Soc. Faraday Trans. 93 (1997) 3045. 12 P. Bopp, G. Jancso, K. Heinzinger, Chem. Phys. Lett. 98 (1983) 129. 13 D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J. Chem. Phys. 110 (1999) 17. 14. H. Dufner, S. M. Kast, J. Brickmann, M. Schlenkrich, J. Comput. Chem. 18 (1997) 15 S. Fritzsche, R. Haberlandt, J. K~irger, H. Pfeifer, K. Heinzinger, Chem. Phys. Lett. 198 (1992) 283. 16. J. K~rger, H. Pfeifer and W. Heink, "Principles and Application of Self-Diffusion Measurements by Nuclear Magnetic Resonance", in Advances in Magnetic Resonance, Vol. 12, Academic Press: New York, 1988. 17. R. Haberlandt, J. K~irger, Chem. Eng. J. 74 (1999) 15. 18. S. Fritzsche, M. Wolfsberg, R. Haberlandt, submitted to Chem. Phys. 19. J. K~irger, J. Phys. Chem. 98 (1991) 5558. 20. S. Jost, N. K. B~ir, S. Fritzsche, R. Haberlandt, J. K~irger, J. Phys. Chem. B 102 (1998) 6375. 21. S. Fritzsche, R. Haberlandt, J. K~irger, H. Pfeiffer, M. Wolfsberg, K. Heinzinger, Chem. Phys. Lett. 198 (1992) 283. 22. C. Bussai, H. Liu, S. Vasenkov, S. Fritzsche, S. Hannongbua, R. Haberlandt, J. K~irger, Appl. Catal. A-Gen, in press. .

.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1987

C o m p a r i s o n of s m a l l size a l u m i n o - and borosilicates o p t i m i s e d by periodic Hartree-Fock A.V. Larina and D.P. Vercauteren Laboratoire de Physico-Chimie Informatique, Facult6s Universitaires Notre Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium, e-mail: a!m:'m__Ca!s-cf:Nn_dp.ac.be apermanent address: Department of Chemistry, Moscow State University, Moscow, B-234, 119899, Russia 1. INTRODUCTION From a practical point of view, borosilicates present an interest as low acid catalysts; promising applications have, for example, already been tested for N-methylaniline synthesis from aniline [1] and Beckmann rearrangement [1-3]. So far, their activity has been interpreted via few theoretical studies mainly limited at the level of isolated clusters [4]. However, borosilicates also offer several advantages for studies at the level of periodic Hartree-Fock (PHF) approaches [5], owing mainly to the small number of atomic orbitals per unit cell as compared to the respective aluminosilicates. The possibility to optimise small aluminosilicate H-forms with PHF, supported by comparison of quadrupole coupling constants (CQQ) of 2H, 170, and 27A1, measured in 2Hy and 2HZSM-5 [6], has indeed already been shown [7, 8]. Important structural distinctions between the dehydrated Na- and H-form borosilicates upon A1 B replacement, i.e., the conservation of the BOa tetrahedra or its transformation to BO3, respectively, was shown both experimentally [9] and theoretically [4]. The transformation is accompanied by the appearance of "internal" silanol groups and by a stronger volume change for the H-form v e r s u s the Na-fonn, as could be verified by PHF calculations considering long range stabilisation effects. The different trigonal BO3 and tetrahedral BO4 moieties can be distinguished by sharp differences of the CQQ values which can serve as "fingerprints" for quantitative determination of the ratio between the two B coordination types in boron containing glasses [10]. But no theoretical PHF assignment of the CQQ values of liB in zeolites has, to our knowledge, been realised yet. 2. COMPUTATIONAL DETAILS Our optimisation strategy of the H-form borosilicates includes three stages: dehydration, A1/B replacement, and Li/H replacement (Figure 1). LiABW (Si/B = 1) and LiEDI (Si/B = 1.5) borosilicates were first optimised (Table 1) at the STO-3G level with PHF [5] as realised for aluminosilicates [8]. Final H-form borosilicate structural parameters are presented in Table 2. Then, single point calculations were considered at the ps-21G*(A1, Si)/6-21G*(H, B, O) level.

1988

a)

Table 1. Symbol, number of atoms , of different B, Si, and O types, of atomic orbitals (AO) per unit cell (UC) b~, and s~nnetr~ ~roup of the cationic forms of borosilicates Name S~rrnbol AtomsBdC nB/nsi/no AO/UC S)rmmetry LiABW LiABW 28/40 1/1/4 352 Pna21 Edingtonite a)

LiEDI

for dehydrated/hydrated forms;

32/b)

1/2/5

434

P21212

dehydrated form at the ps-21 G* level

Initial XRD hydrated LiABW aluminosilicate model ......... ,1, -HaO Optimisation with CRYSTAL code at the STO-3G basis set level

]

Single point testing with ps-21G*(AI, Si)/6-21G*(Li, O) basis set $ AI---) B replacement Optimisation with empirical force field [11]

[

I

Optimisation with CRYSTAL code at the STO'3G basis set level

I

$

$

$

Single point testing with ps-21G*(Si)/6-21G*(Li, B, O) basis set

$

Li--) H replacement

I Calculation of initial H coordinates on the basis of literature for the bridge Si-O(H)-B]

$

Optimisation with CRYSTAL code at the STO-3G basis set level

..]

$

Single point testing with ps-21G*(Si)/6-21G*(H, B, O) basis set Calculation of structural parameters and electronic properties Figure 1. Three-steps optimisation procedure of the H-form borosilicates To characterise each particular atom within each coordination sphere, we considered the quadrupole interactions of each asymmetric nucleus and calculated the CQQ values using a precise estimation of the electrostatic potential and its derivatives with the CRYSTAL code [5]. CQQ values (in MHz) were obtained using the electrostatic field gradient tensor (EFG) elements at the tensor principal axes: CQQ = 2.3496• 102•215

(1)

where the coefficient in the right hand side corresponds to the diagonal element VEz~ of EFG expressed in exau 3, and the nuclear quadrupole moments q are 0.00286, -0.03, -0.0355, and0.02558, barn (1 barn = 10 -28 m 2) for 2H, 7El, 11B, and 170, respectively. The EFG anisotropy ~1 = (IVEyyl - IVExxl)/lVEzzlvalues will also be given below for completeness.

1989 3. R E S U L T S AND D I S C U S S I O N 3.1 S t r u c t u r a l and electrostatic properties The comparison of the electrostatic properties of the H-form alum,no- and borosilicates is an instructive way to explain the lower acidity of borosilicates because they possess the lowest (A1) and highest (B) proton affinities thru the series of substituted frameworks with T = Be, Fe, Ga [ 13]. As observed from Table 2, the favoured H localisation in borosilicate A B W is the same as in aluminosilicate ABW; H is located at the 0(2) outside of the 4-membered (4T) ring window. The energy gaps between the favoured site and the next ones are however larger for the H positions in the borosilicate. Table 2. Geometry of the BrOnsted sites (distances in ~, angles in o) for alum,no- and borosilicate A B W optimised with STO-3G varying the coordinates of H, B, O, A1, and Si. Relative energies (in kcal/mol) are also 1G*(A1, Si)/6-21G*(H, B, O). Parameters 0(2) 0(3) 0(4) O(1) Si-O-A1 initial a) 139.45, 124.87 124.84 143.34 , Si-O-A1opt,raised 139.06, 126.37 126.22 143.32 O-H 0.981 0.979 0.980 0.982 Si-O 1.745 1.768 1.764 1.790 A1-O 1.809 1.828 1.832 1.861 A1-H 2.350 2.457 2.480 2.371 Oc) 9.7 10.4 3.5 16.3 .

.

.

.

.

|

|

i

|

,,,

|

,,,,

,,

H,

Si-O-H A1-O-H AUSTO_3G/AUps_21G. Si-O-B initial u) Si-O-B optimised O-H Si-O B-O B-H ~c) Si-O-H B-O-H AUSTO_3G/AUps_21G, i

,

,,,,

|

,,,,,

,,,,

i

110.0 113.7 112.0 , 118.9 0.0/0.0, 8.810.9 135.29 i'24.i"5 135.56, 130.1..9 0.981 0.985 1.724 ' 1.758 1.647 1.631 2.086 2.223 3.4 ' 16.2 110.8 113.6 113.1 114.1 0.0/0.0i 64.7/59.3 |

,

,, ,,

,,,,,

,,

,

,

.

,,.,

,.,

113.1 j 120.6 i 12.3/3.0 ! 124.40 , 128.61 0.982 1.709 ! 1.653 2.283 i 17.7 |

i

.,

|

|

,

|

,

|

106.1 109.1 22.8/41.4 149.74 147.00 0.973 1.764 1.743 2.202 1.3

114.4 108.2 117.0 104.8 35.9/48.2 75.9/115.4

") coordinates from [12]; b~ coordinates of LiABW first optimised with GLASSFF 2.01 [11] and then partly with CRYSTAL [5]; c~ [3 is the angle of H deflection from the Si-O-A1 plane. The validity of the H-form aluminosilicates optimised with our approach is supported by the structure recently optimised by PHF with Schlegel algorithm [ 14] of the Si-O(H)-A1 moiety in HEDI as well as by comparison of the calculated and experimental CQQ for a series of 2H, 1~O, a~A1 framework atoms [6]. The structural parameters and CQQ of the obtained cationic forms are presented in Tables 3 and 4, respectively. Only for one O in LiABW, we observe a slightly overestimated Si-O-B angle of 148.4 ~ as compared to the data obtained experimentally for borosilicate minerals [ 15-18].

1990 Table 3. Si-O-B angles (in o) and B-Si distances (in/~) in dehydrated borosilicates optimised with STO-3G compared to experimental data for several borosilicates Structure [ Si-O-B [ B-Si Calculated LiABW 123.0, 124.2, 135.8, 148.4 2.85, 2.86 LNDI 122.8, 125.1,140.7, 141.0 2.86, 2.93 Experimental Ca-danburite, B/Si = 3 ") 125.9, 132.2 2.79, 2.83 Howlite b) 127.9, 128.0, 128.1,135.8 2.80, 2.81, 2.81, 2.87 Reedmergnerite ~) 125.0, 135.4, 140.5, 143.1 2.71, 2.85, 2.88, 2.92 Searlesite a) 130.2, 142.6 2.81, 2.92 a) ref. 15; b) ref. 16; c) ref. 17; a) ref. 18 ............... Table 4. Quadrupole coupling constants CQQ (in MHz) and EFG anisotropy 1] for 7Li and ~IB atoms calculated with ps-21G*(A1, Si)/6-21G*(H, B, O) in ABW alumino- and borosilicates Atom ] Position, geometry ] Cqq r] , Calculated 7Li . LiABW(B), LiO3 L -0.361 0.082 LiABW(A1), LiO3 i -0.478 0.728 LiABW(A1) H20, LiO4 , -0.056 0.997 ~ " "-~xper-~entaJ ) LiKSOfl ), LiO4 type 0.025 0.15 LiKSO4 "), LiO4 type 0.0358 0.0 Calculated lXB HABW(B-O2H), BO4 1.265 0.079 HABW(B-O3H), BO4 1.220 0.045 HABW(B-O4H), BO4 1.416 0.030 HABW(B-O1H), BO4 1.674 0.090 HABW(B-OzH), BOab) 2.526 0.141 LiABW(B), BO4 0.282 0.627 ExperimentaF ) Li203 B203, BO4 0.527 0.53 PbO 2B203, BO4 0.805 0.09 PbO 2B203, BO4 0.960 0.05 H-boralite (MFI type) d) . 2.55 0.0-0.2 CaO 2B203, BO3 2.56 0.54 Li20 2B203, BO3 2.6225 0.1653 33LIO 67BZO3, BOa 2.61 0.14 a) ref. 19; b) intermediate silanol structure; ~)ref. 10; d)ref. 9 ,

,

An interesting example of CQQ(~Li) shift by one order of magnitude upon dehydration in the LiABW case is demonstrated for one Li which changes its coordination from tetrahedral LiO4 to planar LiO3 in the same manner as for B upon the ~bridge to silanob~ transformation in boralite (zeolite of the MFI type) [9]. The fourth O atom in the coordination sphere is the one

1991 of a water molecule as confirmed by XRD [12]. PHF optimisation only contracts the LiO4 tetrahedron, i.e., Li-O distances shift from 1.913, 1.942, 1.968 (water), and 1.981 A to 1.814 (water), 1.878, 1.942, and 1.942 ~. Unfortunately, relevant experimental results are related to the dehydrated a3Na forms and cannot thus confirm this effect at the moment. CQQ values in the optimised structures are compared for 170 and 2H atoms in Figures 2 and 3, respectively. For two observed moieties in ABW, one can note a slight increase of CQQ at the Si-O(H)-B position as compared to the ones of Si-O(H)-A1 in aluminosilicates and a stronger increase in absolute value at the Si-O-B moiety with respect to St-O-A1. This difference between the CQQ variations at Si-O(H)-B and Si-O-B correlates well with the Si-O and T-O (T = A1, B) bond length differences. The latter are between 0.18 and 0.24 A for the ABW borosilicates and between 0.122 and 0.21 ~ for LiEDI (not shown in Figure 2) at the SiO-B moiety. For St-O-A1 in aluminosilicates [8], the differences are smaller, 0.10 - 0.15 ~. At the bridged position, the Si-O and T-O bond length differences are closer, i.e., 0.06 - 0.07 for HABW aluminosilicate and 0.02 - 0.13 A for HABW borosilicate (Table 2). I

'

I

lO -

9

9

NAT ~

ABW,,

8 N

li

..

g

--'~

'

I

Si-O(H)-B,,,.,~

V

~i'=

o

.e~,". , o

si-ol.i-A.

~

9 9 =

"~ C-5 -6

~7

9~

EDI

........................................

9

i f

9

Si-O.A! experiment

9

tx

tx

{

9

iAK. . . . . . . . . . . .

9

................... We," @ ........................... ~ Z ............... ~ I " - U - I : I t l I ~ I ~ W

\'"

120

1

J

J

.v ..... ~ ................................. r. ........ .-I

Si-O-Si e x p e r i m e n t 9

" I Si'O'B/LiABW'

-] r_

.~

"r

4

"l

"

I 140

"

x

'

Si-O-T angle, degrees

r

I

"I"

J

7

160

Figure 2. CQQ(I~o) v e r s u s Si-O-T angle (T = B, St, Ai) in H-form aluminosilicates (triangles, squares, circles), ABW borosilicate (stars, diamonds), and LiABW (crosses). Experimental boundaries are for aluminosilicates (dashed lines). Opposite to CQQ(170), absolute CQQ(ZH) values are found to decrease with respect to those of aluminosilicates (Figure 3), which is in agreement with experiment after OH anharmonicity corrections [6]. This behaviour does not correlate with the Si-O and T-O (T = A1, B) bond length differences whose variations explain only the CQQ(170) increase.

3.2 ~> transformation The drastic variations of all structural parameters predicted in [4] upon A1 --> B replacement in the H-forms as compared to the those of the initial Al-framework present a problem for the PHF optimisation. A series of H-form borosilicates was hence constructed to guess a possible favoured structure. The A1 ~ B replacement leads to an anisotropic decrease of the a, b, and c cell vectors, by 6.8, 3.4, and 3.4 %, respectively, in aluminosilicate LiABW. This cell variation was assigned to a change by 5.9 % of the average 1%OI bond lengths (T = B, Si, A1) between the B- and Al-frameworks of LiABW. On the basis of an analogous evaluation of the cell

1992 parameters, we tested two possibilities of the < transformation in HABW borosilicate, i.e., first, a simultaneous increase of the cell volume (with an average IT-O[ distance increase in the <<silanob> model) and second, the conservation of the cell volume. I

'

0,34 -

0,32

9

""m

o

N

I

"

I

"

De ~ ~9 -

"

"1-- 0,28

'

I

'

'

I'

'

I

'

I

"

" " -.

.'<>

__

'

I

0,98

'

I

0,99

'

I'

1,00

[]

Roll ,

"-

'--o. . . . . . . . . . . .

........................................ 0.97

I

ps-21G

and ZSM-5 aluminosilicates 0.20

I

9

I

1,01

'

Angs

I

1,02

'

I

1,03

'

'

a~f..... I

1,04

1

1 05

Figure 3 . CQQ(2H) v e r s u s O-H distance in H-form aluminosilicates (triangles, circles) and ABW borosilicate ( d i a m o n d s for e l o n g a t e d 2H-O(2) bond, s q u a r e s for equilibrium OH lengths) Taking into account that the <> transformation in borosilicate with the isolated cluster model has been theoretically simulated for the proton located in the 4T ring [4], we also considered an initial H-O(3) silanol in the 4T ring. Both zeolite <~silanol>>models, near 0(2) and O(3), with various cell parameters were optimised to coincide with the initial geometry of the silanol group, corresponding to IB-OI = 2.327 A, IB-Sil = 3.799 A, obtained by the isolated cluster approach [4]. But all initial silanol models converged to the usual bridged geometries. An almost planar intermediate one, at the 0(2) atom, obtained after the first optimisation cycle is characterised by a CQO of 2.526 MHz (5th line in "11B" part of Table 4). The latter is two times larger than that corresponding to the tetrahedral B coordination and is in agreement with experimental values for borates and zeolites. We believe that the most reasonable explanation of the larger stability of the bridged moieties comes from the insufficiency of the STO-3G basis set to support the favoured silanol geometry even if this STO-3G drawback has not been discussed in literature. Another problem of the <> modelling with ABW is related with the larger framework density (circa 19) as compared to the one of MFI (circa 18) for which the < transformation has been studied [9]. The larger density can hinder the relaxation and lead to the initial bridged model. That is why our next attempt to simulate the < transformation will be done with the EDI framework whose density is 16.5 and at a higher basis set level.

3.3 Correlation between the electric field at the proton position and the acidity One of the factors in zeofites which can correlate with the acidity is their cavity size as discussed by Jacobs [20] and Dwyer et al. [21]. A quantitative relationship between the norm of the electric field (EF) on the proton and the OH frequency for H-form aluminosilicates (CHA, AFI types) and respective SAPOs models optimised with semi-empirical GULP code

1993 [22] was also shown. As soon as the OH frequency is related to a "softer" or "harder" O-H bond character, the frequency can conventionally be accepted as a measure of the acidity. The basis sets available in CRYSTAL usually lead to strongly overestimated OH frequencies; hence, we discussed the EF behaviour with respect to the Si-O(H)-T angle (T = A1, B). We calculated the norm of the EF at four possible H positions in the HABW borosiiicate and at all H sites in the aluminosilicates [8] (Figure 4). For comparison, we also recalculated the norm of the EF for the CHA and AFI zeolites [22] in order to present them in the same scale. '

0,05

I

"

I

.... " v ~ - _ : 2 ; 0,04

'

I

'

I

'

I

'

I

'

I

NAT (Si/AI = 1.5)

/

............................ ................. .-~_ ~2..... 9 g. - ....

-

z

EDI (Si/AI = 1 . 5 ~

~

-

AFI (SitAI = 3)

~

Cj~'~"~--...~..

~-'-'---r

o CHA (Si/AI = 3) .....

0,03

-,'"

........

...-'"

o

"'"""

= "'''"'~""

CAN (SilAI = 1 )

-r"

ABW (Si/AI = 1)

0,02

--tL-- --~_""

0,01

0,00

o

"~'-----~lC--~--~._._

20

'

I

125

'

I

130

'

ABW (SilB = 1)

I

135

"

I

140

'

I

145

'

I

150

'

I

155

Si-O-T angle, degrees

Figure 4. Norm of the electric field on the protons v e r s u s Si-O-T angle (T = B, AI). Aluminosilicates: this work (triangles, squares, circles), ref. 22 (diamonds, crosses). Borosilicate (stars) Three important features can be observed: (1) a sharp difference by two orders of magnitude between the norm of the EF on the proton in the alumino- and borosilicate of ABW type, (2) higher EF values on the protons in aluminosilicates of higher Si/A1 ratio, (3) no quantitative EF dependence v e r s u s the Si-O-T angle (T = A1, B) for any of the zeolite types studied herein. Our last observation only is confirmed by other calculated results [22]. The smaller values of the EF at the proton for all positions within the H-form borosilicate confirm that this parameter is related to acidity. But this lower EF cannot lead to a higher OH frequency in order to be in agreement with the usual proportionality between the OH frequency and the EF [22]. This suggests that other contributions to the OH frequency than those from the EF can be important for more covalent structures as borosilicates. Anyway, a borosilicate series of wider structural diversity should be considered in order to confirm this trend. 4. CONCLUSIONS Li- and H-form borosilicate models were optimised using the periodic approach at the STO-3G basis set level and their structural parameters angles) were found in agreement with the only available data, i.e., Calculated quadrupole coupling constants for 2H, 7Li, 11B, and 170 nuclei

Hartree-Fock (PHF) (bond lengths, bond for cationic forms. were also discussed.

1994 Similar favoured proton locations, corresponding to the 0(2) atom outside of the 4T ring window, were found in both the ABW alumino- and borosilicates. This position together with the proton located in the 4T ring near 0(3) were considered for the PHF simulation of the <> transformation in the ABW borosilicate as observed experimentally for the MFI type [9]. But all the obtained <<silanol>> models relaxed to the <> ones. This suggests that the STO-3G basis is not sufficient to support the favoured silanol geometry. For the H-forms, the norm of the electric field at the proton for all positions within the borosilicates was lower than those in the respective ABW aluminosilicate. The latter confu'ms that this parameter is thus related to the zeolite acidity that is smaller for borosilicates than for alumino silicates. ACKNOWLEDGEMENTS

The authors wish to thank the FUNDP for the use of the Namur Scientific Computing Facility (SCF) Centre and the Interuniversity Research Program on "Reduced Dimensionality Systems" (PAI/IUAP 4/10) for partial support. REFERENCES

1. L.B. Pierella, O.A. Anunziata, O.A. Orio, Lat. Am. Appl. Res., 24 (1995) 223. 2. J. ROseler, G. Heitmann, W.F. H61derich, Appl. Catal. A, 144 (1996) 319. 3. W.F. H61derich, J. R6seler, G. Heitmann, A.T. Liebens, Catal. Today, 37 (1997) 353. 4. G. Valerio, J. Plevert, A. Goursot, F. Di Renzo, Phys. Chem. Chem. Phys., 2 (2000) 1091. 5. R. Dovesi, V.R. Saunders, C. Roetti, M. Caus?~, N.M. Harrison, R. Orlando, E. Apr?~, CRYSTAL95 1.0, Univ. of Torino, 1996. 6. A.V. Larin, D.P. Vercauteren, Int. J. Quant. Chem., 82 (2001) 182. 7. P. Ugliengo, B. Civalleri, R. Dovesi, C.M. Zicovich-Wilson, Phys. Chem. Chel~ Phys., 1 (1999) 545. 8. A.V. Larin, D.P. Vercauteren, J. Mol. Catal. A, 168 (2001) 123. 9. K.F.M.G.J. Scholle, W.S. Veeman, Zeofites, 5 (1985) 118. 10. P.J. Bray, Inorg. Chim. Acta., 289 (1999) 158. 11. Cerius 2, Version 4.0.0, MSI, San Diego (1997). 12. E. Krogh Andersen, G. Ploug-Sorensen, Zeit. KristaUogr., 176 (1986) 67. 13. E.A. Paukshits, E.N. Yurchenko, Usp. ~ 52 (1983) 426. 14. B. CivaHeri, Ph. D'Arco, R. Orlando, V.R. Saunders, R. Dovesi, Chem. Phys. Lett., 348 (2001) 131. 15. M.W. Phillips, G.V. Gibbs, P.H. Ribbe, Am. Miner., 59 (1974) 79. 16. D.T. Griffen, AI~ Miner., 73 (1988) 1138. [17] M.E. Fleet, Arn~ Miner., 77 (1974) 76. 18. S. Ghose, C. Wan, ArrL Miner., 61 (1976) 123. 19. A.R. Lima, S.-Y. Jeong, J. Phys. Chem. Solids, 62, (2001) 881. 20. P.A. Jacobs, Cat. Rev. Sci. Eng., 24 (1982) 415. 21. J. Dwyer, K. Karim, W. Kayali, D. Millward, P.J.O'Malley, J. Chel~ Soc. Chenl Comn~ (1988) 594. 22. G. Sastre, D.W. Lewis, J. Chem. Soc. Faraday Trans., 94 (1998) 3049.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1995

Monte Carlo Simulation of the Temperature Dependence of Adsorption of Nitrogen and Oxygen by LiLSX Zeolite + Sudhakar R. Jale, Dongmin Shen, Martin Bttlow and Frank R. Fitch BOC Process Plants Technology, 100 Mountain Ave., Murray Hill, NJ 07974, USA. Grand Canonical Monte Carlo simulation of adsorption of nitrogen and oxygen was carried out on the LiLSX zeolite structure, and the results are compared with experimental adsorption data. The influence of temperature on the differential adsorption enthalpies, adsorption capacities, and N2/O2 selectivities is discussed. 1. INTRODUCTION The capability of molecular modeling to predict adsorption properties of zeolites is being explored by many research groups due to both academic and industrial relevance. The simulation of adsorption of various molecular species in zeolites was shown to be in good agreement with experimental data [1]. The forcefield developed by Watanabe, Austin and Stapleton [2], which was eml~ded in the Cerius2 TM simulation package (Aeeelrys), was applied to predict adsorption equill"oria of nitrogen, N2, and oxygen, 02, on various FAU-type zeolites such as CaX, NaX, NaY, LiX [3], and LiLSX [4,5]. The validity of Grand Canonical Monte Carlo, GCMC, simulations to predict adsorption isotherms and differential adsorption enthalpies has been demonstrated for both zeolites [4-6] and aluminophosphates [7]. A limited amount of simulation work has been undertaken to study the influence of temperature on adsorption capacities and enthalpies [8,9]. The heat of adsorption is typically calculated using (i) experimental isotherms measured at different temperatures by applying to those the van't Hoffor Clausius-Clapeyron equatiom, cfi, for example [10], (ii) the sorption isosteric method, e.g., [11], or (iii)calorimetric measurements, e.g., [12]. Since differential adsorption enthalpy data sets are required to design adsorption processes and provide useful information on the nature of interactions between adsorbents and sorbing species, an accurate measurement and understanding of its dependence on temperature are vital. Since, in adsorption processes, the bed temperature varies during the adsorption process, it is also necessary to have the isotherms measured or calculated at different temperatures. This paper addresses several features of the influence of temperature on adsorption properties, in particular differential adsorption enthalpies of N2 and 02 on LiLSX adsorbent. 2. EXPERIMENTAL A full description of the Sorption Isosteric Method, SIM, has been given previously [11 ]. Details of the Monte-Carlo ~ o n procedure and parameters used in the foreetield ~ n + The authors dedicate this paper to Dr. P. Ratnasamy, The Director of the National Chemical Laboratory Ptme, India, on his 60th birthday.

1996 are given in literature [2,4]. When modeling adsorption on LiLSX zeolite, the [3-cages of the FAU structure are blocked by large "dummy" atoms to avoid "creation" of N2 and O2 molecules inside sodalite cages. Simulations were carried out for one million iterations, and the results of first 50,000 steps were rejected. The quadrupole m o m e n t a of N2 and O2 molecules were described by three-point charge models the three fictitious charges being positioned linearly. The values of the charges and distances between them were chosen to match the experimental quadrupole m o m e n t a o f - 1.2 x 10.26 and - 0.40 x 10.26 esu for N2 and 02, respectively. The central "dummy atoms" in N2 and O2 were given charges of + 0.810 and + 0.224, respectively. In an earlier publication [4a], an effective charge of + 0.95 was utilized for the Li cations. Although a charge, + 0.95, on Li cations gives a better match between the experimental and simulated isotherms, the calculated differential adsorption enthalpies were higher than the experimental data. In this study, the charge on Li cations was reduced to a value, +0.92, to provide for a better match with respect to both isotherms and adsorption enthalpies. Although Li cations need to have a charge, +1.0, to compensate for the negative charge of the framework, the effective charge on these cations would be reduced due to some screening of cations by partial hydration, and a certain charge transfer from framework oxygen atoms to cations. Charge neutrality was achieved by adjusting appropriately the charges of framework silicon and aluminum atoms. The simulations yield estimates of the differential adsorption heat, which is calculated from the slope of the plot of total potential energy vs. amount adsorbed. The differential adsorption enthalpy, AH, is calculated by adding the mechanical work term, RT, to the calculated differential adsorption heat [8]. 3. RESULTS AND DISCUSSION Adsorption isotherms were simulated on a model LiLSX structure at various temperatures. The results are presented in Figure 1. As expected, at a given temperature, the amount of N2 adsorbed is always higher than that of O2. Although, both van der Waals and C o u l o m b interactions are responsible for the adsorption of N2 and 02 on LiLSX zeolite, the contribution of C o u l o m b interactions is much greater for N2 than for 02 due to the larger quadrupole m o m e n t u m of N2. The experimental adsorption isotherms reported in Figure 1 were measured on LiLSX crystals synthesized in our laboratory, c f , [13]. Since the charge on Li cation was intentionally decreased to a value, + 0.92, to improve the fit between simulated and experimental values of differential adsorption enthalpy, the calculated adsorbed amounts of N2 and O: are slightly lower than the experimental data at both 298 K and 323 K. The 02 concentrations calculated match the experimental data reasonably well at lower pressures, but they are lower than those at higher pressures. If a charge, + 1.0, is used on Li cations, the calculated N2 concentrations become higher than the experimental data, while the 02 concentrations are higher at lower pressures, and they match well with the experimental data at higher pressures. These discrepancies can be ascribed to a certain inaccuracy in the potential parameters represented by the forcefield expression used. However, for qualitative purposes, the results can be considered as being in good agreement with experimental data. As shown in Figure 1, the amounts of both N2 and O2 adsorbed decrease with increase in temperature, which is the trivial result expected. The rate of decrease in N2 adsorption with

1997 increase in temperature is greater than that for 02, due to a higher adsorption enthalpy for N 2. The simulated NJO2 selectivities (defined as mole ratios of N 2 t o 0 2 at 1000 mbar), which amount to 6.2 and 4.1 at 298 and 323 K, respectively, are comparable with experimental data (7.0 and 5.4, respectively). Since the adsorption capacity decreases stronger for N2 than for 02 with increase in temperature, N2/O2 selectivities decrease from 273 K to 373 K.

~:~3.0

~0.8

E

E 0.6

o.2 0.0

0.0 0

1000 2000 3000 4000 PRESSURE, mbar

0

1000 2000 3000 4000 PRESSURE, mbar

Figure 1. Adsorption isotherms of N 2 (A) and 02 (B) on LiLSX at different temperatures (similated data at 273 K (.), 298 K (..), 323 K (A), 348 K (o) and 373 K (.); experimental data at 298 K (D) and 323 K (A)). Because of the larger quadrupole momentum for N2 its adsorption is much stronger than that of 02, and it is dominated by the Coulomb (electrostatic) term, which leads to a heterogeneous (localized) distribution of adsorbed N 2 molecules over specific adsorption sites [4b]. On the contrary, 02 adsorption is dominated by the Lennard-Jones interaction term, and it leads to a fairly homogeneous distribution of 02 molecules within the microporous void volume. As known for a long time [14a,b], heterogeneity in sorption interaction arises not only from a heterogeneous adsorbent surface but also from the specific nature of the adsorbing species, cf, [ 14c]. The differences in adsorption behavior of N2 and 02 with varying temperature can be illustrated by plots of distribution of adsorption sites (or sorbing species) as exemplified in Figures 2. The dots represent centers of masses of adsorbate positions as generated by simulation. At 275 K, adsorption of N 2 is localized significantly stronger than that of 02. The localized adsorption can be attributed to specific quadrupole-cation charge interactions, which leads to a heterogeneous distribution of adsorbate molecules inside zeolite pores. On the other hand, non-localized type of adsorption can be attributed to prevailing non-specific interaction [ 14a], which leads to an almost homogeneous distribution of adsorbate molecules. Adsorption of N2 is localized at stronger sites (due to Coulomb interaction with cations), in particular, near Li cations in SIII or SIII' sites [4]. As temperature increases, the density of N2 positions

1998 around cationic sites decreases, which leads to an almost homogeneous adsorption that obeys a B o l t z m a n n distribution. Localized adsorption of N2 can be seen even at 375 K. On the

contrary, adsorption of 02 is energetically homogeneous even at 275 K, and it remains that way at higher temperatures. Figure 3 reports the calculated differential adsorption enthalpies for N2 and 02 on the LiLSX structure at various temperatures. At temperature fixed, the differential adsorption enthalpy is higher for N2 than for O2 due to the higher quadrupole m o m e n t u m of N2, which leads to prevailing specific interaction of the C o u l o m b type. For a comparison, the differential adsorption enthalpy data for N2 and 02 on the LiLSX structure as obtained by SIM experiments, is also included in Figure 3. Their error margin represents ca. • 0.1 kJ/mol. For methodical reasons, there is a significant difference in the temperature, at which differential adsorption enthalpies are obtained experimentally either by SIM or simulation. Simulation is carried out at a constant temperature. In SIM experiments, the temperature is varied to obtain an isostere at a given sorption-phase concentration. In particular, temperature has to be decreased as sorption-phase concentration is increased to achieve a higher sorptionphase concentration, and to maintain the isosteric condition [11]. In case of N2, the temperature varies from 250 K at a sorption-phase concentration of ca. 0.02 mmol/g to about 175 K at a concentration of ca. 2.5 mmol/g. For 02, it varies from 160 K for a sorption-phase concentration of ca. 0.2 mmol/g to about 110 K for a concentration of ca. 1.0 mmol/g. This approach is justified as long as there is only a negligible temperature dependence of the differential adsorption enthalpy. This makes a comparison of simulation data with experimental results difficult for a wide range of sorption-phase concentrations. However, a comparison of initial sorption heats, viz., those for concentrations within or close to the corresponding H e n r y regions, becomes more meaningful since the influence of the experimental temperature change is minimized due to a narrow range of temperature to be chosen to measure adsorption isosteres. To minimize the temperature effect described, the simulated "isosteric" heats of N 2 adsorption obtained at 250 K were used for a comparison with the experimental differential adsorption enthalpies. The simulated heat of N2 adsorption on LiLSX crystals at 250 K is by ca. 2 kJ/mol higher than the experimental value for 250 K. In the 02 case, the simulated "isosteric" heat for 160 K should be used to compare with the experimental differential adsorption enthalpy. As it can be seen clearly from Figure 3(B), the heat of 0 2 adsorption predicted at 200 K matches reasonably well experimental data measured at 160 K. One noticeable deviation from experimental data is that simulation over-estimates lateral interaction energies, which leads to somewhat higher differential adsorption enthalpies for 02 with coverage. This trend was found at all temperatures. It is evident from Figure 3 that the simulated differential adsorption enthalpies for both N2 and 02 decrease with increasing temperature, the decrease being less significant for 02. For N2 adsorption, the decrease amounts to ca. 5.5 kJ/mol over a temperature range from 273 to 373 K, while, for 02, it decreases only by 1.5 kJ/mol. The decrease in differential adsorption enthalpy with temperature ought to be attributed to a heterogeneous nature of the adsorption process [14] and the differing influence of temperature towards various types of intermolecular interaction forces, viz., strong influence onto C o u l o m b and weak influence onto L e n n a r d - J o n e s type interactions. As discussed earlier, when temperature is raised, adsorption of N2 becomes more

M:)

2000 homogenous due to a decreased Coulomb-type contribution to the overall interaction energy. This leads to a decrease in differential adsorption enthalp3; and adsorption capacity for N2, as well as in N2/O 2 selectivity with increase in temperature. Since the contribution of Coulomb-type interactions between 02 molecules and cations is comparatively small, the enthalpies of 02 adsorption are significantly lower than those of N2. This also leads to non-localized O2 adsorption even at low temperatures. Because of this homogeneous nature of the adsorption of 02, the temperature dependence of differential adsorption enthalpy is less pronounced than that observed for N2 adsorption. 14 A

B

&

28

13

O

O

E

E

"~ 12 :s

=

11 20 ~ 0.0 !

. . . . . . 1.0 2.0 !

i

SORBED AMOUNT, m m o l / g

10 ' 0.0

'

'

I

0.4

'

'

"

I ' 0.8

SORBED AMOUNT, mmol/g

Figure 3. Differential adsorption enthalpies of N 2 (A) and 02 (B) on LiLSX at different temperatures (Similated data at 200 K (o); 250 K (A), 273 K (*), 298 K (,,), 323 K (A), 348 K (o) and 373 K (.); Experimental data at 298 K ([])). The temperature dependence of adsorption enthalpies has been addressed in many publications, a few of which should be named here. The data sets presented in [8] show that the total potential energy of N2 adsorption is smaller at lower temperatures. Recent studies [15] of an application of the Clausius-Clapeyron equation to various forms of adsorption isotherm models have shown different strengths of temperature dependences of the differential adsorption enthalpy. If the differential adsorption enthalpy does not depend on temperature, the adsorption-phase heat capacity should be close to the gas-phase heat capacity. It was shown [15] that deviations between differential heat capacities for the adsorption phase and molar gas-phase heat capacities were in ranges of (27to67)% and (-10to 10)% for localized and mobile diatomic adsorption, respectively. The temperature dependence of differential adsorption enthalpy was most severe for localized adsorption, low temperature conditions and for strongly adsorbed species. Similar conclusions were drawn from experimental data published more than two decades ago, e.g., [16]. Using a quantum-chemical approach, it was shown that isosteric adsorption heats at various temperatures obey the Boltzmann statistics, which comprises their decrease with increasing temperature, because of a decrease in the ratio of population of sorbing molecules

2001 at stronger sites compared to weaker sites. The density-functional theory also predicts a weak temperature dependence for the differential adsorption enthalpy [17]. Such a dependence was also demonstrated using gas-phase vibrational partition functions [15] and surface-energy site distributions [ 18]. By examining approximations used to determine differential adsorption enthalpies by the

Clausius-Clapeyron equation, it was concluded that the approximations have a pronounced effect on data for sorbing species with relatively low molecular weight, at low relative pressures. This seems to apply for the adsorption of N2 and 02 on LiLSX. As demonstrated in Figure 2, the population of stronger sites decreases as the temperature increases. Consequently, as shown in Figure 3, the differential adsorption enthalpy decreases with increase in temperature. Since the adsorption of 02 is homogeneous even at 273 K, the population of different sites does not vary much with temperature, and, hence, the influence of temperature is not as significant as that observed for N 2. 4. CONCLUSIONS For nitrogen and oxygen adsorption by LiLSX zeolite, adsorption isotherms predicted by GCMC simulations are very similar to experimental data. The simulated differential adsorption enthalpies agree well with experimental data-obtained by SIM, considering the experimental peculiarities involved with the two approaches. The decrease in adsorption capacities and N2/O 2 selectivities and the dependence of differential adsorption enthalpies on temperature are due to a heterogeneous nature of adsorbent-adsorbate interactions. Adsorption of N2 is energetically more heterogeneous and shows a temperature dependence stronger than that of 02. 5. ACKNOWLEDGEMENTS The authors thank Dr. A.F. Ojo as well as R. Wolf and V. Hunter for synthesizing LiLSX zeolite, and Z. Orban for measuring a series of adsorption isotherms. They are grateful to Dr. N. Perelman for valuable suggestions during the course of this work. Last not least, they thank The BOC Group for the permission to publish this work. 6. REFERENCES

1. 2. 3.

A.H. Fuchs and A.K. Cheetham, J. Phy. Chem. B, 105 (2001) 7375. K. Watanabe, N. Austin and M.R. Stapleton, Mol. Simul., 15 (1995) 197. a) A.J. Richards, K. Watanabe, N. Austin, and M.R. Stapleton, J. Porous Mater., 2 (1995) 43. b) J.M. Newsam, C.M. Freeman, A.M. Gorman, and B. Vessal, Chem. Commun., (1996) 1945.

4.

a) S.R. Jale, M. Btilow, F.R. Fitch, N. Perelman and D. Shen, J. Phys. Chem. B, 104 (2000) 5272.

5.

N.D. Hutson, S.C. Zajic and R.T. Yang, Ind. Eng. Chem. Res., 39 (2000) 1775.

b) V.B. Kazansky, M. Billow and E. Tichomirova, Adsorption, 7 (2001) 291.

2002 6.

7. 8. 9. 10. 11.

12.

13.

14. 15.

a) C. Mellot and J. Lignieres, Physical Adsorption: Experiment, Theory and Applications, J. Fraissard (ed.), Kluwer Acad. Publ., 1997, p. 429. b) P. Pullumbi, J. Lignieres, A. Arbouznikov and A. Goursot, Metal-Ligand Interactions in Chemistry, Physics and Biology, N. Russo and D.R. Salahub (eds.), Kluwer Academic Publishers, 2000, p. 393. H. Reichert, W. Schmidt, Y. Grillet, P. LleweUyn, J. Rouquerol and K.K. Unger, Stud. Surf. Sci. Catal., 87 (1994) 517. D.M. Razmus and C.K. Hall, AIChE Journal, 37 (1991) 769. A. J-M. Pellenq, 13. Tavitian, D. Espinat and A.H. Fuchs, Langmuir, 12 (1996) 4768. D. Valenzuela and A. Myers, Adsorption Equilibrium Data Handbook, Prentice-Hall, Englewood Cliffs, NJ, 1989. a) M. 13tilow, Stud. Surf. Sci. Catal., 83 (1994) 209. b) D.M. Shen, and M. Btilow, Micropor. Mesopor. Mat., 22 (1998) 237. c) M. 13tilow, D.M. Shen and S.R. Jale, Appl. Surf. Sci., in press. a) N. Cardona-Martinez, J.A. Dumesic, Adv. Catal. Lett., 38 (1989) 149. b) D. Shen, M. Btilow, F. Siperstein, M. Engelhard and A.L. Myers, Adsorption, 6 (2000) 275. a) C.C. Chao, US Patent No. 4 859 217 (1989). b) C.G. Coe, J.F. Kimer, R. Pierantozzi and T.R. White, US Patent No. 5 152 813 (1992). c) S.U. Rege, R.T. Yang, Ind. Eng. Chem. Res., 36 (1997) 5358. d) C.C. Coe, Access in Nanoporous Materials, T.J. Pinnavaia and M.F. Thorpe (eds.), Plenum Press, New York, 1995. e) D. Shen, M. 13tilow, S.R. Jale, F.R. Fitch and A.F. Ojo, Micropor. Mesopor. Mater., 48 (2001) 211. a)A.V. Kiselev, Intermolecular Interactions in Adsorption and Chromatography, (Russ.), Vyschaja Schkola Publ., Moscow, 1986, pp. 32-47. b)M. Jaroniec and R. Madey, Physical Adsorption On Heterogeneous Solids, Elsevier, Amsterdam, 1988. c) G. De Luca, A. Arbouznikov, A. Goursot and P. Pullumbi, J. Phys. Chem.B, 105 (2001) 4664.

16. a) S.A. A1-Muhtaseb and J.A. Ritter, Ind. Eng. Chem. Res., 37 (1998) 684. b) S.A. A1-Muhtaseb and J.A. Ritter, J. Phys. Chem. B, 103 (1999) 2467. c) S.A. A1-Muhtaseb and J.A. Ritter, Langmuir, 14 (1998) 5317. 17. a) H. Thamm, H. Blank and M. Btilow, Z. physik. Chem., Leipzig, 256 (1975) 395. b) H. Blank, M. Btilow and W. Schirmer, Z. physik. Chem., Leipzig, 260 (1979) 395. 18. A. Clark, The Theory of Adsorption and Catalysis, Academic Press, New York, 1970. 19. W. Rudzinski and D.H. Everett, Adsorption of Gases on Heterogeneous Surfaces, Academic Press, London, 1992. 20. H. Pan, J.A. Ritter and P.B. Balbuena, Ind. Eng. Chem. Res., 37 (1998) 1159.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

D e n s i t y F u n c t i o n a l T h e o r y C a l c u l a t i o n s o f H e n r y ' s constant for N2, Ar m o l e c u l e s in C a - A and C a - L S X Zeolites.

2003

02 and

G. De Luca a, P. Pullumbi b and N. Russo a. aDipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite-Centro d'Eccellenza MURST, Universit/t della Calabria, Via Pietro Bucci, 1-87030 Rende, Italy. bAir Liquide, Centre de Recherche Claude Delorme, 1 Chemin de la Porte des Loges B.P. 126, 78354 Les Loges-en-Josas Cedex, France. Zeolites are widely used in industry as gas-separation adsorbents or heterogeneous catalysts. An understanding of the adsorption properties of these materials (adsorption isotherms, Henry constants and isosteric heats) is vital to efficient separation process design and operation. In this paper we propose a method for the prediction of Henry constants and isosteric heats at zero coverage. By embedding a quantum mechanics calculation (QM) in a classical molecular mechanics (MM) model of the environment, the hybrid QM/MM scheme attempts to incorporate environmental effects at an atomistic level, including influences as accessibility hindrance, electrostatic perturbations and dielectric screening. 1. INTRODUCTION. Zeolitic materials are widely used as adsorbents. Their adsorption properties are determined by their structure, essentially their pore size and number and location of their extraframework cations. The adsorption properties of zeolitic adsorbent with respect to a particular adsorbate are described, at equilibrium, by its adsorption isotherm. The isosteric heat of adsorption, which can be estimated with different experimental techniques, describes the energetic of adsorption. The most accurate experimental method to measure isosteric heats is the calorimetric one [1 ], but it can also be determined from experimental isotherms measured at different temperatures, applying Clausius-Clapeyron equation. The Henry constant can be obtained from the limiting slope of the adsorption isotherm. These techniques, however, are very sensitive to numerical errors of differentiation and extrapolation, which leads to poor agreement between available experimental data [2-4]. The interpretation of adsorption isotherms needs the use of analytical equations in which the surface heterogeneity has been introduced by simple distributions laws [5,6] based on the idea that the adsorption on the heterogeneous surface can be described as a summation over various parts of energetically homogeneous "islands". In this way each part could be treated as a local homogeneous adsorbent, with local adsorption isotherm. The simplest and most common model for dealing with the local adsorption isotherm is the one proposed originally by Langmuir [7]. Recently we have shown that molecular modelling is able to predict qualitative trends of

2004 N2/O2 separation in Ca-LSX and CaA zeolites by coupling QM to MM calculations [8,9]. Here we focus on low-coverage adsorption aiming the assessment of QM/MM adsorption energies for reproducing Henry constants and isosteric heats of adsorption. As we will deal with low-coverage only, we will consider that cations alone are responsible for the gas adsorption in the zeolites. The calculation of just isosteric heats doesn't permit to apprehend the effective limits of QM/MM model because this property cannot be measured with high precision. 2. MODEL DESCRIPTION. We will consider that the adsorption occurs at cations and the interaction energy of an isolated molecule is described by its potential energy E(r), r being its distance from the cation. The equilibrium constant K, representing equilibrium between adsorption and desorption on the surface [M (gas) + S (surface) = MS (adsorbed state)], can be expressed in function of the Gibbs energy: K =

qm (MS) qm (M)

e

- AE/RT

(1)

The variables qm(MS) and qm(M) are the standard molar partition functions of the adsorbed and free molecules, respectively. AE is the difference in molar energies of the adsorbed and gaseous species. The qm and AE values, which are connected with the interaction between the zeolite and a gaseous molecule, can be evaluated theoretically. The temperature dependence of K can be expressed through the van' t Hoff equation: O _

_

ad

dT

(2)

RT 2

Where AH~ is the standard molar enthalpy of adsorption. The prediction of adsorption isotherms requires a model for adsorption, i.e. a function relating the pressure to surface coverage. Langmuir model [7] for monolayer adsorption is defined as: 0-

n n

-

sat

Kp

(3)

l+Kp

Where 0 is the surface coverage, n is the number of molecules adsorbed at the pressure p, and nsat is the saturation number. The model is based on two fundamental assumptions: (i) there is no interaction between molecules adsorbed at different sites; (ii) all sites are equivalent, i.e. the surface is homogeneous. These limitations given, the Langmuir formula, through the Clausius-Clapeyron equation, is generally used to estimate the isosteric heats of adsorption (enthalpy of adsorption at a fixed surface coverage) [4]: AH

ad

= R [ c31np ] a(l/T)

n

'

(4)

2005 Where AHad is the isosteric heat of adsorption for a given number n of adsorbed molecules. It is important to underline that the van' t Hoff equation defines the variation of K respect to the temperature and yields the molar enthalpy of adsorption; indeed the ClausiusClapeyron equation defines the variation of pressure with respect to the temperature for two systems in equilibrium (gas and adsorbed phase) and allows, in this case, to evaluate the isosteric heats of adsorption at a fixed coverage [1 ]. The molar enthalpy of adsorption corresponds to the isosteric heat at the limit of zero coverage. The limiting slope of an experimental adsorption isotherm, characterizes the Henry constant and its dependence with temperature yields the isosteric heat of adsorption at zero coverage [1 ]. Actually, the molar enthalpy of adsorption varies with the temperature, but if the variation is assumed to be small in a given range of temperature, the equation 2 can be applied to calculate isosteric heats at zero coverage as the slope of the linear relation between ink and 1/T. Real solid surfaces may consist of patches of identical adsorption sites, so that the overall adsorption isotherm may be written as [10]: nsit o =

Z

i=l

K.p c.

~

(5)

1 l+K.p 1

Where Ci is the fraction of the total surface, consisting of adsorption sites of the same kind and Ki corresponds to Langmuir constant connected to correspondent site. For this model the limiting slope is positive and finite [6], that is: o lim-= p--}O p

nsit = Z C.K. p=O i=l 1 1

(6)

In this work, according to Langmuir model, we assume that the total saturation capacity (nsat) is equal to the number of cations, because only cations can adsorb one gas molecule. Consequently the all Ci were defined equal to 1/nsat. It is worth to note that the eq 5 can be rewritten in a different way:

(n t / nsitX(ns nsit nsit Kp cY ~=C i_ll+Kp

K p+ E j Z ... Z K .K .K k,,i = 1 i j=2 zl=l zj > zj-1 zl zl + 1 zj

+ nsi

pnsit

X,i =1

s i t _l lZnj s-i t _2zl=l z j-- l nKs i t -K i t(p n7S ilt) [ 1+ in~l t K / np+ Y. ... z j > 5-' .K ) / nJS+]I-]K k,,i = 1 i zl zl+l zj \ i =1 at low pressure this equation may be reduced to"

2006

nsit K.p ~ ~ cZ i=ll+K.P

Ki c 1

1+

\i=1

)

(8)

Ki p

by means of equation 8 is possible to reproduce the adsorption isotherms at low pressure like monosite Langmuir model with K equal to sum Of single Ki. Besides, the limiting slope derived from equation 8 it is equal to equation 6. 3. QM/MM CALCULATIONS. Both considered zeolites, Ca-LSX (low silica X) and Ca-A, have a Si/Al=l and contain 48 calcium cations per unit cell. Whereas the LTA structure accommodates 6 cations per sodalite cage, located in the middle of six-membered rings, the LSX zeolite has only 32 possible calcium ions in six-membered rings (site II), the remaining 16 cations being distributed in sites I and I'. The zeolites structures have been generated by MM simulation. In the present study we have used the simulation protocols recently reported [11] to determine the out off framework cation configuration. The results of the Monte Carlo packing procedure carried out using the aluminosilicate cvff_aug forcefield in conjunction with the Discover 3.2 package for energetic optimizations, compares well with the new grid-based algorithm for cation location within the Cerius2 interface [12]. QM/MM method has been recently used for studying adsorption of gaseous molecules in zeolitic frameworks [8,9,13]. In this study we have adopted a different methodology from the recently reported in [13]. Here we calculate adsorption energy of the gas molecules interacting with each accessible cation. The embedded cluster approach consists of a Ca>-molecule (N2 or 02) system treated quantum chemically, surrounded by the specific environment of each cationic site, i.e., point charges simulating the A1, Si, O, and Ca 2§ zeolite ions. This procedure allows us to take into account long-range effects, which may differ from one cation to another. The QM calculations have been performed within the framework of density functional theory and by using the Becke [14] -Perdew [15] exchange-correlation potentials. Computational details are extensively described in ref. [ 16, 8]. As discussed in these papers, this method needs as input the values of the charges representing the zeolite atoms. At first sight, this is not a trivial problem, since these charges are not directly measurable properties. The embedding charges used in this study, i.e. Si (+2.4), A1 (+1.4), O (-1.2) and Ca (+2.0), are those used for MM simulations. The QM calcium cations were also assumed to bear a +2 charges, i.e. neglecting any zeolite to cation charge transfer. The strategy used in this work aims to evaluate Henry constants and the molar enthalpy of adsorption by using the equations (6) and (8). The multisite Langmuir model, allows us to express the global surface coverage as a sum of individual contributions. Each local equilibrium constant is evaluated as a ratio of partition functions which, written respect to the minimum of interaction energy, gives: K = i

qm,i (MS) q m (M)

e

_ AEi/RT

with

K

o, i

=

qm,i (MS) q m (M)

(9)

2007 qm,i(MS) is the local molar partition function at each cationic site for the adsorbed system, while qm(M) describes the partition function for the gaseous state. More details about the calculation of the qm (M) and qm,i (MS) are described in the ref. [8]. It is worth noting that in this study the partition functions are evaluated quantum chemically, assuming Morse potential for frustrated movements of adsorbed molecules. The thermodynamical functions qm,i and qm depend explicitly on the temperature and their ratio is temperature dependent. The Ki values are the individual equilibrium constants associated with every local isotherm. These constants depend on the Ko,i and the adsorption strength of the site according to equation (9). If all sites are equivalent, the surface is homogeneous and adsorption can be described with a monosite Langmuir model, where the global equilibrium constant K is equal to each equivalent Ki. When the surface is heterogeneous, the global equilibrium constant K could be estimated through the equation (8). Unlike our previous publication [8], in which the global constant K is achieved by fitting the theoretical multisite isotherms, in this work K is calculated analytically. As mentioned above, AH~ and K vary with T, choosing an appropriate AT, around a particular temperature, allows the plot of lnK(T) against 1/T to be linear, yielding the isosteric heat of adsorption in this domain. It is worth underlining that in this approach the accuracy of Henry constants and molar adsorption enthalpy depends on the accuracy of adsorption energies and local Ko,i, and all range of temperatures can be used to evaluate the isosteric heats of adsorption at zero coverage. Moreover, the simplicity of equation 8 allows to understand better what kind of corrections is needed to improve the QM/MM model used. In the temperature range AT, where AH~ is assumed to be constant, the logarithmic term of Ko,i should be constant within a good approximation. This term reflects the different response of the adsorbed molecule to temperature changes, with respect to gaseous molecule. It is clear that an increase of temperature will affect differently the vibrational states of the adsorbed species and the degrees of freedom of the isolated system. Although at high temperatures the adsorbed and gaseous states become less different and Ko,i may be assumed constant, generally this term is not constant for all temperatures. 4. RESULTS AND DISCUSSIONS. The calculated adsorption energies, for N2, 02 and Ar at all accessible sites have been reported in table 1. Compared to the energies obtained in our previous calculations, these are corrected on average of 1.5 kcal/mol in absolute value. The corrections of the QM/MM model necessary to obtain these energies will be the subject of a future publication. Table 1. Maximum and Minimum Adsorption Energies (in kcal/mol) for N2, O2 and Ar calculated for the Accessible Cations of Ca-A and Ca-LSX. Molecule Ca-A Ca-LSX Maximum Minimum Maximum Minimum N2 -8.5 -7.2 -9.5 -5.9 02 -6.2 -5.1 -7.5 -4.3 Ar -3.9 -3.4 -5.2 -2.8 Although the spread of the energies for Ca-LSX is larger than for Ca-A, due to the nonsymmetrical distribution of the 16 no accessible site I/I' cations, we have distinguished,

2008 in the range of 0.4 kcal/mol, four groups of adsorption energies corresponding to the cations in a CaLSX cage. The Henry constants of the same gases estimated according to equation 8 and 9 and using QM/MM adsorption energies reported in table 1, are presented in table 2. Table 2. H emy Constants for N2, 02 and Ar adsorption on Ca-A and.~Ca-LSX (in 10.6 mol/k!~Pa). Molecule Ca-A Ca-LSX Theory Exp. a Theory Exp. a N2 10.4 9.2 24.4 26.0 02 1.7 1.6 3.2 1.9 Ar 1.0 1.4 1.9 1.3 ;'Te-fe~en c ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The experimental reported values [17] have been derived from chromatographic study of the N2, 02 and Ar adsorption on cation exchanged A and X zeolites. Comparison of the calculated Henry constants of table 2 with the experimental ones shows that our approaches give good results for the adsorption of N2 on both zeolites. The Henry constant corresponding to the adsorption of oxygen molecule on Ca-A, agree well with the experimental one, contrary to the constant of 02 on Ca-LSX which is less satisfactory. Finally the calculated Henry constant associated to adsorption of Ar on Ca-LSX can be considered satisfactory, whereas the constant for Ca-A is half of the experimental value. From these results and the energies corrections necessary to obtain the values in table 1, it is possible to assert a partial conclusion: the QMM/MM model adopted in this work has some problems to reproduce the interactions energies lower than 6 kcal/mol, however it reproduces better the interactions higher than 6 kcal/mol (e.g. N2 on Ca-A) with the exception of many heterogeneous surface sites (some sites of CaLSX). However, to confirm this conclusion, a more accurate study of theoretical Koi is necessary. In fact, as reported in figure 1a and b, which show the influence of temperature on the calculated preexponential factors, the accuracy of Koi can be important. Ca-A Oxygen 0.0033 -1.2

.......................................

0.00335

0.0034

, ........................................

0.00345

~. . . . . . . . . .

,

. . . . . . . . . . . . . . .

0.0035

0.00355

, ...................................

0.0036

, ...............................

, .............................

0.00365 ,

0.0037

.......................

-1.4 .... -1.6 ,r

..~

l j ~ t ~ ~

-1.8 -2

..E -2.2 -2.4

........

-~ 11

~---/--~f~~-

-2.6

1/T Ik)

Figure l a Henry constant for Ca-LSX, 02. Koi as a function of temperature (triangles) and Koi calculated at 293 K (circles).

2009 Ca-LSX

0.00325 -1.5

0.0033

0.00335

............................ ~. . . . . . . . . . . . . . . . . . . . . .

,

Argon 0.0034

. . . . . . . . . . . . . . . . ,. . . . . . . . .

0.00345 ,

0.0035 ..........

0.00355

,

........

-1.6 -1.7 -1.8 "'

-1.9

~'

-2

J

-=

-2.1 -2.2 -2.3 -2.4

lrr (K)

Figure 1b. Henry constant for Ca-LSX, Ar. Koi as a function of temperature (triangles) and Koi calculated at 293 K (circles). Through an analysis of theoretical Koi values and effective pre-exponential factor, obtained by fitting the experimental first points of adsorption isotherms and using theoretical Henry constants and isosteric heats at zero coverage, it is possible to see that the theoretical Koi of CaA-N2, CaA-O2 and CaA-Ar agree with these effective pre-exponential parameters but for Ca-LSX systems there are some discrepancy. For this reason more tests are necessary to confirm the previous conclusion. Comparison of calculated isosteric heats at zero coverage and experimental values, reported in table 3, shows that our approach give good results. The experimental values have been derived from two methods: from slopes of van't Hoff plots using Henry constants derived from chromatographic measurements [17], and from application of Clausius-Clapeyron equation with volumetric adsorption measures [4]. Table 3. Computed and experimental isosteric heats at zero coverage (in kcal/mol) for N2, 02 and Ar adso,~ion on CazA and Ca-LSX. Molecule Ca-A Ca-LSX Theory Exp. a'b Theory. Exp:,~. . . . . . N2 7.3 6.7 a 7.5-t; 8.4 9.1 02 4.5 5.0 a 3.4 b 6.0 6.7 Ar 2.9 4.3 a 3.2 b 3.9 5.7 a ~ [ ] 7], refere-n~ce[ - [ ~ . . . . . . . . . . . . . .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

In the first method, the plot of K with respect to 1/T can be easily fitted with a linear function but the error done by using a linear regression over a range of 70 Kelvin can easily reach 1 kcal/mol, according to the system. With the second method the values obtained depend, apart from the temperature range, on the coverage level considered. This precision and approximation being kept, a comparison of calculated and experimental results shows a good agreement for N2 adsorption. The agreement is not so good for Ar, with heats of adsorption derived from chromatographic experiments, whereas the other results are within the acceptable range of error. It is worth noting that if the isosteric heats

2010 at zero coverage are calculated with Koi which do not depend on temperature, as reported in figure 1, a systematic increase of the values is observed, according to the system. 5. CONCLUSIONS. We have presented a method to evaluate Henry constants and molar enthalpy of adsorption for gaseous molecules in zeolites based on the simple Langmuir equation, i.e. equation 8, to reproduce adsorption isotherms at low pressure. The method is based on the QM description of the interaction between the molecule and the cationic site, modeled as an embedded cation. Most of the models used to interpret adsorption isotherms are based on site equilibrium constants, where Koi and AEi are parameters adjusted to reproduce experimental isotherms. The Koi terms, often called affinity parameters, are generally considered to be independent of T and AE. The fact that we do not use empirical parameters and apply the multisite Langmuir model based on simple statistical thermodynamics derivations allows us to retain the physical significance of the thermodynamic quantities involved in the adsorption equilibrium.

Acknowledgements.The work was supported by Universit~t della Calabria. G. De Luca permanent address: Istituto di Ricerca su Membrane e Modellistica di Reattori Chimici, IRMERC-CNR, Via P. Bucci, 1-87030 Rende, Italy. REFERENCES. 1. J.A. Dunne, R. Mariwala, M.Rao, S.Sicar, R.J.Gorte and A.L. Myers, Langmuir, 12 (1996) 5888. 2. H.A. Boniface and D.M. Ruthven, Gas Sep. Purif., 7 (1993) 183. 3. G.A. Soriel, W.H.Granville and W.O.Daly, Chem. Eng. Sci., 38 (1883) 1517. 4. G.W. Miller, K.S. Knaebel and K.G. Ikels, AIChE J., 33 (1987) 194. 5. S. Sicar, Ind. Eng.Chem. Res., 30 (1991) 1032. 6. P. M. Mathias, R. Kumar, J. D. Moyer, J. M. Schork, S. R. Srinivasan, S. R. Auvil and O. Talu, Ind. Eng. Chem. Res., 35 (1996) 2477. 7. I. Langmuir, J. Am. Chem. Soc., 40 (1918) 1361. 8. G. De Luca, A. Arbouznikov, A. Goursot and P. Pullumbi, J. Phys.Chem. B, 105 (2001) 4663. 9. P. Pullumbi, J. Lignieres, A. Arbouznikov, A. Goursot, In Metal-Ligand Interactions in Chemistry, Physics and Biology ; Russo, N. and Salahub D.R. (eds), Kluwer, Dordrecht, 2000. 10. W. Rudzinski and D. H. Everett, Academic Press, Adsorption of Gases on Heterogeneous Surface, 1992. 11. J. Ligni6res, J.M. Newsam, Microporous and mesoporous materials, 28 1999 305 12. Cerius2 9Molecular modeling software, Accelrys Inc., San Diego, USA. 13. F. Yielens and P. Geerlings, J. of Molecular.Catalysis A, 166 (2001) 175. 14. A. D. Becke, P, Phys. Rev. A, 38 (1988), 3098. 15. J. P. Perdew, Phys. Rev. B., 33 (1986), 8822. 16. A.Goursot, V. Vasilyev and A. Arbuznikov, j. Phys. Chem., 101 (1997) 6420. 17. J.A. Martens, D. Ghys, M.Van de Voorde, H. Verelst, G.Baron and P. A. Jacobs, E. F. Vansant (Ed)., Separation Technology, Elsevier, Amsterdam, 1994.

2011

AUTHOR INDEX

Aboukais A. 699, 1197 Acevedo del Monte E. 1737 Acosta D.R. 1039 Agger J.R. 93 Agren P. 159 Aguado J. 77 Agueda VI. 1579 Ag6ndez J. 1267 Aiello R. 45, 423,469, 1165, 1427, 1891 Aiello S. 383 Aizawa T. 739 Akita T. 611 Alberti A. 151, 1923 Alifanti M. 823 Aliyev A.M. 787 Aloi D. 469 Alvarez A.M. 525, 1339 Anastasescu C. 1213 Anderson M.W. 93, 327, 1149 Andrade H.M.C. 343 Armaroli T. 975 Armendfiriz-Herrera H. 1039 Arribas M.A. 1015 Artioli G. 45, 1721 Artok L. 799 Asencio I. 731 Auroux A. 1091 Avalos-Borja M. 1939 B.Nagy J. 279, 287, 295, 469, 477, 503, 541, 927, 1395, 1427 Baerns M. 1141 Baldi M. 967 Bandyopadhyay R. 15 Baran P. 439 Barth J.-O. 69 Basaldella E. 1411 Batista M.S. 983 Battaglia G. 1751, 1759 Bauer E 303 Bazzana S. 117

Bechara R. 1133 Bein T. 223, 1465 Beland E 125 Bellat J.P. 1101, 1571 Bellussi G. 61, 1923 Bengoa J.E 525, 1339 Bentrup U. 1323 Berber-Mendoza S. 1849 Bergaoui L. 903 Bernauer B. 303, 1513, 1521 Berthomieu D. 1899 Berti D. 61 Beta I.A. 1647 Bevilacqua M. 975 Beyer H.K. 1347 Bi Y. 1205 Bilba N. 295 Birjega R. 1331 Bischof C. 911 Bispo J.R.C. 517 Biswas D. 771 Biswas G.K. 771 Bitter J.H. 573 Blanc A. 1395, 1473 Blanco C. 1253 Blasco T. 1331 Blin J.-L. 1687 Bogdanchikova N. 815, 1939 B6hlig H. 1647 B6hlmann W. 1355 BShringer W. 635 Boisen A. 109 Boldi~ M. 1663, 1713 Bonelli B. 143 Bonino E 3 Bonneviot L. 125 Bordiga S. 3, 199, 1963 Borello L. 143 Bortnovsky O. 533, 1505 Botas J.A. 1671 Botella P. 651

2012

Bourdon X. 951 Brabec L. 1505, 1521 BrandSo P. 327 Broctawik E. 453, 1971 Broersma A. 573 Bronic J. 423 Brorson M. 109 Brouca-Cabarrecq C. 1371 Broyer M. 1101 Briickner A. 1141 Brunel D. 1371, 1395, 1473 Bruno M. 279, 287 Bucci M. 1751, 1759 BiJlow M. 1995 Bulut H. 799 Buondonno A. 1751, 1759 Burgio G. 1023 Burtic~ G. 1655 Bus E. 573 Busca G. 967, 975 Bussai C. 1979 Cadoni M. 343 Cagnoli M.V. 525, 1339 Cai Q. 1205 Calabr6 V. 1561 Calcaterra D. 1775 Caldeira C. 359 Calleja G. 1671 Campelo J.M. 781, 1299 Cafiizares P. 707 Cappelletti P. 1775 Caputo D. 1611 Carati A. 191 Carluccio L.C. 61 Carotenuto L. 45 Carpentier J. 1197 Casci J.L. 1149 Catanzaro L. 287 Caullet P. 951 Cavallaro G. 1165 Cavani E 565, 831 Ceglie D. 1759 (~ejka J. 23, 247, 627 Centi G. 477, 503 (~erven~, L. 627 Chelaru C. 951 Chen W. 1237

Chen Y. 795 Chenevarin S. 335 Cheng L.-B. 1007 Choi S. 595 Chu Y.E 595, 887 Chun Y. 175 Ciahotn~, K. 1713 Ciambelli P. 1031 CibriLn S. 847 Clark L.A. 643 Colella A. 1751, 1759 Colella C. 45, 1611, 1751, 1759, 1841 Coluccia S. 1419 Comite P. 967 Constantin C. 1213 Coppola E. 1751, 1759 Corma A. 487, 651 Cormier W.E. 887 Corr~a M.L.S. 517 Cremades Ja6n E 549 Cresi S. 1963 Crisafulli C. 1023 Cruciani G. 151 Curcio S. 1561 Cuypers M. 263 Czuryszkiewicz T. 1117 Czyzniewska J. 335 d'Espinose J.-B. 1173 da Rosa R.G. 461 Dal Pozzo L. 565 Dalea V. 1825 Dalloro L. 191 Danilczuk M. 311 Datka J. 439, 445, 453, 1971 Davido% M. 101 de Castro B. 879 de Jong K.P. 573 De Luca G. 2003 De Luca P. 279, 287 de Lucas A. 723, 731 de Oliveira E.C. 461 D e P a o l a M . 1561 De Rossi S. 715 De Simone P. 1775 de' Gennaro B. 1841 de' Gennaro M. 1775 Decrette A. 1571

2013

Decyk P. 1785 D6de~ek J. 23, 1817 Dellarocca V. 1419 Demontis P. 1931 Derewinski M. 1157 Dexpert-Ghys J. 1371 D i R e n z o E 1057 Diaz G. 815 Diaz I. 1083, 1267, 1275 Dimitrov M. 1245 Dong J. 431, 1553 Dorado E 707, 723, 731 Duc~r6 J.M. 1899 Dumitriu E. 167, 951 Dumrul S. 117, 1497 Egger C.C. 1149 Ei6 M. 1611 Elanany M. 1867 Elangovan S.P. 911 Eltekov A.Y. 1537 Eltekova N.A. 1537 Engler C. 1647 Erdmann K. 755, 1631 Ernst S. 23, 549 Escola J.M. 77 Evmiridis N.P. 807, 839 Fajula E 581, 1057 Fan W. 1553 Fayon E 383 Fechete I. 167, 951 Fejes P. 477, 503 F6rej G. 1907 Ferey G. 1091 Fernandes A. 1371 Fernandez C. 383, 391, 1915 Ferraris G. 715 Ferreira R. 879 Ferro O. 1729, 1877 Fiddy S.G. 1939 Fieres B. 223 Fierro J.L.G. 517 Figueiredo J.L. 879 Fila V. 1513 Finocchio E. 967 Fiorilli S. 143 Fitch A.N. 1963

Fitch ER. 1995 Flego C. 1603 Fleith S. 415 Flytzani-Stephanopoulos M. 1031 Fois E. 1877 Frache A. 151, 343 Freire C. 879 Fricke R. 1323 Fritzsche S. 1947, 1955, 1979 Fr6ba M. 1245, 1695 Frontera P. 45 Frunza L. 1323 Frunza S. 1323 Fry~ovh M. 1521 Fuentes S. 815 Fujiwara M. 1307, 1379 Fukushima T. 1833 Gabelica Z. 215, 1101 G/tbov/t V. 1817 Galarneau A. 1057, 1371 Gallegos N.G. 525, 1339 Galli E. 1705 Gamba A. 1877 Gammon D.W. 619 GaneaR. 1331 Garcia-Sanchez M. 959 Garrone E. 143, 207, 215, 1395 Gaudino M.C. 1031 Gautier S. 391 Gazzoli D. 715 G6d6on A. 1173 Geidel E. 1647 Gener I. 1679 Geobaldo E 143, 215 Gervasini A. 1091 Ghisletti D. 61 Ghorbel A. 903, 935, 943 Giammona G. 1165 Gianotti E. 1419 Gierczyfiska M. 351 Gil B. 439, 453, 1971 Gilles E 1687 Giordano G. 39, 351,477, 503,581,831, 1561 Girard S. 1907 Giuffrida V. 1023 Gleizes A. 1371 Golembiewski R. 1631

2014

GomezA. 815 Gonzfilez E 1253 Gonzfilez-Pefia V. 1283 Gonzfilez-Velasco J.R. 847 Goursot A. 1899 Gramlich V. 415 Grange P. 823 Griboval-Constant A. 1133 Grizzetti R. 45 Gualtieri A.E 1705 Guczi L. 1801 Guenneau E 1173 Guerrero-Coronado R.M. 1849 Guimon C. 167 Gulbifiska M. 1221 Guo H. 999 Gurin V. 815 Guti6rrez EJ. 1671 Guti6rrez-Ortiz J.I. 847 Guzmfin-Castillo M . L . 1039 Haberlandt R. 1947, 1979 Haddad E. 1173 Halfisz J. 927 Hailer G.L. 1261 Han D. 1237 Han X.W. 175, 1481 Han Y.-J. 1109 Hancs6k J. 587, 863 Hannongbua S. 1979 Hannus I. 927 Haouas M. 31 Hartmann M. 375, 549, 911 Haruta M. 611 Hayakawa M. 675 Hecht T. 549 Hedlund J. 1437 Heintz O. 1101 Hensen E.J.M. 271, 959 Hernadi K. 85, 541 Hernando M.J. 1253 Hervieu M. 1091 Hobzovfi R. 1521 Holgado M.J. 1387 Hol16 A. 863 Horvfith W.I. 587 Hoshino M. 53 Hosoda S. 871

Hrabfinek P. 1513, 1521 Hradil J. 1513 Hrenovic J. 1743 Hrubfi I. 247 HsiaoL. 117 Huang W.Y. 1481, 1545 Huang Y. 135 Hudec P. 855, 1587 Hulea T. 951 Hulea V. 951 Hunger B. 1647 Hunger M. 659 Huwe H. 1245 Ibrahim K.M. 1767 Ilhan Y. 159 Imre B. 927 Inagaki S. 53 Iorio G. 1561 Iovi A. 1655 Ito S. 557 Ivanova I.I. 659 Jackson D.B. 1125 Jacobsen C.J.H. 109 Jacquemin J. 699 Jale S.R. 1995 Jaramillo H. 1291 Jentys A. 1619 Jirglovfi H. 1713 Johannsen K. 109 Jolima~tre E. 1571 Joly G. 1679 Josien L. 415 Jost S. 1947 Joyner R.W. 511 Kabe T. 795 Kaliaguine S. 125, 367, 823 Kall6 D. 587, 863, 1047 Kameoka S. 557 Kanda Y. 739 Kang J.-K. 683, 895 Kantarll i.C. 799 Karetina I.V. 1627 K~ger J. 1955 Katada N. 815 Katovic A. 39, 477, 503, 1561

2015 Katranas T.K. 839 Kessler H. 951 Kevan L. 763 Khaddar-Zine S. 935 Khedher I. 943 Khodakov A.Y. 1133 Khvoshchev S.S. 1627 Kikot A. 1411 Kikuchi E. 53 Kikuchi S. 1833 Kim D.-S. 683, 895, 919 Kim S.-C. 683, 895, 919 Kim S.-J. 895, 919 Kim Y.S. 595, 887 King B.L. 1125 Kirby C. 135 Kiricsi I. 85, 541,927, 991, 1801 Kiss J. 541 Kita K. 557 Kitayama Y. 675 Kiyozumi Y. 231, 239, 255 Klass L. 117 KleitzE 1117 Klepel O. 1355, 1647 KnappM. 117 Knops-Gerrits P.P. 263, 1809 Knyazeva E.E. 659 Ko~ifik M. 303, 1505, 1513, 1521, 1663, 1713 Kodama T. 675 K6hn R. 1245, 1695 Komatsu T. 667 Koningsberger D.C. 1885 K6nya Z. 541, 927 Kooli E 255 Kornatowski J. 69, 399, 755, 1631 Ko~ovfi G. 247 Kosslick H. 1141, 1323 Kovaleva N.Yu. 691 Kowalak S. 39, 351 Kozyra P. 445, 1971 Krystl V. 1513, 1521 Kub~nek P. 533 Kubo M. 1867 Kubota Y. 15 Kuliyev A.R. 787 Kumar E 1229, 1307, 1379 Kunimori K. 557 Kuraoka K. 1307, 1379

Kurosaka T.

739

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2016

Magusin P. 959 Mal N.K. 1229, 1307, 1379 Manoilova O.V. 207 Mao Y. 1857 Marchese L. 151, 343, 1419 Marchetti S.G. 517, 525, 1339 Marinas J.M. 781, 1299 Mfirquez-Alvarez C. 1267, 1283 Marra G.L. 1891, 1963 Martinez A. 1015 Martinez E 1109 Martinez H. 167 Martucci A. 151 Marzke R.E 815 Mascarenhas A.J.S. 343 Maselli L. 565 Massiani P. 359 Matsukata M. 53 Mazzei Justin E. 967 M6hn D. 85 Melenovfi L. 1713 Melero J.A. 1109, 1181 Mellot-Draznieks C. 1907 Meloni D. 167 Melzoch K. 1663 Mendoza-Barron J. 1849 Meshkova I.N. 691 M&hivier A. 1571 Mezzogori R. 565 Mhamdi M. 935 Miao W.-R. 1007 Michalik J. 311 Michel D. 1355 Mignard S. 1679 Mikailov R.Z. 787 Mikulcovfi K. 627 Milanesio M. 1891 Millini R. 61, 191, 1923 Min K. 1481 Minchev C. 1245 Minic6 S. 1023 Mintova S. 223, 1465 Mitov I. 1245 Mitsuhashi M. 667 Miyadera T. 557 Miyajima A. 1595 Miyamoto A. 1867 Miyamoto J. 1595

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2017 Ovejero G.

1109

Pachtova O. 303 Pak C. 1261 Palacio L.A. 1291 P~l-Borb61y G. 1347 Palin L. 1891, 1963 P~ilink6 I. 85, 1801 Paneva D. 1245 Pang W. 1205 Papp H. 1355 Parker, Jr. W.O. 61 Parton R. 1809 P~rvulescu V.I. 367, 823, 1213, 1403 Pasqua L. 469, 1165, 1427 Passaglia E. 1705, 1729 Pastore H.O. 343, 461, 1419 Patarin J. 415 Paulidou E.G. 839 Paulin C. 1101 Pavel C.C. 295 Pazzuconi G. 1603 Pefiarroya Mentruit M. 207 Perathoner S. 477, 503 Perego C. 191, 1603 Perego G. 1923 P6rez-Pariente J. 1083, 1267, 1275, 1283 Perlinska J. 311 Perlo P. 1963 Perri C. 279 Pesquera C. 1253 Pestryakov A. 815 Petitto C. 581 Petkov N. 1465 Petranovskii V. 815, 1939 Philippot E. 1371 Piccolo C. 45 Pierro P. 1165 Pietrzyk P. 453 Pinnavaia T.J. 1075 Pino E 477, 503 Pirngruber G. 31 Pistarino C. 967 Pitsch I. 1323 Pizzio L. 1411 PodeR. 1655, 1825 Pode V. 1825 Pokorny J. 1663

Poladly P.E 787 Pomakhina E.B. 659 Ponomoreva O.A. 659 Pop Gr. 1331 Popovici E. 1825 P6ppl A. 375 Prestipino C. 3, 1963 Prihod'ko R. 271 Prins R. 31 Proke~ovh P. 627 Pullumbi P. 1907, 2003 Qiao w. 999 Quartieri s. 1877 Quoineaud A.A. 391 Radovet R. 1655 Rafuzzi B. 1737 Rains J.A. 117 Rangel M.C. 517 Ranjit K.T. 763 Rathouslo) J. 1067, 1457 Rees L.V.C. 1639 Renard G. 1473 Renaud A. 1679 Reschetilowski W. 1315 Rey E 651 Ribeiro M.E 359 Richer R. 135 Ricchiardi G. 3 Rigoreau J. 1679 Rives V. 1387 RizzoC. 191 Rocchia M. 1395 Rocha J. 319, 327, 1915 Rodriguez Delgado M. 207 Rodriguez E, T. 1737 Rodriguez J.M. 77 Rohlfing Y. 1067 Rojasov~ E. 855 Rombi E. 167 Romero A. 723 Romero A.A. 1299 Romero R. 707 Rosenholm J. 1117 Rozwadowski M. 755, 1631 Russo N. 2003 Rybarczyk P. 1141

2018

Sabo L. 855 Sacco, Jr. A. 117, 1497 Sadlo J. 311 Saha S.K. 771 Salas-Castillo P. 1039 Salou M. 231, 255 S~inchez P. 707, 723, 731 Sannino D. 1031 Sano T. 871, 1229, 1595, 1833 Sarijanov E.E. 787 Sariofglan A. 787 Sarv P. 1157 Sasata K. 1867 Sastre E. 1267, 1275, 1283 Sastre G. 1015 Sauer J. 643 Sauerbeck S. 549 Schenkel R. 69 Schmidt I. 109 Schmidt W. 159, 1857 Schneider P. 1587 Sch6nhals A. 1323 Schiith E 159 Schwieger W. 407 Scir6 S. 1023 Selvam T. 407 Seong K.H. 595, 887 Serban S. 1331 Serrano D.P. 77, 1671 SerreC. 1091 Shakhtakhtinsky T.N. 787 Shen D. 1995 Shi J. 1529 Shimizu K. 675 Sierka M. 643 Sierra L. 1291 Siffert S. 699, 1197 ilhanM. 101 Silva M. 879 Simon J.M. 1571 Simon-Masseron A. 415 Sinha A.K. 611 Siska A. 541 Sitkei E. 85 Smie~kovfi A. 855, 1587 Sobalik Z. 533 Sojka Z. 453 Solcov/t O. 1505, 1587

Sotelo J.L. 1109, 1579 Sponer J.E. 533 Spoto G. 3 Stara G. 1931 Staudte B. 439 Stein E.M. 117 Sterte J. 183, 1437, 1449 Stievano L. 359 Stocchi B. 191 Stockenhuber M. 511 Stucky G. 1109 Su B.L. 1213, 1403, 1687 Suboti6 B. 423 Suffritti G.B. 1931 Sugi Y. 15 Sugioka M. 739 Sullivan M.J. 117 Sychev M. 271 Sysel P. 1521 Szauer Gy. 863 Szegedi A. 1347 Szyrnkowiak E. 351 Tabacchi G. 1877 Takami S. 1867 Tamfisi A. 1801 Tanaka H. 1619 Tanaka S. 557 Tanchoux N. 1057 Taouli A. 1315 Taulelle E 1915 Testa E 423, 469, 1165, 1427, 1891 Thibault-Starzyk E 335, 1809 Thommes M. 1695 Thomson S. 159 Tibljas D. 1743 Tiddy G.J.T. 1149 Tkachenko O.P. 511 Tobdn-Cervantes A. 1039 Todinca T. 1655, 1825 Tolvaj G. 587 Tomishige K. 557 Torracca E. 1841 Tosheva L. 183, 1437, 1449 Toufar H. 1687 Trejda M. 1785 Trens P. 1057 Triantafillidis C.S. 807, 839

2019 Trifir6 E 831 Trombetta M. 975 Trujillano R. 1387 Tsagrasouli Z.A. 807 Tsiatouras V.A. 807, 839 Tsoncheva T. 1245 Tsubota S. 611 Tsyganenko A.A. 207 Tuel A. 943 Tun9 Sava~gi 6. 787 Tuoto C.V. 287 Turnes Palomino G. 199, 207 Tvarfi~kovh Z. 533 Uemichi Y. 739 Uguina M.A. 1579 Ulkii S. 799 Uozumi T. 871 Urquieta-Gonzfilez E.A. Ushakova T.M. 691 Utting K.A. 1473 ..

983

Valange S. 215, 1101 Valencia S. 651 Valente A.A. 327 Valkai I. 863 Valverde J.L. 723, 731 Valyon J. 991, 1639 van Bokhoven J.A. 1885 van der Eerden A.M.J. 1885 van Donk S. 573 van Grieken R. 1181 van Santen R.A. 271,959 van Steen E. 619 van Veen J.A.R. 271 Varga Z. 587 Vasalos I.A. 807 Vfizquez P. 1411 Vfizquez-Rodriguez A. 1039 Vercauteren D.P. 1987 Vezzalini G. 1877 Violante D. 287 Visinescu C.M. 367 Viterbo D. 1891 Vlessidis A.G. 839 Voltolini M. 1721 Vondrov~i A. 533 Vuono D. 279, 295

Wang A. 795 Wang G. 603 Wang G.-R. 999, 1007 Wang J. 1529 Wang Q. 747, 1545 Wang Y. 175, 1489, 1545 Wang Y. 795, 1007 Wark M. 1067, 1457 Warzywoda J. 117, 1497 Watson A. 1125 Wei C. 1205 Wei Y. 603 Weidenthaler C. 1857 Weitkamp J. 659 Wellmann H. 1457 West C.M. 117 Wichterlov~i B. 533, 1817 Wilkenh6ner U. 619 WlochJ. 755 W6hrle D. 1067 W6jtowski M. 1221 Wolf G.-U. 1141 Woltermann J. 595, 887 Woo J.Y. 117 Wu Z. 747 Xie G. 175 Xie P. 603 Xie S. 747 Xu L. 603, 747 Xu Y. 1489 Xue C.E 431 XueJ. 175, 1489 Yamada H. 311 Yamana K. 1229 Yan D. 1529 Yan X. 1237 Yan X.W 175, 1189, 1481 Yang X. 1205 Yang Y.K. 683 Yao N. 1261 Yao P. 795 Yashima T. 667 Yeramifin A.A. 525, 1339 Yllmaz S. 799 Yokosuka T. 1867 YuQ. 1545 Yuschenko V.V. 659

2020

Zadrozna G. 399 Zahedi-Niaki M.H. 125, 367, 823 Zanardi S. 1923 Zecchina A. 3, 199, 1963 Zemljanova G.Ju. 1627 Zhang A.M. 1237 Zhen K. 1205 Zheng S. 1619

Zhilinskaya E. 699 Zhu J.H. 175, 1189, 1481, 1489 Zibrowius B. 159 Zidek Z. 855, 1587 Zikfinova A. 1505, 1513 Zilkovfi N. 247 Ziolek M. 1785 Zukal A. 1067

2021

SUBJECT

ab initio calculation 1979 Acetonitrile 335 Acetophenone 667 Acetylene hydration 1047 Acid catalysis 715 Acid sites 383, 439, 1331 Acid sites characterization 247 Acidic zeolite catalysis 643 Acidity 167, 1157, 1315 Acidity of OH groups 1419 Acrylamide 511 Activated clinoptilolite 1655 Acylation 627 Ag exchanged ferrierite 1031 Agglomeration 707 Ag-zeolites 1963 A1 coordination 31 A1 distribution 1817 A1, BZSM-5 279 A1,B-LZ 287 AI/Ce mixed pillars 1253 27A1 MQ MAS NMR 31 Albumin 1537 AI-Cu pillared clays 683 Aldol condensation 667 Alkali metal ion 239 Alkylamine route 215 A1-MCM-22 23 A1-MCM-41 catalysts 1299 A1-MCM-48 755, 1229, 1631 A1PO-34 151 A1PO4-11, crystallization 135 A1PO4-40 1915 A1SBA, Hyperpolarized Xenon 1173 Aluminium coordination 1885 Aluminium distribution 23 Aluminosilicate, mesostructured foams Aluminum species in zeolites 391 A1-ZSM-12, synthesis 247 Aminoacids 1083 Aminopropyl 1395

INDEX

Ammonia 1687, 1713 Ammonia treatment 549 Ammonium 1737 Analcime 423 Anchoring 461 Aniline conversion 1299 Aniline methylation 659 Anomalous XRD 1963 Anthraguinone synthesis 1007 Aromatics 595, 887, 1213 Aromatics hydrogenation 1015 Aromatization 855 Atomic Force Microscopy 93, 1721 Au/Ti-MCM-41 611 Automotive exhaust gases 1611

1075

Base Catalysis 1473 Basic MCM-48 1481 Basic Pt-supported BEA 359 Basic zeolites 549 BEA 469 Beckmann rearrangement 191 Benzene 143 Benzene adsorption 1631 Benzene oxidation 477, 503 Benzothiophene adsorption 1579 Benzoyl chloride 1427 Beta zeolite 799, 1603, 1833 Beta-zeolite catalyst 1007 B-FER 1923 Bifunctional catalysis 603 Bikitaite 1877 Bimetallic catalyst 581 Bimetallic clusters 911 Binary adsorption 1595 Biological phosphorus removal 1743 B-LZ 287 B-MFI 1923 Borosilicate 1987 Boyd Adamson equation 1655 Br6nsted acidity 31,975

2022 BrSnsted sites 3 Brownian motion 1505 B-Silicalite 1891 Butane isomerization 715 Butene skeletal isomerization 573 Butenes oligomerization 831 CA-A zeolite 2003 Calcium oxide 175 Calorimetry 1101, 1627 CA-LSX zeolite 2003 Carbenium ion 643 Carbon nanotubes 541, 1237 Carbonates 359 Catalyst deactivation 715 Catalytic activity 675 Catalytic combustion 1023 Catalytic cracking 77 Catalytic cumene cracking 755 Catalytic oxidation 847, 1205 Catalytic wet oxidation 683 Catanionic Surfactant Self-assembly 1189 Cation selectivity 1841 CCVD 541 Cd ion exchange 1849 Cd-clinoptilolite 1047 CdS and CdSe nanoparticles 1457 Chabazite 1729, 1947 Chemically modified aluminum alkoxide 1283 Chemisorption 1545 Chlorinated VOCs 847 Chlorobenzene 1023 Cigarette 1489 Clay binder 707 Clinoptilolite 1663, 1713, 1849 Clusters 1939 CO adsorption 207 Co exchanged ferrierite 1031 CO hydrogenation 919 Coal Fly Ash 1229 Cobalt catalysts 1133 Co-Fe-Cu mixed oxide 1197 Colloidal faujasite 1267 Competitive ion exchange 1849 Computational methods 1867 Computer modelling 93 Confined liquid crystals 1323 Coordination of iron in MCM-41 1347

Copper 723, 815, 1101 co-template 399 Covalent anchorage 1067 Co-ZSM-5 453, 935 Cr-Beta 1449 Cr-MFI 839 Crosslinking 159 Crystal chemistry 1705, 1729 Crystal growth 93, 117, 223 Crystal growth modelling 423 Crystal structure 1729, 1891, 1923 Crystallization Kinetics 45 Cs-MCM-41, support 525, 1339 Cu-MCM-22 343 Cumene, cracking 1075 Cu-zeolites 1899 Cu-Zn complex 85 CuZSM-5 375, 445, 1971 Cyclohexane oxidation 879 Deactivation by coking 167 Dealumination 1817, 1833 Deep hydrodesulfurization 795 Defect 1595 Defect-free MEL, synthesis 61 Dehydrogenation 807, 839 Dehydroisomerization of n-butane 603 Delaminated zeolites 651 Delamination 69 Denitrogenation 1579 DeNOx 823, 1971 Density functional theory calculations 2003 Desertification 1751 Design of zeolitic materials 1867 Desulfurization 863, 1579 DFT 1899 Diesel cetane 1015 Diffusion 109, 1931, 1955 1,4-dimethylpiperazine 415 Dissolution reactions 1721 Drug delivery systems 1165 Dry gel 15 Dry gel conversion 53 Dyes in Si-MCM-41 1067 Dynamic light scattering 45, 223 Effective diffusivity coefficient Electron spin resonance 763

1825

2023 Eley-Rideal mechanism 557 Enzime modeling 85 Epoxidation 611,943 EPR spectroscopy 453, 699, 1899 ESR 1785 Esterification 787 Ethanol dewatering 1663 Ethylene 1627 Ethylene ammoxidation 935 Ethylenediamine 1923 EXAFS 1939, 1963 Extraframework iron 167 FAB-MS coke analysis 781 Fatty acids esterification 1275 Faujasite 879 Fe- introduction 477, 503 Fe ion-exchanged zeolite 557 Fe species 983 Fe-(A1)MFI 477, 503 Fe,A1-PILC 991 Fe/Ga,A1-MCM-41 1331 Fe/ZSM-5 983 Fe-BEA zeolite 469 Fe-containing molecular sieves 1785 FeH-zeolites 533 Fe-MCM-41 1347 Fe-montmorillonite 991 Ferrierite 199, 573 Fe-ZSM5 511 Field emission scanning electron microscopy 117 Fischer Tropsch synthesis 1133 Fischer-Tropsch reaction 525, 1339 Fluid Catalytic Cracking 807 Fluoride contribution 367 Framework flexibility 1885 Framework vibrations 263 Frequency-response spectra 1639 Friedel-Crafts acylation 799 FT-IR 3, 199, 207, 215, 975, 1419, 1687, 1785, 1809 Functionalization 1181, 1411 71Ga NMR 959 Ga/H-ZSM5 959 Galliation 1833 Gallium phosphates

367

Gallophosphate 415 Gas oil 587 Gas separation 1907 Gold surface 1497 Growth kinetics 1513 Guanidines 1473 Guest-host interactions 1323 ~H / 27A1 decoupling

1915

2H NMR 1687 H2-D2 exchange 959 H202 683 Hafnium silicate 319 H-Beta 383 H-bonding 143 Heats of adsorption 1995 Heavy metals 1825 Heterocyclic compounds 1647 Heteropolyacids 1291 Heteropolycompounds 1411 Hexene 747 Hierarchical porosity 1057 H-MCM-22 635 HY zeolite 967 Hybrid catalyst 771 Hybrid zeolite disc 231 Hydrazoic acid 1793 Hydrocarbons (C6) 1679 Hydrocarbons oxidation 1403 Hydroconversion of aromatics 581 Hydrocracking 771 Hydrodechlorination 927 Hydrogenolysis of tripropylamine 1221 Hydroisomerization 707 Hydrolisis of triglycerides 1561 Hydrophobicity 1039 Hydrotalcite 1197 Hydrothermal Synthesis 1379 Hydrothermal transformation 407 Hydrotreating 587 Hydroxymethylation 565 IMEC 1315 Immobilized complexes 85 In situ 13C MAS NMR 659 In situ Synchrotron XRPD 45 In situ-EPR/UV-Vis spectroscopy Indole adsorption 1579

1141

2024 115In MAS NMR 1355 In-MCM-41 1355 In-situ XAS 1885 Interaction strength 1647 Iodine Indicator Technique 1521 IR spectroscopy 143, 335, 439, 445, 453, 573, 927, 1603 Iron containing, SBA-15 1109 Iron in MCM-41 1347 Iron oxide 1237 Iron phthalocyanines in zeolites 1809 Iron supported catalysts 1339 Iron-doped mesoporous material 517 Iron-doped zeolites 517 Isobutane 823 Isomerization 747, 863 Isomorphous substitution 327, 1331 Isopropanol decomposition 39 ITQ-2 69 Jordan faujasite

1767

K10 montmorillonite 943 Kanemite 255 Kinetic model 303 Kinetics 787 Kinetics of ammonia sorption 1639 Knoevenagel condensation 549 Lanthanides 1371 Large pore zeolites 627 Layer growth 1505 Layered silicate 231, 239 Layered zeolites 69 LDH 1387 Levynite 287 LiLSX 1995 Lipase immobilization 1561 Lithification 1775 Location and motion 1687 Long chain olefins 999 Luminescence 1371 M41S materials 1695 Macrostructures 1437 Macrotemplate 183 Magnesia 1481 Manganese (III) complexes

879

Mass Spectroscopy 903 Mass transport limitations 1793 MCM-22 951 MCM-22, synthesis 53 MCM-36 951 MCM-41 795, 871, 911, 1083, 1101, 1221, 1237, 1315 MCM-41 mesoporous material 1261 MCM-41, modified 1245 MCM-48 1205 MCM-48, modified 1245 Mesoporosity 159 Mesoporous Aluminophosphates 1419 Mesoporous aluminosilicate 1157 Mesoporous films 1457, 1465 Mesoporous materials 1091, 1411, 1671 Mesoporous materials as support 1165 Mesoporous materials, catalysis 487 Mesoporous materials, reactivity 1427 Mesoporous molecular sieves 651 Mesoporous SBA material 1323 Mesoporous silica 1291, 1371 Mesoporous silica SBA-15 739 Mesoporous solid 1173 Mesoporous supports 1561 Mesoporous zeolites 1267 Mesostructured materials 1057 Metallic Cd introduction 1047 3-methoxy-4-hydroxybenzyl alcohol 565 2-Methoxynaphthalene 799 2-methoxyphenol 565 MFI catalyst 831 MFI zincosilicate, catalysis 351 MFI zincosilicate, synthesis 351 MFI zincosilicates 215 MFI-nanocrystals 159 Micelle Templated Silicas 1125, 1395, 1473 Microporosity 1587 Microporous alumina 1283 Microporous materials, catalysis 487 Microporous molecular Sieves 651 Microporous silica 255 Microwave radiation 1481 Middle distillate 771 M-MCM-41 1403 Mo-based sulfides 795 Modelling 1149 Modified crystals 1437

2025 Molecular dynamics 1877, 1931 Molecular modeling 61, 1907 Mono- and bimetallic MCM-41 1213 Monte Carlo simulation 1995 Montmorillonite 1253 Mordenite 335, 407, 731,815 Morpholine 151 Morphology control 399 MQHETCOR 1915 MQ-MAS 391 m-xylene 1267 N20 decomposition 557 Na-magadiite 407 Nanocomposite 191 Nanocrystalline ZSM-5 77 Nanoparticles diffusion 1513 Nanosized zeolites 223 Naphthalene 627, 999 Natural zeolites 1751, 1759 Nb sources 1363 NbMCM-41 1363 n-decane isomerization 911 n-hexane 855 Ni/Y-zeolite 919 Ni-A1 hydrotalcite 1387 Nickel metal distribution 919 Nitrogen adsorption 1587, 1631 Nitrous oxide decomposition 343 N-methylation 1299 NMR 391,469, 1173 N-Nitrosamines 1489 NO 895 NO adsorption 343, 375 NO reduction 983, 991 Noble metal loading 739 Non-hydrothermal synthesis 271 Non-steroid antinflammatory drugs NOx abatement 1031 Numerical simulation 1655

1165

Organic functionalisation 1125 Organised mesoporous alumina 1283 Ortho-benzoyl benzoic acid 1007 Oxide dispersion 1245 Paraffins 1571 Para-selectivity 635

Pb ion exchange 1849 Pd/H-ZSM-5 847 Pd-Fe/Beta zeolite 699 Pellets 279 Pentasil zeolites 1817 1-pentene isomerization 781 Periodic Hartree-Fock 1987 Periodic mesoporous silicas 1133 Phenol 619 Phenol methylation 635 Phenyl functionalised 1275 Phillipsite 1705 Phosphates 823 Phosphomolybdic heterpolyacid 1221 photocatalysis 1457 Photoreduction 763 Pillared clays 903, 1253 ct-pinene transformation 1291 Platinum 1023 Polyimides 1521 Polymerization 691, 871 Polyolefin 77 Pore condensation 1695 Pore expansion 1117 Pore size 1057 Porous aterials 1907 Potential function 1979 Preferred orientation 1465 Propane 839 Propane oxidative dehydrogenation 1141 2-Propanol conversion 755 Propene 611 Propylene 871 Protonic zeolites 975 Pt nano particle 1261 Pt/H-mordenite 863 Pt-Pd/USY 587 PtSn nano particle 1261 PtY PtY-zeolites 927 PtZSM-5 1801 QM/MM embedding 643 Quadrupole coupling constants 1987 Quantum chemical modelling 1971 Quanttma chemical molecular dynamics Quantum mechanics 1809

1867

2026 Radiolysis 311 Raman spectroscopy 263, 1809 Redox activity 533 Reduction by H2 or NaBH4 1801 Reformate 595 Re-hydration 1603 Rho 311 Rhodium 461 Rietveld method 1705 Ring opening 1015 Rock texture 1775 SAPO catalysts 781 SAPO-11 603 Saponite 271 SBA-1 1149 SBA-15 1109, 1181 SCR of NOx 723, 731 Secondary mesopores 1083 Secondary synthesis 1307 Selective catalytic reduction 895 Selective oxidation 1213 Self-transformation of glass 1553 Sepiolite 675 Shape-selective adsorption 1679 Si/A1 815 Silicalite 1595, 1679, 1891 Silicalite-1 191,303, 1521, 1979 Silicalite-1 layer 1513 Silicalite-1 membranes 1505 Silver 1939 Silver clusters 311 Simulation 1947, 1955 Single molecule spectroscopy 1067 Small-pore framework 319 Sodalite 255, 311 Sodium azide in zeolites 1793 Soil amendment 1759 Soil reconstruction 1751 Solid state transformation 231 Solid strong base 175 Solid-state ion exchange 935 Solid-state NMR 135, 383 Solid-state transformation 239 Sorption hysteresis 1695 Spectroscopic Investigation 1395 Spectroscopic techniques 3 Spin coating 1465

Stability 367 Steam reforming of HVOC 967 Steam-assisted conversion 15 Steamed HZSM-5 439 Structural changes 1801 Structural characterization 151 Structural stability 1857 Styrene production 517 Sulfate ion 1039 Sulfated Zirconium 903 Sulfidation 511 Sulfonic groups 1181 Sulfonic MCM-41 1275 Superbasicity 1545 Supported iron catalysts 525 Supported vanadia catalysts 1141 Surface Microtopography 1721 Surface modification 1427 Surface structure 117 Surfactant 1387 Synthesis modification 399 Synthesis optimization 39 Synthetic clays 271 Synthetic phillipsite 1841 TAPO's 125 Template removal 303 Templates 263 Textural properties 831 Thermal Stability 1157 Thiophene hydrodesulfurization activity Thioresistance 581 Time resolved IR spectroscopy 1619 Tin phosphates 1091 Ti-PILC 723 Ti-Sn-MCM-41 1379 Titanium oxo phosphate 1117 Titanium pillared clay 895 titanium-silicalite 109 Titanosilicates 327, 619 Titanosilicates, ionic Exchange 295 Titanosilicates, synthesis 295 TMI coordination 101 Toluene alkylation 951 Toluene oxidation 699, 1197 Toxic elements 1759 TPD 445, 1647 t-plot 1587

739

2027 TPR 359 Transalkylation 887 Transition metals 1205, 1403, 1857 Trichloroethylene decomposition 967 TS-1, mesoporous 109 Urban dump leachates 1737 UV spectroscopy 1603 UV-Vis spectroscopy 23, 199 UV-Vis-NIR spectroscopy 1419 Vanadium 327 Vanadium-K10 943 Vapor phase transport 431 Vesicular mesoporous silica V-MCM-41 1307 VOCs recovery 1671 VPI-7 431

1189

Wastewater treatment 1743, 1767 Wastewater, P removal 1743 Water adsorbed in chabazite 1947 Water adsorption 1631 Water and SO2 resistance 731 Water confined in zeolites 1931

W-band ESR spectroscopy

375

XANES 125 XPS 125 XRD 1785 Zeolite 3A 1663 Zeolite films 1437, 1497, 1553 Zeolite macrostructures 1449 Zeolite particles 1639 Zeolite Seeds, assembly of 1075 Zeolite X 763 Zeolite Y 461 Zeolites as additive 1489 Zeolite-supported magnetite 1825 Zeolitization 1775 Zeotype support 541 Zincophosphate 415 Zirconia 1545 Zirconocene 691 Zn-MFI 39 ZnZSM-5 855 ZSM-5 101,207, 279, 691, 807, 1497, 1571, 1619 ZSM-5 zeolite membrane 1529 ZSM-5, spheres 183

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2029

STUDIES IN SURFACE SClENCEAND CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium J.T.u University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1

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Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11

Volume 12 Volume 13 Volume 14 Volume 15

Preparation of Catalysts I.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, RA. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts II. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedingsofthe Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, R Grange, RJacobs and G. Poncelet Growth and Properties of Metal Clusters.Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t~ de Chimie Physique,Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedingsof an International Symposium, Ecully (Lyon), September 9-11,1980 edited by B. Imelik, C. Naccache,Y. BenTaarit, J.C.Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium,Antwerp, October 13-15,1980 edited by B. Delmon and G.E Frornent New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis,Tokyo, June 30-July4,1980. PartsA and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.l.Yermakov, B.N. Kuznetsov andV.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyfie, September 29-October 3,1980 edited by M. Ldzni~:ka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium,Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, R Meriaudeau, R Gallezot, G.A. Martin and J.C.Vedrine Metal Microstructures in Zeolites. Preparation - Properties-Applications. Proceedings of aWorkshop, Bremen, September 22-24,1982 edited by RA. Jacobs, N.I. Jaeger, RJir~ and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B6nard Vibrations at Surfaces. Proceedings of theThird International Conference, Asilomar, CA, September 1-4,1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets

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Preparation of Catalysts III. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings oftheThird International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, R Grange and RA. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J.Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by RA. Jacobs, N.I. Jaeger, R Ji~,V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, RQ., September 30-October 3,1984 edited by S. Kaliaguine andA. Mahay Catalysis byAcids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. BenTaarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure,Technology andApplication. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. HoEevarand S. Pejovnik Catalytic Polymerization of Olefins. Proceedingsofthe International Symposium on FutureAspects of Olefin Polymerization,Tokyo, July 4-6,1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985.Proceedingsofthe Fourth International Conference, Bowness-on-Windermere, September 15-19,1985 edited by D.A. King, N.M.Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerven# New Developments in Zeolite Science andTechnology. Proceedings of the 7th International Zeolite Conference,Tokyo,August 17-22,1986 edited by Y. Murakami, A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis andAutomotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11,1986 edited by A. Crucq andA. Frennet Preparation of Catalysts IV. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-laNeuve, September 1-4,1986 edited by B. Delmon, R Grange, RA. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by RWissmann Synthesis of High-silicaAluminosilicate Zeolites edited by RA. Jacobs and J.A. Martens Catalyst Deactivation 1987.Proceedings of the 4th International Symposium, Antwerp, September 29-October 1,1987 edited by B. Delmon and G.E Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine

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Volume 47 Volume 48 Volume 49 Volume 50

Volume 51 Volume 52 Volume 53 Volume 54

Methane Conversion. Proceedingsof a Symposium on the Production of Fuelsand Chemicals from Natural Gas,Auckland,April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by RJ. Grobet,W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987.Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of the IUPACSymposium (COPS I), Bad Soden a.Ts.,Apri126-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings ofthe Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P6rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Padl Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings oftheWorldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium,W~rzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo andT. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14,1989. PartsA and B edited by RA. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting,Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G.Anthony New SolidAcids and Bases.Their Catalytic Properties by K.Tanabe,M. Misono,Y. Ono and H. Hattori RecentAdvances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and RJ. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8,1989 edited by D.L.Trimm, S.Akashah, M.Absi-Halabi andA. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono,Y. Moro-oka and S. Kimura

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New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and F.Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts,Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A SpectroscopicAnalysis of Heterogeneous Catalysts. Part A: Methods of SurfaceAnalysis edited by J.L.G. Fie~ro Volume 57B SpectroscopicAnalysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Pdrot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals,Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba andT.Tatsumi Natural Gas Conversion. Proceedings ofthe Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPACSymposium (COPS II),Alicante, May 6-9, 1990 edited by E Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of CatalystsV. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990 edited by G. Poncelet, RA. Jacobs, R Grange and B. Delmon NewTrends in COActivation Volume 64 edited by L. Guczi Catalysis andAdsorption by Zeolites. Proceedings of ZEOCAT90, Leipzig, Volume 65 August 20-23,1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf~ired, September 10-14,1990 edited by L.I. Simdndi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, Apri122-27,1990 edited by R.K. Grasselli andA.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26,1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13,1991 edited by RA. Jacobs, N.I. Jaeger, L. Kubelkovd and B.Wichtedovd Poisoning and Promotion in Catalysis based on Surface Science Concepts and Volume 70 Exper;'ments by M. Kiskinova

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Volume85 Volume86 Volume87

Catalysis andAutomotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13,1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings ofthe 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission.Theory and CurrentApplications edited by S.D. Kevan New Frontiers in Catalysis, PartsA-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, E Solymosi and RT~t~nyi Fluid Catalytic Cracking: Science andTechnology edited by J.S. Magee and M.M. Mitchell, Jr. NewAspects of Spillover Effect in Catalysis. For Development of HighlyActive Catalysts. Proceedings of theThird International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimoto,T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings ofthe 3rd International Symposium, Poitiers, April 5- 8,1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. P(~rot and C. Montassier Catalysis: An IntegratedApproach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, RW.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals ofAdsorption, Kyoto, Japan, May 17-22,1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings oftheThird Natural Gas Conversion Symposium, Sydney, July 4-9,1993 edited by H.E. Curry-Hyde and R.F.Howe New Developments in Selective Oxidation II. Proceedings of the SecondWorld Congress and Fourth European Workshop Meeting, Benalm&dena, Spain, September 20-24,1993 edited by V. Cort6s Corberdn and S.Vic Bell6n Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited byT. Hattori andT.Yashima Zeolites and Related Microporous Materials: State of theArt 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by J.Weitkamp, H.G. Karge, H. Pfeifer andW. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12,1993 edited by J.Rouquerol, E Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger

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Volume 101 Volume 102 Volume 103 Volume 104 Volume 105

Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5,1994 edited by B. Delmon and G.I-. Froment Catalyst Design forTailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design forTailor-made Polyolefins, Kanazawa, Japan, March 10-12,1994 edited by K. Soga and M.Terano Acid-Base Catalysis I1. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori, M. Misono andY. Ono Preparation of CatalystsVI. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe Sixth International Symposium, Louvain-La-Neuve, September 5-8,1994 edited by G. Poncelet, J. Martens, B. Delmon, RA. Jacobs and R Grange Science andTechnology in Catalysis 1994. Proceedings of the SecondTokyo Conference on Advanced Catalytic Science andTechnology,Tokyo, August 21-26,1994 edited by Y. Izumi, H.Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.EVansant, RVan DerVoort and K.C.Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals andAIIoys by V. Ponec and G.C. Bond Catalysis andAutomotive Pollution Control II1.Proceedings of theThird International Symposium (CAPoC3), Brussels, Belgium, April 20-22,1994 edited by A. Frennet and J.-M. Bastin Zeolites: A RefinedTool for Designing Catalytic Sites. Proceedings of the International Symposium, Qu6bec, Canada, October 15-20,1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dqbrowski andV.A.Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings ofthe 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26,1995 edited by M.Absi-Halabi, J. Beshara, H. Qabazard andA. Stanislaus 11th International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 11th ICC, Baltimore, MD, USA, June 30-July 5,1996 edited by J.W. Hightower, W.N. Delgass, E. Iglesia andA.T. Bell RecentAdvances and New Horizons in Zeolite Science andTechnology edited by H. Chon, S.l.Woo and S.-E. Park Semiconductor Nanoclusters - Physical, Chemical, and CatalyticAspects edited by RV. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzi~ski,W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings ofthe 11th International Zeolite Conference, Seoul, Korea, August 12-17,1996 edited by H. Chon, S.-K. Ihm andY.S. Uh

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Volume 119

Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.F. Froment, B. Delmon and P.Grange Natural Gas Conversion IV Proceedings ofthe 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23,1995 edited by M. de Pontes, R.L. Espinoza, C.R Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings ofthe 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12,1996 edited by H.U. Blaser,A. Baiker and R. Pdns Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium,Antwerp, Belgium, September 15-17,1997 edited by G.I-. Froment and K.C.Waugh ThirdWorld Congress on Oxidation Catalysis. Proceedings oftheThirdWorld Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T. Oyama,A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings of the 7th International Symposium, Cancun, Mexico, October 5-8,1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings ofthe 4th International Conference on Spillover, Dalian, China, September 15-18,1997 edited by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings ofthe 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4,1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings ofthe 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11,1997 edited by T. Inui, M.Anpo, K. Izui, S.Yanagida andT.Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis andAutomotive Pollution Control IV. Proceedings ofthe 4th International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997 edited by N. Kruse, A. Frennet and J.-M. Bastin Mesoporous Molecular Sieves 1998 Proceedings of the 1st International Symposium, Baltimore, MD, U.S.A., July 10-12,1998 edited by L.Bonneviot, F.B61and,C. Danumah, S. Giasson and S. Kaliaguine Preparation of Catalysts VII Proceedings ofthe 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4,1998 edited by B. Delmon, RA. Jacobs, R. Maggi, J.A. Martens, R Grange and G. Poncelet Natural Gas ConversionV Proceedings ofthe 5th International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25,1998 edited by A. Parmaliana, D. Sanfilippo, F.Frusteri,A.Vaccari and EArena

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Volume 120A Adsorption and its Applications in Industry and Environmental Protection. Vol I: Applications in Industry edited by A. Dabrowski Volume 120B Adsorption and its Applications in Industry and Environmental Protection. Vol I1:Applications in Environmental Protection edited byA. Dabrowski Volume 121 Science andTechnology in Catalysis 1998 Proceedings of theThirdTokyo Conference in Advanced Catalytic Science and Technology,Tokyo, July 19-24, 1998 edited by H. Hattori and K. Otsuka Volume 122 Reaction Kinetics and the Development of Catalytic Processes Proceedings ofthe International Symposium, Brugge, Belgium,April 19-21,1999 edited by G.F. Froment and K.C.Waugh Volume 123 Catalysis: An Integrated Approach Second, Revised and Enlarged Edition edited by R.A. van Santen, RW.N.M. van Leeuwen, J.A. Moulijn and B.A.Averill Volume 124 Experiments in Catalytic Reaction Engineering by J.M. Berty Volume 125 Porous Materials in Environmentally Friendly Processes Proceedings of the 1st International FEZAConference, Eger, Hungary, September 1-4,1999 edited by I. Kiricsi, G. PdI-Borb61y,J.B. Nagy and H.G. Karge Volume 126 Catalyst Deactivation 1999 Proceedings ofthe 8th International Symposium, Brugge, Belgium, October 10-13,1999 edited by B. Delmon and G.F.Froment Volume 127 Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2nd International Symposium/Tth European Workshop, Antwerpen, Belgium, November 14-17,1999 edited by B. Delmon, G.F.Froment and R Grange Volume 128 Characterisation of Porous SolidsV Proceedings ofthe 5th International Symposium on the Characterisation of Porous Solids (COPS-V), Heidelberg, Germany, May 30- June 2,1999 edited by K.K. Unger, G. Kreysa and J.R Baselt Volume 129 Nanoporous Materials II Proceedings of the 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari, M. Jaroniec andT.J. Pinnavaia Volume 130 12th International Congress on Catalysis Proceedings ofthe 12th ICC, Granada, Spain, July 9-14, 2000 edited byA. Corma, F.V.Melo, S. Mendioroz and J.L.G. Fierro Volume 131 Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Natta and Ring-Opening Metathesis Polymerization byV. Dragutan and R. Streck Volume 132 Proceedings of the Intemational Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8, 2000 25th Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan edited byY. Iwasawa, N. Oyama and H. Kunieda Volume 133 Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings ofthe 3rd International Symposium, Oostende, Belgium,April 22-25, 2001 edited by G.E Froment and K.C.Waugh Volume 134 Fluid Catalytic Cracking V Materials and Technological Innovations edited by M.L. Occelli and R O'Connor

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Volume 137 Volume 138 Volume 139 Volume 140 Volume 141

Volume 142

Zeolites and Mesoporous Materials at the Dawn of the 21st Century. Proceedings of the 13th International Zeolite Conference, Montpellier, France, 8-13 July 2001 edited by A. Galameau, F. di Renzo, F. Fajula and J.Vedrine Natural Gas ConversionVl. Proceedings ofthe 6th Natural Gas Conversion Symposium, June 17-22, 2001, Alaska, USA. edited by J.J. Spivey, E. Iglesia and T.H. Fleisch Introduction to Zeolite Science and Practice. 2ndcompletely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, RA. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces. edited byA. Guerrero-Ruiz and I. Rodriguez-Ramos Catalyst Deactivation 2001 Proceedings ofthe 9th International Symposium, Lexington, KY, USA, October 2001. edited by J.J. Spivey, G.W. Roberts and B.H. Davis Oxide-based Systems at the Crossroads of Chemistry. Second International Workshop, October 8-11, 2000, Como, Italy. Edited by A. Gamba, C. Colella and S. Coluccia Nanoporous Materials III Proceedings of the 3~ International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by A. Sayari and M. Jaroniec Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium Proceedings of the 2nd International FEZA (Federation of the European Zeolite Associations) Conference, Taormina, Italy, September 1-5, 2002 Edited by R. Aiello, G. Giordano and F. Testa

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